STRUCTURE magazine - June 2020

STRUCTURE JUNE 2020

NCSEA | CASE | SEI

TALL BUILDINGS

INSIDE: Evolution of Fire Safety

8

High Capacity Foundations 10 Hudson Yards Seismic Performance of Towers

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Contents JU N E 2020

Columns and Departments 7

Editorial Iconic Global Structures – what did we learn?

8

By Derek Skolnik P.E., Ph.D.

Structural Performance The Evolution of Fire Safety in Supertall Buildings

By James Antell, AIA, P.E.

22 HIGH CAPACITY FOUNDATIONS FOR 110 N WACKER DRIVE

12

Round Timber

By Darren S. Diehm, P.E.

Structural engineers designing the new building at 110 N Wacker Drive, determined that the core foundations would support nearly 40,000 kips

16

20

36

38

Brown University identified an opportunity to relocate the historic Sharpe

32 SEISMIC PERFORMANCE OF SLENDER CORE-ONLY TOWERS

By Mark Sarkisian, S.E., Eric Long, S.E., and David Shook, P.E.

The design and verification of the 500 Folsom seismic force resisting system revealed the importance of conducting a detailed analysis even if code-provisions are satisfied.

June 2020 Bonus Content Additional Content Available Only in the Digital Magazine – www.STRUCTUREmag.org

Code Updates What’s Old Is New Again

By Lori Koch, P.E., and Matthew Hunter

By Dan Knise

CASE Business Practices Coordination and

By David Ruby, P.E., S.E., SECB

40

Historic Structures Bridge Failure in Dixon, Illinois

House. Moving the building opened a site for development while simultaneously preserving the historic fabric that makes the neighborhood unique.

Legal Perspectives Managing the Risks of

Completeness in a BIM Dominated World

29 BIG MOVES FOR THE HISTORIC SHARPE HOUSE

By David J. Odeh, P.E.

By Richard S. Barrow, P.E.

Subconsultant Relationships

immensely to the success of 10 Hudson Yards. The reduced weight of the and foundation reinforcing.

Structural Rehabilitation Fuzzy Wood and Coastal Piles

By Adam Beckmann, P.E., Chris Christoforou, P.E., and Michael Squarzini, P.E.

composite system allowed for reductions in column sizes, beam depths,

By Sean Clifton, P.E., S.E.,

Russell Larsen, P.E., S.E., and Kevin Aswegen, P.E.

socketed drilled shafts of 600 ksf.

The structural and architectural advantages of filigree slabs contributed

By Amelia Baxter and Michaela Harms

Structural Design Performance-Based Wind Design

using high-capacity concrete, a design equivalent end bearing of the rock

26 10 HUDSON YARDS

Building Blocks The Future of Structural

50

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

InFocus Understanding Why

By John A. Dal Pino, S.E.

In Every Issue 4 44 47 48 49

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

On the Cover

The 875 N. Michigan Avenue Building in

Chicago, formerly the John Hancock Center, is a typical example of high-rise construction during the post WWII era. See the full article in the Structural Performance column on page 8.

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.

JUNE 2020

5

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EDITORIAL Iconic Global Structures – what did we learn?

The last great international conference … that I attended

By Derek Skolnik P.E., Ph.D., M.ASCE

D

uring the last two days of September 2019, a group of internationally renowned structural engineers, along with their architectural (or other-related-industry) cohorts, assembled in Dubai to share experiences in building some of the world’s most iconic structures.

The Iconic Global Structures – what can we learn?

Conference was the successful inaugural event jointly organized by IStructE and SEI/ASCE. It aimed to promote an international exchange of essential lessons learned within the structural engineering profession. It was awesome. For those (left) Iconic Global Structures – what can we learn post-conference summary document. Available at who missed it, a post-conference summary report www.asce.org/SEIGlobal (right) The Last Panel. From left: Derek Skolnik, Jiemin Ding, Jiliang Chen, is freely available at www.asce.org/SEIGlobal.

Ron Klemencic, Edward Dionne, John Peronto, Peter Weismantle, and Hendrik Stephan.

I was honored to be a member of the technical committee. The invitation to do so came from a chance encounter perhaps COVID-21 emergence limit attendance? Is handshaking with SEI Director Laura Champion at the 2018 National Conference really done? Will presenters and panelists wear facemasks? Ok, that is on Earthquake Engineering in Los Angeles, CA. Not only did I get enough of my anxiety; I shall now explore the possibility of co-location to help shape the event, but I also had the pleasure to moderate the while working from home. BTW, I have been a remote worker at last session that culminated in a panel discussion with some of our Kinemetrics for over five years and do feel qualified enough to make profession’s brightest stars – see last panel photo on performance- outrageous suggestions. based design. The benefits of networking at conferences are evident.

Virtualized meetings are the obvious alternative when co-location is The post-conference report elegantly summarizes the event with not possible. There can be awkwardness and shortcomings, of course, excellent images of several of the iconic structures. Of course, there but online meetings do get more effective with each iteration. What is no replacement for firsthand experience, but the committee spent never seems to get better, in my opinion, is missing opportunities to considerable effort to highlight the more significant points, and it develop interpersonal relationships and stumbling upon impromptu shows. My assigned task was to divine the most important lessons conversations that foster new ideas. Can these and other officelearned from the many pages of notes. There were many, but we settled intangibles ever sprout organically in a virtual landscape? I do not on these six: influence, collaboration, reflection, constructability, peer know. However, I recently observed something like that in my young review, and sustainability. son collaborating with schoolmates on Fortnite while simultaneously As I ran through the conference report again, I thought about the FaceTiming each other – a dual virtual environment. Imagine an entire current applicability of each lesson learned, given the COVID-19 project team plugged-in to a virtual construction trailer with full-on pandemic. Most of the statements remain true as ever, but there was gamer headsets constantly chatting away but without the trash talk. one red-flag-waving exception – that of the need for co-location of I admit that it would be too chaotic for me to get any work done, collaborative teams throughout a project. but chaos breeds opportunity, as they say.

The lesson learned around collaboration speaks to the use of coThese days I find myself thinking a lot about the potential long-term location and effective communication to enable more collaborative impact from sustained co-isolation or perhaps biennial human migramulti-disciplinary work. Working in the same physical space during tions to home offices. Our profession is resilient, but yes, some global a project fosters shared thinking and new levels of expertise. For activities such as conferences and sizeable international construction example, structural engineers that become fluent in the language of projects will need some adapting. Our challenge is to maintain global other disciplines become invaluable “trade translators.” interconnection throughout.

Between the time of composing the report and this editorial, a whole If you want to help address this challenge, consider joining the SEI lot of bad stuff has happened. It is unsettling to write about the ben- Global Activities Division www.asce.org/SEIGlobal, and efits of conferencing while in lockdown and not knowing what the please check out the Iconic Global Structures conference world is going to be like three months from now. I hope that these summary document.■ thoughts elicit more worry-free chuckles than wood-tapping knuckles, Derek Skolnik is a Senior Project Manager at Kinemetrics, Inc., a world but for me, right now, it feels like it will be some time before we can leader in earthquake monitoring technology and services. He currently serves enjoy international exhibitions like this one. I do wonder what a large as Secretary on the SEI Global Activities Division Executive Committee. conference would look like if it were to take place shortly after lifting social distance mandates. Will the fear of COVID-19 resurgence or

STRUCTURE magazine

J U N E 2 02 0

7

structural PERFORMANCE The Evolution of Fire Safety in Supertall Buildings

By James Antell, AIA, P.E.

A

s structural engineers, our approach to structural fire safety in supertall buildings has evolved along with overall

fire and life safety goals. Structural systems have progressed from the early steel frame towers of the 1970s to current practice incorporating concrete and composite steel/concrete

John Hancock Center, Chicago.

structural elements. This article looks at the relationship between structural engineering practices for tall buildings and how these practices have influenced fire safety strategies for passive and active protection systems in tall buildings over the last 50 years.

Tall buildings have always relied on a combination of passive building elements and active fire detection, alarm/notification, and suppression systems to control fire and its effects. What has changed is how we use these passive and active elements and the intended results of these fire safety strategies. This evolution is the result of both advances in structural and fire safety technology, as well as new safety and security threats that make the integrity of the building structure and the ability to evacuate occupants in a timely manner of the utmost priority.

Fifty years ago, the technology available to us to create fire-safe tall buildings was quite basic. Passive protection of the steel structural elements was provided by sprayed-on cementitious or fiber fireproofing materials. These materials were designed to insulate the steel from the heat of a fire and keep the steel below critical temperatures for a prescribed period (usually 3 hours for the structural frame and 2 hours for floors when tested to a standardized fire exposure test). Exit stairs to evacuate occupants from the fire area to areas of relative safety within the building were provided. Firefighting features such as elevators and firefighting water supplies to facilitate fire department response and manual suppression of fires at altitude were developed. Still missing from fire protection strategies were reliable fire detection, alarm and voice communications, and automatic suppression by sprinklers.

The idea that tall buildings would someday be targets for a broad range of malicious attacks was not a consideration for the first wave of tall buildings constructed in the 1960s and 70s in the U.S.

The Post WWII Era The 1970s witnessed the first, sustained tall building boom in the United States with iconic, tall buildings constructed in Chicago, New York, and other major cities around the country. Most building codes did not include provisions specifically for high-rise buildings. During this time, design practices advanced ahead of codes and standards.

Only after significant experience with high rise design and high-rise fires did codes and standards adopt specific high-rise provisions. In those early days, passive elements were more prevalent than active elements. Fire alarm systems were not considered reliable in fire 8 STRUCTURE magazine

safety design. And, automatic sprinkler systems that, at the time, used thick-walled schedule 40 pipes with screwed fittings that were cut and threaded on-site, were considered too expensive.

The goal of passive elements was to control the spread of fire and to maintain the buildings’ structural integrity during occupant evacuation and manual firefighting operations. Structural systems were primarily structural steel frames with steel deck and concrete floor slabs.

The 875 N. Michigan Avenue Building in Chicago, formerly the John Hancock Center, is a typical example of high-rise construction during this period. The building structure consists of a robust structural steel frame where the perimeter structural system carries significant structural load with perimeter diagonal bracing for lateral loads. Although the robust steel structure itself has significant resistance to heat from a fire by virtue of its high thermal mass, spray-applied cementitious fireproofing was also applied to the structure to provide the required fire resistance of 3 hours to the structural frame and 2 hours to the floors.

Exit stairs, elevator shafts, and mechanical shafts were enclosed with fire-rated non-structural drywall or concrete block construction. In those early designs, the building core was typically not a structural element and did not generally contribute to the structural strength or fire resistance of the structure itself. Active firefighting systems were limited to a water supply. In most cases, this included relatively small on-site tanks and heavily relied upon municipal water supplies pumped to upper levels of the building to serve wet standpipe outlets located in each stair for use by fire department personnel. Those services were considered highly reliable, and the need for the building to operate “off the grid” was not a high priority.

Codes “Catch Up” During the mid-1970s, building regulations in Chicago, New York, and throughout the U.S. “caught up” with the design practices of early tall buildings and mandated specific requirements for tall buildings. The requirements included most of the passive features incorporated into the earlier designs, including structural fire resistance and enclosed egress

stairs. At the time, automatic sprinklers, active smoke control systems, and emergency voice communication systems were not required by these codes.

During the 1980s, the U.S. experienced many significant fires in high-rise buildings, including hotels and office buildings. However, none of those high-rise fires resulted in the overall failure of the buildings’ structure.

In several cases, there was significant localized structural failure where the localized heat of the fire had compromised the steel structural frame with spray-on fireproofing. As a result of those fires, many of the U.S. codes mandated automatic sprinkler protection for all new high-rises and eventually for most existing high-rise buildings. It is widely held that automatic sprinklers are the most effective way of controlling fire size and therefore controlling heat and smoke from a fire. Sprinkler system technology had also advanced considerably at this time, reducing costs and improving constructability.

Petronas Twin Towers and the Kuala Lumpur skyline

Asia 1990s

9/11 Brings Changes As a result of the events of September 11, 2001, the U.S. National Institutes of Science & Technology (NIST) undertook a series of investigations into all aspects of the design and construction of the

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The next tall building boom occurred in Asia during the 1990s. Two iconic projects during this timeframe were the Jin Mao Tower in Shanghai and the Petronas Twin Towers in Kuala Lumpur, Malaysia.

Built contemporaneously, these two projects represent an evolution in the structural design of tall buildings and the associated fire safety strategies of these buildings. In both cases, the buildings were designed

by U.S. based design and construction teams, using a combination of local building regulations and international design standards and practices. In terms of both structural systems and fire safety, both projects were quite advanced for their time.

Both designs incorporated poured concrete structural cores with composite perimeter super columns and steel and concrete floor assemblies. The early use of high strength concrete enhanced the design efficiency of the core and perimeter columns. This structural system was found to have the advantages of both improved structural efficiency and improved ease of constructability. With the concrete core carrying a significant portion of the lateral loads on the building, the structural system had the advantage of allowing the perimeter to be more open to more glass curtain walls and better views for building occupants.

The concrete cores provide a highly fire-resistant enclosure in the center of the towers, separated from the occupied areas of the buildings where fire is likely to occur. All critical life safety egress stairs, firefighting elevators, and critical risers’ power, fire alarm, fire sprinkler, and smoke exhaust are located within the fire-resistant core, protecting them from fire occurring in the perimeter occupied portion of each floor. The concrete core wall also forms a portion of a fire-rated public corridor system surrounding the core and separates occupied office spaces from the common public exits and core elements.

In the case of the Jin Mao tower, the concrete core contains the emergency egress

stairs, firefighting elevator vestibules, passenger elevators and critical fire alarm, emergency communications, smoke extraction, and fire suppression system risers, protecting them from exposure to a fire in the occupied portion of the building. The concrete cores also facilitate safe “refuge” areas required by local building regulations.

In the Petronas Towers, in addition to the emergency evacuation stairs, firefighting elevator vestibules, and critical life safety risers, the concrete cores also contain passenger elevators that can be used in some emergency scenarios to evacuate building occupants from the sky lobby levels. The sky lobbies are designed with limited combustible contents so they can act as safe areas for staged building evacuation during emergencies. The sky bridge has the additional benefit of providing an emergency evacuation route between the towers in case of an emergency affecting the lower portion of one tower.

JUNE 2020

9

World Trade Center towers. The results of Passive fire protection is achieved primarthese investigations, published in the docuily by the building’s concrete structure, ment “NIST NCSTAR 1 Final Report on the which acts as a fire-rated enclosure for all Collapse of the World Trade Center Towers,” exit stairs, firefighting elevators, and evacuincludes “areas in current building and fire ation elevators. Critical fire safety system codes, standards, and practices that warrant risers are incorporated into the building’s revision” including “a list of 30 recommenconcrete core. dations for action in the areas of increased Because of the extensive pace of developstructural integrity, enhanced fire endurance ment in Dubai at the time of construction, of structures, new methods of fire-resistant the building was designed so that it does not design of structures, enhanced active fire rely on municipal water or electric service protection, improved building evacuation, during emergency operations. The fire proimproved emergency response, improved tection water supply is provided from water procedures and practices, and education tanks of 1- or 2-hour capacity located on all and training.” mechanical floors. Water supplying autoThe NIST recommendations included matic sprinklers is fed by gravity to zones changes to structural design standards to prebelow the tanks, so there is no reliance on vent progressive collapse, as well as changes fire pumps to supply necessary water in an to fire endurance of structures, in-service emergency. performance of fireproofing materials, Finally, evacuation elevators are incorporedundancy of active fire protection systems, rated into the design to enable full building and provision for full building evacuation evacuation, if needed, based on a broad range in response to a wide range of emergency of emergency scenarios. Shuttle elevators are scenarios other than fire. operated in “lifeboat” mode to shuttle occuMany of these recommendations were pants from sky lobbies to grade. adopted by model code groups and were Burj Khalifa – currently the world’s tallest building. incorporated into their building and fire The Future: Jeddah Tower codes. The 2009 International Building Code (IBC) adopted a series of new requirements specific to buildings more than 420 feet (128 Currently under construction in western Saudi Arabia, Jeddah Tower, meters) in height. These requirements include: when complete, will be the world’s tallest building at over 1,000 • Reverting to earlier code requirements mandating a fire meters. Structurally, the building is quite similar to the Burj Khalifa, resistance of the structural frame of 3 hours by removing the using a fully reinforced concrete structure for most of the building, allowance for 2-hour ratings on buildings over 128 meters. incorporating concrete core, shear walls in each wing, columns, and • Design of the structure to eliminate the likelihood of concrete floor slabs. Because the building is residential, it is also highly progressive collapse compartmented by concrete corridor walls and demising walls. Passive • Impact-resistant construction materials for walls enclosing fire protection is achieved primarily by the building’s concrete strucexits and elevator shafts ture, which acts as a fire-rated enclosure for all exit stairs, firefighting • Minimum bonding strength for spray-applied fireproofing elevators, and evacuation elevators. Critical fire safety system risers materials, so they are less likely to fail in a fire are incorporated into the building’s core.

