STRUCTURE magazine | February 2017 by structuremag

A Joint Publication of NCSEA | CASE | SEI

STRUCTURE

February 2017 Steel/Cold-Formed Steel Inside: Pterodactyl, Culver City, California

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CONTENTS Features

Columns/Departments EDITORIAL

26 Lone Tree Bridge

7 The Esthetics of Structures By Jon A. Schmidt, P.E., SECB

By Scott Lomax, M.Eng, C.Eng and Kelly Dunn, AIA In its simplest form, the Lone Tree Bridge provides a crossing over Lincoln Avenue in Lone Tree, Colorado. As a project, it is much more…from connecting communities to linking cycling trails to creating a landmark respectful of the beauty of the nearby Rocky Mountains.

STRUCTURAL FORENSICS

10 Lessons Learned from the Bay Bridge Bolt Failure By Thomas Langill, Ph.D. STRUCTURAL DESIGN

30 Restoring New Haven’s East Rock Road Bridge

14 Fracture Case Studies – Part 3 By Paul W. McMullin, S.E., Ph.D. CONSTRUCTION ISSUES

16 What is a 10d Common Nail? AGAIN – Part 2

By Thomas Strnad, P.E. Rehabilitating a bridge to meet LFRD design and an increased load capacity while incorporating historical elements and emphasizing aesthetics led engineers down a path full of challenges. The result – an awardwinning bridge that stayed true to its heritage.

By Williston L. Warren, IV, S.E., SECB INSIGHTS

20 Infrastructure Check-In By Brian J. Leshko, P.E. CODES AND STANDARDS

On the cover The award-winning Pterodactyl is a uniquely engineered office building formed by the intersection of nine rectangular boxes stacked on top or adjacent to each other. Structural clarity is not synonymous with structural redundancy. Exploration finds its own logic. – Eric Owen Moss. See more about the project on page 43. Photos courtesy of Tom Bonner.

22 Current Code and Repair of Damaged Buildings By Zeno Martin, P.E., S.E., Brian Tognetti, and Howard Hill, Ph.D., P.E., S.E. ENGINEER’S NOTEBOOK

32 Mechanical Bridging of Axially Loaded Cold-Formed Steel Studs By Nabil A. Rahman, Ph.D., P.E.

SPOTLIGHT

43 Pterodactyl, Culver City, California HISTORIC STRUCTURES

By Hooman Nastarin, P.E.

35 Williamsburg Bridge By Frank Griggs, Jr., D.Eng., P.E.

STRUCTURAL FORUM

50 ASCE 7-16 Controversy LEGAL PERSPECTIVES

38 Understanding the Difference between Indemnification and Insurance By Gail S. Kelley, P.E., Esq.

IN EVERY ISSUE 8 Advertiser Index 41 Resource Guide (Bridge) 44 NCSEA News 46 SEI Structural Columns 48 CASE in Point

By Jim DeStefano, P.E., AIA STRUCTURAL FORUM

51 ASCE 7 Controversy By Ronald O. Hamburger, S.E., SECB

STRUCTURE magazine

February 2017

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

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Editorial

The Esthetics of Structures new trends, new techniques and current industry issues By Jon A. Schmidt, P.E., SECB, NCSEA Secretary

odern culture tends to associate esthetics primarily with visual appearance, but philosophy has traditionally sought to unify the virtues of beauty, goodness, and truth. How might someone evaluate whether a particular structure achieves these ideal ends? As mentioned in Part 2 of my recent series of “Outside the Box” articles on The Logic of Ingenuity (October 2016, www.structuremag.org/?p=10490), Charles Sanders Peirce provided some assistance to George S. Morison with the latter’s mid-1890s proposal for a span across the Hudson River. Morison’s paper about it, “Suspension Bridges – A Study,” appeared in Transactions of the American Society of Civil Engineers in December 1896 (Vol. 36, pp. 359-416, https://books.google.com/books?id=EFJDAAAAYAAJ&pg=359), accompanied by 66 pages of discussion. As stated on page 400, “A careful investigation of the theory of the stiffening truss has been made on entirely independent lines by Mr. Charles S. Peirce.” Peirce’s own surviving materials related to this project are collected under manuscripts 1357-1360, as maintained by the Houghton Library at Harvard University and cataloged in 1967 by Richard S. Robin. An initiative called the Scalable Peirce Interpretation Network (SPIN) is now posting and, in some cases, transcribing digital images of such unpublished texts online (http://fromthepage.com/collection/show?collection_id=16). 1357 begins with a typescript that almost exactly duplicates pages 398-401 of Morison’s paper, including the portion quoted above; it is not clear whether Morison prepared it and sent it to Peirce, who then kept it in his files, or Peirce prepared it for Morison to include in the ASCE paper. Based on the other contents of the manuscripts, all handwritten by Peirce, his primary task was to prepare a report about the effect of live loads on the structure. There are various partial drafts, lots of detailed calculations, and other miscellaneous fragments; but unfortunately, nothing resembling a complete document. Even so, one surviving draft, which may have been intended to serve as a cover letter, is worthy of being excerpted at length for its answer to the question that I posed above: When, after having agreed to calculate the effects of the loads upon your projected Hudson River bridge, I came to study the plan of it, I became more and more impressed with the honor of being concerned, even in that entirely obscure way, with such an instrument for the elevation of man. For whoever, in allowing his eye of a morning to rest a moment for refreshment on that splendid scene, should catch sight of that bridge and should reflect upon how calmly and simply it performed a great duty, conforming in every detail to the principles of good sense and of sound reason, would certainly receive a moral lesson which would have its effect upon his conduct for all that day. In the absence of reflection, modern psychology informs us that the influence of the sight might perhaps be even more efficient on the whole; for the subconscious mind – that marvellous [sic] power we call instinct, so much greater than the little self – would virtually make such calculations, without the individual being otherwise aware of it than by the sense of beauty and the elevating thoughts that would well up into his consciousness. Now when I came to reckon, as a good mathematician should, what multitudes of men were to be so influenced daily for century after century, if STRUCTURE magazine

not for millennium after millennium, I found the integral sum of good, in proportion as the plan of the bridge was simple and scientifically adequate, to be sufficient to rouse the utmost depths of any man’s earnestness. Distant ages shall rise up and extol the contrivers and the executors of such a monument, as they would have reason to curse ever more and more deeply those who should deface the landscape with a hideous, broken-backed structure that should half intend one thing and half another, perpetually acting to debase the souls of the generations whose eyes it should weary and torture. Would not the total guilt of every man who should lend a hand to such a nuisance be worth his serious consideration? The nineteenth century is destined to be looked back upon as the classical age of engineering – for every art has its classical age, before it shrinks to small ambitions, and every engineer ought to hope that the century may be crowned by some great enduring type of classical simplicity. Every American must desire that such a secular memorial may be placed in New York, though he shudders at the danger of its being converted into an ineffaceable record of stupidity and bad taste. Peirce’s eloquent words pose a worthy challenge to those of us who practice engineering some 120 years later. Do our own structures typically serve as “instruments for the elevation of man” (and woman) by “conforming in every detail to the principles of good sense and of sound reason”? How will the twenty-first century “be looked back upon” by “distant ages” with respect to “the contrivers and executors” of its monuments? Has our profession irrevocably “shrunk to small ambitions” at this point in its history, or is it not yet too late for each of us to produce “some great enduring type of classical simplicity”?▪ Jon A. Schmidt (jschmid@burnsmcd.com) is a Senior Associate Structural Engineer in the Aviation & Federal Group at Burns & McDonnell in Kansas City, Missouri. He serves as Secretary on the NCSEA Board of Directors, chairs the SEI Engineering Philosophy Committee, and shares occasional thoughts at twitter.com/JonAlanSchmidt.

February 2017

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Important news for Bentley Users

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STRUCTURE magazine

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Evans Mountzouris, P.E. The DiSalvo Engineering Group, Ridgefield, CT Greg Schindler, P.E., S.E. KPFF Consulting Engineers, Seattle, WA Stephen P. Schneider, Ph.D., P.E., S.E. BergerABAM, Vancouver, WA John “Buddy” Showalter, P.E. American Wood Council, Leesburg, VA C3 Ink, Publishers A Division of Copper Creek Companies, Inc. 148 Vine St., Reedsburg WI 53959 Phone 608-524-1397 Fax 608-524-4432 publisher@structuremag.org February 2017, Volume 24, Number 2 ISSN 1536-4283. Publications Agreement No. 40675118. Owned by the National Council of Structural Engineers Associations and published in cooperation with CASE and SEI monthly by C3 Ink. The publication is distributed free of charge to members of NCSEA, CASE and SEI; the non-member subscription rate is $75/yr domestic; $40/yr student; $90/yr Canada; $60/yr Canadian student; $135/yr foreign; $90/yr foreign student. For change of address or duplicate copies, contact your member organization(s) or email subscriptions@STRUCTUREmag.org. Note that if you do not notify your member organization, your address will revert back with their next database submittal. Any opinions expressed in STRUCTURE magazine are those of the author(s) and do not necessarily reflect the views of NCSEA, CASE, SEI, C3 Ink, or the STRUCTURE Editorial Board. STRUCTURE® is a registered trademark of National Council of Structural Engineers Associations (NCSEA). Articles may not be reproduced in whole or in part without the written permission of the publisher.

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Structural ForenSicS investigating structures and their components

ome lessons are not learned until after events occur. This was the case with the San Francisco-Oakland Bay Bridge (SFOBB) or Bay Bridge. This bridge, which carries more than 240,000 vehicles per day along Interstate 80, connects the peninsula of San Francisco with the city of Oakland and eastern side of the San Francisco Bay. Initial reports suggested hot-dip galvanizing embrittled the bolts causing a failure. After more research, it was determined the embrittlement was not from the galvanizing but was a much more complex issue.

History of the Bay Bridge In 1989, the series of bridges that make up the Bay Bridge were severely damaged during the infamous Loma Prieta Earthquake that rocked the city and surrounding areas. The most substantial damage occurred when the upper east section of the bridge collapsed onto the lower deck during the height of the heaviest traffic commute, causing heavy damage to traveling vehicles and killing and injuring many. After this earthquake, the western section of the bridge underwent a complete seismic retrofit in 2004, where the bridge was modified to become more resistant to seismic activity and other ground motion. In 2013, the eastern section of the bridge was completely replaced with a new self-anchored suspension bridge (SAS) and opened in September of that year (Figure 1). This allowed the bridge’s cables to be completely attached to the ends of its deck, rather than into the ground, allowing for more flexibility during seismic activity. However, one of the issues with an SAS bridge is the potential for an amplification of stresses during a seismic event where the SAS cables are hit with an increase of pressure to hold up the bridge.

Lessons Learned from the Bay Bridge Bolt Failure By Thomas Langill, Ph.D.

Dr. Thomas Langill has been with the American Galvanizers Association for 22 years as its Technical Director. He is Chairman of three ASTM Subcommittees, including the subcommittee that maintains the standards for the hotdip galvanizing process, A05.13, G01.04 on atmospheric corrosion, and G01.14 on corrosion of construction materials. Dr. Langill is also Secretary of the ASTM Main Committee A05 on MetallicCoated Iron and Steel Products.

Figure 1. San Francisco-Oakland Bay Bridge.

10 February 2017

A New Design Takes Shape Bridge designers and constructors are always concerned with preserving their bridges for as long as possible. To combat the amplification of stresses and to offset the potential of harmonic amplification of the bridge deck, the designers installed shear keys in the new design of the eastern bridge span. Shear keys are blocks of concrete supported by large diameter bolts that are made from ASTM A354 Grade BD material in diameters from two to four inches and have been hot-dip galvanized. The shear keys dampen the seismic energy transmitted to the bridge deck and help prevent damage during an earthquake. They are not intended to support the bridge deck itself but merely play a role in the suppression of forces during a seismic event. Each bolt within the shear key is heat-treated to meet the minimum specification of mechanical properties, and hot-dip galvanized to provide corrosion protection. These bolts were used throughout the Bay Bridge design, not just in the new shear key installation. These anchor rods, A354BD, are specified to have an ultimate tensile strength (Fu) of a minimum of 140,000 psi for bolts with diameters above 2.5 inches and a hardness range of a minimum of 31 HRC and a maximum of 39 HRC. Figures 2a and 2b show the placement of the anchor rods used for the shear keys when the bolts were first put into place in the Pier E2 location. The anchor rods were installed into the shear keys in November 2008, and grouting of the rods began in January 2013. However, these anchor rods were not tensioned until March 2013 because they could not be installed until the superstructure of the bridge was put in place. Because of this, the very bottoms of the anchor rods were purposely damaged to hold the nuts until they could be properly tensioned. The top of the steel pipe sleeve assemblies holding the shear key rods were exposed to the environment

(a)

Figure 2 (a and b). Placement of the anchor rods used for the shear keys when the bolts were first put into place in the Pier E2 location.

during the period before the superstructure of the bridge was installed. This left little clearance between the top of the rods and the bottom of the bridge.

Beginning Signs of Trouble Once the superstructure of the bridge was erected in 2013, and the load transfer was completed, the anchor rods were pre-tensioned to 70% of their minimum specified ultimate tensile strength (Fu). After the first two weeks of tension, 32 of the 96 anchor rods had fractured, all occurring at or near the threaded engagements towards the bottom of the rods. Once the pre-tension level was reduced to 40% Fu, failure of the rods ceased. This resulted in the decision to abandon all 96 of the rods. An alternative anchoring system was successfully designed and installed. Although these anchor rods were no longer in service, their failure raised major concerns about the long-term performance of the remaining A354BD rods throughout the bridge. The fracturing (Figure 3) resulted in three investigations focused on metallurgical testing and failure analysis on two of the fractured rods from the shear keys. It was determined the failure of the rods was a result of hydrogen embrittlement. As a result, the California Department of Transportation undertook a second testing program to further examine the cause of failure and to evaluate the remaining A354BD rods throughout the bridge.

(b)

of IBECA Technologies and Chairman of the ASTM F16 Committee on Fasteners; Rosme Aguilar, Branch Chief of Cal Trans Structural Metals Testing Branch; and Conrad Christensen, Principal and Founder of Christensen Materials Engineering. The design of the bridge created a very low clearance between the rods and the superstructure of the bridge. For testing to continue, the rods had to be removed in small sections by pulling them up as far as possible and sawing them off. This process was very extensive, so only a few rods ultimately were removed for the study. The first batch of A354BD rods, those installed in the shear keys, were not tested through the Magnetic Particle Inspection (MPI) because they were installed before the requirement for MPI testing was added to the contract. The MPI looks for small cracks in the rods. All of the A354BD rods were cleaned by dry blasting to SSPC No. 10 Near-White Blast Cleaning before being dipped into the molten zinc bath, to avoid acid cleaning. By skipping the acid solution, the rods avoided the generation of hydrogen. The rods were galvanized within four hours of blast cleaning to ASTM A123 standards. The bolt threads on the rods can be rolled or cut, and heat treatments must include quenching in oil.

Shear Key Rod Failure Investigation This investigation, which began shortly after the tensioning of the shear key rods, included performing metallurgical testing and failure analysis on the rods. It was conducted by three investigators: Salim Brahimi, President

Figure 3. Fracture surface of 2008 shear key rod.

