STRUCTURE magazine February 2019

STRUCTURE FEBRUARY 2019

NCSEA | CASE | SEI

CFS/ Steel INSIDE: Tall Buildings in Mexico City

18

Composite Steel Deck-Slabs Welded Steel Moment Frames Oakland International Airport

10 15 26

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Baystate Noble Hospital Entryway Renovation Steffian Bradley Architects with BVH Integrated Services Photography by J. Michael Worthington, Jr.

LeMessurier Calls on Tekla Structural Designer for Complex Projects Interoperability and Time Saving Tools

Tekla Structural Designer was developed specifically to maximize collaboration with other project parties, including technicians, fabricators and architects. Its unique functionality enables engineers to integrate the physical design model seamlessly with Tekla Structures or Autodesk Revit, and to round-trip without compromising vital design data.

“We’re able to import geometry from Revit, design in Tekla Structural Designer and export that information for import back into Revit. If an architect makes geometry updates or changes a slab edge, we’ll send those changes back into Tekla Structural Designer, rerun the analysis and design, and push updated design information back into Revit.”

Tekla Structural Design at Work: The Hub on Causeway

For over 55 years, LeMessurier has provided structural engineering services to architects, owners, contractors, developers and artists. Led by the example of legendary structural engineer and founder William LeMessurier, LeMessurier provides the expertise for some of the world’s most elegant and sophisticated designs while remaining true to the enduring laws of science and engineering.

Known for pushing the envelope of the latest technologies and even inventing new ones, LeMessurier engineers solutions responsive to their clients’ visions and reflective of their experience.

An early adopter of technology to improve their designs and workflow, LeMessurier put its own talent to work in the eighties to develop a software solution that did not exist commercially at the time. Their early application adopted the concept of Building Information Modeling (BIM) long before it emerged decades later.

While LeMessurier’s proprietary tool had evolved over three decades into a powerhouse of capability, the decision to evaluate commercial structural design tools was predicated on the looming effort required to modernize its software to leverage emerging platforms, support normalized data structure integration and keep up with code changes.

After a lengthy and thorough comparison of commercial tools that would “fill the shoes” and stack up to the company’s proprietary tool, LeMessurier chose Tekla Structural Designer for its rich capabilities that addressed all of their workflow needs. According to Derek Barnes, Associate at LeMessurier, ” Tekla Structural Designer offered the most features and the best integration of all the products we tested. They also offered us the ability to work closely with their development group to ensure we were getting the most out of the software.”

One Model for Structural Analysis & Design

From Schematic Design through Construction Documents, Tekla Structural Designer allows LeMessurier engineers to work from one single model for structural analysis and design, improving efficiency, workflow, and ultimately saving time. “Our engineers are working more efficiently because they don’t need to switch between multiple software packages for concrete and steel design. Tekla Structural Designer offers better integration of multiple materials than we have seen in any other product,” said Barnes.

LeMessurier engineers use Tekla Structural Designer to create physical, information-rich models that contain the intelligence they need to automate the design of significant portions of their structures and efficiently manage project changes.

TRANSFORMING THE WAY THE WORLD WORKS

“Tekla Structural Designer has streamlined our design process,” said Craig Blanchet, P.E., Vice President of LeMessurier. “Because some of our engineers are no longer doubling as software developers, it allows us to focus their talents on leveraging the features of the software to our advantage. Had we not chosen to adopt Tekla Structural Designer, we would have needed to bring on new staff to update and maintain our in-house software. So Tekla Structural Designer is not just saving us time on projects, it is also saving us overhead.

Efficient, Accurate Loading and Analysis

Tekla Structural Designer automatically generates an underlying and highly sophisticated analytical model from the physical model, allowing LeMessurier engineers to focus more on design than on analytical model management. Regardless of a model’s size or complexity, Tekla Structural Designer’s analytical engine accurately computes forces and displacements for use in design and the assessment of building performance.

“Tekla Structural Designer offers better integration of multiple materials than we have seen in any other product.”

Positioning a large scale mixed-use development next to an active arena, a below grade parking garage, and an interstate highway, and bridging it over two active subway tunnels makes planning, phasing and engineering paramount. Currently under construction, The Hub on Causeway Project will be the final piece in the puzzle that is the site of the original Boston Garden.

Despite being new to the software, LeMessurier decided to use Tekla Structural Designer for significant portions of the project. “Relying on a new program for such a big project was obviously a risk for us, but with the potential for time savings and other efficiencies, we jumped right in with Tekla Structural Designer. It forced us to get familiar the software very quickly.”

“Tekla Structural Designer allowed us to design the bulk of Phase 1 in a single model,” said Barnes. The project incorporates both concrete flat slabs and composite concrete and steel floor framing. “Tekla Structural Designer has the ability to calculate effective widths based on the physical model which is a big time saver,” said Barnes. “On this project, the integration with Revit, along with the composite steel design features enabled us to work more efficiently. Adding the ability to do concrete design in the same model was a bonus because we had both construction types in the same building.”

“Tekla Structural Designer helped this project run more efficiently, and in the end it was a positive experience,” said Blanchet.

“Tekla Structural Designer gives us multiple analysis sets to pull from, which gives us lots of control. Most programs don’t have the capability to do FE and grillage chase-down. For the design of beam supported concrete slabs, Tekla Structural Designer allows us to separate the slab stiffness from the beam stiffness, so if we choose to we can design the beams without considering the influence of the slab. In the same model we can use a separate analysis set to review the floor system with the beams and slab engaged,” said Barnes.

Barnes also shared similar benefits with concrete column design. “Tekla Structural Designer does grillage take-downs floor-by-floor, finds the reactions and applies them to the next floor. This allows us to view column results both for the 3-dimensional effects of the structure as a whole and from the more traditional floor-by-floor load take-down point of view.

Doing both has always required significant manual intervention, but Tekla Structural Designer puts it all in one place.” “We reduce the possibility for human error because with Tekla Structural Designer less user input is required,” said Barnes. “Tekla Structural Designer automatically computes many of the design parameters, such as column unbraced lengths. The assumptions made by the software are typically correct, but we can easily review and override them when necessary.”

“Tekla Structural Designer provided the best fit for our workflow compared to other commercially available software.”

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Contents FEBRUARY 2019

Features

Columns and Departments 7

Editorial Leadership Is Showing Up

By Jon A. Schmidt, P.E., SECB

9

InFocus My Thoughts on the Future of STRUCTURE

By John A. Dal Pino, S.E.

10

Structural Design Composite Steel Deck-Slabs with Supplemental Reinforcing Bars

By Vitaliy Degtyarev, Ph.D., P.E., S.E.

15

Northridge – 25 Years Later Welded Steel Moment Resisting Frames

26 OAKLAND INTERNATIONAL AIRPORT

By Jamison Curry, S.E., Peter Revelli, S.E., and Marko Schotanus, S.E., Ph.D.

This Terminal 1 building’s retrofit scheme introduced new vertical

By Ronald O. Hamburger, S.E., and James O. Malley, S.E.

18

for Tall Buildings in Mexico City – Part 2

seismic force-resisting elements at the perimeter, consisting of special

By Ahmad Rahimian, Ph.D., P.E., S.E., et.al

reinforced concrete shear walls and buckling-restrained braced frames.

A new diaphragm was created at the roof level by the addition of a

Structural Performance Structural Design Challenges

22

Structural Systems Mid-Rise Wood-Frame Buildings

By Richard McLain, P.E., S.E.

horizontal truss system comprised of hollow structural sections.

34

Historic Structures Joseph B. Strauss Bascule Bridge

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

39

Legal Perspectives Indemnification versus Defense – Part 1

By Gail S. Kelley, P.E., Esq.

50

InSights Performance-Based Design is the Future

By Donald O. Dusenberry, P.E., SECB

In Every Issue

30 HISTORIC GOODMAN LIBRARY

By Chris Jonas, S.E., and Steve Heyne, S.E.

The Goodman Library’s stone walls sustained damage in the form

4 42 44 46 48

Advertiser Index Resource Guide – Bridge Resource NCSEA News SEI Update CASE in Point

On the Cover

The Torre Siqueiros project is an example of recent Performance-Based

of cracks during the powerful earthquake that struck in 2014 in

Design of a super tall building in Mexico City. Read how PBD approaches

downtown Napa, California. However, it was the partial collapse

are allowing buildings to reach greater heights and deeper basements in

of its prominent tower that was remarkable. Repairs required a lot of engineering finesse and close attention to historic detail.

the Structural Performance article on page 18. (Project Architects: Bunker Arquitectos and Colonnier y Asociados)

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

F E B R U A R Y 2 019

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EDITORIAL Leadership Is Showing Up

By Jon A. Schmidt, P.E., SECB

I

t is a tremendous honor for me to serve as the 26th President of the National Council of Structural Engineers Associations (NCSEA).

Unlike most (perhaps all) of my predecessors in this office, I am not the owner of my own firm, or a partner, or a principal, or a project manager, or a department head, or a supervisor of any kind. How did an ordinary practicing structural engineer like me wind up in such a lofty position?

Like many in our profession, my undergraduate degree was in civil engineering. Therefore, it was natural for me to join the American Society of Civil Engineers as a freshman in college, a membership that I have retained ever since. In 1998, ASCE established the Structural Engineering Institute and I eagerly signed up as a Charter Member.

I have been active in both ASCE and SEI in various ways over the years, including several leadership roles at the national level.

It must have been only a year or two later that I started receiving a magazine that I had never seen before. It was called STRUCTURE, and it consistently contained articles that struck me as being of immense value to a practicing structural engineer like myself. It also provided information about groups called Structural Engineers Associations; as someone living in Kansas and working in Missouri, I had never heard of those. There was even a National Council of such organizations, and it apparently had a committee for helping people start new ones in their states.

In the summer of 2001, I sent an e-mail to its chair, Marc Barter, asking whether there was any such effort underway in my area. The gist of his reply was, “There is now!” Before long, he had connected me with others who had also expressed interest, and four of us ultimately co-founded the Structural Engineers Association of Kansas & Missouri (SEAKM) – the first, and still the only, explicitly bi-state Member Organization of NCSEA. I attended my first NCSEA Structural Engineering Summit that fall, although we called it the Annual Conference back then. The keynote speaker was Leslie Robertson, who gave an emotional presentation STRUCTURE magazine

about his most famous project, the World Trade Center towers in New York. This was only a few weeks after the tragic events of 9/11.

A couple of months later, I became aware of a new industry initiative called The Infrastructure Security Partnership (TISP). Its purpose was “to minimize the effects of terrorism on the U.S. through effective and efficient planning, design, construction, and operation of the built environment.” I promptly volunteered to represent NCSEA and did so for a few years. During the same time frame, I joined the Advocacy Committee and its Clients & Prospects Subcommittee, becoming chair of the latter in 2003; these are now part of the Communications Committee.

My “big break” came in 2005. Ron Hamburger, who was finishing up his term as NCSEA President, asked me to consider becoming chair of the Editorial Board for STRUCTURE. I was receptive, since I enjoyed writing and editing, and had worked with the publisher on articles previously. I was especially intrigued by the prospect of authoring a regular column on topics of my choosing. I took the job, and then kept it for the next decade, producing 60 bimonthly “InFocus” pieces along the way. During those years, I also participated actively in the NCSEA Structural Licensure Committee.

When I decided to step down from the Editorial Board in 2015, I was looking forward to taking a break; but it was not to be. Brian Dekker, then NCSEA Vice President and Chair of the Nominating Committee, asked me to join the Board of Directors for a two-year term as Secretary. That placed me in the rotation to become Vice President last year, and now President.

The point of reciting this history is to highlight the fact that any practicing structural engineer can become a leader in our profession.

The single most important qualification is simply seeking and seizing ways to get involved. As Richard Weingardt, a prominent structural engineer in Colorado who passed away a few years ago, often emphasized in his many writings and speeches, “The world is run by those who show up.”

One such opportunity is on the horizon within NCSEA: developing a new Strategic Plan over the coming months. The current Mission Statement – “NCSEA advances the practice of structural engineering by representing and strengthening its Member Organizations” – has served us well, helping us focus on what makes NCSEA distinctive.

Having now gotten our institutional house in better order, the next plan needs to be more genuinely strategic, beginning with a Vision Statement that is truly aspirational.

The Board of Directors, Committee Chairs, and staff will be direct participants in the process. However, we need input and energy from all of our constituents to be successful – not just this year, but also into the future. How will you show up and make a difference in our profession and our world?■ Jon A. Schmidt is a Senior Associate Structural Engineer in the Aviation & Federal Group at Burns & McDonnell in Kansas City, Missouri. He serves as President on the NCSEA Board of Directors, was the founding chair of the SEI Engineering Philosophy Committee, and shares occasional thoughts at twitter.com/JonAlanSchmidt. (jschmid@burnsmcd.com)

F E B R U A R Y 2 019

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INFOCUS My Thoughts on the Future of STRUCTURE

By John A. Dal Pino, S.E.

I

am quite honored to have been selected as the next Chair of the STRUCTURE Editorial Board. I joined the Board four years ago and have been privileged to serve under past Chairs Jon Schmidt and Barry Arnold, and to work alongside our dedicated, volunteer Board members. This group has worked diligently to carry on traditions, to refine our internal processes, and to improve the magazine’s content and effectiveness. It takes a lot of effort and commitment to publish a magazine every month, rain or shine, good times or bad. There are no schedule extensions in the magazine industry. I want to thank all of them for what I have learned from them and the examples they have set.

As you may have noticed, we are currently refreshing the magazine’s appearance. Expect to see a new look and style in the coming year. The same quality content you have come to rely on will still be inside. However, the Board is discussing and exploring ideas focused on keeping the content relevant and informative and broadening the appeal of the magazine to all readers – younger and older, experienced and less experienced, owners and nonowners, leaders and future leaders, and more. We must also be responsive to our advertisers because, without them, there would be no magazine. Our goal is for you to stop what you are doing when the magazine lands on your desk and to open it up to see what is inside.

The responsibilities of individual Editorial Board members include developing the topics to be published each year by helping to create a roadmap for us to follow, finding and working with authors in completing the articles and getting them ready for publishing, and occasionally writing articles themselves. In addition to my role change, we are starting 2019 with four new members. Therefore, the content of the magazine will undoubtedly change a bit as it takes on the personality of this Board and reflects its interests, while still focusing on the technical content our readers enjoy.

I like to think of a structural engineer as being much more than just a technical expert. A structural engineer is a critical part of the broader construction industry, and to be effective and valuable must also be familiar with and speak intelligently on a variety of issues: building owner issues – land acquisition, zoning, approvals, financing; construction issues – planning, scheduling, techniques, material and equipment availability, costs; architectural issues – planning, design, code issues. Therefore, I hope to encourage a greater focus on a broader range of topics. However, each Board member is free to develop the content of his/her liking

STRUCTURE engage

STRUCTURE magazine

•

enlighten

•

empower

bounded only by our mission. One of our greatest strengths is our diversity (age, gender, technical specialties, and geographic location), and you should see this reflected in the magazine content. If there are topics you would like to see published in the magazine, please let one of us know. Better yet, please contact us about authoring an article yourself! From personal experience, it is fun and quite rewarding.

A side benefit is well-deserved publicity for you and your firm.

