STRUCTURE magazine - July 2020

STRUCTURE JULY 2020

NCSEA | CASE | SEI

WIND/SEISMIC

INSIDE: Saban Building Conservation 24 Tornado Shelter Design 8 Wind Load Effects on Canopy Systems 12 Resiliency in Coastal Designs 22

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Contents JU LY 2020

22 A COASTAL COMMUNITY’S RESPONSE TO A NATURAL DISASTER

By Lance Watson, P.E., and Tyler Marsh, P.E.

Hurricane Michael made landfall in October 2018. Following the hurricane, local municipalities and communities reacted with resiliency in mind to prepare future structures and developments for similar storm events.

Cover Feature

24 CONSERVATION OF A HISTORIC FAÇADE

By Maria Mohammed, S.E., John Fidler, and David Cocke, S.E.

Investigations into the deterioration, causes, and possible treatments to stabilize and repair the limestone cladding panels of the Saban Building resulted in a unique preservation scheme.

Columns and Departments 7

Editorial The Structural Engineering Office of the Future

28

Impacts

By Stacy Bartoletti, S.E.

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30

Codes and Standards Structural Design and

By Jessica Simon, P.E., and Andrew Dziak, P.E.

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By Medapati Abhinav Reddy

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Structural Systems Enhanced Wind and Seismic Performance of Tall Buildings

By Michael Montgomery, Ph.D., P.Eng, and Constantin Christopoulos, Ph.D., P.Eng

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Structural Design Performance-Based Wind Design

By Roy Denoon, Ph.D., John Kilpatrick, Ph.D., P.Eng, C.Eng, and Donald R. Scott, P.E., S.E.

July 2020 Bonus Content

Structural Performance Roofs of Major Logistic Centers CASE Business Practices My Project is in a Flood Zone… What Do I Do?

Structural Components Wind Load Effects on Canopy Systems

By Marc S. Barter, S.E., P.E., and Roger A. LaBoube, Ph.D., P.E.

By Rafik Gerges, Ph.D., P.E., S.E., Guangle (Tyler) Xu, P.E., and Mohan Cheng

Coordination of ICC 500 Tornado Shelters

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Building Blocks Steel Solution for Envelope Missile

By Kevin H. Chamberlain, P.E.

InSights The January 2020 Puerto Rico Earthquake

By Dr. Kit Miyamoto, S.E.

In Every Issue 4 34 39 40 41

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

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

Technology The Embodied Carbon in Construction Calculator Tool (EC3)

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

JULY 2020

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EDITORIAL The Structural Engineering Office of the Future

By Stacy Bartoletti, S.E.

As

I write this editorial, many communities and cities in the United States are just starting to “re-open” following the Coronavirus pandemic and shelter-in-place orders. My firm, like most, transitioned into a full work-from-home situation in March with only minor issues and challenges.

Our business and employees have adapted remarkably well. We are conducting meetings virtually through video calls, developing and reviewing construction documents by sharing data and models, accessing information through the cloud or virtually from our servers, and participating in new project interviews remotely.

Employees are doing their best to remain engaged and connected with one another.

The success with which we have transitioned into a remote company begs the question of why return to the old way of working in an office and why not maintain an entirely remote workforce? Answering this question and planning for the future is not simple. For our industry, I do not believe we can assume that what we have done for the past three months can continue into perpetuity. While it may sound appealing to eliminate a substantial portion of the real estate expense from our business and allow everyone to work remotely, I do not believe that is the future of the structural engineering office. I believe we will come to the realization that our physical offices are important to the long-term health of our business and our employees. However, I also believe that we will see a much larger fraction of our employees splitting time between the office and working remotely. Below I offer thoughts on a few issues that I am considering with my firm and the reasons why I believe we will continue to have offices.

Electronic Connectivity – This is perhaps the most straightforward issue to address. Clearly, the past few months have shown that technology solutions exist to allow the vast majority of our structural engineering work products to be completed remotely. At my firm, we are planning to make additional investment in our connectivity and hardware needs to give employees the flexibility to work efficiently both in the office and remotely. We believe this investment will pay off in productivity and engaged employees.

Training and Mentorship – As a professional services firm, much of what we do depends upon a highly-skilled staff that requires continuous training and development. Some of that training comes from formal programs, but I believe the vast majority at my firm comes in the form of mentorship and interaction on projects. Training also goes beyond technical work. Our engineers learn how to interact with clients, deal with difficult situations, and communicate with each other by seeing their mentors and co-workers do it. The personal experience and day-to-day immersion cannot be replicated over a video conference. I believe that we still need to come together in our offices to develop future generations of structural engineers effectively and efficiently.

STRUCTURE magazine

Mental Health – I have become more concerned about the mental health of my employees as we have extended remote working longer and longer. As engineers, we have the stigma of being loners and antisocial. In my experience, that is not true. Our staff thrives on interacting with one another and enjoys the friendships that develop in the office. I do not believe it can be replicated in a fully remote environment. As engineers and as humans, we need interaction to maintain mental health. During the first couple of months of remote working, our productivity was high.

Still, I predict it will decrease the longer our employees are isolated and working in a fully remote environment.

Personal/Work-Life Separation – Before the pandemic, a frequent topic of discussion was personal/work-life balance. After three months of shelter-in-place and remote working, I am now hearing more about personal/work-life separation. This has become particularly challenging for employees with young families. While most employees are enjoying their ability to capture the time that would have been spent commuting to the office, I believe most will still want to return to an office for some of their work time, creating a physical and mental separation between personal and work life.

Flexibility – As we get beyond the Coronavirus pandemic, I believe we will be structuring our offices and work schedules for more flexibility. At my company, I expect that most employees will return to working in an office, but we will also be providing more flexibility for them to work remotely.

Company Culture – Company culture is hard to define since every structural engineering company has its own. At my company, the items I have discussed above contribute to our culture. I do not believe our culture can survive long term with a fully remote workforce; it needs face-to-face interactions. A substantial contributor to how we have been able to survive and thrive, short term in a fully remote environment, has been our company culture and relationships built over years of face-to-face interactions. In essence, I believe we are drawing from our culture bank right now, and we will eventually need to replace those resources.

As an answer to my own question, I believe we will maintain our offices, and I believe the majority of our employees at my firm will eventually want to return to the physical office with their co-workers, colleagues, and friends. I also believe that we will need to provide the flexibility and tools necessary to allow our employees to balance their time between the office and remote work.■ 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 02 0

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CODES and STANDARDS Structural Design and Coordination of ICC 500 Tornado Shelters Key Considerations and Lessons Learned

By Jessica Simon, P.E., and Andrew Dziak, P.E.

R

ecent media coverage has highlighted the devastation associated with tornado outbreaks in many urban and suburban areas.

Rapid population growth and urban sprawl in many cities within the central United States have increased the number of structures located within the potential path of these dangerous storms. Tornadoes generate high winds and extreme loads that are significantly higher than typical building design loads.

When tornadoes strike in populated areas, the cost can be devastating in terms of injuries, loss of life, and damage to property. The destructive tornado that struck Joplin, Missouri, in May 2011 injured 1,150 people, killed 158, and caused an estimated $2.8 billion in damage – one of the most expensive on record. In March 2020, a pair of devastating tornadoes passed through the Nashville, Tennessee, area, killing at least 24 people and severely damaging or collapsing hundreds of structures.

In 2014, the second edition of ICC 500, Standard for the Design and Construction of Storm Shelters, was co-published by the International Code Council (ICC) and the National Storm Shelter Association (NSSA).

Starting with the 2015 International Building Code (IBC), certain structures are required to be designed with ICC 500-compliant community tornado shelters. This article provides clarity on when an ICC 500 tornado shelter is required per the IBC and shares lessons learned to help practicing structural engineers design safe and effective tornado shelters.

Tornado Rating Scale It is helpful to have a baseline understanding of the Enhanced Fujita (EF) scale to comprehend the requirements of ICC 500. Within the EF scale, tornadoes are rated from EF0 to EF5 based on observed damage that is then correlated back to an estimated three-second gust wind speed. EF0 tornadoes start at an estimated wind speed of 65 mph and can cause damage, including loss of roof covering materials, gutters, awnings, or siding. An EF3 tornado has estimated wind speeds ranging from 135 mph to 165 mph, and damage may include failed roof structures and multiple collapsed walls. The most devastating tornadoes are rated EF5 and carry estimated wind speeds of 200+ mph. These tornadoes can cause complete destruction of engineered, well-constructed structures.

When is a Shelter Required?

Per IBC 2015, Section 423, ICC 500 tornado shelters are required for structures located within the region of the country designated with a 250-mph shelter design wind speed (Figure 1) that meet one of the following criteria: 1) The structure contains critical emergency operations such as 911 call stations, emergency operations centers, fire, rescue, ambulance, and police stations.

2) The structure is classified as a Group E Occupancy, such as a K-12 school, with an aggregate occupant load of 50 or more.

8 STRUCTURE magazine

Figure 1. Shelter design wind speed map for tornadoes. Ref: ICC 500-2014 Figure 304.2(1)

Design Criteria and Systems ICC 500 tornado shelters must be designed for several types of extreme loads. The design standard requires that tornado shelters be designed to sustain wind loads five to seven times higher than a similarly sized, non-shelter building located on the same site. The minimum roof live load for a tornado shelter is 100 psf, up to five times higher than a non-shelter roof.

Storm shelters must also be designed for debris hazard loads, such as wind-borne debris and laydown, rollover, and collapse loading. For wind-borne debris loading, ICC 500 requires that all components on the envelope of a tornado shelter with a 250-mph design wind speed be tested to resist a 15-pound sawn lumber 2x4 missile shot at a speed of 100 mph for vertical surfaces and 67 mph for horizontal surfaces. FEMA P-361, Third Edition, Part B8, is a helpful reference for practicing engineers to clarify debris impact loading and determine minimum wall and roof thicknesses that meet these requirements.