STRUCTURE magazine

February 2017

The investigation continued with a visual inspection of two failed rods and showed the presence of Denso paste, which is part of the Denso Tape System on the rods that provide extra corrosion protection. The threads and fractured surfaces had remnants of this paste as well as the grout used during installation. This visual inspection revealed that the surface was brittle, and that cracks had developed, grown, and spread throughout the rods progressively. This process exceeded the rod’s structural capacity, ultimately resulting in the final failure. After the visual inspection was complete, the investigation moved forward by using a scanning electron microscopy (SEM) examination of the surfaces across the rod diameter. The results showed intergranular fracture morphology near the threaded roots. As the crack progressed, the morphology became more mixed, causing a sudden fast fracture when the crack reached a critical size, meaning the rod could no longer carry the applied load. The mechanical property testing results were within proper specification values for hardness, ultimate tensile strength, chemical analysis, and microstructural analysis of the hot-dip galvanized coating. However, after conducting analysis with the Charpy V-notch tests, it was found that the test result values (13-18) were below expectations (25-35), meaning the material had failed to reach expected toughness and could be susceptible to embrittlement. The Charpy V-notch test is an impact test that determines the amount of energy that can be absorbed by the material during a fracture. The conclusion of this study stated that the cause of this failure was hydrogen embrittlement combined with the applied load exceeding the susceptibility of the rod material. The steel rods complied with the proper specification, A354BD, but the microstructure

• The 2008 rods would not have failed if they were protected from saltwater Following the Townsend Test, the Raymond test was conducted on two types of small specimens cut from full-size rods. This test is a slow, rising step-load laboratory bend that allows for an examination to determine the susceptibility to hydrogen embrittlement. The results were consistent with the Townsend Test and again proved the failure was a result of hydrogen embrittlement based on their exposed environment, not internal hydrogen. The main results of the study were as follows: the 2008 rods failed by hydrogen embrittlement at the same load (0.70 Fu) that resulted in failure on the bridge. The outcome provided independent confirmation of the result obtained with full diameter rods.

Summary

Figure 4. Townsend test schematic.

was not uniform. The microstructure inhomogeneity resulted in low toughness and marginal ductility, causing the rods to be susceptible to hydrogen embrittlement.

Evaluation of the Selected Fractured Rods After the results of the initial analysis on the A354BD rods had shown signs of hydrogen embrittlement, the various parties responsible for the bridge design were concerned about the potential of hydrogen embrittlement in other A354BD rods throughout the rest of the bridge. Therefore, another study was initiated to test rods throughout the bridge. A variety of rods of different sizes, tension levels, and locations were selected for detailed laboratory testing to determine chemical composition, hardness, and susceptibility to hydrogen embrittlement. All mechanical property tests showed that material properties were generally uniform and within specification requirements. Another Charpy impact toughness test was conducted which showed toughness levels of the majority of the rods within the normal ranges for the material. Only the tests on the samples of the 2008 rods showed lower toughness values. After these tests resulted in normal readings, the Townsend Test for Stress Corrosion Cracking (SCC) was performed on the fulldiameter rods obtained from the various groups of previously selected rods. In this test, the tensile load was increased very slowly (in steps) until a threshold load level was established for the onset of cracking due to hydrogen embrittlement (Figure 4). This slow increase was essential because of the required time it takes for diffusion in detecting the

effects of hydrogen. To properly detect the threshold load for hydrogen entering the steel from the environment due to corrosion (environmental hydrogen), the rods were loaded up to 1.8 million pounds while immersed in saltwater containing 3.5% sodium chloride. The results of this test concluded: • The 2008 rods failed because of hydrogen embrittlement at the same load (0.70 Fu) that led to failure on the bridge with similar fracture characteristic. (This confirms the Townsend Test duplicates the actual performance of the rods.) • The fracture surface on the rods showed where the initiation of the crack occurred. As the cracks progressed, the fractures changed from intergranular to cleavage and finally to ductile fractures that took place on the opposite end of the initial cracks. (These fractures were evident in all the 2008 bolts examined.) • After testing the remaining rods, it was determined that all had threshold loads greater than their design loads, indicating they were not susceptible to hydrogen embrittlement. The next step was to determine if hydrogen was already present in the steel and if it could have contributed to the low threshold of the fractured rods. The Townsend test was repeated in air, without exposure to salt water, to make the determination. This test showed a complete absence of hydrogen embrittlement and resulted in the following: • Failure of the 2008 rods in the wet Townsend Test occurred as a result of environmentally induced hydrogen embrittlement.

STRUCTURE magazine

February 2017

In conclusion, there was no indication that the galvanizing process contributed in any way to the failure of the rods from 2008. The low hydrogen embrittlement threshold of the failed rods was likely due to fabrication methods and thermal treatment of the rods. The results of this study indicate the Bay Bridge rods installed in 2008 failed because of environmentally induced hydrogen embrittlement caused by tensioning above their threshold while simultaneously immersed in water. This created the perfect environment to introduce hydrogen into the steel. There was no evidence that hydrogen was present in the steel before installation or tensioning, nor that internal hydrogen contributed to the A354BD rod failures. The Townsend Tests performed on the A354BD rods confirmed that, without the presence of water, the rods would not have failed. All of the remaining rods on the bridge were tested and proved to have hydrogen embrittlement thresholds higher than their pre-tensioned stress levels and were concluded as safe. The remaining rods were designed to have supplemental corrosion protection measures that include dehumidification, paint systems, or grout. All of these measures will prevent corrosion and the possibility of hydrogen embrittlement as long as the galvanized coating remains intact. Additionally, the development of specific maintenance procedures for the corrosion protection system can provide assurance to the bridge owners and will be specified in the Self-Anchoring Suspension (SAS) Maintenance Manual.▪ The online version of this article contains references. Please visit www.STRUCTUREmag.org.

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Structural DeSign design issues for structural engineers

he previous two STRUCTURE magazine articles (General Principles of Fatigue and Fracture, Part 1, August 2016 and AISC and Damage Tolerance Approaches, Part 2, November 2016), reviewed the fundamental principles of cracking and how to design for fatigue and fracture. This article presents three case studies that illustrate how an engineer can use this guidance to address project challenges. The intent of this article is to move from the theoretical to the practical, and demonstrate that there is a realistic place for the more developed methodologies of fatigue and fracture mitigation.

Northridge Earthquake The 1994 Northridge earthquake had a tremendous impact on the American Institute of Steel Construction’s (AISC) steel code over the past 20 years. After the magnitude 6.7 (Mw) earthquake, inspectors discovered 1,300 fractured moment frame connections in 72 buildings. Naturally, this made many people uncomfortable. To address the fracture issues, the SAC Steel Project (www.sacsteel.org) studied material behavior, connection geometry, and construction practices to figure out what happened and why it happened. Results of the project are widely published and infused throughout current AISC Seismic provisions. One of the questions that came up during the studies was the effect of the welding backup bar. Field erectors preferred leaving them in place because they take time and money to remove. However, they create an inherent notch in the joint. This section uses fracture mechanics to study the impact that leaving the backing bar in place has on joint behavior, and what happens when it is fully fillet welded to the beam flange. In the first condition, the backing bar is tack welded to the column flange and fused to the beam as the weld is deposited, illustrated in Figure 1.

Fracture Case Studies Part 3 By Paul W. McMullin, S.E., Ph.D.

Paul McMullin is a Founding Partner at Ingenium Design in Salt Lake City. He is an Adjunct Professor and the lead editor of the Architect’s Guidebooks to Structures series. Paul can be reached at Paulm@ingeniumdesign.us.

Figure 2. Tack welded stress intensity solution as a function of crack depth.

14 February 2017

Figure 1. Beam to column flange weld in PreNorthridge moment connection.

Note how the backing bar and any lack of fusion at the weld root creates a crack. Using fracture mechanics, one can plot the stress intensity KI as a function of crack depth and far-field stress, shown in Figure 2. Using a fracture toughness of 50 MPa (m)1/2 – a middle ground value – most of the stress intensities are greater for stresses in the yield range (250 MPa to 350 MPa). Even with twice the toughness, it still seems like a poor choice to leave the backing bar in place. What happens when a continuous fillet weld is placed along the bar? Won’t that take care of the problem? There now exists an eccentric crack condition. Looking at Figure 3, notice that about half of the stress intensity values are higher than the assumed toughness. There may be an argument to allow this condition. However, considering the possibility of lower toughness, the certainty of constraint near the web intersection, and the potential for the crack to grow due to low-cycle fatigue, it also seems imprudent to leave the backing bar in place. In the end, a joint where the backing bar is removed, with the weld root gouged out and rewelded, can perform orders of magnitude better than one that has a crack-like lack of fusion in it from the backing bar. This conclusion is born out not only by the analysis but also by a rational view of the problem.

Figure 3. Fully welded stress intensity solution as a function of crack depth.

Figure 4. Assumed crack geometry in the tank wall.

Ammonia Tank The question to answer, on a sizeable ammonia tank, is what stress corrosion cracks need to be repaired and which ones can be left alone. When steel is in contact with ammonia with very low oxygen content, cracks do not grow. However, cracks do grow in tanks when the ammonia is contaminated with air. The tank in question had been out of service for some time and had a number of stress corrosion cracks. The owner wanted to recommission the tank, and hence the project. Utilizing API RP 579 Fitness for Service, the engineers on the project created crack ratio charts that let field crews know which cracks needed repair. Cracks under a certain size for a given aspect ratio, though detected, could remain in place. The effort began by mining Charpy toughness data from material test reports. Using the master curve approach, the engineer correlated Charpy values to fracture toughness K1 values. The correlations are a function of thickness and Charpy energy values. This provided one side of the equation – the other being the stress intensity factor. Utilizing this data, the engineer developed stress intensity solutions based on the basic crack geometry shown in Figure 4. These are from solutions in API 579. Selecting a crack length 2c, a crack depth a is calculated. Doing this for numerous crack lengths, the curves in Figure 5 and Figure 6 are generated. Where the crack depth is greater than the tank wall thickness (Figure 5), a leak-beforebreak condition exists. This approach is good because the tank will leak before rupturing. However, for lower toughness material, like in the weld or heat affected zone, a breakbefore-leak condition existed (Figure 6 ). This is of more concern, given the lack of warning before catastrophic failure. This analysis tells two things. In the base metal, long, shallow cracks need to be repaired, as a break-before-leak condition exists for aspect ratios (a/2c) less than 0.5. In the weld base metal, all cracks of a given size need to be repaired. The engineer can decide what crack size, for a given aspect

Figure 5. Critical crack size, a leak-before-break condition in the tank wall.

Figure 6. Critical crack size, a break-before-leak condition in the weld.

ratio, needs to be repaired by choosing an acceptable safety factor. Finally, perhaps a third lesson: Not every crack is a problem and needs to be repaired.

A key lesson to learn from the bridge crane is the importance of thorough inspection. The stress and fatigue analyses showed the bridge crane was in good shape. However, reality showed a very different picture, one that eventually saved lives. In the end, the principles of damage tolerance can be applied to traditional civil engineering structures in a way that provides clarity to the cracks they may contain. These are rooted in fracture toughness testing, stress intensity factor solutions, fatigue testing, life correlations, and non-destructive testing. These case studies show the approach in utilizing some of these tools and the insight gained through their application. Greater application of these tools to civil engineering structures would lead to increased safety of the structures for which engineers are responsible.▪

Bridge Crane The bridge crane in Figure 7 was one of the dozens in the area that were decommissioned over the years. It was about 100 years old and had experienced somewhere between 5 to 10 million fatigue cycles. The owner wanted to know if the structure was safe before investing in a major electrical upgrade. The study looked at the member forces, AISC fatigue requirements, and non-destructive testing of the eyebars. The force analysis did not identify any problems. The model results matched the Maxwell diagram in the original drawings. The fatigue analysis indicated stresses in most members below the threshold values in AISC of 4.5 ksi. A few members towards the middle of the truss had stresses near 10 ksi. They had failed at one point, causing the truss to lose over a foot of camber. Up to this point, nothing was of major concern. However, enter non-destructive testing (NDT). Before any NDT testing occurred, ironworkers stripped the paint of some key joints and discovered cracks, visible to the naked eye, shown in Figure 8. The phased array ultrasonic and magnetic particle testing found cracks inside and at the surface of a substantial number of joints. The cracks ranged in size from 1/8 to 2½ inches long and 1/64 to 1/32 inches wide. After lengthy discussions and a second engineering opinion, the owner elected to retire the truss – creating a serious operational challenge to the site. Given the size and extent of the cracks and difficulty in repairing eyebars, it was truly the only rational decision.

STRUCTURE magazine

February 2017

Figure 7. Bridge crane with eyebar bottom chord and diagonal members.

Figure 8. Eyebar cracking.

ConstruCtion issues discussion of construction issues and techniques

ave you ever been in a discussion, adversarial or entertaining, with a non-engineer or even an engineer, about what code requirements exist for construction activities? For example, a discussion about the generation of structural calculations for an existing building where the construction does not comply with the permitted construction drawings and, in some cases, with specific code requirements. The justification usually includes the excuse that structural engineers are commonly overly conservative and that the buildings described in the construction documents exceed the minimum requirements of the building code. What follows is a review of what should be considered non-negotiable points. The 2012 International Building Code (IBC), in Section 101.3, states “The purpose of this code is to establish the minimum requirements to provide a reasonable level of safety, public health, and general welfare through structural strength, means of egress facilities, stability, sanitation, adequate light and ventilation, energy conservation, and safety to life and property from fire and other hazards attributed to the built environment and to provide a reasonable level of safety to firefighters and emergency responders during emergency operations.” IBC Section 101.2 states “Scope. The provisions of this code shall apply to the construction, alteration, relocation, enlargement, replacement, repair, equipment, use and occupancy, location, maintenance, removal and demolition of every building or structure or any appurtenances connected or attached to such buildings or structures.” The administration chapter of the code also describes what constitutes violations of the code, that the power of the building official is that of a law enforcement officer, and that the building official is not liable for any injury resulting from an omission in the construction when acting in good faith and without malice. This liability section states, “The building official, member of the board of appeals or employee charged with the enforcement of this code, while acting for the jurisdiction in good faith and without malice in the discharge of the duties required by this code or other pertinent law or ordinance, shall not thereby be civilly or criminally rendered liable personally and is hereby relieved from personal liability for any damage accruing to persons or property as a result of any act or by reason of an act or omission in the discharge of official duties.” IBC Section 104.1, Powers and Duties of Building Official, also states that “The building official is hereby authorized and directed to enforce the provisions of this code. The building official

What is a 10d Common Nail? AGAIN Part 2 By Williston L. Warren, IV, S.E., SECB

Williston L. Warren, IV is Principal Structural Engineer, SESOL, Inc., Newport Beach, California. Treasurer of the National Council of Structural Engineering Associations (NCSEA), Member of the NCSEA Code Advisory Committee (CAC), Chair of the CAC Evaluation Service Committee, Past President of the Structural Engineers Association of California (SEAOC), Member of the Applied Technology Council (ATC) Board of Directors, Board Representative on the Project Review Panel for ATC 58-2, ATC-110 and ATC-124.

16 February 2017

shall have the authority to render interpretations of this code and to adopt policies and procedures in order to clarify the application of its provisions. Such interpretations, policies and procedures shall be in compliance with the intent and purpose of this code. Such policies and procedures shall not have the effect of waiving requirements specifically provided for in this code.” IBC Section 107.3, Examination of Documents: “The building official shall examine or cause to be examined the accompanying submittal documents and shall ascertain by such examination whether the construction indicated and described is in accordance with the requirements of this code and other pertinent laws or ordinances.” Once the code is adopted by city, county, or state ordinances or laws, the following apply to design and construction of every building: 1) The code is the minimum standard for building design and construction. 2) The most restrictive section of the code governs. 3) Any construction omissions or defects are not the building official’s responsibility. 4) Inspections do not lessen the builder’s responsibility for defects in construction. 5) The building official is responsible for determining if the construction is in accordance with the code. 6) Interpretations of the code must be consistent with the intent and purpose of the code. 7) The construction is required to comply with the permitted drawings unless noted by the building official. There doesn’t seem to be a lot of wiggle room in these seven requirements. In the author’s opinion, this looks to be intentional in order to take contractors’ creativeness out of the design documents and construction process. The engineering professionals that defend poor and deficient construction regularly claim that “there is testing that shows…” This claim is used to justify the construction based on these test results, even knowing that these reports are incomplete or inconclusive. For example, after the 1994 Northridge Earthquake, testing of timberframed buildings was performed by universities across the country to examine not only component performance but system behavior during cyclic loadings. These investigations included the determination of the performance of stucco and drywall covered framed walls subjected to cyclic loads and cyclic testing, and force transfer behavior of anchor bolted sill plates of shear panels. One finding for the sill plate testing is that the behavior and capacity for the sill plate configurations without nuts on the anchor bolts are similar to tested configurations with tightly installed anchor bolt nuts. These findings are brought up regularly in discussions of code requirements for anchor

learn-by-doing. The example of a nail attaching two pieces of wood would appear to be a simple example because almost everyone has used a nail to connect two pieces of wood. The previous article on this subject discussed the point of connecting wood structural panels to framing to resist lateral loads with a 10d common nail. The building code and current AWC Special Design Provisions for Wind and Seismic (SDPWS) clearly define the length of a 10d common nail as 3 inches long and the minimum penetration of that 3-inch long nail.