• Redundant water supply sources and risers for automatic One of the distinguishing features of the Jeddah Tower is the use of sprinkler systems highly protected refuge floors every 20 floors, as mandated by local • One additional exit stair for firefighters or elevators designed code. The refuge floors are full floors and, in an emergency, become for emergency occupant evacuation safe areas for occupants’ evacuating the building. Occupants from Many of these enhanced features that were incorporated into high- the 20 floors above each refuge floor can evacuate their zone of the rise buildings on a project-by-project basis were now mandated for building using exit stairs to reach the refuge floor below. Each refuge all buildings over 420 feet. floor is a safe area with minimum connections to other floors and includes independent mechanical systems. During an emergency, they serve as a staging area for elevator evacuation. Shuttle elevators 2000 to the Present are used to evacuate occupants from refuge floors to grade.

The design of Burj Khalifa, currently the world’s tallest building, began in 2003 after the events of 9/11, but before the release of the Conclusion NIST report and the incorporation of the report’s findings into the 2009 IBC. The design does incorporate several features that even- Over the past 50 years, our approach to fire safety in tall buildings has tually became part of the 2009 IBC requirements. The design also evolved considerably, incorporating new technologies to provide buildincorporated some relevant aspects of the local code in conjunction ings that are protected from a broad range of threats from with international codes (IBC) and fire safety enhancements. fire, natural disasters, and malicious acts. As we keep building Structurally, the use of a reinforced concrete structure incorporat- taller, our strategies and technologies will continue to evolve.■ ing a concrete core, shear walls in each wing, columns, and concrete James Antell is a Regional Practice Leader for Telgian Engineering & floor slabs was dictated by the fact that the building is primarily a Consulting. Mr. Antell acts as an advisor to design teams and building residential occupancy and the shape of the building is three wings. owners regarding compliance with local and international fire safety codes Because the building is residential, it is highly compartmented by and standards. (jantell@telgian.com) concrete corridor walls and demising walls.

10 STRUCTURE magazine

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building BLOCKS The Future of Structural Round Timber The Original Mass Timber

By Amelia Baxter and Michaela Harms

As

Architecture, Engineering, and Construction (AEC) decision-makers continue to explore new applications for mass timber products. Forest product innovators are applying decades of existing research toward the scaled commer-

cialization of structural round timber (SRT). The authors of this article predict that rising demand for mass timber products is an enormous opportunity for accelerating the use of SRT columns, spanning members, and trusses.

New Markets

trees are left in the forest longer while the timber stand reaches a more mature stasis. Forest products can thus achieve higher ratings in data-driven certifications such as Life Cycle Analyses (LCAs) and Environmental Product Declarations (EPDs).

New software platforms such as EC3 and Oneclick LCA then glean carbon sequestration rates from EPD and LCA data, drawing the market’s attention to carbon sequestering materials, such as mass timber.

A dramatic rise in North American mass timber markets reflects successful public and private initiatives over the last decade to develop new markets for forest products from sustainable forest management. The U.S. alone has over 700 million acres of private and public forests, some ten percent of which are critically under-managed (Bowyer, 2011). The traditional wood products markets that once supported these forests, such as lumber and pulp, are depressed. The long-term decline New Markets for SRT of the housing and paper industries exacerbates overstocking and requires new diversified markets Well managed forests are defined here as forests folwithin the forest products industry. lowing prescribed plans that successively remove, As government awareness has grown for the role or thin, low-value trees while the remaining forest Figure 1. A Douglas Fir’s ability to commercial construction markets can play in forest capture carbon from the atmosphere grows toward mature high-value harvests. This economies and ecologies, building professionals increases exponentially between 35 to healthy forest management may cost more during and innovative building owners have amplified 60 years of growth. As a forest matures the lifespan of the forest but generally results this call for new engineered wood products. Private toward a 60-year planned harvest, it in more stable ecologies and improved carbon manufacturers have followed suit and invested in adds considerable mass and carbon sequestration. If forest owners, both public and manufacturing capacities for high-tech engineered storage. (Source: PortBlakely.com) private, are expected to manage their forest lands wood solutions such as glued laminated and crossfor healthy ecologies and carbon sequestration, laminated timber. The results of this 21st century “wood zeitgeist” can then they need new markets for the on-going timber thinning of a be seen in tall wooden buildings such as T3 in Minneapolis, Brock forest’s first 30 to 60 years (Figure 1). The most optimal market value Commons in Vancouver, and HoHo in Vienna. of a timber thinning is when it is used as Structural Round Timber New markets for mass timber products can also be attributed to in place of other high-margin mass timber and steel alternatives in the role carbon footprints now play in AEC decision-making and commercial construction. specification. Detailed studies show that well-managed forests result SRT is stronger in bending than an equivalent cross-sectional area in increased carbon uptake. This first occurs as forest fiber growth of milled lumber due to the wood fiber continuity and preservapatterns speed up when trees are thinned of competition, and then as tion of grain orientation (Wolfe, 2000) (Figure 2). Comparing SRT strength data (Wood et al. 1960) to dimensional lumber strength data (Green and Evans 1989) has shown that the coefficient of variability (COV) for SRT is about one-half to two-thirds that of conventional lumber. This is because wood fibers in milled lumber are disrupted and discontinuous, creating stress concentrations and fracture initiation, while wood fibers in round timber flow continuously around knots on the surface. This lower variability in strength leads to higher design values for SRT. Figure 3 illustrates the effects of lower COV on design values. The blue line shows the lower 5th percentile limit for Select Structural Red Pine 2x6 lumber, assuming a normal distribution. Assuming the same mean strength value but half the COV (15%), a distribution and lower 5th percentile for SRT is overlaid with a red line. In this Figure 2. The largest timber (A) that can be milled will be only 17-33% of the example, the ratio of 5th percentiles is 1.60 for round wood versus strength of the log (B). (Source: Wolfe, 2000, image by author).

12 STRUCTURE magazine

x 10-4

Probability Density Function

2 by 6 Select Structural, COV = 32% Est. Roundwood, COV = 15% 3

2

1

0 1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Modulus of Rupture Figure 3. Estimated effects on SRT design values if variability can be lowered through a reliable MSR process.

milled lumber. The tighter distribution allows for more predictable design values based on fewer outliers.

Grading Methods Before exploring the future of SRT, an engineer must look to the historical research of this material. SRT is one of the oldest construction materials used by humans, yet there is still no standardized means of deriving optimal design values for SRT in spanning structures. When U.S. builders bring SRT into commercial applications, structural engineers and building code officials often win approval using significant over-designs of members and connections. This over-design is the result of using design values derived from antiquated visual grading techniques meant to be applied to cantilevered poles and piles loaded axially, not in spanning.

Design values for structural round timbers used in pole or log buildings may be determined using the standards published by ASTM International. The ASTM standard D 3200 refers pole designers to ASTM D 2899, the same standard used to derive design stresses for timber piles. Derivation of design stresses for construction logs used in log homes is covered in ASTM D 3957, which provides a method of establishing stress grades for structural members in any of the most common log configurations. These approaches typically assume regular inscribed geometry with an irregular shape. Additional research has been conducted on developing a machine evaluation system for round timber beams (Green and others 2006).

Despite a substantial body of research demonstrating superior design values for SRT, the International Building Code (IBC) only accepts design values initially established in the 1930s. Current-state visual grading methodologies build on material properties derived from clear wood one-inch-square samples paired with substantial margins of safety. Visual grading methods to-date are better researched, updated, and practiced for milled lumber where grain patterns can be clearly seen, and do not suit SRT in the same manner.

Machine Stress Rated (MSR) grading, on the other hand, is better able to assign optimal and reliable design values to this geometrically variable natural resource stream, thereby improving consistency and production yields. For this reason, the USDA Forest Products Laboratory in Madison, WI, now recommends MSR grading practices when assisting other countries with assigning new allowable design values to lumber resources or assigning design values to yet-unclassified wood species in the U.S.

Figure 4. Red Pine ASTM D198 Flexural Testing at the USDA Forest Products Laboratory, Madison, WI.

The core of MSR grading is the ability to non-destructively measure one or more characteristics from the source material, such as modulus of elasticity (MOE) and specific gravity, to reliably assign memberspecific mechanical properties. MSR grading allows manufacturers to sell material with reliable design values without destructive testing, giving architects, builders, and consumers confidence in the specified material.

Research has shown that longitudinal stress tests on standing trees can assist manufacturing decisions by sorting the poles into strength classes earlier in the supply chain (Wang 2001, 2004 and 2007). The authors of this article have built on this body of research. They are working toward the implementation of such nondestructive evaluation (NDE) practices at the log sort yard and in the manufacturing yard to further reduce the downstream variability of SRT members and ensure that components are used in the highest value application for their strength. (Figure 4). What is more, pairing NDE data collection with ASTM D198 tests (prescribed flexural bending test) demonstrates that SRT can earn assignable design values 25 to 50% higher than the currently established visual grades, which is allowed when NDE data is cross-correlated with destructive test data by species.

The presence of continuous fibers in SRT, the lack of which diminishes the structural capacity of conventional milled lumber, needs to be considered in an optimized SRT grading system. These benefits are not currently attributed to SRT because existing visual grading and ASTM procedures were developed for the use of poles and piles in an exterior environment, or log buildings. Until methods consider spanning applications in enclosed building structures, grading of SRT will continue to result in over-built structures that inhibit cost competitiveness and market expansion.

Quality and Design Values Building a broader commercial market for SRT will require a standardized MSR grade procedure for establishing structural design load values. MSR grade methodologies in the milled lumber industry evaluate lumber using a nondestructive machine test, followed by a visual override of certain characteristics that the machine may not properly evaluate. MSR applies measured pressure to each piece of lumber to determine the modulus of elasticity (MOE), and uses the relationship between MOE and bending strength to assign a design value. The general procedure for establishing the mechanical properties of MSR grade systems is covered by ASTM D 6570. continued on next page JUNE 2020

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Figure 5. Festival Foods Grocery Store (Madison, WI). The red pine SRT trusses span up to 55 feet and support up to 500lbs/lf. The load-bearing Ash Columns support up to 220kip in live and dead loads. Courtesy of Heartland Photography.

The advantage of using MSR grades for logs is twofold. First, the conventional grading system for logs is based on a conservative, mathematical determination of design values; second, MSR grading allows the identification of select pieces that are superior to the average values based on visual grading alone. For example, a study by Green et al. showed that 64% of the 292 machine-graded lodgepole pine logs studied could be assigned an MSR grade of 1.4E-2250Fb, which is 27% stiffer and 80% stronger than the allowable values for No. 1 lodgepole pine logs. Furthermore, nearly 17% of those 292 logs could be assigned an MSR grade of 1.8E-3350Fb, which is 67% stiffer and 2.7 times stronger than allowable values for No. 1 lodgepole pine logs (Green, 2005). The authors, applying Green’s techniques to a study of red pine poles, found similar optimized values at 23% stiffer and 33% stronger than assignable visual grades.

Studies have shown that continuous fibers have a significant benefit to the design value of SRT, but a broad study solely to investigate this would be cost-prohibitive. The authors are instead developing a better approach – a standardized MSR grading procedure that incorporates SRT’s unique factors and optimized strength. NDE techniques that include stress wave velocity, transverse vibration, and static mid-point loading can be used to assign an MSR grade to SRT members. Current MSR lumber grade designations could also be utilized for SRT. This relationship and familiarity will enhance the acceptance of the MSR procedures for logs.

The Future of SRT With cost-effective, reliable, scaled grading procedures that allow for 25 to 80% increases in design values for over half of all timber thinnings removed from managed forest plans, SRT of any species becomes a cost-effective mass timber product. Also, SRT thinnings from the on-going management of forest stands are regionally available in nearly all areas of the U.S. This contributes to their carbon-negative footprint, as does the minimal additional processing required to leverage the innate strength of unmilled trees.

The authors predict that rising demand for mass timber products is an enormous opportunity for SRT columns, spanning members, and trusses. Recent projects across the country include the Festival Foods Grocery Store in Madison, WI (Figure 5), and the Blakely Elementary School (Figure 6) on Bainbridge Island, WA. Both projects include heavy live and dead loads (up to 220 kips on an individual column and 500 lbs/lf on an individual truss) and high seismic conditions in areas of high seismicity. Long spanning trusses are perhaps the earliest and largest potential market for scaling SRT, and examples to-date include 60-foot spans tested at the USDA Forest Products Laboratory.

In addition, early adopters of SRT in new commercial settings are specifying load-bearing columns in a wide range of applications, including K-12 education, retail, workspaces, health and recreation, and university facilities.

An approved MSR grade system, in addition to facilitating scale, will promote the development of industry-standard and accepted procedures to disseminate the superior strength, improved environmental impacts, and rural economic development of SRT. The successful scaling of SRT will create markets for a wider variety of low-value trees.■ The online version of this article has detailed references.

Please visit www.STRUCTUREmag.org.

Amelia Baxter is co-founder and CEO of WholeTrees Architecture and Structures. She has led project teams in several USDA research grants working toward the commercialization of the tree’s natural engineering.

Figure 6. Blakely Elementary School (Bainbridge Island, WA). Fourteen structural White Oak columns interact with a steel superstructure in a Seismic Zone 4.

14 STRUCTURE magazine

Michaela Harms is a sustainability-focused systems-thinker with an array of research and development experience across the wood construction supply chain. Michaela serves as a Building Products Engineer for PFS TECO.

structural DESIGN

Performance-Based Wind Design The Next Frontier

By Sean Clifton, P.E., S.E., Russell Larsen, P.E., S.E., and Kevin Aswegan, P.E.

W

ith the release of the ASCE/SEI Prestandard for PerformanceBased Wind Design (PBWD) in August 2019, the industry has taken an initial step toward implementing a structural engineering technique similar to well-established Performance-Based Seismic Design (PBSD) for the other most common building environmental hazard, wind. The Prestandard outlines an alternative and comprehensive approach to building design for wind loading, which explicitly evaluates occupant comfort, building drift, and extreme wind event behavior. The application of this approach may have the greatest significance to tall building design, particularly in high seismic hazard regions where both seismic and wind load effects control lateral demands.

While PBSD methodologies have been in use worldwide for over 25 years, the development of similar techniques for the design of buildings due to wind hazards has lagged behind. Several concerns have slowed the application to wind design, including duration and directionality of loading, element fatigue, computational methods, wind-tunnel techniques, and dynamic response. The Prestandard was created to address these concerns and chart a path forward for implementation of PBWD.

ASCE/SEI Prestandard The Prestandard for Performance-Based Wind Design presents an alternative to the prescriptive procedures for wind design specified in the nationally adopted ASCE/SEI ASCE 7 standard. The Prestandard calls for performance objectives to be established concerning the relevant building or facility responses meaningful to the owners, occupants, and users of the building or facility. These objectives range from occupant comfort level (detection of objectionable building motion), through serviceability (drift and motion), to strength and safety levels (building strength, damage potential, stability, and reliability). The designer and building stakeholders may apply specific design techniques to determine and demonstrate acceptable building functions across the range of objectives.

The Prestandard recognizes that a detailed evaluation of building response requires a detailed understanding of the relevant wind environment. Therefore, the building analysis and design are predicated on conducting wind-tunnel testing to establish structural loads. The designer then evaluates these loads using one of three methods of linear or nonlinear response history analysis. The three methods are included to give designers a choice between modest additional analysis up to sophisticated levels of additional analysis.

Method 1 The first method requires a linear response history evaluation of wind loads. This analysis can be completed using commercially available analysis platforms.

The Prestandard provides a series of element- and system-level acceptance criteria benchmarked to the linear analysis output. If the linear evaluation indicates elevated demand-to-capacity ratios, as defined in the Prestandard, the designer may be required to 16 STRUCTURE magazine

A storm descends on the Chicago skyline.

perform a nonlinear evaluation of the structure. In this method, the designer is restricted to limited levels of inelasticity within specific structural elements.

Method 2 The second method directly evaluates structural reliability for agreement with the target reliabilities of ASCE 7, Chapter 1. The reliability evaluation requires a nonlinear incremental dynamic evaluation of the building response as input; the analysis findings are then compared with the critical collapse initiation modes specific to the structure. The designer can then determine the resulting reliability of the structural system relative to the target reliabilities required by ASCE 7. In this method, the designer has considerable latitude to identify and demonstrate acceptable structural performance within elastic and inelastic elements.

Method 3

The third method, like Method 2, also directly evaluates a building’s structural reliability but instead uses system nonlinear analysis directly coupled to the wind time-history loads and the uncertainties in structural load and resistance. The high computational demands of Method 3 can be avoided using a structure Shakedown Analysis, which has recently advanced to a point where it is ready for practical use. Shakedown Analysis directly determines reliability through Monte Carlo simulation of the structural response. Method 3 findings can then be compared to the target reliabilities required

by ASCE 7. With this method, the designer has the most significant latitude to identify and demonstrate acceptable structural performance within the structure.

