STRUCTURE AUGUST 2019
NCSEA | CASE | SEI
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Braced Frames Hybrid CLT-Steel Residence Hall Bridge Inspection Frequency
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LeMessurier Calls on Tekla Structural Designer for Complex Projects Interoperability and Time Saving Tools
Tekla Structural Designer was developed specifically to maximize collaboration with other project parties, including technicians, fabricators and architects. Its unique functionality enables engineers to integrate the physical design model seamlessly with Tekla Structures or Autodesk Revit, and to round-trip without compromising vital design data. “We’re able to import geometry from Revit, design in Tekla Structural Designer and export that information for import back into Revit. If an architect makes geometry updates or changes a slab edge, we’ll send those changes back into Tekla Structural Designer, rerun the analysis and design, and push updated design information back into Revit.”
Tekla Structural Design at Work: The Hub on Causeway
For over 55 years, LeMessurier has provided structural engineering services to architects, owners, contractors, developers and artists. Led by the example of legendary structural engineer and founder William LeMessurier, LeMessurier provides the expertise for some of the world’s most elegant and sophisticated designs while remaining true to the enduring laws of science and engineering. Known for pushing the envelope of the latest technologies and even inventing new ones, LeMessurier engineers solutions responsive to their clients’ visions and reflective of their experience. An early adopter of technology to improve their designs and workflow, LeMessurier put its own talent to work in the eighties to develop a software solution that did not exist commercially at the time. Their early application adopted the concept of Building Information Modeling (BIM) long before it emerged decades later. While LeMessurier’s proprietary tool had evolved over three decades into a powerhouse of capability, the decision to evaluate commercial structural design tools was predicated on the looming effort required to modernize its software to leverage emerging platforms, support normalized data structure integration and keep up with code changes. After a lengthy and thorough comparison of commercial tools that would “fill the shoes” and stack up to the company’s proprietary tool, LeMessurier chose Tekla Structural Designer for its rich capabilities that addressed all of their workflow needs. According to Derek Barnes, Associate at LeMessurier, ” Tekla Structural Designer offered the most features and the best integration of all the products we tested. They also offered us the ability to work closely with their development group to ensure we were getting the most out of the software.”
One Model for Structural Analysis & Design
From Schematic Design through Construction Documents, Tekla Structural Designer allows LeMessurier engineers to work from one single model for structural analysis and design, improving efficiency, workflow, and ultimately saving time. “Our engineers are working more efficiently because they don’t need to switch between multiple software packages for concrete and steel design. Tekla Structural Designer offers better integration of multiple materials than we have seen in any other product,” said Barnes. LeMessurier engineers use Tekla Structural Designer to create physical, information-rich models that contain the intelligence they need to automate the design of significant portions of their structures and efficiently manage project changes. TRANSFORMING THE WAY THE WORLD WORKS
“Tekla Structural Designer has streamlined our design process,” said Craig Blanchet, P.E., Vice President of LeMessurier. “Because some of our engineers are no longer doubling as software developers, it allows us to focus their talents on leveraging the features of the software to our advantage. Had we not chosen to adopt Tekla Structural Designer, we would have needed to bring on new staff to update and maintain our in-house software. So Tekla Structural Designer is not just saving us time on projects, it is also saving us overhead.
Efficient, Accurate Loading and Analysis
Tekla Structural Designer automatically generates an underlying and highly sophisticated analytical model from the physical model, allowing LeMessurier engineers to focus more on design than on analytical model management. Regardless of a model’s size or complexity, Tekla Structural Designer’s analytical engine accurately computes forces and displacements for use in design and the assessment of building performance.
“Tekla Structural Designer offers better integration of multiple materials than we have seen in any other product.”
Positioning a large scale mixed-use development next to an active arena, a below grade parking garage, and an interstate highway, and bridging it over two active subway tunnels makes planning, phasing and engineering paramount. Currently under construction, The Hub on Causeway Project will be the final piece in the puzzle that is the site of the original Boston Garden. Despite being new to the software, LeMessurier decided to use Tekla Structural Designer for significant portions of the project. “Relying on a new program for such a big project was obviously a risk for us, but with the potential for time savings and other efficiencies, we jumped right in with Tekla Structural Designer. It forced us to get familiar the software very quickly.” “Tekla Structural Designer allowed us to design the bulk of Phase 1 in a single model,” said Barnes. The project incorporates both concrete flat slabs and composite concrete and steel floor framing. “Tekla Structural Designer has the ability to calculate effective widths based on the physical model which is a big time saver,” said Barnes. “On this project, the integration with Revit, along with the composite steel design features enabled us to work more efficiently. Adding the ability to do concrete design in the same model was a bonus because we had both construction types in the same building.” “Tekla Structural Designer helped this project run more efficiently, and in the end it was a positive experience,” said Blanchet.
“Tekla Structural Designer gives us multiple analysis sets to pull from, which gives us lots of control. Most programs don’t have the capability to do FE and grillage chase-down. For the design of beam supported concrete slabs, Tekla Structural Designer allows us to separate the slab stiffness from the beam stiffness, so if we choose to we can design the beams without considering the influence of the slab. In the same model we can use a separate analysis set to review the floor system with the beams and slab engaged,” said Barnes. Barnes also shared similar benefits with concrete column design. “Tekla Structural Designer does grillage take-downs floor-by-floor, finds the reactions and applies them to the next floor. This allows us to view column results both for the 3-dimensional effects of the structure as a whole and from the more traditional floor-by-floor load take-down point of view. Doing both has always required significant manual intervention, but Tekla Structural Designer puts it all in one place.” “We reduce the possibility for human error because with Tekla Structural Designer less user input is required,” said Barnes. “Tekla Structural Designer automatically computes many of the design parameters, such as column unbraced lengths. The assumptions made by the software are typically correct, but we can easily review and override them when necessary.”
“Tekla Structural Designer provided the best fit for our workflow compared to other commercially available software.”
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4 STRUCTURE magazine
Contents
Cover Feature
AU GUST 2019
Columns and Departments 7
Editorial Advancing Beyond the Technical By Richard C. Boggs, P.E., SECB
8
Practical Solutions Joist Girder Moment Connections By James M. Fisher, Ph.D., P.E.
13
Northridge – 25 Years Later Braced Frames: The Quest for Ductility By Rafael Sabelli, S.E., and Patxi Uriz, Ph.D., P.E.
26 AMAZON SPHERES
18
By Jay Taylor, P.E., S.E., and Robert P. Baxter, P.E., S.E.
Imagine your ideal office space. What if it was a sanctuary
Structural Performance Is Seismic Design by U.S. Codes and Standards Deficient? – Part 2
By S. K. Ghosh, Ph.D.
31
Construction Issues Recommended Details for
from the chaos of corporate office life? Amazon’s Spheres
Reinforced Concrete Construction – Part 3
are a one-of-a-kind workspace that utilizes endless possible
By David A. Fanella, Ph.D., S.E., P.E., and Michael Mota, Ph.D., P.E., SECB
configurations of steel framing inside the repetition of a pentagon shape. Cover graphic courtesy of Magnusson
34
Building Blocks Mass Timber Engineering By Jim DeStefano, P.E., AIA
Klemencic Associates/Michael Dickter. 37
Building Blocks – Sidebar Mass Timber Floor Vibration By Lucas Epp, P.Eng.
38
InFocus A Continuing Discussion on SE3 By John Dal Pino, S.E.
50
InSights Bridge Inspection Frequency By Jennifer C. Laning, P.E.
22 THE HYBRID CLT-STEEL RESIDENCE HALL By David J. Odeh, S.E., P.E., and Paul Kuehnel, P.E.
A new six-story student residence hall at the Rhode Island School of Design incorporates exposed cross-laminated
In Every Issue 4 41 44 46 48
Advertiser Index Resource Guide – Software NCSEA News SEI Update CASE in Point
timber (CLT) panels supported by steel framing. Doing something new required a calculated risk, but the system yielded unexpected benefits.
Publication of any article, image, or advertisement in STRUCTURE® magazine does not constitute endorsement by NCSEA, CASE, SEI, the Publisher, or the Editorial Board, Authors, contributors, and advertisers retain sole responsibility for the content of their submissions. A U G U S T 2 019
5
A Powerful Software Suite for Detailed Analysis & Design of Reinforced Concrete Structures
EDITORIAL Advancing Beyond the Technical By Richard C. Boggs, P.E., SECB, LEED AP
I
n December 2017, renowned structural engineer Leslie E. Robertson delivered a presentation at the annual Connecticut Structural Engineers Coalition holiday event entitled The Structure of Design. While many in the audience were expecting a retrospective on Mr. Robertson’s long career of technical innovation and fascinating tales about the design challenges that he has faced, instead the presentation focused on the importance of building relationships and being willing to assume leadership on one’s projects, in one’s profession, and in life. I reflected on Les’ perspective as I read Jon Schmidt’s editorial in STRUCTURE earlier this year, Leadership Is Showing Up. Jon’s article outlined the story of his involvement in the creation of his state Structural Engineers Association, later in SEI and NCSEA, and ultimately his rise to the position of NCSEA President. Most important, Jon’s article served as an example of one person’s decision to make a mark on his profession at a local and national level, and described the route that he took to do so. Young practitioners of structural engineering find themselves in a role that requires an intense focus on technical issues over a long period to master structural behavior, design, construction, and the tools of our trade. The ultimate objective of this journey is simply to be able to function effectively as a structural engineer – and, of course, to qualify for licensure. Yet, both of these gentlemen made the decision at different points in their lives that technical excellence would not be enough to achieve their professional goals. Technical excellence is essential but otherwise unimportant, to paraphrase Hardy Cross. This is true in the minds of our clients and the general public, who presuppose technical competence on the part of structural engineers unless and until they discover evidence to the contrary. While technical competence is an admission ticket to the profession, successful structural engineers need to be more than just technical experts. They need to find ways to build trust, inspire confidence, and ultimately lead. Much has been written recently about the challenges faced by our institutions of higher learning to prepare students adequately for the workforce. The skyrocketing price of a college degree and intense competition between programs have pressured universities to reduce the scope of their course offerings and lower graduation requirements in the interest of attracting students and controlling costs. At the same time, in the structural engineering field, the increasing automation of design has made familiarity with software for analysis, documentation, and building information modeling essential to graduating structural engineers. So the question has become, what coursework gets eliminated to make room for training in the use of such tools? There is pressure to lighten the requirements for a well-rounded education that includes verbal and written communications, civics, leadership training, and the like. Unfortunately, it is precisely these types of courses and academic diversity that prepare students for STRUCTURE magazine
leadership roles in the future, not only through the content itself but also by facilitating interaction with students with a broad range of skills and interests. The situation is difficult, but not impossible. Fortunately, our professional organizations have done an excellent job of providing vehicles for members to engage with other structural engineers on a local and national level and encouraging them to move beyond the technical realm. Structural Engineers Associations are generally clamoring for volunteers to assist with a whole host of activities, including the code adoption process, program planning, emergency response, advocacy and education, and licensure and other legislative efforts. The committees of CASE, NCSEA and SEI are made up of individuals from around the country who share their knowledge and experience in every aspect of their local activities. Such committees provide an excellent opportunity for structural engineers at any experience level to become involved and rapidly rise to leadership positions in any area that they find particularly compelling. Most importantly, these organizations are full of role models equipped to inspire those with a desire to make a difference in their profession and their community. I urge anyone reading this to find your most burning area of interest and get involved! Our associations and our profession need you to engage. If you do not have a specific area of interest, or don’t think that you have a specific enough set of skills or experience to bring to bear, the NCSEA Structural Engineering Equity and Engagement (SE3) Committee or Communications Committee (CommComm) might be great places to start. Regardless of where you decide to dip your toe in the volunteer pool, you will develop a network of engineers around your state and the country, hone your leadership skills, and get to meet some pretty amazing people with a remarkably wide variety of abilities and interests along the way. The important thing is to make the decision not to be confined to the comfortable, technical zone. There is an old joke that goes like this: What’s the difference between an introverted engineer and an extroverted engineer? The introvert looks at his shoes when he talks to you. The extrovert looks at your shoes when he talks to you. (Pause for uncomfortable laughter.) Like many stereotypes, the quiet “nerd” engineer has some basis in truth, not for all, but for many of us. The shy, analytical introvert is something that lives inside most of us who choose this profession, but we can (and should) choose to move beyond this persona if we want to achieve maximum impact – a concept any engineer can appreciate.■ Richard C. Boggs is a Senior Project Manager at Fuss & O’Neill in Trumbull, CT and a board member of NCSEA. He is a former president of the ACEC/CT Structural Engineers Coalition, served for 15 years as Connecticut’s NCSEA Delegate, and is currently an ACE mentor and Chairman of his town’s sewer commission. A U G U S T 2 019
7
practical SOLUTIONS Joist Girder Moment Connections By James M. Fisher, Ph.D., P.E., Dist.M.ASCE
T
he Steel Joist Institute (SJI) defines a Joist Girder as “... a primary structural loadcarrying member with an open web system designed as a simple span supporting equally spaced concentrated loads of a floor or roof system acting at the panel points of the member and utilizing hot-rolled or cold-formed steel.” Figure 1. Chord bending. If the Joist Girder is not a simple span, or if the loading condition is anything other than concentrated panel point ANSI/SJI 100-2015) and the AISC Specifications (Specification loads, the Specifying Professional must clearly indicate the loading for Structural Steel Buildings, ANSI/AISC 360-10). Before using (i.e., end moments, axial forces, and other required design informa- the Spreadsheets, the user should perform a structural analysis to tion) on the construction documents. determine that the column has the available strength to resist the The “Basic Connection” used by all Joist Girder Manufacturers for applied loads. simple spans is one where the Joist Girder seat rests on the column This article focuses on the Joist Girder Moment Connection to cap, and the bottom chord angles slide between a stabilizer plate. the Strong Axis of a Wide Flange Column (Figure 2, page 10) with The manufacturer designs the Joist Girder seat for the imposed verti- additional information available in the reference manuals and spreadcal concentrated loads. The Basic Connection becomes a moment sheets. The other Spreadsheet tools follow a similar presentation. In connection when the bottom chord of the Joist Girder is welded this detail, the Joist Girder vertical reaction is supported by a stiffened to the stabilizer plate. This connection has very limited moment seat welded to the column flange. If the Joist Girder is modeled as a capacity. Its strength is limited by bending stresses, induced in the truss, the chord forces are obtained directly from the model; however, top chord by load path eccentricities (Figure 1). if Joist Girders are modeled as beam elements, chord forces are deterThe purpose of this article is to discuss moment connection design mined by resolving end moments into force couples. Top chord force tools that are available from SJI at no cost to engineers and detail- is transferred to the column by a top plate field welded to the chord ers. These tools can assist the design professional by making the and to the column cap plate. For Joist Girders framing to both sides design process more timely and complete. SJI provides six different of the column, the top plate is also used to transfer continuity forces spreadsheets to assist in the design of moment connections. Each from one Joist Girder to the other. Bottom chord force is transferred can be used to calculate connection strength based on the necessary to the column via the stabilizer plates. Numerous limit states, which limit states. A reference manual is provided with each Spreadsheet, must be examined, are discussed below. explaining the calculations. Each Spreadsheet provides for the design of a Joist Girder framing into one side or both sides of the column. Top Chord Connection The six connection Spreadsheets are: 1) Connection to the Strong Axis of Wide Flange Columns The required top plate strength is determined from the axial force in 2) Connection to the Strong Axis of Wide Flange Columns – the top chord (Pu = Mr/de); where Mr is the required end moment Intermediate Levels of the Joist Girder and de is taken as the distance from the top of 3) Connection to the Weak Axis of Wide Flange Columns the Joist Girder to the half depth of the bottom chord leg. The 4) Connection to HSS Columns – Top Plate required top plate area is Pu/φFy (φ = 0.90). Plate length is based on 5) Connection to HSS Columns – Knife Plate the required length of fillet welds attaching the plate to the column 6) Connection to Wide Flange Columns – Knife Plates Table of available strength (LRFD) for chord angle sizes. Although the Spreadsheets are specifically written for the design of Unbraced Length Area Angle Size moment connections, they can also L = 4 ft. L = 5 ft. L = 6 ft. L = 7 ft. in.2 be used for cases where Joist Girder 2L 6 x 6 x 1 939 911 879 842 22.0 additional top and bottom chord axial load transfer is required. 2L 6 x 6 x 7/8 825 809 781 749 19.5 It is expected that the spread2L 6 x 6 x 3/4 702 694 679 651 16.9 sheet user is familiar with the SJI 2L 6 x 6 x 5/8 570 564 556 547 14.3 Specifications for Joist Girders (Standard Specification for K-Series, 2L 6 x 6 x 9/16 499 494 487 480 12.9 LH-Series, and DLH-Series Open 2L 2-1/2 x 2-1/2 x 3/16 49 48 41 34 1.80 Web Steel Joists and for Joist Girders, 8 STRUCTURE magazine
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cap plate and the top chord. Shear lag must be checked per the 2010 AISC Specification Table D3.1 “Shear Lag Factors for Connections to Tension Members.” The Spreadsheet requires the top plate to the top chord weld length to be a minimum of two times the width of the top plate. Based on Case 4 in the AISC Manual Table D3.1, U = 1.0 for this condition; thus, shear lag does not reduce the strength of the top plate. For the top plate connection to the column cap, the spreadsheet reduces the strength of the top plate for any shear lag. When the top chord is in tension, the Joist Girder Manufacturer has the responsibility to check the top chord angles for shear lag. Case 2 from Table D3.1 is applicable for this check. For reference, the shear lag factor is calculated for the top chord based on the input of the angle leg size, Btc and the angle thickness, ttc. Shear lag factors greater than 0.92 do not affect the Joist Girders. Providing longer fillet welds will reduce shear lag effects. Many times, the size of the Joist Girder chord angles is unknown when designing the connection. When the chords are subject to axial compression, a reasonable estimate of the angle sizes can be obtained using Table 2-1 in the SJI Technical Digest, Design of Lateral Load Resisting Frames Using Steel Joists and Joist Girders. The digest can be ordered from the SJI Website, www.steeljoist.org. From the structural analysis, the table can be entered with the chord force, unbraced length, to determine the angle size based on the available strength. A representative sample of Table 2-1 is shown in the Table (page 8).
