STRUCTURE magazine | October 2017

October 2017 Bridges

Inside: Great Bridges

A Joint Publication of NCSEA | CASE | SEI

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CONTENTS Columns and Departments

Features

EDITORIAL

7 Six Years of Experience in Three Years

By Corey M. Matsuoka, P.E. OUTSIDE THE BOX

10 Great Bridges

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

26 Eliminating Fancy Footwork

STRUCTURAL DESIGN

14 Masonry FEM/FEA 2.0

By Andrew Loff The use of prefabricated, lightweight, Fiber Reinforced Polymer (FRP) sidewalks solved load capacity and constructability issues for the rehabilitation of the Wilson-Burt Bridge in Newfane, New York.

By Samuel M. Rubenzer, P.E., S.E. STRUCTURAL SYSTEMS

18 5-over-2 Podium Design – Part 2

By Terry Malone P.E., S.E. and Scott Breneman, Ph.D., S.E. STRUCTURAL ANALYSIS

22 Vibration Excitations – Part 2

By David A. Fanella, Ph.D., S.E., P.E. and Michael Mota, Ph.D., P.E., SECB STRUCTURAL PRACTICES

35 Support Restraints and Strength of Post-Tensioned Members – Part 1

By Bijan O. Aalami, Ph.D., S.E., C.Eng.

30 The Appleton Pedestrian Bridge

INSIGHTS

By Marian C. Barth, P.E. and William Goulet, S.E. This 750-foot long bridge is a contemporary, curving, “ribbonlike” structure located on the banks of the Charles River in Boston. Its geometrically challenging form and iconic slenderness called for unique solutions from structural engineers.

38 Shaping the Future of Structural Engineering

By Burcu Akinci, Ph.D. PROFESSIONAL LIABILITY

40 Do’s and Don’ts During Construction

By G. Daniel Bradshaw STRUCTURAL FORUM

51 What “R” are You?

By Justin D. Naser, S.E. and Virginia E. Gilbert

IN EVERY ISSUE 8 Advertiser Index 43 Noteworthy 43 Resource Guide – Seismic 44 NCSEA News 46 SEI Structural Columns 48 CASE in Point

On the cover Considered one of the Great Bridges, the Great Belt East Bridge, Denmark, replaced a ferry crossing from mainland Europe to Scandinavia. With a main span of 5,328 feet, it is the third-longest bridge span in the world. Read about this and other Great Bridges in the Outside the Box article on page 10.

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

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Editorial

Six new trends, Years new techniques of Experience and current industry inissues Three Years

By Corey M. Matsuoka, P.E., Chair CASE Executive Committee

T

a member benefit

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hroughout my career, I have been fortunate to have a always a good idea. The action plans serve number of great mentors that have provided me with two purposes. First, it brings a focus and excellent advice and meaningful opportunities. It started clarity that helps get the mentorship off to right after entering the workforce as I remember my first a good start. Second, it adds accountability boss telling me, “It is great working here, you work three years and to enhance the chances of progress. As time get six years’ experience.” Looking back, I realize that he did not goes by, establish checkpoints where mentees report on their projects mean he would be a great mentor for me, but that he literally meant and discuss any challenges they are facing. These checkpoints are we would be working 80 hours a week. great opportunities to ensure movement and realignment if required.

In all seriousness though, I did greatly benefit from the menIf you are looking for a mentor, don’t be afraid to seek one out. torship and guidance I received. An effective mentor can build Mentors are not going to go out of their way to share their expeconfidence, enhance performance, refine leadership skills, and riences with others if there is not some level of initiative by the expand the networks of their mentee. The mentors I had acted as a other party. Don’t be afraid to ask questions and seek advice from sounding board, drawing on those you admire. At the their experience to offer guidsame time, do whatever you ance, a fresh perspective, and can to get noticed. It should If you are in a leadership position, then it is your insights that I used to navigate not come as a surprise that responsibility to develop the leaders of the future. my career (and sometimes life) management takes better care challenges. I definitely would of those they view as superWhen creating a program, the most important not be in the position I am stars and future leaders. My thing to note is that the mentor and mentee need today without their help. opportunity came a few years So the natural question is… into my career when we had to develop a personal relationship.

How does one take advantage trouble in our Saipan office, of this? and a few of us were called If you are in a leadership position, then it is your responsibility into our Manager’s office. He explained that we had to let go of to develop the leaders of the future. When creating a program, our engineer there, and asked which one of us was willing to go the most important thing to note is that the mentor and mentee to take his place. Next thing I know, I am headed home to pack need to develop a personal relationship. Mentorship cannot be and the next day was on a plane to Saipan for three months. I did forced. All too often, mentor programs fail because management not even know where Saipan was! simply assigns a mentor to a mentee and hopes for the best. For real What I also did not know at the time, but do know now, is that mentorship to succeed, a personal relationship has to be developed I made myself known to management by volunteering and they between the mentor and mentee where the mentor genuinely cares began to go out of their way to look for opportunities for me. In for the development of the mentee. According to research conducted Saipan, they started to teach me about project management and

by Belle Rose Ragins, a professor at the University of Wisconsin- guided me through the details of client service. Being a small Milwaukee and an expert in the field of mentoring, unless mentees office, I also got to eat and participate in conversations with the have a basic relationship with their mentors, there is no discernable ‘old guys.’ I soaked up and listened to what they were saying, difference between mentees and those not mentored. Let me say trying to absorb as much knowledge as I could. When I came that again… Without a relationship, a mentoring expert could find back home, the firm allowed me to manage my own projects and no discernable difference between people who have been mentored my own design group soon after. and those who have not.

If there’s no Saipan trip to volunteer for, there are also a myriad What this says is that it is essen- of opportunities outside of your firm to find mentors; you just tial to find the right fit between have to look. For me, those opportunities came by volunteering mentors and mentees. If start- for leadership positions in organizations such as CASE, SEI, and ing a formal program, I would NCSEA. Within those types of organizations, there are always recommend allowing the men- past presidents and others that are willing to pass on leadership tors and mentees to match up lessons that they have learned along the way. Find a way onto their with each other on their own. committees and boards, then ask as many questions as you can.

This method allows for the best Most will be more than happy to share their knowledge. chance of developing that perSo whether you are interested in mentoring the leaders sonal relationship. of the future or seeking out a mentor of your own, just Once the partnerships are remember there are much better ways to get six years of made, the real work begins. experience in three years than by working 80 hours a week.▪ STRUCTURAL Many times, the mentorships ENGINEERING Corey M. Matsuoka is the Executive Vice-President of SSFM can grow and succeed on their INSTITUTE International, Inc. in Honolulu, Hawaii. He is the chair of the CASE own. To make sure this happens, Executive Committee. He can be reached at cmatsuoka@ssfm.com. having goals and action plans are STRUCTURE magazine

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ADVERTISER INDEX

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October 2017, Volume 24, 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. The publication is distributed free of charge to members of NCSEA, CASE and SEI; the non-member subscription rate is $65/yr domestic; $35/yr student; $90/yr Canada; $60/yr Canadian student; $125/yr foreign; $90/yr foreign student.

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

Great Bridges Remarkable, Signature or Great Bridges?

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

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

To the engineers and builders of the amazing, elegant, and powerful structures commonly known as Great Bridges.

M

ankind has been building bridges since the time of early civilizations.

Today, bridges are all around us – they provide easier, faster, and safer connections between two points. Bridges have evolved from merely utilitarian structures to become symbols of cities, countries, and human progress. Iconic bridge structures embody engineers’ eternal aspiration to greater achievement – longer and taller, stronger and faster. We all admire bridges that span gracefully over a deep ravine, canyon, large river or bay, and rightfully consider them high technical achievements.

However, very few of these bridges have acquired the prestigious title of “Great Bridges.” It may be interesting to find out what makes a Great Bridge.

A Great Bridge is one that can be distinguished from others by something unique or exceptional – longer span, singular beauty, appeal to the general public. Throughout history, people have admired and prized remarkable buildings and structures. As early as about 150 BC, the Seven Wonders of the World defined a grouping of the most renowned constructions of the time. While there were some bridges built as early as 850 BC (like the Caravan Bridge at Smyrna in Turkey, still in use), no bridge was included in the Seven Wonders.

With the development of civilization and correspondingly higher standards of construction, many other buildings and structures were in their turn named as an “Eighth Wonder of the World” in recognition of their high achievement. Among others, these have included the Brooklyn Bridge in 1883 and the Golden Gate Bridge in 1937.

We also often see the term “great” applied to a new architectural or structural project. However, mere months later, few of these achievements are still identified as exceptionally great.

One way to estimate the development of a society is by considering the level of its structural achievement.

The great architectural and structural achievements at different periods of human history are a product of the building knowledge and available construction technology at the time of their creation. There are enormous differences between the construction possibilities in Roman times and those in the last six to seven decades. Therefore, when we review the development of bridges, it would be fair to consider their achievements in relation to the technical level at the period of their creation.

Historical Great Bridges Based on the historical development of construction, remarkable bridges can be subdivided into ancient, medieval, industrial revolution, and modern Great Bridges. Ancient greats include those bridges serving as aqueducts or bridges for military

10 October 2017

conquests. Examples include the Pont du Gard aqueduct (40–60 AD), a three tiered stone bridge, Caesar’s bridges over the Rhine river during his conquest of Gaul (50s BC), and Trajan’s Bridge (104 AD) and Constantine’s Bridge (328 AD), both over the Danube river.

Medieval and Renaissance bridges were often stone bridges serving as crossings within cities, and including buildings and shops on top of the bridge structure. Examples include the Ponte Vecchio in Florence (1564), the Rialto Bridge in Venice (1591) and the “Old” London Bridge (1209).

The bridges of the Industrial Revolution were some of the first long span bridges. These heroic era bridges include: the Menai Strait Suspension Bridge (1826, 577-foot span); the Britannia Bridge (1850, two 459-foot spans, a wrought continuous box girder bridge), also over the Menai Strait; the Clifton Suspension Bridge (1864, 702-foot span) over the Avon Gorge; the Garabit Viaduct (1884, 541-foot span) by Gustave Eiffel; and the Firth of Forth Bridge (1890, two 1,709-foot spans).

High-Level Achievements in Bridges Several bridges are notable for their high level of achievement, but still don’t qualify individually as a great bridge. These include the Verrazano Narrows Bridge, for its span, and the Bosphorus Bridge I, built between two continents. Several cable-stayed bridges should be noted for their long spans, including the Normandy, Sutong, Stonecutters, Maracaibo Bay, Rion Antirion, Millau Viaduct, Sunniberg, and Russky Island Bridges. This structural system, in just 50 years, reached free-spans exceeding 3,280 feet. It is still too soon to tell if any of these bridges will be added to the group of Great Bridges.

“Signature” Bridges Many cities have commissioned eminent engineers to design new “signature” bridges, intended to become symbols of the adjacent city or area. The most prominent bridges identified as “signature” bridges include: Erasmus Bridge, Rotterdam, 1996, a cable-stayed bridge, main span 932 feet; Millennium Bridge, London, 2002, a cable-stayed pedestrian bridge, main span 472 feet; and the Sundial Bridge, CA, 2004, a pedestrian cablestayed bridge, main span 492 feet. Calatrava’s Alamillo Bridge (Seville, Spain, 1992, 656-foot [200 meters] main span) is a beautiful, yet controversial bridge design due to its deviation from basic cable-stayed systems, resulting in high inefficiency.

The notable omission of back span cable-stays creates a dramatic view and contributes to the attractiveness of the bridge, but such a concept

(a)

(b)

Figure 1. a) Golden Gate Bridge; b) Golden Gate Bridge, south tower.

Figure 2. Brooklyn Bridge.

has been discouraged for any bridge not built as a monument.

The self-anchored suspension part of the East span of the SF-Oakland Bay Bridge (2016, 1,263 feet) became an example for an inefficient design concept. The cost, construction material quantities, and building schedule were 7 to 8 times greater than for bridges with similar span but with other structural systems.

All these bridges resulted in very costly projects and proved inefficient in materials and construction time. These “signature” bridges are closer to structural extravagance than to Great Bridges. While these “signature” bridges have interesting, even attractive looking structures and often became symbolic of their locations, none of these exceptional pre-planned structures has gained recognition as a Great Bridge.

Ocean to San Francisco Bay. The bridge’s central span is 4,200 feet (1,280 meters) and was the longest span in the world until 1964 (Figures 1a and b). The bridge created a much-needed link between San Francisco and the Northern Bay Area. The Golden Gate Bridge was designed by chief engineer Joseph Strauss, who assembled a team of experienced bridge engineers. Architect Irving Morrow worked closely with the engineers, making the decisive aesthetical improvements, providing the Art Deco style of the bridge towers and the “International Orange” color, thus contributing to it recognizable image and fame. The combination of a dramatic site and the elegant, powerful structure is essential for considering it one of the greatest bridges ever built. The changing day and night visual conditions, from bright sunlight and shade to an almost opaque layer

Bridge and have been able to preserve the title. Therefore, merely breaking the record for the longest span is not sufficient in and of itself; entering into this prestigious club depends on the long-lasting perception of the public at large.

Modern Great Bridges The fast development of structural analysis and construction technologies in the 20th century provided a significant development and improvement in bridge construction, resulting in the creation of the modern Great Bridges. Some of their forerunners among late-19th-century bridges are also included in this group.

Golden Gate Bridge, San Francisco, California, USA, 1937, spans the Golden Gate Strait, the entrance from the Pacific

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Every new bridge that establishes a record for longest span may be considered a Great Bridge in some way. Breaking the long-span record is a move into new territory; it requires an extremely talented, well-prepared team of designers and builders. Such achievement is only possible by using the highest available level of bridge engineering, some recent innovations and experience gained from previous complicated structures, and challenges and competitions between engineers, architects, companies, and nations. Several sources list the longest bridge spans (absolute or per structural system like suspension, arch, continuous girders). This provides for simple differentiation, as the span length is the only value to be considered. However, just a few of these bridges, holders of recordspan at the time of their completion, have gained the prestigious title of a Great

Figure 3. Akashi-Kaikyo Bridge.

Figure 4. Great Belt East Bridge.

of fog, provide countless view variations for the bridge and its elements.

Brooklyn Bridge, New York, USA, 1883, spans the East River, connecting Manhattan with Brooklyn. The Brooklyn Bridge is a hybrid of a suspension and cable-stayed structure, the first bridge using steel wire cables. It was envisioned and designed by John Roebling. The construction was complicated, with a much longer central span, 1,594 feet (486 meters), than any other bridge at that time. From its opening day, the bridge became one of the landmarks of New York; it is still one of the city’s major attractions (Figure 2, page 11). The link between Manhattan and Brooklyn was essential for the development of Brooklyn. The beautiful bridge, with the Manhattan skyline behind it, is a favorite choice for photography due to the pedestrian access to the bridge deck, from the ground on both sides near the structure, and from vessels on the river.

