STRUCTURE magazine - September 2020

STRUCTURE SEPTEMBER 2020

NCSEA | CASE | SEI

CONCRETE

INSIDE: Temporary Wind Bracing

30

Prefab Rising 22 The District Office 26 Post-Tensioned Concrete Encapsulation 34

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Contents SEPTEM BER 2020

22 PREFAB RISING

By James Lee, P.E., S.E., and Matthew Michnewich, P.E., S.E.

The design-build team for Stanford University’s Escondido Village Graduate Residences selected a prefabricated precast concrete solution to address site logistical challenges and building performance requirements. Leveraging the efficiencies inherent with prefabrication and precast concrete resulted in a resilient structural design.

26 THE DISTRICT OFFICE

By Alexandra Stroud, P.E.

The demand for mass timber continues to grow. The District Office, a 90,000 square foot, six-story building, has proven to be a success story of how thoughtful system planning, along with constructability considerations, can result in a cost-competitive, beautiful building.

Columns and Departments 7

Editorial Professional Association Membership is More Valuable than Ever

34

By Glenn R. Bell, P.E., S.E., C.Eng

9

Emerging Technology Building Code Compliance

By Matthew Olender, P.E., S.I.

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By Mahmut Ekenel, Ph.D., P.E., et. al

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Structural Design Structural Thermal Breaks Structural Rehabilitation Adaptive Reuse of the Apex Hosiery Company Building – Part 4

By D. Matthew Stuart, P.E., S.E., P.Eng

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Structural Systems Tall Timber Buildings

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

30

Structural Performance Earthquake Effects on Temporary Wind Bracing

By James M. Williams, P.E., C.E., S.E., AIA

September 2020 Bonus Content

Education Issues We are All Students Now

By Ben Rosenberg, P.E.

42

By Dritan Topuzi, Ph.D., P.Eng

16

Structural Repair Post-Tensioned Concrete Encapsulation

Structural Forum Tackling Conflict Head-On

By Michael Yost, Esq., and Aaron Mann, Esq.

In Every Issue 4 36 39 40 41

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

On the Cover

3-story tilt-up panel braced to the exterior using temporary

helical piers. See full Structural Performance article on page 30.

Additional Content Available Only in the Digital Magazine – STRUCTUREmag.org

Structural Testing A Look at Discrepancies in Concrete Strength Testing By Alicia Hearns InSights Design of Cross-Laminated Timber Structures for Lateral Loads By Jason Cattelino and Jamie Garcia, P.E. 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.

SEPTEMBER 2020

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A Powerful Software Suite for Detailed Analysis & Design of Reinforced Concrete Structures

EDITORIAL Professional Association Membership is More Valuable than Ever

By Glenn R. Bell, P.E., S.E., C.Eng, FIStructE, F.SEI, F.ASCE

E

ven before the tumultuous events of 2020, things were changing rapidly in our profession. The technological revolution, globalization, major societal challenges requiring structural engineering leadership, and the advancement of a new generation of engineers eager to make a difference in the world have been shifting the landscape for some time. The events of the last six months, however – COVID and our Social Awakening – have imposed on us dramatic change that we never imagined. No doubt that, when the day comes that we can once again congregate and travel more, some things will revert toward what we once thought was normal, but some things will be permanently changed. I believe these changes will make memberships in professional associations more valuable than ever.

Rapidly-Changing Technology and Practice We expect continued transformational development in technology in areas like performance-based design, minimization of embodied carbon in constructed works, and resilience. We also expect professional and business practices to respond to the demand for creativity, value, societal needs, and human-centric design. Gone will be the days when crunching numbers that satisfy code equations are a differentiator in a competitive, dynamic environment. Those who embrace change and keep up will flourish; those who do not will struggle for relevance.

Our professional societies are primary resources for keeping abreast of change and gaining access to resources such as conferences, webinars, live chats, collaboration sites, journals and other publications, codes and standards, and continuing education. As we continue to pivot toward digital delivery, you can access the latest content in an environment customized to your needs.

Connecting with Thought-Leaders Professional committee work, publication, and lecturing are great ways to work with the thought leaders in our profession – building one’s expertise and reputation. Want to become a true and recognized expert in wind loads, reliability-based design, or tsunami effects? Join an ASCE-7 subcommittee and learn from the best.

Digital delivery also increases your direct access to thought leaders.

As SEI President, I receive a constant flow of inquiries from members looking for advice, mentoring, or to find information. Other industry leaders and I work hard to answer every communication.

And, when I was CEO of Simpson Gumpertz & Heger, I was part of a network of company executives who helped each other in issues of company leadership. We built this network through our professional society connections. You can build networks like this too.

joint Vision for the Future of Structural Engineering. Together we are working on several initiatives to advance the profession such as leadership training, advancing structural engineering licensure, and promoting diversity, equity, and inclusion. SEI has a collaborative relationship with IStructE, with whom we have joint efforts in advancing performance-based design and embodied carbon reduction. SEI is also engaging with the other ASCE Institutes in the development of joint standards, and the Institutes will hold their first Virtual Technical Conference in September.

This increasing collaboration amongst organizations means that membership in one can be the gateway to the broader knowledge and network of others, and provide the broader vision that is the antidote for overspecialization.

The Power of a Larger Voice I am inspired by the younger generations’ desire to drive change, not only in our profession but in societal issues that impact our profession. We seek to influence the public’s knowledge and view of our profession. We want to redefine the domain of structural engineering so that we can continue to attract and retain the best and brightest toward a resilient future for our profession. In recent months, our awakening to issues of systemic racism, inequity, and need for diversity has reached new levels of commitment for action. The voice of our professional societies is the collective voice of our members, and we can use the power of that collective voice to achieve our goals.

Cultivating your Professional Community We all need professional communities, and these communities will be increasingly essential to each of us in the future. Digital technology continues to shrink the world, reducing the need for physical proximity, giving us access to the latest and best resources, and allowing us to broaden our horizons. For many of us, those professional communities are through our places of employment, but nurturing your connections through your professional associations can be so much more powerful and durable.

My primary professional community is ASCE/SEI. Through its vast communication channels, the use of which I can tailor to my own needs, I have gained global access to people, other organizations, knowledge, and a platform for sharing ideas that, even after 45 years, is proving of increasing value and importance. How can you better cultivate your professional community towards a more impactful and resilient career?■ Connect: SEI Virtual Events https://bit.ly/3k9GkYK ASCE Collaborate https://bit.ly/3gIJssk

Broadening our Horizons We recognize our need to overcome silos of specialization by collaborating with like-minded organizations in our profession and with other related organizations in and outside our industry. Increasingly, SEI has engaged with CASE and NCSEA, and in 2019 adopted our

STRUCTURE magazine

Glenn R. Bell is SEI President (FY20), a Director of Confidential Reporting on Structural Safety – U.S., a Board member of The Charles Pankow Foundation, and Visiting Scholar at the University of Bath (U.K.).

S E P T E M B E R 2 02 0

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emerging TECHNOLOGY Building Code Compliance 3-D Printed Concrete Walls

By Mahmut Ekenel, Ph.D., P.E., FACI, Melissa Sanchez, S.E., LEED AP, Ali Kazemian, Ph.D., and Berok Khoshnevis, Ph.D.

A

dvances in construction technology create opportunities using new and

innovative methods for the building construction industry, which are fast, efficient, reliable, and cost-effective. One of these methods is three-dimensional (3-D) print-

Figure 1. Layer-based construction using Contour Crafting.

ing construction technology (also known as additive manufacturing) for the job site construction of 3-D concrete structures. Although 3-D printing technology is not new and has been utilized for small-scale part production since the 1980s, it has found its way most recently into the construction industry and is pushing the boundaries of how engineers view and approach building construction.

Construction with 3-D printing technology offers many advantages, including reduction of costs through its single-step construction process by decreasing labor and eliminating the need for concrete forms, reducing construction times, providing affordable dwellings for people in urgent need of shelter, and greener construction by utilizing materials more efficiently and creating less waste. It also provides improved and rapid project planning, reduction of accidents as a result of a reduction in labor, new design possibilities, freedom to create complex designs, and many more advantages. In a research study (Lofgren, 2005), it was shown that formwork typically accounts for 40 to 60 percent of the total construction costs; therefore, formwork elimination by 3-D printing may lead to considerable financial savings. Because of these advantages and the high demand for affordable housing in the United States and other countries, many major city jurisdictions are considering the ability of 3-D printing construction technology to help solve the homelessness crisis.

In the U.S., a system of model building codes is adopted by local jurisdictions who have the authority to enforce construction regulations.

The International Building Code (IBC) Section 104.11 allows for the integration of new construction products, systems, and technologies not explicitly described in the code itself, permitting manufacturers to demonstrate that these products are compliant with the intent of the code. This evaluation is typically accomplished through product testing following established and peer-reviewed acceptance criteria documents. Acceptance criteria documents outline specific product sampling, testing, and quality requirements that must be fulfilled to verify a product is code-compliant. The results of evaluation and testing are summarized in a research report made available to code officials.

As of 2020, there are no provisions for 3-D printing construction technology within the IBC. Therefore, an acceptance criteria document for 3-D concrete walls (AC509) has been developed under IBC Section 104.11 by the ICC Evaluation Service. It states that supporting data, where necessary to assist in the approval of materials

or assemblies not expressly provided for in the code, can consist of accurate research reports from approved sources to demonstrate building code compliance verification. The new AC509 addresses wall construction using 3-D printing, evaluating structural strength, fire safety, and material durability. AC509 also outlines specific product sampling and quality requirements that must be fulfilled to obtain code compliance verification.

Background One of the innovations in 3-D printing technology is the use of Contour Crafting building printing technology, which was developed at the University of Southern California (USC) in 1997. Contour Crafting building printing is a new layered 3-D printing technology that enables the automated construction of entire structures. Contour Crafting uses computer control to exploit the superior surface-forming building printing technology capability of troweling to create smooth and accurate planar and freeform surfaces. Some building elements built using Contour Crafting technology are shown in Figure 1.

There has been a significant number of projects that have used Counter Crafting technology since 1997. However, most of these projects used either mortar or clay as the 3-D printing material.

While clay seems a viable option for non-structural purposes, its mechanical properties may not be as suitable as concrete for building construction. The main advantage of using concrete is that it is the most widely used construction material in the world.

Other than building construction, infrastructure development and planetary construction are two other areas that have been explored regarding the advancement of 3-D printed construction. Different infrastructure elements may be automatically built with variations of the Contour Crafting technology. For example, in a research project (Khoshnevis et al., 2016), sulfur concrete was used as the material for planetary construction purposes. In this process, sulfur S E P T E M B E R 2020

9

is the computer program used to control the 3-D concrete flow and nozzle speed, position, and orientation.

• Proprietary Concrete Core: Selfconsolidating proprietary concrete poured in place between the 3-D printed outer face shells. The proprietary concrete core is either the same as, or a different mixture than, the 3-D concrete mixture.

was pre-melted and then mixed with other ingredients (elemental sulfur, sulfur modifier, coarse aggregate, and fine aggregate) at 150°C, and was kept in the reservoir for one hour until the elemental sulfur was modified. The enhanced articulated robot used in this project, as well as a successfully extruded multi-layer sample (made with sulfur concrete), are shown in Figure 2.

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Acceptance Criteria Test Methods

Material Tests

To qualify under Section 104.11 of IBC, The following definitions were developed Acceptance Criteria, AC509 requires the to be used within AC509: following material tests: • 3-D Automated Construction • Concrete compressive strength testTechnology: Construction-scale 3-D ing in accordance with ASTM C39 printing technology, also known as or ASTM C109 with minimum additive manufacturing or layeraverage 28-day compressive strength by-layer automated construction of 2500 psi (17.2 MPa). technology.

• Concrete slump testing in accor• 3-D Concrete: A proprietary cementidance with ASTM C143 or ASTM tious material that consists of cement, C1611, to be reported for quality fibers, and supplementary cementicontrol purposes. tious materials (if applicable), fine • Freezing and thawing durability and/or coarse aggregate, and admixtesting of both outer face shells and tures (if applicable). core concrete in accordance with • 3-D Concrete Walls: Walls that are Procedure A of ASTM C666 for a Figure 2. Enhanced articulated robot (top) and the printed constructed with the use of 3-D minimum of 300 cycles and minisulfur concrete sample (bottom). printing construction technology by mum durability factor of 80. printing 3-D concrete in layers to create two outer face shells, • Shrinkage and volume change testing in accordance with and then placing a proprietary concrete core between the shells ASTM C157 with the acceptance conditions of 0.064 percent to form a solid wall. If applicable, structural steel reinforcing or 0.050 percent depending on aggregate size and fiber used. can be placed within the proprietary concrete core.

• Because the presence of discontinuous fibers in a 3-D con• 3-D Printer and Software: Computer-controlled equipment, crete mixture may affect the performance of 3-D constructed which includes a frame and rail system used to support and concrete walls, AC509 requires that fibers used in 3-D concrete control the position and orientation of a proprietary nozzle, mix design comply with a consensus acceptance criterion for used to construct 3-D concrete walls. The 3-D printer software quality control.

• Test for minimum and maximum extrusion time intervals. The performance of a 3-D concrete wall may be affected by the time interval U.S Patent No. 10,570,618 between the extrusion of concrete from the printer nozzle for each layer.

Therefore, AC509 developed a procedure using Section 5.2 (Method A) of ASTM E518 to understand the effect Eventually There is of minimum and maximum time a BETTER idea... intervals between extrusion of 3-D concrete layers on the bond between the extrusion layers.

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Structural Performance Tests

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AC509 requires full-scale structural tests for each 3-D concrete wall configuration, with or without structural steel reinforcement, and for each combination of 3-D concrete mixture design to be tested to justify the design provisions. AC509 requires the following details to be considered while preparing test plan: each 3-D concrete mixture design, reinforcing

details (rebar size and spacing), variation in geometry of the 3-D concrete outer shells (such as thickness and width of the extrusion layers) and the proprietary concrete core, and minimum and maximum time intervals between extrusion layers. Wall axial compression tests, wall flexure tests, and wall static in-plane shear tests are required for the justification of structural design provisions.

AC509 also requires that a design criteria report is submitted by a registered or licensed design professional, which must include complete analysis and interpretation of the qualification test results demonstrating that 3-D concrete walls can be designed in accordance with the applicable sections of the IBC. Test data must qualify the design characteristic strengths used in the analysis and design. Any deviation from design must be established in the analysis for inclusion in the final research report.

Average maximum strength is to be reduced by a factor of 3 to determine characteristic strengths and must be used to verify or modify the design equations in applicable codes. Also, per AC509, where loading conditions result in combined transverse and axial loads, the sum of the ratios of actual loads over design loads must not exceed one.

