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STRUCTURE ® magazine (ISSN 1536 4283) is published monthly by The National Council of Structural Engineers Associations (a nonprofit Association), 20 N. Wacker Drive, Suite 750, Chicago, IL 60606 312.649.4600. Application to Mail at Periodicals Postage Prices is Pending at Chicago, IL and additional mailing offices. STRUCTURE magazine, Volume 28, Number 6, © 2021 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, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606.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, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606. POSTMASTER: Send Address changes to STRUCTURE magazine, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606. 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
Features
JU N E 2021
Moment Frames, Fluid Viscous Dampers, and Seismic Instrumentation By Reid Zimmerman, P.E., S.E.
Bellevue’s Innovative New W Hotel Tower By Cary Kopczynski, P.E., S.E., and Mark Whiteley, P.E., S.E.
The David H. Koch Center for Cancer Care By Andrew Altamirano, P.E.
100 MOUNT STREET
Late Game Pivot by Project Team
By Benton Johnson
Cover Feature
The 100 Mount Street building is a project built on efficiencies. Tenant views are maximized with an offset core and 20-foot column-free cantilevered zones. The offset core is balanced by an innovative composite mega-bracing system.
By Tim Carroll, P.E., and Maryann Davis, P.E.
Columns and Departments Editorial
andemic’s Luck: SEI’s Golden Moment P Committing to an Equitable, Diverse, and Inclusive Profession By David O. Prevatt, Ph.D., P.E.
Structural Repair
tabilizing San Francisco’s S Leaning Tower By Ronald O. Hamburger, S.E., Stephen K. Harris, S.E., and Lachezar V. Handzhiyski, S.E.
Geotechnical Issues Tall Building Foundations
By Nathaniel Wagner, Ph.D., P.E., Debra Murphy, P.E., Micaela Largent, EIT, and John Egan, P.E., G.E.
Emerging Technology
Machine Learning Applications By Henry V. Burton, S.E., Ph.D., and Michael Mieler, Ph.D.
Engineer’s Notebook
rror Checking and the Black E Box – Part 3 By Scott N. Jones, S.E.
Structural Sustainability 960 W 7th Street
By Donald W. Davies, P.E., S.E., Kelsey Rose Price, P.E., and Farshad Berahman, Ph.D., C.Eng
Building Blocks
InSights
By Russell H. Davies, P.E.
By Rollin J. Jones, Sarah L. Williams,
rack Patterns Tell the Story C of Glass Breakage
InFocus
Sobering Dose of History A and Humility By Eytan Solomon, P.E.
Structural Connections Detailing Cross-Laminated Timber (CLT) Connections By Jim DeStefano, P.E., AIA
Professional Liability
Do Not Forget Serviceability! By Richard S. Barrow
Structural Foundations
Pile Load Testing for Bored Piles in Soil By Hee Yang Ng, C.Eng, P.E.
Legal Perspectives
onstruction Documents for C International Projects By Gail S. Kelley, P.E., Esq.
an We ‘Grow’ Living Concrete C Alternatives? and Wil V. Srubar III, Ph.D.
Historic Structures
ussey Bridge Disaster, aka B Forest Hills Bridge, 1887 By Frank Griggs, Jr., D.Eng., P.E.
CASE Business Practices hen your client expects you W to pay for change orders. . . By Kevin H. Chamberlain, P.E.
Structural Forum
Meaning and Satisfaction in Structural Engineering By Jim Lintz, P.E., S.E.
In Every Issue
Advertiser Index Resource Guide – Tall Buildings NCSEA News SEI Structural Columns 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. J U N E 2 0 21
EDITORIAL Pandemic’s Luck: SEI’s Golden Moment Committing to an Equitable, Diverse, and Inclusive Profession By David O. Prevatt, Ph.D., P.E., F.SEI, F.ASCE
L
ike many in this COVID moment, I have reflected on life, pre- and post-pandemic. I have come to realize that, were it not for the particularly brutal, televised killing of George Floyd at the hands of law enforcement, I would not be the Inaugural Chair of SEI’s Diversity, Equity and Inclusion Committee. It was a personal and galvanizing call to action born out of my trauma that grew into a need also to be accountable as a professional. Racial violence, specifically anti-Black racial violence, is not new to any of us. My education, two decades of exciting engineering consulting, and the relative privilege of being a researcher and professor have not shielded me from sensing “my uneasy queasies” that I could be harmed anytime I deploy to lead a post-tornado damage assessment survey. Despite professional accomplishments – locally, regionally, and internationally, despite supportive faculty, mentors, and colleagues, I and so many people of color live a part of George Floyd’s terror every day as we navigate this world. The symbolism of 9 minutes and 29 seconds is as significant to structural engineers and society as is the galloping failure of the Tacoma Narrows Bridge. The domination and humiliation of Black men by the police cannot be set aside by us because we professionals are not above the fray. ASCE articulated our societal responsibility in its Code of Ethics, as “first and foremost, (to) protect the health, safety, and welfare of the public,” and that engineers shall “recognize the diverse historical, social, and cultural needs of the community, and incorporate these considerations in their work.” These noble words must be turned into actions. SEI has taken steps to improve our diversity and, alongside others, loudly condemned racism and committed to its eradication – a call for action. Four months after Mr. Floyd was killed, the SEI Board approved the Charter for the Diversity, Equity, and Inclusion Board-level Committee (DEI), and I agreed to serve as its Inaugural Chair. The committee is currently comprised of three members: Vice-Chair Dr. Sara WadiaFascetti, Dr. Malcolm Ammons, and I. This is a worthy beginning.
Why a DEI Committee? That said, as a profession, structural engineers still need to take concrete actions to confront the less pleasant parts of our historical legacy before we can move forward. Like many other professions, American civil engineers have enabled systemic racist practices through our work. Our 20th-century contributions in the design and construction of Whites-only suburban neighborhoods in the 1950s (e.g., the Levittown suburbs of NY, NJ, and PA), supported by Federal housing subsidies, remain a low point. Until Howard Grant became ASCE’s first Black member in 1948, no credible voice could speak on behalf of Black society. At the height of America’s post-World War II economic miracle, houses were supplied for the American middle class – but African Americans need not apply. Civil engineers also designed and built the highways that enabled the new suburban workers to drive rapidly to their downtown offices – without consideration for the social upheavals these highways wrought by the planned destruction of established African American communities in the path of these superhighways, under the rhetoric of “urban renewal”. Our history is directly related to our present, and we need to acknowledge its impact upon who structural engineers are today. Today’s reality is that 85% of structural engineers still work for firms with all-White or majority White leadership (2020 SE3 survey). The number of minority structural engineers is embarrassingly small (<1% Native American, 1.3% African American, 1.8% Middle Eastern or North African, 6% Hispanic, Latino or Latina, and 12% Asian). In my 17 years as a Professor of structural engineering at Tier I research universities, I have never taught any class that had more than 1 or 2 African American students. Rarely have I seen another Black Structural Engineer at our annual engineering conferences.
The symbolism of 9 minutes and 29 seconds is as significant to structural engineers and society as is the galloping failure of the Tacoma Narrows Bridge.
The Diversity, Equity + Inclusion Committee Charge The charge and challenge of the DEI Committee is to advance the structural engineering profession by promoting the adoption of these values into the business, the practice, and the education of structural engineers. Success will be measured by tangible and lasting modifications in how we educate, how and where we do business, how we recruit and mentor structural engineers, and ultimately in how structural engineering firms engage and practice within ALL communities. It is essential to act decisively as we adapt to rapidly changing social norms and cultural expectations in this post-pandemic moment. A more diverse engineering workforce is critical for assuring 21st-century business credibility and success. STRUCTURE magazine
Call to Action This is a call to action, a call to passion and commitment. No Committee can deliver change without our members’ support, effort, and willingness to envision a more inclusive future. We issue a distinctly different call, inviting the leaders of each committed structural engineering firm to nominate one member to join us and become the catalyst within your organizations. Members will work together within the DEI Committee to build the road towards a welcoming, diverse, equitable, and inclusive structural engineering profession. We encourage all interested SEI members to step forward together with us on the DEI Committee. Apply at https://bit.ly/2Rtc7tG.■ David O. Prevatt is the Kisinger Campo & Associates Term Professor and Associate Professor of structural engineering at the University of Florida, Gainesville, FL. He also serves on the SEI Board-level Resilience Committee. J U N E 2 0 21
structural REPAIR Stabilizing San Francisco’s Leaning Tower By Ronald O. Hamburger, S.E., Stephen K. Harris, S.E., and Lachezar V. Handzhiyski, S.E.
L
ocated at 301 Mission Street, the 650-foot-tall Millennium Tower was designed to be San Francisco’s premier residential address. The project geotechnical report predicted 4 to 6 inches of settlement over the project’s life; however, as construction neared completion in 2009, the settlement had already reached 10 inches. As development occurred on adjacent parcels, accompanied by continuous dewatering of the surrounding soils, settlement continued. By 2014, instrumentation installed to monitor the effect of adjacent construction recorded that the mat had dished, settled nearly 14 inches and that the roof had tilted to the northwest a similar amount. As the City of San Francisco threatened to red tag the building, counsel for the developer, Mission Street Development (MSD), retained Simpson Gumpertz & Heger (SGH) to determine if the settlement had damaged the structure and created a safety issue. Although SGH found that settlement had not appreciably affected the structure’s adequacy, in 2015, with settlement exceeding 16 inches, litigation ensued between the homeowners’ association, MSD, the City of San Francisco, and the development teams for adjacent projects. Under the terms of a negotiated settlement, SGH designed a foundation stabilization upgrade that formed the basis for dispute resolution and is currently under construction. This article focuses on the structural aspects of the problem and the upgrade.
Structure Description
Figure 2. Perimeter pile upgrade.
STRUCTURE magazine
The 58-story tower, with a single basement, was constructed together with a structurally separate but functionally interconnected 12-story structure atop a 5-story subterranean garage. Tower floors comprise post-tensioned concrete flat plates supported by a central reinforced concrete bearing wall core and perimeter columns. The structure was prescriptively designed to the criteria of the 1997 Uniform Building Code. Seismic and wind forces are resisted by a dual system comprising special reinforced concrete shear walls at the building core and special reinforced concrete moment resisting frames at the building perimeter. Plan dimensions are approximately 100 feet east to west by 150 feet north to south (Figure 1). Six levels of
Figure 1. Foundation plan highlighting lateral force-resisting elements.
outriggers, consisting of extensions of the north and south core walls to the perimeter moment frames, assist the core in resisting east-west overturning forces. The foundation is conventional to San Francisco high-rises comprising a 10-foot-thick, heavily reinforced concrete mat supported by approximately 940 precast, prestressed concrete piles driven into a dense, silty sand layer present approximately 50 to 85 feet below grade. Bedrock is more than 150 feet below the sand, with layers of clay and alluvium between the rock and sand.
Settlement Cause Extensive geotechnical investigation and analyses were conducted, including three-dimensional modeling of the subsurface conditions and imposed loading using FLAC 3D software, a finite difference method code used by geotechnical engineers. The analyses indicated that settlement resulted from consolidation of a deep-seated clay layer under the 11 ksf imposed tower loading, combined with increased effective stresses in these materials resulting from long-term lowering of the water table for construction of this and adjacent structures. Further, the analyses suggested that, under the effects of secondary compression (creep), settlements could double over the structure’s remaining life.
Perimeter Pile Upgrade During mediation, the homeowners insisted on a substantive retrofit that would arrest settlement and allow tilt recovery. The
Figure 3. New mat extension.
homeowners’ consultant proposed installing several hundred micropiles through the existing mat, extending 250 feet and supported in bedrock, with sequential jacking of the load onto the piles to remove tilt. Unfortunately, the parties could not reach an agreement to fund the substantial estimated cost of this retrofit. As an alternate, SGH proposed underpinning the structure along its north and west sides. The upgrade includes the installation of 52 piles, spaced at approximately 5 feet, drilled into the underlying bedrock beneath the Fremont and Mission Street sidewalks (Figure 2). Following pile installation, an extension of the existing foundation mat is constructed around the piles, which are sleeved through and extend above the mat. Hydraulic jacks are used to transfer 800 kips of load to each pile, removing approximately 20% of the building’s weight from the original foundation and transferring this load directly to the bedrock underlying the consolidating clays. FLAC 3D analysis indicates this retrofit will effectively arrest further settlement along the structure’s north and west sides and allow gradual recovery of tilt through a modest continuing creep settlement of the east and south sides. The 800-kip jacking load was based on preliminary estimates of the existing mat’s flexural and shear capacity and its ability to safely withstand large forces imposed by an external line of piles. As design progressed, SGH implemented capacity-design principles to assure the new piles would not take excessive loading that would overstress the existing mat due to continued building settlement or earthquake shaking. As illustrated in Figure 3, a shear key is excavated at the existing mat edge to expose flexural reinforcing. Mechanical couplers are used to extend the existing reinforcing into the new mat extension. Additional epoxy-adhered dowels are used to assure that the shear strength of the attachment exceeds the flexural capacity of the joint, assuring ductile behavior in the event of overload. Load is transferred to the new mat extension from the piles through hydraulic jacks placed at the top of the piles, as shown in Figure 4. These jacks push downward on the piles and
upwards against a jacking beam that extends across the top of each pile and which is anchored to the mat extension with four 2½-inch diameter high-strength threaded rods at each pile. These rods are selected such that their combined yield strength limits pile load to approximately 1,100 kips, considering material overstrength and strain hardening. Detailed nonlinear finite element analyses confirm the existing mat has the capacity to resist such loading without failure, as does the interconnection of the new and existing mats. In the event of overload associated with unanticipated settlement or earthquake, pile loading is limited by fuse-like yielding of the high-strength rods. The hydraulic jacks have mechanical lock-off capability and can be re-energized if necessary to add or reduce pile loading. A protective vault is constructed around the pile tops and jacks to allow this, if necessary. The upgrade intends to remove some loading from the consolidating clays and transfer it to the underlying bedrock. It is important that the new piles do not transfer load back to the soils above the clay or the clay itself. Figure 5 (page 10) illustrates the pile design, which consists of a concrete-filled 24-inch-diameter steel casing, with a single, central, 103 mm hollow, high strength, coarse thread reinforcing bar, which also serves as a tremie. The pile extends into bedrock as an uncased 20-inch diameter reinforced shaft that transfers load through friction. Above the clay, the 24-inch-diameter pile is installed through a 36-inch-diameter outer casing. The annular space between the inner and outer casings is filled with a low-strength controlled-density fill only after the load is jacked onto the piles, preventing any load transfer in this region. The outer surface of the 24-inch-diameter casing is coated with a friction-reducing material to minimize load transfer directly to the clays. The penetration length into rock and the effectiveness of the friction-reducing coating will be confirmed with an indicator pile, to be tested using Osterberg
Figure 4. Load transfer to piles.
J U N E 2 0 21
Cells within the 20-inch-diameter portion of the shaft. The indicator pile will also incorporate several sets of discrete strain gauges and continuous fiber-optic strain measurement. The upgrade was submitted for permitting as a voluntary seismic upgrade under the San Francisco Existing Buildings Code provisions. This code permits voluntary upgrades of any type, providing
the design team can demonstrate that the upgrade will not reduce the building’s safety, impose more load on existing elements than they can withstand, or create a structural irregularity. SGH conducted extensive nonlinear dynamic analyses using PERFORM 3D software to confirm the project complies with these requirements and demonstrate that the building can meet typical performance criteria for new structures. In addition, a city-appointed team of two structural and two geotechnical engineers performed an independent design review.
Construction and Beyond Construction was initiated in October 2020, with completion scheduled for October 2022. The new piles will be loaded by stressing the hydraulic jacks at all piles simultaneously, in 100 kip increments, with a hold of 24 hours after each load increment to allow monitoring of building response. Following loading, the pressures will be maintained for 1 month while additional monitoring occurs. The design team anticipates, based on analysis, that some substantial rebound of the existing piles will occur as they are unloaded, resulting in loss of effective load in the new piles. If this occurs, the piles will be reloaded to the design level before completing the vault and demobilization from the site. Continuous monitoring of the project will occur for 10 years following construction completion. Monitoring instrumentation includes load cells at each pile, settlement markers on the mat, piezometers and extensometers in boreholes at depth, and survey points on each of the building’s facades and roof. As noted previously, the jacking load can be adjusted as needed to influence the building’s behavior.
Conclusions The Millennium Tower’s settlement and tilting did not pose a safety issue but created unacceptable devaluation of the homeowners’ investments, especially after national news media publicized the problem. SGH’s upgrade design employed conventional underpinning technologies to arrest the settlement, remarkable only for the scale of the project, the public scrutiny that occurred, and the sophisticated analyses the team of structural and geotechnical engineers employed to substantiate the design. Since the Millennium Tower settlement problems emerged, design practice in San Francisco has changed substantially. For example, all high-rise buildings developed since 2009 have used deep foundations extending to rock. Furthermore, the City of San Francisco has instituted administrative procedures requiring independent geotechnical review of all buildings exceeding 240 feet in height.■ See the article on page 12 (Geotechnical Issues) for additional information on the geotechnical aspects of the Millennium Tower. All authors are with Simpson Gumpertz & Heger (SGH). Ronald O. Hamburger is Senior Principal. (rohamburger@sgh.com) Stephen K. Harris is a Principal. (skharris@sgh.com)
Figure 5. Pile design.
STRUCTURE magazine
Lachezar V. Handzhiyski is a Senior Project Manager. (lvhandzhiyski@sgh.com)
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geotechnical ISSUES Tall Building Foundations The SF Millennium Tower Example
By Nathaniel Wagner, Ph.D., P.E., Debra Murphy, P.E., Micaela Largent, EIT, and John Egan, P.E., G.E.
A
s new skyscrapers soar to increasing heights worldwide, the tip an additional investigative budget to improve knowledge of a site. of the iceberg regarding potentially significant issues is hidden Communicating the cost/benefit of reducing uncertainty to clients below the surface. For example, the Millennium Tower (Tower) in is difficult yet is not impossible. Whether the analyses are simple or San Francisco, California, with a height of 645 feet and 58 sto- sophisticated, the quality of the inputs is essential for accurate results; ries, easily falls under the San Francisco stratigraphy and soil properties are readily Department of Building Inspection’s identifiable as critical inputs. Other infor(SFDBI) definition of a tall building. The mation, such as the magnitude and rate of Tower is one such case highlighting that foundation loading, potential interactions communication among development team of construction activities, and long-term members, including engineers, architects, foundation response with adjacent and developers, and regulators, is imperative to nearby structures, are also crucial for such achieving a successful project and illustratevaluations. ing how communication gaps can result in unanticipated issues. The entirety of a tall Communication building ultimately performs as a single integrated system; its design team should Like many other structural materials, soils strive for the same. are highly non-linear. However, soil is For decades, tall buildings in downtown typically reduced to simple springs for San Francisco have successfully been supuse in structural models. Structural engiported on pile or mat foundations, bearing neers need to appreciate the significance in dense late-Pleistocene sands that underthat such simplification may have on the lie artificial fill and soft Holocene sediments results of their analyses. Many geotech(known locally as Young Bay Mud [YBM]). nical engineering firms have advanced Underlying the dense sands is a thick layer computational tools and can provide of Old Bay Clay (OBC) deposited before geotechnical inputs appropriate to and the last ice age. The OBC is a compressible consistent with structural modeling soil that has become lightly overconsolisophistication. dated due to erosion, aging, and lower sea It is essential to understand the intended levels during the late-Pleistocene glacial demands on the soil supporting the period. Very dense Alameda formation is foundation early in the design process. found at the base of the OBC and immediThese demands generally stem from the ately overlying Franciscan Mélange bedrock structure’s characteristics, including size, that is highly weathered and fractured weight, depth below grade, foundation due to the regional tectonic environment. type, loading conditions, and spatial disSchematic representation of these subsurtribution. Performance objectives should face conditions is illustrated in the Figure. also be developed to guide the level of Historically, extending foundations for tall Subsurface stratigraphy beneath the Tower. detail required in the analyses. Once buildings through the OBC to Franciscan project demands are understood, a geobedrock at depths of 250± feet was not necessary. The Tower’s foun- technical investigation can begin with existing information such as dation followed this trend by tipping the piles of its foundation in geologic maps, previous geotechnical investigations, and awareness the dense sand, as depicted in the Figure. The Tower’s settlement has of and experience with historical building performance in the site altered the tenor of design practice and regulation in San Francisco vicinity. In addition, logs of previous borings and lab testing results toward only foundations that extend to bedrock being acceptable are commonly helpful to identify the scope of further subsurface for tall buildings. Although bedrock-supported foundations may be investigation. Still, many geotechnical engineers lament that existing necessary for acceptable performance for some tall buildings, such a boring logs frequently do not include information desired for today’s requirement is not appropriate for every tall building. problems. This highlights that communication issues can arise within Foundation design for tall buildings can be fraught with uncer- the same discipline, and it is important to document one’s findings tainty, primarily due to limited knowledge of subsurface conditions fully and accurately. underlying the site. Typically, geotechnical field exploration occurs After the general configuration of the project is understood, a proper early in the development process, sometimes even before full project field investigation is necessary. Geotechnical teams regularly have conceptualization. Geotechnical engineers are always grateful for tight budgets and time constraints for characterizing the subsurface
STRUCTURE magazine
stratigraphy and its variability. Therefore, it is essential to invest in a subsurface characterization program that explores multiple locations distributed across the site to sufficient depths for designing tall building foundations. Using a combination of exploratory, sampling, and other testing methods, the soil’s mechanical properties can be characterized, including stress history, compressibility, strength, and shear wave velocity. Geotechnical engineers must adamantly express (and justify) these needs to the development team; the team must then act appropriately in the interests of the developer/owner and the public that will use the building. As discussed previously, communication within the project development team benefits from being intradisciplinary. The desired use, cost, and potential occupancy of the building should be considered from the first conceptual designs and with any design changes that occur along the way. Altering the structural materials, structural system, foundation system, or basement depth can create highly interactive issues if not factored into the design of each component of the structure (i.e., broadly, the superstructure and the foundation). Hence the typically iterative design process. For foundations, at a minimum, the bearing capacity, settlement, and factors of safety should be checked for potential configurations. In addition, the project team should address design objectives and performance criteria, including seismic and wind loading and anticipated settlements.
