STRUCTURE magazine - January 2021

STRUCTURE JANUARY 2021

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INSIDE: Hamel Music Center

18

Rheological Properties of Concrete 8 Avoiding Problematic Uses of Slabs 12 Prestressed Concrete Bridge Girders 22

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Contents JAN UARY 2021

Cover Feature

18 HAMEL MUSIC CENTER

By Steve Roloff, P.E.

Little does the musician or eventgoer at the Hamel Music Center know that, to achieve acoustic perfection, three separate buildings needed to fit within a whole other, larger structure. Acoustics and sound isolation were critical to project success; concrete was the material chosen to meet sound treatment and mass requirements.

Columns and Departments 7

Editorial Making the Case for Diversity, Equity, and Inclusion

8

24

By Stacy Bartoletti, S.E.

Structural Testing Rheological Properties of Concrete

34Structural Forum Lessons Not Learned

By N. Subramanian, Ph.D., P.E.

12

By James Lefter, P.E.

Structural Design Avoiding Problematic Uses of Slabs on Ground

16

By Alexander Newman, P.E.

Structural Systems Cross-Laminated Timber Diaphragms

By Jim DeStefano, P.E., AIA, and Rick Way, P.E.

22

Building Blocks Axial Load Limits for Prestressed Piles

By John C. Ryan, Ph.D., P.E., and Timothy W. Mays, Ph.D., P.E.

Structural Rehabilitation Prestressed Concrete Bridge Girders – Part 1

By Dustin Black

January 2021 Bonus Content

In Every Issue 4 27 28 30 32

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

Additional Content Available Only at – STRUCTUREmag.org

InFocus Staying Engaged and Effective While Working Remotely – Part 2 Spotlight The International Spy Museum

By STRUCTURE’s Editorial Board

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

J A N U A R Y 2 0 21

5

EDITORIAL Making the Case for Diversity, Equity, and Inclusion

By Stacy Bartoletti, S.E.

D

iversity, Equity, and Inclusion (DEI) have gained substantial job boards that serve underrepresented groups across many schools and exposure in 2020. Unfortunately, it has taken recent events of regions. We also plan to review our interview practices across the firm racism and abuse of power to bring it to the forefront. I hope the to ensure that we have a consistent, structured interview process and pain and loss associated with these events will not go to waste but will a clear rubric with which to evaluate candidates. Providing structure result in positive change. I do not consider myself an expert on DEI in these areas and tools for our team to shine a light on areas that are and even feel a bit exposed to critique by merely writing this editorial. prone to bias are essential steps toward meeting our DEI objectives.

Still, given my position in my firm and the profession, I feel it is my Work opportunities: The Council works with those responsible for responsibility to talk about DEI and address it. I believe we all need to individual staff assignments to develop a tool to capture the needed consider how we can positively impact experiences and desired experiences DEI as individuals, as firm leaders from all staff. The intent is to have a within our firms, and through our more formal procedure that will allow We need to bring our unconscious professional organizations. for bias in the process to be more As an individual, I believe my first easily identified and avoided. The biases to the forefront when responsibility is to gain knowledge process is also intended to create an and understanding of DEI topics and equal playing field among those with possible and make a conscious issues. When I can, I read about the different work styles, such as those subject, participate in seminars and that are more or less comfortable with effort to mitigate their effects on workshops, and engage in discussions being vocal about advocating for their with an open mind. I recently parown interests. our lives and businesses. ticipated in a firm leader roundtable Inclusion: Starting with inclusion organized by the SE3 Committee of among departments and work styles, the Structural Engineers Association the Council is looking at who is invited of Northern California and had two significant takeaways. First, I to meetings and events, how are they held, which disciplines or personbelieve it matters how we talk about DEI, and the words we use are ality types may not participate, and what we can do to help people feel important. As preparation for the roundtable, we were all asked to that they belong. This work could also extend to firm practices regardwatch a short video titled “Why I’m not a racist is only half of the ing recognition, group meetings, and celebration of project success. story” by Robin DiAngelo. Robin talks about racism and how we as External partnerships and volunteering: The Council is identifying a society have defined the word brings up white defensiveness on the organizations that advance goals related to increasing the diversity of topic. I cannot do proper justice to the short video’s profound nature, people getting into the engineering profession and considering ways so I recommend you search for it and watch it. My second takeaway to encourage and support our employees to participate with these is that we all have biases, and only by learning about them and expos- organizations. The Council has identified the barriers to entry into ing them can we address them. We need to bring our unconscious the profession as an important focus area and will seek to partner with biases to the forefront when possible and make a conscious effort to many of the excellent existing organizations to amplify their efforts. mitigate their effects on our lives and businesses. This is not easy, but At the Professional Organization level, I believe the most important I can admit that I have biases, and I am actively trying to recognize thing we can do is speak with a common voice about DEI. Our them and address them. professional organizations of CASE, NCSEA, and SEI can have a As a firm and firm leader, I have an obligation to make sure that much more significant influence than any of us as individuals or my business is creating a culture of Diversity, Equity, and Inclusion firms, but it takes individuals with a passion to work on behalf of the and taking active measures to address areas of deficiency. At my firm, organizations. In June of this year, the three organizations issued and we have recently established a DEI Council. The Council consists signed a joint call to action to address DEI in our profession. There of a diverse group of employees that are committed to the mission is an amazing amount of work underway to advance the Structural of fostering an environment that attracts the best talent, values and Engineering profession through these organizations, from education recognizes diversity, and enables all employees to contribute and thrive to licensure to leadership development and beyond. We can influence while experiencing a culture of inclusion and belonging. The Council DEI in our profession if we infuse DEI principles and discussion into is tasked with bringing recommendations for change directly to the everyone one of those activities.

CEO and the leadership team. Our DEI Council is early in its work, I encourage you to consider what you can do as an individual and they are currently evaluating four specific areas. to expand your understanding of DEI, challenge your firms to Where and how we hire: The first step in this process is to address how be proactive, and engage with your professional organizations.■ we fill the candidate pool with diverse candidates and how we interStacy Bartoletti is the CEO and Chair of Degenkolb Engineers in San view in a way that seeks to eliminate opportunities for bias to impact Francisco, California, and the Chair of the CASE Executive Committee. our decisions. To impact our candidate pool, we are re-thinking and (sbartoletti@degenkolb.com) expanding our search practices and are looking to identify events and STRUCTURE magazine

J A N U A R Y 2 0 21

7

structural TESTING Rheological Properties of Concrete Are We Testing for Flow Properties Correctly?

By N. Subramanian, Ph.D., P.E., F.ASCE, F.INAE

T

he rheological (flow) properties of concrete are essential for the construction industry, because concrete,

for different elements of a structure, is placed into the formwork while it is in its plastic state. The flow properties affect not only proper concrete placement, consolidation, and finishing but also the hardened state properties such as strength and durability. Concrete that is not correctly placed and consolidated may have defects, such as voids

Figure 1. Drawback of slump test-predicted workability based on concrete rheology.

Reprinted with permission from ACI.

and honeycombing, and be prone to aggregate segregation. Improper placement and consolidation may result in reduced compressive strength and increased permeability, reducing the concrete’s durability and sustainability.

Unfortunately, due to the complex composition of concrete, a decisive method to predict the flow of concrete is not yet available.

The most commonly used workability test methods are based on empirical methodologies, such as slump tests. It is important to note that, at the same slump value, two concretes may exhibit different workability. However, numerous two-point tests have been developed over the years to measure both yield stress and plastic viscosity. The selection of a proper test and interpretation of the obtained results to predict the performance of concrete in the field are not that easy. The American Concrete Institute’s Report on Measurements of Workability and Rheology of Fresh Concrete (ACI 238.1 R-08) provides discussion and guidance.

Test Methods

Why Slump Test is Not Adequate The slump test was developed in the early 1920s when concrete researchers recognized the role of water-cement ratio in concrete strength. The slump test was quickly adopted because of its simplicity. Using the slump test to assess the workability of fresh concrete has become questionable, especially for modern concrete mixtures incorporating chemical admixtures. Its primary deficiency is that two fresh concrete mixtures having the same slump value may behave quite differently at the job site (Figure 1). This is because the standard slump test is related only to the yield stress, and a single parameter alone cannot fully evaluate concrete flow characteristics. Even selfconsolidating concrete (SCC), which is governed by the property of flowing under its own weight, is typically classified based on the results of empirical tests such as slump flow, V-funnel, L-box, and J-ring tests.

Several test procedures for determining workability have been develConcrete as a Bingham Fluid oped in the past for research, mixture proportioning, and field use.

However, only the slump test is widely accepted and prescribed in Research has now established concrete as a Bingham fluid. Based on codes and standards because of its ease of use. Even with the increased this, the flow of concrete is better measured in terms of shear stress knowledge of concrete rheology and the development of several other and shear rate. Many of the new methods recently developed have test methods since the 1920s, the concrete industry is not yet convinced attempted to measure both yield stress and plastic viscosity (Figure 2). to replace the slump test.

Measurement of the rheological parameters of concrete is not easy ACI 238.1 R-08 describes 69 test methods to perform, as concrete has a broad range that could be used to measure the workof particle sizes (from 1 Îźm cement grains ability of concrete, including the details of to 10-20 mm coarse aggregates in normal these test methods, their advantages, and concrete or even 100 mm size of coarse drawbacks. Most test methods for workaggregates used in dams). Also, highability can be classified as single-point and performance concrete may contain a large two-point tests. For example, slump test, number of additives, such as different kinds penetrating rod (Kelly ball, Vicat, and of mineral and chemical admixtures and Wigmore test), and K-slump test are used fibers, which may have different particle to measure the yield stress of concrete. In sizes, and some of the admixtures may intercontrast, Vee-Bee consistometer test, LCL act with each other. Hence, the flow of a apparatus, flow cone test, and Orimet given concrete is usually measured using apparatus are used to measure the plastic Figure 2. Bingham model of concrete workability. one of the many standard tests available viscosity of concrete. that only partially measure the intrinsic flow The stars represent experimental data points.

8 STRUCTURE magazine

properties of the material. Rheological measurements play an essential role in quality control (QC) of many liquid and semi-solid materials.

Two Point Tests It is well established that the flow properties of fresh concrete at low shear rates approximate the Bingham model, expressed as follows: τ = τ0 + µp γ̇ (Equation 1) where τ, τ0, µp, and γ ̇ are the shear stress, yield stress, plastic viscosity, and shear rate, respectively. Compared to a Newtonian model, the Bingham model incorporates Figure 3. Different types of rotational rheometers (Source: Küchenmeister-Lehrheuer and Meyer). a yield stress term τ0, and the viscosity is often replaced Reprinted with permission from Thermo Fisher Scientific. with plastic viscosity (µp). In Equation 1, the shear stress should be measured with at least two different shear rates to estimate rotating relative to the other, permit small specimen size and direct the Bingham constants. Physically, the yield stress represents the force analytical calculation of rheological parameters. In the parallel plate required to initiate flow, while plastic viscosity is a measure of the and coaxial cylinder rheometers, yield stress and plastic viscosity are resistance of the mass to an increase in the rate of flow. calculated directly from the torque and rotation speed. In impeller rheometers, however, yield stress and plastic viscosity cannot be Rotational Rheometers calculated directly. Hence, standard calibration fluids have to be Several attempts have been made in the past to adapt traditional used to establish the relationship between the measured torque and rotational rheometers (e.g., ASTM D7175:15), commonly used in rotation speed. The surfaces of the coaxial cylinder and parallel plate asphalt binder testing, to measure rheological properties of concrete. are usually textured or roughened to prevent slippage between the Rotational rheometers for concrete apply continuous shear to the concrete and the surface of rheometers. concrete specimens through rotational movement at controlled torque Although concrete rheometers have been used with several types of and speed. Rotational methods offer the advantage of being able to concrete, such as SCC and fiber-reinforced concrete, they are not sugshear a sample continuously in order to achieve equilibrium and to gested to be used with stiff concretes. It is also important to note that monitor changes over time. These are basically dynamic tests. From the different rheometers give different absolute values of the rheological measurement of torque and rotation speed, the rheological parameters parameters, but the degree of correlation of both parameters between of yield stress and plastic viscosity can be calculated. Rheometers for concrete must be designed carefully due to the large and different sizes of aggregates used. A view of the different types of standard geometries is shown in Figure 3.

Most geometrical configurations for concrete rheometers are based on Over 990 pages and 140 worked-out examples coaxial cylinders, as shown in Figure 3a providing the proper application of the 2019 and 3b, consisting of an inner cylinder Building Code Requirements for Structural (called a bob), which is inserted into an Concrete (ACI 318-19) provisions for castouter cylinder. As shown, several geomin-place concrete buildings with nonpreetries can be used for the bob, including stressed reinforcement. a solid coaxial cylinder, a coaxial cylinder with recessed bottom (Figure 3b), Features: double gap geometry (Figure 3c), vane, » A simplified roadmap that can be used to navigate and helical impeller (Figure 3d ). In a through the updated ACI 318 requirements coaxial cylinder’s rheometer, the fluid » Step-by-step design procedures and design aids between an inner and outer cylinder that make designing and detailing reinforced is sheared. A torque is applied to the concrete buildings simpler and faster outer cylinder, while the inner cylinder measures the torque. The vane and Download the FREE Rebar Reference App helical impeller geometries can be used for mobile devices! instead of the inner cylinder. In impelFeaturing reinforcing steel data, ler rheometers, a vane or impeller is hook details, field inspection information, bar markings inserted into the concrete and rotated identifier*, development lengths at various speeds in an axial or planetary calculator*, and more!

