STRUCTURE magazine July 2019

STRUCTURE JULY 2019

NCSEA | CASE | SEI

Wind/ Seismic INSIDE: MOT Tower

22

Seismic Design Using Dampers The Krause Gateway Center Seismic Drift

28 30 32

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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

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Contents

Cover Feature

22 MOT TOWER

JU LY 2019

By Onur Ihtiyar, P.E., Hi Sun Choi P.E., and Nickolaus Sundholm

The unique and futuristic shape of MOT Tower has become a new national landmark of Azerbaijan. Innovative structural solutions were developed to support five twisting cubes, each consisting of five office levels and a column-free roof terrace. Advanced structural analyses were used to design and confirm tower strength, safety, and serviceability.

28 RESIDENTIAL SEISMIC DESIGN USING VISCOUS DAMPERS

By Nik Favretto, P.E., and Jillian van Enckevort, S.E.

The task of designing a high-end single-family residence that could withstand a Maximum Considered Earthquake with minor damage was not easy. Advanced structural seismic design (ASSD) increased the seismic performance of the structure with only a small overall construction cost increase.

30 THE KRAUSE GATEWAY CENTER

By Thomas Reynolds, P.E.

The 6-story Krause Gateway Center is framed using structural steel beams and columns and redefines the standard notion of an office building. Structural engineers overcame numerous structural challenges rarely seen in a building of this use and type.

Columns and Departments 7

8

Editorial Why Do I Participate

20

Northridge – 25 Years Later

41

InSights

in Professional Activities?

The California Earthquake

By Stacy Bartoletti, S.E.

Authority

Cast-in Specialty Inserts

By Janiele Maffei, S.E.

By Natasha Zamani, Ph.D., P.E.

Structural Performance Is Seismic Design by U.S. Codes

26

Professional Issues

Code Provisions for Headed

50

Structural Forum

and Standards Deficient? – Part 1

For the Betterment of the Structural

The Importance of

By S. K. Ghosh, Ph.D.

Engineering Profession

Professional Advocacy

By Angelina V. Stasulis, P.E., S.E.

12

Building Blocks

32

ASCE 7-16 Provisions for Lateral

A Story to Brace For

Drift Determination – Part 1

By Michael Gannon, S.E., P.E.

16

By Abdulqader Al-sheikh

Construction Issues

Recommended Details for Reinforced Concrete

Structural Design

36

Structural Practices

Construction – Part 2

Design of Concrete Flat Slabs to Resist Flexure-Induced Punching

By David A. Fanella, Ph.D., S.E., P.E.,

By Ramez B. Gayed, Ph.D., P.Eng.,

and Michael Mota, Ph.D., P.E., SECB

and Amin Ghali, Ph.D., P.Eng.

In Every Issue 4 Advertiser Index 43 Resource Guide – Concrete Products 44 NCSEA News 46 SEI Update 48 CASE in Point

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

J U L Y 2 019

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EDITORIAL Why Do I Participate in Professional Activities?

By Stacy Bartoletti, S.E., Chair CASE Executive Committee

I

have just returned from the American Council of Engineering Companies (ACEC) Annual Meeting and the Council of American Structural Engineers (CASE) Executive Committee Meeting in Washington, D.C. The conference marks the start of my two-year term as Chair of CASE. Given my new role, I reflected upon the time I invest in professional activities. These activities always make me feel invigorated and energized about our profession. Energy and passion can come from several different areas; why choose professional activities?

It might be easier on my personal life, and perhaps better in the short term for my firm, to just keep my head down and focus all of my attention on running a successful business, taking care of clients, and supporting our employees. I fundamentally believe that it would be very short-sighted. Professional associations and their related activities rely heavily on volunteer time from members of the profession and exist to support the needs of the profession and individuals.

Without volunteers and leaders, the contributions these associations can make to the development or our profession and the engineering industry would be substantially diminished. In the long run, I believe that I get more than I give when supporting professional association groups.

Interaction with great people – Over the years, I have met and developed relationships with many great firm leaders and structural engineers. The interaction with these professionals, from across the country, is one of the best reasons to invest significant time in professional activities. Interactions with this extensive network influence my view of the structural engineering world and direction of my firm through my leadership. The network I have built through professional activities acts as a sounding board to understand how other businesses address issues and has even resulted in business opportunities through referrals or partnering.

Learn from the best and brightest – In the early part of my career, I had the opportunity to support a senior principal in our firm in his role as chair of the Structural Engineers Association of California Seismology Committee. I learned more from that activity about the basis for seismic code provisions than I could have from any class or any individual project experience. We are fortunate to participate in a collaborative profession that is highly engaged and attracts some of the best and brightest people in society. More recent interactions with CASE committees have allowed me to enhance my knowledge of business practices, project management, and risk management from numerous other structural engineering leaders. Professional activities are a constant reminder of the intellect, dedication, and high ethical standards of the people in our industry.

STRUCTURE magazine

Give back to the profession – If a rising tide lifts all boats, then the process of professional involvement raises all of us as individual engineers and as firms of practice. As professionals, we should share much of what we do with others and not hold it close because we consider it a competitive advantage.

As an example, my firm has been actively involved in and helped to lead the development of seismic standards for the evaluation of existing buildings for decades. We choose to participate in these activities because we believe it advances the seismic safety of buildings for the benefit of the public and the profession. At the same time, it allows our firm to be at the forefront of seismic safety.

Advance our communities – Many states and communities are currently debating the appropriate performance expectations for buildings during hurricane and seismic events. These communities need our expertise in this debate. We live and work in communities that need dependable, expert advice to make informed policy decisions. Only through participation can we influence results in our own communities.

Grow as a professional – Being a member of a profession such as engineering means that I also need to grow individually and continually learn. I believe this is a fundamental difference between practicing as a professional and just having a job. My participation in professional activities shaped me as much or more than anything else I have done since graduating from college and entering the engineering profession.

Grow business – Generating new business may not be a primary reason for participating in professional activities, but I believe it plays a role in growing and maintaining business for my firm. Is there a better way to gain expertise in specialty areas of structural engineering than to help develop the next generation of standards or code requirements?

Participating in professional activities can help distinguish our firms and develop expertise in niche areas of structural engineering.

As an engineer and CEO, I think a lot about value and return on investment. I believe the long-term value to my firm and my development outweighs the short term cost of professional activities. I also believe that being considered a profession carries with it certain obligations; contributing to the advancement of the profession is one of those obligations. How do you give back to our great profession, and how does your firm support those that want to participate in professional activities?■ Stacy Bartoletti is the CEO and Chair of Degenkolb Engineers in San Francisco, California and the Chair of the CASE Executive Committee. (sbartoletti@degenkolb.com)

J U L Y 2 019

7

structural PERFORMANCE

Is Seismic Design by U.S. Codes and Standards Deficient?

Part 1

By S. K. Ghosh, Ph.D.

A

rticles recently appearing in major newspapers and features run on other media outlets have called into question the seismic performance of buildings designed using U.S. seismic codes and standards.

The primary criticism appears to be that, while the codes and standards prevent the collapse of buildings in strong earthquakes and even provide life safety by allowing people to evacuate safely, they do not ensure the continued functioning of the buildings or the community.

The following is a sampling of relevant comments: NY Times, April 17, 2018: San Francisco’s Big Seismic Gamble The article quotes seismologist Dr. Lucy Jones, formerly of the U.S. Geological Survey, as saying: “When I tell people what the current building code gives them, most people are shocked… Enough buildings will be so badly damaged that people are going to find it too hard to live in L.A. or San Francisco.”

ABC7 Los Angeles, June 7, 2018: Dr. Lucy Jones pushes for Safer Buildings “What we have are buildings that won't kill you. But if it's a total financial loss, well that was your financial choice to make. We're creating disposable buildings. So when you go into the engineering analysis, we can make it 50 percent stronger by adding 1 percent to the cost of construction, which is not very much. There may be even better, more cost-effective ways.

What we're creating right now is such a huge financial vulnerability.

It really impairs the economic future of the state [California]. I think it's something that needs to be done at the state level so that the cities aren't competing with each other on this type of thing.”

If a useful discussion is to occur regarding seismic performance and functionality of buildings following earthquakes, several additional essential aspects need to be brought into the discussion. These include: the performance impact of the large existing stock of vulnerable buildings, the many aspects beyond building design that would impact building functionality, the likely performance of new buildings designed to current building codes, the earthquake performance successes of Japan that might provide guidance, and the already existing process by which seismic design provisions are developed, vetted, and adopted into U.S. building codes. This article discusses these items with the hope that they will be contemplated together, allowing for the development of well-considered and beneficial improvements in seismic design, where needed.

1) The design and construction of the building were approved by the city or county before the adoption of the 1976 edition of the Uniform Building Code and had one or more of the following characteristics: a) Unreinforced masonry lateral force-resisting systems or unreinforced masonry infill walls that interact with the lateral-force-resisting system. b) Concrete buildings with a nonductile lateral-forceresisting system. c) Soft, weak, or open front walls at the ground floor level of multistory light framed buildings.

2) The design and construction of the building were approved by the city or county under the 1995 or earlier edition of the California Building Code and consisted of any of the following structural systems: a) Steel frame buildings with moment frame connections. b) Concrete or masonry buildings with flexible diaphragms. c) Buildings with precast, prestressed, or post-tensioned concrete.

The significance of the 1976 Uniform Building Code (UBC) is that there were significant changes made in the 1973 UBC following observations of structural performance and damage in the 1971 San Fernando earthquake. These changes were enhanced and refined in the 1976 UBC. The significance of the 1995 California Building Code (CBC) is that it is based on the 1994 UBC and the prequalified moment connections at beam-column joints of steel special moment frames of the 1994 and earlier editions of the UBC were found to be deficient (many of them failed) in the 1994 Northridge, CA, earthquake.

Much of the risk to human life and property in earthquakes stems from the existence of the vulnerable buildings listed above.

Effective mitigation of the risk to vulnerable buildings would require widespread retrofitting measures which can be mandated or facilitated only by local (city, county, or state) ordinances. This does not fit into the conventional scope of building codes. If the decision is made to retrofit a building – either because it is required or voluntary – the International Existing Building Code (IEBC), which makes extensive references to ASCE 41, Seismic Evaluation and Retrofit of Existing Buildings, can be utilized for that purpose.

Anticipated Performance of Existing Vulnerable Buildings

Survival of Cities Raises Diverse Challenges

The existing building stock in this country includes a large proportion of vulnerable buildings that were either not designed by a structural engineer at all or designed by older codes with no or inadequate consideration of resistance to lateral (sideways) forces due to wind or earthquakes. In California, there is more knowledge of the existing building stock than in many other parts of the country.

According to California Assembly Bill (AB) 2681, which was vetoed by then-Governor Brown in September 2018, “Potentially vulnerable building” means a building that meets one of the following criteria: 8 STRUCTURE magazine

To quote again from the NY Times article cited above, “The goal of the code, say proponents of a stronger one, should be the survival of cities – strengthening water systems, electrical grids, and cellular networks – not just individual buildings.” The items mentioned are referred to as lifeline infrastructure and are critical to the resiliency of communities, but they are also clearly outside the scope of the nation’s building codes as they are currently constituted and accepted.

Exterior cladding (such as walls, windows, doors, and roofs) and interior non-structural systems (such as suspended ceilings, partitions, fire sprinklers, and communication systems) or components (such as HVAC

equipment) may sustain damage in an earthquake, which leads to a partial or total loss of building function. The lack of integration between performance goals for structural, exterior, and interior systems means that even if the structural system performs well, damage to these nonstructural systems and components may mean the building would not be available for its intended use. It ought to be noted that satisfactory seismic performance of exterior and interior nonstructural systems is already within the purview of the building code. The problem is that these non-structural systems and components often do not benefit from the same level of engineering attention and inspection as the structure itself.

Reasonable Level of Safety 2018 IBC Section 101.3 states: “The purpose of this code is to establish the minimum requirements to provide a reasonable level of safety…” A reasonable level of safety has been interpreted to mean that we expect some level of damage, but less than what would be expected to put lives at appreciable risk when loads are so large (such as in an earthquake) that society has determined it to be economically unjustifiable to prevent all damage.

Two Initiatives Given concerns raised about inadequacies of U.S. seismic codes and standards, two significant initiatives that were recently undertaken merit brief discussion.

California Assembly Bill 1857 Assembly Bill (AB) 1857 was passed by the California legislature but vetoed by then-Governor Brown in September 2018.

This bill would have required the California Building Standards Commission to assemble a functional recovery working group. The bill would have required the working group, by July 1, 2022, to consider whether a “functional recovery” standard is warranted for all or some building occupancy classifications and to investigate the practical means of implementing that standard.

The bill would have required the working group to advise appropriate state agencies to propose appropriate building standards. If it were determined that a functional recovery standard was not warranted, the bill would have required the working group to assist with the development of a document providing guidance to, among others, building owners and local jurisdictions regarding function recovery after a seismic event.

The bill would have authorized the commission to issue regulations based upon the recommendations from the working group. According to the bill, “functional recovery standard” meant a set of enforceable building code provisions and regulations that provide specific design and construction requirements intended to result in 1) a building for which post-earthquake structural and nonstructural capacity is maintained, or 2) can be restored. The restoration was to support the basic intended functions of the building’s pre-earthquake use and occupancy within a maximum acceptable time, where the maximum acceptable time might differ for various uses or occupancies.

The shortcomings of California AB 1857 were that it moved towards requiring that more resources go into construction of new buildings 1) for California only, not making use of the national forums already used for the development of seismic design requirements, 2) without recognizing the broad and complex infrastructure and community aspects to be addressed to achieve continued function of buildings, and 3) without specific discussion of the much larger, likely impact of existing vulnerable buildings. continued on next page

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U.S. Senate – NIST Initiative In May 2017, the U.S. Senate tasked the National Institute of Standards and Technology (NIST) with the development of a plan detailing the basic research, applied research, and implementation activities necessary to develop a new immediate occupancy (IO) building performance objective for commercial and residential buildings.

This led to the release, in August 2018, of NIST Special Publication 1224, Research Needs to Support Immediate Occupancy Building Performance Objective following Natural Hazard Events.

To quote from NIST’s announcement of the report: “After an earthquake, hurricane, tornado or other natural hazard, it’s considered a win if no one gets hurt and buildings stay standing. But an even bigger victory is possible: keeping those structures operational. This outcome could become more likely with improved standards and codes for the construction of residential and commercial buildings, …” It is important to note that NIST’s scope extends beyond earthquakes to all natural hazards.

According to Steven McCabe, Director of the National Earthquake Hazards Reduction Program (NEHRP) at NIST, “Current standards and codes focus on preserving lives by reducing the likelihood of significant building damage or structural collapse from hazards. …But they generally don’t address the additional need to preserve quality of life by keeping buildings habitable and functioning as normally as possible, what we call ‘immediate occupancy.’ The goal of our report is to put the nation on track to achieve this performance outcome.”

The NIST report is organized around four topic areas: building design issues; community considerations; economic and social considerations; and, acceptance and adoption considerations. The report concluded: “New engineering design approaches and construction techniques, combined with considerations of community, social, economic, and acceptance and adoption issues, are needed to improve the performance of commercial and residential buildings and community resilience.”

The report went on to state: “In exploring the research and implementation needs for IO building design and adoption, it has become clear that enhanced building performance is more than a technical problem of how to design and construct buildings that are more resilient to natural hazards. There are multiple complex social, economic, and policy challenges that should also be addressed to ensure that adoption of IO performance objectives is not only viable but would also be successful in meeting goals for increased community resilience to natural hazard events. …The challenge of achieving IO performance is just as much a social and economic matter as it is a technical one.”

Current Codes and Standards Provide More than Life Safety Seismic design for basically all buildings in the United States is done by a standard that is adopted by the International Building Code (IBC).

The standard is ASCE/SEI 7, Minimum Design Loads, and Associated Criteria for Buildings and Other Structures. It assigns every structure to one of four Risk Categories (RCs): • RC I – buildings that pose a low risk to human life in the event of failure (e.g., unoccupied storage facilities and barns).

• RC II – all buildings except those classified as Risk Categories I, III, and IV. (e.g., most commercial and residential buildings).

• RC III – buildings designed to accommodate a large number of occupants (e.g., schools and theatres) or that contain hazardous materials or processes, potentially posing a substantial risk to human life in the event of failure.

• RC IV – buildings classified as essential facilities or that contain hazardous materials or processes, the failure of which could pose a substantial risk to the community.

10 STRUCTURE magazine

Figure 1. Expected seismic performance of buildings assigned to different seismic design categories. Courtesy of FEMA P-1050-1.

