STRUCTURE magazine | August 2021

STRUCTURE AUGUST 2021

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INSIDE: NAVFAC Remote Hangar Construction Over Podiums Steel in Corrosive Environments Meow Wolf Denver Project

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Contents AU GUST 2021

Cover Feature

NAVFAC DELIVERS A RESILIENT REMOTE HANGAR

By Yuriy Mikhaylov, S.E., and Frank K. Humay, Ph.D., S.E.

A new U.S. Marine Corps hangar was recently constructed in Guam. The main hangar doors can fully open, creating an uninterrupted 325-foot-wide opening. The opening required a steel box truss. The primary trusses and box truss were constructed on the U.S. mainland and then broken down into individual members for shipping.

Features

Columns and Departments

MODULAR DESIGN MAKES FOR QUICK CONSTRUCTION

7 Editorial

By Aaron Miller and Erin Spaulding

8 Structural Systems

NCSEA Foundation: Laying the Groundwork for a Bright Future

By Jami Lorenz, P.E.

The Union project, a six-story residential project in Oakland, California, with

Multi-Unit Residential Construction Over Podiums

five prefabricated wood construction

By Matthew Johnson, P.E., Connor Bruns, P.E., S.E.,

levels, 110 market-rate apartments,

and Eric Twomey, P.E., S.E.

parking, and ground-level retail, is the type of modular solution developers are looking for to deliver projects to market faster, without the added costs.

11 Structural Connections Proven Technologies for Fastening to Steel in Corrosive Environments

By Christopher Gill

34 InFocus Engineering Books for Babies

By Linda Kaplan, P.E.

36 Structural Loads A New and Unexpected Roof Snow Drift

By Michael O’Rourke, Ph.D., P.E., and Chris Letchford, Ph.D., CPEng

39 Spotlight The Academy Museum of Motion Pictures 40 Insights Parametric Structural Design for High-Performance Buildings

By Demi Fang and Caitlin Mueller, Ph.D.

MANAGING UNCERTAINTIES WITH COLLABORATION

By Shaun Franklin, P.E., John Jucha, P.E., and Julie Wanzer

For the Meow Wolf Denver project, challenges

included

constraints/solutions,

unique dual

sight design

criteria to accommodate future retrofit, wide elliptical arched entryways, deflection and vibration performance of

various

and more.

structural

components,

12 Practical Solutions Going to New Heights with Cross-Laminated Timber Shaft Design

By Jim Henjum, P.E, S.E., Jeff Jack, P.E., Kelsey West, P.E., and Wilson Antoniuk, P.E.

15 Structural Performance Community Storm Shelter Design – Part 1

By Bradford Russell, AIA, P.E., SECB

18 Historic Structures Blackshear Bridge Failure

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

20 Structural Design Seasoning Checks in Timber

By Kevin Cheung, Ron Anthony, Michelle Kam-Biron, P.E., S.E., SECB, and Bonnie Yang

42 Business Practices This Year, Make “Lean” Your Firm’s Buzzword

By Sarah Scarborough, P.E., S.E.

50 Structural Forum Ethics Instruction…Ideas for Moving Forward

By Scott Civjan, Ph.D., P.E.

In Every Issue

Advertiser Index NEWS of note Resource Guide – Wind/Seismic NCSEA News SEI Structural Columns CASE in Point

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

A U G U S T 2 0 21

EDITORIAL NCSEA Foundation: Laying the Groundwork for a Bright Future

By Jami Lorenz, P.E.

H

aving been involved with NCSEA for the past 14 years, I have always admired and respected the leadership and unification that the organization provides to the structural engineering profession. As a former NCSEA Summit Delegate and President of the Structural Engineers Association of Montana, I quickly started to integrate myself into the NCSEA Summit Committee, and – after a few years – reveled in the new energy and growth the Young Member Group Support Committee and SE3 initiatives infused into the organization. So, in the Fall of 2020, I jumped at the chance to join the NCSEA Board as a Director, soon realizing that I was joining at a promising time in the lifespan of NCSEA, following the establishment of the NCSEA Foundation.

In early 2020, the Foundation was established to herald a new chapter in NCSEA’s support of our profession and the diversity of thought.

Ultimately, the goal of the Foundation is to sponsor initiatives that elevate our profession, provide diversity within our community, bolster our state SEAs, and advance important research and educational programs. As a truly charitable 501(c)3 organization, the Foundation provides donors an avenue for tax-deductible donations with the knowledge that their funds are being directed to these critical and targeted goals.

In the first year, the NCSEA Foundation has already made significant strides in laying the groundwork for advancing these critical initiatives for our future through its Grant Program for SEAs, the first-ever Diversity in Structural Engineering Scholarships, educational content, and funding for industry research. A major focus for NCSEA is to increase engagement and provide resources to our state SEA Member Organizations through the Foundation, building a more robust network and connectivity across the country.

Grant Program The Grant Program was the first initiative taken on by the NCSEA Foundation to assist our Member Organizations in providing new and innovative programs to their members. With all the challenges of 2020, there were some creative and socially relevant ideas coming from the state SEAs. A few of the more innovative grant recipients of 2020 were: • SEAOI (Illinois) awarded funding to create a cohesive library of STEM videos and funding to enhance the association’s Remote Site Visits.

• SEAONC (Northern California) awarded SE3 DEI Firm Leader Cohorts funding to provide a forum in which firm leaders can discuss their commitments to and the challenges in implementing strategies for racial DEI in their firms and the industry.

The complete list can be viewed here www.ncsea.com/awards/grants.

We cannot wait to see what 2021 brings!

Diversity in Structural Engineering Scholarship The first of its kind in our industry, the Diversity in Structural Engineering Scholarship was established to award funding to students who have been traditionally underrepresented in structural STRUCTURE magazine

engineering, including (but not limited to) Black/African Americans, Native/Indigenous Americans, Hispanics/Latinos, and other people of color. Multiple scholarships will be presented annually to junior college students, undergraduate students, and/or graduate students pursuing degrees in structural engineering. With over 50 applicants this year, the Foundation awarded four $3,000 scholarships in 2021, along with free registration for the Structural Engineering Summit. Recipients hailed from Oregon State University, University of Southern California, University of Texas-San Antonio, and Rowan University. Read more about them in the NCSEA News article in this issue ( page 44 ) or here: www.ncsea.com/awards/scholarship. The generation of an endowment is the next step towards ensuring these scholarships can continue well into the future.

Education Another avenue of grant and scholarship fundraising is through providing new and innovative educational content. For example, the Foundation hosted two webinars in 2020 that fall outside the more typical technical education. The Diversity, Equity & Inclusion Webinar Series (www.ncsea.com/resources/dei) and the Economic Outlook for Structural Engineering: 2021 & Beyond event (www.ncsea.com/events/past) pushed engineers outside of the code books to think about the bigger picture in their own firms.

Industry Research Another cornerstone of the Foundation is supporting and funding research incorporating practicality into the theory behind building code development. The first such project was a Wind Engineering Research Program in Support of the ASCE 7 Load Standard. The potential to support research for practicing structural engineers is a big leap for the organization’s support of our industry’s bright future.

What’s Next?

To ensure streamlined operations and the best use of resources in this initial stage of development, the NCSEA Board of Directors currently fills the role of leading the Foundation. However, we are now beginning to reach out to our industry to enlist support, build diversity, and continue to refine our growth strategy. Working in tandem with our state SEA foundations is an essential part of the Foundation’s success, and we will be further defining this partnership as we grow.

I personally think that the Foundation and its primary objectives will be the “magic sauce” to elevate engagement within our national structural engineering community. We hope you join us and bring your ideas to the table!■ Jami Lorenz is a Director on the NCSEA Board of Directors and the Principal of Business Development for DCI Engineers, based in the Bozeman, MT office. (jlorenz@dci-engineers.com)

A U G U S T 2 0 21

structural SYSTEMS Multi-Unit Residential Construction Over Podiums

By Matthew Johnson, P.E., Connor Bruns, P.E., S.E., and Eric Twomey, P.E., S.E.

M

ulti-unit residential construction continues to see an increasing demand for living units above a one- or two-story podium. The demand is primarily driven by maximizing residential unit density within permissible building heights for a particular construction type. Architecturally, the podium’s primary purpose is a horizontal separation between different occupancies, creating open space for parking, retail, or amenity space on the lower floor(s). Yet, for the structural engineer, the podium level is expected to transfer significant loads from the density of the program above the podium to a different structural system below. While “stick-built” construction is historically wood-framed, the prevalence of coldformed steel is increasing the structural demands on the podium level due to the ability to construct as Figure 1. Cold-formed steel construction above a steel podium. many as ten stories of residential (Figure 1). While the design focus is often the structural efficiency of the residential unique – they can be items required for architectural expression, units, due consideration is required for a properly designed, detailed, mechanical, electrical, plumbing, and fire protection (MEP/FP) and constructed podium structure. coordination, or the structural system’s analysis and design.

Podiums can be used to create a tall story at the ground floor for lobby, retail, or restaurant occupancies. However, the tall story can System Overview create complexities for the facade support design. Secondary strucThe podium structure is typically structural steel (Figure 2) or cast- tural steel may be required to support the facade, either through in-place reinforced concrete, though regional variations and/or structured mullions or continuous horizontal framing to support construction cost models might lend themselves to post-tensioned the vertical structure of the exterior wall system. Contract docuconcrete and even precast/prestressed concrete; the latter is not dis- ments should either provide fully designed details for the secondary cussed in this article. Design considerations for these structures are steel or clearly identify the performance requirements and delegate the design to the contractor. Similarly, canopies, overhead doors, louvers, signage, entry vestibules, and other architectural elements may dictate additional steel if they require support between the ground floor and podium level.

With any structure, MEP/FP distribution is a critical coordination step for the design team. Reviewing duct, pipe, and conduit size, plan location, and elevation is essential in designing the structure.

It can require structural elements to move, change depth, or be locally strengthened. This is the same for podium structures and even more critical. Because gravity loads from the residential units accumulate at the podium level, structural members are highly loaded and typically very large. Similarly, mechanical systems are often their largest and/or most densely congregated at the podium level.

Gravity-fed plumbing may run horizontally from the vertical chases in the building to the outlet location at this level. Plumbing lines have slope requirements with minimal flexibility, so the structural system must typically accommodate the plumbing. There can be penetrations through steel or concrete beams or rearranged framing Figure 2. Structural steel podium. layouts to accommodate these needs. The complexity and density

STRUCTURE magazine

of MEP/FP at the podium level play an important role in deciding the structural system of the podium level.

Some factors impact the podium analysis, independent of the structural systems used, that should be considered in developing system concepts. Serviceability requirements can be one of the governing factors. Instantaneous and long-term deflection (if applicable) can create cracking in bearing wall finishes if the deflection is not minimized with an appropriate construction sequencing. From the authors’ experience, limiting the podium level combined superimposed dead and live load deflection to L/1000 effectively minimizes the risk of cracking and other serviceability issues. When analyzing the podium, how and if the walls above are modeled in 3-D analytical software can significantly impact the analysis. If bearing walls are modeled, it is important to confirm that the bearing wall in-plane stiffness does not artificially stiffen the podium structure. Reducing the in-plane stiffness in the analysis model or applying loads as line loads to the podium level are possible solutions to confirm loads are accurately applied to the podium structure.

Structural Steel Structural steel podiums provide many benefits compared to other structural systems; however, like all structural systems, there are limitations. Loading configuration can impact both the steel tonnage and the number of steel members required. If bearing walls above are arranged in a linear, orthogonal, and regularly spaced pattern, steel girders can be located directly below the walls and designed to meet the requisite strength and serviceability limits, allowing the remaining framing at the podium level to be designed only for a single level. Vertical and horizontal plan offsets, and complex loading patterns from the structure above, can necessitate additional steel beams or the slab construction to support each section of wall or applied load; this can lead to a large number of total pieces, complex framing geometry, increased material, and increased number of highly loaded connections.

Structural steel systems are not susceptible to additional long-term deflection considerations. This can be a benefit when considering the likelihood of cracked finishes and other serviceability considerations.

For structural steel podiums, there will be a combination of deep beams sized to support accumulated loads from the structure above

Figure 3. Beam web penetrations in structural steel podium framing.

Framing bears on top of the perimeter columns.

and shallow beams to support just the slab of the podium level occupancy. Since ceiling elevation is typically dictated by the elevation of the bottom flange of the deepest beam, in the absence of soffitted ceilings, these shallow floor beams can create a zone for MEP/FP distribution in the space between the bottom flange of the shallow and deep framing. Inevitably, there will be zones where MEP/FP distribution is required to pass by deeper beams; in these cases, if located near the mid-span of the girders and away from high point loads, beam web penetrations can be designed and incorporated into the fabrication of the steel framing (Figure 3). It is crucial to have a dialog with other design team members and the contractors and key trade partners, if possible, to determine if additional web penetrations should be included in the design. Web penetrations are typically easier to fabricate in the shop than in heavily loaded girders in the field.

Structural steel podiums are typically lighter than concrete solutions, an additional benefit that can reduce foundation loads. Therefore, it is a valuable exercise to compare the total foundation demands for the building with a structural steel podium against alternate podium solutions to determine if cost savings in the foundation systems will be a significant factor in structural system selection.

Though often governed by stiffness considerations, girders supporting multiple floors also accumulate significant loads at their connections.

Contract documents should specify actual connection forces at each member if delegating the connection design. Relying on conventional beam reaction tables may underestimate the connection demand.

For single-story podiums, detailing steel beams to bear on top of the columns will simplify connection design, improve constructability, and reduce eccentricity in the steel column (Figure 3). Continuous beams over columns are also more efficient than single spans; comparable deflections can be achieved with less steel.

Reinforced Concrete

Figure 4. Concrete podium with cast-in-place anchor rods and embedded conduit.

Reinforced concrete (RC) podiums offer a great deal of flexibility in the arrangement of the supported superstructure. Still, they require important considerations related to both initial detailing and longterm performance (Figure 4).

Like structural steel, RC podiums can be designed and detailed with beams and girders at the bearing walls or columns above. However,

A U G U S T 2 0 21

the reinforcement details and significant formwork requirements are rarely cost-effective. Alternately, a deep flat plate can be a costeffective option for the transfer of discrete line or point loads from the structure above. Forming is much simpler, and reinforcement can be increased locally to accommodate high loads due to bearing walls and/or individual columns. Flexibility can be designed and detailed to allow walls/columns to move marginally as the design develops.

Typically, the overall flat plate RC podium is shallower than other materials. The flat plate also allows for reasonably impediment-free distribution of MEP/FP systems below the slab.

The primary concern with RC podiums is long-term deflection, particularly where vertically stiff, multi-story bearing walls are supported.

Providing additional reinforcement to increase the transformed area, increasing the concrete strength to increase the modulus of elasticity, and extending the duration of shoring can mitigate some of the concerns. However, regardless of the additional measures, the importance of long-term deflection to the serviceability of the structure above must be explicitly considered.

There are several secondary considerations for RC podiums. For discrete systems, such as structural steel columns or hold-downs in wood-framed or cold-formed steel shear walls, detailing of the base plate and exposed anchors is a consideration – providing box-outs to recess the baseplate in the slab to not impact floor finishes should be clearly detailed (Figure 4 ). Concrete levelness should be considered for pre-fabricated bearing wall systems detailed and assembled in the shop to design, not as-built, dimensions.

Post-Tensioned Concrete Post-tensioned (PT) concrete is the preferred structural system for podium slabs in certain regions because of cost savings. However, the added economy accompanies heightened trade coordination and, for many practitioners, analytical complexity.

PT concrete slabs are generally more economical than non-prestressed RC when the slab span exceeds 25 feet, a common scenario for parking or retail below a podium. PT slabs can be 10% to 20% thinner and contain 40% to 60% less reinforcement than RC slabs for the same span and loading. Whether these savings offset the additional post-tensioning material and labor costs is highly dependent on the local market, project scale, and contractor experience. A thinner slab at the podium level also translates to less load on foundations, more space for MEP/FP distribution, and taller ceiling heights, if desirable. While long-term deflections can be the Achilles heel of RC slab design, PT slabs are inherently powerful at minimizing deflections and controlling cracks due to their precompression and load balancing.

PT podium design is generally governed by two-way (punching) shear strength at slab-column joints. Increasing the slab thickness to improve shear strength offsets the economy of a PT slab; increasing column sizes may have programmatic impacts. Shear reinforcement is typically a reasonably cost-effective compromise. However, there are instances where shear reinforcement is impractical for heavy loading. In these cases, shear caps are an effective way to increase shear

strength. For PT slabs, shear caps can also be used to reduce flexural stresses because the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318) prescriptive limitations on drop panel dimensions only apply to non-prestressed slabs.

Therefore, practitioners should be aware of significant reductions in precompression stress at shear caps and ensure they stay above the 125-psi code-required minimum.

Trade coordination makes the list of challenges for any structural system, yet PT podium slabs are particularly vulnerable to costly coordination mistakes. With MEP/FP systems collecting and redistributing at the podium level, openings should be coordinated and clearly detailed to avoid banded tendon lines, anchorage zones, and slab-column joints. Electrical engineers also view the PT slab as a viable location for conduit, leading to congestion and heightened risk of either conduit damage, or worse, poor slab performance. Early efforts should be made to hang conduit below the slab or embed it in a concrete drop slab, if necessary, for fire ratings. The podium is also the interface between the PT subcontractor and carpenters or ironworkers building the residential structure above. Ideally, steel embed plates or shear wall hold-down anchors should be cast into the PT slab to avoid post-installed anchors that might damage the PT tendons.

Analytically, PT podium design is likely the most challenging system for practitioners. Load application from the residential floors above can be the most troublesome part of the design process. Bearing walls can be irregularly spaced, staggered with plan offsets and feature concentrated loads from posts and hold-downs. Design is iterative – walls move, framing orientation changes, and wall openings are modified.

Prior to modeling and designing for distributed line loads or concentrated loads, practitioners should study if a conservative equivalent uniform load, supplemented with localized checks, is appropriate for the PT design. As PT design is usually governed by punching shear at the columns, the flexural design for the wall loading pattern may be less critical, allowing some flexibility in design evolution.

Closing The podium structural system for a multi-family residential development can impact nearly every part of the project. System selection, design, and detailing should balance structural efficiency with serviceability, architectural needs, MEP/FP coordination, local contractor experience, and constructability.■ References are included in the PDF version of the article at STRUCTUREmag.org.