Another type of debris hazard load is laydown, rollover, and collapse loading; however, little code guidance is provided to assist practicing engineers when calculating the magnitude of these loads. Generally, these hazards are defined as structures or components that have a fall radius overlapping the footprint of the shelter. Based on the verbiage in ICC 500, structural engineers must rely on judgment to determine these significant and potentially catastrophic loads. At a minimum, it is recommended the shelter be designed for the weight of any laydown, rollover, or collapse hazard multiplied by an impact factor, the magnitude of which is left up to the engineer’s judgment. Without further code guidance, the authors believe it is prudent to consider an impact factor of no less than 2.0. Further guidance on this topic is being considered for inclusion in the 2020 version of ICC 500.

When determining the best structural system for a tornado shelter, the size of the shelter has a significant impact. For smaller shelters, such as those commonly located in municipal facilities with emergency operations functions, fully grouted concrete masonry unit (CMU) walls may be the most economical option.

For these shelters, a roof system composed of structural steel

beams and concrete-topped composite metal deck has proven to be cost-effective.

However, designing connections to transfer the large roof beam reactions directly into the CMU walls can become difficult. Detailing a concrete ring beam around the perimeter of the shelter roof, integral with the CMU wall, has been a successful way to facilitate more effective steel beam connections to the walls (Figure 2).

For larger shelters, commonly located in K-12 schools, concrete walls are frequently Figure 2. Typical concrete ring beam detail. required. A cost consultant should be engaged to provide preliminary cost estimates to help guide the decision regarding which type of concrete wall system would be most economical for a given project. Total precast concrete solutions can also be an economical solution for larger shelters; however, special detailing must be provided at the precast panel joints, and the diaphragm design and connections require careful coordination with the precast engineer. If pursuing this option, a precast manufacturer should be retained to consult on the project during the design phase.

Lessons Learned Having experience with both municipal and educational projects, the authors’ firm has designed many ICC 500 storm shelters. As a result, the following list of lessons learned may help practicing engineers as they navigate through these provisions.

First, it is wise to purchase a copy of the ICC 500 standard with the commentary included and download a free copy of FEMA P-361, Design and Construction Guidance for Community Safe Rooms. Both documents provide helpful guidance and context for many of the provisions in ICC 500.

Overall Project 1) Ensure the Owner engages a peer reviewer early, and ensure the architect builds time into the project schedule to account for the peer-review process. Within ICC 500, an independent peer review is required for all storm shelters that are mandated by IBC 2015. Reviews are required for structural, architectural, and MEP systems, and many times structural calculations are requested by the peer reviewer. Shelter peer reviews commonly take two to three weeks, and the authors have found success scheduling the peer reviews in conjunction with the 50% Construction Documents (CD) and 90% CD deadlines. A signed peer review report must be submitted to the authority having jurisdiction (AHJ) before the issuance of a building permit, so allowing adequate time for peer review is essential to maintaining the project schedule.

2) Encourage the architect to clarify the occupant load for the storm shelter early. In essence, the occupant load requirements can be

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widely interpreted by AHJ’s for both responsible for the construction, municipal and educational projects, fabrication, or installation of any item and the size of the shelter is signifilisted in the QA plan submit a written cantly affected by that decision as the statement of this responsibility that minimum size is dictated by occuacknowledges the special requirements pant load. and quality control measures they will 3) Encourage early study and compreundertake. These written statements hension of the Quality Assurance of responsibility must be submitted (QA) plan requirements by the to the AHJ, the responsible design full design team. ICC 500 requires professional, and the Owner before that a QA plan be developed and the commencement of any work. included within the construction 2) Require a pre-construction meeting documents. This plan must idenspecifically for the storm shelter and tify all main wind force-resisting include an agenda for this meeting systems and wind-resisting compowithin the project specifications. nents along with the observations, 3) Ensure the third-party testing lab is special inspections, and testing engaged in all conversations related Figure 3. Baffling system example. requirements for these elements. to storm shelter inspections early – In addition to structural elements, they are the eyes and ears on-site in there are architectural and MEP components that fall within the many cases. scope of the QA plan.

4) Require a partial-wall CMU mock-up before the start of overall construction if the shelter perimeter walls are CMU, as this Structural generally results in a much higher quality of construction.

5) Consider incorporating qualification-based selection criteria for 1) Drawing organization is critical – separate storm shelter and certain key subcontractors to ensure prior experience with ICC non-shelter requirements on notes, special inspections, plans, 500. A contractor’s previous experience on an ICC 500 storm and details to keep the shelter requirements clear. shelter increases the level of quality dramatically.

2) Provide an expansion joint around the perimeter of the shelter and avoid supporting elevated framing on top of single-story shelters.

3) Include wall elevations for all perimeter shelter walls on Summary drawings and closely coordinate all architectural and MEP penetrations. Check the capacity of opening jambs early, as jamb When working on a project with an ICC 500 tornado shelter, strive to capacity often governs the design. engage clients early in discussions about storm shelter requirements.

4) Design the shelter for an internal pressure coefficient of GCpi = Encourage clients to build additional time into the project schedule +/−0.55 when calculating wind loads, unless atmospheric pressure for peer reviews and to plan for additional costs in the scope-to-budget change (APC) venting is provided to justify an enclosed building phase. Costs associated with a more robust structure and foundations, coefficient of GCpi = +/−0.18. In the authors’ experience, providspecialty architectural and MEP components, and emergency MEP ing APC venting is challenging and generally not relied upon. systems can add up quickly. Lastly, structural engineers should expect to help lead a more detailed coordination effort during the design and Architectural/MEP Coordination construction stages. Those steps, coupled with a thorough understanding 1) Coordinate with the architect and MEP engineers to ensure of both ICC 500-2014 and IBC 2015, will aid in a smoother missile impact test data has been obtained for all opening design and construction process resulting in a storm shelter protective devices such as doors, windows, and louvers. If a capable of withstanding catastrophic weather events.■ qualified component is not available, then a baffling system must be provided; the design of this frequently falls within the See the STRUCTURE archives (www.STRUCTUREmag.org; structural engineer’s scope. See Figure 3 for a simplified depicuse Storm Shelter as a search term) for previous articles on tion of a baffling system for a wall penetration; remember that ICC-500, including: the storm debris trajectory must hit two missile impact resistant surfaces before entering the protected occupant area.

Tornado Shelters in Schools. Harris, STRUCTURE September 2016 2) Coordinate with the architect and MEP engineers to ensure Hurricane-Driven Building Code Enhancements. Knezevich et al., conduit, electrical boxes, fire extinguishers, and/or other compoSTRUCTURE July 2017 nents are not embedded in perimeter shelter walls such that they Tornado Debris and Impact Testing. Throop et al., compromise the minimum wall thickness required to meet the STRUCTURE May 2018 impact resistance requirements.

3) Coordinate with the mechanical engineer to determine if the shelter will be naturally or mechanically ventilated since this will Jessica Simon is a Structural Project Manager and Quality Assurance Manager affect the size and quantity of penetrations through the envelope at JQ Engineering, LLC. She serves on the National Technical Program of the structure.

Committee for ASCE SEI as well as the Design Practices Committee for the

Construction

National Storm Shelter Association (NSSA). (jsimon@jqeng.com)

1) Develop a standard form to help guide contractors in submitting “Contractor’s Responsibility” statements; it is not common practice in many jurisdictions. ICC 500 requires that all contractors

Andrew Dziak is a Structural Engineer at JQ Engineering, LLC. Andrew is an active member of the NSSA and has worked directly with Jessica to develop the firm’s storm shelter standards and specifications. (adziak@jqeng.com)

10 STRUCTURE magazine

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structural COMPONENTS Wind Load Effects on Canopy Systems

By Medapati Abhinav Reddy

C

anopies can either be free-standing structures or can be attached as a structural component to a main

building structure. They can be situated at an entrance of the building, acting as awnings, or they can be located anywhere along the face of the building up to the roof level.

Canopies are not only used for protection of the entrance from dust and rain but also to increase the aesthetic appeal of the overall structure by either becoming integrated into the building or by highlighting it. Hence, there is a need to economically design the size and shape of the canopy and its connections.

Figure 1. Differing wind pressures between short buildings and high-rise buildings.

Codes governing canopies provide limited information dedicated to the design of canopies. For example, the American Society of Civil Engineers’ ASCE 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, does not differentiate between the different types of canopies and recommends that canopies be designed as “Components and Cladding” structures for wind loads. Without accurate guidelines, structural engineers often overestimate loads acting on canopies and design components with increased size, which may often lead to space constraints and reduce the aesthetic appeal of the overall structure. ASCE 7-16 provides a dedicated section for canopy design for buildings with an overall height of less than 60 feet; however, it does not provide for canopy design for high-rise building structures.

This article discusses the effect of wind loads on the canopy systems and provides special considerations and precautions that need to be taken when designing such systems. Here, canopy systems can be defined as the components related to the canopy itself, to its connections to the wall, and the wall connections to the foundation. This discussion indicates the need for a distinction between the design criteria of canopies for low- rise buildings and for high-rise buildings.

Canopies Canopies are the structures attached to the main structure or buildings, which are often subjected to dynamic loads such as wind, seismic, and snow.

These load combinations predominantly govern the design. ASCE 7-16, for buildings not exceeding 60 feet in height, considers an upper surface pressure and a lower surface pressure on a canopy, acting individually in one case and acting simultaneously in a second case, where these two loads are combined to obtain a net pressure on the canopy.

ASCE 7-16 does not provide separate provisions for the design of canopies for high-rise buildings, and that often leads to a conservative approach of 12 STRUCTURE magazine

overestimating loads. This overestimation of loads happens when trying to determine uplift forces caused by wind loads.

Codes have not yet considered the effect of wind for the design of canopies attached to tall buildings. Structural engineers have been left to apply the same principles of design for both low-rise and high-rise buildings. The location of canopies and the shape of buildings are also critical aspects of design. Canopies situated at the corner of L-shaped or irregular buildings would see an increase in upward wind loads due to the torsional effect of wind at corners. The height of the canopy and the height of the parent wall of the building (i.e., the building wall to which the canopy is attached) is a significant contributing factor in estimating the downward pressure acting on the canopy.