RC I and II buildings are designed for earthquakes using an Importance Factor, Ie, of 1.0; RC III buildings are designed using an Ie of 1.25, and RC IV buildings are designed using an Ie of 1.5. This means that RC III buildings (schools) are designed using seismic forces that are 25% higher than those for RC I or II buildings. Moreover, RC IV structures (hospitals, fire stations, police stations) are designed for 50% higher seismic forces than RC I or II buildings. Thus, RC III buildings have 25% more strength to resist lateral forces due to earthquakes than RC I or II buildings, and RC IV structures have 50% more strength.

Seismic design is different in another critical respect for higher risk category structures. ASCE/SEI 7 imposes acceptable limits on interstory drift. Interstory drift is the difference between the lateral deflection expected in the design earthquake at the top of a story minus the same deflection at the bottom of the same story. RC I and II structures are typically permitted an interstory drift up to 2% of story height, RC III structures up to 1.5%, and RC IV structures only up to 1%. This is primarily to minimize damage to nonstructural (architectural, mechanical, and electrical) components.

The combination of the higher importance factor and the tighter drift limit results in a higher level of performance for RC III buildings than for RC I and II buildings, and an even higher level of performance for RC IV buildings (Figure 1). RC I and II buildings can be immediately occupied following frequent earthquakes (ones that are likely to occur once every 50 years or so), provide for life safety (an opportunity for occupants to safely evacuate the building) in the design earthquake (simplistically, an earthquake that is likely to occur once every 500 years), and prevent collapse in the maximum considered earthquake (again, simplistically, an earthquake that is likely to occur once in 2500 years). An RC IV structure (hospitals, fire stations, police stations), on the other hand, remains operational following frequent earthquakes, can be occupied immediately following the design earthquake, and provides for life safety even in the maximum considered earthquake.

The performance of an RC III structure is in between the performances of an RC I or II structure and an RC IV structure.

The critics of current seismic codes and standards are primarily looking for seismic performance for RC II buildings (commercial, residential occupancies) that would be equivalent to, or at least comparable to, the seismic performance of RC IV buildings.■ Part 2 of this series will be published in an upcoming issue of STRUCTURE and will discuss earthquake performance successes and opportunities within already existing code processes.

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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building BLOCKS A Story to Brace For

By Michael Gannon, S.E., P.E.

A

s the building industry forges ahead with technology and innovation, designers are encouraged to develop structures that perform as never before possible. Particularly in zones of high seismic activity, structures with large open spaces and high ceiling heights may not have seemed practical before the introduction of systems such as multi-tiered braced frames (MTBF). However, specific requirements for designing these MTBF systems are now defined in the Seismic Provisions for Structural Steel Buildings, ANSI/AISC 341-16, referred to as the 2016 AISC Seismic Provisions. This provides support both for these structures and for the seismic design industry.

What is a Multi-tiered Braced Frame?

In an MTBF system, two or more panels of bracing are stacked vertically within a single story. As shown in Figure 1, these braces can be built using common configurations, such as X-bracing, V-bracing, and chevron bracing. At intermediate levels between floors, horizontal struts span between each panel and transfer axial loads between the braces. The columns are typically I-shaped members oriented such that column buckling out-of-plane of the frame is about the major axis. The columns are braced in the plane of the frame by the struts, thereby reducing the minor axis unbraced lengths. Along the lines of braced frames, gravity columns outside of the MTBF can also be tied in at each strut level to take advantage of these reduced unbraced lengths. MTBFs are commonly found in buildings likely to have tall story heights, such as stadiums and other sports facilities, industrial steel structures, performing arts centers, airplane hangars, warehouses, and convention halls. Even multistory buildings with individual, tall story heights can make use of this system.

Why Use a Multi-tiered System?

Braced frame systems are extensively used when designing buildings to resist seismic loads; they are inherently stiff and are designed to efficiently dissipate energy from an earthquake by yielding at predetermined locations. However, for structures with tall story heights, it may be impractical to configure a single brace from floor to floor and still effectively resist the lateral loads. For a given bay width, a taller story height results in a longer brace that is oriented at a steep angle relative to the horizontal force. This increased brace slenderness requires a larger member size to resist axial forces and reduces the number of available steel shapes that will meet the seismic ductility requirements found in the 2016 AISC Seismic Provisions. Also, the increased angle between the brace and the floor is not as efficient in its primary purpose of resisting horizontal loading as if it were oriented more horizontally.

For these structures, using an MTBF system in place of a conventional braced frame addresses these concerns while still achieving the same desired building behavior.

How Does an MTBF Behave During an Earthquake?

As with other steel seismic systems, an MTBF is designed so that specific members within the frame yield during large earthquakes.

This yielding, or ductile behavior of these members, absorbs a large amount of energy from the earthquake and dissipates it through 12 STRUCTURE magazine

Figure 1. Typical configurations for MTBF.

inelastic mechanisms such as lengthening, bending, or buckling of the steel. This deformation is designed to occur specifically in the braces of MTBF, which then protects other members such as columns and struts from experiencing similar damage during the earthquake.

Brace buckling typically occurs in the weakest tier based on differences in brace size or tier height. When these factors affecting tier strength are designed to be identical between tiers, small imperfections in material or geometry will instead determine this initial buckling location at any one of the tiers. Buckling will then propagate through the remaining tiers until each compression brace has buckled.

Following this intended buckling behavior, the horizontal compression struts then play a critical role in engaging the tension braces and maintaining a complete load path between floors. The struts and braces essentially behave as a vertical truss spanning between lateral supports at floor diaphragms. Without these struts, the tension braces would impose significant transverse loads into the columns.

What Is Permitted in the 2016 AISC Seismic Provisions?

Before the 2016 AISC Seismic Provisions, an MTBF system was classified as k-bracing and was thus restricted in practice. In a k-bracing configuration, two or more braces frame into a column at a point lacking any lateral support from inframing beams, struts, or a diaphragm.

Therefore, the unbalanced forces from tension braces due to buckled compression braces induce large flexural forces into the column. Without a compression strut to resolve this lateral load, the system is inherently unstable and therefore prohibited in seismic design.

With the publication of the 2016 AISC Seismic Provisions, the MTBF classification is introduced with guidance for designing this system that uses horizontal struts to prevent the flexural forces from developing in the column to the extent that they do in k-frames. The

and in stiffening the frame. Therefore, the AISC Seismic Provisions specify how to properly account for these column demands with additional analysis, design, and detailing. Columns are required to be torsionally braced at each tier level; this is typically satisfied through the connection to a strut that has adequate flexural strength and stiffness to perform this function.

MT-SCBF brace connections are detailed so that buckling occurs either in-plane or out-of-plane of the frame using the same provisions as for designing SCBF braces. Based on the expected buckling behavior, the adjacent columns, struts, and connections are designed to accommodate these deformations at each tier. The resulting rotations impose additional flexure and torsion that must be considered in the design of these supporting struts and columns.

Would an MT-OCBF Have These Same Requirements?

Figure 2. Progression of brace buckling and yielding in MT-SCBF.

Provisions address MTBF systems for use as ordinary concentrically braced frames (OCBF) in Section F1.4c, special concentrically braced frames (SCBF) in Section F2.4e, and buckling-restrained braced frames (BRBF) in Section F4.4d. Note that eccentrically braced frames (EBF) are excluded from this list because it is considered impractical to meet the link beam bracing requirements at intermediate levels with no inframing beams or diaphragm support.

What is so Special About MT-SCBF?

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As one would expect, MTBF systems in regions with the highest level of seismic activity require the most ductility to dissipate earthquake energy. Multi-tiered special concentrically braced frames (MT-SCBF) are designed to accommodate brace buckling at each tier and the effects on adjacent struts, columns, and connections. Braces must be arranged in opposing pairs at each tier level to ensure that, for each direction of loading, one brace in tension has the tensile strength to support the earthquake loads after the other brace in compression has buckled. Figure 1 illustrates these pairs of tension and compression braces at each tier.

Even with the struts resolving the unbalanced horizontal load between tiers, some flexure is still induced into the columns. Because the compression braces in a single MTBF do not all buckle simultaneously, there are discrete moments when individual buckled tiers experience larger horizontal drift than those that have not yet buckled. As shown in Figure 2, this uneven drift at each tier up the height of the column results in column flexural forces in the plane of the frame. The amount of drift at each tier needs to be limited so that the ductile braces do not fracture while cycling between tension and compression during an earthquake. The MTBF column stiffness plays a substantial role in ensuring that tier drifts remain within acceptable levels. The column design must also consider out-of-plane forces, such as the effects of building mass on the structure or braces buckling out of plane, which can contribute to column instability. Overall, whereas other braced frame systems depend primarily on brace behavior, the columns in MT-SCBF are required to perform adequately in flexure

Multi-tiered ordinary concentrically braced frames (MT-OCBF) are used extensively for buildings in regions of low seismicity whose conditions still invoke the AISC Seismic Provisions. Several of the same geometry configurations will apply in accordance with the definition of an MTBF: pairs of braces must be placed in opposing directions at each tier, struts must separate each tier between floors, and each column must be torsionally braced by these struts. However, the system does not need to be as ductile because of the lower level of earthquake energy in these regions, so the design requirements are relaxed when compared to those of MT-SCBF systems.

Design level forces are determined for each member in the frame, and the braces are sized accordingly. The columns, struts, and connections are then sized for these forces with an overstrength factor of 2, and an additional factor of 1.5 applied. The purpose of these increases in design forces is to ensure that any inelastic response occurs in the braces and not in any other elements or connections where such behavior would compromise the stability of the frame.

For the columns, in particular, increasing the required strength is a simplified method of resisting any in-plane flexural demands due to nonuniform brace forces and drifts between tiers.

The lower ductility requirements also allow for the implementation of tension-only bracing, a system commonly used in non-seismic buildings,

J U L Y 2 019

13

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TOGETHER, WE BUILD SOLUTIONS

as long as additional provisions are satisfied. In this for even more possible brace configurations because system, braces are only deemed effective in resisting braces do not need to be arranged in opposing pairs seismic forces when they are loaded in tension, and as with the systems previously discussed. any contribution from compression braces is ignored.

While both compression and tension braces To consider this a practical assumption, only those exhibit ductile behavior through steel yielding, steel shapes with slenderness ratios of 200 or more there still will be a load imbalance between tiers may be used. Because buckling of these slender brace due to the compression strength adjustment factor members does not impact the columns, struts, and applied to all BRBF compression braces. Even if connections to the same extent as in typical frames, all other factors (brace member size, steel grade, those elements do not require the additional 1.5 factor orientation, etc.) are uniformly defined, this on design forces. However, as with MT-SCBF systems, adjustment factor creates a net horizontal load the lack of lateral support at intermediate tiers allows when both a tension and a compression brace for progressive yielding in the frame. The column is frame into a single node. Furthermore, even if therefore designed to resolve a portion of this unbaltwo identical braces are oriented in the same direcanced lateral loading as a flexural force. The AISC tion, the 2016 AISC Seismic Provisions require that Seismic Provisions specify this loading as five percent of Figure 3. MT-BRBF column design loads. each intermediate level in the frame account for the larger brace strength horizontal component above a horizontal notional load equal to 0.5 percent or below each strut; this design consideration captures any potential of the adjacent tier, with the higher strength based on its adjusted differences in the strengths of otherwise identically specified braces due braced strengths. The purpose of these minimum horizontal loadto material yield strength variability. ing requirements is to account for quantifiable load imbalances as well as those caused by varying brace strains, tolerances in BRBF How Does This System Apply to MT-BRBF? steel core cross-sections, and small differences between tested and Multi-tiered buckling-restrained braced frames (MT-BRBF) are installed core yield strengths. unique in that the braces in compression are designed to retain Figure 3 illustrates how these different absolute and notional loads their full strength at large earthquake loads. While SCBF buckled are accounted for in the frame design. The “ABS” annotation between compression braces provide significantly less resistance than tension the lowest two tiers represents an absolute loading that occurs on braces, BRBF compression braces can reach a yield strength similar account of two different specified BRBF cores. The absolute loading to the tension yield strength. This prevents the progressive buckling at the top two tiers is based on varying tier heights that affect the behavior between tiers that is found in other MTBF systems, and horizontal force components for each brace. At the remaining levels, the MT-BRBF is therefore recognized as highly stable. This allows the identical BRBF braces and orientations would not result in load imbalances and instead are designed for notional “NOT” loads. The columns are then designed to resolve these loads in flexure at each tier, as shown in Figure 3.

The First Project Permitted, Approved, and Built in California Using CrossTo not penalize a single column, these Laminated Timber (CLT) loads may be shared between a series of columns so long as the struts account for this load path and the design accounts for the simultaneous compression loading in the column. The same requirement for other MTBF systems to torsionally brace the columns at each strut level applies to MT-BRBF columns as well.

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Brace for the Future With the introduction of MTBF into the AISC Seismic Provisions, designers now have the tools available to expand the scope of buildings that can withstand seismic forces. While the actual magnitude and location of the next big earthquake remain unpredictable, this structural steel system provides the strength and reliability needed to withstand such an event.■ Michael Gannon is a Senior Engineer at the American Institute of Steel Construction and Secretary of the AISC Task Committee on Seismic Systems. (gannon@aisc.org)

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construction ISSUES Recommended Details for Reinforced Concrete Construction Part 2: Beams

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

T

his article is the second in a series on recommended reinforcement details for cast-in-place concrete construction. The first article, on Two-Way Slabs, ran in the June 2019 issue of STRUCTURE.

Detailing Flexural Reinforcement Once the size of the cross-section of a beam has been determined based on serviceability and strength requirements, the required area of flexural reinforcement, As, is calculated by setting the required flexural strength, Mu, equal to the design flexural strength, Mn. The size and number of reinforcing bars must be chosen to (1) provide an area of reinforcement equal to or greater than the amount that is required, and (2) satisfy the minimum and maximum spacing requirements in ACI 318-14, Building Code Requirements for Structural Concrete and Commentary.

Reinforcing bars that are spaced too far apart could result in relatively large flexural crack widths. Thus, the maximum center-to-center spacing, s, of the deformed longitudinal bars to limit crack widths is given by the following equation (see ACI Table 24.3.2): s ≤ lesser of

– 2.5c (40,000 f ) 40,000 12 ( f ) 15

s s

c

Figure 1. Recommended bar extensions for flexural reinforcement in beams subjected to uniformly distributed gravity loads.

and dagg is the nominal maximum size of coarse aggregate in the where fs is the calculated stress in the flexural reinforcement closest concrete mix. Table 3 (page 18) contains the maximum number to the tension face of the section due to service loads and cc is the of bars that can fit in a single layer for various beam widths based least distance from the surface of the reinforcement to the tension on Grade 60 reinforcement, the overall reinforcing bar diameter, face of the member. It is permitted to assume that fs = 2fy ⁄ 3 where 1.5-inch cover to the beam stirrups, dagg = 3⁄4 inch, #3 stirrups used fy is the specified yield strength of the reinforcement. Table 1 con- with #4, #5, and #6 longitudinal bars, and #4 stirrups used for #7 tains values of the minimum number of bars required in a single and larger longitudinal bars. layer for various beam widths based on Grade 60 reinforcement ( fs Selecting the number of longitudinal bars within the limits of = 40,000 psi), cc = 2 inches (1.5-inch cover plus the diameter of a Tables 1 and 3 provides automatic compliance with the ACI 318 #4 stirrup), and the overall longitudinal reinforcing bar diameter requirements for cover and spacing, given the assumptions noted (approximate diameter to the outside deformations of the bar), above. The minimum clear spacing requirements of ACI 318-14, which is given in Table 2.

Section 25.2.1, are also applicable to contact lap splices and adjacent Minimum spacing between the longitudinal bars is required to adequately place the concrete; concrete may not be able to Table 1. Minimum number of reinforcing bars required in a single layer. flow in the voids between the bars if the bars are spaced too closely together, especially with concrete mixes with larger Beam Width (in.) aggregates. According to ACI 318-14, Section 25.2.1, the 12 14 16 18 20 22 24 26 28 30 36 42 48 minimum clear space between reinforcing bars must be at least equal to the greatest of 1 inch, db, or (4dagg /3) where db 2 2 3 3 3 3 3 4 4 4 5 5 6 is the nominal diameter of the longitudinal reinforcing bars 16 STRUCTURE magazine

Figure 2. Splice arrangement for bottom bars in a reinforced concrete beam: a) All bottom bars spliced over the column; b) Separate splice bars provided in the beam-column joints.

splices or bars. Using the largest practical bar sizes that satisfy these requirements usually results in overall cost savings.

ACI 318-14, Section 9.7.3, contains the requirements for the development of reinforcing bars in beams. For beams subjected to uniformly distributed gravity loads where the shape of the moment diagram is known, the development lengths in Figure 1 can be used. These recommended details include the requirements for structural integrity reinforcement in ACI 318-14, Section 9.7.7, and can be used for beams that have been designed using the approximate bending moment coefficients in ACI Table 6.5.2.

The Notes in Figure 1 are as follows: 1) Reinforcement to be anchored to develop fy at the face of the support. (Standard hooks are depicted in Figure 1.)

