The Bridge and Structural Engineer

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B&SE_Volume 46_Number 2_June 2016

The Bridge & Structural Engineer JOURNAL OF THE INDIAN NATIONAL GROUP OF THE INTERNATIONAL ASSOCIATION FOR BRIDGE & STRUCTURAL ENGINEERING

Steel & Composite Bridges

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The Bridge & Structural Engineer

Indian National Group of the International Association for Bridge and Structural Engineering ING - IABSE

Contents :

Volume 46, Number 2 : June 2016

Editorial • From the Desk of Chairman, Editorial Board : Mr. Alok Bhowmick iv • From the Desk of Guest Editor : Mr. Amitabha Ghoshal vi

Special Topic : Steel & Composite Bridges

2. Seismic Response Analysis of Train-Track-Bridge Interaction System to Evaluate Running Safety of Train in Case of High-Speed Railways Biswajit Pal, Anjan Dutta

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3. Design and Construction of Steel through Type Superstructure for Railway Bridges in North-East India Mohan Behl, Sumantra Sengupta

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4. Composite Decks for Long-Span Cable-Stayed Bridge 32 Mike Schlaich, Uwe Burkhardt 5. Selection of Appropriate Steel for Bridges in Indian Conditions 40 Sanjeev Kumar Garg 6. Rehabilitation of Steel Bridges: Design Aspects 50 Utpal K. Ghosh 7. Design Codes for Steel Construction in India–A Brief Review 57 Arijit Guha 8. Slender Section Dilemma – The Indian Perspective 66 V. Karthik 9. Full Scale Load Testing and Performance Evaluation of a Steel-Concrete Composite RoB S. Arul Jayachandran

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10. Evolution of Long Span Railway Bridges N. Bandyopadhyay

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11. Bridges in Steel Across River Hooghly and Bhagirathi in Bengal Delta Amitabha Ghoshal

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Contents

1. Futuristic Composite Bridges 1 Subramanian Narayanan

Panorma 

Office Bearers and Managing Committee – 2016

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List of ING-IABSE Publications

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Forthcoming Events of ING-IABSE

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Volume 46 Number 2 June 2016 i

The Bridge & Structural Engineer JOURNAL OF THE INDIAN NATIONAL GROUP OF THE INTERNATIONAL ASSOCIATION OF BRIDGE & STRUCTURAL ENGINEERING

September 2016 Issue will be a Special Issue with focus on TALL STRUCTURES (Tall Buildings, Chimneys, Silos, TV Towers, Cooling Towers, Transmission Towers)

SALIENT TOPICS TO BE COVERED ARE : 1.

2.

3.

4.

5.

6.

Structural System & Forms Green Building & Smart Cities Wind induced response & EQ resistant design of tall structures Critical Appraisal of Existing Codes & Standards (Indian as well as International) New Construction Materials and Techniques.

Case studies for Design, Construction and Rehabilitation

The Bridge & Structural Engineer JOURNAL OF THE INDIAN NATIONAL GROUP OF THE INTERNATIONAL ASSOCIATION OF BRIDGE & STRUCTURAL ENGINEERING

December 2016 Issue of the Journal will be a Special Issue with focus on PROFESSIONAL ISSUES CONFRONTING THE STRUCTURAL ENGINEERING PROFESSION IN INDIA SALIENT TOPICS TO BE COVERED ARE :

1. Ethical and Professional Issues Confronting Indian Engineers.

2. Need for Regulation of Engineering Profession in India.

3. New Challenges Facing Engineers & Engineering Organizations 4. Role of Civil Engineers in Built Environment 5. Vision 2025 for Engineering Fraternity in India Those interested to contribute Technical Papers on above themes shall submit the abstract by 1st November 2016 and full paper latest by 15th November 2016 in a prescribed format, at email id : ingiabse@bol.net.in .

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The Bridge and Structural Engineer

B&SE: The Bridge and Structural Engineer, is a quarterly journal published by ING-IABSE. It is one of the oldest and the foremost structural engineering Journal of its kind and repute in India. It was founded way back in 1957 and since then the journal is relentlessly disseminating latest technological progress in the spheres of structural engineering and bridging the gap between professionals and academics. Articles in this journal are written by practicing engineers as well as academia from around the world.

Disclaimer :

Editorial Board

All material published in this B&SE journal undergoes peer review to ensure fair balance, objectivity, independence and relevance. The Contents of this journal are however contributions of individual authors and reflect their independent opinions. Neither the members of the editorial board, nor its publishers will be liable for any direct, indirect, consequential, special, exemplary, or other damages arising from any misrepresentation in the papers.

Chair :

The advertisers & the advertisement in this Journal have no influence on editorial content or presentation. The posting of particular advertisement in this journal does not imply endorsement of the product or the company selling them by ING-IABSE, the B&SE Journal or its Editors.

Director, STUP Consultants Pvt. Ltd., New Delhi

Alok Bhowmick, Managing Director, B&S Engineering Consultants Pvt. Ltd., Noida

Members : Mahesh Tandon, Managing Director, Tandon Consultants Pvt. Ltd., New Delhi A K Banerjee, Former Member (Tech) NHAI, New Delhi Harshavardhan Subbarao, Chairman & MD, Construma Consultancy Pvt. Ltd., Mumbai Nirmalya Bandyopadhyay, Jose Kurian, Former Chief Engineer, DTTDC Ltd., New Delhi S C Mehrotra, Chief Executive, Mehro Consultants, New Delhi

Advisors : A D Narain, Former DG (RD) & Additional Secretary to the GOI N K Sinha, Former DG (RD) & Special Secretary to the GOI G Sharan, Former DG (RD) & Special Secretary to the GOI A V Sinha, Former DG (RD) & Special Secretary to the GOI S K Puri, Former DG (RD) & Special Secretary to the GOI

Front Cover : The cover picture shows the recently completed New Jubilee Bridge on the Naihati-Bandel section of Eastern Railway, forming a vital link across the river Hooghly, connecting the two important Railway corridors on both banks of the river. The bridge is one of the rare instances in India, where emphasis has been laid on the aesthetics, in preference to just material economy, and have open web girders continuous over three spans – first time for the Indian Railways. The graceful curve of the top chord is an improvement over the balanced cantilever Old Jubilee Bridge, appreciated by onlookers for its looks for more than 130 years.

R P Indoria, Former DG (RD) & Special Secretary to the GOI S S Chakraborty, Former Chairman, CES (I ) Pvt. Ltd., New Delhi B C Roy, Former Senior Executive Director, JACOBS-CES, Gurgaon Published : Quarterly : March, June, September and December Publisher : ING-IABSE C/o Secretary, Indian National Group of the IABSE IDA Building, Ground Floor (Room Nos. 11 and 12) Jamnagar House, Shahjahan Road New Delhi-110011, India Phone : 91+011+23388132 and 91+011+23386724 E-mail: ingiabse@bol.net.in, ingiabse@hotmail.com, secy.ingiabse@bol.net.in Submission of Papers : All editorial communications should be addressed to Chairman, Editorial Board of Indian National Group of the IABSE, IDA Building, Ground Floor, Jamnagar House, Shahjahan Road, New Delhi – 110011.

Advertising: All enquiries and correspondence in connection with advertising and the Equipments/Materials and Industry News Sections, should be addressed to Shri RK Pandey, Secretary, Indian National Group of the IABSE, IDA Building, Ground Floor, Jamnagar House, Shahjahan Road, New Delhi-110011.

Journal of the Indian National Group of the International Association for Bridge & Structural Engineering

June 2016

The Bridge & Structural Engineer, June 2016

The Bridge and Structural Engineer

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Volume 46 Number 2 June 2016 iii

From the Desk of Chairman, Editorial Board

India is currently the fourth-largest producer of steel after China, Japan and the United States. Rising domestic demand by sectors such as infrastructure, real estate and automobiles has put the Indian steel industry on the world map. The steel Industry is however passing through a challenging phase presently. The demand for steel is at its lowest.

Domestic consumption is severely affected due to lack of activity in infrastructure, as well as in the manufacturing space. The biggest challenge facing the domestic steel industry is to have the per capita steel consumption in India at par with the average global standards.

The new Government at the centre has, however, rekindled hope in the industry. The ambitious infrastructure projects and the thrust in manufacturing through the “Make in India” campaign are steps in the right direction. The plan for smart cities, improved road and rail connectivity by building highways, bridges, dedicated freight corridors and super fast rail corridors have huge potential to spur domestic steel demand. A new steel policy is on the anvil to facilitate increasing its production from 91mil-t per annum currently to 300mil-t per annum. The aspirations are to achieve this target by 2025 and at the same time improve consumption. The policy aims to develop the Indian steel industry into a global leader in terms of production, technology, quality and efficiency.

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Steel is increasingly present in the field of highway/ railway bridges, as the material is guaranteeing a perfect combination between technical performances, safety and sustainability. If steel is used in composite construction, an additional economic potential is developed with the use of cost-effective construction techniques and fast tracked construction procedures. However these supremacies are not truly reflected in the market share of steel consumption for bridges with small and medium spans. For composite bridges with small and medium spans the main competitors are ‘concrete bridges’ and more particularly ‘prestressed concrete bridges’. As drawback of the steel and composite solution, high cost, lack of adequate design experience for achieving economy and optimisation in design particularly with the new suite of available limit state code, lack of availability of structural steel plate of required quality & required thickness, lack of facilities for quality bridge fabrication, lack of availability of quality HSFG bolts and Shear Studs, poor workmanship in the field are considered as some of the primary obstacles in promotion of steel as a potential alternative to concrete. The situation is not helped by Indian Railways, who meet more than 70% of the steel industry’s transportation needs.

Railway designs of steel bridges, mostly carried out by RDSO, are not optimised enough and have lot of flab. This does not help to market the steel

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bridge option in competition with the reinforced or prestressed concrete option.

Keeping the above factors in mind, the editorial board thought it prudent to bring out this special issue titled “STEEL & COMPOSITE BRIDGES”, to take stock of the current design and construction trend on steel and steel-concrete composite bridges in India and also to know the world trend on steel and composite construction.

For this special issue, we are privileged to have Mr Amitabha Ghoshal as our Guest Editor. Mr Ghoshal is the ‘steel man’ of the industry, a well-known personality in India and an expert in the field of steel bridges. Apart from holding many important

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positions in the industry, he is also the Convener of steel bridge committee of Indian Roads Congress (B-5) for more than a decade. There could not have been a better choice of Guest Editor for us & we are thankful to Mr Ghoshal for his role in bringing this issue of the journal to this shape with authors from all around the globe.

Happy Reading !

(ALOK BHOWMICK)

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From the Desk of Guest Editor

It is heartening that the Editorial Board of The Bridge & Structural Engineer chose the theme of this issue to cover Bridges in Steelwork and SteelConcrete Composite construction.

Steel is universally accepted as the most trusted building material for diverse structures and has proved to be the longest lasting material used for Bridges. Steel is also accepted to have the lowest Life-Cycle cost, even though in developing countries like ours the use has been restricted in case of Roadway Bridges, due to its high initial cost.

There has been a spurt in use of Steel in recent times with emphasis on reduced construction time and the need for minimum interference with existing service facilities in urban areas. Steel-Concrete composite construction introduces optimal saving of materials by benefiting from the core strength of both these versatile materials, and ushers in reduction of construction cost. As a result the uses of composite girders have become widely adopted for urban transport structures like Flyovers, ROBs and Viaducts. The ever increasing need for longer obligatory spans at crossings of road corridors and widened highways have indicated use of composite box-girders, which also meet the demand for aesthetic structures that contributes to improved landscape.

This issue has therefore aimed to provide the readers with exposure to the ongoing developmental work, with the hope that such work done by engineers and academics will help them in future planning.

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The papers have been divided into subgroups covering Research, Design, Code Provisions, Case Studies and Historical Retrospective. The eminent contributors have been extremely cooperative and indulgent, despite there own busy schedules, and has allowed this issue to be brought out as per planned timeline.

Papers have been selected with an emphasis on the fundamentals to allow young designers to benefit from the materials provided. The design papers have included step by step approach, making adaptation by readers convenient. Numbers of recent and ongoing structures have been covered in the papers, to make the referencing easy for Indian engineers.

Papers included in the Research subgroup give an indication of the future developments and safety concerns that come in with the need for faster movements in transport sector.

Three of the design papers explain how in each case the design philosophy has evolved and the basic logic, that governed the selection, of the adopted approach. One paper covers the important theme of selection of appropriate grade of steel for service conditions that are extremely divergent in our country.

Two papers on codes, adopted in the country and widely followed for design work, will allow users to recognize the thought process followed in developing the recommendations and also the sectors, where changes are imminent.

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The case studies included in this volume will be of practical use and lays down how, the changes in design principle and new knowledge acquired, are affecting the ultimate product.

The historical perspective on Bridges across the main river, Hooghly, in Bengal delta brings out the logic adopted for development of Bridge designs over the last hundred years and more.

Steel, as a material, is still evolving with changes in metallurgy, acquiring higher strengths and superior properties. Appropriate and optimum use of the new grades of steel can only come with learned discourses and spreading of awareness. By bringing out an issue like this, the ING-IABSE governing body and its Editorial Board are enhancing the

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knowledge base amongst Indian engineers, and deserve thanks and appreciation of the entire industry. I am personally thankful to the Editorial Board and the ING-IABSE secretariat for giving me this task, which I have enjoyed immensely.

It is my sincere hope that the readers will find this issue of interest and future use, and that will be the greatest reward for the contributors, who have used their knowledge, experience and quality time to enrich this issue.

(AMITABHA GHOSHAL)

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Brief Profile of Shri Amitabha Ghoshal Shri Amitabha Ghoshal joined the largest steel bridge design and construction firm in India, The Braithwaite Burn and Jessop Construction Company (BBJ), in 1957 immediately after his graduation from Bengal Engineering College, Sibpur (now IIEST) and worked in the same organisation for 25 years. During this period he had the opportunity of working for almost all the long span Railway bridges in the country, like the Rail cum Road Bridges across Brahmaputra near Guwahati and across Godavari near Rajamundry, Railway bridges across rivers Rupnarayan, Mahanadi, Krishna, Yamuna, Jia Bharali, Ravi, Ulhas, Vapi etc.

He was entrusted with the restoration and rehabilitation of all the war damaged bridges in Bangladesh including the Hardinge Bridge across Lower Ganges and the Bhairab Bridge across Meghna river immediately after the formation of Bangladesh.

He had the opportunity of preparing the tender stage design and costing of the Second Hooghly Bridge at Kolkata (now Vidyasagar Setu), then slated to be the longest cable stayed bridge in the world.

During services with STUP for 35 years he has been responsible for diverse projects in India and 19 countries abroad. He was responsible for number of Steel bridges including the second Rail cum Road bridge across Brahmaputra near Jogighopa, large number of high level bridges for the new railway lines in Manipur and Mizoram and steel viaducts in composite construction in Malayasia. He has been member of the Board of Directors of STUP for ten years and entrusted with development of the organisation in new areas of specialisation. After retirement from active services he is continuing as Advisor to the organisation.

Shri Ghoshal had the rare opportunity of being involved in construction stage of major projects he had led during design stage, thereby allowing him to develop total understanding of the engineering logic.

He had been involved with composite construction in steel and concrete from very early in his career, having designed the first major composite construction viaduct spans in India for the roadway approach of Brahmaputra Bridge at Guwahati, and then applied the same principle for numerous projects in India and abroad. Shri Ghoshal has been assisting IRC in developing codes for steel and steel concrete composite bridges as the Convener of the concerned code committee under IRC for more than ten years.

Shri Ghoshal has been involved with professional and learned societies like IABSE, ASCE, IIBE, IEI, INSDAG CEAI, FIDIC from early days of his career. He had been member of Working Commission on Steel for IABSE for multiple terms and currently member of OSTRAC (Outstanding Structure Awards Committee) of IABSE. He has been felicitated as distinguished professional by IEI, ASCE and his Alma Mater IIEST. He has served as Chairman of Information committee of FIDIC ASPAC and been responsible for publication of ASPAC newsletter for four years. Widely travelled, in connection with his professional responsibilities, he has served actively in more than twelve countries.

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Futuristic Composite Bridges Dr. Subramanian NARAYANAN Consulting Engineer Gaithersburg, MD 20878 USA drnsmani@gmail.com

Summary Three futuristic composite bridges, viz., Bridge in a backpack [using FRP tubes filled with SCC (Self Consolidating Concrete) as the main load bearing elements], Hybrid-Composite Beam (with a FRP shell housing an arch of SCC which is tied at its ends by high strength galvanized pre-stressing strand) and ProCoBeam [with a shear composite dowel connecting bottom steel T- section (with a specially profiled web) with the top concrete T-section] are described, which result in accelerated construction as well as sustainable solutions.

Keywords: Bridges; Bridge-In-A-BackpackTM; FRP composite arch tubes; Fast-track; Green construction, Hybrid-Composite Beam; PreCoBeam; Sustainability.

1. Introduction Bridges are critical links in transportation networks that should be maintained to remain safe and functional during their service lives to enable personal mobility and transport of goods to support the economy and ensure high quality of life [Failure of any crucial bridge not only results in precious loss of lives, injury and huge property loss, but also affects the economy of the region. For example, it was found that the collapse of I-35W Mississippi River Bridge (which was used by more than 140,000 vehicles per day) resulted in huge economic loss to Minnesota, The Bridge and Structural Engineer

Dr. N. Subramanian earned his PhD from IIT, Madras in 1978. He worked in Germany as Alexander von Humboldt Fellow during 1980-82 and 1984. He has 40 years of professional experience which includes teaching, research, and consultancy in India and abroad. Dr. Subramanian has authored 25 books and more than 225 technical papers, published in international and Indian journals and conferences. He has won the Tamil Nadu Scientist Award, the Lifetime Achievement Award from the Indian Concrete Institute (ICI) and the ACCE(I)Nagadi best book award for three of his books. He also served as the past National Vice-President of ICI and ACCE(I).

USA –about $17 million in 2007 and $43 million in 2008]. But demands on most of the bridges have been increasing annually because of growing traffic volumes, higher loads, and aggressive environments (e.g. deicing salts, frequent freeze- thaw cycles, etc.). These conditions, coupled with the inadequate funding allocated for maintenance, have led to the accelerated aging and extensive deterioration of these critical structures [Subramanian, 2011].

Bridges all over the world are not aging gracefully.

The recent US Federal National Bridge Inventory showed that 65,605 bridges were classified as "structurally deficient" and 20,808 as "fracture critical”. Of those, 7,795 were both structurally deficient and fracture critical and have to be replaced sooner to avoid the risk of collapse. More than 30 percent of existing bridges have exceeded their 50year theoretical design life and are in need of various levels of repairs, rehabilitation, or replacement.

Bridges constructed 50 years ago in other countries like India should also be in a similar condition and have to undergo major repair or replacement. Similar to bridge rehabilitation, bridge replacement projects require engineering resources for design, a substantial and complex completion schedule, and considerable costs. Life cycle costs and other economic factors are usually considered when weighing rehabilitation versus replacement costs. Since most of these bridges are still in service and carry huge amount of traffic, Volume 46 Number 2 June 2016 1

conventional bridges or constructional methods can’t be used for their replacement. A fast-track method is necessary to quickly replace an existing bridge, in a way that does not disturb the existing traffic.

Three futuristic composite bridge technologies are described, which not only result in fast-track construction but also in greener bridges with expected life span of more than 100 years, as they are protected from or not vulnerable to corrosion.

2. Composite Arch Bridge System- Bridgein-a-BackpackTM Engineers at Advanced Infrastructure Technologies, Orono, Maine developed a hybrid concrete-composite bridge, which will cut the cost of replacing many existing bridges that also save time and raw materials.

This system was developed by Dr. Habib Dagher and associates of the University of Maine’s Advanced Structures and Composites Center in Orono, Maine, USA and is distributed and marketed by Advance Infrastructure Technologies (AIT), also located in Orono, Maine. This system could be used to repair or replace any existing deteriorating bridge, faster and cheaper. This Composite Arch Bridge System, known as Bridge-In-A-BackpackTM (BiaB), is a lightweight, corrosion resistant system for short to medium span bridge construction using FRP composite arch tubes that act as reinforcement and formwork for cast-inplace concrete. The concept of this system is shown in Fig.1.

rapidly deployable and do not require heavy equipment or large crews needed to handle the weight of traditional construction materials. These FRP tubes are bent around arch forms and infused with resin in the factory and removed from the form within hours and sent to site for ready installation (see Fig, 2a). The resin hardens in 4 hours, creating an arch that is twice as strong as steel. The weight of hollow arch for a 305 mm diameter and 15.24 m span is approximately 91 kg, and the same arch with 381 mm diameter would weigh 113 kg. Hence, two men can easily lift and adjust the arches to their final position, thus minimizing heavy trucking and eliminating heavy equipment (see Fig. 2b).

Fig. 1: Concept of the bridge in a backpack

The main advantage of this system is that the fiber reinforced polymer (FRP) tubes (made of carbon and/ or glass fibers set in marine grade Vinyl Ester Resin), which are made to arch are easily transportable, 2 Volume 46

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Fig. 2: (a) FRP tubes are transported in light trucks and (b) at site the tubes can be carried to the required place by hand labour (AIT)

The three functions of the FRP arch tubes are: (1) Along with the decking, they act as stay-in place form The Bridge and Structural Engineer

Fig. 3: Anchoring the FRP arches in the foundation

for concrete, thus eliminating the need for temporary formwork, (2) They act as “structural reinforcement� for concrete confinement (no steel rebars are used in the superstructure), and enhances concrete performance for safety and structural redundancy, and (3) Provide environmental protection for concrete, thus drastically reducing maintenance requirements.

The predominant structural components of the BiaB system are the arches (ribs) of the bridge made of FRP tubes, which are made rigid by filing them with concrete. These ribs are spaced at a regular interval and are configured to arch over the opening. The installation consists of constructing the foundation, setting the arches in the foundation at regular intervals, and anchoring the arches in the concrete footing (see Fig.3a to Fig. 3c). The first arch is set into place with one person at each end to ensure arches are set in perfect vertical and horizontal alignment. The next arch is set and braced from the previous arch, using the hardware provided by AIT to ensure alignment.

Restraining is accomplished by using AIT provided positioning hardware and fixing the arch end to the abutment at the specified location/elevation. Wooden spacers and ratcheting nylon straps can be used to maintain the specified arch spacing prior to and

(a) Pumping concrete into arches

during FRP deck installation. After all the required numbers of arches are anchored in the foundation, the corrugated FRP decking is attached to the arches using stainless steel fasteners along the spine of the arch (see Fig. 4). A battery drill with clutch may be used taking care not to strip holes when fastening the deck. A panel of deck weighs about 10.5 kg/m, and a 12.2 m panel may weigh around 128 kg, necessitating light equipment during deck installation. These sheets transfer the weight of passing vehicles to the FRP arches, after the bridge is commissioned.

Fig. 4: Attaching FRP decking to the FRP arches

The arches are then filled with self consolidating concrete (SCC), through a hole at the top of each arch.

Note that SCC is selected due to its high flowability, which does not require rodding or any vibration (see Fig. 5). SCC is used with High Range Water Reducers

(b) Funnel boxes direct flow and prevent overflow

Fig.5: Pumping of concrete into the FRP arches

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(a) Dry guniting of concrete deck

(b) Hand placement of RC deck

Fig.6: Concreting the Decking

(HRWR), or superplasticizers, to achieve high flowability. Shrinkage Compensating Admixture (SCA), a viscosity modifying admixture (VMA), a hydration stabilizer (retarder) and 10 mm pea stone aggregate are also specified in the mix (www.ctt.mtu. edu). The company AIT usually provides standard specifications for the concrete mix. A concrete pump truck with a boom that can reach the apex of the arch maybe used to fill the arches with concrete or a traditional concrete bucket may be used.

The deck may then be concreted and cured for 24 hours (see Fig.6) After the arches have cured for 24 hours, the headwall panels may be erected and braced into position (A variety of headwall options are available). Finally the structure may be backfilled using maximum lifts of 300 mm, with the installation of drainage, as appropriate (see Fig. 7).

After backfilling is completed to finish the grade, the guardrails and paving is done to complete the bridge (see Fig. 8). More details of installation of this Bridge-In-A-Backpack TM bridge may be found in AIT Installation handbook (AIT, 2011).

Fig.7: Attachment of Headwalls, Wing walls, and Backfilling

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In 2009, an 8.5 m long bridge of this type was first built by Advanced Infrastructure Technologies (AIT) in Maine, USA, in just 11 days, instead of the usual 2 months, with an expected life of 100 years. The “Bridge in a Backpack” serves three purposes: it is a stay-in-place form for poured concrete; provides exoskeleton reinforcement for existing bridges; and serves as a protective layer for concrete. This bridge is a greener alternative to concrete and steel construction and saves money, reduces fabrication time, lessens transportation costs, accelerates bridge construction, and dramatically reduces lifetime maintenance costs. Recently the State of Vermont Agency of Transportation conducted an assessment of this BiaB system, and found it to be greener than conventional bridge systems [SoV-AoT, 2014].

This patented FRP system has been tested with advanced structural characterization, predictive modeling, and fatigue testing, along with environmental durability tests for UV, fire, and abrasion resistance. All designs are engineered to

Fig. 8: Completed bridge

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exceed American Association of State Highway and Transportation Officials (AASHTO) load standards for single span bridges from 10 m to 20 m. In 2012, AASHTO developed a standard for the Design of Concrete-Filled FRP Tubes for Flexural and Axial Members [AASHTO, 2012, www. countyengineers.org]

strength is provided by steel and concrete. The encapsulating FRP shell provides maximum protection for the steel and concrete ensuring an extended service life and minimal maintenance.

This Composite Arch Bridge System has been used in 18 bridges in USA and beyond. This accelerated bridge construction technology has received the following awards: (a) 2011 AASHTO TIG Focus Technology award by the American Association of State Highway and Transportation Officials (AASHTO), (b) 2011 Charles Pankow Award for Innovation by the American Society of Civil Engineers (ASCE) (c) 2011 Engineering Excellence Awards by the American Council of Engineering Companies (ACEC), and (d) 2010 Most Creative Product Award by the American Composites Manufacturers Association (ACMA).

3. Hybrid –Composite Beam (HCB) Another innovative bridge system, called the Hybrid-Composite Beam (HCB®) and shown in Fig. 9, is a structural member similar to a prestressed concrete beam. The HCB® is a sustainable technology that combines the strength and stiffness of conventional concrete and steel with the lightweight and corrosion resistant advantages of fiber reinforced polymers (FRP). All of the strength and stiffness of the beam is derived from concrete and the high strength galvanized prestressing strand. The FRP (Fiberglass Reinforced Polymer) outer shell- made of quad weave fabric with fibers that are horizontal (0o), vertical (90o) and (± 45o), infused in an epoxy vinyl ester resin matrix, provides shear strength and encapsulates the tension and compression elements.

The compression element is concrete, which is in the shape of an arch and carries compressive load internal to the beam. The concrete used is SCC, which is pumped into a profiled conduit (generally an arch) within the beam shell (Hillman, 2012).

The tension element is the pre-stressing steel reinforcement that runs longitudinally along the length of the beam and ties the two ends of the concrete arch together. Essentially the HCB® is a tied arch in a fiberglass box where 90% of the The Bridge and Structural Engineer

Fig. 9: Concept of Hybrid-Composite Beam

Safety is inherently built into HCB® and the strength capacity has been confirmed by full scale testing and found to consistently exceed the code requirements. By optimizing the inherent qualities of the three components (FRP shell, SCC concrete in compression and tension reinforcement), the HCB allows construction professionals to build better structures that are cost competitive, stronger and require no additional training for their crews.

The underlying concept of the HCB® was conceived by the bridge design engineer Mr. John Hillman, PE, SE in the mid 1990's, who proposed that if a structural member consisting of a concrete arch were tied at the ends and encapsulated in a FRP shell, it would be lightweight, strong and corrosion resistant. During the next ten years, Mr. Hillman developed mathematical modeling of the concept, and tested various small scale prototypes.

After small scale prototypes proved successful, full scale, 10 m long beams designed for Cooper E-80 railroad loading, were tested, which proved the concept again. During 2006 and 2007, he developed a commercially viable fabrication process to build Volume 46 Number 2 June 2016 5

a 10 m railroad bridge. On November 7, 2007, the first known Hybrid Composite Railroad Bridge was tested at the Transportation Technology Center (TTCI) in Pueblo, Colorado, under live railroad loading consisting of two locomotives and 28 fully loaded (145,150 kg each) gondola cars.

Once again, the beams performed according to the model developed and refined over the years by Mr. Hillman.

Since then, several highway bridges have been built using the HCB®, the first one being the High Road Bridge in Lockport Township, Illinois (see Fig. 10). It was designed by Teng & Associates and constructed by Herlihy Mid-Continent Company in 2008. The 17.4 m long, single-span ridge consists of six HCBs supporting a conventional 200 mm thick reinforced concrete deck. The second is the 9.5 m span bridge over Route 23, Cedar Grove Township, New Jersey in 2009. The third highway bridge, the 165 m long, eight span Knickerbocker Bridge in Boothbay Harbor, Maine, was completed in June 2011. Details of these bridges and other HCB® bridges may be found from www.hcbridge. com. The design procedures and installation sequence of HCB may be found in Hillman, 2012.

HCB® received the 2010 Award of Excellence from Engineering News-Record.

The HCB® provides an effective method of replacement of deteriorating bridges. Some of the

inherent benefits of this system are [Hillman, 2012]: 

Straightforward Production: The HCB® is fabricated in a controlled shop setting without any special equipment, expensive molds or handling equipment. Glass fiber reinforcement and steel tension reinforcement placement is done quickly and efficiently, increasing product quality and reliability while reducing fabrication/ labor costs. Moreover, the HCB® does not mandate any complex or new design criteria or changes in construction methods.



Reduced Shipping Costs: An empty HCB® weighs only 10% of a comparable concrete beam making it possible to ship up to six beams on one truck as compared to one beam per truck for precast concrete beams.



Ease of Installation: The HCB® can be quickly installed at the site with only light duty cranes or excavators. HCBs do not require complicated bracing and diaphragms as compared to typical steel framed structures. The simplicity of installation provides advantages to small, local contractors as well as large construction firms.



Sustainable: With a composite exterior, the HCB® product has a high degree of protection and is inherently corrosion-resistant, offering service lives beyond 100 years with little or no maintenance. The HCB® also uses 60% to 80%

Fig. 10: HCB® High Road Bridge – Lockport Township, IL – 17.4 m span, Aug 2008

6 Volume 46

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The Bridge and Structural Engineer

less cement than a comparable concrete beam, thus resulting in lesser carbon footprint. Also, the closed mold, vacuum-assisted resin transfer method (VARTM) used for the manufacture of the composite shells is an environmentally friendly, zero VOC emission manufacturing process. 

Increased Safety: Since HCB® design is controlled by deflection limits; strength capacity typically exceeds code requirements by at least 10% to 60%. The significantly lower mass and high strength energy absorbing FRP shell results in a highly resilient structure under seismic loads.



Low Initial Cost: The HCB® utilizes highercost composite materials only for the shell of the beam. The primary strength and stiffness comes from much lower-cost concrete and steel. This combination results in a cost effective system that is superior to conventional materials and can compete economically on an installed, first cost basis.



Adaptable: The low weight of the HCB® makes it a perfect component for prefabricated modular construction. The rapid replacement of bridges is becoming more important with increasing traffic volumes. In the case of railroad installations, a completed HCB® bridge superstructure is one half the weight of a conventional precast concrete structure.

Prefabricated Railroad Bridge Modules, including the concrete deck, ballast curbs and fall protection can completely replace an existing bridge in a matter of hours, not days.

4. Precobeam The third system is a composite construction of steel and concrete. Unlike regular composite bridge construction, where a steel beam is connected to the concrete slab by means of welded shear connectors, in this system, half depth steel beam cut with specially profiled web is connected to the concrete T-beam, without any welding. Special reinforcement detailing is adopted for the shear transmission. This system is due to the research project called PreCoBeam (Prefabricated Composite Beam), funded by the Research Fund for Coal and Steel (RFCS) of the European Community in 2003, The Bridge and Structural Engineer

to develop a solution using prefabricated elements that would be price-competitive, durable, suitable for integral bridges and decks monolithically connected to a substructure (in order to minimize maintenance) and simple to erect. This concept of PreCoBeam is shown in Fig. 11.

Fig. 11: Concept of PreCoBeam

The result is a composite beam with steel T-sections that act as external reinforcement to a concrete top chord. Steel T sections are obtained from rolled steel profiles that are longitudinally cut, with a special shape, into two T-sections. The special shape of the cut embedded in concrete results in composite dowels, which allow for effective shear transmission between steel and reinforced concrete. A composite dowel is formed by a specific cut steel plate (steel-dowel) and the reinforced concrete that fills the recesses in the steel plate (concrete-dowel). Different types of cuttinggeometries (See Fig. 12) have been developed and were successfully introduced to the market. The Fin-shape (SA) offers high load bearing capacities (however, due to its asymmetric geometry, forces in different directions will result in reduced bearing capacities). In contrast to that the Puzzle (PZ) and Clothoidal-geometry (CL / MCL) have comparable bearing capacities for changing directions of forces due to their symmetrical shape. The Modified Clothoidal-shape (MCL) was found to provide the highest fatigue resistance for cyclic loads due to the smooth cutting radius. The cutting-process may be accomplished by thermal autogenous cutting technologies or by similar techniques that provide comparable material characteristics and fatigue behaviour for the cutting edge. The system was tested at ultimate, serviceability and fatigue limit

Volume 46 Number 2 June 2016 7

(a) Different types of cutting geometries

(b) Typical cutting-geometry for Clotholdalshape (MCL); grey parts are wasted

(c) Clotholdal shape after cutting

(d) Steel-sections alter cutting

Fig. 12: Different types of cutting-geometries and cutting pattern of Clothoidal shape

states in Europe.