The upper surface pressure on a canopy is a direct downward force on the top of the canopy. This occurs when the wind is obstructed by the face of the wall and travels along the face of the wall, causing a downward force on the canopy. Lower surface pressure is often a combination of uplift caused by the wind and roof uplift (suction) acting on the canopy, which results in an upward force on the canopy. Sometimes, both loads can act simultaneously and result in a combined net pressure acting on the canopy.

For a relatively typical rectangular building, the key difference between canopies for short buildings and high-rise buildings is that, for short buildings, canopies are often at or near the roof level. This makes the attached canopy a part of the roof system and has to be designed for roof uplift pressures as well.

However, for high-rise buildings, the parent wall of the building is much taller than for short buildings, which increases the downward force acting on the canopy, as shown in Figure 1. This consideration is significant because engineers often assume greater lower surface pressures and underestimate the downward forces for high rise buildings.

A canopy is often suspended or supported by cables Figure 2. Illustration of a typical canopy connection to the wall. attached to the free end of the cantilever member

of the canopy, as shown in Figure 2. The use of a cable system is preferable by architects because of its aesthetic appearance.

However, it is a drawback because cables are not capable of resisting compression loads or moments, although they are suitable for resisting tension loads.

Structural engineers generally prefer pipe systems in place of cable systems to mitigate some of these drawbacks. But in most cases, pipe sections are expensive to install and aesthetically not preferred. Instead of relying on a cable to resist the compression force, which it cannot, the canopy end connection to the parent wall is designed such that it resists the moment caused by the upward pressures as well as the downward pressures, as shown in Figure 3.

Parent Wall Design

Surface Cladding Design

Figure 3. Illustration of the location where additional reinforcement is required.

Precautions must be taken such that the parent wall can resist the moment forces transmitted by the connection. The wall is often thin and may not be capable of resisting excess moments from the canopy connection reactions. Thus, additional vertical reinforcement can be provided near the tension face of the wall (generally at the inner face of the wall if the connection is made to the outer face or vice versa) to resist the tension caused by the moment acting on the wall, as shown in Figure 3. The reinforcement must be placed along with the typical wall vertical reinforcement before placing the wall. The length of the reinforcement provided must at least exceed the development length required.

As an alternate procedure, the moment due to the wind loads can be distributed over a length of the wall with the help of the stiffener plates or angles. The stiffener plates could transmit the forces from the moment couple over the length of the wall, thereby reducing the concentration of stresses over a small section.

Member Design

Foundation Design The parent wall-to-foundation dowels must not only be designed for compression loads caused by the weight of the wall but also must be designed for tension loads, lateral loads, and over-turning moments caused by the canopy moment connection to the face of the wall. Also, the eccentricity of the embed plates, used for the canopy connection to the face of the wall, must be considered in the design of the foundation wall dowels.

Cable Design A cable with an angle greater than 45 degrees with the horizontal provides the most favorable condition to resist the downward forces or tension forces caused by wind. Also, the connection at either end of the cable is always pinned.

Conclusions It is important to understand code provisions for canopies, as engineers often underestimate the upper surface loads, overestimate the lower surface loads, and usually design for excessive uplift forces.

Consideration of issues involved with pipe and cable support systems also are essential to adequate design.■ The online version of this article contains references.

Please visit www.STRUCTUREmag.org.

Medapati Abhinav Reddy is a Structural Project Engineer at Brockette Davis and Drake in Dallas, TX. (abhinavmedapati95@gmail.com)

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There is always a limit on the size of the canopy framing members. The main cantilever beams that resist the wind loads need to have sufficient size and thickness to resist the moment caused by wind loads. For this situation, a tapered cantilever beam with varying depth works very well. The cantilever depth can increase linearly from the free end of the member to the supported end, providing the required moment capacity.

Most canopies are mono-sloped; as such, the upward forces increase when the slope increases above 30 degrees. No significant increase in upward wind forces has been observed until the slope of the canopy reaches 30 degrees [Suárez, 2012]. If the canopy is situated at the corner of a building, more wind gets trapped underneath the surface of the canopy, thus exerting an upward pressure.

Many canopy systems in buildings are now designed to accommodate glass cladding at the top surface. These glass cladding systems are extremely sensitive to the slightest deflections. These member deflections are often limited to a Span Length (in inches)/480 ratio (i.e., L/480). The glass panels are often subjected to both downward and upward pressures, which can create fatigue in the glass if not uniformly supported by the framing system members, resulting in localization of stresses.

The design of canopy framing members must consider deflections such that they will be within tolerable limits. Side sway deflections in the members caused by wind or seismic forces are often ignored by structural engineers but must be considered, especially when the cladding on the top of the canopy is glass.

JULY 2020

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structural SYSTEMS Enhanced Wind and Seismic Performance of Tall Buildings Viscoelastic Coupling Dampers

By Michael Montgomery, Ph.D., P.Eng, and Constantin Christopoulos, Ph.D., P.Eng

T

all building designers are increasingly facing challenges related to wind and earthquake-induced vibrations, especially as buildings are built taller and more slender.

Frequent windstorms can cause lateral accelerations, which can result in occupant discomfort. Rarer, more severe windstorms and service level earthquakes (SLE) produce large loads in the structure that have to be resisted elastically by the structural members. A primary cause of these vibra- Figure 1. Viscoelastic coupling dampers. tions is the low levels of inherent damping (the ability of structures of the structure. This is efficient and practical for low rise steel frame to absorb vibrational energy and slow down dynamic vibrations) in buildings, but today’s tall and slender reinforced concrete (RC) buildings taller structures. Furthermore, severe earthquakes can cause distributed behave more like cantilevers under lateral loads. The lateral load resisting damage throughout the entire structure in conventionally designed systems of tall buildings are now primarily formed by vertical structural buildings, putting in question their post-earthquake safety and use. elements coupled together with coupling beams or outriggers, which are deformed and stressed in vertical shear. Those heavily stressed coupling members are ideal locations to configure dampers to add distributed Damping Systems damping to high-rise buildings to reduce wind and seismic vibrations.

Damping systems for tall buildings are classified as distributed damping These features led to the development of the Viscoelastic Coupling systems (typically viscoelastic or viscous) or vibration absorbers (typically Dampers (VCDs) at the University of Toronto in collaboration with tuned mass dampers or tuned sloshing dampers). Vibration absorbers are Nippon Steel Engineering and Kinetica (Figure 1). large masses at the top of buildings that, when “tuned,” transfer a portion VCDs consist of multiple layers of solid viscoelastic (VE) material of the energy from the building structure to the vibration absorber. These sandwiched between and bonded to multiple steel plates. Each consystems are only tuned to the fundamental lateral modes of vibration and secutive steel layer is extended out, connected to the opposite side, and are typically only relied on for reducing frequent wind vibrations. This anchored to the structure. As buildings deform due to lateral vibrations, is due to their reduced effectiveness beyond their tight-tuning range and the solid VE material layers are sheared in-between the alternating because of maintenance requirements, such as monitoring water levels consecutive steel plates, providing instantaneous elastic and viscous or checking waterproofing for a tuned sloshing damper or inspection forces. VE material can be modeled simply as a spring and dashpot in of mechanical components in tuned mass parallel. The force in the VE material, FVE dampers. The most significant overall impact (t), at a time, t, is expressed as FVE(t) = kVE on a project is that they occupy large valuable uVE(t) + cVE u̇VE(t), where uVE(t) and u̇VE(t) are space at the top of buildings. the shear deformation and deformation rate, Distributed viscoelastic and viscous damprespectively, at a time t, while kVE and cVE are ers are activated through relative movements the VE material stiffness and damping coefinduced between structural members when ficients. Figure 2 shows the deformed shape a structure sways under wind or earthof a coupled tall building with the solid VE quake loading. When they are optimally material being sheared vertically and dissipatconfigured and designed, they can increase ing lateral vibrations. damping levels in building structures in Because the solid VE material is rigidly both fundamental and higher modes connected to the structure and the dampof vibration and thus be effective for all ing mechanism does not rely on mechanical dynamic loading conditions. components or pins, the solid 3M™ VE material dissipates energy at the molecular level. It, therefore, provides instantaneous viscoelastic Viscoelastic Coupling response even for very small deformations.

Dampers (VCDs) Tests conducted at the University of Toronto Distributed dampers have historically been showed that the solid 3M VE material used configured in shear-type frame buildings in the VCD provided a viscoelastic response either as braces or vertical damping panels, even for deformations of +/-0.003mm or which are engaged by inter-story racking Figure 2. Coupled wall tall building structural kinematics. +/- 0.00011 inch. In seismic areas, where 14 STRUCTURE magazine

the building could be subjected to a rare Coupling Dampers (VCDs) and ii) a earthquake, whereby drifts can become bi-level Tuned Sloshing Damper (TSD) very large as the amplitude of vibrations tank. The building developer selected is increased, the connecting steel members the VCDs for the project after a detailed are capacity designed to yield. This adds comparative technical and financial even more energy dissipation, and capacanalysis of the two systems. A primary ity protects the remaining structure. This advantage of the VCDs was the fact that unique feature allows the damper to be they were integrated within the strucused efficiently for all loading scenarios, fretural system, resulting in an additional quent windstorms, severe windstorms, and 5,000 square feet of usable penthouse frequent earthquakes through to Maximum real estate. Another consideration was Credible Earthquakes (MCEs). that there was no tuning or monitoring, The solid 3M viscoelastic material was and there was no long-term maintenance the first damping material used in strucplan required for the VCDs to ensure tural applications dating back to 1969 performance. and has been utilized in 300 buildings in The structure used 84 identical modular some of the world’s most severe wind and Viscoelastic coupling damper panels, proseismic regions. There is no requirement duced by Nippon Steel USA and 3M. Each for maintenance or monitoring because VCD consisted of 2-VE damper panels of the excellent aging and fatigue characbolted to cast-in-place steel embeds. A total teristics of the 3M VE material. of 42 VCDs replaced 42 RC beams on 21 At the University of Toronto, the 3M levels of the structure. Figure 5 shows the VE material has been thoroughly tested implementation of the dampers in the projto confirm its mechanical properties, Figure 3. Full-scale uniaxial and shear racking VCD tests at the ect. During erection and concrete casting, which agreed very well with the manu- University of Toronto. temporary steel channels were used where facturer’s stated properties. Also, multiple the dampers were to be installed to ensure full-scale VCD test specimens, manufactured by Nippon Steel the correct VCD placement. After the building envelop was completed, Engineering, have been tested uniaxially, confirming the scalability a small three-person crew of ironworkers removed the channels and of the material properties. Multiple full-scale VCDs have been tested installed the VE damper panels with a slip-critical bolted connection. in two different racking configurations to confirm the overall system Drywall was then installed over the VCDs. performance (Figure 3).