Tower Bridge, London, UK, was built from 1886 until 1894. The bridge is a combination of a suspension and a bascule system over the River Thames, with the largest span at 270 feet (82 meters). It is located near the Tower of London, which gave the bridge its name, today an equally recognizable symbol of London. The structure was designed by Sir Horace Jones and supervised by Sir John Wolfe-Barry. It was a great relief for most Londoners, providing a connection between the two sides of the Thames, as the then-existing London Bridge was far insufficient for the demand. The bridge contains two bascules which can be raised for river traffic and pedestrian walkways at 141 feet above the river, a smart design approach that permits pedestrians to use the bridge even when the bascules are raised.

Sydney Harbor Bridge, Sydney, Australia, 1932, is a steel through-arch bridge crossing Sydney Harbor with a main span of 1,650 feet (503 meters). The bridge carries a primary

route between the residential North Shore and the central business district of the city. It was designed and built by the British firm Dorman Long and Co. The beautiful view of the bridge with the harbor and the Sydney Opera House completes the familiar, breathtaking skyline of the city. It is a symbol of Sydney and Australia.

The arch was constructed first and served as a support for the superstructure.

Akashi-Kaikyō Bridge, Japan, 1998, was designed by Honshu-Shikoku Bridge Authority with a span 6,532 feet (1,991 meters). The bridge is a suspension steel structure crossing the Akashi Strait and connecting the city of Kobe on the Japanese island of Honshu to Iwaya on Awaji Island. Since its completion, the bridge holds the world record for longest bridge span (Figure 3). According to some experts, this bridge is the top achievement in the classic suspension bridge system with stiffening trusses largely used and developed in Northern America. The bridge was designed with a two-hinged stiffening girder system, allowing the structure to withstand winds of 178 mph (286 km per hour), earthquakes measuring up to magnitude 8.5, and strong sea currents. The bridge also contains tuned mass dampers that are designed to operate at the resonance frequency of the bridge to reduce seismic and wind forces. Visitors can walk under the bridge deck at the level of lower truss bracing and can go up an elevator to the top of one of the bridge towers.

Great Belt East Bridge, Denmark, 1998, has a main span of 5,328 feet (1,624 meters).

The structure was designed by COWI, Klaus Ostenfeld et. al. The central suspension span is the longest span in Europe and currently the third-longest bridge span in the world (Figure 4). The bridge is part of the Great Belt Fixed Link that runs between the islands of Zealand and Funen, and replaced the ferry service that had been the primary transportation means in the past. The link has reduced travel times

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significantly; previously taking about an hour by ferry, the Great Belt can now be crossed in about ten minutes. The construction of the link and the previously completed Øresund Bridge enabled driving from mainland Europe to Scandinavia.

George Washington Bridge, NYC, New York, USA, 1931. The chief engineer of the bridge was Othmar Ammann, with Cass Gilbert as architect. With a span of 3,500 feet (1,067 meters), this bridge held the longest span record until 1937, at nearly double the span of the previous record holder.

The George Washington Bridge is a double-decked suspension bridge spanning the Hudson River between the Washington Heights neighborhood in the borough of Manhattan in New York City and New Jersey. As of 2015, the George Washington Bridge carries over 106 million vehicles per year, making it the world’s busiest motor vehicle bridge. Originally, the steel towers of the bridge were designed to be encased in concrete and granite. However, the towers were left as exposed steel to reduce costs.

These steel towers, with their distinctive crisscrossed bracing, have become one of the bridge’s most identifiable characteristics. The world renowned architect Le Corbusier said of the exposed steel structure: “The George Washington Bridge over the Hudson is the most beautiful bridge in the world. Made of cables and steel beams, it gleams in the sky like a reversed arch.”

San Francisco-Oakland 1936 Bay Bridge, San Francisco, USA. The San FranciscoOakland Bay Bridge connects San Francisco and Oakland. The bridge is actually a combination of several structures with different systems. Its main part is 10,302 feet (3,140 meters) in length. The West crossing from San Francisco to Yerba Buena Island (YBI) is a twin suspension bridge with central spans of 2,310 feet (704 meters) (Figure 5). The bridge

DO IT ON

WITH

Figure 5. San Francisco – Oakland Bay Bridge, West span.

these bridges are more efficient than the earlier American suspension bridges with steel stiffener trusses, they needed strengthening to accommodate increased traffic loads.

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was designed by Ralph Modjeski, Charles Purcell, et al. and built by the American Bridge Company.

At the time of completion, the bridge was the longest bridge in the world – 8.5 miles including approaches. It featured the second longest suspension span at 2,310 feet (704 meters), the third longest cantilever truss span at 1,401 feet, the deepest pier foundation (243 feet below the water surface at low tide), and the largest bored tunnel. The West crossing was the only major bridge with two consecutive suspension spans. The bridge, with its three principal segments, is listed as “One of the largest and most important historic bridges in the country.” President Herbert Hoover called this project “The greatest bridge yet erected.”

It is not by chance that the designers of the Akashi-Kaikyo Bridge used the same type of main towers as the Bay Bridge.

Humber Estuary Bridge, East Yorkshire, UK, 1981, was developed by Freeman Fox & Partners. With its central span of 4,626 feet (1,410 meters), this bridge is a suspension steel bridge with towers consisting of a pair of hollow vertical concrete columns, each 510 feet (155 meters) tall. The bridge is designed to accommodate constant motion due to winds of 80 mph (129 km/h). The bridge held the record for the world’s longest single-span suspension bridge for 16 years, from its completion in July 1981 until the opening of the Akashi-Kaikyo Bridge. With the Humber Bridge, Freeman Fox & Partners developed their previous design for the Severn Bridge and Bosphorus I; these structures represent the “new modern type of suspension bridges” with shallower aerodynamic (aerofoil) deck and inclined suspenders. While

CE

Conclusions What makes a Great Bridge? A bridge that: • Is an exceptionally designed and built structure; • Is built in locations where the bridge was a longtime dream; • Has to surpass previous records or excels over other structures; • Is attractive, elegant, simple, slender, with a feel of robustness; • Fits harmoniously within and complements its environment; • Is easily accessible; • Is accepted as an engineering and architectural achievement; • Is highly recognized and celebrated by the community; and • Withstands the test of time.

Even when all the requirements are met, the result can be just one more perfectly designed and built structure, but still not recognized as a Great Bridge. It should be understood that there is no recipe for designing and building a Great Bridge; the identification as a Great Bridge is a public recognition. They come into being by a fortunate combination of outstanding qualities of structural design in relation to their natural settings and superior construction achievements.▪ The online version of this article contains references. Please visit www.STRUCTUREmag.org.

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Structural DeSign design issues for structural engineers

D

emonstrating the need for utilizing finite element modeling (FEM) and analysis (FEA) to accurately model, analyze, and efficiently design masonry wall systems was the subject of a STRUCTURE article in May of 2016. Hopefully, the reader used the information in the article and is well on their way to building FEM that include structural masonry elements. With that background, they should be ready to advance their design of masonry with the aid of FEM. With FEM, even the simplest designs are better understood. What is the actual lintel force? What forces should be used to design the masonry jamb?

How does the masonry system deflect under load? Traditional methods (Figure 1) have led to a conservative design that is effective but, with better tools (Figure 2), more efficient and accurate designs are possible.

Masonry Compressive Strength The first place to start in developing a sophisticated FEM is to make every effort to define the material strength of all structural items correctly.

When using FEM, what design strength should be utilized? Since FEM associates load to elements based on their strength/stiffness, accuracy is crucial. Make every effort to model the actual masonry element strength. When designing structural elements with concrete masonry units (CMU), the compressive strength (f'm) of the assemblage of the block, grout, and mortar is crucial to the strength of the element. Sometimes, engineers will mistakenly specify a minimum f'm because the design will work at that value and they are not aware that higher design strengths are available; or, because they believe that masonry of lower strength will be easier to obtain.

Many manufacturers, however, regularly produce stronger blocks resulting in stronger structural members. Block manufacturers have increased the strength of CMU to reduce potential damage to units in production, storage, and site transportation. Knowing the actual strength of the material, and designing with it in mind, can lead to cost savings in both labor and materials. Visit www.forsei.com/cmudata for information on block strengths in several states.

The value of f 'm affects numerous parts of masonry design. The modulus of elasticity of masonry is directly dependent on f'm, and for design, flexure, and shear checks that are performed against some percentage of f'm – a higher f'm means a higher allowable strength. This can mean less reinforcing steel and smaller members for lintel and jamb designs, wall design for axial

Masonry FEM/FEA 2.0 Next Step in Incorporating Masonry in Your FEM

By Samuel M. Rubenzer, P.E., S.E.

Samuel M. Rubenzer is Founder/ Owner of FORSE Consulting, Eau Claire, WI. He can be reached at sam@forseconsulting.com.

14 October 2017

loads and out-of-plane lateral loads, and in-plane loads on shear walls. A higher f'm will also reduce reinforcing lap lengths and, depending on the fastener, increase fastener capacity when connecting to the masonry, again reducing material costs.

Designing with the actual strength of the block that will be used on a project can be very beneficial and cost effective, especially for projects with a lot of openings or other details that create a complex masonry system.

Control Joints Since the purpose of a control joint (CJ) is to provide a bond break that will permit longitudinal movement and relieve horizontal tensile stresses, horizontal reinforcing is generally not continuous through a control joint. Therefore, it makes sense for the structural engineer to locate the joints so that they will have as little impact as possible on wall capacity, within prescribed spacing recommendations. Many times, with limited methods for analyzing elements, structural engineers have used CJ in structural masonry walls to simplify the wall design. Designing simple walls (Figure 3) would be much easier to do with more primitive methods. A wall with a simple opening in the center (Figure 4b, page 16) would require a more advanced tool, such as an FEM. If the wall in Figure 2 is 24 feet long and has an 8-foot wide opening centered within it, reinforcing around the opening will allow the wall to be designed as a 24-foot long perforated shear wall (Figure 2b) as opposed to two separate eight-foot long walls (Figure 2a). This design can be performed in software packages like ETABS 2016, RISA 3D, and RAM Elements.

The National Concrete Masonry Association (NCMA) has prescriptive requirements that recommend that control joints be spaced at 25 feet or 1.5 times the height of the wall, whichever is less. Joints located at areas of stress concentrations such as changes in wall height or thickness, or near wall openings or corners, are also recommended.

Many of these requirements were based on past performance and unreinforced walls, and may or (a)

(b)

Figure 1. a) Traditional approach; b) New approach with FEM.

(a)

(b)

Figure 2. a) CJ next to opening; b) CJ away from opening edge.

Shear Wall Performance It is important to consider control joints during the design of the lateral force resisting system. In Figures 1a and 1b, it is easy to see where to locate CJs to gain capacity in the design. Other locations to gain capacity are in stair and elevator core walls. When eliminating control joints in wall groups, the designer benefits from the stiffness of a box

Hybrid Design

Figure 3. Hybrid design of masonry wall and surrounding frame.

FEM is an excellent way to model masonry infills that are designed to take the shear load, commonly referred to as a hybrid design. With frame members on each side, the masonry in the system can contribute significantly to the overall load resistance of the structure. continued on next page

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October 2017

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may not apply as well to today’s reinforced walls. Paying attention to these requirements and adding some reinforcing can result in the need for fewer construction joints. An alternative to the prescriptive requirements is to use an engineered method, again as outlined by NCMA TEK 10-3. This method accounts for the effect of reinforcing in the wall and may result in options with no control joints in the wall.

One of the likely spots for stress concentrations, and therefore a good candidate for control joints in unreinforced walls, is at wall openings. However, if the opening is essentially surrounded by reinforcing – that is, jamb reinforcing on each side, lintel reinforcing, and sill reinforcing (if applicable) – the wall around the opening can be considered sufficiently strengthened to avoid stress concentrations and control joints can be placed away from the opening to meet the minimum spacing listed above. Control joints should be spaced at least two feet from openings if possible (not required, but recommended) to allow for this reinforcing.

This procedure will also enable the designer to take into account the full length of the wall when checking capacity.

shape rather than just a few individual short walls. For example, a masonry core when analyzed as a box can have a moment of inertia nearly three times greater than if it were analyzed as just the individual walls. Even in a typical wall, it is advantageous to space control joints as far as possible in order to have longer wall panels. A 48-foot long wall, for example, could have four CJs spaced at 12 feet or three joints spaced at 16 feet. If the wall is eight-inch CMU and 20 feet tall with an f'm of 2000 psi, the option with four 12-foot walls has an in-plane design capacity of 4*50 = 200 kips total, while the option with the three 16-foot walls has a capacity of 3*93 = 279 kips. This represents a 39% increase in strength just from spacing the control joints farther apart.

As previously stated, NCMA also has recommendations on eliminating control joints altogether by providing sufficient horizontal reinforcing in the wall. These recommendations are usually met when horizontal reinforcement (joint reinforcement or bar reinforcement) is added, such as when designing walls for seismic or high wind requirements, or walls with many openings where continuous lintel reinforcement can be provided. When designing in areas with low seismic forces and few windows, it is usually not cost-effective to provide enough horizontal reinforcing to eliminate joints.

The in-plane shear (shear wall) capacity of a masonry wall is typically sufficient because of the reinforcing already required from other loading scenarios like out-of-plane bending or axial loading. This is certainly true when you have perimeter walls on all sides of a singlestory building. Shear wall designs for masonry only start to require additional reinforcement for relatively tall buildings or certain other situations with low axial loads and low outof-plane bending.

In every situation, it is important for control joints to be shown on the structural drawings rather than allowing the contractor to locate them in the field during construction. This way, the designer can locate them to provide greater structural stiffness, which can ultimately save money on the project.

Excerpt from ASCE 7-10 Table 12.14-1.

Bearing Wall System

Response Modification Factor, R

SDC B

SDC C

SDC D, E

Special reinforced concrete shear wall

5

Permitted

Permitted

Permitted

Special reinforced masonry shear wall

5

Permitted

Permitted

Permitted

In fact, with Type II hybrid design where the masonry is built tight to the beam above (no gap), the masonry shares in the axial load support with the frame surrounding the wall.

With multiple materials involved, the best tool to account for the load distribution in a thorough structural analysis is an FEM. With box-shaped wall groups, hybrid masonry frames, or other shear wall considerations, having finite element software such as RAM Elements V8i and RISA 3D is essential to appropriately account for masonry’s actual stiffness and design strength. With finite element software, walls with openings can easily be considered as shear walls when they might have been previously ignored because of the complex analysis required. With these programs, engineers can recognize the strength of perforated masonry shear walls, hybrid masonry design, and stair and elevator shafts. As a result, masonry is more effective for masonry shear walls, and more accurate analysis leads to more precise and efficient design. (a)

In seismic design situations, masonry walls do require particular attention to detailing similar to other concrete products, especially as you move from an ordinary reinforced masonry wall to a special reinforced masonry wall. Again, even with seismic loading, there are no provisions in the code to make the wall thicker simply because of seismic design. In fact, a lighter, thinner reinforced wall performs better in seismic regions, so reinforced 8-inch masonry can be a better choice than thicker walls.