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J A N U A R Y 2 0 21

9

any pair of rheometers is good. Differences service life. The significant difference between in absolute values were attributed to factors conventional concrete and HPC is essentially such as wall slip, particle interference, and the use of chemical and mineral admixtures. the use of different materials for calibration.

All aspects of mixture proportioning of HPC, Tattersall (U.K.) designed the first two-point such as cement content and characteristics, rheometer in 1973, with an impeller rotating water-cementitious materials ratio (w/cm), inside the concrete, placed in a vessel. Since supplementary cementitious materials (SCMs), then, it has been commercialized into the chemical admixture dosage and type, aggregate BML viscometer (Iceland, using a serrated properties and content, and gradation play an cylinder), and the IBB rheometer (Canada, important role in its rheology. The Table and using an H-shaped impeller). Other com- Figure 4. Effect of constituent materials on the rheological Figure 4 summarize the effects of these different mercial rheometers are the Bertta Apparatus, properties of typical concrete (Adapted from Banfill, components on the rheology of HPC. Except for the BTRHEOM rheometer (France), the 2006). Reprinted with permission from ACI. the combined WRA + VMA behavior, Figure 4 CEMAGREF-IMG rheometer (France), the shows the effect of individual components only.

UIUC rheometer (the University of Illinois at Urbana-Champaign), and To achieve better quality by limiting segregation, producing a good the ICAR rheometer (the University of Texas at Austin). These different surface finish, minimizing pumping pressure, or controlling formpieces of equipment have software built into them, which converts the work pressure, it is necessary to balance the rheological properties of applied torque and speed to yield stress and plastic viscosity automatically. concrete. Ferraris et al. (2017) suggested the following: These modern controlled-stress rheometers have allowed measurements to 1) Control segregation by increasing the yield stress or plastic be made at significantly lower shear rates than previously possible. These viscosity, keeping in mind that increasing both parameters rheometers show that at low shear rates, plastic viscosity is very high. Some excessively will lead to very stiff concrete. rotational rheometers have been designed to be sufficiently small and 2) Achieve good surface finish by having adequate plastic visrugged for use at job sites. More descriptions and comparisons of these cosity; when it is too low, segregation will occur, and when rheometers are available in a NIST article by Ferraris and Brower (2000). it is too high, air bubbles cannot escape. If the yield stress is increased, consolidate the concrete to remove air.

3) Decrease pumping pressure by decreasing plastic viscosHigh-Performance Concrete (HPC) ity. Decreasing yield stress will not appreciably decrease High-Performance Concrete (HPC) is designed to give optimized pumping pressure. performance characteristics for a given set of materials, usage, and 4) Reduce formwork pressure either by increasing yield stress exposure conditions consistent with strength, durability, workability, and or plastic viscosity.

Summary of parameters influencing HPC rheology (ACI 238.1R-08). Reprinted with permission from ACI.

Parameter

Yield stress

Plastic Viscosity

Increase in cement content

Decrease

Decrease

Increase in water content

Decrease

Decrease

Aggregates Volume fraction

Increase

Increase

Shape

Rounded aggregates perform better than flat, angular and elongated

Texture

Smooth better than rough

Gradation

Uniform gradation and high packing density perform better

Microfines content

Mixed

Mixed

Sand to aggregate ratio

Optimum value

Optimum value

Supplementary Cementitious Materials Fly ash

Decrease

Mixed

GGBS

Mixed

Increase

Silica fume (low dosage)

Decrease

Decrease

Silica fume (high dosage)

Increase

Increase

Admixtures Water-reducing

Decrease

Mixed

Air entraining

Mixed

Decrease

Viscosity modifying

Increase

Increase

Fiber content

Increase

Increase

10 STRUCTURE magazine

Summary Single-point tests like the slump test measure empirically only the yield stress, which alone is not sufficient to evaluate the flow characteristics of concrete. Two-point tests measure both yield stress and plastic viscosity. It is difficult to measure the rheological parameters of concrete as the ingredients of concrete have a broad range of particle sizes. Also, high-performance concrete may contain different kinds of mineral and chemical admixtures, and fibers which again may have different particle sizes. Some admixtures may interact with each other.

The yield stress represents the force required to initiate flow, while plastic viscosity is a measure of the resistance of the mass to an increase in the rate of flow.

Rotational rheometers measure the torque and rotation speed of a bob that circulates inside concrete, based on which yield stress and plastic viscosity are calculated.

Several geometries can be used for the bob. Several commercial rheometers also exist. All aspects of mixture proportioning of HPC play an important role in its rheology.â– References are included in the PDF version of the article at STRUCTUREmag.org.

Dr. N. Subramanian is a retired consulting engineer in Gaithersburg, MD.

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structural DESIGN

Avoiding Problematic Uses of Slabs on Ground Understand the Consequences of Specifying Them to Resist Horizontal and Vertical Loading

By Alexander Newman, P.E., F.ASCE

M

ost structural engineers would not dream of deliberately violating any building-code provisions, but some are doing it on a regular basis – unwittingly. The problem area is concrete slabs cast on the ground. These concrete elements are frequently designed to serve as vertical supports for posts and columns, lateral ties, lateral-load transfer devices, and lateral bracing for walls. There is nothing wrong with relying on concrete slabs for these needs – as long as they are designed as structural slabs, like those in elevated floors, rather than common slabs on ground (SOG) that are relatively thin, reinforced with welded-wire fabric (WWF), if that, and contain control and isolation joints. It is this type of slab that is problematic for structural uses.

Structural Slabs vs. SOG Are all SOG considered structural elements? Generally, the answer is “no,” except as discussed below. The primary purpose of an SOG is to provide a dry, hard, and level surface. In most cases, the building structure does not behave differently if the ground floor is made of bituminous concrete, precast pavers, or even crushed stone. Structural slabs are, well, structural and perform a structural function besides supporting live loads and loads from equipment, racks, and the like. According to the American Concrete Institute’s ACI 318-19 Section 1.4.8: This Code does not apply to design and construction of slabs-onground, unless the slab transmits vertical loads or lateral forces from other portions of the structure to the soil.

The International Building Code (IBC) contains a similar statement.

Put differently, ACI 318-19 Section 13.2.4, Slabs-on-Ground, states: Slabs-on-ground that transmit vertical loads or lateral forces from other parts of the structure to the ground shall be designed and detailed in accordance with applicable provisions of this Code.

Slabs-on-ground that transmit lateral forces as part of the seismicforce-resisting system shall be designed in accordance with 18.13 [seismic provisions].

It is quite clear: The concrete slabs that directly support vertical structural elements (e.g., columns or walls) or are involved in lateral load transfer must be designed as structural elements subject to ACI 318 provisions for structural concrete. The engineers who miss this critical point do so at their own risk.

Designing SOG for Structural Loads Is there a way to avoid designing such concrete slabs as structural concrete elements? After all, ACI 318 references another document, ACI 360R-10, Guide to Design of Slabs-On-Ground. (To avoid confusion on grammar, ACI chooses to hyphenate the term “slabs-on-ground,” but the IBC and the author do not.)

ACI 360R includes a wealth of information on various SOG types, their joints, loads, etc. Yet, there is no escape from complying with structural provisions of ACI 318 for the concrete slab involved in load transfer (see ACI 360R Sections 3.2.4 and 12.1). The latter provides some examples of concrete slabs that must be designed as structural 12 STRUCTURE magazine

members per ACI 318.

These include the slabs supporting vertical loads from columns, posts, and load-bearing walls, as well as Figure 1. Slab on grade with reinforcing bars. the slabs helping resist lateral loads from perimeter foundation walls and pre-engineered metal building columns. Note that, in all these cases, the slabs are subjected to tensile or flexural loading, as opposed to compression or bearing, which could have been resisted by some concrete alternatives such as pavers.

The slab designed for tension or flexure must be properly reinforced.

As the ACI 318-19 Commentary Section R14.1.3 explains, the use of structural plain concrete should be limited to members primarily in compression. Therefore, when the slab resists tension or bending, in most cases it should be designed as a one-way (and sometimes two-way) structural slab. For the sake of simplicity, consider the code provisions for one-way structural slabs, such as those found in elevated concrete floors and roofs. The attributes of a properly designed structural slab are: • An appropriate combination of thickness and primary flexural reinforcement to meet the specified strength and serviceability criteria.

• A minimum amount of reinforcement to resist the effects of temperature changes and concrete shrinkage.

• Structural integrity reinforcement (the new provisions added in ACI 318-19).

More can be learned by examining how these structural provisions would work in SOG.

Minimum Slab Reinforcement For one-way non-prestressed slabs, the minimum areas of flexural reinforcement and the reinforcement required to resist the effects of temperature changes and shrinkage are the same (compare ACI 318-19 Sections 7.6.1 and 24.4.3). In both cases, the minimum bar areas are equal to 0.0018 times the slab's gross area, but the maximum spacing of the bars differs. There are also new provisions for integrity reinforcement. Thus, there are three relevant issues: 1) According to ACI 318-19 Section 7.7.2.3, the maximum spacing of primary flexural reinforcement is equal to the lesser of three times the slab thickness and 18 inches. Still, the spacing is much smaller in most cases. Following Section 24.3.2, the maximum spacing is 12 inches or less [see sidebar (online)].

2) By contrast, the spacing of deformed reinforcement to resist shrinkage and temperature effects is limited to the lesser of five times the slab thickness and 18 inches. Unlike primary flexural reinforcement, which is placed near the tension surface, shrinkage and temperature reinforcement may be placed anywhere in the slab in one or two layers. The Commentary Section R24.4.3.4 states that splices and anchorage of this reinforcement must develop its specified yield strength.

3) ACI 318-19 adds a new Section 7.7.7, which calls for structural integrity reinforcement in cast-in-place one-way slabs. At least one-quarter of the maximum positive-moment reinforcement should continuously extend from one end of the slab to the other. This reinforcement should be anchored at the end supports to develop the bars' yield strength, and it should use Class B tension lap splices or mechanical or welded splices.

This discussion clarifies that all three types of bars envisioned in ACI 318 consist of deformed reinforcement (Figure 1). It might be possible to use its lightweight prefabricated version – deformed WWF mats – but not the plain WWF common in SOG, since bond on smooth steel is not recognized by ACI 318.

Also, the common WWF used in SOG has an insufficient area for structural purposes. For example, using the minimum reinforcing ratio of 0.0018, the cross-sectional area of the ubiquitous 6x6-W2.9xW2.9 WWF (0.058 in2/ft) is not even sufficient for a 3-inch-thick slab.

Instead, to comply with ACI minimum reinforcing ratio and bar spacing requirements, slabs from 4 to 7 inches thick should have at least #4 bars at 12 inches on centers; closer spacing or larger bar sizes are needed for thicker slabs.

Sawcut Joints A typical SOG includes three different types of joints, described in detail in ACI 360R Chapter 5. These are: 1) Construction joints, which define the extent of concrete placed at one time. In SOG, these joints often have greased dowels that permit in-plane movement but resist vertical offset. In structural slabs, reinforcement typically continues through the joints.

2) Isolation joints, which isolate the SOG from the restraint to shrinkage and temperature movements provided by perimeter

Figure 2. Reinforcing bars would make control joints in SOG largely ineffective.

foundation walls and interior columns. As ACI 360R states, “Every effort should be made to avoid tying the slab to any other element of the structure.” The opposite occurs in structural slabs, where isolation joints are rarely used since there must be a load path for structural loads.

3) Control (contraction) joints, which are typically sawcut to weaken the slab at predetermined locations to induce shrinkage cracks to form in straight lines rather than randomly. For the joints to be most effective, reinforcement should stop at each side of them; otherwise, it would undermine the purpose of the joints. Neither elevated slabs nor concrete mats – the elements closest to structural slabs – use control joints where the reinforcement stops. However, an SOG without isolation and control joints or where the deformed reinforcement continues through the joints (as in a structural slab) (Figure 2) is vulnerable to random cracking, as explained in ACI 360R Section 6.2.

Diaphragm Detailing As mentioned in the introduction, and discussed in more detail below, SOG are often expected to act as braces or ties that resist horizontal reactions from building columns under wind and seismic loading.

According to ACI 318-19 Section 18.13.3.2, if these columns form a part of the seismic-force-resisting system (SFRS) in buildings assigned

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J A N U A R Y 2 0 21

13

Figure 3. Slab on grade diaphragm.