The wind tunnel results required an added 0.9% damping in the fundamental mode of vibration. Each VE damper panel consisted of nine (9) VE material layers, each 5mm-thick, bonded between the steel Examples of Design with VCDs plates. The VE dampers were modeled with a spring and dashpot in The following are projects where structural engineers have used the parallel and configured into the Engineer of Record’s (EOR) ETABS VCDs to create value for their clients, in wind-critical regions, seismic- models. The added damping was assessed using free vibration analyses. critical regions, and combined wind and seismic-critical regions Free vibration is readily implemented by inputting a lateral push to the throughout the world. In all instances, the VCDs have allowed engi- building, holding it there, and releasing the load, and then measuring neers to meet design targets economically and improve the dynamic the reduction in peak cycle amplitude over multiple cycles of vibration. performance of the structure. All buildings were designed and anaDuring construction, the dynamic characteristics of the structure lyzed using typical commercial software, such as ETABS, SAP2000, were monitored with accelerometers as the project progressed; localor Perform-3D. Linear springs and dashpots were used to model the ized displacement measurements of the dampers were also taken to mechanical VE damper behavior. Every VCD project has a robust study the robustness of the damping system and to compare the inQA/QC program provided by the damper manufacturers, Nippon situ building behavior to the models. Monitoring showed that the Steel Engineering and 3M, including VE material tests and full-scale predicted added damping was slightly exceeded during a service level VE damper panel production tests. event (5-year return period). Also, the kinematic behavior predicted by the EOR’s ETABS models was accurate even though the building YC Condominiums, Toronto, ON period was shorter than that considered for the wind tunnel studies, Yonge College (YC) Condominiums is a 66-story, slender residential and the peak modal amplitude of vibration during the storm was tower (11-to-1 slenderness) in downtown Toronto, where limiting only 80 mm (approximately 3 inches). wind vibrations caused by frequent windstorms was a crucial aspect Outrigger VCD Configuration of the structural design. The developer’s mandate for the project was to maximize sellable space within the prescribed architectural height, Recently, the VCD system was used in south-east Asia in nine tall which determined the choice of damping system for the project. buildings (Park Central Towers 1 and 2, Seasons Residence Towers In the narrow plan direction, which was critical for lateral vibration, A, B, C and D, Connors Tower, and ParkLinks Towers 1 and 2), the lateral load resisting system consists of two primary coupled RC where the VCDs are intended to improve both the wind and seismic shear walls, along with an RC core and columns (Figure 4, page 16). response of the structure. The general design approach in these areas is Wind tunnel studies indicated that the building would require a to conduct a conventional code design for wind and non-prescriptive supplemental damping system to be added to increase the damping performance-based design using the PEER-TBI and the Los Angeles in the fundamental mode of vibration to improve the level of human Tall Buildings Structural Design Council (LATBSDC) guidelines. comfort for 1 in 1-year and 1 in 10-year wind-induced motions. Because of the significant wind demands, outrigger flag walls are Two damping systems were considered: i) distributed Viscoelastic commonly used to increase the stiffness of the lateral load resisting JULY 2020

15

Figure 4. VCDs in Yonge College condos.

system to reduce wind drifts. The flag walls consist of heavily reinforced concrete beams spanning over the corridors and framing into an RC wall that runs between two residential units. During rarer design level events, such as service earthquakes and design windstorms, the RC beams are designed to remain linear elastic; during rare earthquakes, those RC beams are designed as structural fuses to prevent overload of structural columns and outriggers. Because of this design intent, the beams end up being very strong to resist wind loads; during MCE events, they introduce large shear forces into the core and large axial forces in the columns. The beams are expected to be mostly damaged because of the significant combined axial and shear demands.

VCDs are introduced connecting the RC flag walls to the columns with ductile detailing of the steel connecting elements, following general requirements prescribed in ASCE-341, Seismic Provisions for Structural Steel Buildings, seismic details for Eccentrically Brace Frames (EBF) steel links. The flag walls and columns are capacity-designed to accommodate expected overstrength in the ductile members (Figure 5, online). A slotted connection is provided in the damper panel connection to allow for differential settlement between the core and the columns if required.

Under more frequent windstorms, the dampers add significant damping to the system and reduce drifts. Under design level wind or service level earthquake events, where the structure is intended to respond linearly elastic or essentially elastic, loads are reduced, resulting in structural efficiencies. Under more extreme events such as Maximum Credible Earthquakes (MCE), the ductile connecting elements can yield reliably, ensuring columns, flag walls, and corridor beams are not damaged.

This results in reduced loads on the RC core and columns because of the added damping and a gentler stiffness transition compared to the RC flag walls. It is also a more reliable ductile mechanism compared to the conventional flag wall RC beams. In addition, because of the increased wind and earthquake performance, a number of developers have also advertised the use of the VCDs to emphasize higher structural performance, garnering an estimated 5% additional revenue.

630-meter Seismic-Critical Building in Southeast Asia A 110-story, 630-meter mega tall building, with a total gross floor area of more than 330,000 square meters designed in a highly seismic region in Southeast Asia, has been redesigned with the VCDs (Figure 6, online ).

Even though it is a mega tall structure, the design of the building was governed primarily by seismic loading using a non-prescriptive performance-based approach with PEER-TBI and LATBSDC guidelines. The primary lateral load resisting system consists of a coupled core wall (RC) and a steel truss outrigger system that connects the core with the super columns. There is a secondary lateral load resisting system consisting of a mega frame comprised of the super columns and belt trusses along with the steel truss outrigger system.

16 STRUCTURE magazine

Due to the importance of the tower in the region, size of the tower, and seismicity of the site, there was a desire to improve seismic performance and reduce the cost of the lateral load resisting system. Two design challenges included the large core shear forces requiring significant reinforcing and large overturning moments requiring extremely deep mat foundations. The redesign consisted of the VCDs replacing about 60% of the diagonal RC coupling beams in the core throughout the height of the building, with the VCDs locations optimized for performance with Perform-3-D results targeting the large shear forces and overturning moments. The VCDs provide significant levels of added damping in the first modes of vibration, which reduces the overturning moments. The VCDs also provide added damping in the higher modes of vibration, which reduces the core shear demands significantly. The upfront financial benefits are substantial, with a significant reduction in structural materials and approximately 6 months of expected construction time savings primarily because of reduced complexity of the reinforcing and member size reduction. In addition, because of the added damping and the fact that the VCDs are replacing structural members that are expecting to be heavily damaged during an MCE event, the VCDs inherently increase the resilience and expected damage for all earthquake levels. Also, the use of the VCD enables the developer to market the resilience improvements that are achieved, which is expected to result in increased property value and revenue.

Looking Forward Tall building design is continuing to evolve, and the VCD is a tool that structural engineers can use to solve dynamic loading challenges as designs push new limits. As a robust wind and earthquake damping system, VCDs can be readily implemented by using tall building structural design tools such as commercial software platforms, wind tunnel testing, and performance-based design, resulting in significant structural performance and cost-effectiveness of high-rises.â– Structural Engineer Project Teams ARUP, Magnussen and Klemencic Associates (MKA), PT Gistama, Read Jones Christoffersen (RJC), SY^2, and Thornton Tomasetti (TT).

Michael Montgomery is the Chief Technical Officer at Kinetica in Toronto, Canada. (m.montgomery@kineticadynamics.com) Constantin Christopoulos is the Canada Research Chair in Seismic Resilience of Infrastructure, Professor at the University of Toronto, and a technical advisor to Kinetica. (c.christopoulos@utoronto.ca)

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structural DESIGN Performance-Based Wind Design What is it, and how is it implemented?

By Roy Denoon, Ph.D., M.ASCE, John Kilpatrick, Ph.D., P.Eng, C.Eng, F.ICE., M.ASCE and Donald R. Scott, P.E., S.E., F.SEI, F.ASCE

T

he recent publication of the ASCE/SEI Prestandard for Performance-Based Wind Design (Prestandard), and

the upcoming publication of a Manual of Practice on Design and Performance of Tall Buildings for Wind prepared by an ASCE/SEI Task Committee, make this an apt time to provide an overview of the intent of these documents, the present state-of-the-art in Performance-Based Wind Design (PBWD), and current efforts to update knowledge.

Figure 1. Rainier Square, Seattle – a test case for performance-based design approaches. Courtesy of Magnusson Klemencic Associates / Michael Dickter.

The Prestandard, in its first edition, provides a roadmap to achieving the wind performance objectives specified by ASCE 7 for structural loads and building envelopes while working outside of common prescriptive procedures. The document was compiled by a working group comprised of structural engineers, building envelope engineers, wind engineers, and academics to provide a wind engineering document to complement the PEER TBI Guidelines for Performance-Based Seismic Design of Tall Buildings. The overall goals of PBWD are to allow more efficient designs that meet performance targets for building functionality while reducing property damage from wind events.

The Prestandard provides a set of procedures that can be followed to show compliance with performance objectives for both strength and serviceability in design. It is laid out with the commentary interspersed among the normative text, a move that was made due to the unique nature of the content and in recognition that some areas still require further research before they can be widely applied.

The Main Wind Force Resisting System (MWFRS) portions of the Prestandard are focused on tall buildings (such as that shown in Figure 1). Tall buildings are the class of structures that have the most potential to benefit from PBWD by allowing some inelastic deformation of limited portions of the structural system under extreme wind loads. The chapter on building envelopes, though, is targeted towards all types of buildings where superior performance is required in extreme wind events, such as hospitals, data centers, and other buildings requiring post-disaster functionality. Apart from the building envelope provisions, all applications of PBWD require wind tunnel generated building-specific wind loading inputs.

