STRUCTURE magazine | October 2015

October 2015 Bridges

A Joint Publication of NCSEA | CASE | SEI

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October 2015 34

44 editorial

7 Good Fortune or Planned opportunity?

Feature

Structural Modeling and evaluation

outSide the Box

31 Back of the envelope engineering

By David W. Mykins, P.E.

By Craig E. Barnes, P.E., SECB

inSiGhtS

hiStoriC StruCtureS

9 have You Checked us out lately?

49 the Monongahela river (Wabash) Cantilever

By Maribeth Rizzuto

By Frank Griggs, Jr., D. Eng., P.E.

StruCtural ForenSiCS

StruCtural SPeCiFiCationS

10 Service life Predictions for reinforced Concrete Bridges

By Gina Crevello, Paul Noyce and Charles ‘Chas’ Bransby-Zachary StruCtural deSiGn

14 Bridge Mega-Projects: Quality assured

By Dan Domalik, P.E.

52 the evolution of Structural design Specifications in the united States

By Phillip C. Pierce, P.E. eduCation iSSueS

54 SeFW Promotes SteM activities in Washington State

By Thomas M. Corcoran, P.E., S.E.

ConStruCtion iSSueS

18 accelerated Bridge Construction to rehabilitate aging highway Structures

By Ric Maggenti, P.E. and Kenneth Brown, P.E. PraCtiCal SolutionS

22 Suspension Bridge Cable dehumidification

By Shane R. Beabes, P.E. and Barry R. Colford, C.Eng

and Angela Gottula Twining SPotliGht

59 Floating Cofferdam for repair of the Washington Sr-520 Floating replacement Bridge

By Hamid Fatehi, P.E., S.E. StruCtural ForuM

66 human Factors in Structural Failures

By Akash Rao, P.E., Matt Reichenbach, E.I.T. and David Marcic, S.E., P.E. This article describes the modeling and evaluation of a steel vertical lift highway bridge located in Ontario, Canada. The bridge is considered a Lifetime Bridge; and the intent of the owner was to conduct a bridge evaluation for dead, live, wind, and seismic loads per the Canadian Bridge Code.

40

Feature

Civil War era trestle Bridge Gets advanced 21st Century Makeover

By Andy Loff The push to expand access to the outdoors means agencies have to tackle increasingly difficult bridge projects while finding ways to trim overall expenses. Popular programs like Rails-to-Trails Conservancy are using this balancing act as an opportunity to take a harder look at alternative methods and materials.

44

Feature

the rio abajo Footbridge

By Hendrik Westerink, E.I.T. The 3,000 person community of Rio Abajo in northern Nicaragua lost a vital link to the nearby town of Pueblo Nuevo during Hurricane Mitch in 1998. North American bridge team and committed volunteers from Rio Abajo built a new pedestrian bridge that provides a safe, year-round crossing over the Rio Pueblo River, for the first time in more than 16 years.

By Irfan A. Alvi, P.E. StruCtural rehaBilitation

26 efficient Methods for upgrading or reinforcing existing Bridges

By Roumen V. Mladjov, S.E., P.E.

On the cover Westbound Tower of the William Preston Lane Jr. Memorial Bridge (Chesapeake Bay Bridge).

Publication of any article, image, or advertisement in STRUCTURE® magazine does not constitute endorsement by NCSEA, CASE, SEI, C 3 Ink, or the Editorial Board. Authors, contributors, and advertisers retain sole responsibility for the content of their submissions.

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5

in everY iSSue 8 Advertiser Index 56 Resource Guide (Seismic/Wind) 57 Noteworthy 60 NCSEA News 62 SEI Structural Columns 64 CASE in Point

October 2015

Editorial

Good new trends, new Fortune techniques or and current Planned industry issues Opportunity?

By David W. Mykins, P.E., Chair CASE Executive Committee

“Luck favors the prepared mind”. This adaptation of a quote by Louis Pasteur is one of my favorites because it very succinctly describes the ingredients that make up what many of us consider our good fortune. The truth is, we are presented with opportunities for success all the time. Luck happens when we are prepared to take advantage of those opportunities. And most of those opportunities don’t just happen to us. They are really the result of hard work, combined with careful, thoughtful planning. One example of that can be found by examining those lucky firms that are able to undergo successful ownership transitions.

From the outside, we may be tempted to say things like “How lucky those owners are to have those talented young people in their organization.” Or alternatively, “They sure were lucky to have been hired by that firm.” But was it really luck? Or was it good judgement and the result of years of careful mentoring that produced that next generation of talent? Of course, some of this is due to chance. Random events will lead individuals to decide to live in a certain place and accept one offer over another. But once we’re in the game, we all play a role in making our good fortune happen.

Over the past several years, at conferences around the country, a recurring topic of discussion has been the challenge of ownership transition. Planning for ownership transition is difficult for any firm, but for small firms it presents some special and unique challenges. One inherent truth is that the pool of potential future leaders is smaller than for large firms. So let’s examine a few key considerations to create some “luck” for small firms in finding the next generation of owners.

Most of today’s successful small firms were founded by one or two entrepreneurial engineers with a singular drive and enthusiasm for both engineering and business. They poured their heart and soul into the business and developed a loyal client base that looks to them as the experts in their field. The founders have, in a sense, given birth to the business, nurtured it and watched it grow. And their dream is to see it survive and thrive after they have retired. Often, it is only as they approach retirement that they begin to seriously think about transition. One of the most important lessons in Ownership transition is that it is never too early to start planning for it, but it can be too late. Consider that, in nature, most living things have at birth everything they need to help produce the next generation.

Thus, at the beginning of life, the process of regeneration begins.

Possibly, the planning for ownership transition should begin with the hiring of the first employee.

Another reality is that successful small firms are often identified by the personalities of the founders. Because of this, as these leaders age and look for candidates suitable for ownership, the natural tendency is to look for mirror images of themselves. But as most anyone over the age of 50 can tell you, the Generation X and Generation Y (otherwise known as Millennials) think and work differently than the typical Baby Boomers who started the firms. Perhaps the firms that successfully transfer ownership manage to embrace the strengths of the new generations rather

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than trying to mold them into their image. This means actively engaging younger staff in discussions regarding key management decisions related to the future of the firm. By involving younger members of the firm in this type of dialogue, owners are able to identify individuals that have both the ambition and the talent to lead. This has the added advantage of providing fresh, unique ideas to improve the company’s business practices and having the younger staff’s buy-in to those ideas.

Once you are fortunate enough to have identified the next generation of leaders, you’ll want to effectively communicate the advantages of ownership. These include, principally, the financial and leadership opportunities. According to a recent article on the website Business Insider, “Millennials who watch their elders struggle to escape the creditor want to avoid debt at all costs.” How, then, do you convince them that ownership is a good investment? The first step is to have a valuation formula that is simple to understand, and financial risks and benefits are easily comprehended. An overly complex stock purchase agreement can be a real turn off to a financially unsophisticated potential investor.

In addition to the future financial advantages of ownership, you must promote the opportunity owners have to shape the future of firm. Let’s face it: the major stockholders in a small firm make the key decisions about how the firm operates. This includes short term decisions like promotions, salaries, and bonuses, as well as long term decisions like geographic expansion, mergers and acquisitions. It is important that the individuals targeted for the next generation of leadership understand the responsibility and tremendous opportunity that ownership will provide them, and that they are ready to embrace it with enthusiasm.

These are just a few of the many things that need to be considered in ownership transitions. So where can you find more information and tips? Obviously there are lots of publications with different ideas, and plenty of experts ready to help you. But one important, often overlooked resource, is networking with your peers. That can be difficult, or at least awkward to do locally. But by actively participating in professional organizations like CASE, you have a unique opportunity to talk to other structural engineers from large and small firms across the nation who have successfully navigated the same path you are taking, and can often offer examples of winning strategies. If you are already a member of a national structural engineering organization, take advantage of one of the truly great benefits of membership by taking a more active role in it. Who knows, you might find out that you’re already one of the lucky ones.▪ David W. Mykins is the president and CEO of Stroud, Pence & Associates, a regional structural engineering firm headquartered in Virginia Beach, VA. He is the current chair of the CASE Executive Committee. He can be reached at dmykins@stroudpence.com.

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October 2015

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October 2015

C3 Ink, Publishers A Division of Copper Creek Companies, Inc. 148 Vine St., Reedsburg WI 53959 Phone 608-524-1397 Fax 608-524-4432 publisher@structuremag.org October 2015, Volume 22, Number 10 ISSN 1536-4283. Publications Agreement No. 40675118.

Owned by the National Council of Structural Engineers Associations and published in cooperation with CASE and SEI monthly by C3 Ink. The publication is distributed free of charge to members of NCSEA, CASE and SEI; the non-member subscription rate is $75/yr domestic; $40/yr student; $90/yr Canada; $60/yr Canadian student; $135/yr foreign; $90/yr foreign student.

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STRUCTURE® is a registered trademark of National Council of Structural Engineers Associations (NCSEA). Articles may not be reproduced in whole or in part without the written permission of the publisher.

S

teel has always been reliable as a building material – durable, strong, resilient and sustainable. Steel is also an innovative material, and there are new products and practices available to architects, engineers, building owners, contractors and others to streamline the design process and speed up the time of construction. Here are some examples.

InSIghtS new trends, new techniques and current industry issues

Fire, Acoustic and Thermal Design Solutions To meet the growing demand for new and improved solutions, the number of tested assemblies included in the online tool, A Guide to Fire and Acoustic Data for Cold-Formed Steel Floor, Wall and Roof Assemblies, has been expanded.

The guide is updated and maintained by the Steel Framing Alliance (SFA) and is available to use free of charge at www.steelframing.org. More than 350 assemblies have been captured, with additional solutions added regularly.

The SFA design guide, Thermal Design and Code Compliance for Cold-Formed Steel Walls, 2015 Edition, was just published. It aids designers in meeting and often exceeding the latest requirements in the building codes, and is available to download free of charge at www.steelframing.org. The guide integrates the results of extensive hot box tests of coldformed steel wall assemblies to provide the most up-to-date and accurate methods for determining their thermal performance. It also addresses compliance paths and requirements for the 2015 International Energy Conservation Code® and ASHRAE 90.1-2013, Energy Standard for Buildings Except Low-Rise Residential Buildings.

The design guide’s chapters provide: a) background and overview of the Energy Code, b) base code U-factors for cold-formed steel assemblies, c) methods for determining U-factors of cold-formed steel walls, and d) code compliance options and examples.

New Ways to Connect Technology advancements in cold-formed steel connections that offer lower-cost solutions, increase speed of construction, and eliminate any guesswork relating to connection design are being developed at a rapid pace. Several companies offer these connections.

New cold-formed steel analysis and design software to streamline the design process and advanced panelized systems to reduce construction time are also available to maximize return-on-investment results for owners and developers. More information on these and other innovations is included in an interesting article by Larry Kahaner in the March 2015 issue of STRUCTURE magazine titled Steel and Cold-Formed Steel Construction.

Making it Easy to Design with Steel Many cold-formed steel associations offer technology transfer tools for building design professionals.

An example is the Cold-Formed Steel Engineers Institute (CFSEI), which regularly offers AIAaccredited technical webinars and seminars for PDH credit. CFSEI also issues Technical Notes on specific topics related to durability and corrosion protection, fasteners and connection hardware, component assemblies (trusses and wall panels), floor and joist systems, wall systems, roof and ceiling systems, thermal/fire/acoustics, lateral systems, and more. Issue papers on a variety of topics are available. The annual CFSEI Expo is a two-day event offering technical sessions and networking opportunities for architects, engineers, building officials and contractors. The expo benefits newcomers to cold-formed steel design, as well as those with years of experience. All of these resources are available at www.cfsei.org.

For design professionals who are seeking information on specific projects, a Steel Hotline is available at 1-800-79-STEEL or www.steelframing.org. The Steel Hotline connects inquirers with cold-formed steel framing professionals who can assist them, with hundreds of requests handled each year.

In the building codes arena, AISI is fast-tracking new cold-formed steel standards addressing seismic and energy issues. AISI Standards are now available for downloading free of charge at www.aisistandards.org.

Have You Checked Us Out Lately?

Steel is Worth Another Look!

As a building material, steel has a solid, durable and reliable track record. The steel industry is committed to providing state-of-the-art solutions to the challenges faced by today’s building design and construction professionals. This results in cost-effective and time-saving benefits to the building construction marketplace. Feel free to contact any of the steel organizations for more information.▪

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By Maribeth Rizzuto, LEED AP-BD+C

Maribeth Rizzuto is Managing Director of the Cold-Formed Steel Engineers Institute and Director of Education and Sustainable Construction for the Steel Framing Alliance. She can be reached at mrizzuto@steel.org.

Structural ForenSicS investigating structures and their components

A

Figure 1. Distribution of American Bridges by Age. Courtesy of Nace International.

s the stock of American bridges averages an age of forty three (43) years (Figure 1), it is clearly of great value to understand their remaining service life. When you consider the theoretical design of these bridges was for fifty (50) years, it is inevitable that a high proportion of them will now be deficient. In fact, the number of deficient bridges today is 1 in 9. Within the next ten years this will become 1 in 4.

It is critical to implement service life prediction (SLP) models so an appropriate rationale is incorporated in the decision making process can be made on which bridges need attention first. With aging infrastructure, it is common practice to deal with the most critical conditions on a structure (typically visual failures) in a reactive manner.

This is always more costly and, in extreme cases, tends to be done on an emergency basis due to possible health and safety issues.

SLP models of ageing infrastructure should be implemented to ascertain whether the materials have exceeded or are approaching their minimum acceptable value when routinely maintained.

Service Life Predictions for Reinforced Concrete Bridges

By Gina Crevello, Paul Noyce and Charles ‘Chas’ Bransby-Zachary, MRICS

Gina Crevello has extensive experience in building diagnostics, corrosion diagnostics, material assessments and mitigation. Gina has been involved with the majority of ICCP Systems on landmark structures in the U.S. She may be reached at gcrevello@e2chem.com.

Paul Noyce is professionally trained as an electrical electronic engineer with a further diploma in electrochemistry. Paul has been involved with numerous ICCP systems for historic buildings in England and the U.S. He may be reached at pnoyce@e2chem.com.

Charles ‘Chas’ Bransby-Zachary is a chartered surveyor who has over 18 years of experience in Building and Structures’ Assessment and is regarded as an international expert in the field of Non-Destructive Evaluation [NDE]. He may be reached at cbz@e2chem.com.

Service life is defined by the following three categories (Sommerfield et al. 1986): 1) Technical Service Life – The time in service until a defined unacceptable state is reached 2) Functional Service Life – The time in service until the structure no longer fulfils the functional requirement 3) Economical Service Life – The time in service until the replacement of the structure (or part of it) is more economical than keeping it This article willfocus on Item 1 above, the technical service life, and look at how condition states are defined in order to predict a time to an unacceptable condition. There are a number of recognized condition state methods for bridge owners to classify their structures. The National Bridge Inventory (NBI) from 1988 is typically used, as shown in Table 1.

Some bridge owners adopt their own classification systems, or use other systems as bridge management tools such as AASHTOWare™ Bridge Management software (BrM). These tools allow bridge owners to efficiently monitor costs, schedules, inventories, inspections, performances, displacements, and safety. In addition, AASHTOWare™ helps

Table 1. National bridge inventory condition rating Federal, Highways Administration (FHWA) 1988.

NBI Rating

Description

Repair Action

9

Excellent Condition

None

8

Very Good Condition

None

7

Good Condition

Minor Maintenance

6

Satisfactory Condition

Major Maintenance

5

Fair Condition

Minor Repair

4

Poor Condition

Major Repair

3

Serious Condition

Rehabilitate

2

Critical Condition

Replace

1

Imminent Failure Condition

Close Bridge and Evacuate

0

Failed Condition

Beyond Corrective Action

10 October 2015

Table 2. Corrosion state interpretation of condition state definition.

Condition State (Degree of Damage)

State Definition

0

Initial State, No Degradation

1

⅓ Limit State

2

⅔ Limit State

3

Limit State

4

Post Limit State

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Table 3. Likelihood of corrosion damage as a function of the corrosion potential.

Corrosion Potential (Volts vs. Cu/CuSO4 )

Probability of Corrosion

>-0.200

<10 %

-0.200 to -0.350

Uncertain

<-0.350

>90 %

Understanding Corrosion Condition Corrosion is regarded as the primary cause of deterioration in reinforced concrete structures. Although other damage mechanisms exist, corrosion is the primary mechanism of concrete failure which impacts service life; this, in turn, makes it the number one item to fully understand as it will eventually leave the bridge unsafe.

One method for determining corrosion condition is to use a five state system, defined as shown in Table 2.

This more simple approach facilitates an understanding of the degree of damage of the degradation of reinforcing steel over time, and allows for the classification of condition states.

The procedures typically used to predict the remaining service life can be expressed following the non-destructive evaluation definition (NDE) by Rilem, an international union of laboratories and experts in construction materials, systems and structures, as follows: 1) Identify accurately the root cause of the problem 2) Confirm potential evolution of damage, and if any, at what rate?

3) Determine the severity level of the problem, its location and extent By following the three main requirements of NDE, sufficient data is collected to portray this information in a more simplistic manner by using statistics. Nearly everyone

understands a statement such as “50% of your structure is deficient ” whereas very few understand a comment such as “chloride levels are 1000 parts per million (PPM) and corrosion rates are measured in microAmps per square centimeter (100uA/cm 2).”

Once the value of the NDE survey is demonstrated, it is essential that a sufficient volume of data is collected for each of the structural components of greatest concern. This data is critical in enabling a more accurate prediction. If only 1% of the structure is assessed, accuracy will not be as good as if 10% has been surveyed. An appropriate, representative sample of testing must be conducted in order to achieve satisfactory confidence levels regarding the bridge’s condition; this needs to balance information required to compile the most accurate and appropriate service life predictions, while working within the confines of the budgets available at the investigation phase of a project.

NDE is the key to understanding remaining service life; it forms the foundation of knowledge integral to assessment techniques.

As indicated above, the first requirement is to understand the root cause of the problem and not necessarily the problem itself. All too often, a failed drainage system is left unrepaired when ultimately the resulting water infiltration is the root cause of the corrosion problem.

The second requirement of NDE is to understand the elements of the structure which are unknown. Often, known conditions are easily ascertained visually. As shown in Figure 2 (page 12), as the structure deteriorates, the known (typically visible) conditions increase while the unknown conditions decrease. The key is to identify the time it takes for the condition state to change from one state to

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October 2015

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the next. When assessing limit states, as the structure encroaches the end of limit state three (3), those components that are at or beyond limit state 3 have become obsolete.

This is the basis of service life prediction and, as with nearly all structures, these conditions vary considerably around different components, sometimes even within the same component. An example of this would be a reinforced column positioned in water where conditions vary significantly. This is the key principle in the third requirement of NDE, determine severity, extent and location.

Without gathering information using NDE surveys, it becomes difficult to predict service life with an acceptable level of accuracy.

Gathering as much field data as possible to ultimately allow for a more accurate repair design approach is highly recommended.

As stated earlier, the primary cause of distress to the structure is reinforcement corrosion; an understanding of how this phenomenon will affect service life is critical. This can be achieved in two ways, one by conducting corrosion measurements in the field and secondly by modelling. For corrosion measurements in the field, there are numerous claims about predicting corrosion condition and risk which do not necessarily warrant a corrosion rate measurement. These techniques may provide a guide to understanding where corrosion may be occurring, but they do not quantify the amount of corrosion which is occurring at the present time. Two such examples are Half-Cell Potential and Surface Resistivity measurements. On the contrary, Corrosion Rate measurements measure corrosion current.

Half-Cell Potential measurements, performed in accordance with ASTM C876, provide the risk associated with corrosion and the characteristics indicated in Table 3 (page 11). In some circumstances, these results can be misleading. Because this testing procedure was developed for reinforced concrete

Figure 2. Condition versus Time Relationship.

structures impacted by chlorides or deicing salts, the results of a survey on a bridge suffering from carbonated induced corrosion would yield very different values.