THE LEGACY ADVANTAGE

Other resources also define a 10d nail. In ASTM F-1667 Standard Specifications for Driven Fasteners: Nails, Spikes, and Staples, Table 15 Type I, Style 10 – Common Nails, steel wire, defines a 10d common nail as a length of 3 inches and a diameter of 0.148 inches. Reviewing American Institute of Timber Construction Timber Construction Manual, Second Edition 1974, page 5-65, Table 5.19 describes a 10d common nail as having a length of 3 inches and a wire diameter of 0.148 inches. continued on next page

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bolt nuts and are used to justify a myriad of conditions that are not allowed by the code. This justification dismisses the fact that when this issue has been brought up during the code development process, it was rejected, and the code still states that nuts on anchor rods are necessary. Perhaps what our profession needs is an extensive discussion on construction tolerances and the code provided minimum requirements. If you investigate two out of 10 shear walls, and you find problems at those shear wall locations, is that an indication of non-compliant construction? If you specify concrete reinforcement spacing of 5 inches on center, is 6 inches on center acceptable? Remember, all construction is required to be code compliant, so it is hard to justify that anything else is sufficient. As a profession, we really do not understand the effects construction tolerances have on the engineering behind the drawings. Our profession is continuing to strive to understand material performance under loadings in an attempt to economize and provide for the safety of occupants, and protect the investments made by building owners. We can question whether the constructors of these buildings understand that the engineering design of today is not that of twenty years ago. The loadings are better understood and now have less of a “confusion factor” in them, especially true for those infrequent but high demand loadings such as lateral loads resulting from ground motion and winds. Another issue we should be concerned with is whether a structural component designed 25-plus years ago is going to perform the same or in a similar fashion compared to one designed today. Are we sure they would perform similarly based on the advances in the understanding of expected loading conditions, material behavior, and, in the case of timber, the actual material changes over the past 30 years? These differences would also vary across the materials and loadings. But, do the constructors understand the differences in 25-plus-year-old designs versus current designs in construction or do they see them as the same? Is a reinforced concrete element designed using the allowable stress method versus one designed using ultimate strength going to perform the same? The NCSEA Basic Education Committee has identified timber engineering design as a subject not frequently available in a significant number of universities and as a result, in too many cases, practitioners

OK, you are probably saying, “Can’t you calculate this using the NDS?” Sure you can – but with care. First, you need to consider that the information in the shear wall tables is a product of testing versus the results you get from the calculations, and there is a difference. What is the capacity of a given configuration versus the specified configuration on the permitted drawings? With many materials, this is fairly direct for assemblies consisting of various components, as with concrete where the variables are considered in the analysis, but what about that of an assembly that uses table values out of the code? Frequently, this discussion is about a wood structural panel shear wall specified to have 10d common nails spaced at 4 inches on center on the boundaries and edges. This example was observed to have 21/8-inch long by 0.148-inch diameter nails with one boundary having an average spacing of 3.2 inches on center of a 24-inchlong distance, edge nailing condition of 4.3 inches on center, and yet another boundary nailing found to be an average of 1.65 inches on center. Also, the boundary nailing into the pressure-treated sill plate is not corrosive resistant, as required by the code. So what is the capacity of this assembly? This is one of the hundreds of thousands of incorrectly constructed shear walls that have not been tested because the code requires the builder to comply with the requirement of Section 107.4 which states: “Amended construction documents. Work shall be installed in accordance with the approved construction documents, and any changes made during construction that are not in compliance with the approved construction documents shall be resubmitted for approval as an amended set of construction documents.” (Emphasis added by author.) Is this why this section of the code exists, to eliminate the re-engineering of work (performed by licensed engineers) by constructors that most likely lack the perspective, knowledge, and experience to understand the situation? The discussions do involve a lot of nontechnical individuals, but they have retained structural engineering consultants. They make a circular discussion due to the belief that nail size choices are made in the field by adding the sheathing thickness to the minimum penetration. There have been tests, by various sources, performed on wood structural panel shear walls subjected to cyclic lateral loads and it was observed that withdrawal of the nail is a significant response component. Moreover, according to the codes, the withdrawal capacity is a function of its length. Shorter nails would have less withdrawal capacity to draw on than the required length.

So which is it? If you make a trip to the building supplier and look at nails, you find that nails, independent of diameter, come in lengths of a quarter to eighth-ofan-inch increments. Due to the number of times we observe short nails – just due to the probability – my suggestion is that any engineer designing timber buildings with wood structural panel shear walls to resist lateral loads needs to check nail lengths if 10d common nails are specified and, as the current code does, clearly specify the diameter and length of all fasteners. Now consider a reinforced concrete example of varying spacing of reinforcing bars. With the current understanding of reinforced concrete, analysis tools are available that allow for such variation and determination of constructed capacities. Assemblies such as timber sheathed shear walls do not have these tools. Also remember that the capacities in the code are a result of testing specific configurations, and do not include all combinations and permutations. Another thought, and you may not like it – a new cottage industry of engineers, or perhaps not engineers, has grown to review your submittal and submit a report to the building owner or whoever paid you. That review report could conclude that the original design is overly conservative, requires the building owner to spend more money to build than it should, and you (the Engineer) should help pay the difference because it was because of you that there is a difference. If it is acceptable to have the judicial process, involving judges, juries, lawyers and other “experts,” accept construction that does not comply with 107.4, and the variation is subject to interpretation by those not educated in the profession or by those who improperly build, then why do building departments require that a responsible licensed engineering professional remain responsible for that design? Also, why do we then have to endure plan review processes that vary in both quality and depth? Why are so many of us volunteering time in code development, reviewing proposals, and attending hearings? So is the “reasonable level of safety, public health, and general welfare” purpose of the code so important? Many will say that this is important to engineers and, yes, my discussion has been about items that are only included just to make engineers happy, correct? Moreover, yes, many of these components are included in construction to allow the building to resist loading it may never see in its lifetime. Also, components designed to resist gravity loads get tested often and they are within the understanding of the builders

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because, if not, they fail when walked on. However, the lateral force resisting system is different and, unfortunately, can be a source of “savings” for builders. Most evening news programs include a story about some natural loading conditions on structures such as floods, tornadoes, and occasionally earthquakes; many of us work actively to reduce the “loss of life” aspects of these loadings. With this confusion, it could come down to one of the following: 1) The building code is a collection of recommendations and happy wishes. Adoption of the code by ordinance is merely a pro forma measure. What nail size to be used in all the construction underway nationally is an individual exercise in simple math performed multiple times a day. Nail spacing of shear wall panels does not need to be as described by the drawings – whatever is provided is good enough because the panels really will not ever see the lateral loads for which they are designed. Use of corrosion resistive nails in pressure treated lumber is also unnecessary because, even though the code requires pressure treated lumber on concrete slabs due to water, we all know there really will not be any water ever. And just so you can sleep at night, remember, there is a ten-year statute on construction litigation for the project. 2) OR, is the building code the minimum requirement for construction as it states? And, when it notes that the heads of nails shall be installed with the crown of the nail flush with the sheathing, it means it. Any other requirement that accepts some over-driven nails and some underdriven would require an engineer be observing with calipers to measure each installation when they vary from flush. As for nail lengths, that only one length should be used for each nail size, so the framer does not need someone to oversee the minimum penetration and sheathing thicknesses. Moreover, that nail spacings be installed as specified, not accepting just what ends up in the field. Is there a specific and uniform construction standard, or is it non-standard or non-uniform? Which is it?▪ Part 1 of this series was published in the July 2016 issue of STRUCTURE magazine. Visit www.STRUCTUREmag.org.

InSIghtS new trends, new techniques and current industry issues

ne of the most frequently used “buzz words” in the media lately is Infrastructure, a term used to represent many different things to many different people. One definition of Infrastructure is “the basic physical and organizational structures and facilities (e.g., buildings, roads, and power supplies) needed for the operation of a society or enterprise.” (https://en.oxforddictionaries.com/ definition/infrastructure) The list in the sidebar is by no means exhaustive but serves as a point of discussion. This article will focus on just a few of these elements. The American Society of Civil Engineers’ (ASCE) most recent Report Card for America’s Infrastructure, issued in 2013, gives an overall grade of D+ across 16 categories. In the 1950s, structures and roadways were designed for a lifespan of 50 years. It is no wonder that 20% of the nation’s 900,000+ miles of interstate and major roads are in need of resurfacing or reconstruction, and 25% of the nation’s 600,000+ bridges are either structurally deficient or functionally obsolete. Focusing solely on the country’s highway bridges, one out of nine is rated as structurally deficient and the average age of the 607,380 bridges is 42 years. The Federal Highway Administration (FHWA) estimates that $20.5 billion would need to be invested annually in eliminating the nation’s bridge backlog by 2028, while only $12.8 billion is spent each year. As far as roadways, 42% of America’s major urban highways remain congested. The FHWA estimates that $170 billion in capital investment would be needed annually to improve conditions and performance, while only $91 billion is spent each year. Highway tunnels were not addressed in the ASCE Report Card or the National Bridge Inspection Standards (NBIS); however, the National Tunnel Inspection Standards (NTIS), effective as of August 13, 2015, established regulations for the uniformity of tunnel inspections similar to the NBIS for bridges. The initial inventory of highway tunnels resulted in a total of 473 tunnels identified by owners across the country. These highway tunnels will have initial inspections performed in accordance with the new NTIS regulations to determine their baseline condition for future asset management. In addition to the structural elements and civil elements, many tunnels feature mechanical systems (i.e. ventilation), electrical systems (i.e. lighting), life safety systems (i.e. fire detection and protection), and additional components (signage, SCADA, etc.), which are now being inventoried and inspected every 24 months. Stay tuned for the FHWA report of findings that will follow the uploading of data into the newly created National Tunnel Inventory resulting from the completion of the initial tunnel inspections by August 13, 2017.

Infrastructure Check-In By Brian J. Leshko, P.E.

Brian J. Leshko is a Vice President, Principal Professional Associate and the Infrastructure Inspection Program and Practice Leader with HDR, Inc. in Pittsburgh, Pennsylvania. He is an FHWA National Certified Tunnel Inspector, an FHWA-Certified Bridge Inspection Team Leader, an NHI Certified Instructor (teaching the Tunnel Safety Inspection Course) and a former SPRAT-Certified Level I Rope Access Technician. Brian recently completed 11 years on the STRUCTURE magazine Editorial Board.

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Infrastructure Elements For structural engineers, infrastructure can include the following: Bridges • Highway • Railroad • Transit Tunnels • Highway • Railroad • Transit Hydraulic Structures • Dams • Spillways • Boat Locks • Weirs • Culverts • Pump Stations • Wharves & Piers Pipeline Structures • Petroleum • Water • Wastewater Storage Tanks • Water • Petroleum Towers • Communication • Transmission Power Lines • Wind Turbines Buildings • High-Rise • Stadiums • Facades/Chimneys • Convention Centers • Coliseums • Theaters • Monuments For structural engineers, maintaining the nation’s infrastructure presents a challenge in the face of limited fiscal resources as well as vacillating priorities at the local, state, and federal levels of government. As such, asset maintenance programs are helping to inform asset management initiatives nationwide as owners and maintainers of infrastructure optimize their limited resources. Repair and rehabilitation strategies appear to remain the major focus of funding programs, with the occasional new design of replacement structures the exception to the rule.▪

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Codes and standards updates and discussions related to codes and standards

hen done rationally, maintaining and repairing existing buildings represents an efficient use of resources that should be promoted. Also, reusing and repairing existing construction becomes increasingly important as sustainability becomes a higher priority (SEI 2013). Model building codes change over time, with hundreds of changes every few years. Given such revisions, existing buildings would either require frequent modifications or need to be treated differently. Fortunately, lawful existing building conditions are typically “grandfathered,” which means they can be used without modification. Various incidents such as fires, accidents, and storms cause damage to buildings that often requires repair to maintain conformance with applicable requirements. When damage occurs, the minimum required scope of work must be determined in many cases. Can the building be maintained as it was? What upgrades, if any, must be added to the repairs? Answers to these and similar questions can be found within the code provisions that govern repair of existing buildings. The intent of code repair provisions can be better appreciated by studying their evolution.

Current Code and Repair of Damaged Buildings Are Upgrades Required? By Zeno Martin, P.E., S.E., Brian Tognetti, R.A., and Howard Hill, Ph.D., P.E., S.E.

Zeno Martin is an Associate Principal at Wiss, Janney, Elstner Associates, Inc., Seattle, WA. He can be reached at zmartin@wje.com. Brian Tognetti is an Associate Principal at Wiss, Janney, Elstner Associates, Inc., Bingham Farms, MI. He can be contacted at btognetti@wje.com. Howard Hill is a Senior Principal at Wiss, Janney, Elstner Associates, Inc., Northbrook, IL. He can be reached at hhill@wje.com.

The 50% Rule & Upgrade the Entire Building The very first model codes in the U.S., the Uniform Building Code (UBC 1927), Southern Standard Building Code (SBC 1946), and Basic Building Code of the Building Officials Conference of America (BOCA 1950), contained specific provisions applicable to existing buildings. These stated what was required to be done depending on the type of work performed (i.e., repair, alteration, and change in use) and its cost. For repairs, if the anticipated cost exceeded 50 percent of the building’s value before the damage, then all aspects of the entire building, not just portions affected by damage, needed to be upgraded to meet new construction requirements. Some of these earlier codes included an additional 25 percent repair cost threshold. When the anticipated cost of repair was between 25 and 50 percent of the building’s pre-damage value, then unaffected portions of the building did not have to be upgraded to meet new construction rules. If the cost of the proposed work was less than 25 percent, then in-kind repair was typically allowed. The three model codes maintained these costof-repair based upgrade triggers until the late 1970s. At that time, these percent-rule upgrade triggers were deemed an obstacle to the re-use of existing buildings (Mattera, 2006) and so were largely eliminated.

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Upgrade Only What Was Affected After removal of the general percent-rule triggers, and starting with the 1979 UBC, 1981 BOCA, and 1982 SBC, the extent to which new construction provisions were triggered by repair work was no longer dependent on cost. Instead, the intent was to leave undamaged, unaffected elements alone, and apply new construction rules only to elements of the construction that were affected by the damaging event. This upgrade only what was affected philosophy was promulgated by each of the model codes until they were consolidated into the International Building Code (IBC) in 2000.