The Prestandard provides clear minimum performance objectives and acceptance criteria, with associated mean recurrence intervals (MRIs), for different risk categories of buildings. The performance objectives and associated acceptance criteria are provided for Occupant Comfort, Operational and Continuous Occupancy, Limited Interruption performance objectives for the MWFRS, the building envelope, and nonstructural components and systems.

The Occupant Comfort and Operational performance objectives are evaluated using traditional linear elastic design approaches.

The Operational performance assessments consider drift limits and, importantly, a Deformation Damage Index (DDI), which is a more representative technique for the assessment of racking deformation that is the primary source of damage to internal nonstructural components.

Non-Linear Time-History Analyses (NLTHA) may be utilized for the Continuous Occupancy, Limited Interruption case, to demonstrate that performance objectives are met. The Prestandard outlines three methods by which this can be achieved.

• Method 1 is a deemed-to-comply method based on engineering experience and judgment.

• Method 2 is based on NLTHA of the structure, followed by a conditional probability reliability assessment of the design.

This method provides a slightly more prescriptive approach on how to use NLTHA to validate the design but also recognizes the limitations in current knowledge that may limit its practical use at present.

• Method 3 is based on NLTHA of the structure in conjunction with a dynamic shakedown analysis to evaluate the reliability of the structure. A dynamic shakedown analysis is a very computationally efficient approach that can be applied to PBWD by allowing probabilistic assessment using many time-histories.

Three types of wind tunnel tests are commonly used for the determination of overall wind loads and responses of tall buildings: High-Frequency Pressure Integration (HFPI), High-Frequency Balance (HFB) testing, and aeroelastic testing. The first two of these are what are known as aerodynamic models; they measure the external wind loads applied directly to the model with the dynamic responses calculated analytically after testing. Aeroelastic testing incorporates the structural dynamic properties into the wind tunnel model and directly measures load effects and responses.

18 STRUCTURE magazine

application for PBWD. The HFB approach uses the wind tunnel model as a mechanical integrator with the applied loads measured at the base. While this is a very accurate approach in terms of the overall loads, it does not provide a direct measurement of the distribution and correlation of excitation forces over the height of the building. For many buildings designed using traditional approaches, this is of limited importance. With tall, slender buildings, the overall load effects may be dominated by resonant response, which is a function of the mass distributions and mode shapes.

For PBWD, however, it is necessary to know the distribution of the applied loads. Load distribution can only be measured directly using HFPI. This requirement leads to one of the limitations of the approach. For very tall and slender buildings, especially those with complex geometry, it is not always possible to physically fit sufficient pressure Figure 2. Graph of along-wind and cross-wind responses for a tall, slender building for different wind directions. tubes into the wind tunnel model to accurately capture the simultaneous pressure distributions over the entire building.

One of the critical elements of PBWD for the structural system These very slender buildings are also the type where aeroelastic testing is optimizing the Demand-Capacity Ratios (DCRs) of individual may be needed to capture aerodynamic damping effects. Therefore, key structural components. The techniques that can be used for this composite approaches are likely to be required for particularly tall are similar to those that have been used in long-span roof analyses and slender buildings. where pressure time-histories can be applied directly to the structural Unlike seismic loading, critical wind loads can result from uncormodel based on areas of influence, or influence coefficients can be related excitation mechanisms in multiple directions. For many of the provided by the structural engineer to allow the wind engineer to tall buildings for which the use of PBWD may be most valuable, the quantify critical load effects in members. Of the three test types peak responses may result from cross-wind excitation (also referred described above, only the HFPI approach is amenable to an easy to as vortex shedding) combined with along-wind buffeting. The

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JULY 2020

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Prestandard, to investigate the DCRs of individual members and to determine the critical governing load cases, e.g., serviceability deflections, accelerations, or member strength capacity.

Given the novelty of PBWD, all parties involved in the design and approval of buildings designed using PBWD approaches must be fully aware of the limitations, the risks, and the ramifications of the use of alternate methods. Consequently, independent peer review by qualified professionals is required when using this approach. In these early Figure 3. Standardized CAARC models in the wind tunnel: High-Frequency Balance Model (left) and High-Frequency days, peer review is expected to be Pressure Integration model (right). Courtesy of CPP Wind Engineering. extensive and conducted by a team rates at which the building responds to each of these phenomena of reviewers with individual specialties. The local Authority Having with increases in wind speed are not proportional, and, in the case of Jurisdiction (AHJ) should also be involved from early in the decisioncross-wind response, may exhibit peaks at less than the design wind making process to ensure that design methodologies and reviews will speed. The correlations between wind speed and response can also be acceptable to them. To assist in this process, the Prestandard has vary widely with even small changes in wind direction. Examples of a full chapter on peer review expectations, including scope of work this variation are shown in Figure 2, page 19, where the base moment and guidance on dispute resolution. responses about orthogonal axes are indicated for two wind directions As can be concluded from the discussions above, PBWD may for a tall, slender building. The cross-wind response is particularly not yet be ready for everyday application. The Prestandard has distinct about the x-axis for a wind direction of 310 degrees, where been written with this in mind and specifically highlights current a vortex-shedding peak can be observed at a reference wind speed limitations. Significant progress is, however, being made to create of around 24 m/s, after which the response reduces, before increas- a framework for its use and to fill in the gaps in current ing again at higher wind speeds. Consequently, calculating the total knowledge to facilitate the improved and more efficient probability of exceedance is much more computationally intensive for design of future buildings.■ wind effects than for seismic effects due to the wide variation in building responses to wind.

Recognizing that we currently have minimal experience with the application of PBWD principles to design, a research effort is being conducted under the auspices of the ASCE 7 wind loading subcommittee. Multiple structural engineering firms developed designs for three standardized buildings located in two different wind climates representing New York and Miami. The three buildings are prismatic with two tall towers and one shorter, less dynamically sensitive building.

Pressure time-histories for the buildings were measured in the wind tunnel and will be made available as open-source for reference. Figure 3 shows one of the buildings, the classical CAARC building that has long been used as a reference Figure 4. Comparison of high-frequency balance and high-frequency pressure structure for calibrating and comparing wind tunnels, being integration results for CAARC model. tested using both the HFB and HFPI techniques.

The structural engineers did their preliminary design using Roy Denoon is Vice President and Principal of CPP Wind Engineering. He codified approaches. The dynamic properties from these designs co-authored the CTBUH “Guide to Wind Tunnel Testing of High-Rise Buildings” and were then used in a typical linear elastic analysis using wind edited the Australasian Wind Engineering Society “Quality Assurance Manual for tunnel data to provide updated loads with which the designs Wind Tunnel Testing of Buildings and Structures.” (rdenoon@cppwind.com) were refined. This process followed the typical pattern of wind tunnel testing. A set of results for one set of structural properDonald R. Scott is a Senior Principal at PCS Structural Solutions, Tacoma, WA. He ties is shown in Figure 4, which demonstrates good agreement is also Chair of ASCE 7 Wind Load Subcommittee and Chair of NCSEA Wind between the HFB and HFPI approaches. Figure 4 also shows Engineering Committee. (dscott@pcsstructural.com) the dominance of the cross-wind responses, at 0° and 180°, John Kilpatrick is a Principal at RWDI. John is active in the field of wind engineering relative to the along-wind responses, at 90° and 270°, for an and, from 2012 to 2014, was Chair of the United Kingdom’s Wind Engineering isolated, prismatic tall building of relatively modest slenderness.

Society. (john.kilpatrick@rwdi.com) The structural models and associated pressure time-histories were provided to Dr. Seymour Spence and his research team at All three authors worked together in developing and authoring the ASCE/SEI the University of Michigan. They are currently subjecting the Prestandard on Performance-Based Wind Design. models to shakedown analysis, consistent with Method 3 of the 20 STRUCTURE magazine

A Coastal Community’s Response to a Natural Disaster

Incorporating Resiliency to Coastal Designs in The Florida Panhandle

By Lance Watson, P.E., and Tyler Marsh, P.E. Aerial photograph of Mexico Beach as rebuilding is underway.

H

urricane Michael, a Category 5 hurricane, made landfall along 100-year flood plain. Following Hurricane Michael, it was evident the Florida Panhandle on October 10, 2018, with a direct hit that most structures built in the last 10-15 years, to the specifications between Tyndall Air Force Base and the coastal City of Mexico Beach, recommended in FEMA Technical Bulletin 9, performed as intended leaving a trail of destruction in its path. The damage extended over during the devastating flood event. In most cases, the ground floor sixty miles east and west of the eyewall. walls designed to be frangible were torn The areas affected the most experienced away from the piling supported structures I think most people, including myself, were as the storm surge rose and wave action sustained winds of 161 miles per hour shocked. How could Mother Nature do so (mph), a minimum pressure of 919 milincreased. This allowed for the reduction libars (mb), and a storm surge reaching 19 in surface area in which the hydrostatic much damage in three hours? We had no feet above sea level with additional wave pressure and velocity driven impact were idea at the time that the winds were over action well over this elevation. Hurricane able to act, reducing the overall lateral load Michael was “directly responsible for 16 160 mph and storm surge was over 17 feet. experienced by the pilings supporting the deaths and about $25 billion in damage superstructure above.