Figure 4. Hairpin in slab on grade.

to SDC C-F, the SOG must be designed and detailed as concrete diaphragms with a load path from the point of load to the resisting element. This means that the SOG must be reinforced for in-plane shear and flexure, and the engineer must consider the locations of joints without reinforcement. As in a typical concrete diaphragm, continuous reinforced chords might be required (Figure 3).

foundations expect an SOG with closely-spaced reinforcing bars, without any sawcut joints, and one that is positively connected to the perimeter foundation walls. However, that is the kind of SOG that would be necessary if hairpins were used.

There are multiple solutions for resisting horizontal column reactions that do not rely on any contribution of SOG. Among them are moment-resisting foundations and tie rods designed as grade beams. The author provides the design examples in Foundation and Anchor Design Guide for Metal Building Systems (McGraw-Hill, 2013). These foundations are reliable, but they might cost more than inferior alternatives such as hairpins.

Common Situations Slabs with Hairpins

SOG are sometimes used for lateral support of columns in metal building systems (MBS), a.k.a.

Slabs Supporting Rack Structures pre-engineered metal buildings. MBS typically and Mezzanines include primary moment-resisting gable frames that carry secondary roof and wall members. One Many warehouses and some industrial buildings feature of these frames – and any “rigid” frames for contain multistory rack systems that support shelvthat matter – is that their columns exert vertical ing – and even the roof structure. Rather than and horizontal reactions on the foundations. MBS provide separate foundations at each rack post and are often promoted based on their low cost, and isolate those from the floor slab, the typical solution the pressure is on to use inexpensive foundations is to support them on the SOG. Columns supportfor these buildings as well. ing mezzanines are also frequently placed directly on The cheapest – and the most problematic – Figure 5. Soil-retaining wall braced by SOG. the slab. Several publications address the design of solution relies on hairpins, bent reinforcing bars SOG subjected to concentrated loads, but they do hooked around the column anchor bolts and extending into the slab not focus on the broader implications of using the slabs for structural (Figure 4 ). The bars are assumed to carry the horizontal loading from purposes. Such SOG should be designed as structural slabs or as mats. the column into the SOG and then . . . where? A common assumpSoil-Retaining Walls Braced at the Top tion is that the SOG would somehow act as a tie that transfers the horizontal column reactions to the opposite side of the building or These are often found in loading docks and similar structures where the carry them to the soil by friction. The author has heard yet another surface behind the wall contains an SOG (Figure 5). The slabs bracing idea: the slab carries the horizontal loading to the parallel exterior these walls serve structural purposes and should be designed as such. walls as a diaphragm. Some people do not even go that far. If the Some Unclear Situations hairpins are tied to the slab, we are good, right?

Hairpins are cheap, and it takes little time to design them. But, as What about a concrete or masonry wall carrying relatively light loading discussed above, using an SOG as a tie, brace, or diaphragm makes and supported on a thickened SOG (Figure 6 )? A column bearing it into a structural slab constructed differently from a typical ground- on the spread footing integrally built with an SOG? An exterior loadfloor slab. It is doubtful that those who hope to save money on MBS bearing wall bearing on an integrally placed slab haunch (Figure 7 )?

When an SOG of uniform thickness supports such walls or columns, the slab would clearly be considered structural. Must a thickened area be isolated from the rest of the SOG not to make the slab into a structural element? The codes are silent.■ Sidebar is included in the PDF version of the article at STRUCTUREmag.org.

Figure 6. Thickened slab on grade supporting lightly loaded concrete or CMU wall.

14 STRUCTURE magazine

Figure 7. Slab on grade with integral haunch supporting loadbearing wall.

Alexander Newman is a Forensic and Structural Consultant in the Boston area. He is the author of three engineering reference books and a published fiction writer. (newmanauthor.com)

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structural SYSTEMS Cross-Laminated Timber Diaphragms

By Jim DeStefano, P.E., AIA, F.SEI, and Rick Way, P.E.

C

ross-Laminated Timber (CLT) panels are becoming increasingly common as a roof or floor deck system

in mass timber buildings. The roof and floor deck systems need to be carefully engineered and detailed to serve as diaphragms resisting wind and seismic loads. The diaphragm transmits lateral loads to the vertical lateral load resisting elements – usually shear walls or braced frames.

It is common for CLT floor decks to have a concrete topping slab for sound isolation. The concrete topping slab can be detailed to serve as the lateral load resisting diaphragm, but it is often more practical to engineer the CLT deck to serve as the diaphragm. The American Wood Council’s (AWC) 2021 Special Design Provisions for Wind & Seismic (SDPWS), referenced in the 2021 International Building Code (IBC), contains CLT diaphragm design provisions.

Diaphragm Flexibility

Figure 1. Longitudinal side joint.

The in-plane shear strength of a CLT panel is controlled by the shear strength of the transverse cross laminations. The allowable in-plane shear values are shown in Table 1. Per 2021 SDPWS, CLT panels should be designed to resist 2.0 times the induced shear associated with seismic loads to preclude a non-ductile shear rupture.

The in-plane shear strength of a CLT deck is typically limited by the strength of connections between panels and connections at boundary elements rather than by the strength of the CLT panels. Consequently, careful detailing of the panel connections and fasteners is crucial.

In most instances, it is reasonable to consider CLT floor and roof CLT Connections decks to act as rigid diaphragms with lateral loads distributed to the vertical resisting elements based on their relative stiffness. It is recom- Longitudinal side joints between CLT panels are most commonly mended that the aspect ratio (L/W) of the rigid diaphragm not exceed achieved with plywood splines fastened with common nails. Spline 3:1 if there is a non-composite concrete topping (2½-inch minimum joints deform and dissipate energy during a seismic event, lending thickness) or 2:1 if there is no concrete topping. If the aspect ratio ductility to the diaphragm. If thicker splines are used or fastened exceeds these limits but is not more than 4:1, the diaphragm may be with screws rather than common nails, some ductility is sacrificed. modeled as semi-rigid.

Alternatively, rather than performing a semi-rigid diaphragm Table 1. CLT in-plane shear ASD reference design values (lbf/ft of width). analysis, which can be tedious, the lateral analysis may be run twice – once assuming a rigid diaphragm and once assumCLT Layup Fv (psi) 41⁄8” (3-ply) 67/8” (5-ply) 95/8” (7-ply) ing a flexible diaphragm, and designing for the envelope of E1 135 2385 4769 7154 the two. If the lateral force-resisting elements are similar in E2 180 3180 6359 9539 stiffness and well distributed throughout the footprint, the two solutions will often not be far apart.

E3 110 1943 3886 5829 The CLT decks can be considered flexible diaphragms only E4 175 3091 6182 9274 if calculations demonstrate that the diaphragm’s in-plane E5 150 2650 5299 7949 deflection is more than twice the average drift of the vertical resisting elements. This is outlined in the American Society V1 180 3180 6359 9539 of Civil Engineers’ ASCE/SEI 7-16, Minimum Design Loads V2 135 2385 4769 7154 and Associated Criteria for Buildings and Other Structures, V3 175 3091 6182 9274 Section 12.3.1.3. In no case should the diaphragm aspect V4 135 2385 4769 7154 ratio exceed 4:1.

Diaphragm Strength CLT shear values are based on ANSI/APA PRG-320, Standard for Performance-Rated CrossLaminated Timber. V (in-plane shear strength) = t (thickness of cross plies) × 12” × Fv × 1.6 (CD) / 1.5.

16 STRUCTURE magazine

V5

150

2650

5299

7949

CLT Layup

Fv (psi)

41/2” (3-ply)

71/2” (5-ply)

101/2” (7-ply)

S1

130

2496

4992

7488

S2

150

2880

5760

8640

S3

115

2208

4416

6624

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The allowable unit shear capacity Table 2. Allowable unit shear capacity of splines (lbf/ft). for plywood splines are shown in Spline 10d 10d 10d 2 rows 10d 2 rows 10d Table 2.

Thickness 6”o.c.

4”o.c.

2½”o.c.

4”o.c.

2½” o.c.

Some dynamic testing of spline Wind joints with screw fasteners has 19 been done at the University of ⁄32” 505 672 1007 British Columbia. The research 23 ⁄32” 995 1427 suggests that spline joints with Seismic inclined screws loaded axially typi19 cally exhibit high initial stiffness ⁄32” 360 480 720 and ultimate static capacity but 23 ⁄32” 710 1020 are prone to non-ductile failure.

Source: 2021 SDPWS Tables 4.2A and 4.2B based on “Sheathing” grade wood Spline joints with screws installed structural panels and a 3-inch nominal nailing face. at 90 degrees and loaded in shear exhibited lower initial stiffness and Table 3. Allowable shear capacity of timber screws in CLTs (lbf). ultimate static capacity but failed Fastener Fastener Major Strength Minor Strength in a more ductile fashion.

Thickness Diameter Length Direction Direction Transverse end joints between 41⁄8 ” 1⁄4” (6mm) 77⁄8” (200mm) 348 348 CLT panels must resist the same 5 in-plane shear as the longitudinal ⁄16” (8mm) 77⁄8” (200mm) 456 364 41⁄8” joints. In most instances, the ends of 5 ⁄16” (8mm) 117⁄8” (300mm) 456 364 67⁄8” the CLT panels are supported on a 67⁄8” 3⁄8” (10mm) 117⁄8” (300mm) 669 480 glulam timber beam, and the shear is transmitted through the beam with 95⁄8” 3⁄8” (10mm) 15” (380mm) 669 480 timber screws.

95⁄8” 1⁄2” (12mm) 153⁄4” (400mm) 961 676 Diaphragm boundary elements Source: Heavy & Mass Timber Connections Handbook – MyTiCon. (chords and collectors) are often glulam timber beams. Connections at the ends of timbers serving as boundary elements must be detailed to resist the axial forces induced into them to maintain the continuity of the chords. If the chord forces must be resisted by the CLT panels rather than by timber beams, steel straps are needed at the panel joints.

Per 2021 SDPWS, chord splices should also be designed to resist Figure 2. Transverse end joint.

Figure 3. Diaphragm boundary.

2.0 times the induced seismic chord force to preclude non-ductile behavior. The timber screws fastening the CLT panels to the Jim DeStefano is the President and Rick Way is an Associate with DeStefano boundary timbers need to be spaced to transmit the chord and & Chamberlain, Inc. located in Fairfield, CT. (jimd@dcstructural.com) collector forces.

The allowable shear capacity of timber screws fastening CLT panels to timber beams is shown in Table 3. The values in Table 3 are based on timber screws with a minimum Fyb of 136.6 ksi, a CLT specific gravity of 0.42, and a timber beam specific gravDemos at www.struware.com ity of 0.49. The load is assumed to be applied parallel to the Wind, Seismic, Snow, etc. Struware’s Code Search program calculates these and grain of the timber beam. These values have been increased for other loadings for all codes based on the IBC or ASCE7 in just minutes (see online a load duration factor, CD, of 1.6 and a diaphragm adjustment video). Also calculates wind loads on rooftop equipment, signs, walls, chimneys, factor, Cdi, of 1.1 trussed towers, tanks and more. ($250.00).

The timber screw values contained in Table 3 are controlled by CMU or Tilt-up Concrete Walls Analyze solid walls for out of plane loading and Mode IV fastener yielding in accordance with AWC’s National panel legs next to or between openings by automatically calculating loads to the wall leg from vertical and horizontal loads at the opening. ($75.00 ea) Design Specification® for Wood Construction 12.3.1 as required by 2021 SDPWS. Floor Vibration Program to analyze floors with steel beams and/or steel joist.

Compare up to 4 systems side by side ($75.00).

This article is reprinted, with permission, from the Timber Frame Engineering Council (TFEC) Timber Design Concrete beam/slab Program to provide bending, shear and/or torsional reinforcing.

Quick and easy to use ($45.00).

Guide 2020-19. TFEC documents are available at www.timberframeengineeringcouncil.org.■

J A N U A R Y 2 0 21

17

Hamel Music Center

The Structural Key to Outstanding Performances

By Steve Roloff, P.E., LEED AP

A

balance between the structural, acoustic, and architectural designs resulted in a world-class music facility right on the University of Wisconsin-Madison’s campus. Little does the musician or event-goer know that, to achieve acoustic perfection, three separate buildings were designed – and that these three separate buildings had to fit within a whole other, larger structure.

Experts considered the facility’s structural strength and support, reverberation and sound isolation, acoustical needs, and building materials. As part of the design team, raSmith (structural engineer) worked with Strang, Inc. (architect) and Talaske (acoustics) to provide this efficient design. The project was led by Manhattan-based Holzman Moss Bottino Architecture.

Need for a Modern Facility The Mead Witter School of Music building was constructed in 1969 and had changed little to keep pace with the times. Physical and functional upgrades were needed for both staff and students. Sound isolation was paramount to performances. Predevelopment for a

Figure 1. Street view of Hamel Music Center from University Avenue.

new School of Music facility began in the mid-2000s, but multiple setbacks put the project on hold until 2014.