2) At least the larger of (A+s1/4) or (A+s2 /4) but not less than 2 bars must be continuous or spliced with Class B tension splices or mechanical or welded splices.

3) At least the larger of (A‒s1/6) or (A‒s2 /6) but not less than 2 bars must be continuous or spliced with Class B tension splices or mechanical or welded splices.

4) Closed stirrups in accordance with ACI 318-14, Section 25.7.1.6, or hoops must be provided along the clear span.

5) Where the requirements in Note 2 are not satisfied for beams other than perimeter beams, closed stirrups in accordance with ACI Section 25.7.1.6 or hoops along the clear span must be provided.

For simpler detailing, all the bottom bars are often extended the entire span instead of cutting off a portion of them, as shown in Figure 1.

Lapping of continuous bottom bars at supports often presents congestion and installation Table 2. Overall reinforcing bar diameter. problems. For example, it is common to splice all the bottom Approximate Bar bars over the columns away Diameter to Outside Size from the section of maximum Deformations (in.) positive reinforcement, as 7 ⁄16 #3 shown in Figure 2a. This 9 #4 ⁄16 arrangement is the simplest 11 to detail and is most suitable #5 ⁄16 where the beams are wider 7 #6 ⁄8 than the columns. However, it #7 1 can result in congestion in the beam-column joints. One way #8 11⁄8 to circumvent this issue is to use 1 #9 1 ⁄4 the detail in Figure 2b: splice #10 17⁄16 bars are provided in the joint, which are spliced to the bottom #11 15⁄8 bars on both sides of the joint.

7 #14 1 ⁄8 This arrangement works very #18 21⁄2 well with preassembled beam

cages because no bottom bars pass through the column during installation. Even though this arrangement increases the amount of reinforcing steel that is required, the cost of the additional material may be more than offset by the savings in labor and other costs; it may be the most cost-effective arrangement in certain situations.

Detailing Shear Reinforcement To avoid potential congestion issues at beam-column joints, it is good practice to specify beams that are at least 4 inches wider than the columns into which they frame. As floor systems become shallower (which also leads to overall economy), beams generally need to become wider.

Proper stirrup detailing in wide beams is essential to ensure that the longitudinal flexural reinforcement and the stirrups are fully effective.

Research has shown that locating stirrups solely around the perimeter of a wide beam is not fully effective. Thus, stirrup legs are required in the interior of a wide beam. A common stirrup configuration is illustrated in Figure 3a, where three closed stirrups are provided. One problem with this configuration is that none of the stirrups traverse the full net width (that is, the full beam width minus the total side cover) of the beam. Thus, the overall width of the stirrup arrangement needs to be measured and verified in the field before installation, which translates to extra time and cost. continued on next page

Figure 3. Beam stirrup configurations: a) Three one-piece, closed stirrups distributed across the beam width; b) Alternate stirrup configuration with open stirrups and stirrup caps.

J U L Y 2 019

17

During installation, it is possible for the net width to change when the preassembled cage is hoisted into position by crane; this increases the possibility that the provided cover will be less than that which is required. Another problem may occur where the stirrups are built in place instead of preassembled: one-piece closed stirrups make it challenging to place all the required longitudinal reinforcement in the beam, especially where large, long longitudinal bars must be threaded through the stirrups.

In the configuration illustrated in Figure 3b (page 17), a single, open stirrup is provided that extends the full net width of the beam.

A stirrup cap consisting of a horizontal bar with a 135-degree hook at one end and a 90-degree hook at the other end is provided at the top of the configuration, which also extends the full net width of the beam. Providing a full-width stirrup helps in maintaining the correct concrete cover and facilitates installation of the beam reinforcement: longitudinal bars can be placed easily within the beam from the top before installation of the stirrup cap. Two sets of identical U-stirrups with 135-hooks are shown symmetrically placed within the interior of the beam. This configuration provides a cost-effective way of providing shear reinforcement for wide beams.

Drip Grooves A drip groove or edge in a beam often presents a problem in maintaining the required cover to the reinforcement in the beam (Figure 4a). It is frequently not feasible to increase the concrete cover after the bars in the beam have been detailed. Raising the stirrups from the bottom to achieve the required bottom cover decreases the top cover (Figure 4b). A practical solution is to measure the concrete Table 3. Maximum number of reinforcing bars permitted in a single layer.

Bar Size

Beam Width (in.)

12

14

16

18

20

22

24

26

28

30

36

#4

5

6

7

8

10

11

12

14

15

16

20

#5

4

5

7

8

9

10

11

13

14

15

19

#6

4

5

6

7

8

9

10

11

12

14

17

#7

3

4

5

6

7

8

9

10

11

12

15

#8

3

4

5

6

7

7

8

9

10

11

14

#9

3

4

4

5

6

7

8

8

9

10

12

#10

2

3

4

5

5

6

7

7

8

9

11

#11

2

3

3

4

5

5

6

7

7

8

10

Figure 4. A beam with a drip groove on the bottom soffit: a) Inadequate bottom cover at the drip groove; b) Shifting the reinforcement cage upward causes inadequate top cover; c) Adequate concrete cover provided at both the top and bottom surfaces.

cover to the drip groove and detail the stirrups accordingly, as shown in Figure 4c. This impacts the overall effective depth to the flexural reinforcement and needs to be accounted for in the design.

Beam Intersections Maintaining the proper concrete cover can also be challenging at beam intersections (Figure 5). In particular, layering the top steel in the slab at such intersections can create constructability issues. The sequencing and layering of beam and slab top reinforcement can also create congestion issues. The following sequence for bar placement is one way of avoiding problems associated with these intersections: 1) Erect the reinforcement for the primary beams (bottom bars, stirrups, and top bars) as stand42 48 alone cages and set in place.

2) Place the stirrups (bottom pieces of two-piece 24 28 stirrups) and the bottom bars for the secondary 22 26 beams.

20 23 3) Place the bottom bars for the slab (not depicted in Figure 5 for clarity).

18 21 4) Place the top bars and the top pieces of the two16 19 piece stirrups for the secondary beams.

15 17 5) Place the top bars for the slab.

Additional recommendations for detailing of rein13 15 forced concrete beams, including detailing guidelines 11 13 for torsional reinforcement, steps in beams, and for beams in building assigned to Seismic Design Categories C through F can be found in the CRSI publication Design Guide for Economical Reinforced Concrete Structures.■ 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. Layering of beam and slab reinforcing bars at beam intersections.

18 STRUCTURE magazine

NORTHRIDGE

25 YEARS LATER

The California Earthquake Authority

Providing Residential Earthquake Insurance and Mitigation Programs to the State of California

By Janiele Maffei, S.E.

T

he 1984 California Mandatory Offer Law required insurers to offer earthquake insurance as a condition of participating in the California residential insurance market. After suffering unprecedented insured losses from the 1994 Northridge earthquake, almost all California residential insurers stopped selling homeowner’s insurance throughout the state. California’s legislature stepped in with a solution in 1996 by creating the California Earthquake Authority (CEA), a “public instrumentality” of the state. Through its participating insurers, the CEA provides more than 66% of the residential earthquake insurance market in California. However, nearly 90% of California homeowners do not have earthquake insurance. With a strategic plan to Educate, Mitigate, and Insure Californians, the CEA endeavors to change that.

However, the Mandatory Offer Law does not require that the insured purchase earthquake insurance. California, with two-thirds of the nation’s earthquake risk and about one-fifth of the nation’s mortgage debt, has a residential earthquake insurance take-up rate (percentage of homeowners who purchase insurance) of just over 10 percent.

The approximately $10 billion in residential insured losses (out of $20 billion in total residential losses which represented half of the $40 billion in total damages) resulting from the Northridge earthquake exceeded the premiums collected on earthquake insurance in the previous 30 years. The resulting insurance crisis threatened California’s residential property market. In addition to addressing a serious insurance and mortgage crisis, the legislature re-committed to making earthquake insurance available to California homeowners by refusing to rescind the Mandatory Offer Law. It took over 18 months for the Insurance Commissioner and the legislature to create the California Earthquake Authority (CEA) and enlist the original 13 participating insurers (PIs) to pay an operating-capital levy voluntarily. The CEA was initially thought to be a temporary solution, and it began by writing the minimum, bare-bones coverages specified in its authorizing law.

By law, the CEA is a “public instrumentality” of the State of California that provides basic residential earthquake insurance and manages a residential mitigation program. The publicly managed authority is governed by a three-member governing board consisting of the Governor, the Treasurer, and the Insurance Commissioner, each of whom may name designees to serve as board members in their place.

The Speaker of the Assembly and the Chairperson of the Senate Committee on Rules serve as nonvoting, ex officio members of the board and may name designees to serve in their place.

CEA is privately financed by the investment of PIs who now number over 25 and by the collection of policyholder premiums. The CEA is not part of the State of California’s budget. Nor are CEA’s financial obligations backed by either the state or federal government. The claim paying capacity of the CEA is currently composed of just under $6 billion in capital, $8.5 billion in reinsurance, and other financing mechanisms that total just over $17 billion. This capacity could handle a repeat of the 1906 earthquake or two recurrences of the Northridge earthquake.

At the time of the Northridge earthquake, there was three times the number of insured households in the affected area than there are in 20 STRUCTURE magazine

2019. All of the insured had significantly underpriced earthquake insurance. The CEA enabling legislation mandated that rates be actuarially sound and be determined by the best available science to rectify this serious issue. Since 2004, and with the Governing Board’s support and approval, CEA has worked continuously, under contract, with three widely recognized, commercially available catastrophe-loss models/modelers (known as “cat” models/modelers): • AIR-Worldwide • EQECAT (a wholly-owned subsidiary of Core Logic Solutions, LLC), and • Risk Management Solutions (RMS) All of these firms were founded between 1987 and 1991 when Cat modeling was in its infancy. Cat modeling is a computerized system that generates simulated catastrophic events, such as hurricanes or earthquakes, and calculates the insured loss of those events.

Before the availability of large-capacity computer systems in the late 1980s, exposure was quantified using empirical methods. In 1992, Hurricane Andrew devastated Florida with $26.5 billion in direct damages followed two years later by the Northridge earthquake that caused $20 billion in insured losses (all buildings). Utilizing empirical methods, insurers had dramatically underestimated their exposure to a Northridge-like earthquake. The Northridge earthquake and other disasters demonstrated the need for effective catastrophe-loss modeling.

California earthquake models are continually updated, most recently by the Uniform California Earthquake Rupture Forecast (UCERF3) and Next Generation Attenuation (NGA West 2) models. Committed to funding the development of programs that update earthquake models, the CEA partially funded both UCERF3 and NGA West 2.

The CEA modeling firms recently utilized these new developments and data from international earthquakes to update their models. The results led to changes to some of CEA’s insurance rates that will be effective on July 1, 2019.

Insurance In 2016, with just under $13 billion in claim-paying capacity, the CEA made significant changes to its residential earthquake insurance policy.

Homeowners were offered significantly more choice in coverage and deductibles. For the first time, CEA allowed homeowners to choose between purchasing all of their coverages together under one dwelling deductible or to have separate deductibles for dwelling and personal property. Separate deductibles can be beneficial for the homeowner who has experienced contents damage in a moderate earthquake, as they do not have to meet the larger structural deductible.

Also available since 2016 has been a range of deductibles: 5%; 10%; 15%; 20%; and 25%. These significant changes, combined with as much as a 20% premium hazard reduction discount for homeowners who retrofit their older home, have spurred tremendous growth.

Between 2006 and 2015, the CEA’s average annual policy growth was about 6,700 policies. Since the changes introduced in January 2016, CEA annual policy counts have increased significantly, as shown here:

• 2016: 52,000 • 2017: 90,000 • 2018: 26,000 These increases put the CEA’s policy count over 1,000,000 in late 2017. The CEA estimates that some of the increases in 2017 were due to the increased awareness about the need for insurance to prevent catastrophic financial losses in light of the numerous hurricanes that severely damaged U.S. states and territories, as well as the Mexico earthquakes. These significant increases demonstrate that the new coverage and deductible options are resonating with homeowners looking for earthquake insurance that meets their needs and budgets.

Mitigation The reduction of earthquake damage through mitigation has been a charge of the CEA since its inception. CEA enabling legislation requires the CEA Board to set aside 5% of investment income or five million dollars, whichever is less, each year into a subaccount of the CEA fund called the Loss Mitigation Fund (LMF). The LMF was intended for programs to be applied to supply grants and loans or loan guarantees to dwelling owners who complete seismic retrofits. The fund was intended to serve all Californians, not just policyholders. Unfortunately, the statute that established the LMF did not expressly provide for full and flexible staffing to implement the CEA’s mitigation mandate. Until the arrival of CEA Glenn Pomeroy, various mitigation programs were undertaken with limited success in deploying available funding. In 2008, Mr. Pomeroy and his new Chief Communications Officer, Chris Nance, worked with former FEMA Administrator James Lee Witt to organize workshops throughout the state to determine what mitigation program would best serve California. The CEA, along with other consultants such as Ines Pearce, began to formulate a plan to retrofit unbolted and unbraced cripple wall houses in a program similar to the FEMA Project Impact program established in Seattle, Washington. To effectively manage and promote a statewide seismic retrofit program, the CEA entered into a Joint Powers Authority (JPA) agreement with the California Governor’s Office of Emergency Services in 2011 to create the California Residential Mitigation Program (CRMP).

In 2010, the Governor signed into law AB 2746 (Blakeslee) authorizing the CEA to contract for the services of a Chief Mitigation Officer (CMO). A national search was performed, and the CEA hired Structural Engineer Janiele Maffei as the CMO in May 2011. The bill also authorized the CEA to accept grants and gifts of property and services for the LMF or the related residential retrofit program from federal, state, and local government sources and private sources.

Since 2011, the CEA mitigation department has developed programs to provide support or programs for the following mitigation needs: • Seismic retrofit incentives, • Single-family wood-frame seismic retrofit code development, and • Research on the seismic performance of wood-framed, single-family structures.

The California Residential Mitigation Program (CRMP) was established to carry out mitigation programs to assist California homeowners who wish to seismically retrofit their houses. CRMP’s goal is to provide grants and other types of assistance and incentives for these mitigation efforts.

The first CRMP retrofit incentive program, Earthquake Brace + Bolt: Funds to Strengthen Your Foundation (EBB), was launched by the new CEA mitigation department as a pilot project in September 2013. The EBB provides up to $3,000 to homeowners who complete a qualifying retrofit of their houses. Registration for the 2019 program, which closed on November 13th, set a record with 8,688 homeowners applying – an increase of 15% over the 2018 registration.

As of March 2019, the EBB program has provided over 7,600 grants for code-compliant seismic retrofits of crawlspaces in specific high seismic-hazard areas of California.

CEA Brace and Bolt Program In addition to CRMP and EBB, the CEA mitigation department manages the CEA brace and bolt (CEA BB) program. CEA’s pilot program, authorized by the CEA board in 2017, granted each selected CEA policyholder up to $3,000 toward a retrofit, encouraging them to strengthen their CEA-insured older houses in CEA-identified high-seismic-activity areas. Just under 100 policyholder houses were retrofitted in the pilot program. This CEA BB pilot program was completed in the fall of 2018.

In February 2019, CEA invited 4,000 CEA policyholders with three or more years tenure to register for a seismic retrofit incentive program similar to EBB. The new program encourages retrofit-program participation

by CEA policyholders who may experience upward premium impacts of 15 percent or more on account of the CEA’s recent rate-and-form filing (RFF), now approved by the Department of Insurance to take effect starting July 1, 2019. More than 14,000 qualifying policyholders will be invited into the CEA BB program over a period of one year.

Basic CEA-EBB program eligibility criteria will continue, and the program will be available to CEA policyholders who (1) own a pre1940 house that is within established criteria for a code-compliant EBB retrofit, (2) live in a ZIP Code where there will be an RFF-caused premium increase of 15 percent or greater, and (3) have insured their house with CEA for at least three continuous years.

Future Retrofit Incentive Funding While over 7,500 retrofits is a tremendous accomplishment, the CEA estimates that more than 1 million houses in California’s high-seismichazard areas would qualify for an EBB retrofit. The need far exceeds presently available funding. However, because more funding means more incentive payments for more homeowners, CRMP continues to look beyond present funding sources to find additional EBB funding, including available FEMA Hazard Mitigation Grant Program (HMGP) funds. The primary source of funding has been the CEA Earthquake Loss Mitigation Fund, augmented two years in a row with $3 million from the California general fund. Recently, CEA completed the retrofit of just under 100 homes in Napa using FEMA HMGP grants and has been awarded a new $3 million FEMA HMGP grant which will partially fund the 2019 EBB program. CEA is also awaiting a determination from Cal OES on FEMA-funding through 13 additional HMGP-grant applications.