Current Code – Repair with No Upgrades? Starting with the 2015 versions of IBC, matters governing the repair of existing buildings are addressed almost exclusively by the International Existing Building Code (IEBC). The IEBC has three optional approaches to repair, per Section 101.3: “…to provide flexibility to permit the use of alternative approaches to achieve compliance with minimum requirements…” The applicant is required to select one of three compliance methods, which are termed: Prescriptive, Work Area, and Performance. However, not all alternatives may be available in all circumstances. The Performance Compliance Method is detailed in IEBC Chapter 14. It is the most lenient compliance alternative in that it merely requires repairs to be consistent with pre-damaged construction. It contains no requirements associated with particular building code provisions, regardless of the extent of the damage. According to the associated commentary, it was written in this fashion to accommodate treatment of buildings that cannot be associated with any particular code, and yet were considered suitable for occupancy and use before the subject damage. This situation occurs when a building

pre-dates the jurisdiction’s adoption of codes and there is no documentation as to what standards were used in its construction. The Applicability section of Chapter 14 (1401.2) provides a “prior to” date that defines what structures can be evaluated using its provisions. This section recommends that the date in question “coincide with the effective date of building codes within the jurisdiction.” There is a recommendation that the Performance Compliance Method only apply to buildings constructed before there were any identifiable code provisions being enforced. This makes sense since buildings constructed after codes were put into effect have defined provisions as benchmarks, while buildings that pre-date code enforcement typically do not. In spite of the recommendation to limit application of the Performance Compliance Method to buildings that pre-date code enforcement, some jurisdictions make it effective to a much broader category of buildings, even all existing buildings. In such cases, it is certainly appropriate to use the Performance Compliance Method, which usually comprises the minimum requirements for repairs. Consider the following excerpt from Chapter 14: “An existing building or portion thereof that does not comply with the requirements of this code for new construction shall not

be altered or repaired in such a manner that results in the building being less safe or sanitary than such building is currently.” [2015 IEBC, Section 1401.2.4] The only stated requirement for a repair is that the repaired condition be no less safe or sanitary than it was before the damage being addressed occurred. This is based on the entirely rational premise that, as long as the building was considered safe to use before the damage, restoration to the pre-damage state should be sufficient for continued use. The Prescriptive and Work Area compliance methods also allow like-kind repair of damage, with certain exceptions. For example, the Work Area Compliance Method, in Section 601.2 states, “The work shall not make the building less conforming than it was before the repair was undertaken.” The method then has separate sections outlining requirements for Building Elements and Materials, Fire Protection, Means of Egress, Accessibility, Structural, Electrical, Mechanical, and Plumbing. With the exception of Building Elements and Materials, Structural, Electrical and Plumbing, each of these specific sections repeats the general requirement that the repair shall not make the building less conforming than it was before the repair was undertaken. The

Building Elements and Materials, Structural, Electrical and Plumbing sections also indicate when pre-damage conditions can be recreated or when like materials can be utilized, but also describe situations in which repair to something other than the predamage state is required. Each of the IEBC compliance methods has provisions that dangerous (2015 IEBC Sections 401.3 and 606.1) or unsafe (2015 IEBC Section 1401.3.1) conditions be abated. So if the damage was related to a hazardous or unsafe condition, as defined in 2015 IEBC Section 202, then in-kind repair that would restore such a condition would clearly not be allowed. Substantial Structural Damage Starting with the first IEBC (in 2003), the concept of Substantial Structural Damage (SSD) was introduced as a means for defining when structural repair to something other than the pre-damage condition might be required. In the 2015 IEBC, SSD is defined in Section 202 and is used in the repair provisions by both the Prescriptive (Section 404) and Work Area (Section 606) compliance methods. The SSD threshold is used, in part, as follows: “For damage less than substantial structural damage, the

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damaged elements shall be permitted to be restored to their pre-damage condition.” (2015 IEBC, Section 606.2.1; and similar in Section 404.4) If damage greater than SSD has occurred, then an evaluation is triggered. The outcome of the assessment determines the required scope of the structural-related repairs. If the evaluation establishes compliance of the pre-damage building with the associated criteria, then repairs are allowed to restore the building to its pre-damage state. If the evaluation does not establish compliance of the pre-damage building with the associated criteria, structural-related repairs to something other than the pre-damage state are usually, but not always, required. An important aspect of the SSD provision is to recognize that if such upgrades are necessary as a result of the evaluation, the extent of the upgrades are limited to the structuralrelated work and do not alter the previously discussed scope of the provisions within the IEBC that address the other aspects of the building (i.e., fire protection, means of egress, etc.) Flood Hazard Areas and the Redacted Percent Rule

Too Much Damage to Qualify as “Repair”? Sometimes people reach the wrong conclusion that replacing damaged materials is not a “repair” but rather new construction or an alteration, or that too much damage has occurred to use the repair provisions. This interpretation is incorrect because it is contrary to the code provisions themselves. The 2015 IBC and 2015 IEBC definition of repair in their respective Section 202 is, “The reconstruction or renewal of any part of an existing building for the purpose of its maintenance or to correct damage.” Repair work then, by definition, reconstructs, renews (i.e., restores), or otherwise maintains what was previously there. Within the 2015 IEBC, Section 502 states that repairs “…include the…replacement of damaged materials, elements, equipment or fixtures...” There is no limitation that correcting damage (repair) pertains only to a certain amount of damage. In fact, both the Prescriptive and Work area compliance methods contain provisions to repair buildings

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Although the historic general and wide-reaching percent-rule cost thresholds are not in the IEBC, a limited version is still present within all three IEBC compliance methods when addressing damage to buildings located in identified flood hazard areas. When a building, damaged by any means, is located in what Section 202 of the 2015 IEBC defines as a “flood hazard area,” and the cost of restoring

it to the pre-damage state exceeds 50 percent of the market value of the pre-damaged building, then all aspects pertaining to flood design for the building shall be brought into compliance with requirements for new construction. Similar to the limitations of the SSD upgrades, eclipsing the 50 percent threshold of the flood provisions does not require other non-flood design aspects of the building to be brought into compliance with new construction provisions.

that have sustained substantial damage, such as SSD. A description is offered by NCSEA (2014) that is: “essentially, if the work only ‘fixes’ what was previously there, then it is classified [in building codes] as ‘repair’ work.”

Meeting Current Code Meeting current code in the context of repairing an existing building means to meet the code provisions that control such work. Upgrades to improve aspects of buildings beyond the explicit requirements of the applicable code provisions that apply to the repair of existing buildings (e.g., the 2015 IEBC discussed above) may be recommended, prudent, or a good idea – but are not required in order to repair and maintain buildings. For excellent reasons, the concept of grandfathering has been applied to the repair of damaged buildings since the inception of building code provisions dealing with repair, and continues today. This practice is based on the reasonable and rational notion that the “victim” of an unfortunate event should not have to bear substantial costs to provide a better structure than what would have existed had the event not occurred. The code provisions for repair of buildings have evolved since their beginning almost 90 years ago. There are now fewer upgrades required. For example, for approximately 50 years (from 1927 to the late 1970s) when repairs in excess of fifty (50) percent of the pre-damage value of a building were made to any building within any period of twelve months, the entire building was then required to be made to conform to all requirements for new buildings. In the 2015 IEBC, repair that does not make the building less conforming than it was before the damage occurred, is, with few exceptions, allowed for nearly all aspects. The significant repair-related upgrade requirements are now limited in the 2015 IEBC as follows: • A 50 percent repair cost threshold, which only pertains to flood hazard areas and only triggers upgrade for flood design features. • If SSD occurs, at most, upgrades are limited to specific structural aspects.▪ This article summarizes an ASCE published Technical Paper written by the same authors (Martin et al., 2015). Reprinted with permission from ASCE. The online version of this article contains detailed references. Please visit www.STRUCTUREmag.org.

STRUCTURE magazine

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Lone Tree Bridge By Scott Lomax, M.Eng, C.Eng, MICE and Kelly Dunn, AIA, LEED AP BD+C

Bridge rendering by Fentress Architects.

edestrian bridges capture the imagination and have the potential to transcend conventional design. There is a long history of landmark bridges created by some of the industry’s greatest engineers and architects. However, the overarching aesthetic demands an integrated approach. This article discusses the historical context of signature pedestrian bridges and showcases the tools and processes that facilitate an integrated design approach, using the example of the Lone Tree Bridge in metro Denver, Colorado, designed by Fentress Architects and Thornton Tomasetti.

Background/Historical Context Throughout history, bridges have been seen as pivotal links in infrastructure, the means for the consolidation or expansion of a community, a testament to progress in terms of design and materials, and above all, landmarks. The raw nature of these objects appeals to a wide audience, and enormous civic pride is captured in the expression of a crossing. While there are many historic precedents such as Pont des Arts in Paris or Venice’s Ponte dell’Accademia, in more recent times, there has been a significant increase in the design of signature or landmark pedestrian bridges. Projects such as Gateshead Millennium Bridge in the U.K., Langkawi Sky Bridge in Malaysia, or the Puente de la Mujer in Argentina have raised the profile of pedestrian bridges, and owners are aware of the impact such designs may have on the urban fabric. Aesthetic through expression of the structural behavior is not a revolutionary concept. The work of artists such as Nervi, Candela, Dieste, and Calatrava, among others, resonate through a deeply rooted integration of architecture and structural engineering. Indeed, as Candela once stated, “Structural design possesses more art than it does science.” The attraction of the purity of bridge design has fascinated many famous designers, and the most successful projects are a result of an integrated approach combining architecture, engineering, and an appreciation of constructability.

Approach The success of a project depends on clearly identified goals and objectives, which is more difficult that it sounds. Each project has its unique circumstances and challenges that must be analyzed on a project-specific basis. It starts with basic requirements such as length of span, followed by clearance envelopes, a discussion of cost and schedule, and then on to intangibles such as an improved quality of life and a catalyst for development. From the outset, the challenge is a combination of practicalities, such as constructability or the contextual design of a signature component. The process demands varying skill sets and experience, as well as integration between architect and engineer. For the Lone Tree Bridge, the most STRUCTURE magazine

Puente de la Mujer.

Context plan by Fentress Architects.

basic requirement was to provide a 170-foot crossing over Lincoln Avenue in Lone Tree, Colorado. A closer look showed the need to connect communities and provide an essential link in a network of cycling trails. A more aspirational outlook was to create a landmark that would represent the ambition of the city and be respectful of the amazing natural beauty and vistas of the nearby Rocky Mountains, yet be functional, practical, and within budget. Several concepts were developed, and designs pushed, pulled, and tested. Sometimes this led to minor tweaks, while other times to seismic shifts in the form. Some of the tools available, such as advanced computational modeling (ACM) and 3D printing, were useful during the early stages. However, the principal mode of communication was through hand sketching, preliminary hand calculations, and a mutual respect for the experience and insight that each team member brought to the table.

February 2017

be conventional, which helps the overall economy. The cables help to yield a lightweight structure, but both redundancy and dynamic behavior became key criteria in the design and were studied in detail. The dynamic behavior, in particular, required time-history analysis of user-induced vibrations to simulate occupancy and determine acceleration levels.

Tools

Overall massing model by Fentress Architects.

As the core form of the bridge – an asymmetric cable stay – began to take shape, the analysis and sculpting advanced further. With an efficiency of form founded in the basic layout, the design team focused on the sensitivities of the cable and pylon geometry to create a balance between structural efficiency and art form. Paying homage to the symbolism of the client (the City of Lone Tree), the team refined the pylon to an elementary leaf while retaining the structural integrity. The pylon is essentially a three-dimensional lattice truss, constructed of industry standard elements with a twist in the geometry to create a sculptural form. As the form and vision were consolidated, a number of studies were undertaken to investigate member and material options. The benefits of a lightweight, slender yet stiff structure led to the selection of a steel pylon and deck with a precast concrete walkway. The main legs of the pylon are 24-inch-diameter and 18-inch-diameter for the front and rear legs, respectively. The use of pipe section helped reduce the amount of welding on the project, proved convenient for the intersection of the nodes, and provided a softness to the pylon in keeping with the aesthetic intent. A wall thickness of one-inch was selected to avoid changing thickness, which would incur a splice, and to avoid local stiffening or high stresses at the anchorage connections. This balance of artistry, engineering efficiency, and practical construction was a recurring theme throughout the design and design-assist process. Twin backstays anchor the pylon and the forestay cables splay to support the deck at 24-foot spacing. The 12-foot wide deck is connected to the cables via outriggers that cantilever from the deck sufficiently to ensure the cables do not conflict with the enclosed walkway. The deck is defined by an in-plan truss created by longitudinal edge beams, crossbeams, and diagonal bracing – all using conventional rolled steel members. The main span has an enclosure to protect users in severe weather yet enables one to enjoy the open air and direct sunlight on nice days via a stainless steel mesh on the sides and an ETFE roof. A simple portal frame, supported on the main span deck, provides the infrastructure for the enclosure. As with many pedestrian bridges, the final sizing was a balancing act between strength to carry the imposed loads, stiffness to yield acceptable movements under use, economy through efficiency of sections, type of member and detailing, and attention to the aesthetic vision. The pylon is a signature component, and the member selection and connections were honed with architectural input. The deck was required to be slender for both aesthetics and to maintain the clearance envelope without added depth, which would have a knock-on effect of increasing the approach spans. However, the member types could STRUCTURE magazine

To work fluidly and communicate effectively, the team used a number of different software packages as the project advanced. From early Sketch-up files through to Revit and then Tekla for the fabrication process, the geometry was developed and shared among the team. The team also developed parametric models to create multiple analysis models and test global configurations to optimize the form. The design team used software to import geometry files into analysis models and then export from analysis models back to the geometry files. This enabled direct communication between the architectural and engineering team, and facilitated a smoother process. Tools and ACM were an integral part of the process; however, it is important to note that they did not drive the design intent. The design was conceived, developed, and finalized through sketches, physical models, and drawings. It was an artistic process rather than a mechanical one.

Process + Design Assist The client made a decision to follow a design assist approach and engage a contractor early in the project. Once the design concept was consolidated by Fentress Architects and Thornton Tomasetti, and vetted by the public via consultations, the next hurdle was to ensure the vision was realized within the schedule and budget constraints. The overall project cost was set at $6.8 million, with a design period of eight months and a construction schedule of 12 months. By integrating the contractor into the design process and using Guaranteed Maximum Price (GMP) milestones, the client reduced cost and schedule risk. For a long-span signature project, the design assist process was also invaluable to develop and finalize the design with input from the industry. Conventional project milestones of SD, DD and CD were replaced with IGMP, GMP, and Mill Order. The design team worked live and directly with the contractor/fabricator to develop options and select details, and received immediate feedback on how they would impact schedule and budget. The process helped control the quality of the final product and reduced risk across the project. Design assist is a loosely defined process and can be of enormous benefit if correctly applied. Some key requirements include: • Engagement of the contractor at the correct stage of project development. Balancing sufficient design development to show feasibility, intent, and consolidation of principal

Architectural rendering by Fentress Architects.

February 2017

Local FEM of the pylon base pin.

Stress contour map.

requirements, yet allowing enough latitude for the construction team to have influence. • Selection of the most appropriate construction partner. In addition to cost and experience, the understanding and willingness of the contractor to engage in design assist in a collaborative manner is paramount. The contractor must understand the design goals, and the entire team must be committed to achieving the balance of cost, schedule, and quality. • Respect for the experience and skill set of the various parties. Essentially, for design assist to be successful, the design and construction teams must work toward common goals and value input across the board. Clarity of the objectives and challenges is key, followed by effective communication. For Lone Tree Bridge, general contractor Hamon Construction and steel fabricator King Fabrication joined Fentress Architects and Thornton Tomasetti during Schematic Design. Their input,

knowledge, and expertise were fundamental to advancing the project. A series of charrettes and workshops created a common understanding of the issues, and open lines of communication facilitated the platform to work through solutions. An example of this was the pylon base – a joint that was not only of huge engineering and architectural significance but would also be influenced by the fabrication process and the erection requirements. The use of a sculpted pin connection provided a strong yet artistic architectural expression, and was in keeping with the contractor’s preferred erection scheme whereby the pylon would be assembled flat and rotated into position. The engineering team worked closely with the fabricator and developed local finite element models of connections to optimize the configuration and sizing. Design assist is not necessarily the correct approach for every project; however, in the case of Lone Tree Bridge, where an outstanding team was assembled at the critical project milestones with experienced, committed, and passionate individuals, it has been a success to date.

Summary

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The most successful pedestrian bridge designs are often pure in their concept. There is an elegance to their simplicity and form that transcends conventional architecture and engineering. To deliver such a project requires an integrated process starting at conception and continuing through the design development and construction. It is essential that the design is influenced by architectural, engineering, and construction principles. Lone Tree Bridge is a wonderful example of an educated client; a balanced, experienced and focused design team; the exchange of ideas and tools to communicate effectively between parties; and integration of constructability with the design process.▪

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Restoring New Haven’s East Rock Road Bridge By Thomas Strnad, P.E.

popular community destination in New Haven, Connecticut, East Rock Park is listed on the National Register of Historic Places. The 427-acre park, which attracts visitors year-round for hiking, picnicking, bicycling, boating, and cross-country skiing, features a number of historic buildings, gardens, and structures that date to the late 19th and early 20th centuries. Among these is the circa-1900 East Rock Road Bridge, a steel arch bridge that crosses the Mill River on the west side of the park. The 84-foot-long, single-span bridge carries a two-lane roadway with a 20-foot curb-to-curb width. The bridge is also used by many hikers, runners, and bicyclists, and has two five-foot-wide sidewalks. The superstructure consists of a steel grid deck with infill concrete supported on steel floor beams, columns, and deck arches. The arches are part of the original construction; the remainder of the superstructure was replaced during a rehabilitation project in 1984. The original abutments and wingwalls are gravity-type walls with brown, cut-stone masonry facing. These elements were modified during the 1984 rehabilitation by adding a concrete cap to support the new sidewalk and railing on the wingwalls. The east and west abutments are supported on spread footings and timber pile foundations, respectively. In 2007, the Connecticut Department of Transportation (ConnDOT) performed a routine biennial inspection of the East Rock Road Bridge that led to ratings of “serious” for the bridge deck condition and “poor” for the superstructure. Based on the state’s inspection, the City of New Haven determined that the bridge required a major refurbishment.