Lastly, you might ask why I volunteered to take on the role of Chair? The answer comes in three parts. First, I like to read history, learn new things, ask “why,” and “connect the dots” so that I can make sense of a chaotic and unpredictable world. Through the magazine and its voice, I am better able to help bring relevant and timely information on a variety of topics to our readers and hopefully help them become better, well-rounded structural engineers. Secondly, I get great satisfaction and enjoyment in working with young engineers and helping them advance and succeed in their careers. I heard recently that, for young people, the jobs of the future have not been invented yet, so the four skills essential to being prepared for what may come are: thinking, managing, creating, and communicating. I am not suggesting that engineering will fundamentally change but staying abreast of new trends and future developments is crucial. My personal goal is to work with the Board to create content that speaks to these skills across the broad field of structural engineering. And lastly, it was just my time. As I have moved through my career and my kids have grown up and are about to finish college, I decided it was time for me to attempt to make a significant contribution to our industry. I worked for many years at a firm where this is just what one did in terms of advancing the profession. Volunteering wasn’t a written requirement; it was just expected.

As I mentioned above, the magazine’s content and style may change a bit. Hopefully, you will like it. Please send comments, positive or critical. Article ideas and authors are welcomed too!■ John A. Dal Pino is a Principal with FTF Engineering located in San Francisco, California. He serves as the current Chair of the STRUCTURE Editorial Board. (jdalpino@ftfengineering.com)

MISSION: The Editorial Board will engage, enlighten, and empower structural engineers by publishing interesting, informative, and inspirational content in STRUCTURE magazine that is professionally relevant, technically reliable, and intellectually refreshing.

F E B R U A R Y 2 019

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structural DESIGN

Composite Steel Deck-Slabs with Supplemental Reinforcing Bars

By Vitaliy Degtyarev, Ph.D., P.E., S.E.

C

omposite steel deck-slabs, referred to hereafter as composite slabs, have been successfully used without supplemental reinforcing in buildings with

relatively short spans and typical design loads. As slab spans become longer or slab design loads become heavier, adding reinforcing bars is an effective alternative to making the composite slabs deeper and the steel decks thicker. Properly designed supplemental reinforcing allows for light, slender composite slabs that can span longer distances and results in large open interior spaces. This is in addition to the benefits of conventional composite slabs, such as reductions in construction time and cost. This article discusses different strategies for achieving economical composite slab designs by adding steel reinforcement and gives practical guidelines for the design of composite slabs with supplemental reinforcing bars.

Typical Designs

Figure 1. Floor plan with composite slab and supplemental reinforcing bars.

shows an example of a floor plan with a composite slab including supplemental reinforcing bars. Figure 2 illustrates the typical locations of the top and bottom bars within the composite slab cross-section.

Designers usually specify composite slabs based on composite steel deck load tables developed and published by deck manufacturers. The load tables are generally applicable to simple-span slabs; although, load Top Bars for Slab Continuity tables for continuous slabs with supplemental top bars over interior slab supports are available. Allowable loads published in these load Properly designed top reinforcing bars installed above interior supports tables have been determined based on comparisons of slab capaci- make the composite slab continuous. A composite slab without such bars ties for different limit states with slab internal forces. The American is considered simply supported even when the steel deck is continuous National Standards Institute/Steel Deck Institute C-2017, Standard over several spans. When gravity loads are applied to a composite slab for Composite Steel Floor Deck-Slabs (ANSI/SDI C-2017), gives provi- without top continuity bars, cracks form in the concrete above the interior sions for calculating slab capacities. The internal forces are determined supports, which causes the slab to behave as a series of simply supported from structural analysis of a composite slab as a beam. For simply slabs. The maximum positive bending moment and maximum deflecsupported slabs and uniform loads, the structural analysis consists tion of a continuous slab due to applied loads are considerably smaller of using well-known expressions for maximum shear, moment, and than those in a simply supported slab with the same spans and loads, deflection in a simply supported beam. Slab deflections are calculated which allows for longer continuous spans when compared with simply using the average of cracked and uncracked moments of inertia of the supported slabs of the same depth (Figure 3). transformed section and are compared to required deflection limits.

To achieve a longer span or greater allowable loads for a simply supDesign of Continuous Slabs ported composite slab without reinforcement, the designer should specify a deeper slab, a heavier deck, or greater strengths of the For the analysis, the composite slab is divided into strips based on the materials. Adding properly designed reinforcing bars may be a more support layout and loading conditions. For example, the single-span appealing option, which results in a shallower composite slab formed composite slab area adjacent to the opening in Figure 1 would be one strip, on a lighter steel deck. and the remaining composite slab area, with Reinforcing bars can be added in three three unequal spans, would be another strip. different locations. Top bars above interior Because composite slabs are designed as supports provide composite slab continuone-way slabs, every strip is analyzed as a ity. Bottom bars between composite slab beam accounting for load combinations supports contribute to improved positive required by the building code. moment capacity and allow for establishing For simple cases of approximately equal fire resistance of the slabs. Top bars between spans, applied moments and shears can be composite slab supports help to control determined using moment and shear coeflong-term deflections of the slabs. Figure 1 Figure 2. Typical locations of reinforcing bars in slab cross-section. ficients tabulated in the American Concrete 10 STRUCTURE magazine

Figure 3. Simply-supported and continuous composite slabs.

Institute’s (ACI) Building Code Requirements for Structural Concrete (ACI 318-14) and Commentary (ACI 318R-14), similar to the process for simply supported slabs. Composite slab deflections can be calculated using deflection coefficients for continuous beams with equal spans available in the technical literature. When spans are unequal, the composite slab should be analyzed using a general purpose structural analysis software considering pattern loading and load combinations required by the building code. Figure 4, page 12, shows an example of such analysis and resultant moment, shear, and deflection diagrams.

The calculated composite slab internal forces are compared with composite slab capacities calculated in accordance with ANSI/SDI C-2017. The composite slab’s flexural resistance in positive bending, vertical shear capacity, and the average moment of inertia can be obtained from the deck manufacturer or can be back-calculated

from composite slab allowable loads published in manufacturers’ deck load tables. The negative moment capacity of a composite slab is determined in accordance with ACI 318-14, neglecting the steel deck contribution, as follows: ΦMn = Φfy Atr(dtr - 0.5a) where a = (fy A tr)/(0.85f´c bb) and all other variables are as defined in ACI 318-14 and in Figure 5, page 12.

The negative moment reinforcing detailing – including the concrete cover, reinforcement spacing, and minimum reinforcement area – are determined in accordance with ACI 318-14. The tensile strain in the negative moment reinforcing bars, εt = [(dtr - c)/c]εcu , is required to be at least 0.004 by ACI 318-14 to prevent brittle failures. This requirement may prohibit the use of large amounts of

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Figure 4. An example of analysis results for a continuous slab with unequal spans.

reinforcement in composite slabs with relatively narrow ribs, which will limit the maximum negative moment capacity that the slab can achieve. If that is the case, the tensile strain in the top bars can be increased by providing bottom (compression) reinforcement in the slab ribs in the negative bending moment region. The larger tensile strain will allow for a greater amount of the top reinforcement and a greater negative moment capacity of the slab.

Additional Bottom Bars for Moment Capacity Bottom reinforcement may be provided in composite slab ribs between supports to improve the positive moment capacity of the composite slab.

A greater positive moment capacity may be required to attain longer spans or to accommodate heavier design loads. Adding bottom reinforcement for the higher moment capacity is an alternative to using a heavier-gauge steel deck. This option may be justified where a heavier-gauge steel deck is not available or where the greater moment capacity is required for a relatively small slab area, such as the slab area adjacent to the opening in Figure 1. Instead of introducing a different gage for the steel deck over a small floor area, bottom reinforcing bars can be added to the composite slab to achieve the required moment capacity. However, a heavier-gage steel deck (if available) is generally a better option when a greater moment capacity is required over a large area of the floor. In those cases, the heaviergauge steel deck will eliminate expenses associated with the additional reinforcement installation and will produce longer unshored spans, which may reduce or eliminate the required deck shoring.

The positive moment capacity of a composite slab with supplemental reinforcement can be determined using a general strain analysis (GSA) method outlined in the American National Standards Institute/ American Society of Civil Engineers’ Standard for the Structural Design of Composite Slabs (ANSI/ASCE 3-91). The GSA method is based on the considerations of the composite slab internal-forces equilibrium and strain compatibility, as well as on the constitutive material models for the steel and concrete. Figure 6 illustrates the method. The following equilibrium equations of the internal forces and moments can be written for the composite slab cross section: Fc + Fdt + Fdw + Fdb + Ftr + Fbr = 0 Fc x c + Fdt xdt + Fdw xdw + Fdb xdb + Ftr xtr + Fbr xbr = Mn where Fc, Fdt, Fdw, Fdb, Ftr, and Fbr are internal resultant forces in concrete, steel deck top flange, steel deck web, steel deck bottom flange, top reinforcement, and bottom reinforcement, respectively (positive when in tension and negative when in compression); xc, xdt, xdw, xdb, xtr, and xbr are distances from the neutral axis of the composite section to the internal resultant forces in concrete, steel deck 12 STRUCTURE magazine

top flange, steel deck web, steel deck bottom flange, top reinforcement, and bottom reinforcement, respectively (positive when resultant force is below neutral axis and negative when resultant force is above neutral axis); and Mn is nominal moment capacity of composite section.

The internal forces in the concrete, steel deck and reinforcement are expressed in terms of the internal stresses in the slab components and the areas of the components to which the stresses are applied. Afterward, the stresses in the components are expressed based on the assumed stress-strain relationships for the materials, as functions of strains. The strains in the slab components are related based on the hypothesis of plane sections shown in Figure 6.

The resulting equations of the GSA method are quite cumbersome and generally require a computer to solve.

Some deck manufacturers can provide positive moment capacities of composite slabs with supplemental reinforcement calculated using this method.

It should be noted that either the ultimate flexural strength of the composite slab section or the bond between the steel deck and the concrete may govern the flexural resistance of a composite slab in positive bending. The GSA method, which is based on the assumption of the perfect steel deck-to-concrete bond, allows for determining the ultimate flexural strength. The composite slab flexural capacity governed by the bond should also be checked. The bond-governed strengths of composite slabs are steel-deck-profile specific and can be provided by the deck manufacturer.

Additional Bottom Bars for Fire Resistance Bottom reinforcing bars can also be added between slab supports to establish fire resistance of the composite slab by rational design in accordance with the building code. This approach may be justified when an Underwriters Laboratory (UL)-approved slab design assembly is not available or when the rational design results in a more economical solution when compared with the available UL designs. The 2015 International Building Code (IBC) permits establishing fire resistance of concrete slabs by calculations in accordance with ACI/The Masonry Society Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies (ACI/TMS 216.1-14). To apply ACI/TMS 216.1-14 to composite slabs, the steel deck is neglected in the design for a fire event and the slab is analyzed as a reinforced concrete slab.

Because the steel deck is neglected, the required moment capacity of the concrete slab in positive bending is achieved by adding bottom reinforcement.

In a fire, heat transmission and structural end-point behaviors govern slab design. Based on heat transmission end-point behavior, the concrete slab must meet the minimum required equivalent thickness specified in the building code. The minimum required concrete slab thickness is a function of the required fireresistance rating and concrete type.

Thicker concrete slabs and lesser concrete densiFigure 5. Composite slab design diagrams for ties provide higher negative bending moment calculations.

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fire-resistance ratings. The building code gives guidelines for calculating the equivalent thickness of a concrete slab with ribbed soffit, which is a function of the slab’s cross-sectional shape and dimensions.

Based on structural endpoint behavior, the reduced capacity of the reinforced concrete slab is determined using multiple charts given in ACI/TMS 216.1-14 for Figure 6. Design diagrams for the GSA method. different concrete types and durations of fire exposure, which correspond to fire-resistance ratings. reinforced concrete flexural members showed the beneficial effect of First, temperatures of concrete and bottom reinforcement are deter- compression reinforcement on the reduction of long-term deflecmined. The reinforcement temperature depends on the concrete rib tions. For a load duration of 5 years and more, the composite slab width and the distance from the reinforcement to the bottom of the long-term deflection factor can be calculated using either the ANSI/ slab. Once the reinforcement and concrete temperatures have been ASCE 3-91 equation established, the reduced strengths of the materials are determined λ = [2-1.2(A's /A"s ] ≥ 0.6 using ACI/TMS 216.1-14 charts, which show percentages of the or the ACI 318-14 equation retained material strengths as functions of the material temperatures. λ = 2/(1+50ρ´ ) Finally, the slab’s nominal moment capacity is calculated using the where reduced strengths of the reinforcement and concrete and compared A´s and A"s are areas of steel in compression and tension, respectively; with the unfactored full-service load moment in the concrete slab. ρ´ is the ratio of compression reinforcement.

The bottom reinforcement amount is adjusted as needed for the Therefore, top reinforcing bars between supports reduce composite slab concrete slab to achieve the required moment capacity after the long-term deflections and may be a cost-effective alternative of using required period of fire exposure. a deeper section in the cases where deflections govern composite slab design. Top reinforcing bars have a small effect on the composite slab positive moment capacity and can be conservatively neglected in positive Long-Term Deflection Control moment capacity calculations. If desired, the described GSA According to ANSI/SDI C-2017, additional composite slab deflection method can be used to account for the effect of the top bars due to concrete shrinkage and creep shall be taken into consideration. on the composite slab positive moment capacity.■ The 2015 IBC also requires the long-term deflection of floors due to concrete shrinkage and creep be considered in the composite slab Vitaliy Degtyarev is a Design and Research Engineer with New Millennium design. Deflection requirements rarely govern the design of convenBuilding Systems. (vitaliy.degtyarev@newmill.com) tional composite slabs with relatively short spans and typical design loads. For slender, long-span composite slabs, deflection control is a primary design consideration that often governs composite slab design. Research on the long-term behavior of composite slabs is limited, but the available experimental studies clearly show that deflections of composite slabs increase over time under constant loads due to concrete shrinkage and creep, similarly to deflections of reinforced concrete members.

Due to the limited research on composite slabs, ACI 318’s long-term deflection provisions have been considered applicable to composite slabs.

The long-term deflection of a composite slab is determined by multiplying the calculated instantaneous slab deflection due to sustained loads by the long-term deflection factor, ∆LTD = λ∆i,sust. The longterm deflection factor is a function of the sustained load duration and the amount of reinforcement in the concrete compression zone. Experimental studies on

F E B R U A R Y 2 019

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Welded Steel Moment Resisting Frames The Earthquake-Resisting System that Surprised the Profession

By Ronald O. Hamburger, S.E., and James O. Malley, S.E.

T

his is one of a series describing surprising lessons engineers have learned from earthquakes and,

in particular, the January 17, 1994, Northridge earthquake. Engineers have been learning from earthquakes for more than 100 years by observing the patterns of damage earthquakes have caused in the built environment. Sometimes, things only partly true were part of the lesson, discovering the mistake only after a later earthquake causes unexpected damage. Such

Figure 1. Early steel frame high rises, sole survivors of the SF earthquake and fire.

was indeed the case with steel moment frame structures and the 1994 Northridge earthquake.