Cap Plate to Column Weld The weld of the cap plate to the column must also be determined since the top plate force must be transferred into the column web. The spreadsheet uses the column T-distance as the weld length. On occasion, the base metal strength may be less than the weld strength. If this occurs, the user can select a deeper column (one with a thicker web), an additional weld can be placed beyond the T distance, or any combination of the above.
Column Web Shear The nominal shear strength, Vn, is determined using the provisions of AISC Section G2.1 (φ = 1.0 for rolled shapes when Eq. G2-1 controls, otherwise φ = 0.90). If the web does not have the available strength for shear, then it is generally most economical to either select a deeper W shape or one with a thicker web. The column web shear yielding is checked at the Joist Girder top chord connection independent of the column web panel zone shear.
Stiffened Seat Connection The seat width (Ws) can be determined from the minimum bearing length and (N) from the SJI Specifications (Table 5.4-3). The reaction is located N/2 from the interior edge of the seat. Additionally, for the stiffened seat connection, the stiffener shall be finished to bear under the seat (AISC Steel Construction Manual, Table 10-8).
Column Web Checks The spreadsheet checks the following column web limit states: 1) Web Local Yielding 2) Web Crippling 3) Web Compression Buckling 4) Web Panel Zone Shear 10 STRUCTURE magazine
Figure 2. Joist girder moment connections to the strong axis of wide flange columns. Courtesy of Johnson & Burkholder, Engineers.
The spreadsheet does not check the web panel zone shear below the bottom chord. Web compression buckling is applicable when a pair of singleconcentrated forces are applied at both flanges of a member. This condition does not exist at the exterior columns. When unequal depth Joist Girders frame into both sides of the column web, compression buckling is checked when the stabilizer plates overlap one another. In cases when the web does not have sufficient strength for the compressive or tensile forces delivered by the stabilizer, the strength can be increased by: • Selecting a W Shape with a thicker web. • Adding a stiffener to the web of the column. • Adding a doubler plate.
Bottom Chord Connection The bottom chord of the Joist Girder must be attached to the stabilizer plate to resist and transfer the chord force to the column. Stabilizer plates are typically sized based on a ¾-inch thickness.
Using a ¾-inch plate allows the plate to fit between the bottom chord angles, allowing fillet welds to be made to the heels and toes of the chord angles. Economically, the stabilizer plates can usually be connected to the column using only fillet welds. Stabilizer plates must be welded to the column flange to resist the compression and tension forces. The specifying professional must specify that the Joist Girder bottom chords be a minimum thickness to accommodate the required weld size. As is required for the top chord, the Joist Girder Manufacturer has the responsibility to check the bottom chord angles for shear lag. Case 2 from Table D3.1 is applicable for this check. For reference, the shear lag factor is calculated for the bottom chord based on the input of the angle size, bottom chord leg size, Bbc and bottom chord thickness, tbc. Providing longer length fillet welds will reduce shear lag effects.
Minimum Member Thicknesses Throughout the spreadsheet, checks are made for the minimum thicknesses of base metal to match the weld strength. From the AISC Specification, Section J2.4, the design strength, φRn, and the allowable strength, Rn/Ω, of welded joints shall be the lower value of the base material strength according to the limit states of tensile rupture, shear rupture, and the weld metal strength based on the limit state of rupture. All of the moment connection design tools can be downloaded at no cost from the SJI website: https://goo.gl/mr7MQR. The tool discussed here and the other available tools from the SJI website will assist the designer with the design of Joist Girder Moment Connections.■
Stabilizer Plate Checks
A sample input sheet for the spreadsheet tool is available in the online version of this article; www.STRUCTUREmag.org.
The following strength checks are made: 1) Determine the weld between the bottom chord and the stabilizer. 2) Check the Whitmore width for stabilizer (AISC Manual Section 9-3). 3) Check stabilizer yielding. 4) Check stabilizer Block Shear Rupture Strength. 5) Determine the weld between the stabilizer and the column. The spreadsheet uses the Joist Girder bottom chord forces to determine the weld requirements. Some designers prefer to provide enough weld to develop the full strength of the stabilizer.
Dr. James M. Fisher's structural expertise has resulted in authoring and co-authoring several books and numerous technical publications. Dr. Fisher served as the Chairman of the AISC Specification Committee from 2003 until 2010. He continues to be active on several AISC Specification Task Committees, the AISI Cold-Formed Specification Committee, and ASCE 7. Dr. Fisher serves as the Consulting Engineer to the Steel Joist Institute.
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A U G U S T 2 019
11
NORTHRIDGE
25 YEARS LATER
Braced Frames: The Quest for Ductility By Rafael Sabelli, S.E., and Patxi Uriz, Ph.D., P.E.
T
he Northridge earthquake exposed the need for a more
considered approach to seismic design. In the case of braced frames, this led to a closer examination of the behavior of the system and its components at large drifts. Engineers changed their approach to analysis, proportioning, and detailing of braced frames to avoid connection
Figure 1a. Net section fracture occurring at ductilities less than expected.
Figure 1b. Cover plate details now commonly used to increase brace ductility.
failure and other unfavorable behaviors they had observed. Simultaneously, lessons learned in the research on steel moment resisting frames, such as examination of expected material strength, were applied to the design of braced frames. The postNorthridge era also saw the introduction of the buckling-restrained brace as a useful tool in seismic design.
Background Before the Northridge earthquake, braced frames were not the typical structural system utilized in non-industrial facilities. Past performance of braced steel frames and early research by U.S. and Japanese researchers showed the plastic energy dissipation of these systems to be poor; so moment resisting frames dominated steel construction in high seismic regions. When the 1994 Northridge earthquake exposed unanticipated damage to steel moment resisting frames (SMRF), a period of uncertainty resulted regarding the reliability and proper design methods for SMRFs. In response, the Federal Emergency Management Agency (FEMA) funded research to generate moment-frame design and construction guidelines (a joint venture known as the SAC steel project). These guidelines resulted in costly restrictions and regulations for steel moment frame construction, leading to an increase in steel braced frame popularity. Unfortunately, some of the issues that affected the performance of moment resisting frames in the Northridge Earthquake (and were addressed in the SAC steel project) could also affect the performance of braced frames.
The Problem Due to the relative lack of popularity of braced frames, the Northridge earthquake did not expose many braced frames to intense ground shaking. Six instrumented braced steel buildings were found to have little or no damage, although ground shaking intensities at those locations were reportedly mild (Naeim, 1997, 1998). However, widely publicized reports of a single braced steel building were published and widely reviewed: a four-story office building in North Hollywood.
Designed to the 1980 Los Angeles building code, and constructed in 1986, it represented a fairly modern braced frame design at the time of the event (Kelly, et. al, 2000). All damage was concentrated at the lower floor where most of the braces on a single level fractured at the mid-length or fractured at the connections. Damage to the upper story braces was minor. While this building did not collapse, the lateral resistance on the bottom floor was compromised. In the authors’ opinion, the performance of this single building illuminated two fundamental issues with braced frames (already identified by some researchers): (i) the width-to-thickness ratio requirements (b/t) for braces were insufficient to forestall fracture when subjected to many inelastic cycles, and (ii) the design of connections proved insufficient to prevent their failure. Nearly identical behavior was reported precisely one year later during the 1995 Kobe earthquake. Although designers tend to think of braces as governed by compression, a careful examination of research into factors affecting seismic response reveals that the stability of braced-frame structures is owed almost entirely to the behavior of braces in tension. Braces subject to compression tend to buckle at very low drifts. If these braces and their connections withstand the deformations associated with this buckling, they can maintain their integrity to resist tension forces as the direction of drift reverses. Some energy is dissipated in the first cycle of buckling; but, after that, most of the energy dissipation is a result of brace elongation. Although design requirements mandate that the number of braces in tension and compression be roughly balanced, proportioning the braced frames to have a positive post-yield behavior is not mandated by code and can be challenging. Inadequate proportioning typically results in a concentration of damage in a single story (as observed in the Northridge earthquake), further increasing brace demands. continued on next page A U G U S T 2 019
13
To complicate matters, the inelastic demand on braces with moderate slenderness (KL/r) results in extreme plastic hinge rotations for modest inelastic drifts. Braces buckling out-ofplane experience severe inelastic rotation at midspan. Rotation demands for moment connections (and corresponding section compactness requirements) provide an Figure 2a. Large gusset plate being fabricated. analogy for braces subject to flexural buckling and subsequent plastic-hinge formation, but the brace rotation demands can be much higher. Depending on brace slenderness, the plastic rotation at the mid-span hinge can exceed 10%. Pre-Northridge slenderness limits were inadequate against such extreme rotation demand and were partially responsible for the severe concentrations of damage and brace fractures with far less energy dissipation than desired. A problem mutual to moment frames compounded this: yield strengths were much higher for materials used in braces than was anticipated (FEMA, 2000). Typical hollow structural sections (HSS) common to braced frames utilize a cold-rolling procedure which strain-hardens the steel from a nominal Fy of 46 ksi to nearly 60 ksi. This overstrength results in connections that may not be adequately designed to accommodate brace yield strengths, further limiting the expected resistance provided by the brace when frames undergo inelastic drift.
Finding a Solution The research and design communities began to address the observations in the years following the 1994 Northridge and 1995 Kobe earthquakes. Research began to re-focus attention on assessing the adequacy of existing design provisions and typical practices with primary attention to brace selection, connection design, gusset plate detailing, and exploration of alternative design methods. Research prior to the 1994 Northridge earthquake indicated that there is a delicate interaction between brace slenderness and rotation demands due to buckling (Goel, 1992). The more slender a brace, the lower the plastic deformation demand in the plastic hinge. This interaction was only recently quantified with modern construction detailing practices. However, multiple independent full-scale tests of wide-flange, round HSS and concrete filled tube braces demonstrated that compactness has the greatest influence on brace axial ductility. While it is important to note that concrete-filled tubes greatly enhanced brace performance for very slender sections, the behavior was nevertheless sensitive to compactness. There was no “magic bullet” to bypass this interaction, and compactness criteria were reviewed and updated to reflect this understanding. Studies have shown that more slender braces (KL/r > 100) for shorter period buildings (i.e., less than 1.0 second) may improve post-buckling performance since slender braces sized for their compression capacity will of necessity have very high overstrength when subject to tension. When the ratio of the Ry AFy to the Pcr approaches unity (that is, for stockier braces), the system is likely to have strain-hardening
Figure 2b. Very large gusset plate installation.
sufficient to distribute damage to nearby stories. The use of slender braces, however, provides an additional challenge in that connection forces are relatively large, and the braces in tension can impose large forces on columns. Typical HSS bracing connections utilize a “knifed” gusset plate, resulting in a reduced section immediately adjacent to the gusset plate. While this “net-section” area was known to be the weak link in brace behavior, the reduction in ductility was never quantified. The research demonstrated that typical designs were not ductile, Figure 1a, page 13, whereas reinforced connections performed well. New design provisions mandated reduced sections to be designed to be stronger than the brace, allowing distributed brace yielding. Typical details now include reinforcement plates (Figure 1b, page 13). Designers have sometimes favored large gussets, Figures 2a and 2b, for questionable reasons: reduction of weld size, simplification (or misunderstanding) of “Whitmore yielding,” etc. Such large gussets may result in weld failure and gusset plate separation from the beamcolumn joint. New design requirements mandate this to be remedied by requiring consideration of the design of such connections for flexural forces and rotation capacity. Mitigation of local buckling and design of connections for flexural forces and rotation capacity do not adequately address the severe strength and stiffness deterioration that may occur as a result of brace buckling. This can potentially lead to significant permanent plastic deformations, extended downtime, concentration of damage in a single story, and associated collapse risk. Alternative bracing methods have been developed to address the issues associated with conventional buckling braces. One prominent alternative method is the buckling restrained bracing system. Although used elsewhere, these systems were unused in the U.S. at the time of the Northridge earthquake. Several large-scale tests on full assemblies demonstrated their ability to resist seismic loads in a stable, reliable manner, and nearly eliminate the severe strength and stiffness degradation associated with conventional bracing elements. Today, the buckling restrained braced frame is a common bracing system.