Other Compliance Requirements The following requirements and limitations are included in AC509.

Once more research is available, these limitations may be revised.

1) The 3-D concrete walls are limited to use in one story singleunit residential dwellings.

2) The 3-D concrete walls are limited to non-fire resistance-rated construction unless qualified by testing in accordance with ASTM E119.

3) The 3-D concrete walls used as the lateral-force resisting system are limited to Seismic Design Categories (SDC) A and B only.

4) The foundation and roof, and their anchorage to the 3-D concrete walls, must comply with applicable sections of the IBC.

5) The structural calculations must address the design and detailing of openings and loads on headers.

6) Exterior envelope requirements of the applicable codes have not been evaluated and are outside the scope of this article.

Conclusions The IBC is the predominant building code in the U.S. IBC Section 104.11 allows for alternative materials, design, and methods provided that such alternatives are evaluated to meet the intent of code requirements. Acceptance criteria document AC509 provides the required evaluation criteria and data for quantifying the performance characteristics of 3-D concrete walls constructed using 3-D printing construction technology to satisfy the intent of the building code. The resulting final research (evaluation) report issued per AC509 is intended to demonstrate code compliance and is primarily used by code officials and structural engineers.■ The online version of this article contains references.

Please visit www.STRUCTUREmag.org.

Mahmut Ekenel is a Senior Staff Engineer with ICC-Evaluation Service in Brea, CA. Melissa Sanchez is a Staff Engineer with ICC-Evaluation Service in Brea, CA. Ali Kazemian is a Senior R&D Engineer with Contour Crafting Corporation in Los Angeles, CA. Berok Khoshnevis is the CEO and Founder of Contour Crafting Corporation in Los Angeles, CA.

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S E P T E M B E R 2020

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structural DESIGN Structural Thermal Breaks Preventing Heat Loss Through Concrete Parapets Conventional concrete parapets can form a thermal bridge that dissipates heat energy outside like large cooling fins, aided by cold winds whipping over the rooftop. One solution is to cast a structural thermal break between the parapet and the heated interior structure supporting it.

By Dritan Topuzi, Ph.D., P.Eng, PMP, LEED AP

P

arapets protect the edge of roof assemblies from uplift forces created by winds blowing against and over the building.

However, solving one problem creates another: a thermal bridge the length of the building perimeter that penetrates the insulated building envelope, conducting heat energy from the building's interior support structure and dissipating it into the environment. In addition to energy waste, uninsulated parapets chill the heated interior space adjacent to the building envelope, creating uncomfortable living spaces because of the non-uniform temperature distribution.

The traditional approach to mitigating thermal bridging at parapets is to wrap its exposed surfaces with insulation, making it a part of the heated building mass.

A newer method addresses thermal bridging more effectively: thermally separating or "breaking" the parapet from the heated interior structure by casting structural thermal break modules between the two masses. These thermal breaks are optimized for thermal performance without affecting structural integrity.

Figure 1. Examples of thermal bridging; a) Material thermal bridge; b) Geometric thermal bridge.

does the insulation, a material thermal bridge is created. Examples include balconies, as conductive elements that penetrate the vertical building envelope, and parapets, as elements that penetrate the roof insulation.

Geometric thermal bridging occurs when a heat-emitting surface is larger than the heat-absorbing surface. Building corners are a typical example, but geometric thermal bridging can also affect wall/roof and wall/floor junctions, junctions between windows, walls, and doors, and, of course, parapets. Most of the time, material and geometric thermal bridging occur in concert.

Heat transmittance through thermal bridges can be idealized as linear or point: • Linear transmittance through thermal bridges occurs with disturbances in the continuity of the thermal envelope along a certain length. Typical examples include concrete balcony connections with the floor slab penetrating the wall, outer wall edges, floor supports, and window-to-wall junctions.

• Point transmittance through thermal bridges occurs with disturbances in one spot that penetrate an insulating layer. Examples include steel balconies, canopies, and roof Thermal Bridging – a Primer extensions.

“Thermal bridging in building construction occurs when thermally While heat energy loss is an obvious consequence of thermal bridgconductive materials penetrate the insulation, creating areas of ing, developers are faced with a newer and potentially more significant reduced resistance to heat transfer. These thermal bridges are most outcome: condensation and mold growth. often caused by structural elements that transfer loads from the buildSince older buildings often leaked air profusely, interior humiding envelope back to the building ity levels equalized with low exterior superstructure” (Payette Architects humidity levels, typically between 2015). The results of thermal bridg18% to 25% during winter months. ing include higher heat transfer Forced hot air blowing at or near cold resulting in colder internal surface penetrations, such as balconies and temperatures, higher energy use for parapets, further ensured that the heating and cooling, noncompliance local interior humidity never rose to with building regulations, building the dew point, thus preventing conoccupant discomfort, and potential densation and mold growth. for condensation and mold.

Because modern buildings are airThe two most common types of tight, humidity levels can reach 30% thermal bridging are material and to 40% during winter months. While geometric (Figure 1). When an elecomfortable for occupants, high intement made from a material of high rior humidity near chilled building thermal conductivity penetrates an Figure 2. Condensation on the chilled interior side of uninsulated penetrations can reach the dew point, insulating layer, and the protrusion penetrations supports the growth of mold, which can become airborne years forming condensation that supports conducts heat at a higher rate than before spreading to visible wall and ceiling surfaces. mold growth on adjacent surfaces such 12 STRUCTURE magazine

Figure 3. Left: Continuous parapet connections without thermal break. Right: Parapet connection was thermally broken with a structural thermal break. Courtesy of Schock Isokorb Design Guide [5752].pdf.

as sheetrock, insulation, and any cellulose-based material, particularly in stagnant air cavities (Figure 2). Mold can become airborne years before it migrates to visible surfaces, exposing the developer to potential liability and remediation costs.

A Closer Look at Parapets Parapets play an important architectural and structural role in buildings. However, being at a sensitive location – at the connection of the roof and wall envelope – they are subject to various potential performance issues. In his paper, Parapets: Where Roofs Meet Walls, Joseph W. Lstiburek, Ph.D., mused, “Historically, so many problems have occurred with parapets that we have a name for it: parapetitus.

Thermal bridging is everywhere.”

As a high-mass structural element that penetrates the building at its windiest point, parapets are especially susceptible to thermal bridging and its consequences.

Wrapping Exterior Parapet Surfaces To prevent thermal bridging, architects historically wrapped the perimeter of the wall with an insulation barrier and then wrapped the parapet as well, making it part of the heated building mass (Figure 3). In addition to being costly and minimally effective, this method presented long-term risks.

When a parapet is wrapped, it functions similarly to an insulated flat roof and has many of the same problems. Both roofs and parapets are prone to damage and need repair and maintenance, particularly if railings or other elements breach the insulating layer.

Waterproofing of wrapped parapets rarely provides effective, long-term moisture protection, and leakage can incur significant recurring maintenance costs.

Designers at Payette researched this subject in their article (Payette 2015): “One question we found intriguing was whether it was better to insulate around the parapet – covering all structural interfaces – or underneath it by finding a way to design a structural connection that effectively attaches the parapet after

installation of the insulation. Since there are many variables in the detailing of a parapet, we started with a sensitivity analysis of parapet height normalized to one construction type. Because the degree of impact of the assembly depends on how much of the building we are looking at in conjunction with the parapet, we also normalized on an extracted detail that includes 24 inches (61 cm) in height of inside wall surface and 48 inches (122 cm) in length of inside roof surface.”

The Payette findings were compelling but not surprising. Naturally, the higher the wrapped parapet, the greater the potential for energy loss. Payette also discovered that insulating beneath the parapet – thermally breaking it from the roof – negated the height factor and provided the most effective insulation, even though there was no thermal break specifically designed for parapets at the time. “We tested a commercially available structural thermal break designed for concrete slabs (balconies) and installed this in a vertical orientation. The improvement decreased the heat flow through the assembly by 27%.”

New Parapet Paradigm Today, manufactured structural thermal breaks designed explicitly for parapets are commercially available. They thermally isolate the parapet from the heated interior, while preserving the structural integrity of the connection.

A typical manufactured structural thermal break for parapets (Figure 4) is a fabricated assembly consisting of enhanced insulation and stainless steel reinforcing bars, creating a module capable of transferring the loads from the parapet to the concrete roof slab that supports it, while minimizing thermal conductivity between the two concrete masses (Figure 5). The insulating block, made of graphite-enhanced expanded polystyrene, is roughly 98% less conductive than concrete, and the stainless steel reinforcing bars are approximately 70% less conductive than carbon steel reinforcing bars, effectively reducing heat loss at the penetration by up to 90%.

The vertical reinforcing bars resist tension/ compression and bending moments on the parapet. The interaction between applied bending moments and tension/compression is taken into consideration. The crossed bars provide the necessary shear strength, in either direction, through the tensile strength of these bars.

Figure 4. Structural thermal break for parapets.

Figure 5. Rendering of a structural thermal break cast between a concrete roof slab and concrete parapet.

Other Concrete-to-Concrete Construction In addition to parapets, thermal bridging can occur at balconies when constructed conventionally as structural extensions of interior floor slabs. In cold weather, the balcony conducts heat energy from the heated interior floor slab, dissipating it into the environment while chilling the interior floor opposite the balcony.

Structural thermal breaks designed for balconies are comprised of insulating blocks the same approximate width of the building wall. Stainless steel reinforcing bars are cast into the concrete slab on the interior side SEPTEMBER 2020

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and the concrete balcony on the exterior side, providing the necessary structural strength and stiffness at the connection, working as a truss system.

Similar structural thermal breaks are available for the insulation of slab edges.

Costs vs. Benefits While the manufactured structural thermal breaks come with a higher initial investment, there are benefits in the short- and longterm, which compensate such costs.

Maintenance-free, once installed, the module reportedly simplifies the formwork process, reducing construction costs versus the conventional parapet insulation wrapping method.

Compared to wrappings, it eliminates costs from future restoration related to water penetration (Figure 6).

Compared to non-insulated connections, the addition of structural thermal break elements can achieve a significant reduction in thermal conductivity at the connection area for standard load-bearing scenarios, typically 50 to 90%, translating into overall reductions in energy use for the building.

The reduction in energy to heat the building also allows corresponding reductions in heating system size/capacity, resulting in additional savings on capital equipment and ongoing operation and maintenance of mechanical systems.

Thermal isolation of balconies and slab edges also improves comfort for occupants and provides value for developers by increasing the warmth and usability of interior floor space.

In addition to these tangible values, structural thermal breaks can prove instrumental in complying with tightening building codes that mandate higher energy efficiency and continuous insulation.■ The online version of this article contains references.

Please visit www.STRUCTUREmag.org.

Figure 6. Heat loss and cost comparisons.

Dritan Topuzi is the Product Manager for Schöck North America. (dritan.topuzi@schock-na.com)

Parapet Thermal Breaks Help Attain LEED Gold A major developer in British Columbia, Canada, has been an early adopter of thermal breaks for balconies and shading eyebrows in its Vancouver senior residence projects. Following these successful experiences, the developer employed a similar solution for a subsequent Vancouver project that included parapets in addition to balconies and eyebrows. In part, it helped meet the new LEED Gold certification requirements for new construction.

For the 23-story tower main building and a two-story auxiliary building, 199 residential units and the tower building incorporated 1,820 concreteto-concrete thermal breaks that insulate 5,970 linear feet (1,820m) of balconies from the interior.

Thermally broken parapets form the perimeter of levels two and three in both the main tower and the auxiliary building, for a total 1,100 feet (335.3m).

This was the first time the company employed structural thermal breaks for parapets. The construction managers reported, “We’re always looking to improve on the comfort, efficiency, and sustainability of our facilities. So, this was the perfect setting for us to work on incorporating the thermal breaks for parapets. If you’re going to be an owner/developer and your facility has parapets, this thermal break concept has significant Figure 7. Structural thermal breaks positioned against wooden parapet forms are tied into rooftop reinforcement before casting in concrete. benefit.” (Figure 7 )

14 STRUCTURE magazine

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structural REHABILITATION Adaptive Reuse of the Apex Hosiery Company Building Part 4: Demolition Special Inspection and Post Demolition Assessment

By D. Matthew Stuart, P.E., S.E., P.Eng, F.ASCE, F.SEI, A.NAFE, SECB

T

his four-part series (Part 1, STRUCTURE, November 2019, Part 2, January 2020, Part 3, April 2020) discusses how the collapse of a building during a

demolition operation in Philadelphia in 2013, which resulted in several fatalities, led to the enactment of a City Ordinance to prevent similar future calamities.

As a result of the Ordinance, the author became involved with the structural investigation, review of the Site Safety Demolition Plan, and Demolition Special Inspections associated with the adaptive reuse of the Apex Hosiery Company Building located in Philadelphia.

Demolition Special Inspection The Special Inspector maintained a full-time presence on-site during the demolition operation and submitted the documentation of observations required by the Special Inspection clause of the City Demolition Ordinance. The demolition operations generally proceeded as anticipated; however, during the demolition, two incidents occurred that required special attention.

The first incident involved the demolition contractor cutting a large, unapproved opening in the SMI slab, as shown in Figure 18, to enable equipment to be driven between the floor levels scheduled for demolition. Subsequent assessment and engineering evaluation indicated that the opening had compromised the structural integrity of several top and bottom reinforcing hoops within the bay containing the opening. As a result, the load-carrying capacity of the immediately adjacent surrounding bays was reduced. The solution submitted by Pennoni to correct the temporary structural deficiency involved the installation of shoring posts in the surrounding compromised bays.

The second incident involved a localized progressive collapse of an SMI framed bay down to the foundation, in an area of the building that was scheduled for complete demolition.

The collapse began on the 4th floor as a result of overloading the affected floor framing with an excessively large debris Figure 19. A progressive collapse to the ground floor pile. The collapse then progressed through began on the 4th floor as a result of overloading the the floors below until floor framing with an excessively large debris pile.

16 STRUCTURE magazine

Figure 18. Vehicle access opening cut in the slab during demolition required the installation of shoring posts in the surrounding compromised bays.

coming to rest at the ground floor slab, as shown in Figure 19. No injuries occurred as a result of the localized structural failure, and the demolition contractor was instructed to avoid similar overloading going forward.

Post Demolition Assessment The results of the post-demolition assessment, conducted at the conclusion of the demolition, indicated that, in general, the remaining three-story structure had not been adversely impacted by the demolition of the upper levels and adjacent original facility.

In addition, the few crack monitors that had not been inadvertently damaged by the demolition contractor indicated that the minor movement of the structure documented by the devices had resulted due to thermal forces and not structural duress during the demolition.