The PreCoBeam concept combines the advantages of prefabricated prestressed concrete beams (the upper T of the section) with the steel girders (the lower T of the section). As mentioned earlier, this prefabrication method uses longitudinally cut-half rolled steel beams. A concrete top chord is then added to each element: this first layer is cast in the workshop. A second phase layer is finally cast in-situ to complete the cross section. There are many assembly types, but the two often used types are the Duo- PreCoBeam and Mono- PreCoBeam (see Fig.13). In Duo- PreCoBeam, two halved steel T-beam sections are positioned beside each other and filled with concrete, which ensures a consistent torsional inertia, a more slender section and that the shear connection is nearer to the neutral axis.

Mono- PreCoBeam uses only one halved steel T-beam and calls for a deeper reinforced concrete web. This option is more similar to a prefabricated concrete section, but with better bending moment resistance since the steel section acts like “external reinforcement”. Due to the broad variety of available rolled sections optimized solution for a particular project can be realized; for example using robust flanges for high stiffness or thick webs for high longitudinal shear forces. As composite dowels provide high fatigue resistance, they can be used in road and railway bridges, subjected to cyclic loads.

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Fig. 13: Duo- PreCoBeam and Mono- PreCoBeam

The structural components of a typical monoPreCoBeam are shown in Fig.14 and the components and detailing of typical composite dowel is shown in Fig. 15. More details about the structural behaviour of composite dowels, design recommendations, design rules, example design of Simmerbach Bridge, and construction details of PreCoBeam are given in the Design Guide, 2012. The static and fatigue design of continuous shear connections of PreCoBeam based on recent

The Bridge and Structural Engineer

research is provided by Hechler et al., 2008.

9

8

7

6

5

1) Steel flange 2) Steel web 10

3) Composite dowel

4

4) Precast concrete web

3

5) Prefabricated concrete plate

2

6) In situ concrete plate

1

7) In situ Longitudinal reinforcement 8) Precast Longitudinal reinforcement 9) Transversal shear reinforcement 10) Confinement reinforcement

Fig. 14: Structural Components of a Typical MonoPreCoBeam Notation of Composite-Dowel 1) Steel–Dowel 2) Concrete–Dowel 3) Reinforcement of Concrete–Dowel 4) Dowel–Bose 5) Dowel–Core 6) Dowel–Root 7) Dowel Top 8) Upper Reinforcement 9) Confinement Reinforcement

(a) Components of a Composite Dowel

(b) Typical reinforcement scheme for composite girders

Fig. 15: Components and detailing of composite dowel of PreCoBeam

The advantages of PreCoBeam bridges include the following: The Bridge and Structural Engineer



High safety factor for vehicle impact, especially for bridges with only two girders (shock),



Reduction of coating surface,



Shear connection without fatigue problems,



Simple steel construction nearly without any welding,



Sparse maintenance and easy monitoring

Bridge owners as well as general contractors evinced interest in this innovative construction method due to its effective concept, easy adoption, and the advantages listed above. After it was developed and tested in laboratories, more than 20 bridges (roadway, railway and pedestrian) have been built throughout Europe, demonstrating its viability as an alternative for short-span bridge construction [Zanon et al., 2014].

The 100 year old bridge in the Community of Pöcking which links the village with Lake Starnberg, Germany was reconstructed in 2004 using PreCoBeam technology. This bridge is 16.6-m-long, two-span deck with abutments and one intermediate pier between the tracks. The total deck width is 10.5 m. As the reconstruction was taking place over an existing railway line, a prefabricated solution to minimize traffic disturbance was essential. The entire deck width is supported by only three PreCoBeam elements. Rolled sections HE1000M in S460M steel grade (equivalent to w1000×300×350 in Grade 65) were cut into two halves and recomposed in small open box girders in full length of 32.5 m, and the connection was made with puzzle shaped composite dowels. Using the PreCoBeam technology reduced the construction time significantly, and the three elements were erected in one night [Zanon et al., 2014]. Next, 250 mm thick slab of concrete grade M35/45 was cast in-situ to solidify the three elements- it has to be noted that neither scaffolding nor formwork were required for the construction.

The PreCoBeam technology has also been applied to other bridges on Highway S7 in Poland between 2009 and 2012. Wide decks were used as continuous beams over three or four spans, with a maximum span of 18 m with a construction height of 830 mm (slenderness L/22). PreCoBeam elements were made out of coupled HE1000A/B/M in S355 (equivalent to w1000×300×272/314/350 in Grade 50 steel) with a Volume 46 Number 2 June 2016 9

slab width of 2.4 m, and the prefabrication was done directly on-site by the general contractor.

5. Summary Transportation infrastructure is necessary for the movement of goods and people across the country, and also reflects the economic development of a particular country. Many bridges constructed 50 years ago have exceeded their design age and have to be replaced. In addition, several bridges have deteriorated due to poor maintenance or corrosion. Failure of bridges (as witnessed by the I-35W Mississippi River Bridge in Aug. 2007) will affect the local economy significantly. Hence it is important to replace the ailing bridges quickly and efficiently. Composite bridges are being used extensively, due to their many advantages (Usual composite deck construction consists of steel girders which support a reinforced concrete slab. Composite action is achieved by connecting both materials by shear studs. Transverse bracing over supports provides lateral restraint). Three futuristic composite bridges viz., Bridge in a backpack [using FRP tubes filled with SCC (Self Consolidating Concrete) as the main load bearing elements], Hybrid-Composite Beam (with a FRP shell housing an arch of SCC which is tied at its ends by high strength galvanized pre-stressing strand) and ProCoBeam [with a shear composite dowel connecting bottom steel T- section (with a specially profiled web) with the top concrete T-section] are discussed in this paper, which result in accelerated construction as well as sustainable solutions. As these bridges are protected or prevented from corrosion, they have more than 100 years of active life and result in greener solutions, with significantly reduced CO2 emissions.

References 1. Advanced Infrastructure Technologies (AIT), Bridge-in-a-Backpack Installation Handbook, AIT, Orono Maine, March 18, 2011.

2.

ASHTO, 2012, LFRD Guide Specifications for Design of Concrete-Filled FRP Tubes for Flexural and Axial Members, American Association of State Highway and Transportation Officials, First edition, 2012

3. Design Guide, 2012-Prefabricated Enduring Composite Beams based on innovative Shear Transmission, SSF Ingenieure AG, Germany, 119 pp. [http://www.stb.rwth-aachen.de/ projekte/2005/INTAB/docs/PRECO_English. pdf] 4. HECHLER, O., BERTHELLEMY, J., LORENC, W. , SEIDL, G., and VIEFHUES, E., “Continuous Shear Connectors in Bridge Construction”, The 2008 Composite Construction in Steel and Concrete Conference VI, July 2008, Tabernash, CO, American Society of Civil Engineers, 13 pp.

5. HILLMAN, J.R., “Hybrid-Composite Beam (HCB®)-Design and Maintenance Manual”, Prepared for The Missouri Department of Transportation, Aug. 27, 2012, 41 pp. [http:// aii.transportation.org/Documents/BMDO/HCBdesign-maint-manual.pdf] 6. http://www.aitbridges.com/resources/ 7. http://www.countyengineers.org/events/ annualconf/Documents/2013%20Presentations/ FRP%20Tube%20Bridge%20Clemens.pdf

Acknowledgements

8.

The author wishes to acknowledge the following sources, from which the images are used in this paper: The images of the Bridge-In-A-Backpack TM (BiaB) system have been used from the presentation available at www.countyengineers.org. Images of Hybrid-Composite Beam are from www.nist.gov, and the images of PreCoBeam are from the Design Guide, 2012.

9. http://www.nist.gov/el/construction_ productivity/upload/7-HillmanNIST-MSN-AID-Wksh-New-TechHybridCompBeam-2010-05-18.pdf

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http://www.ctt.mtu.edu/sites/ctt/files/ resources/2013 bridge conference/ katenhus&matheny-bridgeinabackpack.pdf

10. http://www.hcbridge.com/ The Bridge and Structural Engineer

11. http://upcommons.upc.edu/bitstream/ handle/2099.1/8531/00.pdf?sequence=1 12. State of Vermont Agency of Transportation (SoV-AoT) , Assessment of The “Bridge In A Backpack” Bridge System From Advanced Infrastructure Technologies (AIT), Report 2014 – 12, Dec. 2014, 23 pp.

The Bridge and Structural Engineer

13.

SUBRAMANIAN, N., “Transportation Infrastructure Needs and Developments”, New Building Materials & Construction World (NBM & CW), Vol.17, No.3, Sept 2011, pp.106-124.

14. ZANON, R., BERTHELLEMY, J., SEIDL, G., and LORENC, W., “Short Span Solution”, Modern Steel Construction, March 2014, 3pp.

Volume 46 Number 2 June 2016 11

Seismic response analysis of train-track-bridge interaction system to evaluate running safety of train in case of high-speed railways

Biswajit PAL Post Graduate student Indian Institute of Technology, Guwahati Guwahati, Assam, India. biswajitjgec.1992@gmail.com

Anjan DUTTA Professor Indian Institute of Technology, Guwahati Guwahati, Assam, India. adutta@iitg.ernet.in

Anjan Dutta, born 1962, received his civil engineering degree from the Gauhati University.

His main area of research is related to Bridge Engineering.

Biswajit Pal, born 1992, received his civil engineering degree from the West Bengal University of Technology.

Summary This study is intended to evaluate the seismic response of a train-track-bridge interaction system along with assessing the running safety of train.

In regards to this, a rather simplified approach of introducing a complex, realistic bridge model into the coupled system is suggested and the system responses are found out with the co-simulation of MATLAB and ANSYS. Thereafter, in order to investigate the dynamic behaviour of the system, first a single span simply supported bridge of composite section made of concrete deck over steel box girder is taken and the effect of seismic excitation, train speed, bridge vibration and striking time of earthquake on the system responses are shown. Moreover, the running safety of train is also investigated with the help of a safety index, called derailment factor. Finally, a 3-span simply supported and a 3-span continuous bridge model are taken in order to show the effect of different status of train motion simultaneous with 12  Volume 46

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earthquake on train-bridge responses in case of two different bridge model. In addition to that, the effect of those two bridge models on the safety index of running train are also shown. Some useful conclusions are drawn thereafter, indicating the importance of considering the dynamics of bridge-train interaction system in seismic design.

Keywords: train-bridge interaction; seismic analysis; bridge model, co-simulation of MATLAB-ANSYS, running safety, dynamic response.

1.

Introduction

The dynamic effects of moving trains play key role in design of railway bridge. In case of high-speed railways, it would become inevitable to develop a coupled train-bridge model for effective realisation of the dynamic interaction of train and bridge.

Further, the design of high-speed railway requires more strict design conditions than a conventional The Bridge and Structural Engineer

railway since it needs large curve radius and fully closed operation, resulting in much more bridges than a conventional railway. For instance, 1059 km of the 1318 km long (80.5%) Beijing-Shanghai highspeed railway (HSR) line in China are on bridges [1]. Similarly, the total bridge length of the Northto-South Line from Taipei to Kaohsiung, in Taiwan is 73% [1] of the whole line. Increase in length and the number of HSR bridges increase the possibility of a train to experience an earthquake while crossing a bridge. According to previous research [2], the major factor that poses a threat to the safety of bridges is the seismic load. In case of railway bridges, it would be possible that a bridge itself may remain safe during an earthquake. However, while a train crosses a bridge and simultaneously experience an earthquake, the train may not remain stable due to added dynamic effect of bridge due to earthquake. Evidently, the running safety of moving trains over bridges during an earthquake receives a great attention in railway engineering due to the enormous use of bridges as railway supporting structures in case of high-speed trains. Two alarming accidents were already reported within last ten years. In 2004, a Shinkansen high-speed train derailed during the Niigata Chetsu Earthquake while running on viaducts at a speed of 200 km/h [3].

Further, in 2010, a Taiwan high-speed train which was running at a speed of 298 km/h derailed during the Jiasian Earthquake [4]. Therefore, it is required to bring forward the importance of seismic response of a coupled train-bridge interaction system which is currently attracting the attention of researchers all around the world.

Although the reasons for derailment of a train are varied and involve complexity of trainbridge interaction, this issue has been however considered by many researchers. Miyamoto et al.

[5] investigated analytically the safety of railway vehicles under the action of earthquakes using a three-dimensional (3D) simplified vehicle model.

Yang and Wu [6] carried out dynamic response analysis of train-bridge systems subjected to uniform seismic ground motions. 3D train-bridge model was used to investigate the stability of trains considering both running and rest condition using a simply supported bridge model. Further, for the Shinkansen system, efforts were been undertaken by the Japanese Railway Technical Research The Bridge and Structural Engineer

Institute to investigate the running safety of the high-speed bullet train [7]. Kim and Kawatani [8] established an analytical procedure in context to a 3D framework to simulate the dynamic responses of a monorail bridge-train interaction system under moderate earthquakes. Tanabe et al. [9] studied numerically the behaviour of the Shinkansen trains and railway bridges during earthquakes. A method was proposed to capture the interaction between wheel and the rail in both pre-derailment state and post-derailment state during an earthquake. Further, Yau [10] presented a computational framework of interaction analysis for a maglev train traveling over a suspension bridge shaken by earthquakes where the maglev train was considered as a series of maglev masses. He et al. [11] attempted to evaluate the seismic response of the Shinkansen bridgetrain interaction (BTI) system and the conclusions of the study are in agreement with those of Ref. [9]. Recently, Shan et al. [12] used more refined 3D vehicle in order to find out the system responses.

The authors found out the responses with the cosimulation of SIMPACK (a graphical multi-system dynamic software) and ANSYS. In spite of these works, dynamic characteristics of the BTI system subjected to seismic loads needs further attention for the development of simplified approach.

Thus, in order to address train-bridge interaction involving complex bridges, the existing formulations for BTI is modified. Such modification enables the use of existing finite element software like ANSYS for bridge modelling with all possible complexities and solution of BTI problem in MATLAB based code in an efficient manner. However, in order to investigate the dynamic behaviour of the system during earthquake, rather simplified bridge models, initially a simply-supported bridge and thereafter, a 3-span continuous and a three single span simply supported bridge models are taken. The system responses as well as the running safety of train are then found out corresponding to different cases in order to show the effect of earthquake, different motion status of train, dynamic effect of bridge and striking time of earthquake. In addition to that, it is also intended to show that how two different bridge models could affect the system responses and moreover, on the safety index of running train during earthquake.

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2. Dynamic model interaction system

of

vehicle–bridge

This section briefly presents the dynamic bridgetrain interaction (DBTI) model for the evaluation of dynamic response of bridge–train systems. The dynamic model of the vehicle–bridge interaction system is composed of a train subsystem and a bridge subsystem and the two subsystems are linked by the assumed wheel–rail interactions. Within DBTI, bridges are modelled using finite elements, whereas each vehicle in the train is modelled as a multi-body assembly of rigid bodies, lumped masses and springdashpot elements.

2.1

Vehicle model

The train is assumed to consist of a series of independent cars or vehicle element, each of which consists of a car body, two bogies, four wheelsets and the spring-damping suspension systems having 27 degrees of freedoms (DOFs) (Fig. 1). The modelling of a train vehicle is based on the following assumptions: i.

The car body, bogies and wheel-sets of each vehicle are considered as rigid components i.e., the elastic deformations of these components are not taken into consideration.

ii. The suspension systems that link the car body and the bogies (secondary suspension system) and also the bogies and the wheelsets (primary suspension system) are represented by springs with linear property and dampers with viscous property

Fig. 1: Three-dimensional vehicle model: (a) rear view, (b) side view, (c) top view, (d) sign convention

iii. In modelling of car-body and each of the bogies in a vehicle, 5 DOFs are considered as lateral ( yc,b), vertical (yc,b), roll (θxc,b), yaw (θyc,b ), and pitch ( θzc,b) displacement (Fig. 1). On the other hand, for each of the wheel-set, 3 DOFs are 14 Volume 46

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considered, which are lateral (yw ), vertical ( zw), roll (θxw) (Fig. 1). Here, superscripts “c”, “b” and “w” stand for car-body, bogie and wheel-set.

Further, it is also assumed that the wheels of the vehicle axles always remain in contact with the rails i.e., the axles are constrained by the rail and thus there is no relative displacement, velocity or acceleration between the wheel-sets and the railtrack. Therefore, 12 DOFs which are associated with the four axles, become dependent on rail and as a result independent DOFs of a vehicle body become 15, out of total 27 DOFs. With the consideration of above assumptions and by writing the dynamic equation of each component of a vehicle body [11], the global equation of motion of a vehicle body can be written as (1) where the subscript “v” corresponds to the vehicle, u and f are displacement and force vectors and M, C, K are mass, damping, stiffness matrices, respectively.

Here, the force vector f contains interaction forces acting on the vehicle sub-system [8,11] i.e., f is a time dependent vector which is coupled with both bridge and vehicle responses at a particular instant. On the other hand, the matrices M, C, K are constant i.e., these are time independent matrices.

However, it is required to mention that modelling of wheel–rail interaction is a key point of vehicle–bridge interaction problem. Among the various interaction model [13], Kalker creep theory of determination of tangential wheel-rail contact force and Hertz contact theory of determination of normal wheel-rail contact force [14, 15] are considered as the most accurate methods. Instead of accurate mechanics’ concept, this type of model has some shortcomings [16]. Moreover, many researchers [6, 11] concluded that consideration of a well-maintained straight track and under the action of ground motions of moderate intensity, the relative motion between the wheel and rail-track could be neglected i.e., the wheelsets can be considered to be rigidly attached to the rails. Therefore, in order not to violate the assumptions of rigid contact of wheelsets to the track and also the assumption of linearity for tracks and structures as mentioned in Ref. [6], the ground motion is normalized to have a lateral PGA of 0.8 m/s2.

The Bridge and Structural Engineer

−

−

f B and f v f B and f v 2.2 Bridge subsystem model

*

Finite element method is extensively used as a numerical tool in modelling of bridges. It is done by choosing finite elements of beam, plate or shell, the choice of such an element type depends on each bridge configuration. As an example, for modelling of beam-girder types such as for simply supported or continuous box girder types of bridge, 3-D beam elements have widely been adopted by many researchers [6, 15, 17]. In this paper, two-nodded 3-D beam element having six DOFs at each node has been taken to investigate the dynamic behaviour of bridgetrain interaction system. In case of steel composite railway bridges, the track is laid on the bridge without having ballast. Further, ignorance of elastic effect of the rail-track helps one to assume that there is no relative displacement between the rail-track and the bridge deck [16, 17]. In fact, this assumption is reasonable for the work in which the objectives are to study the dynamic behaviour of bridge and vehicle as mentioned in Ref. [13]. Therefore, it wouldn’t be required to treat the rail as a different sub-system as that of train and bridge in the interaction analysis of bridge-train system. Instead, the mass of rail and sleeper can simply be added with the bridge mass to incorporate it in system model. When a bridge is modelled by finite elements (FE), the equation of motion of the bridge system can be written as (2) in which, u = the displacement vector, f = the load vector and M, C and K are the global mass, damping and stiffness matrices of bridge and subscript “B” stand for bridge. When earthquake is not considered, the force vector fB in Eq. (2) contains the wheel-rail interaction force as well as gravity load of train [8, 11, 16].

2.3 Equation of motion of coupled Train-bridge model The coupled equations of motion of the train-bridge interaction system can be derived by combining the Eqs. (1) and (2) as done in Refs. [8, 11, 16], where the two sub-systems are coupled through the interaction force, (3) Here, and are the external load vectors obtained − − by transferring coupled components of f B and f v

*

to the left side of the equation, C B , K B respectively denote the equivalent damping and stiffness matrices of the bridge system taking into account the effect of the train, CBv and KBv are respectively the coupled damping and stiffness matrices of the BTI system.

These equivalent as well as coupled damping and stiffness matrices are formed from the coupled components of interaction forces which have been moved to the left side of system equation. In fact, such shift of the coupled force terms to the left side of equation make the equation solvable at an instant without an iteration. FE analysis of simple bridge and train system can be easily studied using this approach.

However, for a complex bridge, existing FE software should be used for modelling and generation of system matrices. Thus, whenever a complex and realistic railway bridge is considered, it is suggested not to transfer any force term from right to the left side of equation. As a consequence, the force vector then become only time-dependent. It is therefore required to concentrate only on the formation of the force vector at each time step. In fact, this can be done by simply calculating the interaction force along with train’s gravity load at each instant and by distributing these loads to appropriate bridge nodes.

Therefore, one can first make a bridge model using general propose software such as ‘ANSYS’, and import the mass, damping and stiffness matrices of the bridge to MATLAB based code along with the nodal co-ordinates and element connectivity matrices.

Using these imported matrices and position of train at an instant, it would now be possible to develop a MATLAB code through which the system force vector and finally the responses of the system could be found out. This approach, though makes the solution procedure iterative yet makes the formation of system matrices easier in case of a complex bridge model.

In order to place the forces in its suitable position in fB, nodal coordinates, element connectivity and nodal force distribution vectors are used. On the other hand, as far as fv is concerned, it can be formed by putting the specific interaction force term in its suitable DOF. These complete steps are arranged point wise in next section after the inclusion of seismic load.

2.4 Train-bridge interaction model subjected to ground motion When the two interaction systems defined in Eqs. (1)

f B and f v The Bridge and Structural Engineer

C*B , K *B

Volume 46 Number 2 June 2016 15

and (2) are subjected to an earthquake, the problem is solved by considering additional force input [8, 11].

Adopting ū as the input ground acceleration vector, the coupled equations of BTI system under seismic load can be written as (by combining Eqs. (1) and (2) with no shift of the coupled force terms to the left side of the equation)

have a lateral PGA of 0.8 m/s2 as stated in sec. 2.1.

Whenever the lateral ground motion is scaled down in terms of the PGA, the vertical ground motion is also scaled in a proportional manner. Here, the horizontal excitation of the earthquake is applied in the lateral (Y) direction of bridge.

..

..

FB* = FB − M B u g and (4) ..

where, FB* = FB − M B u g and and f v* = f v − M v u g . Thus, the analysis steps can ..be summarized as follows: i.

f * = f − M ug

v v v Construct bridge model in ‘ANSYS’ and export mass, damping and stiffness matrices as well as connectivity and nodal coordinate matrices of bridge from ‘ANSYS’ to ‘MATLAB’.

Construct also the time-independent vehicle mass, damping and stiffness matrices as well,

ii. At time t, knowing the position of each wheel of train, construct the force vector as defined in Eq. (4) in ‘MATLAB’ and solve for bridge responses, iii. With the new bridge responses, update the force vector and solve for vehicle responses, iv. Check for convergence of bridge and vehicle responses. If convergence limit is satisfied, go for the solution at next time t = t+Δt, otherwise go to step (ii) with the updated bridge and vehicle responses.

3.

Numerical study

In this section, several numerical examples are presented through which the effect of earthquake on bridge and train responses are investigated. In sec.

3.1, a simply supported straight bridge of 30 m span is taken and in sec. 3.2, a 3-span continuous (C.B.) and a three single span simply supported (S.B.) bridge having a span of 30 m each are considered.

The section of the bridge is assumed to consist of a concrete deck slab over the steel box girder with cross section as shown in Fig. 2. It is considered that the train consists of ten identical vehicle bodies which moves on a smooth rail track i.e., rail irregularity is not considered. The earthquake data recorded at the free field stations of Taipei during the 1999 ChiChi Earthquake in Taiwan is used. Both horizontal and vertical ground motion data are shown in Fig. 3. The horizontal ground motion is normalised to 16 Volume 46

Number 2 June 2016

Fig. 2: Half cross-section of composite bridge

3.1 Dynamic behaviour of single span simplysupported bridge At first, the typical acceleration time-histories at midspan of bridge in both lateral and vertical direction subjected to simultaneous passage of train and the earthquake excitation are shown in Fig. 2. As a validation, those are roughly comparable with those in Refs. [6, 11]. The time histories are evaluated for a train speed of 200 km/h. Observation of Fig. 4 clearly reveals that the bridge acceleration time-history follows the pattern of ground motion. However, in both the time histories, some contrariness are observed after the entry of train in bridge. The dynamic interaction effect of train and bridge may be influencing the response.

(a) The Bridge and Structural Engineer

(b)

Fig. 5 show that the mid-span acceleration increases with the train speed. On the other hand, though the vertical displacement also increases with speed as can be observed from Fig. 6 (b), no such variation can be observed in case of lateral displacement (Fig. 6 (a)).

The effect of vertical impact increases with speed and hence the vertical displacement while the lateral component is solely governed by the earthquake. The variation in vehicle body acceleration with speed can be observed from at Fig. 7. Increase in speed of train increases both the vertical and lateral acceleration of car body of the train.

Fig.3: Acceleration time-history of Chi-Chi earthquake: (a) Horizontal, (b) vertical

(a) (a)

(b) (b)

Fig.5: Maximum Acceleration of bridge at different speed: (a) Horizontal, (b) vertical

Fig.4: Acceleration time history at mid-span of bridge: (a) Horizontal, (b) vertical

3.1.1 Effect of train speed In order to see the effect of train speed on system responses, maximum value of responses of both bridge and train are plotted in Figs. 5-7 for a speed ranging from 50 to 200 km/hr. It is considered that the PGA of lateral excitation occurs at the instant when 5th vehicle is crossing the bridge. The observed responses are also found comparable with those in Refs. [6, 11].

The Bridge and Structural Engineer

(a)

Volume 46 Number 2 June 2016  17

Of the various types of derailment, the wheel climb derailment, which is most likely to occur, can be quantified by the index derailment factor. This derailment factor is often used by many researchers [3, 14, 15] as a measure of evaluating the running safety of train and is defined as [3] derailment factor = (b) Fig. 6. Absolute maximum displacement of bridge at different speed: (a) Horizontal, (b) vertical

However, the maximum values indicate no regular tendency, which implies the complexity of the trainbridge interaction. In fact, the response of train-bridge system during earthquake depends not only on the dynamic characteristics of bridge and train, but also on the characteristics of ground motion and the time of interaction of each car body with the bridge. From comfort point of view of passengers, the commonly accepted threshold values of vertical and lateral acceleration of car body for high-speed trains are 2 m/s2 and 1.5 m/s2 [15]. Evidently, the vehicle’s peak acceleration in the horizontal direction is beyond the comfort limits. In fact, these limits can be ignored when earthquakes occur.

Q P

(5)

where Q and P are the lateral and vertical contact forces at wheel-rail interface for an individual wheel. Fig. 8 show the variation in maximum values of derailment factor with speed. It is observed that increase in speed increases the maximum value of derailment factor and at a speed of above 140 km/h, it crosses threshold limit.

Thus, the train must decrease its speed to less than 140 km/h or even stop, in order to avoid derailment of train.

3.1.2 Effect of time of occurrence of PGA on vehicle responses and on running safety of train

Fig. 8: Maximum value of derailment factor at different speed

(a)

(b) Fig.7: Maximum Acceleration of car body at different speed: (a) Horizontal, (b) vertical

18  Volume 46

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In order to see the effect of bridge vibration on vehicle responses and derailment factor, two cases are considered. In the first case, it is assumed that the lateral PGA occurs when the 1st vehicle body is crossing the bridge and in the second case, this peak value occurs when the 10th vehicle is crossing bridge.

In order to avoid derailment, a speed of 130 km/h is considered here. The peak values of vertical and lateral acceleration of 1st and 10th vehicle bodies for the above two cases are presented in Table 1 along with derailment factors of the two car bodies. In the first case, when the 1st vehicle is on the bridge and simultaneously lateral PGA occurs, resembles to the second case, when 10th vehicle is on the bridge and The Bridge and Structural Engineer

3.2 Dynamic behaviour in case of Multi-span bridge model

PGA strikes.

Table 1: Effect of striking time of PGA on different properties

In this subsection, the effect of train dynamics on the seismic response of bridge-train system are investigated in case of two different bridge models.

In order to conduct such analysis, the following four cases are selected.

Response types Response Occurrence of PGA of specified of lateral excitation car body when 1st when 10th vehicle on vehicle Response bridge on bridge types Maximum 1st car-body 0,37 0,11 vertical 10th car0,54 0,35 acceleration body (m/s2) Maximum lateral 1st car-body 2,30 1,30 th acceleration 10 car7,10 3,18 (m/s2) body Derailment Factor 1st car-body 0,25 0,03 th 10 car0,83 0,37 body

Case-1: Self wt. of bridge only i.e., earthquake excitation is not considered.

Case-2: No train i.e., in the evaluation of bridge response, the effect of train load is not considered.

Therefore, in this case, earthquake is the only source of excitation.

Case-3: Train standing. The train is assumed to stand on the bridge in such a way that the whole bridge is occupied by the train and subjected to earthquake excitation.

Case-4: Train running. The train is running over the bridge with a speed of 130 km/h. Earthquake with lateral PGA strikes when the 5th vehicle is on the central span of the bridge while the train passing over the bridge.

However, it is the 10th vehicle of first case where the responses as well as derailment factor are observed to be the maximum. This can be attributed to the dynamic effect of bridge along with the vibration due to occurrence of earthquake. Actually, the bridge responses differently with the passage of each vehicle body of train. At the instant when the 1st vehicle enters the bridge, bridge just starts to experience the train load and vibrates accordingly. While for the 10th vehicle, since the bridge has already been subjected to the effect of first nine vehicles, it would be obvious that the bridge will vibrate in somewhat in a different manner. This variation in bridge vibration brings the differences in vehicle responses and in derailment factor while earthquake acts simultaneously.

3.2.1 Evaluation of bridge response: Corresponding to the four cases described above, maximum and RMS values of displacement and acceleration of bridge at middle of central span in both vertical and horizontal direction are shown in Table. 2 and Table 3. As expected, displacement and acceleration of bridge increase significantly when seismic load is introduced.

Table 2: Displacement of bridge in different cases Different cases Response types

Peak

RMS

Case-1

Case-2

Case-3

Case-4

C.B

S.B

C.B

S.B

C.B

S.B

C.B

S.B

Vertical Displacement (mm)

1,8

8,0

21,3

23,1

22,2

26,6

22,9

27,5

Horizontal Displacement (mm)

0,9e-3

0,4e-2

97,1

99,2

97

99,2

97,1

99,3

Vertical Displacement (mm)

0,8

5,4

9,4

10,3

9,9

11,1

10,6

11,9

Horizontal Displacement (mm)

0,37e-3

0,25e2

32,4

34,2

32,3

34,2

33,1

35,4

The Bridge and Structural Engineer

Volume 46 Number 2 June 2016  19

Further, by looking at values corresponding to the cases of No train, Train standing and train running, one could say that bridge response is very sensitive to the seismic excitation. This is proved by the fact that whenever earthquake is introduced, bridge responses amplify sig- nificantly while such notable

amplification is not observed when the train-bridge interaction effect is added over the seismic excitation.

On the other hand, by comparing the results of the cases: No train and Train standing, it would be possible to render that the train system has a damping effect on the acceleration response of bridge.

Table 3: Acceleration of bridge in different cases Different cases Response types

Peak

RMS

Case-1 C.B

Case-3

Case-4

S.B

C.B

S.B

C.B

S.B

C.B

S.B

Vertical Acceleration (m/s2)

0,20

0,37

0,92

1,09

0,70

0,82

1,06

1,32

Horizontal Acceleration (m/s2)

0,0035

0,004

1,42

1,67

1,00

1,27

2,12

3,36

Vertical Acceleration (m/s2)

0,08

0,13

0,21

0,22

0,18

0,19

0,35

0,39

Horizontal Acceleration (m/s2)

0,0012

0,0013

0,44

0,51

0,35

0,39

0,58

0,67

The responses in case of No train show larger maximum and RMS values than those of the case of Train standing. This might be due to the fact that, whenever the train is on the bridge, mass of bridge increases which in turn increases the time period of the system and finally leads to lesser responses.

Further, the dynamic responses of the bridge in the cases of Train standing and Train running indicate a difference. The responses appear in case of Train running are larger than those of Train-standing. It would be the dynamic effect of running train which make such differences.

3.2.2 Evaluation of train response and Derailment factor: Maximum and RMS values of acceleration of the car body in the horizontal and vertical directions for the above mentioned cases are presented in Table 4. It can be observed that the dynamic responses of the car body are in agreement with the bridge response tendencies. Earthquake excitation increases the responses of car body significantly compared to the case of no earthquake. Further, the Train running case gives a larger value of response than the Train standing case which can be attributed to the dynamic effect of running train. On the other hand, when earthquake strikes it’s not only the bridge and train 20  Volume 46

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Number 2 June 2016

responses which are increased. In fact, the derailment factor also increases when earthquake is introduced as can be seen from Table 4. However, if the train could be made stop when there is an earthquake, the derailment factor could be reduced to a substantial extent as can be confirmed from Table 4. On the other hand, comparative study of simply supported and continuous bridge model presented in sec. 3.2 reveals that continuous bridge model instead of simply supported model is used not only to reduce bridge responses, but in addition to that continuous bridge model also could be used to reduce vehicle responses and derailment factor as can be confirmed by looking at Table 2-4.