All authors are with Simpson Gumpertz & Heger.

Matthew Johnson is a Principal. (mhjohnson@sgh.com) Connor Bruns is a Senior Consulting Engineer. (cjbruns@sgh.com) Eric Twomey is a Senior Project Manager. (ejtwomey@sgh.com)

Additional Reading 5-over-2 Podium Design. Malone, STRUCTURE, January and October 2017.

Mid-Rise Wood-Frame Buildings. McLain, STRUCTURE February 2019.

Reaching Higher with Cold-Formed Steel Framing for Podium Structures, Warr, STRUCTURE, March 2019 Steel Stud Bearing Walls. Bruns et al., STRUCTURE, November 2019.

STRUCTURE magazine

structural CONNECTIONS Proven Technologies for Fastening to Steel in Corrosive Environments

By Christopher Gill

C

orrosion of metals used in construction is an age-old problem, the fastening without one with annual costs exceeding $300 billion today in the United accessing the steel backStates alone. In addition to economic losses, corrosion of structural side, and minimal and non-structural elements creates a significant safety risk, particu- disturbance of the base larly with critical connections. Fortunately, technologies have been material coating. developed to help address many of the causes of corrosion of these One example features connections with effective products and processes. a fastener that is driven The type and extent of corrosion are often a function of the envi- into a small pre-drilled ronment. Wet environments in the presence of conductive fluid (i.e., hole in the steel. Forming humidity) can attack the surface of the connection elements (base the hole is accomplished Figure 1. Corrosion of connections is steel, connected material, and accessories such as nuts and washers), by a special stepped drill a common concern. thus causing surface or pitting corrosion. A steel base material could bit that does not penbe carefully coated with a protective coating, only to have that coating etrate through the base steel; simultaneously, the bit removes a small destroyed when a connection is made. For example, effective welding circle of coating or surface rust to prepare for the fastening. The to the steel may require removing the coating, or the welding process blunt-tipped stainless steel fastener is driven into the hole using itself may destroy the coating. Connection methods like through- a battery-actuated tool. The act of driving the fastener point into bolting can cause another form of corrosion called galvanic corrosion the hole displaces some of the base steel, which is then melted by when dissimilar metals are in contact with each other in the presence the friction heat, creating a micro weld. The result is a threaded of a conductive fluid. It should be noted that welding and drilling stud protrusion from the steel, carrying a working load up to 800 can damage the coating both on the connection side of the steel and pounds in tension and up to 850 pounds in shear. Below the threaded the opposite side of the steel (Figure 1). portion, tight against the steel surface, is an Where load transfer requirements are integrated stainless steel washer with a neohigh, welding of the steel members may prene washer bonded to it, which seals and be the only feasible solution. Quality conprotects the uncoated steel surface and prevents trols should be specified to ensure that water infiltration. Even though the stud matecoatings and welds are properly repaired rial and the base steel may be dissimilar, the and re-coated. In many applications, nonprevention of water infiltration helps prevent welded solutions may be appropriate. For galvanic corrosion (Figure 2). example, when framing supplemental steel Also available is a system meeting similar to primary steel, bolting may be indicated. requirements, which can be installed withHowever, in a corrosive environment, the out a battery-actuated drive tool. A hole potential for galvanic corrosion or crevice is drilled with a similar stepped drill bit, corrosion may exist. Additionally, unless which provides the precise drilling depth the hole locations are planned and pre- Figure 2. A welded stud with field repair to coating and removes a small circle of the existing punched in the shop, the process can be damage (left). A properly designed fastening system coating. The blunt tip of this fastener features laborious and expensive. Similarly, the prevents damage (right). high-strength threads. When the fastener is attachment of modular systems (“strut” systems) can be accomplished driven (screwed) into the pre-drilled hole, it cuts fine threads in by through-bolting or clamping. However, the clamping devices may the base steel. Like the system described above, the threaded stud not always be effective, depending on the steel shape that is being protrusion includes a sealing washer, which protects the steel and clamped to, like tube members, for example. prevents water infiltration. Working loads for this fastener can As a structural engineer, you may be called on to design or evalu- be up to 600 pounds. Both systems described above provide a ate critical non-structural connections, such as hangers for utilities ¼-inch or 3⁄8-inch threaded protrusion that can be used to attach and equipment or fastening bar grating to steel on a mezzanine. a baseplate or strut profile or hang a pipe or equipment using a Traditionally, these connections are accomplished with welding, coupler and a threaded rod. through-bolting, or clamping. However, as outlined above, these As long as connections need to be accomplished on construction projects, solutions can invite corrosion. Additionally, they may not be feasible corrosion will continue to cause concern. Manufacturers will in the field if the base steel is too thick to drill or the backside cannot continue to advance the state-of-the-art by developing safer be accessed to complete the connection. Fortunately, technologies have and more practical technologies to address these concerns.■ recently been developed which address corrosion concerns and proChristopher Gill is the Manager of Code and Approvals for Direct vide a practical means for installation. Key features of these products Fastening at Hilti, Inc in Plano, TX. (christopher.gill@hilti.com) include highly corrosion-resistant materials, the ability to complete A U G U S T 2 0 21

practical SOLUTIONS Going to New Heights with Cross-Laminated Timber Shaft Design

By Jim Henjum, P.E, S.E., Jeff Jack, P.E., Kelsey West, P.E., and Wilson Antoniuk, P.E.

T

he building community has rediscovered the oldest building material – wood! Over the past decade, Cross-Laminated Timber (CLT) has gained momentum in the U.S., with over 500 projects currently in design or constructed to date. It is rare for a new building material like CLT to come along so quickly and become an integral part of the structural engineer’s toolbox. CLT has many advantages, including sustainability, speed of construction, strength, and beauty – all of which are the driving forces behind the doubling quantity of projects each year (www.woodworks.org, https://bit.ly/3wQCt8D). In addition, the desire to design sustainable and economic structures has many investors, designers, and builders captivated with CLT and wondering how they can find creative ways to integrate this material into their next project.

Figure 1. CLT elevator shaft.

Elevator and Stair shafts While a large percentage of CLT is being used in horizontal applications (floors and roofs), CLT can also be integrated into vertical applications such as exterior walls, shear walls, and elevator/stair shafts (Figure 1 and 2). Historically, shafts, typically designed and constructed out of concrete or CMU, have been disregarded elements of buildings. CLT, on the other hand, is an alternative approach worth considering. An elevator or stair shaft constructed of CLT offers inherent advantages in fire resistance, aesthetic appeal, and accelerated installation (Figure 3). Considering CLT is “hot off the press,” there are a few items worth discussing further.

Codes and Standards Chapter 6 and 7 of the 2018 International Building Code (IBC) provides a prescriptive path to specify CLT as an approved building material for shaft construction. However, specific tests and documentation may be required from the CLT manufacturer. Code references for shafts and CLT allowance in building construction include: • Chapter 6 of the IBC defines various construction types and allowable materials. CLT shafts meet the 2-hour fire-resistance rating (FRR) requirement for construction Types III, IV, and V. • Chapter 7 of the IBC defines fire-resistance rating requirements. o Section 713 – Shaft Enclosures: Section 713.2 provides the definition of a shaft enclosure to be detailed as a “fire barrier” in accordance with Section 707. o Section 707 – Fire Barriers: Section 707.2 identifies that fire barriers shall be materials permitted by the building type of construction. Section 707.3 further defines the fire-resistance rating requirements indicating that shafts must comply with Section 713.4.

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o Section 713.4 – Fire-Resistance Rating: This code section outlines that shaft enclosures must meet a 2-hour FRR when connecting 4 or more stories. This story requirement includes basements but not mezzanines. Furthermore, shaft enclosures must have an FRR that meets or exceeds the floor assembly it penetrates but does not need to exceed 2-hours. Lastly, it notes that shaft enclosures must meet the requirements of Section 703.2.1. o Sections 703.2 and 703.3 define fire-resistance rating and methods for determining a fire-resistance rating. For CLT, the most popular methods are the ASTM E119 Test and Char Calculations per Chapter 16 of the NDS. Each CLT manufacturer should have testing data to meet the requirements outlined in ASTM E119.

If the shaft is incorporated as part of the Lateral Force Resisting System (LFRS), an Alternative Materials and Methods Request (AMMR) may be required per 2018 IBC Section 104.11. Check with the local Authority Having Jurisdiction (AHJ). Many AHJ’s allow CLT shear walls when low response factors for seismic design are used. Since CLT panels are made of wood, one common misconception is that they can be modeled as flexible. This is the case for conventional light-frame wood structural panel shear walls, but not for rigid CLT panels, which can be fabricated up to 10 feet wide, 52 feet tall, and 123⁄8 inches thick.

If AMMR is required per the AHJ, a common question arises: “What is the best path for achieving approval?” A few suggestions: • Since this application is relatively new to most parties, it can take additional effort and time to get approved. Take this into consideration when determining the design schedule.

• Discuss CLT shaft options with the AHJ early in the design process. Then, if required, complete paperwork (provided by AHJ) to meet Section 104.11 requirements of the 2018 IBC.

• Reference future codes as part of your documentation, including the 2021 IBC, 2021 ANSI/AWC Special Design Provisions for Wind and Seismic (SDPWS), and ASCE 7-22, Minimum Design Loads for Buildings and Other Structures. o ASCE 7-22 provides response factors for CLT to be used as LFRS for seismic design. Aspect ratios and special detailing are required. Response Modification Coefficient (R) equals 3.0 for normal shear walls and 4.0 for high aspect ratio shear walls. For wind-controlled design, the response is not a concern. o 2021 SDPWS provides special detailing and aspect ratios, while items such as in-plane shear and connection capacity values must be calculated based on engineering mechanics.

Check with the CLT manufacturer for published in-plane shear capacity values.

• For seismic-controlled design, try to isolate CLT shaft walls from the main LFRS to lessen the impact on the overall building response.

• Early on, verify that the CLT manufacturer has an ICC-ES report, as this information is typically required by the AHJ, specifically in high seismic regions.

• For a specific example of coordinating with the AHJ, go to the WoodWorks presentation on the Burwell Center for Career Achievement. (reference mark 16:20 in the video https://vimeo.com/427874585)

Design and Detailing Designing and detailing CLT shafts may seem overwhelming, but it can and should be simple. Most CLT manufacturers can support/ assist with design and detailing to meet the gravity, lateral, and fireresistance considerations outlined within this article. In addition, they should assist with acoustical related items and provide design guidance to help optimize the manufacturing and assembly process, which are imperative for a successful CLT project.

The architect and engineer should become familiar with the CLT option early in the design process. To save time and money, it is best practice to integrate CLT shafts into the project during the initial project design and detailing. Regardless, the CLT manufacturer should provide all the information needed to successfully design CLT shafts, whether they are incorporated into the initial design or are substituting another system.

Detailing the CLT shaft is critical to meet minimum code requirements and significantly impacts the ease of installation.

Figure 3. CLT vs. CMU comparison.

Figure 2. CLT stair shaft. Courtesy of Lara Swimmer Photography.

Figure 4 (page 14) identifies an elevator shaft example and simple assembly details used to construct a CLT shaft. Stair shafts will have similar construction details. The CLT panel-to-panel connections are made with readily available partially threaded screws. Prefabricated steel angles placed on adjustable cast-in-place or post-installed concrete anchors can be used for the CLT wall-panel-to-foundation connection. It is essential to have base adjustability to achieve the required elevation and level bearing surface for the CLT wall. Hoist beams, life-line beams, divider beams, and stair landing beams can be supported by wall pockets or face mount hangers. Beyond simple assembly details, there are a few other items architects and engineers should consider.

1) Shaft Size and Height: For shafts that are less than 4 stories, a single CLT panel can be installed to extend from the foundation to the top of the shaft. The number of panels provided on each side of the shaft will depend on the required size of the shaft. In the example provided (Figure 3), installing a single cab elevator shaft with only 5 panels, 4 walls, and a cap is possible. When shafts are 4 stories or more, a horizontal joint will be necessary to stack panels vertically. This is because the wall height will create temporary bracing challenges

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and push the limits of CLT manufacturing capabilities. Installing vertical segments as the other building construction goes up allows for improved job site safety and ease of installation.

2) Fire-Resistance Rating Detail: For 1-hour fire-resistance-rated shafts using NDS Chapter 16 char calculations or ASTM E119 tested panels, the CLT can generally remain exposed.

For a 2-hour fire-resistance-rated shaft, consult the CLT manufacturer to review specific ASTM E-119 detailing. It may be possible to leave the CLT wall panels exposed; however, special detailing considerations will likely be required.

3) Acoustic Details: CLT alone may not provide the requisite Sound Transmission Class (STC) wall rating to meet minimum code values depending on the application. In these cases, additional wall assembly detailing may be necessary. Many times, this detailing is already part of the building system (e.g., a simple stud wall assembly adjacent to the CLT shaft) and will not become a special detailing requirement. Consult your CLT manufacturer for additional direction regarding STC wall rating assemblies

Figure 4. CLT shaft connection details.

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Conclusion As CLT continues to gain traction in the U.S., those within the architectural, engineering, and construction communities continue to seek new ways to integrate this material into projects. With some specific items considered, CLT elevator and stair shafts offer a unique and simple way to incorporate CLT into traditional building structural systems. The key to success is early engagement with the AHJ and the CLT manufacturer to ensure the CLT shaft design process goes smoothly.

For more information on elevator and stair shafts, please visit: https://bit.ly/3hx6uWb.■ Jim Henjum is the Director of Engineering and Design at SmartLam. Jeff Jack is the Mass Timber Technical Lead at RedBuilt.

Kelsey West and Wilson Antoniuk are Mass Timber Sales Specialists at RedBuilt.

structural PERFORMANCE Community Storm Shelter Design Part 1: A Marriage of Codes and Artistry

By Bradford Russell, AIA, P.E., SECB, F.SEI, F.ASCE

T

ornado season in the United States is officially March through June, but tornadoes, including major outbreaks, have been documented in the United States during every month of the year. The U.S. sees more tornado and other high wind activity than any other place on earth. In fact, as many as 1,200 to 1,500 tornados could occur in any given year. Interestingly, the number of tornado events is generally increasing from one year to the next. In 2004, the highest number in recent history in the U.S were recorded, with over 1,800 tornadoes. As recently as 2008 and 2011, 1,700 tornadoes occurred. With the population increasing year after year, this presents community issues needing to be addressed through storm shelter design, documentation, and construction.

The increase in the frequency of tornadoes has caused a heightened need to regulate the protection of the public Total tornadoes per year in the U.S. Provided by NOAA (National Centers for Environmental Information).

A more detailed chart is included in the online version of this article at STRUCTUREmag.org. during these extreme events. The International Code Council (ICC) 500-2014 Standard and Commentary (ICC/National the minimum requirements of building codes. The model building Storm Shelter Association Standard for the Design and Construction of codes do not provide design and construction criteria for life safety Storm Shelters) addresses this. The Standard requires special consider- for sheltering during high wind events, nor do they provide design ations from designers mandating storm shelter design and construction criteria for tornadoes. for emergency operation centers, fire, rescue, ambulance stations, police This article specifically addresses stations, and K-12 education buildings with a capacity of fifty (50) or community storm shelter design more occupants. ICC 500 applies to the design, construction, instal- and documentation requirements lation, and inspection of residential and community storm shelters. By and how the codes are used to mitidefinition, a community shelter is not associated with a single dwelling gate risks from these storm events. unit, as the single dwelling unit has an occupant load of less than 16 The current code for storm shelter and may only be designed for the host building occupants. design of community and residential shelters, ICC 500-2014, is briefly reviewed to look at architecThe Need for Shelter tural considerations and structural Tornado wind loading on a structure has unique performance engineering considerations. Finally, requirements beyond the ordinary high winds experienced by the the documentation requirements structure. The faster tor- for proper storm shelter perfor- Storm shelter signage. nado wind speeds result in mance requirements, as addressed higher loading pressures and by the architect and the structural engineer, are discussed. The next an increased need to protect installment will look at more specific structural engineering issues the occupants from flying and expected requirements in the new ICC 500-2020. debris than typical hurricane or other high wind events.

Architectural Considerations – Shelters are intended to The ‘Art’ of the Storm Shelter provide protection against both wind forces and the The foremost concern of architectural design (and engineering design) impact of windborne debris. of community shelters is occupant safety. From that viewpoint, the The level of occupant pro- elements concerning the necessary occupant floor area space, accestection provided by a sibility, and short-term utilization of the shelter’s features are factors space specifically designed in the design. Occupant safety comes in several forms. First, the as a shelter is intended to community shelter must provide access, in terms of the requirebe much greater than the ment for accessibility of the mobility-challenged and the distance protection provided by any potential occupant must travel to arrive at the refuge. Second, The Art of the shelter. buildings that comply with the shelter must have the area and volume necessary to comfortably A U G U S T 2 0 21

contain those being sheltered and the accommodations, including seating and restroom facilities, necessary to be operational for the occupants. Third, addressed by the architect or other design professional, the shelter design must meet code and ADA requirements along the path of travel (1000 feet maximum) to the shelter from all building spaces being served by the shelter. This makes the art of the storm shelter start with the occupants' safety while meeting the design challenges of the project.

Siting the community shelter above the floodplain is a minimum requirement. The finished floor elevation must be two feet (610 mm) above the flood elevation having one percent (1%) annual chance of being equaled or exceeded in any given year, or simply two feet

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above the one-hundred-year floodplain. This requirement accounts for continued development surrounding the shelter and allows for coverage of the one-hundred-year flood event into the future. This recognizes that, as development continues, the storm level events’ flood elevation also rises due to the increase of impervious cover in the areas surrounding the shelter.

Section 401 of the ICC 500-2014 Standard provides additional criteria for siting community shelters with respect to setting floor elevations above the flood event: 1) The flood elevation, including coastal wave effects, having a 0.2 percent annual chance of being exceeded in any given year; or 2) The flood elevation corresponding to the highest flood elevation if a hazard flood study has not been conducted for the area; or 3) The maximum flood elevation associated with any model hurricane category, including coastal wave effects; or 4) The minimum elevation of the lowest floor required by the Authority Having Jurisdiction (AHJ) for the location where the shelter is installed.