Steel framing was first used for building construction in the 1880s Construction technology changed little in the next 30 years; then when it became a popular construction system for long-span industrial further development was abruptly halted by the Great Depression structures and the new “high rise” buildings that began to spring up followed by World War II. After the war, unreinforced masonry conin major cities. The invention of the elevator safety break by Elisha struction became uneconomical and, in California, was not permitted Otis made high rise construction practical. However, unreinforced by code following the 1933 Long Beach earthquake. Some steel strucmasonry bearing wall construction could not practically extend tures were constructed with cast concrete walls replacing the masonry. beyond 5 or so stories, as the weight of the masonry would exceed However, architectural styles evolved favoring glazed curtain walls, its strength. Engineers turned to a transitional masonry construction and the steel frames had to rely on more compact moment resisting wherein steel frames were provided to bear the building’s weight, connections for lateral resistance. Riveted connections gave way to while relatively thin masonry walls provided enclosure and interior bolted varieties, with WT sections replacing angles and brackets as partitioning. These early steel frames used built-up steel box and wide connection elements. As design forces began to exceed the practical flange sections, constructed of rivet-connected plates and angles. capability of bolted connections, welding, first introduced in the Beam-to-column connections comprised riveted shipbuilding industry, found its way into building top and bottom seat angles as well as heavier construction. By the 1960s, engineers commonly bracketed connections, both of which provided designed connections employing bolted angles or moment-resistance, albeit as partially-restrained, tabs to transfer beam shear forces to columns and partial strength joints. Engineers often designed complete joint penetration groove welds to join the beams as simply supported for gravity loading beam flanges to columns for moment-resistance and fixed-ended for wind resistance and, indeed, (Figure 2). In 1972, Popov and Stephen published the heavier beam-column connections were often research validating the performance capability of designated as “wind connections” on drawings. these connections, and the welded unreinforced The masonry walls which provided most of the flange, bolted web (WUF-B) connection became actual lateral resistance in these structures were the standard for steel moment frame and high-rise considered architectural and neglected. In April construction in the western U.S. 1906, a large number of these structures were Though no structures of this type had yet expesubjected to strong earthquake shaking, and rienced strong earthquakes, in the early 1960s, then fire, in the great San Francisco earthquake. engineers placed language in the Uniform Building Engineers observed the remarkably good perforCode (UBC) requiring that all buildings, in excess mance of these high-rise steel frames compared of 13 stories or 160 feet in height, be provided with with other structures (Figure 1). Similar superior a moment-resisting steel frame capable of resistperformance was observed in later earthquakes, ing at least 25% of the required seismic forces. In including the 1926 Santa Barbara and 1933 Long the building booms of the 1960s, 70s, and 80s, Beach events. engineers designed hundreds of these structures in Figure 2. Typical WUF-B connection.

F E B R U A R Y 2 019

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major west coast cities. In February 1971, downtown Los Angeles experienced moderate shaking from the San Fernando earthquake and engineers observed that the new high-rise buildings performed well. Downtown Los Angeles and San Francisco continued to see moderate shaking in a series of earthquakes in the 1980s, including the 1983 Coalinga, 1984 Morgan Hill, 1987 Whittier-Narrows, and 1989 Loma Prieta earthquakes. In each case, following cursory inspections, steel frame buildings mostly appeared undamaged, bolstering the profession’s confidence. A few fractures were discovered in some buildings, both in 1971 and 1989, but their significance was not understood nor was their presence disseminated. By 1988, most engineers believed they truly knew how to design earthquake-resistant structures and the Welded Steel Moment Frame (WSMF) structure was the gold standard for earthquake resistance. Although some engineers perceived problems with the now typical WUF-B connection, the 1988 UBC required the use of this detail in steel moment frames used for seismic resistance unless the engineer proved other connections would perform adequately, even including a “penalty factor” of 1.25 on the connection forces for other connections. This all changed on the morning of January 17, 1994.

One of the first discoveries of damage was at the Getty Art Museum, a WSMF structure then under construction in the hills between West Los Angeles and Sherman Oaks. Workers returning to the site observed strange cracks (fractures) in the still exposed steel moment connections.

Then, an engineer performing a post-earthquake inspection of the Santa Clarita City Hall was surprised when a fractured bolt fell on his head when he opened a ceiling. Removal of fireproofing revealed similar cracking in that building’s connections. Engineers observed that the U.S. Borax Corporation headquarters building in Valencia and the nearby Southern California Automobile Association Building (Figure 3) had large, permanent interstory drift and found fractured connections in those buildings as well. Fracture patterns varied. In some cases, the fracture severed the welded bottom beam-flangeto-column-flange joint. In others, the fracture progressed into the column flange, forming a loose divot of column flange material. In a limited number of other cases, the fracture propagated into the column and across the column web (Figure 4). In some cases, bolted web connections also failed, shearing bolts or fracturing shear tabs.

Engineers rapidly lost faith in these structures and began performing detailed inspections of many building connections, using visual inspection and ultrasonic testing (UT). Visible connection fractures were found in more than 20 buildings and, in a few cases, more than half the connections were damaged. Potentially more concerning was that UT inspection revealed rejectable flaws, interpreted as incipient cracks, in many buildings (well over 100). An emergency code change removed the prescriptive WUF-B connection from the building code, substituting a requirement to demonstrate, by testing, that moment connection details could perform acceptably, effectively rendering WSMF construction impractical for new buildings. Los Angeles County then adopted a requirement for mandatory inspection of steel buildings within the zone of intense shaking.

Structural engineers desperately needed guidance on inspecting these structures, repairing damage, evaluating and Figure 4. Fracture extending from CJP weld across upgrading undamaged buildings, and designing column web.

16 STRUCTURE magazine

Figure 3. The Southern California Automobile Association Building. To the right, a large permanent drift in the building.

new buildings. The Structural Engineers Association of California (SEAOC), Applied Technology Council (ATC), and California Universities for Earthquake Engineering (CUREE) formed the SAC Joint Venture and entered into a cooperative agreement with the Federal Emergency Management Agency to conduct $12 million of field, laboratory, and analytical research into the problem. This research found that the connection fractures were the result of many factors, including an inherently flawed connection geometry; poor construction; poor welding technique, lack of adequate inspection; use of poorly controlled base and weld filler metals. Research revealed similar damage had occurred in steel buildings in the 1971 San Fernando, 1989 Loma Prieta, and 1993 Landers Big Bear earthquakes.

In January 1995, the Kobe earthquake produced similar damage in Japanese steel buildings. The FEMA/SAC project concluded in 2000 with the publication of a series of guideline and research reports (FEMA 350, FEMA 351, FEMA 352, FEMA 353, FEMA 354 and FEMA 355). These publications resulted in massive changes to design practice, reflected in AISC 341, the introduction of a new steel material specification (ASTM A992), and new design specifications (AISC 358, AWS D1.8). Also, a far more measured approach was instituted for the introduction of new systems and requirements in the building code.

Many lessons were learned from the effort to respond to the damage to WSMF buildings in the Northridge earthquake that hopefully will result in much better performance in future large earthquakes.

However, a larger lesson remains. Structural technology is continually changing and evolving with new code requirements, materials, detailing practices, and systems constantly being introduced. As a profession, we are more careful about the adoption of new approaches on a wide scale basis than we once were. As individuals, engineers must use new approaches with caution, think through the possible pitfalls, and proceed cautiously, lest the next earthquake teaches us unpleasant lessons about our practice.■ The online version of this article contains references.

Please visit www.STRUCTUREmag.org.

Ronald O. Hamburger is a Senior Principal with Simpson Gumpertz & Heger in San Francisco. Following the Northridge earthquake, he served as Project Director for the SAC Steel Project, responsible for the development of engineering guidelines. He is a past President of NCSEA. (rohamburger@sgh.com) James O. Malley is a Senior Principal with Degenkolb Engineers. He was responsible for the analytical and testing investigations performed as part of the SAC Steel Project in response to the Northridge earthquake damage.

Jim is Chair of the AISC Specifications Committee and the Past-Chair of the AISC Seismic Subcommittee. (malley@degenkolb.com)

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structural PERFORMANCE

Structural Design Challenges for Tall Buildings in Mexico City Part 2: A 20-year Perspective

By Ahmad Rahimian, Ph.D., P.E., S.E. F.ASCE, Rodolfo E. Valles Mattox, Ph.D., Esteban Anzola, P.E., Ilya Shleykov, Ph.D., P.E., LEED AP BD+C, Sissy Nikolaou, Ph.D., P.E., D.GE, F.ASCE, and Guillermo Diaz-Fanas, P.E.

I

nnovation and resourcefulness in the engineering of tall buildings are one of the cornerstone requirements of building resilient cities and communities. Successful performance of these structures during extreme natural events is living proof of the value of design that targets resilient performance, enhancing the confidence of the stakeholders and the public for the engineering and technologies used. Mexico City is a mega-city where performance-based design using cutting-edge technologies in the past two decades has been tested repeatedly by large earthquakes, floods, and winds. This city has evolved from the ancient Aztec capital to one of the most prominent modern metropoles of Latin America. The economic growth in recent years has resulted in an increase of the city’s footprint by Figure 9. Paseo de la Reforma development in the past two decades (top). Location of a factor of 100 to accommodate the millions of people that have the iconic towers Torre Mayor and Cuarzo Reforma (bottom). (Top left ©2003-2006 moved into Mexico City from rural areas, generating an everby V. Shmatikov; top right by S. Ruiz). increasing demand for taller buildings and deeper basements.

The design and construction challenges that the City of Mexico avenues that have been home to tall buildings since the 1990s with presents are multi-faceted due to its location in one of the most the development of Torre Mayor, the tallest building at the time of adverse geologic and tectonic environments in the world, as its construction until recently. The opulent urban landscape reflects described in Part 1 of this article (STRUCTURE, December the city’s position as the financial center of Latin America and one 2018). This article discusses specific challenges that continue to of the largest economic powers worldwide.

In this continuously evolving city, tall buildings are towers that grow with the increasing demand, and design solutions that are also advancing with time. Key seismic features of several iconic feature deep basements to accommodate parking spaces and high-rise buildings are presented, and their performance in recent mechanical, electrical, and plumbing demands. It is not uncommajor earthquakes are discussed by the designers and co-authors. mon for a modern tall building in Mexico City to have 9 to 10 The overall resilient behavior of these structures can be attributed to underground levels which, in combination with the city’s unique two fundamental factors: (i) enforcement of higher code design and geology and seismic hazard exposure, imposes its own challenges construction standards following the devastating 1985 Michoacán in design and construction. Specifically, as described in detail Ms8.0 Earthquake, a major event exactly 32 years prior to the in Part 1 of this article, a large portion of Mexico City is built 2017 Mw7.8 Puebla-Morelos Earthquake, and (ii) advancements on a basin formerly occupied by the ancient Lake Texcoco. The in the state of practice for tall buildings using performance-based subsurface conditions are largely comprised of extremely soft engineering that go beyond the minimum code requirements, and compressible lacustrine clay deposits, with unusually high integrate geotechnical and structural performance objectives, and plasticity and natural water content, and low seismic shear waves incorporate seismic protective technologies. velocity of propagation. Since the beginning of the 20th century, these saturated soft deposits have settled approximately 9 meters (30 feet) because of their consolidation due to continuous water Higher Heights and Deeper Depths pumping and exploitation of the aquifers underlying those soil Development of tall buildings over the past two decades has increased strata, and loading from new construction. These factors caused the exponentially to accommodate the rapid social and economic growth city to continue to sink with damaging differential deformations of Mexico City. The complex architecture of the city is a mix of colonial of many of the monuments and structures built in the former lake architecture, modern high-rises, and its grand Paris-style boulevard, area, affecting the city’s architectural heritage.

Paseo de la Reforma, where the national stock market resides (Figure 9).

In Mexico City, the preferred foundation system is a combination of This wide downtown avenue that runs diagonally across the city perimeter slurry walls (known as “Milán” walls that are used during serves as one of its “arteries” and is one of the first and oldest city construction and later become permanent support elements) and deep 18 STRUCTURE magazine

cast in-situ piles and barrettes. The latter in using less steel to achieve the performance extend more than 40 to 50 meters (130 to objectives.

160 feet) below the ground surface to reach This seismic protective system proved its competent soil strata below the soft clay worth immediately after completion of the lake deposits and to accommodate the high tower as the Mw7.5 Tecomán, Colima earthvertical and lateral load demands, often with quake shook the city in January 2003, merely enhancement of post-grouting technologies. a month after it was occupied. Not only did Figure 10 shows the reinforcement of a typithe building survive undamaged, but the cal barrette from a recent high-rise project occupants at the time of the event did not in Mexico City. realize a tremor had occurred. The same was Maintaining serviceability of new buildreported during later events, including the ings in these ever-changing site conditions recent 2017 Mw7.1 Puebla-Morelos earthrequires long-term planning and solutions quake. The creative engineering solution during the design process in anticipation applied in Torre Mayor was far ahead of of settlements of the site and its surroundits time and pioneered the implementation ings. Since the conditions vary depending on of a PBD philosophy for tall buildings in age, loading conditions, and use, attention Mexico City, while comprehensive PBD seisshould be given to the spatial distribution of mic guidelines for tall buildings in the U.S. Figure 10. Reinforcement of a typical barrette from a the rates of settlement (e.g., street areas that recent high-rise project in Mexico City that has a length were developed a half decade later (FEMA, typically have been subsiding at a higher rate of 40 meters (130 feet).

2006; TBI, 2010). To this date, Torre Mayor than the building structures), but also to the is a celebrated iconic tower that is a real-life interaction between buildings that are close to each other and that example of seismic resilience and has enhanced the people’s confidence have different basement depths. This consideration becomes a critical in engineering ingenuity. It is known as the building that people functionality factor for building access points such as ramps, drive- run into, rather than out of when an earthquake strikes (Popular ins, and walkways, and can be addressed by including overbuild into Science, 2003). the access areas which can be gradually adjusted as the surroundings The successful implementation of high-performance design elements settle due to the continuous and non-uniform sinking of the city. in Torre Mayor was followed by advancements in seismic design guidelines that took into consideration the response characteristics of tall buildings. One of the latest towers, the Cuarzo Reforma, located promiSeismic Resilience – nently along Paseo de la Reforma, was completed in 2017 (Figure 12).

Design Examples 20-Years Apart This new development is comprised of two buildings connected with Over the past two decades, multi-hazard resilience considerations have a common base: an iconic 40-story mixed-use tower and a 27-story been integrated by the authors into the structural design of numerous hotel. The design combines experience with tall buildings in the area tall buildings spread across the Mexico City area. Two signature project with PBD developments and the latest technology in seismic energy examples, Torre Mayor and Torre Cuarzo on Paseo de la Reforma, dissipation systems. Friction dampers were designed and installed were designed and constructed within this period. Both projects along the building height. The lateral force-resisting system features incorporate seismic protective systems and were designed after the devastating 1985 Michoacán Ms8.0 Earthquake that resulted in a toll of more than 39,000 deaths and nearly 10,000 building collapses. Since that event, combined with the historic high seismic activity in the region, there have been significant changes in design criteria and a shift in the philosophy of developers and design professionals in applying cutting-edge advancements in earthquake engineering, analytical tools, and protective technologies to safeguard lives and properties.

The first tower that went beyond conventional prescriptive building code objectives, by following a Performance-Based Design (PBD) approach, is the 55-story, 235-meter (774-foot) tall office tower Torre Mayor (Figure 11). From its completion in late 2002 until 2010, Torre Mayor was the tallest building in Latin America. Viscous damper technology, originally developed for military use, was used for the design of a patented diamond configuration (Post, 2003), rather than the typical X pattern that greatly improved the seismic performance of the tower.

The lateral force-resisting system of Torre Mayor is equipped with 96 viscous dampers, effectively dissipating seismic energy and minimizing inelastic demands to structural components.

Strategically located in an overlapping diamond-shaped array, four mega trusses with large-stroke energy dissipation devices Figure 11. Torre Mayor tower, completed in 2003. The tallest building in Latin enhanced the lateral-force-resisting system (Rahimian, 2007).

America until 2010 and the first application of PBD with use of seismic protective The innovative diamond-shaped layout is more efficient, resulting devices in a unique, innovative layout (Rahimian, 2007).

F E B R U A R Y 2 019

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Distrito Federal, OGDF, 2017) and its corresponding design and construction standards (Normas Técnicas Complementarias, OGDF, 2017). A comparison between characteristic records from the 1985 and 2017 events is shown in Figure 6 of Part I of this article for selected strong motion locations and site conditions. The updated design standards mark a significant technical development with respect to the previous 2004 edition. Specifically, the 2017 standards have incorporated explicit performance objectives for different seismic hazard levels, allowing for alternative structural systems that meet the intent of the standards and are approved by the building code enforcement officials in a similar approach to contemporary United States building codes.