The University of Wisconsin-Madison sought to transform the School of Music into a gateway connection from campus to community by creating a new arts corridor highlighted by the Hamel Music Center situated along University Avenue (Figure 1). Next door stands an art museum, University Theatre across the street, and the Wisconsin Union and Wisconsin Union Theater down the block and around the corner.

Acoustics and sound isolation needed to be top tier for exceptional performances. The structural engineering team faced challenges of isolating sound stemming from pedestrian and vehicular traffic (including frequent ambulances) along University Avenue, one of Madison’s most traveled vehicular streets. Additionally, noise pollution was expected from the building’s doors opening and closing, lobby chatter, loud HVAC systems, and music played in adjacent halls. Each of the center’s halls (concert, recital, and rehearsal) and its lobby needed to be isolated from one another, like a vault for acoustics.

Solutions to highly unique problems were developed, such as using acoustical isolation joints essential to providing sound isolation around the three individual halls. An acoustical coffer system, acoustically isolated construction, and other techniques were also employed.

Concrete for Sound Treatment and Strength

Figure 2. The scale of the reverberation chambers on each side of the concert hall is evident during construction, as are the extensive acoustical openings to the chambers and coffers in the walls overall.

18 STRUCTURE magazine

The general structural design that met acoustical and sound isolation requirements was to place three separate buildings within one larger building. The concert, recital, and rehearsal halls are self-supporting, using independent lateral truss systems, and are isolated from the large overall building (including the main lobby and its required support space) via the acoustical isolation joint (AIJ).

The AIJ assembly is just two inches thick. Still, its impact is critical: it essentially separates the concert hall from the rest of the building framing into, and supported by, the surrounding 16-inch concrete walls and serving as part of the structural system. The design firm eliminated direct steel-to-steel contact by isolating components of the AIJ with neoprene bearing pads, washers, and bushings. The acoustical isolation joint provides extra resistance to prevent sound from spreading into and beyond the walls. This, combined with the

thick walls that soar to 70 feet in spots, likened the concert hall to a concrete vault.

The acoustical engineer dictated concrete (to provide resiliency and mass) for the building and the incorporation of six-inch recessed coffers. The engineering firm then provided a structural design in which the mass, the AIJ, and the coffer system all worked in tandem to provide the acoustic level needed for the project.

Wall thickness became a necessity due to the acoustical, architectural, and structural needs. Thus, much coordination for these facets became essential. An acoustical coffer system, designed as a series of strategically sized and strategically placed circles, is seen within the concert hall’s walls and ceiling (Figures 2 and 3). These sizable circles treat sound by absorbing and reflecting it, along with the hidden reverberation chamber areas where sound passes through. Some of these circular holes were designed to penetrate through the concrete wall. Other holes were concave and topped with a dome. A third form of coffers was recessed six inches through the wall.

First, the architect had to define a repetitive pattern in which formwork could be used efficiently in construction. The acousFigure 3. The typical Acoustical Isolation Joint (AIJ) where framing outside of the tical engineer then addressed the size and type of hole that concert hall and chambers was supported by the concrete walls of these spaces. needed to be placed in the pattern. Finally, the structural design had to be engineered to specific standards, which would dictate adjustment in some of the circles. Coordination was essential the coffer recesses in the concrete wall could be added smoothly. for the complex arrangement and location of circles, which addressed Tapered formwork, with a smaller diameter inward and wider outward, an efficient formwork system and met requirements for bouncing was successfully removed from the recesses and concrete edges were sound. Each firm communicated back and forth on which needs smooth. Using self-consolidating concrete for all music halls added to required which resolution. the smooth surfaces and pristine finish. Construction on the full-size Within the walls are three layers of rebar. An inside-facing layer building resumed once testing was completed. (looking toward the inside reverberation chamber wall) comprises #6 bars at 16 inches on-center horizontally and 12 inches vertically. This Atypical Reverb Chambers layer was cut to fit around the circular recesses. Both the middle layer and outside-facing rebar layer are the structural bars of the system. Developing the reverberation chamber designs for this project proved With the coffer system in place, the structural engineer located beams to be an exception, not what a typical music hall requires. The large and designed connections to the concrete. This structural system chambers set the facility apart as a world-class venue. For instance, the supports the 70-foot-tall walls, is extremely stable, and will not be project architects were simultaneously working on the Hamel Music affected under wind load.

Center and design for a music hall on the University of Wisconsin-Eau Going back to the box-within-a-box premise, with the concert hall Claire campus. The size of those reverberation chambers was much designed, other buildings could then be added. This included two smaller than the ones in the Madison facility. reverberation chambers, a recital hall, and a rehearsal hall. The facilStructural engineering contributions were significant, especially ity’s common area had to frame into the concrete walls, which brings given that the concrete chambers had to float in the air. The vast in the AIJ. In order to connect, an embedded plate was placed on volume and height of the concert hall paled compared to the chamthe outside face of the wall and headed wall studs were secured into bers, which were two-story volumes. Framing into the sides of the the thick concrete. Some connections were very heavily loaded, so hall and chambers on all floors used the AIJ. Any components that further coordination was required to ensure recesses did not occur were not part of the concrete walls of the concert hall and chambers at such points. were connected with the AIJ assembly. Inside the chambers, the cofAdditionally, the structural engineer had to avoid extreme loads on fers are covered with cloth to restrict acoustic entry. Interior curtains the steel beam framing into the concrete wall. In this case, the full can also be adjusted to soften the amount of sound reverberating 16 inches was needed for the headed wall studs, so some recesses within the chambers. had to be placed elsewhere. The wall width was determined based on To clarify, the phrase “had to float in the air” means that the chambers calculations; the engineer used local finite element analysis to look were one story (on the east side) and two stories (on the west side) at the stress flow around some of the coffer openings and recesses to up in the air since there was occupied space that was not part of the ensure the amount of rebar was appropriate. concert hall or chambers below them.

Concern arose for concrete breakout during construction when the formwork would be removed. Since structural software did not Structural Design address all needs, the engineer resorted to free-body diagrams and Enhances Building Aesthetics brute-force engineering calculations. When software does not come into play, an opportunity to do things by hand arises. As such, the Unique to the rehearsal hall is its view of bustling University Avenue. design firm engineered a 20-square-foot building mock-up to ensure A corner of this hall’s double walls was cut open and in its place sits a J A N U A R Y 2 0 21

19

as part of the mass required for the acoustic outside box wall system. Molds of the shapes were designed exclusively for this project. A lateral truss system provides the necessary support, enabling the walls to stand freely.

Project Success Delivered

Figure 4. The completed concert hall with acoustical coffer system visible.

double set of windows, providing musicians and pedestrians a view of each other. The design application prevents outside noise from entering the hall and allows music to remain solely within the rehearsal space. Support for the exterior’s zig-zag precast panels floating over this corner condition was accomplished via a steel frame located between the two layers of glass. Each glass layer served as part of the acoustical system by isolating that steel framing.

Both recital and rehearsal halls feature distinctive exterior wall shaping of precast panels sloped in different directions and used

Design and construction occurred between 2014 and 2019 at a total cost of $55.8 million. The new recital hall’s capacity nearly doubles that of the old recital hall.

Unlike its predecessor, the concert hall’s stage comfortably fits a large group of student-musicians (Figure 4).

The isolation of the concert, recital, and rehearsal halls from one another is entirely unlike that of other auditorium and performance hall designs. Indeed, the University of Wisconsin-Madison’s School of Music Hamel Music Center is a rare project that calls for the fine art of acoustics. In honor of the work, the structural design firm has received Engineering Excellence Awards (both state and national) from the American Council of Engineering Companies.■ Steve Roloff is a Structural Group Leader at raSmith and served as the Project Manager and Senior Structural Engineer for the Hamel Music Center project. (steve.roloff@rasmith.com)

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20 STRUCTURE magazine

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structural REHABILITATION Prestressed Concrete Bridge Girders Part 1

By Dustin Black

T

he ability to premanufacture elements helped to proliferate concrete as an essential building material. Rectangular blocks, for example, could be manufactured rapidly and easily transported to make many types of structures,

from steps, sheds, and buildings to intricate tunnels, aqueduct systems, and bridges. These emerging technologies and the infrastructure they enabled have continually reshaped the way communities are organized through more efficient transportation of people and goods. The next step in the evolution of precasting concrete is prestressed concrete.

This article discusses some of the technologies and materials that make modern prestressing techniques achievable. functionality. However, the same strands that make this increase possible have characteristics that need to be designed and monitored Precast prestressed girders are a type of concrete girder that facilitates correctly. The concrete-steel interaction moves from a juxtaposed the rapid construction of a bridge using girders fabricated off-site and partnership to a synergistic relationship. Prestressing strands stimulate then transported and erected into place at the job site. Because these the compressive resistance of concrete long before putting the member girders require little to no falsework, they are a preferred solution for into service, an action that continues to intensify unless mitigated by jobs where construction speed or minimal traffic disother design considerations. Engineers have continruption is required. Prestressed girders are particularly ued to develop design methodologies that harness economical when longer beam lengths are required; the power of prestressing while mitigating the less some types are suitable for spans of up to 200 feet. Prestressing the desirable effects of the inherent material tendenPrestressing the concrete reduces the size of the cies involved. The result is a design process that required cross-section and the depth of the beam. concrete reduces the leverages prestressing benefits and allows for high The smaller cross-section size reduces the self-weight serviceability for the entire design life. of the beam by requiring less concrete. Prestressing size of the required Steel Elements members allows for a lower span-to-depth ratio, which allows for longer spans. The presence of mild cross-section and the Mild steel reinforcement is most commonly specisteel and high-stress tensioning tendons, when propfied to be 60 kips per square inch (ksi). This is erly designed, minimizes cracking and increases the depth of the beam. typical of non-prestressed concrete members, and member's durability. the material itself does not change for prestressed applications.

Post-Tensioned Elements Standard prestressing steel has a 270 ksi ultimate tensile strength.

Post-tensioning is the method of bundling a group of reinforced concrete This steel is used in strands, a group of wires wound helically, or as elements together, after they have been cast and installed in their final a tendon, which can describe a single wire, a strand, or a group of location, to create enough lateral compression that the beam-unit will strands used together. Design stresses in this steel are limited by the resist the desired amount of vertical loading. This is perhaps the most American Concrete Institute (ACI) and the American Association readily available method of prestressing objects. This method requires of State Highway Transportation Officials (AASHTO) specifications that the beam elements are cast with hollow tubes such that prestressing during jacking, immediately after prestress transfer, at anchorage tendons can be installed and loaded in situ. devices and couplers, and at service limit states. The primary characteristics of prestressing steel that must be considered during design Pretensioned Elements are the stress-strain curve and relaxation of the prestressing strands.

Pretensioning is the method of introducing high-strength steel ten- These are behavioral characteristics of the prestressing steel that directly dons to the beam element, stressing them to a predetermined load, correlate to the overall results of the prestressing process. and then casting the concrete around them. Once the concrete has High Strength Concrete gained enough strength, the load is released from the steel tendons, thereby transferring this load to the concrete portion of the composite High strength concrete (HSC) is concrete that has 28-day commember. It is the relative loading of the strands before the concrete pressive strengths of 10 ksi or more. The use of HSC, rather than is cast that gives this method its name. Pretensioned beam elements normal-strength concrete, enables a section of a given size to support are the main focus of this article. larger loads or span longer distances. The improved durability usually associated with HSC increases the lifespan of structures and increases the ability to meet greater future loading demands.

Design of Prestressed Girders High-strength concrete has a higher paste content, but the paste has The introduction of high-stress strands into the tension fibers of a lower water/cement ratio. As a result, the shrinkage of high-strength a concrete member allows for dramatic increases in the member's concrete is about the same as that of normal concrete.

Prestressing Techniques

22 STRUCTURE magazine

Prestressed Concrete Design Considerations

are interdependent. If required, rigorous calculation of the prestress losses should be made following a method supported by research data. However, for conventional construction, such refinement is seldom warranted or even possible at the design stage, since many of the factors are either unknown or beyond the designer’s control.

Raja (2012) notes that the increased modulus of elasticity reduces the elastic shortening due to the prestress force. Further, the longterm deflection due to creep and shrinkage are also reduced. Hence, by using high strength concrete, the prestress losses are significantly reduced, increasing the efficiency of such construction.

AASHTO’s Load and Resistance Factor Design (LRFD) Article 5.9.5.3, based on the National Cooperative Highway Research Program (NCHRP) Report 496, considers the interaction of creep and shrinkage throughout the life of the member. This method breaks up the evaluation of prestressing losses into three distinct periods: 1) at transfer, 2) transfer to bridge deck placement, and 3) bridge deck placement to final time. The first period accounts only

The instantaneous and time-dependent implications of the effects of prestressing steel must be understood and accounted for in the prestressed concrete girder design process.