A Bold Future California and the CEA, born of the Northridge earthquake, have benefited from 25 years without a large, damaging earthquake. This has allowed the CEA to grow and prosper, offering better insurance and mitigation solutions to California residents. However, insurance take-up remains far below what is needed to protect Californians from catastrophe. The bold envisioned-future is one where earthquake loss is reduced through a well-funded mitigation program, and personal finances are protected by affordable catastrophe insurance.■ Janiele Maffei is the Chief Mitigation Officer and Research Director for the California Earthquake Authority who has worked in the earthquake engineering industry for over 35 years. She is responsible for managing activities that support the mitigation of vulnerable residential structures in California against seismic risks. (jmaffei@calquake.com) J U L Y 2 019

21

MOT TOWER STTAC ACKING INN NNOVA OVATIO OVAT ION N

BY THE CASPIAN CASPIA N SEA

By Onur Ihtiyar, P.E., LEED Green Associate, Hi Sun Choi P.E., LEED AP, and Nickolaus Sundholm, M.S.

T

he population of Baku, the capital of Azerbaijan, has rapidly increased in the past decade. The resulting growth in urban activity has transformed the city into the Caspian Sea region’s hub, with new high-rise buildings and lively cultural centers creating a unique, well-balanced, and organically blended futuristic composition. The recently-topped-out 555-foottall Ministry of Taxation (MOT) tower has revitalized the skyline of Baku. It features five twisted stacked cubes that are independently cantilevered from a circular central core. Each cube holds five office floors and a column-free green roof terrace providing a distinct separation from the next cube above. Each floor plan is rotated 1.2 degrees with respect to the floor below, resulting in an astonishing 40 degrees twist overall from bottom to top (Figure 1).

Inherent Structural Challenges At 32 stories, MOT tower is not exceptional in height but it posed particular challenges due to unique features central to its iconic design. First, the separate stacked cubes, and column-free terraces between them, required picking up eight perimeter columns on 50-footlong cantilevers from the central core, with 33-foot cantilevers from the columns to the extreme corners of each floor. In addition to the undulating form of the building, its geolocation within the highly-seismic Caspian Sea region pushed the building into Seismic Design Category D, with Site Class D soil, thus increasing the code-required demand on the structural system. Performance-Based Design was utilized to verify the adequacy of the seismic design and special seismic detailing was provided where required per code. High wind demand was an added challenge; basic wind speed for the region is 118 mph. Internationally accepted codes and specifications for building construction from authorities such as ASCE, ACI, and AISC were used for tower design references.

ETABS and Perform-3D, both CSI analysis software products, were primary tools used for the design of the superstructure.

Transfer System Analysis and Detailing The column-free terraces between adjacent cubes required transferring all column loads at the base of each cube through trusses cantilevered from the core. An innovative design solution for the eight trusses per cube has vertical load from the columns above carried Figure 1. Topped-out tower. Courtesy of Tekfen Construction.

22 STRUCTURE magazine

Figure 2. Primary structural components.

Figure 3. Transfer truss construction. Courtesy of Tekfen Construction.

through steel diagonal web members, while the induced moment from a 50-foot-long cantilevered truss is resisted by a force couple from tension and compression in the slabs integral with truss top and bottom chords. This reflects realistic compatible behavior at chords and slabs (Figure 2).

Cantilever truss chord forces of similar magnitude occur on opposite sides of the tower. Designing the transfer floor slabs as “tension rings” and “compression rings” that balance gravity cantilever chord forces was beneficial, simplifying both load path and connections. Because horizontal forces are designed to bypass the core, truss connections to the core only needed to resist vertical loads and the core wall was not tasked with resolving the large coupling forces generated by the 50-foot cantilevers. Reinforcement of slabs in tension was optimized using nonlinear finite element models with layered shell elements to simulate accurate cracking behavior of the concrete slab under tension loads. In addition, overall stability was analyzed for a variety of unbalanced live load patterns to ensure each cube remained stable and secure under eccentric loading conditions. In these situations, the core resists modest unbalanced forces applied through slab bearing.

Cantilever truss tip deflections received close attention throughout the design process, as excessive deflection would not only become unsightly but could also cause unwanted strain on the façade panels. Since slab tension and compression was relied on for truss chord forces, long-term deflection predictions reflected concrete creep and shrinkage effects over the life of the building. Trusses were detailed, fabricated, and constructed with upward camber to compensate the anticipated large instantaneous deflection at their outer ends (Figure 3). Relative long-term deflections along the perimeter of the floor plate were coordinated with the façade consultant to ensure the façade joint and connections were detailed accordingly.

the transfer trusses (and the wet weight of the slabs) required shoring, an added challenge for the design and construction team.

However, by choosing steel trusses for the main vertical load-transferring element, the amount of shoring required during construction was dramatically reduced. Other options, such as concrete shear or corbel walls, would have required shoring of more than one cube to engage multiple transfer walls to support the added weight of the concrete walls; relieving this requirement allowed for rapid progression of construction for lower cubes and a reduction in the overall construction schedule.

Construction staging and sequence analysis were performed to ensure the construction weight of each cube’s transfer system (steel truss and weight of concrete) could be supported by the transfer system of the cube below. This sequencing analysis served as the controlling load case for transfer truss design strength (Figure 4, page 24 ).

Relying on tension and compression slabs to resist cantilever truss chord forces meant the transfer system was not complete until both tension and compression slabs were cast, cured, and achieved design strength. For this reason,

A centrally located circular reinforced concrete core wall runs the entire height of the building to resist lateral loads and to support gravity loads. The wall transitions in thickness at three points along its height, varying from 6 feet thick at the base to 2 feet at the top, while the core inside radius remains 26 feet throughout. continued on next page

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Construction Sequence Challenges

Lateral Load Resisting System

J U L Y 2 019

23

The walls experience highly concentrated plastic rotation demands, diagonal reinforcement compression stresses at transfer truss diagonal was provided to ensure ductile behavior under bearing points, where columns transfer their cyclic seismic loads. gravity loads back into the core wall. Additional compression stresses will occur during seismic Azerbaijan’s New Twist events when the core walls are resisting high overturning moments. As a result, determining Innovative structural solutions were developed locations that would require proper concrete to support five twisting cubes, each consisting of confinement detailing was essential. five office levels and a column-free roof terrace.

Core wall acceptance followed a PerformanceAdvanced structural analyses, including seismic Based Design approach. Nonlinear Response performance nonlinear analysis and construction History Analysis, using realistic earthquake staging and sequence analysis, were used to design records and post-yield structural element and confirm that tower strength, safety, and serproperties, was used to analyze and guide viceability requirements would be achieved. These detailing of the core wall. Where combined efforts were essential to developing a practical, seismic and gravity loading was found to result constructible design that made this tower possible. in high compressive demands, confinement Upon reaching structural top-out in April 2018, reinforcement meeting ACI 318 requirements Figure 4. Shoring the transfer system. Courtesy the unique and futuristic shape of MOT tower of Tekfen Construction. was provided to enhance concrete performance became a new national landmark of Azerbaijan. under cyclic loadings. These conditions occurred throughout the tower We want to acknowledge the Tekfen Construction team – Bulent height. High confinement was also provided around the transfer trusses Guney, Muharrem Arslan, Baris Altiparmak, Ahmet Cobanoglu, due to the sudden increase of compressive forces in the core wall, as Ertac Yildiz, Alper Celen and Arcan Aksakaloglu – for well as to aid in preventing pull-out of the trusses. their support and contributions to this unique project, as Coupling beams were the major fuse elements to absorb seismic energy well as the Design Architect, FXCollaborative.■ and to limit seismic forces applied to other elements. The nonlinear All authors are employed by Thornton Tomasetti in their New York response history analysis results were beneficial in optimizing their design. headquarters office.

Nonlinear results revealed that most of the coupling beams could be Onur Ihtiyar is a Senior Associate. (oihtiyar@thorntontomasetti.com) designed with conventional reinforcement while still following capacity Hi Sun Choi is a Senior Principal. (hchoi@thorntontomasetti.com) design principles; shear reinforcement is sufficient to have flexural hingNickolaus Sundholm is a Senior Engineer. (nsundholm@thorntontomasetti.com) ing occur at beam ends without shear failure. At floors with significant

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professional ISSUES For the Betterment of the Structural Engineering Profession

T

he CASE, NCSEA, and SEI leadership announce that the three organizations have

jointly endorsed the Vision for the Future of Structural Engineering. The endorsement is the result of a new Memorandum of Understanding that was signed in 2017 by all three organizations to work collaboratively for “…the betterment of the structural engineering profession…”

CASE, NCSEA, SEI leadership at May 7, 2019 meeting where joint Vision received final approval from all three organizations.

Since the creation of the three organizations and working together to jointly launch STRUCTURE magazine in 2006, the organizations have informally collaborated. In May 2006, the organizations held a meeting to address issues facing the SE profession and to avoid “…wasteful duplication of effort…A balance is necessary – some cooperation …” The September 2006 STRUCTURE editorial outlined the discussion and identified 25 shared issues or activities. The SEI Vision for the Future of Structural Engineering and Structural Engineers: A case for change (2013) had its roots in that meeting. Moreover, the 2006 list was a seed for establishing the joint Structural Engineering Licensure Coalition and the Structural Standards Coordination Council that all three organizations support.

In 2018, the SEI Board charged a task committee to review the progress of the 2013 Vision for the Future of Structural Engineering initiatives at the fifth anniversary of adopting the document. It seemed natural that the task committee’s update included a review by CASE and NCSEA. The review request led to a discussion and decision among the three organizations that perhaps an executive summary of the Vision for the Future of Structural Engineering could be adopted by all three organizations. These were the objectives of the new MOU agreement: “to create collaboration within the structural engineering profession, to use our strengths to advocate for our profession, and to ensure there is a future for our profession.” Each organization has specific strengths and initiatives, and collaboration can result in a better future for structural engineering, through a joint Vision.

The collective leadership met, reviewed, and created a joint Vision for the Future of Structural Engineering with 10 long-term goals that all three organizations are committed to investing in and cooperating on. This joint document includes the SEI 2013 Vision for the Future of Structural Engineering initiatives with the addition of two new, key initiatives through input from CASE and NCSEA: “Encouraging

resilience in the built environment, including disaster response planning” and “Promoting diversity within the structural engineering profession.”

These new initiatives were discussed in the 2013 SEI Vision for the Future document but were not identified as specific initiatives. By including diversity and resilience as stated goals, new and existing programs such as SEER (Structural Engineering Emergency Response) and SE3 (Structural Engineering Engagement and Equity) will be more successful in attracting and retaining young structural engineers.

The Vision will now be the starting agenda for biannual joint leadership meetings of CASE, NCSEA, and SEI. Tracking the initiatives will determine progress on how the professional structural engineering organizations are advancing and serving structural engineering. The Vision will also be used to start a discussion within NCSEA as they work on a new strategic plan, give guidance to funding decisions for SEI Futures Fund grants, and assist CASE in collaborating on common causes with related industry organizations in supporting legislation. The Vision is the basis for a long-term strategy to ensure a vibrant and dynamic future for structural engineering as stated in the first initiative: “to develop and position structural engineers as leaders and innovators on project teams and in society.”

“The profession of structural engineering is incredibly fortunate to have three organizations to lead, represent, and strengthen our practice. When CASE, NCSEA, and SEI are aligned on a joint vision and empower their membership to collaborate toward achieving that vision, our profession ensures its viability in an evolving world.” Emily Guglielmo, P.E., S.E, NCSEA Board Secretary “This document is a very significant step for all three organizations and our profession–it will serve as a reminder to our future leaders and members of our common goals and vision. I am very proud to have been involved! ” David Cocke, S.E., SEI President■

Learn about how YOU can engage with the Vision, at www.acec.org/case, www.ncsea.com, www.asce.org/SEI. 26 STRUCTURE magazine

Vision for the Future of Structural Engineering Adopted by CASE on May 7, 2019, NCSEA on April 23, 2019, and SEI on April 27, 2019 CASE, NCSEA, and SEI are united in envisioning a future where, as stewards of the built environment, structural engineers are widely recognized as making key contributions to the advancement of society on a national and global scale.

The profession is at a critical turning point. Increasing complexity, computer automation, onerous contractual requirements, and global interconnectivity are among the trends that are fundamentally changing the practice of structural engineering. The challenge is to foresee the impacts of these trends in a way that reinforces and expands the critical role of structural engineers in improving the safety and well-being of all. This vision is the basis for a long-term strategy to ensure a vibrant and dynamic future.

A unique, fully engaged profession with a strong identity will be realized when structural engineers: • are viewed as an integrating factor in every project, rather than merely part of the supporting cast; • routinely address policy, aesthetics, and finance as important aspects of their role; • are renowned for their creativity and commitment to learning; and • articulate their vision in a way that is understood, embraced, and admired by the public.

To achieve these long-term goals, CASE, NCSEA, and SEI are committed to cooperating on and investing in the following key initiatives: 1) Develop and position structural engineers as leaders and innovators on project teams and in society.

2) Reform structural engineering education.

3) Improve mentoring of young structural engineers.

4) Enhance the professional development of practicing structural engineers.

5) Advocate structural engineering licensure.

6) Implement performance-based codes and standards.

7) Encourage resilience in the built environment, including disaster response planning.

8) Promote diversity within the structural engineering profession.

9) Collaborate on common causes with related industry organizations.

10) Advance the structural engineering profession nationally and globally.

NCSEA

STRUCTURAL ENGINEERING INSTITUTE

National Council of Structural Engineers Associations

Corey Matsuoka, P.E. CASE Chair

Jon A. Schmidt, P.E.,SECB NCSEA President

David W. Cocke, S.E. SEI President J U L Y 2 019

27

RESIDENTIAL SEISMIC DESIGN

USING VISCOUS DAMPERS

Photos courtesy of Treve Johnson

L

ight wood framed residential construction has historically performed well in seismic events. The combination of low mass, redundancy, and damping resulting from damage of finishes allow the buildings to see large inelastic deformations without collapse.

In high-end residential construction, this may no longer be the norm.

Clients are choosing finishes that are heavier and more brittle, such as natural stone veneers and tile floors over lightweight concrete topping.

The amount of glass versus a solid wall is ever increasing. Engineers are pushing the limits of wood as a construction material with long open spans and cantilevers, increased shear wall aspect ratios, and higher ceilings. This results in heavy, brittle residential structures, with less wall length and redundancy, that will probably see significant damage in a moderate earthquake and potentially partial collapse under the maximum considered earthquake (MCE). The resulting financial losses could be enormous, especially when the loss of contents is considered along with the damage to the building.

A few years ago, FTF Engineering was asked to design a highend single-family residence that could withstand an MCE with minor damage. The task was not easy since the architectural design included an open floor plan with tall ceilings, large glass surfaces, a long cantilever deck, and a cantilever roof with a large rectangular opening. For their new home, the owners selected a steeply sloped, wooded site on the Portola Valley Hills, offering expansive views of the entire San Francisco Bay Area. They wanted a modern house without compromising on space and safety, although the site is just over 1 mile from the San Andreas Fault. To realize the owners’ wishes, the firm decided to look beyond the ordinary code residential design and explore all the possibilities that advanced seismic structural design (ASSD) can offer. Considering the topography and the architectural challenges of this three-story building, FTF selected a lateral force 28 STRUCTURE magazine

By Nik Favretto, P.E., and Jillian van Enckevort, S.E.

resisting system that largely improved the expected seismic performance of the structure with a small overall construction cost increase.

In the plan view, the building is a “T” shape. The long leg of the “T” is a two-story concrete structure that houses an impressive 5000-bottle wine cellar and ample storage space in the concrete basement. Above the wine cellar is a 3-car garage over a concrete-on-metal deck floor with a green deck above supported by concrete shear walls that provide the needed strength to this wing of the house. The head of the “T” is the main residence, a 4400-square-foot modern architectural masterpiece.

The sound-insulated basement includes a game room with pool access.

The living quarters are above and include a spacious master suite with a private deck, additional bedrooms, and bathrooms. The 3rd story is for entertainment and includes all the utility spaces, a large lounge surrounded by multi-panel sliding doors that open onto a large cantilevered deck, and a living room with clerestory windows on all four sides. Steel beams and columns span the large, open spaces and frame the clerestory. Wood stud walls and engineered wood joists merely infill the space between the steel structure. Thirteen steel braced frames with viscous fluid dampers,

by Taylor Devices Inc., provide the lateral force resistance for this wing of the house. The frames are hidden inside the stud walls and connected with steel collectors to the diaphragms. Tucked in the back corner of the upper floor is the owner’s private office with a special picture frame to show off one of the dampers. To separate the “stiff leg from the flexible head,” a two-way sliding connection on top of the short concrete wall, shared between the two wings, provides enough displacement for the roof of the main house to move independently from the garage.

