STRUCTURE magazine April 2019

STRUCTURE APRIL 2019

NCSEA | CASE | SEI

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INSIDE: U.S. Olympic Museum

34

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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.”

Want to Evaluate Tekla Structural Designer? tekla.com/TryTekla

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Evans Mountzouris, P.E. The DiSalvo Engineering Group, Danbury, CT John “Buddy” Showalter, P.E. American Wood Council, Leesburg, VA Eytan Solomon, P.E., LEED AP Silman, New York, NY Jeanette M. Torrents, P.E., S.E., LEED AP JVA, Inc., Boulder, CO STRUCTURE® magazine (ISSN 1536 4283) is published monthly by The National Council of Structural Engineers Associations (a nonprofit Association), 645 N. Michigan Ave, Suite 540, Chicago, IL 60611 312.649.4600. Application to Mail at Periodicals Postage Prices is Pending at Chicago, IL and additional mailing offices. STRUCTURE magazine, Volume 26, Number 4, C 2019 by The National Council of Structural Engineers Associations, all rights reserved. Subscription services, back issues and subscription information tel: 312-649-4600, or write to STRUCTURE magazine Circulation, 645 N. Michigan Avenue, Suite 540, Chicago, IL 60611. The publication is distributed to members of The National Council of Structural Engineers Associations through a resolution to its bylaws, and to members of CASE and SEI paid by each organization as nominal price subscription for its members as a benefit of their membership. Yearly Subscription in USA $75; $40 For Students; Canada $90; $60 for Canadian Students; Foreign $135, $90 for foreign students. Editorial Office: Send editorial mail to: STRUCTURE magazine, Attn: Editorial, 645 N. Michigan Avenue, Suite 540, Chicago, IL 60611. POSTMASTER: Send Address changes to STRUCTURE magazine, 645 N. Michigan Avenue, Suite 540, Chicago, IL 60611.

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

Contents APRIL 2019

26 LEAVING A LEGACY

By Sean O’Keefe

The iconic U.S. Olympic Museum’s diverse elevations called for fifteen independent concrete-slab-on-metal-deck elevations, scaling just four stories of construction with no two planes running parallel for long. Structural tolerances are ultra-tight, becoming even less forgiving higher up in the structure. A 3-D point cloud provides an accurate digital record of a physically intangible space. Cover

Cover Feature

graphic courtesy of GE Johnson Construction.

Columns and Departments 7

Editorial Leave the World a Bit Better

32

By Corey M. Matsuoka, P.E.

8

Building Blocks Ultra-High-Performance Concrete

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

35

By Maher K. Tadros, Ph.D., P.E., Adam Sevenker, P.E., and Rick Berry P.E.

12

16

Structural Specifications Specifying Requirements

Historic Structures Willamette River Swing Span 1908

Risk Management Crisis Management

By Randy Lewis

38

CASE Business Practices Including a Claim

for Concrete Mixtures

Validation Clause in your Agreement

By Colin L. Lobo, Ph.D., P.E.

By Bruce Burt, P.E., SECB

Structural Design New Trends in Reinforcing Steel

42

In the News Changes on STRUCTURE’s Editorial Board

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

43 20

Northridge – 25 Years Later Tilt-Up Concrete Wall Anchorage Design

By John W. Lawson, S.E., and David L. McCormick, S.E., P.E., SECB

Spotlight 2400 Market Street Renovation

By Lea Cosenza

50

Structural Forum Improving the Practice of Residential Wood Truss Roof Systems

24

Anchoring for Non-Structural Items

By Christopher Gamache, P.E.

28

By Brent Maxfield, S.E.

Construction Issues Shallow Post-Installed

Structural Practices Do Structural Engineers Design for Rain Loads?

By Michael O’Rourke P.E., Ph.D., and Anthony Longabard

In Every Issue 4 41 44 46 48

Advertiser Index Resource Guide – Engineered Wood Products NCSEA News SEI Update CASE in Point

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.

A P R I L 2 019

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EDITORIAL Leave the World a Bit Better

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

“T

o laugh often and much; to win the respect of intelligent people and the affection of children; to earn the appreciation of honest critics and endure the betrayal of false friends; to appreciate beauty; to find the

best in others; to leave the world a bit better, whether by a healthy child, a garden patch, or a redeemed social condition; to know that even one life has breathed easier because you lived. This is to have succeeded.”

This poem is often mistakenly attributed to Ralph Waldo Emerson.

Many of the words were derived from an essay written by Bessie A. Stanley of Lincoln, Kansas. No matter who wrote it, what is your definition of success? For me, I like to think of these words whenever beginning a new endeavor. Whenever I near the end, I try to reflect on how I did. This will be my last editorial as the Chair of CASE, so here goes…

In my years on the Executive Committee of CASE, I definitely laughed often and much. I have been blessed by meeting some exceptional people who know how to have fun. As CASE started to coordinate with the other disciplines within ACEC (American Council of Engineering Companies), the laughter only got better. Whoever said engineers are boring has never met this group.

We have pictures with animal print bathrobes at a pool bar on the top of a DC Hotel to prove it. This doesn’t mean we ignored what needed to be done.

It just means you can be wearing animal print bathrobes while you’re doing it.

Find the fun in whatever you do; it will make everything better.

Hopefully, through my STRUCTURE editorials and my contributions to CASE, I have won the respect of intelligent people. To people like Andy Rauch and Dave Mykins who came before me, I hope I have lived up to the high standards you have set. To people like Heather Talbert of ACEC who was always there to help and guide us. To people like you, structural engineers who are no exceptions! I do not know of any group more intelligent. I still do not know how I passed my Advanced Structural Dynamics class or how to determine how a building will react in an earthquake. My utmost respect goes out to all of you who figured it out and work hard to design the structures we need and use in our everyday lives.

While laughter and respect are important, what is most important is asking, did I leave CASE a bit better than when I got there? The poem suggests that one way this could be accomplished is with a garden patch. I think that is the perfect metaphor for what we are doing at NCSEA, SEI, and CASE. In the past, the three organizations acted independently from one another and sometimes even in competition.

STRUCTURE magazine

A few years ago, the leadership of each of the three organizations met in Chicago to determine how we could work together. We now meet two times a year to share our initiatives and discuss how these organizations can assist in meeting those objectives.

What I think of as our garden is the shared vision for the future of structural engineering that we are currently working on. As part of that effort, we are also determining what part each organization will play in achieving that vision. I think of each initiative identified in the vision as a seed we will be planting in our garden. Each initiative will be meticulously cultivated until they blossom by one or more of the organizations. Given all of the amazing accomplishments that structural engineers have made to date, it is exciting to think how much farther the industry will go with the three organizations working together. I genuinely believe we are building something special!

On May 8th, 2019, 9:45 am Eastern Time, my twoyear term as CASE Chair will come to an end. Some may say that knowing the exact moment my term is over is proof that I am looking forward to leaving the position. On the contrary, it has been a pleasure and honor to make a small contribution to the premier organization providing risk management and business practices for Structural Engineers. I hope I made CASE a bit better. I know the exact time because I look forward to the accomplishments my successor, Stacy Bartoletti, will surely achieve. He is an incredibly talented leader, and I am excited to see what exciting new heights are reached.

They say the biggest smile is always on the face of the Past-Chair.

In my case, that is true. Not because I am happy my term is over, but because I have learned to appreciate beauty and to find the best in others. I smile because of the lifelong friends I have made along the way, the garden patch we planted, and the members of CASE we have helped to breathe a little easier.■ Corey M. Matsuoka is the Executive Vice-President of SSFM International, Inc. in Honolulu, Hawaii and the chair of the CASE Executive Committee. (cmatsuoka@ssfm.com)

A P R I L 2 019

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building BLOCKS Ultra-High-Performance Concrete A Game Changer

By Maher K. Tadros, Ph.D., P.E., Adam Sevenker, P.E., and Rick Berry, P.E.

U

ltra-high-performance concrete (UHPC) was first introduced as reactive powder

concrete (RPC) in the early 1990s by employees of the French contractor Bouygues. When introduced, it came in two classes: Class 200 MPa (29 ksi) and 800 MPa (116 ksi). In the U.S., several

Figure 2. Components of Ultra-High-Performance Concrete (UHPC).

state departments of transportation have expressed interest in introducing UHPC in their bridge projects, supported by Federal Highway Administration (FHWA) research as well as research done by universities. Most notably, Virginia has produced I-beams with UHPC and Iowa has built two bridges with UHPC beams and one with a UHPC deck. Significant interest has recently been directed at using UHPC in longitudinal joints between precast concrete beams. Use of UHPC in bridges has increased in applications in the U.S. primarily due to leadership by the FHWA. Lafarge Cement Company markets a trade-named, pre-bagged UHPC product called Ductal®, formulated based on the work of the RPC invented by Bouygues. However, its high cost has discouraged owners from implementing the use of this outstanding material in applications beyond initial demonstration projects, many of which had been subsidized by government technology implementation programs.

Recently, several state highway agencies, in collaboration with local universities, have produced concrete mixtures that are much less expensive than the original RPC and Ductal materials. In addition, the Precast/Prestressed Concrete Institute (PCI) has recently commissioned the authors, in partnership with others, to undertake a large UHPC implementation project in which six major precast concrete companies are collaborating to develop their own UHPC mixture proportions and to implement the technology in long precast pretensioned concrete beams for bridges, parking structures, office buildings, and residential buildings. The three-year program will culminate in guidelines for precasters, designers, and owners.

The project is believed to be the first of its kind, as it will result in recommendations for design and production of precast, pretensioned beams as long as 250 feet.

What is UHPC? According to FHWA, “UHPCclass materials are cementitious based composite materials with discontinuous fiber reinforcement, compressive strengths above 21.7 ksi, tensile strengths above 0.72 ksi, and enhanced durability via their discontinuous 8 STRUCTURE magazine

pore structure.” However, this definition is not universally used. The Canadian Standards Association (CSA) includes two categories of UHPC: 120 MPa (17.4 ksi) and 150 MPa (21.7 ksi). The PCI research project specifies compressive strength of 10 ksi (at prestress release) and 18 ksi (at 28 days). The most significant property for structural design using UHPC is the tensile strength and tensile ductility, which are much higher in UHPC due to the presence of steel fibers, compared to conventional concrete. In the PCI research project, it is recommended that the ASTM C1609-determined flexural strength is above 1.5 ksi at first cracking and above 2 ksi at peak value with a significant deflection (ductility) beyond cracking (Figure 1). This high strength allows for much higher shear resistance and the possibility of total elimination of shear reinforcing bars.

The ingredients in UHPC vary. In general, the mixture consists of about 1200 lb/yd3 of portland cement; 250 lb/yd3 of silica fume; 250 lb/yd3 of supplementary cementitious material such as fly ash, ground slag, or silica powder; and 1700 lb/yd3 of fine sand with a maximum grain size of 0.03 inch. The materials are proportioned to produce the highest particle packing density. This, along with a very low water-cementitious materials ratio of 0.16 to 0.20, is the primary source of UHPC’s high compressive strength. The material is highly flowable with the aid of special admixtures. The addition of special steel fibers in the amount of about 2 percent by volume, about 265 lb/yd3, cause UHPC’s high tensile strength and ductility. The fibers are cut from very fine 360 ksi brass coated steel wire. Figure 2 shows an example of the individual materials being used in UHPC. Figure 1. Excellent tensile strength and toughness of UHPC.

Optimize Strength.

Figure 3. Comparison of conventional concrete pile with UHPC pile of the same flexural capacity.

Note, UHPC has higher compressive capacity and better resistance to pile driving effects.

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Pre-bagged UHPC has been selling for over $2000 per cubic yard. Also, fibers cost between $250 and $500 per cubic yard depending on dosage and source of fibers. It is therefore sometimes difficult to justify using materials that cost nearly $3000 per cubic yard. This is why UHPC usage has been limited to joints, where low volume use, although expensive, does not significantly affect the total cost of the project.

Several precasters, under the sponsorship of PCI, are in the process of developing their own mixture proportions resulting in a total materials cost of ChromX rebar strengthens the 49-floor Vizcayne Luxury Condominum Towers in about $600 to $800 per cubic yard. This Downtown Miami - to learn more, visit www.cmc.com/ChromXMiami. lower cost has the potential of making UHPC competitive on a first-cost basis, in addition to the benefit of achieving outstanding durability characteristics Up to 40% less steel 100+ years of Twice the strength of the resulting structures. Trial designs of conventional which means lower labor service life have demonstrated the possibility of with uncoated, reinforcing steel without costs, less congestion, saving about 50 percent of the concrete corrosion resistant sacrificing ductility and better concrete volume, weight, and other associated reinforcing steel (120 ksi vs. 60 ksi) consolidation benefits: savings in shipping, erection, foundation, temporary supports, Contact us at 866.466.7878 to learn how to optimize strength on your next etc. Conventional precast/prestressed project with ChromX. concrete bridge beams sell on average nationally for about $750 per cubic yard. If only 50 percent of the volume MMFX TECHNOLOGIES, A COMMERCIAL METALS COMPANY is used with UHPC, the price per cubic yard can double to $1500 without exceeding the conventional concrete cost. The additional $750 would be more than adequate to cover the cost of production and to allow for Several countries such as Australia, France, Japan, Switzerland, and, some extra risk in using a relatively new material. most recently, Canada have already published similar recommendaSeveral additional hurdles must be overcome for commercial produc- tions, which will prove to be helpful as U.S. codes and standards are tion of large-scale precast, prestressed concrete members. For example, updated to allow for this exciting material. mixing time and concrete delivery to precasting molds must allow quantities as large as 75 cubic yards for a single member without risking Structural Design Considerations the creation of cold joints. Also, structural engineers and architects are being challenged to step forward with creative solutions for full In the U.S., structural design criteria for UHPC have not been fully utilization of this special material. developed. However, enough knowledge exists to perform conservative An effort is urgently needed to publish a design guide for both designs until refinements are published. For a beam element, servicebuilding and bridge products and systems. Again, several attempts are load flexural design can be performed using linear elastic theory with currently underway by FHWA and PCI to produce such documents. the relevant material properties and with prestress losses assumed to

100+

A P R I L 2 019

9

Piles Figure 3 (page 9) shows a standard Florida Department of Transportation (FDOT) pile that is 24 inches square and prestressed with twenty ½-inch-diameter special strands and an equivalent UHPC pile. The conversion is made by blocking out the corners and creating an internal stay-in-place cardboard tube void. The result is over 50% reduction in weight while the flexural capacity remains essentially the same and the axial capacity is significantly increased. Note that the spirals generally required for conventional concrete may be found to be unnecessary due to the presence of high tensile strength fibers.

Pile driving may prove to be less risky due to the material toughness and ability to absorb energy. Figure 4 shows a pile segment, made

by Standard Concrete Products (SCP) of Tampa, being tested in the structural laboratory of FDOT. Figure 4. UHPC pile being tested at the Structures Laboratory of Florida DOT.

be 10% and 20% at initial and final conditions, respectively, and with tensile stress limit assumed equal to 50% of the flexural strength of UHPC. Prestressing is still the primary tension resistance element.

A significant beneficiary of UHPC is shear resistance. It is conservative to assume that the fibers contribute about 1000 psi of shear strength resistance. Considering that concrete resistance, Vc, is on the order of 100 to 400 psi and conventional mild-steel reinforcement resistance is capped at about 800 psi, it is easy to see that shear rebar reinforcement may not be necessary for many applications. Long-term camber is not much different from initial camber, as creep is a small fraction of that in conventional concrete. Live load deflection can be a critical design parameter due to reduced inertia associated with relatively small cross sections.

It is advisable to not reduce the depth of a conventional member in the optimization process without carefully assessing live load deflections.

Example Applications The following describes several concepts being pursued for development of UHPC precast/prestressed products.

Decked Bridge I-Beams Figure 5 shows a conventional bridge system, using an NU (Nebraska University) girder. Eight-foot-deep beams spaced 9 feet apart with a composite cast-in-place deck can span up to 180 feet.