Seventy years gone in just three hours. in the United States,” according to a Following Hurricane Michael, local ~Mexico Beach Mayor Al Cathey report prepared on May 17, 2019, by the municipalities reacted with resiliency in National Hurricane Center. mind to prepare future structures and Upon further investigation of the variation of wind and flood damage developments to withstand the extreme effects of similar storm events. in coastal areas like Mexico Beach, Port St. Joe, Cape San Blas, and For example, the City of Mexico Beach has amended the pre-Hurricane Bay County, the overwhelming majority of homes built to the current Michael flood ordinance, which required structures to be elevated 6th Edition of the Florida Building Code (based off the 2015 IBC), above the FEMA 100-year flood plain. The pre-existing flood ordiand within the guidelines set forth by the FEMA regulations within nance required the lowest horizontal structural member (LHSM) to be the 100-year floodplain and ASCE 7-10, survived the storm with elevated to a minimum of 1 foot above the FEMA designated 100-year minor to moderate damage. Among the variety of observed failure flood elevation within the (VE) flood zone (coastal high hazard area modes, the most common within structures built utilizing the current subject to water including wave action). Furthermore, within the VE Florida Building Code were shear wall failures (ranging from inad- flood zone designation, all solid walls below the minimum LHSM equate sheathing thickness to elevation were required to an insufficient number of fasbe break-away (frangible) It was obvious when you walked the streets that the newer teners), complete lack of shear and were not to exceed 299 walls, excessive scouring, and building code was successful. Unfortunately, the majority of the square feet of enclosed space. impact from flood and wind- houses were older style houses that created a domino effect when All structures within the AE borne debris. The overall load the storm surge came, causing extra damage to newer buildings. flood zone designation (stillpaths and uplift connections water flood elevations and ~Mayor Cathey performed very well, confirmwave effects less than three ing that the minimum design feet) were required to have parameters and installation methods required by the current Florida the finished floor elevation (FFE) elevated to a minimum of 1 foot Building Code are satisfactory for structural integrity and overall above the FEMA designated 100-year flood elevation. There was no public safety if the design wind event is met. FEMA’s Technical limit on the square footage of enclosed space below the FFE elevaBulletin 9 establishes a standard for construction of break-away tion within the AE flood zone, and the only requirement was to (frangible) structures built below the base flood elevation within the install flood vents to equalize hydrostatic pressure in the event of a 22 STRUCTURE magazine

We adopted an eighteen-inch freeboard over the 500-year floodplain. It was hard to ignore what just happened. The Council felt comfortable adding this freeboard to the 500-year floodplain because the only maps we have are the “best available” by FEMA, and these flood elevations are still a work-in-progress.

We do not know what the final flood zones will be. I think Mexico Beach did the right thing to try to give property owners the best protection that we could. ~Mayor Cathey

Performance-based design utilizing a moment frame to maximize openings on a new coastal home in Port St. Joe (Gulf County).

non-velocity flood event. Following Hurricane Michael, the City voted stability and overall resiliency of specific building materials as they to raise the freeboard to 1.5 feet above the worst-case flood elevation. plan to rebuild. Some simply want to know what siding is best, and Within the new ordinance, this is considered to be the maximum some want to know whether to use cast-in-place concrete construcof the following: FEMA VE (100-year), FEMA AE (100-year), and tion, insulated concrete forms (ICF), or conventional wood-framed FEMA X (500-year). construction. Others ask if their proposed structure can be designed In many cases, the 500-year “X” flood zone designation governs to a 180-mph wind-speed. As stated above, the implementation of the the required FFE and LHSM elevations, requiring some structures existing Florida Building Code, in conjunction with performance-based to be elevated up to 5-6 feet above the previous flood ordinance design, provides a resilient design approach that will produce strucrequirements. In addition to the new flood ordinance, the City of tures that perform to their intended durability and overall expected Mexico Beach increased the minimum design ultimate wind speed life and performance. to 140 mph as opposed to the 130-mph requirement What we have in Mexico Beach is remarkable, unlike any other that was in place before Hurricane Michael. This wind speed requirement is compatible with Gulf County, just community. Citizens, business owners, frequent tourists, and citizens to the East of Mexico Beach. Although residents were of surrounding communities showed great resiliency to rebuild the extremely vocal in their concerns with the changes to pieces of what was lost and protect what we still have. the flood ordinance due to the associated increase in construction costs, they were very receptive to the idea ~Mayor Cathey of amending the minimum ultimate design wind speed to 140 mph. Additionally, all structures considered to be substantially The primary concerns with the rebuilding efforts in the areas affected damaged, as described within FEMA substantial damage guidelines, by Hurricane Michael are the availability of contractors, skilled workare required to be either demolished and reconstructed to meet the ers, and sub-contractors. It is not uncommon, nearly eighteen months current codes or undergo improvements to bring the entire structure after Hurricane Michael made landfall, for local Builders and General up to current codes.

Contractors to have an eighteen- to twenty-four-month backlog before In many cases, performance-based design is implemented in coastal they can begin a new project. Local Planning and Building departcommunities and other areas prone to natural disasters like hurricanes. ments, as well as the Florida Department of Environmental Protection This methodology considers a design approach that will protect the (FDEP) and other coastal permitting agencies, have adjusted in a comfunctionality and maintain the intended service and use of structures mendable manner to facilitate the influx in new construction. Within to continue to meet the needs of the owners and users. In the wake of a the entire coastal area impacted by Hurricane Michael, residents, as natural disaster, the term resiliency becomes a key factor in rebuilding a well as municipal and governmental entities, have united to make the community. Most property owners that endured the devastation associ- best of the situation presented. Each week, more and more homes ated with Hurricane Michael want to ensure that, as they rebuild their are being completed, as businesses are also beginning to re-open their homes, businesses, and more, all sensible measures are taken to ensure doors to provide much-needed services to this community. that their investment can withstand a similar storm event. Performancebased design presents the ability to meet the aesthetic and functional Conclusion demands of homeowners and developers for the intended use of the structure while remaining within the established budget. The designer Due to the on-going determination and perseverance of the residents has the comfort level of providing a conservative design to suit the and local government agencies within these coastal communities, the wants and needs of the Owner, while also preparing for the unknown, Florida panhandle is on its way to rebuilding with the same whether it be defective building materials, human construction errors, resiliency that the people of this area have shown time and or natural disasters. This is especially important in Hurricane prone time again since October 10, 2018.■ regions that are subject to high wind-speeds and velocity storm surge because the damage is unpredictable and highly variable. This approach Lance Watson is a Vice President of Southeastern Consulting Engineers in can be suitably utilized with other climatic hazards as well.

Gulf County, Florida.

Many of the homeowners and commercial developers have witnessed, Tyler Marsh is a Vice President of Southeastern Consulting Engineers in Gulf firsthand, the destruction that Hurricane Michael left in its path. It is County, Florida. very common to have an Owner ask questions related to the structural JULY 2020

23

I

n the fall of 2016, a project team began to investigate the deterio- exterior elevations, vertically supported on unpainted mild-steel shelf ration, causes, and possible treatments to stabilize and repair the angles that are welded to embedded steel plates in the concrete wall. As limestone cladding panels of the former May Company department the limestone panels sit on the shelf angles, kerf anchors extend from store (renamed the Saban Building). The building was gradually the horizontal leg of the angle and sit in a kerf slot in the limestone being renovated to form part of the new Academy Museum of panels. Mild-steel dowels provide side-to-side panel connections.

Motion Pictures.

Moisture intrusion through open and cracked mortar panel joints The façade rehabilitation team came late to the project. Previous caused corrosion of the steel shelf angles and kerf dowels. Water intrufaçade engineers sion was exacerbated recommended comby the heavy stone plete demolition panels sitting on top and reconstrucof the window boxes, tion of the historic depressing their thin façade utilizing new steel box ‘tails’ and corrosion-resistant causing the top of supports and 100 the windows to percent waterdrain into the wall.

By Maria Mohammed, S.E., John Fidler, RIBA, and David Cocke, S.E. proofing. But the This condition was Academy Museum made worse by some wished for restoration comof the kerfs not welded to the pliant with the requirements steel box heads (as designed) of the Environmental Impact but instead wire-tied through Statement and other planning holes in the metalwork, thus restrictions relating to the Los affording more water ingress Angeles Cultural Historical to the otherwise unprotected Monument’s protected status. support structure. The voluThe Museum saw the consermetric expansion of the steel, vation approach as aligned from the corrosion spalled with its sustainability and portions of the panel edges, preservation objectives; the caused cracks along some old building, after all, was the panel tops and bottoms. Thus, largest object in the Museum’s Original detail of the historic limestone panels and steel window boxes show embedded the out-of-plane restraint of steel angles and kerf dowels into the limestone panels. newly developing collection. the limestone panels was comIn turn, the City’s Office of Historic Resources also deemed a con- promised. Vertical support at heavily corroded shelf angles posed servative approach to the cladding to be most appropriate, since it falling hazard conditions along the exterior of the building. would preserve original materials as well as the historic character and design of the A. C. Martin masterpiece.

Façade Retrofit Work included reviews of previous reports; visual inspection of the limestone panels, interspersed with protruding steel window Helical friction anchors were proposed to secure the limestone panels shadow boxes and frames; field pull-out testing of helical friction to the exterior concrete walls and replace parts of damaged panels anchors; exploratory openings to observe concealed conditions; in-situ. However, helical anchors have never before been used in corrosion potential assessments; the design of an amended ASTM this application in the City of Los Angeles. The true capacity of the E488 engineering laboratory test to account for the performance of anchors and the connection assembly were determined through mateheadless friction anchors; a trial mockup swing-stage installation of rial testing, following the requirements of ASTM E488 Standard Test replacement panels and Dutchmen; negotiations with Los Angeles Methods for Strength of Anchors in Concrete Elements. Additionally, Department of Building and Safety; the production of full construc- since the material properties of limestone panels used in such applition documentation; and, construction cations were not available, additional administration support services. hydraulic dilation and temperature tests were performed on limestone samples to determine the behavior of the stone Existing Façade Condition when exposed to water or excessive heat.

The historic Saban Building’s exterior The testing procedure mimicked the walls consist of steel beams and colinstallation of helical anchors into umns encased in reinforced concrete, existing configurations of the limestone infilled with reinforced concrete walls. panels with grout backing on the conThe façade consists of 3-inch-thick limecrete wall. Tapping tests of the existing stone panels, approximately 4 feet wide panels determined unbonded grout backby 5 feet tall, each weighing up to 900 ing in some cases. Thus, a worst-case pounds, with 1 inch of grout backing scenario was used for the testing, assumand joints with Type N and S mortar. ing that the grout was unbonded and/ Limestone panels consist of vein-cut Limestone panel edges located along corroded steel shelf or cracked and made no contribution to panels and cross-cut panels across all angles were spalling and cracking. the cladding stability. In place of grout

Conservation of a Historic Façade

24 STRUCTURE magazine

Table of statistical analysis of Helifix anchor testing results.