Additionally, Section 402 requires that the proximity of hazardous materials be addressed. When a community shelter is located within a precautionary zone that includes facilities that manufacture, use, or store hazardous materials, they must be provided with protection from the hazardous material releases as deemed necessary by the Local Emergency Planning Committee (LEPC) and the AHJ. Building occupant load served by the shelter is handled by the shelter area allotment and is guided by the International Building Code (IBC) to meet a design occupant demand. The occupant load determines the area needs of the storm shelter serving the spaces per Table 501.1.1 of the IBC. Because of the larger area requirements, community shelters are almost always designed for multiple purposes, including gymnasiums, cafeterias, assembly halls, or music rooms in public schools.

Critical support systems, structures, equipment, and components necessary to ensure the occupants' health, safety, and well-being are also required. A tornado shelter requires critical support systems for two hours, including functioning bathrooms, fire extinguishers, first-aid kits, lights on back-up or battery power, and ventilation. Natural ventilation is preferred since mechanical equipment is typically exterior and difficult to protect against high-speed winds.

In education facilities, the most efficient shelters function as classrooms or gymnasiums when not used as a shelter. But when parking garages are designated for storm shelter use, the mechanical venting requirements are more easily met with the openness and natural ventilation provided. With more enclosure in the design, mechanical ventilation and lighting become a higher need and invoke additional design requirements.

As the shelter design becomes enclosed, i.e., in a gymnasium or cafeteria, a mechanical ventilation system must be connected to an emergency power system (installed in accordance with the National Fire Protection Association’s NFPA 110 or 111). Ventilation rates must be provided in accordance with the applicable building and mechanical code provisions for the normal use of the space. In tornado shelters, section 702 requires emergency power to be provided for a minimum of two (2) hours. Also, the mechanical exhaust or intakes shall be protected from debris by the provisions of ICC 500-2014, Section 306.3 for exterior wall and roof impact protective systems.

The ‘art’ of the storm shelter starts with the safety of the occupants and continues through comfort while meeting the project's design challenges.

Documentation Requirements To ensure that the shelter is appropriately designed and constructed, the design team produces construction documents thoroughly illustrating to the contractor the completed assemblies and documentation of the properly distributed loading to the subgrade for third-party and other shelter reviewers to validate. All professions involved in the design of the storm shelter will be concerned with identifying the following information within the construction documents for community shelters, usually on a single page in the front of the document package: 1) Type of shelter: residential or community tornado, hurricane, or a combination of both.

2) A statement that the wind design conforms to the provisions of the ICC 500 Standard, with the edition year specified.

3) The shelter design wind speed, mph.

4) The wind exposure category.

5) The internal pressure coefficient, GCpi. 6) The topographic factor, Kzi. 7) The directionality factor, Kd. 8) A statement the shelter has/has not been constructed within an area susceptible to flooding in accordance with Chapter 4 of the Standard.

9) The Design Flood Elevation and Base Flood Elevation for the site.

10) Documentation showing the shelter envelope components will meet the pressure and the missile impact test requirements identified in Chapter 3 and Chapter 8 of the Standard.

11) A floor plan drawing or image indicating the storm shelter's location on a site or within a building or facility, including a drawing or image indicating the entire facility.

Table 501.1.1. Occupant Density – Community Shelters

Type of Shelter

Minimum Required Usable Shelter Floor Area in Square Feet Per Occupant

Tornado Standing or seated

5

Wheelchair

10

Bedridden

30

Parking garage as a storm shelter.

12) A storm shelter section or elevation indicating the height of the storm shelter relative to the finished grade, finished floor, and the host building.

13) The lowest shelter floor elevation and the corresponding datum.

14) The occupant load of the storm shelter.

15) The usable area of the storm shelter.

16) Venting area (sq.in.) provided and locations in the shelter.

17) Calculations for the number of sanitation facilities.

18) Minimum foundation capacity requirements.

19) Shelter installation requirements, including anchor location and the minimum required capacity for each anchor.

20) For hurricane shelters, the rainfall rate of the roof primary drainage system.

21) For hurricane shelters, the rainfall rate of the roof secondary (overflow) drainage system.

22) For hurricane shelters, the rainwater drainage design rainfall rate for facilities subject to rainwater impoundment.

This list addresses multiple professionals. The architect should be prepared to compile this list together on a single reference sheet for review by the LEPC, AHJ, third-party reviewers, and others. Providing this sheet in the front of the document set ensures easier reference.

The architect, and other professionals that have overall design responsibility, will be required to ensure the documents adequately show the load path is thoroughly represented in the detailing of the storm shelter assembly from the roof to walls, to floor diaphragms, into the Main Wind Force Resisting System (MWFRS), through MWFRS connections, and into the foundation.

The Art of the shelter carries into the preparation and creation of a comprehensive set of contract documents by the project design professionals for the contractor to execute from.

Conclusion The frequency and intensity of storms are increasing, as well as the population. As the population increases, so does the likelihood that this elevated weather event will affect greater numbers of occupants and structures, old, young, new, or used. The guidelines and codes will require taking a harder look at occupant safety, structural performance, and the communication of these factors from the design team to the contractors.

The art of the shelter begins with the risk management inherent in the design of the architecture and the engineering. It continues through proper documentation and final construction – when Art and Safety collide. As Walter Wriston is noted as saying, “All of life is the management of risk, not its elimination.”■ As an Architect and Professional Engineer (structural), Bradford Russell promotes his understanding and leadership in both disciplines relating to the built environment. With multiple A/E/C patents from the USPTO, he brings a unique perspective of innovative approaches.

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historic STRUCTURES

Blackshear Bridge Failure

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

B

lackshear, Georgia, is located about 90 miles southwest of Savannah. The Savannah, Florida, and Western Railroad (formerly the Atlantic Gulf Railroad) ran southwesterly from Savannah to Bainbridge and opened in December 1867 after starting construction as early 1859. About one mile northeast of Blackshear, the line crossed the Alabaha (sometimes called the Hurricane) River on an iron bridge that was flanked by long wooden trestles as was common at the time to keep costs low. The trestle that failed was reported to be 300 feet long and 25 feet above the ground. The trestle had been updated two years prior to the disaster with the Engineering News reporting, “The trestle on the Savannah, Florida & Western Ry., on which the dreadful accident occurred, noted last week, is stated by C. S. Wyckoff, formerly an engineer in the service of the company, to have been rebuilt within two years and to have been in thoroughly Inverted W wooden trestle with piles, similar to Blackshear. good condition. The trestle is stated to be of the ‘inverted W type, which is the strongest known to engineering,’ although it is the United States. Even though they only had a life of 8-10 years, certainly not the most approved; and the piling, according to Mr. they were considered an economical choice by many engineers Wyckoff, must have been remarkand railroad companies. Some railably well done, and it is stated by roads put speed restrictions on their him that each pile in the trestle wooden trestles. bridges ‘is driven with a hammer All was well on the line until 9:30 In the creek [it was dry at the weighing 2,200 pounds falling 10 am on March 17, 1888, when the site], all was chaos and confusion. feet until the last blow only moves it Atlantic Coast Fast Mail train out of 1 ⁄8 inch.’ We have heard of such pile Savannah approached the bridge. One The cars were piled on the top of driving in textbooks, but we have newspaper account wrote, never yet seen it on any railway in “The baggage car got off the track each other, and the cries of the America, and we are glad to know about a quarter of a mile before it frightened, injured passengers that it exists somewhere.” reached the bridge at Hurricane Trestles at the time consisted of two river. The baggage car mounted the arose from a caldron of death. or more piles with framing up to a cap track, but the train passed safely beam. Then, on top of the cap beams, over the bridge [main river bridge]. wooden stringers, generally two in Immediately on the other side of the number, and the wooden ties laid on the stringers. bridge, there is a trestle several hundred feet in length. When the Many other railroads around the country continued to use wooden baggage car struck the trestle work, it gave way, and the entire train, trestles as approaches to their iron bridges across major rivers of with the exception of the engine, dropped through and, with the exception of one car, was completely wrecked. The train consisted of a combination car, three baggage cars, a smoking car, one coach, two Pullman sleepers, and the private car of the Lehigh valley…”

Another wrote, “In the creek [it was dry at the site], all was chaos and confusion. The cars were piled on the top of each other, and the cries of the frightened, injured passengers arose from a caldron of death. Nineteen dead bodies were taken from the wreck as soon as help could be organized.

There may be others yet to be found.”

Engineering News wrote, “The front truck of the baggage car became derailed ‘fully a quarter of a mile’ before the bridge over the Hurricane [Alabaha] river was reached, with the train on a downgrade and running at high speed.

The apparent cause was a broken axle. Before the train struck the bridge, several other cars were derailed, and the trucks wobbled Wooden trestle with the recommended steel guardrails, closely spaced ties, and back and forth over the roadbed considerably, never getting very wooden outer rails.

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far out, as is not only distinctly stated but conclusively shown by the fact that the bridge proper was passed in safety by the entire train. The bridge was of considerable length; one account says 500 feet, but ‘the marks of broken axles (more likely, wheels, or the most part) show for several hundred yards on the roadbed and bridge.’

Beyond the bridge, however, was 500 to 800 feet of trestle approach. As soon as the derailed train struck this, it began to go over, but not so rapidly, but that it fell only after passing the first third of the trestle, knocking its apology for a floor all to pieces and demolishing the stretch beyond completely. The engine escaped, but the tender was dragged down with the rest of the train. It appears clear from these facts that the floor was merely the common one of 8- or 9-foot ties spaced 2 feet or so apart and with no pretense of guardrail. If wrong as to this particular floor, we shall be pleased to do justice to it as conspicuously as possible when details reach us, but there are thousands of trestles in the South with just exactly that kind of floor, and plenty of them in the North as well. It is such a paltry economy as compared Harper’s Weekly image. with 12- or 14-foot ties spaced close together and with heavy outside guardrails, that for the most part, its use comes merely rate on a bridge which was suited for half that speed…The question from thoughtlessness and inexperience in operating. We have some of speed modifies all questions of this kind and really should be a drawings in preparation showing how a trestle should be built, which first consideration, though it is often made the last in importance. will help to reinforce this moral, and until then, we pass it. Even with There are miles of weak trestles which the owners cannot afford to such a trestle floor, however, the circumstances make it highly prob- rebuild; there are numerous roads which cannot at once renew even able that good re-railing bridge guards would have put enough of the the floors of these bridges, but they can run slowly over these shaky trucks back on the rails at the entrance of the bridge to have ensured structures. The question is whether people will continue to entrust the safety of the train on the trestle, even if a broken axle was, in fact, their lives to such roads when they bid for patronage by running fast the original cause of the derailment, which is not entirely clear. There trains in spite of the great risks that must be taken.” has been enough money loss from this accident alone, not to speak Newspapers around the country picked up on the disaster that killed of the loss of life, to equip the whole State of Georgia with re-railing 19 and injured 35. Many more would have died, but it was written, guards at every bridge and trestle. We hope and believe that in time “The train caught fire from the stoves, but the heroic presence of mind these repeated lessons will be heeded.” of Engineer Welsh, who leaped from his engine and put out the fire, The magazine had been advocating for stronger decks, outside prevented an awful cremation.” guardrails, and re-railing guards on the approaches for years at the The failure was, therefore, due to a derailment and, in the eyes of time. The speed of the train was estimated at 45 miles per hour, and the Engineering News, a poorly designed trestle with ties too the Railroad Gazette wrote, short, too widely spaced, and with no curb to keep trains on “The tender was derailed 1,200 feet before reaching the bridge, the structure.■ which consists of a bridge proper over Hurricane River and wooden Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, having trestle approaches at either end. The derailment seems not to have restored many 19 t h Century cast and wrought iron bridges. He is now an been promptly discovered, and the whole train passed over the bridge Independent Consulting Engineer. (fgriggsjr@twc.com) and trestle until the tender got within 90 feet of the further abutment, where the derailed truck slewed around and tore up the sleepers. The tender broke loose from the engine and, on reaching the abutment, fell about 20 feet, the rest of the train following it.

The only information we have concerning the speed of the train is the testimony of the porter of the sleeping car, who said it was 45 miles an hour. The officers of the road have found no evidence of a broken wheel or axle, and the track, which is of 60-pound steel, shows no evidence of any defect. The testimony of the pasSTART WRITING YOUR DCI STORY sengers goes to show that the cars, even the strongest of them, were completely crushed in, some of them failing upon others…

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We do not know the real condition of this Hurricane River trestle.

There may have been a guard timber, and the sleepers may have Visit our website been securely blocked to prevent bunching, but the description of for more details the action of the tender truck indicates that the trestle was faulty in both these respects. The train may have been running at a reasonWASHINGTON | OREGON | CALIFORNIA | TEXAS | ALASKA | COLORADO | MONTANA able speed, but the indications are that it was running at a 40-mile

A U G U S T 2 0 21

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structural DESIGN

Seasoning Checks in Timber

By Kevin Cheung, Ron Anthony, FAPT, Michelle Kam-Biron, P.E., S.E., SECB, and Bonnie Yang

A

ll wood is subject to some degree of seasoning, i.e., drying until it acclimates to the humidity conditions of the surrounding atmosphere at in-service conditions. Seasoning occurs when the wood is air-dried, dried in a kiln under controlled conditions, or subject to radio frequency drying. As wood loses (or gains) moisture, it will shrink (or swell) until it reaches equilibrium with the constantly changing level of moisture of its immediate environment. As shown in Figure 1, seasoning checks are separations of the wood fibers that develop along the length of lumber or timber due to shrinkage of the wood as it dries.

Structural Timber Grades Checks are not defects.

Checks, particularly in • Nominal 5” x 5” and larger large, dried timbers, • Commonly sold unseasoned are natural characteris• Seasoning checks are permitted tics of timber and very • Design values are not affected by checks common. To many people, a check may look like a split. However, splits extend through the timber from one face to either the opposite or an adjacent face and are typically the result of rapid drying at the ends of the piece or excessive structural loads.

Checks do not generally affect the design strength of timber; however, these natural characteristics may impact appearance. When a check extends to a timber connection, the connection should be evaluated for fastener-holding capacity and may need to be reinforced or repaired.

Why Do Timbers Check?

In living trees, it is not uncommon for the wood moisture content to exceed 100% when the weight of the water in the wood is greater than the weight of the wood material.

After harvesting, the wood begins to dry and eventually begins to shrink as it dries below the fiber saturation point, typically at 26 to 30% moisture content level. As this shrinkage occurs, the timber dries more quickly on its surfaces and at the ends than in its interior.

Eventually, timbers, wood framing, and millwork protected from the weather on the interior of a structure will reach an equilibrium moisture content of typically between 6 and 13%, depending on the season and geographic location. Because of the cellular structure of wood, until it Figure 1. Timber column with seasoning checks in construction. The timber reaches equilibrium, it shrinks at dif- acclimated to in-service conditions. ferent rates in the cross-section (e.g., breadth and depth) than along the length and a greater rate in all three directions at the ends of the timber. This often causes checks to form at the mid-width on the face or the end of the piece as shrinkage occurs, as shown in Figure 2. The outer fibers become smaller than the internal fibers, and “seasoning checks” resembling fissures or cracks on the surface of the wood or end checks may occur.

A seasoning check develops on the outer face of a timber but does not extend to the opposite or an adjacent face. If it does, it is no longer a check but is called a split. Splits can occur at the end of a timber, where drying is the most rapid or checks join together. Splits along the length of a timber, as opposed to checks, are typically not due to drying but, more often, the result of excessive structural loads applied to the timber.

Effect of Checks on Appearance

Figure 2. Timber beam with checks in a 100-year old construction. The checks were limited in size once the timber acclimated to in-service conditions.

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Before specifying timber for a project, designers and property owners may want to visit several old buildings that contain large timbers to consider the degree of checking to be anticipated over time. Some species have lower shrinkage values and will develop fewer or smaller checks.

Shrinkage coefficients for various wood species can be found in the Wood Handbook published by the U.S. Department of Agriculture.

The rustic appearance of checked timbers has an aesthetic quality that may not meet every designer’s or owner’s expectations. Suppose a large cross-section is required, but seasoning checks are to be minimized. In that case, designers may want to consider specifying a lowshrinkage wood species, a lower moisture content at the time of delivery and installation, or consider a different product, such as glulam timbers. However, when the large size and rustic aesthetic of timbers are central to the design, the checks in the timbers may be consistent with the look desired by the archi- Figure 3a. Boxed heart timber with checks radiating from the Figure 3b. Free of Heart Center Timber with no tect and owner. pith, some that extend to the exposed faces of the timber. large checks extending to the surface.

It should be recognized that some degree of checking is virtually unavoidable in large timbers. After the the pith. Timbers can be specified FOHC “Free of Heart Center” timber reaches equilibrium, several small checks may be visible on the (Figure 3b), which means that the pieces do not contain the pith. face of the timber. Alternatively, a single, wider check may be visible. Such timbers typically demand higher prices and may be limited The number and width of the checks are a function of the anatomical in availability. If desired, the FOHC specification can be added to structure of the wood and how the internal stresses are released as the the plan’s general notes or details. wood dries. There is no difference in performance between numerous The ends of timbers can be protected temporarily with wax or small checks or a single large check other than appearance. This is quite other coatings containing paraffin or sealer to retard the natural apparent when viewing timbers in an older building where the timbers drying process and reduce the likelihood of developing large checks. have been at equilibrium for many years.

End coating is available from some timber-producing mills upon request (through buyer/seller agreement on large orders) or may be undertaken at the job site if seasoning checks have not yet begun to Kerfing develop. End coatings help timbers season more evenly by slowing the drying process during storage and transport before installation.

One method for limiting check development in large timbers is to Radiofrequency (RF) drying can dry the timber without producing saw a narrow, longitudinal kerf (a deep saw cut to the timber) to large checks but at additional cost. Check with local suppliers about the center of the timber surface from the end along the full or partial the availability of RF-dried timbers. length. The kerf serves to allow some movement and relieve stressKerfing dates from medieval times and can be specified in assigned es from drying shrinkage that would otherwise cause the timber to locations to provide drying stress relief to avoid or minimize the check. While kerfing may reduce the strength of the timber, it can be probability of large checks developing on visible surfaces. effective in controlling the development of checks or splits.