Masonry is exceptional at resisting lateral load, as defined by the structural engineers standard for determining loads, ASCE 7. In this document, masonry is not only allowed to be used in seismic loading situations, but it also gives special reinforced masonry walls the same Response Modification Factor value as a special reinforced concrete shear wall in bearing wall situations. When reinforced and detailed properly, masonry walls are equivalent to concrete walls of equal strength, even in the most extreme in-plane shear loading situations that result from seismic events.

Lintels

(b)

Figure 4. a) 3D image of masonry lintel/jamb reinforcement; b) Plate stresses used to determine lintel moment.

Masonry lintel analysis and design in an FEM are evaluating an area of the plate/ wall mesh used for analyzing a masonry wall. With FEM, you do not create a separate entity for a masonry lintel. Rather, the lintel is part of the wall which most accurately represents the real wall structure.

The masonry lintel creates an integral joint with vertical jamb reinforcement. This leads to a more robust design over other lintel solutions that consider the lintel separate from the wall (Figure 4a). The loads used in the design of lintels are actual wall forces generated by the finite element analysis of the wall (Figure 4b).

In the FEM, because of the integral nature of the lintel with the plate elements that represent the wall around the lintel and opening, the forces generated will be similar to a fixed end beam. This will create the need in masonry lintels for top and bottom

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reinforcement due to the positive and negative moment determined from the analysis.

Similar to the discussion above regarding perforated shear walls, the lintel areas of the wall are also participating elements in lateral resistance. This allows for the creation of the perforated shear wall which has more overall capacity than separate shear walls on each side of the openings.

This requires the lintels to be designed for lateral loads as part of the overall integral masonry system.

Conclusion From integrated lintels and hybrid masonry frames to perforated shear walls and boxed wall group shear walls, FEM is the tool necessary to advance the analysis and design of masonry. This is creating new trends in the use of masonry in building design.

Masonry is once again becoming a structural system that engineers across the country can more fully understand and utilize to provide efficient designs for many building types, thanks to new tools from FEM. In the words of G.K. Chesterton: “I am the man who with the utmost daring discovered what had been discovered before.”

Perhaps with the capabilities of FEM, we have rediscovered what has always been obvious – masonry is an excellent structural system.▪

Resources The number one request from the May 2016 article has been actual examples of utilizing masonry in finite element programs. The space to include those examples is not available here, but manuals for specific software programs with these examples in mind can be downloaded. Along with examples of how to incorporate masonry into specific FEA programs, examples for the general topics presented in this article are also provided: Pre-processing and modeling examples 1) How to account for cracking 2) How to account for partial grouting 3) The best way to incorporate CJ locations Post-processing 1) How to extract results 2) How is an FEA reporting lintel forces, and are they right?

3) What is a masonry jamb, and what needs to be considered for design?

You can download these manuals at www.forsei.com/masonry.

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Part 2: Wood Diaphragm and Shear Wall Flexibility

By Terry Malone P.E., S.E. and Scott Breneman, Ph.D., S.E. Terry Malone is Senior Technical Director of Project Resources and Solutions Division at WoodWorks.

He is the author of “The Analysis of Irregular Shaped Structures: Diaphragms and Shear Walls,” published by McGraw-Hill and ICC. He can be reached at terrym@woodworks.org.

Diaphragm Flexibility – Seismic Requirements for considering relative stiffness of diaphragms and shear walls have been in building codes for decades. Section 1604.4 of the 2015 International Building Code (IBC) requires, in Transverse SW Typical

Scott Breneman is a Senior Technical Director of Project Resources and Solutions Division at WoodWorks. He can be reached at scott.breneman@woodworks.org.

Corridor Walls

Typical Unit

Corridor Walls

Typical Unit

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

Ext. SW Typical

Ext. SW Typical

Open Front & Non-open Front Floor Plan w/and w/o offsets

Figure 1. Typical mid-rise multi-story floor plans.

Open Front

5-over-2 Podium Design

Open Front & Non-open Front Floor Plan w/ and w/o offsets 18 October 2017

Non-Open Front

discussion and advances related to structural and component systems

I

part, that the total lateral force shall be distributed to the various vertical elements of the lateral force-resisting system in proportion to their rigidities, considering the rigidity of the horizontal bracing system or diaphragm. In the American Wood Council’s (AWC’s) 2015 Special Design Provisions for Wind and Seismic (SDPWS), Section 4.2.5 gives additional requirements for Wood Structural Panel (WSP) sheathed diaphragms.

Seismic-specific requirements are found in ASCE 7-10, Section 12.3.1, which requires structural analysis to consider the relative stiffness of the diaphragms and the vertical elements of the seismic force-resisting system. Provisions for determining diaphragm flexibility under seismic forces are addressed in IBC Section 1604.4, ASCE 7-10 Section 12.3, and SDPWS Section 4.2.5.

Flexible diaphragms are dealt with in ASCE 7-10, Section 12.3.1.1. Diaphragms constructed of WSP are permitted to be: • Idealized as flexible provided they meet ASCE 7-10 Section 12.3.1.1 (c), which includes meeting allowable story drift limits at each line of lateral force-resistance as noted in the flow chart shown in Figure 2, or • Idealized as flexible where the computed maximum in-plane deflection under lateral load is greater than two times the average story drift of adjoining vertical supporting elements of the lateral force-resisting system, in accordance with Section 12.3.1.3 and Figure 12.3-1.

Distribution to the vertical force-resisting elements for flexible diaphragms is based on tributary area.

Although WSP-sheathed diaphragms are commonly assumed to be idealized as flexible, there can be conditions where a diaphragm does not qualify for flexible diaphragm analysis via ASCE 7-10 Section 12.3.1 (c) – i.e., when shear walls with adequate seismic force capacity are provided, but there is not enough wall stiffness to meet the allowable story drift as a wall line. ASCE 7-10 Commentary, Section C12.3.1.1 – Flexible Diaphragm Condition,

Transverse SW Typical

Structural SyStemS

mportant design considerations and traditional approaches related to the design of a five-story wood-framed structure over a two-story concrete or masonry podium were addressed in Part 1 of this series (January 2017, STRUCTURE). The goal of this article is to help engineers better understand flexibility issues associated with these types of structures and how they can affect the design process.

Complex building shapes and footprints are driving design procedures and code requirements to evolve for all lateral resisting systems and materials. Associated research and full-scale testing are in turn causing some engineers to consider refining their techniques beyond traditional methods of design. Until fairly recently, wood structures tended to be straight forward in shape and size, with ample opportunity for shear walls and structural redundancy.

As increasingly complex building geometries and floor plans similar to those shown in Figure 1 are becoming more prevalent, there becomes a greater need to consider relative stiffness of the lateralresisting elements and their impact on force distribution through the structure. A variety of challenges often occur on projects due to fewer opportunities for shear walls (e.g., more glass and larger openings at exterior wall lines), increased building heights, and multi-story shear wall effects.

Although challenging, efforts are being made to bring greater awareness to these issues and to create guidance for more rational designs.

ASCE7-10 Section 12.3 Wood Diaphragm Flexibility Seismic Section 12.3.1- The structural analysis shall consider the relative stiffnesses of diaphragms and the vertical elements of the lateral force resisting system.

Is any of the following true?

1&2 family Dwelling

Start

1. Steel braced frames 2. Composite steel and

1. Topping of concrete or similar material is not placed over wood structural panel diaphragms except for non-structural topping not greater than 1 ½” thick.

2. Each line of vertical elements of the seismic force-resisting system complies with the allowable story drift of Table 12.12-1.

3. Concrete, masonry, steel

SW or composite concrete and steel shear walls.

Is diaphragm Wood Structural Panels

Idealize as flexible

Light framed construction where all of the following are met:

concrete braced frames

Yes

No

Vertical elements one of the following :

Yes Idealize

as flexible

No

DO IT ON

CE

For other materials, see ASCE7-10 Sections 12.3.1, and 12.3.1.2 Yes

Structural analysis must explicitly include consideration of the stiffness of the diaphragm (i.e. semi-rigid modeling), or calculated as rigid in accordance with 2015 IBC Section 1604.4 or 2015 SDPWS Section 4.2.5.

Maximum diaphragm deflection

Average drift of walls No

Envelope Method Allowed for semi-rigid modelling

Is maximum diaphragm deflection (MDD) >2x average story drift of vertical elements, using the Equivalent Force Procedure of Section 12.8?

Figure 2. Wood diaphragm flexibility – seismic.

similar flexible diaphragm analysis of a typical multi-family central corridor layout, the rigid diaphragm analysis distributes more load to the corridor and transverse walls while reducing the load distribution to the more flexible exterior walls. The loads in the diaphragm see a similar shift between the two analysis methods.

For semi-rigid modeling, the distribution of forces to the vertical resisting elements is based on the relative stiffness of the diaphragm and the vertical resisting elements below, accounting for both shear and flexural deformations.

In lieu of a semi-rigid diaphragm analysis, SDPWS 4.2.5 permits the use of an enveloped analysis, where the diaphragm force distribution to the vertical elements is the larger of the resulting shear forces analyzing the diaphragm as flexible and rigid. A semi-rigid analysis is always an acceptable method of analysis and is considered a valid path to code compliance.

Diaphragm Flexibility – Wind Diaphragm flexibility requirements for wind conditions are embedded within the definitions of ASCE 7-10 Section 26.2 – Definitions, DIAPHRAGM, which states that diaphragms constructed of WSP are permitted to be idealized as flexible. It should be noted that, under wind loading, an open front diaphragm configuration is possible.

Although not required for wind, following SDPWS 4.2.5.2 is considered good engineering practice, including constructing the diaphragm to meet semi-rigid or rigid stiffness requirements and showing that the resulting drift at the edges of the structure can be tolerated. It is also important to note that under

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states that compliance with story drift limits along each line of shear walls is intended as an indicator that the shear walls are substantial enough to share the load on a tributary area basis and not require torsional force redistribution. With very flexible exterior walls, lateral loads will partially shift to the corridor walls, differing from the load distribution by a flexible diaphragm assumption.

Wood diaphragms are sometimes treated as open front or cantilever where a capable shear wall line is not provided on the exterior of the building. Open front structures are covered in SDPWS Section 4.2.5.2. This section requires that, for loading parallel to the open side, diaphragms shall be modeled as semi-rigid or idealized as rigid, and the story drift at each edge of the structure, not just the center of mass, shall not exceed ASCE 7-10 allowable story drift. The applied seismic forces for drift checks of open front diaphragms should include torsion and accidental torsion. The drift calculations shall include shear and bending deformations of the diaphragm and be computed on a strength level basis amplified by Cd. Flexible diaphragms cannot be used for open front structures because they cannot transfer torsional forces.

Rigid diaphragms are addressed in IBC Section 1604.4, ASCE 7-10 Section 12.3.1.2, and SDPWS Section 4.2.5. In accordance with IBC Section 1604.4, a diaphragm is rigid for the purpose of distribution of story shear and torsional moment when the lateral deformation of the diaphragm is less than or equal to two times the average story drift. In a rigid diaphragm analysis, distribution is based on the relative stiffness of the vertical-resisting elements of the story below. Compared to a

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Exterior shear walls

TD1 TD2

• • •

Flexible Semi-rigid Rigid

• •

Semi-rigid Rigid

Horizontal Distribution of Shear

TD1 TD2

Rigid support or Partial support Seismic Loads

I1 Rigid or spring Support ??

I2

I3

Support

Support

Support

Support

Unit with Exterior Wall

Wind Loads as applicable

I2

I3

I1

Unit without Exterior Wall

If flexible diaphragm Condition C

Condition A No SW support

Full support (SW rigid) Condition B Partial support (Decreasing SW stiffness)

Corridor only SW

Varying Degrees of Stiffness Effects of Exterior Walls

Figure 3. Horizontal distribution of shear.

the wind provisions of ASCE 7-10, Chapter 27, Section 27.5.4 – Diaphragm Flexibility, requires that the structural analysis shall consider the stiffness of diaphragms and vertical elements of the main wind force resisting system (MWFRS).

Shear Wall Stiffness Shear wall stiffness can have a significant effect on the distribution of shear forces through the diaphragm. It has become increasingly difficult to find exterior walls that can be used as shear walls due to diminishing wall lengths, larger openings, and building offsets. Force distribution to individual shear wall segments in each line of lateral-force resistance shall provide the same calculated deflection per SDPWS Section 4.3.3.4.1 (i.e., distribution by stiffness), regardless of whether they are inline or offset. Optionally, where the nominal shear capacity of sections exceeding an aspect ratio greater than 2:1 are multiplied by 2bs/h, shear distribution shall be permitted to be taken as proportional to the shear capacities of individual full height wall segments. This method results in the long-used method of distributing load to walls based upon their length.

Shear wall deflections are permitted to be calculated using the familiar SDPWS threepart deflection equation 4.3-1 or four-part equation C4.3.2-1. These equations are based on an idealized single-story shear wall with a horizontal shear force applied at the top of the wall. For example, the first

term, addressing bending, is the deflection resulting from a lateral shear force, but not a bending moment applied at the top of the wall. Extending the basic equations to designs with four, five, or even six stories of WSP-sheathed shear walls is a process of incorporating multi-story wall effects.

Examples of the practical application of multi-story shear wall design can be found in the Structural Engineers Association of California (SEAOC) Structural/Seismic Design Manual – Volume 2 and Woodworks Five-story Wood-frame Structure over Podium Slab. These examples follow what is sometimes called the traditional multi-story shear wall method, as it is a straightforward extension of what has commonly been used for two- and three-story buildings for decades. Other multi-story deflection calculation methods have been discussed, such as by Hohbach and Shiotani and the more recent “mechanics-based” approaches suggested by FPInnovations in Canada. Both of these approaches use a behavioral model where the shear walls essentially cantilever off of their foundation with the floor levels contributing little rotational stiffness in the plane of the walls, predicting higher deflections and more flexible walls than traditional methods. These methods take rational engineering approaches to account for multi-story effects which are not explicitly delineated in codes and standards. The decision to use such methods is currently left up to engineering judgment, preference, or the desire to improve or refine current design practices.

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The combined effects of diaphragm and shear wall flexibility, multi-story shear wall effects, offsets in the diaphragm, and presence of exterior shear walls all affect horizontal distribution of forces within the diaphragm and to the vertical force-resisting elements. Consideration of the relative stiffness of the lateral-resisting elements becomes very important under these conditions.

The partial unit plans in Figure 3, representing highlighted units shown in Figure 1, demonstrate the types of units that might be found in a modern mid-rise, multi-residential structure. One is shown with exterior walls and the other as an open front with no exterior walls. Each unit is shown to have multiple horizontal offsets in the diaphragm.