Loss of prestress can be characterized as that due to instantaneous loss and time-dependent loss. Losses due to anchorage set, friction, and elastic shortening are instantaneous. Losses due to creep, shrinkage, and relaxation are time-dependent. For pretensioned members, prestress loss is due to elastic shortening, creep of concrete, and steel relaxation.

Elastic shortening is the immediate shortening of the concrete member due to the application of prestressing. Prestressing strands are initially tensioned with hydraulic jacks at solid abutments, which causes them to stretch slightly. Concrete is then cast around the tightened strands to form the beam element. Once the concrete has cured sufficiently, the initial prestress is released, which effectively loads the concrete member with the remaining stress within the strands as they try to return to their original length. The result is that the concrete encounters enough stress to buckle slightly and deform longitudinally, resulting in a slightly shorter member than what was cast. As the member shortens, the strands shorten also. Because of this shortening, the strands lose a portion of their internal stress.

Shrinkage loss is associated with the time-dependent change in length of concrete along the direction of the tendons. As concrete shrinks over time, the stands themselves also shrink, causing a reduction of the effectiveness the prestressing strands provide to the member.

Similar to the effect of elastic shortening, shrinkage loss occurs because the prestressing strands are anchored into the concrete itself and must react, i.e., shorten, as the concrete does.

Creep is the stress-dependent change in length of a girder subjected to compression along the direction Figure of prestress loss over time (Source: MDOT 2019). of the tendons.

Relaxation is the stress-dependent gradual decrease of the prestress for the instantaneous effects of elastic shortening when the stress in force over time. The strands have a slightly lower modulus of elasticity. the strands are transferred to the concrete member and is illustrated Prestressing steel encounters more deformation than reinforcing steel by the near-vertical curve between Points 1 and 3 in the Figure. when loaded but has a substantially higher ultimate strength. This The remaining periods include the long-term effects of shrinkage, deformation under loading is commonly referred to as relaxation. creep, and relaxation within the girder. The parabolic curves illustrate Relaxation in steel strands is much akin to the reduction of size in time-dependent losses in the Figure. However, the third period adds concrete members due to creep. The amount of relaxation a strand the effect of differential shrinkage associated with the addition of a encounters is dependent on the amount of initial stress in the strands composite deck section. Deck-only and full-section continuity are relative to the ultimate stress. discussed in the second part of this article in a future issue.

The combination of elastic shortening, shrinkage, creep, and relaxAASHTO uses several assumptions to simplify the prestressing loss ation results in a cumulative deformation on the concrete member calculations. This method assumes 1) normal weight concrete that and a mitigating factor in the prestressing strand's efficacy that must was steam or moist-cured, 2) low-relaxation strands, and 3) average be accounted for in the design. The interaction of elastic shortening, exposure conditions and temperatures. AASHTO 4 also has provishrinkage loss, and creep regarding total prestressing loss can be sum- sions for use with lightweight concrete. marized as follows: bridge girders instantaneously shrink, or shorten, This is the first of two articles providing a high-level overview of under the axial load applied by the prestressing tendons. All concrete the design considerations of prestressed concrete girders. In Part 2, shrinks as internal humidity reduces, and this phenomenon also the discussion continues with fundamental design considerations for begins immediately. Additional instantaneous shortening occurs from internal stress distributions within prestressed concrete girders and the application of the various dead loads (e.g., bridge deck, parapet the methodology for applying those principles.â– walls). On a strain-time curve, a member's flexural loading would be evident by the presence of a jump in strain at the time of loading.

References are included in the PDF version of the Because the dead loads are sustained, the beam continues to deform article at STRUCTUREmag.org. over time. All three of these phenomena cause the entire member to shrink, effectively reducing the stress within the prestressing strands by 25,000 to 50,000 psi in some cases.

Dustin Black is a Design and Operations Engineer at the Michigan Accurate estimation of the total prestress loss requires recognition Department of Transportation. that the time-dependent losses resulting from creep and relaxation J A N U A R Y 2 0 21

23

building BLOCKS Axial Load Limits for Prestressed Piles Reconciling the 2018 IBC, ACI 318-19, and PCI’s 2019 Report

By John C. Ryan, Ph.D., P.E., and Timothy W. Mays, Ph.D., P.E.

T

his is a follow-up to a previous STRUCTURE magazine article titled Rethinking Seismic Ductility (March, 2016). The previ-

ous article presented possible shortcomings associated with the International Building Code (IBC) prescriptive seismic design philosophy used for both auger cast piles and prestressed piles to contrast foundation ductility design with that used for other structural elements. It also provided a side-by-side comparison of design and performance issues associated with auger cast piles and prestressed piles.

Figure 1. Critical moments developed during moment-curvature analysis of prestressed concrete piles.

Recently, there have been significant changes to the seismic design recognized that piles that are significantly stronger than required provisions for prestressed piles. Previous versions of the American to resist seismic demands might not need seismic confinement.

Concrete Institute’s ACI 318, Building Code Requirements for They provided an exception for piles that have been designed using Structural Concrete and Commentary, did not address the seismic seismic overstrength factors. The only drawback of adopting new design of prestressed piles. Now, both ACI 318-19 and the 2018 confinement equations is that the ISU researchers were not satisIBC include matching seismic design criteria for prestressed piles. fied with the amount of moment drop that occurred at a plastic It is understood that these seismic provisions will be excluded from hinge after the onset of cover spalling. To address their concerns, future editions of the IBC now that they fall under the purview ISU researchers proposed axial load limits as an attempt to proof ACI 318. vide what they considered to be a more reliable seismic response.

As to specific provisions, new confinement equations based on research performed at Iowa State University (ISU) are included in 2018 IBC and ACI 318-19 both ACI 318-19 and the 2018 IBC. These equations replace those used since the 2000 IBC, which were based on expected ductility The 2018 IBC and ACI 318-19 prestressed pile provisions present performance that was not wholly quantified in the literature at factored axial load limits for piles. For Seismic Design Categories the time. The ISU research recommends C through F, IBC 1810.3.8.3.4 and ACI the newly adopted confinement equations 18.13.5.10.6 limit the factored axial load with a presentation of results of a comprefor all square piles to 0.2f´cAg. In many cases, hensive study of confinement requirements this limit is less than the factored axial loads for square and octagonal solid piles with traditionally considered for the design of The code committees circular spiral confinement and 2 inches of commonly used 14-inch square piles in areas cover to the spiral. Square piles with square of high seismicity. As discussed above, the adopting the equations also confinement were not included in the scope code committees established the new axial of the research. Overall, the authors agree load limit when adopting the new confinerecognized that piles that that the adoption of the new equations was ment equations. ISU researchers proposed an are significantly stronger an excellent addition to both ACI 318-19 axial load limit of 0.2f´cAg for 14-inch square and the 2018 IBC since the resulting quanpiles with circular strand configurations and than required to resist tity of confinement spiral is now based circular spiral with 2 inches of clear cover. on specified curvature ductility capacities Square piles with square confinement were seismic demands might not at locations of pile hinging (i.e., 18 for not considered in these recent studies. areas of high seismicity and 12 for areas of The basis of the limit on axial load need seismic confinement. moderate seismicity). Another encouragdescribed above can be explained using ing result is that the new equations yield Figure 1. A drop in moment capacity to a a required spiral quantity very similar to what was required by level Mdrop after the first peak moment Mpeak,1 and subsequent to previous versions of the IBC. In other words, the overall cost reaching the second peak moment Mpeak,2 is evident. The previous of prestressed piles changes very little with the adoption of the research showed that the percent moment drop is related to the axial new equations. The code committees adopting the equations also load applied during moment-curvature analysis. The ISU research

“

24 STRUCTURE magazine

suggested that the drop should be limited to approximately 40% of for prescriptive and performance-based design are presented sepathe first peak moment and that the most effective way of adhering rately for piles considered part of the lateral force-resisting system to the 40% limit was to limit the axial load. Therefore, axial load (i.e., bridges and piers) and for piles that are not considered part limits were recommended for all 14-inch pile configurations to of the lateral force-resisting system (i.e., buildings). Note that prevent loss in moment relative to the first peak moment in excess building piles are typically designed to remain elastic during the of approximately 40%, based on pile configurations considered design earthquake event. Buildings are required to be detailed such (i.e., 14-inch square piles with 2 inches cover and round spiral). that seismic damage is ductile and occurring above the foundation The drop in moment reportedly correlated well with another level. The damage that occurs in the lateral force-resisting system desired performance outcome; maintaining a response in which above grade dramatically reduces the maximum force delivered the curvature at the initiation of tension cracking (φ cr) is less to piles. Although performance-based design of bridge and pier than the curvature associated with the initiation of unconfined piling is commonplace in the industry, performance-based design of concrete spalling (φsp), where strain in the outermost unconfined compression fiber equal to 0.004 is taken as the value that spalling would initiate.

To the best of the authors’ knowledge, it does not appear as though recent studies have attempted to define or study the rationale for the concern or to determine if the moment drop would actually result in poor performance of the subject piling. Instead, the authors note that larger moment drops were deemed to be “unacceptable for piles in seismic regions” and “the stability of the pile experiencing significant moment drop may not be dependable.”

As shown in Figure 2, page 26, results from an ongoing Citadel research projUse for all types of concrete and grout applications, from slabs-on-grade to ect on pile ductility suggest that, in containment tanks, multi-story post-tension structures to bridge decks. many cases, increasing the axial load limit from 0.2f´cAg to 0.3f´cAg has a negligible impact on expected seismic ADVANTAGES performance. Note that moment-cur¡ Maximize joint spacing (up to 300 ft, L/W 3:1) ¡ Enhance compressive and flexural strengths vature stability is unaffected in Figure 2, and the area under the moment-curva¡ Prevent shrinkage cracking and curling ¡ Eliminate pour/delay strips ture curve is approximately the same for ¡ Thinner slabs and walls viable ¡ Reduce long-term relaxation of P/T tendons all three axial load variations shown. and shear wall stresses

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The prescriptive procedure is similar to the current procedure presented in the 2018 IBC and ACI 318-19. The performance-based design option is similar to procedures for the seismic design of piles contained in American Society of Civil Engineer’s ASCE/ COPRI 61-14, Seismic Design of Piers and Wharfs, Caltrans Seismic Design Criteria (2019) for bridges, and ASCE/ SEI 41-17, Seismic Evaluation and Retrofit of Existing Buildings. Procedures

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building piles is exceptionally complicated and not commonly used in practice.

When performance-based design is used to design building piles, more economical pile designs are possible. Prescriptive requirements related to spiral quantity, spiral placement, connection detailing, splice detailing, and other design issues are directly modeled to accurately predict, rather than just assume, their performance.

rotation demands. Performance-based design procedures account for moment loss directly.

Option 3

Designing for Axial Loads How can an engineer design prestressed piles for a load above the axial load limit and still meet the code’s intent?

The authors’ opinion is that engineers who wish to design for axial loads in excess of the prescribed limits, while complying with the current axial load limits now established by the 2018 IBC and ACI 318-19, have three Options. Options 1 and 2 below assume that a drop from the ultimate moment in excess of 40% is problematic, as opined by the Iowa State University researchers. Option 3 may provide a more practical approach, but it is not currently available. Current PCI research underway at The Citadel will result in conclusions and practical applications for Option 3 by the end of 2020 or early 2021.

Figure 2. Typical moment (k-in.) vs. curvature (in.-1) plots for a commonly used 14-inch prestressed pile.

The Citadel is currently working on a research project to more closely examine and possibly modify the axial load limits proposed by ISU researchers. The Citadel project considers circular and square spiral with different covers and strand configurations used in practice. The study aims to determine if a more accurate axial load limit can be established and justified by the results. The extent to which the moment strength recovers throughout the moment-curvature response, indicated as Mpeak,2 in Figure 1, is examined as a more significant contributing factor in the pile’s overall stability. This was not a consideration of the previous study.

Conclusion

As a final note, the authors’ goal for this article is to help establish a consistent methodology for the design of different types of piles and provide structural engineers of record with tools that result in Figure 3. Moment-curvature for 14-inch auger cast safe designs that are not unnecessarily pile with 0.20f´c A g axial load. Note drop greater costly. Currently, and as noted above, than 40% and no return to Mpeak,2.

Options 1 and 2 represent the peerreviewed and codified options available Option 1 to the pile designer. However, inconsistency in the design requireThe easiest solution to the axial load limit is to perform a moment- ments for auger cast piles versus prestressed piles is interesting and curvature analysis of the concrete pile cross-section in question should be brought to the reader’s attention. and verify that the moment drop from the ultimate moment is Figure 3 provides the moment-curvature response of a 14-inch less than or equal to 40%. It is likely that many actual condi- auger-cast pile with prescriptive confinement reinforcing. It can be tions (e.g., cover, axial load, and spiral configurations) do not seen that a similarly sized auger-cast pile can be shown to be more violate the ISU researchers’ concern regarding moment drop, susceptible to the moment drop issue without the benefit of a second and the new confinement equation is appropriate for use at loads peak moment. Therefore, applying a consistent methodology would higher than the axial load limit. Valid moment-curvature analytical disallow the use of the represented 14-inch auger-cast pile with code models must account for confined and unconfined concrete and allowed minimum reinforcement. include nonlinear material properties. The required analysis can be performed using readily available commercial software proAcknowledgments grams such as SAP2000. The designer should note that Option 1 may be considered by some as an application of the Alternate The authors wish to thank the Precast/Prestressed Concrete Institute Means and Methods provisions allowed by the governing codes (PCI) for their support. The Citadel research discussed in this and standards and used by engineers to satisfy the intent of the article would not be possible without the financial support code when a particular provision is inappropriate or really doesn’t from PCI and PCI committee members’ input.■ apply. As such, some jurisdictions may require notification when using this approach.