Changing the Code Design practice was quick to react to the observed behavior and research immediately following the Northridge earthquake, and code requirements quickly followed. New seismic design provisions were released as early as 1997, and continuously updated as new and emerging research on braced frames was published. continued on page 16
14 STRUCTURE magazine
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Prior to the Northridge earthquake, seismic design recommendations only contained one category: “concentrically braced frames,” or CBF. Work on improving the behavior of braced frames had been ongoing for several years, resulting in the creation of the “eccentrically braced-frame” system. The 1994 Uniform Building Code (UBC) distinguished between Ordinary Concentrically Braced Frames (OCBF) and Special Concentrically Braced Frames (SCBF), with the later designed for lower forces but subject to much more stringent detailing and proportioning requirements. While OCBF are designed with little expectation of inelastic drift capacity, SCBF are designed to develop an identified plastic mechanism and withstand large seismic drifts. This SCBF mechanism includes brace buckling and tension yielding, and the associated axial (and in some cases flexural) forces required in the beams and columns. The UBC provisions also introduced the concept of post-buckling frame behavior as a design case. Beams in chevron-braced (V-braced or inverted-V-braced) frames were required to be designed for a special load combination that applies forces corresponding to the full tension capacity of one brace in combination with 30% of the compression strength of the opposite brace, creating a significant flexural demand on the beam. Beams were required to be designed to have sufficient strength to resist this flexural demand, although the flexibility introduced was not explicitly considered. AISC 341, Seismic Provisions for Structural Steel Buildings, has become the building-code standard for seismic design of steel systems. AISC 341 has moved (for braced frames) to defining a first-mode plastic mechanism analysis that is to be used to obtain beam, column, and connection design forces. For SCBFs, two analyses are considered: the condition on the cusp of buckling (considering full tension and compression strength), and the condition after buckling (considering the full tension strength in combination with 30% of the compression strength). The first case maximizes connection demands and overturning, while the second case captures flexural demands and forces that only appear as the system changes to a mode of behavior in which braces act predominantly in tension. This represents a continuation of the movement toward a full plastic-mechanism approach to the design of braced frames.
The Future Continuing research has shown ways in which braced-frame behavior may be further improved. Sizemore, Fahnestock, Hines, and Bradley have studied the collapse probability of OCBF and R3BF (R=3 braced frames with no seismic detailing or proportioning requirements, allowed only in areas of relatively low seismicity). Their research included the beneficial effect of a modestly sized moment frame in forestalling or preventing story mechanisms by preventing the severe strength and stiffness degradation. In a similar vein, Simpson and Mahin (2018), Figure 3, have developed a design method to employ a “strongback” truss to prevent the formation of story mechanisms in braced-frame structures. Integration of such an approach into the code regimen of design is problematic, but the benefits in performance are considerable. Work by Roeder, Lumpkin, and Lehman (2011) has helped identify pitfalls in the binary approach that is common in seismic design of braced frames. By designating braces as the fuse and other elements as force controlled, the ductility demands on the brace can be quite high. In their “balanced approach” to design and proportioning, Roeder et al. (2011) have demonstrated that brace ductility demands can be reduced by allowing minor inelasticity in the connections and
16 STRUCTURE magazine
Figure 3. The authors acknowledge their debt to the late Professor Stephen Mahin, (pictured left) whose insights into the seismic response of steel braced frames (among countless other topics) has guided their work for many years.
the beam (in chevron-braced frames). This lower ductility demand translates into greater drift capacity before brace fracture. Performance-based design (PBD) allows for the use of a combination of elements to provide adequate reliability against collapse. For braced frames, the difference between the elastic modes of response and post-buckling modes of response can be quite pronounced. Additionally, nonlinear seismic analysis may also indicate multiple different post-buckling modes; with stiffness degradation, these may be mutually exclusive. Mean values of response quantities corresponding to radically different modes may not be as informative as considering the responses individually. Use of performance-based design to justify non-ductile detailing has been proposed but, in the authors’ opinion, the demonstration of the suitability of such designs serves more as an indication of the limitations of PBD and statistical methods than a demonstration of the reliability of a given design. Use of performance-based design to better understand the possible behaviors of braced frames offers a more promising approach, especially when combined with design interventions (such as the strongback) that would mitigate or preclude unfavorable behaviors.■ The online version of this article contains references. Please visit www.STRUCTUREmag.org. Rafael Sabelli is a Principal and Director of Seismic Design at Walter P Moore. Rafael has earned a Special Achievement Award from AISC, as well as the T.R. Higgins Lectureship award. He is active in the development of seismic design standards for steel systems and is vice-chair of the AISC Seismic Provisions Committee, a member of the ASCE 7 Seismic Task Committee, and the NIST Building Seismic Safety Council’s Provisions Update Committee. Rafael is the chair of the AISC Seismic Design Manual committee and was the Project manager for the 5-volume SEAOC Seismic Design Manual. Rafael is a co-author of Ductile Design of Steel Structures. He has served as Chair of the Seismology Committee of the Structural Engineers Association of California and as President of the Structural Engineers Association of Northern California. Patxi Uriz is a Model Developer at Risk Management Solutions (RMS), where he creates analytical models that inform catastrophe losses. Patxi has also taught graduate-level courses at Stanford University in structural analysis and design and construction of steel structures.
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structural PERFORMANCE Is Seismic Design by U.S. Codes and Standards Deficient? Part 2
By S. K. Ghosh, Ph.D.
Part 1 of this series discussed background information relative to the issues, including an overview of current codes and standards (STRUCTURE, July 2019).
A Success Story The Disaster Prevention Research Institute (DPRI) of Kyoto University issued some revealing statistics following the January 17, 1995, earthquake that hit the Japanese port city of Kobe and surrounding areas (the Great Hanshin earthquake). Figure 2 is drawn based on those statistics. The figure clearly shows that the strongest correlation of damage was with the age of the structure. The correlation can be attributed largely, if not entirely, to important revisions over the past 50 years to the Japanese national building code and related national standards. The Japanese national code, the Building Standard Law of Japan (BSLJ), specifies design loads, allowable stresses, and other requirements. The details of structural design are specified in standards issued by the Architectural Institute of Japan (AIJ). These AIJ standards, prepared separately for each structural material, are supplements to the BSLJ. The 1968 Tokachi-Oki earthquake caused significant damage to buildings, and a revision to the BSLJ reduced the spacing of steel ties in reinforced concrete columns to 4 inches. In 1971, a major revision of the AIJ standard for reinforced concrete incorporated ultimate strength design of beams and columns for shear, including more stringent shear reinforcement requirements. These changes are comparable to significant code changes in the United States following the 1971 San Fernando earthquake in California. Post-1971 reinforced concrete structures performed much better in the 1995 Kobe earthquake than their pre-1971 counterparts, primarily because of the improved shear design of columns, as can be seen in Figure 2. The 1978 Miyagi-ken-Oki earthquake caused significant damage to buildings and led to a 1981 revision of the BSLJ, which introduced a two-phase earthquake-resistant design. The first-phase design (essentially the allowable stress design from the previous BSLJ) is intended to protect a building against loss of function in ground motions expected to occur several times during its lifetime, with peak ground accelerations in the range of 0.08g to 0.10g. The second-phase design is intended to ensure safety under a ground motion expected to occur once in the lifetime of a building, with peak ground accelerations in the range of 0.3g to 0.4g. Post-1981 structures designed by the two-phase procedure performed well in the 1995 Kobe earthquake, as can be seen in Figure 2. There was a remarkable lack of widespread significant structural damage to buildings from ground motions associated with the magnitude 9.0 Tohoku earthquake of March 11, 2011, which caused tsunamis that unleashed major devastation. Significant structural damage was observed only in older buildings predating the 1971 and 1981 code changes mentioned above. 18 STRUCTURE magazine
Figure 2. Correlation of damage observed in the 1995 Kobe earthquake with age of structures.
Although precise definitions of the various damage states are not available, it appears the two-stage design introduced in the BSLJ in 1981 may have the potential to bring designers close to attaining a functional recovery performance objective. Examining this potential in the context of U.S. seismic codes and standards is likely to be beneficial.
Improvements Are Desirable; Simplistic Solutions Are Not the Answer The U.S. Geological Survey (USGS) has issued Fact Sheet 2018-3016: The HayWired Earthquake Scenario – We Can Outsmart Disaster. The scenario anticipates the impacts of a hypothetical magnitude-7.0 earthquake on the Hayward Fault. The fault is along the east side of California’s San Francisco Bay and is among the most active and dangerous in the United States because it runs through a densely urbanized and interconnected region. Studies done for the HayWired scenario showed that: • Even if all buildings in the bay region met current building code, 0.4 percent could collapse, 5 percent could be unsafe to occupy, and 19 percent could have restricted use. • For only a small percentage cost increase, more resilient buildings constructed to more stringent building codes could allow 95 percent of the bay region’s population to remain in their homes and workplaces following such an earthquake. Although many assumptions form the basis of a study such as the above and the numbers are not to be taken literally, the importance attached to building codes and the impact of improvements in building codes should be noted. Governor Brown’s message accompanying his veto of California Assembly Bill 1857 read in part: “The National Institute of Building Science and Technology is in the initial stages of developing an immediate occupancy standard for buildings following a natural disaster. This federal agency is consulting engineers, scientists, and other experts to understand the changes needed to ensure that a building can be used immediately after a natural disaster.
Instead of duplicating this federal process approach is unlikely to be sufficient, as at the state level, it would be wise to let the evidenced from the fact that California's Institute finish its work.” Office of Statewide Health Planning and As noted in Part 1 of this article, NIST’s Development (OSHPD), the state agency charge was “the development of a plan charged with the safety of healthcare facilidetailing the basic research, applied research, ties, has found it necessary to make dozens and implementation activities necessary to of significant modifications to the seismic develop a new immediate occupancy (IO) design provisions of the IBC and ASCE/ building performance objective for comSEI 7 for the design of healthcare facilities mercial and residential buildings.” Despite in California. The use of a higher Ie-value Governor Brown’s claim, NIST is not “in in design does not change the risk category the initial stages of developing an immediate of a building. The risk category, along with occupancy standard for buildings.” the anticipated intensity of seismic ground While the needed research detailed in the motion at the site, determines the Seismic NIST report will take much time and resources Design Category (SDC) of a building, to carry out, if the objective is narrowed down which dictates many important aspects of to functional recovery or immediate occupancy design and detailing. If one is looking for Figure 3. The codes and standards system in the of commercial and residential buildings fola simplistic solution, assign all buildings to United States. lowing the design earthquake of ASCE/SEI RC IV. That would be like being forced to 7 (note that these are not identical objectives; buy expensive insurance, difficult to afford immediate occupancy is somewhat more stringent), that may indeed be and often of questionable benefit, for every building. attainable with the knowledge and the information that is already availSecond, it must be understood that there is always a cost associated able. This will doubtless contribute to community resiliency. However, with enhanced performance. The challenge is going to be to attain a few things are going to be essential to keep in mind. the objective while keeping the total cost increase to a minimum. First, it has been suggested that the use of an importance factor The basis of a 1% increase in construction cost for a 50% increase in Ie of 1.5 for all buildings will take us to, or at least close to, func- lateral strength, as claimed in the Lucy Jones interview, is far from tional recovery or immediate occupancy (see Lucy Jones’ ABC7 clear. In any case, this would only address the primary structure and interview cited in Part 1). The approach was proposed in a docu- not all of the non-structural items that keep a building functional. ment issued by the National Institute of Building Sciences, Natural Third, the best way of accomplishing the outlined objective may Hazard Mitigation Saves – 2017 Interim Report. This kind of an not be by passing a bill in a state legislature. Such a measure is ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org
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A U G U S T 2 019
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preemptive, cannot have a national consensus behind it, and also bypasses long-established procedures for making changes in U.S. building codes and standards.
Regular Order
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There is an established building code development and adoption process in the United States (Figure 3, page 19). State and local building codes, which are the legal codes that must be followed for design and construction, are typically based on a model code. The model code of choice in virtually the entire country today is the IBC, seven editions of which, dated from 2000 to 2018, have been published. A model code organization such as the International Code Council (ICC), the publisher of the IBC, does not have resources to develop code provisions on every aspect of design and construction covered by the building code. Thus, it is common for the model codes to adopt national consensus-based (or ANSI-approved) standards. ASCE/SEI 7 – Minimum Design Loads and Associated Criteria for Buildings and Other Structures and material standards such as ACI 318 – Building Code Requirements for Structural Concrete, TMS 402/602 – Building Code Requirements and Specification for Structural Masonry, AISC 360 – Specification for Structural Steel Buildings, and the National Design Specification® (NDS) for Wood Construction are important standards that are adopted by the IBC for design loads on structures and design and construction provisions for structures made of different materials. ACI 318 and TMS 402 are standards and not codes, even though the word Code appears in their titles. The various standards published by ASTM International are also widely adopted by the model code as well as by many other standards. The seismic design provisions of ASCE/SEI 7 are drawn mostly from a resource document called the NEHRP Recommended Seismic Provisions for New Buildings and Other Structures, funded and published by the Federal Emergency Management Agency (FEMA). Enhancements to the seismic provisions are often based on reports prepared by the Applied Technology Council (ATC). That organization prepares its reports following research or studies sponsored by entities such as NIST or FEMA on particular topics of interest (Figure 3). These reports are then published by FEMA, NIST, or ATC itself.