Table 4: Car-body acceleration and derailment factor in different cases Different cases

Case-1

Case-3

Case-4

Response types

C.B

S.B

C.B

S.B

C.B

S.B

Peak

Vertical acceleration (m/s2)

0,075

0,174

0,34

0,50 0,64

0,69

Horizontal acceleration (m/s2)

0,58 e-3

0,57 e-3

0,83

0,89 5,69

6,43

The Bridge and Structural Engineer

RMS

Peak

4.

Vertical acceleration (m/s2)

0,024

0,052

0,11

0,13 0,12

0,13

Horizontal acceleration (m/s2)

0,1e-3 0,09e-3 0,37

0,39 0,51

0,59

Derailment Factor

0,0011 0,0014

0,09

0,11 0,45

0,69

Conclusions

In this study, seismic analysis of train-bridge interaction system has been performed through the use of ANSYS-MATLAB interface. ‘ANSYS’ software has been used to model the bridge and the time independents components of the bridge model are then exported to a ‘MATLAB’ code in order to perform the analysis. In fact, the suggested way regarding this helps one to easily extend the study to a complex bridge model. Further, different types of bridge models have been considered in order to perform the seismic analysis of train-bridge system.

Initially, a single span simply supported bridge has been taken and the effect of speed on different system responses have been shown. Moreover, the effect of striking time of PGA on vehicle responses and on derailment factor have also been shown. Further, a 3-span simply-supported and 3-span continuous bridge have been taken and simultaneously the effect of earthquake, different motion status of train on system responses have been shown in tabular form.

Besides the advantage of extending the theory to the analysis of a complex bridge in a rather simpler way, it would further be possible to reach at following conclusions through this work: i.

Earthquake excitation contributes more to the system response than what comes from the train only. However, the complexity of train-bridge interaction and the added effect of earthquake over that could significantly affect the system responses.

ii. Increase in speed of train increases the vertical displacement of bridge, while the lateral displacement is solely governed by the seismic excitation. However, the speed of train affect the bridge acceleration in both the lateral and vertical directions. iii. Train response is also amplified with the addition of earthquake effect. Moreover, the speed of train has a significant effect on its response.

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However, there is no a regular tendency of increasing responses always with increasing speed. iv. Increase in speed of train increases the maximum value of derailment factor which serves as a controlling index of running safety of train.

Therefore, if the train could be stopped or its speed can be decreased to a substantial value, derailment of the running train could be avoided. v. The train can act as a damper to the bridge during earthquake. Therefore, a train stands on or run over the bridge during earthquake might not be the critical loading combination to the bridge always. vi. Continuous bridge instead of simply supported bridge, could be used to reduce the vehicle responses and derailment factor along with bridge responses.

5.

References

1.

GUO W.W., XIA H., ROECK D.G., and LIU K., “Integral model for train-track-bridge interaction on the sesia viaduct: dynamic simulation and critical assessment”, Computers & Structures, Vol. 112, 2012, pp. 205-216.

2.

DONG Z.F., GUO J., and WANG J.J., “Review of bridge collapse and prevention measures”, Shanghai Highways, vol. 2, 2009, pp. 30-33.

3.

XIA H., HAN Y., ZHANG N., and GUO W.W., “Dynamic analysis of train-bridge system subjected to non-uniform seismic excitations”.

Earthquake Engineering & Structural Dynamics, Vol. 35, No. 12, 2006, pp. 1563-1579.

4. JU S.H., “Nonlinear analysis of high-speed trains moving on bridges during earthquakes”, Nonlinear Dynamics, Vol. 68, No. 1-2, 2012, pp. 173–183.

5.

MIYAMOTO T., ISHIDA H., and MATSUO M., “Running safety of railway vehicle as earthquake occurs”, Railway Technical Research Institute, Quarterly Reports, Vol. 38, No. 3, 1997.

6.

YANG Y.B., and WU Y.S., “Dynamic stability of trains moving over bridges shaken by earthquakes”, Journal of Sound and Vibration, Vol. 258, No. 1, 2002, pp. 65-94.

Volume 46 Number 2 June 2016 21

MATSUMOTO N., SOGABE M., WAKUI H., and TANABE M., “Running Safety Analysis of Vehicles on Structures Subjected to Earthquake Motion”, QR of RTRI, Vol. 45, No. 3, 2004, pp.

116-122.

12. SHAN D., CUI S., and HUANG Z., “Coupled Vibration Analysis of Vehicle-Bridge System Based on Multi-Boby Dynamics”, Journal of Transportation Technologies, Vol. 3, No. 2, 2013.

8. KIM C.W., and KAWATANI M., “Effect of Train Dynamics on Seismic Response of Steel Monorail Bridges Under Moderate Ground Motion”, Earthquake Engineering & Structural Dynamics, Vol. 35, No. 10, 2006, pp. 12251245.

13. ANTOLIN P., ZHANG N., GOICOLEA J.M., XIA H., ASTIZ M.A., and OLIVA J., "Consideration of nonlinear wheel–rail contact forces for dynamic vehicle–bridge interaction in high-speed railways", Journal of Sound and Vibration, Vol. 332, No. 5, 2013, pp. 1231-1251.

9. TANABE M., MATSUMOTO N., TANABE Y., WAKUI H., SOGABE M., and OKUDA H., “A simple and efficient numerical method for dynamic interaction analysis of a highspeed train and railway structure during an earthquake”, Journal of Computational and Nonlinear Dynamics, Vol. 3, No. 4, 2008, pp.

041002.

14. ZHANG N., XIA H., and ROECK G.D., "Dynamic analysis of train-bridge system under multi-support seismic excitations", Journal of Mechanical Science and Technology, Vol. 24, No. 11, 2010, pp. 2181-2188.

7.

10. YAU J.D., “Interaction response of maglev masses moving on a suspended beam shaken by horizontal ground motion”, Journal of Sound and Vibration, Vol. 329, No. 2, 2010, pp. 171– 188.

11. HE X., KAWATANI M., HAYASHIKAWA T., and MATSUMOTO T., “Numerical analysis on seismic response of Shinkansen bridge-train interaction system under moderate earthquakes”, Earthquake Engineering and Engineering Vibration, Vol. 10, No. 1, 2011, pp. 85-97.

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Number 2 June 2016

15. ZENG Q., and DIMITRAKOPOULOS E.G., “Seismic response analysis of an interacting curved bridge–train system under frequent earthquakes”, Earthquake Engineering & Structural Dynamics, Vol. 45, No. 7, 2016, pp.

1129–1148.

16. ZHANG, N., XIA H., and GUO W., "Vehicle– bridge interaction analysis under high-speed trains", Journal of Sound and Vibration, Vol. 309, No. 3, 2008, pp. 407-425.

17. XIA H., and ZHANG N., "Dynamic analysis of railway bridge under high-speed trains", Computers & Structures, Vol. 83, No. 23, 2005, pp. 1891-1901.

The Bridge and Structural Engineer

Design and construction of steel through type superstructure for Railway Bridges in North-East India

Mohan BEHL Chief Bridge Engineer, Northern Frontier Railway mohanbehl13@gmail.com

Sumantra SENGUPTA Chief Manager (Design) STUP Consultant Pvt. Ltd., Kolkata, India sumantra.sengupta@stupmail.com

Mohan Lal graduated in Civil Engineering from NIT, Kurukshetra in the year 1983. He is presently in-charge of all bridges (including 1115 major & important) in Northeast Frontier Railway.

Till recently he was in-charge of planning & construction of tall bridges in Jiribam-Tupul New Rail project of Indian Railway. His field of expertise is construction of Bridges, Tunnels and Route selection in mountainous regions.

Sumantra Sengupta completed his graduation in Civil Engineering in the year 1990 and post graduation in Structural engineering in 1992, both from Jadavpur University. Since then he is working in STUP Consultants Pvt. Ltd., Kolkata.

His field of expertise is Bridges, Special structures and Seismic analysis.

Summary

Cantilever launching

The paper deliberates on the design and construction methodology of steel open web through type railway bridge superstructure in North-East part of India where the railway line passes through steep rolling hills of Patkai region, eastern trail of Himalaya. Large number of tunnels to penetrate the hills and tall bridges to connect the gorges are needed to lay the railway lines. The region falls under highest seismic zone area of India. Due to the varying height and considerably high piers in seismic prone area, multi modal analysis has been performed using peak ground acceleration (PGA) and acceleration response spectrum obtained from site specific seismic vulnerability study with the available data. Wind analysis has been performed followed by wind tunnel analysis.

1. Introduction

Keywords: Open web through type steel girder, fatigue, Site specific seismic vulnerability study, Multi modal analysis, effect of tall piers on bearing, The Bridge and Structural Engineer

Indian Railway has undertaken the construction work of connecting the capitals of the four North-East states, Manipur, Mizoram, Nagaland and Arunachal Pradesh with rest of the railway link in the country.

The work of Manipur and Mizoram has been started and the designs of 5 tall bridges in Manipur are already complete and 6 tall bridges in Mizoram are in progress.

Construction at site is progressing in full swing and presently piers are being constructed. The length of railway line in Manipur is about 111 km and that of in Mizoram is about 52 km. While the high mountains are penetrated by tunnel, the deep gorges between the mountain ridges are connected by tall bridges. The tallest of such bridges spans over a gorge at about 141 m above its bed level with an overall length about 703 m at rail level. With extensive study and discussion on possible alternative span arrangement of the bridges, Volume 46 Number 2 June 2016  23

considering the parameters like the length of span, type of span, location of the piers, constructability, maintainability and techno-economic factors it was finally decided that main superstructures will be steel open web through type girders of span up to 103.5m (c./c bearing). The critical issues of analysis and design of the superstructure involve preparation of site specific spectrum for seismic design, wind tunnel analysis to ascertain the actual behaviour of the structure in wind, fatigue analysis of superstructure with the latest provision of fatigue. Apart from IRS (Indian Railway Standard), other codes like IS (Indian Standard), IRC (Indian Road Congress), AREMA (American Railway Engineering and Maintenanceof-way Association), UIC (International Union of Railway) and Euro code provisions have been taken into account. The paper presents the steps followed for making these bridges as sustainable structures in a highly seismic zone at optimum cost.

2.

Special features of the bridges

The bridge configuration was selected depending on the special features of the bridges which are discussed here. A technical advisory group (TAG) comprised of a team of experts was formed by railway to discuss on the final configuration of the bridges.

should be of some optimal value and the weight of the overall structure as limited as possible to control the external applied loading on the slope.

Construction material: As the bridges, are located in the remote areas of the country and in spite of the vicinity of the national highway in some of the bridge sites, the general road condition is extremely poor. Supply of high quality construction material cannot be envisaged. Due to very narrow and winding nature of the approaches, the individual chord lengths which are to be transported to the site are also limited and accordingly the joints in superstructure are determined.

Tall Piers in high seismic zone: As the region is under high seismic zone light superstructure was preferred so that less force is drawn to the foundation during seismic and thus only steel superstructure has been considered for alternate bridge configuration study.

RCC or PSC (Pre-Stressed Concrete) superstructures which are comparatively heavier have been eliminated from the alternate bridge configuration study for the same reason. The piers are considered as RCC hollow section. The steel substructure was not considered due its large deflection at the superstructure level in wind which might affect the movement of train.

Geological aspect: From the geotechnical investigation study it was found that the bed material is comprised of mixed type of soil at top 5 to 8 meter underlain by highly to moderately weathered sandstone/ siltstone with almost nil RQD value. The minimum strength of the rock was found 5MPa. These geotechnical aspects lead to elimination of large span bridge like arch etc where the required bearing capacity is very high. The direction of dipping plane of rock however is either horizontal or opposite to the direction of slope which makes the stability aspect favourable.

Serviceable criteria: The serviceability criteria like horizontal angular deformation of rail at the junction of the two spans, vertical deflection of superstructure, vibration of the superstructure during movement of the train etc are critical factors in choosing the configuration of the superstructure. The deformation criteria have been described herein after. The vibration criterion has been taken care by keeping appropriate natural frequency of the superstructure. The detailed analysis of vibration during movement of the train with a range of velocity is beyond the scope of the present paper.

Slope stability: As the area falls under severe seismic zone and the foundations are located on the steep slope of the hill the slope stability is also a critical design issue. Under existing condition the slopes are stable as they are in the present configuration for a considerable period of time. When the construction is being done on the hills by disturbing the ground profile and additional loading is being applied on the hills in the form of foundation supporting the substructure and superstructure the stability of the slope is disturbed. It was targeted that the span arrangement of the bridge

Constructability: Due to the remoteness of the area and poor existing road condition the standard construction methodology need to be adopted. For large span arch configuration, tall towers of height more than 150 m along with cable cranes are needed.

For circular hollow pier slip form construction methodology can be adopted and for open web through type parallel chord superstructure, standard cantilever erection scheme can be adopted. Moreover the length of the members should also be limited for the ease of transport through the hilly winding road.

24  Volume 46

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The Bridge and Structural Engineer

Thus feasible construction method is one of the most important factors which governed the selection of bridge configuration.

Inspection & Maintenance of the structures: The inspection and maintenance of the structures are one of the most critical issues in design. In finalizing the configuration of the structure the same aspects have been kept in mind so that the components of the structures are easily accessible and easy maintenance is possible.

3. Criteria of selection of Superstructure – Different options Among the different configurations of the tallest bridge options like, simply supported through type open web girder, continuous superstructure, steel cantilever arch, balanced cantilever arch and cable stayed options were considered. The alternate options are shown in Plate – I.

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In order to keep the lateral deflection of the track with in the permissible limit the span length vs width of the deck becomes the guiding factor and for single track bridge the optimum span length was worked out. From this criteria large span structure like arch or cable stayed bridge configuration was ruled out. 103.5m span open web through type steel girder superstructure has been chosen as a final superstructure option which gives an optimal solution for the superstructure configuration. Simply supported option has been preferred to continuous one as a choice by Indian Railways from easier maintenance point of view.

All the bridges are designed for Broad Gauge single track carrying 25T maximum axle load. Spherical bearings have been adopted for all the structures.

The seismic restraint blocks are provided at both longitudinal direction and transverse direction to restrain the dislodgment effect.

Volume 46 Number 2 June 2016 25

In the Plate – II the artist’s impression of the tallest bridge is shown.

deflection is limited to 3mm which has been satisfied in the design.

Plate IV Artist’s impression of the tallest bridge Plate - II Vertical deflection: The vertical deflection of the mid-span of the superstructure has some limit as per different codes and are as mentioned hereinafter.

Mid-span vertical deflection of the superstructure (Refer Plate – III)

Change in angular deformation in horizontal direction: The horizontal deflection of mid-span, corresponding change in angular deformation (Refer Plate – V) and the change in radius of curvature are bounded by the code stipulation to maintain the riding comfort and to avoid the derailment of train with a specified velocity.

The check has been done for wind load case under loaded chord condition. The permissible value of the angular deformation is 0.0035 radian.

(a) As per IRS ∆ less than L/600 (clause 4.17 of steel bridge code) (b) As per AREMA ∆ less than L/640 (clause 1.2.5.6 of Chapter 15) (c) As per UIC 776 3R (Figure 1) ∆ DL = 140 mm for 100 m span

∆ LL less than L/900 to L/400 (Clause no. 8, Table-4)

But the deflection should be limited to L/900.

In the present case guideline given in IRS has been followed.

Plate V Track Twist: The twist of the rail track for wind load has (Refer Plate – VI) been calculated and is compared with the limits provided in the code and is found to be within permissible limit. The allowable angle of twist is .0025 ie 7.5mm vertical deflection over 3m length.

Plate - III Change in angular deformation due to vertical deflection: The change in angular deformation in vertical direction as stipulated in UIC (Refer Plate – IV) is also checked. This criterion is provided in order to limit the angular deformation of rail at the junction of two spans for smooth movement of the train. The angular deformation is limited to 0.005 radian and the

26 Volume 46

Number 2 June 2016

Following table gives the comparative deflection parameter values for superstructure with 5.5m and 8.5m c/c distance between two trusses. From the table it is found that 8.5m distance between the two trusses is the only acceptable option.

The Bridge and Structural Engineer

Table 1: Comparative deflection parameters for 5.5m wide and 8.5m wide superstructure

4.

Detail of the Superstructure

Width of the superstructure: The width of the superstructure or the centre to centre distance between the trusses is governed by the stability aspect of the superstructure and the maximum allowable track angular deformation at the two successive spans.

Accordingly it was found that minimum width of superstructure required is 8.5m instead of 5.5m which is standard width of superstructure of single track bridge. The floor system of the superstructure with 5.5m distance between the trusses and the footpaths at the two sides can be considered by inclusion of the footpaths as the part of the floor structural system. However the increase in distance between the trusses make the floor system simple and thus 8.5m width of the superstructure is the most preferable option.

Plate- VII shows the different cross section options of the superstructure.

the advantage of reduced shear carried by the two diagonal members at each section. In case of K truss same advantage is there but the number of verticals is more compared to the double warren truss. The different section sizes used in the different members are also furnished in the same plate.

Section sizes of the truss members: The section sizes of the truss members (Refer Plate – VIII) have been governed from cantilever erection for the end chord members and serviceable criteria or fatigue criteria for the central chords. Due to the above reason the section area of end chords and the central chords are very close to each other.

Configuration of truss: The configuration of the truss can be of Warren type, Pratt type or K type. In the present case the truss has been considered as double warren type as shown in the Plate – VIII which has The Bridge and Structural Engineer

Volume 46 Number 2 June 2016 27

Configuration of the truss sections: The chords are considered as open box sections (Refer Plate – VIII).

The top chord is closed at top and open at bottom whereas the bottom chords are open both at top and bottom. The diagonal members are also box sections similar to bottom chord. The vertical members are built up I section. The box sections are not considered completely closed as the same will make the connection at the joints complicated. The adequate gap in the open end of the box sections eases the joint connection through the gusset plates with rivet or bolts.

5.

Fatigue effect

The fatigue has been considered as per the IRS code 2010 provision. The earlier fatigue provision was for 10 million cycle of operation for all the members of the superstructure irrespective of the member’s 28  Volume 46

Number 2 June 2016

proximity of the wheel of the train. More over the allowable strength in fatigue was related to the ratio of minimum and maximum stresses and it corresponded to the total stress including both DL with the fluctuating load. The present provision differentiates the members with respect to their relationship and effects of wheel loading and as a result it gives more precise requirement. However the allowable strength in fatigue corresponds to the fluctuating load only and the overall effect has been found more stringent compared to the earlier code. The new provision makes the floor system members like stringers, cross girders etc which are directly in contact with the rail or close to the rail, more robust whereas the chords and the diagonals are comparatively lighter. The vertical members which carry the load from cross girders also become heavy due to the same reason. Overall it was found that the present provision of fatigue give rise to The Bridge and Structural Engineer

higher weight of the superstructure spans.

The fatigue strength is governed from serviceability criteria. Due to the above fatigue effect it was found that the grade of steel higher than E250 is not advantageous for the floor system where the members are fully guided from fatigue criteria. For main chords and the diagonals of the truss, where the members are partially guided by fatigue, the grade of steel is E410.

6.

Design of gusset

The gusset design has been performed taking in to consideration the effect of fatigue. It has been found that the some of the central span gussets have been critical in fatigue consideration. All the end gussets have been critical against cantilever launching consideration. The maximum gusset thickness has been considered as 25mm. The mid level node gussets have been considered as 16mm.

7.

Construction methodology

Parallel chord trusses with spans from about 60 m to 130 m are best erected by cantilever method. This would be particularly suitable in the present case, as the bridge crosses a very deep valley and any support from the bed of the valley would be costly both from the length of support involved as well as stability considerations.

Cantilever construction involves using one span as anchor and cantilevering the next span from it using temporary tension links and compression blocks. For starting the erection, where convenient, the first span can be erected on trestles supported on the ground below. Where the depth below the first span is itself high, a temporary span can be erected on the bridge approach and the first span can be cantilevered from this temporary span. After the first span is completed the temporary span is dismantled to be used as second or subsequent span.

A yard is set up at a convenient point not less than 150 m behind the abutment, where pre-assembly is done and materials are fed through a railway track for erection at the proper location. A crane erected on the top-chord and travelling on a temporary track on the chords, assembles the girder piece by piece, the materials being fed through a temporary railway track erected on top of the stringers.

In order to cantilever one span from another, temporary The Bridge and Structural Engineer

links are used at the top chord level and buffer blocks are used at the bottom chord level. During the process of cantilevering, there is a reversal of stresses i.e. the top chords which are normally in compression are subject to tension and the bottom chords normally under tension, are subject to compression. A check is made of the forces in all members at various erection stages to ensure that all permanent members are adequately strong to bear the erection stage stresses.

(This check is made at the design stage itself so that the members are so designed that the erection stresses are taken into account).

The joints (which are generally riveted or bolted) are completed as the erection proceeds. The last few panels of the girder are to be erected very carefully under controlled conditions as the span is very vulnerable at this stage. As soon the girder reaches the pier (or a landing bracket if one is provided in the erection scheme), the girder is partially secured and after the joints are made, the end of the girder is raised by jacking against the pier till the stresses in the connecting link between the anchor span and the span being cantilevered are gradually reduced to zero when the link becomes free. This also signals that the span is now simply supported on the abutment and pier (or the piers 1 and 2) as the case may be.

The erection method in case of a shorter span as anchor span is a little more complicated as it is not feasible to cantilever a 103.5 m girder from a 69 m girder. In Plate – IX and X the sequence of erection of the Bridge No. 164 in Manipur has been shown. A 103.5 m or 69 m girder is erected first on the ground behind the abutment. Using this as anchor span first 69 m span is erected through cantilever launching between Abutment A1 and Pier P1. After the girder reaches pier P1 it is properly supported. Keeping this combination of first anchor span of 103.5 m/ 69 m and next cantilevered span 69 m, the next 103.5 m is cantilevered from the combined assembly. Proper holding down arrangement at A1 location need to be arranged so that the junction between the first 103.5m span and next 69m span cannot move in upward direction due to unbalanced force. Once the 103.5m span between pier P1 and P2 is erected the erection crane is now taken back to the rear end of the anchor span (the 103 m girder) which is now gradually dismantled. Next successive spans can be cantilevered from the earlier 103.5m span already erected.

Volume 46 Number 2 June 2016  29

8.

Wind analysis

Wind force is estimated from the wind load code IS 875 (Part 3). The basic wind speed is considered in the zone as 50 m/sec. The probability factor or risk coefficient (k1) and the topography factor (k2) for all the bridges under consideration are taken as 1.08 and 1.0 respectively. The terrain, height and structure size factor (k3) varies with the height of the structures and the maximum value of the same in these bridges has been found to be 1.24. With the above factors it has been found that for the tallest bridge the maximum wind pressure comes out to be about 270 kg/sqm for the superstructure. For piers depending on the heights, the pressure will reduce. For the smaller piers in any bridge the height factor k3 is estimated by considering the height of the piers measured from the minimum bed level.

9.

Wind tunnel test

The theoretical response as estimated above has been compared with the actual response of the structures in wind by undertaking the wind tunnel test in the National wind tunnel facility at IIT Kanpur. The wind tunnel studies carried out on a scaled down aeroelastic model (Refer Plate – XI). The main objective of the study was to estimate the maximum along-wind and across-wind responses of the Piers of the bridge under simulated flow conditions with and without superstructures. Aero-elastic models of the prototype Piers were designed and fabricated at a model scale of 1:275. The superstructures have also been designed 30  Volume 46

Number 2 June 2016

and fabricated at the same model scale (1:275). The effects of fixed and free bearings are also taken in to consideration in the model. It was found that the model test results conform to the analytical results.

Aero-elastic model of bridge No. 164

Plate – XI

10. Conclusion The design of the superstructure of the bridges has been critical in many ways as the configuration of the overall bridge itself was governed from many critical aspects like pier height, seismicity of the area, geological and geotechnical aspects, access to the site, constructability, maintenance and cost.

The construction methodology of the bridges, transportation of the materials particularly in view of the winding approaches, the limitation of the size of the fabricated steel chord of the superstructure and erection of the superstructure affect the design The Bridge and Structural Engineer

aspect. All the above issues have been taken in to consideration with due importance to each issue and an effort has been made to achieve safe and sound structures with optimum cost.

11. Concerned agencies

-

TAG formed with acknowledged experts

- STUP Consultants Pvt. Ltd. – Consultant to Railway - IIT, Guwahati who was responsible for proof checking

The authors acknowledge the contribution made by various agencies that include

-

IIT, Kharagpur for site specific spectrum generation

-

-

IIT, Kanpur for Aerodynamic model analysis

NF Railway Engineers - Owner

The Bridge and Structural Engineer

Volume 46 Number 2 June 2016  31

Composite decks for long-span cable-stayed bridge Mike SCHLAICH Prof. Dr.sc.techn.

Technische Universität Berlin, Germany www.massivbau.tu-berlin.de schlaich bergermann partner www.sbp.de m.schlaich@sbp.de

Mike Schlaich, born 1960 in Cleveland-Ohio, received his civil engineering and Dr. degree from the ETH Zürich. He is managing director of schlaich bergermann partner and professor at the "Technische Universität" Berlin.

Summary From the beginning, the benefits of composite decks for long spans have been recognized and the design and detailing have become more refined over the years. This development is described in four examples: Second Hooghly Bridge in India (completion 1992), Ting Kau Bridge in Hong Kong (1998), Albert Canal Bridge in Belgium (2013) and last but not least Signature Bridge Delhi in India (2017). All the bridges were designed in the office of the authors.

Keywords: cable-stayed bridges, conceptual design, composite deck, signature bridges, bridge construction, India.

1. Introduction For spans of around 200 m to 1000 m, cable-stayed bridges are usually the most economic choice.

Compared to suspension bridges, cable-stayed bridges offer the advantage that they are selfanchored, even during construction and, therefore, do not required costly counterweights like suspension bridges do.

Furthermore they react with only small deformations to live loads.

However, with increasing span the axial forces in the deck of cable-stayed bridges increase. Therefore composite decks are a very advantageous solution.

Depending on the situation, the welded, bolted or even riveted steel grids can be brought to the site in 32 Volume 46

Number 2 June 2016

Uwe BURKHARDT Project Manager schlaich bergermann partner Berlin, Germany u.burkhardt@sbp.de

Uwe Burkhardt received his structural engineering degree from the University of Stuttgart, Germany.

Since 2001 he has been working for schlaich bergermann partner. He is project manager of the Signature Bridge in New Delhi.

segments on barges or trucks and be assembled by cranes from the bridge deck in the free cantilevering method. The concrete deck can be assembled using the steel grid as a scaffolding or, preferably, using prefabricated slab elements. In the latter case only the joints must be concreted in-situ. In regards to corrosion, the steel girders are easy to inspect and maintain compared to a hollow box section. Because the girders are open and aerated they tend not to rust and there is no interior corrosion.

2. Composite decks for cable-stayed bridges For long-span cable-stayed bridges the weight of the deck is a crucial factor since it dominates the design of the towers and foundations. Compared to the total weight of a 200-500 m long deck the concentrated load of the heavy vehicle is small. While the lightest possible deck is typically a orthotropic steel deck this option is significantly more costly than a concrete or composite deck and does not have the robustness, especially regarding fatigue when compared to concrete or composite solutions. Concrete decks are very robust and stiff but also extremely heavy and typically require post-tensioning to withstand the tension loads. The optimum solution for medium span cable-stayed bridges therefore seems to be a steelcomposite deck, where the steel sustains the tension loads and the concrete supports the compression loads.

The Bridge and Structural Engineer

Composite bridge decks typically use as the upper chord a concrete slab which is connected to the steel beams underneath via shear studs or equivalent measures. The ideal configuration is two cable planes along the edges of the deck slab. This way the concrete slab is put in compression in both directions.

In the transverse direction the cross girders with a typical spacing in between 4 m and 5 m act as simply supported beams with compression in the top (concrete) flange and tension in the bottom (steel) flange. In the longitudinal direction the horizontal component of the cable force, which gradually increases towards the masts, puts the concrete deck under compression as well. This bi-axial compression state saves reinforcement and increases the stiffness of the deck.

necessary. When using precast panels, which span inbetween the cross girders, no formwork is required at all. The joints between the precast panels will be filled with low-shrinkage concrete to create a jointless deck, rigidly connected to the steel girders. Not to mention that with "old" precast panels the creep and shrinkage effects can be strongly reduced.

Finally, small relative displacements between the concrete deck and steel girders and cracking of the concrete slab itself, lead to noticeably higher structural damping, compared to a steel deck or a fully prestressed and hence uncracked concrete cross section. This can be an important factor when dealing with wind dynamics.

Fig. 1: Typical composite deck

It is noteworthy that composite bridge decks are a relatively simple technology, which can be used all over the world. Compared to steel decks the welding requirements are lower for the steel beams since the concrete deck spreads, and hence reduces, the dangerous fatigue loads that originate from the live load on the bridge. The stress changes in the main and cross girders are significantly lower than in the deck plate and top stiffeners of an orthotropic deck.

While reinforced concrete is also deemed as a simple technology, a concrete bridge deck typically requires post tensioning, which goes along with high precision when placing the tendons and a significant amount of quality control during the tensioning process.

During erection, composite decks show several advantages as well. The light steel girders can be prefabricated in the workshop in large units that can be lifted with cranes and directly connected to the final cables, so that no temporary supports or formwork are The Bridge and Structural Engineer

Fig. 2: Installation of steel girders

3. Examples designed bergermann partner

by

schlaich

3.1 Vidyasagar Setu - Second Hooghly Bridge The Second Hooghly Bridge in India, now known under the name Vidyasagar Setu, has been designed Volume 46 Number 2 June 2016  33

in the 70s by Jörg Schlaich and Rudolf Bergermann under the supervision of Fritz Leonhardt. It bridges the Hooghly river and connects the suburb Howrah with central Kolkata. The total length of the bridge is 823 m with a main span of 457 m and a width of 35 m. The construction finally started 1978 but could not finish before 1992 due to logistic problems and other difficulties [1].

3.2 Ting Kau Bridge

Fig. 3: Second Hooghly Bridge (Vidyasagar Setu), Kolkata, India (© Roland Halbe)

The Ting Kau Bridge in Hong Kong is one of the few multispan cable-stayed bridges built so far [2].

An under-water hill offered the opportunity to built a central mast, which led to economical deck span dimensions and a cable supported deck with a length of 1177 m. The design was further governed by the typhoon wind loads in Hong Kong. Aerodynamic stability of the deck for wind speeds up to 95 m/s had to be achieved. A slender deck of only 1,75 m height supported by four cable planes reflects this.

The high wind loads also led to slender masts, shaped for minimum wind resistance, which are stabilised in the transverse direction by cables just like the masts of a sail boat. The bridge was completed in 1998, after only 44 months for design and construction.

Since weldable steel and HSFG bolts were not available at that time in India, only a riveted structure was possible and with the applicable standards an orthotropic steel deck was not desirable. Thus, Vidyasagar Setu was not only record span at the time, it also became the first long-span cable-stayed bridge with a composite deck.

Fig. 5: Ting Kau Bridge, Hong Kong (© Roland Halbe)

The deck consists of three main girders (see Fig. 4).

The two outer ones, with a distance of 29,1 m in the transverse direction, are supported by cables every 12,3 m. The central one redistributes local wheel loads. Every 4,1 m a cross girder connects the main girders. All girders are open I section with a depth of 2 m. The main girders are made of steel equivalent to S355 and the cross girders of a steel similar to S235.

The 23 cm thick in-situ concrete slab is connected to the steel grid by custom made shear blocks with loops.

Fig. 4: In-situ composite deck of the Vidyasagar Setu: isometric view (left) and during construction (right)

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As described above, the extremely high wind loads have been one of the driving parameters for the overall bridge design. In order to minimize the wind loads a very slender deck was required. This was achieved by reducing the span of the cross girders in transverse direction. Therefore, the 40 m wide deck was subdivided into two decks supported by two cable planes each. Now with a span of 18,8 m in transverse direction and a cable spacing of 13,5 m the overall depth of the deck could be reduced down to 1,75 m. With main spans between 400 m and 500 m and simply supported cross girders, a composite deck was the ideal choice.

The main girders are made of welded open L sections to achieve a flush outer surface of the bridge deck while the cross girders are made of welded I sections with a spacing of 4,5 m. Typically, 13,5 m long deck segments have been fully prefabricated in the workshop and delivered to site with barges, lifted from sea level with derrick cranes and connected to the previously installed segment with high strength friction grip bolt connections. The steel segments of the deck have been trial assembled in the yard to check the geometry and rectify fabrication errors.

The Bridge and Structural Engineer

This geometry was restored on site with undersized holes and fitting pins. This assured that no time was lost with lengthy surveys and rectification on site in the windy environment.

Precast reinforced concrete panels with a thickness of 24 cm and 30 cm closer towards the masts have been placed on the steel grid. These panels had a size of approx. 4,5 m by 4,5 m and are made of a concrete grade 60 (equivalent to C50/60 in Eurocode terms).

All the panels have been cast three to six months prior to their installation in order to reduce creep and shrinkage effects.