Before settling on ASSD, a conceptual code-based design was developed that consisted of multiple steel special moment frames and plywood shear walls. The challenges with this design approach were to hide the moment frame columns within the open concept floor

plan and provide enough plywood shear walls without reducing the length of large façade glass surfaces. Both the owners and the architect preferred an alternate solution that would preserve the architectural expression and also provide higher seismic performance, hence the selection of ASSD. The only concern the clients had was the potential for increased construction cost; therefore, the general contractor priced Typical damper installation. both options and the cost of the ASSD represented approximately a 5% increase of total construction cost.

The Plan Checker is your Friend – Small municipalities have limited After the decision was made to change to steel braced frames with resources and are mainly familiar with a conventional residential fluid viscous dampers for the residence, the first step was to determine design plan check. Knowing that, it is essential to include the local how best to integrate the dampers into the lateral system and then run building department early in the design process. This will facilitate a preliminary analysis to determine their effectiveness. Viscous dampers communication between the design and plan check teams and expedite dissipate the energy of a seismic event by forcing a fluid through an the approval process. Many times, small building departments orifice during deflection in either direction along the length of the outsource to third-party plan check agencies, so it is even more damper. The dampers need to deflect to dampen the energy but also critical that they are aware early of the complexity of the design and need to be strong enough to limit the seismic drift to levels that will assign the project plan check to the right person. not damage finishes during the maximum expected earthquake for the Collaboration with the Architect is Key – FTF’s philosophy is to enable site. Studies have shown that most of the damage to finish materials the architectural expression through smart engineering solutions, by occurs beyond 1% drift; therefore, this deflection limit was set for taking a step back and letting the architecture take center stage. Design the dampers. The damper configuration was horizontal, with one end solutions should organically follow the architectural design and work attached to the horizontal diaphragm and the other end supported by in symbiosis with the rest of the building elements. By utilizing small a diagonal brace system. The brace is designed to support the full axial braced frames supplemented with fluid viscous dampers, they could load of the damper, without large elongations, and transfer it to the be hidden inside walls, enabling the architect to implement his and foundation. A sliding connection was used where the brace connects the clients’ vision for this modern residence. to the damper and diaphragm to provide out of plane support.

Work Closely with the Damper Manufacturer and the Contractor – There is The design firm performed a time history analysis on individual frames a lot of coordination in any construction project of this magnitude. Adding using CSI America’s SAP2000 non-linear in dampers, steel braced frames, and green analysis program in accordance with roofs require even more coordination at ASCE 7-10, Chapter 18, Seismic Design the start. The damper manufacturer Requirements for Structures with Damping should be determined early in the design Systems, to determine the size, deflection, process so the capacity, stroke, diameter, and damping coefficients of the damper length, and available end connections frames. For the analysis input, the project can all be taken into consideration while geotechnical consultant prepared a designing the supporting frames. The site-specific ground motion study and frames need to be well coordinated with three pairs of scaled ground motions in the steel manufacturer because of the tight accordance with ASCE 7-10, Chapter tolerances. The wood framing needs to be 16, Seismic Response History Procedures. detailed to permit attachment of finishes on Before the analysis, the seismic mass was either side of the frames while maintaining Lateral analysis model of the residence. distributed to individual frames using the proper clearances between the moving and flexible diaphragm design approach. The non-moving parts. analysis results determined the maximum damper force by adjusting the damper coefficient and velocity exponent until the story drifts ASSD Delivers dropped below 1%, which was later used to custom-manufacture each damper. With the damper frames providing all the lateral resistance Advanced structural seismic design for high-end single-family residential during large seismic events, the exterior-framed walls sheathed in construction largely increases the seismic performance of the structure plywood and the interior drywall surfaces provide enough stiffness with only a small overall construction cost increase. It is essential that to keep the structure stable during small seismic or wind events. engineers have an open dialog with clients and architects at the beginRecognizing the uniqueness of this project, the clients agreed to ning of a project and explain the benefits: a better performance outfit the house with 11 ground motion sensors, including one in each during a seismic event, life safety, financial investment protecdirection at each level of the house and 3 outside near the driveway tion, and easily achievable architectural design goals.■ to serve as a baseline. The sensor work was done by the Center for Engineering Strong Motion Data. To date, these sensors have not Nik Favretto is a Principal in the San Francisco, CA, office of FTF recorded any substantial ground motion data but, with its proximity Engineering and leads the high-end home design market for the firm. to the San Andreas fault, ground motion is imminent. (nfavretto@ftfengineering.com) Dampers in residential construction are somewhat rare, and this project Jillian van Enckevort is an Associate Level Engineer in the San Luis Obispo, CA, was a first for the design team and contractors. Some of the lessons office of FTF Engineering and manages that office. (jvane@ftfengineering.com) learned along the way, implemented by the design team since, include: J U L Y 2 019

29

The

KRAUSE GATEWAY CENTER

By Thomas Reynolds, P.E. Finished Center (looking at the southeast corner of the building).

Not Your Typical Office Building!

30' - 0"

G

W14x74 PG3-1 c=1.25" W14x370

30 STRUCTURE magazine

J

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W14x370

W14x74

2-1/4x4 1/8" = 1'-0"

30' - 0"

PG4-2 c=1.25"

W14x74

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Figure 1. Frame elevation at grid 7.

FRAME ELEVATION AT GRID 7

H

PG5-1 c=1.25"

PG4-2

Super Column 3 S123

30' - 0"

W14x370

F

PG5-1 c=0.50"

W14x370

30' - 0"

W14x74

E

The building structure at and below grade is all cast-in-place concrete supported by over 450 one-hundred-ton axial capacity auger cast concrete piles.

There are a multitude of factors that contributed to the challenges faced when framing the Krause Gateway Center for support of the main gravity and live loads. The second floor of the building was set back from the building façade (which is inset from the building perimeter above) on 3 sides, creating a double height lobby between the ground and 3rd floors. The 3rd and 4th floor were each larger in plan than the one below it. The 5th floor was larger than the 4th and rotated clockwise, about 16 degrees from the buildings face on the west side. The 6th floor followed the geometry of the 5th floor below and included a small “pop-up” for the elevator over-run and public access to the roof above it. Structural steel was chosen for the need to be flexible (both literally and figuratively) and its multi-faceted ability to solve many different problems with one trade to solve these geometric challenges.

The typical slabs are framed with 3 inches of normal weight concrete on a 2-inch 18-gage metal deck spanning 10 feet between floor beams.

The standard floor beams (however few of them there were) are 30 feet long W10x49s with sixty-five ¾-inch-diameter headed shear studs with a 1-inch camber to meet deflection criteria and allow space for MEP runs below them. The girders, also spanning 30 feet, were typically W27x84s with web penetrations to accommodate the MEP runs. At the perimeter of the 3rd floor, a 60-foot column spacing is K L 30' - 0" 30' - 0" maintained to allow for a T.O. STEEL LEVEL 5 PG5-1 PG5-1 c=1.50" EL. 110.67' column-free space around the main entrance lobby.

T.O. STEEL LEVEL 4 PG4-2 This leads to the need to EL. 95.17' transfer out the columns that are spaced at 30 feet T.O. STEEL LEVEL 3 PG3-1 EL. 79.83' on-center at the 4th and 5th floors. However, the floor plate of the 3rd and 4th floor T.O. STEEL LEVEL 2 EL. 60.50' is smaller than that of the 5th floor, so the beams that T.O. SLAB LEVEL 1 transfer out the columns EL. 47.67' at the 3rd and 4th floor all must be hung from the 5th Super Column

ost office buildings are boring, very boring. They are typically rectangular or square in plan, standard 25-foot x 25-foot repeating bays of framing with repetitive floor-to-floor heights, and are not something engineers and architects get too excited about. The Krause Gateway Center in Des Moines, Iowa, does not challenge this notion; it redefines it, completely. The typical constructs of an office building are still present (open floor plan, large conference rooms, on-site parking) but the atypical aspects of the project separate it from the pack.

The 6-story, 100-foot-tall, 160,000-square-foot building is framed using structural steel beams and columns. The double height lobby (29 feet tall), 60-foot and 30-foot column spacing, extreme cantilevers beyond the façade, the transparency of the lower floors (the entire building is wrapped in glass to enhance the view to the sculpture park across the street), and the need for a thin structural sandwich to accommodate the desired floor to floor heights and MEP runs create structural challenges rarely seen in a building of this use and type.

The design of the building followed the 2012 International Building Code (IBC) as stipulated by Article III of the Des Moines, Iowa, Code of Ordinances. Other standard additional design codes such as ASCE 7-10 (Minimum Design Loads for Buildings and Other Structures), ACI 318 (Building Code Requirements for Structural Concrete and Commentary), and the American Institute of Steel Construction (AISC) Design Guides were utilized for loading requirements and material specific requirements.

W14x74

M

floor, which is supported by 62-foot-4-inch-tall unbraced super columns (see the frame elevation in Figure 1). Each of the transfer beams at the 3rd and 4th floor, as well as the ones at the 5th floor supporting all of the building loads from floors 3 and 4, have a strict 27-inch depth restriction. With ultimate moments ranging from 3,000 kip-feet (at the 3rd and 4th floor) to almost 5,000 kip-feet at the 5th floor, standard rolled shapes did not fit the bill and built-up plate girders were utilized.

A typical plate girder at the 3rd floor uses grade 50, 2½-inch-thick 23-inch-wide flanges with a 21-inch-deep by 1-inch-thick web. As noted above, the 3rd and 4th floor were hung from the 5th floor using two 2-inch-diameter 75 ksi solid rods.

The lateral framing for the building is a hybrid of moment frames, braced frames, and cable-tied super columns. A response modification factor of 3 (not specifically detailed for seismic resistance) was utilized for seismic design with 115 mph 3-second gust for Main Wind Force Resisting System (MWFRS) loads. Two cross braced frames are provided at the elevator core of the building, as the transparent glass façade throughout the building does not allow for cross bracing at the perimeter. Moment frames are provided throughout the building to supplement the braced frames. The double height space between the 1st and 3rd floors create 32-foot-tall moment frames, complicating an already inherently flexible lateral load resisting system. The moment frame/braced frame system was not enough to meet code-stipulated seismic drift criteria and project-desired wind drift criteria (building height/360). The design team found a way to strike a balance between the two by using cable ties at the exterior 62-foot-4-inch-tall super columns to help control drift. The four super columns are unbraced columns that span from the ground floor to the 5th (west side of the building) and up to the 6th floor (east side of the building) outside of the perimeter of the 2nd, 3rd and 4th-floor plates (Figure 2). The super columns are custom built up shapes using 50 ksi plates. The additional challenge discovered when using the super columns and attaching tension rods (at an angle from the top to the base) was limiting the amount of load the rods take so that the structure that the rods attached to at the base did not need to be too large. To continue to control drift and limit the amount of load the tension rods could take, the design team worked with TriPyramid and developed connections (from the tension rods to the base structure) using a fuse pin and an internal spring cartridge to limit the loads in the tension rods to 35 kips. The structural team used the non-linear mode in an ETABS analysis model to quantify the noted 35-kip load, and the cable-tied super column’s contribution to the overall systems drift resistance.

Figure 3. Free-floating interior stair between 3rd and 4th floor.

The showpiece of the Krause Gateway Center is the perimeter exterior cantilevered framing, affectionately known as the “nosing” by the design and construction team. The nose is a two-level exterior cantilever built using two channels, 3⁄8-inch-thick plate, C5x9 vertical channels, and two HSS tubes spanning between wide flange beam outriggers. The wide flange beam outriggers cantilever 9 feet 2 inches up to 19 feet out from their main support points (columns or perpendicular beams) and support the 7-foot-long cantilevered nosing. The extreme cantilevers produced significant deflections and torsion at the outrigger endpoint which were mitigated by kinking the outriggers up at their support point anywhere from ¾ inch to 5 inches. A critical factor was the need to attach (in plan) each of 60-foot-long sections of the nose to each other so that the entire system acted together, and the outriggers did not differentially deflect. To accomplish this, the design team worked closely with the construction team to develop a pre-loading program which predeflected the outriggers to their expected fully loaded levels before the nosing was attached and leveled.

The Krause Gateway Center is the headquarters for the Kum & Go Corporation and other local businesses. The finished product provided open site lines, column-free workspaces, and an abundance of natural light that offers stunning views to the surrounding streetscape and park. There are additional structural challenges, too numerous to describe here (the free-floating stairs [Figure 3], façade support, and 1,400 beam penetrations, to name a few), along with the design ambitions described above that enabled the architect’s and owner’s visions to be achieved using structural steel in new ways not typically seen in a standard office building.■ Thomas Reynolds is a Structural Engineer with Silman Associates in New York City. (reynolds@silman.com)

Project Team

Figure 2. The northeast exterior face of the building during construction (note super columns are supporting 5th and 6th floor).

Owner: Kum and Go Architect: Renzo Piano Building Workshop with OPN Architects Structural Engineer: Silman Façade Engineer: Front Construction Manager: Ryan Companies Steel Contractor: LeJeune Steel Company Steel Erector: Danny’s

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structural DESIGN

ASCE 7-16 Provisions for Lateral Drift Determination Part 1: Seismic Drift

By Abdulqader Al-sheikh

T

his article provides an overview of the Provisions in ASCE 7-16, Minimum Design Loads for Buildings and Other Structures, for the determination of seismic drift. The article covers several factors of drift computations, including the fundamental period, scaling modal drift obtained from modal response spectrum analysis, the seismic design base shear, torsional irregularities in structures, and the significance of P-delta effects. It also addresses the effects of elastic lateral deflections of the floor plate, and how to account for inelastic drift of a structure using a deflection amplification factor, and the allowable drifts for different types of seismic forces resisting systems and the risk categories. All section references and equation numbering are from ASCE 7-16.

Story Level 1 F1 = strength-level design earthquake force δe1 = elastic displacement computed under strength-level design earthquake forces δ1 = Cd δe1 / I = amplified displacement Δl = δ1 ≤ Δa (ASCE 7-16 Table 12.12-1)

Story Level 2 F2 = strength-level design earthquake force δe2 = elastic displacement computed under strength-level design earthquake forces δ2 = Cd δe2 / I = amplified displacement Δ2 = δ2 − δ1 ≤ Δa (ASCE 7-16 Table 12.12-1) Figure 1. Story drift determination (ASCE 7-16 Figure 12.8-2).

Fundamental Period The fundamental period of a structure is the main parameter affecting building displacement. The fundamental period used to obtain loading for seismic drift calculations may be either the calculated approximate fundamental period per Section 12.8.2.1 or the calculated actual period from modal analysis. The fundamental period obtained from modal analysis is permitted to exceed the product of the coefficient for the upper limit on the calculated period (Cu) from Table 12.8-1 and the approximate fundamental period, Ta, determined in accordance with Section 12.8.2.1. The elimination of the period upper limit, TaCu, for drift determination is intended to avoid the overestimation of displacement because of the inconsistency between forces and computed fundamental period.

Scaling Determination of drift resulting from modal response spectrum analysis is, to some extent, similar to the equivalent lateral force determined by a statics analysis procedure. The primary difference is that the periods and displacements are known for several modes of natural vibration and combined using one of the recognized combination methods to obtain the final story displacements.

Also, if the base shear calculated in modal analysis (Vt) is less than base shear (V) as calculated by the equivalent lateral force (ELF) procedure, then the story drifts must be scaled up and drifts are multiplied by CsW/Vt. Note that the scaling is only required if the minimum seismic response coefficient controls the base shear, Cs = 0.5 S1/(R/Ie) (Eq. 12.8-6). Modal drift need not be scaled in all other situations. The reason for the scaling requirement is to be consistent with the requirements for design based on the ELF procedure.

32 STRUCTURE magazine

Story Shear Force Seismic design forces, starting with ASCE 7-10 and onward, have been obtained from the code as strength level forces, and drift compliance with allowable drift limits is also computed at strength level forces. However, the design earthquake forces for checking drift may be lower than the forces used for strength design due to the exemption of minimum base shear obtained from V = 0.044SDSIW ≥ 0.01W, and because the fundamental period is not restricted to the upper limit TaCu which results in lower shear forces.

The Cs exemption is especially important for tall buildings that are typically drift-controlled rather than strength-controlled. However, for structures in high seismic regions where S1 is equal to or greater than 0.6g, ASCE 7-16 requires that the minimum base shear controlled

by Cs= 0.5 S1/(R/Ie) (Eq. 12.8-6) be used for the drift determination because it accounts for large displacement of structures.

Elastic Displacement An important aspect of drift analysis is the application of vertical loading during analysis. The inclusion of gravity loads is to maintain consistency between the forces used in drift analysis and stability verification (P – Δ). The gravity load and seismic load combination used in drift analysis is stated in Load Combination 6, Section 2.3.6 of ASCE 7-16: (D+Ev+Eh+0.5L+0.2S). The live load L is permitted to equal 1.0L for occupancies where the live load is more than 100 psf (4.78 kN/m2). Note that allowable load design combinations use strength level seismic load, as indicated in Section 12.8.6 of ASCE 7-16.