20 STRUCTURE magazine
If a functional recovery or an immediate occupancy (IO) objective is to be added to U.S. seismic design requirements, the document in which it needs to be added is ASCE 7. Work leading to such an addition can be done through a coordinated study, such as an ATC or NIST project, or by some other similar means. Some funding is going to be necessary.
Conclusion To claim that the current U.S. seismic codes and standards are deficient is unwarranted. Codes and standards implement decisions made by the structural engineering community a long time ago given perceived societal needs, including economic considerations. Structures designed by these codes and standards are expected (see Recommended Lateral Force Requirements and Commentary, 1996 Edition, by the Seismology Committee of the Structural Engineers Association of California), in general, to be able to: 1) Resist a minor level of earthquake motion without damage. 2) Resist a moderate level of earthquake ground motion without structural damage but possibly with some nonstructural damage. 3) Resist a major level of earthquake ground motion – of an intensity equal to the strongest earthquake either experienced or forecast for the building site – without collapse but possibly with some structural as well as nonstructural damage. It is expected that structural damage, even in a major design level earthquake, will be limited to a repairable level for most structures. In some instances, repair may not be economical. The level of damage depends upon several factors, including the intensity and the duration of ground shaking, age of the structure, structural configuration, type of lateral force-resisting system, materials used in the construction, and construction workmanship. Damage to nonstructural systems and building contents can be much higher than the damage to the building structure. Although the performance expectations are now being stated differently in recognition of the importance of community resiliency, and some advances have been made in this arena, structural and nonstructural damage is still expected in the design earthquake, except possibly in essential facilities. Whether this is still an acceptable basis for a building code is a question that is increasingly raised. A decision, and ways of implementing that decision, should preferably be developed utilizing the established consensus process.■
Acknowledgments Several colleagues were kind enough to review an earlier version of this article and offer comments, many of which were accommodated and improved the article. Special mention must be made of Kelly Cobeen, S.E., whose comments resulted in significant modifications to the earlier version. S. K. Ghosh is President, S. K. Ghosh Associates Inc., Palatine, IL and Aliso Viejo, CA. He is a long-standing member of ASCE Committee 7, Minimum Design Loads for Buildings and Other Structures, and ACI Committee 318, Structural Concrete Building Code. (skghoshinc@gmail.com)
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Architectural rendering. Courtesy of NADAAA, Inc.
THE HYBRID CLT-STEEL RESIDENCE HALL
and NADAAA architects to develop numerous master planning schemes for the site. Faced with outdated floor plans, poor energy efficiency, and a challenging sloped site, the team brainstormed options to rehabilitate and improve the interconnectivity of the structures and create new spaces for housing, art studios, and amenities. RISD ultimately selected a scheme that would transform student life, including a new six-story, 148-bed building and significant renovations to the remaining buildings to bring them into the 21st century. The project had broad sustainability goals that resonate with RISD’s students, faculty, and programs. Working with Shawmut Design and Construction of Providence, the team established a budget for the proposed work based on a large catalog of masonry and precast concrete plank dormitories constructed in the region. The project was called “RISD Quad.”
Answering the Call
In early 2018, RISD approved financing for the Quad but with a new challenge: was it possible to complete the detailed design, construction, and commissioning of the proposed new building in under 18 months to allow phased construction of the renovations without reducing student bed count – all within the original project budget? To respond to this need, RISD assembled an Integrated Project Delivery (IPD) team consisting of Shawmut, NADAAA, Odeh, and other key consultants and subcontractors. The IPD team used a combination of innovative thinking and lean construction management tools to address the challenge.
Rhode Island School of Design By David J. Odeh, S.E., P.E., F.SEI, F.ASCE, and Paul Kuehnel, P.E.
A
new six-story student residence hall in Providence, Rhode Island, incorporates exposed cross-laminated timber (CLT) panels supported by steel framing. By using this structural system in a dormitory for the first time in New England, an Integrated Project Delivery team achieved the client’s lofty goals of beautiful design, environmental sustainability, and an aggressive construction schedule. The erector exceeded all expectations by completing the superstructure construction in under three weeks. Doing something new required a calculated risk by everyone involved, but the system yielded unexpected benefits for the project that the team could never have imagined.
Planning for Growth RISD (pronounced “Riz-Dee”) is among the world’s great art and design institutions. With 2,500 undergraduate and graduate students, the school has studio-based programs in fine arts, architecture, design, and art education. Perched on Providence’s historic College Hill, the school has not constructed a new residence hall in over thirty years. RISD needed more space to house a growing student population and create a better housing experience. Seeking to expand its capacity to house first-year students and revitalize its aging residential quadrangle, RISD engaged Odeh Engineers
22 STRUCTURE magazine
IPD Enables Innovation Teams often fail to innovate because they fear the unknown. They may not seek the best solution available, but instead the solution that offers the least risk to each party. Designers limit risk from professional liability – tried and true solutions offer the most confidence in avoiding blame for errors and omissions. Contractors want designs that they know they can build and offer the least possibility of schedule delays and subcontractor buyout risk. However, risk comes in many forms – most importantly, the risk of building the wrong building that will hinder the client’s mission in the future, long after the designers and contractors have finished their work. When faced with the challenge of building a highly sustainable project to inspire a future generation of students, with a fixed budget and aggressive schedule, the RISD Quad team needed
Structure at north elevation.
to turn to innovative ideas that could unleash productivity gains without compromising the client’s key goals. A critically important aspect of IPD teams is the close collaboration between contractors and designers from the earliest phases of project planning. For the RISD Quad, this collaboration fostered innovative thinking and gave birth to a new concept: using mass timber construction combined with structural steel framing.
CLT floor profile.
Cross-laminated timber (CLT) is a type of engineered wood panel that consists of dimensional lumber boards glued together in a press with structural adhesives. Available in multiple thicknesses, widths, and spans, CLT is strong, lightweight, dimensionally stable, and inherently fire-resistant. CLT members can act as walls, floors, and roofs in combination with timber or other forms of structural support, including steel. In the last few years, CLT has emerged as a very popular structural design option for buildings in the western United States and is growing in popularity. However, very few large-scale projects have been constructed in New England. A relatively small number of material suppliers in the region, combined with the novelty of the product, can make CLT appear to cost more when compared to other structural
floor systems. How then did the RISD Quad team successfully make the leap to CLT for this project? Working in close collaboration, the IPD team first identified multiple design options for consideration, creating concept design sketches for each option suitable for comparison. Contractors, designers, and the owner then gave each option a numerical score for multiple factors, including the speed of construction, durability, sustainability, and operational flexibility. After scoring these advantages, the team compared the estimated costs of each approach. This process, a type of lean construction management tool called “choosing by advantages,” allowed the owner to assess the overall value proposition from each structural system (instead of just the first cost, which often fails to capture key benefits of a specific option). For RISD, the options included a precast concrete plank building with steel supports and a heavy timber post and beam style structure. Ultimately, the team selected the CLT–Steel Hybrid system despite a slightly higher cost premium (approximately 10%) because of the following perceived advantages: • Sustainability. CLT is made from sapling lumber that can be harvested sustainably. Timber framing also sequesters carbon from the atmosphere, since trees consume carbon dioxide as part of the photosynthesis process.
CLT floor structure to be exposed as a ceiling in the residence hall.
Drone photo of the structure under construction.
Choosing a New System
continued on next page
A U G U S T 2 019
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In coordination with the CLT fabricator, the team chose to use MyTicon self-tapping wood screws to fasten the steel framing to the CLT panels. The use of the MyTicon screws was another decision that went on to provide schedule savings to the team. Screws up to ½-inch-diameter could be drilled into the CLT without the need for pre-drilling as compared to a similar diameter lag screw, requiring each screw to be pre-drilled. Thus, the team realized a much faster detailing time frame on site.
Unexpected Benefits
Drone photo of the completed structure.
• Aesthetics. CLT panels have an inherent fire rating, due to the charring effect recognized in the International Building Code (IBC), and can be exposed without fireproofing as a ceiling. This characteristic allowed for savings in drywall for the ceilings and a highly desirable aesthetic look for the building interiors. The architect and owner felt that the exposed wood interiors also echoed themes of sustainability that students experience in RISD’s curriculum. • Speed. Most importantly, the CLT–Steel hybrid system provided a schedule advantage. Working closely with the selected fabricator, Nordic Structures of Quebec, the team optimized the layout of CLT panels to minimize the erection time. Nordic manufactured the 5-ply panels in 8-foot-wide x 50-foot-long spans. Thus, each slab spans the entire width of the building. Due to the lightweight nature of the material, the erector could use a smaller crane compared to precast concrete slabs. Furthermore, since the contractor could field core most of the plumbing penetrations in the panels without reinforcement, the fabricator could significantly reduce shop drawing preparation and review time when compared to a precast concrete plank system.
Optimizing the Design The composition of the floor structure consisted of 5-ply Grade E1 CLT floor panels measuring 6 7⁄8 inches thick. An architectural grade finish was provided on the underside of the CLT panels, which would be exposed as the ceiling in the living areas. Special mitigation measures were required atop the CLT panels to achieve the desired vibration and acoustic characteristics of the floor structure. Directly above the CLT, the floor assembly consisted of a combination of acoustic isolation mats totaling approximately 1 inch in thickness, a 2-inch-thick layer of self-leveling poured gypsum concrete, and finally a vinyl composition tile. This floor assembly met the owner- and code-required STC rating equal to 50. Additionally, the added mass provided by the 2-inch-thick topping of gypsum concrete enhanced the vibration characteristics of the floor system. The triple span continuous CLT floor panels are supported by steel wide flange beams at the exterior of the building and a line of framing down either side of the central corridor. The steel wide flange beams were fabricated with staggered holes in the top flange to allow fastening of the steel to CLT from the underside of the CLT panels. 24 STRUCTURE magazine
HB Welding, the steel and CLT erector, had recently completed another residential building in Providence using precast concrete plank and steel framing. After comparing the two projects, the erector realized some unexpected benefits, including: • Safety. For field modifications and coring, CLT can be cut using a chainsaw. Compared to concrete slabs, the wood panels can generally be cut from the top on one side, and do not induce silica dust hazards. • Logistics. Due to their lightweight (approximately 19 psf, compared to 75-80 psf for precast concrete planks), multiple CLT panels can be shipped on one truck. This resulted in fewer trucks on the site and less required staging area. • Camber. The CLT for the RISD Quad project shipped with almost no camber and the erector could easily align the horizontal joints between adjacent panels using a nailed spline connector. These unexpected benefits, combined with the advantages already recognized by the team to help HB Welding in conjunction with a crew of carpenters, allowed for the erection of the entire superstructure in 2.5 weeks in the middle of winter in Rhode Island. By completing the structure early, other trades could gain access to the space more rapidly and further enhance the project schedule for completion. As of this writing, the project is on track for on-time delivery in August of 2019.
Conclusions The RISD Quad team succeeded in bringing the first hybrid CLT-Steel residence hall to life in New England by prioritizing innovation and working together to achieve a shared vision. An integrated project delivery approach allowed for close collaboration between the designers and constructors – each bringing their best ideas to the table to create a better solution than they ever thought possible. Future residence hall projects in the region, and beyond, may consider this option as a viable alternative when the speed of construction and sustainability are key considerations of the team. For RISD, combining mass timber and steel construction exceeded the client’s goals for a sustainably designed new home for the next generation of students.■ David J. Odeh is a Principal with Odeh Engineers, Inc. (odehdj@odehengineers.com) Paul Kuehnel is a Project Manager and Senior Engineer with Odeh Engineers, Inc. (kuehnelp@odehengineers.com)
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magine your ideal office space. What (think Jules Verne’s Victorian futurism and the does it look like? What if it was a Nautilus submarine) was a term Dale used sanctuary from the chaos of corporate to describe the aesthetic, and it became office life and allowed you to recharge, a source of inspiration for what the relax, and be inspired at any time architectural character, structure, and in the day without having to venstructural details might look like. ture outside the heart of the city? Dale also made it clear that the This is precisely the kind of space team’s goal was to develop a “neverAmazon was dreaming about for been-done-before” approach for their new headquarters campus the structural system to create the in Seattle, Washington. Doing domes. The design needed to be away with drab and uninspired, organic, crystalline, and snowflakeThe Spheres are a one-of-a-kind like with an indiscernible repeating workspace that fuse nature into the pattern or “kit of parts.” corporate world. On a recent visit to the site, Jay Taylor, Magnusson Exploration Klemencic Associates’ (MKA’s) Structural Managing Principal on the For several weeks the team studied archiproject, commented that, of all the projects tectural and structural precedents as well as he has designed, he could not recall one that geometrical and mathematical theories for definexceeded such challenging expectations in every way ing spheres and shell structures. The design team more so than The Spheres. found their solution in, of Figure 1. Sphere lampshade inspiration. At the project kick-off all things, a lampshade! Courtesy of David Trubridge Studio. meeting in early 2013, (Figure 1) A team member Amazon challenged the had seen a spherical lampteam to conceive, design, shade comprised of what and execute a Seattle appeared to be a repeating landmark destination organic pattern. Another for locals and tourists member of the team idenalike. John Savo, NBBJ’s tified the pattern as being Principal-in-Charge on based on a Catalan solid. the project, shared the The theory of Catalan owner’s challenge to solids was developed by the team: “no comproEugene Charles Catalan, mise on vision.” Based a French and Belgian on comments from the mathematician in the late owner, Amazonians, 1800s. and visitors since the Inspired, the design team opening, the design and dug in. They explored contractor team rose multiple possibilities and above the challenge. ultimately settled on a By Jay Taylor, P.E., S.E., and Robert P. Baxter, P.E., S.E. sphere shape characterized by 60 equally sized and Vision shaped pentagons arrayed around a sphere (technically, a 60-faced, Originally referred to as the “Center of Energy,” the Spheres were dual-polyhedron Catalan solid, otherwise known as a pentagonal envisioned as a place where Amazonians could “get away from the hexecontahedron). city” without leaving the neighborhood. A retreat from the day-to-day From a structural engineering perspective, MKA was intrigued by activity, commotion, and noise of the typical office environment, this the repetition of the pentagon shape and its potential efficiency as a “alternative” workspace would provide employees a place to be energized. “kit-of-parts solution.” However, what intrigued and inspired NBBJ The idea of a workspace surrounded by living plants evolved from and the design team was the endless possible configurations of steel the concept of a traditional conservatory into a “centerpiece” project framing that could be placed within each pentagon and the resultant embracing the belief that biophilic design – the incorporation of unique, organic character, making The Spheres a completely one-of-anature into the built environment – would have a positive effect on kind structure. After literally exploring hundreds of options (Figure 2), employees. The lead design principal for NBBJ, Dale Alberda, articu- the team decided on a framing configuration described as looking lated his vision for the project as a modern interpretation of Victorian like a “fighter jet.” When the configuration was inserted into each conservatories. He wanted a building that had a fully integrated façade pentagon and arrayed around each sphere, it created the beautiful, structure, with exposed and expressive joints, details, and connec- organic pattern Dale Alberda had imagined. The client loved the tions – the structure should be visible and beautiful. “Steampunk” concept – now, how to make the design a reality?