underlain by a pile-supported mat. Accordingly, three years after the initial borings, two additional borings were drilled to depths of 200 and 220 feet to help characterize the deeper subsurface soil conditions. The geotechnical analyses were also seemingly robust, suggesting that settlements would occur primarily during and immediately following construction, comparable to the myriad other tall buildings in San Francisco. However, the choice of structural system and material (concrete) and changes of foundation configurations increased the expected foundation stress on the OBC underlying the bearing stratum from well-below to at-or-near its maximum past pressure. However, the OBC was still interpreted to be in the recompression range of stresses characterized by relatively low compressibility. During the design, the geotechnical engineer estimated the range of expected
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For the Tower, documenting and communicating the effects of choices and/ or changes to these components proved a challenge. The geotechnical engineers’ experiences informed them that tall buildings in downtown San Francisco supported on pile or mat foundations bearing in dense to very dense sands at depths less than 100 feet had performed well for decades. At least a dozen buildings of 30 stories or more and heights of 400 feet or more within the general vicinity of the Tower fall into this category. Therefore, the initial field exploration for the Tower consisted of five borings with an average depth of 100 feet. After that investigation, the design was developed as a concrete structure when the price of steel rose dramatically. The Tower’s use was intended as primarily residential, so the structural system was a stiff concrete core connected to outriggers for stability. During design, the foundation system evolved from a compensated foundation with multiple basement levels, initially below-grade levels to ≈35 feet and pilesupported, then levels extending ≈80 feet deep and supported by a thick mat, and ultimately to a single subsurface level
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building settlement to be about four to six inches, even with a rePerformance Monitoring evaluation of the building changes. Since tall buildings are commonly built in dense urban environ- When a building design becomes a reality, Mother Nature takes control. ments, the impact of the construction process and long-term presence Performance monitoring criteria are enforced with quantifiable metrics on the surroundings is required by the building code. Effects of and action limits to which they can be compared. As tall buildings dewatering and excavation, both on-site and to adjacent/nearby become more complex and densely arranged, monitoring before, during, structures, constituted significant and previously unconsidered and after construction should be included both on-site and at adjacent contributions to the Tower settlement. In this case, there was an properties. A baseline survey is a critical point of comparison to validate extended on-site groundwater drawdown during the construc- expectations or departure therefrom. This proved prudent in the case tion of the project’s adjacent below-grade parking and mid-rise of the Tower as designers for subsequent neighboring construction structure. Re-evaluation of the settlement projects continued to monitor and document during construction to account for the on-site settlement of the Tower before breaking ground dewatering resulted in doubling the original at their own sites. However, quantifying things estimated range. Subsequently, groundwater such as the amount of water extracted to keep drawdown from dewatering at nearby conexcavations dry was complicated as gauges were When a building struction sites and stress changes and ground regularly out of service while pumps remained movements from nearby deep excavations active. Excavation and shoring wall movements design becomes further increased the Tower’s settlement. should also be managed and appropriately docuIt is understood that both activities can cause mented. Any assumptions made to manipulate a reality, Mother settlement to occur on and near a site; specifithe data from ordinary interpretation should cally addressing such concerns is repeatedly be thoroughly vetted and documented. The Nature takes control. overlooked. Unfortunately, this is often inadperformance criteria and action levels should equately communicated among the project be set before construction. This is crucial in team; settlements are implicitly considered negligible outside the communicating within the project team, as well as with neighboring building footprint as if the property lines are a rigid boundary building owners. Monitoring ground movement is one thing; but, isolating everything outside. When these effects are considered, doing little or nothing to mitigate or prevent further movement can the issue is communicated in terms of deflections, drawdown, and result in significant deleterious consequences. settlement, terms with which engineers are comfortable, rather than dollars, which is the language of potential damages (including Conclusion attorneys’ fees). Learning to think or translate measurements to dollars will improve communication with clients and their neighbors Communication, communication, communication! Case histories and tenants. offer the engineering community an opportunity to revisit how we approach each project, how the project team interacts and responds to potentially adverse issues that arise during project conceptualization, External Peer Review design, and construction, and underscore the importance of constant While it is good practice for the design team to consider all potential communication in completing a successful project. issues, doing so is complex, and the contributions of independent For the Tower, the choice of a concrete structure combined with a parties should be appreciated. Independent peer review by a quali- reduced number of basement levels resulted in expected increases in founfied interdisciplinary team of professionals should be a given for any dation stresses on the Old Bay Clay underlying the dense bearing stratum unique structure. Considering the complexity, the project team should that was at or near the maximum past pressure. However, the soil was welcome the opportunity to review the consistency and applicability of still interpreted to be in the recompression range of stresses characterized the design to the site and surrounding environment. At a minimum, by lower compressibility. Additionally, Mother Nature got assists from the peer review should ask and receive responses concerning awareness several unanticipated sources, including extended groundwater drawdown of possible issues, design assumptions, appropriate consideration of on-site during construction and at nearby construction sites and stress uncertainties, and the potential impact on nearby structures. As tall changes and ground movements from nearby deep excavations. These buildings continue to become taller, their foundations will consequently events pushed the stress in the soil beyond its maximum stress into the become deeper, and their zone of influence will expand. The peer-review level of greater compressibility, resulting in increased settlement beyond teams must bring an independent perspective that helps expand the that originally estimated during design. Lastly, with the stress state having design team’s focus beyond minimum code compliance to addressing been pushed into virgin compression, secondary compression effects were the uncertainties that could affect the intended performance objectives. triggered that contributed to the Tower settlement. Communication and transparency are crucial to the peer review A voluntary upgrade of the Tower’s foundation system is currently process; it is imperative that the design team appropriately document underway to arrest further settlement and mitigate some of and showcase the final design in a comprehensible manner that does the tilt that has occurred due to differential settlement across not obscure details. The process itself should not be contentious but the Tower footprint.■ rather cordial with the intent of providing a safe product for the public. Structural peer review has been codified in San Francisco for See article on page 8 (Structural Repair) for additional information tall buildings since 2008, although it was commonly implemented on the structural aspects for the upgrade of the Millennium Tower. prior to that, including for the Tower. However, geotechnical peer review was not formally required of tall buildings until 2017, so it Nathaniel Wagner, Debra Murphy, and Micaela Largent are all with Slate was not performed for the Tower. As a result of the settlement issues Geotechnical Consultants, Oakland, CA. experienced at the Tower, the City now requires geotechnical peer John Egan is an Independent Geotechnical Consultant, Danville, CA. review for tall buildings (SFDBI AB-082).
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emerging TECHNOLOGY Machine Learning Applications Hope, Hype, or Hindrance for Structural Engineering
By Henry V. Burton, S.E., Ph.D., and Michael Mieler, Ph.D.
M
achine learning (ML) is a branch of artificial intelligence (AI) that uses algorithms to find patterns in data and make predictions about the future, essentially enabling computers to learn without being programmed explicitly beforehand. While AI and ML have been active fields of study and research since the 1950s, they have exploded in popularity over the past decade. This explosion Figure 1. Overview of the relationship between AI, ML, and DL. is thanks to deep learning (DL), a type of ML that leverages big data and neural networks to tackle a diverse range of structural design, response analysis, and performance assessment. problems, from image recognition and fraud detection to customer These types of models have been shown to be especially useful support chatbots and language translation. Figure 1 shows the rela- when physics-based models, which are often simplified for practitionship between AI, ML, and DL, while Figure 2 shows the three cal purposes, are unable to capture known physical mechanisms. major types of ML. For example, many of the analytical relationships provided in the The recent success of ML applications in areas such as bioengineering, ACI-318 Building Code Requirements for Structural Concrete are medicine, and advertising has been highly visible, creating a domino either fully empirical (i.e., based on regression using experimental effect where others have begun to ask whether their respective fields data) or a hybrid (i.e., engineering equations with empirically of practice, including structural engineering, can be transformed or derived parameters). Therefore, it is worthwhile to examine whether “revolutionized” by ML. Structural engineering researchers began to the predictive performance of these empirical relationships can explore ML applications as early as the late 1980s. However, it is only be improved by using ML models and what, if any, tradeoffs are within the last five years that the community of structural engineer- included in the latter. ing researchers and practitioners has begun to seriously explore ways For illustration purposes, examine Equation 18.10.6.2b of ACI-318in which ML can improve the efficiency and/or accuracy of specific 19, which is used to estimate the drift capacity (i.e., drift associated tasks or solve previously intractable problems. As with other fields, with a 20% peak strength loss) of special structural walls. The equation some have expressed legitimate concern that the potential benefits of was developed by performing linear regression on a dataset of 164 ML to structural engineering are being overhyped and, in the worst physical experiments (Abdullah and Wallace, 2019) using the ratio of case, exploited for marketing purposes. wall neutral axis depth-to-compression zone width, the ratio of wall This article identifies and reviews three areas of current and potential length-to-compression zone width, wall shear stress ratio, and the ML applications in structural engineering and discusses challenges configuration of the boundary zone reinforcement as the predictor (or and opportunities associated with each. The authors conclude with a independent) variables. Using the same dataset and predictors, a drift general discussion of some of the challenges that must be addressed if capacity model was developed using the Extreme Gradient Boosting ML is to be effectively used in structural engineering practice. (XGBoost) machine learning algorithm. In brief, XGBoost is a type of “tree-based” algorithm that works by repeatedly subdividing the dataset based on a set of criteria defined to maximize the accuracy of Improving Empirical Models the resulting predictive model. More details of how this algorithm There is a long history of using statistical models based on experi- works can be found in Huang and Burton (2019). mental or field data to predict various types of parameters used in Figure 3 (page 18) shows a plot of the observed (from the experimental data) versus predicted (by the model) drift capacity values for the linear regression (Figure 3a) and XGBoost (Figure 3b) models. The solid diagonal lines in the two plots represent the locations where the observed and predicted values are the same. Compared to the linear model, the XGBoost data points are more closely aligned along the diagonal, indicating superior predictive performance. A quantitative comparison of the two models can be obtained by computing the Figure 2. Three major types of ML. DX% value, which is the percentage of data points STRUCTURE magazine
Surrogate Models ML models could also potentially be used to reduce the time and effort associated with some computationally expensive structural analysis tasks. For example, performance-based design (PBD) often necessitates detailed nonlinear (geometric and material) analyses to understand structures’ response to extreme loading. However, despite significant research advancements, PBD has not been widely adopted in practice. Even the 2nd generation performancebased earthquake engineering (PBEE) framework, which is considered by many to be the template for PBD, has not seen widespread adoption. This is partly because the state of structural engineering practice is such that most designers rely on linear analysis models
to estimate response demands. The use of nonlinear response history analysis as part of the design process is generally expensive, both computationally and in terms of labor, which often leads to unjustifiable design fees. One strategy for reducing the computational expense and labor of nonlinear analyses is to use surrogate models to estimate response demands. Additional details on the procedure for developing surrogate models can be found in Moradi et al. 2018. The process of creating and implementing surrogate models for structural response estimation is not without its challenges. First, the dataset used to train the ML model is generated using explicit
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with errors (the difference between observed and predicted) that are less than some predefined percentage (X ) of the observed values. The D10% value for the XGBoost model is 76%, which indicates that 76% of the errors in the predictions are within 10% of the observed values. In contrast, the D10% value for the linear model is approximately 53%, which further indicates the improved accuracy provided by the XGBoost algorithm. A defining feature of tree-based ML algorithms (including XGBoost) is that the resulting model cannot be expressed analytically. This creates obvious challenges with interpreting or interrogating the model and even implementing it in a building code or standard. Therefore, one has to weigh the benefit of increased accuracy with the increased complexity of the XGBoost model. The latter can be addressed by developing a software application that implements the relevant ML model and uses visualizations (e.g., plots) to explain the relationship between the input and output variables. The example presented in this section represents just one of several possibilities where the accuracy of existing empirical relationships for predicting structural response, capacity, or performance can be improved using ML models. However, as noted earlier, this often comes at a cost, which is frequently associated with the complexity of ML models. Other areas where ML algorithms are being examined as potential replacements for commonly used empirical models include component failure model classification, natural hazard damage assessment (e.g., predicting earthquake or flood damage to buildings), and structural health monitoring.
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response analyses, which will be comin structures (e.g., cracks in conputationally expensive. However, the crete, loosened bolts), automate increased availability of computathe development of as-built models tional and storage resources within using images, and identify seisthe research community is likely to mic deficiencies. Through informal overcome this challenge. There are inquiries, the authors also found also limitations in terms of the lack that structural engineering pracof generalizability of surrogate models. titioners are using CV to detect ML models are generally good at intercorrosion in offshore structures, polation (making predictions within monitor the structural integrity of the bounds of the dataset). Still, they bridges during construction, and do not perform as well at extrapolaidentify the presence of soft stories tion (making predictions outside the in buildings. bounds of the dataset). Therefore, the Natural language processing application limits of surrogate models (NLP) is a branch of AI focused on should be carefully communicated to empowering computers to extract the users. Lastly, the challenges with useful information from writinterpretation discussed previously are ten text. NLP technologies have also relevant to surrogate models. found widespread applications in In addition to potential applications advertising, litigation tasks, and in PBD, surrogate models can be medicine. Despite being one of the used in regional assessments of natuleast explored ML technologies in ral hazard impacts. More specifically, structural engineering, researchers the earthquake engineering research have begun to investigate the use of community has begun to explore the NLP to rapidly assess the damage to use of surrogate models as part of the buildings caused by natural hazard workflow for regional seismic risk events using text generated by field assessment utilizing the FEMA P-58 inspections. Additionally, practitioPBEE procedure. At this scale, the ners have begun to explore NLP’s inventory size is typically on the order use to catalog building information Figures 3a and 3b. Observed versus predicted drift of tens or hundreds of thousands of by automating text extraction from capacity values; a) linear regression, b) XG Boost model. buildings, such that explicit nonlinear structural drawings. response simulation becomes unfeasible. The Seismic Performance Prediction Program (SP3) (www.hbrisk.com), a commercial online Challenges tool that is used for PBEE assessments, has developed a “structural response prediction engine” that essentially uses the surrogate model For ML applications to become an effective part of structural engineerconcept to enable users to bypass explicit nonlinear response his- ing practice, several challenges must be addressed. The first involves tory analyses. However, the details of the adopted statistical or ML sufficient access to diverse and high-quality data. Most, if not all, methods have not been made publicly available. of the previously described structural engineering applications have utilized relatively small, homogeneous datasets, which implies that the resulting models cannot be generalized. The development of Information Extraction genuinely representative datasets requires that open access repositories ML models for automating information extraction from images, with rigorous quality control be instituted. The research community video, and written text have found widespread applications in several has begun to take the necessary steps towards achieving this vision fields, including engineering, medicine, and different branches of by establishing platforms such as DesignSafe, Structural ImageNet, science. A reasonable argument can be made that, among the vari- and the DataCenterHub (http://datacenterhub.org). ous ML technologies, those that automate information extraction The second challenge involves the “black box” nature of some ML from different media sources hold the greatest promise in structural algorithms, giving rise to interpretation and quality control issues. engineering applications. This is partially supported by the rapid However, it is important to distinguish between those algorithms growth in the popularity of this area of ML-related structural engi- that are truly black boxes (e.g., deep neural networks whose predicneering research within the past three years. Additionally, there is tions often cannot be explained by ML experts) and those that are anecdotal evidence that, unlike the previously discussed areas of somewhat interpretable (e.g., XGBoost) but merely unfamiliar to potential application, these information extraction technologies the structural engineering community. It is reasonable to suggest have started to make their way into practice. that ML not be used by individuals who are completely unfamiliar Computer vision (CV) is a subcategory of AI that gives com- with the algorithms. This is especially true for structural design and puters the ability to extract meaningful information from images analysis tasks where safety and reliability are a primary concern. In and videos. DL algorithms are essential to the functionality of the long term, the inclusion of basic data science courses in university modern CV techniques. For almost a decade, the research com- civil/structural engineering curricula could increase the number of munity has been exploring the use of CV techniques to classify structural engineering practitioners that have a working knowledge structural system and component types, detect defects or damage of some commonly used ML algorithms.
STRUCTURE magazine
The third challenge, which is a side effect of its current popularity, References are included in the is that AI is sometimes exalted as a “cure-all” for problems across difPDF version of the article at STRUCTUREmag.org. ferent domains. As a marketing tool, there is a tendency to lead with the predictive accuracy of various models. A hypothetical example of such a claim is that “our model can predict earthquake damage to Dr. Henry V. Burton is an Associate Professor and the Englekirk Presidential buildings with 95% accuracy.” Important nuances such as the differChair in Structural Engineering in the Department of Civil and Environmental ences between the model development and application context and Engineering at the University of California, Los Angeles. (hvburton@ucla.edu) uncertainties in the prediction outcomes are often lost in such declaraDr. Michael Mieler is a Senior Risk and Resilience Engineer in Arup’s San tions. It is therefore critical that developers of structural engineering Francisco office. (michael.mieler@arup.com) ML technologies communicate openly and honestly with users about limitations and potential pitfalls. This is especially important when the application context has implications for the safety of populations on a broad scale. The fourth challenge centers on determining whether ML is suitable for a Seismic Performance particular structural engineering task. First and foremost, it is important to Prediction Platform examine whether there are tangible benby Haselton Baker Risk Group efits to its application (e.g., increased productivity or improved prediction accuracy relative to existing models). In those cases where there are actual benefits, potential users also need to weigh them against tradeoffs, such as reduced interpretability. Another important consideration is whether the dataset used to train the ML model is within the domain of the potential application. As noted earlier, many ML models are good at Whether for one building or a portfolio of thousands, SP3 enables interpolation but less capable of extrapolating beyond the training dataset, automated, efficient site/building-specific seismic risk assessments. which brings into question their ability Our seismic risk automations enable quick desktop pre-screening to generalize. Lastly, the high predictive accuracy of some ML algorithms up to comprehensive risk assessment, all seamlessly in the same (e.g., deep learning) within the context platform, as well as Resilient Design for Functional Recovery. of training and testing is such that the uncertainty associated with their predictions on “unseen” datasets is often overlooked. The centrality of the role that structural engineers play in ensuring Structural analysis with no Repair costs, including mean and the safety and functionality of the built modeling needed, leveraging a 90th percentile repair costs. environment is such that uncertainty database of millions of nonlinear Functional Recovery Time in design and performance outcomes modeling simulations. cannot be ignored. Assessments for business In summary, the authors’ opinion is that Integrate site soil and hazard. interruption. there are some promising areas where Site/building-specific strength Component-level damage info, ML can provide meaningful benefits and period information. so you can clearly see what drives to the practice of structural engineerrepair costs and downtimes. ing. However, effective implementation Damageable component requires that the community of practiauto-population, incl. detailed Full FEMA P-58 risk results with tioners and researchers come together anchorage strength calculations. data for thousands of earthquake to address some of the challenges that simulations, run in minutes. have been identified in this article. In the meantime, both researchers and practitioners should proceed with “cautious exploration” of ML applications in structural engineering while being mindful of the profession’s commitment to the public’s safety and well-being.■
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engineer's NOTEBOOK Error Checking and the Black Box Part 3
By Scott N. Jones, S.E.
T
his is the final installment of a three-part series (STRUCTURE, April 2020 and May 2020), wherein we discussed the Black Box, why it is critical to our work, and how to control it to make sure we are getting correct and accurate results. This segment takes a highlevel look at strategies for the successful use of engineering software.
Avoid the Temptation to Duck Your Head We are engineers, not technicians! Take some time to look at the big picture. Create a plan. Establish a vision. Great processes and habits are among the best ways to guarantee quality work. Before you start that next project – before you excitedly import that model from Revit (a building information modeling software) into your favorite design software – take a minute to step back and form a strategy. In what ways is this building like others that you have successfully engineered? How can you efficiently and correctly replicate the processes you have used before? And in what ways is it different? What challenges does this building pose that no other building that you have previously designed did? What makes your stomach tighten just a little because you are not sure you understand (at least not yet) how to handle it? What code provisions need to be applied that make you cringe to think you will have to read, decipher, understand, and apply? Now STOP! Take the parts you do not understand and research or ask someone until you do understand. Establish a plan. Create a vision. Create a process. Then, and only then, can you move forward with an excellent design!
How Deep Does the Software Go? Make sure you understand very clearly where the design of the black box ends and your “hand calcs” start. Did that moment frame get checked for strong-column, weak-beam ratio by the program? Or do you need to do that? Did the program assume the cantilevered beam was braced on both the top and bottom flange? Did the model actually design the perimeter beams for their drag and chord forces – or is that on you? Please, please, please resist the temptation to “assume” the software did the check. If you have not seen the output with your own two eyes for proof that the check was made, you have to assume it was not!
Create Standards As engineers, we create standards for many things – CAD standards, redlining standards, calculation packet standards, etc. But does your firm have modeling standards? A clear and concise recipe for creating frequently used model-types can be a godsend, especially for the less experienced engineer. Maybe a typical single-bay moment frame. A multi-span beam. A truss. A recipe that lays out how to create the proper boundary conditions at supports, end conditions, load cases, etc. Why not take the guesswork out of the process and guarantee more predictable results? It is a lot easier to prevent the error by the use of standards than by finding the error through quality control! STRUCTURE magazine
If it seems wrong, it probably is! Check Your Assumed Conservatism at the Door Are my “conservative assumptions” really conservative? We have all had a good laugh ourselves when contractors try to justify what they did without the blessing of the engineer by saying they “way overbuilt it.” And the reason we laugh is that, almost always, the parts that were “overbuilt” and the parts that will experience high structural demand are almost never the same. Unfortunately, this Achilles Heel also seems to apply significantly to less experienced engineers. The author’s stand is that you cannot say that you are covered by “being conservative” on an element (that you did not calculate) unless you have analyzed it in that exact same condition before and have found it to be, by your own experience, conservative. Yes, it is challenging work being a structural engineer with so many calculations! But the lesson that has to be learned is that if you do not know from actual first-hand experience that it is conservative, buckle down and do the extra calculation and prove it. You may be surprised to find how often you were not as conservative as you thought.