However, along a majority of the eastern line of demolition at the northern SMI slab, the demolition contractor did not take adequate precautions to ensure a uniform vertical face of demolition across the depth of the slab, as shown in Figure 20. As a result, a number of top and bottom reinforcing hoops that were intended to remain were destroyed or damaged, as shown in Figure 21. As a result, and similar to that described at the large equipment access opening during the demolition Special Inspection, the load-carrying capacity of the remaining, immediately adjacent bays to the west of the line of demolition was reduced.

This reduction in load-carrying capacity occurred because once a hoop is no longer embedded in a significant portion of concrete anywhere along its circumference, or severed, it can no longer function to resist the radial flexural bending forces in the slab.

Therefore, in an SMI slab when a top hoop is no longer able to function properly in response to negative cantilever flexural action induced by the supported adjacent simple span slabs, the undamaged diagonal and damaged orthogonal (i.e., north-south) positive moment span hoops must resist forces associated with a longer span

Figure 20. The demolition contractor did not take adequate precautions to ensure a uniform vertical demolition face along most of the eastern line of demolition.

Figure 21. Several top and bottom reinforcing hoops that were intended to remain were destroyed or damaged, resulting in reduced load-carrying capacity of the remaining bays adjacent to the demolition line.

because of the reduced effective cantilever distance of the Unit C section of the slab.

While the existing affected bottom hoops of the controlling diagonal span did appear to have enough reserve capacity to allow the slab to span the longer distance under its own dead load adequately, whether or not the slab had enough adequate reserve capacity to provide the required new adaptive reuse minimum live load of 40 psf and dead load of 15 psf for partitions, could not be confirmed. In addition, the load-carrying deficiency at the north-south orthogonal positive moment slab span was further adversely impacted by the loss of embedment or damage at some of the bottom hoops along the erratic line of demolition.

A proposed solution to the damaged conditions along the eastern line of demolition was submitted to Pennoni for review by another engineer engaged by the developer. The proposed solution involved attaching reinforcing bar dowels via mechanical couplers to all of the damaged reinforcing projecting from the line of demolition and encasing the same supplemental reinforcing in a newly formed and poured slab edge along the affected eastern edge of the northern section of the remaining building.

A review of the proposed repairs indicated that, for the framed SMI slab to be restored to its original capacity, the slab in the first bay west of the line of demolition would need to be jacked up to relieve all self-weight stresses from the remaining undamaged orthogonal and diagonal bottom and cantilever top hoops. Relieving the existing dead load from the remaining undamaged hoops would allow for the proper redistribution of the existing dead load to both the undamaged and repaired hoops, as well as ensure proper sharing of the proposed future loads by both the remaining and repaired hoops.

A plan of the required location of the temporary jack shoring was provided by Pennoni, which indicated that jacking of the slab to relieve the dead load must be sequential, starting at the ground level and then proceeding up to the next level above. The slab was to remain jacked up and shored until the repairs had been completed, and the new concrete slab edge had achieved full strength. Once the shoring was in place, the proposed couplers and bar extensions could then be added to the projecting remains of the top and bottom hoops so the same hoops could function again as initially intended.

After the completion of the demolition operation that involved drilling and coring new mechanical and utility penetrations, it was recommended that the location of the hoops be identified to prevent additional damage to all of the embedded and concealed SMI hoops during the renovation work. This was accomplished using groundpenetrating radar (GPR) equipment, which allowed for the location of the hoops to be painted on the surface of the concrete slabs, as shown in Figure 22.

Conclusion As a result of the investigation associated with the adaptive reuse of the Apex Hosiery Company Building located in Philadelphia, a unique type of reinforced concrete flat slab construction, the SMI System, was encountered. The author had previously dealt with this type of construction at another building in Philadelphia. The findings of the investigation assisted with the successful completion of both the partial demolition of the existing structure and the success of the adaptive reuse project.â– Figure 22. GPR equipment was used to locate the embedded SMI hoops, and their locations were painted on top of the concrete slab surfaces to prevent further damage.

D. Matthew Stuart is the Senior Structural Engineer at Pennoni Associates Inc. in Philadelphia, PA. (mstuart@pennoni.com)

S E P T E M B E R 2 02 0

17

structural SYSTEMS Tall Timber Buildings

Where Structural Engineering Meets Fire Engineering

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

T

Table of required noncombustible protection on mass timber elements by construction type.

IV-A

IV-B

IV-C

IV-HT

Interior Surface of Building Elements

Always required; 2 ⁄3 of FRR, 80 minutes minimum

Required with exceptions; 2⁄3 of FRR, 80 minutes minimum

Not required*

Not required*

Exterior Side of Exterior Walls

40 minutes

40 minutes

40 minutes

⁄ ” FRT sheathing or ⁄ ” gypsum board or noncombustible material

Top of Floor (above Mass Timber)

1” minimum

1” minimum

Not required*

Not required*

Shafts

2 ⁄3 of FRR, 80 minutes minimum, inside and outside**

⁄3 of FRR, 80 minutes minimum, inside and outside

40 minutes minimum, inside and outside

Not required*

raditionally, the role of the structural engineer on building

projects has focused on structurerelated tasks – member sizing, connection detailing, general notes, and specifications for structural components. Design criteria such as fire-resistance ratings, acoustics, and

15

aesthetics have primarily been the

2

32 1 2

*Not required by construction type. Other code requirements may apply. **12-story/180-ft limit on the use of mass timber shaft walls.

5/8” Type X gypsum = 40 minutes.

architect’s domain. However, when it comes to mass timber, especially tall timber buildings, the structure often contributes to the building’s passive fire resistance while functioning as an exposed finish. This combination of structure, finish, and fire resistance makes the mass timber design process a necessarily collaborative effort between architect and engineer.

This article discusses some of the unique engineering aspects of tall mass timber structures, particularly as they pertain to fire and life safety. These requirements are based on three new construction types introduced in the 2021 International Building Code (IBC) – Type IV-A, IV-B, and IV-C – which allow up to 18 stories of mass timber.

Changes to the 2021 IBC are also discussed in the Code Updates article (Hunter and Koch), June 2020 online issue.

A baseline requirement for tall timber structures utilizing one of the three new construction types is that they are constructed of either mass timber or noncombustible materials (or a combination thereof ). Mass timber can encompass well known and widely used products such as glue-laminated timber (glulam) and nail-laminated timber (NLT), as well as newer panelized products such as cross-laminated timber (CLT). The definition of mass timber adopted for the 2021 IBC is: Structural elements of Type IV construction primarily of solid, built-up, panelized or engineered wood products that meet minimum cross-section dimensions of Type IV construction.

In practice, mass timber, as defined in the IBC, is an umbrella term that includes heavy timber elements. Materials must meet minimum size

requirements that give them intrinsic fire-resistance to qualify as heavy timber under Type IV-HT. Mass timber systems utilized in Types IV-A, B, and C also have a minimum required fire-resistance rating (FRR).

Exposed vs. Protected Timber When mass timber elements are used in tall timber structures, the construction type definitions contained in IBC Sections 602.4.1, 602.4.2, and 602.4.3 provide guidelines for whether the timber may be exposed on the building’s interior or needs to be covered with noncombustible protection. General allowances for exposed timber are noted as follows and further explained in the Table.

• Type IV-A: No exposed timber is permitted • Type IV-B: Limited exposed timber is permitted as follows: o Ceilings (including integral exposed beams) up to 20% of floor area in a dwelling unit or fire area*, or o Walls (including integral exposed columns) up to 40% of floor area in a dwelling unit or fire area*, or o A combination of each using sum of ratios (actual exposed/ allowable exposed) not to exceed 1.0 • Type IV-C: All exposed timber is permitted* *Exceptions: no exposed timber is allowed at shafts, within concealed spaces, or on the exterior side of exterior walls. Concealed spaces and shafts are discussed in greater depth later.

Contribution of Noncombustible Protection to FRR

Figure 1. Example floor assembly and primary frame FRR design approach.

18 STRUCTURE magazine

When noncombustible protection is required over timber elements, it must provide at least two-thirds of the FRR. Two prescriptive options are presented in IBC 2021 Section 722.7.1 – 25 minutes for each layer of ½-inch Type X gypsum board, and 40 minutes for each layer of 5⁄8-inch Type X gypsum board.

Contribution of Mass Timber to FRR

Wood connections, including connectors, fasteners, and portions of wood members included in the connection design, shall be protected from As noted, mass timber elements requiring fire exposure for the required fire-resistance time. noncombustible protection must achieve a Protection shall be provided by wood, fire-rated minimum of two-thirds of their FRR from gypsum board, other approved materials, or a the noncombustible materials. The remaining combination thereof. one third must come from the inherent fire As noted in NDS Section 16.3, options for resistance of the mass timber element (Figure 1). protecting connections include: In applications where the timber is exposed, • Concealed connections protected by the full FRR required in IBC Table 601 must charring of wood be achieved through the inherent fire resistance • Fire-rated gypsum board Figure 2. Example fire-protected beam to column of the mass timber element.

• Other materials or methods approved by connection. Courtesy of Alexander Schreyer, In either of these circumstances, IBC Section University of Massachusetts, Amherst. the Authority Having Jurisdiction 703.2 provides options for demonstrating Technical Report 10 (TR10), Calculating FRRs through testing in accordance with ASTM E 119, Standard the Fire Resistance of Wood Members and Assemblies (American Wood Test Methods for Fire Tests of Building Construction Materials, (or UL Council, 2018), provides a discussion on connection protection 263, Standard for Fire Tests of Building Construction and Materials) options, including unique design considerations associated with wood or one of six alternatives listed in IBC Section 703.3. as a covering material.

Item 3 of IBC Section 703.3, which permits the use of calculations There are two common approaches to protecting connections for in accordance with Section 722, is frequently used to demonstrate the the required FRR. One method is to entirely conceal the connection FRR of mass timber. Section 722.1 states: The calculated fire resistance of within the structural timber members (Figure 2). Alternatively, conexposed wood members and wood decking shall be permitted in accordance nections may be protected by applied wood or gypsum coverings. with Chapter 16 of ANSI/AWC National Design Specification® for Wood Aesthetics and required member and covering thicknesses are key Construction (NDS®). Chapter 16 of the NDS can be used to calculate considerations when determining the most appropriate solution. up to a 2-hour fire-resistance rating for a variety of exposed wood memSeveral glulam beam-to-column connection fire tests have been bers, including solid sawn, glulam, structural composite lumber, and completed, and additional testing is anticipated as more connections CLT, and is discussed in more depth in the Hunter and Koch article. and project specific requirements are developed. The WoodWorks Successful ASTM E 119 fire tests have been completed on numerous web-based inventory of completed mass timber fire tests is updated mass timber elements and assemblies, achieving fire-resistance ratings of 3 frequently as new tests are completed. hours or more. Additional tests by manufacturers and others are ongoing.

A qualified professional can also be retained to perform an engiTo help building designers compare options, WoodWorks has com- neering analysis on a proposed connection detail to evaluate its piled a web-based inventory of completed mass timber fire tests. The performance under fire conditions. The 2021 IBC includes language Inventory of Fire Resistance-Tested Mass Timber Assemblies & Penetrations specific to the temperature profile of connections in tall timber struc(WoodWorks, 2019) is updated as new tests become available. tures to support such analyses.

For additional information on both the calculation-based method and ASTM E 119 testing method of demonstrating the FRR of mass timber Concealed Spaces elements, see Fire Design of Mass Timber Members (WoodWorks, 2019).

Most mass timber building designers aim to expose as much of the timber as practical and as the code allows; however, there are often areas Connections that require a dropped ceiling (e.g., to conceal mechanical, electrical, In addition to FRR requirements for structural wood members such and plumbing services). Concealed spaces have unique requirements as beams, columns, and panels, connections between members (e.g., to address the potential of fire spread in non-visible areas of a buildbeam-to-column connections) must have sufficient protection to ing. Section 718 of the IBC includes prescriptive requirements for provide the same fire-resistance rating. Section 16.3 of the NDS states: protection and/or compartmentalization of concealed spaces using

Figure 3. Example tall timber assembly options with and without dropped ceilings.

SEPTEMBER 2020

19

draft stopping, fire blocking, sprinklers, and Noncombustible Protection of Mass other means. For information on these requireTimber Shaft Walls ments, see the WoodWorks Q&A, Are sprinklers required in concealed spaces such as floor and roof When used as shaft walls in Type IV-B or IV-C cavities in multi-family wood-frame buildings? buildings (or IV-A buildings that do not exceed (www.woodworks.org) 12 stories or 180 feet), mass timber must be While concealed spaces have not historically covered on both faces with noncombustible been permitted in Type IV structures (per IBC materials, as indicated in the definitions of the Section 602.4), this changes with the 2021 IBC. individual construction types. Mass timber shafts Type IV-HT construction and all three tall mass in Type IV-A and IV-B construction must be timber sub-types will permit concealed spaces protected with noncombustible materials that when protective measures are taken. Specifically, contribute two-thirds of the wall’s required FRR. for tall timber buildings, no combustible mateMass timber shafts in Type IV-C buildings must rials that form the concealed space (i.e., the be protected with noncombustible materials that underside of a mass timber floor or roof panel) provide a minimum of 40 minutes of protection may be left exposed within that space.

(1 layer of 5⁄8-inch Type X gypsum).

Concealed spaces are permitted in construction Figure 4. Example of a tall mass timber structure.

Types IV-A and IV-B when any mass timber Other Considerations Courtesy of Generate Architecture and surface within the concealed space is protected Technologies + MIT – John Klein. with noncombustible materials that contribute In addition to the FRR and contribution of two-thirds of the required FRR. For floors, this typically means 80 noncombustible coverings, designers must consider factors such as minutes of protection (2 layers of 5⁄8-inch Type X gypsum) on the acoustics and structural loads when choosing mass timber elements underside of the mass timber panel. At roofs, the lower FRR results such as shaft wall assemblies for tall wood buildings. For information in a proportionally lower amount of noncombustible FRR contri- on the acoustical design of mass timber wall assemblies, see Acoustics bution. Concealed spaces are permitted in construction Type IV-C and Mass Timber: Room-to-Room Noise Control (McLain, WoodWorks, when any mass timber surface within the concealed space is protected 2018) and its accompanying Inventory of Acoustically-Tested Mass with noncombustible materials that provide 40 minutes of protection Timber Assemblies (WoodWorks, 2019).