AMAZON pheres S An Uncompromised Vision
26 STRUCTURE magazine
entire surface of each sphere. Once the model was fully assembled in Rhino, the geometry was exported to the analysis software along with member sizes and orientations. Then the supplementary façade support elements were generated by a similar process and exported to the analysis model (Figure 3).
Structural Analysis
Figure 2. The evolution of the Catalan design.
Designing the Structure To turn vision into reality, the design team needed to meet several critical goals beyond aesthetics: determine a consistent structural form to enhance constructability and develop a structural analysis methodology for this complex shape.
Establishing a Buildable Geometry The team coined the term “Catalan” to describe the selected pattern within each pentagon. The original Catalan pattern consisted of rectangular box sections curved in both directions at continuously varying curvatures and twisted about their axis – the shape was reasonable for a model generated as part of an architectural study, but challenging and expensive to fabricate, as it required the entire structure to be fabricated from built-up steel plate boxes. One of the first tasks was to engage the experience of the steel fabricator and steel detailer to understand the boundaries of bending typical Hollow Steel Sections (HSS). Once the boundaries were understood, the design team was able to subtly modify the Catalan geometry such that simplified HSS sections could be rolled into a few constant curvatures without warping. By minimizing the extent of the warped sections and simplifying the HSS sections, the design team was able to simplify fabrication significantly. MKA created a detailed analysis model directly derived from the Architect’s geometric surface model using the parametric soft- Figure 3. MKA’s analysis model. ware Rhino/Grasshopper. A single steel Catalan was carefully constructed in the program; then the parametric design tools were used to replicate that Catalan across the
To test the structural integrity of the Spheres, the team engaged several specialty consultants. The wind and snow consultant, RWDI, provided multiple load combinations with various load patterns across the surfaces of the Spheres. The Rhino model was used to organize each of the thousands of façade elements to wind and snow loads for each load case. The loads were then directly exported to the analysis model with zero manual-data-entry required. Per prescriptive code requirements, the steel frame of the Spheres necessitates fireproofing. Faced with using either bulky spray-applied fireproofing or expensive intumescent paint, MKA worked with the fire consultant to develop a performancebased fire case to demonstrate how the highly repetitive framing did not require fireproofing to meet the required performance objectives. Hundreds of analysis cases were run varying the zone of framing where the design fire would occur, thereby weakening the structure. In each of these cases, the framing was able to resist the fire load case per AISC-360, Specification for Structural Steel Buildings. The result? Intumescent paint was needed only on non-redundant structural arch elements, saving money and delivering the architect’s vision for delicate expressed structural steel. A large displacement analysis method was used to ensure the analysis captured any second-order effects due to structural movement. This required all load combinations to be run separately. With a model composed of approximately 16,000 frame elements, after optimization, the 96 primary load cases could be run in about 20 minutes. In addition to typical axial and bending design checks, consideration of two additional effects was necessary. First, the axial force in the curved side plates changes direction as the member curves, and a perpendicular resisting force is required. This perpendicular force causes twist, which must be considered. The second effect was the application of load at the sidewalls of the HSS by connecting elements, which also caused twist. The along-member stresses were combined with perpendicular stresses due to twist to verify the member design. This was checked at all section corners and the middle of all faces, for each load case. Overall, the design of the structure required tens of millions of stress checks. continued on next page
A U G U S T 2 019
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The Final Structure With a clear vision, an established design, and a vetted structural model, the design and contractor team engaged in the process of final detailing, fabrication, and construction (Figure 4). A major contributing factor to the Spheres’ success was the team’s highly collaborative process throughout the project – especially the early collaboration with the steel erector/fabricator. This early engagement with the steel erector helped determine the detail and splice locations for fabricating the largest transportable pieces in the shop, which ultimately sped up the on-site erection with 620 tons of steel erected in a mere 6 weeks! The integrated design team Figure 5. The completed Amazon Spheres. Courtesy of MKA/Michael Dickter. workflows were exceptionally efficient, requiring only 18 sheets of structural drawings for the Catalan structural steel! Also, a pristine example of what can be achieved with a clear extensive use of jigs during fabrication and component assembly vision and seamless team collaboration and communicacontributed to the creation of a “kit of parts.” All assembled Catalans tion (Figure 5).■ were laser-scanned before leaving the fabrication shop. Survey targets were installed on all members before erection, and installed positions Jay Taylor is the leader of MKA's Cultural Specialist Group. Jay has had a were confirmed before temporary connections being replaced by fully 4-year involvement on the Seattle Design Commission, is an Honorary AIA welded connections. The final erected steel was within one thirtySeattle member, and was appointed as Affiliate Instructor at the University second of an inch of NBBJ’s original 3D model. of Washington Department of Architecture. (jtaylor@mka.com) The attention to detail by all consultants involved and the contracRobert Baxter leads MKA’s Advanced Geometry Technical Specialist tor team allowed the project team to deliver on Amazon’s goals of Team. Robert is one of the firm’s most advanced modeling specialists a unique, never-been-done-before structure. A seemingly imposand has designed multiple projects with incredibly complex geometries. sible challenge – to design and construct a beautiful, iconic, and (rbaxter@mka.com) innovative office space while being cost-efficient. The Spheres are
Project Team
Figure 4. Steel fabrication of the Catalan pieces. Courtesy of NBBJ/Sean Airhart.
28 STRUCTURE magazine
Owner: Amazon LLC – Seattle, WA Engineer of Record: Magnusson Klemencic Associates – Seattle, WA Architect of Record: NBBJ – Seattle, WA Façade: Front, Inc. – Brooklyn, NY Contractor: Sellen Construction Company – Seattle, WA Landscape Architect: Site Workshop – Seattle, WA Fabricator: Supreme Steel Portland (dba Canron Western Constructors) – Portland, OR Steel Erector: The Erection Company – Arlington, WA Mechanical/Electrical/Plumbing: WSP USA – Seattle, WA Steel Bender/Roller: Albina Co. – Tualatin, OR Steel Detailer: Angle Detailing Inc. (ADI) – Wilsonville, OR
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construction ISSUES Recommended Details for Reinforced Concrete Construction Part 3: Columns
By David A. Fanella, Ph.D., S.E., P.E., F.ACI, F.ASCE, F.SEI, and Michael Mota, Ph.D., P.E., SECB, F.ACI, F.ASCE, F.SEI
This article is the third in a series (STRUCTURE: Part 1 – June 2019, Part 2 – July 2019) on recommended reinforcement details for cast-in-place concrete construction.
Detailing Longitudinal Reinforcement Once the size of the cross-section and the required area of longitudinal reinforcement have been determined for a reinforced concrete column based on strength requirements, the size and number of longitudinal reinforcing bars must be chosen to provide an area of reinforcement equal to or greater than the amount that is required, and satisfy the minimum and maximum spacing requirements in ACI 318-14, Building Code Requirements for Structural Concrete and Commentary. Columns that have longitudinal reinforcement ratios in the range of 1 to 2% are usually the most economical because concrete resists axial compression forces more cost-effectively than reinforcing steel. It is usually more economical to use larger column sizes with less longitudinal reinforcement. Figure 2. Lap splice location for The minimum number of lonreinforced concrete columns in buildings assigned to SDC A, B, or C. gitudinal bars in a column based
Figure 1. Minimum clear spacing between longitudinal bars.
on the type of transverse reinforcement is given in ACI 318 Section 10.7.3.1 (Table 1). Where longitudinal bars are in a circular arrangement, the orientation of the bars has an impact on the moment strength of a column where less than 8 longitudinal bars are provided; this must be considered in the design. Minimum clear spacing between the longitudinal bars is given in ACI 318 Section 25.2 (Figure 1). In the figure, db is the diameter of the longitudinal bars and dagg is the nominal maximum size of coarse aggregate in the mix. The longitudinal bars must be spaced far enough apart so that concrete can easily flow between the bars. Minimum bar spacing is especially critical at splice locations. To facilitate the selection of the longitudinal bars, Table 2 contains the minimum face dimension of rectangular tied columns with normal lap splices based on the minimum spacing requirements assuming 1.5-inch clear cover to #4 ties. The column face dimensions have been rounded to the nearest inch. Similar tables can be created for other tie bar sizes and circular longitudinal bar arrangements. For columns in ordinary moment frames in buildings assigned to Seismic Design Category (SDC) A or B, or in intermediate moment frames in buildings assigned to SDC C, lap splices of the longitudinal Table 1. Minimum number of longitudinal bars in a column.
Type of Transverse Reinforcement
Minimum Number of Bars
Triangular ties
3
Rectangular or circular ties
4
Spirals
6
Circular hoops for columns of special moment frames
6
Table 2. Minimum face dimension (inches) of rectangular tied columns with normal lap splices.
A U G U S T 2 019
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bars are permitted to occur immediately above the top of the slab, which is the preferred location for ease of construction (Figure 2, page 31). The type of lap splice that must be used depends on the stress in the longitudinal bars due to the factored load combinations (see ACI 318 Section 10.7.5.2). For columns that are part of special moment frames in buildings assigned to SDC D, E, or F, lap splices must be tension lap splices and located within the center half of the column length. These lap splices also must be located away from the ends of the column where spalling of the concrete shell surrounding the transverse reinforcement is likely to occur due to a seismic event (ACI 318 Section 18.7.4.3).
Detailing the Transverse Reinforcement Requirements for columns with tie reinforcement are given in ACI 318 Sections 10.7.6 and 25.7.2, and standard hook dimensions for ties are given in ACI Section 25.3.2. Tie spacing requirements for reinforced concrete columns in buildings assigned to SDC A and B are given in Figure 3. The clear spacing between ties must be at least (4/3)dagg. Depending on the shear strength requirements, the required tie spacing may be less than that in the figure. The provisions of ACI 318 Section 25.7.2.3, which pertain to rectilinear tie configurations and the maximum clear spacing permitted between laterally supported longitudinal bars, are illustrated in Figure 4 and ACI 318 Figure R25.7.2.3a. Lateral support must be provided for longitudinal bars that have a clear spacing greater than 6 inches from a laterally supported bar on each side along the tie. There are numerous ways to arrange ties in a column, and some arrangements are preferred more than others. Consider the arrangements in Figure 5.
Figure 4. Lateral support requirements for longitudinal bars in tied columns.
32 STRUCTURE magazine
Figure 3. Tie requirements for reinforced concrete columns in buildings assigned to SDC A and B.
Table 3. Recommended standard spirals for circular columns.
The arrangements in Figures 5a and 5b are preferred over the arrangement in Figure 5c because 1) the outer confinement tie acts as a template for the ironworker to place the longitudinal bars; 2) it is easier to maintain the required concrete cover using side-form spacers; 3) it is more efficient at preventing displacement of the longitudinal bars while the column cage is being moved into place by the crane; and (4) the tasks that are needed to be completed by the ironworker are simplified, which translates to increased productivity. Transverse reinforcement requirements in columns that are part of intermediate and special moment frames are given in ACI 318 Sections 18.4.3 and 18.7, respectively. Requirements for columns with spiral reinforcement are given in ACI 318 Sections 10.7.6 and 25.7.3. Standard spiral sizes are #3 to #5, and the clear spacing between consecutive turns on a spiral must not exceed 3 inches or be less than the greater of 1 inch or (4/3)dagg. Recommended standard spirals for circular columns with Grade 60 reinforcement and various concrete compressive strengths are given in Table 3. Additional recommendations and guidelines for detailing reinforced concrete columns in buildings assigned to any SDC can be found in the CRSI publications Design Guide for Economical Reinforced Concrete Structures and Design Guide for Reinforced Concrete Columns.â– The online version of this article contains references. Please visit www.STRUCTUREmag.org. David A. Fanella is Senior Director of Engineering at the Concrete Reinforcing Steel Institute. (dfanella@crsi.org) Michael Mota is Vice President of Engineering at the Concrete Reinforcing Steel Institute. (mmota@crsi.org)
Figure 5. Column tie arrangements; a) With outer confinement tie and inner closed tie, b) With outer confinement tie and crosstie, c) With paired overlapping ties.
New ACI 318-19 Now Available The newest edition of ACI’s 318 Building Code Requirements for Structural Concrete and Commentary is now available. The latest edition includes new and updated code provisions along with updated color illustrations for added clarity. FIND AN ACI 318-19 SEMINAR NEAR YOU Learn more about the latest edition of ACI 318 by attending the public seminar “ACI 318-19: Changes to the Concrete Design Standard.” Visit concrete.org/ACI318 for a complete list of dates and locations and register today!
building BLOCKS Mass Timber Engineering
Something Old and Something New By Jim DeStefano, P.E., AIA, F.SEI
M
ass timber is not a new idea, just a new name. Many people have heard the term “mass
timber” batted around in the last few years, but not everyone has a clear understanding of what it means. Mass timber used to be referred to as “heavy timber” and the International Building Code (IBC) classifies it as Type IV construction.
Portland Jetport’s glulam timber roof structure. Courtesy of Robert Benson Photography.