Fig. 6: Deck cross section of Ting Kau Bridge

One key element of a fast and trouble-free erection was the design of the joints between the precast panels. Not only the longitudinal and transverse reinforcement needed to be lapped within a condensed space of the joint, but also a rigid connection between reinforced concrete deck and steel grid below had to be achieved.

Fig. 7: Details at the joint between the precast panels and steel girders

The continuity of the reinforcement was achieved by two overlapping loops. These loops had to fit in between the rows of shear studs already welded to the top flanges of the steel girders in the workshop.

Furthermore, four loops in each panel have been equipped with vertical steel plates that temporarily support the precast panels on the steel girders (see Fig. 7, right). This way the loads could be introduced directly into the central web of the cross girders without creating torsion. This was an important aspect considering that the cross girders alone, without the stabilizing concrete slab, were very prone to lateral torsional buckling. For the connection between the steel girders and the concrete deck shear studs d=19 mm were used for the cross girder and shear blocks The Bridge and Structural Engineer

along the main girders.

Since no relevant experience with this new type of joint was available, full scale tests have been performed, mainly to ensure the serviceability criteria at joints. It could be proven that the crack width does not exceed 0,25 mm under service loads.

3.3 Bridge K03 over Albert Canal The bridge K03 forms part of a large infrastructure project in Belgium, Kempen North South Connection, which comprises several bridges, tunnels and underpasses. The design is characterized by the harp configuration of the cables and a relatively short back span. Its axis crosses the canal at a skew angle of 22°. The harp configuration was chosen in order to have a harmonic overall design with respect to the skewed alignment of the masts. The main span of 122 m has a light composite deck with precast slab elements placed on hollow box steel main girders.

The relatively short back span with no intermediate tension ties requires a heavy and stiff deck for the back span. This leads to a thicker concrete slab and to a stiff frame configuration between the masts and the back span girders in the cable planes. The back span deck is cast monolithically with the southern abutment to make the bridge semi-integral [3]. The design of this bridge was carried out by schlaich bergermann partner, on behalf of the contractor Jan De Nul during a successful design-and-build competition. While a humble landmark for this rather industrial area was desired, the economic aspect and durability was also of major significance, since the winning contractor not only constructs the bridge but also operates it for several years after its completion in 2013.

Fig. 8: Bridge K03 over Albert Canal, Belgium (Š Jan De Nul Group)

In contrast to the other bridges presented in this paper, the bridge K03 over the Albert Canal only carries a total of two lanes in each direction, so that the transverse span between the cable planes needed to be only about 21 m. This lead to a structural depth of 1,5 m for the cross girders and 1,25 m for the two Volume 46 Number 2 June 2016  35

main girders. The main girders of this bridge are formed of welded hollow steel sections compared to the I sections at the other three bridges described in this paper. The reasons for that were manifold. Since the main and cross girders are fully welded together and no bolted splices had to be foreseen the box girders could be welded air-tight so that no additional internal corrosion protection was required. The outer surface area of a box is typically smaller than that of an equivalent I section and there are no dirt accumulations on the bottom flange possible. Such advantages regarding maintenance made this solution the preferred one of the Belgian road authorities.

Last but not least the elevation from below is more fair compared to open sections and it matched architectonically better to the outline design provided by the client: a single box girder with a central cable plane. In the side span the main girders turn into post tensioned concrete upstands.

Fig. 9: Deck cross section of K03 over Albert Canal

The same design that has been developed for the Ting Kau bridge has been used for the joints between the precast panels and the steel girders. Thanks to the comparatively wide top flange of the main girders, which corresponds to the width of in-situ concrete joint (see Fig. 9), it was possible that the reinforcement of the cantilevering precast panels could be anchored in this joint to support the almost 3 m wide service path, without extending the cross girders beyond the main girders. This way a very slim and elegant edge with about 30 cm height was created and the image of a box girder with wings was emphasized.

A large portion of the steel grid, consisting out of main and cross girders, with a size of about 70 m by 30 m could be prefabricated in the workshop and transported to site with large barges. After completion of the concrete side span, the masts and abutments, the steel grid was lifted to temporary towers that rested directly at the quay walls during a remarkably short closure period of the channel. Once connected with the remaining steelwork at the mast and the opposite abutment, the cables with a spacing of 12 m and the skewed precast panels with a typical size of 9 m by 4

36  Volume 46

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m and thickness of 25 cm have been installed step by step beginning from the mast. The cantilever panels have been supported by temporary steel girders (Fig. 10).

Fig. 10: Installation of the precast panels(Š Jan De Nul Group)

Not only a composite deck for the main span was used, also the masts are made out of a steel cross section that was partially filled with concrete. To achieve a very slender mast which still allows interior access, high strength steel was used to carry down the main compression loads. The composite cross section was mainly used to restrain the steel plates from local buckling, so that the full yielding capacity could be used without additional stiffeners and in order to increase the robustness in case of cable failure.

3.4 Signature Bridge Delhi The "Signature Bridge" as the new landmark of New Delhi connects the city Ghaziabad and its surrounding across the river Yamuna to the inner city.

The dynamically shaped pylon consists of 2 inclined legs, which are rigidly connected to the deck girders and bend mid-way. The upper portion of the pylon anchors the back-stay cables as well as the main-span cables, arranged in a harp like manner. The tip of the pylon is created by a 30 m high steel-glass structure, which can be illuminated to create a landmark visible from afar at night [4].

Fig. 11: Virtual image of Ornamental Painting on the Pylon

The Bridge and Structural Engineer

A special detail of the bridge is that the self-weight of the pylon partially balances out the selfweight of the super structure through the eccentric location of its center of gravity with respect to the pivot point of the pylon footing. This serves well to reduce the load on the back-stay cables. The fact that the steel deck modules were bolted together on site instead of welded is an adjustment to the local method of construction.

The construction started 2010, meanwhile all the 13000 to of structural deck and pylon steel have been fabricated and shipped to the construction site. About half of the deck and pylon are erected. The completion is expected in 2017.

Fig. 12: Deck cross section of Signature Bridge Dehli

The asymmetric cable-stayed bridge has a main span of 251 m (corresponding to a symmetric bridge with two pylons of 500m span) and total length of 675 m.

Its composite deck carries 8 lanes (4 in each direction) and is approximately 35 m wide. The main span is supported by lateral cables spaced at 13,5m intervals.

Towards the approaches the same deck section continues with piers supporting it at 36 m intervals.

The height of the steel tower is approximately 150 m. The bridge deck consists of three main girders with a height of 2 m and cross girders at a spacing of 4,5 m, very similar to the Second Hooghly Bridge.

To provide sufficient space for 8 lanes, the two outer main girders, supported by cables, are spaced 32 m apart from each other. The emergency footpath has been placed on 1,5 m long cantilevers outside of the cable planes. All main and cross girders are welded I sections with plate thickness between 20 mm and 100 mm in grade S355. To save material the cross girders have a variable depth with a maximum value of 2 m in the centre and 1,4 m at the connection to the outer main girders. Similar to the Ting Kau Bridge, all splices have been designed as bolted high strength friction grip connections. In addition to that the outer main girders are in butt contact in order to transmit the compression induced by the horizontal component of the cable force via contact and not the bolts. The contact was achieved by machining the end faces of The Bridge and Structural Engineer

each girder which saved a lot of bolt and splice plate weight. No machining was required for the central main girder splices since it mainly redistributes local wheel loads.

A major difference between the bridges presented above and the Signature Bridge Delhi is that the latter is relatively low above water which is relatively shallow outside of the monsoon period. Therefore, it was possible to erect the entire deck on temporary trestles in contrast to the free cantilever method. Thus full composite action, also for dead load, could be achieved, so that the concrete slab is transmitting even more compression force as in the other cases.

This is reflected in the distribution of the concrete slab thickness. Outside of the cable-stayed part the precast reinforced concrete panels have a thickness of 25 cm which gradually increases to 35cm thick panels towards the pylon and ends in a 70cm thick in-situ portion around the pylon legs. The deck panels are made of grade 50 concrete (equivalent to C40/50 in terms of Eurocode) with a size of 4,5 m by 8m to minimize the amount of Longitudinal joints.

Due to the positive experience gained from the joint detail developed for the Ting Kau Bridge the same detailing has been used again. For the areas outside of the cable-stayed part the detail was further developed to allow for a central layer of reinforcement that became necessary close to the pier supports, where the negative bending moment with tension in the top slab governs the design.

The rigid connection between the concrete slab and the steel girders was achieved by shear studs with different diameters for main and cross girders. To transmit out-ouf-plane shear forces the end faces of the panels have been treated in such a way the cementitious grout has been removed and the coarse aggregate became visible. Furthermore, pockets have been foreseen in the end faces of the panels to transmit the significant in-plane forces safely.

4. Summary The numerous advantages of composite decks for long-span cable-stayed bridges have been discussed above: a comparatively light and robust solution can be achieved by using concrete and structural steel both in their ideal configuration: concrete mainly in compression and steel mainly in tension. The cost effectiveness of such a composite deck is further Volume 46 Number 2 June 2016  37

Fig. 13: Deck erection with precast elements (left), joints between precast elements and steel girder (center) and pylon erection (right)

Fig. 14: Schematic deck cross sections in the same scale from top to bottom: Second Hooghly Bridge, Ting Kau Bridge, K03 over Albert Canal, Signature Bridge Delhi

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The Bridge and Structural Engineer

enhanced by the ease of construction. The light steel grid can be transported and installed in large segments and it acts as falsework for an in-situ concrete slab or, even better, as support for precast concrete panels.

The actual configuration of main girders and the arrangement of the cable planes needs to be decided individually depending on the boundary conditions of each project. A composite deck should be always supported along the edges. For deck widths above 25 to 30m an additional central main girder, that distributes local loads over several cross girders, usually pays off. For deck widths beyond 30m and very windy conditions four cable planes should be envisaged in order to reduced the span of the cross girders and hence the overall depth of the bridge deck.

Whether open or closed steel sections, welded or bolted connections are advantageous needs to be evaluated for each situation afresh, mainly depending on the possibility of transporting large segments, onsite welding, lifting capacities, etc. Without doubt the use of precast panels for the deck slab is always advantageous, due to the reduced creep and shrinkage effects and savings in temporary formwork. The

The Bridge and Structural Engineer

hoop joints in between the precast panels, that has been developed and thoroughly tested for the Ting Kau bridge, allow for a fast erection and results in a continuous, durable and robust bridge deck.

5. References 1. SCHLAICH J., BERGERMANN R., "CableStayed Bridges with Composite Stiffening Girders - The Second Hooghly Bridge in Calcutta", Proceedings of the Sino-American Symposium on Bridge and Structural Engineering, Peking, 1982.

2. BERGERMANN R., SCHLAICH M., "The Ting Kau Bridge in Hong Kong", Proceedings IABSE Symposium Kobe, Japan, 1998.

3. K03 bridge over Albert Canal - project description in the internet under www.sbp.de, 2016.

4.

SCHLAICH M., SUBBARAO H., KURIAN J., "A Signature Cable-Stayed Bridge in India - The Yamuna Bridge at Wazirabad in New Delhi", SEI Journal, 1/2013.

Volume 46 Number 2 June 2016  39

Selection of Appropriate Steel for Bridges in Indian Conditions

Dr. Sanjeev Kumar GARG Dy. Chief Engineer, Const.

Northern Railway New Delhi, India skg_ir@rediffmail.com

Sanjeev Kumar Garg, born 1971, received his civil engineering degree from the University of Roorkee, (Now IIT, Roorkee), India. He joined Indian Railway Service of Engineers after completing his Ph.D. from IIT, Roorkee in 1998 and presently working as Dy. Chief Engineer, Construction, Northern Railway, New Delhi (India). He has vast experience in field of design and execution of Steel Rail/ Road Bridges.

Summary Steel is a vital component of all major infrastructure projects these days. Many grade/quality of steel are available in the market as per various codes. Different structures have varying requirements on quality of steel for proper functioning of structure. Fracture toughness is one of the most critical parameter in selection of steel for a particular structure in given climatic conditions. Presently Indian Codes (IS, IRC, IRS) are silent on this aspect of selection of steel. In this paper, attempt has been made to present criteria for selection of appropriate steel for a structure considering various factors and service conditions.

Keywords: Steel, selection, fracture toughness, service temperature, grade, quality, yield stress

1. Introduction Steel has played an important role in development of mankind. The earliest known production of steel are pieces of ironware excavated from an archaeological site in Anatolia (Kaman-Kalehoyuk, Turkey) and are nearly 4,000 years old, dating from 1800 BC [1].

In India, the 1600 year old iron pillar at Mehrauli, New Delhi (Fig. 1) is one of the world’s foremost metallurgical curiosities. The pillar, about seven metres high and weighing more than six tonnes, is considered to be erected by the king Chandragupta II Vikramaditya and represents the metallurgical skills in India at that time.

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Fig.1: 1600 years old Iron Pillar at Mehrauli, India

Since then steel has been a used in different part of the world for various purposes. However, there was no control on quality of steel till end of 18th century.

First control on quality of steel was introduced around The Bridge and Structural Engineer

1885 when British Steel manufacturers formed an association and laid down limits of sulphur and phosphorus in steel. Steel produced in England after 1895 is considered to be controlled steel and steel prior to that is termed as early steel[2]. Since then, a lot of development has taken place in manufacturing of steel. Standards have been developed by many associations, bodies of various countries and more than 8000 types of steel are available with different names under various standards/ specifications.

Structural steel is now a major construction material in most of the big projects. Hence, it requires a clear understanding of various steel designations for making an informed decision while selecting the steel for appropriate use in a structure. Use of improper steel may lead to distress in structure due to fatigue or violation of serviceability requirements.

2.

Steel Standards in India

2.1 Indian standards on steel The first Indian standard on Steel, IS:226[3] (superseded by IS:2062 subsequently) was published in 1962 by BIS (then ISI) for low and medium tensile steel. It has since been revised 7 times in 1969, 1975, 1984, 1992, 1999, 2006 and 2011 with major changes introduced in 2006[4] and 2011 [5]. Further, IS:8500[6] was published in 1977 for high tensile steels. It has since been revised in 1991.

2.2 Recent development in Indian standards on steel Untill1999, the steel strength was being designated based on tensile strength denoted by Fe-XXX [7].

2006 revision has used a duel grade system based on yield stress denoted by E XXX and tensile strength denoted by Fe-XXX[4]. Further, other two IS codes, viz., IS 1977-1996[8] and IS:8500-1991 [6] have been superseded by 2006 revision of IS:2062. Further, till 2009, Grade E-250 to E-450 were having 3 qualities each (A, B and C). Vide amendment No 1 in March 2009, these have been changed to 4 qualities each (A, BR, B0 and C) [4]. It was in fact addition of a new quality BR with redefinition of quality B as B0 (both B and B0 require Charpy Impact Test at 0oC with Min 27 J CVN value requirement whereas for quality BR, Charpy test is not mandatory), however, it is many times being misinterpreted as split of quality B specified earlier and quality BR is being used in place of earlier quality B in old references/ drawings, which The Bridge and Structural Engineer

is not correct.

Further, in 2011 revision [5], Steel grade has been based on yield stress only denoted by E XXX and tensile strength based grade designation has been dispensed with. Presently IS:2062-2011 provides for 8 grades having total 28 grade-quality combinations of steel.

3. Parameters to be considered in Selection of appropriate steel Selection of appropriate type of steel is first critical decision in the process of design and requires knowledge of different grades of steel and their properties along with analysis of intended use for economic and safe design of steel structures. Various parameters to be broadly considered in selection of steel are as follows: 1. Fracture toughness requirement (Type of structural detail - fracture critical, non-fracture critical; Place of structure; Plate Thickness) 2.

Type of loading (static, dynamic)

3.

Plate Thickness

4.

Serviceability requirement

5.

Industry practices

It is amply clear that selection of appropriate type of steel is required at design stage and cannot be left solely at the discretion of executive in the field who may not be well versed with design and selection criteria.

3.1 Fracture toughness requirement All bridges and many other structures are subjected to dynamic loading due to vehicular live load etc. resulting in fatigue in structure. Fracture toughness requirement is of paramount importance for structures subjected to fatigue. Fatigue assessment of structures subject to dynamic loading only checks the stress variations vis-Ă -vis permissible stress variation [9],[10]. Since, brittleness (or ductility) of structure depends upon the temperature during service, required level of fracture toughness is to be ensured at minimum service temperature in order to allow the permissible Volume 46 Number 2 June 2016  41

stress variation (considered in design during fatigue assessment) at minimum service temperature.

Presently, though IS:2062-2011 [4] has specified subqualities for various grades of steels, all major Indian design codes (IRS[11], IRC[9,10] and IS[12]) are silent about fracture toughness requirement for steel.

The sub-qualities given in IS:2062[5] specifies the CVN value requirement at given temperature, which are in fact fracture toughness for respective quality and grade of steel. However, in absence of fracture toughness requirement in respective design codes, the selection of a quality is difficult and left at the discretion of design engineer/ executive. However, in Indian Railways, a thumb rule of using quality B0 below snow line and quality C above snow line is being practised. It is purely on the basis that B0 quality is tested at 0oC and C quality is tested at subzero temperature (-20oC).In scenario described above, considering the increasing use of steel in bridges (especially in cold climates), a systematic guideline based on technical rationale for fracture toughness requirement is required for selection of appropriate steel for use in Bridges.

The Zone 1 (starting zone) specifies minimum service temperature of 0deg F, i.e., -18 deg C, whereas in southern part of India, minimum service temperature is never below zero deg C. If same zones are adopted, it will place southern part of India also in Zone 1 of AASHTO thus avoiding the possible advantage of higher temperature in that area.

Therefore, the tables of fracture toughness requirement given in AASHTO [5] need to be modified to suit Indian Codes and conditions.

3.1.1 AASHTO provisions on fracture toughness ASHTO LRFD Bridge Design Specifications, 2010[13] has provided fracture toughness requirement in terms of Charpy V-Notch fracture energy (hereafter referred as CVN) for bridge steel. For the purpose, Unites States has been divided in 3 temperature zones based on minimum service temperature as given in Table-1. The provisions are is FPS and being given here in FPS (with corresponding values in SI units being given in parenthesis) for better understanding and further handling of the data.

The provisions for fracture toughness requirement are available in AASHTO LRFD Bridge Design Specifications, 2010 [13]. However, it is not practical to follow those provisions directly in Indian context due to following reasons:

Table 1: AASHTO temperature zones for specifying CVN toughness Lowest Anticipated Service Temperature

Temperature Zone

1.

The provisions in AASHTO are in FPS system whereas in India SI system of units is used.

0°F and above (-17,8°C and above)

1

2.

AASHTO provides for different test temperature and corresponding Charpy V-notch (CVN) test values for same grade of steel whereas Indian Code (IS:2062) has fixed CVN value for a specified grade and quality of steel and only test temperature is variable.

-1°F to -30°F (-18,3°C to - 34,4°C)

2

-31°F to -60°F (-35,0°C to - 51,1°C)

3

3. The CVN values even after conversion in SI units are not compatible with those specified in IS:2062-2011 [5].

4.

Test temperatures specified in AASHTO [13] (after conversion in deg C) are different than that specified in IS:2062-2011.

5.

AASHTO [13] specifies 3 temperature zones.

42 Volume 46

Number 2 June 2016

Table 6.6.2-2 of AASHTO LRFD Bridge Design Specifications, 2010[13] specifies the fracture toughness requirement in FPS units and reproduced in Table 2 below for ready reference. For the purpose of fracture toughness requirement, bridge members are primarily categorised as fracture critical and nonfracture critical. Fracture critical members are those, whose failure may be reasonably expected to cause collapse of the bridge and rest are considered as Nonfracture critical members. Members and portions of members that are considered to be fracture critical are required to be designated by the engineer on the drawings.

The Bridge and Structural Engineer

Table 2: AASHTO Table 6.6.2-2 fracture toughness requirements for bridge steels (2010) (FPS Unit) GRADE (Y.P./Y.S.)

Thickness (in)

FRACTURE CRITICAL

NON-FRACTURE CRITICAL

Min. Test Value Energy (ft-lb)

ZONE 1 (ft-lb @ °F)

ZONE 2 (ft-lb @ °F)

ZONE 3 (ft-lb @ °F)

ZONE 1 (ft-lb @ °F)

ZONE 2 (ft-lb @ °F)

ZONE 3 (ft-lb @ °F)

36

t≤4

20

25 @ 70

25 @ 40

25 @ 10

15 @ 70

15 @ 40

15 @ 10

50/50S/50W

t≤2

20

25 @ 70

25 @ 40

25 @ 10

15 @ 70

15 @ 40

15 @ 10

2≤t≤4

24

30 @ 70

30 @ 40

30 @ 10

20 @ 70

20 @ 40

20 @ 10

HPS 50W

t≤4

24

30 @ 10

30 @ 10

30 @ 10

20 @ 10

20 @ 10

20 @ 10

HPS 70W

t≤4

28

35 @ -10

35 @ -10 35 @ -10

25 @ -10

25 @ -10

25 @ -10

HPS 100W

t ≤ 2½

28

35 @ -30

35 @ -30 35 @ -30

25 @ -30

25 @ -30

25 @ -30

2½ ≤ t ≤ 4

36

NP

35 @ -30

35 @ -30

35 @ -30

NP

NP

Note : NP – Not permitted The values in table-2 have been converted from FPS to SI units and given in Table-3. The Grade of steel has been converted in MPa (1 ksi = 6,897 MPa), energy in Joule (1 ft-lb = 1,356 N.m) and temperature in o F (C=(f-32)/1,8).

Table 3: AASHTO Table 6.6.2-2 fracture toughness requirements for bridge steels (2010) (SI Unit) GRADE (Y.S. MPa)

Thickness (mm)

FRACTURE CRITICAL

NON-FRACTURE CRITICAL

Min. Test Value Energy (J)

ZONE 1 (J@0C)

ZONE 2 (J @ 0C)

ZONE 3 (J @ 0C)

ZONE 1 (J @ 0C)

ZONE 2 (J @ 0C)

ZONE 3 (J @ 0C)

250

t ≤ 100

27

34 @ 21

34 @ 4

34 @ -12

20 @ 21

20 @ 4

20 @ -12

350

t ≤ 50

27

34 @ 21

34 @ 4

34 @ -12

20 @ 21

20 @ 4

20 @ -12

50 ≤ t ≤ 100

32

41 @ 21

41 @ 4

41 @ -12

27 @ 21

27 @ 4

27 @ -12

HPS 350

t ≤ 100

32

41 @ -12

41 @ -12

41 @ -12

27 @ -12

27 @ -12

27 @ -12

HPS 480

t ≤ 100

38

47 @ -23

47 @ -23

47 @ -23

34 @ -23

34 @ -23

34 @ -23

HPS 690

t ≤ 63

38

47 @ -34

47 @ -34

47 @ -34

34 @ -34

34 @ -34

34 @ -34

63 ≤ t ≤ 100

49

NP

NP

NP

47 @ -34

47 @ -34

47 @ -34

Note: Yield stress (Y.S.) rounded off to nearest 5 MPa, other values rounded off to nearest integer.

From the above table, it may be noted that none of the CVN requirement corresponds directly to steel grades available in IS:2062-2011. For fracture critical members, CVN requirement is more than 27 J whereas IS:2062-2011 specifies maximum 27 J at test temperature. For non-fracture critical members, though the CVN requirement is 27 J or less, but test temperature is different. So the table-3 also cannot The Bridge and Structural Engineer

be directly referred for steel grades available in India code.

3.1.2 Minimum service temperature of bridges in India Clause 215.2 of IRC-6-2014 [14] specifies that the minimum service temperature of the bridge based on lowest minimum temperature of the area. The Volume 46 Number 2 June 2016 43

minimum shed air temperature of the area may be taken from Fig. 9 of IRC:6 (Reproduced as Fig 2 for ready reference) using which minimum service temperature may be taken as follows: 1.

Snowbound areas -35 oC

2.

Other areas minimum shed temperature - 10 oC

air

From Fig-2, it is noted that shed air temperature in India varies between -7,5oC to 17,5 oC. The corresponding minimum service temperature works out to -17,5oC to 7,5 oC. After considering the snowbound areas, the range for minimum service temperature works out to -35 oC to 7,5oC. The minimum service temperature specified for Zone-1 in AASHTO LRFD code [13] is only 17.8oC whereas, a large part in southern India observes minimum service temperature above 0 oC (Shed air temperature above 10 oC in Fig-2). Using Zone-1 for that area also will result in higher CVN requirement, which may otherwise not be required. Hence, one more zone is required to be added before Zone-1 for adequately covering the Indian Conditions.

3.1.3 Proposed temperature zones for India As discussed in 3.1.2 above that one more Zone is required before Zone-1 of AASHTO to cover the Indian conditions, it is proposed to have 4 zones, as listed in Table 4. To eliminate the ambiguity in nomenclature, the proposed zone have been designated as A, B, C & D, where, Zone A is new zone over AASHTO and Zone B, C & D corresponds to Zone 1, 2 & 3 of AASHTO. Table 4: Proposed temperature zones for specifying CVN toughness Lowest Anticipated Service Temperature

Temperature Zone

Fig.2: Minimum shed air temperature in India

3.1.4 Steel grades available as per IS:2062-2011 in India IS:2062-2011 provides for 8 grades having total 28 grade-quality combinations of steel. These have been given in Table-5 below along with other critical properties of the steel.

Table 5: Grade, quality and properties of Steel as per IS:2062-2011[4] Grade

Quality

Max. Yield stress (MPa)

% Elongation

CVN at test temperature

Carbon content, % (Max)

E 250

A, BR, B0, C

250

23

27

0,22

E 275

A, BR, B0, C

275

22

27

0,22

E 300

A, BR, B0, C

300

22

27

0,20

E 350

A, BR, B0, C

350

22

27

0,20

E 410

A, BR, B0, C

410

20

25

0,20

E 450

A, BR

450

20

20

0,22

Above 0 oC

A

0 oC to -17 °C

B

E 550

A, BR

550

12

15

0,22

E 600

A, BR

600

12

15

0,22

-18 °C to -34 °C

C

E 650

A, BR

650

12

15

0,22

-35 °C to -51 oC

D

44 Volume 46

Number 2 June 2016

Note: Test temperature for CVN values are 25 oC for BR, 0 oC for B0 and -20 oC for C grade The Bridge and Structural Engineer

From perusal of the Table-5, it is noted that E 450 to E 650 grades have only A & BR quality. Further, for grades E 550 to E 650, elongation is only 12% and CVN value at test temperature is also low (15 J). However in Table-2 & 3, very high tensile grades (HPS 50W, 70W and 100W) are High Performance Steel (HPS) only which have an optimized balance of strength, weldability, toughness, ductility, corrosion resistance and formability to give maximum performance in bridge structures while remaining cost-effective [15]. HPS has low levels of carbon (0.11 %) and carbon equivalents to provide good weldability (with reduced or no preheat).HPS has a high level of fracture toughness (AASHTO Zone 3 minimum) and ductility to improve reliability of structure.

Hence, high tensile steel grades (E 450 to E 650) available in IS:2062 cannot be considered as high performance steel. Accordingly, fracture toughness specified in AASHTO for HPS grades cannot be applied for steel grades E 450 to E 650 covered in IS:2062-2011. Moreover, grades E550 to E650 are not considered suitable for bridge work due to low ductility and fracture toughness. Hence, only grades up to E 450 need to be deliberated further.

3.1.5 Test temperature and CVN values for Charpy Impact Test Integrated study of Table-1 & Table-2 indicates that for Non-HPS grades, ASHTO has adopted test temperature as 70oF (39oC) above minimum service temperature for the respective Zone. The same logic can be used to define test temperature for Zones

defined in Table-4 above. Accordingly, the test temperature for CVN value for Zone A, B, C & D works out to 39, 22, 5 and -12 oC for grades E250 to E 350. For grade E-410 and E-450, test temperature for Zone- D may be adopted -23 oC same as for HPS 70W to be on safer side(i.e., 28 oC above minimum service temperature) and 28, 11, -6 for Zone A, B & C. Similarly for CVN values, it is observed that CVN values are constant (in multiple of 5 lbs-ft in FPS units or 6,8 J) across the zones for a given grade and use. The use of original table in FPS is helpful in understanding the pattern of values. CVN value for Non-fracture critical members is 10 lbs-ft or 13.6 J lower that for fracture critical members. The CVN value for different grades of steel for fracture critical members are summarized in Table-6 in both FPS and SI units for better understanding of pattern. Using the details presented above, the fracture requirement for steels in IS:2062-2011 [5]has been worked out and given in Table-7.

Table 6: CVN values for fracture critical members GRADE (Y.P./Y.S.)

Yield Stress MPa

Thickness (mm)

ft-lb

(J)

Similar Grades in IS:2062

36

250

t ≤ 100

25

34

E-250, E-275, E-300

50/50S/50W

350

t ≤ 50

25

34

E-350

50 ≤ t ≤ 100

30

41

HPS 50W

350

t ≤ 100

30

41

HPS 70W

480

t ≤ 100

35

47

E-410, E-450

Table 7: Fracture toughness requirements for steels grades in IS:2062 GRADE

E-250, E-275 & E-300 E 350

E-410 & E 450

Thickness (mm)

FRACTURE CRITICAL

NON-FRACTURE CRITICAL

ZONE A (J @ °C)

ZONE B (J @ °C)

ZONE C (J @ °C)

ZONE D (J @ °C)

ZONE A (J @ °C)

ZONE B (J @ °C)

t ≤ 100

34 @ 39

34 @ 21

34 @ 4

34 @ -12

20 @ 39

t ≤ 50 50 ≤ t ≤ 100 t ≤ 100

34 @ 39 41 @ 39

34 @ 21 41 @ 21

34 @ 4 41 @ 4

34 @ -12 41 @ -12

47 @ 28

47 @ 10

47 @ -7

47 @ -23

The Bridge and Structural Engineer

ZONE D (J @ °C)

20 @ 21

ZONE C(J @ °C) 20 @ 4

20 @ 39 27 @ 39

20 @ 21 27 @ 21

20 @ 4 27 @ 4

20 @ -12 27 @ -12

34 @ 28

34 @ 10

34 @ -7

34 @ -23

20 @ -12

Volume 46 Number 2 June 2016 45

However, both the required CVN values and test temperature in Table-7 are still different from those specified in IS:2062 2011 (27, 25 & 20 J at 25/ -20 o C)) and table is still not usable for Indian Code.

Hence, either of the CVN value or test temperature needs to be made standard by changing the other value accordingly. It is noted that in the vicinity of 0oC, the CVN v/s temperature transition graph in nearly linear.

IS:2062-2006 (now superseded by IS:2062-2011) had specified CVN value at two different temperatures (25 and -20 oC) as follow: E 300 - 50 & 30 J (0.44 J/oC); E 350 & E 410 – 50 & 25 J (0.56 J/oC); E 450 – 45 & 20 J (0.56 J/oC). Using these details, the temperature corresponding to the required CVN value (27, 25 ,20 J) in Table-7 can be interpolated/extrapolated and table can be modified as given in Table-8.

Table 8: Fracture toughness requirements for steels grades in IS:2062[5] at standard temperature GRADE

Thickness (mm)

FRACTURE CRITICAL

NON-FRACTURE CRITICAL

ZONE A (J @ °C)

ZONE B (J @ °C)

ZONE C (J @ °C)

ZONE D (J @ °C)

ZONE A (J @ °C)

ZONE B (J @ °C)

ZONE C(J @ °C)

ZONE D (J @ °C)

E-250, E-275 & E-300

t ≤ 100

27 @ 23

27 @ 5

27 @ -12

27 @ -28

27 @ 55

27 @ 37

27 @ 20

27@ 4

E 350

t ≤ 50

27 @ 26

27 @ 8

27 @ -9

27 @ -25

27 @ 52

27 @ 34

27 @ 17

27 @ 1

50 ≤ t ≤ 100

27 @ 14

27 @ -4

27 @ -21

27 @ -37

27 @ 39

27 @ 21

27 @ 4

27 @ -12

E-410

t ≤ 100

25 @ -12

25 @ -30

25 @ -47

25 @ -63

25 @ 18

25 @ -6

25 @ -23

25 @ -39

E 450

t ≤ 100

20 @ -21

20 @ -40

20 @ -55

20 @ -72

20 @ 3

20 @ -15

20 @ -32

20 @ -48

3.1.6 Proposed table for selection of steel confirming to IS:2062 for Indian Conditions Using table 4 & 8 presented above, appropriate steel can be selected directly out of grades available in IS:2062. To illustrate, for fracture critical member in grade E 350 of a bridge in Zone-B, fracture toughness

requirement as per Table-8 is CVN 27 J @ 8 oC for which quality B0 as per IS:2062 2011 is suitable. For quick referencing, Table-8 has further been converted in terms of quality available in the code for different grades and given in Table-9.