Parting Thoughts Adopting these strategies will help you become a better, wiser, and more accurate engineer. One last thought in parting. A simple but powerful phrase the author often uses around the office is this: “If it seems wrong, it probably is!” This phrase has been coined as a statement to give our engineers permission – even inspiration – to do a double check when things seem just a little off. Its desired effect is to plant the seed of skepticism in the mind of the otherwise confident engineer who has studiously completed the calculations. Does the size of the beam just “feel” too small? Do the wind loads on the parapet seem less than you would expect? Does the depth of the beam and the width of the column look out of proportion now that you see them in the detail, drawn to scale? If it seems wrong, if it feels wrong, it probably is! Give it another look with fresh eyes. Ask a colleague to do a double-check. You will not regret it! Happy error checking!■ Scott N. Jones is a Partner at Wright Engineers in Orange County, California. (sjones@wrightengineers.com) J U N E 2 0 21
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structural SUSTAINABILITY 960 W 7th Street
A New Benchmark for Reducing High-Rise Construction Costs and Carbon Footprints By Donald W. Davies, P.E., S.E., Kelsey Rose Price, P.E., LEED AP BD+C, and Farshad Berahman, Ph.D., C.Eng, MIStructE
H
ow do you successfully design and construct a high-rise residential tower, being mindful of costs while also reducing its environmental impacts? This was a challenge posed to the design team, contractor, and subcontractors behind Brookfield Properties’ 960 W 7th Street project in downtown Los Angeles, California. Currently under construction, the 57-story mixed-use residential tower will include 11 levels of parking above and below grade, street-level retail spaces, and residential amenities on Levels 3, 35, and 57. Upon completion, the 960 W 7th Street tower will rise 614 feet measured to the top of its core. The 960 W 7th Street project in downtown Los Angeles, CA. Courtesy of Brookfield Properties. tower is situated between two existing structures – a parking garage to the west and a retail center to the east and northeast You Can’t Manage What You Don’t Measure – also owned by Brookfield Properties. To successfully deliver this high-rise building, Magnusson Two factors contributed to credibly measuring, reducing, and then Klemencic Associates (MKA) worked with the design and con- reporting the 960 W 7th Street project’s concrete carbon footprint. struction teams to find collaborative solutions that met tight site First, MKA asked for supplier-specific, third-party verified and budget constraints while achieving low-carbon goals – all Environmental Product Declarations (EPDs), which report the without compromising high-quality standards. The team’s collective “Product Stage” embodied carbon of the measured materials. This problem-solving approaches included a double-bottom-line materi- includes the extraction of raw materials, transportation of these als procurement process and a critical look at the supply chain to materials to the manufacturing site, and manufacturing these avoid extra materials and costs. As a result, the project has become materials. EPD collection efforts focused on concrete, rebar, and a benchmark of win-win scenarios for other projects to consider structural steel, which make up the largest carbon footprint of when seeking to reduce both costs and embodied carbon. the building components and represent some of the most effective targets for meaningful embodied-carbon reductions. Second, the project was an early test case for the Embodied Carbon in Construction Calculator (EC3), which was valuable for identifying lower embodied carbon opportunities previously overlooked. EC3 is a growing, free-to-use national database of EPDs that includes a tool for tracking and comparing embodied carbon emissions across construction material suppliers. This allows easy organization of project material quantities and access to EPD carbon emissions data that help compare and reduce overall emissions during the design Often, the structure accounts for the highest embodied carbon. and construction of a project.
STRUCTURE magazine
Incentivizing the Concrete Procurement Process
alternative resources for comparative evaluations. Evaluations included comparing the amount of cement within competing supThe 960 W 7th Street project team started plier mix designs, comparing the carbon with a performance-based concrete mix footprint of those mixes with other similar specification to lower the project’s embodmixes within the EC3 database, and comied carbon footprint. The specification paring the average carbon footprint of the afforded flexibility to both the contractor electrical grid at the rebar mill locations of and the concrete supplier while still defincompeting suppliers. ing critical mix criteria, such as strength, Once MKA had supplier-specific EPDs in shrinkage, modulus of elasticity, and sethand, they could look back at the concrete time for stripping formwork and finishing. mixes initially proposed and compare them As the experts in how to best create their to the final project mixes, which were more material, concrete suppliers were given the carbon optimized, thereby documenting freedom to optimize their unique mixes By minimizing variability, cement content can be reduced. the project’s final design and construction with fewer restrictions. Still, they were reductions. asked to disclose their cost and carbon footprint data prior to bid, Asking for EPDs benefited this project and will impact future projects creating a double-bottom-line competition. within the Los Angeles region that can now take advantage of these Suppliers were informed that the selection process would consider EPDs. It means increased transparency from the local concrete suppliboth cost and carbon levels. Brookfield Properties made it clear that ers and a more straightforward ask when considering supplier-specific cost-effective, low-carbon material procurement was a valued aspect EPDs at early project stages for double-bottom-line comparisons. of this project, creating competition among suppliers to provide the most economical, lowest-carbon concrete available. Winning Concrete Mixes Comparatively, trying to beat preset embodied carbon caps per concrete strength (something considered early on for the specifications but not Asking for EPDs and focusing on embodied carbon led MKA to used) would not have been an incentive to go as low as responsibly pos- early, collaborative discussions with Webcor Builders and National sible and could have led to added cost for requesting something unique. Concrete and an experimentation process to consider new mix options. Instead, contractors and their suppliers were incentivized to find the best MKA noticed each supplier’s “business as usual” mixes had a much approach to lower-carbon concrete while minimizing ownership costs. higher cement content than were seen in other cities for similar mixes This approach also meant the responsibility and liability for the mix within the EC3 database. While MKA did not have the EPD data to performance and construction schedule remained with the construction compare embodied carbon values per mix, looking at the amount of team. Should the mixes not meet specified final strength requirements, cement within each mix provided a good indication of relative carbon responsibility and liability for that outcome clearly remained with the impacts. National Concrete explained some of the challenges of why contractor and their ready-mix supplier, not the owner. this was the case and offered solutions to reduce their mix cement contents if they could adjust their initially proposed mixes. The discussions that followed focused on the local aggregate supply Contractor and Concrete Supplier Selection and its variability. The Los Angeles ready-mix suppliers were compenA thorough and competitive selection process resulted in Webcor sating for this variability with more cement to ensure they achieved Builders being chosen as the 960 W 7th Street project contractor. In addition to showing diligent management of the project’s construction, schedule, and procurement processes, Webcor Builders proposed the early engagement of material suppliers. In addition, they embraced a collaborative relationship with the design team to fine-tune the design for both costs and embodied carbon. One of Webcor Builders’ early moves was to include a request for supplier-specific EPDs from the concrete and rebar suppliers in their bids. As this was a first for the Los Angeles market, it was not surprising that such EPDs were not yet available from most of the bidders. However, by making the delivery of these EPDs a condition of the bid award, the winning concrete supplier, National Ready Mixed Concrete Co., eventually secured these EPDs far in advance of project completion. Since the EPDs were not available for material supplier mix comparisons during the bid selection process, the MKA team had to use Through collaboration, the project team achieved significant carbon savings. J U N E 2 0 21
By creating a temperature-management plan, win-win scenarios were discovered.
the engineer’s design strength requirements. And, since the project’s low-carbon intentions were communicated early enough to give National Concrete time to react, and before finalizing the concrete supply bid awards, a strategy was agreed upon to change the aggregate supply and move to imported and higher quality ‘Orca’ aggregates shipped from the Pacific Northwest. This allowed National Concrete to achieve the same compressive strength (f´c) of their mixes with less overall cement. As a result, the aggregate variability and the mix strength variability were reduced. This was not the first project to use Orca aggregates in Los Angeles. MKA and others have previously encouraged its use for meeting modulus of elasticity and higher concrete strength requirements. But the decision to use Orca aggregates to reduce the 960 W 7th Street project’s carbon footprint was new. It began by asking National Concrete what it could do to support lower-carbon goals, rather than telling National Concrete how to make its concrete. Finding ways to adjust the design criteria and promote collaboration between the design team and the contractor and their supply chain allowed everyone to come out ahead. The carbon footprint of transporting Orca aggregates by barge down the West Coast proved to be a minor carbon impact compared to reducing cement content. This one intervention resulted in a 24% reduction in the project’s total embodied carbon footprint. On the post-tensioned (PT) slab mixes alone, moving to the imported aggregate reduced that mix’s carbon footprint by 47%. Even more impressive, National Concrete achieved this goal at no additional cost to the 960 W 7th Street project. Imported aggregate is more expensive, but using less cement allowed National Concrete to be more cost-competitive, even with the more expensive aggregate. It was the win-win type of strategy the project team was targeting. National Concrete now has on-demand mix design EPDs available for any of their projects. As these are created, they are also digitally uploaded into the EC3 national EPD database.
Hot-Weather Concrete’s Challenge and Solution Another way the project team and MKA collaborated with Webcor Builders and National Concrete involved alternative approaches to STRUCTURE magazine
address hot-weather concrete for the mass concrete foundation pour, which occurred in August 2020 during the sweltering summertime heat. Installation of chiller pipes within the mat to control heat during summer concrete placement was initially proposed. Although this process was used earlier during the construction of a high-rise tower located across the street, it came with a significant price, time detriments, and an embodied-carbon-intensive use of non-recoverable piping and refrigerant cooling. Instead, MKA, Webcor Builders, and National Concrete arrived at a different solution. Step one was a temperature-management plan that included scenario planning for what might occur during the mat pouring and curing. Next, they established criteria for controlled use of ice within the mixes based upon temperatures on the day of the pour. Finally, an early mix-testing program evaluated lower cement mixes and the strength-gain impacts due to heat. In-situ mat thermocouples and pre-established in-situ cylinders for concrete strength testing validation, cured and then extracted from within the mat, were also included. The most valuable strategy for eliminating the chilled piping system from within the mat involved evaluating each step of the material supply-and-delivery chain and looking for opportunities. Ultimately, nature lent a hand, as the team relied upon marine air to cool the materials prior to mixing them at the batch plant. The batch plant was inland and in a much hotter location than the port, where the imported aggregates were first unloaded after being barged down the West Coast. By changing where the aggregates were stored (specifically, leaving them at the port instead of the batch plant until just before use) and under a covered area out of the sun, the batched concrete’s temperature was lowered by 15 to 20 degrees. It was enough to eliminate the need for the chilled piping system, thereby reducing the mat cooling costs by two-thirds.
Conclusion The lower carbon concrete mix strategies presented in this article prevented approximately 13,650 metric tons of verifiable equivalent carbon dioxide from being released into the atmosphere during the project’s construction, based on comparing the first bid to final construction choices. That savings is equal to released carbon dioxide associated with 42,971 barrels of crude oil, or 2,962 vehicles not being on the road for one year. Low-carbon alternatives and cost savings in high-rise construction do not have to be independent objectives. To achieve both, it takes an owner who incentivizes double-bottom-line decision-making and a collaborative engineer, contractor, and material supplier to step across traditional industry silos and agree to work together for common goals. When completed, the 960 W 7th Street project will be an exciting new addition to Los Angeles’ urban core and an example of what is possible when teams agree to collaborate.■ All authors are with Magnusson Klemencic Associates. Donald W. Davies is President. (ddavies@mka.com) Kelsey Rose Price is an Associate. (kprice@mka.com) Farshad Berahman is a Principal. (fberahman@mka.com)
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as the primary lateral force-resisting system. While special reinforced concrete moment frames can be designed prescriptively per ASCE 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Multnomah County sought enhanced seismic performance beyond that achieved by the building’s Risk Category III code designation. Specifically, the owner and design team selected serviceable performance (i.e., essentially elastic) for a 43-year return period earthquake, repairable performance for a 475-year return period earthquake, and a reduced probability of collapse (compared to a standard code building) for the Risk-Targeted Maximum Considered Earthquake (MCER). The 43-year and MCER hazards, and their corresponding performance objectives, are standard criteria in the Pacific Earthquake Engineering Research Center’s (PEER) Tall Buildings Initiative (TBI) Guidelines for Performance-Based Seismic Design of Tall Buildings and the Los Angeles Tall Buildings Structural Design Council’s (LATBSDC) An Alternative Procedure for Seismic Analysis and Design of Tall Buildings Located in the Los Angeles Region. The additional repairable performance objective for a 475-year return period event included limiting inelastic actions (e.g., moment frame beam plastic rotations) to Immediate Occupancy thresholds. Most importantly, it also imposed a stringent residual Figure 1. The completed Multnomah County Central Courthouse. Courtesy of Michael Talbot. drift criterion based on FEMA P-58, Seismic Performance Assessment of Buildings. The nonlinear response history analyses in PERFORM-3D illustrated a weak story deficiency in the prescriptive design procedures for tall concrete moment frames. Strong columnweak beam requirements By Reid Zimmerman, P.E., S.E. in ACI 318-14, Building he new Multnomah County Central Courthouse is an approx- Code Requirements for Structural Concrete, and prohibition of cerimately 300-foot-tall concrete building located in Portland, tain ASCE 7-16 vertical irregularities (i.e., Discontinuity in Lateral Oregon, completed in the summer of 2020 (Figure 1). It includes Strength – Weak and Extreme Weak Story Irregularities) are intended courtrooms, judicial offices, security facilities, and expanded public to control weak stories. However, they do not appear sufficient for spaces. Built to serve as the new central courthouse for Multnomah taller concrete moment frames. Notably, even when each story below County and to replace the historic courthouse several blocks away, is stronger than the one immediately above and strong column-weak the owner and design team selected enhanced, multi-objective seis- beam checks are satisfied, a weak multi-story mechanism can still mic performance goals for the building. Nonlinear response history form (Figure 2). For the Multnomah County Central Courthouse, analyses revealed the unique characteristic of multi-story mechanism the design team suppressed the weak multi-story mechanism through formation in tall concrete moment frames like those in the Multnomah a combination of tuning the story strength profile versus height and County Central Courthouse. The performance-based seismic design utilizing fluid viscous dampers. Interestingly, the team discovered that then leveraged fluid viscous dampers to improve that performance. commonly practiced design decisions (e.g., grouping beam reinforcing Finally, a partnership between the owner, design team, contractor, for detailing simplicity, increasing beam sizes only at certain levels to Oregon Department of Geology and Mineral Industries, and the satisfy wind demands, etc.) could lead story strengths to significantly U.S. Geological Survey succeeded in seismically instrumenting the exceed earthquake demands in certain multi-story bands. Although building with a dense array of accelerometers. intuitive in hindsight, the best performing design was where the story shear earthquake demand-to-capacity ratio was approximately equal at all stories, thus more uniformly distributing inelasticity. Concrete Moment Frames It is also important to note that a weak multi-story mechanism is further The Multnomah County Central Courthouse’s architectural layout exacerbated by ratcheting (i.e., an increase in unrecoverable drift in one did not provide for locations of solid lateral force-resisting system direction with each cycle) due to P-Delta effects under moderate to large elements (e.g., shear walls or braced frames) that stack vertically at story drifts. The fluid viscous dampers were integral in reducing drifts every floor. Additionally, the design team sought to balance the center and therefore preventing P-Delta ratcheting. The damping provided by of rigidity with the center of mass by providing lateral force-resisting the fluid viscous dampers could be visually seen in the nonlinear response system elements in both legs of the L-shaped plan. These constraints history analyses where a “wave” of building deformation due to ground led the team to choose special reinforced concrete moment frames shaking would be gradually damped out as it moved up the structure.
Moment Frames, Fluid Viscous Dampers, and Seismic Instrumentation The Story of the New Multnomah County Central Courthouse
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STRUCTURE magazine
ITT Enidine, confirmed the damper design against this specification, including 100% testing of all devices supplied for the project. KPFF also determined the minimum damper stroke and force capacity based on the greater of the suite maximum and amplified suite mean demand aligned with ASCE 7-16 Chapter 16 criteria, the project’s basis of design, and the peer review team.
Fluid Viscous Dampers
Fluid viscous dampers are seismic protective devices very similar to an automobile’s front struts. Or, if you still remember what it was like to fly on an airplane, the tiny pistons that slowly open the overhead baggage compartments. In more technical terms, fluid viscous dampers are devices that exhibit a force-velocity relationship and minimal force-displacement response. The design team chose fluid viscous Seismic Instrumentation dampers for the Multnomah County Central Courthouse because they dissipate earthquake The Oregon Structural Specialty Code (OSSC), input energy in a manner that minimizes the the governing building code in Oregon, increase in force/acceleration experienced by requires buildings over certain height and floor the building. They also do so without sustaining area combinations to be seismically instrudamage. See Figure 3 (page 28) for construction mented. However, these provisions require the photos of the fluid viscous dampers. minimal instrumentation of one acceleromASCE 7-16 introduced the concept of eter location at each of the “lowest floor level, property modification factors for designing midportion, and near the top of the building.” structures with damping systems. Property Additionally, with only one location per floor, modification factors represent the variation torsion cannot be measured. of expected properties for effects that may Figure 2. Weak multi-story mechanism in a Instrumentation maintenance is left to the impact the damping devices over their design tall moment frame. building owner with practically no city or life (e.g., manufacturing variability, dynamic state agency oversight. As a result, many cycling, aging, etc.). For the fluid viscous dampers in the Multnomah buildings in Oregon that had instrumentation installed based on County Central Courthouse, KPFF specified the property modifi- this code requirement now have unconnected, damaged, or misscation factors and testing by which to establish them. KPFF then ing accelerometers. Unfortunately, when an earthquake does occur, conducted upper- and lower-bound nonlinear response history analy- these buildings will not record the valuable data for which this code ses in PERFORM-3D based on these limits. The damper supplier, requirement exists. continued on next page
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a)
b)
Figure 3. (a) Fluid viscous damper installation, and (b) completed braced frames with two fluid viscous dampers per V-configuration shown at floor level.
The design team brought this reality to the attention of Multnomah County. They then helped connect the County with the Oregon Department of Geology and Mineral Industries (DOGAMI) and the U.S. Geological Survey (USGS). These groups collaborated to instrument the Multnomah County Central Courthouse with 21 accelerometers manufactured by Kinemetrics to measure orthogonal translations and torsion of approximately every third floor, plus vertical at the base. Figure 4a shows two of the installed accelerometers. For perspective, the code minimum instrumentation layout would only have required six accelerometers. DOGAMI provided funding to offset the additional costs above the code minimum system from a fund paid into by projects that take exception to the OSSC instrumentation requirement. USGS performed the installation, connected the system to their real-time National Strong Motion Network, and contracted with Multnomah County to perform future maintenance. After an earthquake, recorded data will be transmitted to USGS within minutes. In the event of loss of power during the earthquake, the building’s uninterruptible power supply system keeps the accelerometers and data collector running. Redundant local backup batteries also exist. If the belt and suspenders fail, the data is stored locally for in-person retrieval. At the time of this writing, the Multnomah County Central Courthouse has not experienced a significant earthquake. However, the instrumentation was functioning during the Mw 5.8 Lone Pine earthquake in eastern California on June 24, 2020. Although the epicenter was almost 700 miles away, the Multnomah County Central Courthouse sensors recorded the event (Figure 4b). This proved to be a valuable opportunity to test the proper functioning of the instrumentation system and double-check the accelerometer installation.
County Central Courthouse design required more communication and technical effort than a prescriptively-designed code-minimum building. However, the advantages of this hard work by the owner, design team, contractor, and collaborators are evident in the final design, contributing to a more resilient Multnomah County for the future.■ Reid Zimmerman is the Technical Director for the Portland, Oregon office of KPFF. (reid.zimmerman@kpff.com)
a)
b)
Conclusion As the first new building in Oregon to utilize fluid viscous dampers and the first performance-based design of a new tall building in the City of Portland, it should be acknowledged that the Multnomah
Project Team Owner: Multnomah County Structural Engineer: KPFF, Portland, OR Architects: SRG Partnership, Portland, OR, and RicciGreene Architects, New York, NY Geotechnical Engineer: GeoDesign Inc., Wilsonville, OR Contractor: Hoffman Construction, Portland, OR Fluid Viscous Damper Supplier: ITT Enidine, Orchard Park, NY STRUCTURE magazine
Figure 4. (a) Installed accelerometers with protective cover temporarily removed, and (b) recorded accelerations from Lone Pine earthquake.
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he dramatic new 41-story W Hotel Tower features luxury on a grand scale. Located in the heart of downtown Bellevue, Washington, within several miles of the headquarters of Microsoft, Amazon, and other global companies, it includes 245 hotel rooms and 231 upscale apartments atop a three-level podium. The podium includes boutique shops, various themed restaurants, a state-of-the-art IMAX theatre, and other recreational amenities for the public, apartment dwellers, and hotel guests. In addition, the six-level subterranean parking structure provides 2,200 spaces and connects to the adjacent Lincoln Square North via a tunnel, creating a massive parking network for over 4,000 vehicles. The structure strategically incorporated many value-added design features, including the world’s most extensive use of a recent structural innovation – Steel Fiber Reinforced Concrete (SFRC) coupling beams. The building utilizes cast-in-place concrete throughout. Slabs are 8-inch-thick post-tensioned flat plates, and columns are 24 × 36 inches at the base, reducing to 24 × 30 inches at upper levels. The seismic and wind force resisting system uses 36-inch-thick shear walls around W Hotel Tower in construction. the core. The foundation is a 10-footthick mat covering the building footprint, bearing on 12,000 psf undisturbed glacial till.
W
BELLEVUE’S INNOVATIVE NEW
Hotel Tower By Cary Kopczynski, P.E., S.E., FACI, FPTI, and Mark Whiteley, P.E., S.E
SFRC link beams.
STRUCTURE magazine
Materials The structure effectively utilized high-strength, high-performance concrete. Columns have a compressive strength of 14,000 psi at the base, reducing to 6,000 psi at upper levels. Shear walls are 8,500 psi, slabs are 6,000 psi, and the foundation mat 5,000 psi. All mixes used blends of cement and Supplementary Cementitious Material (SCM). The foundation mix included minimal cement and high SCM to minimize the heat of hydration. Other mixes used varying quantities of SCMs depending on short and long-term performance requirements. Slab mixes were designed to a maximum drying shrinkage of 0.030% at 28 days to minimize shortening. Concrete strengths of 10,000 psi and higher were specified at 90 days to allow added curing time.
Innovative Seismic Design The seismic system utilized Performance-Based Design (PBD). The use of PBD allowed optimization of seismic energy, forces, and deformations. This system also resulted in shear walls with ductile behavior, a key to good seismic response. Shear walls included an innovative seismic coupling beam design. Instead of traditional diagonal bars, the beams successfully used Steel Fiber Reinforced Concrete (SFRC). In addition to reducing coupling beam rebar by approximately forty percent, SFRC created significant constructability improvements. The lack of diagonal bars eliminated the conflicts and congestion that are often created between diagonal bars and the adjacent boundary element reinforcing. Also, whereas diagonal bars are often field placed, cumbersome, and time-consuming, horizontal bars in SFRC coupling beams can be pre-tied. This accelerates rebar placement and overall shear wall construction.
The SFRC coupling beam design used methodology developed from research performed at the Universities of Michigan and Wisconsin. Fiber dosage, using a commercially available steel fiber product, was 200# per cubic yard. In the future, fiber quantities can be reduced to 170# per cubic yard based on recent tests showing that coupling beam performance at the reduced dosage is substantially unchanged. A hidden benefit of SFRC coupling beams is that penetrations for utilities are much easier to accommodate. Since the concrete shear capacity increases from 3 to 4√fc´, depending on fiber dosage, stirrup quantities can be reduced and the spacing widened. For example, at the W Hotel, stirrup spacing was 5 inches on-center, which allowed 4-inch-diameter penetrations for plumbing, conduit, and fire protection lines. Penetrations larger than 2 inches through traditional diagonally reinforced coupling beams are seldom possible due to heavy rebar congestion. This was only the third use of SFRC for seismic coupling beams to date, all by the same structural engineering team, and the most extensive use anywhere in North America. As with other recent projects where they have been used, SFRC seismic coupling beams were well received by the contractor, who credited them with reduced cost and accelerated construction.