A UHPC-decked I-beam system with the same total superstructure depth and spacing would have a maximum possible span of 265 feet while using a fraction of the total concrete volume. Also, it may be possible to eliminate all shear reinforcement, thus greatly simplifying production.

Inverted Tee Beams for Parking Structures Figure 6 shows a comparison between a recently used inverted tee beam, designed by the Consulting Engineering Group, San Antonio, TX. The beam was required to span 60 feet and to support double tees that also span 60 feet in the perpendicular direction. The UHPC voided inverted-tee beam shown was based on a preliminary design by the authors. It has the potential of being one foot shallower and 65 percent lighter. Note that the web width is expected to gradually increase from 2 inches in the middle third of the beam to 5 inches at the ends. This can be accomplished by varying the expanded polystyrene void form width. With this design, it is possible to eliminate all shear and torsion rebar and to count on the fibers for resistance to these forces.

Figure 5. Conventional concrete versus UHPC bridge I-beam. It is possible to increase span capacity while reducing the unit weight. Also, shear reinforcement can be eliminated.

10 STRUCTURE magazine

Figure 6. Conventional versus UHPC Inverted Tee for a 60- by 60-foot bay in a parking structure.

Observation Tower Figure 7 illustrates an interesting structure being designed for the owner of SCP who is a racehorse fan and who holds an annual charity event for horse racing on his property. He is convinced of the value of UHPC and wants to demonstrate its use by building a precast UHPC tower on his property. The preliminary design of the tower is reflected in Figure 7. Only two types of UHPC elements are used: an octagonal 36-inch hollow pile and 1⁄8th disc floor segments. Eight segments per floor times three floors = 24 identical segments, each requiring less than one cubic yard of UHPC as the floor is only 2 inches thick.

Conclusion UHPC is expected to reach a new level in practice in the next 5 to 10 years. An effort is underway that will allow the material to be economically competitive with conventional concrete on a first cost basis. Life cycle cost will be much more attractive due to the very high durability of this material. Design guidelines for structural design are being developed for possible inclusion in both bridge and building codes and standards. The efforts led by PCI and FHWA are expected to result in fully developed design examples for buildings and bridges. Using UHPC, it will be possible to design a precast concrete multi-story building with 60- by 60-foot column-free bays and to design bridges with spans well beyond the current limits on precast prestressed concrete of 180 to 200 feet.■

Figure 7. Observation tower for horse racing made of precast UHPC elements, except the roofing, railings, and stairs.

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

Adam Sevenker is a Structural Engineer and has worked in the area of bridge design since 2013. (asevenker@econstruct.us) Rick Berry is President of eConstruct USA, LLC. He has been SEOR for over 400 single and multi-story buildings. (rick.berry@econstruct.us)

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Maher K. Tadros is Distinguished Professor Emeritus of civil engineering at the University of Nebraska, and founding partner of eConstruct USA, LLC. He was the principal author of the first edition of the PCI Bridge Design Manual. (maher.tadros@econstruct.us)

A P R I L 2 019

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structural SPECIFICATIONS Specifying Requirements for Concrete Mixtures

By Colin L. Lobo, Ph.D., P.E.

T

echnological advances in concrete-making materials, production equipment, and processes have enhanced the possible innovative uses of concrete in a wide range of applications. Specifications for concrete mixtures, however, continue Figure 1. Illustration of two concrete mixtures with the same w/cm and different paste volume. to be prescriptive, which often limits the ability to develop and use innovative products and construction methods. Specifications Maximum Water-Cementitious Materials Ratio (w/cm) should be structured to leverage the expertise of the various stakeholders to deliver a high-quality structure with the desired service The w/cm of a mixture is an important parameter. For each set of life to the owner. materials, a unique relationship exists between strength and w/cm.

The challenges and opportunities with performance-based speci- Concrete producers use this relationship to proportion their mixtures. fications for concrete have been previously discussed (Lemay et al., One should not assume that this relationship is universal. Two mixtures STRUCTURE, April 2005). A prescriptive specification is one that with the same w/cm but considerably different paste volumes will have includes compositional details of materials or means and methods considerably different properties (Figure 1). The w/cm also impacts the of construction. The intended performance related to a prescriptive permeability or transport properties of concrete, important when durarequirement may or may not be defined. Alternatively, a performance bility is a concern. This property is also significantly improved by using specification defines the needed outcome tied to acceptance criteria SCMs like fly ash, slag cement, and silica fume. A maximum w/cm without detailing how it should be achieved. An important principle should be specified to address an exposure condition that impacts the with performance requirements is the congruence of responsibility durability of concrete. In 2008, ACI 318 established exposure classes and authority. It provides the specific stakeholder the appropriate for four durability categories with applicable maximum w/cm and an authority with assigned responsibility to achieve the desired outcome. associated minimum specified strength. The premise is that, for quality A prescriptive specification, with a stated or undefined performance assurance, w/cm cannot be verified but strength can. outcome, does not. Specifications that combine inconsistent prescripIn 73% of the specifications reviewed in the evaluation, a maximum tive and performance requirements tend to be more problematic. w/cm was specified regardless of the exposure condition. In most cases, In a recent evaluation (Obla et al., 2015), prescriptive requirements the specified strength was not consistent with the specified maximum that are considered onerous by concrete producers were ranked, and the w/cm; for example, 3000 psi and w/cm of 0.40. This sets up an inherent frequency of these requirements was quantified by reviewing a sampling conflict because the strength acceptance criteria do not assure the speciof about 100 specifications for different types of structures. Most of fied w/cm is being supplied. In some cases, the specified w/cm ratio was these prescriptive requirements are not consistent with the standards of lower than 0.40 and did not seem appropriate for the type of member. the American Concrete Institute (ACI) and are discussed here.

The use of w/cm lower than 0.40 is limited to very high strength concrete or for severe exposure conditions and is not too common for concrete buildings. Specifying a low w/cm impacts the cost of the mixture and its Prescriptive Limits on Mixture Proportions workability for constructability. It can also result in a higher cementitious Mixture proportions for concrete should be developed to have the material content and paste volume, thereby increasing the permeability required workability for constructability and be composed of the lowest of the mixture and the potential for cracking due to temperature or volume of a high-quality paste (cementitious materials and water) drying shrinkage. If exposure conditions for a member require the use that will provide the required strength and durability in the structural of a low w/cm, the resulting higher strength of the concrete should be member as required by the design, which includes consideration of the advantageously used when designing the member. exposure conditions. This will require the use of good quality locally While w/cm is one factor that impacts permeability, the beneficial available cements and aggregates with consistent characteristics, use of contribution of SCMs is not recognized. There are performance-based supplementary cementitious materials (SCMs) for improved durability tests that can be used as an alternative to w/cm. and long-term property development, and effective use of chemical Maximum Limits on SCMs admixtures that are compatible and provide the required workability and enhancement of hardened concrete properties. Specifications that There is one condition in ACI 318 that sets maximum limits on detract from achieving these primary objectives should be reviewed for the quantity of SCMs for concrete that will be exposed to cycles of intent and the potential constraints they cause. Some of the prescriptive freezing and thawing and application of deicing chemicals (Exposure constraints on concrete mixtures include the following: Class F3). The limit is intended because of an increased potential for 12 STRUCTURE magazine

Figure 2. Increase in an indication of the permeability of concrete by ASTM C1202 with an increase in cement content (mixtures with 40% slag cement by weight of cementitious materials at w/cm 0.40, 0.47, and 0.55). (Obla et al., 2017)

Figure 3. Increase in the drying shrinkage of concrete by ASTM C157 with an increase in cement content (mixtures with 40% slag cement by weight of cementitious materials at w/cm 0.40, 0.47, and 0.55). (Obla et al., 2017)

scaling in this type of exposure. In reinforced concrete, scaling will drying shrinkage is illustrated in Figure 2 and Figure 3, reduce cover and exacerbate reinforcement corrosion. This exposure respectively. This tends to reduce the amount of SCMs that can be condition is rare in buildings. This limit was stated in 85% of the used and increases the paste volume to impact performance adversely. specifications reviewed in the evaluation. The specification of this limit There are no minimum cement content requirements in ACI standards is either a misunderstanding of the intent or a directive to minimize for buildings. This requirement considerably constrains the ability of a the use of SCMs in concrete. concrete producer to optimize a mixture for the best performance and The use of SCMs for improved durability – reduced permeability limits competitive bids. This requirement, in addition to conservative and resistance to deterioration due to alkali-silica reactions, sulfate requirements for w/cm, results in actual concrete strengths 30 to 40% attack, and other chemicals – is well established. Reduced permeability higher than the specified strength, thereby causing an ineffective design of concrete protects the corrosion of reinforcement. In mass con- that makes the concrete construction more expensive and less sustainable. crete members, increasing the quantity of continued on next page SCMs is the more economical means to reduce the potential for thermal cracking.

The use of SCMs also improves the workability of concrete by achieving higher slump with less water and reducing the potential for segregation. It also supports sustainability initiatives. Resulting slower setting or rate of strength gain can be offset by the judicious use of admixtures Robert Bird Group offers you the opportunity to join us and verified by maturity methods to estimate in-place strength of concrete with age. It is suggested that these limits only be specified for exposure class F3.

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Minimum Cement Content Specifying a minimum cement content for concrete tends to be a historical remnant in many specifications. It was observed in 46% of the specifications reviewed. In many cases, the minimum cement content specified was significantly more than that required for the specified strength.

State highway agencies continue to define classes of concrete by cement content. The intent may be for improved durability, but this is faulty. The adverse impact on the heat of hydration, shrinkage, and permeability by requiring a higher minimum cement content has been documented (Obla et al., 2017). The effect on the transport properties (or permeability) and

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A P R I L 2 019

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Other Issues

• Prohibiting the adjustment of mixture proportions of • Invoking more restrictive requirements for concrete-making approved mixtures – real-time minor adjustments (without materials, like aggregates or fly ash, than what is in the ASTM requesting approval) are needed to maintain quality and specifications – this causes available sources with local history consistency to accommodate variation in source materials to be restricted from use or requires materials to be imported. and environmental conditions.

• Requiring the use of specific brands or sources of materials, like • Restriction on cement alkali content, primarily as an attempt admixtures or cements – producers have established contracts to minimize the potential for deleterious cracking due to alkaliand requiring the use of materials they are not familiar with aggregate reactions (AAR) – ASTM C1778 recognizes that INFO SPECS can cause problems. the use of a low alkali cement does not necessarily prevent the File Name: 18-2650 Ad_Structure_Mar_CRS Corporate Page Size: 5w" x 7.5h" • Specifying the combined grading of aggregates to relatively problem and provides various options to mitigate AAR. PR#: no Job#: 18-2605 Number of Pages: 1 tight bands with the intent to control workability or shrinkage/ • Tight controls Artist: Georgina Morra Email: gmorra@mapei.com Bleed: Yes Amount: .125" on the slump or other fresh concrete properties 114 4 E. Newpor t Center Dr. Dcurling eerfield – B e this a c h , typically F L 3 3 4 4 2 cannot be enforced andAMoften cannot be Colors: CMYK – can impact Date: February 1, 2019 11:09 Process, 4/0 constructability. ACI 301 permits the contractor N Oachieved T E : C O L O R S V with I E W E D Olocal N - S C R E Ematerials N A R E I N T E N D Eor D F during OR VISUAL R E F E R E N C E O N L Y A N D M A Y N O T M A T C H T H E F I N Ato L P Rselect I N T E D P the R O D U Cslump T. production. and document that in a submittal. In today’s The intended performance may not be achieved. concrete, slump is not a measure or control on water content.

• Including references to the Code or non-mandatory guides or state-of-theart reports – these include committee opinions, observations, and list several options, making the reference unclear.

The contractor cannot be responsible for Code requirements by a general reference to ACI 318. Any desired • Concrete Repair Mortars • Epoxy Adhesives requirement from these documents • Corrosion Protection • Decorative Toppings should be explicitly stated in the specification.

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Summary Prescriptive requirements for concrete mixtures in specifications often cause inherent conflicts that can negatively impact constructability, project costs, and deficiencies in the constructed project.

Many prescriptive provisions observed in specifications are not consistent with ACI standards that have been developed through a consensus process. While some of these requirements are prescriptive, they are tied to specific conditions. Reliable performance-based test methods and criteria are evolving and should be the direction of the future. In the meantime, designers should review their specifications and consider addressing some of the items discussed in this article. This will help optimize concrete construction and make it competitive to alternative systems.■ The online version of this article contains references. Please visit www.STRUCTUREmag.org.

Colin L. Lobo is Executive Vice President of the Engineering Division of the National Ready Mixed Concrete Association. He is a member of ACI Committees 318 (building code), 301 (specifications), and 329 (performance requirements for concrete). He is active on various ASTM committees and with transportation organizations. (clobo@nrmca.org)

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structural DESIGN New Trends in Reinforcing Steel High-Strength Reinforcing Bars

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

G

rade 60 reinforcing steel, with a yield strength of 60,000 psi, is the most commonly used grade in North America.

Recent advances have enabled reinforcing steels of higher strengths to be commercially produced.

High-strength reinforcing bars (HSRB) are typically considered to be any reinforcing bar with a yield strength greater than 60,000 psi.

This article presents pertinent information on material properties, the American Concrete Institute’s ACI 318, Building Code Requirements for Structural Concrete and Commentary, requirements and limitations, and some of the main benefits of using HSRB in reinforced concrete building structures.

The following is a brief history of the appearance and adoption of the various grades of reinforcing bars in ASTM specifications and ACI 318: • Grades 33, 40, and 50 were in common use from the early 1900s through the early 1960s.

• Grades 60 and 75 reinforcing bars appeared in 1959 with the publication of ASTM A432 and ASTM A431, respectively.

• The 1963 edition of ACI 318 allowed the use of reinforcing bars with a yield strength of 60,000 psi.

• In 1968, ASTM A615 first appeared, which included Grades 40, 60, and 75 deformed reinforcing bars.

• Grade 75 bars appeared in the 2001 edition of ASTM 955, and Grade 100 bars appeared in the inaugural 2004 edition of ASTM 1035. The 2007 editions of these specifications first appeared in ACI 318-08, with ASTM 1035 containing requirements for both Grade 100 and Grade 120 bars.

• A yield strength of 100,000 psi was permitted for confinement reinforcement in the 2005 edition of ACI 318 for use in non-seismic applications and then in the 2008 edition of ACI 318 for use in seismic applications.

• The 2009 editions of ASTM A615 and ASTM A706 were the first to include requirements for Grade 80 reinforcing bars, which were adopted into the 2011 edition of ACI 318.

Material Properties The design of any reinforced concrete member must satisfy the fundamental requirements for strength and serviceability 16 STRUCTURE magazine

Figure 1. Stress-strain curves for A615 reinforcing bars of Grades 60 and 100.

as prescribed in ACI 318. With respect to reinforcing bars, the basic mechanical properties that are important in achieving safe and serviceable designs are: • Yield strength, fy • Tensile-to-yield strength ratio, fu/fy • Strain (elongation) at tensile strength • Length of yield plateau Depicted in Figure 1 are typical tensile stress-strain curves for ASTM A615 reinforcing bars of Grades 60 and 100. The initial elastic segments of the stress-strain curves are essentially the same for both grades. Also, unlike Grade 60 reinforcing bars, a well-defined yield plateau for the Grade 100 reinforcing bars is not evident.

The stress-strain curves for some types of Grade 100 reinforcing bars can be more rounded than the one shown in Figure 1; these are commonly referred to as round house or continuously yielding curves (Figure 2). After an initial linear-elastic segment, a gradual reduction in stiffness occurs; behavior becomes nonlinear before reaching a yield strength, fy, that is defined by the 0.2% offset method. This is followed by gradual softening until the tensile strength, fu, is reached.

In general, the tensile-to-yield strength ratio, the elongation at tensile strength, and the length of the yield plateau all decrease (or, in the case of the yield plateau, can become nonexistent) as the yield Figure 2. Rounded stress-strain curve for Grade 100 reinforcing bars. strength increases.

Table of limits for non-prestressed deformed reinforcement.