The Prestandard provides a series of structural element and structural system performance targets for evaluating the analysis findings for each method. If the building performance is acceptable, the resulting design may be submitted to the Authority Having Jurisdiction for Peer Review, according to the alternate design provisions of ASCE 7, Chapter 1 (Figure 2).

Furthermore, recognizing that the historic bulk of wind-related losses for tall buildings are due to winddriven rain damage following breaches of the building envelope, the Prestandard provides building envelope Figure 1. ASCE/SEI Prestandard.

Figure 2. Outline of PBWD Main Wind Force Resisting Systems analysis and acceptance methods.

enhancements specifically intended to improve envelope performance. These enhancements include recommendations from envelope industry groups, recommended ASTM testing benchmarks, and recommendations for installation testing and construction observation.

Tall-Building Design While the Prestandard applies to various building types and heights, it is considered most impactful to tall-building design, which is typically dominated by the flexural and dynamic response of the structure. The

methodology can be applied to the design of a variety of Main Wind Force Resisting Systems (MWFRS) with different structural materials. With the MWFRS typically consisting of a third or more of the structural material in a tall building, the use of enhanced design techniques can optimize its material utilization.

The Prestandard outlines a procedure to ensure the building meets the established performance objectives in three primary areas: occupant comfort, operational performance, and building strength. While most current Figure 3. Tall buildings in the high wind building codes do not require and seismic region of the Philippines. wind-tunnel testing or verification of serviceability criteria, the use of PBWD will allow designers and owners to more directly understand the behavior of the building and make adjustments to refine that behavior.

Optimization of the structure for its strength is partially achieved by taking advantage of inherent material overstrength and allowing for limited element yielding in ductile elements that can then dissipate energy and redistribute forces. The Prestandard recommends this be achieved through the use of expected material strengths and demand-tocapacity ratios (DCR) of 1.25 or 1.5, depending on the method used.

Method 1, described above, is the most straightforward and has many similarities to the methodology of PBSD, as outlined in the PEER Tall Building Initiative Guidelines. Method 1 follows these steps:

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1) Conduct a wind-tunnel study (force-controlled) elements in the building to identify building wind-force to require more strength than they would demands otherwise. By implementing PBWD, this 2) Classify the structural components issue is improved and better aligns wind as deformation (ductile) or force and seismic approaches. (brittle) controlled 3) Using results of the wind tunnel Research study, complete a preliminary design of the MWFRS, applyOne of the most critical topics for the ing enhanced design criteria for PBWD process is the inelastic behavior deformation-controlled elements of structural elements subjected to wind 4) Verify the structural components’ demands. Because inelastic behavior has response when subjected to wind not historically been permitted, there is time history records limited research available on this topic.

5) If necessary, conduct nonlinear To address this gap, the MKA Foundation response history analysis to verify sponsored research on conventionally that components meet the accepdetailed reinforced concrete coupling tance criteria beams at UCLA (Abdullah and Wallace).

Figure 4. Example of a building base over-turning from While the process of Method 1 is wind response history showing dynamic response.

The experimental program tested eight straightforward, it involves considerable different test specimens with four different additional design effort beyond conventional approaches – primarily wind-loading protocols. The initial results are positive and suggest the application of wind time histories and conducting the nonlinear that standard concrete coupling beams can resist wind demands with response history analysis. more than 2,000 loading cycles and ductility demands of at least 1.5 Traditional methods to compute the element forces in the struc- with little to no strength degradation. The research results will be ture involve applying a set of static wind loads distributed through published in 2020 and will include nonlinear modeling recommendathe height of the building to a linear-elastic analysis model. In tions such as effective stiffness values and backbone curves. Similar PBWD, the element forces are determined by applying a set of research is underway across the country to evaluate the performance loads in a response history analysis corresponding to various wind of reinforced concrete shear walls, concrete-filled composite-steeldirections. The dynamic response of the structure, considering mass plate shear walls (CF-CPSW), and concrete-encased embedded-steel and stiffness, is then directly captured in the analysis model. The wide-flange coupling beams. global response of the building, subject to a linear response history Further development is also underway for Methods 2 and 3 found analysis, should match the wind tunnel static loading (Figure 4) in the Prestandard. This includes research sponsored by the MKA closely. If results are not similar, further consideration should be Foundation at the University of Michigan (Spence), which seeks to given to response history input and details of the analysis model. publish software to perform the Method 3 analysis.

Enough wind directions must be considered for the response history analysis to fully envelop the response of all components of the Conclusions MWFRS. As a minimum, wind directions should be selected to produce peak base demands in all four quadrants of overturning The most significant advancements of PBWD will be the application of (Mx+ My+, Mx+ My-, Mx- My-, Mx- My+). the methods found in the Prestandard to real building designs, an effort When the preliminary design has been evaluated with linear response currently underway by the authors on several projects nearing complehistory results and components are found to exceed a DCR of 1.0, a tion. Similar to Performance-Based Seismic Design, the first several nonlinear response history analysis is required to verify the response designs will require outside-the-box thinking and open collaboration and acceptance criteria. The development of this more complex model between the structural engineer, wind engineer, building owner, peer requires advanced modeling techniques to capture the nonlinear reviewers, and the local jurisdiction. The publication of the Prestandard behavior of any yielding elements such as coupling beams, shear wall for Performance-Based Wind Design is only the beginning, and flexure, or other deformation-controlled elements. much more knowledge will be gained in the coming months With a properly calibrated nonlinear model, the dynamic behavior of and years regarding how tall buildings respond to wind.■ the building should respond to the changing stiffness as elements yield and forces redistribute. Given that the analysis directly captures the Sean Clifton is a Principal at Magnusson Klemencic Associates and leads dynamic behavior, the Prestandard also allows for the more thoughtful the firm’s work in Southeast Asia. As an active member of MKA's in-house implementation of supplemental damping systems to control buildHigh-Rise Technical Specialist Team, his portfolio reflects numerous high-rise ing movement as well as element forces. Careful consideration of the developments in high-wind and seismic regions. (sclifton@mka.com) reliability, redundancy, and damping properties of these systems is Russell Larsen is a Senior Associate at Magnusson Klemencic Associates. crucial to ensure that performance objectives are met.

He is the leader of MKA’s Wind Technical Specialist Team and has been Significant motivation in applying PBWD in tall buildings design actively involved in the development of the ASCE/SEI Prestandard for comes in high-seismic hazard regions where the wind demands conPerformance-Based Wind Design. (rlarsen@mka.com) trol the design of certain elements. Seismic design principles rely Kevin Aswegan is an Associate at Magnusson Klemencic Associates. He is heavily on the concept of energy dissipation through the yielding the leader of MKA’s Performance-Based Design Technical Specialist Team of ductile (deformation-controlled) elements. When those elements and has been instrumental in the MKA Foundation’s research efforts on are made stronger and stiffer due to the wind demands, they tend to PBWD. (kaswegan@mka.com) dissipate less energy in an earthquake. This can cause other brittle 18 STRUCTURE magazine

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structural REHABILITATION Fuzzy Wood and Coastal Piles

By Richard S. Barrow, P.E., S.I.

I

t is common in Florida to elevate homes on wood piles in coastal zones A and V. Many of those homes are adjacent

to the Atlantic Ocean, the Gulf of Mexico, bays, estuaries, and other bodies containing saltwater. Most of the piles are sheltered under habitable space. Reacting to its environment, the wood has a fuzzy appearance. Why would a wood pile, driven on land or near a body of water, deteriorate at the protected and dry top and not at ground level or the waterline?

To understand the cause of “fuzzy wood” and to provide an appropriate repair, we need go back to high school biology and chemistry classes.

Figure 1. Wood pile deterioration near grade.

Where is Fuzzy Wood Found?

In high school biology class, we learned that all life requires air, food, water, and a sufficiently warm environment or habitat to survive.

Naturally, the fungi, mold, insects, and marine life that inhabit and consume wood piles are no different. A wood pile driven into the soil will tend to degrade at or near the ground surface. At no more than a couple of feet below ground, oxygen levels become insufficient to support most life. At a couple feet above ground there may not be enough moisture to support life (Figure 1).

Similarly, piles driven into water will tend to degrade in the tidal zone or at just above the mean water elevation. In the continuously submerged zone, the low oxygen content is only sufficient to support some organisms (e.g. marine borers). Above the waterline, the amount of moisture is limited by drying of the pile (Figure 2).

Many structures near or in saltwater are elevated on wood piles, which are sheltered under habitable space such that the tops of the piles have minimal exposure to weather. One would expect that any damage to

Figure 2. Wood pile deterioration near waterline.

20 STRUCTURE magazine

Figure 3. Corrosion of bolt due to wicking at the top of an interior pile under a coastal home.

the piles would be on the portions of the pile that are exposed to the elements. Therefore, despite the lack of exposure to salt spray, it has been surprising to find that such sheltered piles often have severely corroded bolts connecting the top of the piles to the wood floor framing (Figure 3). Those piles may also have a furry or fuzzy appearance, which some fittingly refer to as “fuzzy wood” (Figure 4). Why would a wood pile, driven on land or near a body of saltwater, deteriorate at the protected top and not at the ground level or the waterline? To understand the cause of “fuzzy wood” and to provide an appropriate repair for that condition, we must examine the internal structure of wood and the response of that structure to saltwater.

Basic Biology and Chemistry When building with sawn lumber, we must work with what nature has given us. You may recall from high school biology that the cambium layer in a tree is the growth layer, just under the bark, that produces both the inner bark cells of the phloem and the new living wood cells in the xylem. The phloem conducts sugars from the leaves to the roots, sometimes referred to as “shoots to roots.” The xylem is living "sapwood," which conducts water and substances dissolved in water from the roots to the leaves (“roots to shoots”) and provides support for the tree. The yearly growth in the cambium layer results in annual rings which are visible across the trunk of a felled tree. As new xylem (sapwood) is continually formed, the old xylem dies, hardens, and becomes heartwood at the center of the trunk. Heartwood is durable and more resistant to decay. This is because heartwood has lost much or all its ability to transport liquids due to the blocking of its tube-like structure with gums and resins.

The xylem tube structure is visible as grain when wood is cut vertically down its trunk. (Xylem cells are about the size of animal cells, so one cannot see the individual tubes with the naked eye.) A common analogy used in high school biology classes for xylem is that it acts like a “bundle of straws” running parallel to each other up the trunk.

The xylem “straws” allow the wood to draw water upward in a pile in a process known as “wicking.” To wick, it does not matter whether

the xylem is in a live tree or a timber pile.

What matters is the ability of the xylem straws to transport water.

Furthermore, whether the pile is pressure treated or not does not seem to be a significant factor in wicking. This is likely because the limited depth of penetration of wood preservatives does not significantly inhibit the wicking process in a large wood member. It may also be a result of the replacement of oil-based non-preservatives (e.g. creosote) with more environmentally friendly waterborne wood preservatives (e.g. CCA and ACQ). It has been the author’s experience that the amount of heartwood in a pile appears to have the most significant effect on the ability of the pile to wick, as well Figure 4. Fuzzy wood pulled from a jacketed dock pile. as which piles are damaged.

Shifting now to concepts taught in high school chemistry class, we bearing points under beams are typically areas where a waterproof recall that sodium- or potassium-to-chloride ionic bonds are weak, coating or an isolation material is needed to separate bolts and allowing salt molecules to separate in water. As the water evaporates, beams from the pile. the salt molecules join to form cubic salt crystals, which have a higher Piles with significant structural damage or loss of structural capacvolume than the elements did in solution. The same process occurs ity are more challenging to repair. This is because it is expensive, in wood. As saltwater that has wicked into a wood pile evaporates, if not impossible, to replace existing driven wood piles “in-kind” the increase in volume due to the formation of salt crystals generates under an existing structure. Consequently, shoring and repairexpansive internal forces that work to split the wood fibers apart. Over ing only the degraded top portion of the damaged pile may be time, these expansive forces may damage a polymer called lignin, preferable to replacing the entire pile. Figure 5 shows a pile with which binds the xylem tubes together. The damaged wood may gain a significant damage at the top. However, when the pile was cut “fuzzy” appearance with white salt residue on the surface. This process, off, approximately six inches below the notch for bearing of the known as delignification, is analogous to the spalling of concrete due floor beams, the lower portion of the pile was found not to be to the expansive forces resulting from the formation of rust. significantly damaged. A splice can also be designed as a moisture barrier between the new top portion of the pile and the existing bottom portion.

Repair Methods

There is little available literature regarding how to address pile wicking and delignification in wood piles in service. It is the author’s observation that, although damage to wood piles from delignification may be unsightly, it may take a long time for significant structural damage (loss of load carrying capacity) to occur. Structural damage is more likely to occur in elements attached to the wicking pile. In such cases, eliminating evaporation sites where damage to connected members might occur may be all that is needed. Bolt holes and

Conclusion Sawn lumber is a unique building material because it is grown and not manufactured. Therefore, we have to work with the unique properties that nature has given us. One of those properties is that wood is consumed by organisms. Another property is that the structure of the xylem in wood acts as a bundle of straws that can transport water. We tend to think that wood damage is only due to an attack by organisms or exposure to chemicals. However, as discussed, it can also be the result of internal mechanical stresses created by the evaporation and subsequent crystallization of salt dissolved in water. The result of those mechanical stresses is damage to the pile in the form of “fuzzy wood.”

Understanding and properly diagnosing the cause of damage to a wood pile is needed to provide the proper repair.■

Figure 5. Left: Top of pile removed below a notch for floor beam bearing (note fuzzy wood on the right side).

Right: “Fuzzy” wood and annular rings on the bottom portion of a coastal pile cut six inches below a degraded notch for floor beam. (photos are not of the same pile).

Richard S. Barrow is President of Liebl & Barrow Engineering, Inc. in Fort Myers, FL. He is a member of ASCE’s Committee on Forensic Investigations. (rich@lbengineer.com)

JUNE 2020

21

HIGH CAPACITY FOUNDATIONS for 110 N Wacker Drive

By Darren S. Diehm, P.E., D.GE

R

ock is hard. While the degree of hardness varies, it is objectively harder than soil. Geotechnical foundation design has largely concentrated on the analysis of soil.

In-situ site investigation techniques and constitutive modeling efforts are devoted to characterization and performance prediction of non-linear soil behavior. When it comes to rock, the default assumption has been that rock is unyielding, and the design of a foundation supported on it should be based on the material parameters of the structural element.

Such is the case for driven steel piles. Pound a piece of steel into the ground to the point it stops on apparent rock, and most everyone agrees that the pile will fully develop its structural capacity. However, until relatively recently, building codes did not provide similar consideration for drilled shafts. The rock had to be proven to be 110 North Wacker at dusk. capable of supporting the applied point load. Pointing out the numerical fallacy of the argument – that the tip stress of a driven steel pile in service is 7 to 10 times higher than a cast-in-place concrete drilled shaft – invariably resulted in disapproving stares from building officials.

Not to disparage building officials, this point is intended to exemplify the discomfort many in the industry have in intuitively reconciling the axial capacity of foundations. For the compact sections used in piling, the structural capacity is readily determined as the tip area of steel times the allowable material stress. Under the maximum stress limit of the International Building Code (IBC), the commonly used HP 12x53 section has a capacity of about 387 kips. For comparison, an uncased, 4-foot-diameter drilled shaft constructed with 6,000 psi concrete has an axial capacity of slightly more than 3,250 kips – an 8-fold comparative increase on a single element. Change the diameter to 7 feet, and the capacity approaches 10,000 kips. From the viewpoint of the rock, the driven pile is a more significant concern. Neither foundation realistically threatens it with failure, but the tip stress of the pile could locally crush the rock and penetrate the surface. The concentration of the stress over the small pile footprint also increases the localized influence-variations in the rock (e,g., fractures, clay-filled seams, voids) may have on the performance of the element. Despite the higher likelihood of punching failure with piling, designers tend to fixate on the much larger capacity number and determine that the magnitude of the load justifies greater scrutiny and caution in the design of drilled shafts.

22 STRUCTURE magazine

For 110 North Wacker Drive in Chicago, the provided core foundations would support nearly 40,000 kips. The design equivalent end bearing of the rock-socketed drilled shafts was 600 ksf, which is 50% greater than the maximum allowable presumptive bearing capacity in the Chicago Building Code (CBC).

When completed in 2020 after approximately 2½ years of construction, the 56-story tower will have an architectural height of 817 feet. The gross footprint area will encompass slightly less than 1.8M square feet over an occupied height of 752 feet. In a reflection of changing commuter habits, only 110 parking spaces are provided over 2 lower levels.

After a long history of service as a commercial waterway and an industrial transport corridor, the Chicago River, which defines two sides of the Loop, is experiencing a renaissance of redevelopment.