The aspect of functionality and immediate occupancy is also addressed in the 2017 standards, with specific requirements for continued functionality and maintenance of occupancy under frequent events of low to moderate intensity that may occur more than once during the design life of the structure. For the Design Base Earthquake (DBE or DE), no major structural Figure 12. The high-performance Torre Cuarzo tower, with energy dissipation and restoring failures or loss of life should occur; however, significant damage mechanisms that led to resilient behavior during the 2017 earthquake. Courtesy of Richard or deformations may arise that could affect functionality and Meier & Partners Architects LLP, and Diámetro. may require significant repairs. Foundation systems are genera unique restoring mechanism – double-story diagonals with friction ally expected to behave within the elastic range for the frequent events, dampers passing through intermediate horizontal flexural members essentially being designed for higher seismic loads than that of the which contribute to recovering the original geometry of the tower superstructure, recognizing that structural systems can develop higher once energy dissipation devices are activated. The structural elegance forces due to inherent system redundancy and material over-strength. of the system, combined with high-performance devices, supports a In applying an alternative Performance-Based Design approach based unique architectural appearance with a clean, minimalistic geometry. on international developments for tall buildings (TBI, ASCE41), a The presence of dampers, acting as “fuses,” avoided the use of heavy wider range of seismic hazard demands is incorporated, from high connections at bracing diagonals that otherwise would have been required probability and low intensity to very low probability and high intensity, for special concentrically braced frame systems for high Seismic Design with associated performance targets. This PBD approach addresses Categories (SDCs) as per ASCE 7 in United States designs. Although the additional layer of a rare Maximum Considered Earthquake the Mexico City local code does not use the SDC term, it was used as a (MCE) for which the performance objective is within the life safetyreference to provide the required level of detailing based on international to-collapse prevention range, depending on the structural occupancy engineering practice. It further informs PBD procedure which relies on or importance of the structure. Figure 13 depicts the local code seismic the proper detailing of structural components in accordance with ACI performance objectives as they relate to the PBD methodology with (2014) and AISC (2016) in prediction of cyclic nonlinear behavior. explicit and implicit underlain assumptions in both approaches.

This approach ensures both compliance with local code and validation Going beyond the above standards, special aspects related to the of performance objectives for varying earthquake levels.

Mexico City environment should be considered, including: (i) levels Indeed, Torre Cuarzo went through a series of moderate-magnitude of differential and total settlements and lateral deformations; (ii) earthquakes during construction and experienced the far stronger 2017 Soil-Structure Interaction (SSI) – parameters and soil and foundation Mw7.1 Puebla-Morelos earthquake after its completion. Despite being testing needed to derive and incorporate them into the structural in one of the most adverse site locations in the city that recorded the model while addressing strain dependency; (iii) depth at which highest ground motions, the behavior of Torre Cuarzo was excep- the input ground motions will be derived from (e.g., at the ground tional during the 2017 event – as with its almost 15-year senior surface or lower) and applied to the structural model, (iv) drainage neighbor Torre Mayor – and proved the resilience provided by the or corrosive soils issues that may affect the foundation materials, PBD approach and the use of advanced protective technologies.

Evolution of Local Code and PBD Criteria The prescriptive local building code and construction requirements have historically evolved over the years, incorporating lessons learned from actual events such as the 1985 Ms8.0 Michoacán earthquake. The 2017 Mw7.1 Puebla-Morelos earthquake also influenced Mexico City officials to release the most recent edition of the local Building Code (Reglamento de Construcciones del 20 STRUCTURE magazine

Figure 13. Mexican Code seismic performance objectives and correlation with PBD approach and methodology.

such as the pile wall thickness; and (v) interaction with existing adjacent structures, underground structures, or utilities. To address these challenges, a synergy among involved disciplines is needed, as well as the implementation of state-of-the-art PBD methodologies, protective technologies, and analytical tools, all combined with global experience in the design of unique high-performance tall structures. Participation of the designers in the development of codes and guidelines both in Mexico and the United States benefits the advancement of seismic design practice. The Mexican high-rise structures presented in this article, along with over thirty more structures that the authors contributed to the design of, have proven seismically resilient in the past two decades. This is a direct result of compliance with improved local regulations coupled with advances in seismic engineering and ingenuity.

Conclusions Unique geologic conditions and exposure to the high earthquake hazard of Mexico City require rigorous and holistic approaches to the seismic design of tall building structures and considerations that go beyond minimum code requirements. Close collaboration between structural and geotechnical disciplines during the very early design stages has become one of the main cornerstones of building design in Mexico City. The growing complexity of structural systems requires the utilization of stateof-the-art analysis and design techniques to allow better prediction and an overall understanding of building performance for seismic events of varying intensities. Local code, with explicitly stated performance objectives and allowance for alternative analysis and design procedures that meet the intent of the code, provides a framework for implementation of Performance-Based Design philosophy. This leaves room to integrate

international experience and technological developments to offer the best engineering practice for finding innovative solutions for the numerous challenges of the Mexico City environment and to reach greater heights and deeper basements. The real-life testing of this approach with multiple frequent moderate seismic events and the recent 2017 major PueblaMorelos earthquake has proved the resilience of these tall structures, with minimal or no damage and without functionality interruption. For designers, enhanced public trust is a significant benefit; conversations about community resilience now include specific examples of the efficacy of engineering advancements for high-performance building structures, making stakeholders feel safer and more comfortable about future developments.■ The online version of this article contains references.

Please visit www.STRUCTUREmag.org.

Ahmad Rahimian is the USA Director of Buildings Structures, WSP New York. (ahmad.rahimian@wsp.com) Rodolfo E. Valles Mattox is Mexico’s General Director, WSP Mexico. (rodolfo.valles@wsp.com) Esteban Anzola is a Vice President of Buildings Structures, WSP Miami. (esteban.anzola@wsp.com) Ilya Shleykov is a Vice President of Buildings Structures, WSP New York. (ilya.shleykov@wsp.com) Sissy Nikolaou is an Assistant Vice President of the Geotechnical & Tunneling Technical Excellence Center, WSP New York. (sissy.nikolaou@wsp.com) Guillermo Diaz-Fanas is a Senior Technical Principal of the Geotechnical & Tunneling Technical Excellence Center, WSP New York. (guillermo.diazfanas@wsp.com)

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F E B R U A R Y 2 019

21

structural SYSTEMS Mid-Rise Wood-Frame Buildings Safe, Cost Effective, and Sustainable

By Richard McLain, P.E., S.E.

T

here is a reason more developers and building designers are using wood-frame construc-

tion for large mid-rise projects. Designed to code, wood-frame buildings are safe, cost-effective, and sustainable, and increasing the use of wood in these projects can enhance their value proposition.

Podium buildings with up to six stories of wood construction over another construction style (typically concrete) are one of the best ways to add density while keeping costs in check (Figure 1). Podiums allow developers to maximize height and number of stories, while the Figure 1. 5-over-2 podium building in San Diego, CA. Courtesy of Brett Drury. wood-frame portion above is efficient and economical.

Safety is paramount, and wood buildings designed to code are just as Code Acceptance: Achieving the Same safe as code-compliant buildings made from other materials. Building Scale as Type II Buildings codes require all building systems to perform to the same rigorous standards for safety based on the construction type used, and wood systems Of the five construction types in the International Building Code can be designed to meet or exceed code requirements. Wood framing (IBC), mid-rise wood-frame buildings generally fall under Types III offers the added benefits of construction speed, design versatility, and and V. Each is further subdivided into A and B, which have different a light carbon footprint fire-resistance rating requirements (A being more rigorous) and allowStructural engineers are often called upon to help make early project able sizes. For Type III buildings, the IBC allows up to five stories of decisions on material type selection, and many of the factors that go wood construction for residential occupancies and six for business, into these decisions go beyond what might be seen as the normal scope while Type V buildings have a maximum of four stories. of an engineer. WoodWorks provides project support and education There are several options for using wood to more affordably achieve related to the design of wood buildings, and the purpose of this article is the scale of Type II buildings, which are generally steel or concrete. For to help design and construction professionals make informed decisions multi-family occupancies, the allowable height, stories, and per-story about material use. It touches on key topics related to mid-rise wood area of Type IIIA construction are identical to Type IIA. Likewise, design – such as fire, seismic and wind resistance, thermal impacts of Type IIIB can be used to achieve the same height, stories, and area as structural materials, acoustics, and cost.

Type IIB (Table 1).

Across the U.S., podium buildings have typically been 4-over-1 or 5-over-1, meaning they have four or five stories of wood over a singleSustainable and Durable Design level podium. In the 2015 and 2018 IBC, there is no limit on the number When specifying any building material, it is important to consider of podium stories, giving rise to more 5-over-2 buildings. its life cycle environmental impacts. Wood is renewable, and wood products have less embodied energy, are responsible for less air and Cost and Construction Type water pollution, and have a lighter carbon footprint than other commonly used materials. Wood can contribute to a building’s energy International Code Council (ICC) Building Valuation Data is an efficiency and is thought to have a positive impact on the health and effective way to compare the average cost of buildings, which, as shown well-being of occupants. The fact that it is durable and adaptable also in Tables 2 through 4, varies widely by construction type. Table 3 creates opportunities for renovation, re-use, and recycling. highlights the difference between Type IIA, which is typically steel Durability is also an essential principle of sustainable design. However, or concrete, and IIIA, which is typically wood-frame. As noted, both despite many examples of wood buildings that have stood for centuries, have the same allowable heights and areas, but the average Type IIIA wood has a perception issue when it comes to longevity. building costs $20 per square foot less. In Table 4, Type IIB construcA study by the Athena Sustainable Materials Institute looked at the tion is compared to Type IIIB (again the same size) and shows an longevity of buildings in Minneapolis and found that wood buildings $18/square-foot savings for the wood building. were typically the oldest, the majority older than 75 years. In contrast, Factors that make wood-frame construction economical typically 80% of the steel buildings demolished were less than 50 years old include lower material costs, construction speed, and wood’s relatively (FPInnovations 2005). light weight (which can reduce the need for foundation capacity).

22 STRUCTURE magazine

Table 1. Multi-story residential occupancy allowable building sizes.

Construction Type Options: 5 Stories Construction Type

Wood Framing

Steel/Concrete

IIIA

IIA

IIIB

IIB

Construction Type Options: 3-4 Stories Wood Framing VA

VB

Stories

5

5

5

5

4

3

Height (feet)

85

75

85

75

70

60

Allowable Floor Areab (square feet)

72,000

48,000

72,000

48,000

36,000

21,000

Total Building Areac (square feet)

216,000

144,000

216,000

144,000

108,000

63,000

a

Assumes NFPA 13 sprinklers throughout (IBC Table 504.4) Assumes NFPA 13 sprinklers throughout (IBC Table 506.2), multi-story building, no frontage increase c Assumes three or more stories (IBC 506.2.3), no frontage increase a

b

Further savings can be realized with off-site construction, which ranges from prefabricated components and panelized assemblies to full modular units.

Fire and Life Safety

levels of resiliency, resulting in lower damage and higher occupant safety during and following an earthquake or high wind event.

• When compared to normal load durations (e.g., live loading on a floor structure), wood structures can resist 60% higher wind and seismic loads as permitted by a 1.6 load duration factor in the NDS. • Wood buildings tend to be comprised of repetitive framing attached with numerous fasteners and connectors. This creates more load paths and better redundancy, so there is less chance the structure will collapse should some connections fail.

• Wood-frame buildings are relatively lightweight. Since seismic forces are proportional to weight, this means lower seismic forces which could result in further cost savings.

Thermal Advantages While many people know that wood building materials store carbon and typically require less energy to manufacture than other building materials, their advantages from an operational energy perspective can be overlooked.

Thermal bridging is an essential consideration for the design of building envelopes. It references the conduction of heat more readily through structural/support members relative to the adjacent cavity insulation. This, in turn, reduces the overall effective thermal performance of the assembly under consideration. For exterior walls and roofs, the choice of structural framing material has a significant impact on the relative amount of thermal bridging. For example, a 6-inch nominal wall with R-19 batt insulation has an effective R-value of 16 when framed with 2x6 wood studs at 16 inches on center (o.c.). In contrast, the same R-19 batt insulation would have an effective R-value of 7.1 when the wall is framed with 6-inch metal studs at 16 inches o.c.

While energy-efficiency requirements vary, many states adopt the ICC volume of codes that includes the International Energy Conservation Code (IECC). One of the code-compliance paths in the IECC is prescriptive, where walls, floors, and roofs have specific insulation requirements based on framing type and climate zone.

Wood buildings are designed to meet the same level of fire performance as buildings made from other materials. This is achieved by combining various levels of fire resistance, fire class, and fire protection systems.

• Passive fire resistance is related to the degree of passive protection provided to the structure itself. It is usually achieved with a gypsum product, but fire endurance for exposed wood can also be shown through calculations described in Chapter 16 of the American Wood Council’s (AWC’s) National Design Specification® (NDS®) for Wood Construction.

• Fire class is specific to the finishes of a building and addresses criteria such as flame spread and smoke development potential.

• Active fire protection references the active fire protection systems of a building, such as sprinklers and fire/smoke alarms.

Exterior walls must be fire-retardant-treated (FRT) wood or a noncombustible material and require a 2-hour rating when load bearing to achieve the larger size of a Type III building. Exterior walls in a Type VA building may be framed with standard untreated lumber and require a 1-hour fire-resistance rating when load bearing. In both types, wood can be used for interior structural elements. Opportunities to increase allowable size include firewalls, open perimeters, and automatic sprinkler systems.

Although less than 1% of buildTables 2-4. Average cost by construction types. Published $/square-foot of building area. ing fires occur during construction, special care must be taken to protect buildings before required fire protection elements – such as gypsum, fire doors, smoke alarms, and sprinklers – are in place. For best practices, visit www.constructionfiresafety.org.

continued on next page

Wind and Seismic Safety Wood has several characteristics that contribute to effective seismic and wind-resistive design. In addition to meeting code requirements for lateral loading, wood-frame structures can be designed to achieve enhanced F E B R U A R Y 2 019

23

Figure 2. Wood-frame exterior walls in a 5-over-1 project in Virginia.

The IECC recognizes wood’s advantages from a thermal bridging perspective by requiring continuous insulation in metal-frame walls but not wood-frame walls. For example, in IECC 2015 Table C402.1.3, above-grade metal-frame walls in all climate zones are required to have R-13 cavity insulation and some amount of continuous insulation applied to one face of the wall (R-5 or R-7.5 depending on climate zone). However, in climates zones 1-5 (other than zone 5 group R), no continuous insulation is required for wood-frame walls; cavity insulation alone is adequate. The R-20 wood-frame wall is the only option available in the IECC prescriptive wall path using prescribed R values that does not require continuous insulation for above-grade walls.

Occupant Comfort: Exceeding Acoustic Minimums The acoustics of a multi-family/mixed-use wood structure can be designed to meet or exceed minimum requirements, depending on the expectations of the developer, buyer, and tenants.

The IBC specifies minimum Sound Transmission Class and Impact Isolation Class ratings of 50 for assemblies separating dwelling units, and there are many options for acoustically-tested assemblies that meet and surpass code minimums.

In multi-family buildings, unit separation walls must perform multiple objectives. In addition to acoustic performance, they must meet fire separation requirements and serve as part of the lateral-force-resisting system (shear walls). They are often called upon to support hung cabinets or other features and need to take up minimal space.