(1 layer of 5⁄8-inch Type X gypsum). Figure 3, page 19, illustrates the Detailing items such as primary frame connections and assembly allowances and common assembly options. intersections also requires consideration of these factors, and it is often beneficial to discuss proposed details with the Authority Having Jurisdiction early in design. For shaft walls, some projects have utilized Shaft Enclosures mass timber walls that are multiple stories per lift, resulting in adjaProvisions addressing materials permitted in shaft wall construction cent mass timber floor panels being supported by the shaft walls via a can be found in both the shaft enclosures section (713.3) and the ledger. Other projects have used a platform-frame approach, with shaft fire barriers section (707.2) of the code. These sections state that fire walls lifted in one-story increments and adjacent floor panels bearing barriers can be constructed of any material permitted by the building’s on the walls. Speed of construction, structural loading, cumulative type of construction. As noted, Types IV-A, IV-B, and IV-C permit shrinkage and crushing potential, and fire barrier continuity are all the use of mass timber or noncombustible elements. This allowance factors that will help inform the detail selection. also applies to shaft enclosures, with one exception; Section 602.4 of Although not necessarily required to achieve the floor assembly’s FRR, the 2021 IBC notes the following regarding shaft walls in buildings floor assemblies in Types IV-A and IV-B construction require a minitaller than 12 stories or 180 feet: mum 1-inch-thick noncombustible covering. Examples include a poured In buildings of Type IV-A, B, and C, construction with an occupied concrete or gypsum layer, or sheets of noncombustible board products. floor located more than 75 feet above the lowest level of fire department access, up to and including 12 stories or 180 feet above grade plane, Conclusion mass timber interior exit and elevator hoistway enclosures shall be protected in accordance with Section 602.4.1.2. In buildings greater With significant interest in tall timber construction across the than 12 stories or 180 feet above grade plane, interior exit and elevator U.S., it is becoming more important for structural engineers to hoistway enclosures shall be constructed of noncombustible materials. understand the key fire and life safety issues and how they interface Following these code provisions, mass timber may be used as a shaft with structural design. While construction types and fire-resistance enclosure material in tall wood buildings (Figure 4) – except Type ratings are not commonly the domain of structural engineers, this IV-A buildings that exceed 12 stories or 180 feet. is often the case with mass timber projects, and an understanding As stated in IBC Section 713.4, shaft enclosures are required to of these topics can help engineers provide more effective solutions.■ have an FRR of not less than 2 hours when connecting four or more stories. An FRR of not less than 1 hour is required for shaft The online version of this article contains references. enclosures connecting less than four stories. Shaft enclosures are Please visit www.STRUCTUREmag.org. also required to have an FRR not less than the floor assembly penetrated. In all three tall mass timber construction types, the required floor FRR is 2 hours. Shaft enclosures that penetrate a Richard McLain is Senior Technical Director – Tall Wood, at WoodWorks – Wood Products Council. As WoodWorks’ in-house tall wood expert, floor assembly in any of the new types require a 2-hour rating, he provides analysis and technical guidance on architectural, fire and even if the building is less than four stories tall. Shaft walls that life safety, and structural design topics related to tall mass timber projects. are also bearing walls in Type IV-A construction require a 3-hour (ricky.mclain@woodworks.org) FRR per IBC Table 601.

20 STRUCTURE magazine

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W W W. ATC E P OX Y.C O M

Prefab Rising

By James Lee, P.E., S.E., and Matthew Michnewich, P.E., S.E.

Designing Stanford University’s Escondido Village Graduate Residences

G

iven the high cost of rent in the surrounding community and a campus housing capacity of 55 percent of its graduate student population, Stanford University needed to increase its graduate student housing. The goal was to not only provide better housing options for its current students but also continue to attract the best students from around the world. The Escondido Village Graduate Residences (EVGR) at Stanford University adds more than 1,300 units and 2,400 beds, increasing Stanford’s oncampus graduate housing capacity to 75 percent of its graduate student population.

The project primarily consists of four new residential buildings, a collection of six-, eight-, and ten-story towers, totaling more than 1,800,000 square feet. Also included is a new, three-story pavilion building, providing support amenities including a restaurant, brewery, and multipurpose room.

Projects of this magnitude and complexity often present the team with various design and construction challenges, and EVGR was no exception. The existing Escondido Village is the largest graduate community at Stanford and, because it is an active campus, the

Exterior longitudinal moment frame.

22 STRUCTURE magazine

design-build team for the new EVGR was charged with delivering the project as quickly and with minimal disruption and negative impact on the surrounding neighbors as possible. These parameters drove the team to select a prefabricated, precast concrete system, which allows for the majority of building construction to be fabricated off-site within a controlled shop environment, thereby reducing noise, congestion, and field labor. John A. Martin & Associates, Inc. (JAMA) worked in close partnership with Clark Pacific, the precast concrete subcontractor, to develop and design the all-precast concrete structural system for EVGR. Since the project is in a high seismic region, earthquake resistance was an essential objective of the structural design. Instead of a code-prescriptive design approach, Stanford University required a performance-based approach to design the lateral system. The residential buildings are designed to achieve Class 2 seismic performance following Stanford’s Seismic Engineering Guidelines, using the criteria outlined in the ASCE 41-13 standard, Seismic Evaluation and Retrofit of Existing Buildings, to evaluate the components. Class 2 buildings provide enhanced seismic performance and are classified as facilities critical to the operation of the University. In terms of structural performance, the Class 2 designation implies limited damage (corresponding with Damage Control performance in ASCE 41) at the Design Response Spectra (DRS), or BSE-1N equivalent seismic hazard, and enhanced safety (corresponding with Life Safety performance in ASCE 41) at the Maximum Considered Earthquake Spectra (MCE R), or

BSE-2N equivalent seismic hazard.

BSE-1N and BSE-2N have return periods of 475 years and 2475 years, respectively.

Gravity Framing System The building height at the six-, eight-, and ten-story towers is 72 feet, 94 feet, and 116 feet, respectively. The typical floor-to-floor height is 11 feet, with a 17-foot story height on the ground floor. The gravity framing system consists of double tee precast floor panels with 4-inch-thick flanges and 12-inch-wide by 15-inch-deep Diaphragm shear connection detail. ribbed sections. The floor panels are approximately 10.75 feet wide and span up to 35 feet parallel to the longitudinal direction of each the project schedule, and offset the cost of the additional concrete building and span between precast moment frames that extend the and excavation for the larger footings. full width of the building. The three-bay transverse moment frames supporting the floor panels are comprised of 18-inch-wide by 36- or Lateral Framing System 42-inch-deep columns and 24-inch-wide by 25- to 44-inch-deep moment frame beams.

To achieve a Class 2 seismic performance, each residential building is divided into four or five seismically separated structures. Therefore, the “four� residential buildings are actually seventeen individual Foundation System buildings. This improved the seismic performance by avoiding The buildings are supported on continuous spread footings under each horizontal and vertical structural irregularities such as torsional moment frame, sized to support the service loads using site-specific irregularities and reentrant corners. It also simplified the lateral soil properties. The footings were designed using a capacity-based design by standardizing the design of multiple buildings that have approach. That is, the continuous footings were designed to resist the similar configurations. In turn, standardizing the building designs maximum force that the moment frame columns can transmit to the helped optimize the design of the precast panels, minimizing the footings. The thicknesses of the continuous footings were governed by number of custom forms required for the project. the resulting shear forces induced by the expected moment capacities The precast panels that comprise the special moment frame elements of the moment frame columns in combination with the shear forces consist of a continuous beam and discrete column panels in the due to the gravity loads. Rebar congestion was reduced by utilizing transverse direction of the buildings. Longitudinal moment frames 80 ksi reinforcement and by eliminating stirrups in the foundations. consist of two or three columns joined by a top and bottom beam.

This was accomplished by thickening the footings so that the concrete Column continuity is achieved by utilizing grouted couplers and section alone could resist the shear forces. The tradeoff for reducing corrugated rebar sleeves. The transverse beams used low amounts the amount of rebar in the foundations was the requirement of larger of post-tensioning reinforcement to prevent cracking during shipfootings. However, eliminating the labor-intensive work associated ping and were formed with pre-coordinated MEP penetrations. The with configuring and installing numerous stirrups helped accelerate seismic performance evaluation was performed simultaneously with the frame design, driving optimization of the rebar design to maximize ductility through symmetric top and bottom flexural reinforcement, and by sizing beams to strategically align stiffness and strength at locations of high stress.

The use of precast concrete offered advantages to achieving Class 2 seismic performance.

First, Clark Pacific’s preference for repetitive member design meant that every frame could be a moment frame, resulting in a highly redundant system without incurring additional costs. Second, individual panel Diaphragm connection to transverse moment frame detail.

SEPTEMBER 2020

23

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Moment frame panel production.

Transverse moment frame erection.

designs were replicated across different stories and building segments.

This approach led to panel designs optimized for a worst-case location and provided residual strength in other areas. Finally, the architectural expression of the buildings was integrated directly onto the concrete panels in the precast forms, eliminating the need to detail additional façade elements.

A significant source of innovation on this project was the design of an un-topped precast diaphragm. Precast floor-plank to floorplank-edge connections have historically performed poorly in seismic events, creating a dependence on cast-in-place topping slabs for diaphragm action, which in turn becomes the limiting factor for construction speed. For this project, the design team reimagined the precast shear transfer connection as an extension of monolithic concrete behavior instead of using the typical approach of embedded steel elements welded together across planks. The precast planks were connected to each other and to the surrounding frames using interlaced rebar hairpins that lapped each other and hooked around a lacer bar running parallel to the plank edges, drawing from concepts used in precast bridge design. These connections were arranged in a continuous pattern at the plank-to-beam interface and formed at discrete locations between floor planks. The joints were then filled with grout to create a connection that mimics cast-in-place concrete behavior.

Diaphragm chord elements for resisting flexural demands were created with a similar connection of lapping rebar hairpins that develop the strength of the chord bars within the planks. At a handful of particularly tricky locations with high transfer demands, partially stressed posttensioned strands were anchored within the floor planks along the chord length to provide a supplemental source of strength. The post-tensioning

provides additional elastic strength, and the residual unstressed capacity activates when the mild chord reinforcing yields and dissipates energy.

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Construction There are approximately 14,700 prefabricated precast panels that make up the EVGR residential buildings. By working closely with Clark Pacific throughout the design development and construction documentation phases of the project, the designers were able to optimize the designs of the precast panels and minimize the number of forms needed for fabrication. The sharing of BIM models between JAMA and Clark Pacific was essential to the project workflow, as the design team was able to convert Clark Pacific’s Tekla model to Revit smoothly. Sharing models during design ensured that the shop drawing submittals were consistent with the construction documents; in reviewing all the 14,700 panel shop drawings, there were only a handful of corrections noted. The efficient submittal review process enabled the panels to be fabricated and transported to the site in a timely manner. Ultimately, the four residential buildings were installed in only 11 months, realizing an estimated time savings of 30% to 40% when compared to conventional cast in place construction.

The prefabricated precast concrete solution developed by the designbuild team addressed the combination of the site’s logistical challenges and building performance requirements while reducing the construction schedule and minimizing disruption to the campus. Furthermore, leveraging the efficiencies inherent with prefabrication and precast concrete resulted in resilient structural design. EVGR is scheduled to welcome its first set of students this fall at the start of the new school year.■ James Lee is a Project Manager at John A. Martin & Associates, Inc. in Los Angeles, CA. (jlee@johnmartin.com) Matthew Michnewich was formerly a Senior Project Engineer at John A. Martin & Associates, Inc. and has since moved to IDS Group, Inc. in Irvine, CA. (matthew.michnewich@idsgi.com)

Project Team Owner: Stanford University Structural Engineer: John A. Martin & Associates, Inc. (JAMA) General Contractor: Vance Brown Builders Architect: Korth Sunseri Hagey Architects (KSHA) Precast Concrete Subcontractor: Clark Pacific

The District Office A Creative Mass Timber Office Project

By Alexandra Stroud, P.E.

Rendering of the District Office. Courtesy of Hacker Architects.

T

he District Office is a 90,000 square foot, six-story building in Portland, Oregon. System planning and attention to constructability resulted in the use of mass timber in this successful project.

The primary feature of the project is its exposed mass timber structure consisting of cross-laminated timber (CLT) floor panels with long-span glued laminated timber (glulam) beams supported on glulam columns at 10 feet on-center. Located on a half-block site with perimeter dimensions of approximately 100 feet by 200 feet, the rectangular plot lends to an efficient one-way span of the CLT floors.

Above grade, the mass timber gravity frame is tied to special reinforced concrete shear walls with a concrete diaphragm. At ground level, the post-tensioned concrete slab spans over the below-grade parking garage. Once completed, District Office will offer flexible retail, office, and restaurant space to the neighborhood.

Design Elements Optimization of the structure was among early efforts to make the mass timber building a reality. Pricing studies were performed to determine optimum floor assembly; the final selected assembly consists of a 3-inch concrete topping slab over 3-ply CLT panels. The CLT panels support gravity loads while the reinforced concrete topping slab provides diaphragm capacity and contributes to a one-hour fire rating.

The beam spans range from 30 to 40 feet and bear over the full area of the glulam columns such that each column has a single beam framing into it. The long spans and the tight column spacing create an open floor plan with a central colonnade over the length of the building.

The glulam members and the CLT floors are all Douglas Fir-Larch manufactured locally in Oregon with lumber sourced from California.

The design uses the minimum office live load criteria per ASCE 7-10, Minimum Design Loads for Buildings and Other Structures; however, vibrations become an important design consideration when longspan glulam beams and light-weight floors are combined. The floor assembly and glulam beams were analytically modeled to understand cumulative effects, and in-situ testing was performed to verify results.

KPFF, in collaboration with Woodworks and others, is currently 26 STRUCTURE magazine

finalizing a design guide funded by a USDA Forest Service Grant for vibrations in mass timber construction that will incorporate the results from the District Office analysis.

With the tight column spacing above grade, wide, shallow posttensioned transfer beams are used in the ground floor post-tensioned slab to allow for drive aisle clearance and optimized parking in the garage. Column locations in parking garages always require close coordination. To limit the number of transfer beams at the post-tensioned slabs, careful coordination of column locations in the parking garage below was necessary.

Architectural Consideration One of the primary architectural objectives was to fully expose the wood structure. This required connection designs that were aesthetically thoughtful as well as structurally effective. Accommodating fire design was also necessary. The glulam beams and columns meet a

Central colonnade at concrete core.

one-hour fire-resistance rating in accordance with the 2012 National perimeter beams could not be eliminated, upturned beams were used Design Specification® (NDS®) for Wood Construction. An effective to maintain a consistent rough opening. char layer of 1.8 inches was used at all exposed wood surfaces to calculate the Accommodating fire resistance.