Usage

Application

Maximum fy or fyt Permitted for Design Calculations (psi)

Applicable ASTM Specifications

Flexure; Axial Force; Shrinkage and Temperature

Special Seismic Systems*

60,000

Refer to ACI 20.2.2.5

Other

80,000

A615, A706, A955, A996

Special Seismic Systems*

100,000

A615, A706, A955, A996, A1035

Spirals

100,000

A615, A706, A955, A996, A1035

Other

80,000

A615, A706, A955, A996

Special Seismic Systems*

60,000

A615, A706, A955, A996

Spirals

60,000

A615, A706, A955, A996

Shear Friction

60,000

A615, A706, A955, A996

Stirrups; Ties; Hoops

60,000

A615, A706, A955, A996

Longitudinal; Transverse

60,000

A615, A706, A955, A996

Lateral Support of Longitudinal Bars; Concrete Confinement

Shear

Torsion

*Special seismic systems include special moment frames, special structural walls, and all components of special structural walls including coupling beams and wall piers.

Limitations and Requirements Tables 20.2.2.4a and 20.2.2.4b of ACI 318-14 contain the latest requirements and limitations for non-prestressed deformed reinforcement and non-prestressed plain spiral reinforcement, respectively. Limits for non-prestressed deformed reinforcement are given in the Table.

The yield strength of compression reinforcement is limited to 80,000 psi for use in applications other than special seismic systems.

This limit is imposed because bars with yield strengths greater than approximately 80,000 psi will not contribute to increased compression capacity; at a strain of 0.003 at the extreme concrete compression fiber of a reinforced concrete section (the strain assumed at crushing of the concrete), the maximum usable stress in the reinforcing steel would be 87,000 psi based on linear-elastic behavior (ACI 22.2.2.1). Note that Grade 100 longitudinal reinforcement may be used in columns, provided the aforementioned limit of 80,000 psi is used in the calculations in accordance with ACI 318. continued on next page

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the length of the yield region is related to the relative magnitudes of nominal and yield moments; the greater the ratio of nominal to yield moment, the longer the yield region. Inelastic rotation can be developed in reinforced concrete members that do not satisfy this condition, but they behave in a manner significantly different than members that conform to this provision.

In (iii), the required minimum elongations for ASTM A615, Grade 60 reinforcement were added in ACI 318-14 and are the same as those required for ASTM A706, Grade 60 reinforcement.

Benefits and Limitations Utilizing HSRB in concrete members may result in smaller bar sizes and/or a fewer number of bars compared to members reinforced with Grade 60 or lower bars. It may also permit smaller member sizes. By specifying HSRB, the following may be attained: • Lower placement costs • Less congestion, especially at joints • Improved concrete placement and consolidation • Smaller member sizes • More useable space Consider the reinforced concrete column illustrated in Figure 3a.

For architectural reasons, the cross-sectional dimensions are limited to 18 inches. Assuming that the column is non-slender and that it is subjected to a concentric factored axial force Pu = 1,700 kips, the required longitudinal reinforcement using Grade 60 reinforcement is 12- #9 bars ( ρg = 3.7%) with f 'c =10,000 psi. This is a relatively large reinforcement ratio and at locations of lap splices, ρg = 7.4%, which is slightly less than the code-prescribed maximum value of 8%. Additionally, the clear spacing between the longitudinal bars is about 3 inches. The combination of large reinforcement ratio and relatively small clear spacing could cause considerable congestion issues at the joints.

If Grade 80 bars were utilized instead (Figure 3b), 8- #9 bars would be required ( ρg = 2.5%). Not only is the reinforcement ratio more reasonable, but the likelihood of congestion at the joints is also significantly reduced (the clear space between the bars is almost 5.5 inches). A comparison of the interaction diagrams of the column with different bar grades is given in Figure 4.

Figure 3. Reinforced concrete column. (a) Grade 60 reinforcement (b) Grade 80 reinforcement.

The limit of 60,000 psi for shear and torsion reinforcement is intended to control the width of inclined cracks that tend to form in reinforced concrete members subjected to these types of forces.

References to research that supports the use of 100,000 psi reinforcing bars for lateral support of longitudinal bars and concrete confinement, including special seismic systems, can be found in ACI R20.2.2.4.

Only deformed longitudinal bars conforming to (a) and (b) below are permitted by ACI 318-14 in special seismic systems (special moment frames, special structural walls, and all components of special structural walls including coupling beams and wall piers) to resist the effects caused by flexure, axial force, shrinkage, and temperature. Higher grades of reinforcement were not included because, at that time, there was insufficient data to confirm the applicability of existing ACI 318 provisions for members with higher grades of reinforcement. These special systems are required in structures assigned to Seismic Design Category (SDC) D and higher.

Deformed longitudinal reinforcing bars used in structures assigned to SDC D or higher must conform to the following provisions (ACI 20.2.2.5): (a) ASTM A706, Grade 60 (b) ASTM A615, Grade 40 provided the requirements in (i) and (ii) are satisfied; and ASTM A615, Grade 60 provided the requirements in (i) through (iii) are satisfied. (i) Actual yield strength based on mill tests does not exceed fy by more than 18,000 psi. (ii) The ratio of the actual tensile strength to the actual yield strength is at least 1.25. (iii) Minimum elongation in an 8-inch gauge length must be at least 14% for bar sizes #3 through #6, at least 12% for bar sizes #7 through #11, and at least 10% for bar sizes #14 and #18.

In (i), the upper limit is placed on the actual yield strength of the longitudinal reinforcement in special seismic systems because brittle failures in shear or bond could occur if the strength of the reinforcement is substantially higher than that assumed in the design (higher strength reinforcement leads to higher shear and bond stresses).

In (ii), the requirement that the tensile strength of the reinforcement is at least 1.25 times the yield strength assumes that the capability of a structural member to develop inelastic rotation capacity is a function of the length of the yield Figure 4. Interaction diagrams of the columns in Figure 5 with 12- #9 bars region along the axis of the member. It has been shown that (Grade 60) and 8- #9 bars (Grade 80).

18 STRUCTURE magazine

The cross-sectional area of a column with high-strength longitudinal bars may be smaller compared to one with Grade 60 bars, which could translate into more useable space. Figure 5 illustrates how the gross area of a column, Ag , decreases as a function of the yield strength, fy, for various concrete compressive strengths, f 'c , and amounts of longitudinal reinforcement, Ast. It is clear from Figure 5 that combining high-strength reinforcement and high-strength concrete has the greatest impact on decreasing the required column area for a given percentage of longitudinal reinforcement, Ast /Ag . When reinforced concrete members are reduced in cross-section, it is always important to keep in mind the possibility of congestion issues that may accompany the size reduction.

The Future of HSRB It is anticipated that significant changes will be made to Table 20.2.2.4a in the 2019 edition of ACI 318 based on significant experimental data acquired for HSRB in both non-seismic and seismic applications. A public review edition of ACI 318-19 is now available, which highlights these changes.

The latest, most comprehensive information on HSRB can be obtained from the CRSI Technical Note, High-Strength Reinforcing Bars, which can be downloaded for free at www.crsi.org.■ The online version of this article contains references.

Please visit www.STRUCTUREmag.org.

David A. Fanella is Senior Director of Engineering at the Concrete Reinforcing Steel Institute. (dfanella@crsi.org) Figure 5. Required column area, A g, as a function of reinforcing bar yield strength, fy, concrete compressive strength, f'c, and the amount of longitudinal reinforcement, Ast.

Michael Mota is Vice President of Engineering at the Concrete Reinforcing Steel Institute. (mmota@crsi.org)

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NORTHRIDGE

25 YEARS LATER

Tilt-Up Concrete Wall Anchorage Design A Trial and Error Process

By John W. Lawson, S.E., and David L. McCormick, S.E., P.E., SECB

C

oncrete tilt-up buildings are a common subset of a class of building referred to as rigid-wall-flexible-diaphragm (RWFD) structures.

Common diaphragm types are plywood, oriented strand board, or metal deck. Masonry wall buildings with flexible diaphragms are also examples of RWFD buildings. Tilt-ups have performed poorly in past earthquakes, with the primary weakness being the anchorage between walls and roof.

Figure 1. Typical Pre-1973 UBC wall-to-roof anchorage detail (West Coast U.S.).

In the seismically active western United States, design was governed primarily by the Uniform Building Code (UBC) until the advent of the International Building Code (IBC). Before the 1973 UBC, it was common practice by design engineers to use the nailing of the plywood roof diaphragm to the wood ledgers (which are bolted on the inside face of the walls) as the de facto anchorage for out-of-plane wall forces (Figure 1). This indirect tie arrangement relied on the wood ledger in cross-grain bending and the plywood panel in tension near its edge. Cross-grain bending and tension are both very weak material properties of wood. Wall anchorage design forces before the 1973 UBC were specified as 0.2Wp under allowable stress design (ASD), where Wp is the tributary wall weight being anchored. In the 1971 San Fernando earthquake, the wall anchorage in many tilt-up buildings performed poorly. The use of this indirect wall tie led to wood ledgers failing in cross-grain bending and to plywood edge nailing tearing through panel edges from the wall anchorage tension loads.

Partial roof collapses and wall collapses were common in the areas of strong ground motion.

Subsequently, the 1973 UBC and 1976 UBC adopted detailing provisions intended to prevent such failures. Beginning with the 1973 UBC, the requirement for a positive and direct wall anchorage was introduced, and reliance on cross-grain bending in wood was expressly prohibited in the wall anchorage system. Furthermore, to transfer the heavy perimeter walls’ seismic anchorage forces effectively into the main roof diaphragm, the concept of continuous ties or crossties was explicitly required to collect and distribute the anchorage forces uniformly across the diaphragm depth for further distribution to shear walls (Figure 2). Because of the complexity and cost of providing the necessary repetitive crosstie connections across the roof structure, the concept of the sub-diaphragm was introduced in the 1976 UBC as a design approach for transferring forces from the individual wall ties to the continuous crossties. Sub-diaphragms are smaller portions of the main diaphragm, located adjacent to walls, and span between the continuous crossties. The 1976 UBC also increased the wall anchorage design force in areas of high seismicity from 0.2Wp to 0.3Wp through the inclusion of a 1.5 default site factor under ASD procedures.

Figure 2. Diaphragm failure due to inadequate continuous cross-ties for wall anchorage. Courtesy of Doc Nghiem.

Figure 3. Damage to tilt-up concrete building due to loss of wall anchorage during the 1994 Northridge earthquake. Courtesy of EERI.

20 STRUCTURE magazine

Guidelines for Seismic Evaluation and Rehabilitation of Tilt-up Buildings and Other Rigid Wall/Flexible Diaphragm Structures.

ASCE 41 can be used for retrofits that are more comprehensive when it is expected that wall panel deficiencies may exist (e.g., large openings).

There were substantial changes in the design requirements for new tilt-ups after the Northridge earthquake. The first group of changes was made in the 1996 UBC accumulative supplement, and further changes were made in 1997 UBC. Wall anchorage damage to post-1976 UBC buildings in the Northridge earthquake was attributed to several causes. A significant factor was the lack of steel strap connection ductility

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Fewer, but similar, wall-to-roof anchorage failures were observed in the 1984 Morgan Hill and 1989 Loma Prieta earthquakes. More significant was the strong motion data recorded in several undamaged instrumented tilt-up concrete buildings with flexible wood roof diaphragms. Research determined that the roof diaphragm accelerations were amplified three to four times that measured at the ground level, suggesting that wall anchorage forces were likely underestimated by the existing UBC provisions for a design level earthquake. This created the basis for an update in the 1991 UBC, increasing wall tie forces in the center half of the diaphragm span by 50% from 0.3Wp to 0.45Wp for seismic Zone 4 under ASD procedures.

The 1994 Northridge earthquake was the first test of modern, post-1976 UBC provisions for wall anchorage to flexible wood roof diaphragms under very strong shaking.

Hundreds of buildings were severely damaged due to inadequate wall anchorage, often resulting in partial roof collapses as seen in Figure 3. The poor performance of the pre-1973 UBC tilt-up and masonry industrial buildings was not a surprise. However, the amount of damage in buildings designed to more modern codes was unexpected.

Most notably, steel anchorage straps fractured through the net section in tension causing a sudden loss of anchorage strength, damage concentrated at the tops of pilasters due to inadequate reinforcing, and subpurlins with eccentric anchors failed.

At the time of the earthquake, the City of Los Angeles was developing Division 91 for the voluntary retrofit of pre-1976 tilt-ups. After the earthquake, this document was updated and made mandatory for the repair and retrofit of these older tilt-ups. A second voluntary document (Division 96) was developed for post-1976 tilt-ups. Division 91 served as the basis for Appendix Chapter 5 of the 1997 Guidelines for Seismic Retrofit of Existing Buildings (GSREB); GSREB Appendix Chapter 5 eventually became Appendix Chapter 2 of the International Existing Building Code (IEBC). The appendix chapter has served as the basis of many voluntary upgrades as well as ordinances adopted by various jurisdictions. All these documents recognized that overstress of the diaphragm in shear is not a likely source of the collapse of these buildings. The idea of prioritizing the wall anchor system components and de-emphasizing the wall panels and other components when retrofitting is discussed in detail in the Structural Engineers Association of Northern California (SEAONC) document,

A P R I L 2 019

21

The increased wall anchorage forces, as well as the improved detailing and design requirements, were introduced into the 1997 UBC and have changed little under the IBC since then (Figure 4 ). More importantly, these provisions have yet to be tested in the field by a strong earthquake.

Recent numerical modeling studies using nonlinear time history analyses indicate that the wall anchorage force levels are likely appropriate under a design level earthquake (Lawson et al., 2018). However, it is possible that, if the wall anchorage weaknesses are now resolved, unacceptable inelastic behavior may appear in a new part of the structure previously not considered problematic. Currently, one such location under closer scrutiny is the flexible diaphragm.

This research into the expected building performance of RWFD structures under design and maximum considered earthquakes was conducted using numerical modeling. Several building archetypes were subject to a series of incremental dynamic, non-linear time history analyses to evaluate the estimated margin from collapse using FEMA P695, Quantification of Building Seismic Performance Factors. Findings indicate that a different approach to the design methodology can improve the margin against Figure 4. Evolution of UBC/IBC provisions for wall anchorage to flexible diaphragms collapse to acceptable levels, and an alternate (Lawson et. al 2018). design procedure for RWFD buildings utilizing and overstrength to accommodate the very large roof accelera- wood structural panel diaphragms is proposed in FEMA P-1026, tions that occurred. With prior research indicating that rooftop Seismic Design of Rigid Wall–Flexible Diaphragm Buildings: An Alternate accelerations may be three to four times the ground accelera- Procedure. This alternative approach utilizes independent response tion, code writers of the 1997 UBC decided that, instead of modification coefficients R, over-strength factors Ωo, and deflection relying upon connection ductility, it was more appropriate to amplification factors Cd for the diaphragm, in conjunction with a elevate the wall anchorage design forces up to expected levels. two-stage analysis procedure similar to that used for podium buildAs a result, wall anchorage forces at the roof were increased ings. Currently, this alternative design methodology is being evaluated to 0.80Wp using strength-level design provisions (Figure 4). for possible inclusion in the next NEHRP Recommended Seismic Also, connection material-specific load factors (1.4 for steel, 0.85 Provision for New Buildings and Other Structures, and additional for wood, 1.0 concrete/masonry) were specified to obtain more research work is on-going to evaluate similar recommendations with uniform demand-to-capacity ratios within the anchorage connec- steel deck diaphragms. tion considering expected material over-strengths.

With all that has been done in response to the 1994 Northridge Poor quality control was also judged as a major contributor to the earthquake and the recent P-1026 alternate procedure, the hope is observed damaged. Observed deficiencies included: that there will be few surprises with the performance of the newer • Missing ties at tops of pilasters building stock. However, such claims have been made in the past • Missing bolts at the connection of girders to pilasters after a series of code changes have been made or lessons learned have • Slack in installed anchorages including straps and rods been forgotten. It is possible that a new weak link will appear the next • Gross eccentricity due to misalignment of anchors time the ground shakes hard in an area with a large stock of RWFD • Shimming that allowed bending of bolts buildings. And, of course, the older buildings (un-retrofitted • Oversized bolt holes resulting from drilling without a jig to or retrofitted to earlier code provisions) remain a concern keep the drill bit perpendicular to the member until they are appropriately updated.■ Such deficiencies demonstrated the need for diligent inspection and more involvement of the engineer via structural observation.