Because of these and other considerations, many designers use staggered wood unit separation walls. A common construction includes 2x6 wall bottom and top plates with 2x4 studs at 8 inches or 12 inches o.c., alternating from one side of the plates to the other. STC ratings of staggered wood walls in the range of 50-63 can be accomplished.

Maximizing Value with Podium Designs Maximizing the size and area of a wood-frame building is a costeffective way to achieve five or six stories of saleable or rentable space.

Adding a podium increases the value of a project further with retail and parking. Concrete podiums are the most common, and the upper slab typically acts as both a fire separation and structural transfer slab for the framing above.

24 STRUCTURE magazine

Podium-style buildings are a product of the horizontal building separation provision (IBC Section 510.2). Separated by a 3-hour fireresistance-rated horizontal assembly, they are treated in the code as two separate structures, built one on top of the other, to determine area limitations, continuity of firewalls, the allowable number of stories, and type of construction.

For the podium to be considered a separate and distinct building, the podium and construction below must be of Type IA construction and equipped with an NFPA 13 sprinkler system. Occupancies above the podium are permitted to be A, B, M, R or S, while occupancies below are permitted to be any except H. The overall height of the two buildings is measured from grade plane and is limited by the provisions of IBC Chapter 5 (with increases) for the more restrictive of the two buildings.

Wood-frame exterior walls can be used within the podium level(s) (Figure 2) to improve energy efficiency and further reduce costs.

Although Type IA construction (which typically requires all noncombustible framing) is required, there are several opportunities for the use of FRT wood wall framing in Type I and II buildings per IBC Section 603.1. One example is non-rated, non-bearing exterior walls, where using wood offers two advantages: 1) The building enclosure details utilized for the wood-frame building above the podium can also be used below the podium, and 2) The requirements for continuous insulation versus cavity insulation are the same above and below the podium. In contrast, using steel studs below the podium would require continuous insulation, likely causing the need to shift the wall framing into the building to accommodate a thicker thermal barrier.

When a building has an NFPA 13 sprinkler system throughout, only a 1-hour rating is required when separating enclosed parking areas from occupancies such as B, M, and R per IBC Table 508.4. This indicates that a mixed-use building consisting of an enclosed parking area and other occupancies could be entirely framed with wood if allowable building size calculations permit the use of Types III, IV or V construction under a separated occupancies approach. Even if other materials are used in the building (perhaps as lower walls or columns), there is value from a cost perspective in classifying the entire building as the least restrictive construction type as permitted by IBC Section 602.1.1.

Conclusion Wood-frame construction is an excellent option for developers and designers looking to maximize the value of their mid-rise projects.

Building codes require wood structures to meet the same level of safety and performance as structures made from other materials, and wood offers added advantages such as cost savings, construction speed, and sustainability.â– The online version of this article contains references and a list of resources. Please visit www.STRUCTUREmag.org.

Discussion about cold-formed steel framing on podiums will be included in an upcoming issue of STRUCTURE. Richard McLain is a Senior Technical Director in the Project Resources and Solutions Division of WoodWorks. He is Executive Director of the Structural Engineers Association of Vermont and a member of the ASCE Structural Wind Engineering Committee, SEI Blast Protection of Buildings Standards Committee, and NIBS Offsite Construction Council Board. (ricky.mclain@woodworks.org)

A view of the completed project. Courtesy of Thomas Harris Photography.

four primary structures, all of rein-

OAKLAND INTERNATIONAL AIRPORT

forced concrete and supported on

By Jamison Curry, S.E., Peter Revelli, S.E., and Marko Schotanus, S.E., Ph.D.

T

he Terminal 1 renovation at Oakland International Airport

involved a unique structural solution that helped the airport achieve its goals of continued operations, sustainability, and modernization. The original terminal building dates back to the early 1960s, and consisted of

Terminal 1 Security Checkpoint Building Renovation and Retrofit

piles. The terminal was modified over time to follow the changing requirements of air travel, resulting in several additions. The most recent multi-phased renovation and seismic retrofit program started in 2006 with the seismic evaluation of all Terminal 1 buildings. It resulted in multiple construction projects that were delivered using the CM At-Risk method, with a design team led by Bill Olechnowicz of MWA Architects and Turner Construction as the general contractor. Completed projects to date include the seismic retrofit of the concourse and central plant, as well as construction of a new central plant and electrical substation.

The building that is addressed by the seismic retrofit project presented herein functions as a security checkpoint and Port of Oakland offices. It is one of the original four structures and consists of a two-story, above-grade building over a crawl space. The building has plan dimensions of approximately 238 feet by 238 feet. Forty-eight concrete columns, at 34-feet on-center each way, support a normal weight concrete flat slab at the first floor, a lightweight concrete waffle slab at the second floor, and weakly interconnected lightweight precast concrete shells at the roof (Figure 1).

26 STRUCTURE magazine

Figure 1. Original construction. Courtesy of Port of Oakland.

Figure 2. Various design configurations of the cast collar; actual casting on the right.

The vertical elements of the original seismic force-resisting system consisted of concrete walls at the crawl space level, reinforced masonry walls at the first story, and cantilever columns at the second story. The original Air Traffic Control Tower for the airport was located within the footprint of this building but was seismically separated from the two-story building by a joint. The tower was demolished as part of the renovation and retrofit project, its function having been replaced by a new tower located nearby.

The retrofit scheme for the remaining structure introduced new vertical seismic force-resisting elements at the perimeter, consisting of special reinforced concrete shear walls at the crawlspace and first story, and buckling-restrained braced frames at the second story. Columns and local areas of the existing concrete floor slab were improved using FRP. A new diaphragm was created at the roof level by the addition of a horizontal truss system comprised of hollow structural sections connected at column locations using a cast steel column collar.

Because a new lateral system was provided for the existing structure, design criteria were based on a combination of the 2010 California Building Code and ASCE 41-06, Seismic Rehabilitation of Existing Buildings, with the Basic Safety Objective as defined in ASCE 41-06 as the target performance level. Response spectrum analysis was used, and the base shear from the ASCE 41 analysis was scaled up to 85% of the 2010 CBC static base shear to determine seismic demands on both new and existing elements. Steel roof truss elements are designed to remain elastic while developing the overstrength of the Buckling-Restrained Braces (BRBs), assuming simultaneous yielding of the BRBs in the two orthogonal directions.

Truss Connection Challenges A key constraint imposed on the project team was that the security checkpoint located in this building needed to remain operational throughout construction to avoid impacts to the traveling public.

From a structural perspective, the primary challenge involved the connection of the new truss members at the top of the existing columns. This connection took on many shapes during design to meet the varying demands of strength, constructability, and aesthetics. The original connection of the existing roof shells to the columns resulted in a highly congested, reinforced concrete section at the top of the columns. In addition, roof drainage was routed down each column. To avoid drilling through the column, early concepts for the connection considered a built-up section of steel plates to form a square collar around the column. This approach resulted in a considerable volume of weld material at collar corners. Making these welds in the field would have been very costly and time consuming. As an alternative, a steel casting was explored with Pacific Steel Casting of Berkeley, California. A cross section was developed iteratively in consultation with the

architect, contractor, and the foundry that responded to structural requirements, aesthetics, constructability, and production schedules. The collar consisted of two identical halves that are joined around the column in the field, resulting in a total of 96 castings for 48 connection nodes. Welded gusset plates were added to the cast collars for different configurations of brace connections (Figure 2). In collaboration with the contractor, it was decided that welding the cast collar halves together would add complexity in the field, and so the design was further modified to allow field bolting. A C-shaped cross section was revised to a T-shaped one with stiffening ribs at locations where gussets for horizontal truss members could be placed. The T-shape provided better access for bolt installation. This change also facilitated the casting operation since molten metal flows into the pattern (mold) and cools in a more controllable way. Using cast steel became the preferred alternative after mock-ups in the existing structure showed that the shape of the casting could be integrated into the project in an aesthetically pleasing manner. R+C performed a finite-element stress analysis of the casting to determine an acceptable structural configuration. The material chosen for the casting was ASTM 958, Grade SC8620 Class 80/50. This material has properties compatible with ASTM A572 Grade 50, specified for the gusset plates, and ASTM A500, Grade B, specified for the horizontal truss members.

Cast Collar Fabrication Process Olson Steel was the steel fabricator and erector for the project.

They contracted with Cast Connex and Bradken to fabricate the production castings. R+C worked with Cast Connex to further

Figure 3. Olson’s lifting rig – to the left is a casting being lifted into place; the rig is on the right. Note the top of the roof ribs.

F E B R U A R Y 2 019

27

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refine the casting design, specifications, and quality assurance/quality control specifications to achieve the appropriate level of quality and reliability of the casting. In addition, Cast Connex/Bradken performed a finite element analysis for the casting production to show how molten metal flows into the casting and cools to optimize the pattern for production. The castings were fabricated in Bradken’s Missouri facilities. Quality control, performed by Bradken, and quality assurance, performed by the Airport’s testing agent Construction Testing Services Inc./Diversified Services, were implemented at Figure 4. Shell infill – to the left is the installation of light gauge framing; it supports the metal decking shown on the right, Bradken’s production site before creating a level roof plane. Courtesy of MWA Architects. the castings were shipped to Olson Steel. A prototypical casting, the First Article, was tested using assembled. The tolerances required Olson Steel to cut and weld visual inspection, magnetic particle testing (MT), ultrasonic test- one end of the horizontal brace members in the field to account ing (UT), and radiographic testing (RT). Samples of the casting for existing conditions. metal were taken, and tensile tests and Charpy V-notch testing were performed. For the production castings, RT was eliminated Roofing Enhancements and UT frequency was reduced, but the remaining testing was as required for the First Article. Castings were machined to toler- The original 48 hyperbolic paraboloid roof shells formed indiances that would facilitate subsequent steel fabrication; particular vidual “inverted pyramids” supported by a column, resulting in an attention was paid to bearing surfaces at the bolted collar halves undulating roof plane where each shell had to be independently and to surfaces to receive welded gussets. drained. One of the project goals was to create a “flat” roof with a global drainage plane to improve the roofing system’s reliability and simplify its maintenance. The original 1960 design accounted Cast Collar Erection Process for ponding of water in each shell, and so the initial plan for the Olson Steel’s concept for the erection of the cast collars was to retrofit was to fill the shells with EPS and foam – within the pondplace the two halves around columns in the field on the floor ing load allowance – and maintain distributed loading. When the level below the roof, bolt them together, and finally lift them into roofing contractor advocated an alternate infill design consisting place. Each collar was supported from four existing roof shell ribs of a light gauge framing “lid” (Figure 4), R+C developed a detailed using a through-bolted bearing connection. Given that each collar computer model to analyze the significantly different way in which weighed about 3 tons, this was no mean feat. Olson mocked up this scheme applies loads to the shells. In-situ load testing was used an assembly process in their yard using two forklifts, but this to validate the analysis results. proved too difficult to achieve a safe, vertical lift of the collar.

Olsen then devised a custom-made chain-hoist which allowed the Summary collars to be picked at four points and reliably lifted to the top of the column (Figure 3, page 27). The collars were bolted to the Construction on this $85 million phased project started in mid-2014. existing roof ribs and grouted in place to provide good bearing The Port of Oakland moved into their office space in September of between the existing column and the collar. The bracing was then 2017 with the project substantially completed. The renovation and retrofit of Terminal 1 resulted in a sustainable approach to achieving the project’s objectives (since the demolition and replacement of the New Release existing building were avoided), while at the same time affording continued operation of the security checkpoint.

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Jamison Curry is an Associate Principal at R+C. (jcurry@ruthchek.com) Peter Revelli was the Executive Principal and EOR on the project. (prevelli@ruthchek.com) Marko Schotanus is a Senior Associate at R+C. (mschotanus@ruthchek.com)

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Repair an d Retrofit in t he Af term ath of th e 2014 Napa Earthquak e

By Chris Jonas, S.E., and Steve Heyne, S.E.

D

uring the early morning hours of August 24,

h i s toric

GOODMAN LIBRARY

2014, many of Downtown Napa’s historic

buildings sustained moderate to heavy damage due to the powerful 6.0 magnitude earthquake that struck just over 5 miles away to the southwest.

Among those was the iconic Goodman Library located in the heart of the city. All the building’s stone walls sustained damage in the form of cracks, but it was the partial collapse of its prominent tower that was remarkable (Figure 1). Shocked passers-by and news media assembled to view the stone rubble strewn about the sidewalk and to contemplate a “what if ” scenario had the earthquake occurred during the downtown music festival just a half day prior. Barricades were established around the front of the building to protect bystanders from the dangers of large stones that could still come down. The partially-collapsed tower seemed to defy the laws of gravity, and even the smallest aftershock could quickly complete the demolition process, but, to everyone’s disbelief, most of the historic tower remained in place, damaged but

Restoration of the Goodman Library completed December 2017. Courtesy of Justin Lopez Photography.

30 STRUCTURE magazine

not destroyed.

Built in 1901, the historic Goodman Library is a centerpiece of the downtown district in Napa, California. The structure is named for George Goodman, a local banker and philanthropist, who donated land and funds for the construction of the library. Upon his passing, the library was donated to the City of Napa with the caveat that it must be used as a free library; otherwise, ownership would revert to his heirs. Local architect Luther Turton, also the designer of several local churches, designed the Goodman Library in the Richardson Romanesque style and utilized native Napa field stone for construction. With an architectural style more typically associated with places of worship, the library presented a distinct façade amongst the predominantly Victorian-styled brick and wood structures of downtown. The library encompasses 5,400 square feet over two floor levels extending back within a deep, narrow lot. It is constructed with double wall unreinforced stone masonry: two wythes of mortared stone separated by a gap loosely filled with stone chippings and mortar.

The stone exterior of the structure also features a castellated parapet and a stone tower over the main entry that displays a sandstone sign bearing the ‘Goodman’ name. The roof is framed with original repetitive timber trusses that are embedded into the stone walls for support. The interior spaces have maintained much of their original historical finishes, including wood trim, shelving, plaster walls, and pressed tin ceilings (Figure 2).

The Goodman Library is the longest continuously operated library in California, previously housing the Napa County main library until 1971 and now home to the Napa County Historical Society, which has operated a non-circulating research library on site since 1976. The structure was placed on the National Register of Historic Places in January 1974.

Shortly after the earthquake, a state of emergency was declared in Napa thus freeing up federal funding for the County. This funding stream was critical in allowing rehabilitation to take place but also required strict stipulations with respect to the historic aspects of the building. To receive funding, all repairs and strengthening measures required full review by the State Historic Preservation Officer (SHPO) for conformance to the Department of Interior’s National Guidelines for Historic Preservation. Knowing these strict criteria had to be met, the City of Napa hired local historic preservation architect, Stephen Cuddy with Napa Design Partners, to assist with the documentation of damage and preparation of repair specifications.

The need to secure public funding and maintain historic status contributed to a prolonged schedule. It took more than a year before a temporary steel enclosure was finally secured around the damaged 8-foot-tall tower, intended to reduce the risk of harm to adjacent threatened buildings, as well as provide for a safe and complete assessment of damage.

The architectural and structural engineering teams performed an extensive survey of the damage. It was evident upon closer inspection that there was significantly more damage to the stone walls than initially suspected. Each stone and every crack was recorded by hand on building elevations for later conversion to electronic drawings (Figure 3, page 32).

Stones that were damaged or dislodged were noted in detail, as this

process was also required to secure federal funding and to demonstrate compliance with historic standards.

Although the seismic damage was significant, it was also expected.

Unreinforced masonry (URM) buildings are stiff and brittle, with limited ductility and energy dissipation capacity provided by the fracturing of mortar joints and rocking of wall piers. The overarching goal of seismic Figure 1. Library after the earthquake. retrofit work in URM buildings is to address critical life-safety concerns to increase the likelihood that occupants can exit the building after an earthquake.