Mechanical Systems Due to individual connection complexity, the repetition of custom connections Coordination of the mechanical systems became important for both detailing and occurred at an early phase to effectively constructability. The primary beamexpose the structure since the building to-column connection had only four required an organized, pre-determined variations that were used in over 350 path to serve each floor. Two solutions locations. This connection consists of a arose and drove the framing layout: a single pipe extending through the beam central colonnade open to all bays and and CLT panel to the column above, continuous chases above the beams.

Rendering of typical beam-to-column connection. which is possible as there is only one The central colonnade provided a path beam framing into each column. The pipe was designed to cantilever for the main mechanical runs to reach the north and south ends of off the top of the glulam column with any resulting moments resolved the building. The full ceiling height was maximized by embedding in the bearing plate that is attached with long screws into the column steel beams in the depth of the CLT panel. The steel was installed in end grain. The cantilevered pipe column eliminates any hinge behavior, the CLT panels, then covered by concrete topping above and fireproviding lateral stability as well as a load path from column-above protected from below. Keeping the steel beams in-plane with the to column-below through the pipe. As the pipe redirects the load CLT panels allowed the mechanical, electrical, and plumbing (MEP) path, the CLT panel and glulam beam are not loaded perpendicular systems to be located tight to the structure. The mechanical output to grain. This resulted in a simple, repeatable connection protected could successfully turn to run parallel between beams and distribute from fire exposure and hidden from view. to the east and west ends of the building.

The entrance of natural light was also important to the architectural To further integrate the mechanical system, gaps between the CLT design. The floor-to-floor height at the building perimeter was maxi- panels were provided, creating mechanical chases. Small lines, such mized so the windows could extend to the ceiling. Perimeter beams as sprinklers, can distribute to any location in the building without were eliminated where possible, using composite action of the CLT penetrating beams while minimizing visual impact. The floor frampanel and concrete floor to support the exterior wall assemblies. Where ing at the chases are composed of Douglas Fir-Larch plywood panels ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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CLT chases over glulam beams.

Long-span glued laminated timber beams.

spanned by the concrete topping slab. The chase load is re-distributed to adjacent panels via the concrete topping such that the CLT panels are designed for their own load plus tributary load from adjacent chases.

types needs to be limited. It is great to optimize design, but it is not always best for construction.

Future Flexibility One interesting design criterion was flexibility for future tenants. This was an early consideration since exposed timber construction cannot be modified as easily as a hidden structure while still maintaining the architectural intention.

Tenant flexibility was accommodated by incorporating designated spaces for future large floor openings. At each selected location, 7-ply CLT panels were used to span 20 feet with upsized beams and columns. The lateral analysis required evaluation of the diaphragm and shear wall behavior with these distinct portions of floor panels removed at each level.

Lessons Learned from Construction Creating a mock-up of the primary beam-to-column connection was critical. Seeing the build-up of this custom connection provided an understanding of how to improve its constructability, along with discussing with the rest of the design team how it transferred load.

This unique construction perspective provided an understanding of the required tolerances and fit-up that is challenging to get from computer renderings alone. More importantly, the mock-up allowed the team to minimize unforeseen issues before fabrication and construction.

District Office combined concrete and timber construction, each material having very different tolerance limits. This concept was understood during design, but additional tolerance constraints were discovered during construction. For example, the contractor, Andersen Construction, was thoughtful when it came to ordering the glulam beams that framed to the concrete core. They had the glulam beams manufactured long so they could cut each beam to length based on the as-built location of the core. The CLT chases allowed for flexibility within the panel layout. When the concrete core was out-of-plumb, but within concrete tolerance, that dimension could be absorbed within the chase. For future applications, consideration may be given to tightening tolerances specified on unique concrete elements where differential tolerances accumulate and become difficult to accommodate. Detailing needs to be carefully considered where wood frames into concrete regarding both plumbness and levelness.

One of the biggest lessons learned is to keep the wood connections simple. Just like any other construction material, limiting connection type and complexity is helpful for constructability. Reflecting on the detailing process helped with the realization that the number of screw 28 STRUCTURE magazine

Structural Investigations Tenant space for the project architect, Hacker Architects, was included in the scope of the core and shell design for District Office. They desired to feature an honest mass timber staircase, remaining true to the concept of a mass timber building. KPFF teamed with Hacker and Freres Lumber to produce the concept of an all mass plywood panel (MPP) stair. The stair features intricately cut stringer shapes of mass plywood panels that are then combined to form a true wood stair.

Calculations for the stair were unable to be performed due to a lack of product data specific to the unique stringer geometry. KPFF Structural and KPFF Structural Investigation Group (SIG) developed a testing plan conforming to Chapter 17 of the 2014 Oregon Structural Specialty Code for pre-construction load testing. Required loads were applied at various time intervals, as prescribed in the testing plan, and deflection and rebound measurements were taken. Following positive results of pre-construction load testing, a testing report was written and submitted as part of the stair package. This resulted in a code-approved path for the mass plywood panel stairs satisfying Hacker’s desire for a true wood stair.

Looking Forward The demand for mass timber continues to grow as a modern-day construction material and is increasingly being considered by developers and architects. The State of Oregon now recognizes specific building types to accommodate mass timber construction, showing positive progress toward more regular use. District Office has proven to be a success story of how thoughtful system planning, along with constructability considerations, can result in a costcompetitive, beautiful building.â– Alexandra Stroud is an Engineer at KPFF Consulting Engineers in the Portland structural office. (alexandra.stroud@kpff.com)

Project Team Owner: Beam Development & Urban Development + Partners Structural Engineer of Record: KPFF Consulting Engineers Architect of Record: Hacker Architects Contractor: Andersen Construction Mass Timber Supplier: DR Johnson

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structural PERFORMANCE Earthquake Effects on Temporary Wind Bracing

By James M. Williams, P.E., C.E., S.E., AIA, LEED AP

On

March 18th, 2020, a moderate earthquake of magnitude 5.7 hit Magna, Utah, at 7:09 am. In the downtown area, the strongest

shaking lasted 4-6 seconds; however, the shaking was strong enough to be felt for about 20 seconds. For people living in high-rise buildings in downtown Salt Lake City, 17 miles from Magna, the shaking seemed to last

3-story tilt-up panel braced to the exterior using temporary helical piers.

much longer. Fortunately, due to the Covid-19 pandemic and the early hour at which the earthquake occurred, most people were still at home and in bed. The earthquake was reportedly felt as far away as 66-miles from the epicenter.

Hundreds of buildings were damaged. The economic impact of the damage was estimated to be approximately $70M (and $629M in total economic losses). Buildings that were engineered and constructed using modern building codes did very well withstanding this moderate earthquake. Conditions that are often overlooked following earthquake reconnaissance missions include new buildings that are under construction, particularly tilt-up wall panels supported by temporary wind bracing. How did they fair? And what effect would the 2,016 aftershocks, some of which measured as much as magnitude 4.6, have in these scenarios?

After most moderate to major earthquakes, many owners require an engineer to evaluate their building before allowing tenants, employees, and workers to re-occupy the building. In our “sue-happy” society, owners want and need to take precautions against future litigation, even after a moderate earthquake. In the days following the earthquake on March 18th, hundreds of buildings were quickly evaluated by local engineers, so owners had documentation that their buildings were safe.

Owners and builders also required projects that were under construction to be evaluated by an engineer. This was especially true for structures that rely on temporary bracing, such as concrete tilt-up buildings.

Some of the temporary bracing systems observed performed better than others when subjected to the earthquake forces. Some temporary bracing systems experienced little to no damage, while others resulted in significant damage and even total collapse of some tilt-up wall panels. It should be recognized that tilt-up wall panels are temporarily braced during construction until the roof framing (and or floor framing) and diaphragms are constructed and connections made. The temporary bracing is typically designed per the Tilt-Up Concrete Association (TCA) Guideline for Temporary Wind Bracing of Tilt-Up Concrete Panels During Construction, or the provisions of the American Society Of Civil Engineers’ SEI/ASCE 37-02, Design Loads On Structures During Construction. The wind loading for both publications is based on ASCE 7-10 Minimum Design Loads For Buildings And Other Structures. Temporary bracing is typically designed as if the building were a Risk Category 1 structure defined as, “Buildings and other structures that represent a low risk to human life in the event of failure.” The justification is that the temporary bracing is strictly limited to structures that are under construction. During construction, the 30 STRUCTURE magazine

occupant load is much less than what the design occupant load will be once the building is completed and occupied.

Risk Category 1 structures in non-coastal or special wind areas have a design wind speed of 105 mph. ASCE 37-02 allows for a 0.8 reduction factor for projects whose duration of construction is 6 weeks to 1 year, resulting in an adjusted design wind speed reduced to 84 mph. There is a possibility of the construction site being subjected to higher wind speeds and higher pressures. If the actual wind speed exceeds the wind speed used for the design of the temporary bracing, the construction site should be evacuated by all personnel. For these types of structures, wind speeds should be carefully monitored.

Consideration should also be given to potential fall zones if temporary bracing does fail.

The publications used for the design of temporary bracing do not address earthquake forces. With the recent increase of seismic events in many areas, should temporary bracing be considered and designed to withstand seismic forces (or some percentage of the design earthquake forces)? According to bracing publications, an attempt to temporarily brace wall panels against earthquakes is a risk-based determination rather than being considered a life safety issue, and this decision is left to the entity assuming that risk. Unlike wind, earthquakes cannot be predicted and are far less frequent, so they have largely been ignored and unaddressed. Because they cannot be predicted and are not considered a common occurrence, there is, in some cases, a higher risk for loss of life and property during construction. OSHA’s Requirements for Precast Concrete 1926.704(a) states, “Precast concrete wall units, structural framing, and Tilt-Up wall panels shall be adequately supported to prevent overturning and to prevent collapse until permanent connections are completed.” No exceptions are listed for earthquakes (or for higher wind speeds).

At one particular building site (10 miles from the epicenter), a 3-story tilt-up building, with half of the wall panels erected and temporarily braced, was observed. The majority of the panels were braced to the exterior of the building using temporary helical piers to expedite construction. One corner of the building was braced to the interior floor slab due to its proximity to a drive lane. All of the tilt-up wall panels on this site remained standing after the earthquake, but there

was one localized bracing connection failure where the anchor pulled out from the panel. Another concern at this site was that the top of one of the helical piers appeared to have rotated. It was not clear if the earthquake caused this, or if the pier had been installed that way initially. Multiple wall panels moved several inches so that the spacing between panels was no longer uniform, and a couple of panels were no longer perfectly vertical or plumb. Wall panels had to be re-set, and the failed brace needed to be reattached. It should be noted that the 1-inch-diameter adjusting screws at each end of the braces were only partially extended, and the braces and connections were in good condition and quite robust.

At another building site (also 10 miles from the epicenter and 2 miles from the first site), there were 4 tilt-up buildings under construction. All of these buildings were large spec warehouse structures, and all the tilt-up wall panels were braced to the interior floor slabs.

Luckily, at the time of the earthquake, all site personnel were in the construction trailer for a safety meeting. The contractor described the earthquake as a wave passing through the site, leaving destruction in its wake. A majority of the temporary bracing for wall panels, which were perpendicular to the direction of the earthquake (on the north and south ends of the building), failed, resulting in the collapse of many of these wall panels. Some of the remaining wall panels on the north and south ends of the buildings would later collapse during one of the many aftershocks.

One of the buildings had all the structural steel and open web joists (including bridging) erected. There is a buckle-resistant braced (BRB) frame located in the center of the building due to the length of the building. The south half of the building had the metal decking/ diaphragm installed, and all the temporary wind bracing had been removed. The south half of the building had a completed lateral force-resisting system. The north half of the building was missing the diaphragm, and still had temporary wall bracing in place. Due to the east-west orientation of the BRB frames, they did not see any load. The tilt-up wall panels resisted all the earthquake forces. Even though the north half of the building did not have a diaphragm in place, the building did not experience any damage. The north wall remained braced by the temporary panel bracing. Girder beams, welded to cast-in-place embedment plates, and open web joist bridging also helped to brace the walls. The L4 x 4 x ¼ horizontal braces at 6 feet on-center along the wall, specified in the plans, had not yet been installed.

The next building did not have any metal decking or diaphragms installed.

Only a portion of the front and rear (east and west) bays had open web steel joists and girders installed, at the north end of the building. The south wall had collapsed, making it easier to access the building. The building was wide enough that it was safe to walk in the center bays, outside of the wall panel fall zone. It was noted that wall panels that did fall, on this and the remaining buildings, always fell out, away from the building, or away from the braced side of the panels. The

One of several failures of a footplate identified after the earthquake.

braces always prevented the wall panels from falling inward. All of the wall panels that fell were also thrown approximately 5 to 6 feet away from the building. This is important information for contractors to understand as they consider fall zones for future projects.

None of the wall braces buckled or showed any sign of damage except for the adjustable rod at the base of the brace and the footplate.

The ¾-inch-diameter adjusting screws at the bottom of the braces were extended approximately 12 inches, and many had bent. The connections of the braces to the wall panels remained intact, but the connections of the braces to the floor slab had several modes of failure and appeared to be the weak link that resulted in the collapse of some of the wall panels. There were very few places where the slab showed cracking and or spalling at the brace anchor. The majority of the failures had to do with the brace footplate. There were several

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Toppled wall panels thrown approximately 5 to 6 feet.

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locations where the anchor into the floor slab was intact, but the footplate of the brace had slipped off or away from the anchor. This was likely caused by the wave or rolling effect of the earthquake. There were many other locations where the anchor remained attached to the floor slab, but the foot plate itself sheared off, either at the slab anchor or at the bolted connection to the brace itself. All of the foot plate failures appeared to be a brittle shear failure.

In some cases, north and south wall panels fell immediately. Others remained standing, even though the temporary bracing was no longer attached to the slab. Some bracing remained attached. The panels that remained standing were either connected to a roof girder or joist bridging, which helped to continue to brace the walls. Other wall panels remained standing because some panel-to-panel connections had been welded, thereby connecting unbraced panels to adjacent panels that were still partially braced. The wall panels located on the north and south ends of the building, which remained standing and were not sufficiently braced, were a significant concern; most of these fell during a subsequent aftershock.

The front and rear walls along the east and west sides of the building all remained standing, even though some of the brace footplates

32 STRUCTURE magazine

had disengaged or failed. It should be noted that all the temporary bracing was removed and replaced. These walls were parallel to the direction of the earthquake and oriented so that the braces did not see much load. These wall panels did, however, move an inch or so, and many of the panels had to be re-set. Some spandrel beams, which were not fully welded or not fully braced by the roof, experienced some damage and were also replaced.