If the exterior shear walls in the unit on the left can meet allowable story drift, they can be assumed to be stiff enough to allow a diaphragm to be idealized as flexible and the load distributed to these walls can be based on a tributary area basis, as shown in Condition A. Once exterior wall lengths are reduced, or large openings are placed in the walls, they start losing stiffness and transfer more forces to the corridor walls. When this occurs, they provide only partial support, as shown in Condition B. This process could continue to a point where story drift limits cannot be met, which would require a semi-rigid or rigid analysis to be performed. Using rigid analysis is only an option if justified by calculation. With very narrow, flexible exterior shear walls, under some conditions, distribution of forces can result in almost no load going to the exterior walls in the upper stories, in effect creating diaphragm behavior similar to that of a cantilever diaphragm as shown in Condition C. Figure 3 schematically shows that horizontal offsets in the diaphragm could also affect shear force distribution due to changes in diaphragm stiffness brought about by differing depths at the offsets. As diaphragm stiffness decreases, more of the forces are transferred to corridor walls and less to exterior walls. Continuity must be maintained across the offsets to create complete lateral load paths and transfer diaphragm forces to supporting exterior walls.

Conclusion All designs are required to be based on a rational analysis using accepted principals of engineering mechanics. It has become increasingly important to consider the relative stiffness of diaphragms and shear wall multi-story effects as buildings get taller and more complex in shape.▪

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R

einforced concrete floor systems provide adequate resistance to vibration caused by a variety of sources because of their inherent mass and stiffness. General information on sources of vibration and acceptance criteria for typical office and residential occupancies was covered in Part 1 (STRUCTURE, September 2017).

Vibration characteristics of reinforced concrete flat plate and wide-module joist systems were also discussed in Part 1, with useful guidelines to quickly ascertain when these reinforced concrete systems are adequate for various types of vibration excitations.

Flat plate voided concrete systems and two-way joists (waffle slabs) are covered in this second article. Vibrational characteristics of these systems are provided, as are guidelines on how to select the appropriate system based on span, load, and source of vibration.

Vibration Characteristics As noted in Part 1, the stiffness of a floor system plays a key role in its ability to counteract the effects of vibrations. The main component of deflection in a reinforced concrete floor system is from flexure. Stiffness, which is proportional to the inverse of deflection, can be calculated using the modulus of elasticity of the concrete, Ec, and the effective moment of inertia, Ie. The

dynamic modulus of elasticity can be used to calculate floor stiffness, which can conservatively be taken as 1.2 times the static modulus Ec, which is given by ACI 318-14 Equation (19.2.2.1.a).

A flat plate voided concrete slab system is a two-way reinforced concrete system of uniform thickness that contains regularly-spaced, hollow, plastic balls made of high-density, recycled polyethylene (HDPE) inside the concrete (Figure 1).

The plastic balls are commonly referred to as void formers and are usually spherical or ellipsoidal in shape. Void formers are positioned within wire support cages to create modular grids (cage modules), which are locked between the upper and lower reinforcement layers in the concrete slab.

These grids are judiciously located in zones where concrete is not needed and where flexural strength and load transfer to supports are not compromised. Depending on the size and distribution of the void formers, the weight of a flat plate voided concrete slab can be up to 35% lighter than a solid slab of the same thickness.

Flat plate voided concrete slab systems are essentially flat plates with regularly-spaced voids and, thus, can be designed just like any two-way slab system. It was shown in Part 1 that, for flat plate systems, Ie of a panel section can be calculated based on the average effective moments of inertia of the column and middle strips that make up the panel.

Structural analySiS discussing problems, solutions, idiosyncrasies, and applications of various analysis methods

Vibration Excitations

continued on next page

Part 2: How to Select a Reinforced Concrete Floor System

By David A. Fanella, Ph.D., S.E., P.E., F.ACI, F.ASCE and Michael Mota, Ph.D., P.E., SECB, F.ASCE, F.ACI, F.SEI

David A. Fanella is Senior Director of Engineering at the Concrete Reinforcing Steel Institute and can be reached at dfanella@crsi.org.

Michael Mota is Vice President of Engineering at the Concrete Reinforcing Steel Institute and can be reached at mmota@crsi.org.

David and Michael are co-authors of the CRSI publication Design Guide for Vibrations of Reinforced Concrete Floor Systems.

Figure 1. Flat plate voided concrete slab system.

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23

ACI Equation (24.2.3.5a), which is a function of the cracking moment, Mcr, can be used to determine Ie for the design strips. It is recommended to use fr = 4.5λ√f'c instead of fr = 7.5λ√f'c when calculating Mcr because of the relatively low reinforcement ratios in flat plate voided concrete slab systems.

The effective moment of inertia, Ie, is also a function of the gross moment of inertia of the section, Ig. The gross moment of inertia of a flat plate voided concrete slab system is less than that of a solid slab because of the embedded void formers. In general, Ig depends on the shape, size, and spacing of the void formers. Manufacturers’ literature can be used to obtain gross moments of inertia for flat plate voided concrete slab systems.

In the case of two-way joists, the same equation for flat plates can be used to

calculate Ie, with the exception that Mcr is determined using fr = 7.5λ√f'c. It is common to transform a two-way joist system into a two-way system of uniform equivalent thickness. The equivalent thickness is determined by setting the gross moment of inertia of the actual cross-section of the two-way joist system equal to that of an equivalent section that has a uniform thickness, he.

The effective mass (or weight) of a floor system is required when determining its natural frequency. Damping, which is a measure of how quickly vibration will subside and eventually stop, also plays a key role in vibration analysis. These quantities are covered in Part 1.

The natural frequency, fn, of a floor system is related to mass and stiffness and is utilized in all vibration analyses, including

Table 1. Minimum slab thickness/maximum span lengths for flat plate voided concrete systems subjected to walking excitations.

Table 3. Minimum slab thickness/maximum span lengths for flat plate voided concrete systems subjected to rhythmic excitations.

Minimum Slab Thickness (in.)

Maximum Span (ft)

8.0

24.9

9.0

checking that applicable acceptance criteria are satisfied. Simplified techniques can be utilized to determine fn instead of performing a finite element analysis.

In the case of flat plate voided concrete slab systems, the system can be modeled as a thin, isotropic plate, which is free to deflect at any point except the columns.

The equation for fn can be found in the article titled Vibration of Reinforced Concrete Floor Systems (STRUCTURE, April 2015), by the authors. A simplified equation for two-way joists can be found in the same article. In both cases, the approximate fn are at most 10% less than those from a finite element analysis. These equations can be utilized in the preliminary design stage to quickly ascertain whether the floor system is best suited to satisfy the required vibration criteria.

Maximum Span (ft)

Minimum Slab Thickness (in.)

Dancing and Dining

Lively Concert / Sporting Event

Jumping Exercises / Aerobics

8.0

26.0

24.6

21.7

28.2

9.0

28.1

26.4

23.4

10.0

30.5

10.0

30.4

28.9

25.0

11.0

35.0

11.0

32.3

30.6

26.5

11.5

38.1

11.5

33.4

31.6

27.3

12.5

40.0

12.5

35.4

33.3

28.8

13.5

41.9

13.5

37.1

34.9

30.8

15.5

46.6

15.5

41.0

38.5

33.7

17.5

51.2

17.5

44.0

41.2

36.2

19.5

54.6

19.5

46.8

43.7

38.4

21.5

56.9

21.5

49.7

46.4

40.7

23.0

62.5

23.0

51.6

48.1

42.2

Table 2. Minimum total thickness/maximum span lengths for two-way joist systems subjected to walking excitations.

Minimum Joist Depth (in.)*

Maximum Span (ft)

12.5

39.0

14.5

Table 4. Minimum total thickness/maximum span lengths for two-way joist systems subjected to rhythmic excitations.

Maximum Span (ft)

Minimum Joist Depth (in.)*

Dancing and Dining

Lively Concert / Sporting Event

Jumping Exercises / Aerobics

12.5

33.0

30.0

27.0

48.0

14.5

36.0

33.0

27.0

16.5

54.0

16.5

39.0

36.0

30.0

18.5

60.0

18.5

42.0

39.0

33.0

20.5

64.0

20.5

45.0

39.0

33.0

24.5

68.0

24.5

51.0

48.0

42.0

28.5

72.0

28.5

54.0

51.0

45.0

*4.5-in. slab thickness plus thickness of joist rib

*4.5-in. slab thickness plus thickness of joist rib

STRUCTURE magazine

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October 2017

When to Select These Systems As discussed in Part 1, parametric studies were performed to determine the conditions under which vibration acceptance criteria were satisfied for flat plate voided concrete systems and two-way joist systems.

The following assumptions were used in the analyses: • Normal weight concrete with f'c =4,000 psi • Grade 60 reinforcing bars • Superimposed dead load = 10 psf • Live load varies from 40 to 100 psf • Actual live load from 6 to 11 psf • Damping ratio = 0.03 For both floor systems, acceptance criteria for walking excitations are easily met. Maximum span lengths that satisfy the acceptance criteria for walking excitations for flat plate voided concrete systems as a function of slab thickness are given in Table 1. Maximum span lengths for two-way joist systems based on total thickness (4.5-inch slab thickness plus the thickness of the rib) are shown in Table 2. In short, acceptance criteria for walking excitations are satisfied for typical flat plate voided concrete systems and two-way joist systems that satisfy minimum requirements for deflection.

Maximum span lengths for flat plate voided concrete systems based on three types of rhythmic excitations are given in Table 3.

Table 4 contains maximum span lengths for two-way joist systems.

Flat plate voided concrete systems and two-way joist systems can satisfy the acceptance criteria for sensitive equipment that have limiting vibrational velocities over an extensive range. Table 5 contains a summary of the maximum span lengths and required slab thicknesses for flat plate voided concrete systems assuming a fast walking pace.

Given in Table 6 are the maximum span lengths of two-way joist systems as a function of minimum total thickness and a fast walking pace.

The information presented in these tables can be used to quickly ascertain whether a flat plate voided concrete system or a twoway joist system is suitable for a given set of constraints. The results from the parametric study are not meant to take the place of a more refined analysis; the main purpose of the study is to provide information that will assist the design professional in making a rational decision on a suitable reinforced concrete floor system for vibrations.▪ The online version of this article contains references. Please visit www.STRUCTUREmag.org.

Table 5. Minimum slab thickness/maximum span lengths for flat plate voided concrete systems as a function of limiting vibrational velocities V.

V Minimum Slab Maximum (μin./sec ) Thickness (in.) Span (ft)

8,000

4,000

2,000

1,000

500

250

130

STRUCTURE magazine

Table 6. Minimum total thickness/maximum span lengths for two-way joist systems as a function of limiting vibrational velocities V.

V Minimum Joist (μin./sec ) Depth (in.)*

Maximum Span (ft)

8.0

16

12.5

24

9.0

20

14.5

27

10.0

24

12.5

28

16.5

30

15.5

32

18.5

33

17.5

36

20.5

36

19.5

40

24.5

45

21.5

44

28.5

51

23.0

48

12.5

18

8.0

16

14.5

21

10.0

20

16.5

27

12.5

24

18.5

30

13.5

28

20.5

33

15.5

32

24.5

39

17.5

36

28.5

45

21.5

40

23.0

44

12.5

15

10.0

16

14.5

18

12.5

20

16.5

21

15.5

24

18.5

24

17.5

28

20.5

27

19.5

32

24.5

33

21.5

36

28.5

39

23.0

40

14.5

15

11.0

16

18.5

21

13.5

20

20.5

24

17.5

24

24.5

27

19.5

28

21.5

32

28.5

33

12.5

16

16.5

15

15.5

20

18.5

18

19.5

24

20.5

21

23.0

28

24.5

24

15.5

16

28.5

27

17.5

18

18.5

15

19.5

20

24.5

21

21.5

22

28.5

24

23.0

24

20.5

15

17.5

16

24.5

18

19.5

18

28.5

21

23.0

20

25

October 2017

8,000

4,000

2,000

1,000

500

250

130

*4.5-in. slab thickness plus thickness of joist rib

Eliminating Fancy Footwork Adding FRP Composite Sidewalks to Vehicle Bridges

By Andrew Loff

“C

an I walk there from here?” It’s a question more and more Americans are asking these days. A renewed focus on health and a cleaner environment are partially responsible. This new emphasis is developing into an emerging trend among Millennials, who demographers estimate to number 79.8 million, and is influencing the way developers look at city planning. Because of technologic advances, a significant percentage of the nation’s largest living generation is choosing to hang up their car keys and walk, participate in a bicycle sharing system, or hop aboard public transportation to jump start their workday. The age group is drawn to community spaces that include walkability, proximity to public transit, and other amenities. Recent studies on the walkability of U.S. cities found that metros which prioritize pedestrian access – including San Diego, California, Cincinnati and Cleveland, Ohio, and Rochester, New York – will provide future development models for the nation’s 30 largest cities.

People are more likely to bike and walk in communities where infrastructure improvements make these activities convenient.

Improvements include the installation of safer sidewalks, pedestrian crossings, and protected bike lanes. The addition of cantilever sidewalks to existing vehicle bridges is an effective option to upgrade structures that did not previously allow for pedestrian or bicycle use.

Conventional materials have made this challenging for engineers working to maintain an aging infrastructure likely to have weight restrictions. FiberSPAN-C, a prefabricated Fiber Reinforced Polymer (FRP) composite sidewalk, is beginning to catch the eye of design firms looking for lightweight solutions.

While adding minimal dead load, prefabricated FRP composite sidewalks give walkers and two-wheeled travelers safe access to shareduse paths on vehicle bridges. Projects like the Wilson-Burt Bridge in Newfane, New York, are helping the emerging technology gain a foothold for acceptance by demonstrating its benefits. This Niagara County bridge was built in 1939 and designed for an H-15 live load.

The 440-foot long multi-span bridge is located a ½ mile upstream from Burt Dam, a regional hydroelectric power generator, and carries Wilson-Burt Road, a primary east-west route for Newfane, across Eighteen Mile Creek.

STRUCTURE magazine

In 1981, the New York Department of Transportation (NYSDOT) overhauled the structure’s original deck, joints, curbs, and railings.

The project also included a small, concrete cantilever sidewalk on the North side. In 2011, the Niagara County Department of Public Works prepared a proposal for a rehabilitation project based on deterioration of paint, joints, the sidewalk, bearings, abutments, and piers. Greenman-Pedersen Inc., of Babylon, New York, was awarded the project.

In 2014, the closing of the Wilson-Burt Bridge began the process to ready the structure for a $4.2 million rehabilitation project geared to restore the bridge to like new condition, reopen the sidewalk (closed since 2009), and employ cost-efficient measures to preserve the structure.

“The primary concern with the bridge was its non-standard load capacity,” says Brian Carlson, a senior structural engineer for Greenman-Pedersen and project manager for the Wilson-Burt Bridge project. “The bridge was designed for an H-15 live load. Today we design for HS-25 or HL-93 live loads, depending on criteria.”

Upgrading bridge capacity and re-opening its sidewalk were key goals for the county, along with addressing the deterioration of the

26

Figure 1. FRP sandwich construction uses redundant fiberglass shear webs to connect top and bottom face sheets for maximum bending properties and reduced weight.