The elastic displacement is the absolute lateral displacement of any point in the structure relative to its base under strength-level design earthquake forces. The story drift is calculated as the relative elastic

elastic displacement of a story to the story below, as shown in Figure seismic forces resisting systems that have Cd value less than the R-value.

1. However, the points for this comparison vary depending on the A research paper, Deflection Amplification Factor for Seismic Design following: Provisions, concluded that the deflection amplification factors (Cd) in • For a structure where centers of mass align, the story drift is the code are non-conservative and, to accurately capture the inelastic computed based on the center of mass displacements. drift of system, the value of Cd shall be set equal to the value of R. • For a structure where centers of mass do not align (more than 5% eccentricity between the center of mass), the story drift Allowable DriftSPECS Limits INFO may be computed based on a vertical projection of the center FileName:19-1670_Ad_1/2Island RepairSolutions Page7-16, Size: as 5w"shown x 7.5h" bleed of mass of the upper story to the lower story, and relative disThe allowableStructure_July_Bridge drift limits placed

by ASCE in Table 1 Job#: 19-1670 PR#:functions N/A Number Pages: 1 placement of the projected point used. (page 34), are of the risk category andoftype of seismic forces Artist: Georgina Morrasystem.

Email: Th gmorra@mapei.com Bleed: ∆ Yes, forAmount: .125"frame • For structures in Seismic Category resisting e allowable drift limits, moment 1 1 4 4 E . N C, e w pD, o r tE,C or e n tFe rwith D r . horia Deerfield Beach, FL 33442 Date: Junesystems 7, 2019 assigned 10:29 AM to D, E, or F seismicColors: Process, 4/0divided zontal irregularity 1a or 1b, the story drift is computed based designCMYK category are N O T E : C O L O R S V I E W E D O N S C R E E N A R E I N T E N D E D F O R V I S U A L R E F E R E N C E O N L Y A N D M A Y N O T M A T C H T H E F I N A L P R I N T E D P R O D U C T. on the largest difference of aligned points at the edge of the structure.

Inelastic Displacement of Structure

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Inelastic displacement is the actual displacement, obtained by elastic analysis multiplied by a deflection amplification factor, Cd, chosen from Table 12.2-1 based on the type of seismic force resisting system. The inelastic displacement is obtained from the following equation: C δ δx = d xe Ie where: δx : Inelastic Displacement δxe: Elastic Displacement Cd: Deflection Amplification Factor in Table 12.2-1 of ASCE 7-16 Ie: Importance Factor determined in accordance with Section 11.5.1 of ASCE 7-16 As illustrated in Figure 2 (page 34), the deflection amplification factor, Cd, accounts for the increase in displacement due to the inelastic response of a structure that is not determined by elastic analysis and corrects for the reduction of forces introduced by response modification factor R. If a structure remains elastic during an earthquake, the forces developed in the building are elastic (not reduced by R) which results in elastic displacements that do not account for system ductility and overstrength. If a structure is inelastic with initial stiffness equal to that of an elastic structure, then its maximum displacement will be similar to that of an elastic system. On the other hand, structures having shorter fundamental periods are characterized by inelastic drifts larger than elastic drifts of structures of similar initial stiffness.

The deflection amplification factors values set in Table 12.2-1 may underestimate the inelastic drift, especially for

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33

Drift and P-Delta Effects

by the redundancy factor, ρ, since they are governed by drift considerations per Section 12.12.1.1 of ASCE 7-16.

The allowable drift of other structures with Risk Category I and II is approximately 10 times the allowable drift under wind forces. Risk Category III and IV structures have more stringent drift limits to ensure better performance of the structure and serviceability after a seismic event.

P-delta effects must be considered, per Section 12.8.7 of ASCE 7-16, when the stability coefficient per Equation 12.816 and 12.8-17 is less than 0.10. Where the design story drift is obtained from first order analysis, it should be increased by an incremental factor Ad, or a P–Δ analysis shall be conducted to obtain the second-order drift. The incremental factor accounts for the increase in the drift from applied vertical loads on the displaced structure leading to another increment of story drift. The incremental factor can be computed from the following equation: Incremental factor (Ad) = 1 / (1-θ) where θ is the stability coefficient and is determined as follows:

Torsional Irregularity and Drift

Torsional irregularities affect the structure’s drift and must be considered for buildings with diaphragms that are not flexible. The applicability varies depending on the type of horizontal Px ΔIe 0.5 ≤ 0.25 ϑ= , ϑmax = irregularities that occur and the seis- Figure 2. Displacement used to compute drift (NEHRP Vx hsxCd βC d mic design category. While the inherent Recommended Seismic Provisions for New Buildings and torsion must be applied in all cases, Other Structures, FEMA P-1050-1/2015 Edition).

The final story drift when considering accidental torsion does not; it must the P-delta effect is calculated as follows: only be applied for drift of buildings under certain combinations Final story drift, Δ = (Ad) (Δinitial) = (1 / (1- θ)) (Δinitial) of seismic category and irregularity. However, it does need to be Δinitial is the inelastic design story drift obtained from first order analysis applied in all cases to determine if horizontal irregularity exists. as defined in Section 12.8.6 of ASCE 7-16.

Furthermore, a specific subset of structures must also account Another essential point is that the story shear (Vx) and the design for amplification of accidental torsion. Accidental torsion and drift (Δ) used in stability coefficient equation are required to be those amplification considerations for drift are as follows: occurring simultaneously without restriction to the upper limit of the • For structures assigned to Seismic Category B with Type 1b hori- fundamental period, TaCu. zontal structural irregularity, accidental torsion must be applied.

However, torsion amplification is not required.

Conclusion • For structures assigned to Seismic Category C, D, E, and F with Type 1a or Type 1b horizontal structural irregularity, the accidental In addition to the factors that affect drift mentioned above, seismic torsion with amplification must be applied. drift is also sensitive to additional parameters such as the type of soil-structure interaction and effective moment Table 1. Allowable Story Drift (ASCE 7-16). of inertia for the reinforced concrete frame. All of the factors affecting the drift should be thorTable 12.12-1 Allowable Story Drift, Δaa,b oughly addressed because seismic drift criteria Risk Category is one of the governing factors in the selection Structure of the proper lateral structural system. ASCE I or II III IV 7-16 provisions regarding drift evaluation aim to Structures, other than masonry shear ensure the acceptable performance of structures wall structures, four stories or less above by limiting drift. This results in understanding the base as defined in Section 11.2, with c the structural performance of member inelas0.020hsx 0.015hsx 0.025hsx interior walls, partitions, ceilings, and tic strain, system stability, and vulnerability of exterior wall systems that have been nonstructural elements. Buildings subject to designed to accommodate the story drifts earthquakes need drift control to limit damage to partitions, glass, and other Masonry cantilever shear wall structuresd 0.010hsxc 0.010hsxc 0.010hsxc fragile nonstructural components.■ Other masonry shear wall structures 0.007hsxc 0.007hsxc 0.007hsxc All other structures

0.020hsxc

0.015hsx

0.010hsx

hsx is the story height below level x.

For seismic-force-resisting systems solely comprising moment frames in Seismic Design Categories D, E, and F, the allowable story drift shall comply with the requirements of Section 12.12.1.1. c There shall be no drift limit for single-story structures with interior walls, partitions, ceilings, and exterior wall systems that have been designed to accommodate the story drifts. The structure separation requirement of Section 12.12.3 is not waived. d Structures in which the basic structural system consists of masonry shear walls designed as vertical elements cantilevered from their base or foundation support that are so constructed that moment transfer between shear walls (coupling) is negligible. a

b

34 STRUCTURE magazine

The online version of this article contains references. Please visit www.STRUCTUREmag.org.

Abdulqader Al-sheikh is a Structural Design Engineer at AD Engineering Company (AEC) and a Member of Saudi Council of Engineers (SCE). (abdulqader37@gmail.com)

structural PRACTICES Design of Concrete Flat Slabs to Resist Flexure-Induced Punching

By Ramez B. Gayed, M.Sc, Ph.D., P.Eng., and Amin Ghali, M.Sc, Ph.D., P.Eng.

T

his article presents an alternative method for designing concrete flat slabs subjected to flexure-induced punching. The design method meets the requirements of ACI 318-14, Building Code Requirements for Structural Concrete and Commentary, Canadian Standard CSA A23.3-14, Design of Concrete Structures, and recommendations of ACI 421.1R-08, Guide to Shear Reinforcement for Slabs, and ACI 421.2R-10, Guide to Seismic Design of Punching Shear Reinforcement in Flat Plates. The flexural reinforcement above the columns in two orthogonal directions should not be less than a calculated minimum amount to resist flexure-induced punching.

Insufficient flexural reinforcement would induce wide flexure cracks extending deep in the slab thickness and, if these cracks joined a shear crack, a punching failure would result.

Shear Reinforcement Figure 1a shows a sectional plan at d/2 from the compression face of a solid slab reinforced against punching with headed studs or stirrups, with d equal to the average distance from extreme compression fiber to the centroid of tension reinforcement in two directions. The shear force, Vu, and moments, Msc-x and Msc-y, which are transferred from column to slab, are shown in their positive directions in Figure 2; x and y are orthogonal principal axes passing through the centroid of an assumed shear critical section perimeter. Figure 1b shows a typical assembly of vertical headed studs; inclined headed studs are depicted in Figure 1c. The governing Equation 1 applies at inner and outer shear critical sections (Figure 1a). vu ≤ φ vn (Eqn. 1) where vu is the largest absolute value of factored two-way shear stress calculated at the perimeter of the critical section; φvn is the nominal two-way shear strength, multiplied by strength reduction factor φ; in ACI 318-14, φ = 0.75 (equivalent φ = 0.65 in CSA A23.3-14). At the inner critical section, the nominal strength is: vn = vc + vs (Eqn. 2) where vc and vs are the nominal shear strength provided by concrete and shear reinforcement, respectively.

The requirements for flexural reinforcement in codes (which is not reviewed here) may also satisfy Equation 3. To resist flexure-induced punching, the top two-way flexural reinforcement above the column should satisfy Equation 3 (in addition to the code): Vu ≤ φflex Vflex (Eqn. 3) where Vu is the upward factored shear force transferred from column to slab, with eccentricities (Msc-x /Vu) and (Msc-y /Vu), ϕflex 36 STRUCTURE magazine

Figure 1. Stirrups or headed stud shear reinforcement; a) Sectional plan at the middle of d, (b or c), Sectional elevation, vertical or inclined studs (assembled by a rail).

= 0.9 or 0.85 (flexural strength reduction factor in ACI 318-14 or CSA A23.3-14, respectively), and Vflex is the shear force that develops a local flexural yield-line mechanism. The value of Vflex is determined by yield-line analysis. Equation 3 is used to determine a minimum flexural reinforcement at the top of the slab in a zone above the column. The provisions of ACI 318-14 are sufficient for the design of flexural reinforcement for factored loads, and they are often sufficient to satisfy Equation 3. When Equation 3 shows a requirement for additional flexural reinforcement above the column, bottom reinforcement near mid-span can be reduced (while satisfying static equilibrium).

The provisions of ACI 318-14 and CSA A23.3-14 are intended for the crucifix layout of shear reinforcement (Figure 1a). The two codes do not explicitly require satisfying Equation 3 for safety against

induce punching failure. This flexureinduced punching can occur when a pattern of yield-lines develops a local mechanism. A simplified equation for Vflex is derived below.

Equation 4 gives Vflex, corresponding to yield-line mechanism in Figure 3a, in which an isotropic slab is supported on circular columns of area equal to c 2.

The same yield-line pattern is assumed with square columns of sides equal to c; the same pattern is also assumed for other columns with cross-sectional area equal to c 2. Equilibrium of an isolated slab (Figure 3b) gives (by taking moments about the inner edge of the segment):

Figure 2. Positive sign convention of factored internal forces transferred from columns to slabs. The centroid and principal axes are shown for the outer critical section only.

flexure-induced punching. In the crucifix layout, the stud assemblies are placed perpendicular to column faces and, close to each of its corners with spacing between assemblies measured parallel to the column, faces are less than or equal to 2d.

The number of assemblies at a column face is greater than or equal to 1+ [(column width – 2.5D)/(2d)]; where D = stud diameter. A column with a circular cross-section or other shape may be treated as a square or rectangular column of equal area.

Flexure-Induced Punching A general criterion (Equation 3) supplements ACI 318-14. Yielding of the top flexural reinforcement in a zone above a column can

(m' + m) =

(

c Vflex 0.2lc ‒ _ √ π 0.4πlc

)

=

Vflex(1‒2.8c/lc) 2π

(Eqn. 4)

Equation 4 may be rewritten as: Vflex = 2π (m' + m)/(1-2.8c/lc) (Eqn. 5) where m' and m are the absolute values of ultimate flexural strength per unit width provided by the top and bottom reinforcement, respectively, and lc is the center-to-center distance between columns. In isotropic slabs, the value of m' or m is the same in any orientation.

For a non-prestressed slab, m' (or m) can be expressed in terms of flexural reinforcement ratio ρ' (or ρ): m' = ρ' fy d 2[1‒0.59(ρ' f y /f 'c )] (Eqn. 6)

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orthotropic slab within the yield line pattern by an affine isotropic slab with moment resistances m and m', but with linear dimensions in the x-direction multiplied by (α)‒1/2 and Vu transformed to Vu (α)‒1/2. For simplicity, it would be safe to calculate ρ'min for an isotropic slab having the larger span in both directions.

When Vu is eccentric with respect to the centroid of the inner shear critical section, apply Equation 5, setting Vflex = vu bo d; where vu = largest shear stress – in absolute value – at the inner critical section (Equation 11, derived in the examples provided in the online version of this article). The use of Equation 5 when Vu is eccentric errs on the safe side.

Example 1: Minimum Reinforcement for Flexure-Induced Punching Determine the minimum top flexural reinforcement ratio ρ'min of a solid flat plate. Assume: square columns of side c = 0.06 lc; where lc is the center-to-center distance of columns in two orthogonal directions, factored load, qu = (106.1h + 638.1)×10-3 pound/inch2, with h as the slab thickness in inches, Vu = qul 2c (at the centroid of the inner shear critical section) and m = m'/4. Vary h and lc as in Table 1. Take f'c = 4350 pound/inch2; fy = 58×103 pound/inch2; φflex = 0.9. Equations 5-7 give the values of m', ρ', and ρ'min in Table 1. Assume that Vu is at the centroid of the inner shear critical section.

Summary and Conclusions Resisting flexure-induced punching requires a minimum amount of top flexural reinforcement above the column. The minimum reinforcement is determined by yield-line analysis of a local mechanism; the minimum value depends mainly upon Vu and d. Equations are reviewed for the design of headed studs or stirrups as shear reinforcement arranged in a crucifix layout. The shear reinforcement is placed on equally spaced peripheral lines. The number of lines is determined by repetitive calculation of the maximum shear stress at the outer critical section to satisfy the governing equation: vu ≤ φvn (= φvc ; reference is given, with examples). The properties of the shear critical section perimeter are calculated with reference to principal axes.

The shearing force transferred between slab and column is generally combined with a moment. Fractions γvx and γvy of the transferred moment components in x and y directions contribute to the value of vu. The coefficients γvx and γvy are calculated by equations depending upon the shape of the critical section. The design involves repetitive computations, conveniently done with a computer. Ignoring principal axes, leaving out one of the two components of the transferred moment, or arbitrary reduction of γv gives erroneous results. Stud assemblies are placed perpendicular to column faces and close to each of its corners. The spacing between assemblies is less than or equal to 2d (Figure 1a).■

Figure 3. Assumed yield-line pattern for the derivation of Eqn. 5; a) Overall mechanism, b) Forces and moments on one of the segments forming the mechanism.

where f y is the yield strength of flexural reinforcement and f'c is the specified compressive strength of concrete.

Consider a concentric force, Vu, at the centroid of the inner shear critical section. To determine the minimum non-prestressed flexural reinforcement to avoid flexure-induced punching for a given value of Vu, substitute Vflex by the given value in Equation 5 to obtain m'; then, solve Equation 6 for ρ'. To avoid flexure-induced punching (Equation 3), provide a minimum top flexural reinforcement ratio above the column, considering the reduction factor φflex (= 0.9 or 0.85). ρ'min = ρ'/φflex (Eqn. 7) Flexure-induced punching may occur at a load smaller than Vflex, without the full development of a yield-line mechanism. In such a case, Equation 4 gives a value of flexural strength greater than required; a slightly smaller value may be sufficient. For a prestressed slab with known prestressing, the required reinforcement is determined for m' minus the flexural strength provided by the prestressed tendons. The minimum top flexural reinforcement should extend in orthogonal directions to cover the yield-line pattern in Figure 3, plus development lengths. When the spans in x and y directions are different, the reinforcement may be orthotropic. For an orthotropic slab with positive and negative moment resistances m and m' in the x-direction and αm and αm' in the y-direction, substitute the

The online version of this article includes references and detailed examples with additional equations and graphics. Please visit www.STRUCTUREmag.org.