Mass timber is a type of structure made up of big, fat pieces of wood (DLT). CLT has been used in Europe since the 1990s but has only been that, unlike light wood frame construction, burn slowly. Because mass available in North America for a little more than a decade. Today, there timber structures maintain their integrity during a fire without the are five major manufacturers of CLT in North America and one producer need for layers of fire-resistant materials, they are suitable for larger of DLT. The availability of timber panels has stimulated interest and buildings and the IBC recognizes that. excitement in the architectural community for building with mass timber. Timber buildings have been built for over 4,000 years – since Modern mass timber structures typically consist of a glulam timber bronze-age man developed the technology to forge sharp tools that frame supporting CLT or DLT floor and roof panels, often with could hew trees into square timbers and fashion mortise and tenon CLT shear walls. This type of construction is extremely versatile joints. Timber structures were the dominant and is suitable for both large and small strucconstruction type in Europe, Asia, and North tures – anywhere that an architecturally exposed America until 1850 when balloon-frame wood structure is desired. It does not make much structures began to displace timber construcsense to build with timber and then cover it up tion. By the turn of the 20th century, timber with sheetrock and hung ceilings. By the way, construction was nearly extinct and structural industry experts continue to express that mass iron construction was becoming commontimber construction is also very sustainable, place for larger structures. sequesters carbon, and is cost-effective. Following World War II, glulam timber construction became popular for long-span and CLT, NLT, and DLT architecturally exposed applications such as church roofs. At the time, if you wanted a strucCross-Laminated Timber has often been ture that looked like timber, glulam construction described as plywood on steroids. It is made was your only choice. That is no longer true. up of alternating plies of dimension lumber In the 1970s, traditional timber frame conthat has been planed to approximately 13⁄8struction experienced a revival. A small, core inch thickness. Like plywood, each ply is group of timber craftsmen, mostly located in CLT panel layup with alternating plies of oriented perpendicular to its adjacent plies. northern New England, rediscovered and mas- dimension lumber. Courtesy of WoodWorks. Common CLT layups are 3-ply (41⁄8-inch), tered the lost art of crafting timber structures 5-ply (67⁄8-inch), and 7-ply (95⁄8-inch). CLT with intricate joints. By the 1990s, timber panels are typically 8 feet or 10 feet wide and frame construction had re-entered mainstream up to 60 feet long. construction and was displacing glulam conThe structural design of CLT panels is covstruction for architecturally exposed structures. ered in Chapter 10 of the National Design Specification® (NDS®) for Wood Construction. Effective section properties and reference So, What’s New? design values can be found in ANSI/APA PRG Timber panels are new – Cross-Laminated CLT panels are available up to 10 feet wide and 320-2018, Standard for Performance-Rated Timber (CLT) and Dowel Laminated Timber over 60 feet long. Courtesy of WoodWorks. Cross-Laminated Timber. 34 STRUCTURE magazine
T3 Office Building’s Nail Laminated Timber panels on glulam framing. Courtesy of Bergerson Photography.
Mystic Seaport Exhibition Building’s glulam timber connections transfer structural loads in bearing with minimal exposed bolts and steel hardware.
Nail-Laminated Timber (NLT) panels are made up of dimension lumber (typically 2x6s) laid side-by-side and spiked together with nails. NLT is not a new thing and has been used infrequently for over a century. The advantage of NLT construction is that it does not require specialized fabricating equipment to manufacture, and it can even be built on-site. The disadvantage is that NLT panels require a significant number of nails that are labor intensive to install and make it impossible to cut the panels with Computer Numerical Control (CNC) equipment without the embedded nails destroying the cutter-head. Dowel-Laminated Timber panels are the newest alternative. DLT is similar to NLT, except that it contains no nails. Transverse hardwood dowels are used instead of nails to bond the panels. Unlike NLT, DLT panels lend themselves to CNC fabrication.
Many engineers that are inexperienced with timber engineering will attempt to connect timbers in a fashion similar to structural steel construction, with bulky side plates and lots of bolts. While this approach sometimes works, it is seldom the most practical or efficient way to make a timber connection, and it is rarely the most aesthetically pleasing solution for an exposed structure. It is smarter to configure connections and connection hardware so that structural loads are transferred primarily in bearing and bolts are only relied on to resist incidental loads. Timber is an organic material that shrinks and swells seasonally with changes in humidity. Failure to consider timber dimension changes associated with moisture content when designing connections can lead to disappointing (or dangerous) results. Steel gusset plates can restrain dimension change movements resulting in the splitting of the timbers (for more on this, see the February 2004 issue of STRUCTURE).
Reaching for the Sky High-rise construction has long been the exclusive domain of structural steel and reinforced concrete, but that is starting to change. Mass timber is now a player in the high-rise market. Brock Commons in Vancouver is currently the tallest mass timber building in North America, topping out at 18 stories. In the world of tall buildings, 18 stories may not sound all that impressive, but, for wood construction, it is a big deal. It is unlikely that mass timber is going to displace structural steel and reinforced concrete for high-rise construction completely, but we will see more tall, mass timber projects and will probably be seeing a lot more mass timber and structural steel hybrids, especially with anticipated changes coming in the 2021 IBC. The 2021 IBC will permit buildings up to 18 stories to be constructed of mass timber.
Getting Connected Timber engineering is all about the connections. Sizing the timbers and panels is the easy part. Designing timber connections is the challenging part. It has been said that a structure is essentially an assembly of connections that happen to be separated by beams and columns, and that is especially true of timber structures.
Fired Up
It is a common misconception that because wood is combustible, wood buildings perform poorly in a fire. While that may be true of light wood frame construction, it is not at all true of mass timber. Timbers will develop a char layer on the surface when exposed to a flame. The char layer progresses slowly and insulates the wood beneath it from the heat of the fire, permitting the timbers to continue to carry load. When timber structures do eventually fail during a fire, they do not fail suddenly. They typically give firefighters ample warning prior to a collapse by making loud cracking and hissing noises. The exception is, when steel connection hardware is exposed to the fire, the connections will fail suddenly. It is important to protect steel connection hardware either with an intumescent coating or preferably by having all steel hardware embedded inside the timbers where the wood can protect the steel from the fire. Unlike most other structural systems, the fire-resistance rating 18-story Brock Commons. Courtesy of www.naturallywood.com. A U G U S T 2 019
35
of mass timber assemblies or elements is based on a structural analysis rather than on the listed results of an ASTM E119 fire test. Chapter 16 of the NDS has a procedure for calculating the fire-resistance rating. The thickness of the char layer on the timber is stipulated for different time intervals. For instance, if a 1-hour fire rating is required, the NDS stipulates that the char layer on a timber is 1.8-inch-thick after 1 hour of fire exposure. It is then a simple matter of calculating the remaining section properties of a timber with 1.8 inches of wood removed from the exposed perimeter and determining if the reduced section is capable of supporting the applied dead loads with a stress increase to convert allowable stress to ultimate stress. The fire-resistance calculation for a CLT panel is similar, except the stipulated char thicknesses are a little different. A 5-ply CLT is needed if a fire rating is required since there is not much left of a 3-ply CLT if the bottom ply burns away.
Hybridization Mass timber plays well with other structural materials. Often the right structural solution for a project is not a pure mass timber structure, but a hybrid solution. CLT floor and roof panels with glulam joists, structural steel girders and columns, and a concrete topping slab often make for a very efficient structure. The structural steel elements often require an intumescent coating to achieve a fire resistance comparable to the timber elements. For the 18-story Brock Commons project, reinforced concrete shear walls resist wind and seismic loads. Mass timber shear walls could have done the job, but for a variety of reasons, it just made more sense to use concrete. There is no shame in not being a purist.
Design or Delegate
Serviceability Considerations The design of mass timber floor systems is typically not controlled by strength, but by serviceability – principally sound transmission and vibration. Bare timber floors will readily transmit sound, particularly impact sound associated with footfalls. It is common to install an acoustic mat over the timber floor with a non-composite concrete topping slab or gypcrete topping to address sound transmission. Floor vibration associated with foot traffic must be evaluated. Designing to an arbitrary static deflection limit such as L/360 or L/480 will not ensure that a floor structure does not feel bouncy, particularly if the floor structure has a period of vibration less than 9.0 Hertz.
It is not uncommon for a Structural Engineer of Record to delegate design responsibility to a specialty timber engineer engaged by the contractor. Sometimes, responsibility for engineering just the timber connections is delegated, but occasionally an engineer will delegate responsibility for engineering the entire timber structure. In such cases, it is best to engage the specialty timber engineer during the design phase of the project. Otherwise, the bidding process can turn into a circus. So how much of the timber engineering should be performed by the Engineer of Record and how much should be delegated? The answer depends on how much prior experience the engineer has with timber structures. The best advice is for the Engineer of Record to do as much as he is competent to do and delegate only what he is inexperienced at.
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So, where can an engineer turn to get more information on mass timber engineering? The Timber Frame Engineering Council (TFEC) is an organization of structural engineers who specialize in timber. The TFEC has produced a library of documents that offer design guidance, including a Standard for Design of Timber Frame Structures, Code of Standard Practice, master specifications, and technical bulletins, all of which are available free of charge at https://bit.ly/2S7H7fp. The CLT Handbook produced by FPInnovations offers a wealth of practical information. The WoodWorks team is also always willing to offer design assistance and to point engineers and architects in the right direction.■ Jim DeStefano is President of DeStefano & Chamberlain, Inc., located in Fairfield, CT. He is the chair of the TFEC Mass Timber Committee. (jimd@dcstructural.com)
Mass Timber Floor Vibration By Lucas Epp, P.Eng. The maximum span of many floor systems is often controlled by The CLT Handbook (FPInnovations, 2013) presents a formula limiting occupant perception of vibrations rather than strength which limits the span based on a combination of deflection under or deflection. This is the case for long span concrete and steel a unit point load and natural frequency. This criterion, however, floor framing, high-performance, light frame floors, and mass ignores the contribution of different damping levels, the weight timber floors. The unsettling performance of a bouncy floor is of any superimposed mass (such as concrete topping), and any typically caused by resonance added flexibility of supporting between human walking frestructure, all significant factors quencies (1.6-2.0 Hz) and the in vibration performance. The floor's natural frequency. If the CLT Handbook formula results walking pace is a multiple of in the following maximum floor frequency (e.g., walking at spans for typical CLT layups 2.0 Hz causing a floor at 6.0 Hz (ANSI/APA PRG 320, 2018): to vibrate), resonance can occur 41⁄8-inch-thick, 3-ply CLT – 11 and the resulting accelerations feet to 12 feet can be disconcerting, particu67⁄8-inch-thick, 5-ply CLT – 16 larly in longer-span structures feet to 17 feet with low damping. 95⁄8-inch-thick, 7-ply CLT – 20 For mass timber floors, rulefeet to 21 feet of-thumb guidance such as Concrete topping is often limiting the floor’s fundamental required on mass timber floors frequency to above 8.0 Hz, or to generate sufficient acoustical limiting the deflection under separation between floors. This a unit point load, has been topping increases the modal used in an attempt to avoid mass (i.e., participating mass floors which perform poorly. that needs to be excited by However, simply limiting the footfall), directly affecting the natural frequency of the floor accelerations which would be does not guarantee good perfelt on the floor plate. formance. A better estimate of In Europe, the guidance for Recommended peak acceleration tolerance limits for human comfort, after floor performance is the accelmass timber floors in Eurocode Allen and Murray (1993), Design Criterion for Vibrations Due to Walking. erations, which result from 5 (EN 1995-1-1) states that, for walking activities. These accelfloors with a natural frequency erations depend not only on the natural frequency, but also (f ) > 8.0 Hz, the deflection under a 225-pound point load should the amount of floor mass being mobilized, the damping in the be limited to less than 1⁄16 to 1⁄32 of an inch. For floors with f < system, the length of the walking path, and the frequency of 8.0 Hz, the Eurocode recommends calculating accelerations and occurrence. Human sensitivity to accelerations is a grey area and limiting these to 0.5 to 1.5% gravity. recommended limits on permissible accelerations for various use For early-stage design with panels supported on bearing walls, cases vary significantly between different guidelines. (See Figure) the CLT Handbook formula can be used only as an initial estimate Guides such as AISC Design Guide 11 (Vibrations of Steel-Framed of the maximum vibration-controlled span. For panels spanning Structural Systems Due to Human Activity, 2016) provide detailed onto beams, this formula is unconservative and more detailed approaches to calculate accelerations on floor plates. There are also acceleration analysis should be undertaken, as the beams conseveral European guides which provide similar guidelines with tribute significantly to the flexibility of the system. different calculation approaches. Currently, there is ongoing research to develop more accurate simCalculating accelerations on a given floor plate can be done plified criteria, as well as a robust guidance document for vibration using finite element software, but this is an involved analysis design of mass timber floors. The US Mass Timber Floor Vibration and simpler guidance is needed to assist in early stage design Design Guide will be published by WoodWorks in 2020 and will conand scheming. tain more details on both analysis methods and acceptance criteria. For further information and design guidance, refer to TFEC Bulletin 2019-14, Vibration Design of Mass Timber Floor Systems.
Current Design Guidance
For mass timber floors, there are various guidelines in North America and Europe which have been developed to determine allowable vibration-controlled spans.
Lucas Epp is the Manager of Engineering at StructureCraft in Abbotsford, BC. (lepp@structurecraft.com)
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INFOCUS A Continuing Discussion on Structural Engineering Engagement and Equity By John Dal Pino, S.E.