As recently as the 1990s, buildings were deliberately designed with a rear-facing to the river to avoid interaction with what was often a caustic and foul-smelling soup of shipping traffic, pollutants, and stormwater runoff, swirled with occasional untreated discharges from upstream manufacturing. Today, the Chicago River is being embraced as an asset for real estate, commercialization, and tourism. The City of Chicago requires new construction on the North and South Branches to provide open public access to the riverfront. Rather than set back the entire structure, 110 North Wacker was designed with a multi-story,

Foundation drilling. Belling tool (undeployed) on a rig in foreground and soil auger mounted on rig at back. Core barrel behind group of onlookers at right rear of frame.

Site looking south. Upper and Lower Wacker Drive to the east (left). The barge moored to the Chicago River Wall provided laydown areas for material storage and a platform to tie reinforcing cages.

Site looking north. Mud-slab placement in advance of mat slab pour (3,300 cubic yards placed in less than 8 hours) for the building core. Reinforcement projections include interior “bar cassettes” and exterior circular bars.

open-air overhang, which provides the requisite public space while still maximizing the building footprint on the trapezoidal site. An inspired sawtooth floorplate, which can accommodate as many as nine corner offices on the riverside, enables the tower to feature a dual-front facing. Architecturally, both the street and the riverside are front doors to the building. The perimeter structure loading is transferred through the overhang by three trident columns.

For its height, 110 North Wacker is a comparatively heavy building.

Had it been designed 15 or 20 years ago, it likely would have been a light steel frame building as the majority of the older 50- to 60-story high-rises are in Chicago. While code changes and advancements in analytic methods that allow for consideration of an ever-growing list of load combinations can be blamed for some of the increase in building weight, it is the greater use of concrete in the structure and the changes in design approach to provide occupant usage-driven spaces which have had the greatest impact on the foundation demand.

The Chicago subsoils are predominantly composed of large masses of clay and silt derived from the Devonian shale, which occupied the Lake

Michigan basin. The clays were deposited as a series of ground moraines (or till sheets) lying one atop another by the advances and retreats of the continental ice sheet during the Wisconsin glacial period. Geologically, six separate till sheets, representing six advances and retreats of the ice front, have been identified: (from bottom up, typically) the Valparaiso moraine, Tinley moraine, and the Lake Border moraine which is comprised of the Park Ridge, Deerfield, Blodgett, and Highland Park till sheets. Within the project site footprint, the soil profile is about 120 feet thick. It includes 20 feet of urban fill and 40 to 50 feet of compressible and lacustrine clay.

The hard, silty clay and clayey silt of the Tinley Moraine till sheet, which is locally referred to as “hardpan,” is located about 80 feet below the street grade of Upper Wacker Drive. The majority of the 1,300+ high-rises which form the Chicago skyline are supported on belled caisson foundations bearing in this layer. The introduction of in-situ pressuremeter testing increased the allowable bearing capacity in the hardpan from 15 ksf in the 1960s to more than 50 ksf today.

A 20- to a 25-foot-thick layer of silt, sandy silt, and gravel is present below the hardpan, immediately above the bedrock. The Chicago

Figure 2: Generalized soil profile

Log

Lake Border moraines

Glacial Drift

Unit

Niagran Series

Silurian

Name

Description

Fill

VII

Man placed material consisting predominantly of sand-size particles with varying inclusions of cinders, brick, and concrete fragments.

Glacial Lake Bottom

VI

Weathered, over-consolidated clays with low to medium PI. Identified by Peck and Reed as “Desiccated Clay Crust.”

Blodgett and Deerfield

V

Park Ridge

IV

Lacustrine, low plasticity clay with occasional silt and sand seams.

Natural moisture content between 18-22%. Transitional zone of variable thickness.

III

Glacially consolidated low plasticity clay, silty clay and clayey silt. Blow counts in excess of 40 bpf and natural moisture contents below 14%. Locally referred to as “Chicago Hardpan.”

Tinley moraine

Terminal moraine

Pleistocene

Age

Valparaiso II moraine

Dolomite

Compressible, lacustrine clays with low to medium PI. Very soft and soft grey to medium bluish grey.

Ex. Dense Sandy Silt, Silty Gravel and Gravelly Sand with Occasional Cobbles and Boulders.

Ia

Extremely Weathered to Disintegrated Dolomite

Ib

Fresh to moderately weathered, hard to medium, grey to light tan, blocky, slightly to moderately vuggy dolomite and dolomite limestone. Generally nearhorizontal bedding with slightly inclined to near-vertical joints.

Generalized soil profile.

JUNE 2020

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Top of rock isometric surface and contours.

bedrock consists of shallow dolomite formations mainly of Silurian age and deep sandstone and dolomite formations of Cambrian and Ordovician age. The Niagaran series immediately underlies the glacial drift, and it varies in thickness from 240 to 425 feet, generally increasing eastwards towards the Lake Michigan basin. The rock surface in the majority of the Chicago Loop is relatively flat. The relief over the 400-foot site length (north to south) is only 4 to 6 feet.

For permitting, the City of Chicago required each of the 37 socketed drilled shaft foundations to be precored to evaluate the rock. The presumptive bearing capacity provided in the CBC requires that the rock be “solid” to a depth of at least 8 feet below the tip of the rock socket. In the early days of hand-dug shafts, an acceptably solid rock was determined by driving a steel pick into the base material and evaluating the tone of the ring of the strike. This may have contributed to the code language meant to clarify the concept of solid rock further by defining it as sound, unweathered limestone without visible voids.

Setting aside the quibble that the underlying bedrock in Chicago is dolomite rather than limestone, it is essential to note that the building code relies only on qualitative assessment of the rock; it does not include requirements for determination of mechanical properties. Although the presumptive bearing capacity is limited to 400 ksf for end bearing caissons socketed into rock, this is not an intrinsic property of the rock The Geological Strength Index (GSI) was introduced by Evert Hoek and Paul Marinos in 2007 as a characterization system to combine geologic assessment of a rock mass with the results of sampling and lab testing to improve analytic performance prediction of tunnels, slopes, and foundations. Before GSI, engineering parameters for rock were largely qualitative assessments of the degree of intactness or integrity of the parent material. While the compressive strength of the rock could, with proper sampling, be readily measured in the laboratory, there was little consideration given to how the structure of the geology would affect the engineering performance in the field.

Unconfined compressive strength tests were performed on over 100 samples, and a geotechnical strength index was determined for the rock mass based on visual examination of the recovered cores. The rock exhibited a strength distribution consistent with that for other sites sampled in Chicago. Stress relaxation from drilling and sampling can allow microfractures to open in the recovered cores, and minor irregularities in the end flatness of the samples can cause premature breaks, all of which contribute to the large spread of the data. For the upper 10 to 20 feet of the rock, in which any foundations would be supported, a conservative value of the unconfined compressive strength was determined as 10,000 psi. Utilizing 24 STRUCTURE magazine

the Rock Mass Rating system, the dolomite classified as “Good to Very Good” rock and the GSI was determined to be between 55 and 65.

The analysis of the rock socket followed the methodology of Carter and Kulhawy, a modified form of which is provided in the AASHTO Standard Specifications for Highway Bridges. The GSI is incorporated into a material strength constant and an exponential coefficient, and the ultimate end bearing is determined from a power function. The side resistance is determined from the square root of the unconfined compressive strength (normalized by atmospheric pressure) multiplied by a scalar coefficient, which considers roughness of the socket sidewalls. The axial capacity of the socket is effectively determined using only two parameters.

For the dolomite recovered from the precores, the ultimate end bearing was predicted to be 1,200 ksf, and the side resistance was determined as 80 ksf. The allowable equivalent end bearing was estimated to be more than 715 ksf for a 6-foot-long rock socket using a factor of safety of 2. Under the presumptive bearing capacity of the CBC, a 6-foot rock socket has an allowable bearing capacity of 400 ksf.

An Osterberg load test was performed to verify capacity and to provide an index for validation of performance prediction. A 34-inchdiameter load cell (the largest available) was installed near the base of a 7-foot-diameter production caisson, and a biaxial load was applied to evaluate socket end bearing and side friction in accordance with ASTM D1143 Standard Test Methods for Deep Foundations Under Static Axial Compressive Load (Method A). At its maximum capacity, the O-Cell applied a sustained bi-directional load of 9,054 kips on a 36-inch-diameter bearing plate, which produced a combined displacement of approximately 1 inch (0.3 inches upward and 0.7 inches downward). The mobilized side resistance was slightly less than 60 ksf, and the end bearing was approximately 900 ksf.

It is worth noting that the load test was taken to the maximum capacity of the O-Cell without initiation of creep or slip along the rock socket sidewall. The test demonstrated more than 1.5 times the design effective end bearing of the drilled shaft, also with no indication of creep or crushing below the socket tip. Assuming the rock was completely decomposed to a cohesionless soil, the end bearing would be expected to be fully mobilized at a deflection of 4 to 5% of the base diameter. A movement between approximately 3 and 3.5 inches could only be achieved in the rock by punching failure. Due to the thickness of the underlying massive rock, material crushing of the shaft concrete would occur before the end bearing could be mobilized. Osterberg load tests at other sites in Chicago have demonstrated similar results.

By any measure, it should be clear that the axial capacity of rock socketed drilled shafts is under-utilized. While it may be possible to mobilize greater side resistance and even create slip using an O-cell in a smaller diameter socket, end bearing failure is unlikely ever to be demonstrated by load testing.

The static analysis using conservative parameters indicates that the material properties of the concrete control the capacity of the drilled shaft – just as it would be for a driven pile.

For the near future, the 56-stories of 110 North Wacker will continue to hold the title of the highest bearing capacity ever permitted in Chicago at 600 ksf. The next step in foundation performance (say 700 or 800 ksf ) will have to wait until someone starts producing 14,000 psi concrete, and there is a practical need for a drilled shaft that can support 56,000 kips. Until then, the rock will be waiting.■ Darren S. Diehm is a Senior Professional at GEI Consultants, Inc. in Chicago. He is a specialist in the design and analysis of foundations and deep excavations for urban environments. (ddiehm@geiconsultants.com)

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10

HUDSON YARDS FILIGREE IN NYC

By Adam Beckmann, P.E., Chris Christoforou, P.E., LEED AP BD+C, and Michael Squarzini, P.E., LEED AP

M

any engineers would agree that working on America’s largest private real estate development is a significant challenge.

Imagine that the development was situated directly above an active rail yard located in the heart of New York City. That development is none other than Hudson Yards, a 28-acre site stretched across four city blocks (30th to 34th Street, and 10th to 12th Avenue). The site is split into two phases divided by 11th Avenue, which runs directly through the center of the site. This divide creates a western yard and an eastern yard. While the western yard is yet to break ground, the eastern yard is open to the public and fully operational. Located on the eastern yard are both 10 and 30 Hudson Yards, designed by Kohn Pederson Fox (Figure 1). Even with the difference in height, the buildings appear very similar and are designed to complement each other.

While the outward appearances of the towers are similar, their structural systems could not be more different. The 1,269-foot 30 Hudson Yards consists of a concrete slab on composite metal deck supported by steel framing and steel columns, all laterally supported by a steel braced core and outrigger system, typical for a New York City office tower. The 895-foot-tall 10 Hudson Yards, however, is an all concrete office structure, atypical for New York City. It is the only one in New York to feature a gravity system comprised of post-tensioned beams and a filigree slab supported by concrete columns and a concrete core. This unique floor system provides several structural benefits in addition to creating an aesthetically pleasing floorplate, which can remain exposed in loft-type office spaces.

Figure 2. Typical filigree system overview.

26 STRUCTURE magazine

Figure 1. 10 (right) and 30 Hudson Yards (left). Courtesy of Kohn Pederson Fox

Filigree System Overview Filigree slab systems typically utilize 2¼-inch-thick, 8-foot-wide prestressed concrete planks compositely connected to a cast-in-place topping slab. Based on the shear demand of a particular bay, the planks can be obtained with or without polystyrene voids. Voided planks utilize polystyrene blocks adhered to the plank to reduce concrete volume and load on the supporting elements. The voids are located based on the shear demand of the slab and typically concentrated toward the center of the span where demand is low. Cast into the plank is a steel lattice truss that bonds to the field poured concrete, forming the composite filigree slab system.

The precast planks are designed to serve as both formwork for the weight of the wet concrete and to meet the positive moment demand of the composite slab. Before the production of the planks, the design team provides the maximum positive moments to the precast manufacturer. The manufacturer ensures that sufficient prestressed tendons and rebar are provided in the plank to meet the designer’s needs. Often the planks ultimately are set on steel framing or “U” shaped filigree beams (Figure 2) to complete the floor framing system. However, the planks are also commonly used in typical two-way flat plate systems.

While 10 Hudson Yards utilizes the 2¼-inchthick 8-foot-wide prestressed concrete planks, it did not use typical steel or filigree framing. Due to the typical 45-foot by 30-foot bays in the tower and the required clear floor-to-floor height, the design team decided to use a post-tensioned castin-place beam system in conjunction with voided filigree plank (Figure 3) to reduce the structural depth of the floor system. The typical beam size Figure 3. 10 Hudson Yards filigree slab and beam section. utilized in the tower is a 48-inch-wide by 21-inchdeep section; this allows for the use of 26-foot-long precast planks. To meet the required moment demand of the • Coordination of sleeves, penetrations, and inserts takes place slab, the 2¼-inch voided plank was topped with a 6¾-inch-thick before installation of the precast plank, reducing comeback poured-in-place concrete slab (Figure 4 ). The composite 9-inchwork and slab coring. thick system provides similar moment capacity to a traditional • The filigree slab and beam system provide clear, unobstructed concrete reinforced slab of the same depth but at the equivalent spans allowing for high ceilings and the ability for MEP weight of a 7-inch-thick slab. The reduction in mass has a positive services to be easily hidden away into the void space between cascading effect on the structure. The reduction in load reduces supporting beams. demand on the beams, columns, and foundations, which in turn reduces the overall cost of the structure.

System Challenges

System Advantages The filigree system has a significant number of advantages over typical cast-in-place and steel-framed construction.

• The precast planks eliminate the need for formwork and decrease the number of shores per floor, significantly reducing congestion on the floors below active working decks.

• The system significantly reduces the effort and time required during the typical stripping and reshoring cycle. The shoring system used to erect the plank stays in place until the concrete reaches the specified strength. At that point, the shoring system can be dismantled and cycled up to the active deck to be reused on the floor above.

• The voided filigree system reduces concrete volume and overall building mass without sacrificing the moment capacity of the slab system.

As with every floor framing system, there are drawbacks to consider when specifying filigree. The precast plank sizes can be rather large and difficult to maneuver due to their weight and shape, especially in windy conditions. The typical plank on 10 Hudson Yards required a team effort to place, as each plank weighs approximately 6,000 pounds. In addition to the construction workers on deck, each plank requires a crane to maneuver it into place. It is important to consider and coordinate this process well in advance and work it into the construction schedule to limit its effect on the project. The designer needs to also consider the availability of plank in regard to the project location. Casting plants capable of producing filigree plank need to be within an acceptable distance to the project site to recognize the potential economic benefits of the system.

Summary Although not a common system, the structural and architectural advantages of the filigree slabs contributed immensely to the success of 10 Hudson Yards. The reduced weight of the composite system allowed for reductions in column sizes, beam depths, and foundation reinforcing. The reduction in beam depths and column sizes created a more desirable floorplate from both an architectural and leasing perspective when compared to the properties of a traditional cast-inplace or steel framed system. The reduction in concrete volume and reinforcing resulted in measurable monetary savings to the budget.

When evaluating potential concrete and steel floor framing systems on your project team’s next job, consider examining the viability of a filigree system.■ All authors are employed by Thornton Tomasetti, Inc. Adam Beckmann is a Senior Associate in the Newark, NJ office. (abeckmann@thorntontomasetti.com) Chris Christoforou is a Principal and Office Director in the Newark, NJ office. (cchristoforou@thorntontomasetti.com)

Figure 4. Concrete pour on polystyrene filigree plank.

Michael Squarzini is a Managing Principal and Co-CEO in the New York, NY office. (msquarzini@thorntontomasetti.com)

JUNE 2020

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Big Moves Figure 1. Relocation path for Sharpe House.

I

n 2017, Brown University embarked on a multi-phase program to develop a new Performing Arts Center (PAC) at the heart of its campus on the east side of Providence, Rhode Island. The historic fabric of the neighborhood, with its interwoven streets and buildings of various types and vintage, creates both opportunities and challenges for the construction of new, modern facilities. In particular, the PAC would require a large open site for its planned state-of-the-art concert hall.

Rather than demolish existing buildings to create a new building site, Brown’s planners identified an opportunity to relocate a building, called the Sharpe House, to an adjacent site owned by the University (Figure 1). Relocation of this structure would create a suitably sized lot for the new PAC and would have the additional benefit of reestablishing the historic streetscape at its new site.