Table 1. Design Example 1: Calculation of m', ρ' and ρ'min (Eqns. 5-7).

h (inch)

d (inch)

lc (feet)

qu (pound/inch2)

Vu (pound)

m+m' (pound)

m' (pound)

ρ' (%)

ρ'min (%)

6.299

5.118

18.37

1.306

63.51×103

12.1×103

9.7×103

0.68

0.75

7.874

6.496

22.97

1.473

111.9×10

21.4×10

17.1×10

3

0.74

0.83

9.843

8.465

28.71

1.682

199.6×103

38.2×103

30.6×103

0.78

0.87

11.81

10.433

34.45

1.891

323.2×103

61.8×103

49.5×103

0.84

0.93

38 STRUCTURE magazine

3

3

Ramez B. Gayed is an Adjunct Assistant Professor at the University of Calgary, Civil Engineering Department and Senior Structural Engineer, tkIS Canada Inc. (rbgayed@hotmail.com) Amin Ghali is an Emeritus Professor, at the University of Calgary, Department of Civil Engineering. (aghali@ucalgary.ca)

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INSIGHTS Code Provisions for Headed Cast-in Specialty Inserts

By Natasha Zamani, Ph.D., P.E.

H

eaded cast-in specialty inserts are internally-threaded steel shell elements welded to a bearing plate that is cast into concrete members. Hilti’s KCM and Simpson’s Blue Banger Hanger anchors are some examples of headed cast-in specialty inserts (Figure 1). These anchors are used to attach structural and non-structural components. In general, the American Concrete Institute’s (ACI) Building Code Requirements for Structural Concrete (ACI 318) provides design requirements for cast-in anchors in concrete. However, the wide variety of shapes and configurations of specialty inserts make it challenging to prescribe generalized tests and design equations. Hence, they have been excluded from the scope of ACI 318.

These types of inserts are ideally suited for a variety of rod hanging applications such as overhead vertical supports and seismic restraint anchors for suspended mechanical, electrical, plumbing, and fire protection systems. These inserts offer significant time savings over traditional post-installed anchors.

However, design engineers who want to specify these anchors should address the question of code-compliance since these inserts are not explicitly included within the scope of ACI 318. To address this issue, the International Code Council Evaluation Service (ICCES) has developed an Acceptance Criteria for Headed Cast-In Specialty Inserts in Concrete (AC446) to cover the prequalification of headed cast-in-place specialty inserts in concrete components. Headed specialty inserts satisfying the AC446 test program now receive recognition via ICC-ES Evaluation Service Reports (ESR), which provides a path to assure that the insert can be designed in accordance with the ACI 318 criteria (Figure 2). These ESR reports are ESR-4145 for the Hilti KCM anchors and ESR-3707 for the Simpson Blue Banger Hanger anchors. Note that AC446 only covers headed specialty inserts. Therefore, coil anchors, hooked bolts, or other non-headed specialty insert types are not within the scope of AC446.

Code Provisions Testing, in accordance with AC446, establishes the strength of headed cast-in specialty STRUCTURE magazine

inserts based on the strength design provisions of ACI 318. AC446 has requirements for the geometry and head bearing area of inserts to ensure they can achieve equivalency with the bearing behavior of headed studs or headed bolts that comply with the ACI 318 provisions.

These requirements are set to ensure that the specialty insert has a strength that exceeds the calculated concrete breakout strength when the 28-day compressive strength (f’c ) is set at 10,000 pounds per square inch (psi), the maximum compressive strength permitted per ACI 318. Also, the area and thickness of the head shall be large enough to ensure that the bearing stress on the head does not exceed 6f’c and the bending of the head plate under maximum bearing stress is small enough to accommodate the assumption of uniform stress distribution under the head.

After verifying the geometry of the head, the insert is tested in accordance with AC446 provisions to derive tension and shear strength capacities of the cast-in insert. In summary, the AC446 test program determines the following parameters for the headed cast-in specialty insert: • Insert material characterization (ductile or brittle) • Insert strength in tension (Nsa,insert) • Insert strength in seismic tension (Nsa,insert,eq) • Insert strength in shear (Vsa,insert) • Insert strength in seismic shear (Vsa,insert,eq) Since the shear capacity of the insert in concrete cannot be reliably calculated, AC446 requires that shear testing be performed in low strength concrete. However, to determine the tension capacity, the testing is performed in a steel jig. AC446 eliminated all concrete testing in tension for the headed insert by setting a conservative limit on the bearing stress of the head, which requires a generous head size for the insert. In this condition, the only real concern for the insert in tension will be the insert failing prematurely either at the connection of the head to the insert, at the threads, or at the insert steel shaft. These failures can be captured by utilizing a steel jig when testing the insert.

This new qualification procedure in AC446 will give designers increased confidence that they can properly design and specify the

Figure 1. Examples of headed cast-in specialty inserts.

Figure 2. Design path for headed specialty inserts.

inserts and comply with the requirements of the ACI 318. By defining the insert strength and characterization, the nominal static and seismic strength of the insert steel can be reported in AC446. The nominal strength of the threaded rod or bolt, as well as the nominal strength of the concrete, must be calculated in accordance with ACI 318 provisions but should not exceed the values of insert strengths reported in AC446.

Summary The International Code Council Evaluation Service (ICC-ES), Acceptance Criteria for Headed Cast-In Specialty Inserts in Concrete (AC446), establishes requirements for the qualification of headed cast-in specialty inserts and provides a path to designing headed specialty inserts according to ACI 318 anchoring to concrete provisions.■ Natasha Zamani is the Anchor Approval Engineer for Hilti North America. She is responsible for publishing external evaluation reports such as ICC-ES ESR’s and creating technical data for the Hilti North America Anchor Products. (natasha.zamani@hilti.com)

J U L Y 2 019

41

Get Your Tickets Today!

ACI EXCELLENCE IN CONCRETE CONSTRUCTION AWARDS GALA

Monday, October 21, 2019 • Cincinnati, OH, USA • The ACI Concrete Convention & Exposition

Tickets are now available for the ACI Excellence in Concrete Construction Awards Gala. This premier event will celebrate the concrete industry’s most prestigious and innovative projects from around the globe.

Tickets for this event have sold out in previous years. Those who wish to witness the best of the best should purchase tickets soon. Tickets can be purchased for $95 through ACI Convention registration or through the American Concrete Institute’s online store.

Learn more at www.ACIExcellence.org

CONCRETE PRODUCTS guide Commins Manufacturing Inc Phone: 360-378-9484 Web: www.comminsmfg.com Product: AutoTight Tie-Down Systems Description: High Wind and Seismic, Continuous Tie-Down Systems for connecting light frame structures to concrete Footings and PT decks.

Systems carry up to 147 kips. Systems provide strength and precision drift control.

CTS Cement Manufacturing Corporation Phone: 800-929-3030 Web: CTScement.com Product: Rapid Set® Cement Description: Outperforms other concrete repair materials in durability, repetitive loading, chemical attack, permeability, freeze/thaw, abrasion resistance, and shrinkage. Rapid Set gets 3000 psi in one hour, achieving structural or drive-on strength in one hour. Use for concrete repairs, restoration, and new construction projects.

Dlubal Software, Inc. Phone: 267-702-2815 Web: www.dlubal.com Product: RFEM Description: Strength and serviceability limit state designs of reinforced beams, columns, and flat/ curved plates according to ACI 318, CSA A23.3, and other international standards. Advanced capabilities: non-linear analysis of reinforced concrete elements in the cracked state for a realistic view of deformations, stresses, and crack widths for the serviceability limit state.

ENERCALC, Inc. Phone: 800-424-2252 Web: http://enercalc.com Product: Structural Engineering Library/RetainPro/ ENERCALC SE Cloud Description: SEL quickly completes calculations for the design of footings, columns, beams, pedestals, shear walls and other concrete structures. Our new 3-D sketches let you avoid expensive, complicated software. RetainPro provides detailed concrete earth retention design/calculation tools. Our clear, concise reports are ideal for client/agency reviews.

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Phone: 847-966-4357 Web: www.structurepoint.org Product: Concrete Design Software Suite Description: StructurePoint, formerly the PCA Engineering Software Group, offers concrete design software programs updated to ACI 318-14 for concrete buildings, concrete structures, and concrete tanks. Reinforced concrete structural software includes programs for design of columns, bridge piers, beams, girders, one and two-way slabs, shearwalls, tilt-up walls, mats, foundations, tanks, and slabs-on-grade.

Integrity Software, Inc. Phone: 512-372-8991 Web: www.softwaremetering.com Product: SofTrack Description: Save money on monthly, quarterly, and annual Bentley® license fees! Automatic control to prevent over-usage of Bentley licenses. Ensure licensed applications are used within your license limits. Includes support for all Bentley licensing policies. Automatically block usage of products you do not own. Ask about enhanced Autodesk and ArcMAP reporting.

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Phone: 604-273-7737 Web: s-frame.com Product: S-CONCRETE Description: The most efficient concrete design and detailing solution available for columns, beams, and walls with ACI 318-14 support. Run as standalone application or as ICD in S-FRAME which includes continuous beam design. Multi-Story Designer option quickly designs and optimizes large structures analyzed with ETABS®.

Product: Tekla Structures Description: Move from design-oriented to construction-oriented engineering and empower structural engineers to offer improved additional services. Through our open and collaborative software environment, you can work with other disciplines and reduce RFIs. From concept to completion, Tekla software gives you collaboration and control.

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Phone: 800-836-7271 Web: www.tensarcorp.com Product: TriAx Geogrid Description: An advanced product specifically designed for trafficked surfaces. TriAx Geogrid’s multi-directional properties leverage triangular geometry to provide an enhanced level of in-plane stiffness. Its triangular structure, coupled with the improved rib and junction geometry, offers the construction industry an improved alternative to conventional materials and practices.

Phone: 203-857-2200 Web: www.wejit.com Product: Ankr-TITE Wedge Anchor Description: A high-strength, fully threaded, and torque-controlled expansion anchor designed for anchoring into normal-weight concrete, lightweight concrete, and grouted concrete masonry. The AnkrTITE anchor has a unique machine lathed expansion cone that supports a thick three dimple clip engineered to provide high load carrying capability and reduce slip.

Phone: 800-925-5099 Web: www.strongtie.com Product: Stainless-Steel Titen HD® Heavy-Duty Screw Anchor Description: Now available in Type 304 and Type 316 stainless steel, with serrated carbon-steel threads at the tip. Type 316 is optimal for corrosive environments, and Type 304 is a cost-effective solution for less extreme applications where the environment may be wet or damp.

Product: Fabric-Reinforced Cementitious Matrix (FRCM) Description: Combines a high-performance sprayable mortar with a carbon-fiber grid to create a thin structural layer that does not add significant weight or volume to an existing structure. FRCM can be used to repair and strengthen concrete and masonry structures for seismic retrofit or load upgrades. Contact us for design support.

Demos at www.struware.com Wind, Seismic, Snow, etc. Struware’s Code Search program calculates these and other loadings for all codes based on the IBC or ASCE7 in just minutes (see online video). Also calculates wind loads on rooftop equipment, signs, walls, chimneys, trussed towers, tanks and more. ($250.00).

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J U L Y 2 019

43

NCSEA

NCSEA News

National Council of Structural Engineers Associations

Tapping the Power of the Profession

A contribution from NCSEA provided a strong sprint across the fundraising finish line to launch research of high value to the profession.

While tweaks have been made to the ASCE 7 Wind Load provisions over the years, a systematic study using modern wind tunnel test methods for code-based design has not been conducted in more than 40 years, until now. With organizational and individual financial supporters along with NCSEA’s commitment to power the campaign across the finish line, the Charles Pankow Foundation (CPF) tapped into the energy of the profession to secure funding for the much-needed research.

The Structural Engineering Institute (SEI) of ASCE is currently developing the 2022 update to ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Specifically, ASCE 7 Wind Loads Subcommittee chairman Donald R. Scott, P.E., S.E., F.SEI, F.ASCE, has been working to simplify the approach to wind load design including proactively addressing gaps in the standard. “Current pressure coefficients date from mid-1970s, and knowledge of both the role of turbulence on aerodynamic loading and the turbulence levels in the atmospheric boundary layer have evolved considerably since the initial code efforts,” said Scott. The CPF-led industry response quickly provided resources for additional building height aspect ratios to be tested in the wind tunnel. The research, led by Greg Kopp, Ph.D., is underway. The results will inform possible changes to the Main Wind Force Resisting System (MWFRS) provisions in Chapter 27 of ASCE 7, along with opportunities for simplification and consolidation.

A highly-successful joint appeal to the profession from Scott and CPF Board Member Ron Klemencic, P.E., S.E., Hon. AIA, F.SEI, F.ASCE, achieved the funding goals in seven days thanks to the NCSEA Board and others, proving that the right combination of need and resources can yield powerful results. Learn more about this project at bit.ly/grant0419.

Could an NCSEA Grant Kickstart Your SEA's Next Project?

The NCSEA Grant Program was developed in 2015 to award SEAs funding for projects that grow and promote their SEA and the structural engineering field in accordance with the NCSEA Mission Statement: NCSEA advances the practice of structural engineering by representing and strengthening its Member Organizations.

Any NCSEA Member Organization or member of a Member Organization is eligible to submit a grant application, as long as the application has been reviewed and approved by the Member Organization.

Past Grant recipients have used their funds to support endeavors such as hosting symposiums and networking events for members, enhancing mentoring programs, setting up Engineering for the Arts initiatives, and updating dated studies. Some SEAs used grants to purchase items to enhance their outreach efforts.

Visit www.ncsea.com for more information about the 2019 Grant Program and to submit your proposal.

2018 Grant Program Success The Structural Engineers Association of Southern California (SEAOSC) used their grant to fund two Safer Cities® Events that strengthened their role in the community by building connections with local policy makers, educating members and community leaders, and identifying areas of advancement.

Fifty building officials, elected officials, and policy makers were invited to participate in the Safer Cities® Policy Breakfast that addressed how the structural engineering community of Southern CA could support the government's objectives and assist the public to realize safer and more resilient communities. Attendees were provided tools to improve building safety, function, and performance. The input from this interactive breakfast has helped SEAOSC to address these issues.

The second event was the 2019 Safer Cities® Technical Summit. This event provided a technical symposium, training for the CalOES Safety Assessment Program, and promotion of the contributions made by the structural engineering community after an earthquake or other major shocks to local communities.

SEAOI's Work on the Structural Engineering Act Renewal Based on its efforts, the Structural Engineering Association of Illinois (SEAOI) was able to announce, earlier this year, a bill extending the sunset of the Structural Engineering Practice Act. In November of last year, SEAOI was invited to participate in a coalition to renew practice acts that fall under the Illinois Department of Financial and Professional Regulation (IDFPR): Professional Engineers, Architects, and Land Surveyors. SEAOI’s interests were represented by Christine Freisinger, President-Elect; Jan Blok, SEPAC Secretary; and Stephanie Crain, SEAOI Executive Director. The group worked to introduce changes to the Structural Engineering Renewal Act to the Senate and begin the renewal process.

SEAOI is delighted to announce a bill extending the sunset of the Structural Engineering Practice Act from January 1, 2020 to 2030.

SEAOI has been actively involved in the renewal process for several months and has authored several changes which were incorporated into the House amendments. The Senate concurred with the three House amendments on a unanimous vote of 59-0!

The Act has been sent to the Governor and is in the final stages. SEAOI will be providing more detailed updates to the process on their website: www.seaoi.org. If you’d like to review the current Practice Act, you may visit: www.seaoi.org/se-practice-act.

44 STRUCTURE magazine

News from the National Council of Structural Engineers Associations

Excellence and Special Award Entries Due This Month

Submissions for NCSEA's Special Awards and the Excellence in Structural Engineering Awards are due by end of day on July 16, 2019.

NCSEA's Excellence in Structural Engineering Awards annually highlights some of the best examples of structural engineering ingenuity throughout the world. Projects are judged on innovative design, engineering achievement, and creativity. Multiple winners are presented in seven categories with an outstanding winner being chosen and announced at NCSEA's Structural Engineering Summit in Anaheim, California, this November.

NCSEA's Special Awards are presented each year at the Structural Engineering Summit. These awards are presented to NCSEA members who have provided outstanding service and commitment to the association and to the structural engineering field. Special Awards are granted to worthy recipients in four different categories: the NCSEA Service Award, the Robert Cornforth Award, the Susan M. Frey NCSEA Educator Award, and the James Delahay Award.

Visit www.ncsea.com/awards to learn more about NCSEA's awards program as well as the rules and eligibility for each award.

Submissions are due by 11:59 pm CST on Tuesday, July 16, 2019.