Cross-Cut Shear

Cross-Cut Tension

Vein-Cut Tension

Mean (lbs)

716

631

725

COV

31.4%

35.3%

16.9%

Table of design capacities of Helifix anchors.

Test rig assembly shows shear loading on the helical anchor installed through the limestone panel. Courtesy of Specialized Testing Inc.

backing for the test, Teflon slip membranes were placed at the interface between the limestone and the concrete panel so that the true tensile and shear capacity of the helical anchors without the grout backing was tested. Matching Cordova Shelly Limestone panels were obtained from the same Texas quarry system as the original limestone panels and used throughout the testing to mimic the extant façade behavior closely. Pre-test assessments took place to determine which drift (bed orientation) of cross-cut or vein-cut stone would perform the weakest and whether dry or wet saturated stone affected test results; the worst case was adopted for full testing.

ASTM E488 is used for testing anchors with bolt heads, but the project team’s chosen anchor was a headless helical friction anchor that

Cross-Cut Shear

Cross-Cut Tension

Vein-Cut Tension

Lowest Measured Capacity (lbs)

378

378

528

Factor of Safety

4

4

4

Allowable Design Capacity (lbs)

94.5

94.5

132

could not ordinarily be gripped by standard ASTM test apparatus.

Instead of pulling the anchor out of the limestone and the concrete wall, the team decided to pull the limestone off the anchor embedded in the concrete. To do this, the testing laboratory devised and fabricated “steel shoes” to hold the limestone panels that were then pinned to the concrete with the helical friction anchors extending through a hole at the bottom of the steel shoes. The steel shoes were then pulled off the concrete wall panel to mimic tensile loading in the anchors due to out-of-plane seismic loads or were pushed sideways to mimic shear loading in the anchors due to gravity loads.

Both cross-cut and vein-cut limestone panels were tested in shear and tension and, to account for edge distance of the anchors in varying

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The newly restored façade.

Countersunk anchors blend in with natural voids on the stone surface.

sizes of full limestone panels or Dutchmen, 16-inch and 8-inch square panels were used for both tensile and shear tests. The compressive strength of the concrete slab panels used for testing matched the lowest compressive strength of the existing concrete walls. Before the tests, the concrete panels were manually cracked to achieve more realistic cracked concrete behavior.

Once enough tensile and shear tests were performed, results were presented to the Los Angeles Department of Building and Safety’s Building Research Section for approval. The coefficient of variation of the test results was determined to be high enough that the lowest measured capacity of the anchors was to be used as the appropriate anchor capacity. A factor of safety was then applied to the appropriate anchor capacity to determine the required number of anchors for each limestone panel.

Once the tensile and shear capacities of the helical anchors were determined, helical anchor patterns were designed for the weight and seismic load of each of the approximately 1,200 panels throughout the building façade. The patterns were selected based on the size of the limestone panels and Dutchmen. Depending on the location of the limestone panels, the anchors provide either out-of-plane restraint only, or both out-of-plane restraint as well as gravity support. At the former group of limestone panels, gravity support is provided by existing concrete curbs. Given the different anchor capacities for cross-cut and vein cut limestone panels, exterior elevations were prepared to map the type of each limestone panel across the existing façade. The types of panels were identified as vein-cut or cross-cut through close visual inspections of the natural voids in the existing limestone panels.

Anchors were installed through pilot holes drilled through both the limestone and concrete wall. Once countersunk, the final location of the anchors blends in with the natural voids on the limestone surface.

Conclusion

Anchors tested to shear failure showed localized crushing in the limestone in shear.

Courtesy of Specialized Testing Inc.

The limestone façade of the historic Saban Building was one of its most prominent features. The preservation of the façade in place of full replacement contributes to the building’s participation as a museum piece as well as the new home of the Academy Museum of Motion Pictures. The carbon footprint of the adaptive reuse project was significantly reduced with the preservation of the façade as opposed to a replacement scheme, and the falling hazard from damaged limestone panels was addressed through an innovative mechanism where the solution blends in with the natural feature of the façade.

Over 88 percent of the total historic surface area was retained and repaired, while only 12 percent was proposed for the replacement to match the original historic design. Of the original 1939 Texas Cordova Shelly Limestone, out of approximately 1,200 panels: 95.7 percent of panels were retained, cleaned, and repaired; 4.3 percent of panels were repaired by Dutchmen; 1.5 percent were replaced to match existing; and, approximately 40 percent of panels received minor mortar patch repairs. At the Steel Window Boxes and Frames, 100 percent of the panels were retained and repaired.■ A previous version of this paper was published in the 2019 SEAOC Convention Proceedings.

Maria Mohammed is a Project Engineer with Structural Focus in Gardena, CA. John Fidler is with John Fidler Preservation Technology in Los Angeles, CA.

The pattern for helical anchors installed in Dutchmen and partial size panel.

26 STRUCTURE magazine

David Cocke is President of Structural Focus in Gardena, CA.

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building BLOCKS Steel Solution for Envelope Missile Impacts

By Marc S. Barter, S.E., P.E., and Roger A. LaBoube, Ph.D., P.E.

W

hen a major tornado happens, it is all over the news. And, every year, the average person may recall hearing about a dozen or so tornado events, if that. So it might be startling to know that, on average, the number of tornadoes that touch down each year in the United States, according to www.ustornadoes.com, is more than 1,200.

And then there are hurricanes. While fewer in numbers – approximately seven hurricanes strike the U.S. every four years, according to the National Oceanic and Atmospheric Administration (NOAA), and while limited in terms of areas effected, hurricanes are often devastating in terms of loss of life and property and typically last for days instead of minutes. Tornadoes are more likely to cause death due to the lack of warning and the inability of buildings to resist wind forces. The building codes do not mandate that all structures be designed for tornadic wind pressures, only those designated as shelters or safe rooms.

Some states, such as Alabama, now require all new schools and state college buildings to have tornado shelters attached or included within the building. In the case of elementary, middle, and high schools, this mandate is financially burdensome as construction budgets are tight.

Roof systems in these buildings are typically framed with bar joists and metal deck or cold-formed steel trusses. Changing construction methods for the shelter increases costs. It is financially beneficial to stay with steel.

AISC’s new Design Guide 35, Design of Steel-Framed Storm Shelters, summarizes up-to-date design requirements and guidance to incorporate storm shelters or safe rooms using typical industry-standard structural steel products and materials. The design guide presents a discussion regarding storm shelter design for both tornado and hurricane-force winds. Previous roof decking missile impact tests performed at Clemson University considered the performance of the screw attached to bare deck alone. The bare deck was required to absorb all of the energy created by the missile impact. This was not an economical material or installation solution. This article focuses on a steel industry effort to develop an improved solution for protection from missile impacts resulting from tornados.

criteria for wind speed determination and windborne debris criteria.

However, FEMA opts to use the term “safe room” because the design guidance is intended to provide “near-absolute protection” from extreme wind events.

A storm shelter will typically be either an interior room within a building (Figure 1) or a designated wing of a building. However, the concepts presented in Design Guide 35 may also Figure 1. Restroom St. Louis be employed for standalone structures International Airport. or retrofitting of existing structures.

While both ICC and FEMA address both community and residential shelters, Design Guide 35 focuses on community shelters.

Test Protocol

Design Guide 35 summarizes existing test standards. All shelter envelope components must be designed for the impact of windborne debris as evaluated by the debris impact test of ASTM E1886, Standard Test Method for the Performance of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems Impacted

by Missile(s) and Exposed to Cyclic Pressure Differentials. ASTM E1886 defines the test apparatus and test missile, whereas ICC 500 defines the pass/fail criteria.

For tornado shelter design, the test missile is nominally 15 pounds.

The test missile can be any common softwood lumber species, as defined by the American Softwood Lumber Standard, PS 20. The lumber must be grade stamped No. 2 or better and be free of splits, checks, wanes, or other significant defects. Also, the bow or warp of the missile must be such that stretching a string or wire on the board from end to end is within 0.5 inches of the 2x4’s surface over its entire length. Both defects and bow may affect the performance Order of Magnitude of the missile, resulting in the missile absorbing some of the impact, The primary difference in a building’s structural system when designed thereby reducing the force applied to the test specimen. for use as a storm shelter or safe room, as compared to conventional Roof and wall surfaces are delineated based on their inclination construction, is the magnitude of the design from horizontal. Vertical surfaces of the shelter wind forces and the need to withdraw impacts Table 1. Speeds for tornado shelter missile. envelope, i.e., walls, are defined as surfaces of windborne debris. Safe rooms and storm inclined more than 30° from the horizontal.

Design Wind Missile Speed and Shelter shelters are designed to resist higher intensity In contrast, surfaces inclined less than 30° Speed, mph Impact Surface wind speeds, which correspond to higher wind from the horizontal, i.e., roofs, are treated as 80 mph vertical surfaces pressures than buildings designed for typical horizontal surfaces.

130 53 mph horizontal surfaces occupancies, including essential facilities, as A tornado test missile is assumed to impact well as windborne “missiles.” (Note that it is the test specimen at a designated speed, as 84 mph vertical surfaces 160 important to understand that these two criteria summarized by Table 1.

56 mph horizontal surfaces are not concurrently occurring design events.)

ICC 500 Chapter 8 defines the impact loca90 mph vertical surfaces ICC 500, Standard for the Design and tions for wall, roof, and openings, i.e., doors and 200 Construction of Storm Shelters, and FEMA windows.

The impact locations vary with the test 60 mph horizontal surfaces P-361, Safe Rooms for Tornadoes and assembly configuration, as illustrated in Figure 2.

100 mph vertical surfaces 250 Hurricanes: Guidance for Community and For any roof or wall construction, no more 67 mph horizontal surfaces Residential Safe Rooms, employ the same than three impacts are to be made on any one 28 STRUCTURE magazine

test specimen. Where more than three impacts are required, multiple identical test specimens are to be used. ICC 500 defines the pass/fail criteria as follows: • Any perforation of the interior surface of the tested component of the shelter envelope by the missile constitutes failure.

• Specimens or load-bearing fasteners shall not become disengaged or dislodged during the test so as to endanger occupants. The pass criterion is defined as specimens or fasteners failing to penetrate a witness screen comprised of #70 unbleached kraft paper located within five inches of the interior surface of the test specimen.