High-Strength Columns
Elimination of Slab Puddling A significant challenge in high-rise concrete construction is the transfer of high-strength concrete column loads through slabs, often of much lower strength. Traditionally, the high-strength column concrete is “puddled” into the slabs around the columns immediately in advance of the slab pours. Then, as the slab is poured, slab concrete is intermixed with the column concrete that has been puddled in advance. Although effective when properly executed, this is cumbersome and often results in unintended slab cold joints between the two mixes.
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Columns utilized 14,000 psi compressive strength concrete at the base, transitioning to 6,000 psi at the top of the building. This allowed column sizes to be maintained constant throughout the building height, which accelerated construction, reduced cost, and eliminated the need to modify room layouts for varying column sizes. The high-strength concrete in lower levels was specified with ½-inch maximum aggregate and a superplasticizer to facilitate concrete placement through the heavy reinforcing cages.
Steel Fiber Reinforced Concrete (SFRC).
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W Hotel Tower floor slabs.
High strength column concrete placement at slab.
A unique detail was used to eliminate slab puddling. It involved blocking out column cores with a metal screen prior to slab concrete placement, then pouring the columns above through the blocked-out slab areas. This created full column continuity, eliminating the need for puddling.
requirements, contingent upon demonstrating that the proposed design meets code-intended seismic performance. This was accomplished by generating a mathematical structural analysis model used to perform non-linear analyses while considering the stiffness, ductility, and strength of critical structural elements. The project's geotechnical engineer developed seven pairs of site-specific ground motions by matching the source, magnitude, frequency, and duration of the risk-targeted maximum considered earthquake (MCER) spectra, which corresponds to an earthquake with an approximate 2,000-year return period for the project location. Earthquakes from Chile (2010), Tohoku, Japan (2011), and Olympia, Washington (1949) were among the base ground motions used. Typically, a building in the greater Seattle area using a code-prescribed seismic design approach would consider a design earthquake (DE) with roughly a 475-year return period. The project’s performance goals were to evaluate collapse prevention at the larger MCER ground motion, life safety at DE level forces, and to remain essentially elastic during a service level earthquake (SLE) event, with a 43-year return period. The W Hotel Tower is an excellent example of how contemporary architecture and state-of-the-art engineering can be harmoniously combined into exceptional urban buildings. The structural features helped achieve the project’s goals of delivering a quality project on time, on budget, and to the satisfaction of all stakeholders.■
High-Strength Rebar A706 Grade 80 rebar was used in the foundations and all shear walls. This resulted in significant cost savings since the ten percent price premium for Grade 80 vs. Grade 60 was more than offset by the twenty to twenty-five percent reduction in foundation and shear wall rebar tonnage. Grade 80 was also used for all column ties to satisfy the demanding seismic confinement reinforcing requirements for ultra-high-strength concrete. The synergistic combination of Grade 80 and A615 Grade 60, which was used elsewhere, resulted in construction and cost efficiencies throughout the project.
Long-Span Garage Framing Parking beneath high-rise towers is typically framed with short-span systems that match or approximate the framing above. For the W Hotel Tower, parking was framed with a 30- x 50-foot system to allow the use of an angled parking layout, free of columns at the rear bumpers of stalls. An efficient combination of transfer beams and slant columns created the transfer from the parking column grid below to the tower grid above. In addition to the parking improvements, the placement of shotcrete basement walls followed behind the subterranean slab construction, which allowed the superstructure to advance at greater speed.
Performance-Based Design Since the selected lateral force resisting system of Special Reinforced Concrete Shear Walls for the W Hotel was limited to a maximum structural height of 240 feet per ASCE/SEI 7-10, Minimum Design Loads for Buildings and Other Structures, a peer-reviewed PerformanceBased Design (PBD) approach was necessary. PBD is a methodology for creating acceptable alternates to prescriptive building code
STRUCTURE magazine
Cary Kopczynski is CEO and Senior Principal of Cary Kopczynski & Company (CKC). Mr. Kopczynski is President of the American Concrete Institute (ACI). (caryk@ckcps.com) Mark Whiteley is a Senior Principal at CKC. (markw@ckcps.com)
Project Team Owner/Developer: Kemper Development Company Structural Engineer: Cary Kopczynski & Company Architect: HKS Architects / Sclater Architects Contractor: GLY Construction Geotechnical Engineer: Hart Crowser Peer Review: Maffei Structural Engineering
100
Mount Street By Benton Johnson, P.E., S.E.
Benton Johnson, P.E., S.E.
T
he latest addition to North Sydney’s growing business district is 100 Mount Street. The design of the building by SOM and Architectus takes advantage of panoramic views of Sydney Harbour with an integrated approach to architecture and structural engineering. Tenant views are maximized with an offset core and 20-foot column-free cantilevered zones along the North and South face of the building. The offset core is balanced by an innovative composite mega-bracing system which also creates the architectural identity of the project. The building’s state-of-the-art structure was engineered by SOM and Enstruct, the engineer of record. Refer to Figure 1 for typical floor plans. The project was developed by Dexus and Laing O’Rourke, who also served as the main contractor for the project. The developercontractor-led project sought to embed efficient and constructible solutions throughout the building. These guiding principles resulted in the exposed structural concrete core, exposed concrete, and steel mega-bracing system, and soaring closed cavity curtain wall system of the building design appropriate for the mild climate of Sydney. Spring Street bounds the building to the North, Walker Street to the East, Mount Street to the South, and neighboring 80 Mount Street
Figure 2. Aerial construction shot of leading western core and following mega-brace. Courtesy of Ethan Rohloff.
STRUCTURE magazine
Figure 1. Typical low-rise (top) and high-rise (bottom) plans.
to the West. The design response to the site was to locate the building core immediately adjacent to 80 Mount Street, providing unobstructed views and street access in the remaining three directions. The offset core design of the building enhances the tenant experience and connection to the urban environment and poses a structural challenge under wind loading. Seismic activity is not significant in this region and is not a controlling design condition for buildings with regular proportions.
Figure 3. Eastern bracing aligned to floors at 2/3 r d point. Courtesy of Brett Boardman.
North Sydney is situated atop a sandstone plateau over the harbor, approximately 300 feet above sea level. The natural rise in elevation creates an escarpment, a terrain condition that increases wind speeds at the elevated site. This factor, combined with the relatively taller 100 Mount Street compared to surrounding buildings, caused an increase in wind loads as discovered by wind tunnel testing. The offset core design would have led to objectionable torsional building movements if left unchecked. This design challenge led to the inclusion of the exposed mega-bracing system, which balances the offset core and allows the architectural intent of the building to be achieved. Refer to Figure 2 for a construction photograph of the main building systems and immediate building surroundings. The typical floors are framed with bonded post-tensioned concrete band beams and floor decks. The wide/shallow band beams are spaced at 30 feet on-center and span 55 feet to concrete columns and cantilever 20 feet beyond for unobstructed harbor views. The band beams are notched at inflection points for routing of building services, an integrated solution that allows the Figure 4. Concealed sliding central node details. typical floor-to-floor height of the building to be 13 feet 4 inches. The typical framing is supported the interior floor area to be captured as leasable tenant space as by just 10 vertical high-strength 14,000 psi concrete columns. The elevator banks drop off higher up in the building. This approach is offset concrete core to the West both supports the floors and provides highly efficient from a leasing perspective but results in a core with resistance to lateral loading. The design of the core includes elevator marginal torsional resistance. An efficient core structure required cabs that open directly into the floor plates. The core design allows a supplementary lateral system on the East face of the building to balance the inherent torsion and potential movements from eccentric wind loads. The supplementary lateral system also minimized the potential for wind uplift and deep tie-down foundations below the core, which were problematic due to a future tunnel easement near the site. The supplementary lateral load resisting system required high lateral stiffness to balance eccentric wind loading and possible uplift on the core. Mega-bracing of the Eastern concrete columns was selected over a moment frame with tightly spaced columns because of the high stiffness requirements and to minimize obstructions to valuable harbor views. Structural steel was chosen as the bracing material to minimize brace sizes and simplify erection and other construction issues. Steel mega-bracing of the concrete columns forms a composite bracing system, adding design complexity from concrete creep and shrinkage and thermal effects. The mega-bracing was initially proposed to be a chevron pattern or “k-bracing” to mitigate the effects of the composite system. The initial bracing configuration is less stiff than an “x-bracing” and also has Figure 5. View of eastern mega bracing from ground level of Walker Street. Courtesy of Brett Boardman. longer unbraced diagonals which would increase steel J U N E 2 0 21
tonnages. The design team explored alternative bracing patterns and relied upon internal research and typology optimization to select the final pattern – an x-braced frame with a raised central node. The most efficient bracing pattern is an x-brace with the central node located at 3⁄4 of the bracing module height. The bracing at 100 Mount Street is located at the 2⁄3rd module point to align with a floor level and is approximately 10% stiffer than an x-brace. This system was selected to maximize the supplementary support offered to the core. Refer to Figure 3 (page 35) for an elevation photograph showing the x-bracing with central node at the 2⁄3rd point. The mega-bracing geometry is highly efficient in achieving the engineering goals but could resist long-term creep and shrinkage
movements of the concrete columns. However, this self-stressing behavior causes compression loading in the braces, increasing the demands on the bracing by approximately 250% compared to required wind loads. Designing for these increased loads would be inefficient in material consumption and increase brace member sizes which block valuable harbor views. Alternatively, the design team developed a central node connection with an internal sliding mechanism that resists the necessary wind loads but relieves the creep and shrinkage strains. The concealed central sliding node consists of steel finger plates and brackets sandwiching elastomeric bearing pads. Elastomeric bearings are a common structural engineering component that provides vertical support while allowing lateral movement, most common in bridge structures or expansion joints in buildings. The sliding movement is accommodated by the low shear stiffness of rubber components within the bearings while maintaining vertical contact and support between elements. The central sliding node sandwiches elastomeric bearing pads between steel bearing brackets to resist wind load reversals through compression, either in the top or bottom bearing assembly. The size of the bearings and gap between steel node elements were designed to accommodate movements from elastic column shortening, long-term creep and shrinkage, Use for all types of concrete and grout applications, from slabs-on-grade to floor post-tensioning, differential thercontainment tanks, multi-story post-tension structures to bridge decks. mal movements, and other effects. The bearings can be observed in service with removable closure plates and replaced if ADVANTAGES necessary. The result of the design is a ¡ Maximize joint spacing (up to 300 ft, L/W 3:1) ¡ Enhance compressive and flexural strengths concealed node that minimizes loading and resulting sizes of the steel bracing. ¡ Prevent shrinkage cracking and curling ¡ Eliminate pour/delay strips Refer to Figure 4 (page 35) for a diagram ¡ Thinner slabs and walls viable ¡ Reduce long-term relaxation of P/T tendons of the concealed central sliding node and shear wall stresses and Figure 5 (page 35) for a photograph ¡ Reduce reinforcement requirements of the final bracing design and architec¡ Minimize creep and moment ¡ Improve durability and lower permeability tural appearance. ¡ Minimize waterstops 100 Mount Street is a project built on ¡ Increase abrasion resistance 30-40% efficiencies, from the core design and floor leasing to engineering systems and constructability. The building’s architectural design embraces efficient design and proudly displays the composite megabracing as the defining architectural feature of the building. These achievements were made possible by the close collaboration of the design and construction teams striving toward a successful project that embodies state-of-the-art building systems while enhancing the tenant experience and by CTS Cement Manufacturing Corp. urban environment.■
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Benton Johnson is a Director of Structural Engineering for Skidmore, Owings & Merrill in Chicago. (benton.johnson@som.com)
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The David H. Koch Center for Cancer Care Reimagining Resiliency in Healthcare Design By Andrew Altamirano, P.E.
T
he oldest and largest private care institution of its kind in the world, Memorial Sloan Kettering Cancer Center (MSK) has added a high-tech new facility to its roster of treatment centers as well as an architecturally striking building to New York City’s skyline. Opened in January 2020, the David H. Koch Center for Cancer Care towers over the East River on Manhattan’s Upper East Side. The 25-story ambulatory care center is staffed with more than 1,300 clinicians, nurses, and support personnel working A Holistic Approach to Resiliency at the forefront of patient care and treatment. Measuring at 750,000 square feet and 450 feet tall, the building employs some truly impressive The 1.5-acre site is located directly adjacent to FDR Drive and the East design features, which helped create a comfortable patient experience, River and within the FEMA 100-year flood plain. As the building’s design enabled large flexible spaces that support cutting-edge and often hefty progressed in earnest in 2012, New York City was inundated with high equipment, and resulted in one of the most resilient medical facilities winds and a record storm surge from Superstorm Sandy, which starkly anywhere. revealed the region’s vulnerability to future The building’s massing is divided into storms and rising sea levels. The design team smaller volumes to fit into its tight urban learned from the significant effects other health setting, with the organization of each volume care facilities and critical infrastructure experirepresenting different programmatic funcenced during Sandy and its aftermath, including tions. For example, imaging and diagnostic total power loss and damage to below-grade facilities are located on the lower floors. building systems. This knowledge aided the Patient treatment space is in the middle/ team in developing a comprehensive resiliency largest segment of floors, and offices and approach for protecting the facility from the mechanical systems are on the upper floors. A effects of a 500-year flood event, up to 7 feet terracotta-and-glass façade provides patients above the adjacent FDR Drive. and staff plentiful views of the East River Detail of shear wall with embedded steel frame. The building’s design takes two congruous while creating a warm interior environment paths to achieve resiliency: providing sevwith abundant natural light. eral layers of protection for spaces below the The building’s vertical load-carrying strucdesign flood elevation (DFE) while locating tural system consists of concrete on metal deck all critical mechanical and medical systems with composite steel beams supported by steel higher up in the building above the DFE. The columns. Cast-in-place concrete shear walls foundation consists of a continuous 6-foot provide lateral resistance from the base up to concrete mat foundation designed to resist the fifth floor, transitioning to steel braced over 40 feet of hydrostatic uplift (over 2,600 frames above. The shear walls provided both psf of equivalent hydrostatic pressure) while additional stiffness to control wind drift in the simultaneously transferring the building’s tower and increased lateral strength for large weight to bedrock. In addition to traditional seismic forces generated by the heavy loads subgrade foundation waterproofing, the conon the building’s lower floors. The steel frame crete mix design for the foundation mat and was erected first, with temporary steel braced walls included waterproofing concrete admixframes erected at the lower levels and shear tures, which reduce concrete permeability – a studs later cast into the permanent concrete “belt and suspenders” approach to keeping the shear walls. This approach allowed steel to below-grade areas protected. At grade, operarrive onsite earlier and reduced the overall able flood barriers built into the building’s construction schedule while also allowing the foundation walls protect the main patient embedded steel columns to serve as part of the entrance, while drop-down flood barriers longitudinal reinforcement for the concrete shield the service entrance. To provide addishear walls. The embedded diagonal braces tional protection against any adverse water were left in place or field cut as needed for intrusion, all structural steel below the DFE door and mechanical penetrations once the is hot-dipped galvanized, and all concrete concrete walls reached their design strength. Detail of foundation wall with operable flood barrier. reinforcement is epoxy-coated.
STRUCTURE magazine
Holding Up Weighty Systems Supporting the heavy mechanical and medical equipment in the floors above the DFE presented several structural design challenges. Four mechanical floors provide the critical infrastructure to sustain the center, including onsite Con Edison electrical transformer vaults powered by concrete-encased high-voltage conduit from the street. In addition, a pair of two-story-tall rooftop emergency generators provide redundancy to keep building services online in the event of a power outage. The most significant challenge involved supporting linear accelerators (LINACs), typically located at-grade above the DFE. Linear accelerators are an innovative, targeted treatment approach that focuses high-energy subatomic particles on destroying cancer cells while preserving the surrounding tissue. The accelerators sit inside vaults consisting of five-foot-thick walls, which consist of 2 feet of concrete with 1 foot of lead shielding in between. In total, more than 250 tons of lead blocks were used to protect building occupants from radiation emitted from the treatment. In addition, the linear accelerator vaults are located over the building’s loading dock, creating further challenges. Full-story transfer trusses, spanning 60 feet and weighing more than 120 tons each, support these vaults. This results in the cumulative weight of 22 floors of structure above and, hanging below, two MEP floors that include the Con Edison transformer vaults. Due to building code requirements, a 25,000-gallon fuel tank supplying the rooftop generators is the only mechanical system housed in the basement below the DFE. The tank was surrounded by a waterproof concrete enclosure, with submarine-style flood-proof access doors to keep it protected from flooding, which was a widespread issue seen after Superstorm Sandy.
Open, Versatile, and Inviting Patient Spaces The design team employed multiple structural strategies to support the building’s signature setbacks and overhangs on the north, east, and south façades. In the lower and upper blocks, diagonal steel hangers were located within mechanical spaces to efficiently support the heavily loaded cantilevers. However, diagonal hangers compromise the open floor plates needed to achieve the architectural vision on the patient floors. For the patient floors and other publicly accessible spaces, where the building edge cantilevers up to 25 feet beyond the main support columns, the design incorporates multi-story Vierendeel steel moment frames, which provide the desired open space free of exposed structure. Compared to cantilevered beams, Vierendeel frames also offer increased stiffness to meet strict floor vibration limits and help control inter-story vertical deflections, thus reducing the number of façade movement joints.
An illustration of LINAC vaults supported on transfer trusses.
The building’s setbacks create outdoor terraces, including a public green roof and sculpture garden on the sixth floor, which help enhance the patient experience. This required close collaboration with the project landscape architect and curators to develop structural design criteria to support the planters, trees, and sculptures. Sculpture footings and planter retaining walls were designed as isolated elements not directly connected to the structural slab below, providing an uninterrupted waterproofing membrane and rigid insulation between the terrace elements and the building’s envelope below. The building setbacks also required innovative approaches to façade maintenance. Solutions included structural support for hung monorail systems underneath the building setbacks and a large building maintenance unit (BMU) located atop the rooftop screen wall, which transverses on rails and extends its boom outward to a 100-foot maximum reach.
Summary With a warm, comforting environment and state-of-the-art structural design, the David H. Koch Center for Cancer Care at Memorial Sloan Kettering Cancer Center provides patients with innovative treatments since opening last year. The steel-frame structure enables large, open patient spaces while supporting heavy medical and mechanical systems. The building’s holistic approach to resiliency employs multiple flood-resistance strategies, including operable flood barriers, concrete waterproofing admixtures, and steel galvanizing. At the same time, careful design and placement of critical infrastructure provide continuity of service. The center’s flexible structural design will help it provide leading innovation in cancer treatment for decades to come.■ Andrew Altamirano is an Associate with Thornton Tomasetti. (aaltamirano@thorntontomasetti.com)
Project Team
A close-up of a building setback with monorails incorporated into the soffit.
Project Management: MSK Design+Construction Architectural Design: Perkins Eastman Architects and Ennead Architects Structural Engineer: Thornton Tomasetti Construction Manager: Turner Construction J U N E 2 0 21
39
Change of Plans
Construction phase.
Late Game Pivot by Project Team A Big Win for Denver Health By Tim Carroll, P.E., and Maryann Davis, P.E.
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hen Denver Health needed a new Outpatient Medical Center (OMC) to augment their flagship hospital, they brought the design team, including architecture firm (HKS), the structural engineer (S.A. Miro, Inc.), and general contractor (Turner Construction) on board at the same time. In conjunction with the owner and design team, Turner Construction selected several key trade partners using a Target Value Design procurement method during Schematic Design; amongst them, Drake-Williams Steel (Steel Fabricator) and LPR Construction (Erector). This method fostered an atmosphere of collaboration that promoted greater involvement by the contractor and subcontractors to inform the design to meet the owner’s needs and, perhaps more importantly, the budget. Founded in 1860 and known previously as Denver General, Denver Health provides medical care to an estimated one-third of Denver’s population. After a feasibility study found that the city’s needs would exceed the current hospital’s capacity, Denver Health opted to expand the downtown campus. A new Outpatient Medical Center was proposed to expand outpatient services, including a day surgery center, pharmacy, lab, and radiology services. The construction of the seven-story, 293,000-square-foot structure gives the hospital the ability to free up space around the campus to increase capacity for other primary care services. The existing site included a street that, while it was planned to be closed, had underground utilities, which would have been cost-prohibitive to relocate. The building needed to bridge across this street without needing foundations in this congested area.
STRUCTURE magazine
With the design and construction team in place early, each partner was encouraged by the owner and Turner Construction to look for and propose potential innovations to aid the project. Early in the process, DrakeWilliams suggested that SidePlate, a patented field-bolted Moment Frame Design, might be an innovative lateral system for the new OMC. The preferred lateral system in the Denver market for this type of building has traditionally been concrete core shear walls. Despite the team recognizing the potential benefits, a decision was made to proceed with the more familiar concrete core system. During the Design Development phase, the team realized that they needed to reduce the project cost to meet the budget and condense the schedule. This is where the collaborative nature of the team paid off. The extended team met weekly to review the progress of the design and track the budget. The team quickly keyed in on the lateral system as an area of potential savings and revisited possible alternate systems. The team did a comprehensive review, with all affected stakeholders taking part. In addition to the concrete cores that were currently incorporated into the design, the team reviewed conventional welded moment frames, non-proprietary bolted moment frames, and SidePlate.
Lateral System Investigation Miro worked with the engineers at SidePlate to carry out a conceptual review of the building to determine if the structure could utilize this system and to identify any potential cost and schedule benefits for the project. Upon review of the SidePlate option, the team discovered the following benefits: • Cost and Schedule Savings: The labor and material associated with concrete core shear walls were proving costly and timeconsuming. Instead, using a moment frame system allowed shaft walls to be used at the elevator and stair enclosures, saving time and money. Also, by eliminating the cast-in-place concrete core walls, the steel commenced immediately after the foundations were completed, saving weeks in the construction schedule. • Future Flexibility for the Building Owner: Concrete core shear walls or braced frames limit the ability of owners to reconfigure the space of their building to meet future needs. A moment frame system allows for an open floor plan that can remain flexible throughout the life of the building. • Quality Control: Connections between the steel frames and the concrete cores, which can cause difficulty in coordination between trades, were eliminated. With the SidePlate system, precision work is done in the shop with standard plate and angle construction, which meant easy fit-up in the field. In addition, inspection of the welding is performed in the shop and is more accessible and less hazardous than if performed in the field.
• Minimizing Weather Impacts: The construction schedule had the steel fabrication and erection occurring during the winter months. In Colorado, huge swings in the weather are common, and the risk of construction schedule delays due to weather is high. The SidePlate field-bolted system uses fillet welds in the shop, where material can be rotated so that the welds are placed horizontally while in a temperature-controlled environment and with welders working on the ground rather than astride a beam in the air. Having a field-bolted solution minimized the potential schedule delays due to the weather. • Minimized Steel Material: The SidePlate connection provides a stiffer joint and beam-end than conventional non-proprietary field welded or field bolted moment frame connections, which results in reduced steel material in the frame beams and columns and potentially reduced the number of moment frame connections. This reduction allowed the design to be a more economical alternative to the concrete core wall system than a conventional moment frame design. In the final analysis, Turner and their partners realized an estimated $500,000 in cost savings and shortened the construction schedule by six weeks when switching the lateral force-resisting system from the cast-inplace concrete core shear walls to the SidePlate moment frame system.