October 2017

Figure 2. New gray W-8 I-beams bolted on top of the original green W-40 beams demonstrate the lightweight FRP product’s design flexibility to increase clear sidewalk width and support modern traffic requirements.

Figure 3. FiberSPAN panels feature an integral cross-sloped surface for water runoff.

existing piers. With urban services available to the south, coupled with industry and recreational opportunities provided by Lake Ontario to the north, proximity to an urban economy has boosted Newfane’s growth. Wilson-Burt Bridge is the largest bridge crossing owned by Niagara County and is an important farm-to-market road for the local economy.

“We wanted to restore the sidewalk, but it had to be lightweight so that it did not impact the vehicular load-carrying capacity of the bridge,” says Carlson. “We researched a variety of sidewalk systems including concrete, steel grids, and timber. We narrowed our selection to Composite Advantage’s FiberSPAN composite product. The sidewalk’s sandwich construction (fiberglass facing skins on fiberglass webs in foam core) and design flexibility demonstrated it was the best choice” (Figure 1).

Carlson says he contacted Composite Advantage about the project and received comprehensive details on panel construction, materials, and installation protocol, information he used to include FiberSPAN in design plans for Wilson-Burt. Greenman-Pedersen then prepared a unique specification for the FRP sidewalk and gained approval from the NYSDOT. The existing sidewalk needed to be widened to 5 feet, 5¼ inches (Figure 2) to support modern traffic requirements. The project specified an allowable live load of 85 psf for the FRP sidewalk, with deflection limits of L/500 between supports and an uplift load of 30 psf. The sidewalk was designed to withstand a temperature differential of 100°F. Floor beam spacing was set at 10 feet, 10 inches, with a deck cross slope top surface of 1.76 percent for efficient water drainage (Figure 3).

Fabricating panels with a cross slope is easier and more cost-efficient than sloping support beams or installing shims. The 22-foot long FRP panels were prefabricated with a shop applied non-slip surface and internal steel connection points to accommodate attaching the railing directly to the sidewalk panels (Figure 4).

“In addition to increasing sidewalk width, the lightweight characteristics of FRP allowed the Figure 4. FiberSPAN connection clip captures material to maintain a the support stringer. dead load that was less

than the sidewalk’s original weight,” notes Carlson. For a 440-foot length bridge span, dead load for the FRP sidewalk was 69,690 pounds. Live and dead load totaled 275,380 pounds. A concrete deck on steel pan and supports would have added an extra 180,000 pounds to the sidewalk’s dead load.

Following an open bidding process, the job was awarded to heavy highway and site contractor EdBauer Construction, West Seneca, New York.

“We had never used FRP before,” says Bill Bauer, president of EdBauer Construction. “But we decided to go with it for several reasons. Composite Advantage performed a significant portion of the engineering layout and design work for the panels, as well as providing detailed installation procedures. They had their design calculations and drawings verified and signed by an engineer registered in New York. After we removed the existing concrete grid deck and exposed the structural steel, they also came out and performed field measurements in order to fabricate the decking to fit conditions.”

Sidewalk installation was slated to follow restoration and replacement activities on the vehicle bridge. Work included complete removal and reconstruction of the upper portion of the span’s three piers, as well as the upper portion of abutment back walls. Other tasks included lead paint removal and the addition of a new 8-inch waterline for residents. The construction crew suspended a corrugated platform underneath the structure to give workers access to the structural steel.

Bridge joints and deteriorated structural steel cross-frames adjacent to the piers were replaced. High rocker bearings were switched out for seismic isolation bearings on new pedestals. EdBauer engaged BIDCO Marine Group based in Buffalo, New York, to perform underwater concrete repairs to the piers while they took to Eighteen Mile Creek on barges to tackle pier repairs above the waterline. After attaching a structural framework to the pier, workers had to jack up the bridge and remove the pier cap to accommodate concrete repairs and reinstall rebar. The labor-intensive process was repeated for each of the bridge’s three piers.

“Access issues are always on the top of our list of concerns when you are working over water,” says Bauer. Temperature and weather conditions were also worrisome. “We felt a bit apprehensive about the FRP sidewalk,” he continues. “We were working late in the season.

Cold temperatures made working conditions less than ideal. With conventional material, rain and cold weather would have presented a problem. We would have had to heat the concrete or structural steel to install the sidewalk. But the FRP panels went in quickly. We had

STRUCTURE magazine

27

October 2017

Figure 5. Prefabricated, lightweight panels make installation quick and easy.

the deck put on in a week. And because they were laid out correctly, the installation went flawlessly” (Figure 5).

FRP panels arrived at the job site on a flatbed truck to make installation easy. The construction crew was able to unload the panels one at a time to create two stacks of 10 that could be installed from the top down, working west to east.

“We did not have to weld structural steel or pour concrete,” continued Bauer. “We only needed a small piece of equipment to move the panels. Because the bridge had been rehabilitated once before, some of the structural steel was not uniform, but the advantage provided

by the product’s flexibility allowed Composite Advantage to come out and make some adjustments before our final inspection.” (Figure 5) Technical coordination and upfront prefabrication eliminated the potential for major fit rework on-site. Installation and deck connections allow for fit-up tolerances on the bridge to accommodate lack of uniformity.

Niagara County, NYSDOT, and Greenman-Pedersen conducted a walk-through inspection after Wilson-Burt Bridge re-opened in November 2015 and found the FRP sidewalk installed according to specification. Subsequent biennial bridge inspections have found the FRP sidewalk performing to specification.

“Now that we have some background in FRP, I’d love to work with the material again,” says Bauer. “The coordinated effort of Niagara County, Greenman-Pedersen, and Composite Advantage minimized any on-site issues. The project was on time and budget.”

FRP material selection helped Greenman-Pedersen overcome the unique challenges presented by restricted loading conditions, the inability of the work site to accommodate large equipment, and the need for traffic closures to be kept to a minimum. In addition to FRP’s lightweight and quick installation, a non-slip aggregate surface supports pedestrian safety. The material’s zero maintenance characteristic also demonstrates its potential for use in similar projects for urban settings. ▪ Andrew Loff is Vice President and Co-Founder of Composite Advantage and is responsible for all technical operations. Andrew can be reached at aloff@compositeadvantage.com.

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The Appleton Pedestrian Bridge A Multi-Use Path Curving to the Banks of the Charles River

By Marian C. Barth, P.E. and William Goulet, S.E.

Plan view rendering by STV.

T

he Frances “Fanny” Appleton Pedestrian Bridge is a 750foot long, multi-use path located on the banks of the Charles River in Boston, MA. This contemporary, curving, “ribbon-like” structure will be a signature bridge for Boston thanks to its geometrically challenging form. It will comfortably carry pedestrian crowds to and from the famous Boston Pops 4th of July fireworks concert. The pedestrian bridge is comprised of 550 continuous feet of elevated steel superstructure, including the 222-foot main span Vierendeel truss arch.

STV provided engineering design services to the WSC-Joint Venture of J.F. White Contracting Co., Skanska, and Consigli Construction Co., taking a preliminary design concept with unique architectural features to a final design concept. The firm’s work addressed fabrication, constructability, and pedestrian induced vibration performance.

Pedestrian Induced Vibrations As one of the main connections to Boston’s landmark Charles River Esplanade, the bridge is expected to attract many visitors, including half a million people for the city’s annual 4th of July fireworks display.

However, one of the structure’s principal features – its iconic slenderness – also makes it prone to vibrations. Controlling those vibrations in order to maintain pedestrian comfort was a critical challenge during the design phase.

The project team turned to SETRA (Service d’Etudes Techniques des Routes et Autoroutes), a technical guide referenced by AASHTO which provides a straightforward method to assess pedestrian induced vibrations. SETRA defines comfort levels based on structural accelerations. During design, multiple iterations were performed to achieve the “maximum” comfort range while minimizing the need for future supplemental measures, such as installing tuned mass dampers.

Unique Features The bridge consists of more than 550 feet of continuous girders that curve in two directions, branching to stairs and a scenic overlook plaza. The structure’s steel fit-up required careful planning during the design phase, and construction over a busy city street necessitated a detailed erection plan. The team evaluated stresses in the members during both fabrication and erection.

Jointless Bridge

Vierendeel Arch span with Wye pier in the foreground. Rendering by STV.

STRUCTURE magazine

A major challenge for this unique bridge was the fabrication of the steel structure and its overall constructability. The preliminary design concept called for a structure with complex curves, with members meeting at tight acute angles, and welded connections at all member connections. The concept included a closed box section for the longitudinal girders in the approach spans and the main arch span. The team evaluated fabrication and erection tolerances and determined that box sections would be difficult to splice due to the squareness of the boxes and the curved nature of the beams and profile.

These challenges were compounded by the appropriate root gaps required for welding these connections, which would prove to be nearly impossible given the thermal movements of the bridge. The

30

October 2017

Viernendeel Arch Span The main arch span, which was comprised of diverging 18-inch diameter pipe arches and 14-inch HSS spandrel columns, was analyzed as a Vierendeel truss. The “top chord” of the truss is a composite concrete deck with steel tub girders. 3-D modeling was utilized to capture stresses of steel erection for the entire superstructure.

During the design phase, the splay angle (degrees from vertical) of the spandrel columns was adjusted to improve vibrational performance. The adjustment from 7 degrees to 15 degrees stiffened the truss span, thereby reducing acceleration and the bridge’s response to vertical and longitudinal loads. This adjustment gave a substantial improvement with no material cost.

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STRUCTURE magazine

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October 2017

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box girders were changed to a tub girder section of similar size. The tubs provided better access for making the connections, allowing a better fit-up of field splices. Because the webs and top flange are independent, internal diaphragms helped maintain the sectional dimensions. The welded splice was changed to a bolted splice, thereby adding flexibility to field drill along one side of the splice.

The bridge’s superstructure has expansion joints only at the abutments, 550 feet apart along the S shaped route. The live loading for the bridge was increased beyond AASHTO’s standard to 120 pounds per square foot to accommodate estimated crowd loading. The combination of the live load case and temperature effects on the steel expansion and concrete deck transmitted considerable loads to the abutments and the Wye piers, which were rigidly connected to the girders.

The mass of the structure was altered, to improve the overall vibrational response of the bridge, by removing the crown from the bridge deck, switching to a lightweight concrete deck, and changing the compressive strength of the concrete deck. Using a 120 pcf lightweight concrete deck, with foam filled stay-in-place forms, provided noticeable benefits: considerable increases in frequencies, minor improvement in acceleration, and substantial decrease in overall load. When combined with other improvements, such as changes to the steel framing, it shifted the structure out of the susceptible range.

Wye pier casting. Courtesy of Cast Connex.

Tub girder and fascia assembly. Courtesy of Newport Industrial Fabrication.

along the entire length of the concrete deck reveal. While acting as a concrete formwork during construction, the fascia is also architectural.

The fascia plates were assigned as AESS and specified with tighter tolerances to achieve fit-up that will meet welding criteria and decrease the chances of localized waves. The fascia continues the ribbon appearance along the stairs; however, at that point, it becomes an integral side of the structural stair fascia box girder carrying the stair tread loadings.

This member was redesigned to a thinner plate cross-section, making it easier for the fabricator to form. The majority of the plates are curved in at least two planes to meet the defined geometry.

The depth of the fascia box members for the stairs also changed from the original concept. The redesign allowed for easier detailing of connections and fabrication while increasing the stiffness of the lateral restraint that the stairs provide to the main span. Although increased horizontal stiffness was not required based on AASHTO or SETRA criteria, limiting the sway of the main span was a significant improvement. The increased stiffness of these members also reduced the deflection of the structure under service wind conditions. The fascia box girder changes to the stairs improved performance and required no additional material while maintaining the unique aesthetics of the bridge.

Another consideration for improving the vibration performance of the structure was revising its foundation stiffness. Original micro piles were changed to H-piles, resulting in a stiffer axis. Two pier foundations were changed to a three pile foundation with battered piles, altering the frequency of the west approach and lowering the overall response of the main span. Additional battered pile foundations were designed for the stairs which act as a strut, providing lateral restraint to the main span.

Conclusion The Fanny Appleton Pedestrian Bridge uses architectural elements including a ribbon-like fascia running the extent of the bridge, unique Wye piers, and a slender arch span. STV worked closely with members of the design-build team to include cost-effective design modifications, to meet the architectural objectives, and improve constructability and the user experience.

The bridge is currently under construction and is estimated to be completed in mid 2018.▪

Wye Pier The architectural concept included steel Wye approach piers with an inset detail. This geometrically challenging form, which included a 4-inch radius within the arms, led the team to choose a steel casting to create the unique detailing. The base of the pier was formed as a built up section, and the casting was welded to the base. The top cap plates were provided with alignment studs for the tub girders to facilitate weld fit-up.

The piers’ welded connection to the superstructure created stresses within the piers due to temperature expansion and contraction of the girders. The foundation was modeled with an upper and lower range of estimated stiffness to make sure that the pier columns would perform as desired during temperature fluctuations. A separate check using rigid supports was also conducted to determine the effects of the ground freezing during the winter. Pier stiffness and flexibility were then adjusted as needed to meet the acceptable stress range for each pier. Adjustments included: stiffening with internal plates located at the base of the piers, increasing flexibility by revising local grading to increase pier heights, and setting the piers at each end of the structure on bearing pads to allow for rotation at the pile cap interface.

STRUCTURE magazine

32

Marian C. Barth, P.E., is a Project Manager/Senior Structural Engineer with STV Incorporated. She can be reached at Marian.

Barth@stvinc.com.

William Goulet, S.E., is a Project Engineer at STV Incorporated.

He can be reached at William.Goulet@stvinc.com.

Project Team Owner: Massachusetts Department of Conservation and Recreation. Undertaken as part of the Longfellow Bridge Rehabilitation Project by Massachusetts Department of Transportation-Highway Division Engineer of Record: STV, Boston, MA General Contractor (Design-Build): WSC-Joint Venture of J.F. White Contracting Co., Skanska, and Consigli Construction Co., Boston, MA Steel Fabricator: Newport Industrial Fabrication, Newport, ME with steel castings designed and supplied by Cast Connex Corporation, Toronto, Canada

October 2017

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W

hen an unrestrained posttensioned member is stressed, the member will shorten because of the pre-compression imparted by the stressing. If column and wall supports restrain this shortening, part or all of the pre-compression intended for the posttensioned member is diverted to the supports.

The loss of pre-compression to the supports leads, in turn, to a reduction of the member’s moment capacity. This two-part article describes the mechanism of the loss in moment capacity of post-tensioned members due to support restraint and identifies the salient differences between members reinforced with unbonded tendons and those reinforced with bonded tendons. It shows that the moment capacity of members with bonded tendons is less influenced by support restraint, and the moment capacity of members with unbonded tendons relies heavily on the friction developed between the tendon and its sheathing.

Restraint to Shortening Unless the columns and walls that support a post-tensioned member are extremely flexible, they restrain the free shortening of the member when the post-tensioning tendons are stressed.