Sharpe House, originally constructed in 1873 as a three-story residential building, had been used by Brown for many decades as an academic office building. The nearby Peter Greene House, of similar vintage, was relocated as part of a previous project (2007) completed by the same team of builders and engineers. Brown’s plan was to relocate the Sharpe House and combine it with Peter Greene House, including a new connector wing, to create a new home for the History Department.

Figure 2. Drone photography and laser scanning combined to create a point cloud of Sharpe House in its original location.

FOR THE HISTORIC SHARPE HOUSE

By David J. Odeh, S.E., P.E., SECB, F.SEI, F.ASCE

Through careful planning and thoughtful collaboration, a team of engineers, architects, and contractors pulled off this architectural transplant operation in late 2018.

Moving a structure is a multi-step operation that requires close coordination between the structural engineer and architect of record for the project with the moving contractor.

As a first step, Odeh Engineers (Odeh) used reality capture technology to investigate the existing Sharpe House building. The firm’s engineers used aerial drones and laser scanners to create precise three dimensional digital representations – called “point clouds” – of the building’s interior and exterior surfaces (Figure 2). The contractor also removed some building finishes to expose otherwise concealed structural framing elements for scanning and measuring. Using the point clouds and field measurements, the engineers created a parametric building information model of the structural framing systems and building envelope to use for design.

The architect for the project, KITE Architects, then virtually relocated the above-grade portions of the building to the new site to design the new combined facility. KITE also designed renovations and a new deeper basement with usable academic space accessible from the exterior.

Odeh then designed new foundation walls and support frames in the basement level to support the loadbearing walls of the existing building.

The Rhode Island State Building Code, based on the International Building Code (IBC), requires that relocated structures are engineered to support the permanent gravity and lateral loads required for a new building with the same occupancy constructed at the new site. While the gravity load carrying system was sufficient to carry the proposed new occupancy loads, with some localized reinforcement, the lateral load-carrying system required a complete upgrade. New, reinforced masonry shear walls were incorporated into the permanent design, to be installed after the building was relocated.

With the initial design concept ready, the construction team, led by Shawmut Design and Construction, engaged Davis Building Movers to design and implement the moving sequence. Davis worked with the Da Vinci Group, LLC, of Woodbury, NJ, to design the system of temporary supports to release the wood and masonry superstructure from the original stone foundations, roll the house to the new site, and lower it down into its final position where it could be connected to the new permanent foundations and support frames.

Large scale transport of equipment and structures is common in the shipbuilding, bridge construction, and aerospace industries.

These movers make use of specialized equipment, such as computercontrolled “self-propelled modular transporters” (SPMTs), to safely support massive weights on rolling platforms. For example, see The JUNE 2020

29

130 th Street and Torrence Avenue Railroad Truss Roll-In (STRUCTURE, October 2014).

Moving historic structures uses many of the same technologies noted above but requires special considerations unique to buildings. These concerns include the sensitivity of brittle finishes in the existing building, preparation of a suitable path of travel with adequate clearance and ground levelness, and coordination of support structures in the initial and final locations. In general, wood-frame structures are more resilient to the moving process than masonry structures, although both can be relocated. The path of travel can have slopes of up to 5%, must have sufficient clearance from overhead utilities and trees, and must be suitably compacted to distribute wheel loads from the moving equipment. Theoretically, nearly any size building can be relocated by movers, but these practical considerations typically govern the limits of Figure 3. Plan elevation views of the temporary framework and dollies. Courtesy of Da Vinci what is possible for a given project.

Group, LLC and Davis Building Movers.

Based on the initial building study of the Sharpe House, the Da Vinci Group prepared detailed drawings of the temporary design calculations. The entire building weighed approximately 460 support framework that Davis would install in the existing building tons and is 40 x 60 x 50 feet in dimension. basement. Temporary supports included a gridwork of steel beams, and To implement the move, the contractor first installed the gridwork (1) main beams located between the existing building support lines of steel beams in the existing basement supported by temporary and supported by wood cribs, (2) cross beams, and (3) needle beams wood cribs (Figure 4). The load of the structure was then transferred (Figure 3). This temporary framework, once shimmed into positive to the cribs using a system of hydraulic jacks, carefully located and contact with the existing framing, allowed for the existing exterior controlled to balance the load distribution during jacking. With the foundation walls to be demolished to make way for the dollies with building weight now carried by the temporary cribs, the existing hydraulic jacks that were to be rolled into position beneath the main masonry foundation walls were removed and dollies rolled into posibeams. Da Vinci Group and Odeh worked together to coordinate the tion beneath the main beams. A total of 12 independently powered load path and gravity loading assumptions that Da Vinci used in its hydraulic dollies, each capable of turning on its own, were positioned ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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Figure 4. Hydraulic jacks (two left photos); Gridwork of cribs (right).

finishes are especially sensitive to such movements and can be damaged. The Sharpe House move and repositioning of the old structure on new and more level foundations resulted in several cracks in wall and ceiling finishes that were repaired as part of the renovation process. The relocation of Sharpe House demonstrates the possibilities for campus planning and commercial development that structural engineers can enable through creative thinking and close collaboration with an experienced and detail-oriented design and construction team. Moving a building, where feasible, opens up new possibilities for development in urban areas with limited land availability, while simultaneously preserving the historic fabric that makes the city unique and livable. Sea level rise and coastal erosion are also driving more interest in building relocation in affected regions. Brown is now building its new Performing Arts Center on the site freed up by the relocation of Sharpe House, and simultaneously improving the character and connectivity of the neighborhood in Providence.â–

under the house. After transferring the load from the cribs to the dollies, the Sharpe House was ready to roll.

Meanwhile, the contractor completed excavation and footing placement at the new site, and then covered the pathway between the sites with a mat of crushed stone and recycled asphalt. This mat created a stable and level surface for the moving equipment. Dowel bar substitutes were used at the foundations to ensure that protruding reinforcement did not interfere with the moving operation.

The building was first rotated into its new orientation to complete the move, an operation that required pivoting around one corner dolly (Figure 5). During the 90-degree rotation, dollies were individually picked up and spun to point in the right direction. One dolly in the David J. Odeh is a Principal with Odeh Engineers, Inc. northeast corner was the pivot point for the rotation. The contrac(odehdj@odehengineers.com) tor used steel plates along the path of the dollies to maintain level surfaces during the move and distribute the heavy wheel loads from each dolly. Surveyors carefully monitored the final location to place Project Team the house above the new support framing and foundations.

The rotation of the house took about 45 minutes, while the remainOwner: Brown University ing move took about 90 minutes. The building was moved 160 feet Structural Engineer: Odeh Engineers, Inc., at a rate of about 6 inches per second. Readers can view a time-lapse North Providence, RI video of the move at https://bit.ly/2L4Ysmd.

Moving Engineer: Da Vinci Group, LLC, Woodbury, NJ Once the building was in position, the dollies could be removed, and Building Mover: Davis Building Movers, Blue Point, NY the building was re-supported on the temporary cribs. The contractor General Contractor: Shawmut Design and Construction, then constructed the foundation walls and steel support frames in Providence, RI the basement. With the new supports in place, the structure could Architect: KITE Architects, Providence, RI be lowered into its final position and anchored in place to support the permanent loads.

With any move, some degree of nonstructural repairs should be anticipated. Even if extreme care is taken by the mover, some degree of distortion and differential movement occurs in the building during the process of relocation. While the mover typically takes responsibility for the integrity of the building during the moving process, structural engineers must be diligent in reviewing the final installation of the structure on its new supports to ensure that no structural damage or permanent distortion Figure 6. Investigating post-move cracks. has occurred (Figure 6). Historic Figure 5. On the move, with dollies in place.

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Seismic Performance OF SLENDER CORE-ONLY TOWERS

By Mark Sarkisian, S.E., Eric Long, S.E., and David Shook, P.E.

500 FOLSOM

Figure 1. 500 Folsom Tower.

is a new residential high-rise providing needed housing in the densifying urban fabric of the Transbay District of San Francisco. The site was originally part of the Embarcadero Freeway, connecting the Golden Gate Bridge and Bay Bridge, that was heavily damaged in the 1989 Loma Prieta earthquake. The site has been rejuvenated by the San Francisco Transbay Redevelopment Plan and Essex Property Trust. The architectural and structural designs were collaboratively conceived by Skidmore, Owings & Merrill LLP. The design was further enhanced through alliances with small business design collaborators Fougeron Architecture and STRUCTUS. The design of 500 Folsom is a good example for designers of slender-core only towers in high-seismic zones.

Massing maximizes the site’s housing potential with a 42-story, ž million-squarefoot, 537-unit apartment tower. The large podium is 85 feet tall with a 120- x 90-foot tower footprint rising to 420 feet (Figure 1), a floor height of 9.25 feet is used to achieve the preferred density of units. A significant tenant amenity at the podium roof provides sweeping views of the city. The architectural design gives the distinction of shifting blocks balanced with verticality, achieved with energy-reducing shading fins.

The structural system is entirely reinforced concrete, for cost-efficiency, and facilitates a 3-day floor-to-floor leading core concrete construction cycle. Spans were coordinated with the architectural design to facilitate a thin 7-inch-thick post-tensioned slab supported by conventional concrete columns and a core-only lateral force-resisting system. The concrete core had a dimension of 33 feet by 52 feet. Wall thickness ranges from 36 to 24 inches. This system rests on a 6-story below-grade basement, which is founded on a 10-foot-thick reinforced concrete mat foundation over dense sands, rock, and soil-improvements.

Background

Figure 2. Building cut-away, shear wall elevation, and wall vertical reinforcement ratio.

32 STRUCTURE magazine

The building code requires buildings over 240 feet in height to have a dual lateral force-resisting system, which includes a moment frame. The added moment frame is a redundancy provision, in part due to the limitation of code-prescribed design methods that utilize linear methods such as response spectrum. Performance-based seismic design (PBSD) guidelines by PEER/ TBI have become the standard for tall building design on the West Coast.

These provisions create a rational method for validating seismic force-resisting systems that take specific exceptions to the building code. For 500 Folsom, the dual-system requirement and a slight reduction in vertical reinforcement at the hinge zone where the only exceptions.

As required by the San Francisco Building Code, a peer-review panel was formed with experts in reinforced concrete analysis (professor), design (practitioner), and seismology. The panel is responsible for reviewing the design criteria, analysis results, and final design of the seismic force-resisting system, including drawings and calculations. Presentations are made to resolve comments by the peer-review panel. The deliverable of the peer-review panel is a letter to the City of San Francisco, giving a summary of their findings.

Design and Performance The core-only approach has been analytically verified to provide adequate lateral system strength and stiffness on many projects. It is proportioned to meet all non-exempted seismic requirements using code-prescribed linear response spectrum analysis. The behavior observed in response spectrum analysis is favorable.

PEER/TBI requirements are intended to meet minimum building code provisions with a few enhancements considered appropriate for tall building design. ASCE 7-16, Minimum Design Loads for Buildings and Other Structures, has formalized the use of nonlinear response history analysis (NLRHA) for taking exception to building code requirements under the guidance of a peer review panel, Figure 3. Design revisions and improved performance. much like PEER/TBI, but only meeting the minimum code requirements. A comparison of three significant Furthermore, many structural designers using NLRHA have observed differences between ASCE 7-16 and PEER/TBI are: higher shear demands than prescribed by response-spectrum code design. • ASCE 7-16 Chapter 16.4.1.2 permits the average of peak drift This finding is partially attributed to a code-based design methodology results under MCE-level demands to be 4% for a shear wall build- that presumes all modes experience uniform energy dissipation. As ing. PEER/TBI requires the average of peak drifts affirmed by NLRHA, the deformations associated with higher modes to be 3%. often experience less energy dissipation and, therefore, higher shear • ASCE 7-16 has no limit for individual ground motion results, demands, often 2-4 times response-spectrum code-based designs. In but PEER/TBI limits individual ground motion results to the most recent version of ACI 318-19, Building Code Requirements for 4.5% drift or less.

Structural Concrete and Commentary, this effect has been addressed by a • ASCE 7-16 has no residual drift limits. PEER/TBI limits dynamic amplification factor for shear in slender shear walls. average residual drifts to 1% and individual ground motion As part of the PBSD process, a detailed nonlinear response history residual drifts to 1.5%. analysis was conducted. The analysis was conducted using PERFORM After completion of the code-based design, it can be beneficial to the 3D and includes detailed nonlinear modeling of shear walls, link beams, design team to create summarized plots describing wall vertical rein- and slabs represented as equivalent frames. A robust set of 22 MCEforcement ratio over the building height (Figure 2). This information level linearly scaled ground motions with a high level of dispersion was compiled for all vertical reinforcement in the core and link beam was developed. Conditional mean spectra were used for each set of 11 shear strength. Gradual changes in core-wall vertical reinforcement ground motions to target short period and long period demands. At the are essential. Also, consistent link beam strength over the height of primary period of the structure (5.5 seconds), some ground motions the tower is necessary to avoid local concentrations. were 120% to 170% of the MCE target spectra. In other words, at Traditionally, designers have preferred energy dissipation in core the fundamental period of the tower, some ground motions were up wall buildings to primarily be from a hinge (a focused area of vertical to 70% higher than the MCE level demands. This is not unreasonable wall reinforcement yielding) complemented by the yielding of link since MCE is not an absolute maximum, and it is not unreasonable to beams. When evaluating the seismic performance of slender core- consider a few individual ground motions greater than MCE with some only residential buildings using NLRHA, many designers have not below the MCE, such that the average meets the MCE. observed this behavior. For slender core-only buildings, it is common for most of the energy dissipation to come from link-beam yielding Analysis Results and minimal energy dissipation to come from shear wall vertical reinforcement yielding. Typical residential towers stand unique Upon review of analysis results, ground motions within 120% of MCE from typical office towers in that the floor-to-floor heights of typical at the target period performed well, and behaviors were well aligned residential towers are noticeably lower. This results in shallow link with expectations resulting from response spectrum results. Isolating beams (9.5 feet is a typical residential tower floor height, and 13 results of ground motions less than 120% of MCE, all key behavior feet or more is a typical office building floor height). The shallower results such as drifts, link beam rotations, slab rotations, and wall shear link beams, common in residential towers, provide lower wall-to- were reasonable. The difficulty arose from 3 ground motions that were wall coupling and result in higher link beam rotational demands. between 120% and 170% of MCE at the fundamental period. These The lower cumulative strength of the shallower link beams over the ground motions caused unacceptable link beam rotations and drifts. height of the building limits the formation of a concentrated wall 500 Folsom is a compelling case in that the minimum requirements hinge at the dynamic base of the core wall. While the behavior is of ASCE 7-16 could be considered satisfied with these responses certainly acceptable, it is different than anticipated by the code- but would not have met the requirements of PEER/TBI without a prescribed design procedures. revision to the code-based reinforcement design. Since the criteria JUNE 2020

33

was PEER/TBI, designers evaluated a variety of design alterations which included: • Widening openings on the east and west sides of the core to reduce link beam rotation magnitudes.

• Increase link beam reinforcement from 6√f´c to the code maximum of 8√f´c. This change increased the link beam diagonals from #11s to #14s.

•Increase vertical reinforcement in the upper floors near the roof solid wall regions to reduce yielding.

• Reduce vertical reinforcement near the top of the podium to help encourage a more distinct plastic Figure 4. Proposed mapping of NLTH analysis result to ACI 318-14 Chapter hinge in the wall vertical reinforcement in the 18.10.6.4 boundary detailing provisions. higher magnitude events.

These targeted changes resulted in overall cost reductions and perfor3) Ordinary Boundary with 8-inch Spacing (ACI 318-14 mance improvements. These changes did not significantly change the 18.10.6.5-No Yielding): response to ground motions under 120% of MCE. Still, the changes Limited bar buckling restraint in tension and did result in significant improvements for ground motions greater no confinement in compression. than 120%, as shown in Figure 3, page 33. These results indicate the T: 2x yield > εt ≥ 1x yield importance of designing with NLRHA using prescribed guides such C: 0.001 > εc as PEER/TBI. Through the incorporation of NLRHA, important Tensile Strain Limits design improvements were identified to achieve the design intent Unrestrained bar = 1x yield conforming to PEER/TBI and the San Francisco Building Code.

8" tie spacing = 2x yield The high levels of dispersion in the ground motion demands is an 6" tie spacing = 10x yield important consideration for resilience as well. Both ASCE 7-16 and Compressive Strain Limits PEER/TBI permit the use of spectral matching. As often applied, Ordinary (8") Limit = 0.003/2*/1.5** = 0.001 spectral matching reduces demands to MCE level demands at all Ordinary (6") Limit = 0.004/2* = 0.002 periods, limiting designer understanding of performance at higher Special Limit = 0.013/2* = 0.006 levels of shaking, which may occur. Resilient designs should continue *Per Wallace, 2007 to generally perform well even beyond MCE level demands but, **Force-controlled action without evaluations beyond MCE, this behavior cannot be conBased on performance, a mapping of NLTH results and ACI firmed. The higher levels of dispersion using spectral scaling in the 318-14 design provisions are utilized to distribute the three types of 500 Folsom design allowed designers, through a few modest changes boundary zone types throughout the tower (Figure 4). The boundary to the design, to achieve reasonable performance beyond MCE level zone type above was mapped to each boundary on each floor based demands and reduce costs. on average tension and compression strains under MCE demands.