Inherently, retrofit work does help protect the structure and limit the damage, but not avoid it altogether. The Goodman Library underwent two retrofits prior to the Napa Earthquake. In the 1970s, a tie-rod diaphragm system was added above the roof in an apparent attempt to anchor and support the stone parapets, mitigating one of the most significant hazards to public safety. In 2004, a more extensive retrofit and rehabilitation was performed by the City of Napa in advance of its upcoming URM retrofit ordinance. The retrofit design was based on the 1997 Uniform Code for Building Conservation (UCBC) with a design base shear of 0.10g (ASD) and out-of-plane wall anchorage demands of approximately 0.66g and 1.06g (ULT) at the 2nd floor and roof, respectively. Comparing these loads to the actual seismic event in 2014, the acceleration for the building was likely between 0.90g and 1.20g (based on a ground motion monitoring device less than ¼ mile away) (FEMA P-1024). The ASCE 7-10 design base earthquake acceleration is 1.35g for the building. Using a response modification factor of R = 1.5 for an ordinary plain masonry shear wall structure, the resulting design acceleration is 0.90g, though the California Historic Building Code (CHBC) permits further reduction.

The 2004 retrofit performed quite well during the 2014 seismic event, despite the overall ragged appearance of the building immediately following the earthquake: the transverse concrete shear walls exhibited little to no signs of distress, as did the roof and floor diaphragms and wall anchors. The historic stone walls, which acted as the building’s shear walls in the longitudinal direction and at the rear of the building in the transverse direction, performed as expected, dissipating energy by cracking along joint lines.

The focus of the post-earthquake damage investigation was the tower, where the partial collapse allowed large stones to fall 30 feet to the sidewalk below. Exploration within the tower cavity revealed minimal existing lightweight steel frames for support. These frames, likely part of the 1970s retrofit work, lacked adequate stiffness to prevent substantial displacements of the stone. Additionally, the rock anchors from the steel frame to the stone masonry were an expansion style anchor embedded only into the inner wythe, leaving the outer wythe unsupported. Not only was a new, stronger frame needed within the tower, but external bracing from the backside of the tower to the roof framing was also recommended to reduce displacement of the tower and alleviate the deficiency. Before embarking on the design, however, SHPO required a visual assessment of the impact of the proposed external steel braces from street-view. Approval was granted after a simulation using BIM

Figure 2. Original interior finishes. Courtesy of Justin Lopez Photography.

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modeling was developed to demonstrate the permeable and softer, allowing the masonry bracing had little visual impact on the streetto “breathe” and to prevent fracturing of the level view of the historic façade. stone units themselves during an earthquake.

The new steel space frame, composed of The replacement mortar must also match the angles and channels, was designed to fit historic mortar in color, texture, and tooling. within the core of the vulnerable tower, As an added benefit, the grout injection of acting as a backbone (Figure 4). Provisions the cracks filled voids present between the from ASCE7-10 for a braced non-structural individual wythes of the stone wall, bonding cantilever element were used as the basis of them together. design, the same as would be used for a braced Additional damage was observed during parapet or chimney. It is worth mentionan investigation of the attic area. Several ing that the CHBC requires parapets and existing timber roof trusses, whose chord similar exterior decorations to comply with members were separated at the heels, were regular code requirements for anchorage and repaired using plywood gusset plates. As an strength, recognizing the public safety hazard added measure of precaution, the oversized these elements present. Though inserting a pockets in the stone walls receiving and stiff space frame could result in higher accelsupporting the trusses were fully grouted erations at the tower, it was the preferred and pointed. Anchoring the new tower Figure 3. Extensive documentation of damage. solution to limit local displacement which braces to the roof required several of the could lead to wide, cracked mortar joints original trusses to be reinforced with steel and then dislodged falling stones. Detailing side plates at chord-to-web connections to of the frame was explicitly designed for flexprovide additional capacity, and chords were ibility of installation by skilled masons from braced for increased compression loads. The Rainbow Waterproofing & Restoration Inc. steel beams added to distribute forces from As individual layers of stones were reinthe braces to the trusses were detailed as stalled in their original positions around the relatively short and light pieces due to the tower, heavy steel straps were laid within the limited access to the attic area; opening up horizontal mortar beds, interconnecting the the roof was not a viable option due to cost interior and exterior wythes, and attached to and risk of exposure to weather. the steel frame with self-drilling screws (or As a building occupant, the Napa County field welds) at specified locations. The closelyHistorical Society was integral in the restoraspaced straps were attached to the stones with tion process and maintaining the historical two vertical adhesive anchors and hidden accuracy of the library. All materials were reused within the bed joints of the tower walls. or replaced in kind where possible. Wooden This versatile connection detailing allowed fixtures and trim were cataloged for repair and for flexibility during the installation by the reinstallation, damaged terra cotta columns at masons as they reconstructed the tower over the entry were recreated, dislodged tin ceilings several months. During reinstallation, some were reinstalled, and the damaged sandstone stones were deemed too damaged for reuse. sign at the top of the tower was repaired and Fortunately, a nearby historic URM building restored. Work on the Goodman Library was Figure 4. New steel space frame designed to fit recently completed a remodel and had leftover completed in December 2017, three years after within tower core. matching stones available, allowing the tower the earthquake. With the addition of a new to be rebuilt entirely from local, historically accurate materials. support structure at the tower, restored historical stone construction, The dual-wythe construction of the exterior walls of the library strengthened critical connections, and rejuvenated finishes inside and out, posed added challenges. During the 3-month assessment process, the Goodman Library will continue to be an integral part of Napa’s history the exterior damage was apparent and abundant, but damage to the for years to come. Of note, the library restoration project has been recoginterior wythe remained hidden beneath wood-framed furring walls nized on both local and national levels, having won the American Public with historic wood lath, wood trim, and plaster finishes. Rather than Works Association (APWA) National Award for Historical Restoration & specify that all interior historic wall finishes be removed to allow Preservation as well as preservation awards from the Structural inspections to occur, the design team assumed that the exterior wythe Engineers Association of Northern California (SEAONC) and damage was mirrored at the interior. Localized areas of plaster were California Preservation Foundation (CPF).■ selectively demolished to check the stability of the interior stone wall Chris Jonas is a Principal Engineer with ZFA Structural Engineers in Napa, wythe and review the extent of structural damage. Fortunately, the CA. (chrisj@zfa.com) assumption was verified, and interior furring walls were only specified to be removed at isolated areas reflecting exterior damage, saving time Steve Heyne is an Associate with ZFA Structural Engineers in Santa Rosa, and cost, as well as maintaining the historic finishes as best possible.

CA. (steveh@zfa.com) Segments of the stone walls that were displaced and significantly cracked Both Chris Jonas and Steve Heyne provided repairs and rehabilitation were to be documented, then disassembled and rebuilt to historic standesign on many private and public historic and non-historic buildings dards. Other less damaged areas were repointed and grout-injected with post-earthquake and assisted the Applied Technology Council with their historically accurate lime-based grout to regain the structural integrity preparation of the FEMA P-1024 document regarding the performance of of the wall. Contrary to popular belief, replacement mortar and grout buildings and nonstructural components in the wake of the Napa Earthquake. should contain low amounts of cement compared to lime so that it is 32 STRUCTURE magazine

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historic STRUCTURES Joseph B. Strauss Bascule Bridge

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

A

fter graduating from the University of Cincinnati in 1892, Joseph Strauss (STRUCTURE, October 2012) became a draftsman for the New Jersey Steel and Iron Company at Trenton, NJ. At the end of the year, Joseph went to the Lassig Bridge and Iron Company of Chicago for two years, working as a detailer, inspector, estimator, and designer. He then spent two years as a designer for the Sanitary District of Chicago, which was in the process of building a drainage channel from Chicago to Lockport using water from Lake Michigan. The project was finished in 1899 and Strauss became a Principal Assistant Engineer in charge of the Chicago office of Modjeski & Angier.

He was charged with studying the use of bascule (from the French meaning seesaw) bridges across the Chicago River that would not need a mid-river swing pier, as the War Department declared swing bridges with a central pier were obstacles to the free movement of shipping.

Modjeski had been basing his bascule designs on some of the early bascules spans by William Scherzer and John Page. The earliest bascules used cast iron ingots as counterweights. Strauss presented Modjeski a design that would use concrete as counterweights, with his own system of links to lift the bridge. Modjeski did not like the plan, so Strauss resigned and went to work for the Rall Bascule Bridge Company, serving as Chief Engineer. After a year, Strauss left Rall and opened up his own office in Chicago under the name of Strauss Bascule & Concrete Bridge Company. At this time, 1902, the City of Chicago had developed an improved plan for bascule bridges and built its first one, a trunnion bascule, at Clybourn place with the counterweight under the approach to the span.

The first bascule span Strauss built on his own was for the Wheeling and Lake Erie Railroad (W. & L. E. or Wabash) in Cleveland, Ohio, over the Cuyahoga River.

He wrote of its conception, “High cost was, in fact, characteristic of all types of bascule bridges, due principally to the use of iron as counterweight material, and also in the underneath design to the necessity of deep pits. Elimination of these pits or frequently merely making them shallow, to reduce their cost, meant curtailment of the counterweight arm and consequent increase of counterweight to a prohibitive degree. As a result, in 1901 the limit of length and weight 34 STRUCTURE magazine

Winning design for Copenhagen Bridge.

of bascule spans and consequently the field concrete counterweight by all designers of of usefulness, though greater than in 1893, both bascule and lift bridges. Although thus when the Metropolitan Elevated Scherzer first applied in the overhead type of bridge Bridge was built, was still greatly restricted. in 1904, the pin-connected concrete counIn that year the writer, who had been terweight was originally proposed by the associated with the work on practically all writer for the underneath counterweight types of bascules in use, undertook the task type of bascule in 1901 in a series of plans of removing the limitations above referred which became the subject of much discusto by selecting of the available types that sion and which later were embodied in which promised greatest efficiency and by patent application No. 738,954, applied for modifying the counterweighting mechain December, 1902, and issued September, nism in such manner as to reduce the cost 1903.” without sacrificing efficiency. Determining Its span was 150 feet, and it opened in upon the ‘fixed trunnion’ type as the most October 1905. Engineering News reported dependable, he substituted concrete for the on the proposed design in 1904 as follows, cast-iron counterweights, effecting [sic] at “The Strauss bridge is of the trunnion once a saving in cost proportional to the type, but differs from other designs in its relative cost of the two materials… arrangement of the counterweight, by The counterweight is further supported by which the use of cast-iron weights and means of a rigid member termed a ‘counwatertight tail pits is dispensed with, thus terweight link,’ one end being pivoted to reducing the first cost and maintenance the counterweight and the other to the expenses. The tail end of the bridge is made fixed tower, forming the trunnion support. short enough to clear the water line, and This link is parallel and equal in length to at its extreme end are pins on which the a line joining the main and counterweight counterweight is carried… A pair of braced trunnions forming the so-called ‘parallel links, pin-connected to the fixed portion link’ counterweight system, which obtains of the bridge, guides the box, the motion through all the various forms of the writer’s types of bascule… It will be evident that if the counterweight trunnions are correctly located it is immaterial where the actual center of gravity of the counterweight mass is located. It can either be located above the roadway, as just described, or below the roadway…without raising the center of gravity (i.e., the trunnions) of the structure as a whole.

The result in the case of the W. & L. E. bridge was a saving of approximately $20,000 in first Wheeling and Lake Erie Railroad Bascule. Counterweight system cost and the ultimate adoption of where box moved down and towards the end of the span.

narrow channel, causing eddying currents, hindering navigation, and obstructing the flow, while the draw protection, if not maintained, causes damage to passing boats, with resultant expensive lawsuits.

2) Future addition of immediately adjoining bridges or additional track on either or both sides of existing bascule bridges is obtainable, which is impossible in case swing bridges are used.

3) For small vessels, the bascule need only open far enough to give sufficient vertical

clearance, whereas the swing bridge must be opened fully in any event. The reason for this is that the entire width of navigable channel must be clear to permit passage, which is obviously impossible if the swing bridge be only partly opened.

4) As a corollary it follows that a quicker passage of vessels and resumption of traffic results than with the swing bridge.

5) In a double-leaf bridge the bascule offers an effective barrier against highway traffic going off the roadway into the river,

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of which is downward and forward as the bascule leaf opens. The action is that of a parallel link mechanism, in which the main leaf trusses, the links, the fixed portion, and the counterweight box are parallel and equal elements… It also permits the elimination of all curved structural work and track girders, which tends to greater economy in design, greater’ accuracy in shop work, and greater speed in construction and erection...

The builders of this bridge state that… the Wabash Ry. Bridge at Cleveland… the Strauss design proved its economy by having less material in both substructure and superstructure than was called for in any of the other designs of bascule bridges proposed by competing bidders.”

To sell his design, he placed a fullpage advertisement of this bridge, and his patented ribbed concrete bridge, in the March 1906 issue of Engineering World: A Weekly Technical Journal of Civil, Mechanical, Electrical, Mining, and Architectural Engineering. This overhead counterweight trunnion bascule bridge was the first of its kind built by Strauss.

In his later Album of Bridge Designs, he wrote, “The illustration on the preceding page is a view of the first Strauss Bascule Bridge, erected in 1904, for the Wheeling & Lake Erie Railroad, at Cleveland, Ohio. It is a 150-foot singleleaf, single-track span, and represents the first marked advance in bascule bridge construction. Of its advantages the principal one was economy, due partly to the first application of concrete as a counterweight for bascule bridges.

Up to that date, counter-weighting had been effected with cast iron or more expensive materials. The substitution of concrete involved novel means of supporting same, but the cost of concrete being (weight for weight) much less than that of cast iron, the resultant effect was a material reduction in cost, and it is a matter of record that in this first bridge a saving of twenty-five thousand dollars in construction cost was realized.”

He sold his idea for a bascule bridge rather than a swing bridge by making nine points. “In general, the advantages of the bascule over the swing may be summed up as follows: 1) The center pier and draw protection of the swing bridge are eliminated.

These are especially objectionable in

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Portal as designed.

a class of accidents common with the swing bridge.

6) The bascule can be erected without interruption to land or water traffic.

7) The bascule does not encroach on adjacent property or docks and does not isolate the operator from shore when open.

8) In railroad bridges, mitered rail joints can be used without the necessity of raillifting devices.

9) All stresses are statically determinate, ordinarily in the swing bridge continuous beam action takes place.” [Note, in his first patent application, he had 18 claims to originality] The Chief Engineer for the Wabash Railroad Company wrote, “I consider the bridge lately put up for the Wabash Railroad Company at

Portal as built in 1909.

Cleveland the cheapest and best lift bridge on the market today. It is so well balanced that it can be stopped in any position and will remain there after the brakes have been released. I cannot illustrate this better than by saying that the addition of two or three hundred pounds of weight in the counterweight box makes difference enough in the balance of the bridge to affect the electric current sufficiently to be indicated by the ammeter reading. I understand that the bridge has been looked at by many Engineers, and all those who are unbiased have declared it to be the best structure of its kind they have seen.”

With this bridge, modified over time, Strauss began his career as the leading designer and builder of bascule bridges. He wrote, “but not until two years after the completion and

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Seattle

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thorough demonstration of the W. & L. E. Bridge was the great economic advantage of the all-concrete counterweight recognized by the engineering profession, in general, and universally adopted for all types of lift bridges. The W. & L. E. design, while having rendered excellent service at Cleveland, has been superseded by later and more economical forms which the parallel link counterweight system has taken.”

In 1906, he won an international competition to design a monumental Bridge across the Inner Harbour in Copenhagen, Denmark, called the Knippel Bridge.