Table 9: Selection table for steel confirming to IS:2062-2011 in Indian Conditions GRADE

Thickness FRACTURE CRITICAL (mm)

NON-FRACTURE CRITICAL

ZONE A (J @ °C)

ZONE B (J @ °C)

ZONE C (J @ °C)

ZONE D (J @ °C)

ZONE A (J @ °C)

ZONE B (J @ °C)

ZONE C(J @ °C)

ZONE D (J @ °C)

E-250, E-275 & E-300

t ≤ 100

BR

B0

C

C*

BR

BR

B0

B0

E 350

t ≤ 50

BR

B0

C

C*

BR

BR

B0

B0

50 ≤ t ≤ 100

B0

C

C

C*

BR

B0

B0

C

E-410

t ≤ 100

C

C*

NP

NP

B0

C

C*

C*

E 450

t ≤ 100

C

C*

NP

NP

B0

C

C*

NP

NOTE: i)

ii)

iii)

NP – Not Permitted, quality BR with Charpy impact test C* - quality C with test temperature at -40oC (As permitted in IS:2062 [5])

46 Volume 46

Number 2 June 2016

The Bridge and Structural Engineer

3.2 Type of loading The fluctuation in stress level, usually caused by dynamic loading due to vehicular live load, results in fatigue in steel structures. The fatigue assessment of steel structures is required in such cases. The permitted stress range in fatigue assessment is independent of grade of steel [9, 10, 12]. Hence, available stress range for live load stresses variations remains same irrespective of grade of steel. This in-turn suggests that extra strength available in high tensile steel can primarily be used for increased static load (dead load, SIDL etc.) component only. Therefore, use of high tensile steel is economical when ratio of (dead load + SIDL) to live load is high, i.e., effect of dynamic load is relatively less on structure in comparison to static loads. On the contrary, if dynamic loads are high in comparison to static loads, then permitted stress range in fatigue assessment is more likely to govern the design and it may not be possible to fully utilize the strength of high tensile steel and use of a lower grade steel may work out to be more economical. The above deliberation also indicates the importance of fatigue assessment in high tensile steel.

3.3 Plate Thickness Plate thickness in various parts of the bridge structure

is a subject matter of calculation and finalised based on different design checks. It may look prima-facie that any plate thickness can be selected based on design requirement and it is partially correct also.

However, besides market availability issues (which is not a point of discussion here), it has effect on economy of structure. The yield stress for a particular grade of steel (to be used in design) depends upon the plate thickness, it being lower for higher thickness.

Table-2 of IS:2062-2011 [5] provides yield stress for different grade and thickness of steel sections. The summery of same is given in Table-10. In the table marginal gain has been calculated as extra strength available due to use of higher plate thickness in comparison to increase in plate thickness; e.g., for plates of E-250 grade; available yield strength of 20 and 22 mm thick plate are 5000 and 5280 N/mm respectively, however, the available yield strength of 22 mm plate on prorata basis works (in comparison to 20 mm plate) out to 5500 N/mm, hence, while using 22 mm plate, marginal gain in strength over 20 mm plate is only 280 N/mm against 500 N/mm, i.e., 56% of increase in thickness. It may be noted that marginal gain for 22, 25, 45 and 50 mm plate are in the range of 37-66%, 71-85%, 63-86% and 79-82% respectively.

Further, above marginal gains are considering that only single size plates are used in section.

Table 10: Yield stress of steel as per IS:2062-2011[5] Grade designation

Plate Thickness (mm)

% decrease in yield stress over 20 mm plate thickness

Marginal gain* in increasing plate thickness over 20/40 mm

< 20

20-40

> 40

20-40

> 40

22 mm

25 mm

45 mm

50 mm

E-250

250

240

230

4,0%

8,0%

56%

80%

63%

79%

E-275

275

265

255

3,6%

7,3 %

60%

82%

66%

81%

E-300

300

290

280

3,3 %

6,7 %

63%

83%

69%

83%

E-350

350

330

320

5,7%

8,6%

37%

71%

73%

85%

E-410

410

390

380

4,9%

7,3 %

46%

76%

77%

87%

E-450

450

430

420

4,4 %

6,7 %

51%

78%

79%

88%

E-550

550

530

520

3,6 %

5,5 %

60%

82%

83%

91%

E-600

600

580

570

3,3 %

5,0 %

63%

83%

84%

91%

E-650

650

630

620

3,1 %

4,6 %

66%

85%

86%

92%

The Bridge and Structural Engineer

Volume 46 Number 2 June 2016â&#x20AC;&#x192; 47

If section has multiple sizes of plates than yield stress of complete section gets reduced which will further reduce the marginal gain, even section capacity may Fig. 3: Typical section of top reduce in-spite of chord of open web increase in sectional area. To illustrate, Fig. 3 presents a typical section of top chord of an open web girder with E-250 grade steel. The Area of girder is 298 cm2 with yield capacity of 7450 kN. If in attempt to increase the area of section, bottom flange thickness is increased to 22 mm; the area increases to303 cm2, however, its yield capacity gets reduced to 7272 kN due to reduction in yield stress from 250 MPa to 240 MPa. In this example, in-spite of increase in sectional area, there is reduction in section capacity due to reduction in yield stress as a consequence of using 22 mm plate in built-up section. Hence, the use of 22 and 45 mm plates should be preferably avoided in built-up sections for economical design.

Due diligence also need to be exercised while using 25 mm plates for economical design.

Besides the above, as already discussed in 3.1 above, plate thickness also governs the fracture toughness requirement in some cases. Use of plates requiring higher quality (A, 0, BR, C) may increase the cost of structure.

3.4 Serviceability requirement Generally deflection checks are exercised during design of bridges. If deflection in road bridges increases beyond the prescribed limits (total deflection should not be more than span/600 as per Clause 604.3.2 of IRC:22-2015 [9] and Clause 504.5 of IRC:24-2010[10], the deflection due to dead and superimposed load can be offset by providing the suitable camber in superstructure. In such case, the deflection due to live load and impact should not exceed span/800 for road bridges. For railway bridges, the limit of total deflection is span/600 as per Clause 4.7 of Indian Railway Steel Bridge Code 2003 [11].

However, camber is mandatory for railway bridges of more than 30 m spans.

The deflection limit is same for all type of steel.

48â&#x20AC;&#x192; Volume 46

Number 2 June 2016

However, in case of high tensile steel due to higher permissible stresses, higher stress margins are available for satisfying the live load requirement thus causing higher strains under live load leading to higher live load deflections. Therefore, full utilisation of high tensile steel may be restricted by serviceability requirement (deflection criteria). The effect is likely to be more pronounced in case of very high tensile steel (E-410 and above). Hence, due diligence needs to be exercised for checking the structure in serviceability criteria while using high tensile steel.

3.5 Industry practices Industry practices play an important role in quality of manufactured/fabricated steel items. Before selecting the steel, one should enquire about the fabrication practices being adopted in the area. Use of high tensile steel requires more stringent quality control during fabrication. The manufacturing/ fabrication process should be such that they do not alter the basic micro structure of steel. Any lapse, though its effect may not be prima-facie or at all visible after fabrication, may alter the micro structure of steel making the steel brittle or fracture prone. The gravity of the above may be understood from the fact that Clause 7.3 of IS:2062-2011 [5] prohibits surface repairs using welding followed by grinding for grade designation E 250C, E 275C and E 300 to E 650.

Some of such practises are as follows: 1.

Not preheating the steel before welding (where required)

2.

Excessive heating temperature)

3.

Using stray welds for fitment of members etc

4.

Welder not qualified for the specified job

5.

Repair of damaged surface by welding

6.

Plugging of extra holes by welding

(over

the

specified

7. Non-removal of Heat Affected Zone after gas cutting 8.

Hot bending of plates in place of cold bending (wherever specified) or vice versa

9.

Improper handling of steel during fabrication

If any of the above in generally being practised in area, while using high tensile steel or quality C steels, stringent quality control with increase The Bridge and Structural Engineer

supervision should be enforced otherwise use of high tensile steel should be avoided.

4. Conclusion Selection of appropriate steel for the bridge is a critical activity involving consideration of many parameters.

In the present paper, attempt has been made to deliberate on the issue and parameters requiring attention have been identified. Besides fracture toughness requirement, effect of type of loading (static, dynamic), plate thickness, serviceability requirement and industry practices on selection of steel has been discussed.

It has been noted that presently there is no guideline for fracture toughness requirement in Indian design codes. For the construction of bridges in low temperature zones, these are essentially required.

Though such guidelines are available in AASHTO LRFD Bridge Design Specifications, 2010 [13]; however, due to various reasons, these cannot be used directly in Indian Conditions. In the paper, an attempt has been made to make AASHTO guidelines usable for Indian Conditions & codes by suitably modifying them. A table for selection of steel quality for a grade and minimum service temperature of bridge has been proposed for easy referencing.

6.

Bureau of Indian Standards, IS 8500 - Structural Steel – Microalloyed (Medium and High Strength Qualities) - Specifications, New Delhi, 1991.

7. Bureau of Indian Standards, IS 2062 – Steel for General Structural Purposes - Specification (Fifth Revision), New Delhi – 1999.

8. Bureau of Indian Standards, IS 1977 –Low Tensile Structural Steels, New Delhi, 1996.

9. Indian Road Congress, IRC:22 – Standard Specifications and Code of Practice for Road Bridges-Section VI - Composite Construction (Limit States Design) (Third Revision), New Delhi – 2015.

10. Indian Road Congress, IRC:24 – Standard Specifications and Code of Practice for Road Bridges, Steel Road Bridges (Limit State Method) (Third Revision), New Delhi – 2010.

11. Research Designs and Standards Organisation, Indian Railway Standard Code of Practice for the Design of Steel or Wrought Iron Bridges Carrying Rail, Road or Pedestrain Traffic (Steel Bridge Code, Lucknow-2003).

5. References

12. Bureau of Indian Standards, IS 800 - General Construction in Steel — Code of Practice, New Delhi,2007.

1. "Ironware piece unearthed from Turkey found to be oldest steel". The Hindu(Chennai, India).

2009-03-26.

13. American Association of State Highway and Transportation Officials, AASHTO LRFD Bridge Design Specifications, 5th Edition, 2010.

2. RDSO, Guidelines for assessment of Residual Life of Early Steel/ Wrought Iron Bridges, Lucknow, 2001.

14. Indian Road Congress, IRC:6 – Standard Specifications and Code of Practice for Road Bridges, Section:II, Loads and Stresses , New Delhi – 2014.

3.

Bureau of Indian Standards, IS 226 - Specification for structural steel (standard quality) (fifth revision), New Delhi, 1975.

4.

Bureau of Indian Standards, IS 2062 – Hot Rolled Low, Medium and High Tensile Structural Steel (Sixth Revision), New Delhi – 2006.

5. Bureau of Indian Standards, IS 2062 – Hot Rolled Medium and High Tensile Structural Steel (Seventh Revision), New Delhi – 2011.

The Bridge and Structural Engineer

15. MISTRY V. C., "High Performance Steel For Highway Bridges". Proceedings - Advanced Materials for Construction of Bridges, Buildings, and Other Structures, Vol. III, Engineering Conferences International, 2003, pp. 1-7.

16. WRIGHT W. J., “Steel Bridge Design Handbook: Bridge Steels and Their Mechanical Properties, Federal Highway Administration, New Jersey, 2012.

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Rehabilitation of Steel Bridges: Design Aspects Utpal K. GHOSH Consulting Chartered Engineer uptalmanjula@hotmail.com

Graduated in Civil Engineering from Calcutta University in 1954, Utpal K. Ghosh participated in design, construction and management of a variety of projects including bridges and industrial structures in several countries. He has written three technical books on steel bridges and design of welded steel structures.

Summary

final solution is then taken up.

Issues that require special consideration for the design of rehabilitation of steel bridges have been discussed. Starting with the nature of inadequacies normally encountered, the deliberations continue with the assessment of actual damages in bridges.

The different rehabilitation strategies and activities during concept design stage are discussed. Issues relevant to detail design and importance of clear and unambiguous drawings and specifications have been highlighted. The paper concludes with remarks on the importance of the innovativeness and ingenuity of the engineers in charge of rehabilitation schemes. Apart from these issues, other factors that influence the design are: available down-time, interruptions to the flow of traffic, safety, economy, and the practicability of the scheme.

Before going into the design work, it is necessary to discuss certain other issues which are relevant for preparing the rehabilitation scheme. These are:

Keywords: Stress corrosion, lamellar separation, fatigue, brittleness, impact, stress path, fracture critical member, orthotropic steel deck, pre-stressing, link member, buffer, redundancy, locked-in stress.

1. Introduction

 Inadequacies 

Damage assessment



Rehabilitation strategies

2. Inadequacies The inadequacies which lead to the necessity of rehabilitation maybe broadly divided into the following categories: 

Deterioration of individual members due to natural or man-made causes. Examples: corrosion, vehicular collision, accident, effects of war etc.



A bridge structure may need strengthening or modification due to introduction of new loading standard or design criteria.



An individual member or the structural system may be inherently deficient, needing strengthening.



Geometrical inadequacy due to changed traffic demand. Examples are: wider carriageway requirement, changeover to broad gauge railway track or electrified traction system.

Topics discussed in the present text relate to design aspects for the development of rehabilitation schemes of steel bridges.

Design work is generally carried out in two stages: 

Concept stage



Detail design stage

During concept stage, a number of schemes are examined, out of which two or three alternative schemes are identified. These are then subjected to rigorous analysis and review. Finally the most attractive solution is selected. Design work for this 50 Volume 46

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It is quite common to encounter more than one of these inadequacies in the same bridge.

3.

Damage assessment

The issues that need special attention during this activity are discussed in the following paragraphs.

3.1 History of the bridge The first work for rehabilitation of a steel bridge is to The Bridge and Structural Engineer

study the history of the bridge, and its environment.

The date of construction and history of subsequent repair work or replacement of major members are very important in formulating the rehabilitation scheme.

Knowledge of age provides some very useful information. Some of these are: 3.1.1 Type of material This may give some idea about the type of steel or iron used in the construction and may influence whether a particular type of repair work will be feasible or not. For example, steel with high carbon or silicon content is not easy to weld and has more chance of cracking due to welding than low carbon steel. Similarly wrought iron is prone to lamellar separation, particularly when heated. Also, cast iron cannot be welded. In such cases welding should be avoided, although this may appear to provide an easy solution.Modern methods of material testing should be used for identifying the properties of the materials used.

3.1.2 Loadings and stresses Age may also give an idea about the loadings and stresses considered for the original design, on the basis of the codes of practices prevalent at the time of construction. In some cases, sections provided may have been more than adequate. Also, modern analysis by computer and design by limit state theory may reveal that despite physical deterioration of some members, the bridge is still capable of taking even higher loads than those for which this was originally designed.

responsible for corrosion of bridge elements.

3.2.2 Salinity in the atmosphere Presence of salinity in the atmosphere also affects the bridge elements from corrosion point of view.

3.2.3 Pollutants in the atmosphere Corrosive fumes or discharge of chemical effluence from industrial units situated nearby a steel bridge may cause corrosion to bridge elements.

3.2.4 Proximity with adverse conditions Low clearance between the lowest point of the bridge and the highest flood level (HFL) makes the members located at the lower levels considerably more susceptible to corrosion. Similarly any source of heat or fumes or other adverse conditions should also be considered.

Rehabilitation schemes should consider these hazards and recommend appropriate protective measures.

3.3 Condition of the bridge For the design of an effective rehabilitation scheme, the first step is to understand the present condition of the various members of the bridge by carrying out a special inspection. This inspection comprises of the following main activities: a)

Measurement of the overall dimensions and other parameters of the existing structure including sections and thicknesses of all members.

b) Ascertaining the defects and deteriorations suffered by different members.

3.1.3 Fatigue life

c)

The period of construction may give an idea about the number and magnitude of load cycles the bridge has been subjected to, thereby assessing the fatigue life of the structure.

d) Observation of the behaviour of members and cracks when subjected to vibratory loads.

3.2 Location Review of the surroundings of the location is relevant for the development of the rehabilitation schemes.

Some of the important aspects are: 3.2.1 Moisture in the environment Presence of water spray or moisture in the vicinity, such as waterfall or marshy wet land is very often The Bridge and Structural Engineer

Examination of joints and fastenings.

The first activity enables capacity assessment of the distressed structure through analysis and design, while the balance activities focus on the defects for the purpose of repair and rehabilitation of individual members.

3.4 Corrosion and Fatigue Generally distress in steel bridges may be attributed to three main reasons, viz., atmospheric corrosion, stress corrosion, and fatigue. Although these reasons are strictly separate mechanisms, these are somewhat Volume 46 Number 2 June 2016â&#x20AC;&#x192; 51

inter-related, since any one of these may trigger one or both of the other two mechanisms, which may jointly (or individually) cause distress in steel.

3.4.1 Atmospheric corrosion Atmospheric corrosion is an electrochemical process of flow of electricity and chemical changes in steel occurring in stages. It is to be particularly noted that corrosion occurs only in the concurrent presence of water and oxygen. In the absence of either, corrosion cannot occur. Also, all the corrosion occurs in the metal itself and not at the surface mill scale. Thus, the direct effect of corrosion is the loss of area of the steel component. This increases stress in the component and indirectly makes the member vulnerable to stress corrosion and fatigue cracking.

3.4.2 Stress corrosion In corrosive environment, members with high tensile stress are likely to suffer higher corrosion. This phenomenon is commonly referred to as â&#x20AC;&#x2DC;stress corrosionâ&#x20AC;&#x2122;. As the cross sectional area of an already highly stressed member is reduced due to corrosion, the resultant increase in stress may initiate crack.

Such cracks are found mostly in specific areas such as eye bars and pin connections in suspension and cable stayed bridges, where the details are such as to cause high concentration of stress.

3.4.3 Fatigue In bridges carrying heavy moving loads steel elements are subjected to high fluctuation of stresses. This fluctuation of stresses reduces the ultimate strength of steel considerably as compared to static load applied gradually. Thus, a member may be able to withstand a single application of design load, but may fail at stress levels much lower than the yield stress, if the same load is repeated a large number of times. This phenomenon of progressive localised permanent structural change due to fluctuating stresses that may initiate and propagate cracks in the member after sufficient number of fluctuations is termed fatigue failure.

This reduction in strength is dependent on two factors viz., number of load repetitions (cycles) and stress variations due to these loads. Fatigue failure occurs at the tension zone of members and may be initiated by stress concentrations at notches, sudden change in cross-sections and sharp corners.

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In welded joints the fatigue strength of steel tends to be reduced due to pronounced changes in the structure (hard grain formation) in and around heat affected zone (HAZ) and lowering of ductility of the steel. As a result welded bridges are more prone to fatigue cracks than riveted ones. Consequently there have been many instances of fatigue cracks in welded bridges. Also, a crack developed in one component of a riveted or bolted connection normally stops at the hole, and does not travel beyond to the next component. In case of welded joint, however, crack developed at the weld tends to progress and may affect both the connecting components, thereby damaging the entire member.

3.4.4 Brittleness of steel Brittleness in steel is characterized by failure of the material as a result of rapid crack propagation with very little plastic deformation at a stress level below the yield stress of the material. As the energy required for initiating brittle fracture is rather low, such crack can be initiated from internal residual stress from welding, without even any externally applied force.

Brittle fractures occur suddenly with little or no warning signs.

The factors that generally influence brittleness are: a)

Metallurgical feature:

Depending on their chemical composition, heat treatment or mechanical working during production, some steel may be more brittle than others.

b)

Temperature of steel in service:

Structural steel undergoes a ductile-to-brittle transition as the temperature falls. Thus fracture may occur even at lower stresses when the ambient temperature drops, say below freezing point. Geographical location of a steel bridge is thus very important from this point of view.

c)

Service condition:

Certain distribution pattern of force field tends to make steel susceptible to brittle fracture.

Examples are locations of stress concentrations due to abrupt change in section, notches and cracks etc. Also, thick and wide plates and deep webs in plate girders carry higher chance of brittle fracture due to their complex internal The Bridge and Structural Engineer

stress pattern when loaded. The other causes of brittleness are cold working on steel during fabrication and also rapid rate of loading (impact).

Thus, understanding brittle fracture behaviour of steel is particularly important while developing a rehabilitation scheme.

4.

Rehabilitation strategies

The problems to be resolved for rehabilitation of each individual bridge are to some extent unique and should not be generalized. Nevertheless, some common strategies, which are discussed below, may be considered for developing rehabilitation schemes.

4.1 Repair of deficient members If a particular member is found to be deficient, or damaged, this may be strengthened by adding suitable material on to it. Alternatively, the entire member may be replaced by a new member of the required strength.

Care should, however, be taken to check that the connection details are sufficiently strong to transmit the required forces. Very often the connection details play important role in such repair work.

4.2 Introduction of new member(s) to the system

structural system is by introducing counter balancing forces in the system by means of external post tensioning tendons. The system would induce new stresses in the structure and reduce the effects of the existing dead and live loads on the structure, thereby increasing the live load capacity.

In some cases a combination of the above strategies may be employed to achieve the required results.

5.

During concept stage the problems and the various options for rehabilitation are considered in detail leading to the next stage of detail design. A few relevant points need to be considered in this context: a) It is necessary to have a clear understanding of the causes of the problems in order to arrive at an effective remedial solution. b) While developing alternative schemes, the practical aspect of implementation needs to be considered. Also a solution needing special equipment should be avoided in preference to standard equipment which would be easily available. c)

Rehabilitation work is generally labour intensive and cost of new material is only a fraction of the total cost. Therefore a solution which is straight forward and can be implemented easily by using minimum specialist labour force, even at the cost of more expensive material input, is almost always the most economical solution.

d)

In order to minimize disruption of traffic, partially shop fabricated units should be supplied to site in knocked-down condition, to be assembled and fixed at site.

6.

Detail design

Capacity of a compression member can be increased by introducing new structural member(s), thereby reducing its effective length. In such a case connection details need special attention.

4.3 Reduction of dead load Reduction of dead load of a bridge would increase its live-load carrying capacity. One example of this method is to replace the existing reinforced concrete deck system by orthotropic steel deck system. This aspect should be explored while preparing the conceptual schemes.

4.4 Modification of structural system Capacity of a bridge can be increased by modifying the basic structural system. For example, simply supported spans of longitudinal beams of a deck system can be converted to continuous beams, by suitably modifying the end connections of these beams. Adding new supports to a bridge will reduce the span, thereby increasing the load-carrying capacity of the bridge. One other example of modification of The Bridge and Structural Engineer

Concept design

After a few viable conceptual schemes are identified, these are then subjected to more rigorous analysis and design work. A final scheme is then selected for implementation. A few relevant aspects need to be considered in this connection.

6.1 Relieving dead load stresses Members of an erected bridge are already subjected to the dead load effects of the bridge. Therefore, it is desirable to relieve the members of this dead load Volume 46 Number 2 June 2016â&#x20AC;&#x192; 53

stress temporarily prior to undertaking rehabilitation work. Otherwise, the existing members would continue to be stressed to the extent of dead load effect. Consequently the capacity of the newly added reinforcing material will remain under-utillised, as this cannot reach the permissible stress level without over stressing the existing members. In case it is not practicable to relieve the dead load, the new material should be considered to share the live load only. In any case, proper analysis should be done to avoid over stressing of the existing members.

There are a few methods for relieving dead load stresses of an existing bridge structure. The most common method is to jack up the girder at a few locations and provide temporary support underneath.

Where temporary supports under the bridge cannot be provided (as in the case of a flowing waterway), the bridge may be supported from temporary girders erected above the existing girders by means of suitable hanging arrangements. External pre-stressing is also a very efficient method of relieving dead load particularly for cases with large heights and flowing waterway.

An interesting method of relieving the dead load stresses was adopted in the rehabilitation of Hardinge Bridge in Bangladesh which suffered serious damage during the brief war in 1971. The bottom cord and some diagonal members of the downstream truss (105m span) of the ninth span were damaged by missile attack and the span needed rehabilitation. In order to relieve the dead load stresses, the damaged truss was temporarily anchored to the adjacent downstream trusses with the help of pin-connected link members.

The truss was initially jacked up by adopting barge flotation principle by deploying two steel barges with water tight bulk heads. Prior to jacking up operation the barges were filled with water and the truss was supported at two nodal points of the bottom chord by means of steel trestles fixed on the barges. As water was pumped out of the two barges simultaneously, the span moved back to its original position prior to the damage. In this condition specially designed and fabricated steel link members were connected between the ends of the top chord of the downstream truss of the damaged span with the corresponding top chord ends of the adjacent trusses. Buffers were also fixed between the ends of the corresponding bottom chords to counterbalance the compressive force due 54â&#x20AC;&#x192; Volume 46

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to possible cantilever behaviour of the damaged truss.

After the anchor spans supported the dead load of the truss, the repair and replacement of the damaged members were undertaken. See Figure 1.

Fig. 1: Rehabilitation of Hardinge Bridge, Bangladesh

6.2 Load carrying mechanism Load carrying mechanism is an important issue in determining rehabilitation strategy to be adopted for a damaged bridge structure. A redundant bridge structure has, within itself, multiple load carrying mechanisms, so that if one mechanism fails or is weakened, the load will automatically redistribute itself amongst the other members, and the structure as a whole will still not collapse. A non-redundant structure, on the other hand does not have multiple load carrying system and consequently failure of one member (fracture critical member) may cause collapse of the structure. Therefore, determination of redundancy and identification of fracture critical member require indepth study of the existing structural system. Similarly, when a damaged or inadequate joint is difficult to repair or rehabilitate, providing alternative load paths for transfer of loads may be considered as a solution.

Inadequacies related to load carrying mechanism and inherent structural deficiency is illustrated by a case study of the top lateral bracing system of a railway bridge over Sungai Kerayung in Kuala Lumpur, Malaysia. This double track Pratt truss bridge is about 31.3 m span. Before the rehabilitation work in 1995, the top lateral bracing system consisted of cross beams fixed over the top chords at node points, with diagonal bracings between the top flanges in four out of six bays only. These cross beams extended about 1.2 m beyond the top chords and were connected to the verticals from outside by external knee bracings.

There were no portal bracings at raker ends. Also some of the diagonals and bottom chords consisted of two separate sections without any lacings between these sections. As a result the bridge used to oscillate significantly during passage of trains. The bridge was The Bridge and Structural Engineer

rehabilitated in 1995 by introducing new top lateral bracing system between the existing cross beams.

Also the rigidity of the system was improved by strengthening the connections of the existing cross beams with the top chords and by adding internal sway bracings. As regards portal bracings, the traditional system of portal knee bracings on the end rakers was not feasible, as this would have interfered with the minimum structure clearance envelope for the future locomotives and requirements of proposed electrification of the track. This unusual problem called for an unusual solution. To satisfy the clearance requirements, the portal bracings were placed above the top chords. An open web portal system with knee bracket was fixed on raised support stools positioned along the same axes as the rakers. In addition, new lacings were introduced in the bottom chords and diagonals as required to augment the stiffness of the system. With the introduction of these additional secondary members, the oscillation of the bridge during normal running of trains was substantially reduced. Figure 2 illustrates the scheme.

Fig. 2: Rehabilitation of Railway Bridge over Sungai Kerayung, Malaysia

6.3 Effects of fatigue Causes and harmful effects of fatigue in steel bridges have been discussed in a previous section. In order to reduce these harmful effects, proper and careful examination should be done during detail design stage, particularly in the fracture critical members.

The following aspects need particular attention of the designer: 1.

Details which are likely to produce severe stress concentrations should be avoided.

2.

Gradual changes in sections (avoiding re-entrant notch like corners) should be adopted.

3.

Butt welds are preferable to fillet welds.

The Bridge and Structural Engineer

4.

Double sided fillet welds are preferable to single sided fillet welds.

5. Fillet weld across the direction of principal stresses in tension members should be avoided.

6.

Intersections of longitudinal and transverse fillet welds should be avoided.

7.

Continuous fillet welds should be recommended in preference to intermittent fillet welds, except for connecting intermediate stiffeners to webs of beams and plate girders.

8.

Intermittent butt welds should not be used.

9.

Size of fillet welds should not be larger than is required from design consideration.

10. It is recommended to provide multiple load path (redundancy) to avoid collapse of the structure in case one element fails due to fatigue.

6.4 Connections Defective or loose rivets in an existing riveted connection may need replacement by new fasteners such as close tolerance bolts or high strength fiction grip (HSFG) bolts. Care should be taken to ensure that the new fastener system is compatible with the existing system. These new bolts are to be designed to share both dead and live loads along with the existing rivets and the joints should be checked accordingly.

As far as possible welding should be avoided in existing riveted connections as the welds are likely to carry the entire load in case of slippage in the riveted connection. If used, welding should be designed to transfer the entire load. However, weldability of the parent materialis required to be checked first.

In case of welded bridges additional plates or sections may be added on to the existing members. Care should be taken to detail the joints properly and supervise the field welds closely. There are instances of damages to bridges caused by indiscriminate and improper welding during rehabilitation process. In order to avoid such unwelcome situation, the designer should try to develop a scheme with bolted connections instead of welded connections.

6.5 Effects of eccentricity It is common practice to strengthen a deficient member by adding additional plate or section on to it.

In such cases care should be taken to ensure that the Volume 46 Number 2 June 2016â&#x20AC;&#x192; 55

centre of gravity of the resultant strengthened section coincides, as far as possible, with the centre of gravity of the original section,to avoid secondary stresses induced due to the effects of eccentricity. In case it is not possible to achieve this, the effects of eccentricity should be considered in the design.

6.6 Locked-in stresses In cases of vehicular collision or derailment in railway bridges, very often individual members are damaged due to local buckling. If these members are straightened in situby heating or by mechanical methods, residual locked-in stresses are likely to remain in the straightened member. To avoid this, the damaged member should be first taken out from the structure, thereby relieving the locked-in stresses and then straightened prior to final fitment into the structure. If the damage is considerable, it may be a better solution to replace the damaged member by a new one.

7.

Drawing and specifications

It is imperative that the design drawings and specifications prepared by the designer should be

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clear and unambiguous. All necessary details and preferably the implementation sequence of the proposed rehabilitation scheme should be clearly indicated in the drawings. The specifications should also be very exhaustive and should cover as many problems as possible, which may come up at the time of implementation at site.

8.

Concluding remarks

Design of rehabilitation scheme for steel bridges always demands a great deal of ingenuity and innovativeness. Selection of a rehabilitation scheme depends largely on the various design aspects discussed in the preceding sections. Some other factors such as available down-time, interruptions to the flow of traffic, safety, economy and above all the practical aspect of implementation at site play important role in the selection process. It is always preferable to select a solution which is easy to implement requiring minimal special equipment or specialist workforce even at the cost of higher material input. Proper rehabilitation scheme can extend the service life of the bridge considerably.

The Bridge and Structural Engineer

Design Codes for Steel Construction in India – A Brief Review Arijit GUHA Assistant General Manager (Civil & Structural) Institute for Steel Development & Growth (INSDAG) Kolkata, India. guha_arijit@rediffmail.com

Summary India has been popularly using the erstwhile working stress method of design and construction practices from the early days in all major forms of construction.

Though in reinforced concrete construction the modern limit states method was introduced in India long back in 1984 by publishing its code IS: 456 – 1984, steel construction utilised these rational design practices only from 2007 when the basic code of practice in general construction in steel, IS: 800 was revised from working stress to limit states method and published by the Bureau of Indian Standards in 2007.

Other codes, like code on steel based construction in bridges viz. IRC:24–2010 and IRC: 22–2008, to name a few, were subsequently published by the Indian Roads Congress soon followed keeping Indian conditions in mind, following the basic principles as laid down in IS: 800. This paper touches down on the very basic nuances of limit states design and critically discusses the major factors that govern the merits of the latest codes in India using the limit states method for steel construction.

Keywords: Allowable stress, Limit state, cold-formed, factor of safety, fatigue, composite construction.

1. Introduction The state-of-the-art design finds its way into practice through specifications and stipulations of relevant codes. In India, several development works has taken place for improving the material properties of steel, yet the design is uneconomical at times due to nonavailability of efficient sections. Some design codes The Bridge and Structural Engineer

Arijit Guha was born in 1964. He received his Civil Engineering degree from Jadavpur University, Kolkata and did his M. E. from BITS, Pilani. He worked for Consultants Company like DCPL, McNally Bharat, before Joining INSDAG. His main area work is Preparation of Codes for Bureau of Indian Standards and Indian Roads Congress & Steel Application Research.

have been updated and others are being updated and modified as a continuous process incorporating the results from the various researches and developments being carried out at the various R & D Centres in the country.

The earlier edition of IS: 800, which was prepared in 1984 and reaffirmed in 1991, was out-dated. This code was based on Allowable Stress Design, which was in vogue till the 1960’s all over the world.

The modern Limit State Method developed and adapted in advanced countries in the early 1970’s is technologically improved and results in a more rational design. Considering that the current practice all over the world is based on Limit State Method (LSM) or Load and Resistance Factor Design (LRFD) Method, it was found essential during the year 2002 – 2003 that the code of practice for use of steel in general construction should be modified to LSM similar to what had been done for reinforced concrete structure codes back in 1984, while maintaining Allowable Stress Design as a transition alternative. The code was thus prepared and published by the Bureau of Indian Standards (BIS) in 2007.

The Indian Construction is most often guided and controlled by steel, cement as the prime material of construction. Cement requires a healthy partnership with aggregates and steel to form the structural element called concrete. Steel, on the other hand has not only an advantage of partnering with concrete but also can go alone as an individual structural element.

In order to reap the advantages of steel the whole supply chain needs to be in place. Use of steel as a Volume 46 Number 2 June 2016 57

preferred material for design can be increased with modern rational codes, updated with the scientific researches, and also which are user friendly etc. The design engineers will then be inclined in deciding on using steel. This will automatically increase the steel consumption in the country which plays a direct role in the country’s GDP. The market has a huge potential in the rural sector. In order to tap this untapped scope and potential, various measures and policies are being framed mainly by the manufacturers as also by the government. In order to match with this expectation, other related parameters need to be in line too. Modernising design and construction codes are one of them.