Historical Research, Modern Analysis The city selected the firm of Dewberry as the prime consultant to perform an in-depth inspection and design of the bridge rehabilitation. STRUCTURE magazine

The consulting team also included William Kenny Associates, LLC, for Wetlands delineation, Martinez Couch & Associates, LLC, for the site survey, and Archeological & Historical Services for archeological consulting. City officials challenged Dewberry’s engineers to develop a design that complied with Federal Highway Administration (FHWA), ConnDOT, and AASHTO guidelines and specifications, and in particular the AASHTO LFRD Bridge Design Manual with the HL-93 design vehicle. This led to the final design capacity of 36 tons, as compared to the previous weight limit restrictions of 17 tons and 24 tons for trucks and tractor-trailers, respectively. In conjunction with these design standards, the design was required to emphasize aesthetics and incorporate historical elements into the process. The process began with a review of the original, circa-1900 engineering plans and an effort to supplement information missing from those plans. Dewberry also inspected the bridge to document section losses and current conditions. This required the use of a specialized tracked vehicle to inspect the bridge from the riverbed, as the bridge’s weight restrictions prohibited the use of an under-bridge inspection vehicle located on the bridge deck. Geotechnical investigations included excavating test pits in front of the abutments, verifying existing foundation details, and performing non-destructive testing on the original timber piles to assess their condition and verify their adequacy for design scour events. The excavations extended down to the bottom of footings to observe the tops of the timber piles, which were still in excellent condition. The testing enabled Dewberry to verify the capacity of the original foundation and its ability to carry increased loads. The firm also performed hydrologic, hydraulic, and scour analyses. The design team obtained the required permits from city and state regulatory agencies. Throughout the duration of the project, the

February 2017

The rehabilitation replicated the ornamental pedestrian rail system and added decorative lighting.

Construction views showing erection of rehabilitated steel arches.

city and the consulting team maintained a robust public outreach program, including a project-specific website and three public information meetings. An initial proposal to widen the roadway by four feet to comply with current AASHTO standards met with some concern from community members and the Connecticut Commission on Culture and Tourism, which sought to maintain the historic character of the bridge and limit speeding. Dewberry successfully obtained an exception from FHWA to keep the existing roadway width.

many careful measurements to determine the thickness of shim plates required to install the floor beams at the proper roadway elevations on top of the bridge. • Some fragile elements, such as the original arch cast iron bearing assemblies, could not be re-used as they were damaged during removal operations. These elements were replaced in kind with new steel bearing assemblies. Also, after blast cleaning, the team determined that the deterioration in some steel members was significant. These discoveries required quick action to develop repairs or new details to accommodate these elements. • The design combined the ornamental pedestrian rail system, containing lattice bars and rosettes, with a crash-tested bridge rail system. This resulted in a safe and aesthetically pleasing solution. The contractor also took many measurements and installed shim plates to ensure that the railing posts were installed vertically, and the railing was aligned properly.

Complex Issues The $2.1 million rehabilitation of the East Rock Road Bridge required that the superstructure be completely removed. ROTHA Contracting Company, Inc., led the construction effort, aided by several specialty contractors. The historic arches and the circa1984 columns were transported to Boston Bridge and Steel, Inc., Massachusetts, where they were dismantled, blast cleaned, repaired, strengthened, and painted. Southington Metal Fabricating Company provided the rail fabrication and ADF Industries, Inc., served as the rail erector. The strengthened bridge elements were then transported from Boston back to the site, where they were erected in their original location. The bridge construction was completed with the installation of new floor beams and a steel grid deck partially filled with concrete, which was selected to reduce loads on the arch while at the same time providing a paved riding surface. The project addressed several complex issues, including: • The existing arches were riveted I-section members consisting of a web plate, flange angles and cover plates, with lower steel material properties resulting in insufficient capacity to meet current standards for legal loads. When the bridge was disassembled, the arch pieces were sent to Boston Bridge and Steel, where the contractor removed the rivets and cover plates and installed thicker cover plates on the top and bottom flanges to increase their capacity. The contractor blast cleaned all of the pieces, removed lead paint, repaired deteriorated steel, and painted each piece. The shop fabricated the new floor beams to support the bridge deck and shipped the pieces back to New Haven for reinstallation. All of the original arch pins were replaced as part of the reconstruction. Because the arch strengthening resulted in a deeper section, ROTHA Contracting Company took STRUCTURE magazine

Award-Winning Design The rehabilitation of New Haven’s East Rock Road Bridge over the Mill River, as the structure is formally known, reopened in 2015 and was well received by city officials and community members. The bridge continues to contribute to the historic ambiance of East Rock Park. In addition to the engineering solution that preserved the ornamental rail system, the project incorporated decorative lighting designed by city staff as well as new wayfinding signs, landscaping, and brownstone masonry facing on the concrete surfaces of the new lighting pedestals and bridge rail end walls. The project was awarded a 2016 Engineering Excellence Award from the American Council of Engineering Companies (ACEC) of Connecticut, in recognition of the engineering challenges addressed during the rehabilitation as well as the care taken to maintain the structure’s historic integrity. Identical plaques on either side of the bridge credit the design and construction team, noting that the project was undertaken to “meet modern traffic loads and return the bridge closer to its original splendor… Care was taken to respect the historical setting of East Rock Park in the shadow of East Rock itself.”▪

Thomas Strnad, P.E., is a Senior Bridge Engineer in the New Haven, Connecticut, office of Dewberry. February 2017

EnginEEr’s notEbook aids for the structural engineer’s toolbox

his article provides a better understanding of the design requirements and methods to laterally brace (bridge) axially loaded cold-formed steel stud walls. Cold-formed Steel (CFS) studs provide a cost effective and extremely efficient structural solution for the typical mid-rise building. In recent decades, CFS design has evolved tremendously as the behavior and design constraints of the material continue to be better defined through comprehensive research and testing. As our understanding of the behavior of CFS studs continues to evolve, the height of a typical mid-rise CFS structure continues to increase. Thus, it is more critical to the integrity of the structure that these heavily loaded studs be adequately braced. Global buckling of an axially loaded stud can occur in one of three modes: flexural buckling, torsional buckling, or flexural-torsional buckling. Bridging is used within the plane of the wall to prevent global buckling and specific performance requirements of the bridging must be maintained. The bridging methods described herein represent a mechanical bracing design consistent with an “all-steel design” approach. The allsteel design approach indicates that the CFS studs rely on bridging for stability and that bracing by structural sheathing or gypsum wallboard is not typically considered for axial load stability in the design. One primary reason that CFS walls in mid-rise buildings require an all-steel design bridging approach is that the lower floors may go unsheathed for weeks at a time during construction. Also, for axially loaded walls, it is an industry practice to disregard gypsum sheathing as a structural brace because of durability issues associated with possible water damage. It should be noted that bridging requirements for studs loaded laterally, perpendicular to the plane of the wall, are not discussed herein. Brace forces for studs with combined axial and lateral loading are additive, and the designer is encouraged to refer to The American Iron and Steel Institute (AISI) design guide, Cold-Formed Steel Framing Design Guide (AISI D110-16) where a design example showing the interaction check for combined loading condition can be found.

Mechanical Bridging of Axially Loaded Cold-Formed Steel Studs By Nabil A. Rahman, Ph.D., P.E.

Nabil A. Rahman is the Director of Engineering and R&D for The Steel Network, Inc. and a Principal at FDR Engineers in Durham, NC. He is the current chairman of ASCE-SEI Committee on Cold-Formed Steel Members. He serves as a member of the Committee on Specification and Committee on Framing of the American Iron and Steel Institute (AISI), and a member of ASCE Committee on Disproportionate Collapse. He can be reached at nabil@steelnetwork.com.

Design Requirements The design requirements for the bridging components of axially loaded cold-formed steel studs are described in the North American Specification for the Design of Cold-Formed Steel Structural Members, AISI S100-12, Section D3.3, and Section B3.1 of the North American Standard

32 February 2017

for Cold-Formed Steel Framing – Wall Stud Design, AISI S211-12. AISI S100 Section D.3.3, Bracing of Axially Loaded Compression Members, gives both a strength and stiffness design approach. The required strength of the brace to restrain lateral translation at a brace point for an individual compression member is given as: Prb = 0.01Pra (Eq. D3.3-1) The required brace stiffness for the ASD design method is given as: βrb =

2[4 – (2/n)] (ΩPra), Ω = 2.0 Lb (Eq. D3.3-2a)

While the required brace stiffness for LRFD and LSD design methods is given as: 2[4 – (2/n)] Pra ( ), φ = 0.75 (LRFD) or Lb φ 0.70 (LSD) (Eq. D3.3-2b) where, Prb = Required brace strength (brace force) to brace a single compression member with an axial load Pra Pra = Required compressive axial strength [compressive axial force] of individual concentrically loaded compression member to be braced βrb = Minimum required brace stiffness to brace a single compression member n = Number of equally spaced intermediate brace locations Lb = Distance between braces on individual concentrically loaded compression member to be braced, AISI S211 Section B3.1, Intermediate Brace Design, states that: “For axially loaded members, each intermediate brace shall be designed for 2% of the design compression force in the member.” When using AISI S211 approach, there is no explicit brace stiffness requirement compared to the approach in AISI S100. As a result, AISI S211 uses a more conservative strength requirement, 2%, compared to 1% in equation D3.3-1 in AISI S100. βrb =

Bridging Methods Several bracing concepts are available to effectively brace axially loaded CFS against both flexural and torsional buckling modes. The methods of bracing may be categorized into three groups: tension systems, tension-compression systems, and compression systems. Regardless of the type of bracing used, the bridging must be effectively continuous between anchorage points. Where bridging is spliced, engineered splice details are required to maintain the performance requirements of the bridging along the length of the entire wall.

Figure 1. Flat strap “tension” bridging system.

Tension System In a tension system (Figure 1), the bridging member is designed to resist the stud buckling by means of pure tension. An example of a pure tension system consists of flat straps attached to both flanges of the stud with blocking at intervals along the wall to provide resistance to the rotation tendency of the studs within the wall. This type of bridging is advantageous in scenarios requiring a significant number of mechanical and electrical utilities within the wall plane. However, installation requires access to both sides of the wall, and the flat straps must be installed tight enough to provide the required stiffness. The flat straps are typically attached to each stud flange with screws. The blocking is set at the required intervals (typically first and last stud spaces and at 8 to 12 feet on center) to resist the rotation and is attached to the flat straps on each side of the wall as shown in Figure 1.

Tension-Compression Systems

Figure 3. Continuous blocking “compression” bridging system.

the stud punch-outs must align horizontally for the channel to be inserted continuously throughout the wall length.

S100 and AISI S211 bracing design alternatives that are available to designers.

Compression-Only System A compression-only system is capable of resisting stud flexural and torsional buckling using compression only (Figure 3). The compression system may offer higher brace stiffness compared to the tension and the tension-compression bridging systems. Load and stiffness capacities of the three bracing concepts can be achieved through engineering calculations. Proprietary bridging members offer the advantage that they have also been tested. Refer to Cold-Formed Steel Engineers Institute TN W400-16, www.cfsei.org, for a numerical example problem illustrating the design of a bridging member. The design example provides the detailed calculations for bracing (bridging) and bridging anchorage of axially loaded studs, and compares the AISI

In Summation To ensure the integrity of the structure, the design engineer must fully understand the behavior and bracing requirements of a CFS load bearing stud. An integral part of the bracing requirements is the bridging required in the wall to prevent the tendency of a cold-formed steel stud to buckle about its weak axis under increasing load. For the bridging to be effective, it must be anchored periodically as the accumulation of the bridging force approaches the allowable capacity of the bridging method used. The contents of this article have offered an overview of the current code requirements for the design of the common framing methods of achieving the bracing requirements. The reader is encouraged to refer to the standards referenced above for additional information.▪

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A bridging system having the capability to resist the buckling of the stud in either tension or compression is illustrated in Figure 2. An example of this type of system is a cold-rolled channel inserted through the punch-outs of the studs with a clip attaching the bridging member to each stud. The clip transfers the load induced from the flexural and torsional buckling of the stud into the bridging line. A static analysis suggests that the cold-rolled channel functions in 50% compression and 50% tension between “anchorage” points. This concept is illustrated in a design example given in CFSEI Tech Note W400-16. The typical cold-rolled channel member used for a load bearing stud wall application is a 150U50-54, 33 ksi section. The advantage of this bridging method is that it requires access to only one side of the stud wall for installation. The disadvantage is that

Figure 2. Cold-rolled channel “tension-compression” bridging system.

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significant structures of the past

Historic structures

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

he Williamsburg Bridge across the East River in New York City is now over 100 years old. After a complete rehabilitation, it is still considered by some the “Ugly Duckling” of suspension bridges. In May 1883, the Brooklyn Bridge opened over the East River between New York City and Brooklyn. Even before the Brooklyn Bridge opened, however, residents of Williamsburg created an organization to pressure politicians for a new bridge. No real action resulted from this pressure. It was into this period of inaction that Frederick Uhlman and his associates from the Brooklyn Elevated Railway moved in early 1892. They put together a strong base of support in the legislature. On March 9, 1892, the East River Bridge Company, a private corporation, received a charter from the State of New York to build two bridges to form a loop transit line connecting Brooklyn and Manhattan. After the passage of the bill, the company retained George B. Cornell in April 1892 as its chief engineer. He planned a suspension bridge, similar to the Brooklyn Bridge, with a span of 1,620 feet and a width of 106 feet to accommodate four tracks for passenger trains, two driveways, and a promenade. The towers would be 280 feet high with a clearance over high water of 135 feet, the same as required for the Brooklyn Bridge. Cornell proposed that the towers be made of masonry to a height of 180 feet above the water level and that the remaining 100 feet be made of steel. The company submitted plans for both bridges to the Board of Alderman of Brooklyn in early October, and they approved the plans on November 22, 1892. The company also went before a Board of Engineers representing the Secretary of War for approval. To the surprise of many, the Board recommended a clearance of 145 feet to the Secretary of War. The company appealed this recommendation and requested a hearing with the Secretary of War. In mid-January, 1893, the Secretary approved a clearance of 140 feet, but little work was done for over a year. Based on the slow progress made by Uhlman, Brooklyn Mayor Charles Schieren pushed for a bridge owned by the two Cities. He told the New York Times, “We propose to build a bridge similar to the Brooklyn Bridge, and one that will be a thing of beauty and a

joy forever. I have not consulted Manhattan Mayor Strong yet in regard to the matter, but I think he will acquiesce in anything that is for the benefit of the two cities.” Mayor William L. Strong went along and together they presented their proposal to the Legislature, who approved it on May 27, 1895. Commissioners were appointed and met with the Mayors of both cities for the first time on June 26, 1895. Their first problem was selecting a Chief Engineer. From the multitude of engineers available, eleven applied including Leffert L. Buck, Virgil Bogue, George Morison, William Burr, A. P. Boller and others. The choice evidently came down quickly to Leffert L. Buck, George Morison, and Virgil Bogue. Buck (STRUCTURE magazine, December 2010) was selected to design what was to be the longest and most heavily loaded suspension bridge in the world. His decision was to concentrate on one bridge, not the two bridges originally chartered. On August 7, the commissioners and Buck met with Uhlman and Cornell to discuss the bridge. Uhlman indicated he did not see how the engineers of the Commission could plan a bridge that would not interfere with his company’s rights. For the rights of his company, he wanted $200,000 which he eventually got. On January 23, 1896, Buck submitted his report recommending a six-track layout, based on the original plans for four tracks, two roadways, and a promenade. He decided “the best arrangement seems to be to place the elevated tracks in the middle, and to have two trolley tracks on each side of them.” The commissioners, after much discussion, unanimously resolved to “construct a bridge, not exceeding 118 feet in width, with six tracks thereon, two for elevated railroad service and four for the surface railroad service, with all the necessary approaches and switches, and terminals – all tracks to be on the same level at the centre of said bridge, and to build a promenade for pedestrians over said tracks.” O. F. Nichols, Buck’s Rensselaer classmate, was appointed Buck’s first assistant engineer. On February 6, 1896, an engraving of Buck’s bridge was printed in the New York Times. The article said that the main span would be 1,700 feet and it would be the “Stiffest Suspension Span in the World and the Longest.” The “estimated cost of the bridge is $12,000,000

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Williamsburg Bridge.