Surface Resistivity is a measurement of the resistance of the concrete and not corrosion of the actual steel. Resistivity measurements provide a mapping of lower resistance areas, which may be associated with moisture and thus potentially high chloride ion levels.

Both Half-Cell Potential and Corrosion Rate assessments are typically restricted to smaller, localized test zones, due frequently to the nature of the data. The tests also require a connection directly to the embedded reinforcement and require the steel under assessment be continuous. Both test methods require targeted exposures to be made at the surface, which often limits the test zone sizes due to the logistical challenges involved in coring or probing a bridge that is in service.

A number of additional NDE techniques, such as Surface Penetrating Radar (SPR), magnetics, acoustics and infrared thermal imaging are also available. In most cases, data can be collected more rapidly and can cover larger areas with these techniques, providing the inspection team and the bridge owner with a more comprehensive data set across the structure.

(Note: As technology improves, a new method called a backscatter X-ray is also gathering momentum in the inspection industry, making X-ray images clearer and the equipment more portable for site deployment.)

Combining data sets from the various techniques discussed above and cross correlating the information is very important.

Reliance on data collected using just one or two techniques can lead to inaccurate data, leaving the assessment open to misinterpretation and ultimately the development of

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October 2015

poor repair designs. The best possible testing scenario is to use techniques such as HalfCell Potential and Corrosion Rate, which do provide focused information regarding the condition of embedded reinforcement, in concert with some or all of the other more rapid NDE techniques.

It is also important to consider the bridge’s construction, in addition to its condition.

When reinforced concrete (RC) bridges are constructed, they rarely conform perfectly to their original design intent. This leaves the condition testing results and any subsequent repair designs open to failure where assumptions are made regarding the placement of the embedded reinforcement. Techniques such as SPR, when combined with metal detection and sometimes X-rays, not only provide vital condition related information, but also, critically, information regarding the as-built arrangement of the bridge.

Service Life Modelling The Corrosion Rate measurements technique is fundamental when using models to predict service life. Different techniques of corrosion rate testing exist and there are a number of commercially available pieces of equipment. Corrosion rate measurements provide a corrosion current relative to a surface area being tested.

The most common technique in measuring the corrosion current is the linear polarization resistance (LPR) measurement. This measurement relies on the slope of the current versus voltage (ΔI applied/ΔE) response of the corroding interface at or near its natural free corroding potential. This slope is related to the corrosion current by the Stern-Geary equation.

In order to calculate the actual weight loss, corrosion current (Icorr) is substituted into

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Figure 3. Corrosion initiation probability model.

Second Law of Diffusion. This calculates the chloride penetration until the time of depassivation, adjusted to allow for the time dependency of the chloride diffusivity as seen in Figure 3.

Conclusions Predicting the service life of reinforced concrete bridges is a complex subject and requires a very careful balance of NDE, physical probing, coring and lab testing. Most importantly, accurate predictions require experienced corrosion experts who can collate this information and determine the following as accurately as possible: • The As-Built Arrangement (does the bridge conform to the original design intent) • The Existing Condition (mainly, what is the condition/corrosion level of embedded steel reinforcement) With this information, accurate service life predictions can be made that will target not only the visible deterioration on a bridge, but also the unknown chemical conditions occurring which impact corrosion and subsequent performance. Corrosion is often slow to initiate, and hidden conditions can be occurring which are not yet significant enough to result in cracking and spalling. These unseen conditions are ultimately the most important area of the bridge to understand in order to achieve the best possible service life of the structure.

By being predictive in service life models and understanding corrosion degradation, one can be proactive in the repair approach.▪

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October 2015

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Faraday’s equation. Faraday’s equation for steel loss equates to 1uA/cm2 = 11.6 microns per year (um/yr-1) or 0.457 mils per year (mpy). It should be noted that rust growth can average up to three times the volume of the section loss, which means for the example above, 34.8 um/yr-1 or 8.226 mpy would be the actual volume of corrosion product.

Laboratory tests (Rodriguez, 1994) found that 15-40 um gave rise to cracking on bars with a cover/diameter ratio between 2 and 4.

After all of the data is gathered via NDE (and targeted probing/coring for verification), the information is collated and is used to help create the service life models.

Multiple corrosion service life models are available for existing structures: These models provide an understanding of when corrosion will initiate and, once initiated, when propagation will impact performance leading to various levels of degradation. One of these methods is to look at the reduction in steel cross section where the corrosion current (Icorr) is converted into the reduction in the diameter of the reinforcing steel. The results are then converted into a service life prediction by modelling the effects of reducing the cross section of the reinforcement on the load capacity of the reinforced concrete structure. Models can also address the ingress of contaminants, such as chlorides and sulfates, and chemical reactions, such as carbonation, and when thresholds at the steel will be reached.

For new structures, numerous models are available. One model for corrosion initiation commonly used is a simulation of Fick’s

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Structural DeSign design issues for structural engineers

Y

ou may remember the days when Quality Assurance meant a simple review by the Engineer-of-Record immediately before sealing the drawings. That seal alone was the documented evidence of quality. Clients hired engineers for their expertise and expected them to get it right the first time. It was up to the engineers to develop habits (often unwritten) to ensure the quality of their work.

Today’s projects and today’s clients are more demanding. Projects are larger, more complex, and involve more disciplines. Clients establish shorter schedules while enforcing more requirements on the engineer. And the informal review by a senior engineer has been replaced by a far more rigorous and well-documented Quality Assurance (QA) program. Engineering firms of all sizes have responded by creating internal Quality Management Systems (QMS) that are implemented by teams of dedicated quality professionals. While the senior review is still at the heart of most engineering quality systems, today’s projects benefit from a far more robust and complete approach.

This is especially true on large bridge mega-projects.

Clients typically establish specific expectations for a mega-project quality plan. While every project is different, certain key components are frequently part of these plans. Detailed checking procedures, Quality Control (QC) reviews, audits, and continuous improvement measures are some of these core quality components.

Bridge Mega-Projects: Quality Assured

By Dan Domalik, P.E., CQA, CMQ/OE

Detailed Checking Dan Domalik is a Design Quality Control Manager at HDR Engineering, Inc. He is a registered Professional Engineer, Certified Quality Auditor, and Certified Manager of Quality/ Organizational Excellence.

Dan may be reached at Daniel.Domalik@hdrinc.com.

“The devil is in the details” certainly applies to structural design. “Minor” details such as a formula in a spreadsheet, the units used in a calculation, or a note on a drawing have caused significant problems in design and in construction. The detailed check ensures a second knowledgeable person agrees with each detail of the work. A check of each calculation and each drawing before starting a senior QC review can prevent costly rework in design and in the field.

Detailed checking can be performed and documented in a variety of ways. However, most effective checking procedures share certain characteristics: • Comprehensive – Detailed checks are not spot checks. Rather, they include all calculations, drawings, specifications, reports, and any other engineering deliverables.

• Performed by the designer’s peers – Checking is typically done by an engineer with a similar level of experience as the designer – one who is capable of creating the design that is checked. Although this may be a senior engineer, the checker is most often a mid-level engineer.

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Figure 1. Checking and review process.

• Documented with mark-ups of the deliverable – A record of the “conversation” between the designer and checker must be retained as the documentation of the detailed check. This generally consists of color-coded mark-ups (e.g. yellow highlighter indicates agreement, red edits indicated proposed changes) on a Check Print of the document. Check Print stamps are used to identify the designer and checker, and the dates they completed their checking tasks. The checking procedure defined in the quality plan for the project outlines the color-coding system and processes that must be followed to complete a detailed check.

The end result of the detailed check is a corrected document that is mutually acceptable to two individuals – designer and checker. This is the document that is provided to reviewers in the subsequent Quality Control process. Figure 1 shows the roles of each person in the detailed checking process.

Quality Control Reviews Quality Control reviews provide a higher-level evaluation of the adequacy of the checked documents. Senior engineers with experience designing and managing the work being reviewed provide comments that are based on the project requirements and their own experience. The QC reviewer is someone with proven engineering judgment in the work. Unlike detailed checks, QC reviews are not focused on details such as mathematical correctness. They are meant to reveal issues with the overall design approach taken, to identify discrepancies between contract requirements and the work, and to discover signs of flaws that might have eluded the detailed checker(s).

Some projects document these QC reviews with mark-ups, similar to the process followed for detailed checks. However, this can cause confusion between the separate and distinct check and review processes. And direct markups can be a less effective way to document the higher-level input provided by QC reviewers.

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Figure 2. Review comment form.

• Quality Integration Review – While the Interdisciplinary Review evaluates consistency between disciplines, the Quality Integration Review evaluates consistency between design teams or firms. Most applicable on larger, multifirm projects, this review reveals and corrects the inevitable uniqueness in design work produced by teams from different companies or in different geographic regions.

• Constructability Review – This review is performed by the contractor team on design-build projects. It gives the contractor an ability to improve the efficiency of the design given the planned means and methods of building the project. A maintenance review may be part of this review if the contract includes a long-term maintenance component.

Tools such as Bentley’s Bluebeam Revu™ allow for simultaneous reviews to be performed in a shared virtual environment. This approach is essential for collecting review comments and closing them in an efficient manner. The back-and-forth process between designer and reviewer is shown in Figure 1.

It is not unusual for mega-projects to require independent calculations, especially for structural analysis and design. Independent design calculations typically involve two separate design teams creating parallel models and designs based on a common set of given data.

When independent designs are required, the teams must not coordinate or compare work until they have completed their separate designs. Only when each team is fi nished should the results be compared and reconciled. The independence of this approach, while costly, results in a robust design that was independently achieved by two teams.

Audits Checking and reviews control the quality of the work product, and are generally defined as Quality Control (QC) processes.

Quality audits assure process compliance

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A better way to document review comments is in comment/response tables. This format provides significant advantages that are better suited to the importance and formality of QC reviews: • Each comment is tagged with a unique identifier • A written response is provided for each comment • Status codes (Agree or Disagree, Open or Closed, etc.) can be assigned to each comment/response • Resolution between the designer and reviewer can be explicitly documented for each comment • All of the information above can be sorted and filtered for tracking • Multiple reviews can be consolidated into a single review database for improved oversight and reporting of the review process See Figure 2 for an example template used to document review comments, responses, and final closure.

Mega-projects often include several stakeholder groups and multiple disciplines beyond structures. This diversity of input usually requires multiple types of review to capture all relevant comments. The list below provides examples of the review types that are frequently found on mega-projects: • Quality Control Review (or Discipline Review) – This review is disciplinespecific – a senior bridge engineer reviewing bridge documents, for example. It is focused on the technical correctness and completeness of the documents within that discipline.

• Interdisciplinary Review – Senior experts from each discipline review other discipline’s work for potential conflicts with their discipline. This could be a senior roadway engineer reviewing bridge documents while the senior bridge engineer reviews roadway documents. The intent is to break through the discipline silos and ensure the complete set of design documents work as a unified whole.

and effectiveness. As such, they are part of the Quality Assurance (QA) process. Despite the image of IRS agents combing through your receipts, quality audits are essential and (slightly) less painful steps in producing complete design submittals. Even the most diligent designers, checkers, and reviewers can inadvertently omit a required quality step or piece of documentation. Since quality plan requirements are generally contract requirements, these process discrepancies must be corrected. In many cases, audits are required as the basis of a formal quality certification document that supplements the engineer’s seal.

QA auditors are generally independent of the design, checking, and review processes.

Although they are usually engineers, audits can be performed by other persons who can interpret the quality requirements and engineering work product. Auditing is not a technical evaluation of the work product. It is a confirmation that the quality plan processes were fully implemented and adequate documentation exists. Auditors often document their findings on a checklist that contains questions such as: • Were all documents detail checked?

• Were all required reviews performed?

• Were the appropriate personnel used for the design, checking, and review tasks?

• Were all resolved checker mark-ups and reviewer comments closed and incorporated into the final documents?

• Were any late changes made to the documents and not checked or reviewed?

• Does adequate documentation exist in the form of Check Prints and review comments/response forms?

The value of QA audits is most apparent when a process issue triggers additional checks or reviews that result in content corrections to the documents. QA audits are based on the notion that effective quality processes, consistently applied, improve the quality of the product. When designers, checkers, and reviewers understand their work will be audited, it encourages a consistent application of the quality processes. The old quality axiom applies: “What gets measured gets done”.

Continuous Improvement Detailed checks, quality reviews, and quality audits will not happen by accident.

The quality manager of a mega-project is responsible for a vast array of processes, as well as the quality of the product itself. In order to maintain a high level of quality and process compliance, the quality manager implements continuous improvement

processes such as quality training, quality metrics, and corrective/preventative actions.

This “quality management toolbox” motivates everyone to improve their work from project start to finish.

Structural designers cannot implement quality processes if they haven’t been made aware of their existence. Quality plan implementation starts with quality training. This training is generally most effective when presented inperson by the quality manager. However, large teams may rely on videoconferencing and conference calls as well, so remote designers can stay aware of the project’s quality processes. The quality manager should develop documents that can be used both during and after the training sessions – PowerPoint slideshows, process flowcharts, example documents, QC/QA directives, etc. Quality training attendance must be tracked and a short quiz should be used to ensure trainees have retained the important material. This is no small effort, but it is well worth it to avoid hearing this response in a quality audit: “But no one told me!”

Your car’s dashboard tracks driving metrics such as your speed, the amount of fuel left in the gas tank, and warnings about engine problems. As the driver, you use this information to determine when to slow down, when to fill your gas tank, and when to get your car serviced. Quality metrics serve a similar function to your car’s dashboard. Quality metrics such as the time in review, the number of staff trained, and the number of review comments received measure the quality health of the project. Long review periods might indicate poor quality of work entering review or an overzealous reviewer. Reductions in the percentage of staff trained on quality indicate a need to increase the frequency/reach of the training. Increases or decreases in the number of review comments might indicate a change in the depth of the reviews or the quality of the work itself. If the quality manager is not tracking these metrics, it is difficult to know when corrective actions are needed. Regular quality reports to project management, that contain these metrics, are easily interpreted and are powerful management tools.

Corrective/Preventive Actions (CPA) processes are used to diagnose and treat quality issues that occur on mega-projects. We are all human beings, and human beings make mistakes. The quality metrics described earlier can make it easier to see when a mistake becomes the sign of a larger trend. A complete QC/QA plan must have a proactive plan for mitigating the extent and severity of mistakes. CPA plans usually contain the following elements:

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• Problem Definition – A clear problem statement must be defined to ensure there is no confusion about the issue at hand. A concise, specific problem statement is the start of the CPA process.

• Root Cause Analysis (RCA) – If we treat only the symptom and fail to address the root of the problem, we can expect the problem to continue. Good quality managers can help the project team drill down into the true source of a problem. It is this problem source – the root cause – that must be addressed to prevent the recurrence of the issue.

Quality tools such as “Five Why’s” exercises or Fishbone Diagrams can help direct the RCA. • Corrective Actions – These are actions taken to fix the issues already caused by the problem. They are backward looking efforts to make sure the final work is ultimately correct, even if rework and/or repair is needed to get there. Corrective actions could include resubmittal of design documents or retrofit designs.

• Preventative Actions – CPA’s are valuable learning experiences. While we cannot change what happened in the past, we can take mitigating steps to reduce the likelihood of future problems. These preventative actions are the forward-looking changes to processes, people, or both to maximize positive outcomes in the future.

Examples of preventative actions might be changes to the quality plan, new technical directives, or changes in project personnel.

Summary Clients have always expected that engineers work fast and design it right the first time.

However, the days of relying on an engineer’s seal as evidence of quality are over.

Expectations are higher than ever and now include formal QC/QA processes with complete documentation. The definition of failure has expanded beyond technical problems and now includes failure to follow the proper processes or provide the necessary documentation. In this environment, megaproject quality has grown into a more robust and formal element of overall project success.

The checking, review, audit, and continuous improvement processes included in these projects ensure that engineers continue to exceed client expectations and deliver outstanding designs.▪

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ConstruCtion issues discussion of construction issues and techniques

I

n 1967, the San Mateo/Hayward Bridge incorporated the United States’ first orthotropic steel bridge deck on a major bridge, winning the American Society of Civil Engineers’ (ASCE) Outstanding Civil Engineering Achievement Award (OCEA). The mile long orthotropic steel deck is included within the six lane wide, two mile long steel high-rise portion of the seven mile long bridge. The two mile high-rise, spanning a navigation channel, was designed per the American Institute of Steel Construction’s (AISC) 1963 Design Manual. The San Mateo/Hayward Bridge is the longest bridge in the San Francisco Bay Area and the 25th longest in the world.

The riding surface placed in 1967 was an epoxymodified asphalt concrete (EAC), and was now in need of replacement. Replacement materials for the riding surface were narrowed down to the original EAC that had performed so well and a premixed polyester concrete developed by Caltrans that has been performing well on concrete decks since 1983.

One major concern for replacing the riding surface was taking the bridge out of service during installation. This is a critical bridge connecting the East Bay Area communities to San Francisco, which made construction techniques even more important. It was shown that polyester concrete could be more adaptable to placement within tight time constraints.

Construction was performed primarily during two continuous 55-hour shifts over two weekends in May of 2015. The successful bidder was a joint venture of two California-based prime contractors pooling their capital and labor resources so as to focus on completing the tasks of removing the EAC and replacing it with a premixed polyester concrete on six lane-miles of riding surface.

Detailed contractor planning and coordination of the vast amount of skilled labor and equipment on the bridge at one time, and owner inspection of the work with material quality assurance testing to ensure manufacturer’s quality control were vital to success. In contrast, the original 1967 surface placement time was three weeks at eight hours per day.

Accelerated Bridge Construction to Rehabilitate Aging Highway Structures Replacing Riding Surface to Maintain Structural Integrity and Durability of an Orthotropic Deck

By Ric Maggenti, P.E. and Kenneth Brown, P.E.

Ric Maggenti is a Professional Engineer in California Government working for Caltrans since 1983. He is both a Senior Bridge Engineer and a Senior Materials & Research Engineer currently acting as an Office Chief of a San Francisco Bay Area Toll Bridge Design Office.

Ken Brown is the Office Chief (Supervising Bridge Engineer) of Structure Maintenance and Investigations – Toll Bridges. In his position, Mr. Brown directs the work of 20 engineers responsible for the inspection, rehabilitation design and general custodianship of the major toll bridges in the San Francisco Bay Area.

The San Mateo/Hayward Bridge The bridge consists of a five mile long concrete trestle on the east side, and a two mile long high-rise steel structure over the navigational channel on the west end. Approximately ½ mile on the approach and departure sides of the high-rise consists of steel box girders and a lightweight concrete deck. The center portion, roughly 1 mile long, consists of two steel box girders, floor beams, and a steel deck stiffened with open longitudinal ribs. Welded together they

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Figure 1. View from under showing ribs, girders and floor beams welded to deck plate.

form a composite structure. With the steel deck acting as the top flange for the ribs, beams and girders, this meets the definition of an orthotropic bridge, which is derived from the terms orthogonal meaning having properties at 90 degree angles and anisotropic meaning directionally dependent properties. The steel deck is on the fourteen 292-foot long spans, the main channel span of 750 feet, and the two anchor spans of 375 feet each (Figure 1).

Need for Riding Surface Orthotropic steel decks created a need for a riding surface material that bonds well to steel and is capable of conforming to a desired ride quality.

It must also protect the steel from corrosion.

The 1967 riding surface was an EAC, a thermoplastic asphalt modified with a thermosetting epoxy polymer. Deck sections were field bolted with splice plates requiring a riding surface thickness large enough to cover the bolt heads. This 2.25-inch thick riding surface paved in two lifts was chosen following years of laboratory and field evaluations. After painting the deck with a zinc-rich paint as an added precaution, an epoxyasphalt bond coat was applied to adhere the riding surface to the steel. The EAC was paved using conventional methods and techniques used for placing a typical asphalt concrete roadway.