As stated earlier, IS-800, the umbrella code for general structural steel design was prepared long back. In the meantime the methodology of design of steel structures had undergone major changes due to two decades of research all over the world. Since an out-dated code would be detrimental to the very purpose of the code of practice itself, the basic code for design of steel structures needed updating using recent research findings and practices in developed countries. Thus, the code was revised under the supervision of an expert committee constituted by the Bureau of Indian standards. The code had ultimately been published in February 24, 2007.

Almost all advanced countries are now taking advantage of efficient code stipulations, and the current practice all over the world is based on either Limit State Method (LSM) or Load and Resistance Factor Design (LRFD) method. Table 1, shows design format of steel structures adopted in some of the countries.

Table 1: Countries and their Design Format.

Australia, Canada, China, Limit State Method (LSM) Europe, U K, Japan USA Load and Resistance Factor Design (LRFD) and Allowable Stress Design Method (ASD) India Limit State Method (LSM) [Recently Adopted]

2.

Modern Indian Codes

As discussed above, the major change in design philosophy was achieved in India with the introduction 58 Volume 46

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of Limit States Method as the general method of design and attributing the existing Allowable Stress Method of design gradually redundant.

2.1 Design Philosophies As discussed earlier all modern Indian codes have adopted the Limit states method of design as the premier design philosophy, be it reinforced concrete construction, steel construction or steel-concrete composite construction for all types of structure including general buildings and structures as well as for bridges and their supports. However, it is important to understand the differing philosophies of Allowable Stress Method of Design and Limit State Method / LRFD as they apply to design of steel structures.

2.1.1 Allowable Stress Method With the development of linear elastic theories in the 19th century the stress-strain behaviour of new materials like wrought iron & mild steel could be accurately represented. These theories enabled indeterminate structures to be analysed and the distribution of bending and shear stresses to be computed correctly. The first attainment of yield stress of steel was generally taken to be the onset of failure. The limitations due to non-linearity and buckling were neglected.

The basic form of calculations took the form of verifying that the stresses caused by the characteristic loads must be less than an “allowable stress”, which was a fraction of the yield stress.

Simply put, factual ≤ fallowable and the allowable stress is given by

Yield stress Allowable stress = –––––––––––––

Factor of safety Thus, the allowable stress has been defined in terms of a “factor of safety", which represented a margin for overload and other unknown factors which could be tolerated by the structure. The factor of safety (Fs) used in the allowable stress design method, however, is fixed. This means that no matter how variable the loads are, in terms of either frequency or magnitude, the factor of safety is always the same. This unique value of Fs under varied conditions or situations makes the design rigid and takes away the advantage of flexibility and more optimised aspects of design.

These deficiencies as well as advanced knowledge of The Bridge and Structural Engineer

strength of material beyond yield point and its plastic plateau led to the development of an alternative to the ASD based on the limit states of a material.

In general, each member in a structure is checked for a number of different combinations of loading. The value of factor of safety in most cases is taken to be around 1.67. Many loads vary with time and these needed to be allowed for. It is unnecessarily severe to consider the effects of all loads acting simultaneously with their full design value, while maintaining the same factor of safety or safety factor. Using the same factor of safety or safety factor when loads act in combination would result in uneconomic designs. A typical example of a set of load combinations is given below, which accounts for the fact that the dead load, live load and wind load are all unlikely to act on the structure simultaneously at their maximum values: (Stress due to dead load + live load) < Allowable stress (Stress due to dead load + wind load) < Allowable stress (Stress due to dead load + live load + wind) < 1.33 times allowable stress.

In practice there are severe limitations to this approach.

These are the consequences of material non-linearity, non-linear behaviour of elements in the post-buckled state and the ability of the steel components to tolerate high theoretical elastic stresses by yielding locally and redistributing the loads. Moreover the elastic theory does not readily allow for redistribution of loads from one member to another in a statically indeterminate structure.

2.1.2 Limit State Method The limit state method of design was, in part, developed to address the drawbacks to the allowable stress method of design mentioned earlier. Allowable stress method suffers from the inability of the factor of safety to adequately address the variable nature of loading conditions. Limit state method makes use of the plastic range of material for the design of structural members and incorporates load factors to take into account of the variability of loading configurations.

Thus, a rational but variable factor of safety in different structural performance enables to use steel efficiently The Bridge and Structural Engineer

and economically in different structural systems to withstand tension, compression etc. Limit State Method of design considers the good performance of steel in tension compared to compression and specifies variable factors. The main advantage of the limit state method is that it takes into account this variance by defining limit states, which address strength and serviceability. According to this method a structure or part of it is considered unfit for use when it exceeds the limit states, beyond which it infringes one of the criteria governing its performance or use.

The two limit states are classified as the Ultimate Limit State and Serviceability Limit State. The ultimate limit states take care of the safe operation and adequacy of the structure from strength point of view. The criteria which are used to define the ultimate limit state are yielding, plastic strength, fatigue, buckling etc. serviceability limit state takes care of the performance and behaviour of the structure during its service period. Deflection, vibration, drift etc. are considered as serviceability criteria.

Limit state method considers the critical local buckling stress of the constituent plate element of a beam. This enables to enhance resistance of plate elements to local buckling by suitably reducing the slenderness ratio. Hence it is possible to develop the full flexural moment capacity of the member or the Limit State in Flexure. A very important introduction in the latest code to qualify the limit states method of design is the introduction of section classifications based on the b/t or d/t ratios of the individual elements of a section. In the allowable stress design method there was one single restriction regarding the limiting b/t or d/t ratio. In the LSM the sections are classified as plastic, compact, semi-compact or slender based on the above ratios. The limiting load or moment carrying capacities of a section are derived based on the type of section. This section classification becomes essential as the moment capacities of each of these sections takes different values, whereas in the existing allowable stress design based on IS: 8001984, the extreme fibre stress is restricted to 0.66fy irrespective of slenderness ratio of the constituent plate elements.

In LSM, the factored loads, in different combinations, are applied to the structure to determine the load effects. The latter are then compared with the design strength of the elements.

Volume 46 Number 2 June 2016â&#x20AC;&#x192; 59

S* ≤ R* where S* is the calculated factored load effect on the element (like bending moment, shear force etc) and R* is the calculated factored resistance of the element being checked, and is a function of the nominal value of the material yield strength. S* is a function of the combined effects of factored dead, live and wind loads (Other loads – if applicable, are also considered).

In accordance with the above concepts, the safety format used in Limit State Codes is based on probable maximum load and probable minimum strengths, so that a consistent level of safety is achieved. Thus, the design requirements are expressed as follows: Sd ≤ Rd where Sd = Design value of internal forces and moments caused by the design Loads, Fd

Fd = γf * Characteristic Loads.

γf

= a load factor which is determined on probabilistic basis

Rd = Characteristic Value of Resistance

γm where γm = a material factor, which is also determined on a ‘probabilistic basis’. This probabilistic values vary from code to code depending upon local conditions, for example the partial load factor γf for Dead load as per IS: 800 is 1.5, for dead and live load combination, whereas, it is 1.35 as per British code under similar load combination. Similarly the partial safety factor for material against yield stress and buckling, γmo, which is 1.1 in IS: 800 have been adopted as per Euro Code, whereas it is 1.0 as per BS: 5950, 1.11 for AS: 4100 and AISC: 360. Again the partial safety factor for material against ultimate stress, γm1, is 1.25 in IS: 800, adopted as per Euro Code, whereas it is 1.2 as per BS: 5950, 1.31 for AS: 4100 and AISC: 360.

It should be noted that γf makes allowance for possible deviation of loads and the reduced possibility of all loads acting together. On the other hand γm allows for uncertainties of element behaviour and possible strength reduction due to manufacturing tolerances and imperfections in the material. To summarise the above, a civil engineering designer has to ensure that the structures and facilities he designs are (i) fit for their purpose (ii) safe and (iii) economical and durable.

Thus safety is one of the paramount responsibilities 60 Volume 46

Number 2 June 2016

of the designer. However, it is difficult to assess at the design stage how safe a proposed design will actually be consistent with economy. There is, in fact, a great deal of uncertainty about many factors, which influence both safety and economy. Firstly, there is a natural variability in the material strengths and secondly it is impossible to predict the loading, which a structure (e.g. a building) may be subjected to on a future occasion. Thus uncertainties affecting the safety of a structure are due to 

uncertainty about loading



uncertainty about material strength and



uncertainty about structural dimensions and behaviour.

These uncertainties together make it impossible for a designer to guarantee that a structure will be absolutely safe. All that the designer could ensure is that the risk of failure is extremely small, despite the uncertainties.

An illustration of the statistical meaning of safety is given in Fig. 1.

Fig. 1: Statistical Meaning of Safety

2.2 Discussion on Latest Steel Codes All steel codes, which have been recently formulated or revised from its earlier version, have been prepared keeping in mind the development that has taken place globally in terms of both design practices as well as modern fabrication & construction practices.

IS: 800 – 2007, which is the basic code for design and construction of all steel structures in India has been revised from its earlier allowable stress method version based on stipulations laid down in the Euro codes with modifications in various parameters which depend on geographical location of India and normal Indian construction practices which in turn is dependent on resources available in India. Few other codes have been revised or formulated in the recent The Bridge and Structural Engineer

past and few are under preparation, which are for use keeping the latest trends in steel construction as mentioned below.

1. The code for design of cold formed steel structure, IS: 801 is presently under preparation and is also based primarily on the corresponding euro codes keeping in mind the type of cold formed steel and sections manufactured in India.

2. IS: 11384-1985 which is the general code for design and construction for composite structures is also being revised by the BIS at the moment.

3.

IS: 806, the code for design of tubular structure was using the erstwhile allowable stress design method, but IS: 800 – 2007 has included design of tubular steel structures using the limit states method as prescribed in it.

4.

IS: 12778, which is the code for parallel flange sections have now been included in IS: 808, which is under finalisation and will be published soon. This revised IS: 808 will not only include the parallel flange sections but will also indicate the plastic section modulus of all sections which is must for designers for designing sections using the limit states method.

5. For the bridges, mainly for highway, the two major codes IRC: 24–2010, which caters to steel bridges and IRC: 22–2015 which stipulates design philosophy for steel-concrete composite bridges have been prepared/revised based on stipulations laid down in IS: 800.

Additionally guidebook for design of Steelconcrete composite box girder bridges are also under preparation which will be very useful considering the fact that many of the obligatory spans of major urban and highway flyovers are coming up as composite box girders.

While assessing the highlights of most of these modern codes mainly the IS: 800, one comes across the various new additions or modifications which have been introduced in these codes which are at par with international codes and standards and are far more rational corresponding to earlier practices. A few of them are worth discussing.

2.2.1 Design against Fatigue Over the years the concept of fatigue in a structure and its remedial and preventive treatments has undergone The Bridge and Structural Engineer

major changes with the development of new design techniques all over the world. The revised code which has extensively dealt with design for fatigue stresses by devoting one full chapter for fatigue analysis and design, has adopted various design parameters based on the popular Euro Code, and has shifted away from the procedures and concepts that are prevalent for design of welded steel structures against fatigue as per the existing relevant code, i.e. IS: 1024.

The existing code for fatigue assessment of welded steel structure subjected to dynamic or rolling loads IS: 1024 – 1979 has not been revised for the last 26 years. This code caters to working stress method of approach for fatigue design. As per the general norm this code also has various Classes of Construction Details, namely from “Class A” to “Class G”. The basic concept of analysis for fatigue as per this code is the “stress ratio” concept, which had been in practice since long. According to this code, working stresses needs to be reduced, where necessary, to allow for the effects of fatigue. Allowance for fatigue also requires to be made for combination of stresses due to dead, live, impact, and centrifugal forces, including secondary effects due to eccentricity of connections and off-joint loading in latticed structures. However stresses due to wind, temperature, longitudinal and nosing force and secondary stresses due to elastic deformations and joint rigidity may be ignored in considering fatigue.

Elements of a structure may be subjected to a very large variety of stress cycles varying both in ranges given by stress ratio (fmin / fmax) and in magnitude fmax of maximum stress. For various detail class, there are limiting values of fmax, corresponding to different design stress ratios, fmin / fmax and N (i.e., number of stress cycles for the entire design period). These limiting values corresponding to various loading classes have been tabulated in the code IS: 1024 1979. The actual fmax is kept within the permissible value (i.e. permissible fmax) as obtained from the tables.

The above code also stipulates equivalent stress fe, both for combination cases involving shear plus bending and shear plus bending and bearing, which requires to be checked against the limited values as stipulated in the above mentioned tables. The equivalent stress for combined forces as mentioned in IS: 1024 – 1979 are, Volume 46 Number 2 June 2016 61

2.2.2 Design against Seismic Forces

 For shear and bending,

(1)

 For shear, bending and bearing,

(2)

Where,

fbt

fbc compressive bending stress

fq

tensile bending stress bearing stress

A simple approach has been adopted in IS: 800 – 2007 in line with the Euro codes for the fatigue design of elements or members subjected to moving loads. This code and subsequently all other codes mainly for bridges like IRC: 22 and IRC: 24 adopt the modern approach of “stress range” as are prevalent in the developed nations instead of the existing “stress ratio” concept as followed by the earlier Indian code IS:1024 – 1979. For the purpose of design against fatigue, similar to European and American standards, different details (of members and connections) are classified under different fatigue class. The permissible design stress range or the fatigue strength corresponding to various number of cycles, are given for each fatigue class.

In this new concept the “Stress Range” which is given as fmax – fmin is calculated for a given dynamic or moving loads on a structural elements and is checked against a permissible set of limits which depend on the nature of the element, the nature of the joint at which fatigue is being assessed and the nature of the fatigue stress. Factors influencing fatigue behaviour are mainly stress range, stress concentration, rate of cyclic loading, residual stresses, size, geometry, environment, temperature, previous stress history.

Broadly speaking, for a structure fmin corresponds to stress under only static loads and fmax corresponds to the total stress on the structure including dynamic or rolling loads. IS: 800 – 2007 has been prepared along the same line as Euro code EC4 and has extensively dealt with fatigue calculations. The other important and widely used codes are the British code BS 5400: Part 10, which extensively deals with fatigue and the AASHTO specifications as followed in the United States of America.

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Since IS: 800 is the basic code for design in steel structures, stipulations laid down in this code are binding for all steel structures and are generally adopted for other codes, e.g. codes for bridges, chimneys, silos bunkers etc.

As far as design of steel structures against earthquake is concerned, though IS: 1893 (all four parts) lays down specifications for analysis of structures against earthquake, the latest IS: 800 stipulates guidance specifically for design of steel structures in terms of separate load combinations to be used while designing a structure for combination of various loads such as Dead Load, live load etc. The response reduction factor have also been generally categorised for different types of framing system. Different joint system like use of welded joints, bolted joints with either simple bolts or HSFG bolts has also been included in tune with modern construction practices and as mentioned in other international codes.

Detail guidance on design of various types of framing system with steel such as Ordinary Concentrically Braced Frames (OCBF), Special Concentrically Braced Frames (SCBF), Eccentrically Braced Frames (EBF), Ordinary Moment Frames (OMF) and Special Moment Frames (SMF) have been provided.

Various connection details like connections between braced members and bracings and beam to column connections for moment frames have been illustrated in details with diagrams for proper understanding of the designer.

2.2.3 Fire Resistance Since properties of steel vary with temperature, resistance of structure in case of fire has been dealt in detail in IS: 800 – 2007. The major properties and their effects under temperature have been indicated in the code for design of steel members and structure as a whole under severe fire conditions. For example the temperature variation is as given below.

where fy(T)

= yield stress of steel at To C

fy(20) = yield stress of steel at 20o C (room temperature) The Bridge and Structural Engineer

T

= temperature of the steel in oC

2.3 Steel-concrete Composite Construction Latest Construction Technique

as

Steel-concrete composite construction combines the compressive strength of Concrete with the tensile strength of Steel to evolve an effective and economic structural system. Over the years this specialized field of construction has become more and more popular in the western world and has developed into a multifaceted design and construction technique.

Steel-concrete composite construction is only recently getting popularized in the Indian scenario, but is still limited to commercial structures like multiplexes, some industrial structures and a few multi-storied constructions. This type of construction has more recently gained major popularity in the bridge sector in India, mostly in the eastern region in Kolkata and in the northern sector in New Delhi in the form of flyovers.

In steel-concrete composite construction, structural steel work is typically used together with concrete; for example, steel beams with concrete floor slabs for taking floor loads. The same applies to buildings, road bridges, where concrete decks are normally preferred. Steel and concrete have almost the same thermal expansion apart from an ideal combination of strengths. Hence, these essentially different materials are completely compatible and complementary to each other. Members made of structural steel and concrete can be used as composite structures so that they act together and concrete is subjected to compressive stresses and steel takes the tensile stresses.

Limit State Analysis as well as Load Resistant Factor Design adopted by UK and USA respectively has also contributed so far to uneconomical design. With the introduction of IRC:22 â&#x20AC;&#x201C; 2015, for road bridges, construction of steel-concrete composite bridges are expected to increase. In the eastern and northern part of the country these girders have already started to be constructed due to its efficient economic, geometric and structural advantages.

Fig. 2: Typical Composite Girder

Fig. 3: Composite floor system using profiled sheets

2.3.1 Typical Composite Sections and Design Codes

Composite Slab

Composite sections may be in beams, slabs or columns.

The typical sections are as shown in the figures 2, 3 and 4. The basic code for design of steel-concrete composite buildings is IS: 11384 â&#x20AC;&#x201C; 1985. This code stipulates the design procedure for flexure members only. This code is under revision at the moment and would include design principles for not only beams but also composite slabs and columns.

Traditional steel - concrete composite floors consist of rolled or built-up structural steel beams and cast in-situ concrete floors connected together using shear connectors in such a manner that they would act monolithically (Fig.2).

The pertinent factor which had governed the design of steel and composite sections, have mainly been the use of triangular stress distribution as per elastic theory instead of rectangular stress block based on plastic analysis. Non-availability of codes permitting

The Bridge and Structural Engineer

More recently, composite floors using profiled sheet decking have become very popular in the West for high-rise office buildings. Composite deck slabs are particularly competitive where the concrete floor has to be completed quickly and where medium level of fire protection to steel work is sufficient.

However, composite slabs with profiled decking are unsuitable when there is heavy concentrated loading

Volume 46 Number 2 June 2016â&#x20AC;&#x192; 63

or dynamic loading in structures such as bridges. A typical composite floor system using profiled sheets is shown in Fig.3. There is presently no Indian standard covering the design of composite floor systems using profiled sheeting. The structural behaviour of these floors is similar to a reinforced concrete slab, with the steel sheeting acting as the tension reinforcement. The main structural and other benefits of using composite floors with profiled steel decking are:  Considerable

savings in weight of steel components are a typical benefit of Composite construction over non-composite construction.



Greater stiffness of composite beams results in shallower depths for the same span. Hence lower storey heights are adequate resulting in savings in cladding costs, reduction in wind loading and savings in foundation costs.



Faster rate of construction.

3.

Recent Case Studies

Some buildings using the steel concrete composite construction technology and stipulations indicated in IS:800 – 2007 and draft IS: 11384 (under preparation) are as shown below in Fig 5, 6, and 7. All these buildings have been built using steel-concrete composite sectional elements. The B+G+4 INSDAG building has composite beams and steel columns. The 2B+G+4 Handloom house which was constructed by HSCL for the Ministry of Handloom has composite beams and steel-encased concrete columns whereas one tower of the Indira Pariyabaran Bhawan which is 21 metres by 19 metres with no internal columns has been constructed with composite beams, composite slabs and steel-encased concrete columns.

The steel decking performs a number of roles, such as: 

It supports loads during construction and acts as a working platform



It develops adequate composite action with concrete to resist the imposed loading



It transfers in-plane loading by diaphragm action to vertical bracing or shear walls



It stabilizes the compression flanges of the beams against lateral buckling, until concrete hardens.



It reduces the volume of concrete in tension zone



It distributes shrinkage strains, thus preventing serious cracking of concrete.

Fig. 5: New Office of Institute for Steel Development & Growth

Composite Columns Composite columns may be either steel encased concrete sections or concrete in-filled steel sections.

Standard composite sections using steel encased concrete construction are as shown below: Fig. 6: Indira Pariyabaran Bhawan

Fig. 4: Fully and partially concrete encased columns

64 Volume 46

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The Bridge and Structural Engineer

a structure are rational and economical and do not border around the absurd.

5.

Discussions, Conclusions Acknowledgements

and

Codes and standards are the backbone for design and construction for general as well as special structures.

Though the basic theories are laid down in various text books and references, the code stipulations not only covers the basic fundamentals based on which the design has to be made but also lays down the guidelines and limitations for a particular design or construction based on various conditions associated with the geographical location of the structure and the function of the structure itself.

Fig. 7: Handloom House

4.

Future Actions on Codes

The codes have been revised or prepared mostly to make them at par with other international codes.

There are still some grey area in the codes which may be improved in the near future after proper feedback from users as well as from research and experimental data.

As an example, the design of single angle members in compression connected by one leg, the constants that are indicated in the code are totally experimental and it becomes difficult for designers to understand the basic theory behind the design process. Also the codes are silent about the number of stress cycles that need to be used for design of structures carrying moving loads. These are mainly due to the lack of statistical data. These data need to be developed and incorporated in codes so that the final design of

The Bridge and Structural Engineer

As described in this paper, India is at the moment, moving forward keeping pace with the latest design and construction technologies prevalent across the world mostly in the developed nation. Hence it is imperative that its codes should also be in tune with all international codes. Various research are taking place in the major research institutes in the country, in terms of material development and technological development in design and construction, Being a country with financial constraints it has to also depend on studies and research that have already taken place in developed countries. Thus most of the codes that have been developed in the last 10 to 15 years or that are under preparation are mostly a synergy of research findings both in India and abroad and are therefore most illustratively and accurately conducive for Indian design and construction spectrum.

6. References 1.

T. K. BANDYOPADHYAY & ARIJIT GUHA, “Code Stipulations – Indian & International Codes and Revision of IS 800” – Course at S V University College of Engineering, Tirupati 2003.

2. IIT, MADRAS, ANNA UNIVERSITY, MADRAS & INSDAG, KOLKATA, “Teaching Resource for Structural Steel Design, Volume 1, 2 & 3” 3.

R. P. JOHNSON & R. J. BUCKBY., “Composite Structures of Steel & Concrete – Volume – 1”

Volume 46 Number 2 June 2016 65

SLENDER SECTION DILEMMA – THE INDIAN PERSPECTIVE V. KARTHIK Structural Engineer Delhi NCR, India arsenal4karthik@gmail.com

Summary The failure of a section is influenced by the local geometry of individual parts. A structure would fail either after reaching its plastic capacity or even before reaching the elastic limit due to local buckling based on the proportioning of cross section of individual elements. The design of elements which fail before reaching the elastic limit has been at the centre of many debates in India ever since the Limit state philosophy for steel structures was introduced. This was so because neither the IRC : 24 nor IRC : 22 adequately catered for the cases where it was possible to use such sections safely to achieve better economy.

This paper intends to deal with slender webs of plate girders and how it is interpreted in different codes elsewhere and its comparison to how it is dealt with in India.

Keywords: Slender section, class 4 section

1. Introduction The use of slender sections in steel/steel concrete composite bridges has been a part of foreign codes like BS5950-1:2000, EN 1993-1-1:2005, etc. But the codal provisions of IRC 22 & 24 have caused significant restrictions in the use of such sections mainly because of the lack of description inhibiting an engineer to design and detail such sections. This paper tries to compare the different British code and Eurocode to arrive at a logical conclusion as to what has been missed in translation (some implicit and other explicit) when the new Indian limit state codes were formulated.

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V Karthik, born 1987, received his bachelors in civil engineering from Gujarat University, India in 2008 and Masters in Civil (structures) engineering from Thapar university, Patiala, India in 2011. He is a practising engineer with focus on bridges.

The need for such a comparison owes its roots to the fact that usually in bridges the shear forces are predominantly low and a slender web is usually sufficient to cater for low shear forces in bridges. But, with the restrictions of the Indian limit state codes the webs have to be designed with a much more stocky section then previously done in working stress method leading to a possible loss of economy. This paper tries to explore a logical reasoning behind it and tries to find out whether such a drastic change has arose out a design requirement of Limit state philosophy or if it a result of our codes getting “Lost in translation”.

The choice of BS 5950-1:2000 & EN 1993-11:2005 for comparison to the present Indian codes owes to the fact that the present limit state code was seemingly adopted from the BS 5950-1:2000 (which is now withdrawn) and the code that replaced the old BS 5950-1:2000 i.e., EN 1993-1-1:2005.

2.

Dealing with slender sections

Different codes around the world have different ways to deal with slender sections. Two principle ways in which it is dealt with are : (a) The reduced section method : In this method, a slender section is converted to an effective elastic (Semi compact) section. This requires omitting some portion of web to arrive at a reduced section. This is probably the easier approach and hence, very widely used. (b) The reduced stress method : In this method, the full section is considered effective but the yield stress fy of the section is reduced to convert a The Bridge and Structural Engineer

slender section into an equivalent effective elastic (Semi compact) section.

In this paper, the first methodology is used to compare and contrast the different codes as it is the only method described in Indian codes.

The sample cross section used for calculation is kept the same for all codes. The top/bottom flange are kept as 175 mm x 12mm so as to keep them as plastic section as the primary aim of this paper is to deal with slender webs. The web is kept as 1500 mm x 8mm which will make it a slender section in all the codes used to compare. The grade of steel (fy) is taken as 275 MPa for all cases and the value of ε is modified accordingly.

plate girders in bridges there is negligible axial force in the member.

Step 2 Effective Section (As per Clause 3.6.2.4) The effective width of compression zone under pure bending is given by : beff

For a doubly symmetric section subject to pure bending, fcw = ftw. Hence, beff

= 60εt

= 60 x 1 x 8

= 480mm

Now, the effective width distribution is 0.4 beff from top = 192 mm and 0.6 beff from neutral axis = 288 mm as shown below :

Fig. 1 : Showing the basic section used

2.1 BS 5950 – 1 : 2000 Step 1 Cross section classification (As per table 11) d/tw

=

1500/8 = 187.5mm

Limiting Value for class 3 :

d/tw

=

120ε / (1+2r2) but ≥ 40ε

=

but ≥ 40 x 1

=

120

=

120 < 187.5

but ≥ 40

Hence, the Web is Slender

Fig. 2 : Showing the basic section as per BS 5950 – 1 : 2000

The process of calculating the effective section is iterative. It requires assuming a certain value of the width of ineffective zone and finding out the value of position of neutral axis from bottom. If this value is coherent with the overall width then the value assumed is correct otherwise the width of ineffective zone is changed. This process is repeated till all the three values i.e., (1) The width of the ineffective zone (bx), (2) Position of neutral axis from bottom (yb) and (3) Overall width become coherent with each other.

Note : Here, r2 is assumed as 0 because normally for The Bridge and Structural Engineer

Volume 46 Number 2 June 2016 67

Trial 1 : Considering the width of web in excess of the class 3 section limit as ineffective d / tw

= 187.5 > 120

Width in excess of class 3 section :  bx

= (187.5 – 120) x tw

= 67.5 x 8

= 540 mm

 bt

= 1500 - 192 - bx - 288

= 480 mm

Trial 2 : Reduce the value bx to 350 mm  bt

= 1500 – 192 – 350 - 288

= 670 mm

Now, yb = (175 x 12 x 6) + (958 x 8 x 491) + (192 x 8 x 1416) + (175 x 12 x 1518) (175 x 12) + (958 x 8) + (192 x 8) + (175 x 12) =

12600 + 3763024 + 2174976 + 3187800 2100 + 7664 + 1536 + 2100 9138400 13400 = 681.97 mm =

Therefore, yb + 0.6 beff + bx + 0.4 beff + tf = 681.97 + 288 + 350 + 192 + 12 = 1523.97 mm ≅ 1524 mm Hence, The Width of ineffective zone bx = 350 mm 2.2 EN1993-1-1: 2005 2.2.1 Without Longitudinal Stiffener Step 1 Cross section classification (As per table 5.2) d/tw

= 187.5 mm

Limiting value of for class 3 : Fig. 3 : Showing the effective section as per BS 5950 – 1 : 2000

Hence, yb = ay a [(175 x 12 x 6) + (768 x 8 ) x 396] + (192 x 8) x 1416 + (175 x 12) x 1518 = (175 x 12) + (768 x 8) + (192 x 8) + (175 x 12) 12600 + 2433024 + 2174976 + 3187800 = 2100 + 6144 + 1536 + 2100 =

d/tw

= 124ε = 124 x

= 114.627< 187.5

Hence, web is slender.

Step 2 Effective Section (As per clause 4.4, EN 1993-1-5:2006) Aceff

= ρAc

7808400 11880

= 657.27mm from bottom Therefore, yb + 0.6 beff + bx + 0.4 beff + tf = 657.27 + 288 + 540 + 192 + 12 = 1689.27 > 1524 mm This means that the value of ineffective zone taken as 540 mm is high and needs to be reduced.

Number 2 June 2016

= 0.633

Therefore, as per table 4.1 beff = ρbc

= 0.633 bc

 be1 = 0.4 beff = 0.4 x 0.633

 be2

68 Volume 46

= 0.2532 bc

= 189.9 mm = 0.6 beff = 0.6 x 0.633 = 0.3798 bc The Bridge and Structural Engineer

= 284.9 mm

Hence, The width of ineffective zone bx = (1 – 0.633) bc

= 0.367 x 750

= 275.2 mm

Fig. 6 : Showing the effective section as per EN1993-1-1: 2005 (with longitudinal stiffener)

By adding a longitudinal stiffener from compression flange at a distance of 2/5 of the distance from compression flange to neutral axis almost the whole web becomes effective.

2.3 IRC: 24-2010 Fig. 4 : Showing the effective section as per EN1993-1-1: 2005 (without longitudinal stiffener)

Step 1 Cross section classification (As per table 2)

2.2.2 With Longitudinal Stiffener By adding a longitudinal stiffener in the compressive zone, an increase in the effectiveness of the compressive zone occurs. In this example, a longitudinal stiffener is added at a distance from compression flange equal to 2/5 of the distance from compression flange to neutral axis. The section for the longitudinal stiffener is chosen as 75 x 8 mm. The increase in effectiveness of section due to the addition of the longitudinal stiffener can be illustrated by the following figure :

= 120.3 < 187.5

Hence, web is slender.

Step 2 Effective Section (As per clause 503.7.2) The width of ineffective zone bx = [187.5 –

120.13] x tw

= 67.37 x 8

= 538.96 mm

Fig. 5 : Showing the basic section as per EN1993-1-1: 2005 (with longitudinal stiffener) Fig. 7 : Showing the effective section as per IRC: 24-2010

The Bridge and Structural Engineer

Volume 46 Number 2 June 2016 69

2.4 IRC: 22-2015 Step 1 Cross section classification (As per table 2)

Limited value for class 3 :

= 120.3 < 187.5

Hence, web is slender.

Step 2 Effective Section (As per clause 603.1.1) This code does not deal with these types of section.

Unfortunately the author has found that this is misinterpreted by many as slender sections are not to be used in steel concrete composite bridges. This code is under revision and the author understands that the same calculation/procedure as done in IRC 24 for calculating effective width will be valid for IRC 22.

3. Conclusion This paper tried to answer two questions that are pertinent to the Indian scenario. The first question was whether a slender section should be permitted or not and the second question being if yes, then how to deal with them.

Slender sections are elaborately explained and are an integral part of the foreign codes and there seems to be no ambiguity in whether such sections can be permitted or not. Based on the comparison done in this paper, the author believes that slender should be made part of the Indian codes.

Now, the methodology for using such sections is elaborately documented in the eurocode for sections without/with longitudinal stiffener but the parent code from which the Indian bridge codes were seemingly adopted (BS 5950 – 1 : 2000) gave only procedure

70 Volume 46

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for designing the sections without longitudinal stiffeners. The author believes that this is because it is implicitly understood that the plate girders with longitudinal stiffeners are very rare in buildings and hence not dealt with in BS 5950 – 1 : 2000. It is worth noting that sections with longitudinal stiffeners are an integral part of the British bridge code BS 5400-3 : 2000.

For the design of sections without longitudinal stiffeners, the method adopted in BS 5950 – 1 : 2000 seems apt and the same can be adopted by the Indian codes.

For the design of sections with longitudinal stiffeners, there are two approaches that are used. The first approach (traditional approach) is the empirical based stiffness approach in which a nominal stiffness required for the stiffener is calculated and if that stiffness is provided then the whole section is considered effective. The second approach is the strength approach as adopted by EN1993-1-1: 2005 in which an effective section along with the stiffener is calculated. The Indian bridge codes have adopted the first approach but it is not explicitly mentioned anywhere that if the nominal stiffness is provided then the whole section is considered effective (class 3) and hence, the confusion as to how to deal with sections with longitudinal stiffeners. The author feels that if this fact is explicitly mentioned then that would tie the slender sections with longitudinal stiffeners and the whole confusion can be done away with.

4. References 1.

IRC : 24 – 2010

2.

IRC : 22 – 2015

3.

BS 5950-1 : 2000

4.

BS 5400-3 : 2000

5.

EN 1993-1-1 : 2005

6.