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

Option 2 In accordance with procedures defined in the 2019 PCI Recommended Practice for Design, Manufacture, and Installation of Prestressed Concrete Piling, designers can design the piles using performancebased design. Performance-based design procedures ensure adequate confinement is placed along the length of the pile by providing a spiral that results in plastic rotation capacities exceeding plastic 26 STRUCTURE magazine

John C. Ryan is an Assistant Professor at The Citadel in Charleston, SC. (jryan8@citadel.edu) Timothy W. Mays is a Professor at The Citadel in Charleston, SC. (timothy.mays@citadel.edu)

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27

NCSEA NCSEA News

National Council of Structural Engineers Associations

2020 Virtual Summit Delivers Education, Technology Solutions to Industry NCSEA rallied against the struggles of 2020 and held its first virtual Structural Engineering Summit during the months of October and November. The 2020 Summit was to be held in Las Vegas at the MGM Grand, but due to national restrictions, this year’s summit was transitioned to the web. This landmark event included three days of live-streamed presentations, three days of Bonus Content presentations with live speaker interaction, access to the Virtual Exhibit Hall for two months, and interactive virtual lounges allowing attendees to interact. By offering the Summit virtually, NCSEA had attendees from Canada, India, and New Zealand.

This year’s attendees had the opportunity to earn up to 26 hours of continuing education, the most NCSEA has ever been able to offer during the Summit.

The expert-led sessions focused on the future of the AEC industry, mass timber, wind loads, resilience, racial equity, codes, and so much more. In addition to virtual education, there were plenty of opportunities for live speaker interaction with attendees during and then after the presentation. The Summit also hosted Interactive Lounges allowing attendees daily opportunities to discuss a wide range of topics, meet other SEs from near and far, share some ideas, have a laugh or two, and make new contacts.

An exciting addition to this year’s event was the expanded access to the virtual Exhibit Hall. Not only was the Hall open from early October to late November, but it was open to everyone, not just Summit registrants. With twenty-five exhibitors, an impressive amount of engagement was recorded during the six weeks the Exhibit Hall was open: over 7,000 total visits to Exhibitor booths and over 2,800 downloads, and our exhibitors gave away over 60 raffle prizes.

The NCSEA Awards celebration was also held virtually this year. This event annually highlights the NCSEA Special Awards, honoring NCSEA members who have provided outstanding service and commitment to the association and to the structural engineering field, as well as the Excellence in Structural Engineering Awards, showcasing some of the best examples of structural engineering ingenuity throughout the world. This year’s Special Awards were presented to: • NCSEA Service Award to Brian Dekker, P.E., S.E., for the work he has put toward the betterment of NCSEA that is beyond the norm of volunteerism.

• Robert Cornforth Award to Jerry Maly, P.E., for his exceptional dedication and exemplary service to an NCSEA Member Organization and to the profession.

• Susan M. Frey NCSEA Educator Award to Duane K. Miller, Sc.D., P.E., for his genuine interest in, and extraordinary talent for, effective instruction to practicing structural engineers.

• James M. Delahay Award to Kevin Moore, P.E., S.E., SECB, for his outstanding contributions toward the development of building codes and standards.

In addition, the Excellence in Structural Engineering Awards were presented for exceptional structural engineering projects in eight categories: New Buildings up to $30 Million, New Buildings $30 Million to $80 Million, New Buildings $80 Million to $200 Million, New Buildings over $200 Million, New Bridges or Transportation Structures, Forensic/Renovation/Retrofit/Rehabilitation Structures up to $20 Million, Forensic/Renovation/Retrofit/Rehabilitation Structures over $20 Million, and Other Structures. For more information on both of these awards programs and to view the awards presentations, visit www.ncsea.com/awards.

Next year’s Structural Engineering Summit will be held in New York at the Hilton Midtown, October 12-15, 2021. Information on attending and exhibiting will be available soon at www.ncsea.com.

28 STRUCTURE magazine

Speakers and chat during a live Q&A.

Booths at the Virtual Exhibit Hall.

Attendees of the Interactive Lounge following the session: How Do We Progress Towards Racial Equity in the Structural Engineering Community?

News from the National Council of Structural Engineers Associations

Thank You to the Speakers of the 2020 Summit

NCSEA would like to thank all of the speakers that participated in the 2020 Virtual Structural Engineering Summit. When planning an event such as this, it is imperative to gain the participation of experts in the field. Their willingness to share their time over the last couple of months, along with their expertise in their designated areas, was critical to the success of this event. In much of the way we have all had to overcome what 2020 has thrown at us, our speakers stepped up in a way we never requested of them before. They mastered new tech, adapted to present to no one but a camera, and then exceeded expectations by interacting virtually with attendees. Our expert speakers are instrumental to the Summit and we cannot thank them enough for going above and beyond to provide a phenomenal experience to this year's attendees.

Start Studying for the Next PE Structural Exam Today

The next NCEES PE Structural Exam is coming up on April 22 and 23. Start preparing with NCSEA now! NCSEA’s on-demand course provides the most economical PE Structural Exam Preparation Course available. The course includes 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.

Students also have access to a virtual classroom exclusively for course attendees! Ask the instructors directly whenever questions arise. This NCEES PE Structural Exam Preparation Course allows you to study at your pace but with instant access to the material and instructors.

Several registration options are available, visit www.ncsea.com to register yourself or to learn more about special group pricing!

Sizing Up Seismic Design: A Webinar Series Highlighting Examples from SEAOC’s Seismic Design Manual Based on IBC 2018/ASCE 7-22 The Structural Engineers Association of California (SEAOC) The event will begin on January 14, 2021 and run as follows: and NCSEA have teamed up to deliver a brand new Web- • January 14, 2021 – Introduction to the Seismic Design Manual, Based Seminar Series for you on the newly-published Seismic Provisions in ASCE 7-22 and 2018 IBC, and Wood Diaphragms Design Manual. This web-based Seismic Design Seminar will • January 21, 2021 – Concrete Shear Walls and Diaphragm Design be delivered over six weeks in six 1.5 hour webinars by some • January 28, 2021 – Pile Foundations and Grade Beams of the industry’s best and brightest minds.

Each presentation will provide examples of IBC 2018/ASCE • February 4, 2021 – Steel Special Moment Frames 7-22 problems that include a discussion of the provisions in • February 11, 2021 – Buckling Restrained Braced Frame the standard, speaking to big-picture concepts and the nuts, • February 18, 2021 – Wood Hotel Building (On Foundation) bolts, and nails of implementation.

The registration fee for the Seminar is $495 for members ($800 for nonmembers), which includes all 9 hours of education. Each individual webinar can be purchased separately for $195 for members ($250 for nonmembers). The Seismic Design Manual may also be purchased separately through ICC. Recordings of the webinars will be available to those that purchased single webinars or the Bundle, to view at any time after the live seminar.

The entire series will provide 9.0 PDHs; 1.5 per individual webinar. To learn more and to register, visit www.ncsea.com.

NCSEA Webinars

Register by visiting www.ncsea.com.

January 19, 2021

When Reroofing Requires a Lateral Analysis

January 26, 2021

Special Inspection and Structural Observation

Dale Statler, P.E., and Jerry Maly, P.E.

Violeta D. Tihova, P.E., S.E.

This webinar recounts the origin and evolution of the provision since its introduction, discusses fundamental flaws in its requirements, and argues for the limitation of its applicability to repairs that can be made to correct visible deterioration and/or deficiencies that are readily observed and remedied in the normal course of a roof replacement, or specific geographic regions or building types known to have extraordinary roof diaphragm vulnerabilities.

Special inspections and structural observations are components of the overall quality assurance plan. What is the role and responsibilities of the SER and other parties involved in quality assurance?

The presentation will outline best practices in specifying special inspections and structural observation.

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

J A N U A R Y 2 0 21

29

SEI Update

SEI Online

Check out the Latest SEI Virtual Events www.asce.org/SEI/virtual-events

• #SEILive Conversations with Leaders on Hot Topics in Structural Engineering • NEW – SEI Standards series See the full program on page 35 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 pm - 2:30 pm US ET. Do not Miss the First in the Series: Thursday, January 21 - ASCE/SEI 48 Design of Steel Transmission Pole Structures Registration deadline January 19, 10:00 pm US ET. Individual session: Member $49, Nonmember $99. Student member: Free registration.

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NEW in the ASCE Bookstore and Library www.asce.org

Design and Performance of Tall Buildings for Wind Task Committee for the Design and Performance of Tall Buildings; edited by Preetam, Biswas P.E. Provides a framework for the design of tall buildings for wind, based on the current state-of-practice in tall building structural design and wind tunnel testing.

SEI Graduate Student Chapters

SEI encourages graduate students to establish local SEI Graduate Student Chapters (GSCs) to enhance the education of students preparing to become structural engineers and engage student members in SEI for a successful transition from college to career. GSCs organize and manage visiting speakers, prospective student events, field trips, participate in SEI, perform outreach activities, and more.

Arman Tatar is a Ph.D. candidate in Structural Engineering at Michigan Technological University and Chair of the SEI GSC Leadership Council. His goals are to expand GSC outreach to other universities that have not yet joined SEI, engage with the structural engineering graduate student community to address their needs, and provide online services for the structural engineering graduate students. Learn about SEI Grad Student Chapters and how to start one at www.asce.org/SEILocal.

SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle 30 STRUCTURE magazine

News of the Structural Engineering Institute of ASCE Join us this year in celebrating 25 years of SEI – advancing and serving structural engineering!

Advancing the Profession

SEI SE2050 Commitment Program The SEI Structural Engineers 2050 Commitment Program (SE 2050 ) provides structural engineers resources to aid the profession in doing its part to get to net-zero by 2050.

The SE 2050 Commitment goal is to provide structural engineers with the necessary tools and resources to contribute and track projects toward the vision of net-zero embodied carbon buildings by 2050 through three strategies: Plan, Implement, and Share. After a firm formally signs onto the commitment, they will need to create an Embodied Carbon Action Plan (ECAP). The firm’s ECAP will center around four main topics: an embodied carbon education plan, a reporting plan, reduction strategies, and advocacy. Firms will then implement their ECAP with the support of educational resources and tools accessible through www.SE2050.org.

Firms will input projects’ embodied carbon measurements into the SE 2050 database. After adequate embodied carbon data has been collected for different regions and building types, embodied carbon benchmarks and reduction targets will be developed.

Learn more and view embodied carbon resources at www.SE2050.org.

ASCE 7-16 Supplement #2 Public Comment through January 11, 2021

Public comments on Supplement 2 for ASCE/SEI 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, are invited through January 11, 2021. This Supplement updates two sections of the standard: Section 12.9.1.5, which clarifies Horizontal Shear Distribution provisions for torsional effects, and Section 16.4.2.1, which updates Force-Controlled Actions provisions to align with industry standards, specifically the 2017 PEER TBI Guideline. To participate in the public comment period, contact James Neckel, ASCE Codes and Standards Coordinator, at jneckel@asce.org.

Public Comment Invited on New Professional Standard ASCE/COS 73-XX Standard Requirements for Sustainable Infrastructure through January 25, 2021

The components and outcomes described in the chapters of this standard are intended to guide sustainable infrastructure development through the entire life-cycle process. Leadership shall encourage transformative development of the infrastructure solution at the earliest stages; consider and analyze all reasonable alternatives; and consider natural, no-construction, and constructed project solutions. For constructed project solutions, the entire life cycle of the project shall be considered within the context of this standard.

Accessing the Public Comment System will require using or creating an ASCE web user account. Questions? Contact James Neckel at jneckel@asce.org.

Call for Members – New Standard for Design of Prestressed Concrete Transmission Pole Structures

Development of the new standard will begin in 2021, chaired by Ken Sharpless, P.E., F.SEI, F.ASCE. Practicing engineers, researchers, building officials, contractors, and construction product representatives are encouraged to apply online by January 31. Learn more at www.asce.org/SEI.

Errata

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.

J A N U A R Y 2 0 21

31

CASE in Point Did you know?