– $8,000,000 for the bridge proper and $4,000,000 for the approaches.” Buck and the commissioners went back to the War Department to obtain approval for the 135-foot clearance granted the Brooklyn Bridge, which was finally given on February 28. Buck concluded that a 1,600-foot “continuous stiffened suspended structure” was the best and added, “There is no more durable bridge than a well-built suspension bridge, and said there is no more enduring reputation than that of the man who will sometime in the future build one of large size as it should be built.” He described his bridge as follows: The towers are of steel columns thoroughly braced, eight columns to the tower on each side of the river. The distribution of the roadways and tracks were to be passed through the tower, the tracks in the middle portions or openings and the roadways to pass through between the columns on that side of the tower. This fixed the width of the tower at the roadway. The cradling of cables before mentioned and position of the trusses fixed the position of the columns at the top of the towers, as it was desirable to have each pair of columns rest directly under the saddles in which the cables were to rest. This arrangement gave considerable batter to the tower above the roadway in a transverse direction. In the longitudinal direction, the tower at the roadway had a width of 40 feet longitudinally, and at the top of the tower about a width of 20 feet was required; and the saddles are 19 feet long and between centers of the columns at the top longitudinally there is a distance of 15 feet, making a batter in that direction… On July 22, 1896, the plans were approved. The caissons were to be sunk by the pneumatic method. On top of the caisson, the masonry would be placed. Unlike the Brooklyn Bridge, the masonry only extended a short distance above the river level. The anchorages were to be approximately 150 feet by 180 feet in plan and 75 feet high. continued on next page

In May 1897, the commissioners approved a slightly revised plan. At that time, Buck estimated that he could build the bridge for $7,510,000 and, “if all the contracts for the bridge are completed” in accordance with his schedule, would open on January 1, 1900. Contracts for the foundation, caissons, and stonework of the two towers and the two anchorages were let in 1897 for a sum of over $2.4 million dollars. Work on the tower foundations began in the late summer and early fall of 1897. They were completed in September 1898. Work on the anchorages also started in late 1897, with the Manhattan anchorage completed in June 1898 and the Brooklyn anchorage completed in December 1899. In 1898, in a general election, the Greater New York Act was passed that merged the cities of New York, Brooklyn, Queens, the East Bronx, and Staten Island. The commissioners and Buck were of the opinion that this change in governmental structure would have no impact on the construction of the bridge. They were wrong. The new Mayor, Robert A. VanWyck, relieved the commissioners and replaced them with an entirely new set of commissioners on January 20, 1898. The New York Times reported, “Engineer Buck, who was an engineer for the old commission, will probably be retained. President Nixon said that the new commission had every confidence in Mr. Buck and that there was no reason for a change.” The cities did not always make sufficient funding available but, in spite of this and other delays, the tower piers and anchorages went on without much delay. The contract to spin the 18½-inch diameter cables and attach all the suspenders was awarded to John A. Roebling Son’s Company on January 1, 1900, for the sum of $1,398,000. Buck made many changes to what Washington Roebling used at the Brooklyn Bridge on his cables and suspenders. Instead of galvanizing the wires, he took other steps to minimize corrosion. His first step was to have the wire soaked in hot linseed oil at the mill. After the wire was spun into a strand consisting of 281 wires and banded, he had all the voids filled with what he called a “special anti-oxidation filling.” When the strands were all banded into a single cable, he had the voids filled with the same material. The next step was to attach the suspender castings tightly to the cables every 20 feet. Instead of wrapping the cables with wire as Roebling had done, Buck decided to encase the wire in a 1/16-inch thick steel jacket overlapped to prevent the penetration of water. Contracts were let for the steel towers to the New Jersey Steel and Iron Company on February 21, 1899, in the amount of $1,220,230. The Pennsylvania Steel Company

won the contract for the suspended steel structure and both approaches at a price of $1,123,400. The project was primarily a Rensselaer Polytechnic Institute project with Buck and Nichols as engineers, and John A. Roebling & Sons with Washington Roebling still active in the company along with his son who was also a Rensselaer graduate. John V. W. Reynders from the Pennsylvania Steel Company was also a Rensselaer man. The deck structure was much heavier than that at the Brooklyn Bridge. The four cables were placed at the ends of long, deep plate girders. The cables started 34 feet apart at the end of the anchorage, converging to 22 feet apart on top of the towers. From there they converged to 4 feet apart at mid span. Wire spinning would not begin until November 27, 1901. Once commenced, however, the cable spinning was carried to completion in only seven months, the last wire laid on June 27, 1902. Once again things were going well for Buck and the bridge when, on January 1, 1902, Seth Low became Mayor of New York. He, in turn, appointed Gustav Lindenthal as Bridge Commissioner with full authority for the bridges planned and under construction in the city. One of Lindenthal’s first actions was to reorganize “his office by appointing Mr. L. L. Buck as chief engineer of the entire department, with Mr. R. S. Buck as a principal assistant engineer and chief engineer of the East River Bridges No. 3 and 4 (Blackwell’s Island and Manhattan Bridges)…Mr. O. F. Nichols remains in charge of the New East River Bridge with the rank of principal assistant engineer...” It became immediately clear that Lindenthal and the Buck group did not see eye to eye on the design of large bridges, particularly the East River Bridge. R. S. Buck resigned on May 1, 1902. Leffert Buck, with the support of many New York Engineers, vocally and in the press, challenged Lindenthal, who threatened to fire him. On May 3, 1902, Mayor Low, to clarify responsibilities and resolve the fighting, issued the following notice: “The Bridge Commissioner has appointed Mr. Leffert L. Buck consulting engineer of the Williamsburg Bridge, at a salary of $7,500. This is in accordance with an understanding mutually agreed upon in consultation with the Mayor and Mr. Buck. The intention is to assure to Mr. Buck the same relation to this bridge, as its engineer, which he has occupied from the beginning.” Other than a reduction in salary of $2,500, Buck was still in charge of the bridge with Nichols as his assistant. O. F. Nichols was fired by Lindenthal on July 1 for his statements against Lindenthal’s change in the design of the Manhattan Bridge. One of the few problems of this period occurred on November 10, 1902, when a

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worker’s shanty on top of the Manhattan tower caught fire. Lindenthal appointed a commission consisting of L. L. Buck, George Morison, and C. C. Schneider to report “as to the extent and manner in which repairs shall be made to the steel wire cables and the other steelwork...” After testing the wires, they determined the strength of the outside cable had been reduced by only 2.5%. Since the wire as furnished was 12% stronger than specified, they determined that the cables were still stronger than needed.

Bridge Opening The opening of the bridge on December 19, 1903, was a great success featuring a large parade in the afternoon. The New York Times wrote, “NEW BRIDGE IN A GLORY OF FIRE; Wind-Up of Opening Ceremonies a Brilliant Scene. BIG FLEET IN PARADE – Daylight Dedication Ceremonies and Night Spectacle Witnessed by Immense Crowds – Enthusiasm on Both Sides of the River.” Buck would not ride with Lindenthal near the head of the parade and instead rode in his own carriage with his wife. When he got to the spot where the ceremonies were to be held, he and O. F. Nichols took a seat well away from the speaker’s stand. It was now time for Mayor Low to accept the bridge, but to whom would he give credit for the design, Buck, Lindenthal, Nichols or would he simply not mention the designers at all? To the surprise of many, Low was very complementary to Buck, mentioning his Civil War Experience and more. This tribute to Buck surpassed that given to most bridge engineers at or since that time. The foot walks opened later, on April 24, 1904. It was estimated that over 25,000 people crossed between 10:00 am and 3:00 pm on that date. The first cars of the Metropolitan system crossed the bridge on the last day of 1904. Riding on that first car were many engineers and dignitaries, including “Chief Engineer and Mrs. Lefferts Buck.” The Bridge was named a National Historic Civil Engineering Landmark in 2009 and remained the longest span suspension bridge in the world for 21 years, until the Bear Mountain Bridge across the Hudson River opened in 1924 with a span of 1,631 feet.▪ Dr. Frank Griggs specializes in the restoration of historic bridges, having restored many 19 th Century cast and wrought iron bridges. He was formerly Director of Historic Bridge Programs for Clough, Harbour & Associates LLP in Albany, NY, and is now an Independent Consulting Engineer. Dr. Griggs can be reached at fgriggsjr@verizon.net.

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discussion of legal issues of interest to structural engineers

Understanding the Difference between Indemnification and Insurance By Gail S. Kelley, P.E., Esq.

ndemnification clauses in design agreements are often considered to be “boilerplate” – something to be read quickly (if at all) after the parties have agreed on the scope of work and compensation. However, if a claim arises from the engineer’s services, an overly broad indemnification clause can create an uninsurable and potentially costly liability for the engineer. The article Understanding Indemnification Clauses published in the January 2017 issue of STRUCTURE provided an overview of indemnification clauses. This article takes a closer look at indemnification clauses and compares indemnification with insurance. In many design agreements, the insurance and indemnification obligations are in the same section, which can create confusion. The agreement may further confuse the issue by requiring that the Indemnitees (the parties being indemnified) be listed as “additional insureds” on some of the engineer’s insurance policies. While both insurance and indemnification provide financial protection to the covered individuals, it is important to understand the difference between the obligations.

Additional Insureds The insurance obligations in a design agreement generally consist of the policies that the engineer is required to carry and the limits of each policy. The policies typically required are Commercial General Liability (CGL); Commercial Automobile Liability; Workers’ Compensation / Employers’ Liability, and Professional Liability Insurance (PLI). The agreement may also state that various entities must be named as additional insureds on certain policies. When an entity is an additional insured on another party’s insurance policy, it is covered by the policy under essentially the same terms as the Named Insured (the party that the policy was issued to), subject to any restrictions in the additional insured endorsement. Often, an engineer will be required to name its Client and the Client’s lender (when the Client is the Owner) as additional insureds on its CGL policy. If a claim is filed against the Additional Insured for injury or property damage suffered by a third party, and

the injury or property damage was caused, at least in part, by the Named Insured, the Additional Insured will be covered under the policy, subject to the terms and limits of the policy and any restrictions in the endorsement. The Additional Insured is covered even if the Additional Insured’s negligence was primarily responsible for the claim. However, most claims against an engineer will fall under its PLI, particularly if the engineer is not providing construction administration or doing work such as surveying or condition assessments which require the engineer to be on site. PLI policies do not allow additional insureds to be added to the policy; if the Client is performing design work or other professional services that could contribute to a negligence claim, it needs to be covered under its own PLI policy. Since the Client cannot file a claim directly under the engineer’s PLI, most design agreements require the engineer to indemnify its Client against claims caused by the engineer’s negligence. Indemnification clauses are typically written such that they apply to claims arising under the engineer’s CGL insurance as well as its PLI policy. This provides additional protection to a Client who has been named as an additional insured on the engineer’s CGL policy. For example, if the engineer’s employee was injured while working on-site and filed a claim alleging that the Client was partially responsible, the Client could either file a claim under the engineer’s CGL insurance or seek indemnification from the engineer. However, the indemnification obligation is completely independent of the engineer’s insurance. In particular, naming the Client as an additional insured does not provide insurance against the indemnification clause. The extent of the protection provided to an additional insured is determined by the wording of the additional insured endorsement and the other terms of the insurance policy, not the wording of the indemnity clause.

The Indemnification Obligation The indemnification obligation is between the engineer and its Client; if the engineer

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agrees to indemnify the Client for claims that are not covered by insurance, the engineer will be responsible for the claims itself. As an example, PLI only covers claims to the extent they are caused by the engineer’s negligence. If the engineer agrees to indemnify the Client for “all claims arising from its services,” it could be liable for the entire claim, even if the claim was partly caused by the Client or a third-party. The portion of the claim that was not caused by the engineer’s negligence would not be covered by PLI. Likewise, PLI does not cover defense of claims against indemnified parties; an indemnification clause that requires the engineer to defend claims arising from its professional services can expose the engineer to uninsurable risk. An example of a well-written indemnification clause is the one in AIA C401, Standard Form of Agreement Between Architect and Consultant, which is often used when the structural engineer is providing its services as a subconsultant to the Architect. § 8.3 The Consultant shall indemnify and hold the Architect and the Architect’s officers and employees harmless from and against damages, losses and judgments arising from claims by third parties, including reasonable attorneys’ fees and expenses recoverable under applicable law, but only to the extent they are caused by the negligent acts or omissions of the Consultant, its employees and its consultants in the performance of professional services under this Agreement. The indemnification clause in DBIA 540, Standard Form of Agreement Between DesignBuilder and Consultant, can also be used as a model; however, two changes are recommended, as shown below. An “agent” can be almost anyone with a connection to the

Owner; many risk management consultants recommend not providing indemnification to such an ill-defined universe of entities. Also, attorneys’ fees should be explicitly limited to those that are reasonable in terms of the claim. 10.2.1 Design Consultant, to the fullest extent permitted by law, shall indemnify and hold harmless Owner, DB and their officers, directors, and employees from and against losses, and damages including reasonable attorneys’ fees and expenses, for bodily injury, sickness or death, and property damage or destruction (other than to the Work itself ) to the extent resulting from the negligent acts or omissions of Design Consultant, anyone employed directly or indirectly by any of them or anyone for whose acts any of them may be liable. It should be noted that, unlike the indemnification clause in the AIA C401, under the DBIA 540 the Indemnitee is entitled to attorneys’ fees to the extent the claim resulted from the engineer’s negligence, even if the fees are not recoverable under state law. Under the law in some states, a successful plaintiff in a negligence action is entitled to recover its attorneys’ fees, but this is not true in all states. PLI generally will not cover attorneys’ fees

unless they are recoverable under state law, but many clients will not accept the limitation that attorneys’ fees are only indemnified to the extent recoverable under state law and will require language similar to that of the DBIA 540. Depending on state law and the terms of the engineer’s PLI, this language can result in an uninsurable risk.

A Caution for Contracts Governed by California Law Under California law, an agreement to indemnify a claim arising from a design or construction project includes a duty to defend, unless there is an explicit disclaimer. If the project is in California or the parties have agreed that the design agreement will be governed by California law, the indemnification clauses cited above should be qualified by the addition of a sentence such as: “The obligation to indemnify shall not extend to the defense of professional liability claims.”

Conclusion Unfortunately, indemnification clauses are often extremely long and difficult to understand. Nevertheless, the wording of

the indemnification clause in every design agreement should be examined closely to determine whether the indemnification obligations will be covered by insurance. If the Client insists on wording that will result in uninsurable risks, the engineer should consider requiring that there be a limitation on its liability.▪ Gail S. Kelley is a LEED AP as well as a professional engineer and licensed attorney in Maryland and the District of Columbia. Her practice focuses on reviewing and negotiating design agreements for architects and engineers. She is the author of Construction Law: An Introduction for Engineers, Architects, and Contractors, published by Wiley & Sons. Ms. Kelley can be reached at Gail. Kelley.Esq@gmail.com.

Disclaimer: The information in this article is for educational purposes only and is not legal advice. Readers should not act or refrain from acting based on this article without seeking appropriate legal or other professional advice as to their particular circumstances.