EN 1993-1-5 : 2006

The Bridge and Structural Engineer

Full scale load testing and performance evaluation of a steel-concrete composite RoB Arul Jayachandran obtained his Masters and Doctoral degrees from IIT Madras in the area of stability design of steel structures. After twenty one years of research at Structural Engineering Research Centre, Chennai, in the area steel structures, he joined IIT Madras as a faculty in the Structural Engineering Laboratory, Department of Civil Engineering. He is the convener of the BIS committee for IS:800(2007), member of committees for IS:801, IS:802 and IS:806, IRC Committee B5. He has thirty international journal publications in steel structures and he is an Editorial Board member of the journal Advanced Steel Construction. He is a life educator member of American Institute of Steel Construction, International Welding Institute Committee XV on Fatigue design of steel structures. His research interests include steel-concrete composites and tubular structures design.

S. Arul JAYACHANDRAN A/Professor, Indian Institute of Technology Madras, Chennai 600 036

Abstract This paper presents an on-site testing procedure carried out to evaluate the as built design capacity of the steel –concrete composite RoB between pier P1 and P2 at chainage: KM. 587.832 in Madurai – Kanyakumari section of NH-7, for 70R and two lanes class A loadings. The design of the RoB was submitted by M/s IRCON International Limited.

The team from IITM led by the author, designed an external proof load system which not only creates the maximum deign bending moment but also the share of the moment on girders G1 to G5. The maximum shear was checked analytically. For this test, IRC - SP 51 along with IRC - SP- 37 and IRC -6-2010 are the guiding documents. Test was conducted by applying the design load in increments on the bridge and the measurements were taken for deflection and strains. The measurements of deflection were taken for a further period of 24 hours after the removal of the entire load. The net rebound in the mid span deflection after deducting the temperature variations was more than 88% of the maximum loaded girder and an average of 90% in the mid span of all the other girders. Hence, based on the stipulations of IRC - SP The Bridge and Structural Engineer

51, the RoB design is demonstrated to have passed the proof load test.

1.

Introduction to the RoB

The RoB is a steel concrete composite structure between P1 and P2 at chainage KM 587.832. It has a span length of 52.612m between the two piers P1 and P2 and has a skew of 62º. The superstructure consists of five longitudinal steel plate girders at a spacing of 1.5 m centre to centre, and an overall depth of 2.75m (Fig.1 and Fig.2) . The carriage way width is 7.5m between the both side crash barriers. 1.5 m wide footpath is proposed on outer side of the carriage way (near Grid G5, Refer Fig. no. 2). The overall width of the super structure is 10.25m (Fig.3). There are eleven intermediate cross diaphragms,normal to the traffic direction and two skew end diaphragms at bearing location. Two edge beams (ISMB 600) at the outer edge of deck are supported on cantilevered diaphragms. A RCC deck slab of thickness 200mm (M40) is connected through shear connector comprising of longitudinal and transverse steel girders, and ISMB 600 beam. This superstructure rests on pot-cum PTFE bearings at piers P1 and P2.

The STADD model of the RoB is shown in Fig.4.

Volume 46 Number 2 June 2016 71

2.

Testing Procedure

2.1 Instrumentation Strain and girder deflection reading were taken for 24 hours prior to testing day to take into the effect the diurnal temperature variations. The instrumentation included 20 strain gauges and 3 thermo-couples (Fig.5). 15 benchmark stations with steel inset plates (Fig.6), were made on the ground below the level of the sleepers.

Fig.1: View of RoB from below

Fig.5: Strain Guage Instruments

Fig.2: Plan showing arrangement of steel girder

Fig.3: Cross section of ROB

Fig. 4: Arrangement of girders in STAAD Model

72â&#x20AC;&#x192; Volume 46

Number 2 June 2016

Fig.6: Laser gauge to measure deflection of the girders from bench mark

2.2 Temperature Variation Measurements The deflection due to temperature expansions/ contractions on middle span and 1/3 spans from either supports were taken using a high precision LASER distance meter of 0.1 mm accuracy. These measurements were recorded every hour for 24 hours.

The variations due to temperature were insignificant, however they are needed for net deflection calculation.

The Bridge and Structural Engineer

2.3 Loading Test Before starting the loading,two trucks weighing 10 ton(bricks)were driven on the bridge for the initial settlement. The bearings are checked whether they are released. An external proof load system was designed based on the design document, to create the maximum design bending moment on the girders G1 to G5. Three alternative arrangements were explored for achieving this maximum design moment. An initial idea for using multiple fully loaded multi axle Taurus was abandoned, since it was not feasible to maintain this load for continuous 24 hours as the tyres will start deflating. Unavailability of containers of various capacities caused the team to avoid using containers as well. An arrangement to achieve the design load using cement bricks was finally adopted (Fig. 7), since that seemed to be the most efficient, safe and feasible option. A single patch of bricks (3m×1m×2m)having a load of 10 ton, is arranged in different patterns to attain 25%, 50%, 80% and 100% of the maximum design moment. The patch load arrangement for creating 80% of the design load and 100 % of the design load along with the STADD models is presented in Figs. 8-11.The maximum design moment due to two way class AA loading and 70R loading as per the original design is 5874kNm.It is seen from Figs. 8 and 10, the loading arrangement is entirely different for 80% and 100%. The load reduction from 100% to 80% is NOT proportionally carried out.

Fig.7: Pictorial view of the brick loading adopted on ROB

The load that produced 25% of the maximum bending moment was designed using patch loadings of cement bricks of about 30 ton, which gave a maximum bending moment of 1650kNm including an impact factor of 1.154. This moment is approximately 25% of the maximum design bending moment produced by two way class AA loading and 70R loading. The load that produced 50% of the maximum bending moment

The Bridge and Structural Engineer

was designed using patch loadings of cement bricks of about 60 ton, which gave a maximum bending moment of 3200kNm including an impact factor of 1.154. This moment is approximately 50% of the maximum design bending moment produced by two way class AA loading and 70R loading. The load that produced 100% of the maximum bending moment was designed using patch loadings of cement bricks of about 110 ton, which gave a maximum bending moment of 6200kNm including an impact factor of 1.154. This moment is approximately 100% of the maximum design bending moment produced by two way class AA loading and 70R loading. This loading arrangement is shown in Figs.8 -11.

Fig.8: Arrangement of patch load for 80% maximum design bending moment

Fig.9: STAAD model of the arrangement of patch load for 80% maximum design bending moment

Fig.10: Arrangement of patch load for 100% maximum design bending moment

Fig.11: STAAD model of the arrangement of patch load for 100% maximum design bending moment

Volume 46 Number 2 June 2016 73

Measurements of deflection and strains for 25%, 50%, 80% and 100% of bending moment values were taken and were found to be linear. Following this, the patch load for 100% bending moment was retained for further 24 hours.

Table 1: Deflection measurements at mid-span offive girders of RoB during loading and unloading % of BM 0 25 50 80 100 80 50 25 0

G1 0 3.7 8.8 12.7 17.8 17.4 12 5.6 2.1

Deflection (mm) G2 G3 G4 0 0 0 4.3 3 2.7 7.6 8.1 7.9 10.3 11.7 11 16.1 13.7 11.9 14.9 15.1 13.3 11.1 10 9.2 4.7 3.8 3.9 0.8 1.2 1.4

G5 0 3.6 7.1 10.3 10.9 13.6 8.6 2.9 1.5

middle span and 1/3 spans from either support. The deflection measurements were taken for a further period of 24 hours after removal of the entire load.

Typical deflection reading at the mid-span of all the five girders are presented in Table 1. A typical plot of these deflections for girder 1 and girder 3 is presented in Fig.12 and Fig.13.If one interprets the Figs.12 and 13, it is seen that during unloading there is no reduction in deflections corresponding to the reduction in loading. This may possibly due to the reason that bearings being new, the release of the girder is partially affected. It is also seen from Table 1, there is an increase in deflection in some girders when the load is reduced. This is due to the fact that the load from 100% and 80 % is NOT proportionally reduced, but was created using a completely a new set of loading arrangement and obviously the load share of the girders will change. However, the net rebound in the mid span deflection after deducting the temperature variations was more than 88 % of the maximum loaded girder and an average of 90% in the mid span of all girders.

3.

Fig.12: Plot of deflection obtained in G1 at various points on the ROB for different BM

Concluding Remarks

This paper describe a full scale test carried our as per the stipulations of IRC - SP 51 for a steel concrete compositeRoB. Based on the experimental evidence of the tests on the RoB, the steel concrete composite RoB between P1 and P2 at chainage KM 587.832 in the Madurai - Kanyakumari section of NH-7 in Samayanallur, Tamilnadu, has been demonstrated to be structurally safe for public traffic for 70R and two lane class A loading.

4. Acknowledgements

Fig.13: Plot of deflection obtained in G3 at various points on the ROB for different BM

At the end of the loading period, unloading was done such that patch load of 80%, 50%, 25%, and 0% of bending moment remained. The measurements were taken for deflection and strains for each loading at 74â&#x20AC;&#x192; Volume 46

Number 2 June 2016

IITM would like to thank M/s IRCON for the opportunity given to work for a public cause and also for their kind co-operation and support. The help during testing came from Dr. Arun Menon, Asst.

Professor, IITM, Mr.Ajith M.Tech (Struct IITM), Project Officer, Mr. Vishal, Scholar, Mr. Sivaprasad, Scholar, Mr. T. Rajkumar, Technical Officer, IITM, which is gratefully acknowledged.

5. References 1.

Drawing No.CBE/GU2/224/2010/SHEET 2 TO 10 AND SHEET 21 TO 32

2. Design

document

â&#x20AC;&#x201C;

243/ROB/DN/-104/

The Bridge and Structural Engineer

R3, Design of Composite Super Structure between Pier P1 and P2 for ROB at Chainage : KM.587.832.

3. IS 800:2007, Indian Standard, General construction in steel - code of practice, (Third Revision).BIS 2007.

The Bridge and Structural Engineer

4.

IRC:6-2010, Standard Specifications and Code of Practice for Road Bridges

5. IRC:37, Guidelines for Evaluation of Load Carrying Capacity of Bridges 6.

IRC:51, Guidelines for load testing of Bridges

Volume 46 Number 2 June 2016â&#x20AC;&#x192; 75

Evolution of Long Span Railway Bridges N. BANDYOPADHYAY Director STUP Consultants Pvt. Ltd. nbandyo@stupmail.com

Summary This paper discusses the changes that have taken place in design and construction of long span railway bridges in India over last sixty years.

Keywords: steel bridge; IRS Bridge Rules; Fatigue Assessment; Rail-cum-Road Bridge.

1. Introduction Traditionally in India Steel Bridges have been adopted for Railway crossing over wide rivers. Road bridges in India are mostly constructed with prestressed concrete. This paper examines the factors that have affected the evolution of steel railway bridges in India over the last sixty years.

N. Bandyopadhyay received his Civil Engineering degree from Calcutta University in 1974. After completing his post graduation studies, he has been working in Consulting Engineering profession and engaged in design of Bridges and other Major Structures

compared to building a road bridge. Hence a number of rail-cum-road bridges have been built across the major rivers like Ganga, Godavari and Brahmaputra.

Figure 1 shows a recent such bridge over Ganga opened at Patna.

Due to adoption of heavier wagons, the loading standards have been upgraded over the years. The design rules also have been rationalized. Higher grades of structural steel have become available.

Fabrication and erection methods have also improved with the introduction of welding and HSFG bolts as well as high capacity cranes and other erection tools. The implications of these factors in design of steel bridges have been discussed in the subsequent sections.

2.

Fig. 1 : 4.5 km long Rail-cum-Road Bridge over Ganga at Patna

The effort and cost involved in construction of bridge foundations in wide rivers like Ganga or Brahmaputra is high. Due to scour in the erodible alluvium strata the depth of foundation is often 60-70m. Naturally longer span superstructures are adopted to achieve overall economy. Depending on the depth of foundation 100m â&#x20AC;&#x201C; 120m open web steel girders are adopted. The additional expense for putting a deck for vehicular traffic at top or intermediate level works out small 76â&#x20AC;&#x192; Volume 46

Number 2 June 2016

Loading Standard

With the development of country and associated increase in freight traffic, wagons capable of carrying heavier cargo have been introduced along with more powerful locomotives to carry them. This has necessitated adoption of heavier loading standard in IRS bridge rules.

When India became independent the loading standards on Broad Gauge lines was BGML-1926, where the EUDL (Equivalent Uniformly Distributed Load) was 75.2 KN/m and the formula for computing Impact Factor, CDA (Coefficient for Dynamic Augment) was 20/(14+L). This was replaced by Modified Broad Gauge Loading, MBG-87 where the EUDL became 80.9 KN/m and the formula for CDA got modified to 0.15+8/(6+L). This resulted in increase in Design Bending Moments between 5-8%. Thereafter with The Bridge and Structural Engineer

Table 1: Tonnage of Railway Standard Spans

further increase in loaded wagon tonnage, a new loading standard 25t Axle – 2008 was introduced. The EUDL for this loading standard is 91.5 KN/m. The resulting increase in Design Bending Moment was of the order of additional 10% with respect to MBG87 loading. Figure-2 shows the variation of Design Bending Moments with different loading standards.

Sl No

Span (m)

1 2 3

45.7 61.0 76.2

MBG Riveted 114.5 196.0 292.0

25t Axle Welded 151.0 260.0 344.0

Fig. 3: Rajendra Setu over river Ganga at Mokama

3.

Fig. 2: Design Bending Moments for different IRS loading standards

In addition a special loading standard, HM (Heavy Mineral) was introduced for the routes that generally carry ores in special types of wagons with 30t axle load. The design Bending Moments are about 45% higher than the same with MBG-87 loading. A similar loading standard with 32.5t Axle load is being adopted for bridges in the designated DFC (Dedicated Freight Corridor) lines being built by DFCC. The increase in specification of live load has a direct impact on the design of floor system (stringers & cross girders) as in non-ballasted deck 95% of the design moments of these elements are contributed by the action of live loads. It also adversely affects the fatigue load verification (discussed in the next section). With the increase in weight of floor system as well as increase in live load bending moments, the truss elements also require heavier sections. The tonnages of riveted standard spans under MBG-87 loading are compared with the same for welded spans designed for 25t Axle loading in Table 1. It may be noted that the differences in tonnage are not due to only change in loading specification but also includes other factors discussed herein after.

The Bridge and Structural Engineer

Design Procedure

IRS Steel Bridge Code is based on working stress design method, although the IRS Concrete Bridge Design Code had adopted Limit State Design philosophy. As most of the steel bridge components are slender and design is based on Buckling criteria, the initiation of failure generally takes place at a stress level below the yield stress. The design of most members is effectively guided by Elastic Buckling criteria, which is the limit state of failure.

One of the significant amendments in IRS Steel Bridge Code has been the rationalization of fatigue assessment procedure in the line of EuroCode-3. The causative factors for the revision are: With the improvement of signalling system, the frequency of train movement has increased significantly. Traffic models have been developed as the basis for fatigue assessment (Tables 6.1 through 6.4 in Appendix ‘G’).

Heavier Live Load is plying on the bridge increasing the stress range.

The adoption of welded structures is adding to the rigidity and thereby development of secondary stresses. These are classified as Detail Category in Table 9.

This amendment brought out in 2010 completely changed the fatigue calculation procedure. Prior to Volume 46 Number 2 June 2016 77

this amendment the permissible stress were based on the ratio of minimum stress to maximum stress (fmin/ fmax) and the number of loading cycles, which were specified as 2 million cycles or 10 million cycles.

In the revised provision the permissible stress limits have not been modified but a restriction has been put on the stress range (i.e the difference between the minimum and maximum stresses due to live load).

The effect of this revision of Appendix ‘G’ is shown in the worked out example hereinafter. It can be seen from there that the result is almost 20% increase in the tonnage of floor system.

3.1 Example of Effect of amendment of Fatigue clause

Hence the section is acceptable.

The calculations are repeated herein under as per cl 12.5 (Assessment for Simplified load models) of revised Appendix-G : Stress Range due to LL, ΔσP = 610.0/9.615= 63.45 MPa The damage effects of stress range spectrum is represented by equivalent stress range related to 2 million cycles as per cl 12.5.2.4 as Δσ E,2 = λ * Φ * ΔσP where, λ

is the damage equivalence factor

The design of a few members of a 103.5m single BG track welded Open Web girder is worked out. The loading standard for this bridge designed in 2005 was MBG-87. The Double Warren type truss has 18 panels of 5.75m. The truss centres are 5.5m. One member of floor system (e.g. Cross Girder) and one member of truss (Central Bottom Chord L7-L9) are selected as example.

Φ

is dynamic impact factor (1 + CDA)

3.1.1 Cross Girder

λ2 is a factor that takes into account annual traffic volume in million tonnes. It is defined in cl 12.5.4.4.

The cross girder is a 1.4m deep built-up I-section made up of 1368x10 web plate and 300x16 flange plates. The section modulus works out to 9.615x106 mm3. Steel Grade : Mild Steel (Y.S > 260MPa) Span =

5.75m

CDA =

0.543

Bending Moment due to DL = 60.8 KNm Bending Moment due to LL = 610.0 KNm (without CDA) Bending Moment due to LL = 941.3 KNm (including CDA) Maximun Bending Stress =

104.2 MPa

fmin (due to DL only) =

6.3 MPa

fmin / fmax =

0.061

As per the pre 2010 provisions of code, the permissible stresses from fatigue consideration considering 10 million cycles, Class “C” connection and fmin/ fmax=0.061 are Ptension = 116 MPa Pcompr = 154 MPa 78 Volume 46

Number 2 June 2016

λ = λ1 * λ2 * λ3 * λ4 subject to λ ≤ 1.4 The damage equivalent factors are defined in cl 12.5.4 as under: λ1 depends on the base length of longest loop of influence line diagram and explained in cl 12.5.4.2 and 12.5.4.3

λ3 is a factor that takes into account the design life of the bridge and is defined in cl 12.5.4.5 λ4 is a factor to be taken into account when bridge structure is loaded on more than one track and is defined in cl 12.5.4.6 A&C slip 18 states that fatigue assessment shall be made for a standard design life of 100 years for a standard GMT of 50.

Using the above rules λ1 = 1.254 corresponding to loaded length of 11.5m & MBG loading (Table 7.1) λ2 = 0.5193 * 500.2036 = 1.1517, corresponding to 50 GMT traffic volume λ3 = 1.0 corresponding to design life of 100 years λ4 = 1.0 corresponding to single track bridge λ = λ1 * λ2 * λ3 * λ4 = 1.44 > 1.4 Hence λ = 1.4 Δσ E,2 = λ * Φ * ΔσP = 1.4 * 1.543 * 63.45 = 137.06 MPa The Bridge and Structural Engineer

As per cl 12.5.3, the following criteria has to be satisfied γFf * ΔσE,2 ≤ Δσc / γMf The values of partial safety factors γFf and γMf are defined in cl 11.4 as under: γFf = 1.00

λ4 = 1.0 corresponding to single track bridge λ = λ1 * λ2 * λ3 * λ4 = 0.84 < 1.4 Equivalent stress range related to 2 million cycles as per cl 12.5.2.4 Δσ E,2 = λ * Φ * ΔσP = 0.84 * 1.224 * 55.04 = 56.64 MPa

γMf = 1.15 The detail category corresponding to class “C” connection is 100 Putting the values in cl 12.5.3 ΔσE,2 ≤ Δσc / ( γMf * γFf) ≤ 86.96 MPa (100/1.15) It can be seen from above that the design of the cross girder based on earlier code is grossly inadequate with respect to the present Appendix-G. The tonnage of the cross girder has to be increased by approximately 25% to comply with the current version of the code.

Same is also true for the stringer.

3.1.2

λ3 = 1.0 corresponding to design life of 100 years

Central Bottom Chord

The bottom chord is a built-up channel section of net cross sectional area 79900 mm2.

Steel Grade :

Fe 490B (E350)

CDA =

0.224

Considering a detail category of 100 Δσc / ( γMf * γFf) = 100 / 1.15 = 86.96 MPa > 56.64 MPa (OK) Thus the chord section satisfies the revised fatigue assessment procedure.

Generally, the effect of this revision is more pronounced on the floor system which has a relatively shorter loaded length and hence a higher value of λ1.

The increase in sizes of stringers and cross girders also increases the overall weight of the span necessitating adoption of heavier truss members. The example calculations have been worked out by the simplified load model method (cl 12.5). Application of detail calculation procedure given in the code would result in a more rational design.

Axial Force due to DL = 3170 KN Axial Force due to LL = 4398 KN (without CDA) Axial Force due to LL = 5383 KN (with CDA) Minimum Stress, fmin = 39.67 MPa (due to DL) Maximum Stress, fmax = 107.04 MPa (DL+LL+CDA) fmin / fmax =

0.37

The stresses satisfy the codal requirements of earlier version of Appendix-G The calculations are repeated herein under as per cl 12.5 (Assessment for Simplified load models) of revised Appendix-G : Stress Range due to LL, ΔσP = 4398x103/79900= 55.04 MPa λ1 = 0.73 corresponding to loaded length of 103.5m & MBG loading (Table 7.1) λ2 = 0.5193 * 500.2036 = 1.1517, corresponding to 50 GMT traffic volume The Bridge and Structural Engineer

Fig. 4: 2 km long bridge over river Mahanadi

4. Material, Methods

Fabrication

and

Erection

The availability of different grades of steel has increased over the passage of time. The fabrication and erection techniques have also evolved over the time. This has changed the way in which steel bridges are constructed.

4.1 Material Earlier only one grade of steel i.e. Mild Steel to IS226 was widely available. Higher grades of steel had Volume 46 Number 2 June 2016 79

to be specially arranged. However, at present steel of different grades are available. For bridge construction mild steel (E250) to medium tensile grade (E410) are available in different quality. Normally adoption of higher grade steel would reduce the section sizes because of higher permissible stresses and thereby reduce the self weight of the structure which is substantial for bridges with longer span. Design of members whose permanent stresses are low compared to the same due to fluctuating loads (live loads) are generally governed by fatigue consideration as the permissible stresses are independent of steel grade. For such members like the components of floor system adoption of high tensile steel does not produce any economic design. However for truss members where the stresses due to permanent loads constitute 40-50% of total stresses, the adoption of high tensile steel results in economic design because of reduction of self weight of the structure. Hence the current practice is to adopt E250 steel in floor system and E350 / E410 steel in truss members to achieve an economic solution. It may be kept in mind that adoption of high tensile steel and consequently higher stresses increases the deflection due to Live Load of the structure compared to the design with lower grade of steel.

Fig. 5: 2.3 km long Jogighopa Rail-cum-Road Bridge over river Brahmaputra

4.2 Fabrication Traditionally all connections, both shop and field used to be done by rivets. With the availability of welding, steel members in transportable size were made by shop welding. The site connections continued to be made with rivets. The quality and reliability of rivets depends on the skill of the riveter. Because of the 80â&#x20AC;&#x192; Volume 46

Number 2 June 2016

hazards and safety issues associated with method of riveting, speed and in-situ quality control issues, internationally riveting got replaced with bolting. The introduction of High Tensile Friction Grip (HSFG) bolts provided a faster and more reliable way of jointing. The tightening of the bolts with a calibrated torque wrench ensures proper installation of all bolts to identical grip force. The current practice is to make all field connections with HSFG bolts. The makeup of members has also evolved with the change of fabrication process. For example, earlier it was quite common to make up a higher thickness plate by stitching two plates with rivets whereas with welded members a single thicker plate is adopted.

4.3 Erection Methods

Fig. 6: 4.9km long Rail-cum-road bridge over river Brahmaputra at Bogibeel

Traditionally, long span railway bridges in India used to be erected by â&#x20AC;&#x2DC;Cantilever Constructionâ&#x20AC;&#x2122; method. It required equipments of small capacity. The members as well the erection crane could be easily transported.

However, during this method of erection, the forces in the truss chord members during erection take opposite sense of the forces developed during service condition. That means the members designed to resist tension during service condition develop compressive force and vice versa. Hence, the sizes of some of the members may have to be modified to withstand the forces during erection. Specifically the bottom is subjected to secondary bending besides being subjected to compressive force and they become heavier than the requirement of service condition.

This strengthening for erection condition adds upto weight of the girder. With the availability of higher capacity equipments, the erection methods have also changed. With the application of large capacity jacks, ten spans, each fully welded at bank under strict quality control and weighing nearly 1900 tonnes each have been pulled in one go in the Bogibeel The Bridge and Structural Engineer

Rail-cum-Road bridge being constructed over river Brahmaputra. This has significantly reduced the total erection time and contributed to safety issue.

Table 2: Salient Features of some 400 feet span Rail-cum-Road Bridges over major rivers Sl No

Over River

Bridge Name

Length

Year Completed

Loading Tonnage of One Span

1

Ganga

Rajendra Setu Mokama

2 Km

1958

Single Track BG + Two Lane Road

1160

2

Brahmaputra

Saraighat (Guwahati)

1.3 Km

1962

Single Track BG + Two Lane Road

1080

3

Brahmaputra

Narana2.3 Km rayan Setu – Jogighopa

1994

Double BG Tracks + Four Lane Road

1640

4

Ganga

Patna

4.5 Km

2016

Double BG Tracks + Four Lane Road

1980

5

Brahmaputra

Bogibeel

4.9 Km

2017

Double ~1900 BG Tracks + Three Lane Road

The Bridge and Structural Engineer

5. Discussions The effect of all the changes discussed above have resulted in increase in girder weight in spite of adopting higher grade steel in chord members in some of the longer span bridges. It can be observed from Table 1 that weight of the standard spans have increased between 20-30%. The increase is due to combination of increase in loading, changes in design produces as well as adoption of welded members. It caters for the heavier and more frequent loading.

The salient features of five long Rail-cum-Road Bridges with 400’ (122m) span built over the last sixty years are shown in Table 2. The bridges at Mokama and Saraighat carried single BG track and weighed around 1100 T/span. The Jogighopa bridge has two BG tracks besides the road way and the weight of steelwork in one span is 1640T. These three bridges are of riveted construction. In Patna bridge the members are welded while the joints are riveted.

It also carried two BG tracks and the tonnage of one span including ‘strengthening’ is 1980T. All the five bridges have been erected by traditional ‘Cantilever Construction’ Method. The Bogibeel bridge span is fully welded (including joints) and assembled at site.

Thereafter, ten spans at a time have been incrementally launched (push launch) using launching nose at both ends. With temporary strengthening of the forward span, the tonnage of one fully welded span is about 1900T.

6. References 1.

RDSO , “IRS Bridge Rules”

2.

RDSO.,“IRS Steel Bridge Code”

Volume 46 Number 2 June 2016 81

BRIDGES IN STEEL ACROSS RIVER HOOGHLY AND BHAGIRATHI IN BENGAL DELTA Amitabha GHOSHAL

Amitabha Ghoshal graduated in Civil Engineering from Calcutta University.

He had been Director & Vice President of STUP Consultants Pvt. Ltd., and prior to that Head of Projects at B.B.J. Construction Co., Kolkata. He has varied experience of Steel Bridges including retrofitting and rehabilitation of such structures.

Chief Advisor to Board of Directors STUP Consultants Pvt. Ltd. India gamitabha@yahoo.com

1. Summary

than 500 metres below ground

Hooghly and Bhagirathi are the same river, with changed name, flowing to the Bay of Bengal through West Bengal. Study of the nine bridges across the river, built over 130 years, present an interesting technical appraisal of the engineering logic that made the structures very different from each other. This paper analyses the basic technical factors, including technological progress, that dictated the choice of structural form of the bridge superstructures in steel construction.

 the spread of the water mass that has grown as the river gets into flat terrain approaching the confluence

Keywords: Steel Superstructure, engineering logic, river hydraulics, construction

 very considerable scour around pier supports in the river bed

2. Introduction River Ganges presently flows into Bangladesh and reaches the Bay of Bengal. Historically the river used to flow along the alignment of the present day Bhagirathi, which takes the name Hooghly after crossing Tribeni, about sixty km upstream of Kolkata.

Bhagirathi and Hooghly have been the lifeline of the lower West Bengal of today, with huge transport of goods through the river from the days when Rail and Road transport had not become significant contributors to economy. This river alignment divides the landmass into eastern and western banks, and the need for bridges across the river had been a permanent feature at all times- and is continuing till date.

Constructing bridges across this river has always been a challenge to Engineers with –  the alluvial soil and silt deposits beyond more 82 Volume 46

Number 2 June 2016

 the tidal variations of water level- very considerable near Kolkata, but reducing in magnitude upstream with increasing resistance from bed and the banks  the huge deposition of silt as the velocity reduces with widening of the river profile;

 and the obstruction piers cause to movement of vessels, sea-going near the Port of Kolkata and inland vessels upstream.

These problems and the nature of crossing required by the proposed traffic across the river, have dictated the choice of bridge type, again in keeping with the technology available to the Engineers of the respective period. A study of the seven bridges, built across the river in steel construction, across a period of almost 130 years, for Railway as also Roadway, gives interesting insight into the progress of technology and ingenuity of Engineers entrusted with each task.

3.

The Steel Bridges in this section

Altogether eight bridges have been constructed across the river till date and six of them have used steelwork for the superstructure, the ninth one under construction also uses steel girders.

The Bridge and Structural Engineer

Brief description of these bridges is given in Table-1:

4.

Basic Parameters for Bridge Design

The Bridge and Structural Engineer

Volume 46 Number 2 June 2016â&#x20AC;&#x192; 83

84 Volume 46

Number 2 June 2016

The Bridge and Structural Engineer

1938

2003

1926

1882

Kolkata (near Howrah Station)

Dakshineswar, Bally

Naihati-Bandel

Kalyani

Nabadwip

Azimgunje

2nd Vivekananda Bridge (Nivedita Setu)

Bally Bridge Dakshineswar, (Vivekananda Setu) Bally

Naihati-Bandel

Howrah Bridge (Rabindra Setu)

Old Jubilee Bridge

New Jubilee Bridge

Kalyani Bridge (Iswar Gupta Setu)

Nabadwip Bridge (under construction)

Azimgunje Bridge

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

2006

2012

1970

2005

1972

Second Kolkata (near Hooghly Bridge Fort Williams) (Vivekananda Setu)

3x103m Rail Bridge single BG

5x103m+2x30m Rail Bridge single BG

5 nos. 120m span Road Bridge – two lane

2x132m+1x150m Rail Bridge two BG

2x164m+1x37m Rail Bridge single BG

Superstructure

Well foundation Piers in Concrete

Well Foundation Concrete Piers

Well Foundation Concrete Piers

Well Foundation Concrete Piers

Well Foundation

Well foundation

Superstructure: Cable & PSC Foundation : Concrete

Superstructure : Steel Foundation : Concrete

Superstructure : Steel & Cable Foundation : Concrete

Materials

Open web steel girders in Double Warren construction

Open web steel girders in Double Warren construction

Balanced Cantilever PSC Construction

Open web steel girders continuous over piers for three spans.

Three spans steel open web girders in balanced cantilever arrangement

Superstructure: Steel Pier : Concrete Foundation : Concrete

Superstructure: Steel Pier : Concrete Foundation : Concrete

Superstructure: Concrete Pier : Conrcete Foundation : Concrete

Superstructure: Steel Foundation : Concrete

Superstructure: Steel Foundation : Concrete

Hog backed open web steel Superstructure: Steel girders with concrete deck for Foundation : Concrete Roadway, and steel grillage for Railway tracks

Extradosed design with cables and single box PSC segmental construction

Caissons for Tower Balanced Cantilever Superstructure type in Steel Concrete End Blocks K-Truss construction

Caissons for Tower Cable Stayed Bridge Concrete End Fan Type Cable arrangement Blocks

Foundation & Substructure

7x110m+2x30m Well Foundation Road-cum-Rail Bridge Stone & Brick Two BG track + two Piers single lane road

7x110m+2x55m Deck 30m wide Road Bridge six lane

2x100m+1x450m Deck 30m wide Road Bridge six lane

2x180m+1x450m Deck 30m wide Road Bridge six lane

Project Span Arrangement/ started Usage

(1)

Location

Name

Sl. No.

Arranged location -wise along river Downstream to Upstream

Table 1 : Bridges Across River Hooghly / Bhagirathi In Bengal Delta

safe and dependable means of transporting men and goods. The bridges across the river, therefore, had to be designed to allow free and convenient passage of river crafts during all seasons, irrespective of weather condition. The bridges had to provide vertical clearance above the highest high flood level, inclusive of tidal effect, for the river crafts to move underneath unhampered. This requirement has varied with time, till codal provisions made implement able recommendations for designers to follow.

However, in the case of Second Hooghly Bridge (Vidyasagar Setu) a much higher clearance have been adopted, considering the threat of an ocean going ship, entering the Kidderpore docks, located immediately downstream, losing control and hit the deck of the bridge, causing another Maracaibo Bridge like disaster.

Design of a bridge is essentially dictated by the site conditions, construction convenience and the user demands on the structure. (i)

Foundation Condition : In case of the Lower Ganges alluvial deposits, the foundation of the bridge is able to carry vertical loads by transferring the load on to soil, primarily through skin friction, with insignificant contribution from end-bearing. The soil strata is not able to offer much horizontal resistance. In such soil, with high water table, deep excavation is not possible, ruling out open foundation. Pile foundations were, till recently, possible with only medium diameter, not exceeding 1 m.

Further with very low silt factor, the depth of scour around foundations become substantial, and the moment on pile shaft become high due to effect of horizontal loads from Braking/Tractive effect of vehicles and wind/seismic effect.