Generally, special boundary zones were used between ground and podium hinge and also lined the east and west core openings to Shear Wall Boundary Confinement the roof. Ordinary boundaries with 6-inch and 8-inch spacing were Nonlinear wall elements in NLRHA output strain at numerous locations used through the height of the tower, with select zones near the of the core walls on all floors. This information cannot be obtained with two-third-point of the height having special boundary elements. response spectrum analysis but can be greatly beneficial in specifying boundary zone detailing. At the time of design, ACI 318-14 prescribes Conclusions the type and location of boundary elements based on a simple shear wall, but core walls are much more complex. Mapping of compression The design and verification of the 500 Folsom seismic force-resisting and tension strain demands to boundary zone types could be immensely system with NLRHA revealed the importance of conducting a detailed helpful in increasing resilience while reducing costs. nonlinear analysis even if code-provisions are satisfied. Furthermore, ACI 318-14 Section 18.10.6.4.2 provides boundary zone detailing component behaviors can be understood in greater detail, producing requirements for three different types of boundary zones: special greater resiliency and reducing costs.■ boundary zone, ordinary boundary zone with a 6-inch spacing of ties, and ordinary boundary zone with an 8-inch spacing of ties.

The online version of this article contains additional 1) Special Boundaries (ACI 318-14 18.10.6.4): graphics. (www.STRUCTUREmag.org) Bar buckling restraint in tension and ductile confinement in compression.

Mark Sarkisian is a Partner of Structural and Seismic Engineering at Skidmore, Owings & Merrill LLP, and a member of ACI. (mark.sarkisian@som.com) T: 10x yield ≥ εt ≥ 2x yield C: 0.006 ≥ εc ≥ 0.002 Eric Long is a Director in the San Francisco office of Skidmore, Owings & 2) Ordinary Boundary with 6-inch Spacing (ACI 318-14 Merrill L.L.P. (eric.long@som.com) 18.10.6.5-Yielding): David Shook is a Director and Structural Engineer with Skidmore, Owings & Bar buckling restraint in tension and very modest Merrill LLP, in San Francisco. David is a co-author of a book, published by the confinement in compression.

Council on Tall Buildings and Urban Habitats, bringing performance-based T: 10x yield ≥ εt ≥ 2x yield design principles to an international audience. (david.shook@som.com) C: 0.002 > εc ≥ 0.001

34 STRUCTURE magazine

legal PERSPECTIVES Managing the Risks of Subconsultant Relationships

By Dan Knise

E

ngineers have a responsibility not only for their actions or errors but also for the actions/errors of those subconsultants or subcontractors who work under their direction.

While there is much focus on managing an engineering firm’s own risk, all too often there is not sufficient attention paid to its potential vicarious liability, or so-called inherited risk, attendant to utilizing subconsultants.

Developing and sticking to a process to manage subconsultant/subcontractor relationships can help minimize this risk and further enhance a firm’s success.

What are the components of an effective subconsultant risk management program?

Many point to three critical steps for any engineering firm to take: 1) Work only with firms that have a proven track record of success, are financially sound, practice good risk management/quality assurance, and are known to deliver their work on time and budget; 2) Utilize a well-thought-out subconsultant agreement that clearly spells out the relationship between the parties, delineates the scope of the work expected, and includes appropriate indemnification and other pertinent requirements; and 3) Establish and utilize an appropriate requirement for insurances the subconsultant must carry before beginning work on your behalf.

Effective Subconsultant Selection Engineering firms must have a well-defined process for evaluating and selecting appropriate subconsultants. This typically involves collecting some necessary information for evaluating the firm’s capabilities and qualifications, management approach, quality control/risk management commitment, financial stability, etc. Many firms use a simple questionnaire to collect this data and then maintain an active database to track the information and make it available to project managers and senior executives.

References can play a crucial role in determining appropriate partners, as can 36 STRUCTURE magazine

feedback from contractors, local attorneys, and others in the engineering and construction community.

This should include input about the experience of staff members who may have worked with the potential partner firm in the past.

Furthermore, firms should assess the qualifications and experience of the prospective subconsultant’s key team members to be assigned to the project.

Utilize Good Subcontract Hygiene The second leg of this three-legged stool involves practicing good contract hygiene when working with subconsultants. This includes: • Having a written agreement signed by both parties; • Including a clear scope of services; • Being sure to flow down any unique or additional requirements from the project owner to the subconsultants, including any indemnification and, if appropriate, insurance requirements; and • Specifying payment terms and any dispute resolution procedures; some like arbitration to avoid the courts, while others feel that requiring a lawsuit may reduce the likelihood of being sued.

Having a written, executed contract with a subconsultant can often make a significant difference in who pays a claim (or defends one). Firms that lack standard contracts for these arrangements might begin by checking the prime-subconsultant forms available from the American Council of Engineering Companies (ACEC) or the Engineers Joint Contract Documents Committee (EJCDC).

One item to flag if the subconsultant offers their standard contract: inadequate limitation of liability clauses that may leave you holding the bag. Some subconsultants argue the smaller scope of their work or lesser compensation justifies a limitation of liability (e.g., to no more than the fees they are paid).

The problem with this approach is that, unless the engineering firm hiring the

subconsultant has the same limitation from their client, this could shift risk to you with no recourse against the subconsultant that created the issues and/or caused the damages.

Require Minimum Insurances Before Work Begins Including thoughtful, comprehensive minimum insurance requirements into your subconsultant agreement is another essential approach to help minimize risk when hiring subconsultants. Some key items to consider are: • What insurance policies should be required? There may be variations in required insurances, depending on the type of work the subconsultant will perform, the nature of the project, and any requirements in the prime agreement with the project owner (which must flow down). Nonetheless, several basic building blocks of an effective insurance program should always be required. These include: o Commercial General Liability (CGL); o Automobile Liability (or non-owned and hired auto liability as part of the CGL policy); o Workers’ Compensation and Employer’s Liability; o Umbrella/Excess Liability (may be optional on very small projects); o Professional Liability (also known as Errors & Omissions coverage); and o Pollution Liability (often combined with professional liability).

Other insurance coverages may also be required, depending on the subconsultant’s services, such as drone (unmanned aviation

begin work until they provide certificates of insurance that meet all agreed-upon coverage requirements.

Taking Key Risk Management Steps Pays Off Effective risk management is integral to an engineering firm’s success. Taking these three steps – selecting an appropriate, reputable, and capable subconsultant, utilizing

a solid contract, and requiring appropriate insurance – will help minimize risks from these arrangements and enhance the likelihood of success.■

Dan Knise is Chair and CEO at Ames & Gough, a specialty insurance brokerage serving A/E firms, law firms and professional organizations.

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vehicles), marine, cyber/network security liability, etc.

• What Limits to require? This is a trickier question (and, in any event, should be noted as minimum insurance requirements), but some basic standards for a typical commercial or infrastructure project might be: o Commercial General Liability: $1 million per occurrence/$2 million general aggregate and products/completed operations; o Automobile Liability: $1 million combined single limit or equivalent; o Workers’ Compensation/ Employer’s Liability: Statutory and $500,000 per illness, injury, or disease for employer’s liability; and o Professional and Pollution Liability: Varies, but ideally at least $1 million per claim and aggregate.

Obviously, on larger or more complex projects or with regard to specific services that are riskier, these minimum limits may increase dramatically (to as much as $5 million or $10 million or more).

• Other Insurance Requirements?

A well-thought-out subconsultant insurance specification must also include several other items.

While there are too many to mention here, a few of the more important are: o Additional Insured Status for the Engineer hiring the subconsultant on both the CGL and Automobile Liability policies; o Primary and Non-Contributory Status as part of being an Additional Insured; o Waiver of Subrogation on the CGL, Auto, Workers’ Compensation/Employer’s Liability; and o Notice of Cancellation or NonRenewal to be provided to the engineering firm if the insurer cancels any of the policies.

Another critical step is having a compliance procedure to collect and review certificates of insurance from each subconsultant to ensure that they are genuinely complying with the insurance requirements of their contract, as well as to make sure certificates are entirely up to date. Do not allow any subconsultants to

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37

CASE business practices Coordination and Completeness in a BIM Dominated World

By David Ruby, P.E., S.E., SECB, F.ASCE

I

n earlier times, when computers were neither available or essential, one objective of the structural design process was to discover a computational method which was elegant, simple, and appropriately accurate. When such a process was identified, it was recorded as an expedient approach to solving a recurring structural design problem. Thus, quick “Rules of Thumb” became essential resources for the structural engineer.

As computer software has proliferated, become more comprehensive, and been made very user friendly, the importance of Rules of Thumb and approximate methods has been diminished. It has been argued that, with computational speed and ease of application of computer methods, the need for approximations and Rules of Thumb no longer exists. However, equally imposing arguments can be made for the value of these quick approaches, such as: • The structural engineer should have tools to make on-the-spot intelligent decisions.

• A reasonable solution is often required as computer input.

• The validity of the computer output should be verified with rational approximations.

The above paragraph is a direct quote from a February 2000 article in Modern Steel Construction entitled “Rules of Thumb for Steel Design.”

Twenty years later, the Rules of Thumb still apply regardless of the vocabulary and modified delivery methods: - Building Information Model - BIM Execution Plan - COBie (Construction Operations Building Information Exchange) - Clash Detection - Integrated Project Delivery (IPD) - Level of Development - Model Element Author - Polygonal Modeling - Virtual Construction No matter the means of communication, our responsibilities have not changed: review and confirmation of the results of our analysis, transfer of the design results to construction documents, and compliance of construction documents with governing codes. Technology has allowed the design process to be completed in greater detail, moved us from 2-D to 3-D, advanced our analysis and design capabilities, and allowed 38 STRUCTURE magazine

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today’s structural engineers to achieve structural feats never dreamed of in decades past. However, we have not always viewed technology for what it is not – it is neither all-inclusive nor self-sufficient. It is only a tool, a tool capable of enhancing the process, but a tool that will never replace a competent structural engineer.

In a high-rise building design, technology often utilizes rigid diaphragms to distribute the lateral forces to the lateral force-resisting systems, complete the structural analysis, determine the structure’s member forces, or, in an extreme case, will utilize the rigid diaphragm to account for the lack of a well-defined load path. However, the computations necessary to resolve those related diaphragm forces are beyond the capability of technology. They are and always will be the structural designer’s responsibility to recognize and perform.

Similarly, technology analyzes the structure based on the structural engineer’s assumed boundary conditions. Subsequently, the structural engineer reviews the construction documents to ensure the design concept is shown, essential elements are identified, and the scope and quality of work to be performed is communicated. The engineer must also ensure that construction details comply with the initial design assumptions and vice versa.

Twenty years later, the industry’s expectations related to the quality and purpose of our construction documents have not changed.

These expectations are clearly stated in CASE Document 962D, Guideline Addressing the Coordination and Completeness of Structural Construction Documents, initially published in 2003 and revised in 2013.

962D states that quality can be characterized as: • Meeting the requirement of the owner as to functional adequacy; completion on time and within budget; life-cycle costs; and operation and maintenance.

• Meeting the requirements of the design professional as to the provision of the well-defined scope of work; budget to assemble and use a qualified, trained, and experienced staff; budget to obtain adequate field information prior to design; provisions for timely decisions by the owner and design professional; and, contract to perform necessary work at a fair fee with adequate time allowance.

• Meeting the requirements of the constructor as to the provision of contract plans, specifications, and other documents prepared in sufficient detail to permit the constructor to prepare priced proposal or competitive bid; timely decisions by the owner and design professional on authorization and

processing of change orders; fair and Technology does not compensate for missing leaving no time for innovation and resulting timely interpretation of contract require- elements, such as incomplete load paths, base in unanswered questions about the braced ments from field design and inspection fixity, or member stability. Too often, design vs. moment frame lateral system; column staff; and, contract for the performance documents are developed directly from the through forces; lateral-force-resisting system of work on a reasonable schedule which analysis model without an intermittent review description; special requirements of the design permits a reasonable profit. or constructability input. Load paths may go concept and/or shoring necessary to maintain • Meeting the requirements of regulatory undefined, unique features of the design con- the final structure’s positioning. agencies (the public) as to public safety cept, lateral-force-resisting system, or material Many elements of the design/construction and health; environmental considerations; relationships may not be identified or commu- process have been impacted by technology. protection of public property including nicated, and all result in contractor-initiated Software has replaced manual methods, utilities; and conforcomputer screens have mance with applicable replaced drawing boards, The three C’s > Essential ingredients for successful laws, regulations, simple solutions may not codes, and policies. be sufficient to check comproject execution in a BIM dominated world: 962D states the Purpose of plex structures, and hand the Construction Documents: analysis is out of the quesConstructability through Collaborative design “Documents, including tion. Building models are building information generated directly from the and open and shared Communication models, drawings, and analysis model, which in specifications, are the turn creates the constructools structural engineers use to com- RFIs with an all too familiar response: “But, It tion documents through digitized printers. municate the elements of the design of Worked in the Model!” It has been suggested Even estimates are automated, and engineerstructures to contractors. Contractors use that Albert Einstein stated, “I fear the day that ing team members rarely ask themselves, the Documents to develop and submit technology will surpass our human interaction. “How can I improve the process?” bids for construction of the structure and The world will have a generation of idiots.”

Until the advent of Building Information then, if selected, to implement the design.

What is evident after the author’s 60 years Modeling, the basic process had remained In order for the bid to be accurate, the in the industry? My Many Rules of Thumb virtually unchanged.

Documents must describe in sufficient still apply: Now, however, design professionals have detail the elements of the structure to be - The devil is in the details; a limitless opportunity to expand their role built, the quality with which it is to be - The lightest structure is not always within the process, create their own Rules of built, and any special requirements govthe least cost; Thumb, and harness technology to improve erning its construction. Regardless of the - Load always goes to stiffness; the quality and content of their construction format, the Documents must be devel- Shop time is always less costly and documents. They can do so by engaging in a oped to a sufficient level of completeness more efficient than field time; collaborative environment and truly focusing and coordination so that contractors can, - Structural steel cost is not related to on infusing construction knowledge and expewithin customary time constraints, develop pounds per square foot or based on rience (constructability) through collaborative a price, submit a bid, and, after award of the dollars per ton. It is directly related to problem solving and design development. contract, build the structure in a manner material choice, complexity of details, Such a focus on the process will make them consistent with their understanding of and manhours to fabricate and erect; much more than providers of information the scope of the Documents at the time and, and technical solutions. of bidding. Inherent in this process are - When an ironworker offers a comment, Communication within and between the issues of what is customary in terms it is time for the engineer to listen. the design and construction communities of the level of detail and coordination of Cost is impacted by our design decisions through integrated design teams will provide the Documents and the degree of scrutiny related to complexity, the economy of scale, answers to those unanswered questions. required of the bidder. For example, is a bay size, member and material selection, Silos of influence will disappear, and jointly steel subcontractor able to rely solely on bolted vs. welded options, shop vs. field weld- developed, collaborative solutions will become the structural documents, or must he also ing, and shop vs. field assembly. It is also the order of the day. review the architectural and mechanical impacted by truss vs. girder “do’s and don’ts,” And all stakeholders will benefit. documents? If the contractor is required lateral system, column size, and the design Remember: Communication is the to review documents created by multiple decisions related to strength vs. deformation goal. It is not for you to know and disciplines, how much effort would it be vs. serviceability. All these considerations have the contractor to find out!■ reasonable to expect of the contractor in been ignored by advanced technology. comprehending the totality of the design?

Also, technology has led to compressed The online version of this article Inadequate communication results in schedules and reduced budgets. This precludes contains references. Please visit budget and schedule overruns, disap- the consideration of structural alternatives, www.STRUCTUREmag.org. pointed owners, and a potential risk to material substitutions, or the development the safety of building occupants and the of difficult or complex connections, so they David Ruby, Chairman / Founding Principal, public. Successful communication is critical are delegated. Perhaps, in some circumRuby+Associates, Inc. authored AISC’s Design for the protection of public safety, which stances, the better solution is to modify the Guide #23, Constructability for Structural Steel is a structural engineer’s first priority as a framing concept. However, the owner and Buildings. (druby@rubyandassociates.com) professional.” architect have established a fixed release date JUNE 2020

39

historic STRUCTURES Bridge Failure in Dixon, Illinois

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

D

ixon is located about 97 miles west of Chicago on the southwesterly flowing Rock

River, a tributary of the Mississippi. A wooden bridge crossed the river at Dixon for years but was frequently washed out in floods. In 1868, it was decided that an iron toll bridge was required, and the Mayor solicited bids. Several committees of the City Council visited bridges and bridge works in Chicago, Detroit, Cleveland, and elsewhere. A total of 11 bridge companies submitted proposals for 45 different bridge styles and lengths from wood to

Truesdale Patent No. 24,068.

iron. Some of the most well-known firms were John A. Roebling; Smith, Latrobe & Co.; Zeneas King; and L. B. Boomer. The lowest price was $15,000 for a wooden bridge up to $50,000 for an iron bridge. The Dixon City council, after several votes, selected a design by Lucius E. Truesdale for $40 per lineal foot. The Dixon Telegraph wrote that the Council chose Truesdale’s plan,“taking into consideration expense, as well as the form and comeliness of the structure.” Later, it was charged that some of the Council members were bribed, but that charge was never proven. Truesdale had received several patents for bridges and was proposing to build one according to Patent No. 24,068 for a truss bridge, issued on May 17, 1859. It was very similar to patent no. 21,388 issued on August 31, 1858, also for a truss bridge.