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Spotlight

Pterodactyl, Culver City, California By Hooman Nastarin, P.E. NAST Enterprises Corp. was an Outstanding Award Winner for its Pterodactyl project in the 2016 NCSEA Annual Excellence in Structural Engineering Awards Program in the Category – New Buildings under $10M.

he “Pterodactyl” is a uniquely engineered office building, conceived for an advertising agency, constructed above a previously designed four-story parking garage structure in Culver City, Los Angeles. The Pterodactyl is formed by the intersection of nine rectangular boxes stacked on top or adjacent to each other, connected by interior second-floor bridges, and supported on the steel column grids extended from the parking structure. The Pterodactyl and the parking structure are both made of structural steel members, strategically positioned to satisfy demanding architectural design as well as mechanical, plumbing, and structural requirements. During the design development, Mr. Nastarin and his colleagues determined that only Structural Steel offered the flexibility and strength to both support and accommodate the slick profiles of the visible shell. The entirely open, main floor, with full height glass enclosures, provides uninterrupted views of the surrounding cities. The picturesque windows at the upper floor carefully highlight expansive scenic views to the east, north, and west. The south facing walls, however, are constructed with metal studs covered with fire-retardant, treaded plywood to receive Rheinzink panels. The partial mezzanine level and interconnecting bridges are made of composite steel beams with concrete filled metal deck, complementing the orientation of the boxes and rigorously following the award-winning architectural design of Eric Owen Moss. Through countless work study sessions, the feasibility and constructability of each element were carefully examined, while realizing numerous architectural features in each box.

Challenges Placing a profoundly irregular Pterodactyl over a pre-existing building presented several structural challenges. The structural design team at Nast Enterprises was limited by the size and orientation of the parking garage’s existing wide flange columns. Through close collaboration with the architectural team, each

box was strategically reinforced internally or was interconnected to others to redirect the loads to stronger columns. On the other hand, the differential rigidities of each box, in conjunction with their inter-connecting mezzanine space, had to be fully optimized to allow for utility and mechanical spaces. The boxes were initially designed individually, using conservative stiffness assumptions. Then the boxes were brought together and reevaluated as a whole for compatibility. The office building is laterally flexible in contrast with the very rigid parking building and the concrete block elevator shaft. Various slip connections and separation joints are installed to control inter-level movements, especially at the three access stairs unevenly stretching down to the top of the parking structure. Another unique feature of the office building is the open balconies and the executive offices, extending beyond the west face of the garage, over and beyond the access ramps. The design of the structural system at each projection, against numerous degrees of freedom, was particularly challenging. This was due to the asymmetrical shape, location, and stiffness of the supports, conforming to ramp clearances, styling of the building envelope, and the location of the windows.

The System The initial task of supporting the blended yet individual boxes, which comprise the Pterodactyl, on eighteen (18) existing columns was achieved by utilizing more than thirty (30) distinctive “Ring” like steel frames to carry the “Primary” and “Secondary” members, elegantly showcased within the interior design. The shape and interaction of the steel frames allowed for much of the needed flexibility to resolve the most challenging aspect of this building. The columns’ sizes and orientation were based on the demands of the parking building. The Pterodactyl’s distinctive modular design did not particularly align with the support points. An intricate stiffness sharing system of rings connected via main and

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February 2017

Courtesy of Tom Bonner

secondary structural members was utilized to further normalize the loads away from the weaker columns and to help control the vibration and deflection of the boxes. Within the overhangs, continuous cantilevered members placed at corners and in the floor slabs provided support for the steel “Rings” at the far end of the overhangs, which also functioned as stabilizers. The functionality of the secondary members was tested in various load patterns against gravity as well as lateral, rotational, and racking movement.

Constructability Aggressive value engineering and coordination studies were performed on numerous elements to reduce the weight of material, evaluate the visual aspect of the connections, and reconcile the sculptural demand and financial feasibility of the entire project. The intricacy of the project’s design demanded close coordination and interaction of the design teams from initial concept through detailing, to shop drawing production and implementation. NAST Enterprises worked very closely with the teams at Eric Owen Moss Architects, as well as with Samitaur Constructs (the developer), to make this building possible. The primary structural engineering, and architectural and fabrication, software used for the Pterodactyl were RISA-3D, Auto Cad, and Digital Project. The engineers at Nast Enterprises developed apps to maintain efficient bidirectional communication between the structural and architectural models. The firm has presented Lectures at SCI-Arc, UCLA and Woodbury University schools of architecture on methodologies used for design and implementation of the Pterodactyl.▪ Hooman Nastarin is the President of NAST Enterprises Corp. Consulting Structural Engineering Services in Los Angeles, California. He can be reached at Hooman@nastenterprises.com.

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SEA Update

Florida Takes Their Conference on the Road

Sara Guthrie, FSEA Executive Director

In an effort to reach more of our members, the Florida Structural Engineers Association began hosting a traveling statewide seminar. In just a few short years, this annual meeting has become the main fundraiser for the organization. This seminar travels to three different locations within the state: South Florida, Central Florida, and Northeast Florida. Hosting daily seminars in three locations ensures that our members do not have to travel a long distance to connect with other members of the organization. The process of securing adequate education for the three-day event starts early. FSEA’s Continuing Education Chairman, Carlos Sanz, with the assistance of Roberto Hernandez, past Southern Florida Chapter President, begins the search for the right seminar at the beginning of the year. Seminars with topics like significant changes to ASCE 7-16, AISC, masonry, steel, or changes to the Florida Building Code are all presentations that our association is interested in attending and have been well received in the past. It is important that our attendees value these seminars so every seminar provides them with PDH credit. FSEA works hard to accommodate each attendee, speaker, and sponsor. Because the seminars are given on three consecutive days, our chapter officers are busy organizing (or even providing) travel for speakers or sponsors to the next location, assisting with setup/tear down, and confirming food orders, guaranteeing everything runs smoothly. Marketing for the annual seminar starts several months before the event. We typically start by sending out “Save the Date” flyers to all of our members. Later, after all the logistics have been worked out, we send out another mailing with the specifics for each location, general cost of attendance, time to allot for the seminar, etc. We have found success with our mailings, as the annual seminar is still widely received. Each year we recruit several sponsors to take part in the event. Our attendees benefit by widening their horizons in terms of products or services, but our sponsors also gain a great deal by participating. For their efforts, the sponsors are given the opportunity to present to our attendees at any given seminar; sponsors can choose to support one, two, or three days of our seminar. Along with the NCSEA mission, it is important that we strengthen the member organization by growing our membership. In trying to do this, we provide non-members the opportunity to join FSEA, at a discounted rate, by signing up for the seminar. This effort has shown great success, as we receive approximately 20 new members a year with this promotion. Since we started this traveling seminar, our gross revenues have increased by 283%. Our members look forward to this seminar every year. Our speakers do not mind the traveling. We have a good turnout with sponsors. We will continue to do our yearly statewide seminar, as it has proved to be a win-win situation for all.

NCSEA News

SE Exam Review Course

Prepare for the Structural Engineering Exam Designed by NCSEA and leading structural engineers, this targeted course will help you advance on exam day. Single Course

NCSEA Member: Nonmember:

Both Courses

$500 $600

$800 $1000

Group pricing available by calling 312-469-4600

NCSEA SE Review Course Dates Vertical: March 4 – 5, 2017 Lateral: March 18 – 19, 2017 Registration for these live online courses include: • Over 30 hours of seminars led by notable experts • Updates to important codes and references • Recommended publication guide • Recordings available 24/7 after the course

February 2017

Join Fellow Experts The 2017 NCSEA Structural Engineering Summit Committee is seeking 75 minute presentations that deliver pertinent and useful information that the attendees can apply in their structural engineering practices. Submissions on best-design practices, new codes and standards, recent projects, advanced analysis techniques and other topics that would be of interest to practicing structural engineers are desired. The 2017 Summit will feature education specific to the practicing structural engineer, in both technical and non-technical tracks. All sessions will include time for a Q & A.

SAVE THE DATE October 11th –14th, 2017 Washington, D.C. The can’t-miss event for the practicing structural engineer.

To submit an abstract, please fill out the form located on www.ncsea.com.

Call for Volunteers

Join an NCSEA Committee

NCSEA News

Call for Abstracts

3 jam-packed days of educational & networking opportunities:

NCSEA Member Organization members may apply for NCSEA committee positions throughout the year using the Volunteer Application on ncsea.com. Once submitted, the application will be reviewed to ensure NCSEA Member Organization membership and then forwarded to the committee chair(s) for review. Currently, the following committees have openings for new members: • Basic Education Committee: Corresponding Members • Communications Committee • Continuing Education Committee: All volunteers, with emphasis on Young Member Group representatives. • Structural Engineer Emergency Response (SEER) Committee: Qualified Members from the North Midwest, Northwest and Southwest U.S. • Young Members Support Group Committee

News from the National Council of Structural Engineers Associations

• Learn from experts • Connect with fellow practicing engineers from across the U.S. • Visit with providers of the newest in technology and services at our bustling tradeshow

Even if the committee you want to join doesn’t show an opening, we suggest you still apply as committee needs regularly change. Most committees admit new members on a rolling basis while others add members only once per year. The application can be found on www.ncsea.com/committees

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February 2017

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More detailed information on the webinars and a registration link can be found at www.ncsea.com.Subscriptions that include both live and recorded webinars are available for NCSEA members! A library of over 150 Recorded Webinars is now available online 24/7/365. Webinars provide 1.5 hours of continuing education, approved for CE credit in all 50 states.

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March 9, 2017 Updated Concrete Repair Code and Companion Guide In 2013, ACI published the Code Requirements for Evaluation, Repair, and Rehabilitation of Concrete Buildings and Commentary as a standard for the repair of existing concrete structures. This presentation will provide an overview of the code including a discussion of the significant changes in ACI 562-16. There will be an emphasis on the updated version of the guide which includes enhanced project examples based on ACI 562-16. Jay Paul, S.E., FACI

April 4, 2017 Special Inspections for Masonry This presentation will provide an insight on the new requirements for field and lab testing technicians, and the expected improvement in masonry quality control as a result of the new requirements. John Chrysler, P.E.

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February 21, 2017 Designing for Hot-Dip Galvanizing With new, and innovative designs now specifying batch hot-dip galvanizing, it is important for architects, engineers, fabricators, detailers, and other designers to understand the best design practices. This webinar will assist you in the design and integration of batch hotdip galvanizing for corrosion protection in your next project. Alana Hochstein

March 16, 2017 Draining Low-Sloped Roof Structures – Rain Issues for the Structural Engineer Provisions in the current ASCE 7 as well as other applicable design codes can cause the design professional to bear unnecessary risks to denied roofing warranty claims and roof collapses. This webinar will illustrate several case studies of rain-induced collapses, and the perceived responsibility of the Structural Engineer. John Lawson, S.E.

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The Newsletter of the Structural Engineering Institute of ASCE

Structural Columns

Registration Now Open The Premiere Event for Structural Engineering Come for the innovative solutions and cutting-edge knowledge, leave with connections and resources to advance your career. Register before February 15, 2017, to receive the best rates. Be inspired by the extraordinary keynote speakers, network with your colleagues, and earn PDHs. Celebrate the Future of Structural Engineering at the Special Friday Night Reception Join us for a celebration of the future of structural engineering at the Denver Art Museum. Ashraf Habibullah, president and CEO of Computers & Structures, Inc., will host a special dinner reception on Friday, April 7, in conjunction with the Structures Congress 2017. Join Ashraf for an evening of fun, inspiration and fabulous prizes. You will need to purchase a ticket to attend, with 100% of ticket sales benefiting the SEI Futures Fund. Ticket includes endlessly flowing champagne, live entertainment, full dinner, hosted bar, and amazing raffle prizes. Regular tickets are $30, Student tickets are $10, and will be available through March 31, 2017.

Four Sessions Featuring ASCE 7-16 The Structures Congress offers 120 technical sessions on all aspects of the profession. This year we are offering a series of four integrated ASCE 7-16 sessions on Friday, April 7. Attendees will receive a comprehensive overview of the new standard including seismic, wind, and tsunami. • ASCE 7-16 Overview What’s New in the 2016 Edition • ASCE 7-16 Seismic: Learn from the Experts • ASCE 7-16 Wind: Learn from the Experts • ASCE 7-16 Tsunami: The New Resiliency Approach and Design Provisions We expect the convention hotel to sell out well in advance of the official cutoff day, so book your room now. Convention Hotel Hyatt Regency Denver 650 15th Street Denver, CO 80202 Visit the congress website at www.structurescongress.org for more information, download the Preliminary Program, and to register.

Preliminary Vote on ICC Adoption of ASCE 7-16 is Positive ASCE Week Orlando Florida Earn up to 42 PDHs in one week

Don’t miss ASCE Week, March 26 – 31, 2017, at the Wyndham Grand Orlando Resort Bonnet Creek. ASCE Week offers ASCE’s most popular face-to-face seminars in one location. Structural seminars include Designing Nonbuilding Structures Using ASCE/SEI 7-16; Earthquake-Induced Ground Motions; Application of Soil-Structure Interaction to Buildings and Bridges; Financial Management for the Professional Engineer; Investigation, Analysis, and Remediation of Building Failures; Public-Private Partnerships for Transportation Infrastructure; Seismic Loads for Buildings and Other Structures (newly updated for ASCE 7-16). In addition, there will be a special behind-the-scenes tour of Disney. Register by March 3 for special discounts. Learn more at www.asce.org/asceweek.

STRUCTURE magazine

Governmental members of the International Code Council voted to adopt ASCE 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures as an International Code Council reference standard. It is important to note that the voting results are preliminary. The ballot must be certified by ICC’s governing rules before it becomes official. Thank you to all ASCE and SEI members who contacted their ICC voting members in support of ASCE 7-16 adoption. Read the full story at ASCE News at http://news.asce.org/ icc-vote-points-to-asce-7-16-adoption-as-reference-standard.

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

February 2017

Webinar on SEI Benefits, Opportunities, and Vision Initiatives

David Odeh, SEI past president, and Laura Champion, SEI Director, were recently interviewed by Redshift for an article on diversity in engineering. They and others discuss some of the challenges in the profession and opportunities that encouraging diversity presents. Read the article at https://redshift.autodesk.com/diversity-in-engineering.

Listen to the November 29 webinar by SEI President Andy Herrmann and SEI Young Professional Committee Chair Linda Kaplan for a review of SEI member benefits and opportunities, including special offerings for students and professionals to make the most of membership in SEI/ASCE and an update on SEI Vision for the Future of SE initiatives. View the webinar at https://cc.readytalk.com/cc/playback/Playback.do?id=7vckhs.

SEI Local Activities

The SEI St. Louis Chapter and Geo-Institute St. Louis Chapter joined forces to host the first local interdisciplinary conference on geotechnical and structural topics at the Geo-Structures Confluence 2016. This event combined the chapter’s annual Geo-Confluence and SEI Day events for this year. The November 4, 2016, event attracted 200 attendees, featured a joint morning session and two afternoon breakout tracks with topics ranging from case studies on the Wrigley Field project to code related topics of extreme loads. Geotechnical and structural engineers had the chance to mingle and squabble at a stunning Exhibitors Hall and the star-studded panel discussion; Soil is Not a Spring; Buildings are Not a Load. Panelists included SEI’s past-president, David Odeh, G-I’s past-president Kord Wissmann, Terry Holman (Turner Construction), Tom Cooling (AECOM), and Ted Pruess (Independent Consultant). Visit the chapter website at http://sections.asce.org/stlouis/SEI/Home.htm for more information.

Mohawk Hudson Chapter The SEI Mohawk-Hudson Chapter hosted their 6th annual Structures Day. This conference featured technical seminars, an award presentation, and swearing in new officers. Learn more on the SEI News web page at www.asce.org/structural-engineering/ structural-engineering-news. STRUCTURE magazine

Metropolitan (NYC) Chapter Welcome to the new SEI Metropolitan Chapter. The chapter is planning to host four technical lectures from October 2016 through March 2017, and the Spring Seminar Series in May. The 2017 Spring Seminar Series, to be held at the New York Public Library at Lincoln Center, will consist of four evenings of technical lectures and keynote presentations with dinner provided. Participants will be able to earn up to 8 PDHs.