Coordination Challenges As one might expect, this change was not without its challenges. The change in the lateral force-resisting system resolved the budget and schedule issues but triggered architectural and structural design modifications to accommodate the system change. The team's decision to change the design came in late October of 2017, at the completion of the Design Development phase. With a Construction Document submittal deadline of mid-December 2017, the team needed to react quickly, and in a coordinated manner, to keep the project on schedule. Much of the structural design work to establish frame locations and frame member sizes occurred during the initial vetting process of the system. Once the decision was made to proceed with the alternate design, SidePlate engineers continued to work closely with Miro to coordinate the moment frame design with the overall building design. This included any conditions that might impact the design of the moment frame connections, such as gravity cantilevers, drag members, and any gravity connections with large vertical reactions. For example, the structure spanned over Seventh Avenue at the third floor and required large, 85-foot-long, 9-foot-tall plate
9-foot-tall plate girder spanning over Seventh Avenue.
girders that supported the third level and the levels above. One of the moment frames supports the south end of the plate girders. However, connecting the plate girders directly to the SidePlate connections would have imposed large gravity reactions on the connections resulting in an impractical and cost-prohibitive design. The team decided to provide bearing haunches connecting the plate girders to the moment frame columns below the SidePlate connections. This allowed the plate girders to bypass the connections and not impact their design. The primary architectural consideration was the physical size of the moment frame columns. The moment frame system required deeper columns (often about 24 inches) than the typical gravity column sections (W10, W12, or W14) used with the concrete core lateral system. Typically, exam rooms and doctor’s offices have uniformity in their dimensions to utilize standardized equipment, cabinetry, and furniture. To accommodate the deeper columns, the architects jogged the walls to surround the columns and utilized custom counters and cabinetry. When it came time to produce Construction Documents, SidePlate provided Miro with stamped and signed drawing sheets to be included with the overall structural drawing package. These drawings included details and schedules of the moment frame connections. Joint-specific connection IDs, referencing the SidePlate details and schedules, were provided by SidePlate to be placed on the moment frame elevations.
Conclusion Denver Health’s new OMC opened to the public in early 2021, providing the residents of Denver with a state-of-the-art facility, just when it is needed most. The ability of the design and construction team to meet the needs of Denver Health was made possible by the atmosphere created on the project by the entire team. With the early involvement of key trade partners and the team’s willing cooperation and commitment to seeking out, vetting, and rapidly implementing innovative solutions, the team successfully navigated through a significant, late-game design change and came away with a big win for Denver Health.■ Tim Carroll is Mountain West Regional Engineer for SidePlate Systems, Inc. (tcarroll@sideplate.com)
9-foot-tall plate girder spanning over Seventh Ave (shown in purple), SidePlate connection (shown in blue), and bearing haunch (shown in green).
Maryann Davis is Senior Estimator for Drake-Williams Steel Mountain Division. (mdavis@dwsteel.com)
J U N E 2 0 21
building BLOCKS Crack Patterns Tell the Story of Glass Breakage The Fast and the Slow By Russell H. Davies, P.E.
T
he crack patterns in broken glass and their fracture surface details describe the origination and energy intensity that caused breakage. This article discusses basic fracture technology of flat glass in architectural and structural glass assemblies, illustrating characteristic fracture surfaces and crack patterns. The objective is to provide information to engineers investigating glass breakage and tips for specifiers to avoid glass breakage problems.
Glass Material Soda-lime silica flat glass sheets consist of silica sand, sodium carbonate, lime, metal oxides, and recycled glass. Heated to 1500 degrees Celsius, the mixture forms into a continuous ribbon “floated” on a long bath of liquid tin. An annealing process slowly cools the material to release internal stresses, resulting in the most common manufactured form of architectural flat glass. Unlike solid material crystalline lattice structures with ionic bonds, such as ceramics, glass is an amorphous solid (i.e., not a supercooled fluid) relying on covalent bonds. Covalent bonds share electrons between atoms, typically breaking between 150 and 400 kilojoules per mole (kJ/mole). Glass covalent bond breakage energy is in the high range of 435 kJ/mole. In crystalline solids, ionic bonds transfer electrons between ions, typically breaking between 600 to 4000 kJ/mole. Whereas glass and ceramics share multiple performance properties, ceramics can have solid molecular geometry (from ionic bonding). Despite the difference in molecular geometry, the theoretical compressive strength of glass is in the same high range as metals and ceramics and metal materials. Glass ideally can develop up to 17 Gigapascals (GPa) or 2,466 kilopounds-force per square-inch (ksi) compressive strength. Glass tensile strength, however, is typically in the range of 10% its compressive strength. This is because, despite its apparent transparency, smoothness, and clarity, freshly manufactured glass is proliferated with surface imperfections unobservable to the naked eye. The severity and distribution of these microscopic “flaws” control the tensile strength of glass in combination with its characteristic Fracture Toughness.
Figure 1. Critical stress intensity factor.
fails suddenly with little elastic deformation and no significant plastic deformation; it breaks in a sudden, brittle manner. Surface flaws concentrate stresses and, combined with a low Critical Stress Intensity Factor, reduce glass strength. Moisture and tensile load cycling can accelerate flaw growth. Newly manufactured glass is typically stronger than “weathered” glass, which has additional flaws due to handling and exposure to moisture and/or cycled loading conditions. Figure 1 tabulates theoretical weathered glass breakage stress, with red text highlighting values for typical flaw size and Critical Stress Intensity Factor ranges. Critical glass strength values are established based on material testing and statistical models to standardize safe working limits. Standards, such as ASTM E-1300, Standard Practice for Determining the Load Resistance of Glass in Buildings, and British Standard (BS) Euro norm (EN), Glass in Buildings: Determination of the Bending Strength of Glass, establish “allowable” glass edge and surface strength values used in building design and construction. ASTM C1036, Standard Specification for Flat Glass, establishes architectural glass thickness tolerances and minimum optical quality, including limits on body, surface, and edge flaws. C1036 sets the standard for manufactured annealed (AN) glass. ASTM C1048, Standard Specification for Heat-Strengthened (HS) and Fully Tempered (FT)
Basic Glass Fracture Mechanics Fracture Toughness is a material’s resistance to the opening of a sharp surface imperfection (e.g., a crack or fissure) where propagation occurs suddenly, uncontrollably, and quickly. Fracture Toughness is measured by the Critical Stress Intensity Factor, KIC. The Mode 1 (i.e., crack opening – Figure 1) Critical Stress Intensity Factor for glass is between 0.5 and 1.0 Megapascals – square-root meter (MPa-m½) or between 455 and 910 pounds-force per square inch square-root inch (psi-in½). Steel alloys have a value between 50 and 200 MPa-m½ (between 45.5 and 182.2 Ksi-in½), up to 400-times that of glass. With its relatively high Critical Stress Intensity Factor range, ductile steel alloys will undergo significant plastic deformation, stretching under tensile loading before breaking. However, unlike ductile steel, glass STRUCTURE magazine
Figure 2. Initial fracture surface and associated stress. Courtesy Vitro Architectural Glass.
Flat Glass, establishes the range of Residual The Initial Fracture Surface provides critiCompressive Surface Stress (RCSS) applied to cal information to describe glass breakage, AN glass by a heating and quenching process including the mode of failure (mechanical to produce HS and FT glass kinds. Through or thermal) and the tensile stress at glass this process, HS glass achieves approximately breakage. The mirror radius “R” provides a double the tensile strength of AN glass, while geometric dimension to calculate the stress at FT achieves four-times or more of the tensile breakage through the formula, σ = 1950/R½. strength of AN glass. In addition to heatBeyond the Initial Fracture Surface, cracks treated glass products, chemically tempered form with characteristic surface “Wallner” glass is available for flat and curved glass lines (Figure 3), which are a succession of applications. concave-shaped (primary/secondary) or Breakage is the conversion of the critical curving (tertiary) lines produced when a tensile stress into elastic strain energy at a crack front is momentarily altered by an surface or edge flaw. This strain energy breaks elastic pulse. The direction of crack propaSilicon monoxide Si-O covalent bonds, thus gation proceeds from the concave to the increasing the flaw size and advancing it as convex side of these lines. Primary Wallner a fracture through the glass body until the lines indicate discontinuities before fracture strain energy is expended. Glass breakage is occurs. Secondary Wallner lines indicate due to one or the combination of two types the crack front as it approaches terminal of elastic strain energies applied: external Figure 3. Wallner lines. Courtesy Vitro Architectural Glass. velocity. Tertiary Wallner lines form due and internal. Breakage due to applied exterto an external shock or pulsed energy, trignal elastic strain energy occurs in AN, HS, and FT glasses due to gering the onset of continuing crack growth. ASTM C1256 – 93 mechanical and temperature loading. Breakage due to internal elastic (Re-approved 2019), Standard Practice for Interpreting Glass Fracture strain energy occurs when RCSS (introduced to HS and FT glass Surface Features, provides an excellent reference with descriptions and during fabrication) is released into the glass body. When HS and FT images of fracture surfaces. glasses break due to applied stresses, the released elastic strain energy Crack patterns provide significant clues to describe glass breakage includes the sum of the applied external and internal stresses. The when coupled with information about the Initial Fracture Surface. In combination of these two elastic strain energies can be significantly AN and chemically-tempered glass, cracks form as single or branchhigher than that of applied elastic strain energy by itself. ing lines. In HS glass, cracks form as multiple single and joining The length, characteristics, and quantity of glass fracture are directly branching lines. In FT glass, cracks form across all surfaces, creating related to the strain energy required to relieve applied external and/or a multitude of small fragments. internal stresses. The associated fracture velocity is also a function of In all flat glass, low-stress cracks, due to mechanical stress (e.g., the strain energy, material stiffness, and density. In Harold Edgerton’s flexural), typically occur oriented non-perpendicular to glass edges Death of a Lightbulb / .30 Cal. Bullet (1936), a 0.30 caliber bullet is and often non-perpendicular to glass surfaces. High-stress mechanical shown passing through a lightbulb. The speed of the gun’s report is cracks have similar orientations with branching cracks that break away approximately 343 meters per second (m/s) or 1,125 feet per second (ft/ from the initial crack within two inches of the initial fracture surface. sec), the bullet’s speed is approximately 800 m/s or 2,624 ft/sec, and the On the other hand, low-stress thermal cracks are typically oriented terminal velocity of fracture development is approximately 1,500 m/s perpendicular to glass edges and surfaces; high thermal stress cracks or 4,921 ft/sec. In the release of high elastic strain energy, glass breakage have a similar orientation with branching cracks that branch from the can occur far quicker than is discernable to the eye. Glass fracture can initial crack within two inches of the initial fracture surface (Figure 4). also progress at a low rate, progressing over weeks or months. In mechanical and impact cracks, AN low-stress edge cracks form as meandering single/branching lines. High-stress edge and surface Initial Fracture Surfaces and Breakage Patterns cracks form as many radiating lines with circumferential rings. Crack Breakage originates from an Initial Fracture Surface with distinct characteristics, described as follows (Figure 2): • The origin is a single point in the glass material from which a crack system originates; • The mirror is a smooth, radiused area surrounding the origin; • The mist is a frosted band surrounding the mirror; • The hackle area extends from the mist band as ragged planes radiating away from the origin and towards the crack system. The mirror’s smooth fracture surface forms at a low velocity and quickens from the mist through the hackle area as the fracture propagates into the surrounding glass body.
Figure 4. Typical mechanical and thermal edge crack geometries. J U N E 2 0 21
patterns in AN, HS, and FT flat architectural glass (Figure 5) are combinations and permutations of these typical configurations. Special considerations for glass breakage relate to thermal stresses and spontaneous breakage. When AN glass heats up under direct or reflected solar radiation, glass central regions expand relative to colder edges. Thermally-induced hoop stresses can create tensile stress concentrations at critical edge flaws. AN glass is susceptible to thermal stress breakage if installed in partially shaded conditions, with integral or nearby heat-reflective or absorptive surfaces, and/or when glazed into materials that act as a heat sink. ASTM E2341, Standard Practice for Determining the Resistance of Single Glazed Annealed Architectural Flat Glass to Thermal Loadings, provides guidance to determine the potential for thermal stress breakage. Residual Compressive Surface
Stresses in HS and FT glass assemblies typically make them resistant to thermal stress-induced breakage. Spontaneous breakage can occur due to surface damage propagating through RCSS surfaces under cyclic loading or due to Nickel Sulfide (NiS) stone inclusion (or other batch impurities) in the manufactured glass material. NiS stone inclusions undergo an expansive phase change in their crystalline structure that, due to the relatively quick quenching of FT glass, has the potential to occur sometime after fabrication. HS glass quenching occurs slowly enough to allow NiS stone phase change and, like AN, does not suffer from spontaneous fracture due to NiS stone inclusions. The expansion in a NiS particle can cause a fracture in the body, which, when propagated to FT glass surfaces, suddenly releases RCSS throughout the glass body, causing breakage. The resulting starburst crack pattern features a pair of central pentagonal or hexagonal “cat-eye” or “butterfly” shaped particles whose shared fracture plane contains the NiS stone and the fracture origin. Not all cat-eye fracture patterns are due to NiS stone inclusions. Laboratory analysis such as Scanning Electron Microscopy (SEM) can positively identify impurities in the glass body to confirm the cause of spontaneous breakage.
Breakage Examples
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The following projects provide examples where the crack pattern and initial fracture surface were vital in determining the glass breakage story.
STRUCTURE magazine
Structural Laminated Glass Cantilever Tread Staircase This project includes a multi-layered structural laminated glass cantilever tread staircase in a private residence (Figure 6). The tread specification considered conservative static and dynamic stair loads, plus post-breakage capacity for safety, using AN glass laminated with a stiff structural interlayer material. Installation details included set-screw level adjustment with a maximum setting torque specification into padded brackets, with each tread independently installed and test-loaded. One tread top glass lite cracked in a single line across the lite at an applied load significantly lower than as-designed. Recovery of the failed tread allowed investigators to measure the fracture mirror and to determine that the glass fractured at flexural stress over 7,000 psi, whereas the design allowed for no more than 1,500 psi. Further review determined that the supplied glass treads incorporated a flexible non-structural lamination interlayer material in place of the specified stiffer structural interlayer, resulting in glass non-composite performance and breakage at lower loading than as-designed.
cause of breakage. The possible single or combined cause of breakage included manufactured flaws, damage during handling and installation, over-constraint in the base support shoe due to tight tolerances, glass-to-metal contact, and mechanical loading from stair use. In this breakage example, initial glass fracture was a lowor high-strain energy event due to an unknown cause, followed by low-strain energy dissipating over an extended period.
Tips to Avoid Glass Breakage
Figure 5. Typical flat glass crack patterns.
Replacing and load-testing all treads with glass laminated with the specified structural interlayer provided a resolution to the breakage problem. In this breakage example, initial glass fracture was a high strain energy event dissipating quickly due to excess loading of an improperly supplied laminated glass assembly.
Curving Staircase Laminated Glass Handrail This project includes a residential curving staircase with large chemically-tempered, laminated glass handrails (Figure 7 ). The design called for small joint finish dimensions and tight tolerances for handrail installation into base support shoes on the stair structure. Three months after completion, a single-line crack emerged within one of the laminated lites from the inner stringer base shoe. The crack propagated slowly over a period of 6 months, extending from the base shoe toward the handrail top edge. The owner was reluctant to remove the glass, and the installer was confident that removal would destroy the initial fracture surface. The slowly growing crack featured long, smooth fracture surfaces spaced by shorter groupings of tertiary Wallner lines. These fracture surface characteristics indicated intermittent decelerated and accelerated crack growth consistent with the owner’s stair use. Without access to the initial fracture surface, there was no way to confirm the exact
Figure 6. Example 1 – Structural laminated glass cantilever tread staircase.
The story of glass breakage is documented precisely in crack patterns and their fracture surface characteristics. As useful as these indications are for breakage investigators, designers and specifiers are more interested in avoiding breakage altogether. Here are five tips to assist in avoiding glass breakage in design: 1) Design glass for loads to meet strength and serviceability requirements via standard methodology or first-principles analysis, including consideration for post-breakage performance. 2) Avoid AN glass partial shading. 3) Specify HS glass unless FT glass is required to meet code or contract requirements. When specifying FT glass, heat-soak to reduce the likelihood of spontaneous breakage. 4) Allow for required construction tolerances and support movement. 5) Avoid glass contact with hard objects during fabrication, handling, transportation, and installation. If breakage does occur, the following steps are essential to maintain safety and to determine the cause: 1) Control and make the area safe. 2) Engage a glass expert to review the situation. 3) Document and identify the crack pattern in-situ. 4) Preserve the fracture origin for analysis. 5) Provide design and installation history.
Conclusion Crack patterns in broken glass, along with their fracture surface characteristics, provide a detailed account of the breakage story. Given the importance of glass in architecture and building construction, a basic engineering knowledge of material properties and fracture behavior is key to its proper specification and installation.■ Russell H. Davies is Principal at AVRO Consult Engineering, P.C. in New York.
Figure 7. Example 2 – Curving staircase laminated glass handrail. J U N E 2 0 21
INFOCUS A Sobering Dose of History and Humility By Eytan Solomon, P.E., LEED AP
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nevitably, designers respond to crisis with… design. The world of 2020-2021 is grappling with a devastating pandemic, protests against racism and inequity, and demands to shift resources: from policing to community building, from fossil fuels to green energy, from shareholders to stakeholders, from short-term profit motivation to sustainability and resilience. These are lofty and noble ideals viewed from our current reality. Designers – namely the architects, structural engineers, and civil engineers of our industry – are hard at work, pondering how to design and construct our way through these challenges. And, rightly, we should. But, critically, we must also remember the ugly truth of history, when past “good intentions’’ have led to disastrous outcomes. From the 1930s to 1960s, Robert Moses led public construction in the New York City metropolitan area. He pushed forward public works projects viewed by the dominant society as providing needed improvements and renewal, and in so doing, creating highways, new buildings, and suburban housing, resulting in jobs for designers and engineers. Unfortunately, the side effects were sprawl and the razing of poor neighborhoods (e.g., East Tremont in the Bronx, as vividly described in The Power Broker by Robert Caro), while hindering mass transit, equitable housing, and access to parks for non-white people. Today, we struggle to undo the misdeeds of that era. Moses’s New York is just one example of the type of ‘’urban renewal’’ that would later be judged as misguided. A few more examples: In the early 1900s, racial covenants in Los Angeles limited where non-whites could purchase property. One of those few places was an area known as Chavez Ravine, a close-knit community of primarily Mexican-heritage families. After the 1949 National Housing Act, the Los Angeles (LA) City Housing Authority made the harsh and dubious decision to classify Chavez Ravine as a “slum,” and the city began evicting residents to make way for, ironically, a mass housing project. Ultimately, political disagreements among city lawmakers stalled the housing project, and the owner of the (formerly Brooklyn) Dodgers swooped in to buy the land for a new stadium. In 1959, LA sheriff’s deputies physically dragged the last holdout families from their houses, with STRUCTURE magazine
Educate yourself about both the bad and good sides of history. Buildings along the riverfront which were torn down to make way for memorial parkway, St. Louis, Missouri.
the traumatic images plastered on television and newspaper front pages. The origin of St. Louis’s Gateway Arch – a structural engineering marvel, to be sure – is also rather troubling. The city’s central riverfront had been home to a thriving African-American community, with historic blocks of cast-iron buildings reminiscent of Manhattan’s SoHo district. In 1935, white business leaders proposed a massive monument to Thomas Jefferson and the Louisiana Purchase. They rigged a bond measure vote (including tens of thousands of phony ballots) to ensure its authorization. In 1939, demolitions began, and ultimately 486 structures over 40 square blocks were bulldozed – including 290 businesses, mainly small factories employing some 5,000 workers. The area sat desolate during WW II, but post-war highway development infused federal dollars (aided by fellow Missourian President Harry Truman) to complete the project. This permitted highways to run through downtown St. Louis, which further cut off local residents (mainly poor and non-white) from access to the waterfront and future park. In 1964, construction of the Arch was underway. Still, African-Americans were essentially excluded from working on the construction teams: finally, protesters climbed the Arch while it was still under construction and, ultimately, won promises for fairer hiring at the site. But the damage was already done.
Similar stories of brutal “renewal” occurred at the development of Philadelphia’s Independence Mall in the 1950s, San Francisco’s Western Addition in the 1950s (which resulted in the loss of many of the famous Victorian buildings), and elsewhere. Granted, these examples were developed around societal philosophies and norms (at least for those in power) of their times. However, that does not mean that history should not inform our decisions today. For our present and future, I have no magic spell or secret recipe to avoid a repeat of past mistakes… except to get involved! You probably know as much about the issues involved as the so-called experts. Come to the debate with facts and ideas. If necessary, educate yourself about both the bad and good sides of history. Then insert yourself into current policy debates and decisions in your locale. Lean on your politicians to get your opinions heard and acted on. Apply your engineering approach to see the reality of the issues and what alternatives exist before designing what others would think the future to be. And carry with you a dose of humility: About the past mistakes – yes, mistakes – of architecture and engineering.■ Eytan Solomon is an Associate at Silman in New York City and a member of STRUCTURE’s Editorial Board. (solomon@silman.com) J U N E 2 0 21
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structural CONNECTIONS Detailing Cross-Laminated Timber (CLT) Connections By Jim DeStefano, P.E., AIA, F.SEI
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ross-Laminated Timber (CLT) panels are commonly used in mass timber structures. As with any structural eleFigure 1. Surface spline joint. Figure 2. Half-lap joint. ment, proper detailing of the connections is crucial. Structures with practical connection details are usually allowable strength values. Still, values as high as 5.0 have been used cost-efficient and easy to fabricate and assemble. In contrast, poorly for certain seismic applications. conceived connection details often result in an overly costly structure Exercise caution using long, fully threaded screws, which can restrain plagued with difficulty. timber volume change associated with fluctuations in moisture conThere is little guidance currently available on detailing CLT con- tent. Such restraint can result in the splitting of timbers or rupture nections, although the CLT Handbook provides some guidance. of the fastener. It is not unusual for engineers with limited experience designing In detailing CLT connections, consideration should be given to timber structures to rely heavily on exposed steel hardware to the ease of fabrication. There are a variety of Computer Numerical make CLT connections. Exposed hardware is not only visually Control (CNC) machines used to fabricate CLT panels, and each has undesirable but also performs poorly in a fire, making it unsuit- its limitations. Some machines can only make cuts from one side of able for a structure that is required to have a fire-resistance rating. the panel, and thus the panel must be flipped over in the plant to It is not difficult to detail CLT connections with little more than make cuts on the opposite side. Consequently, it is more cost-effective screw heads exposed. to use connection details that allow all cuts to be made from the top Self-drilling structural screws are the most common and versatile side. For instance, an internal spline joint is more straightforward to fastener for CLT connections. They are proprietary products, and their fabricate than a tongue-and-groove joint. It is also efficient to be able structural properties are usually documented in the manufacturer’s to rout grooves or rabbets on the edge of a long CLT billet before technical literature and product evaluation reports. Be cautious using it is cut into individual pieces. So, it makes sense to have consistent products that do not have North American product evaluation reports. panel edge details. Since some screws are manufactured in Europe, it sometimes requires CLT connections should be detailed in a manner that allows any an interpreter to decipher the technical literature, which may cause qualified CLT supplier to bid on the project. If a particular supplier an inaccurate interpretation. When only ultimate strength values are has been selected for a project, it is wise to consult with them on what provided, it is customary to use a factor of safety of 3.0 to establish connection types they can execute most efficiently.