As a result the only acceptable foundation type in this situation has been Caissons/ Well foundation sunk to large depths. All the bridges across the river has therefore been built by adopting Well Foundations and in the case of Howrah Bridge and Vivekananda Setu large Caisson foundations.

The cost of such foundations are high and, therefore, to achieve cost optimization, the number of foundations have been reduced, leading to large spans.

The foundation condition has effect on the type of bridge superstructure that can be adopted.

With weak subsoil, differential settlement of foundations is a distinct possibility and structures therefore need to be flexible enough, not to be affected by same. Most of the bridges have therefore been designed as determinate structuresSimply Supported spans or as Balanced Cantilever Spans. Cable supported bridges are inherently flexible and therefore could be built in such situation. Only recently, continuous spans have been adopted for the new Jubilee bridge, for the first time, for Indian Railways.

(iii) Navigation and river hydraulics :

The clear navigable space needed for free movement of river crafts controls the free span to be adopted for the bridge. In case of Howrah bridge (Rabindra Setu) and Second Hooghly Bridge (Vidyasagar Setu) however there was the additional compulsion of not disturbing the dredged shipping lanes and siltation pattern of the hydraulically sensitive Hooghly river in this stretch. The Commissioners of the Port of Kolkata (KPT now) laid the strict condition that 450 metre stretch of river profile can not be encroached by any form of foundation construction and that resulted into the clear span of these two bridges to be 450 M.

Only about 12 km upstream, the Bally Bridge (Vivekananda Setu) and the recent Nivedita Setu have spans of 110 M. centres of Piers.

(iv) Technology Status :

Availability of materials and change in technology affects the type of bridge considerably and same is evident while studying the bridges in this area.

The foundation of Bally Bridge, completed in 1932 used stone and brick, but the Howrah Bridge used concrete and steel structure for the foundation.

For Bally Bridge, the 110 M trussed Girders were of hog backed shape, which was compatible

(ii) Vertical Clearance :

Hooghly/ Bhagirathi river route has been, from time immemorial, the most economic,

The Bridge and Structural Engineer

Volume 46 Number 2 June 2016â&#x20AC;&#x192; 85

with the erection scheme, by flotation, adopted.

Bridges across Bhagirathi near Azimgunge and Nabadwip, taken up recently, adopted double warren trusses with flat back to facilitate cantilever process of erection and faster fabrication.

Howrah Bridge, constructed during the Second World War and completed in 1943, was designed and built as a Cantilever-Suspension scheme, a determinate structure unaffected by differential settlement, the appropriate technology for a long span of 450 M. at that period. Other known forms for long span- Arch and Suspension bridges were not suitable for this site with soft foundation condition.

Second Hooghly Bridge, conceived in 1971, also with mandatory 450M span, was designed as a Cable Stayed bridge, slated to become the longest bridge of that type in the world if completed within five years of planned completion time.

This design offered enough flexibility to address the foundation related problems and at the same time ensured huge reduction in material consumption.

Old Jubilee Bridge, conceived in 1880 and completed in 1887, was built as a statically determinate Balanced Cantilever span- the new bridge completed recently has been designed as a three span continuous girder, duly accounting for estimated differential settlement effect, using computer programs available today.

A closer study of the design features, adopted for all these unique long span bridges, gives a greater clarity on the ingenuous technological adaptations that have gone to make each of them landmark structures.

5. Design Features of Chronologically Listed

the

Bridges

Prior to all the bridges, that are listed here, was the legendary Old Howrah Bridge, built on Pontoons banded together and anchored to provide vehicular access to the Howrah Station. Designed by Sir Bradford Leslie, the bridge also provided access for large river crafts that used to move along the river, by introducing a removable central section of 60 M., in steel Semi-through type truss construction 86â&#x20AC;&#x192; Volume 46

Number 2 June 2016

that kept the Roadway traffic on hold, when in use.

This Pontoon bridge, opened to traffic in 1874 and designed for a 25 year life, served the citizens for 68 years before it was replaced by the present Howrah Bridge.

Jubilee Bridge (Old) This bridge, designed also by Sir Bradford Leslie, has the distinction of being the first permanent bridge across the river, and had been functioning as a railway crossing till being decommissioned on 17.04.2016, when the traffic was diverted to the new bridge alongside.

Designed as an unusual Balanced Cantilever structure, it had the central span supported on one-third points of its 110 M. length, by two piers on well foundations spaced at centre to centre distance of 36.6 M. On either end of this central span, two 128 M. trussed girders rested, making the end gaps 165 M. each, permitting free movement of river crafts. The end abutments rested on well foundations.

The bridge was originally designed for twin railway tracks with 3.66M (12 ft) Track centres but had to be converted later to single Broad Gauge (5ft 6 in) track.

The Bridge and Structural Engineer

The top chords of the Pratt trusses (N-shaped) were profiled similar to a Bow, and gave the bridge an awe inspiring aesthetic look. The riveted girders were fabricated in UK and had curved profiled gussets to enhance the visual appeal. The quality of fabrication and riveting are difficult to match, with all the advanced tools available today, and the bridge merits preservation for posterity as a heritage structure.

on the piers by using the tidal variations. The well foundations and piers on top were constructed and the girders erected, by an Indian Contractor Rai Bahadur Jagmal Raja Chauhan. As an appreciation of his work, that allowed completion of the bridge in a period of only six years, the bridge sports plaques bearing his name besides those of Brathwaite & Co and the first train that crossed the bridge was named Jagmal Raja Express, a rare acknowledgment of native talent in colonial days.

Howrah Bridge ( Rabindra Setu)

Bally Bridge (Vivekananda Setu) Initially named Willingdon Bridge, tracing the connection to the then Viceroy of India who inaugurated it in 1932, this was the first link to the city of Kolkata and the Sealdah Station.

Rapid increase of motorized traffic in early twentieth century rendered the floating pontoon bridge across Hooghly inadequate. Pressure built up for a high level bridge capable of addressing the movements of road traffic as also the river traffic, that could only have limited duration movement, when the moveable steel span of pontoon bridge was opened. The responsibility for a new bridge was given to The Commissioners for the Port of Calcutta (CPC), who were custodians for the river besides the Ports, that were the busiest in the country at that time.

The bridge has seven main spans of 110 M. and two side spans of 30 M at either end. The main spans are Petit type, hog backed shape, with sub divided N formation trusses, popular for long span girders at that period. Designed to carry twin BG tracks inside the trusses and single lane road traffic on the outer flanks, the cross-girders of the deck are connected below the bottom chord of the trusses, the roadway being supported on the cantilevered stretch of the cross-girder.

The riveted bridge girders were fabricated at the Kolkata works of Braithwaite & Co, assembled on the river bank and floated in position for erection The Bridge and Structural Engineer

CPC entrusted the design task on their retained Volume 46 Number 2 June 2016â&#x20AC;&#x192; 87

consultants Rendel Palmer Tritton & Partnes of London (RPT), keeping the central span at 450 M., with no obstruction in the river waterway and a vertical clearance above high tide level of 12 M. The alignment was fixed close to the Old Howrah Bridge (Pontoon bridge), as Howrah Station was the destination of major part of the traffic at that time.

With over-built approaches on the Kolkata side and poor soil conditions, options of Arch or Suspension bridges were ruled out.

Tenders were floated with the official design of Balanced Cantilever Suspension bridge with K truss formation and contracts were invited with option for alternative designs. Bidders offered number of alternatives and some with futuristic features like partly cable supported truss and, even, orthotropic deck. Contract was finally awarded to Cleaveland Bridge of Britain based on the official design, but with the understanding that the fabrication and erection will be given to the company newly formed by three local fabricators, namely The Braithwaite Burn and Jessop Construction Co Ltd (BBJ). It was further agreed that medium tensile steel of E 350 grade, used for the first time in the country, will be allowed to be supplied from the steel plant of TATA at Jamshedpur.

Only 6000 tons of special quality steel came from Britain and 22000 tons were supplied by TATA, who incorporated necessary changes in their plant. Work was started in 1938 and completed in 1942.

The superstructure design was extremely innovative for its time, made to suit the compulsions imposed by the restricted construction site. The balanced cantilevers at two ends are centered around 92 M. high steel Towers and the rear anchor section has length of 99 M. with the cantilever arm at 140 M. The cantilever arms at either end support the central suspended span of 170 M. The huge uplift force at the 88â&#x20AC;&#x192; Volume 46

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back of the short anchor span was balanced by huge concrete anchor blocks at the ends- the uplift being transferred to the concrete block by a structural frame.

The balanced cantilever truss followed closely the Bending Moments imposed, and that made this huge bridge aesthetically appealing, making it the Icon of Kolkata. The road deck 32 M. wide and suitable for six lane traffic, including Tram tracks, were hung from the main structure through pin connected hangars at the plane of the truss. Expansion joints, in the deck, were provided at the end of the suspended span. Substantial Bracings were provided along the top profile of the truss and along the bottom horizontal plane, apart from adequate Cross Bracings, to address the very high wind velocity that can develop along the river channel which is straight for a long reach.

The two main towers are supported on 25M.x55M. caissons with 21 chambers, close to the bank, and were sunk to depths of about 30 M., with the help of pneumatic sinking equipment that allows digging of soil in near dry condition by raising the internal pressure more than twice the atmospheric pressure.

The foundation work was done by Hindusthan Construction Co of Bombay, a first use of pneumatic sinking in the country.

The girders were fabricated in the Kolkata works of the three component firms of BBJ and erection was done by local work force and engineers, under the guidance of well known engineer H Shirley Smith, who was deputed by the main contractor as Clerk of Works (a designation that meant The Engineer in those days) by the main contractor.

The erection of the trusses with curved top profile required the deployment of specially designed Creeper Crane that could adjust its base to remain horizontal at all slopes of the top chord, and even allowed it to cross the tower and move forward to a reverse sloped top chord. The cantilevering of the suspended span The Bridge and Structural Engineer

from the balanced cantilever section required special arrangements. The closure of the span at centre, with cantilevers from both side moving in at drooping angles, required use of built in hydraulic jacks and pulling devices.

Built with technology, much of which used for the first time in India (and the world), this bridge remains as a testimony to the skill of Engineers and workers, who worked with dedicated zeal and completed the bridge in less than four years- and that during the Second World War, when logistics were in shambles.

same firm that built Howrah Bridge), and they took the services of Freeman Fox and Partners (FFP) of England for the concept design, while the development design and erection scheme was done by BBJ design office. The offer was based on facilities and capacities that were available locally. The bid was for Cable Stayed Span with 450 M central span and two 2x90M side spans. As the clearance above Highest Tide level was 34 M. the towers became 135 M. high.

Second Hooghly Bridge (Vidyasagar Setu) Thirty years after the opening of the Howrah Bridge, work was started for a second bridge in Kolkata in 1972, about two km downstream. Even though the river width at the location was 630 M., it was reckoned that 180 M. stretch on Howrah bank was shallow waters and the main span can be retained as 450 M.

Port Authorities of Kolkata, being the custodian of the river, was given the task of planning and commissioning the bridge, that was required to be 810 M. long, with the central span at 450 M. and the two side spans 180 M. each, such that one end is on the Howrah side bank and the Kolkata side end is 180 M. behind the Tower which was to be located on the Kolkata bank.

The bridge was put to tender as an Engineering, Procurement and Construction (EPC) contract, leaving the design to be responsibility of the contractor.

The successful bidder was a consortium, where responsibility of superstructure rested on BBJ (the The Bridge and Structural Engineer

At that point of time, the 450 M. central span would have been the longest in world, beating the underconstruction longest span in Scotland by 40%. The design provided for rectangular riveted box girder construction for the towers and also for the two spine girders along the two cable planes on either side of the 32 M. deck. The concrete deck slab was to be supported by a grid of stringers and cross girders, all I beams composite with the concrete deck.

The work was awarded in May 1972 and then the bridge was handed over to a new entity, Hooghly River Bridge Commissioners (HRBC), formed especially for the project.

HRBC appointed the firm of Dr Leonhardt of Germany as proof consultant. BBJ retained the services of FFP as their consultant. There were many arguments on the suitability of the design of the bridge (prompted Volume 46 Number 2 June 2016â&#x20AC;&#x192; 89

by series of failures of Cable Stayed Box Girder Bridges across the world, and some of them had FFP as consultant) and finally it was decided that Dr. Leonhardt supported by Schlaich Bergerman und Partners (SBP) will become the principal designer and FFP will act as the checking designer. As Dr. Leonhardt retired from active service, Dr Jorg Schlaich and his firm SBP took over the responsibility for design. The contract became diluted from the original EPC basis.

bridge got much delayed, initially due to changes made for the various stakeholders (e.g., Port Trust to HRBC, main designer and checking designer role), and effect of all these on the Contract entered with the main contractor Bhagirathi Bridge Construction Company, BBCC- a joint venture between BBJ and Gammon India Ltd., and then due to various external factors including uneven and inadequate flow of fund.

Once the construction of the caisson foundations was over, the superstructure work was completed as per plans.

There was, however, an unfortunate delay due to an accidental failure of the crane used for the erection of the Howrah side tower. Once that problem got sorted out the erection proceeded unhindered and the bridge deck was closed in August 1992.

There were some basic changes in the design; the major amongst them was that the spine girders were replaced by a concrete steel composite deck, with three I girders as steel component, for carrying the compressive forces imposed by cable stayed design.

This arrangement allowed the composite deck to function during the erection stage itself, perhaps the first instance in the world.

By the time the construction work was completed, the bridge originally contemplated to be the longest cable stayed span in the world, had been overtaken by many more such bridges in length, as this form of bridge construction was found to be the ideal solution for bridges of spans above 150m extending to 1000m.

Azimgunge Bridge A new railway link was planned to provide connectivity between the railway corridors on either bank of Bhagirathi near Azimgunge and that required introduction of this new bridge.

The side spans were changed to single 180 M. span in place of 2x90 M. spans, to ensure uplift only at the ends.

The cable arrangement at the top of tower was changed from pure fan type arrangement to an hybrid arrangement, by providing a top section that anchored cables at different levels. Jacking of cables and adjustments were provided for from within the top section which was carefully detailed to provide same facility.

The cables, originally designed as spiral cables, were changed to parallel wire stay cables.

The entire bridge was designed as riveted structure, considering local availability of technology- and perhaps will feature as last of the great bridges that have been designed entirely as riveted structure.

The commencement of actual construction of the 90â&#x20AC;&#x192; Volume 46

Number 2 June 2016

The foundation at this location, almost 275 km upstream of Kolkata also needed well foundations resting at a depth of 42m below HFl, and therefore long non-standard spans were considered necessary.

After studying the topography and the cross section of the river at the alignment of the crossing, it was decided to adopt 3x 103 M spans for this bridge. The maximum pier height is 14m and the standing water depth gets to be 11m maximum.

Considering the optimum depth of the trussed girder to be more than 10 M., it was decided to adopt double The Bridge and Structural Engineer

warren girders, as will allow the shear in the diagonals to be halved and the buckling ratio to be controlled such that high permissible stress range is available for members in compression.

Considering that the river profile with large depth of water extends the entire bridge length, erection by cantilevering method required erection of an anchor span on temporary basis on trestles. After erection of the first cantilevered span, the temporary span was dismantled and reused as a permanent span. The chords of the girders were designed both for service condition and for erection condition, when there is reversal of forces. The component members are of welded construction, and the site connections were done with riveting.

compared to vertical forces.

The central span rises 45 M. above the bearing level at the centre, whereas the side spans have a much lower profile, giving the span an attractive aesthetic look. E410 grade steel has been used for the chord members, whereas E250 grade steel has been used for web members, deck components and bracings. The weight of the 417 M. continuous span with riveted joints is 6600 MT.

New Jubilee Bridge The Naihati- Bandel section of Eastern Railway provides an important link across the river, connecting the railway corridors on both banks of the river. The old bridge using steel of early grade (1980s) had been kept in use by imposing severe speed restriction and very few trains were able to use this link.

The new bridge was planned as a twin track BG line through type truss, with span configuration of two 133 M. and a central span of 150 M., continuous over four piers resting on well foundation. The bridge, located 22 M. downstream of the old bridge, has not followed the location of the existing foundations, even though in close proximity.

This bridge has the unique distinction of being the first major bow string type continuous superstructure in Indian Railways, a decision that was arrived after intense deliberations with the International consultant appointed by Railways and examination of many alternative superstructure models.

The adoption of continuous span helped in reducing the number of bearings and therefore long term maintenance cost. Spherical bearings were adopted considering the imposition of very high lateral forces The Bridge and Structural Engineer

Special planning had to be done to ensure camber for the span that had reversal of forces due to its continuity. Erection of the bridge, having water flow below two of the spans through out the year and with tidal variation of 4 M., presented strong challenges.

Erection was done in three separate sections- the Naihati end span was erected on temporary trestles, partly supported on temporary piles. Bandel side span, that has the navigation channel throughout the year and have deep water with high speed of flow, was erected on an enabling girder, that had to be floated in place before commencement of erection.

After completion of the side spans, erection of the high central section was taken up with the help of the same enabling girder, duly extended to match the larger span. Cranes were mounted on the enabling girder for cantilever erection from both ends with special monitoring of the deflection at the tip of cantilever. Temporary trestles were put on top of the enabling girder to counter the deflections by jacking and provision was kept for putting kentledges on the side spans to control the central closing operation.

There were many first time use of new technology in this bridge which has provided rich knowledge bank to bridge designers of the country, and will help in building many more challenging bridges in the future.

Open to traffic on 17th of April in 2016, the completion Volume 46 Number 2 June 2016â&#x20AC;&#x192; 91

of the new bridge also signaled closure of the adjacent Old Jubilee Bridge.

The river regime, in this stretch, is unstable, threatening scour of the banks and viaduct spans had been added after construction has proceeded considerably, to avoid erosion of embankments.

6. Conclusion

Nabadwip Bridge This bridge, now under construction, has five 103 M. spans and three 30 M. side spans, for single BG track across. Built on Well foundations and with RCC piers that carry the open web steel trusses, the bridge has viaduct approaches on the eastern bank.

The Girders for 103 M span are of Double-Warren type and have been designed for cantilever erection.

While identical in shape and dimensions with the Azimgunge Bridge, the span designed for recently changed Fatigue clause, weighs considerably more.

92â&#x20AC;&#x192; Volume 46

Number 2 June 2016

Study of the seven bridges with steel superstructure, over a period of 130 years, and spaced out along the river for more than 200 km, gives a good understanding of the varied design approach followed by designers of different generations. With improvement of quality of steel, adoption of welding for stitching the components and greater understanding of the fatigue behavior, the shapes and weights of spans are changing over time.

Engineers today have new soft wares for analysis and detailing to address the challenges being faced with widening frontiers of knowledge. Concerns for sustainability and environmental degradation have become essential inputs for designers of bridges, across the world.

The Bridge and Structural Engineer

INDIAN NATIONAL GROUP OF THE IABSE OFFICE BEARERS AND MANAGING COMMITTEE – 2016 Chairman

Honorary Secretary

1. Shri DO Tawade, Chairman, ING-IABSE) & Member (Tech), National Highways Authority of India

11. Shri RK Pandey, Member (Projects), National Highways Authority of India

Vice-Chairmen

2. Shri BN Singh, Chief Engineer (CoordinatorIII), Ministry of Road Transport and Highways 3.

Shri Alok Bhowmick, Managing Director, B&S Engineering Consultants Pvt Ltd

4. Shri AKS Chauhan, C.O.O., GR Infraprojects Ltd

Honorary Treasurer

5. The Director General (Road Development) & Special Secretary to the Government of India, Ministry of Road Transport and Highways

Honorary Members

6.

7.

Shri Ninan Koshi, Honorary Member, IABSE & Former Director General (Road Development) & Addl. Secretary Prof SS Chakraborty, Honorary Member & Past Vice-President, IABSE

Persons represented ING on the Executive Committee and Technical Committee of the IABSE

Members of the Executive Committee

12. Shri AD Narain, Former Director General (Road Development) & Addl. Secretary 13. Shri AK Banerjee, Former Member (Technical), NHAI 14. Shri AV Sinha, Former Director General (Road Development) & Special Secretary 15. Shri G Sharan, Former Director General (Road Development) & Special Secretary 16. Shri RP Indoria, Former Director General (Road Development) & Special Secretary 17. Dr Lakshmi Parameswaran, Chief Scientist, Bridges & Structures Div., CSIR-Central Road Research Institute 18. Shri Ashwinikumar B Thakur, Group Engineer, Atkins India 19. Shri Sarvagya Kumar Srivastava, Engineer-inChief (Projects), Govt of Delhi 20. Dr Mahesh Kumar, Engineer Member, Delhi Development Authority

Secretariat

8. Dr Harshavardhan Subbarao, Vice President & Member, Technical Committee of IABSE & Chairman and Managing Director, Construma Consultancy Pvt Ltd

21. Shri RK Pandey, Member (Projects), National Highways Authority of India

Past Member of the Executive Committee and Technical Committee of IABSE

22. Shri Ashish Asati, General Manager, National Highways Authority of India

9.

Prof SS Chakraborty, Honorary Member & Past Vice-President, IABSE

23. Shri KB Sharma, Under Secretary, Indian National Group of the IABSE

10. Dr BC Roy, Past Vice President & Member, Technical Committee, IABSE The Bridge and Structural Engineer

Volume 46 Number 2 June 2016 93

MEMBERS OF THE MANAGING COMMITTEE – 2016 Rule-9 (a): A representative of the Union Ministry of Road Transport and Highways 1.

Shri DO Tawade, Chief Engineer (CoordinatorII), Ministry of Road Transport & Highways

Rule-9 (b): A representative each of the Union Ministries/Central Government Departments making annual contribution towards the funds of the Indian National Group of IABSE as determined by the Executive Committee from time to time 2.

CPWD - nomination awaited

3.

NHAI - nomination awaited

4.

Ministry of Railways - nomination awaited

Rule-9 (c): A representative each of the State Public Works Departments/Union Territories making annual contribution towards the funds of the Indian National Group of IABSE as determined by the Executive Committee from time to time 5.

Govt of Andhra Pradesh – nomination awaited

6. Govt of Arunachal Pradesh – nomination awaited 7. Shri Ajoy Chandra Bordoloi, Commissioner & Special Secretary to the Govt of Assam 8.

Govt of Bihar – nomination awaited

9.

Govt of Chattisgarh – nomination awaited

10. Shri Sarvagya Kumar Srivastava, Engineer-inChief (Projects), Govt of Delhi 11. Govt of Goa – nomination awaited 12. Govt of Gujarat – nomination awaited 13. Govt of Haryana – nomination awaited

19. Govt of Madhya Pradesh – nomination awaited 20. Dr DT Thube, Chief Engineer, Govt of Maharashtra 21. Shri O Nabakishore Singh, Additional Chief Secretary (Works), Govt of Manipur 22. Govt of Meghalaya – nomination awaited 23. Shri Lalmuankima Henry, Chief Engineer (Buildings), Govt of Mizoram 24. Govt of Nagaland – nomination awaited 25. Govt of Orissa – nomination awaited 26. Govt of Punjab – nomination awaited 27. Govt of Sikkim – nomination awaited 28. Govt of Tamil Nadu – nomination awaited 29. Govt of Tripura – nomination awaited 30. Govt of Uttar Pradesh – nomination awaited 31. Govt of Uttarakhand – nomination awaited 32. Govt of West Bengal – nomination awaited 33. Shri Mukesh Anand, Chief Engineer, Union Territory Chandigarh Rule-9 (d): A representative each of the Collective Members making annual contribution towards the funds of the Indian National Group of IABSE as determined by the Executive Committee from time to time 34. Major VC Verma, Director (Mktg), Oriental Structural Engineers Pvt Rule-9 (e): Ten representatives of Individual and Collective Members 35. Shri G Sharan, Former DG (RD) & Special Secretary

14. Govt of Himachal Pradesh – nomination awaited

36. Shri AK Banerjee, Former Member (Technical), NHAI

15. Govt of Jammu & Kashmir – nomination awaited

36. Shri AV Sinha, Former DG (RD) & Special Secretary

16. Govt of Jharkhand – nomination awaited 17. Govt of Karnataka – nomination awaited

38. Shri RP Indoria, Former DG (RD) & Special Secretary

18. Shri KP Prabhakaran, Chief Engineer, Govt of Kerala

39. Shri Atul D Bhobe, Managing Director, SN Bhobe & Associates Pvt Ltd

94 Volume 46

Number 2 June 2016

The Bridge and Structural Engineer

40. Shri NK Sinha, Former DG (RD) & Special Secretary 41. Dr Lakshmi Parameswaran, Chief Scientist, Bridges & Structures Div, CSIR-Central Road Research Institute 42. Shri Rakesh Kapoor, General Manager, Holtech Consulting Pvt Ltd 43. Shri Ashwinikumar B Thakur, Group Engineer, Atkins India Rule-9 (f): Four representatives of Bridge and Structural Engineering Firms

50. Shri Krishna Sandepudi, Vice President, Aarvee Associates Architects Engineers & Consultants Pvt Ltd 51. Shri Bageshwar Prasad, CEO (Delhi Region), Construma Consultancy Pvt Ltd 52. Shri Alok Bhowmick, Managing Director, B&S Engineering Consultants Pvt Ltd. Rule-9 (i): Honorary Treasurer of the Indian National Group of IABSE 53. The Director General (Road Development) & Special Secretary to the Govt of India

44. Shri MV Jatkar, Executive Director (Technical), Gammon India Ltd

Rule-9 (j): Past-Chairman of the Society, for a period of three years, after they vacate their Chairmanship

45. The Managing Director, UP State Bridge Corporation Ltd

Rule-9 (k): Secretary of the Indian National Group of IABSE.

46. Shri T Srinivasan, Vice President & Head Ports, Tunnels & Special Bridges, Larsen & Toubro Ltd

Rule-9 (l): Persons who have been awarded Honorary Membership of the Parent Body

Rule-9 (g): Two representatives of the Engineering Colleges / Technical Institutes / Universities /Research Institutes 47. Prof AK Goel, Director, Indian Railways, Pune 48. Shri VL Patankar, Director, Indian Academy of Highway Engineers Rule-9 (h): Four representatives Engineering Firms

of

Consulting

49. Shri AD Narain, President, ICT Pvt Ltd

The Bridge and Structural Engineer

54. Shri RK Pandey

55. Shri Ninan Koshi 56. Prof SS Chakraborty Rule-9 (m): Persons represented ING on the Executive Committee and Technical Committee of the IABSE 57. Dr Harshavardhan Subbarao Rule-9 (n): Past Members of the Executive Committee and Technical Committee of the IABSE 58. Prof SS Chakraborty 59. Dr BC Roy

Volume 46 Number 2 June 2016â&#x20AC;&#x192; 95

LIST OF ING-IABSE PUBLICATIONS AVAILABLE FOR SALE Sl. No.

Price Rs

Postage Rs

1

Technical Presentation on “Maintenance and Rehabilitation of Bridge Structures” held at New Delhi in November 1994

Name of the Publications

200/-

100/-

2

Seminar Report on “Elevated Transport Corridors” Hard Copy Out of Stock – CD is available held at Maysore (Karnataka) on 27th and 28th June 2014 Themes of the Seminar are as under:Technical Session - I Planning Preparation, Economic benefits & Value Engineering Technical Session - II Design and Construction Technical Session - III Operation and Maintenance Technical Session - IV Case Studies

250/-

100/-

3

36th IABSE Symposium on “Long Span Bridges and Roofs – Development, Design and Implementation”– held at Kolkata from 24th to 27th September 2013

250/-

100/-

4

Vol.43 No.3, September 2013 Special Issue – Urban Flyovers (Structure, Architecture, Sustainability)

250/-

100/-

5

Vol.43 No.4, December 2013 Hard Copy Out of Stock – CD is available Special Issue – Bearings, Expansion Joints & STUs for Bridges (Selection, Design, Testing Installation, Maintenance)

250/-

100/-

6

Vol.44 No.1, March 2014 –Special Issue – Building Structures

500/-

100/-

7

Vol.44 No.2, June 2014 Special Issue – Codes & Standards in Structural Engineering–(Developments & Need for Improvement)

500/-

100/-

8

Vol.44 No.3, September 2014–Special Issue – Reinforced Soil Walls–(Current Practices & Future Directions)

500/-

100/-

9

Vol.44 No.4, December 2014 – Special Issue – Structural Failures–(and Lessons Learnt)

500/-

100/-

10

Vol.45 No.1, March 2015 – Special Issue – Earthquake Resistant Design of Structures

500/-

100/-

11

Vol.45 No.2, June 2015 Hard Copy Out of Stock – CD is available Special Issue – Strengthening, Repair & Rehabilitation of Structures

250/-

100/-

12

Vol.45 No.3, September 2015 – Special Issue – Aesthetics of Structures

500/-

100/-

13

Vol.45 No.4, December 2015 –Special Issue – Geotechniques & Foundations Design of Structures

500/-

100/-

14

Vol.46 No.1, March 2016–Special Issue – Enabling works, Formworks & Scaffolding Systems

500/-

100/-

Note: These Publications are available on cash payment or through cheque drawn in favour of the "Secretary, Indian National Group of the IABSE, New Delhi" at the following address:

The Secretary, Indian National Group of the IABSE, IDA Building, Ground Floor, Room No.12 Jamnagar House, Shahjahan Road, New Delhi -110011 Tel: 91-11-23388132, 23386724 E-mail: ingiabse@bol.net.in; ingiabse@hotmail.com

96 Volume 46

Number 2 June 2016

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The Bridge and Structural Engineer

Volume 46 Number 2 June 2016â&#x20AC;&#x192; 97

With Best Compliments From :

Reliance Infrastructure Ltd Mumbai

98â&#x20AC;&#x192; Volume 46

Number 2 June 2016

The Bridge and Structural Engineer

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The Bridge and Structural Engineer

Volume 46 Number 2 June 2016 99

FORTHCOMING EVENTS OF ING-IABSE 1. The Indian National Group of the International Association for Bridge and Structural Engineering (ING-IABSE) is organising a Seminar on “Urban Transport Corridors” in co-operation with Government of Andhra Pradesh, Transport, Roads and Buildings Department at Visakhapatnam on 21st and 22nd October 2016.

The Seminar will have four Technical Sessions covering each theme in one Session as per the following: i) Policy and Planning

• Unified Metropolitan Transport Development Authority

• Planning for Multi-modal Transport for Urban Corridors

• Transit Oriented Development including Land Use Planning ii) System and Engineering

•

Demand and Supply Management in Urban Transport

•

Infrastructure Requirement for Integrated Urban Transport

•

Use of ITS – Coordination, Efficiency, Monitoring, etc. in Urban Transport

•

Safety and Security

iii) Financing

•

Innovative Financing for Urban Transport Corridor

•

Congestion Charging for Demand Management (including Parking)

iv) Case Studies

•

Metro

•

Mono-Rail/LRT

•

BRTS

•

Intermediate Public Transport (Auto, Taxi etc.)

Technical papers under various themes are invited for inclusion in the Seminar Report. The paper should be neatly printed including figures, tables etc. on A4 size paper with 25 mm margin on all side using 11 size Font (Times New Roman).

Those who are interested to contribute a paper, kindly send their paper (maximum 9 pages plus one cover sheet) by 20th September 2016 at the following address. Selected authors will be invited to present their papers in the Seminar.

Shri RK Pandey Secretary Indian National Group of the IABSE IDA Building, Ground Floor, Room No.12 Jamnagar House, Shahjahan Road New Delhi-110011 Telefax: 011-23388132  Phone: 011-23386724  E-mail: ingiabse@bol.net.in, ingiabse@hotmail.com GENERAL INFORMATION Venue :

Hotel Novotel, Beach Road, Maharani Peta, Visakhapatnam (Andhra Pradesh)

Inauguration & Registration : 21st October 2016 Seminar : 21st and 22nd October 2016 Registration Registration is to be done by paying the required registration fees to the “Secretary, Indian National Group of the IABSE, New Delhi” through a cheque preferably by 14th October 2016. This will help in making advance arrangements.

Registration Fees Fee Rs

Service Tax 15%

Total Rs

Members, ING-IABSE

3000/-

450/-

3450/-

Non-Members

4000/-

600/-

4600/-

1000/-

150/-

1150/-

Young Engineers (under 35 years)

Service Tax: 14.00%, Swachh Tax Cess 00.50%, Krishi Kalyan Cess 00.50% Note: Collective Members of ING-IABSE are entitled to nominate two delegates at member’s rates.

2. The Indian National Group of the International Association for Bridge and Structural Engineering (ING-IABSE) is also organising a Workshop on “Code of Practice for Concrete Roads Bridges: IRC:112:2011” at Mumbai on 18th and 19th November 2016.

100 Volume 46

Number 2 June 2016

The Bridge and Structural Engineer

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With Best Compliments from :

Intercontinental Consultants and Technocrats Private Limited

For any information please contact us at :

A-8 & A-9, Green Park, New Delhi-110 016, India, Ph. : +91-11-40863000, Fax : 91-11-26855252 E-mail: business@ictonline.com, Website : www.ictonline.com

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B&SE_Volume 46_Number 2_June 2016

The Bridge & Structural Engineer JOURNAL OF THE INDIAN NATIONAL GROUP OF THE INTERNATIONAL ASSOCIATION FOR BRIDGE & STRUCTURAL ENGINEERING

Steel & Composite Bridges