2019 Structural Engineering Summit

November 12–15, 2019 · Disneyland ® Hotel · Anaheim, CA

The Summit draws the best of the structural engineering field together for high-quality education by expert speakers, a leading trade show with over 60 exhibitors, and compelling peer-to-peer networking at a variety of events and receptions. Join us for this growing and dynamic event designed to advance the industry! Visit www.ncsea.com to learn more about this year's Summit and to reserve your room at the Disneyland® Hotel, rooms are going fast.

NCSEA Webinars

Register by visiting www.ncsea.com.

July 11, 2019

Strength Design of Masonry Walls – The Code & Beyond John Hochwalt, P.E.

The design of masonry walls for in-plane and out-of-plane loads using the strength design provisions of TMS 402-16 will be discussed. Rules of thumb for initial proportioning of masonry walls will be shared, the strength design provisions will be reviewed, and some new and lesser used provisions that can allow for more efficient wall designs will be highlighted.

July 25, 2019

Engineered Design of Masonry Veneers John Hochwalt, P.E.

While historically anchored veneers and their attachment to the exterior wall have been designed prescriptively, engineered design of veneers is becoming more and more common to allow the use of veneers beyond the prescriptive limitations of the code and to meet increasingly stringent energy code requirements.

August 6, 2019

CFS Lateral Design Using New AISI S240 and S400 Jeff Ellis, P.E., S.E., SECB

This presentation will provide insight into the lateral design provisions from the new AISI S240 and S400 CFS framing standards. These new standards have been adopted by the 2018 IBC and represent the latest consensus provisions for effective and efficient CFS lateral design.

Courses award 1.5 hours of continuing education after the completion of a quiz. Diamond Review approved in all 50 states.

J U L Y 2 019

45

SEI Update Learning / Networking

NEW on ASCE Collaborate: Integrated Buildings and Structures

Use your SEI/ASCE member login to access highly-rated Special Sessions from Structures Congress on Conceptual Design; Improving Presentation Skills: Science Not Communicated is Science Not Done; and more. https://collaborate.asce.org/integratedstructures/home

SEI Local Leaders Conference

Local SEI Chapter Chairs: Save the date for the SEI Local Leaders Conference, October 24-26 at ASCE in Reston, VA, for best practices and leadership training. If you are a local SEI Chapter Chair and are not on the local SEI leaders email list, contact Suzanne Fisher sfisher@asce.org. Connect with your local SEI professional or Grad Student Chapter at www.asce.org/SEILocal.

STRUCTURAL ENGINEERING INSTITUTE

STRUCTURES CONGRESS 2020

Sponsor/Exhibit to showcase new technology and products

www.structurescongress.org

St. Louis, Missouri I April 5-8

Prepare for Fall Exams with ASCE P.E./S.E. Live Exam Review Courses

Interactive, expert-led classes begin in August. Group rates are available for two or more engineers at the same location. Sign up at www.asce.org/live_exam_reviews.

Register today!

Membership

SEI Sustaining Organization Membership

SEI Elite Sustaining Organization Members:

Reach more than 30,000 SEI members year-round with SEI Sustaining Organization Membership.

Demonstrate your commitment to excellence, and show your support for SEI to advance and serve the structural engineering profession. Learn more and join today at www.asce.org/SEI-Sustaining-Org-Membership.

ASCE Younger Member Leadership Symposia (YMLS) Three Opportunities July-August YMLS is a three-day experiential leadership workshop open to all ASCE Younger Members, age 35 and under, and focuses on early-career professional skills development to help members succeed and lead in the workplace. Tours and special events during the weekend offer additional networking with peers from around the country. Space is limited and fills up quickly. www.asce.org/event/2019/younger-member-leadership-symposia.

SEI Online

SEI News

Read the latest news at www.asce.org/SEI. 46 STRUCTURE magazine

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

News of the Structural Engineering Institute of ASCE Advancing the Profession

Congratulations to the 2019 SEI Fellows recognized at Structures Congress in Orlando

Learn more and apply, nominate, or serve as a reference for 2020 at www.asce.org/SEIFellows.

Vision for the Future of Structural Engineering

CASE, NCSEA, and SEI are united in envisioning a future where, as stewards of the built environment, structural engineers are widely recognized as making critical contributions to the advancement of society on a national and global scale. Learn more on page 40 of this issue of STRUCTURE. View the complete Vision Update and SEI annual meeting at www.asce.org/SEI.

NEW Confidential Reporting on Structural Safety Launched in the U.S. by SEI

Confidential Reporting on Structural Safety (CROSS), originated in the UK in 2005 (www.structural-safety.org), is a system designed to improve standards of practice through learning from structural failures and incidents. It was modeled after the US Aviation Safety Reporting System designed by NASA. Through www.CROSS-US.org, individuals may now confidentially submit reports of structural failures, near misses, and similar incidents.

The reports are first anonymized and de-identified, and then forwarded to a team of distinguished subject matter experts for review and comment on lessons learned. The reports with analysis commentary are then made available on the website to all, free of charge. CROSS also tracks trends, publishes summary or theme reports, and may direct particular issues to organizations, such as code-writing bodies, that may be in a position to improve standards of practice.

CROSS has been very effective in the UK in improving practice to reduce the incidence of failures. Anyone with an interest in structural safety is encouraged to submit reports and to avail themselves of the valuable information posted.

Give to the SEI Futures Fund and Leverage 4-for-1 Match Donate to the SEI Futures Fund from now 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. Learn about programs supported, and make your donation today! www.asce.org/SEIFuturesFund.

Investing in the Future of Our Profession

Errata

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.

J U L Y 2 019

47

CASE in Point Did you know?

CASE has tools and practice guidelines to help deal with a wide variety of business scenarios that structural engineering firms face daily.

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

The following documents/templates are recommended to review/use if your firm needs to update its current Quality Assurance Program, or incorporate a new program into the firm culture: 962: National Practice Guidelines for the Structural Engineer of Record 962-B: National Practice Guideline for Specialty Structural Engineers 962-C: Guidelines for International Building Code Mandated Special Inspections and Tests and Quality Assurance 962-D: Guideline addressing Coordination and Completeness of Structural Construction Documents Tool 1-2: Developing a Culture of Quality

Tool 2-1: Tool 2-4: Tool 4-1: Tool 4-2: Tool 4-3: Tool 4-4: Tool 4-5: Tool 9-2: Tool 10-1: Tool 10-2:

Risk Evaluation Checklist Project Risk Management Plan Status Report Template Project Kick-off Meeting Agenda Sample Correspondence Letters Phone Conversation Log Project Communication Matrix Quality Assurance Plan Site Visit Cards Construction Administration Log

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

Pathways to Executive Leadership – Class Four Registration Open!

A practical, focused program for new leaders facing the challenges of a continuously evolving business environment.

To be successful at taking on higher levels of leadership responsibility and prepare for the demands of being owners, new practice builders need specific and relevant training in the intricacies of leading an A/E firm in ever-changing, always uncertain economic times.

Pathways to Executive Leadership is an intensive leadership program for early-career elites and promising mid-career professionals with 8-12 years of experience who are just beginning to lead and think strategically about their practices and careers. The reality-based curriculum focuses on the core skills necessary to think strategically in their markets, build effective teams, and deliver great service for their most valued clients.

Target Audience: Pathways to Executive Leadership fills a vital gap and creates a strong connection between ACEC’s Business of Design Consulting curriculum and the Senior Executive’s Institute capstone

program. It targets those who are making the transition between managing one team (e.g., project managers) to those managing managers and multiple teams. This program is designed to establish habits for long-term high performance and to create a trusted, national network of colleagues with which to make the journey.

Flow of Learning: Budding practice builders face challenges daily and require new skills to manage people and the uncertainty of a continuously evolving business environment. Pathways to Executive Leadership will lead participants through a practical curriculum focused on becoming more balanced in their personal and professional life, more influential in team development, coaching, and client relationships, and more strategic in their business relationships to build a strong client portfolio.

For more information, contact Katie Goodman, 202-682-4332, or kgoodman@acec.org.

To register, https://programs.acec.org/2019-pathways.

Catch Up on your Summer Reading!

When you’re packing for summer travel – think light for business insights!

With a range of topics from proposal writing to project delivery, these digital resources are a perfect addition to your business library – instantly available in PDF, MOBI, or E-PUB – at a great price.

Download one or all!

• Can I Borrow Your Watch? A Beginner’s Guide to Succeeding in a Professional Consulting Organization • 33 Proven Secrets to Writing Successful Client-Centered Proposals • Construction Management at Risk, Second Edition • Project Delivery Systems Owner’s Manual, Second Edition • Winning Strategies for A/E/C Firms: An Executive’s Guide to Maximizing Growth and Profitability, Third Edition • Win More Work: How to Write Winning A/E/C Proposals

To purchase these and other resources go to www.acec.org/bookstore.

48 STRUCTURE magazine

News of the Council of American Structural Engineers CASE Risk Management Tools Available Foundation 3: Planning – Plan to be Claims Free

• Must have a plan for the firm in order to be claims free.

• Train staff to plan, then implement the plan.

• The plan needs to be simple, understandable, and inclusive to be effective.

• Have Policies and Procedures that are workable and followed.

• Communicate and repetitively reinforce the plan.

• The plan may need to adjust as conditions change.

Tool 3-1: A Risk Management Program Planning Structure

This tool is designed to help a Firm Principal design a Risk Management Program for his or her firm.

The tool consists of a grid template that will help focus thoughts on where risk may arise in various aspects of engineering practice and how to mitigate those risks. Once the risk factor is identified, then a policy and procedure for how to respond to that risk is developed. This tool contains 10 sample risk factors with accompanying policies and procedures to illustrate how one might get started. The tool is designed to insert custom risks and policies to tailor it to individual firms.

Tool 3-2: Staffing and Revenue Projection

Firms are provided a simple to use and easy to manipulate spreadsheetbased tool for predicting the staff that will be necessary to complete both “booked” and “potential” projects. The spreadsheet can be further utilized to track historical staffing demands to assist with future staffing and revenue projections.

Tool 3-4: Project Work Plan Templates

Foundation 4: Communication – Communicate to match expectations with perceptions.

• A high percentage of claims occur because of poor communication.

• Be proactive in communications. Not reactive.

• Create an atmosphere for good and open communication.

• If in doubt, communicate early and often.

• Select the best method of communication (email may not always be the best approach).

• Communicate effectively.

Tool 4-1: Status Template Report

This tool provides an organized plan for keeping your clients informed and happy. This project status report is intended to be sent to your Client, the Owner, and any other stakeholder you would like to keep informed about the project status.

Tool 4-2: Project Kick-Off Meeting Agenda

Effective communication is one of the keys to successful risk management. Often, we place a significant amount of effort and care into communication with our clients, owners, and external stakeholders.

With all that effort, it is easy to take for granted communication with our internal stakeholders – the structural design team. If a project is not started correctly, there is a good chance that the project will not be executed correctly either. Tool 4-2 is designed to help the Structural Engineer communicate the information that is vital to the success of the structural design team and start the project off correctly.

Tool 4-3: Sample Correspondence Guidelines

Preparing and maintaining a proper Project Work Plan is a fundamental responsibility of a project manager. Work Plans document project delivery strategies and communicate them to team members.

Project Managers will use this template to create a project Work Plan that will be stored with the project documents.

CASE Tool 4-3 intends to make it faster and easier to access correspondence with appropriate verbiage addressing commonly encountered situations that can increase your risk. The sample correspondence contained within this tool is intended to be sent to the Client, Owner, Sub-consultant, Building Official, Employee, etc., to keep them informed about a particular facet of a project or their employment.

Tool 3-5: Staffing Schedule Suite (New, 2019)

Tool 4-4: Phone Conversation Log

This tool includes multiple staffing schedule templates to help firms ensure they have the right human resources assigned to tasks. By effectively projecting and balancing workloads, firms can maximize employee productivity and profit by reducing employee burnout and turnover. This tool helps firms answer the following questions: • What are our employees working on do-to-day and week-to-week?

• Do we have enough work to keep our people busy, productive, and profitable?

• Do we have enough staff members to complete current assignments on-time?

Poor communication is frequently listed among the top reasons for lawsuits and claims. It is the intent of this tool to make it faster and easier to record and document phone conversations.

Tool 4-5: Project Communication Matrix

This tool provides an easy to use and efficient way to (1) establish and maintain project-specific communication standards and (2) document key project-specific deadlines and program/coordination decisions that can be communicated to a client or team member for verification.

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

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

J U L Y 2 019

49

structural FORUM The Importance of Professional Advocacy

By Angelina V. Stasulis, P.E., S.E.

A

successful engineer is, in all likelihood, an unknown engineer. Short of significant failures, the general public is unaware of the time and thought that goes into a good design.

Most assume the success of our infrastructure is due to building codes or architects, without any knowledge of the layers of security provided by structural engineers. This lack of awareness has far-reaching consequences for the profession, the most significant of which is a decided undervaluing of structural engineering. Structural engineering, as a profession, needs a marketing plan.

Marketing is not a foreign concept to structural engineers; most of us already do it with clients. Where we fail as a profession is in spreading our message beyond the architect/ engineer/contractor (AEC) industry. As a result, structural engineers are poorly compensated for high-risk/responsibility/stress, and society at large fails to recognize these individuals’ importance and dedication to public safety. Politicians, uninformed and uneducated about the profession, are easily led astray by inadequate or self-serving advisors regarding policies that affect the engineering community (for example, structural licensure). Even building owners often fail to communicate performancebased requirements, assuming buildings will automatically meet their beyond-thecode requirements (for example, vibration requirements for sensitive equipment).

Without an understanding of professional licensure qualifications and necessary knowledge for each distinct discipline of engineering, unqualified individuals may be employed to perform structural design and analysis outside of their area of competence, putting public safety and welfare at risk.

All of these things can be mitigated by the profession improving its market position and visibility.

The struggle to be understood is not unique to structural engineers. However, we are miles behind our peers in the AEC industry in terms of public understanding and recognition.

Contractors have the benefit of the public directly observing their work and instantly connecting their profession with what they do. Architects usually serve as the project manager for the design team, organizing efforts and communicating directly with the client, which positions them to be the public face of a building project’s design team. They create the 50 STRUCTURE magazine

final picture that the public sees in Ways to Advertise and Market media releases, the rendering sign at the job site, and, ultimately, the • Talk to your contractor about including the public’s drive-by experiences. While Structural Engineer of Record (SEOR) on job the public does not understand all site rendering signs. of the complexities of either of • Provide outreach materials for students and these professions, they do, at least, parents that explain structural engineering acknowledge their existence and in more specific terms that others can undercontribution to building projects. stand, rather than “design structural elements.”

Structural engineers need to • Write nontechnical, general structural join the game and emphasize the summaries of local projects for newspapers and importance of our role in how other local media outlets. buildings get built in this public • Have your local Member Organization narrative. Most people attribute take a stance on legislation that impacts the reliable building safety and perprofession (licensure, regulations, etc.) and formance to “the building code” publish your position statement. as if it were a prescriptive, definitive solution for all structures. The public gives little thought to what we do and an interest in our profession from the next even less to the fact that it needs to be done generation but fail to communicate the at all. We need to highlight the expertise critical connection between gumdrop and required to effectively apply the provisions of spaghetti structures on shake tables and how the building code and the need for engineer- structural engineers truly design the strucing judgment in cases not explicitly outlined tural system of a real building under service therein, particularly serviceability condi- loads. Our primary mission as an industry is tions. We need to explain the importance to design safe and economical structures that of regional and project-specific experience, effectively realize the vision of the architect and why prototype plans for a structure in and owner. We are better able to deliver on Michigan cannot simply be duplicated for this goal when we are empowered with the another franchise location in Oregon or support of a public that is educated Florida. We need to call attention to what in both what we do and how we we can provide to contractors in terms of achieve it.■ construction administration services, like shop drawing review and field fixes, and NCSEA Communications Committee why these matter. We need to do a better invites you to participate in the first Member job instilling respect and appreciation for Organization Challenge to inform and the technical knowledge we bring to the educate other industries, professions, and the non-technical masses. We need to join the general public about structural engineering! race for public attention or fall victim to Your Structural Engineering Association the devaluation of our profession as society (SEA) can help improve the visibility and moves closer to a fully automated world, recognition of practicing structural engineers where critical thought is not just unrequired with outreach through news articles, videos, but is unappreciated. blogs, and any other creative content to spread If we do not spread the word about what we the message about our profession and its do and why it is important, we enable society critical role in society. as a whole to continue to ignore our contriVisit www.ncsea.com/challenge for more butions. Most current advocacy efforts are information and to start your member focused on too specific an audience to have organization’s submission packet. the broad, general impact we need. Sharing project spotlights via industry publications fosters awareness of what others are doing Angelina V. Stasulis is a practicing Structural Engineer in Atlanta, GA. She serves as in the industry, but these articles generally a voting member of the NCSEA External reach a technical audience, not the general Communications Committee and is also the public. Science Technology Engineering and Publicity Chair for SEAOG. Math (STEM) outreach programs cultivate J U L Y 2 019

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