All the wall panels that did fall had to be repoured. The embedment connections between panels appeared to be severely damaged, but the damage occurred from the panels falling and not from the earthquake forces themselves. When the panels fell, the connection angles between the footing and the wall panels failed in tension. The legs of the connection angles were still welded to the footing and the wall panel but were simply pulled in half due to the large moment at the base of the panels once the braces had failed. There were some decorative bump-outs in some of the walls that remained standing, which also helped to brace adjacent panels. The damage to the remaining buildings was identical to this second building.

All the temporary bracing was replaced as mandated by the bracing engineer and supplier as a result of the earthquake. Wall panels and spandrel beams that fell were also repoured. All the connections required additional special inspection, as did the structural steel and open web joists. Footings and floor slabs fared well and did not require repairs. Each project experienced considerable delays and increased costs due to the earthquake. Each building had to be assessed by a third-party engineer and architect for insurance claims. Earthquake damage remediation plans and processes also had to be generated.

The weak link in the temporary wind bracing is the brace footplate when braces are in tension from being subjected to the sudden impact of an earthquake force. The same failure mechanism (footplate fracture) most likely occurs at higher wind speeds as well. Increasing the adjusting screw diameter would also reduce or omit the bending that occurred. The argument still exists as to whether temporary bracing should be designed for any seismic force at all. Perhaps temporary bracing should be designed to withstand the forces associated with a moderate earthquake of magnitude 5.5 to 6.0, which results in slight property damage, since they occur approximately 500 times a year and are more common than most people realize. As the codes evolve towards a performance-based design, perhaps temporary bracing should do the same in some areas of the country where there is the likelihood of an earthquake (or of higher wind speeds). The cost of repairs and delays are substantial and could easily justify the cost for more resilient bracing.

In this case, had the earthquake occurred later in the day, there is a high likelihood that there would have been a loss of life.■ James M. Williams is President of AE URBIA & J.M. Williams and Associates, Inc. He is a member of the IBC General Code Development Committee, is HUD’s Residential Resilience Seismic Task Group Leader, and has served on the AIA’s Codes and Standards Committee, as well as being a past president of the SEAU and an executive board member of the TCA.

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structural REPAIR

Post-Tensioned Concrete Encapsulation The South Florida Grease Cap Debate

By Matthew Olender, P.E., S.I.

H

igh-rise residential concrete construction in South Florida typically consists of unbonded post-tensioned concrete slab construction. A

key component to the durability of these slabs is the finishing of the stressing pockets for the post-tensioned tendons, which has proven to be problematic for contractors, especially in South Florida. Tendon finishing consists of cutting/greasing cable ends and capping and filling the pockets with grout.

This article presents both sides of an ongoing debate that is the focus of many lawsuits in condominiums throughout South Florida. The dispute involves improper finishing of post-tensioned tendons and whether or not the tendon encapsulation requires a repair.

Plaintiffs (i.e., building owners) argue for compliant construction. Thus, the owners argue that every post-tension stressing pocket on a building must be opened, cables properly trimmed, and new grease caps installed. Frequently, there are thousands of stressing pockets on a high-rise condominium resulting in lawsuits in the millions of dollars.

Defendants (i.e., design professionals, inspectors, contractors, subcontractors, etc.) argue that if it is not broken then do not fix it. The tendons are meeting their intended function of supporting the building loads and not showing signs of distress, so opening thousands of pockets is unnecessary and wasteful.

The author discusses the current codes and proposes engineering solutions for resolving these disputes.

Background Currently, in South Florida, many high-rise residential buildings are undergoing costly repairs to correct defectively installed post-tensioned systems. Many of these buildings are 5-10 years old and show no signs of distress. Are these repairs necessary, or are they economic waste?

This is the question that is currently being debated among engineers and attorneys, resulting in costly lawsuits. To answer the question, an understanding of how post-tensioned systems work is important.

Figure 2. Post-tensioned components, including grease caps, tendons, anchors, and wedges.

34 STRUCTURE magazine

Figure 1. Post-tension slab showing the layout of cables (blue).

Post-tensioned systems consist of a series of cables embedded within concrete slabs that, once stressed, support the loads imposed on the slabs (Figure 1). An unbonded system uses tendons placed within sheathing so that, after the concrete hardens, the tendons can move along the sheathing within the slab without bonding to the concrete.

Once the concrete hardens and reaches a specified strength, the tendons are stressed and locked into place using wedges (Figure 2). The tendons elongate during stressing, the excess length is trimmed off, and the tendon ends are greased and capped. This is where the problems occur. Often, the tendons are not trimmed short enough for the caps to fit properly. Contractors either do not install the caps covering the anchor tightly or force them into place, resulting in the cable ends puncturing the caps (Figure 3). In either case, the construction is defective and does not meet the encapsulation watertight requirements of ACI 318-14 and ACI 423.7-14. Watertight encapsulation of the post-tension tendon intends to protect the tendon from corrosion failure so that the tendon functions for the useful life of the structure.

Plaintiff Approach Experts engaged on behalf of condominium associations are tasked with identifying construction and design deficiencies as these properties change hands from developer to association-controlled. In South Florida, it has become standard operating procedure to open post-tension pockets during these investigations, whether the building shows symptoms of distress or not. During these investigations, the encapsulation of the post-tension tendons is frequently found deficient, with poorly installed or missing grease caps. After the grout, grease, and caps are removed, the tendon tails and anchorages are evaluated. The tendons and their anchorages are found to be undamaged and functioning as intended, despite the improper grease cap installation. Some plaintiff experts make the argument that the tendons will not last for their intended useful life and argue that all pockets on the building must be opened and repaired. The repairs that the plaintiff experts recommend typically consist of non-structural repairs of trimming cable ends, regreasing the cables/anchorages, recapping the tendons, and regrouting the pockets.

This is a very conservative approach to the issue and does not consider the significant useful life remaining in the post-tensioned system, even with improperly installed grease caps. The repairs are very disruptive to the residents and often take a year or more to complete.

vulnerable portions of the building, such as balconies or eyebrows, where Defendants in these cases are quick to point out regular exposure to the environment that construction is imperfect and that there occurs. The disadvantage of stressing are no damages that the Plaintiffs can identify. from the interior is that it is usually The experts argue that the post-tension system more time consuming and costly, at will last the intended useful life without repair. least initially.

Some experts take samples of the grout and send 2) Plan for access to improve installation. them for laboratory testing, which can then be There is better awareness in the industry used to calculate the remaining useful life of the of the importance of proper grease cap post-tensioning system. Other experts evaluate installation. Proper grease cap installathe tendons/anchorages after the grout/grease/ Figure 3. Punctured grease cap forced into place tion requires good access to the stressing caps have been removed, finding no corrosion over tendon tail that is too long. pocket. In the past, contractors would within the pockets. The defense points out that remove their access to the pockets after the Plaintiff’s repair approach is non-structural stressing was complete before cutting, and ignores the significant useful life remaingreasing, capping, and grouting the ing in the system. Thus, the Plaintiff’s repair pockets. The contractors frequently is unnecessary and wasteful. Frequently, the would finish the pockets by standing/ defense expert opinion is that no repairs are kneeling on the slab edge and reachnecessary. ing over to conduct work by feel. This practice led to many of the encapsulation problems that have been discussed Compromise? in this article. Increased awareness of encapsulation issues has significantly The author proposes the following approach, reduced the finishing-by-feel technique. which has currently been accepted by the Figure 4. Ghosting and corrosion of posttension (PT) pockets, which is an indicator of an By leaving the access in place for a few applicable building departments and imple- underlying issue with the post-tension tendons. days after stressing, contractors can mented in at least two condominium projects vastly improve the finishing of the in South Florida. stressing pockets. Also, inspectors can get better access to the 1) Visually evaluate the exterior of the building to identify signs pockets to verify that the installations are proper. of distress in the post-tensioning system, including cracks 3) Take advantage of new tools. Improvements in post-tension in the stucco, grease staining, corrosion, and displaced or technology have simplified the cutting/capping process of delaminated stucco (Figure 4). tendons. Tendon finishing has evolved from torch cutting 2) Remove the stucco and evaluate the grout if any of the items of tendons to using shears and plasma cutting, which has in step 1 are identified. resulted in improved accuracy/consistency of cutting to the 3) Remove the grout if it is cracked, broken, or has grease and proper length. Further, improvements in grease caps have corrosion staining on it. made installation easier. Caps now have indicators for the 4) Evaluate the encapsulation and condition of the tendon if the installer to make sure they have been installed correctly. grout is removed.

Examples include threaded caps screwed into place, tabs on 5) Recap, grease, and grout the pockets that are opened. Repair the push style caps indicating when a cap is installed correctly, any damaged/broken tendons. and deeper caps that can accommodate longer cables.

6) Do not open any pockets that show no signs of distress.

7) Recommend that the Association conduct maintenance in a Improvements are occurring on new construction projects, and, with timely fashion, specifically repairing any cracks in the stucco continued focus, costly litigations regarding this issue will become and surrounding concrete. a thing of the past.

The repairs to the post-tensioning system can be drastically reduced and be substantially less costly and disruptive to the occupants using Conclusions this methodology while reducing the risks associated with the improper grease cap installation.

Expensive building-wide repairs are being contemplated and implemented in many recently constructed condominiums in South Florida.

These repairs are often wasteful and unnecessary and can be sigBest Practices for New Construction nificantly minimized by implementing engineering evaluations and The best way to handle encapsulation issues is to avoid them alto- regular building maintenance.â– gether. With some simple improvements in design, construction, and technology, these costly litigations are hopefully becoming a thing of The online version of this article contains references. the past. Three recommendations are as follows: Please visit www.STRUCTUREmag.org.

1) Design the slabs to be stressed inside the building via pour strips, elevator shafts, stairwells, etc. on the interior of the building where the building envelope protects the stressing Matthew Olender is a Principal Structural Engineer at Thornton Tomasetti, ends. These are much less likely to be exposed to the environInc. in Fort Lauderdale, Florida, and an Adjunct Professor at the University ment. However, the vast majority of high-rise condominium post-tensioned floor slabs are stressed from the exterior slab of Miami. (molender@thorntontomasetti.com) edges. Many of these stressing pockets occur at highly

Defense Approach

SEPTEMBER 2020

35

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education ISSUES We are All Students Now Learning How to Teach Remotely

By Ben Rosenberg, P.E., LEED AP

I

taught Concrete Structures in the Architecture program at the Rhode Island School of Design (RISD) in the Spring 2020 semester. The course was envisioned by my predecessor and maintained by me as a technical and qualitative exploration of designing concrete, with a series of rigorous but practical design examples for different elements. The lectures included case studies of buildings, discussions of the theory behind concrete design, and sample problems solved on the chalkboards. At the end of the semester, there would be a concrete casting project where the students would split into groups, build their own formwork, mix concrete using materials provided to them, and cast simple home furnishings. It was to be genuine engagement with concrete on every structural level, providing an understanding of concrete structural design and materials that would translate into their studio courses and, eventually, into their work in the industry. And I was teaching a group of incredibly gifted and creative students.

The first third of the semester was terrific, and we were really hitting our rhythm as a class during that time. Then, over about a five-day period that fell between lectures, everything changed in the region. I remember sitting on my couch watching CNN and seeing the NBA postpone its season and thinking how wild that was. Five days later, my wife and I were both working from home, our kids were home with us, that week’s lecture was canceled, and all future lectures were going to be conducted remotely – brave new world.

So what did we do, the students and me?

We did what all good engineers and architects do: we adapted. Engineers adapt all the time.

Project budgets and schedules change, deadlines get pushed or moved up, unexpected existing conditions are uncovered, surprise emails pop up on Wednesday, and there goes your week. As do my colleagues, I pride myself on the ability to adapt to the dictates and demands of a project. The mission was clear: adapt to the new reality of remote teaching to best serve my students and ensure that I could fulfill their expectations, and mine, of what my concrete course should be.

Content-wise, I found remote teaching successful. We met every week on Zoom at 38 STRUCTURE magazine

the regularly scheduled period, with lectures recorded so that students in other time zones could view when able. I delivered lectures with the same content and even added more multimedia use of videos, with screen sharing and flipping between windows easy and intuitive (a video of precast plank manufacturing in Spain set to techno music was a particular favorite). I found it relatively easy to speak during the lectures and take the students through the material. I did not use a digital

“

There was minimal difference between comprehension and retention for remote lectures versus in-person lectures. pad that allowed me to solve problems by hand in real-time, instead relying on writing out problems ahead of time and scanning them in, but there is no reason I could not switch for future classes. In-class quizzes were necessarily eliminated, and the midterm exam was take-home, but that exam and the homework assignments were comparable to those planned before going remote. I was pleased that the content of the course was not significantly watered down, which was admittedly easier than for more hands-on classes.

But for me, the success and pleasure of teaching are standing in front of the students in the classroom. I have had mentors in college and professionally that taught me the importance of good public speaking and how to engage your audience to communicate effectively. Knowing the material and being able to deliver it and use your resources effectively is one part. But understanding the audience is just as critical. This is what I truly love about

teaching – taking the temperature of the room, reading the students’ body language, making subtle changes in delivery and pace in response to what I perceive in the audience. This is such a crucial element of my teaching style, and it is the thing that translates the least to remote teaching. Knowing that students were watching the lecture from a variety of places, I did not mandate students keep their video or audio on, and it was very odd to talk into the silence for three hours. This is different than in the professional world where clients and colleagues almost always have their video on. The human connection on those calls is much stronger and leads to necessary interaction, something that was missing in my remote lectures. I also know how easy it is to lose focus or get distracted watching remote videos, live or otherwise. For future remote semesters and with more planning, engaging the students more and determining ways to have more people with live video would greatly help the class feel more personal.

Based on a review of assignments and exams (and assuming students completed the work on their own), there was minimal difference between comprehension and retention for remote lectures versus in-person lectures. I did not fear that the course material would go unabsorbed. There is still something vital about being taught the material as opposed to learning it yourself, so remote teaching absolutely serves its desired purpose. There are simply lessons learned to increase the enjoyment and fulfillment of the course next time.

Concrete is not a uniform material. Almost every aspect of concrete can be changed by varying the materials and processes used in its creation. That variance determines the success of the finished product. So it was for those of us teaching courses in the spring of 2020. Vary the method of instruction, vary the assignments, the projects, the interaction, the design examples, and come out with a successful course and students who value the time they have spent with you. Do it a little on the fly, creatively, unexpectedly, and perhaps without precedent? Well, we are engineers – it is what we do.■ Ben Rosenberg is a Principal at Silman based in their Boston office. (rosenberg@silman.com)

SEPTEMBER 2020

NCSEA News NCSEA

News from the National Council of Structural Engineers Associations

National Council of Structural Engineers Associations

2020 Structural Engineering Summit Goes All Virtual Last month, NCSEA announced the expert-led slate for the 2020 Structural Engineering Summit. NCSEA is now announcing that this year's event will go all virtual! The Virtual Summit will include three days of live streamed presentations, three days of Bonus Content presentations with live speaker interaction, access to our virtual trade show (which opens in October), and opportunities to interact with fellow attendees. Sessions will include such topics as the future of the AEC industry, serviceability design, timber, tornado wind loads, post-disaster safety assessment, posttensioned concrete design, resilience, and even topics that cover our changing world like work flexibility in relation to COVID-19, racial equity, and how to lead a multi-generational workforce.