The material was strong and tough enough to withstand traffic wear and differential bridge movements, without cracking or de-bonding for 30+ years. By 2002, cracking was observed and in some places the EAC was shoving, indicating it had lost bond. More and more maintenance was required and the potential for deck corrosion increased.

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Figure 2. Deck being shotblasted after removal of old EAC. Note: Transverse bolted splice plate in foreground.

The requirements for a replacement surface needed to meet the 1967 objectives plus some additional criteria. Bonding to steel has taken on an additional importance since this bridge was designed, as an effective bond between the steel and overlay increases stiffness by composite action thus lowering fatigue stresses in the steel deck. Per AASHTO 2012: “the wearing surface should be regarded as an integral part of the total orthotropic deck system and shall be specified to be bonded to the top of the deck plate…the wearing surface shall be assumed to be composite with the deck plate, regardless of whether or not the deck plate is designed on that basis.”

The original targeted minimum bond strength by direct pull out was 200 psi. Much higher bond values are now achievable. Furthermore, accelerated construction to minimize the impact on traffic was now a new criterion.

Laboratory Evaluation Caltrans initiated a research project with the University of Missouri (U of M), which had previous experience evaluating Missouri DOT’s resurfacing options for the orthotropic Poplar Street Bridge. Caltrans chose two alternatives to be investigated – EAC and the standard/widely used polyester overlay system Caltrans developed for concrete bridge decks. The testing evaluated the following: 1) Fatigue characteristics at the steel/ riding surface interface,

2) Structural enhancement the riding surface material provided to the deck that in turn lowers fatigue stresses particularly at welds, 3) Static and dynamic load characteristics, and 4) Structural behavior over a range of ambient temperatures.

The U of M tests were tailored around analytical load/deflection behavior of the San Mateo/ Hayward Bridge. The results of the research indicated the polyester concrete would perform as well as EAC. The polyester concrete, having a higher modulus of elasticity, added more structural integrity while being 15% lighter than EAC, which was also an advantage.

One variation of the standard polyester concrete system was the addition of zinc powder to the prime coat, which enabled skipping the zinc-rich paint. The zinc powder conformed to the requirements for zinc powder of zincrich paint. Comparing State-specified zinc-rich paint with a zinc-rich polyester prime coat showed similar corrosion protection with samples subjected to a salt spray chamber.

Accelerated Bridge Construction Issues The greatest advantage of polyester concrete was construction within a tight time constraint. The risk of compromising the material during installation would be greater with EAC than with polyester concrete.

EAC is centrally batched and delivered in large dump trucks. Temperature and time

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Figure 3. Continuous mixer/batching truck discharging into paver. Prepared surface has a coating of the methacrylate prime coat containing 85% by weight zinc powder as a corrosion inhibitor.

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restrictions after batching are critical to proper polymerization of the epoxy component, the time/temperature sensitive to ambient temperature. Once batched into a truck, little flexibility in timing for proper placement is available. At any given time, multiple truckloads of EAC are on hand, making any unexpected stop or even just a change in the rate of paving problematic.

Additionally, since EAC is batched off site, longer delays would be expected before restarting after an unexpected stop to avoid having even more amounts of potentially compromised material. During EAC construction on the Self-Anchored Suspension (SAS) portion of the San Francisco/Oakland Bay Bridge, to facilitate production rates, the minimum time required before a dump truck could discharge was relaxed. However, this in turn required several more days of cure time to prevent premature damage due to inadequate polymerization of the epoxy component. The EAC specifications used for the SAS orthotropic deck required 48 hours after placement just for cure, when EAC was placed within specification limits for time and temperature. Also, a surface treatment operation was needed to provide friction.

Conversely, a polyester concrete can be batched and mixed by continuous mixer and immediately discharged into a moving paver’s hopper, resulting in no more than the small amount of material in the hopper needing attention at any given time. The paving rate can be easily adjusted since the batching STRUCTURE magazine

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rate is instantaneously proportioned to the paving rate. Since stopping and starting the paving operation is a non-issue due to material integrity, unexpected problems requiring stopping/starting of paving could be easily resolved. Control of the added initiator system (hardener) and accelerators allow for regulating working time and strength gain under very diverse temperature ranges.

Such control expedites a traffic-ready surface in as little as 1.5 hours during ambient temperatures ranging from 50-100 degrees Fahrenheit. Quick set time also facilitated construction traffic as work progressed. The surface is textured with tine tools during placement and will last for decades, rendering friction as a non-issue.

Field Evaluations Experience of other State DOTs was also sought. Portland, Oregon’s Fremont Bridge has an orthotropic upper deck which opened in 1973 with an EAC riding surface. In 2011, the original surface, which is less than 40% of the area of the San Mateo/Hayward Bridge, was removed and a new EAC was placed over several weekend closures. With the cooperation of Oregon DOT, the construction during one of those weekends was observed. This provided direct communication and a first-hand look regarding the risks, anticipated and unforeseen, that ODOT managed. Though Caltrans ended up using a different material, much was

learned and incorporated into the San Mateo/Hayward Bridge plans, specifications and estimates for the rehabilitation project. Given Missouri DOT’s relationship with U of M, MoDOT used polyester with the zinc-rich prime coat to repair large sections of the Poplar Street Bridge riding surface that had come loose. The Poplar Street Bridge was completed shortly after the San Mateo Bridge and has been resurfaced several times. MoDOT was more than just pleased with these repairs.

After gaining experience and evaluation of some smaller scale field installations, a small test patch was completed on the bridge, providing an in situ evaluation of the riding surface’s performance. With the success of the test patch, Caltrans set out to replace the aged EAC with polyester concrete. The new riding surface was completed during two weekends in May of 2015.

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Figure 4. Top: Profilogragh measurements ensuring contract compliance of quality ride. Bottom: Joint venture contractor foremen crossing paths in middle of bridge. One contractor worked eastbound lanes while the other worked westbound lanes.

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On Friday, May 8, 2015 at 8 PM, a few lanes were shut down and equipment was moved onto the bridge. At 10 PM, the entire bridge was closed. Tractor trailer dump trucks were lined up on both sides of the high-rise portion of the bridge.

With the noise of pavement-breakers, excavators, loaders, and a variety of other heavy and hand equipment, the removal of 11 million pounds of 48 year-old EAC began (Figure 2, page 19). After midnight, loaded volumetric batched mixing trucks rolled onto the deck discharging polyester concrete into the pavers (Figure 3). Just prior, the deck was abrasively cleaned by self-propelled shot blasters followed by hand-held “sand” blasting.

The prepared surface was checked for compliance with SSPC-SP 6/NACE. A liberal coat of methacrylate containing 85% by weight of zinc powder was then applied to the surface. Removal, deck preparation, and paving continued on through the weekend – the last of the paving completed shortly after midnight on May 11. A little more than half the bridge was resurfaced and, after clean-up and temporary striping, the bridge was opened prior to the scheduled 5 AM deadline. The process was repeated starting Friday, May 22 and by 3 AM on May 25 the bridge, now with a new riding surface, was opened to traffic.

Eleven volumetric mixer trucks, modified specifically to place polyester

concrete, and four pavers modified for the same (no more than two pavers were in operation at any given time) were on hand.

Twenty-six tanker trailers, each containing 40,000 pounds of polyester resin meeting all contract compliance tests, were delivered.

To ensure a quality material, the supplier supplemented the Quality Assurance testing by Caltrans with additional Quality Control tests, some of which were not even on contractually-required vender properties.

Approximately 64,000 pounds. of zinc for galvanic protection are on the deck as part of the prime coat. The surface was tested for compliance with ride quality smoothness by profilograph (Figure 4), while pull off testing indicated nearly 900 psi on 2-inch diameter cores. Friction values were well above the required 0.35.

With adequate inspection, testing and oversight, coupled with the contractors’ planning and attention to detail, this Accelerated Bridge Construction project was successfully completed without compromise. If the material exceeds its life expectancy, as did the first material, the next replacement and subsequent interruptions to traffic won’t be needed until after the centennial celebrations of the Eisenhower Interstate Highway System.▪

Practical SolutionS solutions for the practicing structural engineer Figure 1. Cable dehumidification process.

Suspension Bridge Cable Dehumidification A Matter of Life and Health

By Shane R. Beabes, P.E., M.SEI, M.ASCE and Barry R. Colford, BSc, C.Eng, FICE

Shane R. Beabes is an Associate Vice President and Technical Leader in preservation for AECOM Complex Bridge Practice in North America. Shane is AECOM’s Project Manager for the cable and anchorage dehumidification for the William Preston Lane Jr. Memorial (Bay) Bridge, Maryland – the first cable dehumidification project in North America. He can be reached at Shane.Beabes@aecom.com.

Barry R. Colford is a Vice President and Preservation Practice Leader for Complex Bridges with AECOM in North America.

Barry is formerly the Chief Engineer and Bridgemaster of the Forth Road Bridge, Edinburgh, Scotland. He can be reached at Barry.Colford@aecom.com.

History The U.S. has the largest inventory of long-span suspension bridges in the world, and the problem of corrosion within the main cables of these bridges has been recognized for some time. As early as 1968, corrosion was found in the outer wires of the cables on the Golden Gate Bridge, and in 1978 the U.S. Grant Bridge over the Ohio River was closed after severe corrosion was detected in the main cables. In the 1980s, broken wires prompted rehabilitation efforts on the main cables of the Bear Mountain and Mid-Hudson Bridges in NY. In 1990, corrosion was discovered on the outer main cable wires of a bridge in Japan that was just seven years old. Further inspections revealed corrosion in the main cables of other suspension bridges in Japan that were even younger.

Investigations ensued, and accelerated testing led engineers to conclude that, even with improved wrapping and sealing, it was not possible to make a cable completely watertight – water, a cable’s worst enemy, would always find its way into the

22 October 2015

cable. The idea was born that if a suitable dry-state environment was maintained by some artificial means, it would be a promising way to protect the cable against the spread of corrosion. The development of main cable dehumidification stemmed from this work.

Cable Dehumidification Main cable dehumidification involves injecting dried-air into the cable microenvironment and allowing the air to permeate into the interstitial spaces (voids) between the individual cable-wires.

The dried-air collects the trapped water before releasing the moisture-laden air through exhaust ports (Figure 1). Cable dehumidification addresses the root cause of corrosion by removing one of the two necessary components of the corrosion process – water – which is far easier than removing the other component – oxygen.

The premise of cable dehumidification is to protect the individual cable wires through the control of humidity within the cable. This is a long-proven technique dating back to the first half of the twentieth century, where work was carried out by W.H.J. Vernon and later by H.H. Uhlig of the MIT Corrosion Laboratory.

The results of corrosion studies indicate that if the relative humidity (RH) is kept below 60%, the corrosion rate dramatically decreases, and below 40%, corrosion practically ceases (Figure 2).

Rate of Corrosion

S

tretching down from the tower tops forming the main support for the load on the bridge, the cables of the suspension bridge are loosely akin to the human spine. Both cable and spine must be strong enough to withstand their load for a lifetime, responding to their imposed demands 24 hours a day, 7 days a week, and 365 days a year – without respite.

As such, both cable and spine demand long-term proactive care and protection. Fail to take care and ignore the signs of ailment, and catastrophe could be right around the corner when least expected.

Main cable dehumidification is as much a prescription for an ailing cable as it is preventative care for a healthy cable. As a doctor may say that it is never too early to live a healthy lifestyle, a suspension bridge owner must institute the same logic for the cables – reflecting on the thought that it may never be too early to dehumidify.

0

20

40 60 80 Relative Humidity %

Figure 2. Rate of corrosion vs. relative humidity (background: 4 stages of cable wire corrosion).

100

Figure 3. Traditional cable protection system.

100

Relative Humidity %

Cable dehumidification is an active system for cable management similar to the routine daily workouts for a healthy body. The implementation of a series of injection and exhaust sleeves along the length of the cable, supported by a system of dehumidified air maintained below the threshold 40% RH, supplies a continuous lifeline of dried-air to combat corrosion. This can be contrasted with the original passive system of protection for most main cables, which typically included galvanized cable wires with a layer of red lead or zinc paste below a soft-annealed galvanized wire wrapping and paint system (Figure 3). Dehumidification also contrasts the historic approach to retrofitting cables with the added protection of oiling – an expensive process of unwrapping and wedging the cable wires apart and pouring in specially formulated oil in the hopes of protecting the cable against continued corrosion.

The older methods of protection have proven unsuccessful and may be seen as dressing a bad wound with a Band-Aid, whereas, cable dehumidification is a holistic approach addressing the root cause of cable-ailment.

80 60

Cumulative Wire Breaks Average Cable RH

40 20 0 0

100

200

300

400

Weeks

Fracture Mechanics of Cable Wires

Figure 4. Reduction in cumulative wires breaks on dehumified cables.

To arrest the corrosion process, it is essential to remove water – both in the form of trapped water in the cable and water that inevitably finds its way into the cable over time.

Historical data on existing bridge cables that have been retrofitted with a dehumidification system demonstrate a marked reduction and near-cessation of wire breaks over time (Figure 4), illustrating the effectiveness of main cable dehumidification on the overall health of the cables.

Monitoring the Vital Signs of the Cable Assessing the condition of the cables by visual and hands-on inspections can be supplemented by acoustic monitoring.

Depending on the condition of the cable, internal cable inspections typically occur but once a decade and focus on just a few locations along the cable. The estimated strength of the cables therefore requires statistical extrapolation from the relatively small amount of data collected. Cable wires gradually deteriorate through corrosion, crack initiation, and crack growth, culminating in a sudden break. This releases energy which can be detected through a network of acoustic sensors.

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Some bridge owners have installed acoustic monitoring systems that listen 24/7 to the whole length of the cables for breaking wires.

This provides valuable and in some cases crucial important long-term information on the structural health of main cables.

Two key pieces of data are generated by the acoustic monitoring – hotspots indicating significant wire-break activity and the longterm trend of cumulative wire breaks. This data enables the bridge owner to have an increased knowledge and confidence in the actual health of the cables. continued on next page

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Cable wires are usually about 0.196 inch in diameter (nearly the size of a No.2 pencil) and have a tensile strength of 225,000 psi. The wires are usually galvanized, pulled across the spans, and then compacted together to form a near circular cable cross-section, comprised of thousands of individual wires, or tens-ofthousands in longer-span bridges.

There are many contributing factors to cable corrosion and loss of strength.

However, it is the initiation and propagation of cracks that ultimately cause cable wires to break. This process starts with the water that has collected within the cable reacting with atmospheric pollutants leading to zinc depletion – the degradation of the galvanized coating on the cable wires. Once the zinc protection is depleted, corrosion pitting will occur; some of the pits develop cracks, which then grow into the cable wire cross-section. The very high strength of the steel used in cable wire makes it more brittle in nature and more susceptible to hydrogen embrittlement and associated cracking. Hydrogen embrittlement is a result of hydrogen at the subatomic level migrating into the steel matrix, causing the cable wire to become brittle and prone to wire cracking and fracture at normal levels of working stress.

Figure 5. Representative AECOM cable dehumidification projects – Forth Road Bridge, UK (left), WPL Bay Bridge, Stevensville, MD (center), S. 10 th Street Bridge, Pittsburgh, PA (right).

Achieving the Life Expectancy of the Bridge Maintaining the overall health of the main cables is crucial to achieving the designlife of the bridge. Historically, suspension bridge cable design has been developed without a full and proper understanding of the potential rate of deterioration. As critical as this is, it is still not effectively reflected in design guidance.

The long time-horizon associated with discovering cable durability problems has required suspension bridge owners to take a long-term view of monitoring such problems and installing countermeasures. It has also stimulated the sharing of knowledge across the suspension bridge engineering community so that lessons are learned.

Cable strength and durability are influenced by many complex interrelated factors including construction quality, long-term performance of constituent materials, and human responses to these. As main cables are the primary load carrying elements of the structure, it is important that their condition is known and closely monitored.

Diminishing tensile strength factor of safety is the main concern for suspension bridge owners and generally drives the implementation of preservation strategies.

In most cases, if main cables are to remain sufficiently durable and still have an adequate tensile strength factor of safety for their target design life of 100+ years, it is necessary to provide active corrosion protection in the form of cable and anchorage dehumidification. Installing cable dehumidification can prolong the service life of suspension bridges, sustain the cables at a reasonable cost, and maximize the return on public investment for these critical assets – results which strongly align with many of the objectives of suspension bridge owners.

Monitoring the Trends As a doctor would monitor the latest advances in the treatment of illness, bridge owners must keep their fingers on the pulse of the latest techniques to protect the health of the cables.

It is becoming evident that the only effective form of positive intervention that can be implemented to preserve the condition of the main cables is dehumidification.

Although first implemented in Japan almost 18 years ago, application of cable dehumidification has steadily increased, with installation on bridges in the UK, Scandinavia, South Korea, and China. The trend for main cable dehumidification crossed the pond with its first implementation in the US on the William Preston Lane Jr. Memorial (Bay) Bridge in Maryland. Studies began in 2011 and culminated with system commissioning on the WB Bay Bridge in early 2014, and anticipated commissioning on the EB Bay Bridge in 2015. Numerous other cable dehumidification projects are underway in the U.S., including the Delaware Memorial Bridge and the S. 10th Street Bridge (Figure 5).

Maintaining the Health of the Bridge The implementation of cable dehumidification should not be considered in isolation, but rather as part of an overall holistic approach to suspension bridge maintenance, utilizing modern technologies and proven strategies for all suspension bridge components.

Such a strategies should seek to maximize the benefit of maintenance and capital improvement work, with special emphasis on long-term suspension system reliability. For example, weigh-in-motion studies originally intended to address fatigue can and should be leveraged to identify bridge-specific loading which can, in turn, be used to refine cable tensile strength factor-of-safety calculations.

A thorough understanding of cable health will

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also inform decisions related to the timing and type (weight) of deck replacement, often the most costly and disruptive work that a suspension bridge owner will undertake.

A practical understanding of cable wire behavior and overall cable health should also drive routine maintenance activities, including worker training across multiple trades and disciplines.

This will ensure that power and communication systems, drainage, and other civil works all function to sustain long-term suspension system reliability.

Conclusion There are approximately fifty main suspension bridges in the U.S. with span lengths greater than 700 feet. About fifty-percent of these bridges are over 75 years old. Coupled with an average bridge sufficiency rating of approximately 46 out of 100, suspension bridge owners are challenged with repairing, rehabilitating and preserving these critical assets – particularly during a time when there is a lack of long-term transportation funding.

Cable corrosion on suspension bridges is insidious. Corrosion will begin slowly and may not be readily apparent through external examination of the cables. By the time the cables are opened for internal inspection, significant corrosion may have already occurred. Of even greater concern is that wires may have already cracked and a number may already be broken, leading to reduced cable reliability and strength.

Unfortunately, cable corrosion cannot be reversed; however, through an active cable and bridge management strategy, main cable dehumidification can be leveraged to practically arrest corrosion in the cables and prolong the life of a major infrastructure asset.▪ The online version of this article contains detailed references. Please visit www.STRUCTUREmag.org.

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Structural rehabilitation renovation and restoration of existing structures

T

he U.S. Department of Transportation has determined that the country has too many existing bridges that need replacing or upgrading, rehabilitating or retrofitting. At the end of 2014, according to the National Bridge Inventory, more than 145,800 highway bridges are listed as structurally deficient or functionally obsolete; this is about 27% of all bridges based on total bridge deck areas. In some states, the situation is even worse; Figure 1. for example, in California the deficient bridges are 34.6%, and in New York State they are 59.8%!