CASE has tools to help firms deal with a wide variety of business scenarios. Whether your firm needs to establish new procedures or simply update established programs, CASE has the tools you need! If your firm needs to update its current Risk Management Program, or establish a program within the firm, the following CASE documents will guide employees: 962-H: Tool 1-1: Tool 1-2: Tool 2-1: Tool 2-4: Tool 3-1: Tool 3-4: Tool 5-6:

National Practice Guideline on Project and Business Risk Management Create a Culture for Managing Risks and Preventing Claims Developing a Culture of Quality A Risk Evaluation Checklist Project Risk Management Plan A Risk Management Program Planning Structure Project Work Plan Templates Lesson Learned

CASE Popular Guideline Updated!!

CASE 962-D: A Guideline Addressing Coordination and Completeness of Structural Construction Documents

CASE has released a comprehensive update to its popular Guideline Addressing Coordination and Completeness of Structural Construction Documents. The guideline will assist the structural engineer of record (SER) and everyone involved with building design and construction in improving the process by which the owner is provided with a successfully completed project. Their intent is to help the practicing structural engineer understand the importance of preparing coordinated and complete construction documents and to provide guidance and direction toward achieving that goal.

This guideline focuses on the degree of completeness required in the structural construction documents (Documents) to achieve a “successfully completed project” and on the communication and coordination required to reach that goal. They do not attempt to encompass the details of engineering design; instead, they provide a framework for the SER to develop a quality management process.

The coordination and completeness of Documents vary substantially within the structural engineering profession and among the various professional disciplines comprising the design team. The SER’s goal should be meeting both the owner’s and the contractor’s needs by producing a complete and coordinated set of Documents. Owners and contractors generally understand that some changes will occur because they realize that no set of Documents is perfect. The SER must focus on completeness, coordination, constructability, and reducing errors to minimize potential changes.

An overall comprehensive update was done to the document to keep up with best business practices and current industry standards.

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

Donate to the CASE Scholarship Fund!

The ACEC Coalition of American Structural Engineers (CASE) is currently seeking contributions to help make the structural engineering scholarship program a success. The CASE scholarship, administered by the ACEC College of Fellows, is awarded to a student seeking a Bachelor’s degree, at minimum, in an ABET-accredited engineering program. Since 2009, the CASE Scholarship program has given $34,500 to help engineering students pave their way to a bright future in structural engineering.

We have all witnessed the stiff competition from other disciplines and professions eager to obtain the best and brightest young talent from a dwindling pool of engineering graduates. One way to enhance the ability of students in pursuing their dreams to become professional engineers is to offer incentives in educational support.

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

32 STRUCTURE magazine

News of the Coalition of American Structural Engineers WANTED: Engineers to Lead, Direct, Engage with CASE Committees!

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

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

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

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

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

Toolkit Committee – 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’ regular 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 (mternieden@acec.org), subject line CASE Committee: • 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!

NEW – Strategies for Developing a Respectful, Diverse, and Inclusive Workplace Culture Employers are under significant scrutiny for the environment that exists in their workplaces.

In recent years, #MeToo, systemic racism, gender inequality, generational differences, and negative behaviors in the workplace have posed enormous challenges for managers that, if ignored, can result in lack of engagement, attrition, and lawsuits.

This course is designed to help those in management positions learn how to address these challenges by developing a culture of respect and inclusion. When respect thrives in the workplace, so does an engaged staff commit to excellence.

This four-module course combines the scheduling ease of video learning and the immediacy and intensity of a live classroom.

• Access recorded lectures anytime via computer, tablet, or smartphone • Attend weekly live discussions with the instructor via WebEx Meeting • Work together on small group assignments Participants will earn a minimum of 8 PDHs too!

The program begins on January 25, 2021. Only 40 Seats Available Register today at https://education.acec.org/diweb/catalog/item?id=6096116

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

J A N U A R Y 2 0 21

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structural FORUM Lessons Not Learned

By James Lefter, P.E.

T

here are many Lessons not Learned in spite of experience. Thankfully there are also examples of Lessons Learned through experience that have contributed to meeting technical and ethical responsibilities. As engineers, we should pursue the latter. The most tragic and devastating LESSON NOT LEARNED in our lifetime is the undoing of all of the bitter lessons learned through experience in battling pandemics.

Today’s coronavirus pandemic has wreaked havoc, bringing death, economic depression, widespread disease, unemployment, education disruption, and more.

In 2005, WHO published a checklist for controlling a pandemic: preparation, surveillance, case investigation and treatment, and preventing the spread of the disease in the community. Despite strong advocacy by the medical community, the U.S. has been reluctant to relearn this lesson.

Every four years, the American Society of Civil Engineers (ASCE) issues a report card on Infrastructure investment needs. It covers 16 categories, ranging from hazardous waste to bridges and dams to parks and recreation. The report card effort originated from a 1988 congressionally chartered report that graded America’s Infrastructure as a C. ASCE independently continued the grading in 1998. Since then, the U.S. performance has not risen above its current grade of D+.

A $2 trillion total funding gap is estimated.

The public is aware of these problems, but political leaders seem intent on lowering taxes and depleting the available funding. In the past, economic recovery largely depended on government infrastructure investment to provide employment and rebuild the economy.

Proposals are being considered by the current legislature to fund infrastructure as an economic stimulus; time will tell whether the lesson was learned.

OSHA Construction Industry Reports At job sites, OSHA has reported that the construction industry, with only 5% of the total industry workforce, suffers more than 21% of on-the-job deaths. The disproportionate risk is most often caused by falls, electrocution, struck-by-object accidents, and caught-in-between mishaps. Despite OSHA standards developed for worker protection, deficiencies still exist in worker training, site safety practices, planning, and worker risk acceptance.

34 STRUCTURE magazine

WTC Disaster

“Experience is a dear teacher, but some will learn from no other.”

- BEN FRANKLIN

Employing lessons learned from the 1946 airplane strike on the Empire State Building in New York City, the World Trade Center (WTC) Towers were designed structurally to withstand a jet plane impact. The designers knew that most of the Empire State Building deaths were due to fire. The WTC Towers were designed to resist impact by small planes.

The FEMA study into the collapse indicated that the structures could have remained standing following the jet impact for an extended period had it not been for the fires.

Unfortunately, the collapse’s primary driver was due to the fire and failures of the fire protection systems, from the type, adhesion, and thickness of the fireproofing to damage to passive systems caused by the impact and inaccessibility to firefighters.

Based on the WTC experience, new recommendations from the National Institute for Standards and Technology (NIST) have been adopted in model building codes. These changes include additional exit stairways, supports for passive fireproofing systems, increased bond strength for fireproofing, and higher fire resistance ratings for primary structural frames of buildings 420 feet and taller.

Most public buildings post code-required warnings that occupants should not use the elevators in a fire emergency. Many handicapped and disabled persons cannot use stairs and must congregate in designated areas awaiting rescue crews to carry them out. The WTC disaster refuted this policy. New building codes permit the use of elevators in a fire emergency if the elevators are in a standalone smoke-proof hoist way, have emergency power, and have manual operation by a firefighter.

San Fernando Earthquake In the aftermath of the 1971 San Fernando earthquake that destroyed an old Veterans Administration (VA) hospital and killed 46 patients and staff, the VA Administrator challenged the staff: “this must never happen again.” As a result, the VA issued its new earthquake design code in 1972. It was the first code to require a structural seismic design based on drift, rather than force, a concept now widely accepted. It provided not only provisions for the design of the structural and other building systems, but also for special access to the facility and backup utilities so that a VA hospital

would remain operational after an event. This was chiefly because it was expected that firefighting and rescue equipment could not reach VA sites immediately after a large earthquake.

The VA also initiated a program to evaluate and reinforce its 160 existing hospitals. Those found to be too vulnerable to strengthen were evacuated and demolished.

The U.S. State Department followed the 1972 VA design code for its new Embassy in Haiti. Hit by the 2010 Haiti earthquake that claimed over 250,000 lives, the Embassy building survived the earthquake without damage and provided community assistance in the aftermath.

Today, many building codes require that some structures be designed to resist attacks, as well as earthquakes and other natural disasters. As a general rule, many technical measures in the current International Building Code (IBC) for high-risk seismic zones can also be useful in mitigating terrorist attacks.

Structural Engineers Generally, building codes are based on observed experience and may not foresee challenges on a specific project. Few owners, operating on tight budgets, encourage design that exceeds code requirements. There may be a conflict between an owner’s cost concerns and the engineer’s prime ethical concerns, or design responsibilities by contract may fall outside the structural engineer’s purview.

A critical Lesson Learned, especially for younger engineers, is to review specific issues with construction workers at the job site. They often know about construction and repair methods that may shed light on the issue at hand.

Finally, all engineers are encouraged to participate in professional society meetings where many important technical issues are discussed, share both successes and failures with other engineers, and utilize these lessons learned in future design and construction.■ James Lefter (retired) was a Visiting Professor at the University of Illinois and Virginia Tech. He held Senior Executive Service positions in the Office of Facilities Veterans Administration. He served on the American Concrete Institute Committee for Building Code Requirements (ACI 318) and as Director of the Learning From Earthquakes Program of the Earthquake Engineering Research Institute.

JANUARY 2021

LIVE VIRTUAL S SESSIONS ON NEW SEI STANDARDS Join for exclusive interaction with expert ASCE/SEI Standard developers on state-of-the-market updates.

NOT AN ASCE/SEI MEMBER?

Participants will gain an understanding of the technical revisions and review a design example. Attendees are encouraged to join and participate in Live Q&A. These sessions are intended for structural engineers, students, and the entire project stakeholder team.

JOIN AT GO.ASCE.ORG/

STANDARDS AND ENTER PROMO CODE STRUCM21

Each session is LIVE and only available 1:00 - 2:30 pm US ET: • JANUARY 21, ASCE/SEI 48 Design of Steel Transmission Pole Structures– Hurry! Registration deadline January 19, 10:00 pm US ET. • MARCH 18, ASCE/SEI 43 Seismic Design Criteria for Structures, Systems, and Components in Nuclear Facilities • MAY 20, ASCE/SEI 49 Wind Tunnel Testing for Buildings and Other Structures • JULY 15, ASCE/SEI 72 Athletic Field Lighting • SEPTEMBER 16, ASCE/SEI 59 Blast Protection of Buildings • NOVEMBER 18, ASCE/SEI 8 Specification for the Design of Cold-Formed Stainless Steel Structural Members Individual session: Member $49, Nonmember $99. Student member: free registration.

REGISTER NOW AT https://cutt.ly/9hQDTEo #SEIStandardsSeries

Celebrating 25 years advancing and serving structural engineering

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ASCE/SEI MEMBERSHIP AND THE EXCLUSIVE MEMBER RATE ON THESE SESSIONS, PUBLICATIONS AND ALL PRODUCTS AND SERVICES. FREE MEMBER BENEFITS: • 10 Free Annual PDHs – Valued at over $1,000.

• AccessEngineering - Online reference tool for practicing and academic engineers.

• ASCE Collaborate, Career Path Series, and more

STRUCTURE J A N UA RY 2021 |

Bonus Content

International Spy Museum

IN FOCUS Staying Engaged and Effective While Working Remotely – Part 2

By STRUCTURE’s Editorial Board Members

The article represents a collaborative effort by members of the STRUCTURE magazine Editorial Board. Text enclosed in quotes denotes personal experiences during the COVID-19 pandemic.

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orking at home provides unique flexibility that you do not have at an office.

“It allows you to enjoy the sights and sounds of nature, be it the birds at the feeder hung on the patio or the flowers in the backyard. You can eat lunch with your family, spend time with a pet, defrost something for dinner ahead of time, or throw in a load of laundry rather than do it on the weekend.”

Try to use the benefits of being at home as motivation, rather than sources of distractions or excuses for not getting your work finished.

Some people may lack self-discipline, but, maybe even for those folks, the opportunity for the “30 minutes of exercise daily” will be the ticket. Remember that if you cannot effectively work from home, you will miss out on these benefits.

Defining the Workday When it is time to get to work, and after any family needs have been attended to, it is essential to create a routine and define a workday that replicates the “normal” office experience as much as possible. No one suggests that wearing work clothes at home is necessary, but the best results can be achieved if a separate home office can be created. If two people are working from home, this

may be difficult. “A separate office or space of your own can be closed off at the end of the day, just like leaving the actual office.

If you normally leave the office at lunch, do so at home as well, if possible, even if it just means going out for a walk or a quick errand.” Our brains need a chance to relax from thinking about work. Burn out will occur if the typical 8-hour day becomes 10 or more hours because one can never “leave the office.”

First, set a time for the start of the workday. While hitting the snooze button is very inviting, the alarm should be used to set the day in motion and establish a time for finishing the day, so don’t ignore it. Secondly, get up and dressed for the day as you would normally. Sure, the suit jacket is most likely unnecessary, but maintaining a routine of personal hygiene helps to stay motivated. You will most likely be interacting with colleagues and clients quite often on virtual calls, and you will appear more

With roommates, spouses, children, or pets sharing the same home during regular work hours, it can be challenging.