Checking and kerfing in preservative-treated timber may be deeper than the depth of penetration of the treatment, exposing untreated wood to conditions that might lead to decay. If kerfing is to be used, it is How to Reduce Checking? recommended that it be done before the preservative treatment process.

New timbers of most structures, dried to end-use conditions, are generally not available at the point of sale because of the time it Timber Grading Rules takes for them to dry, and end-use and Checks conditions are generally unknown by the mill. As timbers dry after millTimbers (Figure 4 ) are commonly milled ing, the wood shrinks, often resulting and graded in the unseasoned condition in checking and, occasionally, end before drying. Checks and end splits splitting. are accounted for when the timbers are If desired, there are several options graded, primarily for appearance, not for reducing the amount of checking structural capacity. to be expected in timbers. Timbers can In standard grading rules terminology, be seasoned and allowed to slowly air members with nominal dimensions of 5 dry under moderate conditions before inches x 5 inches and larger are classified installation (not exposed to direct sunas either Beams and Stringers or Posts light, high temperatures, or extremely and Timbers for all species listed in the low relative humidity). grading rules except southern pine. For Timbers that contain the pith southern pine, all of the larger structural (center of the tree) are called boxed members are called Timbers. heart (Figure 3a) and tend to develop The working stresses for strength more checks than timbers without Figure 4. New timbers that have been bundled after visual grading. grades include adjustments of clear A U G U S T 2 0 21

wood strength properties to make Table of permitted checks and splits in grading rules for new timbers. allowance for sizes and locations POSTS and TIMBERS nominal 5” x 5” and larger width* not more than 2” greater than thickness of checks and splits and other Select Checks – Seasoning checks, single or opposite each other, with a sum total equal to natural strength-reducing charStructural ½ the thickness of the piece. acteristics (such as knots) that are Splits – Splits equal in length to ¾ the thickness of the piece or equivalent of end checks. permitted in the particular grade.

The working stresses, and conNo. 1 Checks – Seasoning checks, single or opposite each other, with a sum total equal to sequently the allowances for ½ the thickness. tree-growth and manufacturing Splits – Splits equal in length to width of the piece or equivalent of end checks. characteristics, vary with specific No. 2 Checks – Seasoning checks. strength grades of a given species.

Splits – Medium** or equivalent end checks.

The corresponding permissible BEAMS and STRINGERS nominal 5” and thicker width* more than 2” greater than thickness characteristics are specified in the grading rules. The design values Select Checks – Seasoning checks, single or opposite each other, with a sum total equal to for horizontal shear require no Structural ¼ the thickness. reductions because checks are Splits – Splits equal in length to ½ the width of the piece or equivalent of end checks. already considered in the gradNo. 1 Checks – Seasoning checks, single or opposite each other, with a sum total equal to ing rules. ¼ the thickness.

In the standard lumber grading Splits – Equal in length to the width of the piece or equivalent of end checks. rules, seasoning checks are allowed in all grades of structural timbers No. 2 Checks – Seasoning checks. at the time of grading at the mills Splits – Medium** or equivalent end checks. as listed. Checks and splits are *Width is the larger of the cross-sectional dimension. Thickness is the smaller of the cross-section dimension. considered time-altering charac**A medium split is equal in length to twice the width of the piece and in no case exceeds 1⁄6 the length. teristics, so they may develop or get larger after grading at the mills until the timber reaches equilib- develop into a through-split that essentially separates the timber rium at in-service conditions. The development of checks does not into two pieces. mean that the timber is defective; only that the wood has dried and Checks also do not affect members stressed in axial tension unless the is reaching equilibrium with the environment of the structure. checks are at an angle to the grain of the piece and tend to extend across Permitted checks and splits in grading rules for new timbers (Western adjacent faces so as to separate the piece in two. This is more of an issue Lumber Grading Rules 2017, Western Wood Products Association) are with lower structural grades of timber with large knots or a steep slope of given in the Table for new timbers based on appearance requirements. grain (where the wood fibers are not parallel to the long axis of the piece).

Seasoning checks are considered to affect shear stress in bending members, which is seldom the governing design value. Structural Effect of Checks on Design Values timber design values for Shear Parallel-to-Grain are published in Other than appearance, seasoning checks do not affect the design the National Design Specification (NDS®) for Wood Construction®, strength of a member stressed in axial compression unless checks Supplement Table 4D. The published shear design values are determined according to ASTM D245-06, Standard Practice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber.

Fv values for horizontal shear listed in Table 4D are independent of the structural grade but are dependent upon wood species. The design values are derived from the shear strength of clear straight-grained wood in green (unseasoned) condition for each species (or group of species) determined from strength values published in ASTM D2555-17, Standard Practice for Establishing Clear Wood Strength Values, and based on the concept of the lower 5% exclusion limit of the strength distribution.

The shear strength of clear, straight-grained wood is adjusted for the combination of normal load duration and a factor of safety. The adjustment included in the design value is 2.1 for softwood species and 2.3 for hardwood species.

The adjusted shear strength from clear, straight-grained wood is then subject to a strength ratio adjustment from ASTM D245 for horizontal shear used to establish the shear design values published in the NDS. The strength ratio value of 0.5 in ASTM D245 is used as the maximum reduction in shear strength to account for splits and checks due to drying Figure 5. Seasoning check that developed into a split due to a bolted connection in service. The designer need not account for the presence or where tension perpendicular to grain stresses results from the bolt acting as a wedge.

STRUCTURE magazine

development of checks in the timber; the reference design values already incorporate the reduction.

Repair of Timbers with Checks As discussed earlier, once placed in service, timber attains a moisture content commensurate with ambient conditions of temperature and relative humidity regardless of its moisture content at the time of installation. The length of time to reach equilibrium depends on the size of the timber, the initial moisture content, and the in-service temperature and relative humidity. As ambient conditions change throughout the year, minor shrinkage or swelling of the timber will cause checks to open or close, respectively. Such changes are seldom visible to the naked eye.

Seeing fresh wood inside a check or a split indicates that it likely developed recently. If the building is not recently constructed, a new check/split or a new extension of an old check/split should be investigated to determine why it has developed long after the wood reached equilibrium Figure 6. Seasoning checks that reveal the natural characteristics of large timbers are often desired moisture content. It may be due to a change of interior for a rustic appearance. environmental conditions, such as the addition of insulation or changes in heating and cooling systems. the preventive measures discussed earlier can be used. However, Rapid drying frequently exacerbates end checks and the develop- installing stitch bolts or self-tapping screws to reinforce checks ment of splits. If checks develop into splits that occur in line with a or splits should not be placed in-line with close spacing, or the timber fastener, the possible effect on joint strength can be estimated checking or splitting may be exacerbated as the wood continues by whether the opening of the end split is equal to the amount to shrink and swell due to minor seasonal fluctuations. In tenof shrinkage for that species and change in moisture content or is sion and compression splices, separate splice plates for each row greater than expected. If the opening is greater than should occur due of fasteners or saw kerfs between longitudinal rows of fasteners to shrinkage alone, it indicates a possible wedging action from the might be advantageous. force applied by the fastener (Figure 5). This wedging action can be The repair of timber members with checks (and splits), when needed, corrected by boring a hole across the end of the piece and installing should not cause any reduction to the structural capacity of the a stitch bolt or inserting a self-tapping screw through the check or member, if possible. The repair should not restrict the timber member split to reinforce the connection. from shrinkage/swelling movements due to seasonal changes in ambiContractors prefer screws for their fast installation and reduced ent conditions, including those from the heating/cooling systems that impact on the remaining amount of wood relative to drilling a hole for result in changes in wood moisture content. the bolt(s). It is most effective to install the screws where the wedging For timbers exposed to the elements that have checks, the checks action force is on the fastener compressing the wood. To attempt to should not be filled with a sealant. Leaving the checks open allows close the separation completely would tend to crush the wood fibers moisture to drain or evaporate if the timber gets wet. Filling the check around the fastener and introduce undesirable tension perpendicular to with any sealant potentially traps moisture that can lead to decay. grain stresses. The NDS specifies avoidance of configurations leading to their initiation since tension-perpendicular-to-grain design values Summary and Conclusion for wood are not published.

If a large variance of greater than 10 percent from highest to lowest Building with timbers can satisfy high load requirements while promoisture content during and after construction is anticipated, viding a rustic appearance (Figure 6). Seasoning checks are natural characteristics that develop as timbers dry. Checks are accounted for when timbers are graded, primarily for appearance, not Shrinkage and Fasteners structural capacity, as the design values for structural timber grades are not affected by checks.■ Since timber shrinks as it seasons until it reaches equilibrium moisture content, it is particularly important that timber structures, when built using unseasoned/green timber, be inspected for tightness of fasteners during the early life of the structure. This is done so that the capacity of the connections is not reduced by the development of a gap between timber members in a joint as a result of shrinkage. After a timber reaches equilibrium moisture content, it will typically continue to have small dimensional changes due to seasonal variations in temperature and relative humidity, although these are generally inconsequential.

Kevin Cheung is the Chief Engineer at Western Wood Products Association. (kcheung@wwpa.org) Ron Anthony is the President and Wood Scientist at Anthony & Associates, Inc. (woodguy@anthony-associates.com) Michelle Kam-Biron is a Mass Timber Specialist at Structurlam Mass Timber Corporation. (mkambiron@structurlam.com) Bonnie Yang is the Sales Engineer at Freres Lumber Co., Inc. (zhuooyang@gmail.com)

A U G U S T 2 0 21

MODULAR DESIGN Courtesy of David Baker Architects

MAKES FOR QUICK CONSTRUCTION

Bay Area Project Redefines Traditional Build Methods By Aaron Miller and Erin Spaulding

W

ith housing demands continuing to grow throughout the Bay Area, developers are looking for ways to deliver projects to market faster, without the added costs. One such solution is multistory modular construction.

The Union project, a six-story residential building in Oakland, California, with five prefabricated wood construction levels, 110 market-rate apartments, parking, and ground-level retail, is that type of modular solution.

Situated near the West Oakland BART Station, The Union offers access to public transportation and valuable housing for a strained market.

While this method of project delivery (also referred to as Factory-Built Housing in the State of California) can reduce construction schedules in half, it requires careful coordination between the design team, on-site contractor, and manufacturer to successfully execute its advantage: speed.

and beams supported by load-bearing stud walls for gravity and wood sheathed diaphragms and shear walls for lateral. The difference with modular construction is that each module comes with a floor, ceiling, and at least four walls. When stacked on top and next to each other, the assemblies are double-wide, and the workers lose access to make connections in some areas. Therefore, attention is required in the design to consider and understand access, sequencing, and the factory process, as well as designing for typical joists, beam elements, and studs.

With modular construction, two construction projects are happening in sequence. While the on-site portions are being built, the factory is building modules. A factory’s capacity is considered for timing with the site construction schedule, including site preparation, foundations, and, where included, the podium construction.

Modular Construction

Factory Process

Design for modular housing varies widely on materials used, whether the units are entirely built out pre-installation or finished on-site, installation limitations, and the specific manufacturer. That said, factories are predominantly using wood framing because it has historically been the most common material for prefabricated housing.

Many of these multi-story modular factories have transitioned from prefabricated homes, education and worksite trailers, man-camps, etc.

Modular construction utilizes conventional gravity and lateral systems familiar to a multi-story wood residential building, including joists

For factory processes, it is necessary to understand how the workstations are set up. These stations include floor framing, ceiling/roof framing, bearing wall framing, non-bearing wall framing, MEP installation, and vertical and horizontal finishes. Understanding the factory scope of work, completed or partial assemblies, and level of finish is important prior to design completion to provide proper load paths and connections made in the field. For example, the ceiling is built separately from the walls in the factory, typically with the ceiling gypsum already installed. Therefore, the ceiling needs to incorporate plywood bearing strips to ensure the ceiling gypsum is not in the load path and crushed when it is lifted onto the walls or when other live loads are added in the future.

The Union project manufacturer, Factory OS, located in northern California, built four modules a day on average. This rapid schedule requires prompt coordination and resolution for construction questions that arise during factory production. An issue identified at the floor station (often the first station) may no longer be accessible after 48 hours. It would need to be resolved after the module has finished its other stations and moved through the factory. During this project, the modular Structural Engineer of Record (DCI Engineers) and Factory OS directly communicated to quickly resolve issues and questions while keeping the rest of the design team and contractor in the loop. Confirmation RFIs were issued shortly after resolution for official documentation of these changes.

Figure 1 shows examples of a standard detail replicated in the construction documents, clearly identifying the in-factory work

Figure 1. Typical horizontal and vertical assembly at the module interface. Factory scope (left) and site scope (right) shown in two details.

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in the first detail and the necessary materials and attachments in the second detail for the required on-site work.

Connection Access Challenges

Putting it all together is the last piece of the puzzle. Although making the on-site Construction Sequence connections is considered “the last step,” it It is not uncommon to have two Engineersneeds to be considered early in the design of-Record (EOR) for a single project with the because access is extremely limited in moduscope delineated between supporting struclar construction compared to a traditional ture and modular structure. For The Union, site-built wood structure. Specifically, the Murphy Burr Curry (MBC) designed the supunits typically arrive with interior finishes porting structure and the site-built steel exterior completed, including walls, ceiling, floor stair. As the modular EOR, DCI designed and finishes, paint, trim and interior fixtures, specified the anchors, including size and spacand appliances, so there is no available access ing, to connect bearing and shear walls to the from the units' interior without planning on transfer slab. Standard 5⁄8-inch-diameter anchor unsightly access points or areas requiring bolts secured flat 2x members that the modules field finishing. Instead, connections must be Figure 2. EOR scope delineation at the concrete podium. would later bear on. To resist uplift forces at accessed from the exterior of the individual shear walls, custom steel connection plates for lightly loaded hold-downs units and corridors left partially unfinished. were designed by DCI, while MBC provided the embeds for attachments.

For The Union project, the only place to make connections were between The attachment methods for the tie-down system (TDS) in the heavily the units during crane-setting the modules, from the corridor and from loaded corridor shear walls were also provided by DCI, which includes the exterior where the modules interface after the set was concluded. DCI panels to access the TDS-to-concrete embeds (once again designed by designed the hold-down connections in the modules' long direction to MBC) that would have otherwise been inaccessible after the module was be accessed from the exterior or the corridor to avoid additional conneclifted into place. Embeds were provided because the modular set sequence tions between the modular units and subsequent delay of the crane set. prohibited the use of conventional TDS anchor bolts, which would have This was accomplished by designing the mateline shear wall (long sides protruded from the slab and interfered with setting the module. of modules) to exterior wall interfaces (corners of modules) so that shear While the site EOR designed the steel embeds required for the custom could be transferred to the exterior studs for hold-down connections, plate and TDS hold-down connections, DCI agreed to specify the welds which had to be accomplished in the factory during wall installation. from the components above. Figure 2 shows a typical delineation in A significant connection that required intense coordination was the the scope between site EOR and modular EOR. Understanding these floor rim joist connection to the sleeper at the long interface between scope delineations is unique to modular construction and extends to modules. As shown in Figure 1, the upper right module (indicated as the factory versus field installation. Other challenges for this project box D) is the last module placed in this detail, and there is no way to included designing in a high seismic zone like the Bay Area and the provide a connection from the floor rim to the sleeper below. In this required ductility of connections. Modular systems share these chal- detail, the crane set sequence was coordinated between the engineer, lenges with traditional field-built wood construction. Still, they require modular fabricator, and general contractor to ensure the module Box special consideration for access because the connections are made after C contained the shear wall at this interface. This way, shear clips could the fully finished modules are installed at the construction site. be installed from the floor rim joist to the flat 2x sleeper below, which Before construction even began, the team considered design for delivery is then plate nailed into module Box A for a positive load path. The and getting the general contractor involved earlier for feedback on the only time to install the shear clips was during the crane set, while the module set sequence. This required coordinating the on-site structural floor rim was accessible, after which they became inaccessible when connections with the crane set sequence, which needed verification by the the next adjacent module was set in the sequence.

General Contractor (GC) as soon as possible. Based on crane availability, As a result of the coordinated efforts, The Union capitalized on location, and space around the site, the GC may request a crane set order modular construction to expedite housing to the Bay Area’s highdifferent from initial design assumptions. For The Union, the assumed demand market. While the speed of modular construction has its set direction during design was east to west for the large building, but it obvious advantages, it takes a combination of advanced ended up going west to east for access and staging, requiring engineers communication, multi-discipline coordination, and a unique to switch the shear wall locations in the modular units after the build- level of trust between project stakeholders to succeed.■ ing permit was issued. Late changes like this required a resubmittal to Aaron Miller is a Senior Project Manager at DCI Engineers and is based in the State jurisdiction for approval. The modules were set starting at the the firm’s corporate headquarters in Seattle. Aaron conducts informational site-built exterior stair (designed by site EOR) and then outward because presentations about modular construction to AEC professionals and is an of the new set order. However, the exterior stair had to be built after the active member of the American Society of Civil Engineers. module set to avoid interference with the crane swing.

Erin Spaulding is a Communications Specialist in DCI Engineers’ corporate To maximize the overlap in the construction schedule, thus saving total headquarters in Seattle, a freelance writer, and an FAA-certified drone pilot. construction time, the modular units' delivery time and the contractor schedule for having the site ready on time is critical. This requires frequent Project Team GC and modular manufacturer coordination, such as weekly coordination meetings on delivery timing and crane sequence. This is to ensure that Owner: Holliday Development Supporting Structures: if the timeline needs to be pushed, the coordinated schedules still work.

Modular Engineer: DCI Engineers Murphy Burr Curry Ideally, modules will arrive as the site is finished; however, the next goal is Architect: David Baker Architects Factory: Factory OS to have the modules ready and staged close by for immediate installation Modular Consultant: Prefab Contractor: Cannon if there is a delay on the site.

A U G U S T 2 0 21

NAVFAC Delivers a Resilient Remote Hangar

By Yuriy Mikhaylov, S.E., and Frank K. Humay, Ph.D., S.E.