Steel bridges are relatively easy to modify, reinThe existing infrastructure is getting old, not force or widen. One additional advantage of such only because of the years of service, but also due bridges is that many built during the period from to the complex modernization of transportation. 1920 to 1960 incorporated the strongest available Regardless of the desire to improve infrastructure, steel at the time of construction. For example, the the allocated funding cannot be sufficient to replace San Francisco – Oakland Bay Bridge, completed or repair and upgrade so many structures. Therefore, in 1936, had higher-strength nickel (Grade 380 using efficient and economical solutions is essential MPa) and silicon steel (Grade 311 MPa) in the for the better use of the limited funding available. In East Crossing, accounting for 62% of the steel used some cases, old bridges have historical significance for the entire bridge and 72% of its cantilever secthat makes it even more nec- tion. Even the carbon steel used in this bridge had essary to consider retrofit in strength (Grade 255 MPa) comparable to today’s lieu of replacement.

ASTM A36. The same bridge had high-strength The most typical defi- cable steel (Grade 828 MPa) for its West Crossing ciencies for old bridges are suspension cables. usually one of the followThe use of higher-strength steel, along with ing: insufficient live load the fact that many of these bridges have served capacity, narrow traffic transportation needs well for decades, suggests lanes, low clearance, need that it would be more efficient to strengthen for more traffic lanes or shoulders, unsafe struc- and modernize such existing structures, rather tural materials or connections. In most cases, than demolishing and replacing them with new there is a combination of two or more of these structures. This article discusses modification items. While the need for additional traffic lanes options for reinforcing existing bridges, utilizcan sometimes be addressed with a new parallel ing the specifics of their static systems. These bridge, most of the other issues require structural options are for steel-framed bridges; however, modifications to, or complete replacement of, the most of them are also applicable in principle for existing structure. concrete bridges.

Efficient Methods for Upgrading or Reinforcing Existing Bridges

By Roumen V. Mladjov, S.E., P.E.

Roumen V. Mladjov has more than 52 years in structural and bridge engineering, and in construction management; his main interests are structural performance, efficiency and economy. He can be reached at rmladjov@gmail.com.

The online version of this article contains detailed references. Please visit www.STRUCTUREmag.org.

Figure 2.

26 October 2015

Replacing Existing Deck with Lighter System When it is required to add more live load capacity to an existing structure, the conventional way for retrofitting is to reinforce all elements of the bridge that need strengthening. This involves substantial material, labor and time, making this method relatively inefficient. If, for example, the bridge has to be upgraded for an additional 25% load capacity, it means that all elements have to be reinforced with added steel plates or other shapes, using welding or high-strength bolting to connect them to the existing bridge members. To comply with the required 25% strengthening and to compensate for the added self-weight, it will likely require a roughly 35% increase of the cross section (and weight) for all existing bridge elements.

A more efficient approach is to replace the usually existing reinforced concrete slabs with lighter systems such as orthotropic steel decks, thus reducing the self-weight of the bridge. This approach was used very successfully in 1978 to 1986 on the re-decking of the George Washington Bridge (New York) and the Golden Gate Bridge (San Francisco).

This can reduce the dead load of the deck, “buying” extra structural capacity or relieving

Figure 3.

overstress situations. This technique would make foundations easier to upgrade, or might even circumvent strengthening of piers and foundations entirely.

Generally, such replacement with a steel orthotropic deck provides a savings from 1.38 to 2.65 MPa (140 to 270 kg/m2) in the self-weight of the structure; this then

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The moment demand is reduced from 85% to 27% of the maximum moment (from uniform loads) for the simply supported spans, depending on the section location (Figure 3, page 27). Additional savings may be achieved through careful configuration of the additional truss members at the zones of support, adding higher “nodes” at the supports to the originally flat trusses with parallel top and bottom chords, thus further reducing the axial force demand for the chord members within these zones.

Reinforcing Existing Cantilever Truss Systems and Arch Bridges

Figure 4.

becomes an extra live load allowance. A detailed study of this approach finds the savings to be approximately 35% of the original bridge weight. For the George Washington Bridge, it was 43%; for the Golden Gate Bridge, it was 32.5%. Even more efficient are substitutions of the old concrete deck with extruded aluminum deck panels, or more recently, with sandwich plate systems (SPS). These even lighter deck systems produce a weight savings of up to 60%.

Converting From Simply Supported to Continuous Spans Considering that many longer-span bridges are multi-bay simply supported trusses or cantilever trusses, one efficient and relatively simple method of reinforcement is to transform them into continuous truss systems.

This approach consists of interconnecting the adjacent simple spans and converting them to a continuous system on four, five or more spans depending on the desired length between expansion joints.

The improvement in capacity is a result of “moving” the maximum bending demand (Mmax = wL2/8) from the center of each span to the much shorter zone at the middle support (Figure 1, page 26). In addition, engineers can use a higher depth of the continuous truss at the middle support zone, reducing the chord loads.

An example is shown in Figure 2 (page 26), which comes from a study for strengthening an existing bridge at Healdsburg, California, by transforming two single-span through-trusses into a two-bay continuous truss.

It should be noted that such transformations require careful analysis of all elements

of the bridge, including the piers and their foundations, which might also require reinforcing. For example, the middle support of a two-bay continuous truss will be subject to higher loads, not only because of the increased capacity triggered by the retrofitting, but also because of the increased reaction at the middle support of a continuous system. Furthermore, the forces in the bottom truss chords near the middle support of the continuous system are changing from tension to compression, which may require additional modifications.

The effect of converting several simple spans into a continuous truss system is even more significant, as the bending moment demands (and related chord axial forces) are smaller along the entire length of the transformed system compared to the existing simple spans.

Figure 5a.

Figure 5b.

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Cantilever truss systems are typically threespan bridges consisting of two side spans (anchor arms), two cantilevered portions extending toward the center, and an interconnecting “suspended” central portion.

This system was very popular at the end of the 19th and beginning of the 20th centuries, and allowed engineers to achieve the longest central spans for that time – up to 1800 feet (549 meters) (Quebec Bridge, 1917).

A rational modification for these old bridges is to add a cable-stayed type of reinforcement utilizing the middle bridge supports. The added cable-stayed system introduces pre-stressing upward forces at the tips of the cantilever arms (Figure 4) and relieves the existing system of a significant portion of the vertical loads. These upward forces, opposite to the forces in the

original schemes, reduce significantly the stresses in the existing system, providing additional load capacity. The “cable-stayed” reinforcing can be pre-stressed to about 75% to 85% of the dead load reactions at the main span cantilevers, therefore relieving the existing cantilevered structure of about 60% of the total loads.

The same basic concept can be used for strengthening existing arch bridges. The most simple and efficient option is to add suspension-type supporting systems below the deck (Figure 5a). This is a possible solution regardless of whether the arches are above or below the traffic deck, as long as there is sufficient clearance for the additional structure. In this option, the suspension catenary system has vertical posts in compression, unlike regular suspension bridges with vertical cables in tension.

In cases where there is insufficient clearance, a cable-stay system developed both above and below the deck can be the most efficient solution (Figure 5b). This option requires adding pylons at the arch supports and, if necessary, reinforcing the piers below the deck at these locations.

General Considerations

When considering replacement of a deficient bridge structure, the owner or local transportation department should always consider possible alternative options for reinforcement. Strengthening an existing structure is a challenging task, and while many engineers would prefer to work on a “clean” new replacement structure, upgrading as described above may provide significant savings in costs, construction materials and time. Even more important

is the reduction of the “carbon footprint” by avoiding the demolition of the existing bridge and significantly reducing the number of new elements used in comparison with an entirely new structure.

As the number of deficient structures continues to grow, the best structural project from a sustainability standpoint is the one that uses the minimum material and total energy. Therefore, a retrofitted structure is by definition more sustainable (and environment-preserving) than an entirely new replacement structure.▪

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These methods have several advantages when compared to traditional reinforcement of existing bridges: • They reduce the strength demand on almost 100% of the bridge elements, and thus increase the available live load capacity.

• The added elements can be practically independent of the existing elements, allowing for much easier retrofitting details and construction sequence.

• Construction work may be done with minimal impact on existing bridge traffic.

If necessary, some members can be additionally strengthened by judiciously adding plates, channels or other shapes, connecting them with bolts and/or welds.

Moreover, compromised original rivets can be replaced with high-strength bolts.

When an enlargement of the bridge deck is necessary, the construction retrofitting is more complicated; however, even for these conditions, the upgrading is simpler and faster than using traditional methods.

Depending on the specific conditions in a bridge reinforcement project, the engineer may decide to combine modification of the superstructure and replacement of the deck with a lighter system.

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E

ngineering is not all WL2/8… or is it?

Unless they’ve been to a university with a good cooperative education program where they acquired some good practical work experience, many young engineers leave school with only an academic impression of structural engineering, based mainly on the core university technical curriculum. (To refresh yourself on what the suggested curriculum is today, visit www.STRUCTUREmag.org and click the Resource tab.) Over my almost 50 years of practice experience, I have learned quite the contrary – that structural engineering is a multifaceted gem. When I left school, I had no idea I would be designing seismic mounts and containment vessels to move priceless art objects around the world, and other such things. Have you ever thought, as you passed by objects like those shown in Figure 1 and Figure 2, about the structural engineering that went into supporting that artist’s concept? This article deals with yet another interesting commission – a cable supported tree.

Cable Supported Tree A client called and said, “I have an artist who needs to support some trees in an open space so that the public can view the trees and the roots as if they were walking through the ground beneath the objects. Can you do it for me?”

Before I agreed to get involved, I learned, through discussions with the artist, that the client thought the trees and roots could be supported on a cable system having a 12-inch on-center grid, and that the artist would of course like the small diameter cables to be nearly “invisible”. If you’re like me, you’d probably think, sounds like an interesting project, let’s do it. A major drawback – I was on vacation with only my cell phone calculator and my client says “time is of the essence” (always is) and that he needs to make a presentation tomorrow.

Outside the BOx highlighting the out-of-theordinary within the realm of structural engineering

Figure 1.

Where does one start on such an unusual commission?

What does the root mass look like? How many trees are there and what do they weigh? What does “nearly invisible” really mean? What do these cables look like and what do they connect to? How much cable sag is the artist willing to tolerate?

Fortunately, my client is experienced in dealing with unusual circumstances and can ferret out much of the detail, thus making the engineering task a more straightforward process. Nonetheless, I need to start somewhere and my client isn’t a structural engineer, so he doesn’t think like I do. He could, however, readily provide space constraints, site location, and through the artist, a general idea of the number of trees, approximate locations and root structure. continued on next page

Back of the Envelope Engineering A Structural Engineer’s Experience

By Craig E. Barnes, P.E., SECB

Craig E. Barnes is the Founding Principal of CBI Consulting Inc. Craig is also a member of the STRUCTURE Editorial Board. Mr. Barnes can be reached via email at cbarnes@ cbiconsultinginc.com.

Live Matter exhibition at Harvard’s Radcliffe Institute for Advanced Study. Concept and design by Rosetta S. Elkin.

Photos by Kevin Grady.

Figure 2.

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But if you think this is the time to put “pencil to paper”, which in today’s world means opening up a sophisticated three-dimensional computer program and embarking on a dizzying array of what-ifs (and I don’t knows)…. you’re wrong! This is a perfect time to fall back on “back of the envelope” engineering.

Simple calculations and some approximate cable sizing will quickly tell me whether this artist’s idea is as unusual as it first seemed, or actually something that I can pull off in a reasonable way in the time permitted. If I had been at the office, I might have gone half-way and dusted off an old textbook (or class notes on cables if I had them, which I don’t) as a refresher. But remember I am on vacation, so I can’t do that anyway. I recall what cable theory is WL2/8, so I should be good to go with some approximate figures and cable loads. I figure the young engineers back at the office can refine the idea later and “prove” I am right. After a quick Google search on cables and what they might look like (wire rope, aircraft cable, piano wire, non-metallic systems, etc.) and who makes them, I am ready for a chat with my client not only about cable options but also membrane versus mesh versus grid supports.

In the short time span of two hours, my client, now armed with knowledge that he didn’t have when he first called me, is in a far better position to discuss with the artist how art and structure come together. Their discussion came back to me for refinement of the system and hand sketches (Figure 3) to be used by my client and the artist.

Vacation is over and, with the continued desire on the part of the artist and my client (the institution) to move forward, a field visit was established to view how the cable loads would be transferred to the existing surrounding structure. I knew in advance that the presentation space had been used previously for art shows and that the walls were drywall

Figure 3.

on plywood supported by cold formed studs.

This of course was insufficient data to be able to determine what load support capacity the system possessed, but provided me with comfort that the facility had been built with the anticipation of supporting some level of wall loads. Once on site, I was advised that the structure on three sides of the room was available for cable connection. However, on the fourth side, the room needed to be narrowed by the erection of a fourth wall containing a 10-foot opening on one end to create a controlled entry for viewers. Fortunately, the design cable system could be connected to the walls approximately 18 inches from the top so that what turned out to be rather flimsy 20 gauge metal studs could be loaded more in shear than bending, and my design would still

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work. In addition, the artist was amenable to taking the concentrated loads directly to the structure above. The design of the new wall and the entry header were easy to accomplish, since it was up to me.

When all was said and done, the cable system and the “back of the envelope” design process was, in our opinion, as interesting as the art work itself.▪

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Structural

Modeling & > > > > evaluation Vertical Lift Highway Bridge in Canada

By Akash Rao, P.E., LEED AP BD+C, Matt Reichenbach, E.I.T. and David Marcic, S.E., P.E. In this article, detailed description of the modeling and evaluation of a steel vertical lift highway bridge located in Ontario, Canada is presented. The bridge spans a major transportation waterway for ships and barges carrying 40 million tons of valuable cargo annually, and continues to be a major factor in the growth of cities like Toronto, Cleveland, Detroit, and Chicago. Hence, it is considered a Lifeline Bridge for all evaluation purposes.

A

typical configuration of a vertical lift bridge is shown in Figure 1. The towers at each end of the lift span contain sheaves over which wire ropes pass. These ropes connect to the corners of the lift span and to the concrete counterweights. The lift span accommodates the motors that drive the operating drums (all housed within an operator house and machinery room), and set a series of uphaul and downhaul ropes into motion.

The ropes, sheaves, and counterweights work similar to an elevator.

The counterweights balance the lift span and minimize the power needed to operate the bridge. Hence, the dead load is nearly balanced in the seated condition, and only live load is resisted by the lift span bearings. During the lift operation, the lift span and counterweights move along guides attached to the front column of the tower. The guides restrain global movements during operation but permit thermal movements.

The two-lane, two-way bridge under consideration was constructed in 1930 and is composed of two 70-foot tower spans and a 208-foot lift span. The floor system consists of a concrete deck on crossbeams, supported by stringers and floorbeams. The deck is composite with the crossbeams and floorbeams.

The intent of the owner was to conduct a bridge evaluation for dead, live, wind, and seismic loads as per Canadian Bridge Code CSA S6-14 (henceforth called “the Code”). CSiBridge software was used to develop two models, namely a three-dimensional Wind and Seismic Model (M1) and a simplified Live Load Model (M2), to accurately capture the inherent difference in response of a bridge under lateral and vertical loads. Section properties and member capacities for all members were computed based on as-built drawings and the code.

Figure 1. Configuration of a vertical lift bridge.

Wind and Seismic Model (M1) The M1 model evaluates the bridge structure under wind and seismic forces, and provides demand-to-capacity ratios for two operating conditions: lift span-seated and lift span-raised. The structure was evaluated under ultimate limit states in accordance with several lateral STRUCTURE magazine

Figure 2. Three-dimensional M1 model of the bridge.

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October 2015

Figure 3. 5% damped response spectrum used for seismic evaluation.

load combinations derived from the Code. The model consists of main truss and tower members, laterals and braces, ropes, sheaves, counterweights, guides, and bearings as shown in Figure 2. These elements were modeled by utilizing a series of frame elements, links, and shells. The model treats the structure as linear and elastic due to the adopted wind and seismic analyses.

Figure 4. Dominant transverse vibration mode for the span-seated condition (T = 3.80 seconds).

Guides of Lift Span and Counterweights Guides provide a load path for wind and seismic loads, from the lift span and counterweights to the front tower columns. That load path is dependent on the directionality of the applied load and the restraint provided by the guide. The guide is only effective at transferring forces when the lateral load causes the lift span and/or the counterweights to displace and the guide(s) to contact the front column. The displacements allowed by these guides are in the range of ½-inch to 1 inch.

Additionally, some guides are able to restrain movement in the longitudinal and transverse directions, whereas others only restrain transverse movement. Accurately modeling these effects is vital to representing the response of the structure to wind and seismic loads from all directions.

The guides were modeled as linear spring elements. A distinction was made between guides in contact with the front column and guides not in contact by adjusting the stiffness parameters of the springs.

The stiffness of the spring elements was iterated in order to reasonably match the expected movement at these locations to simulate its expected nonlinear behavior.

Wind Analysis The wind loads were developed based on a return period of 50 years and the applicable coefficients prescribed in the Code. The loads were applied as equivalent horizontal and vertical static loads, uniformly distributed across the lift span truss, tower members, counterweights, sheaves, and operator house.

For regular bridge structures with a short vertical profile, the applied wind pressure is assumed constant and a uniform exposure coefficient is assigned. Because the lift bridge under evaluation is tall, the structure was divided into three different wind exposure zones.

Although the bridge was originally designed for 30 pounds-persquare-foot wind load, current Code requirements result in a higher wind load. Hence, after evaluation, it was found that several of the tower members and portal frames were inadequate in combined axial and flexure. It was recommended that these members be strengthened or replaced during the next major rehabilitation.

STRUCTURE magazine

Figure 5. Dominant longitudinal vibration mode for the span-seated condition (T = 1.62 seconds).

Seismic Analysis An Elastic Dynamic Analysis (EDA), and more specifically a multimode elastic response spectrum analysis, was selected as the appropriate means for evaluation. This is a force-based approach, wherein the forces derived from the elastic response spectrum corresponding to a 2475year return period were compared directly to the elastic capacities of the truss members. Location-specific, 5%-damped spectral acceleration values were adopted from the Canadian Building Code and Site Class A (Hard rock) parameters were used to develop the response spectrum (as shown in Figure 3) for this lifeline bridge. Primary bridge modes were computed using Eigenvector analysis assuming 5% damping, and combined using the CQC method to determine response forces.

The structure is relatively flexible, as a significant portion of the seismic mass is suspended from the tower by ropes; the dominant vibration periods range from 1 second to 5 seconds (as shown in Figures 4 and 5). The suspended masses also tend to dampen the seismic response of the tower, and the bridge is located in a generally low seismic region. Consequently, the structure was found to have no deficiencies for seismic loading. continued on next page

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Live Load Model (M2)

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The M1 model was simplified by deleting sway frames, laterals, and tower frame members above the first level of the top chord, since initial runs of the live load on the M1 model showed negligible forces in these members. The resulting M2 model is used for live load rating of the truss members and floor system.

Stringers were added as simply-supported members between the floorbeams. The axial constraint was released so that no axial forces were induced in the stringers as a by-product of analysis. Since the shear connection of the floorbeam to the truss bottom chord results in significant fixity, no end releases were applied to the floorbeams in the model. The span supports were modeled as hinged at one end

and rollers on the other end. The final live load model framing is shown in Figure 6.

Modeling the Dead Loads The dead loads from the asphalt wearing surface, deck, crossbeams, and stringers were applied as individual line load cases on the stringers, since the Code applies separate load factors to these dead load components. The self-weight of the floorbeam was modeled, and forces in the floorbeam due to the above cases were derived from the stringer.

The original as-built plans provided dead load concentrations on the panel points of the lift span and the towers. These were input as point loads after adjusting for the already applied floor system loads.

The dead load forces in the truss members were then determined for two cases. a) Counterweight active: To simulate this case, an upward force equal to one-quarter of the span weight (minus 1 kip) is applied to the top chord corners of the lift span, to represent the upward pull on the span transferred from the counterweight by the ropes. b) Counterweight under repair: This is a special service condition of the vertical lift bridge, where the counterweights/ropes are under repair and are removed or independently supported. Hence, the rope is inactive and the bridge essentially behaves like a fixed bridge spanning between the supports. Maximum dead load reactions are experienced at the lift span supports.