The online version of this article contains references.

Detailing and design requirement improvements made because of Please visit www.STRUCTUREmag.org. post-Northridge observations included: • Recognition of two-way action of wall panels due to the John W. Lawson is Associate Professor in Architectural Engineering at Cal Poly, stiffening effect created by pilasters causing increased wall San Luis Obispo. (jwlawson@calpoly.edu) anchorage load reactions at the pilaster top.

David L. McCormick is a Senior Principal at Simpson Gumpertz & Heger in • Prohibition of the use of one-sided eccentric wall anchors San Francisco. (dlmccormick@sgh.com) unless demonstrated adequate by calculation.

22 STRUCTURE magazine

The ACI Collection Online ACI 318-19 being added June 2019

With nearly 50 codes and specifications plus 200+ practices (including all guides and reports), the online version of the ACI Collection of Concrete Codes, Specifications, and Practices is the most comprehensive, always updated, and largest single source of information on concrete design, construction, and materials.

Always updated Inch-pound and S.I. units Historical editions of codes and specifications New titles as they are published The online version of the ACI Collection is structured for individual users—visit www.concrete.org/publications/ collectiononline for additional information. For multi-user options, please visit www.concrete.org/multi.

construction ISSUES Shallow Post-Installed Anchoring for Non-Structural Items

By Christopher Gamache, P.E.

P

ost-installed anchoring systems are very common, cost-effective methods for attaching both structural and non-structural elements to concrete base materials. Non-structural elements, such as fire sprinkler pipes, electrical conduit and cable trays, and heating, ventilating, and air conditioning (HVAC) equipment and ductwork are especially suited for post-installed anchor fastening. Building Integrated Modeling (BIM) has made cast-in anchors more common for non-structural elements hung from the ceiling, but post-installed fixations still dominate because mechanical and electrical locations can move after the concrete is cast.

While non-structural elements are not needed to support the main structure, their anchorage to concrete still requires the same level of safety as a design for a structural building element connection. Thus, it is still a requirement to design non-structural element connections to concrete using the anchor design provisions of Chapter 17 of ACI 318-14, Building Code Requirements for Structural Concrete and Commentary (ACI 318-14R). Non-structural elements will still have dead loads, live loads, and seismic loading conditions to consider.

Many non-structural elements are hung from a concrete ceiling.

Many concrete ceiling/floor systems consist of post-tension slabs or hollow-core precast concrete components. These concrete ceiling/ floor systems can have prestressing steel cables that have ¾-inch

cover from the underside of the concrete surface. This makes it difficult to attach items to the ceiling with a post-installed anchor due to the inability of the installer to locate the cables Example of a post-installed anchor with when drilling.

To help contractors with internal threads for non-structural hanging attachments in concrete with applications. prestressing steel cables and ¾-inch cover, post-installed anchor manufacturers have developed shallow embedment anchors that are intended to have ¾-inch drilled holes that will avoid the steel cables, no matter where the installer drills into the concrete ceiling.

Unfortunately, prior to October of 2017, the International Code Council Evaluation Service’s (ICC-ES) Acceptance Criteria (AC) for Mechanical Anchors in Concrete Elements (AC 193) did not permit testing and design of post-installed anchors with an embedment depth, hef, less than 1½ inches, except for redundant fastening designs which then permitted a 1-inch embedment depth. AC 193 is based on the American Concrete Institute’s (ACI) Qualification of Post-Installed Mechanical Anchors in Concrete and Commentary (ACI 355.2), which references ACI 318-11 Appendix D (now Chapter 17 in ACI 318-14).

Redundant fastenings, per AC 193, are applications where multiple anchors support linear elements (i.e., sprinkler pipes) Table of strength reduction factors, as modified by AC193 for shallow anchors. that will redistribute loads to neighboring anchors in case one (anchor governed by concrete breakout, side-face blowout, pullout, or pryout strength) anchor fails or exhibits excessive deflection. In all cases, this Condition A Condition B would restrict anchors with a ¾-inch embedment depth, even though the installer needed this shallow embedment to miss (i) Shear Loads the steel cables in the concrete. n/a 0.45 Thus, in October of 2017, AC 193 was changed to include (ii) Tension Loads provisions to allow embedment depths from ¾-inch to (Post-installed anchors with category determined from ACI 355.2) 1½-inch (or 1-inch for redundant fastening). However, AC 193 did not just extend the minimum embedment depth range Category 1 Low sensitivity to to ¾-inch. Anchors with a shallow embedment depth do not Installation and n/a 0.45 perform the same as anchors that have a 1½-inch embedment high reliability depth (and deeper). Thus, AC 193 incorporated five changes Category 2 Medium sensitivity for anchors that have a shallow embedment depth: to Installation and n/a 0.35 1) Near the surface, concrete does not have as consistent medium reliability a performance for anchoring when compared with Category 3 High sensitivity the interior of the concrete. This can be attributed to to Installation and n/a 0.25 a higher concentration of the concrete paste near the low reliability surface, along with shrinkage cracking and exposure to Condition A, per ACI 318 Chapter 17, applies where supplementary reinforcement is presenvironmental conditions. Thus, the concrete breakout ent to increase the concrete capacity. Condition B, per ACI 318 Chapter 17, applies where model in ACI 318-14 Chapter 17 still applies, but the supplementary reinforcement is not present. variability of the results has shown to be experimentally Categories 1, 2, and 3, per ACI 318 Chapter 17, indicate the sensitivity to installation and the higher. AC 193 lowered the strength reduction factors reliability of the fastening. Category 1 indicates a low sensitivity to installation variables and indicated in ACI 318-14 Section 17.3.3(c) to the values a high reliability in the fastening, while Category 3 indicates a high sensitivity to installation variables and a lower reliability in the fastening. shown in the Table.

24 STRUCTURE magazine

used for attaching a portion of the struc2) Similar to Item 1), experimental tural and load resisting system frames. testing has shown that shallow 5) Experimental tests have shown that anchors also perform worse when shallow anchors have a concrete breakthey are installed into the top out cone angle less than 35 degrees. surface of the concrete as opposed This means that, proportionally, the to the formed side of the concrete. cone would be larger than the concrete An anchor installed in the nonbreakout cone for deeper anchors. formed side of the concrete could This also means that anchors need have as much as a 30% reduction to be spaced farther apart from each in capacity when compared with other and the edge of the concrete to an anchor installed in the formed develop their full strength. AC 193 side. The formed side will typically requires a minimum spacing of have a higher concentration of Traditional hollow core slab with prestressing steel cable.

4hef (4 x 0.75-inch = 3-inch spacing for aggregate and the non-formed side will typically have a much higher concentration of concrete a ¾-inch embedment anchor) and a minimum edge distance of paste. Thus, without more testing, AC 193 is limiting the 2hef (2 x 0.75 = 1.5-inch edge distance) for shallow anchors. application to the formed side of a concrete member. This is In summary, to address the needs of contractors who install post-installed satisfactory for most non-structural items that are installed anchors for non-structural building components in concrete members in the ceiling. containing prestressing steel cables that may have a ¾-inch cover, ICC-ES 3) Similar to Items 1) and 2) above, experimentation has only AC 193 was revised to permit a shallow embedment depth no been performed in dry, interior conditions. There are conless than ¾-inch. This gives installers a high level of flexibility cerns that shallow anchors could have reductions in capacity while still maintaining a high level of safety and reliability.■ since the environmental conditions affect the concrete surface.

Thus, without more testing, AC 193 is limiting the applicaChristopher Gamache is the Manager of Approvals and Project tion to dry, interior conditions. Again, this is acceptable for Engineering/Anchors for Hilti North America. He is responsible for creating most non-structural items that are installed in an interior the technical data for the Hilti North American Product Technical Guide, concrete ceiling.

Volume 2, for Anchor Fastening and publishing external evaluation reports 4) AC 193 is limiting the use of shallow anchors for non-strucsuch as ICC-ES ESR’s. (christopher.gamache@hilti.com) tural applications. This means that the anchors should not be ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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The exterior plaza is designed to frame year-round views of Pikes Peak and welcome visitors from around the globe.

I

LEAVING A

n athletics, the gold medalist Museum is a process that can only is recognized as a champion be accomplished through a spirit of among all of humankind. Legacies continuous improvement. in construction or design are “Normally, once you have things harder to quantify, but when figured out, construction becomes the new United States Olympic routine,” says Project Manager, Museum opens in Colorado John McCorkle. “I do not see that Springs, Colorado, it will cerhappening here. We are continutainly be meaningful for design ally questioning how we do things. architect, Diller Scofidio + Renfro The ingenuity of this structure (DS+R), and General Contractor/ demands constant collaboration Construction Manager, GE with designers, builders, fabricaJohnson Construction Company. tors, and installers. Everyone will be “The incredible architecture we learning all the way until the end.” are delivering is challenging all of The team is pre-thinking and Pushing the Envelope to Deliver the Iconic us to think beyond boundaries,” rethinking every move by incorsays GE Johnson Superintendent, porating a 3-D point cloud that U.S. Olympic Museum in Colorado Springs Tim Redfern, an industry veteran provides an accurate digital

By Sean O’Keefe of more than 25 years. record of the physically intanDS+R’s design for the U.S. gible space. All subcontractors Olympic Museum (USOM) are required to use the point idealizes athletic motion by organizing its programs – galleries, cloud to develop approvable shop drawings. The point cloud auditorium, and administrative spaces – twisting and stretching is integrated with the BIM model, which draws from several centrifugally around an atrium space. Circulation is organic; spi- computer-aided designs, graphics, engineering, and manufacturing raling down from the top floor through the museum’s galleries, programs, along with the discipline-specific platforms of a variety the folded planes of the building’s superstructure create helical of different subcontractors. Integrated work plans developed with volumes of space circling the open center core. subcontractors define every aspect of each construction activ“The dynamic building form defies typiity including who, what, when, and cal construction. Thinking outside of the where. Most importantly, plans will box is not an adequate description of what detail how each piece is assembled, we’re doing to make this happen,” shares verified, and validated for accuracy Redfern. The design’s diverse elevations against the overall model as tasks called for fifteen independent concretecomplete. Looking beyond typical slab-on-metal-deck elevations, scaling just clash detection, the fully detailed four stories of construction with no two steel fabrication model allows the planes running parallel for long. Structural clearances of each structural framing tolerances are ultra-tight, becoming even member to be independently checked less forgiving higher up in the structure – to make sure the design’s distinctive the opposite of most builds. The exterior shell of diverging planes and scaled frame tolerance is two inches, while intemetal skin reads as intended. rior frame tolerances are only a quarter “There will be a high-level of scrutiny of an inch, with just an eighth of an inch on a building like this because of the of deflection. Controlling precise place- A full-scale mock-up of an exterior wall section was built on site to iconic architecture,” says McCorkle of ment of every piece of an exceptionally troubleshoot assembly and streamline sequencing for the exterior the pressure on GE Johnson to deliver intricate puzzle like the U.S. Olympic skin configuration. the signature design.

L egacy

26 STRUCTURE magazine

The museum’s unique exterior skin aptly illustrates the intricate precision of purpose and combination of expertise required to succeed at the Olympic level. The facade will be covered in more than 9,000 individual diamond-shaped anodized aluminum panels that interlock to form a single, beveled surface with integrated drainage channels. In total, an estimated 27,000 anchor points will attach the exterior wall sections to the structural frame. The specific details of every panel from backing materials, sheathing, and The twisting, helical design takes inspiration from the twisting form of a discus thrower but before release. waterproofing will all be independently analyzed within the model because, seemingly, every panel is either uniquely each placement can be checked against the point cloud to verify shaped, placed, or attached. GE Johnson brought highly specialized alignment accuracy. subcontractors who had previous experience with similar configuraThinking outside of the box has not been limited to solving challenges tions onto the team to achieve the use of these unusual building on the outside of the building. Placing the museum’s extremely large, materials and intricate assembly processes. yet whisper quiet, air handling unit presented a series of sequencing “Premium-quality construction is always a collaboration,” says challenges with a ripple effect that will likely continue to reverberate.

McCorkle. “Delivering this design uncompromised means getting “It is low-speed, high-volume and is by far the largest air handler I have out of the comfort zone and seeking capabilities beyond our own.” ever put in,” says Redfern enthusiastically. “The size dictates a basement Early in the problem-solving process GE Johnson worked with placement, which meant installing it before we put in the structural steel design architect DS+R, architect of record Anderson Mason Dale, for level one.” Once installed, this unorthodox situation left the massive and structural engineer KL&A on refining the micro-framing system (and expensive) unit unprotected from the weather until the floor above that attaches exterior wall sections to the structure. To address the it could be dried in. Complicating matters, structural engineering indicomplex sub-framing of the skin through a design-assist collabo- cated that the concrete floor slabs across the building’s many elevations ration, the team decided to work with Radius Track, renowned should be poured from the top down to deflect loading. Waiting until the for developing curved, cold-formed steel framing, to develop a museum’s 15 elevations were poured and cured would greatly extend the buildable system. Through continual collaboration, the team was exposure period for the mechanical system, presenting significant risk and able to optimize wind-girt supports, which increased certainty and an extremely difficult situation to rework if the unit was damaged. “We repeatability in installation while encouraged the owner and design team also decreasing costs overall. to install terrazzo on the first level floors Even as the structure reaches its instead of stained concrete so that floor highest elevation, preconstruction placement could be moved up in the activities continue. For the specialschedule, increasing protection of the ized subcontractors developing a AHU and equipment below.” sequence of efficiently attaching Placing the air handling unit first also the exterior skin to the structure, required fireproofing the basement nothing is more valuable than before setting structural steel, one of the full-scale exterior wall section several conditions which make mulbeing erected on-site. McCorkle tiple mobilizations of key trades likely and Redfern estimate that the throughout construction.

20- x 20-foot mock-up wall sec- Wedged between a now-industrial zone and a through-town rail yard, the “We have been empowered to use tion will require more than 1,000 new USOM strives to be a catalyst for area redevelopment when complete. ingenuity to solve complex challenges labor hours to assemble and will at every turn on a very, very cool buildlikely cost more than $150,000 to ing,” finishes Redfern. “GE Johnson is build. Eight different subcontracusing anything and everything we can tors must delicately interlace their to build this right. Pushing boundwork through a maze of structural aries, gaining outside expertise, and framing, light-gauge framing, asking more of oneself than others waterproofing, drainage, glazing, will is the Olympic spirit and aluminum panels. Identifying this museum is being built components within wall sections to honor.”■ that can be prefabricated off-site, like the micro-framing system Sean O’Keefe has more than 18 and laser cutting framing plates, years of experience articulating the complexities, challenges, and increases quality control and supcamaraderie of construction and ports repeatable processes during design. He writes Built Environment construction. Each component stories for owners, architects, builders, is individually numbered like a The building’s structural steel skeleton will ultimately be enveloped in more and product manufacturers. giant model, indicating where, than 9,000 diamond-shaped anodized aluminum panels, giving the finished (sean@sokpr.com) how, and to what it attaches, and façade an other-worldly quality.

A P R I L 2 019

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structural PRACTICES Do Structural Engineers Design for Rain Loads?

By Michael O’Rourke P.E., Ph.D., and Anthony Longabard

D

uring its lifetime, a building roof is subjected to a number of different structural loads – roof dead loads and roof live loads (principally snow, wind, and rain). Depending upon the location, one of these will be the controlling roof live load. For a building in northern Vermont, snow is likely the controlling roof live load; in northern Mississippi, it may be rain. For locations such as northern Vermont where the snow load is generally larger than the rain or wind loads, one expects more snow-related structural problems. Similarly, one expects few snow-related structural problems in northern Mississippi where the snow load is small in comparison to wind or rain loads. That is, one expects more snowrelated collapses in places where the snow load is comparatively large and fewer in places where the snow load is comparatively small.