The double leaf bascule measured 106 feet 6 inches from operating pinion to operating pinion. When closed, the arch halves connected and the bridge acted like an arch bridge under live loading. It totaled 238 feet 10 inches in length with flanking plate girder spans. The decorated concrete counterweights were located between flanking ornamental towers and dropped down to stop traffic when the bridge was opened. This was a significant project and cost approximately $230,000.

He became a fierce enforcer of his many patents for bascule bridges and was successful in suing several major cities for bascule bridges built by others that he claimed infringed on his patents. These early bridges, along with his patents, made Strauss the leading designer and builder of bascule bridges that competed with the lift spans of J. A. L. Waddell and swing spans of Alfred P. Boller and others.

In August 1906, he linked up with the Wallace-Coates Engineering Company.

Henceforth, Strauss would concentrate on the design and Wallace-Coats on the construction of the bridges. He went on to obtain 15 bridge patents, with the last two for the Golden Gate Suspension Bridge for which he was the Chief Engineer. The reader is urged to read Strauss’ online paper to the 2nd Pan American Scientific Congress 1917, Vol. 6, pages 304-322, in which he traces the evolution of his bascule designs.■ 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. (fgriggsjr@twc.com)

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legal PERSPECTIVES Indemnification versus Defense Part 2: Why the Difference Matters

By Gail S. Kelley, P.E., Esq., LEED AP

T

he previous article in this series (STRUCTURE, November 2018) looked at the concept of indemnification.

Stated simply, to indemnify someone means to financially protect them against specified claims from third parties. The party providing the financial protection (the “Indemnitor”) can be required to pay the amount of an award or settlement for a claim in place of the party being protected (the “Indemnitee”), or reimburse the Indemnitee for amounts the Indemnitee has already paid. The indemnification clause in many engineering agreements is similar to that found in § 8.1.2 AIA C401, Standard Form of Agreement between Architect and Consultant.

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.

A key point concerning indemnification is that the indemnification obligation does not arise until liability has been determined through either litigation or arbitration, or the parties agree to a settlement. This follows logically from the definition of indemnification – if it is determined that the Indemnitee is not liable for the claim, the Indemnitee will not need to pay any monies and hence will not need to be reimbursed. Likewise, if the indemnification is only to the extent the claim was caused by the negligence of the Engineer and it is determined that the Engineer was not responsible for the claimant’s injuries or damage, the Engineer would not owe a duty of indemnification.

The Defense Obligation While the indemnification obligation does not arise until liability has been determined, if a claim is brought against a party that the Engineer has agreed to indemnify, the indemnified party will likely incur considerable costs

for attorneys’ fees, court filings, and expert witnesses before the trial or arbitration even begins.

As a result, clients may require that the indemnification clause also include a duty to defend against claims “arising from the Engineer’s services.” If the Engineer has a duty to defend the Indemnitees, the Engineer would be responsible for an Indemnitee’s costs of defense as soon as a claim is filed, even if it were ultimately found that the Indemnitee had no liability for the claim or the damage was caused by someone other than the Engineer.

Indemnification clauses requiring defense can be extremely far-reaching; for example: To the extent permitted by law, Engineer will indemnify, defend and hold Owner and Architect harmless against and from all claims, damages, judgments, fines, penalties, and costs arising out of or in any way connected with the Engineer’s Services.

The words “arising out of or in any way connected with” the Engineer’s services can be interpreted very broadly. As long as the claim is written in such a way that it appears the claimant’s injuries arose from the Engineer’s services, the Engineer could be held responsible for the Indemnitee’s defense.

Defense of an Indemnified Party Unless an Engineer provides services that require them to spend a considerable amount of time at the project site, virtually all claims against the Engineer will fall under its Professional Liability Insurance (PLI). In cases where the Engineer is working on site and there is an accident involving the Engineer’s tools or equipment, or instructions that the Engineer has given, the resulting claim could fall under the Engineer’s Commercial General Liability (CGL) claim. However, even in such cases, the Engineer’s CGL carrier will generally try to deny coverage, alleging that the claim arose from the Engineer’s professional services and is thus subject to the professional services exclusion of the CGL policy.

Which policy a claim falls under can be very significant – CGL insurance will cover the defense of an indemnified party; PLI will not.

This is not something the Engineer can change through an endorsement to its PLI policy or by changing insurance carriers. While professional liability policies cover the defense of the insured party (the Engineer), they will not cover the defense of an indemnified party. An Engineer who agrees to a defense obligation will likely end up paying for the defense of the indemnified party out of their own pocket.

Editing the Indemnification Clause If the Client insists that the indemnification clause include a defense obligation, the Engineer can try to edit the clause such that the Engineer is not agreeing to uninsurable liability. Two such options for editing are: • change the word “defend ” to “defend (except for professional liability claims)”; or, • change the word “defend ” to “defend to the extent covered by Engineer’s insurance” If the Client does not agree to either of these changes, the Engineer can leave the word “defend” but add a sentence at the end of the indemnification clause stating: The Engineer’s defense obligation shall not extend to professional liability claims; however, the Engineer shall reimburse the indemnified party for reasonable attorneys’ fees and legal costs to the extent such claims are caused by the Engineer’s negligence or willful misconduct.

While the above wording could leave the Engineer responsible for the attorneys’ fees that it is required to reimburse, the obligation would not arise until liability for the claim had been determined. This means that the F E B R U A R Y 2 019

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Engineer avoids responsibility for defense of claims where the Engineer had no liability for the damages.

The Engineer can also propose a bifurcated indemnification clause that separates indemnification for professional liability from indemnification for general liability. A simple example of such a clause would be: 7.A Professional Liability. The Engineer shall indemnify and hold Client harmless for losses, damages, and costs arising from third-party claims to the extent such claims are caused by the Engineer’s negligence or willful misconduct in the provision of its professional services.

7.B General Liability. Except for those claims covered under section 7.A, the Engineer shall indemnify, defend, and hold Client harmless from and against allegations and claims arising from the Engineer’s acts, errors, or omissions.

Separating the indemnification for professional and general liability also allows the Engineer to agree to a broader indemnification for claims that do not arise from their professional services. While coverage under a PLI policy is limited to the extent of the Engineer’s negligence, both CGL and auto policies will cover the entire claim, provided the claim arose from the Engineer’s services.

This is reflected in the wording of 7.B (above), which indemnifies against “allegations and claims arising from the Engineer’s acts, errors or omissions,” rather than “to the extent caused by the Engineer’s negligence.” Although the difference in the wording may seem trivial, it can be quite significant with respect to how a court will interpret the indemnification obligation.

the Engineer should not be held responsible for the Client’s defense costs.

Reimbursement by the Client

As discussed above, indemnification and defense, while related, are separate obligations. However, under California law, based on the cases Crawford v. Weather Shield Manufacturing, 44 Cal.4th 541 (2008) and UDC Universal Development L.P v. CH2M Hill, 181 Cal.App.4th 10 (2010), if a design professional agrees to indemnify another party, there is an implied duty to defend. Unless the duty to defend is explicitly disclaimed, the design professional can be required to defend the indemnified party against claims arising from the design professional’s services.

The California legislature revised the code section that addresses the enforceability of indemnification requirements for design professionals (California Civil Code § 2782.8) such that, for contracts entered into on or after January 1, 2018, a design professional cannot be required to indemnify or defend another party “except to the extent the claims against the indemnitee arise out of, pertain to, or relate to the negligence, recklessness, or willful misconduct of the design professional.”

If the client will not delete the defense obligation for professional liability claims, the Engineer can propose a requirement that the Client reimburses the Engineer if the allegations of the Engineer’s negligence are false.

An example would be: Client shall reimburse the Engineer’s reasonable attorneys’ fees and legal costs incurred in defending the Client against professional liability claims, to the extent such claims are not caused by the Engineer’s negligence or willful misconduct.

This is the least desirable of the suggested changes, as it means the Engineer will be responsible for covering the Client’s defense fees until liability is established, at which point the Engineer can request reimbursement. However, it is also the hardest for the Client to argue against. If the Client has required the Engineer to defend it against a claim and the claim was not caused by the Engineer’s negligence or willful misconduct, 40 STRUCTURE magazine

Willful Misconduct The AIA C401 indemnification clause limits the indemnification obligation to the extent of the Engineer’s negligence, which is what will be covered by the Engineer’s professional liability insurance. However, like the reimbursement provision described above, many indemnification clauses also require indemnification to the extent the Engineer’s willful misconduct caused the claim. Claims due to willful misconduct are not covered by professional liability insurance – professional liability insurance is designed to cover claims due to negligence (unintentional mistakes) as opposed to intentional misconduct. However, in terms of basic fairness, it is not reasonable for the Engineer to refuse to indemnify another party when the Engineer’s willful misconduct caused the claim. On a positive note, claims against engineers based on willful misconduct are extremely unlikely.

Although the wording of court holdings on willful misconduct can vary, depending on the circumstances surrounding the claim, a party claiming willful misconduct must generally show that the other party has intentionally acted or failed to act knowing that his or her conduct will probably result in injury or damage.

Contracts Governed by California Law

Section 2782.8 also states that “In no event shall the cost to defend charged to the design professional exceed the design professional’s proportionate percentage of fault.”

However, the revised code section is silent on the implied duty to defend, which means the duty probably still exists. Thus, unless the duty is explicitly disclaimed, the design professional can be required to defend an indemnitee; once liability is determined, the design professional would have to seek reimbursement to the extent it was not liable for the claim. The wording of the disclaimer can be quite simple; it is enough to add the statement “The Engineer shall not be required to defend the Client against professional liability claims.”

The 2017 edition of AIA C401 added an explicit disclaimer to its indemnification clause; the clause now includes the statement: “The Consultant’s obligation to indemnify and hold harmless the Architect and its officers and employees does not include a duty to defend.”

While the addition of the disclaimer to the standard agreement is helpful when the agreement is governed by California law, it can be confusing to those who do not understand the reason for its inclusion.

Conclusion The indemnification clause is often the most difficult clause to negotiate in a design agreement, as most clients seek to limit their exposure to claims. Although the indemnification clause is a legitimate means of allocating risk between the parties, engineers should be careful about agreeing to obligations that will not be covered by insurance. In particular, engineers should be aware that the defense of indemnified parties will generally not be covered by professional liability insurance.■ 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.

Gail S. Kelley is an 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. (gail.kelley.esq@gmail.com)

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42 STRUCTURE magazine

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NCSEA News Call for 2019 Structural Engineering Summit Abstracts The NCSEA Structural Engineering Summit Committee is seeking presentations for the 2019 Summit in Anaheim, California, November 12-15, 2019. Ideal presentations will deliver pertinent and useful information that is specific to the practicing structural engineer, in both technical and non-technical tracks. Sessions will be 45-90 minutes total, including time for Q & A. Submissions on best-design practices, new codes and standards, recent projects, advanced analysis techniques, management, and business practices would be of interest. Other desired topics mentioned in the 2018 post-Summit survey include: • • • • •

Case Studies Lessons Learned Snow Loads Ethics Bettering Communication

• • • •

Accounting Client Growth Connection Design Wind Design

The online form must be completed in full by March 30, 2019. Speakers will be notified of abstract acceptance by April 30, 2019. Visit www.ncsea.com to complete the form now. Speakers will be provided with required guidelines after acceptance of abstract. Standard AV equipment will be in the room: podium, laptop, wireless microphone, projector, and screen. All speakers receive free registration on the day of their presentation.

Volunteer for an NCSEA Committee in 2019

NCSEA has a variety of committees that work to further the association’s mission to constantly improve the level of standard of practice of the structural engineering profession throughout the United States, and to provide an identifiable resource for those needing communication with the profession.

NCSEA SEA members may apply for committee positions throughout the year using the online Volunteer Application. Most committees admit new members on a rolling basis while others add members only once per year. Once submitted, the application will be reviewed to confirm Member Organization/SEA membership and then forwarded to the committee chair(s) for review. Please expect a response within 30 days.

Visit www.ncsea.com/committees to learn about NCSEA's Committees and to complete a volunteer application.

2019 EXCELLENCE IN STRUCTURAL ENGINEERING AWARDS The NCSEA Excellence in Structural Engineering Awards annually highlight some of the best examples of structural engineering ingenuity throughout the world. Projects are judged on innovative design, engineering achievement, and creativity. Structural engineers and structural engineering firms are encouraged to enter. Learn more by visiting www.ncsea.com!

Engineering Structural Glass Design Guide Authored by Marcin March, P.E., CEng, MIStructE, and Franklin Lancaster, P.E, and published with the assistance of the NCSEA Publications Committee, the Engineering Structural Glass Design Guide is aimed at structural engineers who are experienced in designing building structures and elements using traditional materials but with little to no experience in using glass to transfer forces. The purpose of this Design Guide is to provide the Engineering Professional with sufficient background knowledge and current methods to determine the specification of glass elements in buildings. The intent of this Guide is to collate relevant design references, requirements, and analysis methods into a single source for easy reference.

The NCSEA Engineering Structural Glass Design Guide is available as an electronic version as well as paperback. Visit www.ncsea.com to learn more about this publication.

44 STRUCTURE magazine

News from the National Council of Structural Engineers Associations

Prepare for April's SE Exam with NCSEA The Best Instructors. The Best Material. Available to you immediately!

NCSEA's restructured SE Refresher & Exam Review course is completely on-demand. Review course materials and watch the recordings when it is convenient for YOU. This on-demand course provides the most economical SE Exam Preparation Course available. The course includes 30 hours of instruction: 9 Vertical Sessions and 11 Lateral. The course will give you preparation tips and problem-solving skills to pass the exam. All lectures are up-to-date on the most current codes, with handouts and quizzes available.

PLUS ... students have access to a virtual classroom exclusively for course attendees! Ask the instructors directly whenever questions arise. This SE Exam Preparation Course allows you to study at your pace but with instant access to the material and instructors. Several registration options are available; visit www.ncsea.com to register yourself or to learn more about special group pricing!

Public Outreach Challenge

Save the Date for NCSEA's Structural Engineering Summit November 12-15, 2019 Disneyland ÂŽ Hotel Anaheim, CA Learn more about the NCSEA Summit by visiting www.ncsea.com!

Communications Committee Hosts New Competition for SEAs. Can structural engineers improve the public visibility and recognition of the profession? The NCSEA Communications Committee thinks so, and wants to encourage you and your SEA to participate. Through the creation and distribution of information and content via news articles, videos, blogs, and a multitude of other methods, SEAs can spread the message about our profession and its critical role in society.

The NCSEA Communications Committee invites NCSEA Member Organizations to participate in the very first Member Organization Public Outreach Challenge. The goal is to inform and educate other industries, professions, and the general public about Structural Engineering.

Learn more about the challenge and eligibility by visiting www.ncsea.com/challenge.

NCSEA Webinars

Register by visiting www.ncsea.com.

February 12, 2019

Insurance and Indemnification: What You Don’t Know Can Cost You Gail Kelley, P.E., Esq. This webinar will reduce the confusion that can arise when trying to decipher the sometimes very-creative wording of an indemnification clause by taking a step back and looking at what it actually means to indemnify another party.

February 26, 2019

Code Compliance & Existing Structures Patrick Sparks, P.E. This course will explore key factors to consider when evaluating and rehabilitating existing structures, and will discuss the application of various approaches to understanding existing structural systems.

March 14, 2019

Economical Design of Insulating Concrete Form Walls (ICFs) Lionel A. Lemay, P.E., S.E., LEED AP, and Shawn Lothrop, P.E., S.E. The presentation will discuss preliminary wall sizing and placement along with structural design considerations including design details and construction inspections. It will also provide guidance on how to minimize the cost of construction to take full advantage of these benefits, resulting in long-term value for building owners.