An envelope of these two cases was taken as the dead load force in the lift span truss members. This methodology of finding critical dead load forces is unique to vertical lift bridges.

Modeling the Live Loads The aspects of live loads and evaluation levels described below are unique to the Code. The Code presents the Ontariospecific truck load of 625 kilonewtons (140.5 kips) and the Ontario-specific lane load as shown in Figures 7 and 8.

The uniformly distributed load of 9 kilonewton per meter (0.62 kips per foot) corresponding to a Class A highway was determined based on the average daily traffic and average daily truck traffic data provided by the owner. Evaluation Level 1 was chosen for the first run, which represents vehicle trains in normal traffic. If the bridge members did not rate for Level 1, then Level 2 or Level 3 live loads were applied, which represent 4 axles and 3 axles of the truck, respectively.

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October 2015

Figure 6. Three-dimensional M2 model showing trusses, stringers and floorbeams.

Load Factors for Dead and Live Loads Another unique aspect of the Canadian Code is the selection of load factors based on target reliability indices. The target reliability index in turn is based on three factors. a) System behavior categories S1, S2, and S3: Correlates the member failure to total collapse, partial failure, and local failure of the bridge, respectively. S1 for the truss members and S3 for the floor system were chosen. b) Element behavior categories E1, E2, and E3: Correlates the loss of capacity of a member to sudden failure, sudden failure with post-failure capacity, and gradual failure of the member, respectively. E3 was chosen for the truss members and the floor system. c) Inspection level categories I1, I2, and I3: Correlates the inspection effort for a member to the involvement of the evaluator during the inspection. I2 was chosen since all members were inspected to the satisfaction of the evaluator.

As per the Code tables, the above three factors resulted in a reliability index of 3.25 for the truss members and 2.75 for the floor system members, and were then used to find the load factors for input into the rating equation.

Figure 7. Canadian Bridge Code truck load model.

Live Load Rating The Live Load Rating factor for a member, F, is given by the equation: F = Resistance – Effect due to ∑(Dead loads × Dead load factors) Effect due to governing (Live Load × Live Load factor) The effect of dead loads and live loads were obtained from the bridge model output. Rating factor F was then calculated. All members of the trusses and the floor system were found to be adequate under live loads.

Figure 8. Canadian Bridge Code lane load model.

Conclusion This article provides some of the unique aspects of modeling and analyzing vertical lift bridges, and briefly highlights the specific aspects of the Canadian Bridge Code. Any structural engineer tasked with modeling lift bridges should pay special attention to dead loads on the lift span; these loads play a critical role in the balance of the bridge, live load ratings, and seismic analyses. The operating conditions of the lift span affect the dead load forces and behavior under lateral loads. Lastly, the inherent flexibility of the towers and the directional-dependent load path through the front column guides also influence the behavior under lateral loads.▪ STRUCTURE magazine

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Akash Rao, P.E., LEED AP BD+C, is a Structural Engineer and can be reached at arao@hardesty-hanover.com.

Matt Reichenbach E.I.T., is a Structural Engineer and can be reached at mreichenbach@hardesty-hanover.com.

David Marcic, S.E., P.E., is a Principal Associate and can be reached at dmarcic@hardesty-hanover.com.

All three authors work in the Annapolis, MD office of Hardesty & Hanover.

October 2015

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Figure 1. The abandoned H&BT trestle bridge was a bottleneck in the township’s plans to extend its network of trails.

Civil War Era TrEsTlE BridgE Gets AdvAnced 21st century MAkeover

By Andy Loff

T

he U.S. earned a D+ in infrastructure, with bridges scoring a C+ and public parks and recreation a C-. The American Society of Civil Engineers (ASCE) released these grades in 2013 as part of the most recent Report Card for America’s Infrastructure. The grades are based on criteria including capacity, conditions, funding, operational maintenance and future needs.

Parks and recreation contributes approximately $646 billion to the nation’s economy, but state and local budgets supporting these activities continue to decline despite the growing popularity of programs like Rails-to-Trails Conservancy. The organization’s goal is that 90 percent of Americans will live within three miles of a trail system by 2020. The push to expand access to the outdoors means agencies have to tackle increasingly difficult bridge projects while finding ways to trim overall expenses. This balancing act has many organizations taking a harder look at alternatives like Fiber Reinforced Polymer (FRP) composite bridge decks, which tend to last longer than steel or timber and often require little or no maintenance.

Creating successful multi-use pathways can, among other things, include restoration of abandoned structures like the Huntingdon and Broad Top Rail Trail (H&BT) trestle bridge located in Rockhill, Pennsylvania. Built in the 1860s, the bridge spans the Raystown Branch of the Juniata River, the largest and longest tributary for the waterway in south-central Pennsylvania (Figure 1).

The Broad Top Township’s Board of Supervisors considered several factors to develop an action plan, where future needs called for a bridge deck product with minimal maintenance costs. The new deck had to meet Rails-to-Trails network application requirements for an aesthetically pleasing appearance that could blend with the area’s natural surroundings. Easy installation was equally important, due to the work site’s remote location.

The board evaluated timber and concrete deck options but took notice of a fiberglass alternative due to a nearby trail project. Research confirmed the composites’ corrosion resistance and lighter weight, making a fiberglass deck the frontrunner for the job. An open bidding process identified multiple suppliers.

STRUCTURE magazine

As a first step, the selected manufacturer (Composite Advantage) identified trouble spots. The deck-to-girder connections proved problematic because, like many railroad bridges, H&BT was constructed with rivet heads on top of its girder flanges. Anchoring the bridge deck to the steel truss was also infeasible due to the amount of corrosion on the aging metal. Rivets and splice plates on the steel structure’s surface created varying elevations. A curve at one end of the bridge added an additional design challenge.

The selected product’s flexibility allowed it to be custom molded to accommodate this bridge structure profile. This made field survey measurements an important component of the design process. In this case, a 2-inch spacer or “step” was integrally molded into the deck’s underside to ensure a flush seating surface that would clear the rivet heads (Figure 2). Because the original superstructure design called for two girders spaced at 10 feet, a 5-inch bridge deck depth was required to span this distance and meet the criteria of 85 psf uniform live load with a maximum deflection of L/500; H-10

Figure 2. The integrally molded step secures the deck panel to the steel superstructure without the need for secondary shim pieces.

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October 2015

vehicle live load; wind uplift load of 30 psf; railing load of 45 lb. per ft.; and deck self-weight of 7 psf. With FRP, the structural properties could be directionally tailored where the H&BT bridge deck was designed for greater stiffness in the span direction versus the longitudinal direction.

When a project requires additional features like supporting the railing system, these detailing requirements should be coordinated into the deck design and fabrication upfront. Loads generated by pedestrians leaning on the rail were designed to be transmitted through the deck to the bridge superstructure (Figure 3). This approach simplified field work and on-site construction for a significant time savings.

Also, incorporating the railing system into the FRP deck eliminated the need to weld or bolt to the existing, historic steel. Pre-fit on the manufacturer’s fabrication floor, the railing was field-installed with nothing more than setting and mounting posts.

Steel bridges are designed to American Association of State Highway and Transportation Officials (AASHTO) standards. There are no applicable design codes for FRP. The material falls instead under an AASHTO special provisions clause, which details allowable stress design and safety requirements. Design calculations and testing correlations were used to determine needed performance properties for each individual project.

In addition to supporting the design gravity loads and limiting deflections, the project called for an environmental durability factor of 0.90. Strains in the panels under dead load only could not exceed 10 percent, and combined dead and live loads could not exceed 20 percent of the ultimate strength of the FRP material. The H&BT deck was designed for a minimum fatigue life of 2 million load cycles.

With conventional structures using traditional construction materials, the engineer of record is typically responsible for the structural design and detailing. Due to the unique nature and performance of composites, the FRP manufacturer generally performs these tasks

Figure 3. FRP is robust enough to handle rail post attachment load.

Figure 4. CA performs bending test on FRP deck panel to design loads and safety factors.

Figure 5. The lighter weight, high-strength FRP panels allow construction workers to maneuver FRP panels to the work site with just a telehandler.

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October 2015

in-house. In the case of the HB&T bridge deck, a full-scale test was performed to validate the FRP design before submitting findings to the design consultant for final approval, and then to the Pennsylvania Department of Transportation (Figure 4, page 41).

The structure was located 400 yards from the trail head. FRP panels and the railing were pre-assembled at the trail head, then trailered through a small canyon to the bridge site. Studs were welded to the superstructure to fasten deck panels to the top of the structure. The lighter weight FRP panels, each 23 feet long by 13 feet wide, were easily moved and placed with a small telehandler (Figures 5, page 41, and 6 ).

Total bridge length was 350 feet.

The H&BT is part of a new 4-mile section of public trail that connects Hopewell Borough and Tatesville, and intersects with Riddlesburg and Cooper public parks. The bridge opened in November 2014 (Figure 7). Although the Broad Top board estimated three to four weeks for project completion, with careful preparation and upfront fabrication the use of FRP pared installation time down to just four days.▪ Andrew Loff is Vice President of Composite Advantage and is responsible for all technical operations. He was previously Manager of Molding Technology at the National Composite Center before co-founding Composite Advantage in 2006. Andrew can be reached at aloff@compositeadvantage.com.

Figure 6. FiberSPAN panels fastened to the top of the bridge’s steel superstructure.

Figure 7. Blending with its natural surroundings, the FRP bridge deck gives pedestrians safe passage across the river and access to new trail sections.

STRUCTURE magazine

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October 2015

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THE

RIO ABAJO FOOTBRIDGE

Reducing Rural Isolation in Nicaragua

By Hendrik Westerink, E.I.T.

F

ermin Garcia Espinoza and the 3,000 person community of Rio Abajo in northern Nicaragua are celebrating a safe way to cross the Rio Pueblo Nuevo (River) for the first time in more than 16 years, thanks to a new pedestrian suspension bridge. Fermin, a Rio Abajo resident and father of five children, was one of several committed volunteers who worked with a team of North American bridge professionals from Buckland & Taylor | COWI and Kiewit Bridge & Marine to build the footbridge. Construction of the bridge, which is owned by the Municipality of Pueblo Nuevo and District of Rio Abajo, was coordinated by Bridges to Prosperity, a non-profit organization dedicated to reducing rural isolation through the construction of footbridges for communities in need. Rio Abajo was one of these communities in need, its access to major markets, secondary schools and healthcare services in the nearby town of Pueblo Nuevo impeded by the Rio Pueblo Nuevo during the rainy season between June and November. An existing bridge across the river was destroyed

by Hurricane Mitch in 1998 and, since then, Rio Abajo residents had been forced to swim or wade across the river to reach Pueblo Nuevo.

The only alternative was to complete a lengthy 5 mile walk to the town of Condega. Fermin remembers the daily problems associated with the lack of year-round access. The river crossing was risky during the peak of rainy season and items that were left on one side of the river so they wouldn’t get wet or ruined, such as shirts and papers, were often stolen.

The new pedestrian bridge now provides Fermin and the rest of Rio Abajo with safe, year-round access across the river.

The new Rio Abajo Footbridge spans 265 feet over the Rio Pueblo Nuevo (River). Courtesy of Buckland & Taylor | COWI.

For more information, please visit www.worldbank.org/ transport/transportresults/headline/rural-access.html or www.bridgestoprosperity.org/wherewework/central-america/ nicaragua/rio-abajo

A Standardized, Efficient Bridge The Rio Abajo footbridge closely matches Bridges to Prosperity’s standard suspension bridge design. Pedestrians, motorcyclists and livestock access the four foot wide bridge via masonry and castin-place concrete ramps at each abutment, and cross the river on a cable-supported deck composed of planks cut from tempisque, a local Nicaraguan hardwood tree. The planks are bolted to transverse double angle steel floor beams that transfer loads from the deck surface to the main cables through vertical steel bar hangers. The geometry of the walking surface is controlled by the length of the hangers, which are cut and bent on site to specific lengths to achieve STRUCTURE magazine

Fermin Garcia Espinoza paints steel bar hangers for the Rio Abajo footbridge.

Courtesy of Ania Giffin.

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October 2015

Build team members install deck planks made of tempisque, a local Nicaraguan hardwood. Courtesy of Abigail Conover.

Community volunteers and bridge professionals team up to move the steel pipe towers. Courtesy of Abigail Conover.

the desired bridge profile. Fencing attached to the steel bar hangers ensures a safe crossing for bridge users. The main suspension cables, each consisting of three sheathed post-tensioning strands, pass over 30-foot-tall steel pipe towers to anchor into an at-grade reinforced concrete transition block. The towers are supported on reinforced concrete pedestals, and the cable loads from the transition block are transferred to a buried deadman anchor block.

Once the bridge foundations had been constructed by Rio Abajo community members with assistance from Bridges to Prosperity staff, a twelve person team of bridge engineers and constructors from Buckland & Taylor | COWI and Kiewit Bridge & Marine travelled to Rio Abajo in March 2015 to help the community construct the bridge superstructure in just eight construction days. The bridge was constructed during Rio Abajo’s dry season, which allowed the team to use the dry riverbed for site access. Upon arrival on site, the team of local volunteers and bridge professionals constructed temporary scaffolding towers that were used to erect and temporarily support the bridge’s steel pipe towers. Following tower erection, the main suspension cables were set to their correct elevation by measuring the distance from the top of the tower to the lowest point of the cable sag at mid-span and anchoring the cables to the anchor block. Preassembled steel bar hangers and transverse floor beams were installed, starting at each tower and working towards the middle using a pull cable. The hangers were attached to the pull cable at three foot intervals and slowly released down the slope of the main cable until the first hanger reached mid-span and the pull cable between each hanger was taut. This method proved efficient for installing the 81 hanger and floor beam assemblies to their required spacing. The bridge was completed by installing deck planks, fencing, and placing concrete for a transition ramp between the superstructure and approach ramps.

of more than 160 footbridges since its inception, typically partnering with student volunteers, corporate partners, and local governments.

The footbridges act as catalysts for poverty-reduction by providing isolated communities with year-round access to healthcare, educational opportunities and major markets. To maximize the long-term sustainability of its bridge building programs, Bridges to Prosperity trains local masons, construction managers, and engineers about bridge construction, maintenance and rehabilitation.

Kiewit Bridge & Marine joined Bridges to Prosperity’s Corporate Partnership program in 2013 and Buckland & Taylor, a COWI

The Rio Abajo footbridge was constructed as part of Bridges to Prosperity’s Corporate Partnership program, which enables engineering and construction companies to help mitigate the worldwide problem of rural isolation. Bridges to Prosperity is a Denver-based non-profit organization founded in 2001 to address the global need for safe and reliable footbridge access across otherwise-impassible rivers. The World Bank, which tracks access to transport through the Rural Access Index (RAI), estimates that more than one billion of the world’s rural population do not have adequate access to transportation services. Bridges to Prosperity has been involved in the construction STRUCTURE magazine

45

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October 2015

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Teaming Up to Reduce Rural Isolation

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North America company headquartered in North Vancouver, Canada joined in 2014.

The two companies teamed up on the Rio Abajo bridge project by each sponsoring a portion of the required construction materials and sending staff to the bridge site to help the local community construct the bridge superstructure. The partnership of a bridge engineer and constructor proved valuable to the project’s success: Buckland & Taylor’s experience with the design and erection engineering of cable-supported structures was beneficial in providing a design review of the structure and its detailing before it was built, and helping with quality management during construction.

Kiewit Bridge & Marine’s extensive construction experience allowed the Rio Abajo staff to successfully manage the site safety, work planning and resource allocation for the duration of the project.

Schoolchildren cross the Rio Abajo Footbridge after the bridge inauguration. Courtesy of Buckland &

Rio Abajo’s Bridge

Taylor | COWI.

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The Rio Abajo bridge superstructure could not have been completed in just eight construction days without the enthusiasm and dedication of the volunteers from Rio Abajo.

More than 150 families in the Rio Abajo community donated labor or supplies to the bridge project; one volunteer donated 30 days of his time to work on the bridge foundations and superstructure. The volunteers donned personal protective equipment, many for the first

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Bridge Inauguration The inauguration of the Rio Abajo footbridge, after its successful completion, was a joyful occasion for the 3,000 residents of Rio Abajo as they celebrated a year-round crossing over the Rio Pueblo Nuevo for the first time in more than 16 years. Members of the build team, local politicians, and the Rio Abajo community gathered at the bridge site for speeches, artistic performances, a ribboncutting, and even a piñata hung from the bridge. Don Bergman, Buckland & Taylor’s Senior Project Director and Vice President of Major Projects, travelled to Rio Abajo as part of the build team and remarked at the bridge inauguration ceremony “I’ve worked on many larger bridges around the world, but none have provided the sense of joy and satisfaction that the Rio Abajo bridge has.” That sense of joy and satisfaction is shared by the Rio Abajo community, including the Garcia family; Fermin and his family can cross the bridge to reach markets, schools and hospitals instead of having to swim or wade across the Rio Pueblo Nuevo. Fermin’s 17-year-old son Joel hopes to become a civil engineer, perhaps to design and build the next generation of footbridges for Nicaragua’s rural population.▪

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time in their lives, to safely work alongside the sponsoring companies’ staff. Many of the children from Rio Abajo were fascinated by the construction process and, after school was done for the day, contributed to the bridge by painting the steel bar hangers and assembling the crossbeams and hangers. The community was able to successfully work alongside the bridge professionals, despite the linguistic and cultural differences between the two groups. Much of the communication between the Spanish-speaking community volunteers and English-speaking bridge professionals was through a few bilingual members of the build team and several local Peace Corps volunteers. Rudimentary Spanish and hand gestures were also successfully used to convey bridge construction concepts. The high level of local involvement during bridge construction gave the Rio Abajo community a strong sense of ownership of the bridge, and means that the community will be more inclined to maintain and repair the bridge.

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October 2015

Hendrik Westerink, E.I.T., is a Junior Bridge Designer at Buckland & Taylor | COWI in North Vancouver, Canada.

He may be reached at hewk@b-t.com.

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T

he Pittsburgh, Carnegie & Western Railroad, commonly called the Wabash line, had tracks in Illinois, Indiana and Ohio generally running southwesterly from Toledo to St. Louis, but there was interest to connect to Pittsburg as a step in becoming a transcontinental railroad. The plan was to connect with the tracks of the Wheeling and Lake Erie Railroad at Toledo and then run southeasterly towards Wheeling on the Ohio River, where a short branch line would be built from Pittsburg Junction to Mingo on the Ohio River. From that point, there were two options: The first was to cross the Ohio River on a major bridge and then cross the mountainous regions of the panhandle of W. Virginia and south western Pennsylvania and enter into Pittsburg with a major bridge across the Monongahela River, the southern boundary of Pittsburgh. This line would require many bridges and tunnels, but it was a direct east-west line to the City. The second route would take the line along the west bank of the Ohio River northerly to a large bend in the river, and then to the south and east to Pittsburgh with a major bridge over the Allegheny River to their terminal. This line was cheaper to build but longer in length.

The Bridge Company, called the WabashPittsburgh Terminal Company, chose the southern route which meant trains had to cross the Monongahela and Ohio Rivers with two large

Anchor pier detail.

Historic structures significant structures of the past Completed bridge, terminal on left side of post card.

bridges, the larger one across the Monongahela.

The president of the line, Joseph Ramsey, in association with George Gould (son of Jay Gould) had great plans for the line and felt the traffic generated would more than pay for the expensive construction, then estimated at $20,000,000.

Ramsey/Gould needed permission to enter Pittsburgh by way of a bridge over the Monongahela River and to build a terminal in the heart of downtown Pittsburgh. The city was considered Pennsylvania Railroad country at the time. Even though “… besieged by rivals, arraigned by the city, delayed by the politics of municipal councils, hindered by unexpected natural obstacles and by accidents, strikes, and even peremptorily stopped by the Supreme Court of Pennsylvania, they gained an entrance into Pittsburgh.” The City Council, at the time unhappy with the Pennsylvania Railroad, approved an ordinance on February 4, 1904.