Floor and Roof Panel Connections
Figure 3. Platform frame butt joint.
STRUCTURE magazine
Figure 4. Cleated platform frame joint.
The most common and versatile edge connection for a floor or roof panel is the surface spline joint (Figure 1). A shallow rabbet is routed in the top of each mating panel, and a narrow plywood strip is nailed across the joint in the field. Nails are typically used rather than screws, except when they are loaded in pullout. The plywood splines are typically ¾-inch to 7⁄8-inch-thick and approximately 6 inches wide (splines can be up to 10 inches wide when an aggressive nailing pattern is required). The diaphragm strength is controlled by the spline thickness and nailing pattern
since the CLT panels are substantially stiffer and stronger than the splines. Where the floor or roof is part of a fire-rated assembly, it is essential that the panels fit tightly. Fire-stopping sealant should be used where there is a gap unless there is a concrete topping over the panels. The half-lap joint (Figure 2) is also commonly used to join CLT panels. The joint is secured with screws. The half-lap joint is more costly to fabricate than a surface spline joint, and it reduces the effective width of the panels slightly. It has the benefit of being able to transfer modest loads across the joint. An example of an application where a half-lap joint would be appropriate is Figure 5. Cleated sill plate. adjacent to a cantilever where the joint must resist uplift. If the load at the joint is substantial, there could be a tendency for the panel to split, and reinforcing screws should be considered.
Bearing Wall Connections CLT bearing walls supporting CLT floor and roof panels are well suited to multi-family residential projects. Platform framing is common for low and mid-rise buildings. The simplest platform detail is a butt joint (Figure 3). The butt joint requires minimal fabrication effort since the panels are merely square cut. However, it is an inefficient joint to erect since the positioning of the panels can be tedious and consumes crane time. The butt joint also requires a large number of toe screws or exposed steel angle connectors. A cleated joint (Figure 4) is more practical than a butt joint. A 2x cleat ripped to a width equal to the space between the face plies of the wall panel is used to position the upper wall panel and provide a point of attachment. The joint is assembled with short screws, and no extraneous hardware is required. The connection to the foundation also utilizes a 2x cleat (Figure 5). The bottom of the CLT wall panel should never be allowed to come in direct contact with the concrete foundations and should bear on a pressure-treated sill.
Figure 6. Tongue-and-groove wall panel joint.
Figure 7. Internal spline wall panel joint.
Joints between panels are commonly either half-lap joints or tongue-and-groove joints (Figure 6). Both a half-lap joint and a tongue must be cut from both faces of the panel. This reduces the effective width of the panel. An internal spline joint (Figure 7) is a practical alternative to the tongue-and-groove. The groove in the edge of the panel can be cut without needing to flip the panel over in the shop, and there is no reduction in effective panel width. A 1¾ x 5½ LVL is a good choice for a spline. Butt joints are often used at wall corners and intersections. Although simple to fabricate, butt joints tend not to fit tightly and can open up when panels season and shrink. Very small gaps can become problematic and can result in a breach of an acoustic or fire-resistant barrier. A rabbeted corner joint (Figure 8) with half-laps cut in the edge of each panel is more practical than a butt joint. It results in a neater joint that tends to stay tight. A cleated joint (Figure 9) is a smarter choice than a butt joint where wall panels intersect.
Connection Design Responsibility It is customary for the Structural Engineer of Record (SER) to be responsible for the design and detailing of mass timber connections. This is different from what is common practice for structural steel or glulam timber structures, where the SER indicates reactions or member forces and delegates connection design to the fabricator. Engineers who lack experience designing mass timber connections should require the mass timber contractor to engage a qualified timber specialty engineer to provide those services.■ This article originally ran in the Timber Frame Engineering Council (TFEC) Timber Design Guide 2019-15. It is reprinted with permission. TFEC documents are available at www.timberframeengineeringcouncil.org. Jim DeStefano is the President of DeStefano & Chamberlain, Inc., located in Fairfield, CT. (jimd@dcstructural.com)
Figure 8. Rabbeted wall corner joint.
Figure 9. Cleated wall intersection joint.
J U N E 2 0 21
professional LIABILITY Do Not Forget Serviceability By Richard S. Barrow, P.E., S.I.
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s structural engineers, we have many responsibilities. Our most important responsibility is to hold paramount the safety and health of the public; therefore, our primary focus is on designing sound structures with adequate strength and stability. Another responsibility that we have is to meet the owner’s expectations regarding the performance and durability of his or her structure. For many years, structural engineers have addressed those two responsibilities by designing for two limit states: strength and serviceability. The American Society of Civil Engineers’ standard ASCE 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, defines a limit state as “a condition beyond which a structure or member becomes unfit for service and is judged either to be no longer useful for its intended function (serviceability limit state) or to be unsafe (strength limit state).” The consequences for exceeding the two limit states differ significantly. For example, strength limit state failures range from design deficiencies to structural collapses; whereas, a serviceability issue may be no more than annoying floor vibrations. Nonetheless, serviceability issues may have severe economic consequences. As noted in the commentary of ASCE 7-16, some serviceability failures can lead to safety issues, such as cladding falling off of a building due to excessive movement of the structure. Therefore, the term “failure” should not just be limited to deficient strength and stability of the structure. So what can structural engineers do to reduce their exposure to serviceability claims? Four suggestions to help with serviceability claims are briefly discussed below.
Understand Project Expectations A serviceability failure typically means that the structure did not perform as intended or desired by the owner or the users of the structure and is typically a contractual matter. Consequently, the structural engineer must understand what the owner wants or expects and what the parties have agreed to provide. That means asking questions during the proposal and design phases and providing what some refer to as “expectation management.” STRUCTURE magazine
Some issues that need to be addressed upfront regarding the owner’s or the client’s expectations include: a) What is the intended use of the structure? This is an important issue for designing for strength and safety and designing for service issues, such as control of deflections and vibrations. b) Does the owner need flexibility for future uses, such as a change of occupancy or to increase the potential lifespan of the structure? c) Are the owner’s and architect’s expectations regarding fees, design and construction schedules, and construction costs reasonable? If a tight budget or short schedule may affect your ability to meet the project expectations, it may be best to walk away from such a project before getting started. d) What is the scope of engineering services and the quality of work to be contracted? Be careful about providing warranties or guarantees for engineering services. Short of contractual agreements to the contrary, engineering services are held to the accepted standard of care, which is the care that structural engineers ordinarily provide under similar circumstances, not perfection. e) If the owner or architect does not want to contract construction phase services, such as answering RFIs, shop drawing review, and site observations, are they aware of the importance of those services? Those tasks help to confirm that the contractor understands the work as well as provide a last opportunity to correct any design issues that may become evident. f ) Barring code-required inspection criteria, the structural engineer typically has no legal duty to inspect the construction unless contracted to do so. Therefore, the engineer may need to explain to the owner or architect the benefits of site observations. For example, periodic site visits by the engineer can head off problems before they happen. Site observations cost the owner a little more money upfront, but more often than not pay for themselves in the long run with higher quality construction.
Structural design of elevated floor slab met the strength requirements, but the large cracks were not compatible with the proposed floor finishes.
g) Agree upon your role and do not take on responsibilities that are not yours. Construction projects involve many parties with differing skills and responsibilities. The contractor is responsible for the “means and methods of construction;” therefore, structural engineers must avoid supervising or controlling the contractor’s work. Design professionals are responsible for their design, approval of modifications to their design, and the acceptability of substitutions. For successful projects, design professionals must work with contractors and understand the difference between prudent assistance and control of the work. When it comes to serviceability issues, it may be better to define the final result desired rather than precisely specifying how specialty engineers, testing companies, and contractors can achieve that result. h) Some serviceability issues may not immediately present themselves; therefore, documenting discussions and decisions made may prove to be very helpful in mitigating disputes which arise years after completion of the project.
Structural Movements and Vibrations For serviceability design, the most important question regarding structural movement is: “What are the deflection, drift, and expansion/contraction requirements for nonstructural items?” Given that a number of
issues may result from structural movements, occupant, the vibrations can be quite distress- durability recommendations for various envithe structural engineer needs to determine ing. Therefore, like all other serviceability ronments. ASCE 7-16 and AISC 360-16, acceptable deflection and drift limits to mini- issues, it is best to address vibration control Specification for Structural Steel Buildings, also mize damage to non-structural items on the during the design phase. provides recommendations for addressing project. At a minimum, the building code durability issues. deflection and drift requirements need to Consider Durability be met. Other acceptable movement criteria The Standard of Care and Maintenance may be obtained from the manufacturer or engineer of non-structural items, or refer- How does the owner feel about initial costs All designs and construction are required to ence documents such as those provided by versus future maintenance costs? Investors meet the code minimum requirements, but is the American Concrete Institute (ACI), the may not be greatly concerned about long that enough? Building codes primarily address American Institute of Steel Construction term maintenance. In contrast, the end- minimum strength requirements and, to a (AISC), and the American Society of Civil users typically are more concerned because much lesser degree, serviceability requireEngineers (ASCE). they will have to deal with the maintenance ments. For example, Section 1.3.2 of ASCE The appropriate service loads need to be and the associated possible loss of use of the 7-16 states: used in drift calculations and the nature of the structure. In the typical scenario where the Structural systems, and members thereof, damage due to drift needs to be considered, owner contracts with the architect and the shall be designed under service loads to have or the design can become overly conservative architect subcontracts a structural engineer, adequate stiffness to limit deflections, lateral for wind loads. For example, for Occupancy it is very helpful if the architect provides a drift, vibration, or any other deformations Category II buildings or structures defined “responsibility to maintain” clause in their that adversely affect the intended use and in ASCE 7-16, strength design is based upon contract with the owner. Such clauses put performance of buildings and other struca mean return interval (MRI) of tures based on requirements set forth 700 years for the wind event. Drift in the applicable codes and stancalculations should be performed dards, or as specified in the project using a wind speed from one of design criteria. (Bold text added for the 10-, 25-, 50-, or 100-year emphasis) MRI maps provided in the comThe terms “adequate” and “adversely” mentary, Appendix CC, of ASCE make serviceability design subject to 7-16. Similarly, deflection calcua fair degree of engineering judglations due to wind are typically ment and differing opinions. Such based upon 0.42 (0.6 ASD wind gray areas, coupled with an evolving load factor x 0.7 pressure reducstandard of care, increase the sustion factor) times the component ceptibility of serviceability issues for and cladding wind pressures, which litigation. Following the serviceabilcorresponds to a 10 year MRI. ity recommendations of recognized Building code deflection requireindustry standards, along with ments to prevent damage to knowledge and experience as to how non-structural items, such as partistructures behave, can go a long way tions and floor tile, due to gravity towards meeting the standard of care loads are typically based on span- Falling object hazard due to a failed roof membrane over a concrete slab. for serviceability issues. to-deflection ratios. Those ratios are The most important task for strucbased upon dead and live loads applied after the owner on notice that the structure cannot tural engineers is to prevent a failure due to installing the item in question with consid- be neglected for years and then claim faulty exceeding a strength limit state. However, eration of creep due to sustained loads. For construction or design defects when, in fact, structural engineers must also be aware that brittle materials, consideration should be given a failure to maintain the structure properly claims and economic consequences can also to limiting the maximum allowable movement was the reason for the damage. result from exceeding a serviceability limit state. regardless of the span of the element, the story How is the structure going to be water- Understanding project expectations, addressheight, or the building height. For example, proofed? It is the author’s experience in Florida ing serviceability and durability in design, and some sources indicate that deflections for some that water intrusion is the most common ser- meeting the standard of care for engineers can masonry shall be limited to the lesser of the viceability issue. Avoiding design details that help the structural engineer reduce the chance span/600 or 0.3 inches. tend to trap water and providing sufficient of claims for serviceability issues.■ Modern “electronic” offices are problem- positive drainage go a long way to preventing atic when it comes to floor vibrations. They water intrusion damage. Water intrusion can References are included in the PDF commonly have long floor spans with few lead to serviceability issues (i.e., damage to version at STRUCTUREmag.org. permanent partitions or heavy paper filing non-structural items) and strength issues (i.e., systems to dampen vibrations. The addition deterioration of structural elements). Richard S. Barrow is the President of Liebl & of fitness areas further exacerbates this situaWhat is the exposure environment of the Barrow Engineering based in Fort Myers, FL. He tion. One complication in resolving vibration structure? Use industry-standard guidelines, is a member of ASCE’s Committee on Forensic issues is that sensitivity to vibrations is highly such as ACI’s exposure classifications and Investigations and ACI’s Committee 311: subjective. The vibrations may be mildly the American Wood Protection Association’s Inspection of Concrete. (rich@lbengineer.com) annoying to one occupant, while to another (AWPA) Use Category System, which provide J U N E 2 0 21
structural FOUNDATIONS Pile Load Testing for Bored Piles in Soil A Brief Summary
By Hee Yang Ng, MIStructE, C.Eng, P.E.
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ored piles, also referred to as auger-cast piles, are large-diameter at intervals throughout the length of the pile so that the force for cast-in-place concrete piles used commonly to support buildings each segment can be calculated and the pile’s load transfer can be when column loads are high. The design of bored piles requires the obtained. The same pile installation method should be used to install designer or the project’s geotechnical engineer to the working piles for the project. To adequately estimate the load-carrying capacity of the pile based verify the shaft friction and end-bearing capacity, on the ground conditions at the project site. One preliminary piles are typically loaded to 3 times way to predict the pile capacity is by carrying out the working load. calculations using soil parameters obtained from Working pile tests are conducted during piling site investigation data. While many pile design work to verify the quality and workmanship of methods are highly empirical, pile capacity can also the installed piles. These tests verify that installed be affected by other factors such as the method of piles perform as-designed to meet strength (abilinstallation, quality of workmanship, and construcity to carry load) and serviceability criteria (with tion materials used. Therefore, pile load testing is acceptable settlement). Working piles are loaded an essential aspect of pile design that should not progressively, with the load and pile head settlebe overlooked. ment monitored and recorded. The acceptance This article summarizes pile load testing (static criteria for working piles are settlement limits at type using a kentledge) for bored piles in soil and Figure 1. Generalized load-settlement certain loads. For example, in CP4 (SS CP4 Code highlights some key aspects designers should look curve of a pile. of practice for foundations, Singapore Standard), for when reviewing and interpreting pile load test the settlement cannot exceed 0.59 inches (15mm) results. For more information on pile design, the at 1.5 times the working load or 0.98 inches reader is directed to Pile Structural Capacity – A (25mm) at 2 times the working load. The higher Comparison of Three Design Codes published in the allowable value of settlement at 2 times of workMarch 2020 issue of STRUCTURE. ing load is to recognize that, at higher loads, the rate of increase in settlement is likely to be greater. Recommendations for the number of tests are Types of Load Tests shown in Table 1. There are essentially two main types of pile load tests: the preliminary pile test and the working Load-Settlement Curve pile test. A preliminary pile test is typically carried out before the piling works commence while the A simplified load-settlement curve is shown in Figure 1. working pile test is carried out during piling or Generally, the curve is non-linear, and the stiffwhen substantial piling works are completed. When ness of the response tends to degrade or soften as using limit state design (e.g., Eurocode 7-EC7), the the load gets higher. This is expected because, as preliminary pile test and working pile test allow the load increases, the pile approaches “failure” in designers to adopt less conservative partial factors Figure 2. The difference in mobilization geotechnical terms. Some codes define the “failure” of shaft friction and end-bearing. when reducing unfactored shaft friction and endof a pile as reaching a settlement of 10% of pile bearing, resulting in a more economical design. Similarly, when using diameter. For large diameter piles, this criterion becomes somewhat LRFD design or ASSHTO specifications, more load testing may allow lenient if the serviceability limit state is considered. For example, a a higher resistance factor (φ) to be used due to the increased reliability. 39.37-inch-diameter (1000mm) bored pile has an allowable settleThe preliminary pile test verifies the design parameters prior to the ment of 3.94 inches (100mm). The settlement acceptance criterion start of piling work. Therefore, the test pile is instrumented so that has to be stringent because pile settlement can result in a differential assumed values of shaft friction and end-bearing could be verified. A settlement between supports, which is a grave concern due to the borehole close to the pile location is necessary to ascertain the type amplification of tilt for a tall superstructure. In addition, buildand layering of the soil for verification. Strain gauges are installed ing settlements may disrupt utilities and services (e.g., pipes and cables). Therefore, a much smaller value of the allowable settlement is usually required. Table 1. Number of pile load tests. Piles rarely “fail” with regard to vertical structural capacity. In a case Preliminary pile test 1 pile or 0.5% of the total number of study presented later, a quick calculation shows that the axial stress working piles, whichever is greater. in the pile at its rated working load is on the order of about 8 to 10 Working pile test 2 piles or 1% of the total number of workMPa (or 1.2 to 1.5 ksi). ing piles or 1 for every 50 meters length of A critical aspect of pile design is shown in Figure 2, where the proposed building, whichever is greater. load on a pile installed in soil is resisted by a combination of shaft
STRUCTURE magazine
friction and end-bearing. As the pile is loaded, shaft friction increases pile with its load-settlement path running along the linear-elastic gradually and can become fully developed by a small movement portion of the curve will likely return to zero load with minimal of approximately 0.5% of pile diameter, say residual settlement. 0.2 to 0.4 inches (5mm to 10mm). However, Sometimes it can be helpful to compare the gradient a large movement is required for end-bearing of the load-settlement curve against the elastic shortto be fully developed, typically in the range of ening gradient (PL/AE line). For example, the elastic 5% of pile diameter. This means that to achieve shortening of a bored pile without any confinement its full end-bearing value, the pile must settle is given by PL/AE, where P is the applied load, L is more, which may not be desirable. For this the length of pile, A is the cross-sectional area, and E reason, designers tend to select more conservais Young’s Modulus. tive values for end-bearing, and codes impose The load-settlement gradient of a friction pile a higher factor of safety. should generally be similar or even very slightly Figure 3 shows some examples of load-settlesteeper compared to the elastic shortening gradient. ment curves of piles with anomalies compared This is because of the presence of soil confinement to a pile expected to perform normally. The and shaft resistance. However, suppose the gradient red curve is abnormal because of kinks in the of the load-settlement of a pile is gentler than the curve, possibly resulting from pile defects or Figure 3. Examples of “abnormal” elastic shortening gradient. In that case, it may be load-settlement curve of a pile. unexpected ground conditions that cause the a case of less friction being developed (e.g., softer pile to settle abruptly when loaded. This inability to sustain load at soils) along the shaft or a case of stiffer soils located only at the localized portions of the curve is not acceptable. The purple curve lower end of the pile. shows normalcy up to a point and degrades abruptly, finding a plateau with a lower load and increasing settlement. This might Calculation of Pile Settlement occur if the pile base is poor or suddenly loses support coupled with a loss of a portion of shaft friction or deterioration of shaft There are various methods to estimate anticipated pile settlement in friction. One example is a case of cavities, archetypal in limestone design. One of the easiest ways to calculate pile settlement by hand is areas. Lastly, the blue curve is abnormal due to its inability to reach by Vesic (1977), as shown in Figure 5 (page 54). In this method, the the expected design load and experiences excessive settlement too pile head settlement is the sum of three components, namely axial early. This might occur for a pile that has fully maximized the shaft compression (ws), the settlement at pile toe due to shaft load (wps), friction and cannot develop the end-bearing, resulting in a “plung- and settlement at pile toe due to end-bearing load (wpp). ing” type of failure. Axial compression in soil can be simplified by assuming it as 75% of unconfined elastic shortening (PL/AE). The formula given by Vesic uses the sum of end-bearing load and a fraction (0.5 to 0.67) Case Study of shaft friction load in place of axial load in the unconfined elastic A 31.5-inch-diameter (800mm) bored pile was installed to a depth shortening formula to calculate the axial shortening of the pile in of 85.3 feet (26m) below ground in alluvial soil. These soils were soil. Settlements due to shaft load and end-bearing load are given by formed by transported material deposited and cemented over time. the ratio of shaft design working load (Qs) and end-bearing design The working load for such a pile is 450 tons (4000 kN). Please note working load (Qb) against the ultimate bearing capacity at the pile that pile capacities provided by geotechnical engineers are always toe (Qo), normalized by the relevant dimension (pile length L and stated as allowable loads (ASD) or working loads, typically with a pile diameter d, respectively) and multiplied by Cp and Cs, empirical safety factor of two against a geotechnical failure. coefficients subject to variability. Because of this, it is appropriate to Figure 4 shows the load-settlement curve for an instrumented pre- study the sensitivity to this variability during the design process and liminary pile test carried out for the 31.5-inch-diameter (800mm) benchmark against past experience in similar ground for similar piles bored pile. The pile was loaded to 3 times the working load so the before relying too much on the result. full value for shaft friction could be developed. During the applicaWhat is important to note in using this method is that the value tion of the lower range loads (red line), the curve is approximately of settlement calculated is highly dependent on what value of Cp is linear and somewhat elastic. This means that the pile is behaving selected and the value of ultimate base capacity qo. The method recogwell, and the response has not reached a state of yielding. When nizes that sand normally has a higher ultimate bearing capacity than loads are higher (green line), it can be seen that the gradient of the curve has decreased, meaning stiffness has degraded. This behavior is analogous to materials approaching yield when a higher load range increases settlement. After reaching 3 times the design working load, the pile was unloaded (purple line). The unloading line usually has a steeper gradient because the pile loading process stiffens the soil around the pile. Lastly, the final portion of unloading to zero load (blue line) shows a very slight stiffness reduction similar to reaching yield during the loading process. However, this may not be obvious or observed in all tests. After unloading, it is helpful to examine the residual settlement (0.43 inches [11mm] in Figure 4). This is the settlement that is permanent and cannot be recovered. A high value of residual settlement usually means that the pile has been loaded beyond yield, resulting in large permanent deformation. Conversely, a lightly loaded Figure 4. Preliminary pile test result. J U N E 2 0 21
clay. For example, the limit on end-bearing shaft friction is reached, the rest of the load can be 40,000 kPa compared to 15,000 kPa would be taken by increasing end-bearing, for sand and clay, respectively. Therefore, the which comes at the expense of a much higher Cp value for clay is correspondingly lower comsettlement. For loads up to the working load, pared to sand. as the design of the pile has already taken into It can be noted that most pile settlement consideration developing very little end-bearcomes from axial compression and base seting and mainly relying on shaft friction, Vesic’s tlement. Because of this, designers who need equation predicts settlement quite accurately, as to control settlement might want to consider seen in this example, when using an artificially increasing pile stiffness (e.g., higher grade conhigh qo to minimize the contribution from crete and increased steel reinforcement area) component wpp. and improving the pile toe condition (e.g., base However, when using a value of qo = 3,000 grouting). Another strategy is to design the pile kN/m2, which is the actual value used in the as a pure friction pile (i.e., there is no reliance pile design, Vesic’s equation over-predicts settleon end-bearing to achieve the pile capacity). ment significantly. Therefore, designers need In such an instance, it is possible to reduce the to be cautioned on the sensitivity of qo and overall safety factor on shaft friction to 1.3. notice that qo may also vary due to size effects. From Figure 6, it can be observed that the pile This illustration is not meant to be an attempt settlement at working load is approximately to validate or invalidate Vesic’s equation for equal to the unconfined elastic compression any case. Designers still need to consider the (i.e., PL/AE). This can be a very crude approxi- Figure 5. Pile settlement calculation by Vesic (1977). design assumptions, ground conditions, past mation to estimate pile settlement without any experience, sensitivity, etc., when calibrating further computation. However, for a friction-only pile, the settlement this equation for use in a new situation. should be halved. This assumes the settlement due to shaft load is small and there is zero settlement due to end-bearing load. Thus, the Other Points to Note only component left is the axial pile compression which reduces to about 0.5PL/AE, according to Vesic’s equation. In loading a test pile, a kentledge reaction system (stacking up concrete Figure 6 shows a comparison of the load settlement curve predicted blocks or steel plates as a counterweight) is customarily used. However, using Vesic’s equation against data from an ultimate load test for the the process is slow, requires significant space, and is not practical for same 31.5-inch-diameter (800mm) bored pile with a working load very high loadings. Nonetheless, many designers prefer this method due of 4,000 kN. It can be seen that the Vesic equation predicted the pile to the easy interpretation of test results. The stacking of high loads on settlement quite well (by assuming qo = 40,000 kN/m2), but only up to a small area requires the ground bearing capacity to be checked. There the working load of 4,000 kN (at about 0.2-inch [5mm] settlement). have been instances of kentledge collapses due to inadequate bearing Beyond that, the predicted settlement is much higher than the actual capacity. Sometimes, it is not practical to test a large diameter pile due settlement. The reason for this is the conservative assumption of to the high loads. Some codes (e.g., Eurocode 7-EC7) allow results of a maximum shaft friction. Using Vesic’s equation, once the maximum pile load test to be extrapolated for a pile diameter not exceeding 2 times.