T

Figure 2. North elevation of the completed hangar.

he U.S. Pacific island territory of Guam is undergoing a massive transformation as part of a multi-billion-dollar realignment of Okinawa-based U.S. Marines throughout the Pacific. Once completed, the new Marine Corps Base Guam will be in the village of Dededo, but supporting facilities are being constructed at various locations around the island. One such facility is the first U.S. Marine Corps aviation support and maintenance hangar on Guam and is located at the North Ramp of Andersen Air Force Base. The new hangar is a 72,500-square-foot, $53.7 million facility that supports Marine Corps aviation squadrons. The project was delivered by Naval Facilities Engineering System Command as a design-build procurement. Designing for resiliency is essential on this remote island – regularly subjected to strong typhoons, large earthquakes, and a highly corrosive tropical environment. Equally important is ensuring the design can be efficiently constructed using limited available local labor and resources.

pockets outboard of the hangar bay, four panels on each side. The door pocket enclosures support and protect the door panels and provide shear walls to stabilize the building’s front in the longitudinal direction (Figure 3).

The long span across the main hangar door opening, combined with heavy concrete roof and precast concrete cladding loads, required a steel box truss that protrudes 15 feet above the main hangar roof.

This step-up creates a parapet-like effect on the hangar’s main roof, resulting in extremely high uplift and downward wind pressures.

The hangar bay’s interior has an overhead bridge crane with very stringent deflection criteria for the supporting structure (less than a couple of inches on spans greater than 100 feet), and the maximum variance between the crane rail supports cannot exceed ½ inch. Careful coordination with the crane supplier and steel fabricator was essential to meet these requirements.

Building Description

NAVFAC required the entire exterior building shell to be constructed from concrete to enhance long-term durability and reduce future maintenance needs. Given this directive and high seismicity, the design-build team’s most efficient option was a bearing wall system using cast-in-place concrete shear walls supported on shallow foundations. The main hangar bay roof is framed with steel wide flange purlins, metal deck, and concrete topping supported by structural steel trusses. The trusses are supported by a concrete wall at the back of the hangar and a structural steel box truss, comprised of heavy wide-flange sections, spanning across the hangar main door (Figure 4). Steel towers support the box truss on each end of the door opening, and 65-foot-tall reinforced concrete shear walls enclose the sides of the hangar bay. All structural steel, metal deck, and high-strength bolts are hot-dip galvanized and finished with a high-performance coating system.

Behind the main hangar bay, the two-story administrative and shops portion of the building is framed with precast concrete double tees with a composite concrete topping spanning the building’s width.

Concrete shear walls support the double tees.

The overall facility is 414 feet long by 176 feet wide by 65 feet tall and consists of three areas: the hangar bay, the shop and maintenance area, and administrative spaces. The single-story hangar bay has interior clear dimensions of 325 feet by 125 feet by 39 feet (to the bottom of the bridge crane) with 46-foot-wide door pockets on each side of the door opening (Figure 1). The facility’s shop maintenance and administrative portions are located at the rear of the hangar in a two-story structure (Figure 2). The two-story structure’s exterior dimensions are 272 feet long by 50 feet wide by 33 feet tall.

The main hangar doors can fully open, creating an uninterrupted 325-foot-wide opening. The 40-foot-wide door panels stack into

Primary Structural Systems

Typhoon Winds and High Seismic Criteria

Figure 1. Interior of the completed hangar bay.

STRUCTURE magazine

The hangar is designed in accordance with the 2012 International Building Code (IBC) and the American Society of Civil Engineers’ ASCE 7-10, Minimum Design Loads for Buildings and Other Structures, for a 3-second gust wind speed of 195 miles per hour and seismic accelerations of SS = 2.79g and S1 = 0.68g. Wind speed-up effects due to the project’s proximity to a coastal cliff (topographic factor, Kzt, of 1.45) and the hangar designation as partially enclosed significantly increase the design wind pressures. Maximum components and cladding roof uplift

pressures exceed 400 pounds per square foot. The structure is assigned to seismic design category D with a seismic response coefficient, Cs, equal to 0.37. Addressing both extremely high wind and seismic forces in a long-span structure is a unique challenge that requires a careful balance to arrive at an optimal structural system. One example is the addition of concrete topping to counteract high roof uplift loads. Whereas it is beneficial for resisting typhoon winds, the added mass has the opposite impact on seismic performance by increasing the building’s inertial forces. The optimum amount of concrete thickness was determined through analysis and design iterations.

Constructability Challenges The constructability of the structural system using limited, locally available labor and resources was a major consideration. Since there are no steel fabricators on Guam, all steel was fabricated, galvanized, and shipped from the continental U.S. Careful coordination between the steel fabricator, erector, galvanizer, and design team was essential, which included fast-tracking the structural steel package to accommodate shipping to Guam. The primary trusses and box truss were constructed on the U.S. mainland and then broken down into individual members for shipping to minimize potential fit-up issues once in Guam.

A second challenge was the limited number of qualified welders in Guam. This was addressed by designing bolted steel connections wherever possible. Heavily loaded connections for the box truss required a considerable number of bolts, the largest of which had over 400 hundred bolts in one connection (Figure 5, see cover).

Construction of the hangar door pockets presented the third challenge. Since the entire hangar must be fully accessible, the door panels slide into pockets outside the hangar footprint. The individual door panels slide between the steel columns supporting the box truss, and, as a result, the columns have large unbraced lengths. Using steel braced frames on each side of the door pocket for lateral force-resistance was considered but ultimately abandoned due to difficulties meeting the stringent high seismic requirements with members having such long unbraced lengths. The final design consists of very thick special reinforced concrete walls that form the door pocket shell.

With wind pressures exceeding 200 pounds per square foot, the out-of-plane stability of the 65-foot-tall hangar side walls posed the fourth challenge. Three different options were initially explored: one-way walls spanning vertically from foundation to roof, two-way spanning walls with concrete pilasters, and two-way spanning walls braced within the wall clear height to the roof. The contractor selected the third choice to minimize the thickness and reinforcement in the walls without complicating the formwork. Five lines of sway frames,

Figure 3. South elevation of the completed hangar.

equally spaced across the hangar’s width, are used to brace the walls to the roof diaphragm.

The 325-foot-long box truss was designed and fabricated with a significant initial camber at mid-span that was expected to flatten as the box truss was progressively loaded. The camber was induced during fabrication by precisely adjusting the geometry of the individual members and connections. This large camber presented the final challenge – how to best sequence erection to ensure that, once the structure is complete, the estimated final camber is attained and the precast cladding panels are level and properly aligned. Developing a workable strategy involved close coordination between the design team, general contractor, cladding manufacturer, and precast erector.

First, the box truss was erected and supported using six shoring towers.

Next, the main roof trusses, purlins, and metal deck were set, and the concrete topping poured. The weight of the roof structure removed 6.25 inches of camber. The box truss was then preloaded with concrete blocks that approximated the precast panels’ weight to force the box truss to deflect into its final position (Figure 5, see cover). After each precast panel was aligned and erected, the concrete blocks representing the weight of that panel were removed. The box truss deflections were carefully monitored in the field over several months, and adjustments were made during construction where required.

As one of the most remote places in the U.S., Guam has been a strategic center for U.S. military operations in the Pacific for decades.

The island is regularly subjected to powerful typhoons and large earthquakes, and, with relief support far away, the buildings in Guam must be durable, self-sufficient, and resilient. The Guam Aircraft Maintenance Hangar is one such resilient facility that supports forward operations and maintenance functions for the U.S. Marine Corps now and in the future.■ All photos courtesy of Pernix Guam, LLC. Yuriy Mikhaylov is a Project Manager and Special Inspector at BASE and is based in its Guam office. (ymikhaylov@baseengr.com) Frank K. Humay is Vice President at BASE and is based in its Honolulu office. (fkh@baseengr.com)

Project Team

Figure 4. Box truss during construction.

Executor/Client: Naval Facilities Engineering System Command – Pacific Structural Engineer of Record: BASE Construction Manager/Contract Administrator: OICC Marine Corps Marianas Special Inspector of Record: BASE Architect of Record: BRPH Companies, Inc. General Contractor: Pernix Guam, LLC Precast Concrete Supplier: Rocky Mountain Precast, Guam Steel Fabricator: R.F. Stearns, Inc. A U G U S T 2 0 21

NEWS of note John Chrysler Retires as STRUCTURE’s Masonry Advisor

J

ohn Chrysler, P.E., FTMS, is stepping down as the masonry advisor to STRUCTURE® magazine’s Editorial Board.

John has been in the masonry industry for over 50 years, evenly split working for a large masonry contractor and, most recently, as Executive Director of the Masonry Institute of America (MIA).

John is a licensed P.E in California and John Chrysler, P.E. Arizona and a Certified Structural Masonry Special Inspector for both ICC and the State of California.

While at MIA, John has served in several roles including President of The Masonry Society, member of the Board of Directors, and many committees. John is currently Chair of TMS 402/602, Building Code Requirements for Masonry Structures. Along the way, John has shown a passion for masonry Quality Assurance by serving on the ICC SMSI and DSA SMSI Exam Development Committees

and authoring several publications, including the Reinforced Concrete Masonry Construction Inspectors Handbook. Regarding his tenure as masonry advisor for STRUCTURE, John said, “It has been my privilege to participate with my esteemed colleagues of STRUCTURE to recognize masonry as an excellent structural material, but it is time for me to pass the baton to the next generation. I do not doubt that Nick Lang will do an excellent job representing our industry”.

Nicholas Lang, P.E., is the Vice President of Business Development for the National Concrete Masonry Association, where he oversees technical education, development of technical resources, and communications. He is active in many technical associations. Nick is the Chair of ASTM Subcommittee C15.03 on Concrete Masonry Units and Related Units, the chair of ASTM Subcommittee C12.07/C15.07 on Laboratory Accreditation, and a member of various ASTM Committees including C12 on Mortar and Grout, E05 on Fire Standards, E06 on Performance of Buildings, and the ASTM Committee on

Standards (COS). In addition, he is a member of The Masonry Society and the American Concrete Institute and was previously the chair of ACI/TMS Committee 216 on Fire Resistance.

Mr. Lang has a bachelor’s Nicholas Lang, P.E. degree in Materials Science and Engineering from the University of Pittsburgh and is a registered professional engineer in Maryland.

John Dal Pino, STRUCTURE’s Editorial Chair, had this to say, “John was a valuable member of the Editorial Board and a true professional in every sense of the word. We will greatly miss his contributions and expertise. On several occasions, he was able to, nearly single-handedly, fill our May Masonrythemed issue with articles. We are looking forward to working with Nick, John’s handpicked successor.”

Please join STRUCTURE magazine in congratulating John Chrysler on his service and welcoming Nicholas Lang to the team.■

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Figure 1. The curved elevated roadways surround the Meow Wolf building on all sides at the third level; as a result, the shape of the building is defined by them.

Managing Uncertainties with

COLLABORATION The Meow Wolf Denver Project

U

ncertainty and risk are inherent components of any design and construction project. Often, the project team’s management of that uncertainty and how the team behaves during the design and construction process determine the project’s overall outcome.

Although this concept may seem obvious to project teams, successfully implementing this integration on complex projects proves challenging. By building trust and setting the stage for an integrated and collaborative team environment, the Meow Wolf project team banded together to mitigate uncertainties and solve the engineering challenges of this distinctive project.

Project Overview The Meow Wolf Denver structure is five stories of steel framing, plus roof, supported by drilled pier foundations. As a Build-to-Suit (BTS) development, the structure was designed to meet the needs of the tenant, Meow Wolf, a Santa Fe-based arts and entertainment company. Meow Wolf creates immersive, interactive experiences to transport audiences into fantastic realms of story and exploration.

This immersive experience for the building design started with the site selection.

One of the primary project constraints is the site’s location underneath a series of elevated roadways. These elevated roadways (Colfax Avenue and Interstate-25) are two major transportation thoroughfares in the heart of downtown Denver. The building’s shape is defined by these curved, elevated roadways that surround it on all sides at the third floor. The exterior skin is inset from the roadways by 10 feet (Figure 1).

By Shaun Franklin, P.E., MBA, John Jucha, P.E., and Julie Wanzer, LEED AP

The other primary design constraint is that the BTS design needed to allow for a multiphase design process to be erected in a single seamless flow. Meow Wolf ’s vision for the experience included catwalks, raised platforms, spiral and rolled stairs, and steel art structures throughout the space. Large column-free spaces were a requirement to accommodate these interior components.

Collaboration with Continuity A collaborative project mindset is ultimately carried out by the project team members and relies upon external and internal team interactions. Working closely with the designers and sculptors within the Meow Wolf organization, Shears Adkins Rockmore (SA+R) architects led the design team. Turner Construction was engaged as the CM/GC early in conceptual design to provide feedback and constructability input. In addition to external collaboration among various team members, internal collaboration was accomplished within KL&A itself. Unlike a more typical consulting engineering project role, KL&A’s scope for Meow Wolf extended beyond just the engineering portion and included three separate design and construction contracts.

KL&A provided design engineering services for the core and shell (C&S) for the building owner and developer, engineering design for the Tenant Improvements (TI), and finally, engineering services for many of the art exhibits and installations. In addition, given the complexity of the building and the inseparable nature of the final exhibits, KL&A was also selected to provide, as builder, the steel construction on the C&S, TI, and a few of the exhibits. This continuity within A U G U S T 2 0 21

Figure 2. Main entry elevation, detailing the elliptical arched entryways.

KL&A’s team for the overlapping phases of the project allowed for a seamless collaborative design and construction process.

Dual Design Criteria Challenges The project team was tasked with designing the structure for not only interactive art installations but also to have the capacity to be a storage facility for possible future use. This dual functionality created some unique design criteria challenges, including a large building grid, heavy storage floor loads (250 psf un-reduceable live load), and large floor openings that would need to be infilled for the conversion to occur. In addition, for the initial program, Meow Wolf expressed the need for anchor exhibits within the museum that would stand multiple stories high. This created the need for long-span floor beams and design loads to accommodate structures within the structure. The design loads for the multi-story exhibits and large-scale rockwork theming surpassed the storage load requirements in many cases.

Peter Kelly, P.E. from KL&A, and Project Engineer on the Meow Wolf project, described this explorative design criteria process as a lesson

Figure 3. The interweaving catwalks and viewing platforms caused the structural design team to consider the effects of differential and compound deflections along these catwalks.

STRUCTURE magazine

learned. “A big lesson learned for me was that unique challenges are really tough to solve in a vacuum. Often the challenges on this project were so unique that the problems themselves were tough to define.

Having the whole design and construction team meeting together with the owner, frequently, to define the ultimate goal of the design and determine the most efficient way to achieve that goal was critical.”

Distinguishable Structural Design Elements Composite steel construction was chosen to accommodate the need for a versatile structure that can support multiple occupancies, unique geometry, and future retrofit. The heavy column loads resulting from the open building grid and heavy storage loads required concrete drilled pier foundations. The first floor is slab-on-grade over rammed aggregate pier ground improvement, employed due to an abundance of undocumented fill that would otherwise require a costly 15-foot over-excavation. The main wind force resisting system (MWFRS) combines ordinary concentric braced frames and ordinary reinforced concrete shear walls at the stair cores. The stair cores were constructed using the Vulcraft RediCor system to speed construction and eliminate the need for formwork adjacent to the elevated roadways.

Another distinguishable structural design element is the three-story open volumes in the C&S structure that Meow Wolf will use for their most dramatic anchor exhibits. The primary wide-flange columns along the building perimeter in these areas are unbraced from Level 3 to the Roof with horizontal HSS wind girts dividing the span of the cold-formed steel stud exterior wall. Wind columns were added between primary building columns to reduce the horizontal wind girts’ span while also taking some wind load off the primary columns. For the future storage facility design case, the floor openings would be infilled with composite steel construction and wind girts removed, taking the wind load demand off the primary building columns and reducing their unbraced length but adding substantial axial load. The multitude of load cases and boundary conditions that varied between the Meow Wolf and storage facility cases were analyzed and enveloped to arrive at the final design.

One of the most iconic architectural moments as one approaches the building is the elliptical arched entryways carved from the exterior CMU wall. The main lobby entrance is a stunning 50 feet wide x 19 feet tall CMU arch with a glass storefront infill (Figure 2). The length of the surrounding structure on each side of the arch was

Reflecting on the project in its entirety, Evan Forbes, P.E., S.E., Construction Manager for KL&A, concluded, “We learned so much about the critical nature of internal and external collaboration. It was a team effort to not only interpret and protect the vision of these talented artists but also make sure the building is structurally safe, achieves the project goals, and is constructed efficiently.”■ Shaun Franklin is the Business Operations Officer at KL&A, Inc. (sfranklin@klaa.com) John Jucha is an EOR & Associate at KL&A Inc. (jjucha@klaa.com) Julie Wanzer is the Owner of Business Rewritten. (julie@increasingmarketvalue.com)

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insufficient to carry reactions associated with masonry arching action, so the design solution needed to bear the weight of the CMU while also bracing the wall and storefront from out of plane loads. HSS wind girts were faceted around the arch in place of expensive rolled structural members, tying into HSS wind columns that aligned with the storefront mullions for concealment. Bent plates with vertical legs of varying lengths were then attached to the HSS girts to reach and support the bottom of CMU. The facets of the steel support system were coordinated with a storefront manufacturer to ensure a tight fit.

The finishing touch was a plasma-cut steel fascia covering the head of the storefront to complete the curved aesthetic.

Another structural design consideration that necessitated a high degree of team collaboration was the deflection and vibration performance of the serpentine catwalks. The catwalks and viewing platforms, which are part of the TI design, ramp and weave throughout the three-story open volumes in the C&S structure (Figure 3, page 32). The structural design team considered the effects of differential and compound deflections along these catwalks. Some are attached to the building columns, and others hung from the roof above or posted down to a transfer beam below.

The geometry and amount of column transfers complicated the vibration analysis significantly as well. RISA 3D design models of the catwalks were created to run modal analyses and check their natural frequencies in both the vertical and horizontal directions against the recommended design criteria of the American Institute of Steel Construction’s (AISC) Design Guide #11, Vibrations of Steel-Framed Structural Systems Due to Human Activity. As a result, beam sizes were adjusted to meet the vertical criteria. The team found that using the reinforced concrete topping as a diaphragm was the most efficient method of meeting the desired horizontal vibration criteria. At some locations, the curvature of the catwalks was extreme enough to compromise the ability of the catwalks to behave as diaphragms. Consequently, HSS moment frames were added to provide intermediate lateral stiffness.