S
TRUCTURE published a series of So, let’s work to make the profession better Based on the 2016 SE3 Survey data, more three articles in 2017 written by the but, at the outset, we should acknowledge than 50% of respondents reported having Structural Engineering Engagement and that we have it pretty good compared to other thought about leaving the profession at some Equity (SE3) Committee of the Structural jobs where people punch a clock, open the point. Men and women alike noted that they Engineers Association of Northern California store in the morning, or start teaching school desired higher pay, less over-time, flexible ben(SEAONC). These articles presented the every morning at 8:00 am sharp. efits, flexible daily or weekly hours, the ability results of a 2016 nationwide to work remotely, and matersurvey of engineers on the curnity/paternity leave. Women rent conditions in the workplace rated “better work-life balance” about both engagement (broadly most highly, while men rated defined as satisfaction) and equity “higher pay” as the top reason. (broadly defined as fairness) and From a firm perspective, the offered ideas for improved career industry has become more development, retention, pay competitive than ever. Large and benefits, access to opporfirms are expanding; smaller tunities, and work-life-balance. firms are leveraging technology STRUCTURE has also published to work outside their normal articles on the importance of mengeographic boundaries. Fees toring and how to do it effectively have stayed relatively constant, (see Anderson, April 2018, and while buildings have become Grogan and Anesta, May 2014). larger, more complex, and more The survey showed that many expensive. Technology has made engineers expressed displeasure in work easier, but, with that, several facets of the workplace enviclient expectations for speed of ...it is essential that we all work toward ronment, which means there are design and construction have real issues to discuss and address. risen. Paper drawings have been creating a more rewarding and fulfilling workplace for Avoidance and dismissal is not a replaced with the new-normal engineers, one that is complementary to an equally good plan of action. Therefore, I of complicated REVIT/BIM support the efforts of SE3 (now models, provided of course for rewarding and fulfilling personal life. an NCSEA committee with many no additional cost. While the MOs starting their own commitindustry is doing well today and tees) and believe that it is essential that we all At the heart of the matter is a clash between profits are strong (or should be), money is as work toward creating a more rewarding and two competing interests: those of the individ- tight as ever, averaged over an entire business fulfilling workplace for engineers, one that is ual and those of the firm. Firms that navigate cycle. Recruiting and retaining talent is critical. complementary to an equally rewarding and this clash directly, equitably, and reach some It can be challenging to get it all to function fulfilling personal life. At the same time, firms level of compromise will come out on top by smoothly. The bottom line is that financial must be structured to succeed. retaining their staff. The firms that do not will pressures on the firm can make it difficult to I am both an engineer and a current project go by the wayside. accommodate the wishes of the staff. manager (and a former large firm shareholder) From the employee perspective, life is getting Firms that embrace change will be successful and feel a bit conflicted trying to fulfill myself more complicated and demanding (longer in terms of recruiting and retaining talent, and my engineers, as well as my firm. Some and more stressful commutes, work travel, which after all is the core of any engineering of the best practices proposed by the SE3 two-working-parent households, helping business, all while addressing client needs 2016 Report are win-win no-brainers (see aging parents, etc.). Engineers are human efficiently and economically. www.se3project.org/best-practices.html), beings and want to enjoy a rich personal life so most firms should seriously consider imple- outside the office. Based on the 2016 SE3 Gender and Pay Equity menting them. However, I see the other side survey data, engineers also expect equitable of the coin, too; other changes require more advancement, reasonable workplace expecta- There is no place in today’s workplace for disdebate and discussion. I enjoy the profession, tions about effort and hours, flexible work crimination or bias in determining pay. This is the lifestyle I have been able to achieve, and hours and schedules, and the ability to work not a uniquely American issue but, as a counthe workplace flexibility that I have enjoyed. from home or remotely to suit their situation. try, we seem to be having the conversation, 38 STRUCTURE magazine
addressing the issue, and working to bring this relic to an end. Our country is a great beneficiary of this trend, and we are all better off. However, for engineering firms, we must recognize that an individual’s level of pay will be based on their overall value to the firm, considering several factors, many of them subjective. Firms that set clear benchmarks and expectations can minimize implicit bias and will have better relationships with employees and be perceived as being fair. In evaluating pay equity, it is important to remember that, early in an engineer’s career, technical skills are most important. As an engineer’s career progresses, client management, networking skills, business development ability, and project management skills are most sought after and rewarded. These skills are not taught in the university yet must be acquired and honed. Although smart firms provide training equally for all interested, ultimately, as professionals, it is up to the individual to develop the necessary skills and earn the pay they desire.
Salary and Benefits Pay was one of the top factors for engineers that reported considering leaving the profession. Honestly, I think engineers do pretty well financially compared to other professions requiring a college degree. The hard truth is overall salary levels and benefits are a function of marketplace competition between firms (design fees), the value of services provided to clients (firms in niche markets or with unique abilities do better), and the supply and demand of engineers. Every employee, regardless of industry, would like to be paid more and get regular salary increases. However, engineering firms can only pay out what they consistently generate, after accounting for non-salary operating costs, needed investments, and a reasonable profit. Firm owners can sometimes be their own worst enemies in their relationships with employees by competing for jobs based on lowest fee or by over-valuing their own expected return, leaving less for salaries and benefits. If you are at one of those firms, and if salary and benefits are really important, advocating for what you are worth is the first step, but changing firms is also an option. Some firms do better than others because of superior management and services offered, and employees benefit. Seek them out.
Work-Life Balance Today’s norm for hourly industrial employees is 40 hours per work week with paid overtime, supported by a host of regulatory labor protections. Although engineering is a service sector field, the expectation of engineers is
much the same. However, engineering is a demanding profession, and getting the work done in 40 hours is, more often than not, impossible. From my personal experience, most people are willing to put in a little extra to meet deadlines. However, over an extended period, more than about 10% overtime is not sustainable and leads to stress and work-life imbalance. Amazingly, some employers take advantage of this dedication (knowingly or without much thought). However, eventually, people burn out and may quit. Unless the business model is to only hire entry-level staff and replace them as needed, the loss of skilled and trained staff due to overwork or rigidity in schedule can have devastating impacts on the firm’s bottom line. My approach to staff has always been to give staff the responsibility to get the job done with flexibility as to how and when they do their work. This allows life to be attended to consistent with project demands. I monitor booked overtime since too much is not good. If an engineer needs to take a few hours off to take care of a personal matter, I let them. My advice is that firms be flexible and pay attention. The bond between firm and employee will be strengthened, and the engineer will likely more than repay you.
Working Remotely Working remotely is easier than ever with technology and secure connectivity. Engineers greatly appreciate this flexibility. The ability to work remotely is likely to be a net positive and also contributes to a healthy work-life balance. Additionally, it can be a way to employ talented staff who are not in the geographic locale of the office. As highlighted in the 2016 SE3 Survey, working remotely is also a two-edged sword. Some engineers reported being critical of co-workers that utilize flexibility benefits like working remotely. Engineers who desire to work remotely must recognize that it is more valuable to the firm for the engineer to be in the office fostering better communications, stronger personal relationships, and higher overall team productivity. As a result, the engineer who works remotely on a regular basis may suffer some consequences in terms of salary and career advancement. This trade-off may be positive for both parties, but each party must understand the issues and enter into the set-up with eyes wide open.
Mentoring In the past, on the job training (a form of mentoring) was achieved by observing senior staff or during casual conversations
with coworkers. Most owners and managers developed their soft skills and habits in the workplace by observing the owners and managers they worked with, and so on. Today, mentoring has become more formal and organized and involves seeking out mentors either inside or outside the firm. The new model should yield better results. Firms that lack formal mentoring programs should set them up to satisfy the need. Avoiding the issue is not a plan. Programs will expose engineers to sought-after project management skills. Mentoring should help the mentee in terms of career advancement and job satisfaction so, logically, the engineer should be happier and be more likely to stay with, and improve, the firm. If you find that engineers are not too interested in your firm’s owners and managers as mentors, that should tell you something!
The Pursuit of Happiness Logic would suggest that engineers work better, are more dedicated, and are more likely to stay when they are happy and like their work experience. Be a good listener and observer; coach engineers and empower them to be successful. Show your appreciation for a job well done. And remember to have some fun along the way. Summary Points 1) Treat everyone equally, fairly, and with respect. Compensate them according to their value and merit using clearly communicated and measurable benchmarks to ensure fairness and eliminate misunderstandings. 2) Structure and conduct the firm’s business to maximize profits and available funds for salary and benefits. 3) Create a workplace that balances work and life. Good staff is naturally dedicated, but owners and managers need to be realistic about the workload. Be flexible. 4) Make working remotely simple and easy. Consider using technology to attract and retain staff outside your geographic area. 5) Set up a mentoring program and be a good mentor if asked. 6) Work hard, but take time to thank people and show genuine appreciation.■ John A. Dal Pino is a Principal with FTF Engineering located in San Francisco, California. He serves as the Chair of the STRUCTURE Editorial Board. (jdalpino@ftfengineering.com)
A U G U S T 2 019
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2019 Structural Engineering Summit – November 12-15 Your Opportunity to Mix Fun, Structural Engineering, and the Magic of Disney!
NCSEA's Structural Engineering Summit draws the best of the structural engineering field together for an event that has been designed by structural engineers to advance the industry. The 2019 Summit is taking place at the Disneyland Hotel in Anaheim, California. NCSEA has secured group rates at the Disneyland Hotel as well as Disney's Grand Californian. Rooms are available before and after the Summit (per hotel availability), giving you the option to fit in some leisure time. There is a lot to look forward to at this year's Summit! First, we have changed the schedule to begin on Tuesday and end Friday afternoon; this brand new format will allow for more education and less overlap. Each day will host at least one keynote speaker, covering topics such as the © Disney future of consulting engineers, the importance communication has on success, the economic risk that comes from protecting communities from earthquakes, the good that comes from connecting populations to important resources, and why structural engineers should develop better interpersonal skills to further their career. The 2019 Summit will also host the very first national Structural Engineering Engagement and Equity (SE3) symposium. Learn more about this event and how to register by visiting www.ncsea.com. In addition to the 16 hours of continuing education that is available, there are also several networking opportunities. To welcome everyone on Tuesday night, NCSEA will be hosting a Grand Opening Reception for all attendees; immediately following that is a reception sponsored by the Structural Engineers Association of California (SEAOC)! On Wednesday night, A Celebration of Structural Engineering is back. This extravagant event, hosted by Computers and Structures, Inc., will celebrate the immeasurable contributions of structural engineers. Finally, Thursday evening will close with NCSEA's Awards Celebration. This event has been redesigned for 2019. It will begin with cocktails and the awards, and will be followed by a festive Velvet Rope After Party with dinner, entertainment, and music, allowing for opportunities to network with fellow Summit attendees, the Excellence finalists/winners, and recipients of the Special Awards. Join us for the best practical education with expert speakers, a leading trade show, and compelling peer-to-peer networking at an event designed to advance the industry. Visit www.ncsea.com to view the complete schedule and to register today! Hotel rooms are being reserved quicker than ever, do not miss this chance to be a part of this growing and dynamic event.
Students Race the Clock in AISC’s Steel Bridge Competition In early June, the American Institute of Steel Construction (AISC) held their Student Steel Bridge Competition at Southern Illinois University. The focus of the competition is to extend students' classroom knowledge to a practical and hands-on steel-design project that grows their interpersonal and professional skills, encourages innovation, and fosters impactful relationships between students and industry professionals. In all, 41 teams of the country’s top student engineers qualified for the national finals, which were held in early June on the campus of Southern Illinois University in Carbondale, IL. Their challenge: build a lightweight 25-foot-long steel bridge that supports 2,500 pounds – as quickly as possible. This year’s design also called for an offset footing on one side of the “river.” Students demonstrated an array of creative solutions to meet these design requirements. Photo provided by AISC “There is so much energy from the students on competition day,” said Christina Harber, S.E., P.E., director of education at AISC. “The level of commitment that they have for learning, achievement, and teamwork is astounding. Their passion for this program will translate into successful careers as engineers.” Lafayette College took first place overall, assembling their bridge in a staggering 3.8 minutes. The team also won first place in the categories of stiffness, construction economy, and structural efficiency. The University of Alaska Anchorage took home the Frank J. Hatfield Ingenuity Award for an eye-catching bridge design that met the challenge of the offset pier placement by using an asymmetrical combination single arch and deck truss design. The Rules Committee also recognized the team from Purdue Northwest for their respect and professionalism during the competition and their positive attitude in the face of adversity, sending the team home with the Robert E. Shaw, Jr. Spirit of the Competition Award. NCSEA worked with AISC as a Supporting Affiliate, providing any needed assistance as well as additional promotion of the competition to our membership. We were glad to help raise awareness with our Member Organizations and connect AISC to local engineers and volunteers. NCSEA looks forward to working with AISC for next year's competition! To learn more about the competition and to watch a recap of the Student Steel Bridge Competition National Finals, visit bit.ly/aiscbridge.
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News from the National Council of Structural Engineers Associations
Prepare for the SE Exam with NCSEA
The Best Instructors. The Best Material. Available to you immediately! NCSEA's restructured SE Refresher and Exam Review course is completely on-demand. Review course materials and watch the recordings when it is convenient for YOU. This on-demand course provides the most economical SE Exam Preparation Course available. The course includes 30 hours of instruction: 9 Vertical Sessions and 11 Lateral. The course will give you 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. PLUS ... students have access to a virtual classroom exclusively for course attendees! Ask the instructors directly whenever questions arise. This SE 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!
YMGSC's Fifth Annual Trivia Night Was A Major Success Each year NCSEA's Young Member Group Support Committee hosts a web-based trivia night for young member groups (YMGs) across the country. This year's event hosted thirteen teams from nine different states! The questions were split between engineering and general knowledge. The night kicked off with a round on masonry, went global in rounds 2 and 3 with World Trivia and "Guess that Airport" before the final round of "Steel Construction Basics," "filled with curve balls that surely made even the most recent PE taker scratch their heads," said Justin Sharkey of SEAONY's YMG. Tennessee's YMG was crowned the overall champion, winning by just a single point. In addition to the friendly competition, each young member group was able to give a brief presentation of their recent activity and upcoming projects. And to sum that up, there are a lot of great things occuring across the county and the YMGs are very busy making them happen!
Public Outreach Competition
Can structural engineers improve the public visibility and recognition of the profession? The NCSEA Communications Committee thinks so, and wants to encourage you and your SEA to participate. NCSEA Member Organizations are invited to participate in the very first Member Organization Public Outreach Challenge. The goal is to inform and educate other industries, professions, and the general public about Structural Engineering. The process is simple, SEAs and SEA members are already creating content; compile it, submit to NCSEA by September 8, 2019, and after review, a winner will be chosen! Learn more about the challenge and content eligibility by visiting www.ncsea.com/challenge.
Timber-Strong Design Build Competition Coming to NCSEA Summit
In partnership with the American Wood Council, the APA-Engineered Wood Association, and Simpson Strong-Tie, NCSEA is bringing the Timber-Strong Design Build™ to the 2019 Summit in Anaheim, California! In conjunction with the Summit, this “hands-on” opportunity for university engineering students is intended to give real-world experience in both planning and building a wooden structure. Student teams will prepare a project complete with a preliminary design, material cost estimates, structural calculations, and estimated carbon footprint. The activity will provide an opportunity for engineering students to experience the full spectrum of designing and building a real project within a team environment. Along with the support of the American Wood Council, key sponsors include APA – The Engineered Wood Association and Simpson Strong-Tie. For more information visit www.ncsea.com/timber or contact NCSEA at ncsea@ncsea.com.
NCSEA Webinars
Register by visiting www.ncsea.com.
August 15, 2019
September 19, 2019
James Jeffrey Griffin, Ph.D., P.E., PMP
Howard Birnberg
Tilt-Up Panels: Stressed-Out, But Rock-Steady: A Review of the Stresses Experienced by Tilt-Up Panels
Transitioning Project Managers to Firm Leaders
August 22, 2019
September 24, 2019
Howard Birnberg
F. Dirk Heidbrink, P.E.
How to Be an Effective Project Manager
Load Testing of Existing Structures
Courses award 1.5 hours of continuing education after the completion of a quiz. Diamond Review approved in all 50 states. A U G U S T 2 019
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SEI Update Learning / Networking
NEW Structural Engineering Channel Podcast Check out the Structural Engineering Channel Podcast by Anthony Fasano with interviews of SEI leaders on the state and future of structural engineering. http://bit.ly/TSECEp02
Last day to register: September 3 The conference includes an exclusive presentation on Dubai Municipality efforts to make Dubai Seismic Smart and a live demo of DB-SAFE – the Dubai Municipality earthquake safety application. Attendees will also appreciate real-time data from buildings such as Burj Khalifa and an optional walking tour of the municipality campus. View the IStructE article “How do we achieve global interoperability in a complex world?” by Martin Powell and Glenn Bell at https://bit.ly/2KVtgrA.