The War Department approved a bridge height of seventy feet over the Monongahela River. The railroads chief engineer, J. W. Paterson, picked the firm of Boller & Hodge to design both bridges.

Alfred Pancoast [A. P.] Boller and Henry Wilson Hodge, both Rensselaer Polytechnic Institute graduates, had excellent experience in designing bridges, but had not designed any major cantilever bridges at the time.

It was necessary to build a bridge which did not obstruct the channel during construction, and had spans long enough and high enough off the river surface so as to permit continued river traffic. The answer to this set of constraints in the early 1900s was a cantilever bridge.

The foundations for the main piers, placed by means of pneumatic caissons, were located at the edge of the low water line and were set on solid slate rock about 42 feet below low water level. The anchorages were also set on solid rock at about the same elevation, and constructed in a manner similar to other cantilevers with steel eye-bars embedded in concrete. Boller & Hodge, however, had the lower part of their anchor rods cast rigidly in the concrete of the anchorage and the upper part contained in a well which permitted them to pivot and permit horizontal movement

STRUCTURE magazine

The Monongahela River (Wabash) Cantilever

49

By Frank Griggs, Jr., Dist. M. ASCE, D. Eng., P.E., P.L.S.

Dr. Griggs specializes in the restoration of historic bridges, having restored many 19 th Century cast and wrought iron bridges. He was formerly Director of Historic Bridge Programs for Clough, Harbour & Associates LLP in Albany, NY, and is now an independent Consulting Engineer.

Dr. Griggs can be reached at fgriggsjr@verizon.net.

of the end of the anchor span. Usually this movement was permitted at the top of the anchorage pier by having an exposed link from the pier at the end of the anchor arm.

The bridge was designed to carry two tracks, and at the time of its completion would be the longest cantilever in the North with a span of 812 feet. At the time of its design, the Blackwell’s Island (Queensboro) Bridge over the East River in New York was being re-designed and the Quebec Bridge over the St. Lawrence River was still under design, so George Morison’s Memphis Bridge across the Mississippi River, with its 790-foot span, was the longest cantilever span railroad bridge.

The superstructure was unprecedented in terms of its size and complexity. Boller and Hodge wrote, given the site constraints, …the anchor and lever arms of the cantilever are unequal, and are so proportioned that under dead load only, the reaction at the anchor piers is reduced nearly to zero. In the suspended span and at the ends of the cantilevers arms, the panels are uniformly 30 feet long, but where the truss is deeper, near the shore pier, the panels are increased to 40 feet, so that the main diagonals cross the two panels throughout at nearly uniform inclination of about 45 degrees and the difference in panel lengths is not noticeable. As the stresses in the main vertical posts over the shore piers are very large, it was deemed advisable to make each post in two parallel halves, which were separated 12 feet on centers and connected by horizontal longitudinal braces without diagonals, thus making a sort of narrow tower which is more easily constructed, and permitted a reduction of stress in the pins, more convenient connections for the assembled members, and a better distribution of the loads on the masonry.

span was 360 feet. The main diagonals crossed two panels, with additional short diagonals as necessary. The top chord of the anchor and cantilever arm was made of steel eye-bars that were the largest ever used in a bridge in the United States up to that time and were made out of plates 14- by 2-inches in section. The American Bridge Company, which received the contract to supply and erect the steel, built a new plant at Ambridge, Pennsylvania especially to form the eye-bars. The trusses were spaced 32 feet apart to accommodate the two tracks.

With the span lengths fixed, the loads determined and the truss geometry fixed, the

Wabash Bridge anchor span and tower under construction.

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The anchor span lengths were 346 feet, the cantilever arms were 220 feet and the suspended

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structure was statically determinate. Boller & Hodge used graphical analysis to determine the member forces for purposes of design.

Instead of using actual loads at each panel point, they developed, possibly for the first time, the use of unit loads at each panel point.

The technique is simple, straight forward, and for anyone who has actually prepared a “strain (load) sheet” for other than a simple truss knows that it was a time-saver.

The anchor spans were erected on false work and the cantilever arms and suspended span by cantilever methods. Boller and Hodge, working with American Bridge, laid out a detailed plan

Two travelers erecting suspended span.

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Truss pattern.

feet and held the title of the longest railroad bridge in the country and third longest in the world. It retained that status until the

Queensboro Bridge across the East River opened in 1909. The line was abandoned and the bridge removed in 1948.▪

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of erection, “…the operations were performed in accordance with eighteen diagrams numbered and lettered so as to show the sequence of all steps and having notes directing the blocking, wedging and any special provisions to be made at special joints.” They used two, specially designed, steel travelers so that work progressed simultaneously from both sides of the bridge. The travelers ran inside the main trusses and had a reach of a panel and a half. Most of the steel was delivered to the bridge site by barge from a staging yard located two miles upstream. The result of this planning was a bridge that went together like, as they said at the Kentucky River Bridge, “a Springfield musket.”

The erection went smoothly except for a tragic accident in which a bottom chord buckled under the weight of the traveler, cantilever arm and half of the suspended span. Ten men were killed when the suspended span was nearing completion on October 19, 1903. This accident occurred “… while the traveler was working well within its capacity, the lower chord of the cantilever truss collapsed and the over hang revolved downwards against the bridge trusses. After careful investigation the only reason to which failure can be ascribed is that the bottom chord of the overhang in the panel next to the traveler tower had become injured a short time before by a blow, probably from the 2-ton steel ram used in driving pins.” This blow apparently buckled some of the lattice bars and forced the chord out of position. When it was subject to live load the stress was greatly increased by the eccentricity and the member failed under a static load, well within its normal capacity and much less than was safely sustained before and afterwards by it and the duplicate traveler on the opposite side of the river. Other than this, the erection of the bridge by workmen of the American Bridge Company was rapid and safe.

The bridge weighed over 14,000,000 pounds, with the largest member lifted being the main vertical posts at the shore piers that were fabricated in two parts.

When built in 1904, the bridge exceeded the span of the Memphis Bridge by 22

Structural SpecificationS updates and discussions on structural specifications

M

uch has been written over many years concerning the development of the Load and Resistance Factor Design (LRFD) methodology used for structural design in the United States, yet many practicing engineers have had minimal exposure to how LRFD evolved from the Allowable Stress Design (ASD) methodology. This article is a simplified synopsis of that evolution and concludes with an assessment of its consequences.

ASD for Structures Prior to the advent of the means for structural analysis, element sizing was based on force and failure capacity – if some element failed, it was simply replaced with a larger element. One could consider this a strength methodology.

When measuring internal stresses became possible, engineers developed interest in understanding elastic methods of analysis with its accompanying determination of stresses.

The sizing of elements was then based on the comparison of predicted stresses in a structural element to an assumed failure stress of the element divided by a factor of safety. This philosophy was used for the three most common construction materials: concrete, metal and wood. The determination of failure stresses and appropriate factors of safety were unique to each material, but were all developed using similar approaches.

Due to the unique characteristics of wood, this article will focus on concrete and steel elements.

The sizing of elements was based on groupings of different loads – self-weight (i.e., dead load), live load (e.g., vehicular loading), wind load, etc. – and considered a probability of occurrence dependent on the grouping. In the case of a bridge structure, an element might have been sized for a group load of dead plus live load compared to 100% of the allowable stress. The element may also have been checked for stresses caused by a combination of dead plus wind load at 125% of the allowable stress. Elements of building structures were sized similarly in accordance with the loads acting thereon. Although this design philosophy had recognized deficiencies and weaknesses, it served well and was employed late into the twentieth century.

The Evolution of Structural Design Specifications in the United States

By Phillip C. Pierce, P.E., F.ASCE

Phillip C. Pierce is a Senior Principal Engineer with CHA Consulting, Inc. in Albany, New York. He can be reached at ppierce@CHAConsulting.com.

LRFD for Buildings A simplistic explanation of the cause for the next step in design methodology was the massive damage to buildings and bridges in Europe during both World Wars. Material shortages led

52 October 2015

European engineers to use material more efficiently in the design of replacement structures.

Because concrete was more readily available than steel, the first material specification to evolve was that for concrete, and the first portion of the construction industry to be affected was that for buildings, represented today by the American Concrete Institute (ACI) in the United States.

Research in the 1930s and 1940s led to the development of a new design methodology termed Ultimate Strength Design, named in part because concrete does not behave in a linear elastic manner. There were two major modifications from the ASD methodology. The first addressed the grouping of multiple types of loads, each having its own load duration, timing and potential for overload. Different factors were incorporated for each type of load. More predictable loads (e.g., dead load) have a lower load factor, while more variable loads (e.g., live, wind or snow) have a higher load factor. The second modification introduced a “resistance” or “capacity reduction” factor to downgrade the theoretical (nominal) capacity of an element to account for variation in material, analysis/design assumptions and equations, fabrication, and erection. While these modifications could have continued with a comparison of stresses, this new methodology changed to a comparison of strengths.

The 1963 edition of ACI’s Building Code Requirements for Reinforced Concrete prompted a rapid transition from Working Stress Design to Ultimate Strength Design. ACI introduced the “Strength Method” in its 1971 edition, while identifying Working Stress as an “alternate method.” While still identified as the “Strength Method,” this methodology is generally equivalent to today’s popular generic reference to LRFD. The steel buildings industry followed a similar but slightly delayed path. The American Institute of Steel Construction (AISC) first published its Standard Specification for Structural Steel for Buildings in 1923, based on the allowable stress methodology. While ultimate strength methodology had been considered for quite some time, AISC delayed its adoption until much later in the twentieth century. By the time it did so, its popular naming convention had changed. It was not until 1986 that AISC published its first Load and Resistance Factor Design Specification for Structural Steel Buildings. The 2005 edition unified the provisions presented in the 1989 Specification for Structural Steel Buildings: Allowable Stress Design and Plastic Design and the 1999 Load and Resistance Factor Design Specification for Structural Steel Buildings.

LRFD for Bridges The American Association of State Highway and Transportation Officials (AASHTO) governs

bridge design in the United States. While the industry generally uses research and specifications developed by ACI for concrete elements and AISC for steel elements, AASHTO traditionally has recognized that bridges should be designed somewhat differently than buildings. Bridges are subject to more rigorous and aggressive environmental effects, and their live loading is typically more variable than that of buildings. Accordingly, in general terms, bridge design necessitates larger factors of safety and higher capacity reduction factors.

The first national highway bridge specifications were published in 1931, based on ASD methodology. Unlike ACI and AISC building specifications, bridge specifications had an interim step in their transition from ASD to LRFD. AASHTO adopted “Load Factor Design” (LFD) in 1973 for concrete and steel elements. LFD incorporated the concept of variability of individual loads and based the sizing of elements on strength rather than stresses. AASHTO still accepted ASD methodology for bridge design in its Standard Specifications for Highway Bridges until the 17th edition in 2002. Through extensive effort in the late 1980s and early 1990s, AASHTO developed and adopted LRFD

for bridge design with its first publication in 1994. From 1994 until the final edition of the Standard Specifications in 2002, LRFD was promoted as the intended replacement methodology. AASHTO no longer updates provisions for design using ASD methodology. Similar to AISC’s LRFD provisions or ACI’s strength provisions, AASHTO’s LRFD includes both factored loads and resistance factor reductions in nominal capacity. The adoption of LRFD in bridge specifications required a significant effort to update provisions for live loading and relied on the extensive use of statistical methods.

Consequences of the ASD to LRFD Evolution These changes in design methodology were evolving as computers steadily became more available, allowing for more efficient and refined evaluations and design. Separate from the element capacity aspects of design specifications, the governing codes for loads have also become much more refined and comprehensive. Using modern specifications and codes, it is impractical to design structures (buildings or bridges) without software. The extreme refinement of loading

and proliferation of load combinations has expanded to an excessive degree, as if it is desirable and possible for results to reflect reality precisely.

In light of the ever-increasing pressure to design larger and more complex structures, one might be inclined to advocate for the most comprehensive and sophisticated specifications and codes. However, the vast majority of structures are still relatively small. Current complicated and involved specifications and codes make it more difficult for less experienced designers to judge whether the results make sense or where they may be suspect.

Given that the “ship has sailed” regarding the evolution of design specifications, it is incumbent on senior staff to mentor those with less experience, so that they consider carefully the assumptions made in computer modeling and use alternative methods to determine if element sizing is appropriate.

Now and in the future, it is also incumbent on those actively engaged in the design of structures to take an increased interest in proposed additional modifications and refinements of codes and specifications, in order to ensure that engineering remains a true profession and does not turn into a fully mechanized service industry.▪

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Education issuEs

core requirements and lifelong learning for structural engineers

SEFW Promotes STEM Activities in Washington State

By Thomas M. Corcoran, P.E., S.E. and Angela Gottula Twining

A

core element of the Structural Engineers Foundation of Washington (SEFW) mission is to support an educational outreach program that provides financial assistance to Washington State high schools with quality science, technology, engineering and math (STEM) programs.

Washington’s Edmonds School District was awarded financial assistance to support their STEM Program, and as a result SEFW promoted structural engineering in their STEM programs. Throughout his career, SEFW Director Tom Corcoran has visited several high school programs across Washington state, promoting structural engineering to students, and he recognized the quality of the Mountlake Terrace High School (MTHS) STEM program as well-deserving of SEFW resources.

A collaborative process was established between Corcoran and MTHS STEM teachers Craig DeVine and James Wilson, one in which they discussed two STEM projects:

A team at Mountlake Terrance High School stands on their wooden bridge structure and demonstrates its load capacity.

Structural engineer, Dan Sloat (right), gets into the action while mentoring several Mountlake Terrace High School students as they design a FIRST Robotics Competition entry.

a wood bridge building competition and the ever-popular FIRST Robotics program.

SEFW fully funded the proposed projects and provided structural mentors from Seattle Chapter’s SEAW Younger Member Forum (YMF) to help students with their projects during and after class.

Ninety students in the MTHS Principles of Engineering course were tasked in groups to design and construct 6-foot-long wood-framed truss bridges designed to carry a minimum of 150 pounds. In the past, students built shortspan balsa wood bridges in an effort to learn statics concepts, so the SEFW funding this year allowed for an improvement to the bridge building program and many opportunities for advanced learning.

At the project onset, the teachers envisioned young engineers in the industry working with the students to improve their bridge designs and to help them understand likely failure mechanisms. YMF engineers Gino Mazzotti and Brent Olson fulfilled that role, mentoring students during the design process.

Once the students were familiar with free body diagrams and the method of joints, principles currently taught in the class, Mazzotti and Olson presented information about additional advanced bridge topics to consider.

Through the presentation and discussions, the students were given insight into the engineering profession and challenges many young practicing engineers face today.

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Next, the students were tasked to design and build their bridges. Modeling in MD Solids helped students determine the member forces and whether members would fail in tension, compression, or buckling. Upon completion, the bridges were tested. To the surprise of many students, several bridges weighing less than four pounds supported a load of more than 150 pounds. Some bridges even held loads of 600 pounds before failing.

Bridges were not the only structures built at MTHS this year. Every year, the after-school FIRST Robotics Competition (FRC) team builds a robot, and SEFW realized it could also incorporate structural engineering practices into the classroom by assisting the robotics team.

YMF member Dan Sloat joined the MTHS group of 25 students in the FRC program, working four nights a week and Saturdays for six weeks to build a 120-pound robot that is fast, agile, and competitive in games.

The design of the frames and moving arms for robots like these require thoughtful engineering to be both light and strong enough to endure the demands of competition. Sloat’s experience and structural engineering abilities provided insight for the design of the robot structure and its moving arms.

Sloat’s role included teaching students how to use tools as well as discussing design and fabrication options with them. Part of the project was designing and constructing a lifting mechanism and supporting tower for

SEFW was created as a 501(c)(3) charitable organization to advance the profession of structural engineering through scholarship, research, education, and outreach. SEFW promotes a major lecture event every fall, funds the SEAW scholarship program, and is always seeking more opportunities to support and promote the structural engineering profession. SEFW has eight directors on the Board, plus one part-time administrator and a communications liaison.

If any organization is interested in starting a local SEF program, the Board of SEFW would be happy to share lessons learned and can be reached at admin@sefw.org.

SEFW Director Tom Corcoran helps three Mountlake Terrace High School students work on their wood bridge structure.

their robot, which needed to stack plastic crates and bins.

The MTHS FRC team competed through several regional events, and made it to the 2015 state championship event in Cheney, WA, held in April.

In addition to mentoring for the specific engineering design activities, the three YMF volunteer mentors, as well as Tom Corcoran of the SEFW Board, were able to share their

education and work experience as structural engineers, encouraging students to consider the structural engineering profession.

In total, 115 STEM-oriented high school students in the cutting-edge MTHS STEM program learned about structural engineering from passionate, local engineering professionals. SEFW truly lived up to its mission to promote structural engineering by partnering with the MTHS program.▪

Tom Corcoran presently leads the Structural Engineering group at Integrus Architecture, serving as Principal since 2006. He is a Past President of SEAW Seattle Chapter and serves on the SEFW Board of Directors. Tom may be contacted at tcorcoran@integrusarch.com.

Angela Gottula Twining has been involved with SEFW for several years, and is currently the SEFW Administrator. Angela may be contacted at admin@sefw.org.

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Nominal shear capacities of diaphragms and shear walls provided for reference assemblies.

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Applied Science International, LLC Phone: 919-645-4090 Email: sscoba@appliedscienceint.com Web: www.appliedscienceint.com Product: Extreme Loading for Structures Description: Study the 3D behavior of structures through both the continuum and discrete stages of loading.

Includes static and dynamic loads. Automated plastic hinges, buckling & post-buckling, crack propagation, membrane action & P-Delta effect, and separation of elements makes analysis more efficient and accurate.

Bentley Systems Phone: 800-236-8539 Email: structural@bentley.com Web: www.bentley.com Product: STAAD(X) Description: Comprehensive analysis and design of monopoles, self-supporting and guyed communication towers through physical modeling and parametric tools, ensuring minimum user interaction.

CADRE Analytic Phone: 425-392-4309 Email: jimhaynes@cadreanalytic.com Web: www.cadreanalytic.com Product: CADRE Pro Description: Solves for internal loads, stresses, displacements, and natural modes. Provides specialized tools for wind and hydrostatic loading and complete seismic analyses features including spectrum development. Includes special code checking for steel, aluminum, and wood structures.

CAST CONNEX Phone: 416-806-3521 Email: info@castconnex.com Web: www.castconnex.com Product: High Strength Connectors™ and Scorpion™ Yielding Connectors Description: High Strength Connectors simplify and improve brace member connections in seismicresistant concentrically braced frames. Scorpion Yielding Connectors are modular, replaceable, standardized hysteretic fuses that provide enhanced ductility and improved performance in the retrofit of seismically deficient structures, or for use in the Seismic Force Resisting System of new structures.

Decon USA Inc. Phone: 707-996-5954 Email: frank@deconusa.com Web: www.deconusa.com Product: JORDAHL® Anchor Channels Description: Hot rolled Anchor Channels are embedded in concrete and used to securely transfer high loads. Main application is for flexible connections of glazing panels to high-rise buildings. Anchor Channels with welded-on rebar or corner pieces are available.

Gripple Inc. Phone: 630-406-0600 Email: e.balsamo@gripple.com Web: www.grippleseismic.com Product: Gripple® Seismic Cable Bracing Description: A seismic solutions provider for nonstructural equipment and components. Provides a complete seismic solution – from on-site engineering services, to Gripple Seismic Cable Bracing Systems, to vibration isolation products such as spring mounts, isolators, hangers, and pads.

Hardy Frames Phone: 800-754-3030 Email: dlopp@mii.com Web: www.hardyframe.com Product: Hardy Frame® Panels, Brace Frames and Special Moment Frames Description: Hardy Frame Shear Wall Systems lead the industry in strength, stiffness and ductility to resist lateral loads, protect against damage and dissipate energy.