Get Involved in Local SEI Activities Join your local SEI Chapter, Graduate Student Chapter (GSC), or Structural Technical Groups (STG) to connect with colleagues, take advantage of local opportunities for lifelong learning, and advance structural engineering in your area. If there is not a SEI Chapter, GSC, or STG in your area, review the simple steps to form a SEI Chapter at www.asce.org/structural-engineering/sei-local-groups. Local Chapters serve member technical and professional needs. SEI GSCs prepare students for a successful career transition. SEI supports Chapters with opportunities to learn about new initiatives and best practices, and network with other leaders – including annual funded SEI Local Leader Conference, technical tour and training. SEI Chapters receive Chapter logo/ branding, complimentary webinar, and more.

February 2017

The Newsletter of the Structural Engineering Institute of ASCE

St Louis Chapter

Structural Columns

Diversity in Engineering Article in Redshift

CASE in Point

The Newsletter of the Council of American Structural Engineers

CASE Risk Management Tools Available Foundation 1: Culture – Create a Culture of Managing Risks & Preventing Claims • Structural engineering is a high-risk profession • All firms can have professional liability claims • Claims cost money, time, reputation, clients, and staff • Firms must commit to managing risks • Commitment must include management, staff, and clients. All individuals must make a commitment • Quality must take high precedence • Legal environment is always changing Tool 1-1 Create a Culture for Managing Risks and Reducing Claims The most comprehensive CASE tool that provides sample templates and presentations that aid in creating a culture of risk management throughout the firm. Tool 1-2 Developing a Culture of Quality This tool was developed to identify some ways to drive quality into a firm’s culture. It is recognized that every firm will develop its own approach to developing a culture of quality but following these 10 key areas offer a substantial starting point. The tool includes a white paper and customizable PowerPoint presentation to facilitate overall discussion. Foundation 2: Prevention & Proactivity – Act with Preventative Techniques, Don’t Just React. • Anticipate problems and situations that may occur • Identify potential risks and mitigate before they are a problem • Develop processes to prevent errors, duplication, and misunderstanding • Take positive actions at the beginning of projects and don’t procrastinate • Many conditions can be altered by positive actions

Tool 2-1 A Risk Evaluation Checklist Don’t overlook anything! A sample itemized list of things you should look for when evaluating a prospective project. Tool 2-2 Interview Guide Getting “the right people on the bus” is one of the most important things we can do to mitigate risk management and yet we never learn about interviewing skills in school. It is the second tool related to the Second Foundation of Risk Management, Prevention and Proactivity. The tool will help your firm conduct higher quality interviews and standardize the process among all your staff. Tool 2-3 Employee Evaluation Templates This tool is intended to assist the structural engineering office in the task of evaluating employee performance. The evaluations provide a method to assess employee performance and serve as an integral part of the company’s risk management program. Tool 2-4 Project Risk Management Plan This plan will walk you through the methodology for managing your project risks, along with a few common project risks and templates on how to record and track them. Tool 2-5 Insurance Management: Minimize Your Professional Liability Premium This tool is designed as a guide to help you provide critical additional information to the underwriter to differentiate your firm from the pack. You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

Donate to the CASE Scholarship Fund! Share Innovative Ideas! Does your firm have an innovative idea or method of practice? Looking to get more involved on 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! STRUCTURE magazine

The ACEC Council of American Structural Engineers (CASE) is currently seeking contributions to help make the structural engineering scholarship program a success. The CASE scholarship, administered by the ACEC College of Fellows, is awarded to a student seeking a Bachelor’s degree, at a minimum, in an ABET-accredited engineering program. Since 2009, the CASE Scholarship program has given $20,000 to help engineering students pave their way to a bright future in structural engineering. We have all witnessed the stiff competition from other disciplines and professions eager to obtain the best and brightest young talent from a dwindling pool of engineering graduates. One way to enhance the ability of students in pursuing their dreams to become professional engineers is to offer incentives in educational support. Your monetary support is vital in helping CASE and ACEC increase scholarships to those students who are the future of our industry. All donations toward the program may be eligible for tax deduction, and you do not have to be an ACEC member to donate! Contact Heather Talbert at htalbert@acec.org to donate. February 2017

CASE in Point

CASE Risk Management Convocation in Denver, CO April 7, 2017 The CASE Risk Management Convocation will be held in conjunction with the Structures Congress at the Hyatt Regency Denver and Colorado Convention Center in Denver, CO April 6 – 8, 2017. For more information and updates go to www.structurescongress.org. The following CASE Convocation sessions are scheduled to take place on Friday, April 7: 8:00 am – 9:30 am

Contractual Risk Transfers for Professionals: Mastering Indemnity, Insurance and the Standard of Care Moderator/Speaker: Ryan J. Kohler, Collins, Collins, Muir + Stewart, LLP

2:00 pm – 3:30 pm

Projects with the Largest Losses and Claim Frequency Moderator: Mr. Timothy J. Corbett, SmartRisk Speaker: Brian Stewart, Esq., Collins, Collins, Muir + Stewart, LLP

4:00 pm – 5:30 pm

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

Public-Private Partnerships and Design-Build: Opportunities and Risks for Consulting Engineers P3 and DB approaches on public infrastructure projects continue to increase, leaving consulting engineers with more questions than ever. That is why, to make conscientious and prudent decisions about P3 and DB project opportunities and risks, you need access to reliable expertise and the latest knowledge. The second edition of Public-Private Partnerships and DesignBuild: Opportunities and Risks for Consulting Engineers presents new industry information and experience on P3 and DB approaches and offers timely recommendations about the rewards, challenges, and risk exposures for engineering firms looking to succeed in today’s still evolving P3 and DB project work environment. New to the Second Edition: • DB opportunities, roles, risks, and contracting practices • Dispute resolution processes in P3s and DB • Contractual availability and sound implementation of fair and appropriate dispute resolution processes in the assessment and management of P3 and DB projects STRUCTURE magazine

Readers will also find updated information on risk allocation and professional liability issues specific to P3 and DB projects. Edited by David Hatem and Patricia Gary of Donovan Hatem LLP – along with the contributions of 14 subject matter experts – Public-Private Partnerships and Design-Build: Opportunities and Risks for Consulting Engineers, Second Edition provides an objective, realistic and practical resource for you to make informed and balanced judgments about pursuing P3 and DB projects. NCSEA members can contact Heather Talbert about obtaining this book at the ACEC Member Price of $99. Heather can be emailed at htalbert@acec.org.

February 2017

CASE is a part of the American Council of Engineering Companies

10:00 am – 11:30 am Construction Administration as a Risk Management Tool Moderator / Speaker: Daniel T. Buelow, Willis Towers Watson

Structural Forum

opinions on topics of current importance to structural engineers

ASCE 7-16 Controversy A Long Overdue Wake-up Call By Jim DeStefano, P.E., AIA, F.SEI

have been watching, with some interest as the recent drama unfolded, the effort to block the adoption of the American Society of Civil Engineers’ ASCE 7-16 into the 2018 International Building Code (IBC). I was particularly amused to see the way that the structural engineering community has rallied in defense of a standard that they openly despise. If you get more than two structural engineers in a room, it is only a matter of time before they start complaining about the latest edition of ASCE 7 and the misery that it has brought to their practice. Has ASCE 7 improved the practice of structural engineering or the lives of structural engineers? The answer is easy and not particularly controversial. There have been many editorials written about the misery that ASCE 7 has brought to the practice of structural engineering, yet I do not recall ever seeing an editorial extolling the virtues of the standard. When I first started practicing forty years ago, the building code section on structural loading was somewhat brief and only filled a few pages. Although the loading provisions were easy to understand and interpret, they were not sufficient. The American National Standards Institute (ANSI) Standard 58.1, first released in 1972, was a huge improvement. It contained all of the important stuff that had been missing from previous building codes, such as snow drift loads and a rational approach to wind pressures, yet it was still easy to understand and use. When ASCE 7-88 replaced ANSI 58.1-82, the loading provisions became more complex and less intuitive. It has been downhill ever since. Today, structural engineers must spend a disproportionate amount of their time determining the loading criteria for their projects rather than designing the structures. Has ASCE 7 improved the safety of structures? The justification for more complex loading provisions has always been that better, more accurate loading data results in safer structures, but is that really true? There is not much evidence to support that argument. Building structures that were designed before 1988 do not seem to be collapsing. Those

buildings that do fail during extreme events, such as hurricanes, blizzards, and earthquakes, are mostly non-engineered and pre-engineered structures with flawed designs. Several years ago, the Structural Engineering Institute/ Business and Professional Activities Division (SEI-BPAD) committee embarked on a trial design program. A group of experienced structural engineers was asked to solve a handful of routine design problems requiring the application of ASCE 7. The results were distressing. The answers were so scattered that they did not fit into a bell curve and the committee members could not even agree on what the correct answers were. The conclusion was obvious. Overly complex loading provisions have increased the risk that an engineer will misinterpret the loading provisions and under design a structure. Do we need a cookbook for structural engineering? There seems to be a belief, held by many engineers that serve on standards committees, that building code adopted standards should be written as cookbooks that prescribe each step that an engineer takes in designing a structure. This kind of thinking has had a deleterious effect on the profession and tends to stifle innovation and the application of sound engineering principles. We should not need a cookbook to tell us how to design a structure. What we really need is stability in our building codes! It is reasonable to expect codes and standards to be improved, refined, and to be made more understandable with each new edition. Revisions must be made to make confusing provisions easier to understand and apply. However, when each new edition of ASCE 7 unveils an entirely different way of calculating wind loads, or maybe six different ways to calculate wind loads, it only results in chaos and instability. Can everything that we have been doing up until now really be that wrong? Do we really need to relearn how to calculate loads every six years? Should the structural engineering community be a rubber stamp for new standards? Every time a new edition of ASCE 7 is

released, everybody complains and gripes. Then they suck it up and buckle down to try and learn the new provisions. Like good sheep, we all go along. Recently, other construction industry groups like the National Association of Home Builders (NAHB) and the National Roofing Contractors Association (NRCA) have taken a close look at some of the provisions in ASCE 7-16 and found the standard to be unreasonable and out of touch. Could it be that they are right? The structural engineering community reacted defensively. We may feel that it is our profession that is being attacked – how dare these guys suggest that a standard produced by ASCE not be adopted into the IBC. Where do we go from here? Maybe it is time to take back our profession – make structural engineering great again. Despite all the grumbling, the ASCE 7 committee has not gotten the message. We need a reasonable and practical standard for calculating loading criteria that does not keep changing. I do not mean to belittle or demean the hard work that has gone into writing the ASCE 7 standard. I have served on SEI standards committees, and I know the effort that goes into them. However, the standards committee needs to be sensitive to all of the unnecessary hard work and lost profits they have generated for all of us that are trying to make a living designing structures. We cannot turn back the clock to 1982 and go back to the ANSI 58.1 standard, but it would not be so bad if we did. Maybe those guys at NAHB and NRCA have the right idea and are not really anarchists. If we want to take back our profession, a grassroots movement is needed. Not just at the ICC hearings, but at every state level. If we, as structural engineers, start lobbying to delete ASCE 7 from our local state building codes in favor of simple, understandable loading provisions, maybe then our message will be heard.▪ Jim DeStefano is the President of DeStefano & Chamberlain, Inc. located in Fairfield, CT. He is the past-pastchairman of the STRUCTURE magazine editorial board. Jim can be reached at jimd@dcstructural.com.

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

February 2017

opinions on topics of current importance to structural engineers

Structural Forum

ASCE 7 Controversy A Rebuttal By Ronald O. Hamburger, S.E., SECB, FSEI

im DeStefano raises many good points as to the complexity of the building codes in general and the ASCE 7 standard in particular. I have made these same arguments many times over the years, in this same magazine and other venues. However, the challenges to adoption of ASCE 7-16 had nothing to do with code complexity or changes in design procedures. Rather, these challenges were about two things: 1) significantly increased values of wind pressure coefficients at areas of discontinuities on roofs, the principal concern of the roofing industry; and 2) changes to site class coefficients for long period structures on soft soil sites, causing an increase in seismic design values for some structures. Countering these increases in design conservatism, the wind speed maps have been revised based on the availability of long-term wind data from hundreds of stations, allowing substantial reductions in design wind speeds and design wind loads across most of the U.S. In fact, except in exposure D, limited to a 600-foot wide strip along the Atlantic and Gulf coasts, these speed reductions mostly counter the change in cladding coefficients and allow substantial reductions in the required strength of the main wind force resisting system. Further reductions in wind load can be obtained by accounting for reduced air density at high elevation sites, allowing substantial reductions in wind pressures in places like Denver and Reno. What else has changed? Well, the snow loading Chapter has indeed become longer and more complex. How? Instead of the so-called “case study” zones on the maps in mountainous regions, the Standard now provide tables with specific ground snow load values for most major communities in the affected areas. Thicker document? Yes. Easier to use? Yes. Other important changes include addition of a chapter on tsunami-resistant design, an Appendix on performance-based fire-effects design, and a substantial update of the seismic nonlinear response history procedures bringing them in line with procedures commonly

used in the Western U.S. The seismic isolation and energy dissipation procedures have been harmonized with those in ASCE 41, which also has been updated to adopt the new response history procedures. The rain load procedures have been made substantially clearer and easy to apply. Another change engineers will likely find useful is the availability of an electronic, web-based version of the standard and a companion tool that will enable determination of mapped values of snow, seismic, and wind loading parameters from a single source. This tool will also enable construction of transects to facilitate computation of topography coefficients for wind pressures. Engineers will be able to annotate their personal copies and index them to find frequently used criteria. This aside, I agree that the Standard is far larger, more complex and challenging to use than the design criteria specified by building codes 40 years ago when Jim and I first entered practice. The complexity has slowly grown for several reasons, including, as Jim suggests, a desire to over-prescribe the design procedures rather than allowing engineers to use basic knowledge and judgment to determine loads and other facets of design. At the start of this cycle, I made a significant effort to reverse this, simplify the procedures, and eliminate prescription. At one point I pushed for a two-volume standard; one containing basic procedures that would apply to the design of most ordinary buildings, and the other containing more complex procedures used only a fraction of the time. The basic procedures would have included criteria for dead and live loads, snow loads for buildings of simple geometry, the simplified wind procedure, and the equivalent lateral force procedure for seismic. All other procedures, used only a fraction of the time, if ever, would have appeared in the second volume. We felt most engineers would use only the first volume, which they would find short and user-friendly. Those engineers who design more complex structures would go to the second volume, where

the more elegant procedures would reside. Ultimately this concept was discouraged by ASCE staff as being confusing, since some loads, such as wind and seismic, would have chapters in multiple volumes. Perhaps we will find a way to do this in future editions As noted, there is little doubt the codes are complex. In addition to the tendency to overprescribe calculation procedures, previously discussed, there are other reasons for this complexity. Most engineers state they want the codes to be simple, reliable, and result in economical construction. My opinion is that you can satisfy only two of these at a time. The codes of 40 years ago were simple, less economical than today’s requirements, and far less reliable. Using today’s standards, you can still design simply and the design will be reliable. However, the resulting design likely will not be economical. Our standards have been developed assuming most engineers would prefer to use more complex procedures that are both economical and reliable. In the end, complex evolving codes and standards do place a burden on engineers. We cannot complacently leave school thinking that we know everything that we will ever have to know. Instead, we have to keep current with developments in our field, learn new procedures, and yes, do more work. Of course, 40 years ago, the electronic slide-rule calculator was just becoming a mainstay. Today we have untold power at our fingertips in the form of personal computers, with far more power than the IBM and Sperry mainframes of 40 years ago, to help us deal with the complexity. Do we really want to go back to the world of the 1970s? I do not think so.▪ Ronald O. Hamburger is a Senior Principal at Simpson Gumpertz & Heger in San Francisco. He has been active in the development of seismic requirements of building codes and standards since the 1980s. He presently chairs the ASCE 7 Committee. Ron can be reached at rohamburger@sgh.com.

February 2017

STRUCTURE magazine | February 2017

Articles inside

LEGAL PERSPECTIVES

CONSTRUCTION ISSUES

CODES AND STANDARDS

STRUCTURAL FORUM

STRUCTURAL FORUM

HISTORIC STRUCTURES

STRUCTURAL FORENSICS

ENGINEER’S NOTEBOOK