Conclusion For buildings with high column loads, bored piles are often adopted. However, due to the customary practice and compliance with regulatory requirements, pile load tests are almost always required. The provision of pile load tests allows the designer to verify design parameters, design more economical piles, check the quality of materials and workmanship, and ultimately show that the performance meets load-settlement requirements. It is important to understand the difference in development rate for shaft friction and end-bearing. The load-settlement curve is an important plot to assist designers in understanding the pile behavior. Pile settlement can be easily calculated using Vesic’s equations but, just like any settlement calculations, the value cannot be taken to be precise. It is only a means to offer insights to the designer as a basis for comparison when carrying out a pile load test. It is essential to scrutinize the load-displacement curve of a pile-load test to be satisfied that the behavior is within expectation.■ References are included in the PDF version of the article at STRUCTUREmag.org. Hee Yang Ng is a Principal Engineer with a building control agency Figure 6. Comparison of load settlement curve.
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legal PERSPECTIVES Construction Documents for International Projects By Gail S. Kelley, P.E., Esq.
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lthough most engineers in the U.S. have at least a passing acquaintance with American Institute of Architects (AIA) documents, few seem familiar with FIDIC documents. Because FIDIC documents are often used on construction projects where the participants are from different countries, engineers pursuing international projects would do well to learn about them. FIDIC is an acronym for the Fédération Internationale des Ingénieurs-Conseils (the International Federation of Consulting Engineers), a worldwide association of consulting engineers. The members of FIDIC are the consulting engineer associations in member countries; the United States is represented by the American Council of Engineering Companies (ACEC). FIDIC is best known for its contract forms, which are almost universally referred to by the color of the book cover. The most widely used form is the “Red Book” – Conditions of Contract for Works of Civil Engineering Construction. The Red Book is part of the three-book “Rainbow Suite”; the other two books are the “Yellow Book” – Conditions of Contract for Plant and Design-Build for Electrical and Mechanical Plant and for Building and Engineering Works Designed by the Contractor, and the “Silver Book” – Conditions of Contract for EPC/Turnkey Projects. The Rainbow Suite thus covers owner-designed projects (the Red Book), design-build projects (the Yellow Book), and engineer-procureconstruct/turnkey projects (the Silver Book). The first edition of the Rainbow Suite was released in 1999, the second in 2017. The numbering is somewhat confusing since there had been several editions of the Red and Yellow books before the first edition of the Rainbow Suite in 1999. However, both the Red and Yellow books were reformatted so that the three documents in the Rainbow Suite would have a standard format. In addition, there were significant changes to the wording; to the extent possible, the wording of the documents was standardized. Thus, the 1999 editions are referred to as the first edition Red, Yellow and Silver books; the 2017 release is referred to as the second edition.
The FIDIC Books While commonly referred to as the “FIDIC contracts,” most of the FIDIC documents are actually General Conditions, similar to STRUCTURE magazine
AIA A201, General Conditions of the Contract for Construction. This is reflected in the book titles. However, each book also contains a lengthy section with guidance for the preparation of “Particular Conditions,” including Part A (Contract Data) and Part B (Special Conditions). Contract drafters can use the Special Conditions to change, add to, or delete provisions in the General Conditions. In addition, the books include notes on the preparation of the Tender (Bid) documents, as well as Advisory Notes for projects that are using Building Information Modelling (BIM). There are also sample forms for securities such as performance bonds, Letters of Tender, Contract Agreements, and Dispute Resolution Agreements. It is worth noting that the FIDIC publications are referred to as “books,” and, given the length of the publications, this is not unreasonable. For example, the 2017 edition of the Red Book is 228 pages; while this includes the guidance notes and sample forms, the General Conditions themselves are 106 pages long. The length of the General Conditions reflects the fact that project participants from different countries may have different understandings of what is customary or reasonable in terms of contract terms and what particular wording means. Thus, the documents are much more detailed than what might be expected when all of the project participants are from the same country. Other FIDIC construction forms include the “Green Book,” a Short Form of Contract for smaller projects; the “Gold Book,” Conditions of Contract for Design, Build, and Operate Projects; the “Turquoise Book,” Form of Contract for Dredging and Reclamation Work; the “Emerald Book,” Conditions of Contract for Underground Works; and subcontract forms for use with the Red and Yellow Books. The “Orange Book,” Conditions of Contract for Design-Build and Turnkey, has generally been replaced by the Silver Book but is still used by those who prefer its terms to those of the Silver Book. As is common in international construction contracts, the FIDIC construction forms refer to the party contracting for the project as “the Employer.” This can confuse those accustomed to U.S. contract forms where the party contracting for the project is generally referred to as “the Owner,” even when it is a tenant rather than the owner of the property in question.
The White Book While the documents in the Rainbow Suite are the best known FIDIC forms, the form that engineers are most likely to work under is the “White Book,” the Client/Consultant Model Service Agreement, which was first published in 1990. The 5th edition (2017) addresses, at least in part, criticisms of the 4th edition and is generally seen as a significant improvement. One such criticism was the vagueness of the standard of care; the 4th edition simply required the Consultant to use “reasonable skill and care.” The 5th edition defines the standard of care as the “reasonable skill, care, and diligence to be expected from a Consultant experienced in the provision of such services for projects of similar size, nature, and complexity.” Contracts created using the White Book are between the purchaser of the services, referred to as the ‘Client’ (rather than the Employer, as with the FIDIC construction contracts), and the supplier of the services, referred to as the ‘Consultant.’ FIDIC also publishes a sub-consultancy agreement for use when the Consultant engages a sub-consultant to provide part of the services.
Conclusion While not common in North America, FIDIC forms are standard on international projects, particularly those funded by multilateral development banks. In the last few years, FIDIC has signed agreements with several such banks; under these agreements, FIDIC has granted the banks a license to refer to specified FIDIC contract forms and use them as part of the standard bidding documents for projects financed by the banks. It can thus be expected that the use of FIDIC documents will continue to increase.■ Disclaimer: The information in this article is for educational purposes only and is not legal advice. Readers should not act or refrain from acting based on this article without seeking appropriate legal or other professional advice as to their particular circumstances. Gail S. Kelley is a LEED AP and a professional engineer and licensed attorney in Maryland and the District of Columbia. She is the author of “Construction Law: An Introduction for Engineers, Architects, and Contractors.” (gail.kelley.esq@gmail.com) J U N E 2 0 21
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INSIGHTS Can We ‘Grow’ Living Concrete Alternatives? By Rollin J. Jones, Sarah L. Williams, and Wil V. Srubar III, Ph.D.
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oncrete has been used for thousands of years. Today, it is the most utilized construction material in the world. Modern concrete is so ubiquitous because of its unmatched compressive strength, fresh-state malleability, and long-term durability, as well as the global availability of raw material resources. The use of concrete is not without its drawbacks. Cement production alone is responsible for 8% of global carbon dioxide (CO2) emissions. Numerous efforts within the cement and concrete industry have targeted reductions in the environmental footprint of concrete. Strategies such as the use of supplementary cementitious materials, like fly ash, slag, calcined clay, and other natural pozzolans, reduce embodied carbon and increase the service life of concrete materials. While companies such as Blue Planet and Minus Materials aim to produce aggregates from CO2, we are a long way off from achieving carbon neutrality for cement and concrete products. At the University of Colorado Boulder, an interdisciplinary team of researchers took inspiration from nature to address the fundamental challenge of producing a carbon-neutral – or even carbon-storing – concrete. They asked, “Is it possible to ‘grow’ a concrete alternative instead? If so, could we keep it alive?”
Self-Healing Bioconcrete An interesting crossover between biotechnology and concrete has been occurring over the past few decades. Self-healing concrete materials, which utilize microbially induced calcium carbonate precipitation (MICP), originated at the Delft University of Technology in the Netherlands and have since been the focus of extensive research worldwide. Self-healing concrete, in principle, is concrete designed to seal its own cracks. Bacteria capable of MICP are mixed into concrete along with the nutrients required for survival. As cracks form, the bacteria produce calcium carbonate minerals, which subsequently fill the cracks. This self-sealing approach could be particularly effective for preventing further exposure and subsequent corrosion of rebar, which is detrimental to the longevity of reinforced concrete structures. While MICP is an effective crack-repair method for concrete, the local environment within concrete is very harsh. It does not take long for most bacteria to lose their ability to reproduce and produce minerals. In fact, less STRUCTURE magazine
than 0.5% of the initial bacterial inoculum has been shown to last more than 30 days.
The Future Led by Professors Wil Srubar, Sherri Cook, Jeff Cameron, and Mija Hubler at the University of Colorado Boulder, the interdisciplinary research team ‘grew’ portland cement-free concrete materials instead. In one of the approaches, researchers engineered a concretelike material with an internal environment more suitable for microbes. In these living building materials (LBMs), photosynthetic marine cyanobacteria capable of MICP were encapsulated in a biocompatible, hydrogelsand matrix. It was demonstrated that the minerals produced by the bacteria significantly improved the mechanical properties of the materials compared to bricks without cyanobacteria. The result was a ‘grown’ composite material with a compressive strength (f´c) of approximately 500 psi – on par with a low-strength cementitious mortar and higher than that of adobe brick. Microbial viability within these materials substantially exceeds that of traditional self-healing concrete. Nine percent of the initial inoculum survived for 30 days in the cyanobacterial mortar. These mortars represent a new materials paradigm, in which microorganisms can participate in material manufacturing and persist within the resultant material, thereby imparting a biological functionality to an otherwise inert structural material.
Other Biological Functions Maximizing long-term microbial viability opens the door for a multitude of biological functions to be engineered into structural materials, like carbon sequestration, chemical sensing and signaling, self-healing, and self-regeneration. In the University’s seminal work, successive self-regeneration was explored. Researchers took one brick, split it into two, fed it fresh nutrients, and effectively ‘grew’ two new bricks from one parent generation. This was accomplished two subsequent times so that one parent resulted in eight living child bricks. This study demonstrated that the manufacture of building materials does not have to be limited to making one steel beam or one concrete mix at a time. In the future, the manufacture of materials will be possible exponentially by harnessing the exponential growth of bacteria.
Sample of a ‘grown’ living building material (LBM).
Initial Conclusions The research efforts are only beginning to scratch the surface of possibilities. Other living organisms, for example, could impart a myriad of biological functionalities, like bioluminescence. Different bacteria species or co-cultures of bacteria could work together in a multifunctional way to self-regenerate, self-heal, and/or sense and respond to external stimuli, such as light, heat, pressure, and magnetism. If nature can do it, living materials can be engineered to do it, too. Considering the challenges humans will face in the 2020s and beyond – climate change, disaster resilience, infrastructure, and space exploration – we should look to the power of biology and living materials at the building scale to help solve some of the most critical environmental crises of our time. By doing so, we could harness the ability of living organisms to create materials that would help us build more sustainable communities – both on earth and beyond – that better blur the boundaries and bring harmony to the built environment and natural world.■ Rollin J. Jones is a Ph.D. student in the Civil, Environmental, and Architectural Engineering department at the University of Colorado Boulder. (rollin.jones@colorado.edu) Sarah L. Williams is a Ph.D. student in the Materials Science and Engineering Program at the University of Colorado Boulder. (sarah.l.williams@colorado.edu) Wil V. Srubar III is an Associate Professor at the University of Colorado Boulder and Principal Investigator of the Living Materials Laboratory. (wsrubar@colorado.edu) J U N E 2 0 21
historic STRUCTURES Bussey Bridge Disaster, aka Forest Hills Bridge, 1887 By Frank Griggs, Jr., Dist. M.ASCE, D.Eng, P.E., P.L.S.
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he Boston & Providence Railroad built the Dedham Branch run- acted occasionning southwesterly from Boston towards Dedham, Massachusetts. ally as consulting The 120-foot-span Bussey Bridge was located six-miles out of Boston engineer; I was and crossed South Street in Roslindale at a 45-degree skew angle. at the Lawrence Originally a wooden bridge, sized for two tracks but only carrying Scientific School one, it sat on masonry abutments. It was called the “Tin Bridge” as at Cambridge one Bussey Bridge – Truss A in foreground, Whipple in background. the wood was covered with tin to minimize the threat of fire. As the year; then went to wood decayed, one of the wooden trusses was replaced with an iron Detroit, where I was employed by the Detroit Bridge & Iron Works Whipple Truss bridge, and the deck structure was supported by this to design their bridges; designed one across the Mississippi river hybrid bridge. In 1876, the railroad decided to replace the remaining where C. B. & Q. Railroad crosses it, at that time the longest drawwooden truss with another iron truss. Engineering News described bridge in the world; built several other bridges on that road, and the the bridge as follows, Illinois Central road, and in several of the Western States; afterward “In 1869, one of the trusses was replaced by a wrought iron…by went South for one winter, returned North, and was employed by C. H. Parker, the other wooden truss being retained. Finally, in July the Moseley Iron Works, and the New England Iron Company as 1876, this iron truss was shifted to the opposite side of the bridge engineer; then built some bridges on my own account at least until from which it had originally been, and another wrought-iron truss this bridge was built… can’t tell how many I built before the Bussey of entirely different design and dimensions was built…and the two bridge; think it was somewhere in the neighborhood of half a dozen.” trusses united together by laterals into one bridge. The first built It was possible to place the cross beams on top of the Whipple truss, B, was a double intersection Pratt truss [Whipple]. It measures Truss since its top chord was built of a flat plate and channels. The 104 feet center to center of end pins, and had 16 panels of 6 feet 6 top chord of the Pratt Truss was built up of phoenix sections coninches each and a depth of 12 feet 6 inches, center to center of pins. nected with cast iron junction blocks, so it was not possible to place The end posts and top chord were built of plates and angles... The the cross beams on top of them. Hewins designed a special cast iron intermediate posts were 8-inch I-beams. The diagonal rods had one junction box that enabled him to have crossbeam hangers suspended pin-end connected to the top chord and one screw-end connected from a pin in the box. by a casting to the bottom chord, and the bottom chord consisted Since these hangers were inside the cast-iron box, they were not of four 6-inch bars, varying in thickness from ¾ to 7⁄8 inches. Truss readily accessible for inspection. Hewins used the Phoenix Iron A measured 104 feet center to center of end pins, had four panels of Works sections for his tension members and top chord sections. The 26 feet each, and a depth of about 16 feet. It had pin connections, rest of the ironwork was done by the Trenton Iron & Steel Company diagonal end posts, and built closed columns, to his design. Apparently, the railroad company with cast-iron connections. The bridge was about relied entirely on Hewins to design and build 18 or 20 feet wide.” the bridge with no outside inspection. Hewins The designer of the bridge was Edmund H. testified he test-loaded the bridge with two of the Hewins. He offered to build it for $4,500 as “a heaviest locomotives on the line and the bridge thoroughly first-class structure in every respect, only deflected 5⁄16 – 8⁄16 inch. Hewins estimated his including a special brand of iron for all tension new truss carried from ¾ to 4⁄5 of the total load members, superior exactness of manufacture, because the track was closest to that truss, and no and in strength to be fully up to that specified.” member, tension or compression, was subjected In his later testimony to a Coroner’s inquest, he to a stress greater than 10,000 psi. testified, Over the years of its use, some passengers “My name is Edmund H. Hewins; …was the expressed concern about the safety of the bridge. builder of a portion of this bridge. The contract The Railroad, however, later testified, “We have required me to build a truss to be placed upon the Cast Iron Junction Box, hangers and cross beams, a competent engineer in the employ of the comwesterly side of the bridge, or on the side nearest trussing rods under beams, and diagonals pany who examines the tracks and bridges several Boston, replacing an iron truss which had been dropping down from the top pin not shown. times a year. Bussey Bridge, along with the rest, there for some years, and which was to be placed has been examined recently, and no adverse report on the easterly side of the bridge; and I was to furnish a floor system. has reached the officials. For aught the company knew, that bridge That was done in the spring or early summer of 1876. I was in business was perfectly sound. That it gave way is certain, and this was no doubt for myself; my first experience in building iron bridges was with the the cause of the accident.” Detroit Bridge & Iron Works of Detroit, Mich., I think in 1863; I am That was the situation until the morning of March 14, 1887, when not sure whether this wasn’t the last bridge that I built; since then I have a northerly bound train with nine passenger cars crossed the bridge at
STRUCTURE magazine
a speed of about 15 miles per hour, and the bridge collapsed, killing 26 people. The engineer, Walter White, who had run trains on the line for over 30 years later testified, “As we approached Tin Bridge, there was no appearance whatever of danger. The bridge lay as solid and safe as ever, the span across showing no weakness, and gradually the train approached. The engine and tender had passed when I looked backward at the cars behind me...However, as I cast a glance at the train behind, I saw the first car swing inward and topple over as though about to fall, and while I still looked, amazed and bewildered, the second and the third cars tipped over in similar positions, and all finally jumped the track. The engine kept to the rails, however, and I turned for a moment to slack my engine. When I looked back, and the time consumed was a very brief minute, of the nine cars but three remained in sight, and the cloud of dust which rose prophetic over the bridge told to a certainty the fate of the remainder.” Newspapers ran very descriptive stories of the disaster, and the Boston Globe sent Henry Prichard, a local engineer, to the site to report on the disaster. Professor George Swain of MIT visited the site shortly after and wrote, “In looking at one of these hangers, where Aftermath. one floor beam was hung at the end joint of the upper chord, I found that the hangers were defective and had been largely rusted off. These hangers were made with a weld, and the weld seemed to be in some places imperfect, and it seemed to me extremely probable that at this joint where the hangers were broken, the original rupture might have occurred… There seems to be no doubt that the quality of the material was imperfect in some places… The angle of skew of the bridge was very large. The skew bridge is more difficult to design correctly than a straight one, but it is perfectly easy to make a skew bridge perfectly strong… The hanging of the floor beams to the upper chord of a deck bridge is a fault in design and very easily avoided.” George Vose testified, “I think that bridge, in its general plan and in its details, was a standing invitation to be knocked to pieces, and I think the immediate cause of the trouble was those broken links…The thing was waiting to tumble down. That is my opinion of that bridge.” The Railroad Commissioners of Massachusetts, consisting of three men, had a 420-page report with illustrations. They interviewed many people, including engineers Henry Manley, George Swain, George Vose, Edward Philbrick, and Thomas Doane. They concluded, “As it happened, the accident was not caused by defects of the system, but the management is none-the-less censurable for its longcontinued neglect to remove this undoubted element of danger. The contract for the rebuilding of the bridge in 1876 was made without proper examination as to the standing of the contractor. Those who acted for the corporation in making the contract had not sufficient knowledge of iron bridge building to enable them to pass intelligently upon the design and specifications. The design and specifications for the bridge were not such as should have been accepted. The bridge was constructed practically without superintendence on the part of the corporation, and the corporation neglected to preserve a copy of the specifications, drawings, and strain sheets. Notwithstanding the repeated warnings of the board, the spaces between the ties on this bridge were far too great for safety; and,
notwithstanding the recommendation of the board in 1881, no suitable guardrails or guard timbers were placed upon the bridge. The disaster and the facts which have been disclosed impose a grave responsibility on the Board of Directors. It is their duty, by the most searching inquiry, to ascertain forthwith whether any other work has been done in a like negligent and incompetent manner, whether in other matters reasonable and well-proved precautions against accidents have been ignored or neglected and whether false economy has been practiced and safety sacrificed… As bridges embody many possibilities of danger, it is proper that special means should be taken to secure careful, competent, and faithful construction, and a thorough and scientific examination of them by the railroads at regular intervals, followed by a thorough State inspection… The board recommends the passage of an act requiring every railroad, at least once in two years, to have a thorough examination of all bridges on its lines made by a competent and experienced Civil Engineer, who shall report in writing to the corporation and to the Board of Railroad Commissioners the results of his examination, his conclusions, and recommendations.” Engineering News published many articles on the failure and was very critical of the Railroad and Hewins. An article on March 19th entitled, The Second Ashtabula Disaster concluded, “As for the bridge itself, we would not forget, nor lead others to forget, that the Boston & Providence Railroad has been and is, in the main, a truly enlightened, liberal, and well-officered and managed corporation...But we need not point out to engineers that a more grotesque and fantastic parody on the accepted canons of good practice than this strange structure could hardly be found unless in its now illustrious predecessor, the one-iron-truss-onewooden-truss combination… Nothing in the catastrophe tends to show that the trusses themselves were too weak, and there has been no time for making the necessary computations. But the badness of their design in many details, we may almost say in every detail, will be at once evident…The hangers…were so boxed in with cast-iron that a real inspection must have been difficult or impossible after erection, but the badness of the fragments now in our office, at least, must have been obtrusively evident during erection.” The Boston Globe summarized the bridge as, “Bad in contract, bad in make, bad in testing, and very bad in general.” Like in other disasters, lawsuits were settled by the Boston & Providence Railroad. The 23 deaths and 100 injuries resulted in claims of $450,000 and almost bankrupted the company. Old Colony took it over in 1888. Like the Dixon Bridge Collapse and the Ashtabula Bridge failure, this disaster was partly due to the poor decisions made by the engineer. The reader is encouraged to read the Special Report by the Massachusetts Board of Railroad Commissioners to the Legislature in Relation to the Disaster on Monday, March 12, 1887, for more on the disaster, available online in Public Document No. 14, 1888, contained in the 17th Annual Report of the Railroad Commissioners January 1886.■ Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, having restored many 19 th Century cast and wrought iron bridges. He is now an Independent Consulting Engineer. (fgriggsjr@twc.com) J U N E 2 0 21
NCSEA
NCSEA News
National Council of Structural Engineers Associations
2021 Leadership Retreat Delivers to SEAs
In April, NCSEA held its 2nd Virtual SEA Leadership Retreat, an annual event that provides educational and social opportunities to strengthen the SEAs. This year's event hosted 21 sessions over the course of 3 days to over 160 attendees. Each day included interactive educational sessions, as well as committee interaction sessions that provided attendees the opportunity to connect with fellow committee members or learn more about a committee they were interested in joining. Attendees heard presentations on Engagement in the New Normal, revenue streams, leadership development, outreach, and diversity, equity, and inclusion. The event also included a wrap-up of 2020 with Past President, Emily Guglielmo, and an introduction to the next year with 2021-2022 Board President, Ed Quesenberry, which included videos honoring Jon Schmidt (Past President, 2019-2020 and President, 2018-2019) and Emily Guglielmo (President 20202021) for their service on the NCSEA Board. Curious about the Leadership Retreat? All recordings from the event have been added to the NCSEA website and can be viewed by visiting www.ncsea.com/events/leadership.