The comparisons below show that building losses reported by FM Global for snow and wind hazards are consistent with the above expectation; for a specific hazard at locations where the magnitude of the hazard is large, more losses are generated. However, surprisingly, this expectation does not hold true for the rain load hazard.

Roof Live Load Losses Across the United States, dollar losses due to rain, in the period from 2007 to 2017, were 58% of those due to snow. In the same period, dollar losses due to wind (primarily hurricanes) were about 470% of those due to snow. Although rain losses were the smaller of the three, they were not negligible. Rain losses in Texas and Arizona (excluding rainfall during hurricanes) were nearly equal to snow losses in New England. Note that the loss data presented herein was based on a review of losses reported by clients of commercial and industrial FM Global between 2007 and 2017. Dollar losses were indexed to 2017 to ensure comparisons were independent of inflation.

Losses due to Snow As shown in the Table, states located in the northeast experienced the greatest snow-related losses followed by midwestern and western states, then southwestern and southeastern states. This is consistent with the ground snow load map in Chapter 7 of the American Society of Civil Engineer’s ASCE 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. That is, snow losses increased where the snow hazard was highest and decreased at locations where the hazard was smaller. Compared to the mean of the 2007 – 2017 period, snow losses fluctuated on an annual basis by +186%/-89% in terms of quantity and +222%/-95% in terms of total cost. As one might expect, snow losses were higher in particularly snowy winters.

Losses due to Wind States bordering the Atlantic Ocean and the Gulf of Mexico are known to experience high wind speeds due to hurricanes. This is reflected in the Basic Wind Speed maps in Chapter 26 of ASCE 7-16. For most of the contiguous U.S., the basic wind speed for Risk Category II structures varies from roughly 95 miles per hour (mph) 28 STRUCTURE magazine

Rain collapse at an FM Global insured location in the southern United States.

in California to roughly 110 mph in the upper Midwest. However, the wind contours range from 115 to 180 mph along the Atlantic and Gulf Coasts. Since the wind pressure is proportional to the square of the wind speed, the design wind pressure for the Florida Keys is roughly three times that for most other places in the contiguous U.S. Losses due to hurricanes in these states during the 2007 to 2017 time frame accounted for approximately 80% by total cost and 68% by total quantity compared to all wind losses experienced by FM Global insured locations throughout the United States. Losses due to hurricanes in Texas and Florida alone accounted for more than 58% by total cost and 32% by total quantity. Moreover, the average dollar loss for hurricanes in TX and FL was four times greater than the typical “straight line” wind loss averaged across all states. Hence, like the snow hazard, insured losses due to wind are highest where the wind hazard is most significant, and vice-versa.

Losses due to Rain The rain load hazard in ASCE 7-16 is a 15-minute duration, 100-year Mean Recurrence Interval (MRI) event. The area in the continental U.S. with the highest rain load hazard is the Southeast (Louisiana to North Carolina) with state average rain hazards ranging from roughly 6.6 to 8.2 inches/hour. The area with the next highest state average rain hazard (5.5 to 8.0 in/hr.) is the Midwest (Texas to the Dakotas). These are followed in order by the Northeast (Virginia to Maine, 5.1 to 6.4 in/hr.), the Southwest (New Mexico to Utah, 3.5 to 6.0 in/hr.) and the West.

The Table also shows the rank ordering of regions by rain load losses.

The region with the largest rain losses is the Southeast – a photograph on one such loss is presented in the Figure. This is not unexpected since this region has the highest rain load hazard. However, the rest of the ranking (i.e., ranking numbers 2 through 5) does not make sense. The Southwest and West regions have lower rain hazard values than the Northeast and the Midwest, yet they experience higher losses.

Note that the Southeast and Southwest regions, which make up approximately 30% of the land mass of the country, accounted for nearly 60% of all rain losses. As noted above, this is reasonable for the Southeast, which experiences the highest rain intensities in the United States; however, it is neither logical nor straightforward that Southwestern states followed as the region with next greatest rain-related losses. Furthermore, many losses in the Southeast that involved hurricanes, which typically bring a significant amount of rainfall, were filtered and categorized as wind losses. Losses were categorized in this manner because the wind associated with the tropical cyclone functioned as the initiating factor by way of breaching portions of waterproofing elements

of the components and cladding. Secondary to Table of snow and rain losses by region. engineer is too low. Note that, in this regard, a breach, rainwater directly entered the build- (Rank #1 means highest losses). the relative lack of rain load losses in the ing envelope. This secondary rainwater likely Northeast and Midwest is likely due to the Rank Snow Hazard Rain Hazard would not have led to a loss without the damage comparatively large design snow load for these 1 Northeast Southeast from wind. regions. The robust structural resistance due 2 Midwest Southwest Also, rain-related losses occurred 50% more to wintertime snow loads, which structural frequently than snow-related losses, and nonengineers routinely consider, is available in 3 West West collapse liquid damage (i.e., from snow or rain) the spring and summer to accommodate rain 4 Southwest Northeast losses occurred 4 times more frequently for rain loads which structural engineers apparently 5 Southeast Midwest than for snow (i.e., a leaky roof versus an eave routinely neglect. ice dam). An analysis of loss variation caused by There is also limited anecdotal evidence that rain compared to the mean of the 2007 – 2017 period showed collapses the authors’ contention is correct. The authors asked a principal of a fluctuated on an annual basis by +83%/-71% in terms of quantity medium sized structural engineering firm in the Northeast if the firm and +76%/-51% in terms of dollars. This shows less annual variability designed for rain loads. The principal responded, “Yes, if we think it is compared to snow losses. That is, the year-to-year rain losses are more needed.” This suggests that they do not routinely determine rain load consistent, while snow losses peak in particularly snowy winters. values, but rely upon snow loads as the “surrogate” for the uncalculated rain loads. In a discussion of rain loads with a principal at a large prominent structural engineering firm headquartered in the Northeast, the Rain Loads in Building Design principal indicated that, in an impromptu internal survey of 10 to 20 In summary, the above analysis showed rain losses to be the outlier people, the majority said that rain loads were not routinely considered among the natural hazards discussed. Unlike snow and wind losses, in their building designs in heavy snow load areas. Finally, when asked rain losses occurred with more frequency and consistency in portions how frequently buildings are designed for rain loads, a retired public of the country where the hazard is not necessarily the highest. sector structural engineer in upstate New York responded, “never.”

So, why did the entire southern United States experience the majority of rain losses when the midwest, north-central, and portions of the Possible Reasons north have a comparable rain hazard?

The authors contend that, unlike snow and wind loads, structural There are three possible reasons or explanations for the apparent lack of engineers do not adequately consider rain loads in building design. consideration of rain loads in United States structural engineering practice.

As discussed in more detail below, either rain loads are ignored by Hazard Level: Rain loads have always been part of the ASCE 7 Load the structural engineer or the rain hazard level used by the structural Standard. However, until recently, the actual hazard level (storm ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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duration and return period) has not been specified. In ASCE 7-88 (the first edition of ASCE 7), the roof drainage system was required to meet the provisions of the “code having jurisdiction.” The Commentary mentions the 1982 Building BOCA (Building Officials and Code Administrators International) Basic Plumbing Code and its required hazard level of a one-hour duration, 100-year return period event.

Also mentioned is the 1975 National Building Code of Canada which used a 15-minute duration, 10-year return period event.

ASCE 7-10 also defers to the code having jurisdiction, while the Commentary mentions a 1-hour/100-year event from BOCA 1993 and Factory Mutual Engineering (1991), a 15-minute/100-year event for secondary drains in the 1991 Southern Building Code, and a 15-minute/10-year event in the 1980 National Building Code. This ambiguity in required rain loads hazard level was thankfully eliminated in the ASCE 7-16 Load Standard. The code language identifies a 15-minute/100-year event as the design basis for secondary drains, while the Commentary clarifies the difference between requirements of the 2012 International Plumbing Code (1-hour/100-year) for primary roof drains and those in ASCE 7 (15-minute/100-year) for secondary roof drains.

Hence, prior to local adoption of ASCE 7-16, a practicing structural engineer may well have assumed that rain loads should be based on a 1-hour/100-year event or a 15-minute/10-year event. Note that rainfall intensity (in inches per hour) for the 15-minute/100-year event is typically 2 to 2.5 times larger than that for the 1-hour/100-year event. The corresponding ratio for the 15-minute/10-year event is roughly 1.5.

The rain load is comprised of two parts, the static head and the hydraulic head. Since the hydraulic head is nominally proportional to the design rain intensity, if the static head is low and the hydraulic head was based on a 1-hour/100-year event, structural collapse of a roof experiencing the 15-minute/100-year event would not be unexpected. That is, design for an unrealistically low rain hazard would appear in an insurance loss compilation as a design lacking consideration of the rain hazard.

Out of Sight, Out of Mind: For the past 40 or 50 years, it has been considered good practice to list the structural design loads on building plans. For example, since its inception in 2000, the International Building Code (IBC), Section 1603, has required that structural

30 STRUCTURE magazine

design loads be clearly indicated on the construction documents.

Until recently, floor live load, roof live load, roof snow load, wind load, earthquake design data, and flood loads were required to be listed on the construction documents. Note that although rain loads are covered in IBC section 1611, they were not required to be listed on the construction documents. In this sense, rain loads seemed to be in a special category of loads, not important enough to be listed on building plans. It is possible that structural engineers may have mistakenly assumed that, since rain loads did not need to be listed, their inclusion in the structural design process was somehow optional.

Also, since design rain loads were not required to be listed, it was difficult for the local building official to confirm whether the structural engineer properly considered them.

Fortunately, this potential misunderstanding has been rectified. The 2018 version of IBC will require rain loads to be listed, along with the other structural loads, on the construction documents.

Bad Timing: Rain loads are unique in that the magnitude of the load is a function of decisions made by other building professionals.

That is, the rain load is a function of the size of the drainage area for a given secondary drain or outlet, as well as the location of the secondary outlet within its drainage area. Also, unless a schedule for the architect/plumbing decisions is made at an early-on project meeting, the structural design of the roof may occur before the drainage area and secondary outlet information is available.

In such cases, the structural engineer may assume that the “plumbing engineer will handle it.” Alternately, the structural engineer may place a note on the structural plans indicating that the roof was designed for a rain load of xx psf. In the first case, the plumbing engineer may not realize that the hazard level for the secondary roof outlets is higher than that for the primary roof outlets. Also, it is highly unlikely that the plumbing engineer would properly check for ponding instability.

In the second case, the structural engineer is relying on the architect to read the “cover your backside” note and appreciate its import. In either case, the potential for inadequate secondary drainage system design and resulting structural collapse is generally consistent with the apparent lack of consideration of rain loads in U.S. structural engineering practice.

In relation to roof drainage information needed to calculate rain loads, it is best practice to discuss the issue at an early-on project meeting. A deadline for the roof drainage information to be sent to the structural engineer should be agreed upon.

It is the authors’ opinion that the structural engineer is best positioned to actually perform the rain load calculations and subsequent evaluation of potential ponding instability.■ Michael O’Rourke is a Professor of Civil Engineering at Rensselaer. He has been chair of the ASCE 7 Rain and Snow Load subcommittee since 1997. (orourm@rpi.edu) Anthony Longabard is a Staff Engineering Specialist in the Construction and Natural Hazards section of Engineering Standards at FM Global. He is the subject matter expert for several construction-related FM Global Data Sheets and is also a member of the ASCE 7 subcommittee on Snow and Rain Loads. (anthony.longabard@fmglobal.com)

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historic STRUCTURES Willamette River Swing Span 1908

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

A

lfred P. Boller’s Thames River Swing Bridge (STRUCTURE, February 2019) remained the longest swing span in the world until J. A. L. Waddell’s Omaha Bridge over the Missouri River in 1893 with its 520-foot span. Waddell would build a bridge in 1903, very similar end-to-end to his 1893 span, with a slightly shorter span. It was not until 1908 that Ralph Modjeski built his Willamette River Swing Bridge in Portland, Oregon, with a 521-foot span and the title of the longest swing bridge was wrested from the Omaha twin swing bridges of Waddell.

The Willamette River is a tributary of the Willamette River Bridge, longest swing span in the world at the time, Modjeski 1910.

Columbia River and separates the city of Portland, Oregon. Modjeski was charged with building bridges across the The bridges were also planned to serve the Oregon Railroad and Columbia River and the Willamette River connecting Portland Navigation Company. with Vancouver, Washington, and the north. The entire bridge Modjeski, who apprenticed under Morison, did not become involved project was just shy of five miles long. Modjeski wrote, “To effect with the project until the fall of 1905 (Morison died in 1903) when an entrance into Portland, the Spokane Portland & Seattle Railway he was approached by the Spokane, Portland & Seattle Railway.

Company is building two bridges, one at Vancouver, across the He had already established a national reputation from the Thebes Columbia River, and one near St. Johns, across the Willamette Bridge across the Mississippi River in 1905. He was also to become, River. The two bridges cross the streams very nearly at right angles during the construction of his bridges, one of the Engineers on the and are practically on the same tangent. The peninsula between reconstruction of the Quebec Bridge across the St. Lawrence, replacthe two bridges will be crossed with a deep cut. Both bridges are ing the collapsed span by Theodore Cooper and the Phoenix Bridge double track and designed for heavy modern loading without Company in 1907. roadways. The Vancouver bridge may be divided into three parts, He generally accepted the earlier layout of the spans with minor as follows: The Washington channel bridge, the Shaw’s or Hayden’s modifications and the use of the partially completed swing span island viaduct, and the Oregon Slough bridge…The Willamette pier on the Columbia River. The Port of Portland approved the River bridge is symmetrical with regard to the center of the pivot Willamette swing span in April 1906 and the War Department pier. The large draw span, 521 feet center-to-center of end pins, approved it in June 1906. The War Department specified the span will be the largest in the world when completed. On each side of length in terms of the horizontal clearance required. It is likely the draw span are two fixed spans approximately 269 feet center- that Modjeski wanted his bridge to be the longest in the world, to-center of piers, making four fixed spans in all. On each end so he had his design result in the 521 feet total length of swing of the bridge is an 80-foot deck plate girder approach span. The span. He described the design and construction of the bridges in total length of this bridge is 1,762 feet 3 inches.” a 1910 30-page report with plates entitled, The Vancouver-Portland Construction had started in 1890 on a single track bridge, but work Bridges, a Report to Mr. Howard Elliott, President of the Northern stopped on the project after a swing span pier by George S. Morison Pacific Railroad Company, and to Mr. John F. Stevens, President of in the Columbia River was placed. The Great Northern Railway and the Spokane, Portland & Seattle Railway Company. He also pubNorthern Pacific Railway, fierce competitors, formed the Spokane, lished a paper on the project for the Railway Age, published on Portland & Seattle Railway Company to build the bridge in 1905. March 20, 1908.

Modjeski called his bridge “the longest, and probably the heaviest, draw span in the world.” On each side of this were two fixed spans of 269 feet each, and an 80-foot deck plate-girder approach span. While only a tad longer than the Omaha Bridge, it held the record for a swing span until the Fort Madison Bridge (1927) with its 525-foot span crossed the Mississippi River.

The draw span pier was constructed of plain mass concrete, octagonal in section, 47 feet 8 inches across, with a two-course ashlar coping. The bridge was one of Willamette River Swing Span, Modjeski 1910. the largest, mostly rim-bearing swing bridges built at a 32 STRUCTURE magazine

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time when most engineers preThis article is the last of the ferred center bearing. Modjeski author’s series on Historic Bridges. stated his design called for 5/6 It started with Timothy Palmer’s of the draw’s weight to bear on Newburyport Bridge across the the rim and 1/6 on the center Merrimack River and closes with bearing, where some other Ralph Modjeski’s Willamette engineers recommended equal River Bridge in Portland, distribution of load between Oregon. In upcoming issues of the rim and center bearing.