SEI Local Leaders Conference October 24-26 at ASCE in Reston, VA
Local SEI Chapter Chairs: Join us to share insights from your local Chapter, learn about new initiatives, best practices, tools and resources to serve Chapters, and take advantage of a special, new opportunity for Leadership Facilitation Skills Bootcamp brought to you by the SEI Futures Fund. If you are a local SEI Chapter Chair and are not on the local SEI leaders email list, contact Suzanne Fisher at sfisher@asce.org.
SEI Grad Student Chapter Best Practice at Northeastern University https://bit.ly/2YxGEWa
Thank You
Thank you to all those that participated at Structures Congress in Orlando and made it a great conference – presenters and participants, sponsors and exhibitors, and to Ashraf Habibullah and CSI for special program funding and Celebrating the Future of SE! Check out the full album of Structures Congress 2019 photos on SEI Facebook https://bit.ly/2xqy0N1.
STRUCTURAL ENGINEERING INSTITUTE
STRUCTURES CONGRESS 2020 St. Louis, Missouri I April 5-8
Sponsor/Exhibit to showcase your brand.
Apply for an SEI Futures Fund Student/Young Professional Scholarship to participate. www.structurescongress.org
Errata 46 STRUCTURE magazine
SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI-Errata. If you would like to submit errata, contact Jon Esslinger at jesslinger@asce.org.
News of the Structural Engineering Institute of ASCE Advancing the Profession
Give to the SEI Futures Fund and Leverage a 4 for 1 Match Donate to the SEI Futures Fund through August 2019 to support the Future of Structural Engineering, and it will be matched 4 for 1 with a generous gift of up to $40,000 from Ashraf Habibullah, CEO of Computers & Structures, Inc. www.asce.org/SEIFuturesFund Learn about SEI Futures Fund Impact on Standards Development at https://bit.ly/2FPkyag.
Verifying Performance-Based Design of Transmission Towers Through Full-Scale Testing Performance-Based Design (as defined by SEI) is “a process that enables the development of structures that will have predictable performance when subjected to defined loading.” Transmission structures are designed to meet both structural and electrical performance criteria. The electrical transmission industry is unique from others that design and construct structures in that new, latticed steel tower designs are often tested at full-scale to verify that they meet structural performance criteria. Transmission engineers also obtain a degree of confidence by understanding structural performance through full-scale testing… Read more at https://bit.ly/2FPkyag.
By Aaron Darby, P.E., M.ASCE
SEI at 20-Something and Into the Future
In the late 1990s, ASCE formed the Structural Engineering Institute by integrating the Structural Technical and Codes & Standards Divisions and later expanded to include Business and Professional and Local Activities. At the time, I was Chair of the ASCE Structures Division Executive Committee. Understandable tensions among members, due to the changes were overcome by goodwill and good management. With the encouragement and collaboration of members and staff, a strategic plan was published in early 1999. The plan attracted previously disconnected members and activities and began to breathe life into the new Institute. Twenty years later, few can remember the time when structural engineering in ASCE was scattered with separate journals, conferences, and leadership… Read more at https://bit.ly/2FPkyag.
By Jeremy Isenberg, P.E., Ph.D., NAE, F.SEI, Dist.M.ASCE
Membership
Welcome SEI Mexico Chapter
SEI welcomes the first International Chapter: the SEI Mexico Chapter! We look forward to their involvement. As they work to start efforts, they are meeting once a month, organizing a campaign to attract new members, and planning their first conference. www.asce.org/SEILocal
Make Sure You Receive SEI Member Benefits
Don’t miss exclusive SEI member benefits such as free journal articles via SEI Update monthly e-news. Update your email preferences for your ASCE/SEI membership to ensure you receive it. Visit www.asce.org/myprofile or call ASCE Customer Service at 800-548-ASCE (2723).
SEI Online
SEI News
Read the latest news at www.asce.org/SEI.
SEI Standards
Visit www.asce.org/SEIStandards to: • View ASCE 7 development cycle • Submit proposals to revise ASCE 7
SEI on Twitter
Follow us: @ASCE_SEI
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CASE in Point Did you know? CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills young engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need! CASE has several tools available for firms to use to enhance their internal policies and procedures – from office policy guides to employee reviews. Tool 1-1 Tool 1-2 Tool 1-3 Tool 2-2 Tool 2-3 Tool 2-5
Create a Culture for Managing Risks and Preventing Claims Developing a Culture of Quality Sample Policy Guide Interview Guide and Template Employee Evaluation Templates Insurance Management
Tool 3-2 Tool 3-5 Tool 4-3 Tool 5-2 Tool 5-3 Tool 5-5
Staffing and Revenue Projection Staffing Schedule Suite Sample Correspondence Guidelines Milestone Checklist for Young Engineers Managing the Use of Computers and Software Project Management Training
CASE Summer Planning Meeting Update CASE convenes two membership meetings a year for continuing education, networking, and to allow their committees to meet faceto-face and interact across all CASE activities. Over two dozen CASE members and guests attended the recent planning meeting in Atlanta, GA, June 13-14, making this another well attended and productive meeting. During the meeting, members had the opportunity to hear about the Mercedes Benz Stadium Structural Design and Delivery from Richard Saunders, S.E., P.E., and Matt Breidenthal, P.E., S.E., LEED AP, from HOK Atlanta, GA; engage in a discussion with Linda Bauer Darr, ACEC President and CEO, regarding the future direction for ACEC and Coalitions; and attend break-out sessions with the CASE Contracts, Guidelines, Toolkit, and Programs and Communications Committees. CASE needs your help to keep up with the fast-paced changes that are moving our industry forward. If you have an interest in getting involved in one of the areas listed below, please contact the chair for more information or contact Heather Talbert, htalbert@acec.org or 202-682-4377. Current initiatives include: I. Contracts Committee – Brent Wright (brent@wrighteng.net) • CASE Commentary A on AIA Document C401 to legal review • Reviewing CASE Agreements #1 through Agreement #12 II. Guidelines Committee – Kevin Chamberlain (kevinc@dcstructural.com) Finishing the following new documents: • Structural Engineer’s Guide to Working with a Geotechnical Engineer • Seismic Engineering Business Practices for the Structural Engineer • Guideline 976-E: Commentary on ASCE Wind Design Procedures Updating the following current documents: • Guideline 962-D: Guideline Addressing Coordination and Completion of Structural Construction Documents • Guideline 976-C: Commentary on Code of Standard Practice for Steel Buildings and Bridges
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III. Programs and Communications Committee – Nils Ericson (nericson@m2structural.com) • Submitted two session topics for the 2020 NASCC Steel Conference • Submitted session topic for 2019 NCSEA Summit • Submitted Session topic for 2020 Structures Congress • Finalized CASE’s three sessions at the 2019 ACEC Fall Conference • Discussed schedule changes and speakers/topics for the 2020 CASE Winter Meeting: Thursday night local project highlight, Friday morning breakfast roundtable, Friday lunch War Story IV. Toolkit Committee – Brent White (brentw@arwengineers.com) • Finished updates to the current tool: Tool 5-1: Guide to the Practice of Structural Engineering • Finished the following new document: Tool 5-6: Lessons Learned • Discussed and worked on several new tools that will be worked on for the rest of the year
News of the Council of American Structural Engineers CASE Risk Management Tools Available Foundation 5: Educate all of the Players in the Process
• Educate management, staff, and clients • Have established education and training policies and procedures • Coaching and mentoring is an important part of the education process
Tool 5-1: A Guide to the Practice of Structural Engineering (UPDATED, 2019)
Tool 5-5: Project Management Training
A management training program that provides a formal indoctrination into a firm’s preferred management practices and the firm’s values as reflected in those practices can be a powerful tool that can increase the firm’s effectiveness and profitability. This tool consists of a sample syllabus in the form of a matrix that lists subject matters in rows and objectives in columns. The matrix is broken up by individual spreadsheet tabs that contain material for each phase of a project.
Intended to teach structural engineers the business of being a consulting structural engineer and things they may not have learned in college. While the target audience for this tool is the young engineer with 0-3 years of experience, it also serves as a useful reminder for engineers of any age or experience. The Guide also contains a test at the end of the document to measure how much was learned and retained. Other sections deal with getting and starting projects, schematic design, design development, construction documents, third-party review, contractor selection/project pricing/delivery methods, construction administration, project accounting and billing, and professional ethics.
Foundation 6: Scope: Develop and Manage a Clearly Defined Scope of Services
Tool 5-2: Milestone Checklist for Young Engineers
This tool should be used when a structural engineer is asked by the prime consultant or owner to provide input on sub-consultant selection and scope of work, or when the structural engineer is required to retain the sub-consultant directly.
The tool will help your engineers understand what engineering and leadership skills are required to become a competent engineer. It will also provide managers a tool to evaluate engineering staff.
A well-developed scope of services: • Avoids later misunderstandings • Provides clarity for future additional services • Clarifies the work effort and fee • Establishes many contract terms • Functions as a guide for staff • Helps to mitigate claims
Tool 6-2: Scope of Work for Engaging Sub-Consultants
Tool 5-3: Managing the Use of Computers and Software in the Structural Engineering Office
Computers and engineering software are used in every structural engineering office. It is often a struggle to manage and supervise these tools. Software availability is in constant flux, software packages are continually updated and revised, and few software packages fully meet the needs of any office. This tool is intended to assist the structural engineering office in the task of managing computers and software.
Tool 5-4: Negotiation Talking Points
This tool provides an outline of items for your consideration when you are in a situation in which you are pressured to agree to lower fees. The text is subdivided into situations that are commonly experienced in our profession. This document is purely advisory and designed to assist you in your individual negotiations and business practices.
You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.
Capture Planning for the Win
Capture Planning for the Win is a step-by-step, scalable capture and pursuit management guide that will help you evaluate and improve your firm’s business development capabilities. Whether you’re pursuing public- or private-sector work, this guide equips your firm with a client-focused toolbox for identifying and understanding key stakeholders and decision makers, as well as developing and reinforcing competitive positions to help you write winning proposals – and even sole-source work more frequently. Capture Planning for the Win also includes customizable MS Word templates for benchmarking business development activities, capture planning, and effective Go/No-Gos. To access this publication, go to www.acec.org/bookstore.
Follow ACEC Coalitions on Twitter – @ACECCoalitions. A U G U S T 2 019
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INSIGHTS Bridge Inspection Frequency How Can We Better Utilize Limited Resources? By Jennifer C. Laning, P.E.
B
ridge inspection frequency is mandated by the Federal Highway Administration (FHWA) metrics in their National Bridge Inspection Standards (NBIS) and the Code of Federal Regulations. The mandate on a regular frequency of inspection is how we ensure safety. That said, we operate under the current reality of limited resources. The relationship between frequency and resources is complicated. Resources are not only funding; they also include people and equipment. These all relate to the key components of any project: scope, schedule, and budget. So how do limitations on these resources affect the bridge inspection industry and the decisions we make about inspection frequency? It is a challenge. When seeking to reduce the demands caused by already limited resources, there must be a way to change the inspection frequency so that resources can be applied elsewhere. We cannot make cost a justifiable reason to extend or reduce frequency due to the high priority for safety. However, safety is often a basis for changing the frequency to shorter timeframes. Frequency can be affected by a decrease or worsening of the condition state (i.e., a rating of “4” results in more frequent inspections) or the presence of certain elements (i.e., Fracture Critical Members (FCM) results in more frequent inspections). While it is justifiable to change inspection frequency because of the poor condition of the bridge, the frequency should not be stretched based solely on a good condition rating; some circumstances increase deterioration or affect the condition that would go unnoticed or unmonitored in the between-inspection timeframe.
Bridge inspection.
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It is also essential to consider how the availability of people and equipment impact the industry. Availability is often the critical path for inspection plans, so stretching the frequency would ease the demand on staff resourcing as well as the tight demand for available Bridge inspection. access equipment. There are notable limitations throughout the industry that ultimately saved lives or preserved caused by the lack of qualified people to per- structures. However, we can better utilize the form inspections. Longer frequencies would resources we have to prioritize the structures also potentially ease the impacts to the travel- that should receive our attention. There has ing public since bridge inspections typically been a considerable improvement in formalrequire lane closures or restrictions. izing processes for prioritizing infrastructure What if we look at a risk management preservation investment, but we must decide concept to develop informed decisions for on how to evaluate priorities on the inspection justifying stretching out the frequency? side. There needs to be a common baseline, Frequency change all boils down to trying and risk management tools are a way to get to manage risk (i.e., bridges in poor condi- there. The decades of information that we tion are inspected more often, for example). have been collecting about our bridge infraIn 2009, the author participated in a paper structure can be utilized to help make these proposing a basis for evaluating the risk for decisions and preserve not only the infrabridge management. In this proposition, structure itself but be more efficient with our the paper looked at three things: condition, limited resources. exposure (what the bridge might experience, Other thoughts for improving how we do like deicing salts or proximity to an indus- business in bridge inspection include ideas trial area), and importance (is the bridge on such as utilizing more innovative technologies a critical route or in a rural area). By looking like drones, using non-destructive evaluation at not only condition but also the hazards a for decks, and placing more consideration on bridge might be exposed to and how critical the inspectability of signature or complex the bridge is to the overall system, the risk structures during construction (e.g. adding can be better understood. Note that there are catwalks and tie off points and ensuring undoubtedly similar risk management propo- structures can be accessed with commercially sitions currently being studied, so this is an available equipment). Furthermore, we need example. However, the author does propose to do more to expose engineering students to that thinking along these lines is the best way NBIS when in college, making people aware to make educated and of the importance of NBIS to public safety informed decisions and that it exists as a career path within civil/ about frequency, which structural engineering. subsequently can The goal for the industry should be to condirectly impact the cost sider bridge inspection within the larger lens of performing inspec- of how we can take the information provided tions as well as other and make smart decisions on more effectively limited resources. using resources to preserve bridge Inspections are essen- structures and keep the traveling tial and, in the author’s public safe.■ personal experience, Jennifer C. Laning is Associate Vice President decisions have been and Bridge Inspection Practice Leader at arrived at based on Pennoni. (jlaning@pennoni.com) inspection findings A U G U S T 2 019
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