The member does not receive the full amount of design pre-compression from the tendons if it is not allowed to shorten without restraint. If the supports prevent all shortening, the entire post-tensioning force is diverted to the supports, leaving the member with no pre-compression.

Failure to account for this pre-compression decrease can lead to cracking when tensile stresses develop due to the expected and unavoidable shrinkage of the concrete. In addition to possible aesthetic objections, restraint cracks can allow leakage and can expose the reinforcement to corrosive elements. More importantly, restraint cracks can reduce the contribution of the posttensioning tendons to the strength capacity of the member.

The extent of the restraint cracking in a posttensioned member depends on several factors including the stiffness of the supports, which is the focus of this article. Figure 1 illustrates two extremes. In Figure 1a, a post-tensioned member on very flexible supports shortens under the pre-compression, forcing the supports to follow the member’s movement. This forced bending can cause cracking in the supports. At the other extreme, in Figure 1b, a member on very stiff supports is prevented from shortening; restraint cracks can develop in the member as it shortens due to shrinkage of the concrete.

Figure 2 (page 36) shows a typical restraint crack. Restraint cracks are most pronounced at the first level of a concrete structure, due to the restraint from the foundation. There is less cracking at higher levels. Restraint cracks are often long in comparison to span lengths; they typically extend beyond the length of a panel and through the entire depth of the member.

They occur at points of weakness, such as where non-prestressed reinforcement is reduced or terminated, or where there is a reduction in the member’s cross-sectional area.

Experienced design engineers are aware of the possibility of restraint cracking and its consequences. They use a number of measures that allow the post-tensioned member to shorten while minimizing the cracking and its effects in either the member or its supports.

Structural PracticeS practical knowledge beyond the textbook

Support Restraints and Strength of Post-Tensioned Members Impact of Support Restraint on Member Strength Figure 3 (page 36) illustrates the mechanism by which post-tensioning tendons contribute to the strength of a member when there is no support restraint. This will be contrasted to the case in Figure 4 (page 36), where the member is subject to support restraint.

For the member in Figure 3, the strength demand at the cut section shown in Figure 3b consists of

Part 1

By Bijan O. Aalami, Ph.D., S.E., C. Eng.

Bijan O. Aalami is Professor Emeritus at San Francisco State University and Principal of the ADAPT Corporation.

He is the author of "PostTensioned Buildings; Design and Construction." He may be reached at bijan@adaptsoft.com.

Figure 1. Effects of support restraint on member cracking.

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Figure 2. An example of a restraint crack. Cracks typically extend through the depth of the member.

Figure 3. Post-tensioned member with no support restraint to shortening.

Figure 4. Post-tensioned member with support restraint. F3 is the support restraint, which is modeled with a spring as shown in part (b).

the moment (M ), shear (V ) and axial force (N ). The demand actions M, V, and N are in static equilibrium with the forces acting on the segment of the member. For the safety of the structure, the resistance that develops at the face of the cut from the forces T, C, and V should not be less than the demand actions M, V, and N. Since the member is assumed to be on rollers, the reaction at the support shown in Figure 3b is limited to a vertical force. There are no horizontal restraints at the supports, so there is no horizontal force demand for the cut segment (N = 0 in Figure 3b). The forces developed at the face of the cut must balance the force demand for equilibrium of the segment, namely V, M, and N. The resistance to the demand moment (M ) at the face of the cut is developed by the tendon force (T ) and the compression force (C ) in the concrete: T=C Equation 1 M = Tz Equation 2 Where z is the moment arm of the forces at the face of the cut. Because there is no restraint to shortening from the supports, the entire tendon force T is available to resist the demand moment M. In Figure 4, the member is attached to supports that restrain it from shortening when the tendons are stressed. The following definitions apply to Figure 4 and the remainder of this article: F = force in the tendon at ultimate limit state (strength condition); F2 = force in the tendon at service condition; F3 = restraint of support at service condition.

The member is modeled as shown in Figure 4b with the springs attached to each end of the member representing the restraint of the supports to the shortening of the member. When the tendons are stressed, the supports absorb part of the post-tensioning force, marked F3 in Figure 4c, where the magnitude of F3 depends on the stiffness of the supports. Note that there is a moment at the end of the member due to the shift of the restraining force (F3) at the support from the support/member interface to the centroid of the member, as shown in Figure 4b. However, since this moment is not relevant to the current discussion, it is not shown in the figure.

The procedure followed above for the member in Figure 3 will be used to determine the contribution of the tendon force to the safety of the restrained member. Figure 4c is the free body diagram of the left segment of the member. The demand actions for the equilibrium of the segment are, as before, M, V, and N. In this case, however, from the equilibrium of the forces in the horizontal direction, we have: N = F3 Equation 3 Thus, in addition to the moment (M ) and shear (V ), there is a net axial tension (F3) that must be resisted by the actions developed at the face of the cut. Equilibrium of the forces on the segment requires that: C = F2 – F3 Equation 4 Hence, the resisting moment at the face of the cut is: ~ (F2 – F3 )z Equation 5 M= The approximation sign (~) is used because while the force (F3) actually acts at

the interface between the support and the member, it is assumed to act at the centroid of the member for this discussion. The discussion is a simplification of the mechanism for the development of resistance in a posttensioned member, designed to show the effect of support restraint.

In Figure 4, the force in the tendon under service conditions is shown as F2. With an increase in the applied load, there is an increase in tendon strain, which in turn results in an increase in tendon force. At the ultimate limit state, the force in the tendon is F2 + δF2, where δF2 is the increase in tendon force due to elongation of the tendon. The amount of the increase depends on whether the tendon is bonded or unbonded. For bonded tendons, the increase is localized at the crack and can bring the tendon’s stress to its ultimate strength (fpu). For unbonded tendons, the increase in force is typically considerably less because the increase in strain is distributed along the length of the tendon.

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Conclusion In summary, when a member is restrained at supports, the post-tensioning force available to resist the demand moment (M ) is reduced.

The magnitude of this reduction depends on the relative stiffness of the restraining supports and the post-tensioned member. The second part of this article, in an upcoming issue, will examine the case of significant support restraint where the entire post-tensioning force is diverted to the supports, leading to restraint cracks.▪

InSIghtS new trends, new techniques and current industry issues

T

he increased use of drone technology over the last several years has had a positive impact on the field of civil engineering. Drone-based sensing technologies enable engineers to inspect largescale infrastructure systems faster, and from angles that were previously inaccessible. Drones equipped with reality capture technologies, such as cameras and scanners, promise to be an effective apprentice to inspectors and engineers. These “assistants” will be collecting spatial data about large infrastructure systems at the resolution needed to give engineers a complete picture of infrastructure conditions during both construction and inspection.

When pieces of essential infrastructure, like bridges, need to be inspected or repaired, the process often causes significant disruptions in the community. Shutting down a full bridge, or even just a few lanes, for the weeks or months

provide comprehensive data about the full spatial condition of an infrastructure system.

As drone mounted reality capture technologies have advanced, it has opened the door to safer, more efficient, and more accurate methods of data collection to support inspection. These aspects enable such systems to act as the inspectors’ apprentice, supporting many decisions made throughout the life cycle of an infrastructure system.

Research conducted at Carnegie Mellon and Northeastern University, called Arial Robotic Infrastructure Analyst (ARIA), aims to streamline these drone-based data capture technologies and create an effective engineer’s apprentice. This interdisciplinary multi-university research uses a tabletop-sized drone, with onboard photo and video capture techniques and state of the art laser scanners, to create a high-resolution, fully immersive 3D model of a bridge, which can then safely be analyzed by an inspector on the ground. The goal is to perform an overall assessment quickly and alert inspectors to particular problem areas. From there, the human inspector can go in and more thoroughly assess problem areas, without having to inspect every inch of the bridge physically.

This process allows the collection of inspection data much more quickly, safely, and autonomously. From the ground, an inspector can instruct the ARIA drone to collect data covering the entirety of an infrastructure system or focus on just a specific portion. Algorithms then process the collected point cloud and image data and identify problem areas on which inspectors/ engineers can concentrate. This way, inspectors/ engineers can not only do a virtual inspection but also have access to processed data that highlights possible damage. Because the drone does not need to use the roadway, bridges can remain open during the inspection, and any necessary shutdowns are dramatically shorter. Moreover, it is not only bridges; ARIA can be used to inspect dams, transmission lines, or any complex, largescale infrastructure where speed and safety are a high priority.

Shaping the Future of Structural Engineering Humans and Drones Must Work Together

By Burcu Akinci, Ph.D.

Dr. Burcu Akinci is a Professor of Civil & Environmental Engineering at Carnegie Mellon University in Pittsburgh and currently serves as the co-director of the Pennsylvania Smarter Infrastructure Incubator. She can be reached at bakinci@cmu.edu.

needed for manual inspection, data collection, and assessment can have negative impacts on traffic patterns and greatly increase travel times.

As such, the general practice up until this point has been to perform these checks only during low traffic periods, resulting in stretched out project timelines and turning what could normally take a week into months of on and off disruption.

Traditional bridge inspection methods come with safety risks. Surveying the underside of a bridge often requires hanging inspectors over the side using service cranes, sometimes hundreds of feet above the ground. On top of that, many parts of the inspection rely on the inspector’s judgment and experience to accurately assess the bridge’s condition, rendering the collected data inherently subjective. Finally, manual data collection targets specific areas where issues are observed or considered most likely and naturally cannot

ARIA robot fitted with various sensors and an onboard computer, to aid autonomous navigation as well as collection of laser scans and images.

38 October 2017

inspection times, will only be possible if policymakers choose to keep up with these new capabilities.

In addition to time and safety concerns, the sheer wealth of collected data from drones will allow those in charge of maintaining the infrastructure to focus their energy better – and their money – on the areas of most desperate need. It is possible to make operations, View of the steel girder railroad bridge at the Robot City Roundhouse maintenance, and retrofit (Hazelwood, PA), used as one of the testbeds in this research project. investments most effectively As this technology proliferates, regulations utilizing accurate and more comprehensive data. will need to change not only as they relate to While the benefits do show great promise for drone usage, but in other aspects of inspection the future of inspection, the technology is not as well. Under current practices, a human without its downsides. When large amounts of inspector must be in physical contact with a data are introduced, people tend to over-rely on bridge during an inspection for it to be con- the data. In this case, the worst thing you can sidered reliable. However, as drones become do is to assume that the technology is 100% able to not only fly and scan, but to latch onto comprehensive and accurate – because it may an infrastructure system for other kinds of not be. ARIA is not designed to replace a human sensing as well, inspectors could perform full inspector’s expertise and analysis, but instead to inspections remotely, using the drone or other be a more objective and comprehensive data robotics systems as their eyes, ears, and hands. collection and assessment tool for engineers.

This kind of advancement, which would fur- As with any technology, when people blindly ther increase safety for inspectors and decrease follow the data, the margin for error increases.

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For example, if the drone were to create a full scan of the bridge, there may be small areas of the bridge, either underneath or between supports, which may be occluded from the drone’s view. Any damage in these areas would not be reflected in the data. If inspectors were to take the data at face value, instead of using it as a starting point from which to complete a faster and more accurate inspection, they would report no damage to the bridge, when in reality, it is entirely possible there is damage.

Hence, inspectors need to be equipped with a good understanding of what these technologies can or cannot do in order to make the most of the technology advantage while not jeopardizing assessment results.

Future adopters of drone technology will have to strike a balance. These tools have the ability to provide faster, safer, and more accurate data. However, they are just that – tools. If we allow the data to replace the inspector, we run the risk of misrepresenting bridge health, defeating the purpose of data collection, and allowing infrastructure conditions to worsen. If used properly, however, drone technology can help us maintain large infrastructure systems with greater speed, safety, and reliability – at a much lower cost.▪

Professional liability

issues affecting the structural engineering profession

Do’s and Don’ts During Construction

By G. Daniel Bradshaw, CPCU, MBA

W

hile working with design professionals for the past 29 years as a Professional Liability Agent, the author has learned a great deal about the construction observation process. The most important lesson – how structural engineers can improve their construction observation processes to reduce risk and reduce the chance of being involved in litigation. This article examines common issues structural engineers frequently encounter during construction and offers suggestions for improvement.

Pre-Site Visit Issues Substitutions Contractors love to make substitutions. Because substitutions during construction may pose a risk to the project and the design team, structural engineers need to have formal procedures in place to control the number of substitutions and define a method for evaluating them. The formal procedure must clearly define the process that a contractor must follow to request and obtain approval for a substitution. The approved procedure should require a formal submittal which includes the manufacturer’s literature, complete product specifications, test data, and approvals for each proposed substitution. It should also include a statement containing reasons and justifications for requesting the substitution.

Value Engineering Often projects receive some form of value engineering. The author’s clients describe the best time for value engineering as early in the design process; however, it is surprising to learn that projects usually get some form of “value engineering” before or during construction. In most cases, the value engineering is done out of desperation by an owner or contractor to save costs.

In many cases, when value engineering is done during construction, the cost-saving substitutions become the subject of costly claims later on. Unfortunately, too often, an owner spends a lot of money and time to fix a problem after construction begins by hiring experts, lawyers, and so on. However, they are not willing to spend money or time for a proper solution during design. Structural engineers need to continually educate owners, contractors, and architects that real value can be obtained if value engineering is conducted during design and not as a cost reduction during construction. Structural engineers

also need to call the activity value engineering at the end of the design process and during construction “cheapen engineering,” as that is often what it actually is.

Communications XL Catlin, a professional insurance provider, gathers information each time a professional liability claim is presented. This information forms the basis of a very large database of claims that provides insights into the causes of claims. For each claim, they strive to determine which “nontechnical” areas fit the claim. “Non-technical” issues are typically the things an engineer does in his or her practice that were not taught in school. A large “non-technical” issue identified in over 35% of claims is poor communication.

Structural engineers should strive to improve communications on projects by implementing a weekly status report. This report can be sent to the appropriate people inside and outside the firm, and clients and owners. It highlights the status of the project, items of concern, and rationale for all decisions that were made to-date.

Issues That Arise While on a Site Visit Do Not Be Forced to Make Quick Decisions One of the author’s first lessons as a young agent working with a seasoned Architect was to never make a decision quickly when pressed by a contractor or a client over the phone, during meetings, or during a site visit. This architect pointed out that he had learned that, every time he was pressed to make a change, he later had to change his mind and tell the contractor or others that the proposed change would not work. Too often something that looks attractive while under pressure to make a decision can quickly become a problem later. Better answers, he said, are found after hours of contemplating solutions to the problem and considering their impacts on the overall design. He recommends telling the contractor and client that he would take the issue under advisement and a decision will be made in approximately 24 hours, if the change is feasible. This gives time to review the change and consider what impacts it might have on other aspects of the design.

Always Walk the Site with the Contractor When conducting a site visit, check in at the site trailer and with the project superintendent.