STRUCTURE, the author will He designed the draw span be describing the most prominent with ten 24-foot 6-inch-width bridge failures in North America panels on each side of a 31-foot starting with the Rider Bridge center tower, going against failure on the New York and Erie prevailing bridge design philosRailroad in 1850 and ending with ophy which preferred varying the Minneapolis Bridge failure in panel widths. The draw span 2007. Many lessons can be learned had two sloping top chord from today’s bridge engineers by members followed by six-panel the knowledge of past failures and straight top chords and a fivetheir causes. As George Santayana, panel hump over the center. It Edmund Burke, and others was designed to function as a have said, “those who balanced cantilever span when don’t know history are Listing of long span Swing Bridges, Merriman and Jacoby 1907. open and as two simple spans doomed to repeat it.”■ when closed and in a locked position.

Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, There were two independent sources of power provided. One having restored many 19 th Century cast and wrought iron bridges. He was two 79 HP electric motors that could swing the span ninety was formerly Director of Historic Bridge Programs for Clough, Harbour degrees in 1¼ minutes. The second was a gasoline engine of 165HP. & Associates LLP in Albany, NY, and is now an Independent Consulting It was directly connected with a generator and was intended to Engineer. (fgriggsjr@twc.com) act as auxiliary power for the electric motors whose normal main power came from land-based sources.

As an additional precaution, Modjeski designed a capstan system with which he estimated ten men could swing the draw ninety degrees in twenty minutes.

Structural work was completed in July 1908, and the span swung for the first time in October. The first train passed over the bridge on October 23, 1908.

The entire bridge cost over $1,000,000 and the swing span weighed 4,600,000 pounds. It was replaced in 1989 with a 516-foot-long lift span, at a cost of $38 million, to provide greater clearance for shipping.

Merriman and Jacoby, in their A Textbook on Roofs and Bridges Part IV. Higher Structures, gave a listing of the major long span, greater than 400-foot, swing bridges built between 1875 and KPFF is an Equal Opportunity Employer www.kpff.com 1907. A. P. Boller (STRUCTURE, November 2012), one of the leading swing bridge proponents, designed bridges 4, 5, 6, 24, 25 as well as several across the Harlem River of shorter span.

Seattle Los Angeles St. Louis Ralph Modjeski built bridges 1 and 10. J. Seattle San Des Moines Tacoma LongFrancisco Beach Chicago UC San Diego Health A. L. Waddell (STRUCTURE, February Tacoma Los Angeles St. Louis Lacey Irvine Louisville Koman Family Outpatient Pavilion Lacey Long Beach Chicago San Diego, CA Portland San Diego New York 2007), even though primarily a lift bridge Portland Irvine Louisville DC Eugene Salt Lake City Washington specialist, built bridges 2, 3, 18. George Eugene San New York Sacramento BoiseDiego S. Morison (STRUCTURE, February Sacramento Boise PHOTO BY TOM BONNER PHOTOGRAPHY San Francisco Des Moines 2008) designed bridges 12, 16, 17.

IN ARCHITECTURE

A P R I L 2 019

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risk MANAGEMENT Crisis Management Plan for the Worst

By Randy Lewis, CPCU

I

n the event of a crisis, your firm may need to defend itself in both the legal and public relations arenas.

Like any other business sector, the design and construction industry must be prepared to deal with crises such as natural disasters, data breaches, sudden financial setbacks, or even workplace violence. However, the prospect of a project-related crisis, such as a catastrophic failure resulting in an injury or death, is the type of event that keeps A/Es up at night.

If not handled properly, a crisis can inflict real harm on your firm, resulting in business interruption, drawn-out lawsuits, financial loss, and damage to your reputation.

What can you do? Every crisis is unique and comes with its own set of issues; however, there are steps you can take to prepare for, manage, and recover from even the most serious and disruptive events. Here are some suggestions, drawn from “lessons learned” by design professionals. While these steps contemplate a project-related crisis, many apply to all types of emergencies:

Before a Crisis Hits

dollars in discovery expenses in the event of a lawsuit.

• Work to strengthen relationships with your business partners. While most firms actively seek strong business ties, it is worth remembering that doing so can help cushion the damage from many major crises.

Clients, contractors, and consultants who have long-term, mutually beneficial relationships are more likely to work together to resolve disputes, less likely to throw each other under the bus, and more eager to work together in the future.

When Trouble Strikes You will need to act immediately and simultaneously on several fronts. Many firms are astounded by the speed at which chaos envelops them. Even the most levelheaded person has trouble thinking straight in the midst of a full-blown emergency. Having your crisis management plan and team in place can help you respond quickly and effectively.

• Think safety first. Your first consideration should be the safety of your employees and the public. If your staff is on the scene and the situation calls for it, contact emergency services, clear the area, and make sure your

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• Develop a crisis management plan so your firm can launch an effective response.

When developing a plan, try to anticipate high-risk scenarios and think through the steps your firm should take in the event of these scenarios. List resources (e.g., key personnel, insurers, lawyers, experts, government agencies) to be contacted and mobilized in the first hour, the first day, and the first week. Do not forget to include contact information as well as access information for computer systems, social media, teleconferencing, and other key accounts you may need to use. (You will see additional suggestions below.

Also see the sidebar, Crisis Management Resources.)

• Select and train a crisis management team. In the event of a crisis, the team will issue notifications, control and coordinate the dissemination and collection of information, and conduct an investigation. At least two team members should be senior firm members, one of whom will serve as team leader.

• Train your employees. Everyone in your firm should know what to do – and what not to do – when there is a crisis. (This includes not sharing on social media unless specifically instructed to.) Make the training part of new employee orientation.

• Run emergency drills to test the crisis management plan, the staff, and your team.

• Identify a public relations firm experienced in crisis management. The owner of a large design firm told us this was one of the most important calls he made when faced with a crisis. However, do not wait until there is a problem. It is far better to identify and interview crisis management firms before you need them, rather than after.

• Develop and maintain a list of forensic specialists you can call on. You may want an expert in the appropriate discipline to give you a private assessment of your firm’s risk exposures. Be sure to check references.

• Consider implementing document management software. These are essentially electronic filing cabinets that organize digital and paper documents.

Well-organized systems allow you and your legal team to locate and review documents from all project participants quickly. This can save many thousands of

A P R I L 2 019

35

employees are all accounted for. Do what is needed to keep everyone safe.

• Convene your crisis management team.

You may need to include additional members with specific expertise. Appoint a team leader, preferably a team member who does not have project involvement.

• Contact your insurance broker and insurers. Your broker and insurance carrier can offer guidance in the event of a natural disaster or project related crisis.

Include them early and follow their advice.

• Call your lawyer. While your insurance carrier will likely appoint a lawyer for you, you may want to contact your own lawyer as well. Insist on a close working relationship between lawyers retained by your carrier and separately by your firm.

• Direct employees to cease all outside communication about the incident, including with family and friends, and especially on social media. Tell them to refer all requests for information to your media contact (see the next bullet). You may want to make some exceptions if social media can help in a crisis.

• Designate a trusted member of the firm as sole media contact and refer all requests for information to him or her.

• Contact the public relations/crisis management firm as soon as possible. These professionals can move quickly to help craft public statements and think through potential landmines. A PR firm’s assistance may be crucial not only during the crisis but throughout any ensuing legal battles and recovery period.

• Develop a few simple talking points.

Working with your lawyers, the public relations firm, and your crisis management team, decide on the best, most reasoned approach.

It is essential to have an appropriate statement for the media that does not create liability for your firm or alienate the media.

• Reach out to your project client, contractor, key project team members, subconsultants, and other clients if appropriate.

Brief them on the situation (as much as you can), remind them that you value your relationship, and reinforce the importance of addressing the issue and working together to move ahead with the project.

• Do not accept (or assign) blame. Firms often accept responsibility for a problem too quickly and before all the facts are known. Remember, even if you have made a mistake, you may not be liable for all of the alleged damages; often there are concurrent causes.

• Move quickly to preserve critical evidence. Carefully preserve and document 36 STRUCTURE magazine

key evidence while the facts Crisis Management Resources are fresh in everyone’s mind.

Assemble project files from all www.axaxl.com/dp sources, including individually www.axaxl.com/dp-ca maintained files, archived files, and site files. If appropriate, Crisis Management Checklist take photos and videos of the site as soon as possible. Advise Policy Highlights – U.S. and Canada subconsultants to begin collectCrisis Management: Mastering the Skills to Prevent ing their files. Unless otherwise Disasters, Harvard Business School Press instructed by your lawyer, be mindful of creating documents Crisis Management: Planning for the Inevitable that can be used against your

by Steven Fink Reporting Claims the Right Way, firm in a lawsuit.

February 2017 Communiqué • Begin your investigation.

Review project documents, including agreements and scope of services. your media contact, who should continue Retain forensic consultants as approprito work with the public relations firm and ate. At your lawyer’s direction, conduct your lawyer. and document interviews of all employ• Continue to provide high-quality service ees who were present at the crisis site or to your other clients. During the demands who have firsthand information regarding of a big lawsuit, it is easy to let other work the situation. (If you might have liability, slide. But your firm’s reputation – and perit is essential to establish a lawyer-client haps its health – will depend on the quality relationship early to allow your firm to of service you provide to everyone. conduct a full investigation without fear that the information will later be used When It Is Time against you.) to Move Forward • Meet with your employees as soon as possible. Make sure they know what • Conduct a crisis management team happened, what your firm plans to do debriefing to document the process for about it, and what you need from them. future reference and examine lessons There may be incorrect information in learned. Ask, “What went right? What the media, and they will need to know could we have done better? How can we how the crisis will affect the firm. Be as improve?” Again, consult with your lawyer forthcoming and as positive as you can, first to determine how best to document and continue to keep them informed. (or not document) the issues so you can • Take care of your people. Do whatever it have a candid discussion about improvtakes to see to their needs, which may include ing your firm’s practices without creating providing grief counseling and time off. liability down the road.

• Move to mitigate losses. Working with • Update your crisis management plan and input from the project team, including revisit it at least annually. contractors and the owner, consider the • Find a way to mark the end of the crisis. impact on the project schedule, and initiate If a celebration is not appropriate, hold an a damage assessment. Identify long-leadinformal staff meeting to explain where time material/component needs. Work things stand, discuss the plan to move quickly to make a plan to minimize losses forward, and thank everyone who helped. and restart the project as soon as possible.

Finally, remember that you can survive a crisis. Many firms have weathered major disasters and come back to thrive. The key is to During a Lawsuit have a plan, the right resources at the • Meet often with your entire firm. After ready, and a dogged determination consulting with your lawyer, keep employ- to endure.■ ees apprised of what is happening and instruct them how to deal with inquiries.

Randy Lewis, CPCU, brings over 20 years They may have concerns about the future working in the insurance industry. Currently of the firm and their livelihoods; stay posiRandy manages the risk management and client tive and confident. education programs of AXA XL, a recognized • Stick to the script. Do not be tempted to leader in the AEC community. respond to finger-pointing by other parties, (randy.lewis@axaxl.com) including the media. Refer all queries to

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CASE business practices Including a Claim Validation Clause in your Agreement

By Bruce Burt, P.E., SECB

I

“

you, the design professional, did not meet a “standard of care.” State statutes also vary on what is meant by standard of care, but it is generally understood as the care and skill that is ordinarily exercised by other members of the engineering profession in performing professional engineering services under similar circumstances.

You need not reside or do business in one of the fourteen states with a merit threshold to obtain some assurance that a negligence claim brought against you is valid. You can include a “Claim Validation” clause in your professional services agreement. Even if you

“

f you work as a structural engineer long enough, you will probably have the misfortune of defending yourself against a baseless legal claim. We have all heard stories about firms forced to spend thousands of dollars in attorney fees to defend themselves against a nuisance lawsuit, or to make it go away.

There are many ways to protect your firm from legal claims, including claims of questionable merit. Some methods include: following a comprehensive quality assurance program, working under suitably written contracts, managing client expectations, and choosing the right clients in the first place.

These measures can help reduce the risk of a lawsuit. However, what if you have developed a quality deliverable and still find yourself the subject of a costly-to-defend lawsuit?

Even after you’ve performed your services in a manner consistent with the care and skill exercised by other design professionals, your client may sue you.

So why would a client choose to litigate?

One obvious reason is that your client may have a legitimate claim due to a design error or omission. Professional liability insurance is available specifically for that possibility.

However, in most states, your client can sue you whether or not the case has legal merit. There are twelve states (Arizona, California, Colorado, Georgia, Maryland, Minnesota, Nevada, New Jersey, Oregon, Pennsylvania, South Carolina, and Texas)

There are many ways to protect your firm from legal claims, including claims of questionable merit.

that have enacted legislation requiring some form of certification of the existence of a fundamental basis for a negligence claim.

Two other states, Hawaii and Kansas, require a claim be submitted to a screening panel to assess its validity.

While the specifics of state statutes differ in terms of scope and application, all state “certificate of merit” statutes require the opinion of a third-party design professional that a claim is factually supportable. Usually, this opinion is obtained by the plaintiff through his or her attorney and must indicate that

38 STRUCTURE magazine

reside in states with some form of certificate of merit requirement, including a claim validation clause may be advantageous, since you would not have to rely on a favorable application of a state statute for protection against unmeritorious claims.

There are some drawbacks to including a claim validation clause in your contract, including a potential for alienating clients.

Selectively including the clause in agreements with new clients is a way to prevent antagonizing longstanding ones. Moreover, it is important to choose the wording of your

clause carefully. An unduly strict clause could be struck down in a court ruling if it is interpreted as discouraging legitimate claims. A carefully worded clause emphasizing “Claim Validation” might be looked on more favorably by your client and the courts than a clause that might be construed as a deterrence to pursuing legitimate claims.

Even if you choose not to include a claims validation clause, you should at least ensure that the contract you sign includes wording properly defining your standard of care.

Avoid contracts that include words like “best” or “highest,” or otherwise attempt to elevate the standard of care. Not only does an elevated standard of care increase your liability exposure, virtually all professional liability insurance companies limit claims coverage to breaches of the commonly defined standard of care.

As with any term written into your client agreement, a claim validation clause should be carefully reviewed by your legal counsel to ensure it is enforceable and does not conflict with state statute.■ Bruce Burt is Vice President of Engineering with Ruby+Associates, Inc., a constructability-focused structural engineering firm located in Bingham Farms, Michigan. He is a member of the CASE Contracts Committee. (bburt@rubyandassociates.com)

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Listings are provided as a courtesy, STRUCTURE magazine is not responsible for errors.

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in the NEWS Changes on STRUCTURE’s Editorial Board Retiring from the STRUCTURE Editorial Board is Timothy M. Gilbert, P.E., S.E., SECB. Timothy served as an NCSEA representative on the Board. He continues his involvement on the NCSEA Structural Licensure Committee and the NCSEA Internal Communication Subcommittee.

Tim has been involved in many positions at the Structural Engineers Association of Ohio (SEAoO), along with the American Society for Civil Engineers and the Association of Iron and Steel Technology. In his role as a Project Specialist for TimkenSteel, he supports the civil and structural engineering work required by this manufacturer of specialty alloy steels used in high-stress fatigue inducing circumstances or under extreme environmental conditions.

Replacing Timothy as an NCSEA representative is Jeannette M. Torrents, P.E., S.E., LEED AP. Jeannette is a Senior Project Manager at JVA, Inc. in Boulder, Colorado. She has experience with a wide variety of materials including steel; cast-in-place, tilt-up, and post-tensioned concrete; wood; and masonry. When not designing buildings, Jeannette serves on the Wind Load, Snow Load, and Scholarship Committees of the Structural Engineers Association of Colorado (SEAC).

Please join STRUCTURE magazine in congratulating Timothy Gilbert on his service and wishing him well in his future endeavors and welcoming Jeannette Torrents to the Editorial Board team.■

To see a list of current Editorial Board members visit STRUCTUREmag.org.