The Virtual Summit will feature a keynote panel showcasing a leading architect [Vibhuti (Vickie) Harris, HKS, Inc.], contractor [Greg Gidez, Hensel Phelps] and structural engineer [Glenn Bell, Simpson Gumpertz & Heger (retired)] to discuss what the future looks like from their perspective. What’s Happening with the Future of the AEC Industry will focus on the trends, technologies, and innovations that will shape the structural engineering profession and the entire Architectural, Engineering, and Construction (AEC) industry in the future. Also featured will be a keynote presentation led by, professional speaker and generational expert, Matt Havens.

Leading the Human Way: How to Stop Acting Your Age and Lead

a Multi-Generational Workforce will share a unique approach to solving all of your generational issues in the workplace (and at home) by rediscovering the similarities among us and learning to lead the Human Way. Registration for the Summit is open now! All registration fees are refundable up until 10/16/2020 so you can register worry-free to receive the best rate.

Your registration to the Summit includes: • 25 Hours of Education Available (Most Hours Offered Ever!)

• 17 Hours Live-Streamed and On-Demand Education • 8 Hours of Bonus Content • Virtual Trade Show Solutions • Open October through the end of November!

• Easy Virtual Access to Leading Industry Suppliers • Set Appointments, Review Latest Materials, and More • Unique Networking Opportunities • Virtual Lounges will be open for topic driven peer-to-peer networking, trivia games, and more!

The 2020 Summit will provide the same great education, created by structural engineers for practicing structural engineers, delivered straight to your desk! Learn more and register by visiting www.ncsea.com.

Prepare for the PE Structural Exam with NCSEA! SE Refresher and Exam Review Course

The Best Instructors. The Best Material. Available to you instantly. Learn more: www.ncsea.com

Apply Now: Young Member Awards Young Member Summit Scholarships NCSEA awards deserving up-and-coming leaders in our industry Young Member Scholarships to attend the NCSEA Structural Engineering Summit. This year's recipients will receive free registration to the Virtual Summit which includes all the great benefits above! The scholarship competition is open to any current member of an NCSEA Member Organization who is under the age of 36.

Young Member Group of the Year Award The emphasis of this award is to recognize Young Member Groups that are providing a benefit to their young members, member organization, and communities. Each finalist will each receive one complimentary registration to the 2020 Virtual Summit.

Learn more about these awards by visiting www.ncsea.com.

NCSEA Webinars

Increase the Benefit of Your Education Program

NCSEA's Diamond Review Program was created to evaluate the quality of continuing education courses, seminars, and conferences geared toward structural engineers. After the education is evaluated and Diamond Review Approved, structural engineer attendees are eligible to receive PDHs in all 50 U.S. states. Diamond Review approval is one of the key values behind NCSEA's high-quality, expert-led webinar program, which has only grown in success since adopting the process. This program can be beneficial for associations and suppliers alike.

To submit education, visit www.ncsea.com. When submitting a course, you will need a detailed outline of the program, presentation materials, speaker qualifications, and the number of continuing education credits to be awarded upon completion of the course.

Register by visiting www.ncsea.com

September 10, 2020

September 22, 2020

October 1, 2020

Gwenyth Searer, P.E., S.E.

Seth Thomas, P.E.

Rafael Sabelli, S.E.

Gravity Loads and Photovoltaic Panels

How to Design for Tsunamis: The ASCE 7-16 Tsunami Provisions and Project Examples

Design of Chevron Connections

Courses award 1.5 hours of Diamond Review-approved continuing education after the completion of a quiz.

S E P T E M B E R 2 02 0

39

SEI Update News of the Structural Engineering Institute of ASCE Check out the NEW SEI Virtual Events Page www.asce.org/structural-engineering/virtual-events

• Monthly #SEILIVE Chat on Instagram • Career Path Series: Insights with Glenn Bell and SE Industry Leaders Join for engaging discussions for every level of structural engineer: from where to begin to possibilities beyond principal. Live sessions are free for ASCE/SEI Members, but space is limited. Register today! #SEICareerPaths SEI/ASCE Members have free access to July and August sessions and resources online.

Session 3: Advancing – Project Engineer to Project Manager – Tuesday, September 22, 1 pm US ET Emily Guglielmo, P.E., F.SEI, M.ASCE; and Victor Van Santen, P.E., F.SEI, M.ASCE Session 4: Evolving – To Principal and Beyond – Tuesday, October 20, 1 pm US ET Joe DiPompeo, P.E., F.SEI, F.ASCE; and Anne Ellis, P.E., F.ASCE

ASCE Virtual Technical Conference 2020

Register Now! Multi-disciplinary technical event powered by ASCE Institutes. Earn PDHs, interact with presenters and peers, and access full library of recorded sessions.

Don’t miss sessions on:  Making the Case for Conceptual Design  San Francisco Millennium Tower Retrofit  Sustainability, Embodied Carbon and SE 2050  High-Performance Buildings  Building Hoover Dam  Diversity and Inclusion in Construction  Atlanta Botanical Gardens Walkway Collapse Investigation  Resilience in Transportation and Development  Offshore Wind Energy Development Challenges and  Do I Really Need to Include Sea Level Rise in Design?

Opportunities  The Future of AEC

NEW in the ASCE Bookstore and Library https://bit.ly/3adFWns

• Aesthetic Design of Electric Transmission Structures Prepared by the Task Committee on Aesthetic Design of Electric Transmission Structures • Composite Special Moment Frames Wide Flange Beam to Concrete-Filled Steel Column Connections Congratulations and thank you to the Committees that produce these publications!

Confidential Reporting on Structural Safety CROSS-US Check out newsletter, failure reports, and case studies, and consider submitting a confidential report. Sign-up for notifications at www.cross-us.org.

SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle Errata 40 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 skills engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need!

CASE has several sample contracts, practice guidelines, and risk management tools for firms to use in enhancing their construction management practices.

CASE #4 – CASE #6 – CASE #7 – CASE #8 – CASE #12 –

An Agreement Between Client and Structural Engineer for Special Inspection Services An Agreement Between Client and Structural Engineer for a Structural Condition Assessment An Agreement for Structural Peer Review Services An Agreement Between Client and Structural Engineer for Forensic Engineering (Expert) Services An Agreement Between Structural Engineer of Record (SER) and Contractor for Transfer of Digital Data or Building Informational Model File Commentary C – Commentary on AIA Document A201, “General Conditions of the Contract for Construction,” 2017 edition CASE 962-D – CASE 962-E – CASE 962-F – CASE 962-G – Tool 2-4 Tool 4-3 Tool 9-1 Tool 10-1 Tool 10-2

A Guideline Addressing Coordination and Completeness of Structural Construction Documents Self-Study Guide for the Performance of Site Visits During Construction A Guideline Addressing the Bidding and Construction Administration Phases for the Structural Engineer Guidelines for Performing Project Specific Peer Reviews on Structural Projects Project Risk Management Plan Sample Correspondence Guidelines A Guideline Addressing Coordination and Completeness of Structural Construction Documents Site Visit Cards Construction Administration Log

NEW CASE Publication Released!

CASE Tool 2-6: Structural Engineer Job Descriptions When targeted to people outside the firm, well-written job descriptions entice the most qualified people to apply for jobs with your firm. To get the most qualified candidates, list both quantitative and qualitative requirements such as experience, education, and desired personality traits.

These types of qualifications help to eliminate undesirable candidates.

When targeted to people inside the firm, job descriptions can be utilized as a powerful management tool. The details contained in well-written job descriptions form the basis for developing a clear understanding between the employee and the manager of what is expected of the employee.

Managers can also use the terms in the job description to determine how the employees performed when conducting performance appraisals. The criteria used for performance evaluations ideally would match the expectations listed in the employee’s job description. The job description for the position above the employee’s current position can be used to explain what is required for that person to earn a promotion.

The job descriptions contained within this tool are intended to be used as a template for you to create job descriptions for your firm.

Word files are provided with detailed descriptions along with a matrix with abbreviated descriptions when comparing engineering levels.

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

CASE Goes Virtual!

With in-person meetings at a stand-still, CASE members still found a way to engage with each other and with other ACEC Coalition members. As part of the first ACEC Coalition Virtual Education Series, held August 6-7, CASE members learned about: • Efforts ACEC is undertaking with the Administration on an infrastructure package; being involved in political affairs with congressional leaders through virtual town halls and participating in the PAC

• How to handle unconscious bias in the workplace • Joined with mechanical and electrical engineers in discussions about the PPP loans, back to office procedures, client networking, business development, and diversity/inclusion discussion follow-up from the previous day session CASE committees all met the afternoon of the August 7th virtually.

Look in next month’s edition for a re-cap of all committee work and what new/updated publications are coming to an inbox near you!

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

SEPTEMBER 2020

41

structural FORUM Tackling Conflict Head-On

By Michael Yost, Esq., and Aaron Mann, Esq.

E

ngineering is a profession based on truth and integrity. Firms and clients rely on an engineer’s expertise, and they trust their engineers to conduct themselves ethically.

Unfortunately, we have seen first-hand where lapses in ethical judgment, resulting from project issues, cut short an accomplished career. These lapses usually come from otherwise decent and honest people, and their actions appear inexplicable. In many circumstances, those lapses can be traced to the same cause – conflict avoidance.

There is a certain amount of conflict in every profession. How you handle that conflict and address our natural aversion to conflict, can make all the difference. In the context of engineering, conflict can arise in many different situations: • Missed project deadlines; • Unmet client expectations; • Disagreements between project team members; and, • Pressures to accommodate project needs, budget, or other factors.

On some occasions, the pressure from those conflicts builds up in the minds of people to the point that they no longer can see the right path forward. And that sometimes includes doing unethical things, including lying and data falsification. In these situations, practitioners often fall into the same trap: 1) The engineer delays the tough or awkward conversation. This is often accomplished by telling a small lie to try to explain the delay (“I am almost done,” when most of the project work has not yet even begun). They think that, if they can buy themselves more time, they can solve the larger problem; but, 2) The problem snowballs because they now have to address both the original conflict AND cannot reveal that they lied in the first place. Rather than diminishing the level of conflict, they have increased it dramatically.

They likely will not be able to resolve the original conflict successfully and now have the additional pressure of not being truthful. So they convince themselves that the only way out of the predicament is a solution that involves an ethical breach, such as data falsification.

Someone who is otherwise well-intentioned could fall into this trap to avoid telling the client they were further behind in their schedule than they initially let on. Likewise, it is not 42 STRUCTURE magazine

difficult to imagine – when they have backed themselves into this corner – that they might tell other lies, manipulate data, or engage in other unethical behavior to avoid other hard conversations with clients or management.

Consequently, engineers must explore how to prevent these types of behaviors from ever occurring in the first place.

“

Culture is ultimately not created by what people hear but by what they see every day.

Do you view ethics training as being analogous to safety training? Consider this, while there are rules regarding jobsite safety, try to focus on the hazards that led to the creation of the rules. It is one thing to say “wear a hard hat because Rule 7 requires one.” It is another to say, “there are lots of things out there that could hurt you. We do not want that to happen, so please wear a hard hat.”

Likewise, it is not enough to simply say “do not lie to clients” or “do not water down your professional opinions because a client wants you to.” In typical situations, most people agree they should not lie to clients, and they will not change their professional opinions.

Yet these are often the same people who – when placed in an extremely stressful situation – may take a very different path. Engineers must focus on the hazards and ask: What are the day-to-day pressures put on your employees? It is important to train your employees not only in required technical skills but in conflict management skills as well. This will help them defuse some of the conflicts inherent in their work. As an added benefit, those soft skills can make them a more valuable consultant to their client. Having a senior person in the role of a project and client management mentor is key to the development of these skills. That mentor can also act as an internal counselor for your employee if their project has gotten off track (i.e., at the first sign of potential conflict).

Are you asking your employees to do the impossible? Life in a professional services firm is never easy, and professionals are often expected to carry a full workload in addition to other responsibilities. But are you stretching your employees too thin? If your employees are being asked to maintain an unsustainable pace consistently, they will find ways to justify cutting corners. This is compounded by the fact that complaining about having too much on your plate is often (incorrectly) viewed as a sign of weakness. We cannot be surprised when someone who truly has more on their plate than they can handle chooses less than ideal ways to resolve their situation.

Are your employees empowered to deliver bad news safely? If your management staff has a “yeller” who consistently responds to bad news with aggression and conflict, employees are going to stop providing necessary information to that manager. It is that simple. That is not to say that the problems will disappear – they will just be hidden. That is also not to say that unforced errors such as missed deadlines should be immediately forgiven. But if employees cannot deliver bad news safely, then the problems will remain unaddressed and fester.

What behavior is modeled within your office? Culture is ultimately not created by what people hear but by what they see every day. The example set, good and bad, will be emulated by others. Consequently, your organization needs to demonstrate ethical behavior in addition to ongoing ethics awareness discussions. This means a continuing conversation from leadership about ethics, and it means training employees to prepare themselves for hard conversations.

Most importantly, it means a consistent demonstration of ethical behavior at all levels of your organization. Training is not enough.

Hard conversations are a part of life. Each of us owes it to our employees to focus on the hazards and help prepare them for what lies ahead.■ Michael Yost is a Senior Vice President and the Chief Legal Officer of Terracon Consultants in Olathe, Kansas. Aaron Mann is a Principal and Senior Attorney with Terracon.

Both authors have presented on ethics and conflict avoidance to numerous organizations, including ACEC, ASCE, ACI, and GBA.

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STRUCTURE SEPTEMBER 2020

Bonus Content

structural TESTING A Look at Discrepancies in Concrete Strength Testing

By Alicia Hearns

As

a general contractor or subcontractor working on a concrete structure, one of the most important tasks is ensuring that the

concrete has been properly cured and its quality has been tested according to applicable standards. This is of the utmost importance for quality control and quality assurance purposes. Equally important is selecting an appropriate and accurate method for monitoring the strength of in-place

concrete. Unfortunately, popular methods of testing concrete quality, especially compressive strength, are frequently subject to discrepancies.