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If possible, conduct the visit with a person who has the authority over the project or site. Doing so ensures that any issues that are discovered are also observed by an authority figure responsible for the construction.

Photographs Should Focus on the Problem Photographs are valuable tools and provide reminders from a site visit. Photographs can increase an engineer’s exposure to risk if used incorrectly. Do not take pictures of the overall site which may include problems that were not noticed. Photographs should focus only on the problem areas identified at the time and that need to be emphasized. Sometimes, an overview picture of surroundings or assemblies are needed to provide context, but always zoom in and focus only on the specific issue that is of concern.

Write the Construction Observation Report as Soon as Possible Take notes during the site visit, and then commit the site visit report to paper or a computer file as soon as possible so that any observed issues are fresh and easy to recall. A template can help record routine items such as the date, the weather, trades on the site during the visit, and other general items. Steer away from generalizations such as “everything looks great,” and use detailed descriptions about the items to describe any issues observed and how they should be corrected. When writing the site visit report, always be specific and clear.

Go Through Proper Channels to Stop Work When construction specifications and details do not appear to have been followed, do not be quick to issue a stop work order. Typically, the engineer’s contract is silent on authority to stop work. Contractually and practically, it is better to have the right to “advise the client” to stop the work when it appears construction does not meet the design specifications. Seek other ways to keep other parts of the project on schedule. Stopping the work outright can have huge financial consequences for the owner, contractor, and structural engineer.

Special Inspections Special inspections may be required by the project specifications. However, promptly receiving the special inspection reports can be a challenge.

A time frame for receiving special inspection

reports should be addressed in the construction documents and during a pre-construction meeting. As a tool for improving communications on a project, a Time Frame Chart and a Communication Hierarchy chart can be more useful than just a written document specifying the procedure and time frame to be followed.

Safety Issues

What to Do if You Spot Trouble Often, Professional Liability Carriers can provide behind-the-scenes assistance to diffuse many types of problems. Tools, like an informal mediation session with the parties early on, can diffuse an issue that could become the subject of a costly Professional Liability Claim. Hiring another expert to review the structural engineer’s work is a good option when intense finger pointing

about a design issue arises. Having an attorney review a particular stance that was taken on an issue can provide an engineer with confidence to stand up for what they believe is the right course of action.

Keep in mind, as symptoms of problems are recognized, if the client is demanding money or services to “fix a problem,” this request might also meet the definition of a Professional Liability claim. It may trigger a duty to put the structural engineer’s insurance carrier on notice. When in doubt, contact a Professional Liability Agent or Broker, tell them what has been recognized, and seek their advice on what to do next.

Using some of the ideas in this article and ideas from other professionals, an engineer can develop a standardized approach to site visits, site reports, and how to spot trouble areas. This standardization helps avoid problems and can lead to more effective site visits and procedural consistency.▪ G. Daniel Bradshaw, is a professional liability specialist in Bountiful, Utah.

He has served on the board and is a Past President of the Professional Liability Agents Network (PLAN), an association of agencies and brokerages serving design firms in the U.S and Canada.

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During a site visit, safety issues may be observed.

How they are dealt with is a critical procedure.

Most structural engineers have very good practical knowledge and no experience or background in OSHA or other safety training. If there is a construction related injury, everyone with anything to do with the project could find themselves at risk to the injured worker. It is likely that the contract the structural engineer is working under contains no responsibility for job site safety or contractor means and methods. This is the reason why the method in which an engineer deals with an apparent safety issue on the site is so important. If safety issues are noticed, the structural engineer should report them to the on-site project superintendent, using language that clearly states that the observation was made by a professional who is not an expert in safety or means and methods. The site visit report must also include some detail about what was observed that appeared unsafe, what, if anything, the structural engineer did, and who was alerted about the issue. The report should be sent to the structural engineer’s client as soon as possible, so they are aware and can make appropriate notifications to the contractor. However, beware that the act of reporting does not change the structural engineer’s contractual role and reclassify that role as an inadvertent onsite “safety expert.” Safety on the site is a contractor responsibility, so it is important that an engineer not assume that role. This area of job site safety should be addressed at the firm level and procedures established within the firm so that each person visiting a site acts in the same manner to perceived safety issues.

• Construction costs over budget.

• Project construction behind schedule.

• The client or their representatives exhibit a curt attitude toward the structural engineer.

• An abnormal amount of RFI requests from the contractor.

• Payment issues, such as the owner not making timely payments to the design team or contractor, or the architect not paying sub-consultants in a timely fashion.

• Turnover of project superintendents.

• Turnover of project managers within the design team.

• The client hires another engineer to look at issues on the project.

Recognize Problems Early A significant challenge for many structural engineers is recognizing symptoms of problems during construction that have the potential to blow up into bigger problems, or worse, a professional liability claim. The following are good reference points that signal if project issues are spinning out of control: • Finger pointing; various trades or sub-consultants blaming others for problems.

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news and information

noteWortHy

Dilip Khatri Retires from STRUCTURE’s Editorial Board

D

ilip Khatri, Ph.D., S.E., is stepping down as a member of the STRUCTURE magazine Editorial Board. Dilip joined the Editorial Board in the fall of 2012 as one of NCSEA’s representatives. Dilip is the Principal of Khatri International Inc, Civil and Structural Engineers, based in Las Vegas, NV, and Pasadena, CA. He was a Professor of Civil Engineering at Cal Poly Pomona for 10 years.

Barry Arnold, P.E., S.E., SECB, Chair of the STRUCTURE magazine Editorial Board, had this to say about Dilip’s departure: “Dilip has served faithfully on the Editorial Board for 5 years. Dilip’s dedication and commitment to the magazine and the profession are commendable and he will be missed.”

Regarding his tenure on the Board, Dilip commented, “Working with STRUCTURE magazine has been a truly rewarding experience. I have learned so much and made new friends in the process. My time on the Board has sharpened my writing and editorial skills for use in future endeavors. I am grateful to this Board for welcoming me and allowing me to grow professionally. Thank you sincerely to this great profession that has done so much for so many.”

seisMiC/WinD GUiDe

Timothy M. Gilbert, P.E., S.E., SECB, will replace Mr. Khatri as an NCSEA representative.

Timothy is a Project Specialist for TimeknSteel Corp. in Canton, Ohio, who has been practicing engineering for over 30 years. In his current role, he is responsible for overseeing the civil and structural engineering for new facilities and facility modifications. Timothy also is a member of the Structural Engineers Association of Ohio (SEAoO) and is currently Past President and Chair of the Licensure Committee. He is a corresponding member of the ASCE Committee on Licensure and active with the ASCE-SEI Professional Activities Committee.

Barry said this about Mr. Gilbert’s appointment to the Editorial Board: “I am pleased to welcome Tim to the Editorial Board. He brings plenty of experience with writing and editing, and comes highly recommended by his peers. The readers of STRUCTURE magazine will recognize Tim’s name because of the many articles he has had published in the magazine. He will be a wonderful addition to the STRUCTURE magazine team.”

Please join STRUCTURE magazine in congratulating Dilip Khatri on his service and welcoming Timothy Gilbert to the team.▪

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for full descriptions.

Resource Guide forms for the remaining 2017 and new 2018 Editorial Calendars are now available on the website, www.STRUCTUREmag.org .

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STRUCTURE® magazine is not responsible for errors.

2017 NCSEA Special Awards Honorees

NCSEA News

News form the National Council of Structural Engineers Associations

The following awards will be presented at the Awards Banquet on October 13th during the 2017 NCSEA Structural Engineering Summit in Washington, D.C.

James M. Delahay Award

NCSEA Service Award

The James M. Delahay Award is presented at the recommendation of the NCSEA Code Advisory Committee to recognize outstanding individual contributions towards the development of building codes and standards. It is given in the spirit of its namesake, a person who made a long and lasting contribution to the code development process.

The NCSEA Service Award is presented to an individual or individuals who have worked for the betterment of NCSEA to a degree that is beyond the norm of volunteerism. It is given to someone who has made a clear and indisputable contribution to the organization and therefore to the profession.

Michael O’Rourke, Ph.D., P.E., received his B.S. in Civil Engineering from the Illinois Institute of Technology and his M.S. and Ph.D. from Northwestern University.

During most of his 43 years on the faculty in Civil Engineering at Rensselaer Polytechnic Institute, he has been involved in snow load research sponsored by the U.S. Army Cold Regions Research and Engineering lab, the National Bureau of Standards, the National Science Foundation, the Fire Protection Research Foundation, the Metal Building Manufacturers Association, and FM Global. This research work has resulted in publication of three dozen snow loading papers in refereed journals and conference proceedings. He is the author of four Guides to the Snow Load Provisions of ASCE 7 (7-02 thru 7-16) and co-author of the recently released “Snow Loads for Solar Paneled Roofs”.

James O. Malley, S.E., is a Senior Principal with Degenkolb Engineers.

He received both his Bachelors and Masters Degrees from the University of California at Berkeley. Mr. Malley has over 30 years of experience in the seismic design, evaluation, and rehabilitation of building structures.

Over his career, Jim has been deeply involved in serving the profession through structural engineers associations and related technical committees. He is former President of the NCSEA Board of Directors (2010-2011), served as SEAONC President in 2000-2001, SEAOC President in 2003-2004, and was named a SEAOC Fellow in 2007. Most recently, he served on the search committee for the new NCSEA executive director. James Malley is active in a number of other organizations, including currently serving as vice-president of EERI. His involvement and eagerness to share his knowledge demonstrates his service and dedication to the profession.

Robert Cornforth Award

Susan M. Frey NCSEA Educator Award

The Robert Cornforth Award is presented to an individual for exceptional dedication and exemplary service to an NCSEA Member Organization, as well as to the structural engineering profession.

The Susan M. Frey NCSEA Educator Award, established to honor the memory of Sue Frey, one of NCSEA’s finest educators, is presented to an individual who has a genuine interest in, and extraordinary talent for, effective instruction for practicing structural engineers.

Theodore E. (Ted) Smith, P.E., S.E., Co-founder of Smith & Huston, Inc., Consulting Engineers (previously Ballinger & Smith Consulting Engineers), graduated from the University of Washington in 1971 with a BSCE. His early work focused on commercial and residential structures, and later grew to include investigations of structural damage and defects.

Mr. Smith recently completed his term as SEAW President after previously holding the position for the Seattle chapter. He served SEAW and the Seattle Chapter as Vice President and was named the Seattle Chapter’s Engineer of the Year. He was a member of SEAW’s Structural Exam Committee for over 30 years, chaired the Membership Committee for ten, and helped plan conferences while on the Planning Committee.

He is also a founding member, and current Treasurer, of the Structural Engineers Foundation of Washington, an organization through SEAW dedicated to furthering structural engineering through scholarships, education, research, and outreach.

Ed Huston, P.E., S.E., is a Principal of Smith & Huston, Inc., Consulting Engineers and has over 40 years of experience in structural design, evaluation, investigation, and code and standards development.

Ed is former President of the Board of Directors of NCSEA and chair of the Code Advisory Committee – General Engineering Subcommittee. He has served as President of the Board of Direcctors of the Applied Technology Council (ATC). He was the Lead Technical Consultant for the development of ATC 45, A Field Manual for Safety Evaluations of Buildings after Windstorms and Floods.

Ed is frequently asked to speak on wind, seismic, masonry, wood, code development, and liability issues. He is a soughtafter speaker for his ability to draw on his career as a practicing engineer and as a contributor to the code-development process to illustrate the ways that our professional codes and standards affect the practicing engineer.

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October 2017

NCSEA Structural Engineering Engagement and Equity Committee

NCSEA has established a new Resilience Subcommittee of its Code Advisory Committee to represent structural engineers within the emerging resilience movement. The committee will develop materials for engineers and SEAs and will engage with efforts by NIST, FEMA, ASCE, ICC, AIA, and others. The committee will operate with one Chair and up to two Voting Members from each of the four regions.

The eight initial voting members that have been selected are: • East: Chris Cerino (NY), Rebecca Laberenne (NY) • North: Chad O’Donnell (WI) • South: Cliff Jones (TX), Scott Lawson (GA) • West: Josh Gebelein (CA), Holly Janowicz (CO), Mark Pierepiekarz (WA) Now that voting members have been chosen, each SEA will be invited to name a non-voting Corresponding Member. The committee will hold their first meeting at the NCSEA Summit on Wednesday, October 11th.

More information about the committee can be found at www.ncsea.com/committees/resilience or by emailing, Resilience Committee Chair, David Bonowitz, at dbonowitz@att.net.

NCSEA Supports NEHRP Reauthorization The Earthquake Engineering Research Institute (EERI) has worked with California Senator Diane Feinstein’s office to draft language for a Reauthorization Bill for the National Earthquake Hazard Reduction Program (NEHRP) that was recently introduced to Congress. The Senator’s office has incorporated feedback from many organizations with varied points of view, and engaged Republican Co-sponsorship.

The NEHRP program, now 40 years old, has been instrumental in coordinating and advancing our federal government’s efforts to advance science and engineering associated with earthquakes, develop and implement techniques to make our nation safer and more resilient, and support the efforts of states, communities, and the public to be prepared for earthquakes.

NCSEA supports this bill and agrees that reauthorization is needed to provide important updates and to highlight the importance of earthquake hazard mitigation research, outreach, and implementation.

NCSEA Webinars

This California Office of Emergency Services (CalOES) Safety Assessment Program, presented by NCSEA, is one of only two post-disaster assessment programs that will be compliant with the requirements of the Federal Resource Typing Standards for engineer emergency responders.

Register now on www.ncsea.com!

Earn PDHs while Reviewing SE Basics!

October 24, 2017 Understanding and Interpreting Geotechnical Reports Trent Parkhill, P.E. November 9, 2017 Findings from the SEAONC Structural Engineering Engagement and Equity (SE3) 2016 Study Nick Sharrow-Groves, P.E. and Angie Sommer, S.E.

NCSEA’s SE Review & Refresher Course is available as recordings! The entire course can be purchased as a complete bundle (Vertical and Lateral), Lateral or Vertical Packages, or as single recordings. This online review course features over 28 hours of up-to-date information and offers PDHs when you view recordings (except in New York). Developed by NCSEA and leading structural engineers, this costeffective course aims to refresh engineers on the foundations of Structural Engineering while also preparing for the SE Exam.

December 7, 2017 Special Inspection for Wood Construction - An Overview for Engineers and Inspectors Tim Hart, S.E., LEED AP Visit www.ncsea.com for descriptions & registration.

Courses award 1.5 hours of continuing education after the completion of a quiz. Diamond Review approved in all 50 States.

STRUCTURE magazine

CalOES Safety Assessment Program - November 3, 2017

Visit www.ncsea.com for more information.

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October 2017

News from the National Council of Structural Engineers Associations

NCSEA is proud to announce the creation of its newest committee, the Structural Engineering Engagement and Equity (SE3) Committee. Started by the Structural Engineers Association of Northern California (SEAONC) in 2015, SE3 is expanding to the national level for the profession’s collective benefit. The national SE3 Committee will continue the work of investigating engagement and equity in the structural engineering profession, but aims to broaden the group’s scope.