Designed by Structural Engineers for Structural Engineers
SAVE THE DATE: October 13-15, 2021 STRUCTURAL ENGINEERING SUMMIT
Virtual + Hilton Midtown · New York, NY More Information: www.ncsea.com
Call for NCSEA Committee Volunteers Are you interested in volunteering with NCSEA? The Council depends on its members to get involved to help advance our mission and further develop our partnership. Our volunteers help educate on codes and standards, develop publications, create courses, advocate for safe structures and post-disaster recovery, and so much more. If you are a new volunteer interested in serving on an NCSEA committee, please visit www.ncsea.com to complete the Committee Volunteer Application. Most committees admit new members on a rolling basis while others add members only once per year. The following committees are actively seeking new members: The Structural Licensure Committee works with state Structural Engineers Associations to influence the adoption of consistent licensing laws and rules in the interest of public safety, especially relating to licensure of structural engineers. Individuals interested in this committee should have a passion for structural licensure, and the unique responsibility structural engineers have to protect the safety, health, and welfare of the public. The Sustainable Design Committee promotes sustainable design practices within the profession through leadership, advocacy, outreach and education. The primary objectives of this committee's members include advocating for the inclusion of sustainable design within the practice of structural engineering, helping to form sustainable design committees on the local level, creating and disseminating educational material, advocating for policy and code development, and more. More information about NCSEA committees can be found by visiting www.ncsea.com/committees.
Enhance Your Education Program, Become Diamond Review Approved NCSEA's Diamond Review Program was developed to evaluate and approve education providers as well as continuing education courses, seminars, and conferences for the structural engineering profession. Once Diamond Review Approved, structural engineer attendees are eligible to receive PDHs in all 50 U.S. states. The Diamond Review Program is open to suppliers interested in offering technical education to their structural engineer customers, and SEAs that deliver annual conferences, monthly meetings, and/or webinars (Diamond Review Approved education can add tangible value for their membership to your SEA). Learn more and submit an application by visiting www.ncsea.com/education/diamondreview. STRUCTURE magazine
News from the National Council of Structural Engineers Associations
Awards Webinar Series | June 9, 16, 23, and 30 Sponsored by Atlas Tube
Join this free event to learn about Outstanding Project Winners from the 2020 Excellence in Structural Engineering Awards. This program will highlight some of the best examples of structural engineering ingenuity throughout the world. Learn more and register on www.ncsea.com.
ICE Block I, Buehler: June 9
Mackinac Bridge Paint Platforms, Ruby+Associates: June 23
The Academy Museum of Motion Pictures, Burro Happold: June 30
Highest-Quality Webinars at Incredible Value
Secure access to NCSEA's high-quality webinars at an incredible value with the Webinar Subscription Plan. With at least 30 live webinars per year and a recorded library of over 170 webinars, NCSEA's Webinar Subscription Plan is designed for you and your firm! Webinars are available whenever, wherever you need them, and can be viewed by multiple users at the same office, together or remote. Subscribe now by visiting www.ncsea.com and don't miss another webinar in 2021!
NCSEA Webinars June 22, 2021
Register on www.ncsea.com
Strategies for Increasing Productivity and Streamlining Workflows Sarah Scarborough, S.E.
Now that you've spent the last decade mastering BIM integration at your firm, it's time to start thinking about the next generation of production workflows. With the industry warning of impending technology disruptions, structural engineers must stay current on the tools available to maximize internal efficiencies and practice Lean workflows. During this discussion, we will look at software and concepts that are continuously improving the way the industry operates. July 20, 2021
Kansas City Hyatt Walkway Collapse: Reflections after 40 Years Gary J. Klein, P.E., S.E.
On July 17, 1981, the first and third-level walkways through the lobby of the Kansas City Hyatt Regency Hotel collapsed during a tea dance, killing 113. This presentation will review the events surrounding the collapse and describe the ensuing investigations by the National Bureau of Standards and Wiss, Janney, Elstner Associates, Inc. The impact of this tragic event on the industry will also be discussed. Courses award 1.5 hours of Diamond Review-approved continuing education after the completion of a quiz.
Visit bit.ly/ESCNcareerfair for more information
J U N E 2 0 21
SEI Update Membership
Renew your Membership by June 15 to Vote in July Election to SEI Board
Your SEI Membership must be current (dues fully paid) in order to vote in the SEI online election for the SEI Board of Governors in July. Make sure to renew and ensure your email is up to date by June 15. Members above the student grade are eligible to vote, so if you are graduating make sure to upgrade. Renew at www.asce.org or contact ASCE Customer Service at memrec@asce.org or 1-703-295-6300.
Tony’s Top Tips to Recruit Members By Tony Lau, P.E., ENV SP, F.ASCE, ASCE Society Director, Region 8
For a strong future for our association and profession, membership growth is a critical focus – to help students transition to professional members, to retain and engage younger members, and to encourage members to recruit new members. The ASCE Board has set a goal of 175,000 members by our 175th anniversary in 2027. We need your help to get there so we can do more to advance our profession and support our members! Because recruiting members is most effective as personal outreach, I challenge you to set a goal to recruit at least one member each year for the next five years. These are my top tips for success: 1) Develop a list of five colleagues you believe would find value in ASCE/SEI and/or would become a member to support our profession. Value can be different things to different people. One person may find the technical offerings of greatest value while another may want to support ASCE’s effort on the Report Card for America’s Infrastructure, etc. You know best what it would take for your potential recruits to join ASCE/SEI. 2) Schedule a meeting with your colleague to share ASCE/SEI and start the recruiting process. Virtual or in-person meetings work best. Do not try to recruit solely by email. We all get way too many emails, and it will just get deleted. 3) One of the most important things after the initial meeting is to follow up. More than one follow-up may be needed because we are all so busy and joining ASCE/SEI may not be a top priority for them. Reminders are helpful. Like a collegiate athlete is not fully recruited until they sign a letter of intent, new ASCE/SEI members are not fully recruited until they pay their dues. Once you have convinced your colleague to join ASCE, make it easy for them. Direct them to www.asce.org/join and remind them to add SEI. As an extra incentive, you can win prizes by recruiting new members and members that have let their membership lapse. Refer potential members through the ASCE Member-Get-A-Member program https://info.asce.org/mgam and for each of your referred colleagues that joins ASCE using the link provided to them, you will receive a $50 Amazon gift card. If you have the most successful referred members at the end of 2021, you will receive a $500 Amazon gift card. See website for details. Thank you for your membership and all your efforts!
Learning / Networking
Call for Proposals for Structures Congress April 20-23, 2022 in Atlanta SEI invites abstracts and sessions on topics of interest to structural engineers at every level of their career. Submissions due June 15, 2021, 11 pm US ET. Learn more at www.structurescongress.org.
Join us this year in celebrating 25 years of SEI – advancing and serving structural engineering!
Errata STRUCTURE magazine
SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI-Errata. If you would like to submit errata, contact Kelly Dooley at kdooley@asce.org.
News of the Structural Engineering Institute of ASCE Advancing the Profession
Public Comment for ASCE/SEI 7-22
Public Comment for the 2022 edition of ASCE/SEI 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures will be open mid-June through July. The comment deadline is July 31, 2021. Accessing the Public Comment System will require using or creating an ASCE user account. Details are available at www.asce.org/structural-engineering/asce-7-and-sei-standards For additional questions contact ASCE Codes and Standards Senior Administrator James Neckel at jneckel@asce.org.
NEW Collaborative Reporting for Safer Structures – CROSS US Resulting from a major UK initiative to expand capability and reach of the CROSS system, CROSS-US has a new name and website. Share knowledge to help create a safer built environment. Share or browse structural safety info or share a safety issue. Sign up for email and newsletter updates. www.cross-safety.org/us
SEI Online
Access Editor’s Choice Selections for ASCE Journals
Papers are open to registered ASCE Library users (must be logged in). Through June 30: Insights into Ineffectiveness of Gender Equality and Diversity Initiatives in Project-Based Organizations in Management in Engineering (https://doi.org/10.1061/(ASCE)ME.1943-5479.0000893) Through July 31: Comparison between Probabilistic and Possibilistic Approaches for Structural Uncertainty Analysis in Structural Design & Construction (https://doi.org/10.1061/(ASCE)SC.1943-5576.0000556)
SEI Virtual Events
www.asce.org/SEI/virtual-events • #SEILive Conversations Conversations with Leaders in Structural Engineering – Wednesday, June 9, 12:30 pm US ET on Diversity • SEI Standards Series Join live, virtual sessions for exclusive interaction with expert ASCE/SEI Standard developers on state-of-themarket updates. Participants will learn about technical revisions and review a design example. Each session is LIVE and only available 1:00 - 2:30 pm US ET. JULY 15 – ASCE/SEI 72 Athletic Field Lighting SEPTEMBER 16 – ASCE/SEI 59 Blast Protection of Buildings NOVEMBER 18 – ASCE/SEI 8 Specification for the Design of Cold-Formed Stainless Steel Structural Members Individual session: Member $49, Nonmember $99. Student member: Free registration. REGISTER NOW at https://cutt.ly/9hQDTEo
ASCE 7 Online A faster, easier way to work with Standard ASCE 7
Get Started at asce7.online
SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle J U N E 2 0 21
CASE in Point News of the Coalition of American Structural Engineers CASE Tools and Resources 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 their engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need! CASE has several tools available for firms to use to enhance their business development processes: CASE 962-F CASE 962-H CASE 976-A Tool 5-4 Tool 7-1 Tool 7-2
A Guideline Addressing the Bidding and Construction Administration Phases for the Structural Engineer National Practice Guideline on Project and Business Risk Management Commentary on Value-Based Compensation for Structural Engineers Negotiation Talking Points Client Evaluation Fee Development You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.
WANTED: Engineers to Lead, Direct, and Engage with CASE Committees! If you are looking for ways to expand and strengthen your business skillset, look no further than serving on one (or more!) CASE Committees. Join us to sharpen your leadership skills – promote your talent and expertise – to help guide CASE programs, services, and publications. We currently have openings on all CASE Committees: Contracts – responsible for developing and maintaining contracts to assist practicing engineers with risk management. Guidelines – responsible for developing and maintaining national guidelines of practice for structural engineers. Programs – responsible for developing program themes for conferences and sessions that enhances and highlights the profession of structural engineering. Toolkit – responsible for developing and maintaining the tools related to CASE’s Ten Foundations of Risk Management program. To apply, your firm should: • Be a current member of ACEC • Be a member of the Coalition of American Structural Engineers (CASE); or be willing to join the Coalition. • Be able to attend the groups’ normal face-to-face meetings each year: August, February (hotel, travel partially reimbursable) • Be available to engage with the committees via email and video/conference call • Have some specific experience and/or expertise to contribute to the group Please submit the following information to Michelle Kroeger, Coalitions Director (mkroeger@acecl.org): • Letter of interest indicating which committee • Brief bio (no more than a page) Thank you for your interest in contributing to advancing the structural engineering profession!
Follow ACEC Coalitions on Twitter – @ACECCoalitions. STRUCTURE magazine
CASE business practices When your client expects you to pay for change orders. . . By Kevin H. Chamberlain, P.E.
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ASE has recently updated CASE 962-D: A Guideline Addressing Coordination and Completeness of Structural Construction Documents. It remains one of the most popular publications, and for a good reason. The AEC industry continues to identify the lack of quality of structural drawings as a significant issue, even as we approach 20 years since 962-D was created. Allegations of deficiencies in documents are often associated with a change order proposed by a contractor and an Owner looking for someone to blame. Then, of course, the next shoe to drop is the Owner’s expectation that the structural engineer will pay for these change orders! Here are a few things to consider: 1) Take a deep breath. When confronted with a claim, do not react emotionally. If you are a project manager, loop in your firm’s ownership and discuss the background for the change order. If you are an owner, talk to your partners. A fresh set of eyes and a calm second opinion is crucial. 2) A wolf in sheep’s clothing that looks like a bargain. A claim is not always in the form of a letter from an attorney or a summons. Often, an Owner withholds payment of professional services invoices to “pay” for a change order that they allege is due to a design error or omission. Is this a demand for money or services? That’s a claim in the eyes of your insurance company, which requires you to notify them promptly. However, the firm may want to cut its losses and accept the payment reduction if it is cheaper than a claim. And it never appears on your claims history. This creates a business dilemma for the firm: a) accept the payment reduction as the least costly way to get out of the situation and risk insurance coverage, or b) notify your professional liability carrier of a potential claim. Not notifying them is a no-brainer for your insurance company – you did not notify them of the claim in a timely fashion and engaged in negotiations without their input. If, later on, you need their coverage, it will be denied. Do not assume the Owner will be forever appreciative – once they realize the cash drawer has opened, they may be back to raid it again. If you’re willing to pay for change orders, you’ve created a dangerous precedent for this client and potentially all of your other clients. STRUCTURE magazine
3) An allegation is just that. Not all claims have merit, and those with some basis still require investigation. Attorneys encourage us to word our contracts such that any indemnification provisions refer to negligence that has been “determined” (i.e., in court), not just “alleged.” 4) Yes, there are bogus change orders. You will inevitably have a contractor submit change orders for work clearly delineated in the bid documents. It happens a lot. There is a book, Contractor’s Guide to Change Orders, which educates contractors on how to get paid extra for work that is already in their contract. Buy the book; it’s eye-opening and will prepare you to look for the warning signs. 5) Omissions are different than errors. If you forgot to show a relieving angle on the north side of the building, and it had to be added in the field, the Owner would have paid for it anyway, correct? Of course. If then you pay for it, it is as if the Owner got it for free. Attorneys will argue that their client lost the competitive edge of a bid process and paid a premium for the work. It is a negotiating point, but take a firm stance that the Owner needs to pay for their building out of their own pocket. 6) No design is perfect. Our work is not expected to be perfect 100% of the time, only that it meets the industry standard of care. That includes making some mistakes. Do not let your clients hold you to an unreasonable and unrealistic performance standard – and make sure you do not agree to such a standard in your contracts. 7) Who is the favorite child? Realize that many Owners have a better relationship with the contractor than with you. Some owners may automatically believe contractors over the engineer, feeling they have more in common with contractors. Forging a solid and lasting relationship with the Owner over several projects is the best investment you can make in having common ground instead of favoritism. 8) When all else fails . . . put it in your contract. One firm has taken the approach of adding a clause to their standard contract
disclaiming responsibility for any change order. No, it may not survive contract negotiation, but give it a try. Such a clause could help to discourage bogus claims. 9) The risk/reward curve. A $10,000 back charge to a GC whose contract is $10 million is a far cry from a $10,000 demand to a structural engineer with a $20,000 fee. The person with the mouse’s share of the fee should not take the lion’s share of the liability. Educate your clients on that concept. 10) Not everyone is your friend. As engineers, we are glass half full people, and it is easy to get lulled into thinking everyone is playing on the same team. Beware of the contractor who is real chummy asking for substitutions or design changes. Is the Owner’s Representative adversarial and landed the assignment mainly because they excel at “cost recovery”? They are not looking out for your interests. 11) Take a seat at the table. If you get the opportunity to advise the Owner on selecting the contractor or Owner’s Representative – take it. Recommend against parties who have burned you before. Sharks do not change their fins. 12) When in doubt, take a hike. Think of it as deciding whether to hire your client, not the reverse. Is the Owner notorious for back-charging the design team on their projects? Sometimes the best project is the one you never take to begin with.■ Kevin H. Chamberlain is the CEO and Principal of DeStefano & Chamberlain, Inc. in Fairfield, CT, and the Chair-Elect of CASE. (kevinc@dcstructural.com)
J U N E 2 0 21
structural FORUM Meaning and Satisfaction in Structural Engineering By Jim Lintz, P.E., S.E., LEED AP BD+C
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here seems to be a lot of angst among structural engineers about our profession, a general notion that we do not get the credit that we deserve or the proper amount of respect from the public. It is as if we work in the Rodney Dangerfield of professions, “I don’t get no respect.” However, having recently read Samuel C. Florman’s book, The Existential Pleasures of Engineering, first published in 1976, it is clear that our concerns for our profession are not new. Many of the concerns Florman had 44 years ago continue to this day, such as not attracting enough of the best and brightest students, engineering education being too technical, lack of passion for our work, lack of exposure for engineers in popular culture such as novels or movies, and not getting the same respect as other professions. Since then, other concerns have developed, such as not retaining talent, not adopting new technology quickly enough, and being replaced by technology. While all of these concerns may be valid, our profession has remained an indelible and essential part of society. To keep our profession attractive to the next generation and respected by society, we should let others know why our jobs are worthwhile. Instead of consistently highlighting our concerns, we should be letting college students, young engineers, and our communities know how our careers bring satisfaction and meaning to our lives. Ask a group of structural engineers what they like about their jobs, and the first item on most lists will be the joy it brings to see their work transformed into real-world structures. Creating something tangible from an idea is fundamental to being human. Enhancing this is the knowledge that our work improves society. Exactly how lives will be improved is clear when designing a new children’s hospital, but what about designing a new warehouse? Rest assured, no one would invest the enormous amount of capital it takes to construct the projects we design without a solid belief that their investment will pay off. Via free markets, the only way for this to occur is for each party to improve the other’s life through a product, service, or payment. Otherwise, no deal would be made. Fortunately, en route to the final transaction, many other lives will be made better as well. That warehouse will provide temporary jobs to construction workers and new, longer-lasting jobs for warehouse STRUCTURE magazine
workers. It will provide storage for products and may be the reason that your new online purchase can be delivered the next day and at no cost, instead of next week. All of which improves the quality of life. The modern world has been, to no small degree, designed by engineers. As structural engineers, we play an essential role. Consider the simple warehouse again. A structural engineer was involved in designing the bones of the building. A mechanical engineer then designed the HVAC system, an electrical engineer the lighting, a plumbing engineer the piping, a fire protection engineer the sprinklers, a civil engineer the grading, and a geotechnical engineer the site itself. If we broaden our perspective, we realize other engineers designed the light bulbs, the mechanical equipment, the dock levelers, and so on. And more engineers designed the forklifts, racking systems, and computers for the building. We can further consider the electrical grid, the water and wastewater systems, and the roads and bridges leading to the warehouse, all designed by engineers. Considering for just a moment all of the engineering that went into making one simple warehouse functional, we should be proud to be a part of that process, humbled that it takes so many, and grateful to live at a time and place where this is possible. As structural engineers, we are engaged in creative work nearly every day. Society typically assigns the term creative to sculptors, painters, musicians, and authors, but not engineers. This is a mistake. Our job is to turn ideas into real-life structures. That is pure creativity. The constraints upon our work do not stifle creativity; they focus it. Developing novel, elegant solutions to complex problems is at the heart of our profession. Whether by new computer code to streamline calculations or by sketchpad to design a complicated connection, solving challenging problems brings deep satisfaction. This comes from understanding the solution in our mind and seeing the final built structure standing strong against a storm. To be a successful engineer, one needs to understand mathematics and science at a
To keep our profession attractive to the next generation ... we should let others know why our jobs are worthwhile. level that is well beyond the norm of society. While this may set us apart, it is not something that we should shrink from. With this understanding comes both enjoyment and responsibility. Having a better understanding of the physical world allows us to appreciate and enjoy the wonders of both the built and natural environment. Driving down the road and understanding the forces in the cables of a bridge or the elasticity of a branch swaying in the breeze enhances our experience of the world in a way unbeknownst to those around us. There may be times when we feel that society is not giving us the credit we are due. If that is getting you down, try creating a photo gallery on your phone of projects you have designed. Something you could show an acquaintance who wonders what exactly you do. Or stream some old Mythbusters episodes to enjoy the fun of engineering again. Or maybe grab some drinks after work with your co-workers and share old stories. You will be glad you did. Modern society would not exist without the work of scientists and engineers whose understanding of the laws of nature allows us to build great cities, travel quickly between them, and communicate across vast distances without going anywhere. Fulfilling our responsibility to carry on this incredible human project brings meaning to our work and our lives. What more can someone ask of their career than for it to instill meaning in their own lives and for it to improve the lives of those around them?■ Jim Lintz is a Structural Engineer at LJB Inc. in Dayton, OH. (jlintz@ljbinc.com) J U N E 2 0 21
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