Truesdale wrote, “The nature of my invenelongation of all the horizontal chords, that tion relates to bridges constructed of iron, any longitudinal tension will be transmitted or and it consists in the use and combination distributed through all the horizontal chords; of a double series of horizontal ribs or chords thus relieving that part which lies in the plane with a series of diagonal and vertical braces of pressure from undue strain.” The truss was a or their equivalents, by means of which the high, 15-foot pony truss in that Truesdale had strain and tension of the various parts under no overhead cross bracing. He claimed to get a rolling weight is in a measure neutralized by his lateral stiffness by having his vertical posts the tendency of these parts to distribute them widen out from top to bottom, as shown on more evenly throughout the whole structure, his sheet 2 of drawings. thereby giving greater strength, rigidity, and The bridge consisted of five spans (with firmness to the bridge with less weight of lengths of 132 feet), a deck width of 18 feet, material than is obtained by any other known and two sidewalks with a width of 5 feet each. modes of constructing them.” He also wrote, The total cost of the bridge was $75,000, “These horizontal chords, which are made of with $31,512 going to Truesdale for the iron wrought or malleable iron and run through the superstructure. The bridge opened on January entire length of the bridge, serve the purpose 21, 1869, to a grand celebration with a local of distributing the tension or strain which may paper writing, “A structure more truly grand be exerted on any particular portion of the Truesdale Patent No. 24,068, Sheet 2. and beautiful to the eye can be found in no bridge equally throughout the whole, for as western city and we presume in no eastern the action of a heavy pressure on any part of the structure is to depress one.” Another paper wrote that the City Council thanked Truesdale, it in a vertical plane, and as such a depression can only be effected by “for the promptness, energy, and faithfulness, and for his gentlemanly an elongation of the baselines of the bridge, thus creating a longitudi- courtesy.” Before the opening, the bridge was test loaded in a fashion nal tension and strain, it follows that in a bridge constructed on this by placing “four harnessed teams hauling stone, a load of flour, and a plan, as the baseline can not be sensibly elongated without a similar large group of bystanders, all weighing at least 45 tons.”

40 STRUCTURE magazine

It was later made public that the City Engineer, L. Stanton, had advised against accepting the bridge since the builder did not follow the specifications. He had also been against awarding the bridge to Truesdale, indicating that he “never knew any engineer or practical bridge builder, living or dead, that approved of the plan upon which the Truesdale Bridge was built on,” and it was his opinion that “the City Council who favored the Truesdale bridge were influenced by the opinions of the citizens, rather than by practical bridge builders. Mr. Truesdale was a sharp, shrewd man, who could make a favorable impression upon men; enthusiastic in favor of his own plan and carried the citizens with him.” Dixon Bridge showing the height of un-braced trusses.

Stanton was dismissed due to his opposition before completion of the bridge and was not invited to be present when and had always been regarded as worthless by those whose opinion the load testing was made. Later, in the press, he gave many reasons why was regarded as valuable.” he opposed the bridge and its construction.

“It is not a bridge at all. His method of construction is utterly at All seemed well, even though a Truesdale bridge in Elgin, Illinois, fault. The bridge is not worth that, snapping his fingers.” collapsed on December 7, 1868. And, after being rebuilt, it collapsed Mr. Herman said the bridge was constructed on a wrong principle. again on July 5, 1869. One newspaper deflecting blame from Truesdale “The iron was spread over too much ground, and the bracing was wrote, “The foundations of the structure must have been tampered very defective. There were five chords in the Truesdale bridge, the with by some evil-disposed persons,” which was very unlikely. middle ones being perfectly useless, simply superfluous, while the On May 4, 1873, only four years after its opening, The Dixon other inside ones were not much better.”

Bridge collapsed into the river. On that day, Reverend Pratt, a Baptist The Tribune also reported, “Some sank to rise no more. Some were minister, was baptizing members of his flock in the river on the north killed before they touched the water. Some were entangled in the side and west of the bridge, as he had done several times in the past. debris. Some jumped from the bridge to the river and swam ashore.

A large crowd, estimated at 200 people, mainly women and children, The weak generally succumbed.” The truss “fell over with the weight gathered on the 5-foot sidewalk. Some young men climbed up on the westerly truss to get a better view. The bridge tender, Henry Strong, was concerned about the number of people bunched together on the ® sidewalk and was reported to have ordered some off the bridge. After baptizing two members and preparing to baptize the third, people later reported they had heard a cracking of the iron and that the bridge dropped out from under them. The collapse threw bystanders BETTER PERFORMANCE into the water, with the truss falling over onto them and trapping many of them under the ironwork. The end span fell in its entirety in the river, and several other spans dropped partially into the river.

The Chicago Tribune ran almost a full two-page article on the failure, including an engraving of the bridge intact and collapsed. They included such statements as, “The cause of all this loss of life, and the broken hearts and desolated homes that it entails was the adoption by the city authorities of a bridge of unsound construction built in defiance of the true principles of engineering…”

“The murderous instrument – The bridge which was the cause of this accident – was built in 1868 at a cost of $83,000…the people were rather pleased with it at first, just as a child is tickled with a new toy. It was neat, light, and airy, very becoming to the river –a sort of fashionable, stylish bridge that looked very well but wore very badly.”

“It was very top-heavy. The heavy trusses were too much for the rather light foundation.”

“That these bridges are perilous to life is the opinion of the best engineers. The following relative to the diabolical Truesdale plan shows in what estimation it is held by those competent to judge.”

The reporter interviewed several Chicago Engineers who told him: “While the bridge was built pretty and light, and to the eye was ZERO LOOSENESS light and charming, it was practically useless…”

“Mr. Truesdale is no engineer at all. His methods of construction PH: (360) 378-9484 – WWW.COMMINSMFG.COM showed an ignorance of the fundamental principles of mechanics,

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41

Collapsed spans.

and imprisoned the doomed in an iron cage, with which they sunk and from which there was no escape.” Despite heroic actions by many bystanders to save them, 46 people, 37 of whom were women, lost their lives on that fateful day.

As with any failure, people wanted to know who was to blame for the collapse. Some blamed the City Council while some stated that Truesdale had gotten the project by bribing the councilors. A Coroner’s Jury was set up on May 7 to determine the cause of the failure. They obtained testimony from many locals, all of whom expressed their long-time concern about the safety of the bridge with one saying, “the bridge was a humbug, it was not strong enough to hold itself up,” and another saying, “unequal distribution of heavy weight threw (the bridge) out of perpendicular and let it down,” which was undoubtedly true. A newspaper concluded that Truesdale, “had the means to push his invention, and was thwarted only in the presence of men of science, who again and again declared it dangerous and useless.

Every railroad company rejected it on sight.” The Coroner’s Jury concluded, amongst other things, “the council erred in judgment in selecting the Truesdale Bridge.”

The Scientific American wrote, in part: “From the information gleaned regarding the superstructure, there is little question but that its theory of construction was wrong and the material poor and clearly inadequate. The principle of the Truesdale patent, upon which it was based, is to lock joint all supports... The metalwork throughout the whole fabric was exceptionally frail…But when the facts are on record, not only of the falling of a structure (its counterpart) but of the pronounced opinions of experts that this very fabric was unsafe, the fault must be plainly attributed to neglect…Too much light and cast iron is employed, and the lock joint arrangement so weakens the metal that its full strength cannot be gained...Here was a structure which any competent engineer should have been able to perceive at a glance was improperly built and unsafe, even were he not aware of the experience of others with its defects. Yet we are told that a city council examined it and were suspicious of its strength, and still it was allowed to remain.”

Charles Macdonald, a leading bridge engineer, wrote a letter to the editor of the Dixon Sun, stating in part: “I have been favored with the perusal of your very enterprising paper containing a list of the bids received for the construction of a bridge at Dixon to replace the one unhappily the cause of such widespread grief in your midst, and having previously read with 42 STRUCTURE magazine

deep interest your account of that disaster. I am induced to make a few suggestions to the people of Dixon through your columns as to the problem they have to solve in building the new bridge…

First then, you employ a thoroughly competent engineer; you will tell him in general terms what kind of a bridge you require and then give him full power to draw up detailed specifications, in order that all proposals may be compared by one standard. After the bids have been carefully examined, you will require from him a report upon their different merits with especial reasons for any preference he may have for a particular design. And then, as businessmen, you will be in a position to decide as to who shall build your bridge…

With a selection made in such a way, there will be satisfaction in preparing plans and estimates for the new bridge at Dixon, and the people will stand a chance of getting full value for their investment.”

Truesdale wrote a letter to the Springfield Republican on May 22, 1873, and a condensed version was picked up by the New York Tribune, stating, “I know that I never made a better bridge than the one at Dixon in every particular. After the completion of the bridge, a trial of its strength was made, when each span was loaded with teams and people all weighing at least 200,000 pounds…The bridge would have sustained a weight of 250,000 pounds with safety, and yet it is said to have fallen with less than 15,000 pounds. If some of the bolts had been loosened in the top chord near their connections and weights placed on the catwalk, then the bridge would have fallen precisely as it is said to have fallen; and I cannot conceive of any other cause.”

He concluded his defense, saying, “It is nearly 18 years since I began building iron bridges and the Elgin and Dixon bridges are the only ones that have fallen, and no loss of life except at Dixon. Can as much be said of any other plan?” The answer to his rhetorical question was yes, and most, if not all of, the engineers of the time agreed the Truesdale Bridge was a poorly designed structure.

The ASCE, at its 5th Annual Convention in Louisville, KY, held only three weeks after the disaster, Resolved, “in view of the late calamitous disaster of the falling of the bridge at Dixon, IL, and other casualties of a similar character…that a committee be appointed to report at the next Annual Convention the most practicable means of averting such accidents.” The committee was a four-star committee consisting of 11 of the leading engineers of their time.

Unfortunately, it was not a unanimous solution. Different members of the committee submitted four reports. James Eads and C. Shaler Smith concluded, in part, two years later, “After a careful examination into the causes of the most disastrous accidents which have occurred during the past few years, it finds that they can readily be divided into three different classes. First, where bridges are erected by incompetent or corrupt builders, and accepted by incompetent or corrupt railway or municipal officials…”

The failure of the Dixon bridges can be traced back to its poor design and use of iron. While men like Squire Whipple had designed bridges by forcing the loads to pass through as few members as possible, Truesdale had added members. His truss was not analyzable by any means then known to the profession and, instead, was a “rule of thumb” design. The loading at failure was similar to the Albion Bridge disaster (STRUCTURE, March 2020) when one sidewalk and side of the bridge was loaded with people to view a tightrope walker.■ 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)

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NCSEA News NCSEA

News from the National Council of Structural Engineers Associations

National Council of Structural Engineers Associations

Call for Entries: 2020 Excellence in Structural Engineering Awards NCSEA's Excellence in Structural Engineering Awards annually highlight some of the best examples of structural engineering ingenuity throughout the world. Projects are judged on innovative design, engineering achievement, and creativity. The awards are presented to multiple winners with an outstanding winner chosen for each of the following categories: • New Buildings < $30 Million • New Buildings $30 Million to $80 Million • New Buildings $80 Million to $200 Million Rufus 2.0 Spheres – Magnusson Klemencic Associates • New Buildings Over $200 Million 2019 New Buildings $20 Million to $100 Million Outstanding Award Winner • New Bridges or Transportation Structures • Forensic/Renovation/Retrofit/Rehabilitation Structures < $20 Million Entries are due on July 14, 2020. More information about the awards along with submission instructions can be found • Forensic/Renovation/Retrofit/Rehabilitation Structures > $20 Million on www.ncsea.com.

• Other Structures

Volunteer for an NCSEA Committee

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

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

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

The Success of the 2020 SE3 Survey Relies on YOU! The NCSEA SE3 Committee is currently administering its third nationwide survey of structural engineers across the profession. With over 2,400 respondents by the beginning of May, the most commonly asked questions about the 2020 SE3 Survey include: • WHO should take this survey?

• WHY should I take this survey (again)?

• HOW is it going to benefit the profession?

Every structural engineer is invited to participate regardless of race, gender, age, or job title. The SE3 mission applies to every structural engineer. Initial survey demographics show that 72% of respondents are male and 25% are female; roughly 79% are white Caucasian, 11% are Asian, and 5.5% are Hispanic; and the average age of a respondent is 39.5 years old with a median age of 36.

As a structural engineer, you are part of a larger community and this is an opportunity to have your voice heard. Preliminary results of the survey suggest that respondents believe the top two (2) challenges facing the profession are inadequate compensation relative to similar professions and retention of engineers due to stress and burnout.

By participating in every survey cycle, the SE3 committee can track trends, identify areas that need and/or have shown improvement, and facilitate conversations towards change. The committee retains questions regarding demographics, career satisfaction, compensation, and work-life balance as a baseline to the survey. However, new questions are incorporated based on popular demand and current events.

The success of the SE3 survey relies on participation of ALL structural engineers. For more information, check out www.SE3committee.com.

NCSEA Webinars June 18, 2020

Register by visiting www.ncsea.com

Frequently Misunderstood Seismic Provisions Emily Guglielmo, S.E., P.E.

June 25, 2020

Seismic Assessment and Intervention for Historic Structures: An Exemplary Case Study Terrence Paret, P.E., S.E., and Jeffrey M. Rautenberg, Ph.D., P.E., S.E.

July 7, 2020

Existing Buildings and the "10% Rule" – Are We in Agreement?

Kevin O’Connell, S.E., and Daniel Zepeda, S.E.

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J U N E 2 02 0

47

SEI Update News of the Structural Engineering Institute of ASCE Resources

Confidential Reporting on Structural Safety CROSS-US â&#x20AC;&#x201C; First newsletter available at www.cross-us.org with failure reports and case studies. Confidential reports on structural safety hazards, including any arising from lockdown or consequences of the COVID-19 pandemic, are invited. Sign-up to receive the free newsletter.

Report from Iconic Global Structures Conference in Dubai September 2019 www.asce.org/SEIGlobal Determination of Pressure Coefficients for High Rise Buildings of Different Aspect Ratios The Final Report is available â&#x20AC;&#x201C; joint effort of SEI/ASCE and Charles Pankow Foundation www.asce.org/SEIStandards Structures Congress 2020 Proceedings are available for free access at ascelibrary.org 2019 SEI Annual Report, Advancing the Vision for the Future of Structural Engineering, now available at www.asce.org/SEI Remember to take advantage of your member benefit: 10 ASCE Free PDHs

Join us to celebrate 25 years of SEI! Students and Young Professionals: Apply for scholarship to participate.

Sponsor/Exhibit and reach more than 1,000 industry professionals.

Contact Sean Scully at sscully@asce.org www.structurescongress.org #Structures21

Access Electrical Transmission and Substation Structures Books, Standards, Proceedings, and Journal Papers at https://ascelibrary.org/ets2020p.

SEI Online

SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle Errata 48 STRUCTURE magazine

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

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

CASE in Point News of the Coalition of American Structural Engineers Did you know?

CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills young engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need!

CASE has several tools available for firms to use 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 and promote your talent and expertise to help guide CASE programs, services, and publications.

We currently have openings on all CASE Committees: Contracts – Committee is responsible for developing and maintaining contracts to assist practicing engineers with risk management.

Guidelines – Committee will be responsible for developing and maintaining national guidelines of practice for structural engineers.

Programs – Committee is responsible for developing program themes for conferences and sessions that enhance and highlight the profession of structural engineering.

Toolkit – Committee will be 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’ usual 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 Heather Talbert, Coalitions Director (htalbert@acec.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.

JUNE 2020

49

INFOCUS Understanding Why

By John A. Dal Pino, S.E.

I

t has been my personal observation, albeit in a limited geographical area, that some people are clearly not understanding the COVID-19

public health messages inundating all of us. Either that or the messages are just too opaque or generalized (one might say dumbed down) as to leave many people bewildered and confused and trying their best, but not understanding why. No need to go into the details and the many possible examples, but we have all probably seen some misunderstandings. Most people are wearing some kind of face-covering while out in public. Still, I often see people briskly driving alone in a car while wearing an ordinary bandana around their face (not an at-risk individual wearing an N95 surgical mask) with the windows rolled up, and the AC turned on. I suppose this is better than not wearing the face-covering, but it seems like the health messages regarding cause-and-effect and the critical do’s and don’ts are just not getting through. I keep thinking that what is needed is to have an engineer presenting the message!