This committee will focus on improving the engagement and retention of structural engineering professionals and promoting equity within the profession. Their mission is to enhance the future of structural engineering and facilitate a discussion forum for engineers to seek out and provide mentorship, support, and share stories of work culture.

SE3 needs leaders and advocates from across the country to lend their unique voices to develop resources. Please contact the committee at se3@ncsea.com to get involved and to join their monthly phone conferences. Learn more about the NCSEA SE3 committee at www.ncsea.com/committees/equity.

Resilience Committee

NCSEA News

New NCSEA Committees Announced

Structural Columns

The Newsletter of the Structural Engineering Institute of ASCE

SEI Online

Participate in the SEI Trial Design Problem on Snow Loads by October 15 To test your practice, and help improve codes and standards. A good exercise to organize at a firm lunch or local group activity.

View the problem, submit your solution, and encourage your colleagues to participate. http://bit.ly/2j1HBG3

Work Faster and Smarter in ASCE 7 Online A new interactive, user-friendly platform; saves time and money. Features such as redlining to quickly spot changes in current and previous editions, corporate and personal note taking, and advanced search features combine to make it easier to work with. Individual and corporate subscriptions available.

Contact asce7tools@asce.org.

The ASCE 7 Hazard Tool has Launched!

Basic and premium access are available. Learn more at asce7hazardtool.online.

Apply for O.H. Ammann Research Fellowship in Structural Engineering by November 1

Awarded annually to encourage new knowledge in structural design and construction. Award is at least $5,000. Learn more and apply at www.asce.org/structural-engineering/ammann-research-fellowship.

Learning / Networking

Check out Presentations by ASCE 7 Leaders • October 9 at the ASCE Annual Convention in New Orleans, www.asceconvention.org.

• October 12 at the NCSEA Summit in Washington, D.C.

We are planning a great program for you – details coming soon at www.structurescongress.org!

Sign up now to exhibit and/or sponsor at Structures Congress, and the Electrical Transmission & Substation Structures Congress November 4 – 8, 2018, in Atlanta. Maximize your visibility in advance of the event and with more than a thousand industry professionals on site. Contact Sean Scully at sscully@asce.org or 703-295-6154.

SEI Student Career Networking Event April 20 at Structures Congress in Ft. Worth

Employers: Sign up now to participate, be included in event promotions, and receive student profiles in advance of the event. www.asce.org/SEI-Sustaining-Org-Membership.

Students: Plan now to connect one-on-one with employers for SE positions and internships. Learn more at www.asce.org/SEI-Students.

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October 2017

Survey for Young Professionals (35 and younger) Participate in the SE Education and Practice Survey to identify and better serve your needs at http://bit.ly/2ftmibV. Take advantage of these programs made possible through the generous support of the SEI Futures Fund www.asce.org/SEIFuturesFund.

NEW Scholarship for Full-time Students – Support to Participate and Get Involved at Structures Congress Learn more and apply by January 5 at www.asce.org/SEI-Students.

SEI Young Professional Scholarship A scholarship for professionals 35 and younger to participate at Structures Congress. Apply by December 1 at www.asce.org/SEI. “Participating at Structures Congress helped me understand the importance of getting involved with SEI, and how it could help me to not only Dream Big but also to make those dreams a reality. And, it helped me to understand there is no better time than the present to begin working with SEI, preparing, and making the future a better world.” Zane Wells, P.E., M.ASCE

SEI Member Benefits Join SEI for innovative solutions and learning, to connect with leaders and colleagues, and enjoy these benefits: • Technical and professional news through SEI Update, STRUCTURE®, Modern Steel Construction, and Civil Engineering Magazine • Member rates on SEI/ASCE conferences, continuing education, and publications • ASCE benefits, including small firm general and professional liability insurance, and 5 Free PDHs/yr • Get involved in an SEI Committee or Chapter effort Learn more and join or renew at www.asce.org/SEI.

SEI SUSTAINING ORGANIZATION MEMBERSHIP STRUCTURAL ENGINEERING INSTITUTE

Reach more than 30,000 SEI Members Year-Round with SEI Sustaining Organization Membership Show your support for SEI to advance and serve the structural engineering profession.

Any organization can join as a Sustaining Organization Member, including design firms, corporations, associations, government agencies, universities and other organizations that support the mission and objectives of SEI. Demonstrate your commitment to excellence in structural engineering.

Learn more and join today at www.asce.org/SEI-Sustaining-Org-Membership.

Advance to SEI Fellow The SEI Fellow (F.SEI) grade distinguishes members as leaders and mentors in the profession.

View criteria and apply by November 1 at www.asce.org/SEIFellows.

STRUCTURE magazine

Errata SEI Standards Supplements and Errata including ASCE 7 See www.asce.org/SEI-Errata. If you would like to submit errata, contact Jon Esslinger at jesslinger@asce.org.

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The Newsletter of the Structural Engineering Institute of ASCE

Membership

Structural Columns

Students and Young Professionals

The Newsletter of the Council of American Structural Engineers

CASE in Point

CASE Risk Management Convocation in Fort Worth, TX

The CASE Risk Management Convocation will be held in conjunction with the Structures Congress in Fort Worth, TX, April 19 – 21, 2018. For more information and updates go to www.structurescongress.org.

The following CASE Convocation sessions are scheduled to take place on Friday, April 20: 9:30 am – 10:30 am Managing Design Professionals’ Risk in the Design and Construction of Property Line Building Structures Moderator: Benjamin M Cornelius, Leslie E. Robertson Associates, R.L.L.P. Speaker: Kriton A. Pantelidis, Esq., Welby, Brady & Greenblatt, LLP 11:00 am – 12:30 pm The Good and the Bad of Delegated Design: How to Work With/As a Specialty Structural Engineer Moderator/Speaker: Kevin Chamberlain, DeStefano & Chamberlain Inc. 1:30 pm – 3:30 pm Construction Dispute Resolution through Forensic Engineering Moderator/Speaker: Benjamin M Cornelius, Leslie E. Robertson Associates, R.L.L.P. 3:30 pm – 5:00 pm Tackling Today’s Business Practice Challenges – A Structural Engineering Roundtable Moderator: Corey Matsuoka P.E., SSFM International, Inc.

COMING SOON!! CASE’s Business of Structural Engineering Seminar Once again, CASE is sponsoring the industry’s only seminar dedicated solely to improving your firm’s business practices and risk management strategies.

Join us and learn about The Business of Structural Engineering. Train and Collaborate with industry leaders and project managers from firms of all sizes, and improve your structural engineering practice. Immerse yourself in topics designed to help engineers learn better ways of reducing areas of risk and liability on projects while learning about tools to implement better business practices within your firm.

The Seminar is geared towards Owners, Principals, Project Managers, and Risk Managers – if you are concerned with risk management, new trends, and profitability, you cannot afford to miss this event!

Registration for the event will open Mid-March; seats will be limited.

For more information about this seminar, contact Heather Talbert, htalbert@acec.org or 202-682-4377.

Looking for Innovative Ideas!

Does your firm have an innovative idea or method of practice? Seeking to get more involved in short duration projects?

We are inviting you to “share the wealth” and submit a proposal for a web seminar topic, publication, or education session you would like to see CASE present at an upcoming conference. Our forms are easy to use, and you may submit your information via email. Go to www.acec.org/coalitions and click on the icon for Idea Sharing to get started.

Questions? Contact us at 202-682-4332 or email Katie Goodman at kgoodman@ acec.org. We look forward to helping you put your best ideas in front of eager new faces!

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October 2017

The 2018 CASE Winter Planning Meeting is scheduled for February 1 and 2, 2018, in Austin, TX. 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. Agenda will be published in early December!

Also at this meeting, the CASE Executive Committee plans to update the CASE Strategic Plan.

CASE in Point

CASE Winter Planning Meeting – SAVE THE DATE!!

Brand New Manual for New Consulting Engineers Fast Becoming an HR Favorite for New Hires

CASE Member Recommendations The following documents are ones that CASE members use the most in their businesses.

Practice Guidelines: CASE 962: National Practice Guidelines for the Structural Engineer of Record CASE 962-D: A Guideline Addressing Coordination and Completeness of Structural Construction CASE 962-H: National Practice Guideline on Project and Business Risk Sample Contact Documents: CASE #1: An Agreement for the Provision of Limited Professional Services CASE #2: An Agreement Between Client and Structural Engineer of Record for Professional Services CASE #3: An Agreement Between Structural Engineer of Record and Consulting Design Professional for Services Sample Toolkits: CASE 1-1: Create a Culture for Managing Risks and Preventing Claims CASE 1-2: Developing a Culture of Quality CASE 3-2: Staffing and Revenue Projection Tool CASE 3-4: Project Work Plan Template You can purchase these and the other Risk Management Tools at www.acec.org/coalitions/coalition-publications.

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

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October 2017

CASE is a part of the American Council of Engineering Companies

ACEC’s newest best-seller, “Can I Borrow Your Watch?” A Beginner’s Guide to Succeeding in a Professional Consulting Organization offers new engineers a head start in the business of professional consulting.

This essential guide is tailored to the unique needs of engineering firms, and the skills and experiences rookie consultants need to be successful in a large organization, including: • Proposal Preparation • Financial Management • Client Relationships • Project Management • Staff Management With over 140 pages of consulting expertise, this resource is the perfect addition to any new staffer’s welcome pack or in-house orientation. It can even be a useful resource for more seasoned engineers looking to refine their skills.

To order this book, go to www.acec.org/bookstore. Bulk ordering is available. For more information, contact Maureen Brown (mbrown@acec.org).

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Get the whole story of Bentley’s comprehensive solutions at: www.bentley.com/STAAD © 2017 Bentley Systems, Incorporated. Bentley, the “B” Bentley logo, and STAAD are registered trademarks of Bentley Systems, Incorporated. Other brands and product names are trademarks of their respective owners.

opinions on topics of current importance to structural engineers

Structural Forum

What “R” are You?

By Justin D. Naser, S.E. and Virginia E. Gilbert, RN, MA, EdS

S

tructural engineers understand the significant role ductility plays in the design of structural elements to resist earthquake forces. We recognize structural systems with higher ductility perform better than less ductile systems in an earthquake. The building codes characterize different lateral force resisting systems by their ability to yield, deform, and absorb energy under load. The ductility factor, or “R” factor, is critical in determining design loads and in understanding the response a structure may go through during ground shaking.

Although we often implement the principle of ductility in the structures we design, we probably rarely apply our knowledge of ductility to ourselves. How do we react when we are pushed to our limit? What ability do we have to stretch and adjust to the rigors of life as a structural engineer?

Similar to the different building systems we use, each of us has different thresholds for the amount of stress we can tolerate before becoming overloaded. Unlike buildings, these thresholds can be modified and may vary within the same individual throughout the day, and from day to day and week to week. Many factors determine how ductile we are to the stressors we face, like genetics, predispositions, personality, physical well-being, relationships, work and civic related responsibilities, and more. Often, a person’s perception of a situation is a stronger indicator of how well they adapt to stress, than what the stressors are. For instance, some individuals thrive on living on the edge and look for adventure wherever and whenever they can find it. Others prefer more controlled, predictable, safer lives.

The structural engineering profession has an abundance of stressors. Whether it is a looming deadline, a difficult design or construction issue, or meeting a client’s demands, we face stressful situations continually. Stress is not all bad. In fact, stress is a necessary, normal, and natural part of life. Stress motivates us.

Stress provides variety and stimulus. Stress may make life exciting and interesting. The object is not to try to eliminate stress from our lives, but to manage it effectively.

Most, if not all, structural engineers have experienced the pressure of a big deadline.

Many times, we receive a boost of adrenaline as the deadline approaches. This helps us work late into the night and work unreasonable hours to meet the client’s or employer’s demands. This dose of adrenaline, however, is only good for the short term. The danger comes when the body does not have time to rebound. If pressing deadlines and long hours persist week after week, the body may release other chemicals that can start to cause serious physical damage. When stress becomes a regular part of our work routine, we may find ourselves overdosing on our own body chemistry. Stress then becomes distress and may lead to disease.

Stress and its accompanying surges of adrenaline cannot be removed from our lives; nor, because of its beneficial effects, do we want them to be removed. The problem becomes how to manage stress effectively, or in structural engineering terms, increase our ductility.

Below are some suggestions.

1) Exercise is the number one natural stress release. It helps to balance the body’s chemistry. Exercises that are meaningful, consistent, and fun keep the mind motivated and the body in shape.

2) Good nutrition builds a strong body and mind. Proper weight and foods can reduce stress on joints and reduce inflammation throughout the body. Small, simple, frequent meals are better than large meals, eaten infrequently. High sugar snacks and empty calorie foods high in fats and salt tax the body’s chemistry.

3) Sufficient sleep reduces stress. The body requires rest to restore its physiological and processing functions. Without adequate sleep, the mind and body can be stretched beyond the ability to cope.

4) Just as important as the connections in our buildings that distribute loads to other members (especially in yielding conditions), our connections with other individuals are vital. When you need support, be willing and wise enough to seek it. Trying to do everything on your own can be overwhelming and is ill-advised.

Do not try to go beyond your expertise.

Have a team approach to life. Being able to connect with others involves developing

good communication skills. Giving and receiving clearly communicated information reduces stress.

5) Breathe slowly, deeply, and rhythmically.

Holding the breath or breathing too rapidly (without exercising or undue physical exertion) causes the alarm system to trigger in the brain and adrenaline to be released.

If this becomes an ongoing pattern, a chronic stress response is created. Take inventory at regular intervals and assess your breathing.

6) Be aware of your thoughts and your selftalk. Stop repetitive negative thoughts.

Avoid expecting perfection from yourself.

7) Perfectionism, unchecked, can lead to distress manifesting itself as anxiety, depression, obsessive compulsivity, and even suicide.

8) Avoid desk rage. Angry thoughts expressed or unexpressed are toxic. Developing good interpersonal skills and respectful relationships does much to reduce role conflicts and burnout. Learn to resolve problems quickly and tactfully.

9) Live fully, in the moment. Put distractions aside, especially when you want to spend time visiting with someone or simply relaxing. Make time for the things that matter most and that you truly enjoy.

This is not an exhaustive list. Hopefully, we can recognize habits and tendencies that are helpful or damaging to our ability to deal with stress. Taking incremental steps to improve allows us to expand our thresholds, increase our ductility, and continue to successfully meet not only the demands of our profession but those that come from all areas of our lives.▪ Justin D. Naser (justinn@arwengineers .com) is a Principal at ARW Engineers and currently serves on the CASE programs committee. Justin was recently named 2017 Engineer of the Year for the State of Utah.

Virginia E. Gilbert (vegil.09@gmail.com) is the Director of the Responsive Living Foundation and is a registered nurse and education specialist. Virginia has devoted a majority of her life to the study and teaching of stress management.

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, the Publisher, or the STRUCTURE® magazine Editorial Board.

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