STRUCTURE magazine

professional if you act more professionally. Third, have a designated work desk that only has work-related material around it. Establish a to-do list for the day or week and, once you get settled in at your desk, start tackling it. Th e usual stream of emails, calls, and chats will keep you engaged and hopefully result in a stream of completed tasks that is motivating. Everyone feels good when they finish something. Finally, accept that there will be some distractions throughout the day, just like there are at the offi ce. “If you expect to focus from 8 to 5 without distractions, then you will end up frustrated when life does not work that way.

Instead, set out to work knowing that you may take a few more breaks than you would at the office or that you may need a significant break in the middle of the day to attend to family issues. These may seem like longer days, but don’t forget that you didn’t have a commute, and you should have taken some breaks along the way!”

TECHNOLOGY CAN ALSO PRESENT DIFFICULTIES THAT REDUCE EFFECTIVENESS AND PRODUCTIVITY AND RESULT IN LENGTHENED WORKDAYS.

Engagement with Others At Work Since we are working in “bubbles” or in “silos,” more senior staff should make an extra effort to engage with the younger staff. The younger staff cannot ask casual questions as easily as when they are at the office, so make sure they know they can contact you at any time during the day, even after normal work hours, if something important comes up. If the younger project staff is still working, then you need to be too. Consider setting up office hours when staff knows they can speak with you. Nothing can be more frustrating and discouraging for a young engineer than to feel stranded or left alone when they need help.

Environment Like everyone else that is new to working from home, there is a learning curve in determining what works for each person. Having

a comfortable, quiet work environment with some natural light is no less important at home than in a traditional office. Having a room that can be closed off from the rest of the house and free from distractions is ideal. It will take time to optimize your home office to work effectively; you may need to buy a worktable or standing desk and a comfortable desk chair. Remember to get up and walk around from time to time to stretch and move. You did this at the office unconsciously as part of work, but you have to force yourself to do it at home.

Obviously, with roommates, spouses, children, or pets sharing the same home during regular work hours, it can be challenging. “We love them dearly, but working at home with young kids, particularly small ones, can be a slow-moving disaster. Most of us are just not set up at home for private, uninterrupted telephone or video conference calls at all times during the workday. Realistically, we may never be. Interruptions from kids are no longer unexpected and can be a source of bonding with most people since they know what you are going through, and they are glad it isn’t them.”

On the other hand, it can get out of hand too. “Like screaming kids on the airplane or in a restaurant, you can only push it so far.”

While it is not something to make a habit out of, an emergency video or snack (having ice cream or popsicles on hand seems to work) might buy some time in a pinch, and parents should remember to cut themselves some slack for doing so. Try not to feel guilty. Also, don’t make ‘perfection’ the enemy of the ‘best I could do’ under the circumstances. Setting up a scheduled rotation with your spouse, the neighbors, or grandparents (if they live nearby) might also work to free up several hours a day.” Understand that you may find yourself working at odd hours or adjusting from an ideal schedule. Communicate this clearly with your supervisor, and as long as you are completing tasks on time and within budget, they will likely accommodate you. It is important for everyone, senior or young, children or not, to try and remain empathetic during this time. Those trying to work at home with young children may not be enjoying the situation either!

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External and internal communications are all leaning more heavily on video conferencing.

Giving a technical presentation, making a pitch to a client, and conducting a performance review are all vastly different over video conference versus in person.

Technology can also present difficulties that reduce effectiveness and productivity and result in lengthened workdays. “Home internet and wireless routers often just do not work as well as those in most office environments, particularly in major metropolitan areas.

Downloading and uploading large files over home systems takes more time. Even if a connection directly to the office system via a VPN is possible, the process can still be limited by the equipment at home. Also, many remote workers are now relying on the use of personal cell phones for work.” Your firm will likely need to invest more in computers and computer systems and possibly subsidize the cost, or outright purchase, home internet upgrades for you, better personal cell phone plans, and office equipment like scanners and printers.

The unwritten rule for many firms is that management does not care how the work is done so long as it gets done. But if the effort required is much greater when working remotely due to various obstacles and inefficiencies, this could result in lengthened workdays, employee burn-out, and displeasure. Management needs to

STRUCTURE magazine

be very attuned to these kinds of issues and proactively deal with them to re-assure staff that every possible effort is being made to help them. It is important for management to remember that work is just a part of life (about ⅓), and everything else their staff does outside of working hours in a COVID world is more complicated and stressful.

Communication and Engagement with Clients During economic downturns, the best practice is to increase marketing and business development efforts, not decrease them. Making contact with clients and potential clients to share a lead is appreciated, even if they are not likely to bring you work directly. Since we rarely see our contacts in person now, the tendency for reluctant marketers, like many engineers, is to assume that you are invading the client’s privacy, so you send an email instead or, worse yet, do nothing. Getting out of your comfort zone and being bold (for an engineer) is the best approach. “What do you really have to lose? The firms that are most likely to emerge from the pandemic in strong positions will make additional marketing efforts by doubling or tripling the number of calls they would have made pre-pandemic.

Some of these calls will be to new contacts, but some ought to be to people you haven’t spoken with in a long time.” You will be pleasantly surprised at the positive reception you will get if you have information to share and are willing to strategize on how you can team with your clients in a way to make you both more successful.

These calls to check-in and offer resources can also be applied to your internal clientele or coworkers. When at the office, it is easy to ask others for help with tasks or technology challenges.

Calling or emailing junior staff to ask for help may be perceived as unnecessarily delegating or shifting burdens and can lead to resentment. “Remember, the staff can’t see that you are hard at work on something else. Instead, call them and offer to help them

with something that you don’t ordinarily do, like calculations.”

Take to the opportunity to strengthen your interactions with the staff and forge stronger relationships.

External and internal communications are all leaning more heavily on video conferencing. Giving a technical presentation, making a pitch to a client, and conducting a performance review are all vastly different over video conference versus in person. On the recipient’s end, there is a temptation to disengage or half-engage, turn off the camera, or get distracted by something else. Can this be countered on the deliverer’s end? “Politely encourage people to turn on their cameras and over-prepare with a lot of questions to ask the recipient in case of dead air. Consider sending information in advance to establish that a conversation is expected rather than a one-way conversation. But, as with all effective presentations, the best way to keep someone’s attention is to have something very interesting and pertinent to say that is specifically targeted to the audience.” Do some homework in advance so you can reduce the “boilerplate” and get right to the point with the information the audience needs to hear. Also, consider the software platform utilized for these interactions, specifically presentations. Different formats provide different opportunities for participant engagement. Learn them and use them.

The temptation to disengage or half-engage increases when the video conference numbers go beyond two or three people, depending on the situation. “It’s not the same as sitting around a conference table and reading body language, so be prepared to give more active verbal signals than normal. If the number of questions is low, or the audience seems to be quiet (which is the tendency), then be prepared to ask a question to start the conversation.” While more people in a meeting can be efficient for conveying raw information, fewer people in the meeting can be more effective, much like a well-run in-person interview where there is clearly a leader running the show supported by staff.

“Even with all the technology at our disposal, don’t forget the power of a care package or a hand-written letter. Sending something tangible to a client or a staff sets you apart and creates a lasting impression.”

The top reason given was they missed meetings (who knew?) and connecting with colleagues face-to-face. Three out of four said they missed the people. 55% said collaborating is more challenging when working remotely, and 51% said staying up-to-date on the work of others is more difficult from home.

Perhaps surprisingly, Millennials (aged 24 to 38) and Gen Z (younger than 24) workers found themselves less productive and less satisfied working from home. These workers found the experience more challenging, more stressful, and less productive than their older peers.

Conclusion We hope that you found this two-part series worthwhile and that you have discovered several takeaways that you can implement in your office today, in the future, or when employees are generally working remotely. There is no doubt that some people like working remotely or from home, but we think that the surveys we have referenced and the Editorial Board’s experiences show that more people prefer being in the office at least most of the time.

There is no denying that humans are social creatures, and they like being together. Computer technology, primarily video conferencing, has made it possible to be separated, but we have all felt that, while we can still do our work, it is just not the same.■

Gensler Survey The mega consulting firm Gensler published its U.S. Work from Home Survey on May 26, 2020. From April 16 to May 4, they surveyed over 2,300 full-time U.S. office workers at companies with 100 or more employees. This was one to two months after most stay-athome orders were put into effect. They found that only 12% of U.S. workers wanted to work from home full-time and that 70% wanted to work in the office most of the week.

only 12% of U.S. workers wanted to work from home full-time and 70% wanted to work in the office most of the week.

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SPOTLIGHT The International Spy Museum

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esigned by renowned architects Rogers Stirk Harbour + Partners, and local architects Hickok Cole, the International Spy Museum at L’Enfant Plaza in Washington, DC, is a 130,000-square-foot, distinctively designed steel building with museum exhibition, office, retail, classroom, and event spaces. The project site at L’Enfant Plaza was chosen in part to create a pedestrian connection between the National Mall and the newly developed Southwest Waterfront in downtown Washington, DC. The building’s innovative design incorporates exposed, “L-shaped” red-painted columns constructed from grade 50 steel plates along the south and west faces that slope at an angle of approximately 2.5-vertical:1-horizontal and are part of the building’s gravity load-carrying system. Columns taper and have reduced depths at the top, where structural demand diminishes, to reduce material cost and for aesthetics. The entire building was designed as a space frame with floor beams and their connections able to resist in-plane forces in addition to shear connection forces to achieve the architectural intent of not continuing the sloping columns to the ground.

The red-sloping columns and their connections to the primary building structure are entirely exposed to view. An innovative connection was developed where the flanges of the steel floor beams were coped just inboard of the facade, the beam webs were reinforced and extended through the façade, and the beams and sloping columns were connected with 1½-inch-diameter slip critical A490 bolts. This provided gravity support for the floors while also providing bracing points for the sloping red columns. All members are Architecturally Exposed Structural Steel (AESS) and intumescent painted. A fullscale mock-up of this connection at level 04 was built and reviewed in the steel fabricator’s shop early in the design phase to ensure structural and aesthetic compliance.

The typical museum floor construction is composite steel beams with concrete on metal deck. RAM Steel and FloorVibe were used to size steel beams for strength and serviceability, and vibrational performance was carefully considered due to the 55½-foot beam spans.

Also, duct openings through beam webs were coordinated with the design team to maximize ceiling heights.

The sloping geometry of building columns at the south and west faces causes an STRUCTURE magazine

inherent lateral drift from gravity loads. Further compounding this issue, museum programming prevented the placement of traditional braced frames near the sloping south and west building faces, resulting in significant diaphragm torsion.

Full story-depth “hat trusses” were placed above the highest museum level within the mechanical plant in line with primary building core braced frames to help address these

The museum is tailored to present state-of-the-art exhibits and installations to promote public understanding of intelligence and espionage. issues. A three-story, sloping braced frame that is discontinuous to ground was used at the south edge, located just inboard of the façade, to avoid intrusion on museum space.

During construction, SK&A worked closely with the contractors to establish a construction sequence to limit lateral building drifts due to the unbalanced sloping nature of the west and south building faces.

A glass veil lines the west edge of the building and is supported with gravity and lateral connections attached directly to the sloping red columns. Within the space of the glass veil, there is an intricate series of monumental stairs and platforms constructed from AESS members of varying shapes and profiles, all of which had expressed connections to the built-up AESS building columns. Steady-state analysis with SAP2000 was conducted to evaluate the vibrational performance of the entire system.

An events space is located at level 07, which includes a 23-foot cantilever protruding above the roadway below to provide striking views of the Capitol Building. A combination of cantilevered beams, perimeter built-up steel box beams for torsional rigidity, and 3-inchdiameter stainless steel diagonal tension rods were used to alleviate vibrational concerns for potential rhythmic excitations.

Due to existing site constraints, the eastern column line of the new museum does not align with the existing columns below.

The structural solution for this was to construct post-installed concrete transfer girders beneath the existing concrete slab, which bear on new concrete jackets at the existing

columns. Three-dimensional finite element modeling with ETABS and SAP2000 was conducted to analyze the effects of these transfer conditions on the core braced frame designs and overall lateral building drifts.

The museum structure is built on top of an existing four-story concrete structure containing retail, government space, and parking, which remained active and occupied throughout construction. Existing concrete columns were strengthened with post-installed concrete jackets. Unique column jackets were designed on a case-by-case basis to minimize disruptions to existing spaces below.

The existing spread footings were not adequate to support new loads and needed to be strengthened. Micropiles were expensive, especially given the site constraints, and so spread footing enlargements were designed wherever possible based on structural demand and soil properties. Where required due to large loads, hollow-core micropiles approximately 50 feet in length were used.

This distinctively designed steel building is a beacon for its museum, event, retail, office, and instructional spaces along the Southwest Waterfront. As the major draw, the museum is tailored to present state-ofthe-art exhibits and installations to promote public understanding of intelligence and espionage.■ SK&A was an Outstanding Award Winner for the International Spy Museum project in NCSEA’s 2020 Annual Excellence in Structural Engineering Awards Program in the Category – New Buildings $80M to $200M.

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