Community Impact As the project team itself relied on collaboration among internal and external parties, the Meow Wolf Denver museum also serves as a collaborative mechanism within the local community. Lucian Connole from Meow Wolf commented, “I think Meow Wolf is going to be an exciting, galvanizing part of the Denver community because it invites play and discovery for everyone, as well as being a platform for local artists to showcase their work. In that way, it should create opportunities for dialogue about individual and communal expression.”

A U G U S T 2 0 21

INFOCUS Engineering Books for Babies A Review of Some Interesting Titles

By Linda Kaplan, P.E.

G

oodnight Moon (HarperCollins). Chica Chica Boom Boom (Little Simon). The Very Hungry Caterpillar (World of Eric Carle). Great classics of children’s literature written for the youngest babies and gifted to new parents regularly. But how about Future Engineer or Baby Loves Structural Engineering? Also excellent books, written for the youngest babies, but not nearly as well known and primarily gifted to or from an engineer. These deserve a place on every young reader’s shelf, not just those with a direct connection to the field of engineering.

You might be wondering how this relates to the mission of STRUCTURE magazine. We strive to “engage, enlighten, and empower structural engineers.” This includes contributing to and encouraging the expansion of the profession and engaging non-traditional populations, such as babies. Just as many of our editorial board members are parents with small children, so are many of our readers; and if you are not, you likely know someone who is.

When speaking to students, especially at the high school or early college level, it is common to hear, “I don’t know what an engineer is/does.” We may be missing out on some of the best talent simply because the students do not know the possibilities of engineering, and therefore do not pursue the field. Unless they have a family member working in engineering, or an unusually good high school guidance counselor, they are likely never introduced to it. Education and outreach at the high school level are clearly very important to combat this issue. However, engineering books introduced to babies can have a positive impact as well.

Just knowing what an engineer is, even at the most basic level, might be all the push a student needs to investigate further.

This obviously takes the long view, but we suspect it is easier to tackle the issue by getting to the young in their formative years rather than trying for a mid-course correction or a turn at the last moment.

Future Engineer by Lori Alexander (Cartwheel Books) introduces the concept of an Engineer

by drawing parallels to babies’ natural interests, making the field approachable to anyone.

This simple board book features brightly colored paired cartoon drawings of the Engineer working and Baby doing something similar in their own way. A full range of diverse characters, both for the Engineer and for Baby, keep this book current and allow almost any child to recognize themselves within the pages. The story remains neutral to all types of engineering, but the last page does have simple descriptions of six common engineering fields.

Baby Loves Structural Engineering by Ruth Spiro (Charlesbridge) takes a narrower approach, focusing solely on structural engineering, specifically building a house. It appeals to the natural curiosity and basic block-building interests of many babies.

The story follows Baby as they build a house with blocks and uses slightly more technical language than other books. Throughout the story, baby remains a single character, never given an assigned gender, allowing the focus to remain on the building. The house has a foundation that must hold up the walls and all the loads inside; walls must hold up the roof and help stay strong no matter the weather, and a roof keeps everyone safe and dry. It even goes so far as to label the structure as dead load and the cartoon pigs living inside the house as live load. This early introduction of “Engineers want to know how the language of engineering is valuable in making the field approachable as the things work. So does Baby.” child grows. When the language and simple concepts are familiar, even in “Engineers make things that a basic sense, the “scary” factor of an unknown field is greatly diminished. help others. Baby loves to help!”

Possibly more mainstream are children’s books focused on trucks and

STRUCTURE magazine

construction equipment. While not as directly about engineering, these stories still spark interest in how “Where do giant cranes sleep things are built and the construction activities children see around when they’ve lifted their last them, opening the opportunity for further conversation. For example, beams? Do their moms pick a kid getting excited about seeing an them up and rock them excavator or crane on the side of the road may someday be the construcand wish them sweet truck tion engineer directing them.

Where Do Diggers Sleep at Night? dreams?”

by Brianna Caplan Sayres (Random House Books for Young Readers) is a simple rhyming bedtime book introducing different types of trucks and construction equipment. together, and the book is written more at a toddler or older toddler The characters here are the trucks; young trucks are presented level than the others. A series of books have followed, including gender neutral with an almost even split of mom and dad trucks Mighty, Mighty Construction Site, Three Cheers for Kid McGear!, and putting them to bed. Each rhyme gives some information about the Construction Site Mission: Demolition, which expands on the idea of purpose of the subject truck but does not get technical, and the focus job site teamwork and introduces more trucks to the crew, includremains on bedtime. This book does a good job of mixing in less ing a much needed female truck, as the first book, unfortunately, common construction presents all five trucks vehicles with trucks that as male. As a series, the children may be more concepts presented give “Scooping gravel, dirt, familiar with, like the a nice overview of the garbage truck, helping complex construction and sand, Excavator broaden horizons and field and equipment interests. at an age-appropriate shapes the land.”

Goodnight, Goodnight, level, potentially sparkConstruction Site by ing future interest.

“A few more holes to Sherri Duskey Rinker With so many chiland Tom Lichtenheld dren’s books available, dig and soon he’ll roll (Chronicle Books) tells it is easy to write off to bed beneath the the story of five truck ones about engineering friends as they finish up as “niche” or “gag gifts,” moon.” their site work for the assuming they will only day and go to bed. The appeal to parents already rhymes here are slightly working in the profeslonger and include far more details about the purpose and parts of sion. Remember that these books are written for children and not each truck and its role on the construction site. just those whose mom, dad, aunt, or uncle are already exposing The focus here is again on construction rather than engineering them to engineering. Kids are listening and learning about the design or problem-solving. The story provides an opportunity world and opportunities through books. Let’s present them all to learn about the individual trucks and discuss how they work with the opportunity to learn about engineering while they are young so that it is not new or scary when they are older.

Next month, we will discuss books written for preschool and early elementary-aged future engineers. These focus more on the prob“Baby likes to build. Engineers lem solving, teamwork, and design process do, too.” involved in engineering. The gender split also plays a more prominent role in these books as well.■ “What is Baby building?

A house! A house is a structure. It has a foundation, walls, and a roof.”

Linda Kaplan is a Project Engineer with Pennoni in Pittsburgh, PA. She has 2 daughters, ages 3½ and 1½. If anyone has pictures of a Blue Excavator, the older one keeps asking. (lkaplan@pennoni.com)

A U G U S T 2 0 21

structural LOADS A New and Unexpected Roof Snow Drift

By Michael O’Rourke, Ph.D., P.E., and Chris Letchford, Ph.D., F.IEAust, F.SEI, F.ASCE, CPEng

O

ne could argue that drift loads are the most important snow load since they account for roughly 75% of all snow-related structural problems. The various types of roof snowdrifts are reasonably well understood. However, a new snowdrift was recently observed downwind of a run of roof-top refrigeration piping, which did not seem consistent with our current understanding of snowdrift formation. In this article, the drift formation processes for four common snowdrifts – leeward roof step drifts, gable roof drifts, windward roof step drift, and parapet wall/roof projection drifts – are reviewed as well as the apparent formation process for the new roof-top piping-run drift. Finally, an explanation for the apparent inconsistency between the new piping-run drift and common drifts is provided.

Figure 1. Growth of leeward roof step drift with time, t1 < t2 < t3. Wind from left to right.

Leeward Roof Step Drifts

Gable Roof Drifts

Leeward roof step drifts are comparatively straightforward. The Although often referred to as unbalanced loads, gable roof drifts are trapping efficiency (percent of windblown snow arriving at the actually across-the-ridge leeward drifts. Water flume studies suggeometric irregularity which remains at gest that the trapping efficiency of gable the geometric irregularity) is taken to be roof drifts is nominally the same as that for about 50%, based on water flume studleeward roof step drifts. In the American ies. The fetch is the horizontal extent of Society of Civil Engineers’ ASCE 7, the roof upwind of the step. These drifts Minimum Design Loads and Associated have a nominal right triangular shape Criteria for Buildings and Other Structures, throughout the drift formation process, the cross-sectional area of a gable roof drift with the peak drift depth located adjais close to and based upon that for a leeward cent to the step, as shown in Figure 1. roof step drift with the upwind fetch being The drift slope is initially about 1 on 4 the eave to ridge distance for the gable.

(14º), which is thought to be the average Gable roof drifts have a non-right trior typical angle of repose for drifted snow. angular shape, as shown in Figure 2.

This slope is maintained while the drift Figure 2. Expected shape of gable roof drift. Wind from The top surface is more or less flat, with height is less than the step size. When the left to right. the bottom surface matching the roof slope drift reaches the top of the step, the slope and the downwind surface having a slope begins to flatten as the drift continues to grow downwind. When the approximating the angle of repose of drifted snow. Although observed slope reaches about 1 on 8 (7º), the drift shape becomes streamlined, gable roof drifts have a non-right triangular shape described above, they and drift growth nominally terminates as the geometric irregularity has are currently approximated in ASCE 7 by a rectangular surcharge, with been eliminated. That is, absent the snowdrift, mean wind stream- an aspect ratio being a function of the roof slope – longer horizontal lines attach to the lower level roof at roughly 8 roof steps downwind. extent and shallower depth for lower roof slopes.

Gable roof drifts typically do not form if the roof slope is too shallow (less than ½ on 12) or too steep (greater than 7 on 12).

At the lower limit, the roof is so flat that there is no flow separation at the ridgeline, and the attached flow eliminates the aerodynamic shadow where the drift would form. This is generally consistent with the maximum angle of about 4° for the expanding portion of a venturi tube or diffuser to avoid flow separation. Note that the total change in roof pitch at the ridge (upslope at ½ on 12 to downslope at ½ on 12) is about 5°. At the upper limit (greater than 7 on 12), the roof is steeper than the maximum (unlikely to be exceeded) angle of repose for drifted snow (taken to be about 30°). In this case, there would be flow separation at the ridge, and an area of aerodynamic shadow would form downwind of the ridge, but the wind transported snow particle will either roll off or Figure 3. Windward roof step drift shape – drift at parapet wall similar. Wind from left to right. not “stick” to the steep roof.

STRUCTURE magazine

Windward Roof Step Drifts The formation process for windward roof step drifts is more complex than that for leeward drifts because of the presence of a separated flow region and an eddy or trapped vortex upwind of the step that initially drives snow out of that region.

The initial drift shape is non-right triangular, as shown in Figure 3. For 3-D obstructions such as poles or posts, the eddy is termed a horseshoe vortex because of its plan shape. The trapped Figure 4. Snowdrift at a non-elevated Roof Top Unit (RTU). Wind from left to right. vortex initially prevents snow accumulation in this region and leads to the peak drift initially being about one step Unexpected Drift height upwind of the step.

During this initial phase of windward drift formation (Phase I), The authors became aware of an unusual roof-top snowdrift during the wind-transported snow layer, e.g., the saltating snow particles, a recent forensic investigation in the Midwest. As shown in Figure 5, are within the wind streamlines that notionally enter the upwind the snowdrift formed downwind of a run of roof-top cooling pipes. separated region. As such, nearly all of the snow particles stay Somewhat surprisingly, the total load, in pounds per foot (lbs./ft.), for upwind of the wall, and hence, the trapping efficiency becomes the piping-run drift was comparable to that for a leeward roof step drift nominally 100%. with the same upwind fetch and design ground snow load. There were If strong winds persist (wind speeds greater than about 10 mph), several reasons why the drift was both unusual and unexpected. First the windward drift upwind of the step continues to grow, forming of all, the drift was centered about 17 feet downwind of the downwind a snow ramp that effectively “fills in” the upwind separation region. edge of the piping-run. Hence, unlike the well-known leeward roof step This process weakens the trapped vortex and eventually eliminates and gable roof drifts described above, the downwind piping-run drift was it. Then the saltating snow particles join the wind streamlines flow- well removed from the geometric irregularity (i.e., the piping run itself). ing over the step. At this point, the 100% trapping efficiency phase This contrasts with the common leeward drifts described above, which (Phase I) ends, the trapped vortex at the step begins to fill with snow, form adjacent to the geometric irregularity. Furthermore, the vertical and the trapping efficiency drops to approximately 20% (80% of the gap between the bottom of the pipes (pipe diameter nominally 6 to 12 saltating snow particles flow over the step). During this next phase, inches) and the top of the balanced snow surface was about 2 feet. This the windward drift’s shape morphs from its initial non-right triangular seems to contradict the recent ASCE 7 provisions for elevating RTUs to shape (Phase I) to a right triangular shape (Phase II), with the peak avoid windward and leeward drift formation. drift depth being at the step.

It should be mentioned that, as part of the forensic investigation, it If strong winds continue and the snow source remains un-depleted, seems that one structural engineer involved with refrigerated storage the Phase II windward drift becomes aerodynamically streamlined facilities design had observed similar roof-top piping drifts in the past. when the drift reaches the top of the wall and the slope is about 1 on A 2003 report by Ronald Tabler, Controlling Blowing and Drifting Snow, 8. At this point, the geometric irregularity has been removed and prepared for the National Cooperative Highway Research Program for additional drift formation ceases. the Transportation Research Board of the National Academies, presents photographs of drifts downwind of box beam and W beam roadside guard rails. Hence, the snowdrift shown in Figure 5 is not an aberration.

Parapet Wall and Roof Projection Drifts At first, the authors thought the piping-run drift to be an anomaly.

By their nature, the snowdrifts at parapet walls and upwind of However, over time, a plausible explanation for the drift’s “unusual” roof-top units (RTUs) are windward drifts. For RTUs, drifts also aspects developed. form simultaneously at the downwind side, as shown in Figure 4.

One would initially anticipate that snow drifting around roof-top However, for simplicity, ASCE 7 specifies the upwind side windward piping would behave similarly to drifting around RTUs. Both provide drift for that location as well. Recently, ASCE 7 has addressed the an obstruction to roof-top wind flow. However, since the piping’s issue of mitigation of snow drifting at RTUs. In particular, if the crosswind extent is much larger than an RTU, the piping is nominally RTU is raised 2 feet above the balanced snow, this vertical gap is a 2-D obstruction rather than a typical RTU, which would be considassumed to prevent drift formation at the RTU. This provision is ered a 3-D obstruction. Wind flow over the top and around the sides advantageous if a smaller RTU on an existing roof is replaced by a of a typical RTU results in two aerodynamic shadow regions – one new, larger, and heavier RTU. immediately upwind and the other immediately downwind where drift formation occurs. A downwind region of reduced wind speed – a wake region – exists at an RTU, but the downwind extent of the wake region behind the piping-run extends further downwind than that of the RTU. As opposed to forming an adjacent aerodynamic shadow, piping-run drifts form well downstream of the obstruction due to the reduced speed and increased turbulence in the wake.

In relation to the drift mitigation gap for a raised RTU, the wind flows beneath the unit as well as over the top and around the sides. The gap and resulting flow below an elevated RTU eliminates the aerodynamic shadow regions and hence eliminates drift formation on both the upwind and downwind sides Figure 5. Observed piping-run drift. Wind from left to right. of the RTU. Artic buildings are often placed on stilts for this A U G U S T 2 0 21

very purpose. This is consistent with the observed behavior of the elevated piping run; specifically, there was no upwind or downwind drift accumulation immediately adjacent to the obstruction.

As mentioned above, in relation to Figure 5, the gap between the piping run and the snow surface is about 2 feet. Hence, the wake region behind the pipes was reasonably close to the saltating snow particle layer adjacent to the snow surface. If the gap between the piping run and the snow surface were much larger (say 10 feet or more), the wake region would be well above the saltating snow layer. For such a case, this type of snowdrift would be much smaller or non-existent.

Summary This article describes a newly observed roof-top piping-run drift and compares it to common windward and leeward drifts in ASCE 7.

Since there is currently only a single reasonably well-documented case history, the specific influence of certain key parameters is not well understood. If and when additional case histories become available, future versions of ASCE 7 may well address this new and interesting snowdrift.■ Michael O’Rourke has been a Professor in the Civil Engineering Department at Rensselaer Polytechnic Institute since 1974. He served as the Chair of the ASCE 7 Snow and Rain Subcommittee from 1997-2017 and currently serves as the Vice-Chair. (orourm@rpi.edu) Chris Letchford is an international expert in Wind Engineering with experience documenting wind-induced structural failures, simulating novel wind phenomena, and codifying findings for practicing engineers in codes and standards. (letchc@rpi.edu)

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CAST CONNEX Phone: 416-806-3521 Email: info@castconnex.com Web: www.castconnex.com Product: High Strength Connectors™, Cast Bolted Brackets, and Scorpion™ Yielding Connectors Description: High Strength Connectors and Cast Bolted Brackets simplify and improve connections in seismic-resistant concentrically braced frames and moment resisting frames, respectively. Scorpion Yielding Connectors are modular, replaceable, standardized hysteretic fuses for enhanced ductility and improved performance of seismically deficient structures or in the Seismic Force Resisting System of new structures.

STRUCTURE magazine

ENERCALC, Inc.

ENERCALC

Phone: 800-424-2252 Email: info@enercalc.com Web: https://enercalc.com Product: ENERCALC/Structural Engineering Library/ENERCALC Cloud/ RetainPro (retired) Description: SEL automatically incorporates seismic loads in load combinations, including the vertical component, redundancy, and system overstrength factors, as applicable. SEL supports ASCE 7’s Base Shear, Demands on NonStructural Components, and Wall Anchorage.

SEL Build 20 subscriptions now include RetainPro's retaining wall modules – including the substantially upgraded Segmental Retaining Wall module.

IES, Inc. Phone: 406-586-8988 Email: info@iesweb.com Web: wwww.iesweb.com Product: IES Building Suite Description: For less than $200/month, get easy tools to help with lateral design for wind or seismic loading. Tackle foundations, structural frames, and connections. VisualAnalysis provides practical ways to load structures and understand behavior. Design your next project with the IES Building Suite.

RISA Phone: 949-951-5815 Email: benf@risa.com Web: risa.com Product: RISA-3D Description: Feeling overwhelmed with the latest seismic design procedures? RISA-3D has you covered with seismic detailing features including full AISC-341/358 code checks and buckling restrained braces from Corebrace.

Using the automated seismic load generator or built-in dynamic response spectra and time history analysis/design, get designs and reports that meet all your needs.

SkyCiv Phone: 800-838-0899 Email: trevor.solie@skyciv.com Web: skyciv.com/wind-load-calculator Product: Wind/Snow Load Generator Description: Get rid of your design criteria Excel spreadsheet with the SkyCiv Load Generator.