A world without building codes would be like a world ignoring the COVID-19 virus and the associated public health messages, and just letting nature take its course. During extreme wind or earthquake events, or in heavily loaded buildings, some buildings would collapse and some buildings would stand. The smart people who hired a competent structural engineer to find out the why would do the best since they would have the soundest information and employ best practices.

Some other people would be fortunate and survive, mostly by luck.

The leaders of most countries decided long ago that having and enforcing a building code was necessary to avoid large numbers of casualties in building failures and to maintain public confidence in the built environment. Over time, society developed a large number of rules to guide structural engineers in the design of buildings in response to evolving conditions and new knowledge. Apparently, engineers or their professional organizations could not be left up to their own devices and figure it out for themselves. I am not sure I agree with this conclusion based on our excellent collective record, but that ship has sailed as they say. The building code provisions have become so numerous and pervasive that I suspect that many structural engineers do not truly understand what these provisions are intended to prevent and just blindly follow them. Whether the “bad” result is avoided by “meeting the code” is someone else’s concern. Some engineers do understand the “why.”

There is a parallel with COVID-19. There will be a new normal until either an effective vaccine is developed or society just gets comfortable with the health risk (that didn’t hurt too much did it?) and reduced economic activity. It will involve many rules for the workplace and social interaction for all areas outside the home. Staying home if you are sick (no heroes at work anymore), waving instead of shaking hands, maintain physical distancing, wearing a good mask at times, and more will define the new normal. Unfortunately, like understanding the building code, some people will understand the “why” behind the rules and when we can bend the rules without increasing risk. However, most people will not know the “why,” and they will blindly do what they are told, take more extreme and unnecessary precautions, or skirt the whole deal (like some building owners do with the building 50 STRUCTURE magazine

code). I can envision a comical situation similar to two cars coming face to face on a narrow street, where two people maintaining the recommended 6-foot social distance come face-to-face in a narrow aisle and freeze waiting to see who backs up first. The sensible thing will be to assess the situation, judge the risk as rather low, and just walk past each other quickly. Only time will tell.

The common thread through all of this, whether it is building or personal safety, is the need for superior knowledge, knowledge of the “why” and the ability to clearly and effectively communicate complex issues in a way that gets real buy-in and true understanding, not just blind allegiance and rule-following. The best engineers can do this.

In a time of uncertainty and stress, quacks, snake oil salesmen, witch doctors, carnival barkers, and talk show hosts (have I listed them all?) are seen by many as trustworthy, real subject-matter experts, depending on where one gets his or her information.

By “best” engineer, I do not mean the smartest since there is more to it than just knowing everything and having it locked up securely in one’s head. Or necessarily the smoothest communicator, since talk not backed up with knowledge and experience (the “why”) leaves people confused. My personal experience with COVID-19 messaging is that many politicians fall into this second category. So by “best,” I mean those engineers who understand the technical issues and can present the information clearly, logically, and in a manner geared to the experience and knowledge of the listener. One size does not fit all in this regard.

The engineer needs to use a lot of communication tools and styles to tell the story effectively and be able to change the approach on the fly.

Perhaps, at the end of this, engineers will shine as experts and show their true worth as honest brokers without bias or personal agendas.

An industry dedicated to helping their clients make informed decisions after understanding the hazards and risks, the options available to them, and what state-of-the-art engineering can achieve – collectively, the why – will help protect life and property.■ John A. Dal Pino is a Principal with FTF Engineering located in San Francisco, California. He serves as the Chair of the STRUCTURE Editorial Board. (jdalpino@ftfengineering.com)

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Bonus Content

code UPDATES What’s Old Is New Again

How Tall Mass Timber Touches the Sky and Matches the Fire Performance of Traditional Non-Combustible Construction Types

By Lori Koch, P.E., and Matthew Hunter, BCO

W

ith the adoption of the 2021 International Building Code (IBC), municipalities across the United States will have the ability to build wood buildings taller than

ever before. With three new types of construction, Type IV-A, IV-B, and IV-C, mass timber buildings will allow design professionals to erect wood buildings up to 18 stories in height.

Figure 1 represents the maximum permitted number of stories under the Type IV-A code change. While these structures are constructed from wood, they are not conventional light-frame construction, and the structural behavior and fire resistance of mass timber structures are not comparable to light-frame construction.

The new provisions for Type IV-A, IV-B, and IV-C construction were developed by the International Code Council’s (ICC) Tall Wood Building (TWB) Ad Hoc Committee. One of the main objectives of the TWB was to develop code provisions that incorporated the inherent fire resistance of mass timber wood structural components into the overall fire-resistance rating (FRR) for the three new construction types. One-third of the required Fire Resistance Rating (FRR) must come from the structural mass of the timber itself, while the other two-thirds must come from noncombustible protection (typically fire-rated Type X gypsum board).

Under the provisions in the 2021 IBC Section 602.4.2.2.2 for noncombustible protection in Type IV-B construction, up to 20% of the ceiling or 40% of the walls are permitted to be exposed wood, based on the floor area in any dwelling unit or fire area. Even though limited amounts of exposed wood surfaces are permitted, each building element must still meet the FRR required for Type IV-B without noncombustible protection. This means the design professional must size the section in question to account for wood charring. The charring rate of wood is non-linear, but the char depth, achar, can be easily quantified using procedures from Chapter 16 of the American Wood Council’s (AWC) 2018 National Design Specification® (NDS®) for Wood Construction, which is referenced in the IBC. Further details about the Chapter 16 provisions of the NDS can be found in AWC’s Technical Report 10 (TR-10): Calculating the Fire Resistance of Wood

Figure 2. Effective char depth, aeff, from 2018 NDS Chapter 16.

STRUCTURE magazine

Figure 1. Brock Commons-Student Housing, Vancouver, BC. Currently the tallest mass timber building in North America at 18 stories.

Members and Assemblies, which is referenced in the NDS Commentary to Chapter 16.

For structural calculations, the effective char depth, aeff, is estimated to be 20% deeper than achar. This accounts for reduced strength and stiffness in the elevated temperature zone in the wood behind the char layer that has not charred yet but has been heated due to the fire. Tables 16.2.1A and 16.2.1B from the NDS (Figure 2) provide aeff values for use in fire resistance calculations of exposed wood members and exposed cross-laminated timber slabs, respectively, for fire exposure durations of up to 2 hours, based on a nominal char rate of 1.5 inches/hr.

While TR10 compiles extensive test data and calculations for wood members and assemblies tested in accordance with ASTM E119 Standard Test Methods for Fire Tests of Building Construction and Materials as required by the IBC. Additional non-standard fire tests were conducted in support of tall mass timber code changes.

Photos from full scale, two-story compartment tests conducted at the U.S. Alcohol, Tobacco, Firearms, and Explosives (ATF) Research Laboratory in Beltsville, Maryland, are shown in Figure 3. The photos reveal a char layer and uncharred wood, which was insulated from the fire by the char layer.

The IBC addresses equivalent fire risk by regulating features that affect fire performance; taller buildings or combustible construction need more stringent fire protection measures. Unlike noncombustible

Design Example

Figure 3. Left, Oblique of 5-ply CLT panel from exposed, ATF Test #3; Right, Crosssection showing char removed and remaining laminations.

construction, which has no limitations on height or number of stories (Type I-A), the maximum height and number of stories for the tallest Type IV-A building are limited to 270 feet and eighteen (18) stories. For an 18-story building constructed out of mass timber, a three-hour fire-resistance rating is required for the primary structural frame. As an example, in a Type IV-A Business occupancy building constructed of mass timber, the primary structural frame members will need three (3) layers of 5⁄8 inch, Type X drywall for 120 minutes of fire resistance, plus an additional 60 minutes of fire resistance from the increased section of the mass timber itself. These requirements are especially crucial to the structural engineer or other design professionals because, depending on the structural support system, it may be possible to reduce the FRR of the secondary structural members to two (2) hours.

Both of the taller types of mass timber buildings (IV-A and IV-B) mandate the use of a minimum one-inch-thick noncombustible floor topping. In the new 2021 IBC, Table 601 Fire-Resistance Rating Requirements for Building Elements (Figure 4 ), the various requirements for both the primary structural frame and the floor construction and associated secondary members is evident. The new Tall Mass Timber construction performance requirements more closely resemble the Type I-A and I-B FRR than the traditional Type IV-HT classification, which must only meet the minimum dimensions associated with the construction type.

This example demonstrates calculations for the required section dimensions for a 2-hour structural fire resistance time when subjected to an ASTM E119 fire exposure test. ASTM E119 uses a standard time and temperature curve that is repeatable in the laboratory and is the referenced standard fire exposure for determining fire resistance ratings in the IBC. A CLT bearing wall with an unbraced height of L = 120 inches loaded in compression in the strong-axis is analyzed.

The design loads are: Live load = wlive = 14,000 plf Dead load (including estimated self-weight) = wdead = 6,150 plf Walls above are supported on a CLT floor slab and aligned with a CLT wall below. Sealing of wall joints with fire-rated caulk restricts hot gases from venting through half-lap joints at the edges of CLT panel sections (another new requirement for mass timber construction).

Compression Design Wall loads: Pdead = 6,150 lb/ft of width (dead load) Plive = 14,000 lb/ft of width (live load) Ptotal = Pdead + Plive = 20,200 lb/ft of width (total load) From ANSI/APA PRG 320-18, select a 7-ply CLT panel made from 13⁄8-inch x 3½-inch lumber boards (CLT thickness of 95⁄8 inches). For CLT grade E1, tabulated properties are taken from ANSI/APA PRG 320-18 Tables A1 and A2.

Fc,0 = 1,800 psi Reference compression stress (PRG 320 Table A1) FbSeff,0 = 18,375 ft-lb/ft of width Reference bending moment (PRG 320 Table A2)

Fire Design Compliance with the upcoming 2021 IBC provisions necessitates providing the required FRR for all mass timber elements. In the 2021 IBC, Type IV-C construction will allow fully exposed mass timber elements in buildings up to 85 feet in height, while requiring a 2-hour FRR for the structural frame. The NDS gives provisions for the calculation of fire resistance for exposed wood members, including cross-laminated timber (CLT), in Chapter 16. NDS Table 16.2.1B (Figure 2) provides effective char depths for CLT manufactured in accordance with ANSI/APA PRG 320: Standard for Performance-Rated Cross-Laminated Timber. NDS Chapter 16 is referenced in the IBC as an acceptable method of determining fire resistance and contains many important provisions for doing so. The NDS provisions for fire resistance calculations are only conducted using allowable stress design (ASD), since ASD is the primary method covered in ASTM E119.

Figure 4. Updated building element fire resistance table per 2021 IBC. JUNE 2020 BONUS CONTENT

EIeff,0 = 1,089 x 106 psi/ft of width Reference bending stiffness (PRG 320 Table A2) GAeff,0 = 1.4 x 106 lb/ft of width Reference shear stiffness, lb/ft of width (PRG 320 Table A2) L = 120 in.

Wall length

Calculate Effective Wall Compression Capacity Area parallel to grain for a 7-ply panel is based on 4 plies @ 13/8-inchthick, per foot of panel width (NDS 10.3.1).

Aparallel = 4 * 1.375 * 12 = 66 in2/ft of width Pc = Fc0 ⋅Aparallel = 118,800 lb/ft of width Effective Wall Compression Capacity (NDS 10.3.1)

Calculate Apparent Wall Bending Stiffness Using NDS Eqn. 10.4-1, the apparent bending stiffness can be calculated. Assume pinned-pinned end fixity.

Ks = 11.8 Shear deformation adjustment factor (NDS Table 10.4.1.1) EIe f f 0

EIapp = 1+

K s .EIeff 0

= 665.106 psi/ft of width

GAeff 0 .L EIapp is adjusted per NDS Appendix D and Appendix H to determine EIapp-min (NDS C10.4.1) EIapp-min = 0.5184 EIapp = 345 x 106 psi/ft of width 2

Calculate Adjusted Allowable Wall Capacity Assume all NDS adjustment factors equal 1.0 (CD = Ct = CM = 1.0).

EIapp_min PcE = π2⋅ = 236⋅103 lb/ft of width (NDC3.7.1.5) L2 Pc*= Pc ⋅ CD ⋅ CM ⋅ Ct = 118,800 lb/ft of width (NDS C3.7.1.5) αc = PcE / Pc* = 2 (NDS C3.7.1.5) c = 0.9 (NDS C3.7.1.5) 2 (1+αc) (1+αc) (α ) Cp = − − c = 0.9 (NDS C3.7.1.5) 2.c 2.c c Pc´ = Pc* × CP 109,400 lb/ft of width Pc´ > Ptotal so capacity is sufficient

√(

)( )

Fire Design Mass loss due to charring will be neglected, so loading is unchanged.

Calculating char depth per NDS Table 16.2.1B gives an aeff = 3.8 in.

For this example, char penetrates the first three laminations, including the first two strong axis laminations. The contribution of the uncharred crossing layer to buckling resistance is minimal, so it will be neglected. The post-char wall can be designed as an eccentrically loaded 3-ply CLT column.

3-ply panel properties for CLT grade E1 panel are in ANSI/APA PRG 320-18 Annex A. Fc,0 = 1,800 psi Reference compression stress (PRG 320 Table A1) FbSeff,0 = 4,525 ft-lb/ft of width Reference bending moment (PRG 320 Table A2) EIeff,0 = 115 x 106 psi/ft of width Reference bending stiffness (PRG 320 Table A2) GAeff,0 = 0.46 x 106 lb/ft of width Reference shear stiffness, lb/ft of width (PRG 320 Table A2) Area parallel to grain is equal to breadth (b) x depth (d) of the remaining two strong axis plies.

Aparallel = 2 * 12 * 1.375 = 33 in2/ft of width (NDS 10.3.1) STRUCTURE magazine

Pc = Fc0 ⋅ Aparallel = 59,400 lb/ft of width (NDS 10.3.1)

Calculate Apparent Wall Bending Stiffness

Using NDS Eqn. 10.4-1, the apparent bending stiffness can be calculated. Assume pinned-pinned end fixity.

EIe f f 0 = 95.4 .106 psi/ft of width EIapp = K s .EIe ff 0 1+ GAe ff 0 .L2 The method of adjustment of EIapp per NDS Appendix D and Appendix H to determine EIapp-min is unchanged.

EIapp-min = 0.5184 EIapp = 49.5 × 106 psi/ft of width Using the general form of the Euler buckling equation, with the “f ” subscript denoting fire design. (NDS C3.7.1.5): (π2) EIapp_min PcE_ f = 2.03 = 68.8 .103 lb/ft of width (NDS C3.7.1.5) L2 Pc_f * = 2.58*Pc = 153,300 lb/ft of width (NDS C3.7.1.5) αc = PcE_f /Pc_f * = 0.4 (NDS C3.7.1.5) c = 0.9 (NDS C3.7.1.5) Cp =

√(

)( ) 2

(1+αc) (1+αc) (α ) − − c = 0.419 (NDS C3.7.1.5) 2.c 2.c c

Pf´ = Pc_f * × CP = 64,200 lb/ft of width Pf´ > Ptotal so capacity is sufficient Initially, the wall is assumed to be loaded concentrically; however, as one side of the wall chars,the load becomes eccentric. The eccentricity in this example is half the difference in depth between a 7-ply CLT member and a 3-ply member. e = (9.625 – 4.125) / 2 = 2.75

Calculate Resisting Moment Assume all applicable NDS adjustment factors = 1.0 CL = 1.0 M´f = 2.85 . FbSeff0 . CL = 12,900 ft-lb/ft of width = 154,755 in-lb/ ft of width (NDS 16.2.2) Using the general form of NDS Eqn. 15.4-3 for wood columns: (Ptotal .e). 1+0.234 Ptotal Ptotal 2 PcE_f Interaction := = 0.64 + Pf́ P total (Mf́ ). 1− PcE_f The interaction equation is less than 1.0, so the design is sufficient.

( )

(

( )) ( ( ))

Conclusion In summary, the changes to the IBC will permit mid- to high-rise buildings to be constructed for all occupancy types, provided that the stringent fire-resistant construction details and active and passive fire protection are installed following the 2021 IBC. The addition of the new construction Types IV-A, IV-B, and IV-C will provide designers with the flexibility to use new materials to engineer buildings that are safe, efficient, and sustainable.

For additional information on Tall Mass Timber and the Tall Wood Building Ad Hoc Committee, as well as additional information on the rigorous fire testing performed at the ATF, please refer to www.awc.org/tallmasstimber.■ Lori Koch is the Manager of Educational Outreach for the American Wood Council.

Matthew Hunter is the Northeast Regional Manager for the American Wood Council.

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