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SPOTLIGHT Warehouse-to-Retail

2400 Market Street Renovation and Vertical Expansion

By Lea Cosenza

A

daptive re-use of buildings is commonplace. For this project in Philadelphia, PA, The Harman Group and the Varenhorst/ Gensler architectural design team were challenged with adapting a turn of the century, occupied, five-story concrete automobile warehouse building with diagonal column grids and 40-inch-diameter columns into functional office/retail space. Additionally, while occupied, the team had to complete a five-story steel framed overbuild; cut out the existing concrete core; rebuild two lateral concrete cores supported on hundreds of micro-piles; remove a column supporting 1,700 kips of load; and, insert a 6,000-squarefoot atrium and a new full building length promenade for an incredible experience on the urban waterfront.

Built around 1920, the existing automobile warehouse, encompassing a whole city block, is a two-way concrete flat plate with concrete columns (mostly circular) that have capitals and drop panels. The grid is on a diagonal pattern called the Morrow System, and the bay spacing is roughly thirty feet square. The existing columns are supported on spread footings 14 feet below the slab on grade.

After PMC purchased the building in 2014, multiple iterations of design were assessed until Aramark signed a lease in 2016 to take over the top five vertically expanded floors for their new corporate headquarters. Fastrack, PMC’s owned contractor, along with architecture firms Varenhorst and Gensler, and The Harman Group, structural engineer, made up the team.

Overbuild The vertical expansion is structural steel with a concrete-slab-on-metal-deck floor system, and steel columns and beams. The new columns are set above the existing columns, creating a diagonal pattern with large bay spacing and large cantilevers.

Two concrete cores sitting on hundreds of micropiles act as the lateral load resisting system for the existing building and the overbuild. Oversized openings to permit room for formwork and construction were cut into the existing floors, requiring careful analysis of the existing concrete structure. Shear walls were formed and placed, formwork removed, then a continuous concrete corbel was installed off the walls to re-support the existing floor slabs and tie the existing building into the

new shear walls for lateral support.

In some locations, existing beams, both steel and concrete, were also re-supported on the new shear walls. The existing beams were overcut to allow for formwork and construction, and extended with intricate connection details to attach to the new shear walls.

Interior Fitout Work

The Harman Group was an Award Winner for its Aramark Headquarters project in the 2018 Annual Excellence in Structural Engineering Awards Program in the Category – Forensic/ Renovation/Retrofit/Rehabilitation Structures over $20M.

Throughout the design, additional tenants signed up for the floors in the existing building, including The Fitler Club, a high-end club that was set to occupy three floors. The Club’s space would feature an event ballroom, a bowling alley, a fitness center, a restaurant, and hotel rooms. Further structural challenges were faced to provide a space fit for this tenant.

Significant upgrades were made to the ground floor, first floor, and second floor for The Fitler Club. Also, a new 16,000-square-foot, two-way, flat plate concrete mezzanine was required for the fitness center. This mezzanine is supported by a combination of the existing columns, new steel columns and hangers, and new concrete square columns sitting on new micropiles. At the existing columns, the floor is supported with a continuous shelf plate attached to the circular columns with collar plates affixed with adhesive anchors. A small 1,200-square-foot mezzanine was also required above the bowling alley to create a trophy room.

The event space on the ground floor required a large open area for dancing. One of the existing concrete columns, supporting 10 floors, was removed to provide the open area. Two new, six-foot-deep steel trusses spanning 50 feet were installed to support a steel transfer girder that carries the column supporting the floors above with a service load of 1680 kips.

These trusses, supported on each end by new steel columns on micropile foundations, also provided support for the temporary shoring columns installed at the floors above. After the jacking process, the ground floor column was removed and the transfer girder, spanning from truss to truss, was installed beneath the column above, completing the clear space required below. This construction was done while a tenant occupied the floor above the truss and the vertical expansion was being fit out by Aramark.

Additional Challenges Serving as a main entrance and connection of Market Street to Chestnut Street, the promenade is constructed of steel that cantilevers out from the existing building. New foundations were not possible because of a major CSX rail line adjacent to the building. Large steel plates are attached to the existing concrete columns with adhesive anchors, and cantilevered steel beams are supported with braces to the lower part of the columns below.

Connections to the existing round columns proved especially challenging. One of the connections of the steel beams to the existing round column utilized a large collar plate fastened with adhesive anchors. The connection was not possible at the top of the column because of the capital. Therefore, the collar plate was wrapped around the concrete column on the floor above, and two steel rods pass through the existing floor to hang the new steel beam below. Underneath the 5th floor/ existing roof, this detail could not be used because there was no concrete column above.

Thus, the concrete capital was cut out and a new steel beam was installed over the concrete column. Half of the steel structure was in place above when the capital was cut. Shoring was designed to support the steel columns above to erect the new steel transfer girder.

The design team for 2400 Market Street incorporated complex solutions to revitalize this landmark building, now forming a link between the office corridor of West Market Street and Philadelphia’s University City neighborhood.■ Lea Cosenza is an Associate and Project Manager at The Harman Group in King of Prussia, PA. (lcosenza@harmangroup.com) A P R I L 2 019

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NCSEA

NCSEA News

National Council of Structural Engineers Associations SEAOI Builds Interest at STEM Expo in Chicago Suburbs

The DuPage Area STEM Expo, in conjunction with national Engineers Week, is a specialized event designed to promote the awareness of professional and educational opportunities provided among engineering and STEM fields.

The 35th Annual DuPage Area STEM Expo (formerly known as DuPage Area Engineers Week) was held on February 23, 2019, sponsored by the School of Applied Technology at the Illinois Institute of Technology’s Rice Campus in Wheaton, Illinois. The rainy weather did not dissuade attendees of all ages from partaking in a variety of displays and presentations offered by engineers and scientists from across the greater Chicago area.

Structural engineers were well represented by SEAOI and its member volunteers, who offered explanations on everything from ‘shapes’ to ‘materials’ in the designing of better buildings and bridges. SEAOI member and past-president of the National Council of Structural Engineers Associations (NCSEA) Brian Dekker, engaged his audiences with several presentations of the “Last Brick Standing.” Using LEGOs® to build towers, they were then tested on a custom designed “shake table” – two at a time – to see which design could withstand the simulated earthquake. Dekker then used opportunities during the actual “test” as teaching moments – offering an engineering perspective on why certain buildings “survived” while others did not.

What did you or your SEA do for Engineers Week? Consider sharing your student outreach efforts with NCSEA by emailing ncsea@ncsea.com. For more structural engineering student outreach ideas, visit NCSEA’s STEM Resource page on www.ncsea.com.

Brian Dekker, NCSEA Past President

Helping to Expanding Design Build Student Competition The National Council of Structural Engineers Associations (NCSEA) in partnership with the American Wood Council is bringing the Timber-Strong Design Build™ to Anaheim, California, during the 2019 Summit. In conjunction with the ever-growing annual Structural Engineering Summit, this “hands-on” opportunity for university engineering students is intended to give real-world experience in both planning and building a wooden structure. Student teams will prepare a project complete with a preliminary design, material cost estimates, structural calculations, and estimated carbon footprint.

The activity will provide an opportunity for engineering students to experience the full spectrum of designing and building a real project within a team environment.

The building of the wood structure portion of the competition will take place at the NCSEA Structural Engineering Summit in Anaheim, California, on November 12, 2019, at the Disneyland® Hotel.

NCSEA Member and Senior Director of Education for the American Wood Council, Michelle Kam-Biron noted, “AWC is excited to be partnering with NCSEA. Getting the word out about the competition, as well as increasing the level of student engagement, will be easier via the network of forty-four (44) Member Organizations across the country.”

Along with the support of the American Wood Council, key sponsors include APA – The Engineered Wood Association and Simpson Strong-Tie®. For more information visit www.ncsea.com/timber or contact NCSEA at ncsea@ncsea.com.

NCSEA Excellence in Structural Engineering Awards Each year, at the Structural Engineering Summit, NCSEA awards the Excellence in Structural Engineering Awards. This program annually highlights some of the best examples of structural engineering ingenuity throughout the world. Projects are judged on innovative design, engineering achievement, and creativity. Multiple winners are presented in seven categories with an outstanding winner being chosen and announced at NCSEA's Structural Engineering Summit in Anaheim, California, this November.

Entries are due by 11:59 pm on July 16, 2019. Structural engineers and structural engineering firms are encouraged to enter. More information about the awards, along with submission instructions, can be found in the Awards section of www.ncsea.com.

44 STRUCTURE magazine

News from the National Council of Structural Engineers Associations

AISC Steel Bridge Competition Comes to NCSEA Members The American Institute of Steel Construction’s (AISC) extremely successful Student Steel Bridge Competition (SSBC) is tapping NCSEA and its Member Organizations (SEAs) beginning in 2019. Through its active SEAs, covering 44 states and the District of Columbia, NCSEA has a network of engineer members that have local access to various universities with an engineering curriculum. AISC will be accessing this network to assist and support the current year’s events while helping to grow the competition.

“The SSBC needs access to the profession to assist with local and regional technical guidance,” says Al Spada, NCSEA’s Executive Director. “The Member Organizations will be there to offer focused assistance on an as needed basis when universities require personal involvement of a structural engineer.” Assistance will also include connecting AISC with potential judges and volunteers as requested by AISC.

SAVE THE DATE

AISC's Director of Education, Christina Harber, S.E., P.E., says "AISC is pleased to have NCSEA as a supporting affiliate of the Student Steel Bridge Competition program. We look forward to exposing students to the professionalism, technical expertise, and experience of SEA members that volunteer to mentor individual schools or judge at competitions around the country."

Started in 1987, the popularity of the SSBC has grown, with approximately two hundred schools competing each year across the country.

NCSEA is proud to be a partner offering volunteer structural engineering support while helping the AISC event to thrive and grow.

For more information, visit https://goo.gl/xFiLX7 or contact NCSEA at ncsea@ncsea.com.

2019

STRUCTURAL ENGINEERING SUMMIT

November 12–15, 2019 · Disneyland® Hotel · Anaheim, CA

NCSEA Webinars

Register by visiting www.ncsea.com.

April 18, 2019

Masonry Infill Design Charles J. Tucker, P.E., Ph.D.

The 2016 TMS 402, Building Code Requirements for Masonry Structures, contains Appendix B: Design of Masonry Infills to address their design and construction. This course will cover the behavior of masonry infills and the TMS 402 design provisions.

May 2, 2019

Session I: Bridge Design – Introduction to AASHTO Tony Shkurti, Ph.D., S.E., P.E.

This session is geared towards structural engineers that are not familiar with AASHTO and are interested in learning more for several reasons, including preparing for the NCEES Structural Engineering (SE) exam. Focus will be in explaining the basic loads that are different from building codes, including the application of Dead Load based on their sequence, Live Load, Wind, and Thermal.

May 9, 2019

Session II: Bridge Design – Approximate Methods in the Design of Bridges Tony Shkurti, Ph.D., S.E., P.E.

This second session is intended to go into more depth in the approach to design of bridges. It will explain approximate methods for calculating demand on bridges due to Live Loads, using both the Lever Rule method as well as the approximate tables for calculating live load distribution for the design of the superstructure and substructure.

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

A P R I L 2 019

45

SEI Update Students and Young Professionals

Student & Young Professional Scholarship Recipients

Thanks to the SEI Futures Fund – www.asce.org/SEIFuturesFund – and generous donors in collaboration with the ASCE Foundation, we welcome the following SEI Student & Young Professional Scholarship recipients to participate and get involved at Structures Congress in Orlando:

Students Bryam Astudillo, S.M.ASCE, University of Cuenca, Ecuador Cody Beairsto, S.M.ASCE, Oregon State University Jared Blount, S.M.ASCE, University of South Alabama James Bumstead, S.M.ASCE, University of Pittsburgh Jacob Choate, S.M.ASCE, University of Oklahoma Kanchan Devkota, S.M.ASCE, University of Nebraska-Lincoln Sophie Engel, S.M.ASCE, University of Pittsburgh Lissette Fernandez, S.M.ASCE, Clarkson University Savannah Howie, S.M.ASCE, University of South Alabama William Hughes, S.M.ASCE, University of Connecticut Maria Kozdroy, S.M.ASCE, Rensselaer Polytechnic Institute Maddie McQuillan, S.M.ASCE, Villanova University Pranavesh Panakkal, S.M.ASCE, Rice University Sebastian Pozo, S.M.ASCE, Universidad de Cuenca, Ecuador Amir Safiey, S.M.ASCE, Clemson University Nidhi Shah, S.M.ASCE, Stevens Institute of Technology Annabel Shephard, S.M.ASCE, Oregon State University Ali Shokrgozar, S.M.ASCE, Idaho State University Manuel Velasco, S.M.ASCE, University of Illinois at Chicago Adam Werntz, S.M.ASCE, University of Illinois at Urbana-Champaign

Young Professionals Lisa Marie Anderson, P.E., F.SEI, F.ASCE, Reston, VA Andrea Atkins, A.M.ASCE, Toronto, ON, Canada Luis Felipe Duque, EIT, A.M.ASCE, Boulder, CO Sayantani Dutta, EIT, Aff.M.ASCE, Arlington Heights, IL Paul Michael Evans, P.E., S.E., M.ASCE, New York, NY Eric Herbert, P.E., M.ASCE, Englewood, CO Jeena Rachel Jayamon, Ph.D., A.M.ASCE, Brookline MA Swarna Karuppiah, EIT, A.M.ASCE, Dallas, TX Robert Keene, A.M.ASCE, San Francisco, CA Se Yun Kim, EIT, A.M.ASCE, Queens, NY Avial Lumagui, A.M.ASCE, Glen Allen, VA Rebecca McGowan, EIT, A.M.ASCE, Grand Island, NY William Michalski, EIT, A.M.ASCE, Shrewsbury, MA Renee Oats, Ph.D., A.M.ASCE, Baltimore, MD Mathew Picardal, P.E., M.ASCE, Santa Ana, CA Danielle Schroeder, EIT, A.M.ASCE, Jenkintown, PA Max Stephens, A.M.ASCE, Pittsburgh, PA Maryanne Clare Wachter, A.M.ASCE, Tarrytown, NY

Young Professional Teaching Faculty Warda Ashraf, Ph.D., A.M.ASCE, University of Maine Negar Elhami Khorasani, Ph.D., A.M.ASCE, University of Buffalo Burcu Guldur, A.M.ASCE, Hacettepe University, Ankara, Turkey Ji Yun Lee, A.M.ASCE, Washington State University Jian Li, Ph.D., M.ASCE, University of Kansas Mustafa Mashal, Ph.D., P.E., C.Eng, CPEng, M.ASCE, Idaho State University Kyle Tousignant, A.M.ASCE, Dalhousie University, Halifax, NS, Canada 46 STRUCTURE magazine

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News of the Structural Engineering Institute of ASCE

PMENT H

Experience the Premier Event in Structural Engineering JOIN US IN ORLANDO

WHAT’S THE NEXT BEST THING TO ATTENDING IN PERSON?

• NEW: Performance-Based Design Trends, New Smart Technologies, Career Development, and Leadership Skills • View program detail and use interactive planner online

• A fantastic evening Celebrating the Future of Structural Engineers hosted by CSI

Joint International Conference:

Collaboration Efficiency Safety •

•

Dubai, UAE | 29-30 September 2019

Choose from: • Behavior and Design of Diaphragms, Chords and Collectors for Steel Buildings

• Special Sessions: Grenfell Tower | Workshop on Conceptual Design | Improve your Communication/ Presentation Skills

• Register at www.structurescongress.org

Livestream up to three sessions on Friday, April 26, US Eastern Time:

• Structural Optimization in Everyday Design Tasks • Diverse and Illustrative Bridge Case Studies

LIVESTREAM REGISTRATION DEADLINE APRIL 16.

Learn more and register at Livestream Made Possible By structurescongress.org/livestream

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. Keynote speakers will share their experience of high volume occupancy, unusual structures where typical codes do not apply, and performance based design (PBD) of tall buildings.

Register now to receive early booking discount: https://structuresdubai2019.cvent.com

Visit SEI at NASCC Steel Conference April 3-5 in St. Louis

Visit us at Booth 211 for new member benefits and opportunities, and enter to win a six-month individual subscription to ASCE 7 Online.

Errata

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

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

A P R I L 2 019

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!