The Panel shape permits recessed fixtures and the ability to insulate. New installations include back-to-back for double capacity and reinforced anchorage solutions.

Hilti, Inc. Phone: 800-879-8000 Email: us-sales@hilti.com Web: www.us.hilti.com Product: PROFIS Anchor and PROFIS Rebar Software Description: PROFIS Anchor performs seismic calculations for Hilti post-installed anchor systems using the anchoring to concrete provisions of ACI 318. PROFIS Rebar performs development length calculations for Hilti adhesive anchor systems and post-installed reinforcing bars using ACI 318-11 seismic provisions for special moment frames and special structural walls.

All Resource Guides and Updates for the 2016 Editorial Calendar are now available on the website, www.STRUCTUREmag.org.

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October 2015

Powers Fasteners Phone: 845-230-7533 Email: Mark.Ziegler@sbdinc.com Web: www.powers.com Product: Powers Submittal Generator (PSG) Description: PSG is a submittal and substitution online tool that helps contractors create submittal packages in just minutes. In only a few simple steps users can include all applicable code reports and technical detail. Contact us for a free demonstration.

RedBuilt Phone: 866-859-6757 Email: csprung@redbuilt.com Web: www.redbuilt.com Product: Open-web Trusses Description: Tested lateral load capacities to frame seismic details without the use of strap ties. RedBuilt’s clips may be welded or nailed to bearings in most project applications where straps are specified, for CMU walls, or wider on-center spacings of 32 inches or more.

RISA Technologies Phone: 949-951-5815 Email: amberf@risa.com Web: www.risa.com Product: RISA-3D Description: Overwhelmed with the latest seismic design procedures? RISA-3D has you covered with seismic detailing features including full AISC 341/358 code checks. Whether you’re using RISA3D’s automated seismic load generator, or the built-in dynamic response spectra analysis/design capabilities, you’ll get designs and reports that will meet all your needs.

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news and information Craig E. Barnes, P.E., SECB, Founding Principal of CBI Consulting Inc. is stepping down as a member of the STRUCTURE® magazine Editorial Board.

Craig is a Founding Member of STRUCTURE magazine and served as one of NCSEA’s representatives on the Board since its inception. As an engineer registered in both the civil and structural fields, Mr. Barnes has over 50 years of experience designing, coordinating, and managing structural and civil engineering projects throughout the United States.

Jon Schmidt, Chair of the STRUCTURE magazine Editorial Board, had this to say on Craig’s departure: “Craig Barnes has been involved in the production of STRUCTURE magazine since long before I joined the Editorial Board myself. It has been great to work with him over all these years, and I wish him well on whatever comes next. I have no doubt that he will continue to make significant contributions to the advancement of the structural engineering profession.”

Regarding his tenure on the Board, Craig commented, “I have enjoyed every minute of participation on the Editorial Board, working with the other Board Members, processing articles, as well as writing.

After who knows how many years, it is time to let someone else have the fun! Jon has been such a great leader and made it easy to stay on.

Although I am leaving the Board, I will continue to keep on writing for this wonderful magazine.

Jessica Mandrick, P.E., LEED AP will replace Mr. Barnes. Ms. Mandrick is an Associate at Gilsanz Murray Steficek (GMS). At GMS, Product: Strong-Rod™ Systems Description: Anchor tiedown systems for shearwall overturning restraint and uplift restraint for roofs address many of the design challenges specifically associated with light-frame, multi-story buildings that must withstand seismic activity or wind events. Our engineers can help optimize your designs with tested, code-listed solutions.

Standards Design Group Phone: 800-366-5585 Email: info@standardsdesgin.com Web: www.standardsdesign.com Product: Wind Loads on Structures 4 Description: Performs computations in ASCE 7-98, 02 or 05, Section 6 and ASCE 7-10, Chapters 26-31, computes wind loads by analytical method rather than the simplified method, provides basic wind speeds from a built-in version of the wind speed, allows the user to enter wind speed. Numerous specialty calculators.

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Noteworthy

Jessica has worked on many civic projects in New York City including Stapleton Public Library, art installations at the Guggenheim Museum, the new Amur Leopard Enclosure at the Staten Island Zoo, and several theaters. She has provided structural engineering services for a number of buildings located in hurricane and flood hazard areas. She served on the ASCE 24 Urban Flood Study team after hurricane Sandy and also assisted with the DOB Sandy recovery efforts. Ms. Mandrick is licensed as a Civil Engineer in the States of California and New York, and is certified as an ICC Special Inspector.

She is an active member of the ASCE Structural Engineering Institute’s Young Professionals Committee, a mentor with the ACE program, and annually participates with GMS in the Construction Competition.

She remains active in art, working in welded steel and bronze at the Art Student’s League of New York.

Jon Schmidt said this about Ms. Mandrick’s appointment: “I am pleased to welcome Jessica Mandrick to the editorial board. She has plenty of experience with writing and editing for her firm and comes highly recommended by her colleagues, so I have no doubt that she will be a wonderful addition to the team. In fact, she has already hit the ground running.”

Please join the STRUCTURE magazine Editorial Board in congratulating Craig Barnes on his many years of service to the magazine and welcoming Jessica Mandrick to the team.

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StructurePoint Phone: 847-966-4357 Email: info@structurepoint.org Web: www.StructurePoint.org Product: Reinforced Concrete Design Software Description: spColumn – design of shear walls, bridge piers and typical framing elements in buildings and structures. spWall – design and analysis of cast-in-place reinforced concrete walls, tilt-up walls, ICF walls, and precast architectural and load-bearing panels. spColumn complements spWall by generating axial/flexure (P-M) diagrams suitable for shear wall design.

Listings are provided as a courtesy. STRUCTURE® magazine is not responsible for errors. See more companies in the 2015 Annual Trade Show issue!

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Product: Tedds Description: Automate wind and seismic calculations, and perform member designs. Built-in library of calculations allows quick calculations ASCE 7 wind and seismic forces. Use one of our component design modules to design beams, columns and foundations.

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award winners and outstanding projects

Spotlight

Floating Cofferdam for Repair of the Washington SR-520 Floating Replacement Bridge

By Hamid Fatehi, P.E., S.E. COWI (formerly Ben C. Gerwick Inc.) was an Outstanding Award Winner for the Floating Cofferdam for Repair of the Washington State SR-520 Floating Replacement Bridge project in the 2014 NCSEA Annual Excellence in Structural Engineering awards program (Category – New Bridges and Transportation Structures).

T

he SR 520 Bridge crosses Lake Washington, linking Seattle and its neighboring cities to the East.

The floating section of the bridge is more than 1.4 miles long, making it the longest floating bridge in the world. Afloat for more than 50 years, the four-lane bridge is often clogged by traffic and vulnerable to storms.

The State of Washington is replacing the bridge with a new six-lane floating bridge, which includes 21 longitudinal concrete pontoons.

Each longitudinal pontoon is approximately at 75 feet wide, 360 feet long and 28 feet tall.

The first phase of cast-in-place in-the-yard construction resulted in structural cracking at the ends of four concrete pontoons due to design error. Following discovery of the cracking, COWI (formerly Ben C. Gerwick Inc.) was retained by Kiewit/General/Manson, a Joint Venture (KGM is the design-build contractor building and assembling the replacement bridge) to develop a repair plan that ensures that the bridge will meet the performance requirements. COWI’s repair plan consists of multiple repair measures including crack injections with epoxy, transverse post-tensioning, waterproofing membrane, and carbon-fiber reinforced plastic wrap. A floating steel cofferdam was built to allow the repair work to be completed for the floating pontoons in a dry environment. As a design consultant to KGM, COWI completed the design of the cofferdam.

Design Innovations The cofferdam weighs approximately 600 tons and is 96 feet wide, 44 feet long and 35.5 feet tall.

The cofferdam design includes many innovative features in order to meet the project requirements such as safety, operability, watertightness, and adequate contingency. Features include: • As a one-of-a-kind floating dry dock, the cofferdam has a highly asymmetric layout. The design includes ballast tanks that are strategically located inside the cofferdam so that the center of gravity aligns with the center of buoyancy.

• The ballast tanks and floating tanks are integrated into the structural framings of the cofferdam, which helps to achieve structural efficiency and substantial cost saving.

• An elaborate seal system was developed to ensure a complete watertight seal of the cofferdam, and to create a dry work environment for the pontoon repair inside the cofferdam at 26 feet below the lake water surface.

• Two hydraulically activated sliding gates are provided to enclose the sides of the cofferdam. The sliding gates are 26.5 feet tall and slide on UHMW bearing pads.

• Seven steel trusses are attached to the bottom of the cofferdam in order to provide its required strength. The cofferdam can be quickly raised vertically by picking up the trusses with a hydraulic jack system attached to flexifloats.

• Once the cofferdam is installed, it is subjected to buoyancy of 6,400 kips and must withstand a 5-year storm. The hydraulic pressures are transmitted to the pontoon through 18 hydraulic rams. Several threedimensional finite element models of the cofferdam, bridge pontoons and launching barge were created to analyze the load effects for (1) the cofferdam operations and installation, (2) launching off a barge in a sideway manner, and (3) emergency removal of the cofferdam in case of an accident.

Fabrication and Launch The cofferdam was assembled on the floating barge equipped with cofferdam supporting frames and a launch system. The launch rail system consisted of two steel wide-flange rocker beams with pins and hydraulic jacks to conduct a side launch from the barge.

After the cofferdam was assembled on the barge, it was towed through the Ballard Locks to Lake Washington. On November 26, 2013,

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the cofferdam was launched sideways off the barge into the lake. Hydraulic jacks pushed the cofferdam across the barge and onto two rocker beams. As the cofferdam passed beyond the pin of the rocker beam, the rocker beams rotated about a hinged connection and allowed the cofferdam to slide into the lake. A ballasting sequence and plan for the barge at different stages of the launching process were developed by performing floating stability analysis.

Cofferdam Installation The cofferdam installation process starts with adding ballast water to the ballast tanks to increase its submergence so that it could float underneath the bridge pontoon with adequate underkeel clearance. A hydraulic jacking system reacting on two flexifloat assemblies was used to pull the cofferdam vertically into direct contact with the bridge pontoon and to compress the perimeter rubber seal of the cofferdam. Two sliding gates were moved by hydraulic jacks to close side openings between the cofferdam and the bridge pontoon. This resulted in an enclosure that allows for dewatering of the cofferdam.

Water from the cofferdam was pumped into ballast tanks and Baker tanks on the top deck of the bridge pontoon, allowing the floating cofferdam and the bridge pontoon to stay level.

Summary Up against a tight schedule, the designer and contractor completed the design and construction of the cofferdam within 8 months. The design and construction were fully integrated in a design-build process and underwent progressive review and check by the Owner, WSDOT. Close collaboration among the designer, the contractor, and the Owner was the key to the project success.▪ Hamid Fatehi is Chief Project Manager at COWI Marine, Oakland, CA. He can be reached at hmdf@cowi.com.

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NCSEA News

News form the National Council of Structural Engineers Associations

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The world around us is constantly changing, and the structural engineering profession is changing, too. It is essential, therefore, that NCSEA change as well, in order to provide the services and opportunities the Member Organizations (MOs) need and want. Of course, change isn’t easy. For NCSEA, it involved digging deep into core philosophies and examining bedrock principles, to get at the root of who we are and why we exist.

James C. Collins, one of the authors of Built to Last: Successful Habits of Visionary Companies, stated that “contrary to popular wisdom, the proper first response to a changing world is not to ask, ‘how should we change?’ but rather to ask ‘what do we stand for and why do we exist?’” He indicates that a review of bedrock principles, and a close self-examination of guiding beliefs and long-standing traditions, may not only be useful, but mandatory, if a company or organization is to remain relevant, healthy, and vibrant in the future.

With respect for the past leaders of NCSEA, who provided guidance and labored diligently, the current NCSEA Board began a lengthy process of re-examining the needs of NCSEA and, just as importantly, the needs of the MOs. One year ago, the MOs were sent a 3-question survey containing the following questions: What are the three most significant problems facing your member organization? What are the three things you believe NCSEA is doing well? and, What could NCSEA do to improve its support of your member organization? NCSEA received over 300 responses to this 3-question survey. The answers to these bedrock questions provided the Board with a better understanding of the problems the MOs face and how NCSEA can best support them.

With MO responses in hand, NCSEA began a six-month exploration and evaluation process. During the first strategic planning meeting in October 2014, a new Vision Statement and Mission Statement were created, providing NCSEA with an updated direction and purpose. NCSEA’s new Vision Statement (direction) is: The National Council of Structural Engineers Associations will be recognized as the leading advocate for the practice of structural engineering. NCSEA’s new Mission Statement (purpose) is: NCSEA advances the practice of structural engineering by representing and strengthening its Member Organizations. NCSEA’s strengths, weakness, opportunities, and threats were discussed, and groups were organized to focus on four strategic initiatives expected to yield the greatest benefit for the organization and the MOs. The four areas of focus were as follows: ORGANIZATION – To assess NCSEA’s organizational structure and revise it, as needed, to effectively execute the Mission and Vision statements; DELEGATE MODEL – Improve the effectiveness and engagement of NCSEA Delegates; COMMUNICATION – Streamline and improve communications between NCSEA and its Member Organizations; and, FINANCIAL – Create a fiscally secure future for NCSEA by investigating options, opportunities, and threats that may affect NCSEA’s financial sustainability.

During this planning phase, goals and priorities were set, budgets were generated, deadlines were established, and tasks were assigned. The process of making the Strategic Plan a reality could then begin in earnest, i.e., the extensive changes to the Summit and Business meeting, while other changes will take STRUCTURE magazine

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time because they are dependent on other goals or additional resources.

During the implementation phase, I realized that a strategic plan, if fixed and unalterable, could quickly become useless and irrelevant – a millstone hanging around the neck of future leaders of NCSEA. A strategic plan must be a living document to remain valid. As the world and profession continue to change, rigidity in its strategic plan could result in NCSEA’s inability to quickly and adequately respond to the needs of the profession and the MOs. While the bedrock principles which NCSEA was founded on remain fundamentally unchanged, it is expected that NCSEA’s goals and priorities will need to be modified on a regular basis. Therein lies the problem with creating a plan that reaches into the future – change is inevitable.

This past year was the busiest year I have had while serving on the NCSEA Board. NCSEA’s Past Presidents, Committee Chairs, Volunteers, and Board members put in, I conservatively estimate, a thousand hours on this Strategic Plan. To help make it a reality, NCSEA Executive Director Jeanne Vogelzang, and Office Staff, provided their full support and backup to ensure the plan’s success. It is because of these dedicated individuals that NCSEA and the new 2015 Strategic Plan will be a success.

Barry Arnold President, NCSEA Board of Directors

NCSEA VISION STATEMENT The National Council of Structural Engineers Associations will be recognized as the leading advocate for the practice of structural engineering.

NCSEA MISSION STATEMENT NCSEA advances the practice of structural engineering by representing and strengthening its Member Organizations.

ORGANIZATIONAL STRUCTURE Mission: To assess NCSEA’s organizational structure and revise it, as needed, to effectively execute the Mission and Vision statements DELEGATE MODEL Mission: Improve the effectiveness and engagement of NCSEA Delegates NCSEA and MO COMMUNICATIONS Mission: Streamline and improve communications between NCSEA and its Member Organizations such that all practicing structural engineers will desire to become active participants of their Member Organizations and NCSEA and become an integral resource for the structural engineering profession FINANCIAL SUSTAINABILITY Mission: Create a fiscally secure future for NCSEA by investigating options, opportunities, and threats that may affect NCSEA’s financial sustainability October 2015

NCSEA

National Council of Structural Engineers Associations

2016 W����� L��������� F���� MARCH 10 & 11 Coronado Island Marriott San Diego, CA

The NCSEA Winter Leadership Forum draws principals and leaders from a diverse group of structural engineering firms to engage in thought-provoking sessions, roundtables, and networking. In 2016, the focus will be on managing risk professionally, collaboratively and transparently. The first day will consist of interactive discussions on managing the various risks every firm faces every day. The second day will be a discussion of the various risks faced on projects and some of the preventative measures you can take. Three to four presentations of actual claims will be presented, with a discussion of what went wrong (if anything) and how some of these claims could have been avoided.

O N F RIDAY :

Project Delivery Workshop–This interactive workshop will explore virtually every aspect of modern project delivery: formal methods, with an examination of advantages and disadvantages for each; details of various contractual tools and techniques; legislative history of QBS for design firms and future QBS prospects for the construction team; and a deep dive into results of major studies that document what works and what doesn’t in the design and construction industry.

Speaker: Dale Munhall, Architect, Director of Construction Phase Services, Leo A Daly

An exploration of the differences in E & O insurance policies and how to conform to insurance requirements. Have your questions answered on Best Ratings, prior acts coverage, pre-claim assistance and cost, whether coverage is limited to the settlement amount if the insured refuses settlement, and more.

Speakers: Dan Bradshaw, CPCU, Benchmark Insurance Agency, Inc., & Craig Coburn, Attorney, Richards Brandt Miller Nelson

Legal advice, as well as give and take, on how to achieve your business objectives while minimizing the risks of litigation. Topics will include: Hiring the Best Candidate...Lawfully!; Properly Classifying and Paying Your Workers; Preventing Discrimination and Harassment in the Workplace; The Handbook: Informing Employees of Policies and Procedures; Disciplining the Problem Employee; and What to Expect when Facing a Lawsuit.

Speaker: Staci Ketay Rotman, Attorney, Franczek Radelet P.C.

Claims Sharing: Three firms will present what happened to them when they were sued for professional negligence. They will tell their stories about the projects, the allegations, the outcome and the lessons learned; but the outcome and lessons learned will not be revealed until after WLF attendees have had their own opportunity to predict the outcome.

Moderator: John Tawresey, S.E., Retired VP & CFO, KPFF Speakers: Seasoned [been sued] structural engineers and defense counsel

News from the National Council of Structural Engineers Associations

O N T HURSDAY :

NCSEA News

Save The Date

Coronado Island Marriott, March 10-11, 2016 Stunning views of the San Diego skyline across the bay make Coronado Island Marriott Resort & Spa a comfortable atmosphere for relaxation and renewal. The Resort includes a variety of luxurious on-site features, a full-service health spa, three heated pools and convenience to beautiful sandy beaches, shopping and restaurants at Ferry Landing. The NCSEA Winter Leadership Forum room rate for the Coronado Island Marriott Resort is $239 with a complimentary resort fee (a $25 value).

A panel of Degenkolb Engineers: David Conneville, S.E.; John Dal Pino, S.E.; Mahmoud Hachem, S.E.; Kirk Johnston, S.E., LEED AP; and Roger Parra, S.E. October 27, 2015 Significant Changes to the Wind Provisions of ASCE 7-10

T. Eric Stafford, P.E., Eric Stafford & Associates

November 10, 2015 Designing for Wind Loads Using the Directional Procedure in ASCE 7

T. Eric Stafford, P.E., Eric Stafford & Associates

More detailed information on the webinars and a registration link can be found at www.ncsea.com.

Subscriptions are available!

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Diamond Reviewed

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1.5 hours of continuing education. Approved for CE credit in all 50 states through the NCSEA Diamond Review Program. www.ncsea.com.

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October 22, 2015 Observations & Reflections on the 2014 South Napa Earthquake

November 3, 2015 Calculating & Applying Design Wind Loads on Buildings Using the Envelope Procedure in ASCE 7

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Second ATC-SEI Conference on Improving the Seismic Performance of Existing Buildings and Other Structures

Structural Columns

The Newsletter of the Structural Engineering Institute of ASCE

December 10 –12, 2015, San Francisco, California The ATC-SEI Conference on Improving the Seismic Performance of Existing Buildings and Other Structures is an opportunity for structural engineers, business owners, and users of ASCE seismic standards to learn the latest in seismic evaluation and rehabilitation.