Cylinder Break Tests If the project is anything like most other concrete construction sites, break tests are likely used to monitor the strength of newly placed concrete. This practice has remained mostly unchanged since the early 19th century. There are two types of specimens that field technicians collect to test the strength of concrete: standard-cured cylinders and field-cured cylinders. These samples are cast and cured according to ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, and are tested for compressive strength, most often by a third-party testing laboratory.

As the name suggests, field-cured cylinders are subject to the same temperature and relative humidity conditions that the completed structure will experience in its environment. Unlike standard-cured cylinders, field-cured specimens are kept right beside the concrete slabs on site.

They are predominantly used for determining whether a structure is ready for critical operations like removing formwork or post-tensioning.

In standard or lab curing, concrete cylinders are sent to the lab where they are stored in curing tanks or rooms which are subjected to curing conditions outlined in the ASTM standard and the project’s specifications. Standard-cured cylinders are generally tested 28 days after the concrete is placed for quality control and standard acceptance purposes.

Although cylinder break tests are the most widely accepted method of compressive strength testing, they are frequently associated with testing discrepancies that are not often genuinely representative of in-situ concrete elements. Curing conditions, the surface area of the cylinders compared to the onsite concrete element, and transportation to the laboratory of field-cured specimens are all factors that can skew the setting, hardening, and strength performance of the samples in comparison to the actual structural elements made from the same concrete material.

Field-cured cylinders, onsite.

Standard-Cured Cylinders Even though the process of testing cylinders is fully standardized, there has been a considerable amount of “bad” or low breaks recorded when standard specifications are not properly followed on site. The American Concrete Institute’s (ACI) pertinent specifications (ACI 318-14, 301-16, and 311.6-09) state that acceptance test specimens need to be standard-cured in accordance with ASTM C31. After the cylinders are molded, ambient temperature and humidity are to be monitored and maintained. Test specimens are required to be stored in a temperature range of 60°F to 80°F (16°C to 27°C) for a period of up to 48 hours (subject to change based on the type of concrete). Moisture and relative humidity loss are prevented by storing the samples in a moisture-filled environment, which is typically a cooler installed on site. Improper temperature and relative humidity control at the initial stages of the cylinder life can result in inaccurate strength data when testing occurs at later ages.

Furthermore, as standard-cured cylinders are subject to these strict curing conditions, they largely do not reflect the in-situ concrete but rather verify the QA/QC of the concrete’s mix design to ensure it meets specifications.

Field-Cured Cylinders On most construction sites, field-cured concrete samples are tested for strength at various ages during the first week after the concrete is poured to decide when to allow formwork removal. Usually, if the concrete reaches 75% of its designed strength, the structural engineers allow for the stripping of forms.

If samples are not properly prepared and rodded according to ASTM standards, there may be voids or aggregate segregation in the samples.

As a result, the concrete cylinder will have a lower measured strength when compressed.

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Furthermore, despite being placed near concrete elements on site, field-cured cylinders are often not subject to the same ambient conditions as placed concrete; test cylinders are often kept in the shade or a trailer. As a result, the strength value when the cylinder is broken will not represent that of the actual structure, affecting its quality in the long run. A main contributing factor to this is the surface area and size of the placed concrete in comparison to the field-cured cylinders. This difference significantly alters the curing temperature of field-cured specimens, affecting their Penetration resistance test. ability to represent the placed concrete.

Following the initial curing onsite, the specimens for standard-cured and field-cured are transported to the laboratory. Due to their relatively low strength, early-age specimens are highly susceptible to mechanical damage if not protected from jolting, which is typically experienced during transportation. Improper handling of specimens, especially at an early age, makes them more vulnerable and susceptible to micro-cracking. Therefore, when the samples are broken, these voids created by the micro-cracking affect the strength value tested, resulting in a low cylinder break that does not represent the strength of the in-situ concrete.

ASTM C31/C31M outlines the details for the safe transport of the specimens, with transportation time not exceeding 4 hours. These specimens need to be monitored for exposure conditions during the transport phase. This includes protection from freezing with efficient insulation materials, including; being wrapped in plastic, wet burlap, or surrounding them with wet sand to prevent moisture loss.

Even when handled with caution, transportation is still an issue.

Test specimens that are placed in the back of trucks with no plastic caps or cushioning are subject to moisture loss and structural integrity. Although transportation is not a technical process, it is an essential step in ensuring the laboratory receives quality test specimens.

After the concrete specimens have been curing for a specific number of days [3, 7, 14 days], they are prepped by the lab for breaking.

This involves either grinding the ends of the cylinder so that they are parallel or capping them. Grinding the ends of a break is done to ensure continuity in contact while the load is applied. Capping cylinders, according to ASTM C617, is utilized when grinding is not possible. If the concrete cylinder is not prepared with care and attention before breaking, the cylinder will not properly break when a load is applied, and the strength value that results will, therefore, show up as a low break.

Alternative Methods As a result of the discrepancies associated with concrete cylinder testing, alternative strength testing methods are used.

Rebound Hammer or Schmidt Hammer (ASTM C805) A spring release mechanism is used to activate a hammer, which impacts a plunger to drive into the surface of the concrete. The rebound distance from the hammer to the surface of the concrete is given a value from 10 to 100. This measurement is then correlated to the concrete strength. A rebound or Schmidt hammer is relatively easy to use and can be done directly on site. However, pre-calibration

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using cored samples is required for accurate measurements. Additionally, test results can be skewed by surface conditions and the presence of large aggregates or rebar below the testing location.

Penetration Resistance Test (ASTM C803) A device drives a small pin or probe into the surface of the concrete to complete a penetration resistance test. The force used to penetrate the surface and the depth of the hole is correlated to the strength of the in-place concrete. Penetration resistance test is also relatively easy to use and can be done directly on site. Data gathered using this technique is significantly affected by surface conditions as well as the type of form and aggregates used. Furthermore, precalibration is necessary using multiple concrete samples for accurate strength measurements.

Ultrasonic Pulse Velocity (ASTM C597) This technique determines the velocity of a pulse of vibrational energy through a slab. The ease at which this energy makes its way through the slab provides measurements regarding the concrete’s elasticity, resistance to deformation or stress, and density. This data is then correlated to the slab’s strength. Ultrasonic pulse velocity is a nondestructive testing technique that can also be used to detect flaws within the concrete, such as cracks and honeycombing. Unfortunately, this technique is highly influenced by the presence of reinforcements, aggregates, and moisture in the concrete element. It also requires calibration with multiple samples for accurate testing.

Pullout Test (ASTM C900) The main principle behind this test is to pull the concrete using a metal rod that is cast-in-place or post-installed in the concrete. The pulled conical shape, in combination with the force required to pull the concrete, is correlated to compressive strength. The pullout test is easy to use and can be performed on both new and old constructions. However, this test involves crushing or damaging the concrete.

A large number of test samples are needed at different locations of the slab for accurate results.

Drilled Core (ASTM C42) A core drill is used to extract hardened concrete from the slab. These samples are then compressed in a machine to monitor the strength of the in-situ concrete. These samples are considered more accurate than field-cured specimens because the concrete that is tested for strength has been subjected to the actual thermal history and curing conditions of the in-place slab. However, this is a destructive technique that requires damaging the structural integrity of the slab. The locations of the cores need to be repaired afterward. A lab must be used to obtain strength data.

Wireless Maturity Sensors (ASTM C1074) This technique is based on the principle that concrete strength is directly related to its hydration temperature history. Wireless sensors are placed within the concrete formwork, secured on the rebar, before pouring. Temperature data is collected by the sensor

Choosing the Right Test

SmartRock Wireless Maturity Sensor.

and uploaded to an app in any smart device using a wireless connection. This information is used to calculate the compressive strength of the in-situ concrete element based on the maturity equation that has been set up in the app. Compressive strength data is given in real-time. As a result, the data is considered more accurate and reliable as the sensors are embedded directly in the formwork, meaning they are subject to the same curing conditions as the in-situ concrete element. This also means no time is wasted on site waiting for results from a third-party lab. However, testing concrete strength using the maturity method requires a one-time calibration, for each concrete mix, to establish a maturity curve using cylinder break tests.

Tests like the rebound hammer and penetration resistance technique, while easy to perform, are considered less accurate than other testing methods. This is because they do not examine the center of the concrete element, only the curing conditions directly below the surface of the slab. Practices, such as the ultrasonic pulse velocity method and the pullout test, are more challenging to perform as their calibration process is lengthy, requiring a large number of sample specimens to obtain accurate data. As destructive testing techniques, the drilled core and cast-in-place cylinder methods are more expensive and take longer to perform, resulting in more time needed in the project schedule. Comparatively, with the maturity method, strength data is available in real-time directly on site, allowing for well-informed and quick decision-making.

Decisions in choosing a testing method may simply come down to what the engineer is accustomed to and what information is needed. However, the accuracy of these tests and the time they take to obtain strength data are significant factors that are not always taken into consideration as comprehensively as they should. The accuracy of the chosen technique can lead to future durability and performance issues of the concrete structure. Furthermore, choosing a technique that takes additional time to receive strength data can be detrimental to project deadlines, negatively impacting productivity on the job site. Conversely, choosing the right tool can positively impact project timelines and allow the project to be completed below budget.■ Alicia Hearns is a Content Marketing Specialist with Giatec Scientific Inc. (alicia.hearns@giatec.ca)

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INSIGHTS Design of Cross-Laminated Timber Structures for Lateral Loads

By Jason Cattelino and Jamie Garcia, P.E.

C

ross-laminated timber (CLT) is taking the building industry by storm and has put wood back into the spotlight as a sustainable construction material that provides occupants with physical and psychological benefits. Its prominence in North America is fundamentally changing approaches to design, manufacturing, and construction. As a relatively new wood product in the United States, design standards and performancebased testing data are lacking or non-existent.

Structural engineers are forced to rely on simplified or rudimentary design approaches, particularly when it comes to the lateral design of CLT diaphragms and shear walls.

U.S. Design Standardization CLT was first adopted into the 2015 International Building Code (IBC) and is permitted for use in gravity systems when designed using procedures outlined in the National Design Specification (NDS®) for Wood Construction. Extensive fire testing over recent years has driven wider acceptance of CLT structures among code officials, and the upcoming 2021 edition of the IBC will allow for mass timber buildings up to 18-stories tall. Despite this most recent amendment, the design standards fail to address CLT for use in Lateral Force Resisting Systems (LFRS), and engineers must employ an Alternate Methods and Materials Request (AMMR) to comply with building regulations. Without a prescriptive code path or design standards, engineers have no choice but to rely on overly conservative design approaches or state-ofthe-art research with limited data to justify the strength and performance of CLT LFRS. Consequently, local building jurisdictions may be reluctant to allow CLT, particularly in high seismic and wind areas, without a sound understanding of the mechanics-based approach to design.

Lateral Design Principles CLT diaphragms and shear walls can be effectively designed using a capacity-based approach, similar to other common building materials such as concrete or steel. The premise of this approach is to control the level of force in a structure by understanding

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Cross-laminated timber was utilized as the roof diaphragm and shear walls at the G.K. Butterfield Transportation Center in Greenville, North Carolina.

performance characteristics and deformation under seismic or wind events. Isolated areas within the lateral system are designed to dissipate energy by undergoing permanent deformation while all other areas are “capacity protected” with overstrength factors. In wood design, connections dissipate energy through fastener yielding using specific failure modes in the NDS. Brittle wood failure modes are avoided. Due to the high in-plane shear strength and stiffness of CLT panels, connection deformation becomes the most significant component of total deformation and the primary influencer of overall diaphragm or shear wall performance.

CLT Diaphragms Using a capacity-based design approach, CLT diaphragms offer numerous design and construction efficiencies. Currently underutilized by many engineers, in part due to the lack of design standards, CLT panels can be designed as diaphragm chord members, drags, and struts by resisting a combination of bending and axial forces from gravity and lateral loads.

Additionally, design professionals may control diaphragm response and performance through

proper connection detailing at panel joints and supports, such as compatible deflection or isolation from vertical elements of the LFRS to accommodate lateral drift. Since many of the connections for CLT diaphragms utilize self-tapping screws and other plate connectors, building officials can easily identify and review the lateral load path throughout the construction process.

ASCE 7 Minimum Design Loads for Buildings and Other Structures and the IBC define diaphragm rigidity for the purpose of force distribution to vertical elements of the LFRS, such as shear walls. Based on the interaction between relatively stiff CLT panels and flexible connectors, diaphragms may be idealized as rigid or flexible depending on the type, layout, and stiffness of vertical elements in the LFRS. Using a modified equation for wood panel sheathed diaphragms in the American Wood Council’s Special Design Provisions for Wind and Seismic (SDPWS), flexibility assumptions for CLT diaphragms can be verified using direct deflection calculations. For diaphragms containing irregularities, such as discontinuities or reentrant corners, overall performance and deflection cannot be predicted using

simple equations. In these cases, a semi-rigid analysis should be performed using Finite Element models. However, this approach is highly dependent on software capabilities, as well as available product data, and may not be economical or feasible for many projects.

For a more prescriptive design approach, it is common practice for engineers to design members and connections for worst-case loads derived from both flexible and rigid analyses while addressing deflection using a simplified approach.

CLT Shear Walls As with CLT diaphragms, CLT shear walls present their unique design challenges. When subject to in-plane lateral loads, CLT walls primarily engage in rocking behavior due to the high in-plane shear strength and stiffness of the panels relative to the flexible connectors used to adjoin them. Full-scale performance testing of CLT shear walls is limited and focused mainly on tall wood buildings rather

than code development for prescriptive design.

As such, engineers utilize industry-accepted limits on wall height-to-width aspect ratios to encourage this known “rocking” response.

The lag between performance testing and prescriptive design guidance has forced many engineers to fall back on mechanics-based approaches previously developed for woodsheathed shear walls (perforated, segmented, and force transfer around openings) to calculate CLT shear wall capacity. While these approaches tend to be conservative for CLT, complex force interactions, especially around wall openings, provide a hurdle to prescriptive, yet more precise, analysis methods.

Conclusion Many notable projects have been completed in the United States, illustrating that CLT is a revolutionary building material for the construction industry with many economic and environmental benefits. However, current design standards are notably lacking,

specifically when it comes to the lateral design of CLT. As the mass timber industry matures, architects and engineers must push the boundaries of design with CLT. Investing in additional performance testing and research is critical for the development of tools and resources necessary for more accurate and efficient CLT designs.■ The online version of this article contains references. Please visit www.STRUCTUREmag.org.

Jason Cattelino is a Project Engineer for SmartLam North America, the first CLT manufacturer in the U.S. (jason.cattelino@smartlam.com) Jamie Garcia is a Principal and Project Manager at Eclipse Engineering, P.C., in Bend, Oregon. (jgarcia@eeimt.com)

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