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

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45

SEI Update Membership

Call for Members

NEW SEI Global Activities Committees

Members interested in SEI Global Activities are invited to apply to join these new standing committees: • Global Professional Development & Qualifications Committee • Inter-Organizational Collaboration Committee • Global Credentials Committee Learn more and apply at www.asce.org/SEIGlobal.

SEI Sustainability Committee

The SEI Sustainability Committee develops resources for structural engineers and other building professionals to help them address the environmental impacts of structural materials and systems. The committee has published three technical reports: Sustainability Guidelines for the Structural Engineer, Structural Materials and Global Climate, and Whole Building Life Cycle Assessment: Reference Building Structure and Strategies, and written numerous articles. www.seisustainability.org.

Committee working groups lead technical efforts in areas such as climate change, life-cycle assessment, disaster resilience, thermal bridging, and infrastructure. To join this collaborative group of structural engineers from across the country, working in industry, academia, and industry organizations, apply at www.asce.org/structural-engineering/sei-tad-committee-application.

For more info, contact committee Co-chairs Megan.Stringer@holmesstructures.com or Mark Webster mdwebster@sgh.com.

Join or Renew SEI/ASCE

For innovative solutions and learning, to connect with leaders and colleagues, and to enjoy member benefits such as SEI Member Update monthly e-news opportunities and resources – visit www.asce.org/myprofile or call ASCE Customer Service at 800-548-ASCE (2723).

Advancing the Profession

Announcing 2019 SEI Fellows Recognized as Leaders and Mentors in the Profession Join us to congratulate them at Structures Congress in April: Lisa Anderson, P.E., F.SEI, M.ASCE Michele Barbato, Ph.D., P.E., C.Eng, F.SEI, M.ASCE Michel Bruneau, Ph.D., P.E., P.Eng, F.SEI, F.ASCE Robert Driver, Ph.D., P.Eng, F.SEI, M.ASCE Gregory Fenves, Ph.D., NAE, F.SEI, F.ASCE Thomas Heausler, P.E., S.E., F.SEI, M.ASCE Camille Issa, Ph.D., P.E., F.SEI, F.ASCE Susan Jorgensen, P.E., F.SEI, M.ASCE

Carl Josephson, P.E., S.E., F.SEI, M.ASCE Chun Lau, P.E., P.Eng, S.E., F.SEI, M.ASCE Abbie Liel, Ph.D., P.E., F.SEI, M.ASCE Daniel Linzell, Ph.D., P.E., F.SEI, F.ASCE Kevin Mackie, Ph.D., P.E., F.SEI, M.ASCE Gian Andrea Rassati, Ph.D., P.E., C.Eng, F.SEI, M.ASCE Frederick Rutz, Ph.D., P.E., S.E., F.SEI, M.ASCE Partha Sarkar, Ph.D., F.SEI, M.ASCE

Ignasius Seilie, P.E., P.Eng, S.E., F.SEI, M.ASCE Kenneth Sharpless, P.E., F.SEI, F.ASCE Arpan Tailor, P.E., S.E., F.SEI, F.ASCE John van de Lindt, Ph.D., F.SEI, F.ASCE Michael West, P.E., F.SEI, F.ASCE Gergis William, Ph.D., P.E., F.SEI, F.ASCE

Learn more at www.asce.org/SEIFellows.

SEI Online

Follow SEI on Twitter @ASCE_SEI

Errata 46 STRUCTURE magazine

SEI Standards

Visit www.asce.org/SEIStandards to: • View ASCE 7-22 Committee Meeting schedule and archive • Submit proposals to revise ASCE 7

SEI News Read the latest news items at www.asce.org/SEI

SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI-Errata.

If you would like to submit errata, contact Jon Esslinger at jesslinger@asce.org.

News of the Structural Engineering Institute of ASCE Learning / Networking

Thank you to Premier Sponsor Computers & Structures, Inc. for funding programs on leadership, innovation, career development, and more, to inspire and develop the next generation of structural engineering leaders.

Register for the best rate by February 13 and make sure to include your tickets for the CSI Special Evening Reception April 26 www.structurescongress.org.

Electrical Transmission and Substation Structures Conference Wrap-up

It was a powerhouse SEI Electrical Transmission and Substation Structures Conference, November 4-8 in Atlanta – with more than 1,400 attending the expert technical program and enjoying a sold-out exhibit hall with 115 booths and fun events. Dedicated to Strengthening Critical Infrastructure, ETS attracts structural/civil engineers, suppliers, and many more from the electric power, foundation, and construction industries. Thank you, attendees, speakers, sponsors, exhibitors, planning committee, and staff! Save the date for the next ETS September 19-23, 2021 in Orlando www.etsconference.org.

SEI San Francisco Chapter hosts Orthotropic Bridge Conference

The SEI San Francisco Chapter announces the 5th International Orthotropic Bridge Conference (5OBC), August 14-16, 2019, at Santa Clara University, in partnership with Santa Clara University and the Capital Branch of the ASCE Sacramento Section.

5OBC encompasses the interests of bridge owners, contractors, designers, researchers, maintenance personnel, and vendors involved with orthotropic bridges. New topics include Orthotropic System for Non-Bridge Structures (e.g., Marine, Offshore, Terrestrial or Extraterrestrial structures) and Use of Orthotropic Concept for Non-Steel Materials (e.g., Aluminum, Carbon fiber, etc.).

Learn more about program, abstract submissions and registration at www.5obc.org.

Joint International Conference:

Collaboration Efficiency Safety •

•

Dubai, UAE | 29-30 September 2019

Iconic Global Structures:

what can we learn?

Join structural engineers and project stakeholders to explore the successes and challenges of constructing nine complex structures across the world. Participate in panel discussions with industry experts to share best practice and promote the highest standard of engineering globally.

Learn more at — https://structuresdubai2019.cvent.com

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47

CASE in Point Did you know?

CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its Risk Management Program, keep track of the skills young engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need! CASE has several tools available for firms to use for managing projects and training staff: Tool 3-2 Tool 3-4 Tool 3-5 Tool 4-1

Staffing and Revenue Projection Project Work Plan Templates Staffing Schedule Suite Status Report Template

Tool 4-2 Tool 4-5 Tool 4-6 Tool 5-5

Project Kick-off Meeting Agenda Project Communications Matrix Project Team Coordination Project Management Training Guide

CASE Practice Guidelines Currently Available CASE 962 – National Practice Guidelines for the Structural Engineer of Record (Updated in 2018)

CASE 962-B – National Practice Guidelines for Specialty Structural Engineers

CASE 962-A – National Practice Guidelines for the Preparation of Structural Engineering Reports for Buildings

CASE 962-C – Guidelines for International Building Code-Mandated Special Inspections

The purpose of this document is to give firms and their employees a guide for establishing Consulting Structural Engineering Services and to provide a basis for dealing with Clients, generally, and negotiating Contracts in particular. Since the Structural Engineer of Record (SER) is normally a member of a multidiscipline design team, this document describes the relationships that customarily exist between the SER and the other team members, especially the team leader. Further, this Guideline promotes an enhanced Quality of Professional Consulting Structural Engineering Services while also providing a basis for negotiating a fair and reasonable compensation. Additionally, it provides a basis for Clients to better understand and determine the Scope of Services that the Structural Engineer of Record should be retained to provide.

The purpose of this document is to provide the structural engineer a guide for not only conducting conditional surveys, code reviews, special purpose investigations, and related reports for buildings but includes descriptions of the services to aid with the client risk management communication issues. This Guideline is intended to promote and enhance the quality of engineering reports. A section of this Guideline deals specifically with outlines for various reports.

While it is not intended to establish a specific format for reports, it is believed there may be certain minimal information that should be contained in a report. The Appendix includes disclaimer language which identifies statements one might consider to clarify the depth of responsibility accepted by the report writer.

This document has been prepared to supplement CASE’s National Practice Guidelines for the Structural Engineer of Record by defining the concept of a specialty structural engineer and the interrelation between the specialty structural engineer and the Structural Engineer of Record. CASE encourages the concept of one Structural Engineer of Record for an entire project. However, for many if not most projects, there may be portions of the project that will be designed by different specialty structural engineers. The primary purpose of this document is to better define the relationships between the SER and the SSE and to outline the usual duties and responsibilities related to specific trades. This is done for the benefit of the owners, the PDP, the SER, the SSE and the other members of the construction team. The goal is to help create positive coordination and cooperation among the various parties.

The CASE Guidelines Committee has developed three distinct versions of the Guidelines for International Building Code-Mandated Special Inspections covering the following IBC Code Updates: 2012, 2015, 2018.

The Guideline describes the roles and responsibilities of the parties involved in the special inspection and testing process, how to prepare a special inspection and testing program, the necessary qualifications of the special inspectors, how to conduct the program, and who should pay for the special inspections and test. The Appendix contains sample forms for specifying special inspections and tests and sample letters to be filed with code-enforcement agencies after the program is completed.

You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

48 STRUCTURE magazine

News of the Council of American Structural Engineers NEW!! CASE Tool 3-5: Staffing Schedule Suite By effectively projecting and balancing workloads, firms can maximize employee productivity and profit by reducing employee burnout and turnover.

This tool helps firms answer the following questions: • What are our employees working on day-to-day and week-to-week?

• Do we have enough work to keep our people busy, productive, and profitable?

• Do we have enough staff members to complete current assignments on-time?

You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

Share Innovative Ideas!

Does your firm have an innovative idea or method of practice?

Are you looking to get more involved in short duration projects?

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

Questions? Contact us at 202-682-4377 or email Heather Talbert at htalbert@acec.org. We look forward to helping you put your best ideas in front of eager new faces!

Donate to the CASE Scholarship Fund!

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 $29,000 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 for 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 don’t have to be an ACEC member to donate! Contact Heather Talbert at htalbert@acec.org to donate.

Fresh EJCDC Contracts to Meet Modern Market Demands

EJCDC’s newly released 2018 Constructions (C-Series) Documents are a significant modernization, revision, and expansion of the 2013 C-series and now the state-of-the-art in construction contract documents.

The updated edition comprises 25 integrated documents, including: • Fundamental contract documents such as the Standard General Conditions, the Small Project agreement, and Supplementary Conditions • Forms for gathering information needed to draft bidding documents • Instructions for bidders and a standard bid form • Bonds including bid, performance, warranty (new for 2018), and payment bonds • Administrative forms, such as change orders and a certificate of substantial completion EJCDC C-700, Standard General Conditions of the Construction Contract, has been extensively refreshed and updated, too. The new EJCDC 2018 C-Series also includes expanded and updated, “Notes to Users” and “Guidelines for Use” to provide more specific instructions and it eliminates the need for notary and corporate seals.

You can purchase these and other EJCDC documents at www.acec.org/bookstore.

F E B R U A R Y 2 019

49

INSIGHTS Performance-Based Design is the Future

By Donald O. Dusenberry, P.E., SECB, F.SEI, F.ASCE.

W

e, as structural engineers, often find ourselves confined by a situation we created for ourselves. We operate in a design environment with easy access to information, have the ability to collect and analyze extensive data, and have access to robust and reliable analysis and design software. Nevertheless, prescriptive codes and standards we have developed prevent us from fully leveraging these capabilities. As we face multiple goals for our designs – safety, economy, serviceability, sustainability, and robustness – we can be constrained to follow a prescriptive path to a solution that often does not optimally satisfy any of them.

Although nearly all modern design specifications strive to achieve some level of performance, they do not establish specific performance levels. Rather, design conformance to prescriptive criteria on materials, configuration, detailing, strength, and stiffness is implicitly taken as evidence the desired performance will be achieved. We believe our structures are safe, but we rarely know the true safety margins or whether other design solutions would provide superior performance.

To make matters worse, we rarely state, or even know, what performance levels we should strive to achieve.

As a result, we are not able to apply our full capabilities to the design process. We are evolving into masters of the Code, who add value by being able to navigate the complexity of prescriptive provisions rapidly, rather than by developing creative and innovative solutions to multi-faceted problems. The societies we serve are not getting maximum value from their limited resources of time, money, energy, and materials. Instead, they are getting designs that are constrained by prescriptive codes that attempt to address all conditions for all structures serving all purposes, with uncertain reliability because design by prescription neither quantifies nor directly evaluates performance.

Performance-based design is founded on the premise that structural systems must meet specific performance objectives. Specific performance expectations are set for the completed design, and processes are prescribed in minimal terms. Performance-based design, therefore, reverses the design process by defining the end goal as the starting point. The engineer then engages creativity and innovation employing science and principles of structural and material mechanics, unencumbered by unnecessary and in many cases counterproductive prescriptive requirements, to identify optimal solutions to multiple, and sometimes competing, objectives. The design is completed by demonstrating complying performance through analysis, simulation, testing, or a combination thereof.

Defined performance objectives are keys to the process because they establish the expectations for the design. The profession needs to describe for the public the damage levels or service states that are attainable for a variety of hazards acting at specific intensities on structures serving certain purposes and occupancies. For instance, engineering practice following performance-based approaches requires quantitative criteria such as 1) the structure should have less than a 10% chance of collapse given the occurrence of the Maximum Considered Earthquake and 2) no more than one wind event in 10 years should cause swaying troubling to occupants. Setting these performance goals and agreeing on them among the stakeholders will be challenging but critical to the process.

50 STRUCTURE magazine

Performance-based design offers several advantages over prescriptive design. First, properly executed performance-based approaches enable desired performance to be attained with greater confidence and expectations of reliability mainly because of the focus on the damage states. Second, since the performance objectives for the design are explicitly defined, the stakeholders can select the expected performance levels that are appropriate and satisfy their own needs. Third, since performance is evaluated directly as part of the engineering process, engineers need not be limited by requirements to conform to prescriptive solutions, thereby allowing for innovation and creativity using new materials and systems, and using existing materials and systems in new ways.

Structural engineers will be able to declare the expected performance of individual building designs and demonstrate compliance by working with all stakeholders. Structural engineers will be able to innovate and develop designs that respond to needs for resilience, robustness, and sustainability. Performance-based design approaches in civil engineering will allow structural engineers to influence broader public debates, beyond the design of individual structures, to impact public safety, welfare, and resilience at the site, community, regional, and national levels. Structural engineers will advance their roles to become more informed and critical partners in the design team and the community.

While performance-based design processes are routine in many engineering disciplines, they are unfamiliar to most of the stakeholders in the construction industry. The process demands more of structural engineers, including a better understanding of risk assessment and management. Peer reviews likely will be vital to the validation process. However, performance-based engineering approaches encourage research and development, and innovative engineering processes. The result is the freedom to solve harder problems with better structures.

Performance-based design approaches are not needed for most structures. In the future, we could easily have dual code approaches for structural design. Design of routine structures could default to prescriptive requirements, with a performance-based option for those interested in exploring its benefits. However, performance-based design processes should become an accepted protocol for complicated, high-value, and mission-critical structures (e.g., hospitals, emergency facilities and shelters, high-rise and iconic buildings, etc.), since the communities they serve will benefit from the innovation and creativity performance-based approaches foster.

To understand more about performance-based design and to learn what the Structural Engineering Institute is pursuing to advance performance-based design, see the report from which this article is adapted: Advocating for Performance-Based Design (April 5, 2018, Task Committee Report to the Structural Engineering Institute Board of Governors, https://goo.gl/XwiFBG).â–

This article is adapted by Donald O. Dusenberry. Mr. Dusenberry is a Consulting Principal of Simpson Gumpertz & Heger Inc., Waltham, MA. He has spent decades developing codes and standards in common use by structural engineers and presently chairs the Structural Engineering Institute Committee to Advance Performance-based Structural Engineering.

F E B R U A R Y 2 019

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