If your firm needs to update its current Risk Management Program or establish a program within the firm, the following documents will guide employees: 962-H: Tool 1-1: Tool 2-1: Tool 2-4: Tool 3-1:

National Practice Guideline on Project and Business Risk Management Create a Culture for Managing Risks and Preventing Claims A Risk Evaluation Checklist Project Risk Management Plan A Risk Management Program Planning Structure

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

Save on CASE Membership!

Can you ever really be too successful? Keep your business thriving – no matter what your competition or the economy is doing – and say YES to membership in ACEC’s Council of American Structural Engineers (CASE). An “Association within an Association” that complements your ACEC National benefits.

CASE, the oldest of ACEC’s four discipline-specific Coalitions, is a professional community for, of, and by structural engineers who want relevant, useful information – on BIM, international building codes, risk management, and more – to run their businesses better.

Join CASE today, and you’ll qualify for: • Education: CASE offers a track of 3 dedicated education sessions at both the ACEC Fall Conference and Annual Spring Convention to keep members current with best practices and trends in structural engineering. As a member, you will also receive a discounted rate to ACEC webinars focused on structural engineering issues. CASE also provides education sessions at the AISC Steel Conference and the ASCE-SEI Structures Congress.

• Resources: Coalition members get free access to over 145 contracts, tools, and publications (a total value of over $5,000! ). CASE developed over 65 documents geared toward structural engineering firms. You will also receive our weekly A/E/C Digest, an online compilation of current articles and information about our industry.

• Advocacy: Your voice matters! Coalition members are often the first ones contacted to share their expertise with Congress and government agencies in response to current legislation and relevant regulatory agendas.

As an active member of CASE, I have found great networking opportunities within the structural engineering industry and consistently reference Coalition documents to aid in my business’ development.

Save $75 off your first year’s dues through June 30, 2020! Join CASE by April 30, 2019, and get 14 months for the price of 12!

Questions? Contact CASE’s Executive Director, Heather Talbert, at 202-682-4377 or email her at htalbert@acec.org.

Together, we can take your business to the next level!

Corey Matsuoka CASE Chairman SSFM International

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 in educational support.

Your monetary support is vital in helping CASE and ACEC increase scholarships to those students who are the future of our industry. All donations toward the program may be eligible for tax deduction, and you don’t have to be an ACEC member to donate! Contact Heather Talbert at htalbert@acec.org to donate.

48 STRUCTURE magazine

News of the Council of American Structural Engineers CASE Member Firms Win Engineering Excellence Grand, Honor Awards

Congratulations go out to CASE Member firm Magnusson Klemenic Associates, Inc. for winning a Grand Award. Magnusson Klemenic Associates, Inc’s highlighted project, Salesforce Tower in San Francisco, CA, is a finalist for the Grand Conceptor Award awarded at the 52nd Engineering Excellence Awards Gala held during the upcoming ACEC Annual Convention., May 5-8, 2019, Washington, DC. CASE Member Firm Walter P Moore won an Honor Award for their joint venture project, TPA Automated People Mover and ConRAC, Tampa International Airport, Tampa, FL.

CASE Contracts – Usage Guide Structural Engineer is Retained • CASE Contract #1 – An Agreement for the Provision of Limited Professional Services. This agreement is intended for use for small projects or investigations of limited scope and time duration.

• CASE Contract #2 – An Agreement Between Client and Structural Engineer of Record for Professional Services. This agreement is intended for use when the client, e.g., owner, contractor developer, etc., wishes to retain the Structural Engineer of Record directly. This agreement may also be used with a client who is an architect when the architect-owner agreement is not an AIA agreement.

• CASE Contract #4 – An Agreement between Client and Structural Engineer for Special Inspection Services. This agreement is intended for use when the Structural Engineer is hired directly by the Owner to provide Special Inspection services.

• CASE Contract #5 – An Agreement for Structural Peer Review Services. This agreement is intended for use when performing a peer review for an Owner or another entity and includes responsibilities and limitations.

• CASE Contract #6 – Agreement for Use with and Commentary on AIA Document C401 “Standard Form of Agreement Between Architect and Consultant,” 2007 Edition. This document is intended for use as a letter-form of the agreement which adopts the AIA C401 by reference. This Agreement is intended for use when the owner-architect agreement is an AIA B-series. A scope of services is included.

The purpose of the commentary is to point out provisions that merit special attention.

• CASE Contract #8 – An Agreement Between Client and Specialty Structural Engineer

for Professional Services. This agreement is intended for use when the structural engineer is hired directly by a contractor or sub-contractor for work to be included in a project where you are not the Structural Engineer of Record.

• CASE Contract #12 – An Agreement Between Client and Structural Engineer for Forensic Engineering (Expert) Services. This agreement is intended for use when the engineer is engaged as a forensic expert, primarily when the Structural Engineer is engaged as an expert in the resolution of construction disputes. It can be adapted to other circumstances where the Structural Engineer is a qualified expert.

• CASE Contract #13 – An Agreement between Owner and Structural Engineer as Prime Design Professional. This agreement is intended for use when the Structural Engineer serves as the Prime Design Professional.

• CASE Contract #16 – An Agreement Between Client and Structural Engineer for a Structural Condition Assessment. This agreement is intended for use when providing a structural condition assessment.

Structural Engineer is Retaining Additional Entity • CASE Contract #3 – An Agreement Between Structural Engineer of Record and Design Professional for Services. This agreement is intended for use when the Structural Engineer of Record, when serving in the role of Prime Design Professional or as a Consultant, retains the services of a subconsultant or architect.

• CASE Contract #9 – An Agreement Between Structural Engineer of Record and Testing Laboratory. This document is intended for use when the structural engineer retains testing services.

• CASE Contract #10 – An Agreement Between Structural Engineer of Record and Geotechnical Engineer of Record. This agreement is intended for use when the Structural Engineer of Record retains geotechnical engineering services. It can also be altered for use as an agreement between an Owner and the Geotechnical Engineer of Record.

Other Situations • CASE Contract #7 – Commentary on AIA Document A295 – 2008 “General Conditions of the Contract for Integrated Project Delivery,” 2008 Edition. This document provides commentary on AIA Document A295 Integrated Project Delivery.

• CASE Contract #11 – An Agreement Between Structural Engineer of Record (SER) And Contractor for Transfer of Digital Data (Computer Aided Drafting (CAD) or Building Information Model (BIM)) Files. This agreement is intended for use when transferring CAD or BIM files to others.

• CASE Contract #14A – Supplemental Form A, Additional Services Form. This document is a form for providing additional services to an existing agreement.

• CASE Contract #14B – Standard Form for Request for Information. This document is a standard Request for Information (RFI) form that can be included in the bid documents for use by contractors and subcontractors.

• CASE Contract #15 – Commentary on AIA Document A201 “General Conditions of the Contract for Construction,” 2007 Edition.

This document provides Commentary on AIA document A201 sections which merit special attention.

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

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

A P R I L 2 019

49

structural FORUM Improving the Practice of Residential Wood Truss Roof Systems

By Brent Maxfield, S.E.

T

his article is a follow-up from the author’s article, Code Requirements for Residential Roof Trusses, in the March 2019 issue of STRUCTURE. (The terms in this article beginning with capital letters are defined in Section 2.2 of ANSI/TPI 1-2014, National Design Standard for Metal Plate Connected Wood Truss Construction, published by the Truss Plate Institute (TPI) – www.tpinst.org).

The ANSI/TPI 1 standard (incorporated into the International Building Code (IBC) and International Residential Code (IRC) by reference) defines the responsibilities of the various parties associated with the design, fabrication, and erection of metal plate connected wood Trusses. If all responsibilities were adhered to, the system would work. It is my opinion that very seldom are all the responsibilities fulfilled as intended.

The Building Designer, according to ANSI/ TPI 1, is defined as “Owner of the Building or the Person that contracts with the Owner for the design of the Building Structural System and/or who is responsible for the preparation of the Construction Documents.” The ANSI/ TPI 1 Standard places technical responsibilities on the Building Designer regarding the information required for the design of the Trusses and the review of the Truss Submittal Package, yet the IRC allows this individual to be someone other than an engineer. I feel that, because of these technical responsibilities, the code should require that an engineer perform them. I am not arguing that the entire residential structure is required to be designed by an engineer, only the roof truss system and its associated load paths.

The Truss Designer (defined by ANSI/TPI 1 as the “Person responsible for the preparation of the Truss Design Drawings”) has very specific and limited responsibilities under the ANSI/TPI 1 Standard. This individual is responsible for ensuring that each truss is designed for the loads provided by the Building Designer on the Construction Documents. It is the Building Designer who must provide the loading diagrams for every truss on the project (including the snow drift loading diagrams). The Building Designer also has the responsibility of ensuring that all components of the roof system function as intended.

50 STRUCTURE magazine

Based on my experience observing residential wood truss projects, it is my opinion that all wood Truss Manufacturers should have engineers on staff who oversee all projects. These engineers should be responsible for calculating the loads (including snow drift) on every Truss using the design loads provided on the Construction Documents. They should be the ones stamping the Truss Design Drawings and preparing an engineered Truss bracing plan that coordinates the location of each truss and shows the required Truss bracing for all Trusses of the roof system. I feel that the current system of stamping only individual Truss Design Drawings should not be allowed because the engineer stamping the drawings is not required to understand or be responsible for the entire roof system. The stamp gives a false sense of security.

Let me share with you a few of my personal experiences.

Experience and Opinions On a project for a close family member for which I was the Building Designer, the Truss Submittal Package had each Truss Design Drawing stamped by the Truss Designer.

Rather than a “general conformance” review (see ANSI/TPI 1 2.3.2.3), my review was very thorough. During my review, I discovered that the girder Truss did not have enough load in the web members, even though the loading on the Truss seemed correct. I called the Truss Manufacturer to explain my concern. I was told that it was done correctly and it was stamped by the Truss Designer.

Following an impassioned discussion, the

Truss Manufacturer reluctantly agreed to let me talk to the Truss Designer directly.

Once I explained my concerns to the Truss Designer (an engineer), he quickly understood the problem and said that the technician had used a top chord bearing end jack truss instead of a bottom chord bearing end jack.

The issue was quickly resolved with the Truss Manufacturer. Had I not performed a thorough review, the girder Trusses would have been severely under-designed. The engineer stamp on the Truss Design Drawings gave a false sense of security because the location of the load provided to the Truss Designer was not correct.

On the same project, I discovered that one of the girder Trusses was not constructed per the Truss Design Drawing. The roof was erected with a critical error. After alerting the Truss Manufacturer of the error, they said they would get an engineered fix for the issue.

I got a stamped “fix” from the same Truss Designer. The fix was inadequate. During a conversation with the Truss Designer, I explained the problem with his stamped fix, and he said, “That is not what I was told by the Truss Manufacturer.” Here again, the Truss Designer relied on information provided by someone who was not an engineer and provided a stamped drawing that did not work.

Once the Truss Designer understood the issue, he quickly came up with a proper fix.

Individual Truss Design Drawings show webs that must be braced, but the Truss Designer does not coordinate the continuous lateral bracing with adjacent Trusses. Many times, webs do not align. ANSI/TPI 1 and BCSIB3 place the responsibility of stabilizing the

Continuous Lateral Bracing on the Building Designer. In my experiences, Truss bracing is rarely installed correctly. Very few residential projects have structural observation of the Truss bracing installation.

The technicians working for the Truss Manufacturer, who input the Truss framing and loads into the computer software, are almost always not engineers nor is there an engineer in the facility. These technicians are the ones who usually calculate the loads, including the snow drift, for each Truss. At times, they make changes to the Truss framing shown on the Construction Documents to make a more economical system, but they may not be qualified to evaluate the impacts the changes could have on the structural system. Most Building Designers believe that they are delegating the responsibility for an entire roof system, but ANSI/TPI 1 makes it clear that the Truss Designer is only responsible for the design of individual Trusses using loads provided by the Building Designer. The burden to verify the Truss loading diagrams transferred from the Truss Manufacturer to the Truss Designer rests with the Building Designer.

The Truss Submittal Package is required by IRC Section R802.10.1 to be submitted and approved by the Building Official prior to Truss installation. This critical step is not happening in most parts of the country. Building Officials must have better enforcement of this requirement.

My Recommendations As I expressed above, I believe that Truss Manufacturers should have engineers on staff who oversee the Truss design and stamp a coordinated Truss system, engineered for the requirements provided on the Construction Documents. This is what is done by the steel joist industry. I feel that the stamp by a Truss Designer who is only stamping individual Truss components should not be allowed.

This change is not likely to happen anytime soon, so engineers must work within the current system. I make the following recommendations.

1) Roof System a. Delegate the design of the roof system. Understand that the Truss Designer will not do more than stamp individual Truss Design Drawings. This delegated design will be from an additional engineer.

Require that the Truss Manufacturer engage a structural engineer (separate from the Truss Designer) who will be responsible for the design of

the wood Truss roof system. This engineer will 1) Be responsible for the Truss layout and will work with the Building Designer if the Truss layout is changed from the layout shown on the construction documents; 2) Oversee the calculation of the loads on each Truss including the snow drift loads; 3) Ensure that the loads for every individual Truss provided to the Truss Designer are correct; 4) Provide a bracing plan that coordinates the web bracing for every Truss and clearly shows how and where the diagonal bracing should be located for the Continuous Lateral Bracing, or where “L” or “T” bracing should be used; 5) Be responsible for the Trussto-Truss connections; and 6) Stamp the Truss bracing plan. Alternatively, you could do the following: b. Perform the responsibilities listed above and ensure that your fee is adequate to provide this oversight.

Most of these are required by the IRC and the IBC. 2) Web Bracing a. Become familiar with the SBCA Building Component Safety Information (BCSI) Guide to Good Practice for Handling, Installing & Bracing of Metal Plate Connected Wood Trusses. Require that all Truss webs that require bracing as shown on the Truss Design Drawings use an “L” or “T” brace, with an exception to allow horizontal bracing only if 3 or more webs align and a diagonal brace is provided from the top chord to the bottom chord. Provide the details for these braces on the Construction Documents.

3) Truss Submittal Package a. Insist that you review the Truss Submittal Package prior to Truss installation and ensure that the items as noted above are carefully reviewed.

Until the requirements in ANSI/TPI 1 are changed, engineers must be more vigilant in their responsibilities as outlined above, or they must clearly and specifically delegate the responsibility to a third engineer who will function as a wood Truss roof system engineer.■ Brent Maxfield is a Civil/Structural Engineer with the Special Projects Department of The Church of Jesus Christ of Latter-day Saints in Salt Lake City, Utah. (maxfieldba@churchofJesusChrist.org)

Possible Scope of Work for a Delegated Wood Truss Roof System Engineer 1) Provide a Truss placement plan that clearly shows the dimensioned location of all Trusses, clearly labeled.

2) Provide Truss-to-Truss connection requirements clearly labeled. Each Truss-to-Truss connection will identify the calculated load, the specific connector model to be used, and the number and type of nails or screws that must be used with the connector.

3) Show details for the anchorage of the Trusses to the supporting structure as indicated on the Construction Documents.

(Remember that, per ANSI/TPI 1, the Building Designer is responsible for specifying the Truss-to-structure anchorage.)

4) Provide anchorage of gable end Trusses and the required out-ofplane reinforcement for these gable end Trusses.

5) Show locations of field blocking to maintain the proper load path.

6) Provide all other elements and details necessary to certify that the erected Trusses will act as an entire system capable of transferring the roof loads through the system to the elements providing resistance.

7) Certify that the loads provided for each Truss to the Truss Designer by the Truss Manufacturer are in conformance with the loading requirements provided in the Construction Documents. This includes certifying that the snow drift loads were properly calculated and applied to each Truss.

8) Provide a bracing plan that is coordinated with every individual Truss Design Drawing. Show all necessary permanent bracing of Truss webs and the Truss bottom chords. Show locations of Continuous Lateral Bracing and provide details and the locations of diagonal bracing. Also, provide details of the brace connections. Indicate which bracing is to be accommodated with “T” or “L” bracing with appropriate details.

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