Earn up to 14 Professional Development Hours (PDHs) Who Should Attend • Structural engineers • Civil engineers • Bridge engineers • Business owners • Researchers working in the structural engineering discipline • Students • Users of ASCE/SEI 31, Seismic Evaluation of Existing Buildings, and ASCE/SEI 41, Seismic Rehabilitation of Existing Buildings • Members of ASCE/SEI • Professional engineers looking for additional PDH opportunities

Special opportunity to honor the Champions of Earthquake Resilience in a spectacular historic landmark. The San Francisco Maritime Museum is a marvel of art-deco architecture with classic terrazzo floors, and fanciful murals and mosaics, with sweeping views of San Francisco Bay, Alcatraz, Sausalito, and the historic ships at Hyde Street Pier. All proceeds will benefit the ATC Henry J. Degenkolb Endowment Fund and the SEI Futures Fund.

Visit the conference website at www.atc-sei.org for complete information and to register.

Advance To SEI Fellow

Young Professional Scholarship

The SEI Fellow grade of membership recognizes accomplished SEI members as leaders and mentors in the structural engineering profession. The benefits of becoming an SEI Fellow include recognition via SEI communications and at the annual Structures Congress along with a distinctive SEI Fellow wall plaque and pin, and use of the F.SEI designation. SEI members who meet the SEI Fellow criteria are encouraged to submit application packages online by November 1st to advance to the SEI Fellow grade of membership and be recognized at the Geotechnical & Structural Engineering Congress, February 14 – 17, 2016 in Phoenix, Arizona. Visit the Fellows webpage at www.asce.org/structural-engineering/sei-fellows to learn more.

Apply for the SEI Young Professional (age 35 and younger) Scholarship to the Geotechnical & Structural Engineering Congress, February 14 – 17, 2016, in Phoenix, Arizona.

SEI is committed to the future of structural engineering and offers a scholarship for Young Professionals to participate and get involved at the annual Congress. Many find this event to be a career-changing and energizing experience, opening up networking opportunities and expanding horizons to new and emerging trends. Applications are on the SEI website at www.asce.org/structural-engineering/sei-young-professionalsscholarship-application/ and are due November 2nd.

SEI Futures Fund

Probabilistic Mechanics and Reliability Conference

REGISTRATION NOW OPEN Each day will begin with two compelling Keynote Speakers and will include multiple opportunities for networking with colleagues and leaders in the field.

Champions of Earthquake Resilience Awards Dinner Benefiting ATC Endowment Fund and SEI Futures Fund

The SEI Futures Fund Board recently met and approved the following strategic initiative funding proposals for FY16. Your gift of support provides critical funding for these visionary efforts that invest in the future of structural engineering: • Workshop for Committee for the Reform of Structural Engineering Education (CRoSE) • SEI Global Activities Division Executive Committee – initial meetings • SEI BoG Task Committee to evaluate and recommend SEI global initiatives • Joint Congress Registrations for Young Professional Scholarship Recipients • Research evidence to support the promotion of SE licensure • Local Chapter Webinars Learn more and invest in the future of structural engineering at www.asce.org/SEIFuturesFund. Gifts are fully deductible for income tax purposes.

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Call For Abstracts Deadline for Submission is October 15th The Probabilistic Mechanics and Reliability Conference, PMC 2016, will examine all aspects of probabilistic mechanics, reliability, risk analysis and uncertainty quantification relevant to the assessment and design of structural, mechanical, marine, aerospace, geotechnical, environmental engineering and civil infrastructure systems. The conference will be co-located with EMI 2016, the annual conference of the Engineering Mechanics Institute, May 22 – 25, 2016 at Vanderbilt University in Nashville, Tennessee.

The conference is chaired by Prof. Sankaran Mahadevan and Prof. Caglar Oskay and co-sponsored by SEI. Visit the conference website at www.vanderbilt.edu/emipmc2016 by October 15th to submit an abstract.

October 2015

February 14 –17, 2016, Phoenix, Arizona Connect | Collaborate | Build

REACH SEI MEMBERS

Increase your exposure to more than 25,000 SEI members through www.asce.org/SEI, SEI Update e-newsletter, STRUCTURE magazine, and at SEI conferences year round.

www.asce.org/SEI-Sustaining-Org-Membership

STRUCTURE magazine

Errata SEI posts up-to-date errata information for our publications at www.asce.org/SEI. Click on “Publications” on our menu, and select “Errata.” If you have any errata that you would like to submit, please email it to Jon Esslinger at jesslinger@asce.org.

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October 2015

The Newsletter of the Structural Engineering Institute of ASCE

The Geo-Institute (G-I) and Structural Engineering Institute Student and Young Professionals Programs (SEI) of the American Society of Civil Engineers (ASCE) are Sunday, February 14 coming together to create this fi rst-of-its-kind event. By com• Meet the Leaders Event – Meet with top leaders bining the best of both Institutes’ annual conferences into one through a series of roundtable discussions. unique conference, you will profi t from unmatched networking • Student And Young Professionals Reception – opportunities with colleagues within and across disciplines.

Exclusive event for emerging professionals to network with each other and industry leaders.

Pre-conference Short Courses • Welcome Reception – Students and Young Professionals Begin your Geotechnical and Structural Engineering Congress are welcome at this general congress event.

2016 experience with a full-day, or one or two half-day short Monday, February 15 courses on Sunday, February 14, 2016. Professional Development • Geo-Structure Student Challenge – A wide variety Hours (PDHs) for completion of the Short Courses are availof competitions for geotechnical and structural able. Space is limited, so register early. engineering students.

Full-Day Courses: 7:45 A.M. – 4:45 P.M. o Wall – Challenges student teams to construct a model EARN 8 PDHS mechanically stabilized earth wall and cantilevered load SC1 / Recent Developments in Ground Improvement – frame supported on piles. Teams will compete to create Tools Every Geotechnical Engineer Should Have the lightest systems capable of carrying the design loads.

SC2 / Geotechnical and Structural Instrumentation o Prediction – Challenges students to predict the results and Monitoring During Construction of a large scale field soil-structure interaction field test. o Video – Challenges students to prepare short SC3 / Bridge Scour informational videos around the theme of soil/ SC4 / Risk Assessment in Geotechnical and structure interaction.

Structural Engineering • Graduate Student Career Fair And Networking Event – SC5 / FRP Composites for Structural and Invitation only event that enables students to network with Geotechnical Infrastructure prominent companies.

Morning Half-Day Courses: 7:45 A.M. – 12:00 P.M. EARN 4 PDHS SC6 / Cold-Formed Steel–History, Design, and Innovation CASE Spring Risk Management SC7 / Introduction to the 2016 Edition ASCE 7 Minimum Convocation Design Loads for Buildings and Other Structures SC8 / Heave Prediction for Pier Foundation in The Council of American Structural Engineers (CASE) is a Expansive Soils national association representing more than 200 structural Afternoon Half-Day Courses: 12:30 – 4:45 P.M. engineering fi rms dedicated to making structural engineerEARN 4 PDHS ing a fair, profitable, and robust industry. Several technical SC9 / Seismic Design of Diaphragms sessions were organized by CASE, including Soil/Structure SC10 / New Structural and Geotechnical Seismic Design Interaction: Dialogue Between Engineers to Create Good Soil Requirements in the 2015 NEHRP Provision Reports, Characteristics of Higher Performing Design Firms, and SC11 / Data Acquisition Basics – Setting Up a Simple Tackling Today’s Business Practice Challenges – A Structural Automated Instrumentation System for Geotechnical Engineering Roundtable. and Structural Monitoring Want to learn more about what to expect? Download free papers from last year’s GI and SEI congress.

Remember to use the congress hashtag: #GeoSEI2016 Visit the Joint Congress website at www.Geo-Structures.org WITH SEI SUSTAINING ORGANIZATION MEMBERSHIP for complete information and to register.

Structural Columns

REGISTRATION NOW OPEN Geotechnical & Structural Engineering Congress 2016

CASE in Point

The Newsletter of the Council of American Structural Engineers

CASE Practice Guidelines Available CASE 976-A Commentary on Value-Based Compensation for Structural Engineers

CASE 976-D Commentary on 2010 & 2015 Code of Standard Practice for Steel Joists and Joist Girders

The importance of receiving adequate fees for structural services is vital for the engineering practice to thrive. If fees are not adequate, the structural engineering professional becomes a commodity; libraries are not maintained, computer software and equipment becomes out-dated and the quality of our product declines significantly.

Value Based Compensation is based on the concept that there are specific services, which may vary from project to project, that provide valuable information to the client and whose impact on the success of the project is far in excess of the prevailing hourly rates. Value Based Compensation is based on the increased value or savings these innovative structural services will contribute to the project. As a result, the primary beneficiary of an innovative design or a concept is the owner, but the innovative engineer is adequately compensated for his knowledge and expertise in lieu of his time

The specification of joists and Joist Girders can provide an economical structural solution, but there are very specific requirements that must be understood by all parties. The updated 2010 SJI COSP provides a more practical approach to specifying joists, to introduce new design terms for use by the structural engineer, and to identify and clarify topics that may have been subject to varying interpretation in the past.

The more recently released 2015 SJI COSP provides additional clarifications and minor revisions.

This commentary provides observations and analysis of the revisions and additions in both documents, and discusses specific aspects of the COSP that have a direct impact on the structural engineer’s practice of specifying steel joists. A familiarity and understanding of the entire SJI COSP is necessary to ensure the proper design and documentation of steel joists and Joist Girders. However, the commentary discussion highlights sections of particular interest to the specifying structural engineer.

CASE 976-C Commentary on Code of Standard Practice for Steel Buildings and Bridges The 2010 COSP addresses many recent changes in the practice of designing, purchasing, fabricating and erecting structural steel, and is therefore a continuation of the trend of past improvements and developments of this standard. It is important to note that the Structural Engineer can change any of the requirements of the Code of Standard Practice by specifying an alternative in the Contract Documents.

This document discusses the list of changes published in the preface of the 2010 Edition and provides some commentary to these changes. This document also addresses areas of the COSP that may not be well understood by some SERs, but will likely have an impact on the structural engineer’s practice of designing and specifying structural steel.

You can purchase these and the other Risk Management Tools at www.acec.org/coalitions/coalition-publications.

CASE Winter Planning Meeting – SAVE THE DATE The 2016 CASE Winter Planning Meeting is scheduled for February 11–12 in Phoenix, AZ. If you are interested in attending the meeting or have any suggested topics/ideas from a firm perspective for the committees to pursue, please contact Heather Talbert at htalbert@acec.org.

WANTED

Engineers to Lead, Direct, and Get Involved with CASE Committees!

If you’re looking for ways to expand and strengthen your business skillset, look no further than serving on one (or more!)

CASE Committees. Join us to sharpen your leadership skills (promote your talent and expertise) to help guide CASE programs, services, and publications.

We have a committee ready for your service: • Risk Management Toolkit Committee: Develops and maintains documents such as business practices manuals and policies for engineers under CASE’s Ten Foundations for Risk Management.

Please submit the following information to htalbert@acec.org: • Letter of interest • Brief bio (no more than 2 paragraphs) STRUCTURE magazine

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Expectations and Requirements To apply, you should • be a current member of the Council of American Structural Engineers (CASE) • be able to attend the groups’ two face-to-face meetings per year: August, February (hotel, travel partially reimbursed) • be available to engage with the working group via email and conference call • have some specific experience and/or expertise to contribute to the group Thank you for your interest in contributing to your professional association!

October 2015

The CASE Risk Management Convocation will be held in conjunction with the joint Geo-Institute /Structures Congress at the Sheraton Phoenix Downtown and Phoenix Convention Center in Phoenix, AZ February 14 – 16, 2016. For more information and updates go to www.geo-structures.org.

The following CASE Convocation sessions are scheduled to take place on Monday, Feb. 15: 10:00 AM – 11:30 AM Soil/Structure Interaction: Dialogue between Engineers to Create Good Soil Reports Moderator: Mr. Brent L. White, S.E., ARW Engineers Panel Speakers: Structural Engineer Panelist: Michael Murphy, P.E., m2 Structural Geotechnical Panelist: William M. Camp, III, P.E., D.GE., S&ME, Inc.

New Amazon Portal Knowledge is power – and your firm’s greatest asset. Whether it’s keeping ahead of the competition or improving your bottomline, beefing up your firm’s know-how can only help. And laying your hands on trustworthy A/E and business resources is about to become a whole lot easier.

In mid-August, ACEC launched its new webstore, the ACEC Business Resource Center, on the Amazon e-commerce platform.

Now ACEC members, as well as A/E professionals worldwide, can enjoy fast access to hundreds of engineering and general business resources published by ACEC and other publishers through one convenient hub. As an added benefit, current Amazon Prime members can continue to enjoy the privileges of Prime membership – including free 2-day shipping – when making purchases at the ACEC Business Resource Center.

Visit the ACEC AMAZON Portal at www.acec.org/publications/amazon.

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

3:00 PM – 4:30 PM Tackling Today’s Business Practice Challenges–A Structural Engineering Roundtable Moderator: David W. Mykins, P.E., Stroud Pence & Associates

ACEC Business Insights Winning Strategies for A/E/C Firms: An Executive’s Guide to Maximizing Growth and Profitability, Third Edition Also available in MOBI (Amazon Kindle) and EPUB (B&N Nook, Sony Reader, IPhone/IPad/IPod, Android, and other e-readers/apps.

Churn and change – it’s the landscape of the A/E/C industries today. Technology is transforming business operations.

Competition is keener. Services commoditization remains a constant threat. Clients are demanding performance while focusing on efficiencies. Author Clare Ross in his book, Winning Strategies for A/E/C Firms: An Executive’s Guide to Maximizing Growth and Profitability, offers an experienced look at how smart strategic thinking can help firms manage change and grow strong by focusing on opportunities that offer the greatest chance for long-term success and profitability.

Access these publications and more at the ACEC Bookstore: www.booksforengineers.com.

STRUCTURE magazine

Win More Work: How to Write Winning A/E/C Proposals Also available in MOBI (Amazon Kindle) and EPUB (B&N Nook, Sony Reader, IPhone/IPad/IPod, Android, and other e-readers/apps.

Over the past decade, A/E/C firms have seen a spike in the number of competitors vying for the same work. The crowded field makes it difficult to get noticed, remembered, and selected by decision-makers. Win More Work: How to Write Winning A/E/C Proposals offers an experienced look at how to write memorable proposals. Being memorable is the key to success.

This book will help architects, engineers, construction executives, and their marketing professionals understand how to write proposals that can double their win rate – thereby freeing them up to provide better service to customers, reduce marketing costs from bad pursuits, or simply spend more time with family and fewer Saturdays at the office. This book is for the novice proposal writer as well as those with decades of experience. Read this book to understand how to create memorable proposals that win more work.

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CASE is a part of the American Council of Engineering Companies

1:00 PM – 2:30 PM Characteristics of Higher Performing Design Firms Moderator/Speaker: Mr. Timothy J. Corbett, SmartRisk

CASE in Point

CASE Risk Management Convocation in Phoenix, AZ

Structural Forum

opinions on topics of current importance to structural engineers

Human Factors in Structural Failures

By Irfan A. Alvi, P.E.

W

hile relatively uncommon, structural failures continue to occur, sometimes with catastrophic consequences.

Investigations of such failures have typically focused on the physical factors involved, which is understandable given the technical orientation and background of engineers.

However, the design, construction, and management of structures always involve physical and human factors, and this broader dynamic system is responsible for both the safety and failure of structures.

Moreover, because structural behavior is deterministically governed by mechanistic physical laws, with no possibility of physical “mistakes,” we can assert that failure of structures – in the sense of not fulfilling human design intentions – is fundamentally due to human factors; i.e., humans falling short in various ways.

The propensity toward failure is determined by the balance of factors that contribute to failure versus safety. The human factors contributing to failure include three categories of primary drivers: • Pressure from non-safety goals, such as achieving functional design, reducing cost, increasing profit, meeting schedules, engaging in competition, building and maintaining relationships, pursuing political objectives, and following personal agendas.

• Human fallibility and limitations due to misperception, faulty memory, incompleteness of information, lack of knowledge, unreliability of intuition, inaccuracy of models, cognitive biases operating at a subconscious level, use of heuristic shortcuts, adverse effects of emotions, and fatigue.

• Complexity, resulting from multiple interactions of multiple components, which exacerbates the other drivers and can result in nonlinearly large effects from small causes, as well as difficulties in modeling, predicting, and controlling structural behavior.

These primary drivers of failure lead to various types of human errors – e.g., slips, lapses, and mistakes – as well as compromised risk

management due to ignorance, complacency, and overconfidence.

A fundamental human factor that helps prevent failures is safety culture, which entails individuals at all levels in organizations placing value on safety, having a humble and vigilant attitude, and conscientiously implementing best practices. With respect to general design features, these best practices include conservative safety margins; structural redundancy, robustness, and resilience; and controllable failure modes. Organizational and professional best practices include: • Sufficient staffing and reasonable schedules.

• Peer review and cross-checking.

• Thorough documentation and effective information-sharing, including allowing dissent, in order to ‘connect the dots’ on project issues.

• Creating teams who bring in diverse perspectives, while also having effective and continuous leadership.

• Recognizing knowledge limitations, deferring to expertise, and engaging in training.

• Using checklists.

• Careful structural modeling and use of software.

• Meeting professional, ethical, and legal/ regulatory standards.

• Learning from failures.

• Promptly and effectively detecting, investigating, and responding to warning signs, including after extreme events and during “quiet periods.”

To apply this framework briefly to a case study, consider the failure of the Quebec Bridge in 1907, which collapsed during construction and resulted in 75 fatalities. Drivers of failure for this steel cantilever truss bridge included: • Excessive cost cutting.

• Schedule pressure due to a substantial financial penalty for delayed completion of construction.

• Cozy and deferential relationships towards an eminent Chief Engineer, Theodore Cooper, who was undercompensated for his services, in poor health, wanted control of the project but never visited the

site during construction, displayed considerable hubris, and likely wanted this project to be the crowning achievement of his career.

• Lack of other sufficiently experienced engineers on the project team.

• Inaccurate models for the capacity of built-up compression members.

• Complexities associated with designing and building what was then the longest cantilever bridge in the world, located in a harsh and icy river environment.

Some additional enablers of failure, in terms of not following best practices, included: • Unconservative safety margins due to excessively high allowable stresses, and not updating an initial dead load assumption that was about 18% too low.

• Lack of meaningful peer review of the design.

• Misinterpreting and denying numerous warning signs during construction, which began more than two months before the failure, such as readily visible and growing bridge member deflections.

• Poor communications among the project team, such as ignoring concerns expressed by laborers.

These factors collectively resulted in poor risk management due to ignorance, complacency, and overconfidence, to the extent of producing a technical and human tragedy.

In summary, structural failures can usually be fundamentally attributed to human factors at both individual and group levels. Understanding these factors requires going beyond identifying “human errors” and assigning blame, in order to also carefully consider the systemic pressures, tradeoffs, complexities, and uncertainties which powerfully influence human decisions and drive a “drift into failure.” To deal with these challenges, successful engineers and teams have a shared family of traits, the most central of which is safety culture, which results in a humble and vigilant preoccupation with avoiding failure, as well as implementation of best practices. By doing our part in exhibiting these traits, all of us involved in structural safety can contribute to reducing the occurrence of failures.▪ Irfan A. Alvi (ialvi@alviassociates.com), is President & Chief Engineer of Alvi Associates, Inc. in Towson, Maryland, and a member of the SEI Engineering Philosophy Committee.

Structural Forum is intended to stimulate thoughtful dialogue and debate among structural engineers and other participants in the design and construction process. Any opinions expressed in Structural Forum are those of the author(s) and do not necessarily reflect the views of NCSEA, CASE, SEI, C 3 Ink, or the STRUCTURE® magazine Editorial Board.

STRUCTURE magazine

66

October 2015