Quickly generate wind and snow design loads for your structure. Take advantage of SkyCiv’s analysis and design tools to complete your structural workflow, directly from an internet browser with no installation necessary.

Trimble MAX USA Corp.

Phone: 800-223-4293 Email: yasaba@maxusacorp.com Web: www.maxusacorp.com Product: PowerLite® System Description: Power beyond the limits of standard 100 PSI pneumatic tools with the PowerLite system. Designed with a lightweight body and engineered for heavy-duty applications, PowerLite tools are built to shoot through steel, concrete, and engineered woods.

Phone: 678-737-7379 Email: jodi.hendrixson@trimble.com Web: www.tekla.com/us Product: Tekla Tedds Description: A powerful design program for automating wind and seismic calculations and performing member designs. Built-in library of calculations for quickly calculating ASCE 7 wind and seismic forces for any structure and component design modules, beams, columns, and foundation designs. Link modules together to create a professional report for review submittals.

SPOTLIGHT The Academy Museum of Motion Pictures

D

esigned by the Renzo Piano Building Workshop, the Academy Museum of Motion Pictures consists of a major renovation to the 1930s May Company Building (renamed the Saban Building) plus a soaring spherical addition that includes the 1,000seat Geffen Theater. The Saban building is connected to the new theater with three steel and glass bridges.

Connecting this base-isolated structure with an existing building introduced significant challenges for Buro Happold, the project’s structural engineer, as well as MEP engineer, lighting and environmental designers, energy modelers, and IT consultant. The two buildings required very different structural solutions.

The theater and terrace are composed of reinforced concrete. To construct the curves, a partial steel structure was introduced to support the exterior precast concrete panels, which then provided formwork for the structural shotcrete on the inside – a time-saving strategy.

A glass-curtain-wall canopy encloses the terrace, completing the spherical form.

For the 150-foot-diameter, orb-shaped spherical addition, in-depth analysis argued for an unusual base-isolation system. The theater is supported by four mega-columns, seismically isolated from the ground. The unique design employs just eight base-isolators set 15 feet above grade and exposed as a design element. Base-isolators allow the Sphere to move up to 30 inches in any direction during an earthquake. The Saban Building’s retrofitting is designed to limit movement to address the seismic challenges.

Because the buildings are designed to move so differently during an earthquake, building components between the two structures – including circulation bridges and stairs – required flexible connections to allow one building to move and the other to stand still.

STRUCTURE magazine

The bridges – the main pathways to and from the Saban Building and Geffen – are anchored to the Saban Building and designed to pivot and slide, moving with the Sphere during an earthquake. At the Saban Building, the bridges connect to pivot and bearing connections that provide vertical and horizontal restraint but also allow the bridges to rotate about a vertical axis when the Sphere moves during an earthquake. At one corner of each bridge, a cylindrical “pivot” connection acts as the center of rotation and locks the bridge to the Saban Building. There is a simple bearing connection at the other corner of each bridge, allowing the bridge to move freely in horizontal directions.

At the Sphere, the bridges connect with sliding tracks that provide restraint in the vertical and horizontal east-west directions and move freely in the north-south direction. When the Sphere moves east or west, the tracks push on the bridges and cause them to rotate about the pivot connections at the Saban Building.

When the Sphere moves north or south, the bridges slide along the tracks, allowing the theater to move freely without applying any load to the bridges. All the connections were custom designed to accommodate the forces and displacements that were determined during the structural analyses of the Sphere, the Saban Building, and the bridges. The connections are also architecturally exposed, and, as such, form and function were both given equal weight during the design process.

The two stair towers, one on either side of the Sphere, are designed to provide egress to the Geffen Theater and Dolby Terrace in the event of an emergency. However, the stair towers sit on the ground. They must also connect to the sides of the Sphere at multiple levels – a significant design challenge because of the differential movement that is anticipated between the Sphere and the ground during an earthquake, up to 60 inches in any direction. Buro Happold developed a structural solution that decouples the gravity and lateral load-carrying systems of the stairs.

Vertical gravity loads are transferred to the ground through long slender columns that anchor to

“stationary” foundations. Lateral earthquake loads are transferred to the Sphere through beams at each floor level. These beams also push on the stairs when the Sphere moves on its isolators. Lastly, hinges are introduced at the top and bottom of each column to prevent lateral loads from being transferred through them and accommodate the Sphere’s movement without damaging the stair structure.

In an earthquake, the Sphere moves on the base isolators below; these hinges allow the stair columns to “lean over” so that the upper portion of the stairs can go along for the ride.

These joints are hinged in two perpendicular axes, which effectively allows rotation and translation in any direction. The column hinges that were developed for the stairs were custom designed to accommodate the anticipated displacements of the Sphere and the calculated gravity forces from the stairs above. The finished column hinges were cast from high-grade carbon and stainless-steel alloys, providing a streamlined construction and fabrication process and ensured a highquality final product that is aesthetically pleasing and will perform over the life of the building.

The Academy Museum of Motion Pictures will be the world’s premier institution devoted to the art and science of movies and moviemaking.

The unique design of the spherical addition, the renovation of the Saban building, connecting the two structures, and an unusual base-isolated system all contribute to this unique Museum’s appeal.■ Buro Happold was an Outstanding Award Winner for the Academy Museum of Motion Pictures project in the 2020 Annual Excellence in Structural Engineering Awards Program in the Category – New Buildings over $200M. A U G U S T 2 0 21

INSIGHTS Parametric Structural Design for High-Performance Buildings

By Demi Fang and Caitlin Mueller, Ph.D.

T

he climate crisis has shifted priorities in all sectors. For structural engineers, improving performance, such as reducing emissions embodied in structural materials, can improve low-carbon building design. Parametric design can enhance current structural design methods by enabling designers to more readily explore the design space, the space of available design solutions, and optimize within it for single or multiple objectives. This exploration can reveal high-performance or optimal structural solutions that may otherwise have been overlooked.

While many architects have started using parametric design methods in recent years, there are untapped Parametric design space visualized for a three-panel truss, showing embodied carbon associated with various opportunities for structural engi- options across geometries and materials. neers to use such approaches to Parametric Design and enhance collaborations with architects and In addition, specific structural metrics for play a more active role in the design pro- material efficiency might include volume or Optimization cess. This article presents both theoretical embodied carbon of materials used. background and practical tips for structural Typical engineering workflows already The parametric design space also offers engineers to implement parametric design incorporate some degree of parametric opportunities for automated optimizain their work. design. For example, most spreadsheets of tion, which returns the best combination engineering calculations follow a parametric of design variables according to specified framework. The cells that the user updates performance metrics, called objectives Parametric Design and with project data are the design variables, in optimization frameworks. Established Implementation while the rest of the spreadsheet may use structural optimization methods include Parametric design is a framework that allows formulas to calculate the single engineering member size, shape, and topology optia design to vary along different quantita- solution associated with the input variables. mization, but parametric design and tive parameters, called design variables. The user can manually explore the design optimization can generally be applied to a Ideally, these design variables capture the space by changing the design variables within range of building problem types. extent of all possible solutions, also known the bounds of the problem.

It is not always practical to select the as the design space. For example, given a Architects are increasingly utilizing tools theoretically optimal design. There may be building massing, the column grid spacings like Grasshopper for Rhino 3D or Dynamo qualitative or hard-to-quantify considerations of a structure may be parameters, with each for Revit to create parametric models directly not accounted for in the objective functions, design variable bounded by feasible spans. into the stages of design and modeling. Both such as the visual preference of the archiThese variables may be continuous, such Grasshopper and Dynamo are visual pro- tectural designers. And, the theoretically as any angle between 20 and 45 degrees, gramming tools that enable users to define optimal solution may not be compatible with or discrete, such as the integer number of geometries based on quantitative parameters current manufacturing or construction methpanels in a truss. and rules. In addition, both tools can con- ods. Because of these inevitable limitations, The value of a parametric framework in nect directly to structural analysis software to navigating the design space offers a powerengineering is systematically comparing compute metrics. By gaining proficiency in ful way to flexibly consider a collection of design alternatives according to one or more these parametric tools, structural engineers better-performing designs in a less automated performance metrics. For example, perfor- can enhance early-stage collaborations and way. In addition, navigation methods can mance metrics can include occupancy, air design decisions with the architects who account for multiple objectives, as in multiflow rate, or energy loads in building design. use them. objective optimization, and offer ways for STRUCTURE magazine

human engineers and designers to interact with generative algorithms.

This is practical because designs within a comparable range of the optimal solutions are often still valuable engineering solutions.

The Figure shows the design space of a 3-panel steel truss loaded at its midpoint; despite determining a geometry that attains minimal emissions, the design space reveals various designs that perform within 15% of the global optimum. Furthermore, if alternate materials are considered during early-stage design, one can see that substituting timber for compression members results in even more options that outperform the optimal steel design. This exploration could be made more realistic by adding a model for economic and environmental cost of connections, but the latter is typically small relative to the rest of the structure.

Navigating the Design Space

The Power of Parametric Design For climate-conscious structural engineering, an understanding of performance in conceptual building design

References are included in the PDF version of the article at STRUCTUREmag.org.

Demi Fang is a Ph.D. candidate in Building Technology at MIT’s Department of Architecture. (dfang@mit.edu) Caitlin Mueller is an Associate Professor at MIT in the Building Technology Program within the Architecture and Civil and Environmental Engineering departments. She directs the Digital Structures Research Group, whose work focuses on new computational methods for designing and constructing innovative structures in the built environment.

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How do you find the regions of high performance in the design space? For lowdimensional problems, this can be done by plotting a solution’s objective score against different sets of design variables and finding regions of variable settings that result in higher performance, as the figure shows.

For more complex problems, more advanced tools for exploring the design space are available. One such tool is a plug-in developed by the MIT Digital Structures Research Group for Grasshopper, called Design Space Exploration (DSE). Given a parametric model in Grasshopper, components of DSE can be used to sample and record solutions in the design space, create approximate surrogate models, evaluate variable importance, and find optimal solutions with automated, interactive, and multi-objective approaches. This tool is available free and open-source (see online References). More recent academic research seeks to enhance these methods with new advances in computing, including VR and sketch interfaces, incorporation of historical design data, and deep learning of relationships between variables and performance (see online References).

is critical. Parametric design space exploration enables engineers to discover comparably high-performing solutions systematically that may not have been previously considered. In addition, the ability to offer multiple solutions in collaborative early-stage design is not only an asset to structural engineers everywhere but can also enhance the structural engineering field at large.■

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A U G U S T 2 0 21

business PRACTICES This Year, Make “Lean” Your Firm’s Buzzword

By Sarah Scarborough, P.E., S.E.

S

tructural engineering firms should embrace Lean thinking, tools, and techniques to reduce excess waste in processes and produce a higher quality product for clients. The Lean Construction Institute (LCI) recognizes the 6 Tenets of Lean Construction: Respect for People, Process & Flow, Eliminate Waste, Generation of Value, Continuous Improvement, and Optimize the Whole.

Implementing a Lean philosophy at your firm might be easier than you think. Simple steps can incorporate these Tenets into your firm’s daily internal practices.

needs and contributions are valued and heard and feel empowered to take ownership of their tasks.

Teamwork, communication, and process problem solving are critical to providing a successful project.

Stop Wasting Time

Waste (Tenet #3) is any effort or resource utilization that does not Generate Value (Tenet #4).

Additional meetings such as huddles are not necessarily a waste.

As long as these meetings are intentional and focused, they can generate value. It requires effort to identify and remove waste from your project workflow. Not Take Stock of Internal Six tenets of lean construction. (www.leanconstruction.org/about-us/lci-tenets) all waste can be eliminated, but Processes standardizing processes and creatEverything we do is a function of a process, staff needs. Project huddles may look a ing a culture of Continuous Improvement whether it is formalized or not. The key is to little different today with the aid of Zoom (Tenet #5) can identify opportunities to understand your current Processes & Flows or Microsoft Teams, but virtual huddles remove waste.

(Tenet #2), or lack thereof, and determine can still be highly effective. The frequency Work smarter, not harder. We must be effiwhat works well, what can be improved, and of project huddles varies based on project cient and effective in how we work. Be aware what needs to be developed. Ask yourself the size, schedule, team preferences, etc. For of opportunities to eliminate unnecessary following: longer project schedules, a weekly or bi- steps in a process and the time spent waiting • How do you track work in progress on monthly huddle may be appropriate. Still, on information and work to eliminate defects a project? as deadlines approach, a daily huddle is and reduce rework.

• How does information get dissemilikely needed to make sure all team memnated to the firm? bers have the information they need to meet Break out Sticky Notes • How do changes get made to your the deadline. standard drafting library?

Discussion topics in a 5 to10 minute huddle Every project is different, but there is always For any process to be successful, there must include: a process to follow when efficiently managing be buy-in from all users of the process. There • What did you do since the last huddle? and executing a project. As a supplement does not need to be 100% agreement on every • What are you doing before the next to your project huddles, step up your to-do step in the process, but there must be agreehuddle? list with a Kanban board to provide a colment that everyone will use and respect the • What is holding you up? laborative and visual way to track project process. Remember – no process is ever finalProject huddles help ensure all team mem- tasks and progress. ized; there is always room for change and bers have the resources to get their job done A basic Kanban Board can be achieved with improvement. The concept of process and flow and identify any roadblocks that may be in sticky notes, a blank wall, three columns; is integral to Respect for People (Tenet #1). the way of delivering their contributions the to-do column, the work-in-progress Your people must be empowered to be critical to the project on time. The project huddle column, and the done column. Define every of the processes and voice ideas and opinions promotes respect for other team members by task on the project with its own sticky note for changes. making the entire project team aware of each and assign it to a team member. Prioritize individual’s constraints, how the tasks of one tasks by moving sticky notes around the person may impact another team member, board and set internal milestones to keep Join the Huddle and the overall schedule of the project. Team the team on track. The Kanban board helps Project huddles that include all internal leaders must convey that this is a forum eliminate waste of Transportation or the team members are a visible way to demon- for identifying project-related concerns or unnecessary flow of information. With the strate that your firm values and prioritizes problems. Team members must feel that their visual aid of the Kanban board, only those

STRUCTURE magazine

tasks ready to be tackled are moved to the work in progress column.

This practice also aids in reducing the waste of Waiting for information delivery or completing a preceding task. The Kanban board shows which tasks must be completed prior to other tasks and can help identify tasks that are stuck waiting for information, prompting the Team Leader to get answers to move the project forward.

Reflect on Project Performance Make time for reflection after completion of a project. A project close-out discussion with the entire team is a great way to generate ideas for better processes, increase the value of future projects, and identify areas that need improvement. Ask everyone their thoughts on what went well and what can be improved going forward.

There must be honest and open communication to get the most out of this discussion.

Openly discussing roadblocks and conflicts as a team allows the group to take action toward improvement for the next project.

Database of Lessons Learned

Save time and generate value by engaging tech-savvy staff to explore the world of add-ins and tools available that interact with 3-D analysis and drafting software – from pyRevit to Bluebeam Studio. Unfortunately, the newest waste is Skills and Underutilized Talent. Do not make the mistake of ignoring peoples’ skills, creativity, or knowledge on a project or their potential contribution to your firm. Instead, build your staff’s capabilities and provide them with growth opportunities for the sake of current and future projects.

Know the Why Lean thinking encourages Optimizing the Whole (Tenet #6). This concept requires constant reflection and analysis to determine if implementation has made an impact.

To analyze if you are generating value, you must first Know the Why. Why are you changing or implementing a new process?

Are you looking to make a task easier? Are you looking to accomplish something faster?

Identify the metric to measure so you can refer to that after process implementation and determine if you are meeting those metrics. Utilizing one of the most powerful tools you have – your own data – can aid in this process.

The Author’s Go-To Efficiency Tools 1. Collaboration App – Microsoft Teams 2. Internal Database – SharePoint 3. Kanban Board – Microsoft Planner 4. Revit Add-In – pyRevit, Ideate 5. Data Analytics – PowerBI 6. Visual Programming for Revit – Dynamo 7. Site Visit Management – PlanGrid 8. PDF Markups – Bluebeam Revu 9. PDF Collaboration – Bluebeam Studio 10. Digital Note Taking – OneNote A Lean philosophy will help your firm make the most of its people and project expertise, keeping old clients coming back and creating opportunities for happy new clients.■

What internal practices can you Lean on today that will help influence your firm’s future successes?

Sarah Scarborough is the Quality Assurance Manager at PES Structural Engineers. She is heavily involved in the firm’s continuing education and onboarding programs and focuses on developing and implementing company processes and standards. (sscarborough@pesengineers.com)

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Everyone probably has a library of technical references and CAD/Revit standards, but how can you take that resource bank a step further?

Close-out discussions help set the scene for continuous improvement through reflection on the project and individual performance, generating opportunities for additional knowledge sharing and training.

Takeaways from these discussions should be appropriately documented, and any action items should be addressed and communicated to the rest of the company.

Establishing an internal database of lessons learned that all company members can contribute to and that becomes a searchable tool for all staff can help foster the culture of continuous improvement.

The practice of documenting common issues or lessons learned can save time on the next project and can guide the next person performing the task.

Similarly, an internal database of sample calculations and design guides creates efficiencies from project to project. No calculation is too complex or too simple to provide a valuable starting point for someone else. Opportunities to contribute to internal databases should be encouraged and welcomed at any time.

Utilize Efficiency Tools and Staff

A U G U S T 2 0 21

NCSEA

NCSEA News

National Council of Structural Engineers Associations

Recipients of First-Ever Diversity in Structural Engineering Scholarship

The NCSEA Diversity in Structural Engineering Scholarship was established by the NCSEA Foundation in 2020 to award funding to students who have been traditionally underrepresented in structural engineering. In the first year of this scholarship, NCSEA received an abundance of qualified applicants and is proud to announce the first-ever recipients of this program. Congratulations to each of the following recipients who each received a $3,000 scholarship, an invitation to attend the 2021 Structural Engineering Summit as a guest of NCSEA, and a $1000 travel stipend generously sponsored by Computers & Structures Inc. Aime Nacoulma, Oregon State University, is pursuing a bachelor's in civil engineering.

He is passionate about structures and plans to embrace the field of structural engineering with the goal to work on public infrastructures such as hospitals or schools.

Aime is building his leadership experience with active involvement in the ASCE student chapter, STEM Leaders program, and the Student Government at OSU.

Jessica Brown, University of Southern California, is a senior studying Civil Engineering. As a native of the Washington, DC area, as well as a student in Los Angeles, Jessica has seen how infrastructure affects the lives of city residents. Living in these cities has inspired her to use her curiosity, love for problem solving, and passion for the public's well-being to create impactful solutions to complex urban problems.

Jessica Gonzalez, University of Texas at San Antonio, is a graduate student conducting research aimed at quantifying changes in the seismic collapse risk of concrete structures associated with switching from conventional reinforcing bars to Niobiumbased. She is looking forward to becoming a licensed SE to help her community by considering the balance of safety, economy, and sustainability. Jessica wishes to influence young minority women to conquer their dreams no matter the present stereotypes.

Juan Vera-Bedolla, Rowan University, is a rising senior Civil and Environmental Engineering student. Juan plans to get his Master’s in Structural Engineering through the Masters program at Lehigh University.

His interest is in designing new buildings and reconstructing infrastructure. Juan's goal is to work across the globe helping to improve the infrastructure in developing countries.

Are You Ready for the Next NCEES PE Structural Exam?

The next NCEES PE Structural Exam is October 21 and 22; start preparing with NCSEA’s on-demand course. The SE Refresher & Exam Review Course is the most economical PE Structural Exam Preparation Course available with 30 hours of instruction, preparation tips, and problem-solving skills to pass the exam. All lectures are up-to-date on the most current codes, with handouts and quizzes available.

This NCEES PE Structural Exam Preparation Course allows you to study at your pace but with instant access to the material and instructors through the exclusive virtual classroom. Several registration options are available; visit www.ncsea.com to learn more.

Now Accepting: 2021 Young Member Award Applications Young Member Summit Scholarships

Young Member Group of the Year Award

Each year NCSEA awards Scholarships to deserving up-and-coming Young Member leaders in the industry to attend the Structural Engineering Summit. This year's recipients will receive free registration to the Summit which includes all educational sessions, access to the trade show, and an invitation to the NCSEA Awards Event, as well as a travel stipend of $1,000 to use toward transportation and hotel costs. The scholarship competition is open to any current member of an NCSEA Member Organization who is under the age of 36.

This award recognizes Young Member Groups that are providing a benefit to their young members, member organization, and communities. Each finalist will receive complimentary registration to send a representative to the Structural Engineering Summit.

Finalist groups also will receive a $1,000 travel stipend to use toward transportation and hotel costs. The winning Group will be announced at the Summit and will receive an additional $2,500 for their Young Member Group to use for future activities.

The funds for these awards have been generously sponsored by Computers & Structures Inc. To learn more and apply, visit www.ncsea.com.

STRUCTURE magazine

News from the National Council of Structural Engineers Associations Will We See You in New York? | 2021 Structural Engineering Summit The Summit will host attendees in New York City at the Hilton Midtown on October 12-15, featuring an industry leading Trade Show, unique networking opportunities, and, of course, remarkable education led by industry experts. Our host hotel, the Hilton Midtown, is located in the heart of midtown Manhattan just a short distance from Central Park, Rockefeller Center, Radio City Music Hall, the Museum of Modern Art, and Carnegie Hall, and just a few blocks away from Times Square, Broadway theaters, and the renowned Fifth Avenue shopping district. Reservations can be made at the Hilton Midtown with NCSEA's guest rate until September 20, 2021. Visit the Summit section of www.ncsea.com to reserve your room.

NCSEA is overjoyed to bring attendees back together in-person this year, and we hope to see you in New York. But we are also hosting a virtual Summit alongside the in-person event.

Whether attending in-person or virtually, attendees will have access to equally immersive and exceptional events. The Virtual Summit will span from September 27 to October 21 featuring the same high-quality education, unique networking opportunities, and the ability to connect with industry leading service and software providers. The entire slate of education is now live on www.ncsea.com featuring topics like steel design, structural fire design, seismic design and resilience, licensure, and more! As 2021 marks the 20th anniversary of 9/11, this year's opening keynote will feature panelists who were involved in the initial recovery efforts and the subsequent building performance study. 20 Years After 9/11: Lessons Learned & More to Research will discuss the response of the engineering community, building performance, structural design, fire protection, emergency response legal issues, and building code changes, focusing on what has changed and what changes are still recommended. During Thursday's keynote, attendees will learn how to create levity in the workplace with tools to help reach personal and professional potential through the power of fun.

Join us for this one-of-a-kind event that draws the best of the structural engineering profession, highlights structural engineering innovation and ingenuity, honors outstanding service and commitment to the organization, and illuminates the necessity of the practicing structural engineer. Register at www.ncsea.com.

Over 30 Live Webinars + Recorded Library = Affordable PDHs

NCSEA's Webinar Subscription Plan is available to members and nonmembers alike. By subscribing to NCSEA webinars, you are subscribing to the highest-quality webinars developed by leading experts at an incredible value (as low as $30/hour). With at least 30 live webinars per year and a recorded library of over 170 webinars, NCSEA's Webinar Subscription Plan is designed for the individual engineer as well as the firm; no matter the size, this subscription plan can work for you!

Webinars are available whenever, wherever you need them. Multiple users at the same office, together or remote, can take advantage!

Subscribe now by visiting www.ncsea.com and don't miss another webinar in 2021!

NCSEA Webinars

Register on www.ncsea.com

August 5, 2021

August 26, 2021

Ben Nelson, P.E.

Jennifer Laning, P.E.

A Common Sense Approach to “Deferred Submittals & Delegated Design” August 19, 2021

The Basics of Facade Access Equipment: Design & Testing

Inspectability Design: Bridge Life Cycle Cost Savings September 16, 2021

Timber Engineering for Structural Engineers Jim DeStefano, P.E., AIA, F.SEI

Gwenyth Searer

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

follow @NCSEA on social media for the latest news & events!

A U G U S T 2 0 21

SEI Update Advancing the Profession

SE2050 Commitment Update

Learn about the impact of embodied carbon in structures on climate change and join the movement to get to net zero emissions! www.se2050.org

NEW ASCE Manual of Practice 142: Structural Design for Physical Security

Evolving threats from bombings, civil disturbances, biological attacks, and other malevolent events have raised the importance of physical security at buildings of all types, from traditional municipal buildings to “soft” targets like cultural venues and stadiums. Structural Design for Physical Security provides an overview of the typical design considerations encountered in new construction and renovation of facilities for physical security.

Editors Van Eepoel and Gallant note that “the constant evolution in aggressor profile, threat tactics, and types, combined with advances in the field of mitigation strategies, were the driving force behind this manual of practice, developed as a guide for protective design professionals.” This book serves as a replacement for the 1999 technical report Structural Design for Physical Security: State of the Practice and is intended to provide a roadmap for designers and engineers involved in physical security to better account for the extent of damage and plan upgrades that will maintain structural integrity.

Key topics include threat determination, how structural loadings are derived for the determined threats, design of structural components, function and selection of structural systems, as well as window and façade components, retrofitting, testing methodologies, and bridge security. This book will be a valuable resource to structural engineers and design professionals involved with projects that have physical security concerns related to explosive, ballistic, forced entry, and hostile vehicle threats. www.asce.org

Preparing for Wind and Water

Subscribe to Daily Newsletter at source.asce.org

Engineers have played an essential role in preparing cities and states for the 2021 hurricane season. Their efforts to protect the public and public infrastructure range from sea walls to sewer systems, with an emphasis on sustainability. Among the projects is the Sabine Pass to Galveston Bay Coastal Storm Risk Management program, which will involve raising approximately 5.5 miles of the existing 27.8 miles of earthen levees and replacing approximately 5.7 miles of flood wall in Galveston, Texas. By Robert L. Reid Read the article at https://source.asce.org/engineers-help-prepare-for-hurricane-season.

One source for all your news in an all-new digital platform and AI-based newsletter.

Join us this year in celebrating 25 years of SEI – advancing and serving structural engineering!

Errata STRUCTURE magazine

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

If you would like to submit errata, contact Kelly Dooley at kdooley@asce.org.

News of the Structural Engineering Institute of ASCE Membership

Welcome to new SEI Graduate Student Chapters Congrats on establishing SEI Grad Student Chapters to: • American University of Sharjah (UAE) • University at Buffalo • University of Cincinnati Local SEI Chapters and Graduate Student Chapters serve member technical and professional needs through events, field visits, construction tours, scholarships, K-12 outreach, and more.

Graduate Student Chapters engage students, enhancing education, and transition to practicing professionals. Learn more at www.asce.org/SEILocal.

Congratulations to the 2021 ASCE Distinguished Members SEI Members Highlighted Lilia A. Abron, Ph.D., P.E., BCEE, NAE, Dist.M.ASCE Gregory B. Baecher, Ph.D., Dist.M.ASCE Roberto Ballarini, Ph.D., P.E., F.EMI, Dist.M.ASCE Glenn R. Bell, P.E., S.E., F.SEI, Dist.M.ASCE John D. Hooper, P.E., S.E., NAC, F.SEI, Dist.M.ASCE

Joe D. Manous Jr., Ph.D., P.E., D.WRE, F.EWRI, Dist.M.ASCE Satish Nagarajaiah, Ph.D., F.SEI, Dist.M.ASCE Julio A. Ramirez, Ph.D., P.E., Dist.M.ASCE Lucio Soibelman, Ph.D., P.Eng, NAC, Dist.M.ASCE Kelvin C.P. Wang, Ph.D., P.E., Dist.M.ASCE

Read more at https://source.asce.org/asce-honor-its-2021-distinguished-members

SEI Online

SEI Virtual Events

www.asce.org/SEI/virtual-events • #SEILive Conversation with Leaders on Code Development August 4 • SEI Standards Series Join live, virtual sessions for exclusive interaction with expert ASCE/SEI Standard developers on state-of-the-market updates. Participants will learn about technical revisions and review a design example. Attendees are encouraged to join and participate in Live Q&A. Each session is LIVE and only available 1:00 – 2:30 pm US ET. SEPTEMBER 16 – ASCE/SEI 59 Blast Protection of Buildings NOVEMBER 18 – ASCE/SEI 8 Specification for the Design of Cold-Formed Stainless Steel Structural Members Individual session: Member $49, Nonmember $99. Student member: Free registration.

REGISTER NOW at https://cutt.ly/9hQDTEo. • View the SEI Annual Meeting on SEI YouTube For updates including Performance-Based Design/Resilience, CROSS-US Collaborative Reporting for Safer Structures, and SE2050 Commitment to Net Zero.

SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle A U G U S T 2 0 21

CASE in Point CASE Tools and Resources Did you know? CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural

engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its risk management program, or keep track of the skills engineers are learning at each level of experience – CASE has the tools you need!

The following documents/templates are recommended to review/use if your firm needs to update its current Quality Assurance Program, or incorporate a new program into the firm culture: 962-A: National Practice Guidelines for the Structural Engineer of Record 962-B: National Practice Guideline for Specialty Structural Engineers 962-C: Guidelines for International Building Code Mandated Special Inspections and Tests and Quality Assurance 962-D: Guideline Addressing Coordination and Completeness of Structural Construction Documents Tool 1-2: Developing a Culture of Quality Tool 2-4: Project Risk Management Plan Tool 4-2: Project Kick-off Meeting Agenda Tool 4-4: Phone Conversation Log Tool 9-2: Quality Assurance Plan Tool 10-2: Construction Administration Log Tool 2-1: Risk Evaluation Checklist Tool 4-1: Status Report Template Tool 4-3: Sample Correspondence Letters Tool 4-5: Project Communication Matrix Tool 10-1: Site Visit Cards

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 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 a well written job description 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 employee 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 to create job descriptions specific to 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.

STRUCTURE magazine

News of the Coalition of American Structural Engineers ACEC Coalitions Summer Meeting August 9-10, 2021 | Nashville, TN

Join your peers at the ACEC Coalitions Summer Meeting where you will participate in candid roundtables and take a closer look at key issues like cybersecurity and risk management through expert-led breakout sessions.

Highlights include: • Networking Opportunities • Coalition-Specific Roundtables • Breakout sessions on Cyber Security and Risk Management Earn 3.5 PDHs

For more information and to register – https://programs.acec.org/coalitions-summer-2021.

Questions? Contact Michelle Kroeger at coalitions@acec.org.

WANTED: Engineers to Lead, Direct, and Engage with CASE Committees!

If you are looking for ways to expand and strengthen your business skillset, look no further than serving on one (or more!) CASE Committees.

Join us to sharpen your leadership skills – promote your talent and expertise – to help guide CASE programs, services, and publications.

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

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

Programs – responsible for developing program themes for conferences and sessions that enhances and highlights the profession of structural engineering.

Toolkit – responsible for developing and maintaining the tools related to CASE’s Ten Foundations of Risk Management program.

To apply, your firm should: • Be a current member of ACEC • Be a member of the Coalition of American Structural Engineers (CASE); or be willing to join the Coalition.

• Be able to attend the groups’ normal face-to-face meetings each year: August, February (hotel, travel partially reimbursable) • Be available to engage with the committees via email and video/conference call • Have some specific experience and/or expertise to contribute to the group Please submit the following information to Michelle Kroeger, Coalitions Director (mkroeger@acec.org): • Letter of interest indicating which committee • Brief bio (no more than a page) Thank you for your interest in contributing to advancing the structural engineering profession!

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

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structural FORUM Ethics Instruction…Ideas for Moving Forward

By Scott Civjan, Ph.D., P.E.

T

he July 2021 Structural Forum article presented the general state of ethics instruction and some shortcomings. This second article offers ideas that might better influence engineering ethics instruction.

Modifying personal behavior begins with understanding how we make decisions and the broader impact of our personal decisions. Unfortunately, ethics curricula rarely approach the topic at the personal level, leaving a disconnect between assessing “correct” behavior and acknowledging the personal reactions at the moment the decision is made.

Many decisions are made or heavily influenced through gut feel, reflex, and “norms.”

Other situations allow time to contemplate but can still be heavily influenced by our initial reaction. How many times have we seen something that did not seem quite right, decide to move on, and then later worry that we should have said something? Assessing questionable decisions from past experience, no matter how small the consequence, can train response to future situations. With reflection, we can change our behavior or decide to repeat our response if we see no consequences of our decision. The latter is especially true if a precedent was set or followed. Company culture can dominate these decisions, allowing questionable actions via a slow, imperceptible shift resulting from erratically enforced rules or a tendency to avoid communication or conflict. Ethical lapses can be accelerated by placing insufficiently trained early-career engineers in positions inspecting the work of people who have more experience. How can we prepare engineers to make sound decisions and open lines of communication when an ethical dilemma arises?

Awareness of our personal decision process is a start. We make decisions based on past experiences and values, adapting in new situations. Through evaluating day-to-day decisions, understanding how they become routine, and examining our reactions to decisions that affected us, we can prepare ourselves for future decisions. We can learn to react proportionately and minimize unwarranted whistleblower actions and decision avoidance.

Codes of Ethics case studies are not always straightforward. When a new engineer sees a calculation or field practice that they think is incorrect, but senior personnel tells them that it is typical, they face an ethical STRUCTURE magazine

dilemma. There is uncertainty in whether the situation is understood completely, variation from expectation is justified, and sufficient information exists to override seniority opinion. The ethical decision has less to do with Code of Ethics criteria and more to do with whether to defer to the experience or explanations of others. Do you risk stopping a job until you can learn more, or risk allowing job continuation?

Who should you communicate with when making your decision? The decision is more difficult when direct implications to public safety, technology issues, or risk communication are uncertain. Specific statements from Codes of Ethics may be difficult to apply, but contemplating how decisions are made and comfort level with previous decisions can modify future behavior.

Incorporating other perspectives is also critical. Engineering projects can have competing goals; maximizing profit, meeting schedule, minimizing risk, or mitigating environmental and societal impacts.

Depending on your role in a project, any of these could be the primary decision driver.

The impact on others may not be apparent. Other stakeholders may feel similarly about a competing goal. It becomes easy to assume that peripheral issues fall outside of your responsibility or that you should defer to someone else. Differing perspectives are always present, including amongst different disciplines working on a project, ownerengineer-architect-contractor relationships, user and public concerns. Acknowledging different local/regional/international norms, getting support from all stakeholders, and thinking about voiceless stakeholders are all essential, though not equally applicable to all projects.

What seems like an ethical dilemma to a new engineer is often due to not understanding the implications of a decision. Other times it could be an eye-opener to senior members of a company to be asked why something has become common practice. Therefore, conversations about ethics are important to develop clear communication and expectations.

When discussing ethics with students, coworkers, or mentees, consider the following: • Discuss decision-making processes that different people may use. How do you make decisions (immediate and long-term)?

• Start with immediately relatable scenarios and slowly/incrementally expand situations to those they have not experienced. Minimize arms-length discussions of ethical decisions and include reflective components through “how did you respond to” prompts about previous decisions.

• Discuss the influence of peer pressure and office culture on decisions.

Acknowledge that these develop over time and can result in ethical fading (failing to realize that there is an ethical component in a decision.)

• Incorporate diverse perspectives in discussions and acknowledge the effects of implicit bias in decisions. Acknowledge the reliance on the dominant culture for ethical values and the potential to marginalize other perspectives.

• Discuss tradeoffs between short and long-term interests.

• Include social justice and equity in the decision process and discuss impacts to the company, owner, and project interests. Discuss competing interests.

• Focus on a continuum of ethical decisions (daily life, work, global impact).

This includes breaking down the compartmentalizing of ethical topics as having distinct personal versus societal impacts. Instead, discuss these topics as a continuum where ever-widening perspectives are included in the decision.

• Provide ASCE/NSPE Codes of Ethics as a separate topic representing a minimum threshold of ethical responsibility.

Further categorization is needed to use case studies effectively. For instance, identifying cases based on the experience of the decision-maker and personal versus societal dilemmas would be useful. In addition, evaluate how relatable the case study scenarios are to the audience and organize them in incremental imaginative leaps.

The next goal would be to develop examples to fill scenario gaps and provide incremental instruction from current personal experience through a career.■ Scott Civjan is a Professor at the University of Massachusetts Amherst Department of Civil and Environmental Engineering. He teaches classes in structural engineering, including design classes, where he has been introducing and modifying ethics content.

A U G U S T 2 0 21

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