• The permanent deformation of an interior surface of the test specimen shall not exceed three inches.

• Excessive spalling shall not occur, if applicable.

Figure 2. Typical Test Specimens (Excerpted from the ICC 500: ICC/NSSA Standard for the Design and Construction of Storm Shelters: Copyright 2014. Washington, D.C.: International Code Council.

Reproduced with permission. All rights reserved. www.ICCSAFE.org.)

Industry Impact Tests The goal of a 2016 industry-supported missile impact test program was to assess the performance of more cost-effective assemblies than had been previously tested. The 2016 tests were performed at Texas Tech’s National Wind Institute Debris Impact Facility and utilized common steel construction methods and materials (Figure 3): • 18 and 20 gage, 1.5-inch-wide rib steel metal deck (commonly referred to as Type B deck) • 12K5 open web steel joists • HSS 6×3×1⁄8 • Nail base insulation consisting of 3-inch polyisocyanurate with 1-inch spacers and 5⁄8-inch CDX plywood.

Five test series were performed: • Series 1 – 20 gage decking supported on open web steel joists • Series 2 – 18 gage decking supported on open web steel joists • Series 3 – 18 gage decking supported on HSS • Series 4 – 18 gage decking supported on open web steel joists • Series 5 – 18 gage decking supported on HSS Series 1, 2, and 3 were exploratory tests. The test missile penetrated the Series 1, 20 gage decking; thus, subsequent testing focused on the 18 gage decking. Series 4 and 5 were duplicate tests to verify the performance for the 18 gage decking.

Design Guide 35 summarized the research recommendations.

Based on the test performance, Series 4 and 5 were deemed to be adequate for horizontal (roof assemblies with a slope of 30° or less) applications for design wind velocities of 250 mph. Series 4 assembly was deemed acceptable for vertical (wall or roof assemblies having a slope over 30°) applications for design velocities of 250 mph. Series 5 assembly was also deemed acceptable for vertical applications if a minimum 5⁄8-inch-thick gypsum board was employed as an interior finish. A screw dislodged during the test, but the kraft paper was not in place during the tests. The

researchers judged that the gypsum board would be adequate to capture the screw that dislodged. Typically, the gypsum board would be required to achieve the building code required membrane fire protection.

Design Guide 35 addresses the most current requirements and considerations for storm shelter and safe room design. It should prove to be an invaluable resource and push your next shelter design project to be as safe and cost-effective as possible. The design guide provides the design engineer with alternative economic systems for inclusion in a tornado shelter design. For those projects that employ steel bar joists, metal deck, and an insulated nail base, it allows a tested assembly to be incorporated into the overall building design.

Schools utilizing masonry walls, steel bar joists, and metal roof decks can be designed with the same materials throughout. Changes in the area of the building designated as a tornado shelter consist of using the same tradesmen to increase the robustness of the construction.

This is accomplished through the closer spacing of bar joists, the use of slightly heavier metal deck, and thicker masonry walls with more grout and reinforcing steel. This approach compares favorably with the use of alternative construction, which may require additional subcontractors and tradespeople.

Steel has the properties conducive to the utilization of steel-based roof and wall assemblies to achieve economical shelter design. Engineers have always been able to design these structures for strength, supporting their designs with calculations for bending stresses, shear, and pullout and pullover of fasteners. The missing link, so to speak, was the reaction of the assemblies to missile impact. With the recent testing for missile impact that utilized the entire roof and wall assembly to absorb the energy, economical steel-based shelter design can be technically justified. AISC and its industry partners have given the design engineer the tools needed in the form of Design Guide 35. The guide is available at www.aisc.org/dg, where you can also access AISC’s entire library of Design Guides.■ Marc Barter is President of Barter & Associates, Inc., a structural engineering firm in Mobile, Alabama. (mbarter@barterse.com)

Figure 3. Typical test specimen. Courtesy of Ken Charles and Texas Tech.

Roger LaBoube is Curator’s Distinguished Teaching Professor Emeritus of Civil Engineering and Director of the Wei-Wen Yu Center for Cold-Formed Steel Structures at the Missouri University of Science and Technology. Roger is active in the American Iron and Steel Institute’s Committee on Framing Standards and the Committee on Specifications. (laboube@mst.edu)

JULY 2020

29

structural PERFORMANCE Roofs of Major Logistic Centers Is the Wind Blowing in the Right Direction?

By Rafik Gerges, Ph.D., P.E., S.E., Guangle (Tyler) Xu, P.E. and Mohan Cheng

T

he ASCE 7-16, Minimum Design Loads for Buildings and Other Structures, has been published in accordance with

the International Building Code (IBC 2018), incorporating updates regarding wind load calculations from ASCE 7-10.

This article relates to wind uplift on flat and gable roofs of major logistic centers with slopes ≤ 7 degrees and buildings

Figure 1. Basic wind speed (mph).

≤60 feet in height. The article focuses on the wind uplift loads on the roof elements of joists and girders. For joist wind uplift loads, the method of Components and Cladding in Chapter 30 of ASCE 7 is adopted. For girders, considering the effective wind area is larger than 700 square feet for typical major logistic centers, the Main Wind Force Resisting System (MWFRS) method in Chapter 27 is adopted.

The wind load updates can affect the design of roof joist and girder elements. The sizes and bracing required can be reduced if the wind uplift loads are smaller. The updates are examined by selecting multiple cities in the U.S. and comparing the results. A case study in the city of North Las Vegas is also presented to show the influence on the design of roof joists and girders.

The first significant change is the zone divisions. Another major change is the external pressure coefficients with respect to the effective wind areas. Figure 2 shows the comparison of external coefficient values between ASCE 7-16 and ASCE 7-10 at the center zone. In Figure 2, if the effective area is 500 square feet, the value can be reduced by 40% and, when the area becomes more, the reduction can be up to 60%.

Basic Wind Speed Comparisons

Joist Framing Wind Uplift and Pressure Comparisons

In ASCE 7-16, the basic wind speeds are updated for risk category II buildings, and many cities see a significant reduction. Figure 1 summarizes the differences for risk category II buildings in major U.S. cities.

The values are derived directly from the maps, and special wind zones are not considered. For Houston, Miami, New Orleans, and New York City, the basic wind speeds remain the same. For the remaining major U.S. cities, the basic wind speeds are decreased by 1.7% to 11.8%.

The roof External Pressure Coefficient (GCp) comparison in this article is only limited to ASCE 7-16 Figure 30.3-2A, which is updated from ASCE 7-10 Figure 30.4-2A, for components and cladding of buildings with height less than 60 feet and gable roofs with θ ≤ 7 degrees.

The method used to calculate wind uplift and downward pressure for the design of joist framing is adopted from ASCE 7-16 and 7-10 Chapter 30, Wind Loads: Components and Cladding. The comparison is based on an effective wind area of 400 square feet. For ASCE 7-10, the building type selected is Building Enclosed. For ASCE 7-16, two building types are considered, Building Enclosed and Building Partially Enclosed.

Figure 3 shows the roof joist wind uplift pressure ratio between ASCE 7-16 Enclosed Building and 7-10 Enclosed Building. Note in Figure 3 that all the cities see a significant decrease of 10% to 40% due to the reduction of external pressure coefficients.

Figure 4 shows the roof joist wind uplift pressure ratio between ASCE 7-16 Partially Enclosed Building and 7-10 Enclosed Building. Due to

Figure 2. External pressure coefficient comparison for center zone

Figure 3. Roof joist wind uplift pressure ratio.

Roof External Uplift Pressure Coefficients

30 STRUCTURE magazine

Figure 4. Roof joist wind uplift pressure ratio.

Figure 6. Roof girder wind uplift pressure ratio.

the higher internal pressure coefficient, Charleston, Houston, Miami, New Orleans, New York City, and Philadelphia see an increase from 5% to 27%. However, Atlanta, Boston, Chicago, Columbus, Dallas, Denver, Las Vegas, Los Angeles, Memphis, Phoenix, Portland, San Antonio, and Seattle still see up to 15% reduction.

Why compare ASCE 7-16 partially enclosed to ASCE 7-10 enclosed? For many buildings, structural engineers consider “enclosed” as the default. However, “partially enclosed” can give more flexibility when considering future uses of a building. A minimal increase in the pressure ratio between enclosed and partially enclosed may provide additional options by using ASCE 7-16 partially enclosed.

Figure 5. Roof girder wind uplift pressure ratio.

roofing, plywood, rafters, sprinklers, and member self-weight. No miscellaneous loads are considered.

Figures 7, 8, 9, and 10 (page 32), show four different comparisons of roof element wind net uplift. They are joists under ASCE 7-16 Enclosed Building vs. 7-10 Enclosed Building, joists under ASCE 7-16 Partially Enclosed Building vs. 7-10 Enclosed Building, joist girders under ASCE 7-16 Enclosed Building vs. 7-10 Enclosed Building, and joist girders under ASCE 7-16 Partially Enclosed Building vs. 7-10 Enclosed Building. The values of the case study reflect the analysis presented above.

For the center zone, the joists see a reduction for both comparisons.

There is a significant reduction for girder joists when comparing ASCE 7-16 Enclosed Building and 7-10 Enclosed Building and slight increase between ASCE 7-16 Partially Enclosed Building and 7-10 Enclosed Building. continued on next page

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Per ASCE 7-16 and ASCE 7-10 30.2.3, component and cladding elements with a tributary area greater than 700 square feet shall be permitted to be designed using MWFRS provisions. Assuming the roof girders have a tributary area larger than 700 square feet, which is very common for industrial buildings, the roof uplift pressure calculation can be based on Chapter 27, Wind Loads on Buildings-MWFRS. Figure 5 shows the roof girder wind uplift pressure ratio between ASCE 7-16 Enclosed Building and 7-10 Enclosed Building. From Figure 5, note that Houston, Miami, New Orleans, and New York City have the same value, whereas all the rest of the cities see a decrease of 5% to 15%.

Figure 6 shows the roof girder wind uplift pressure ratio between ASCE 7-16 Partially Enclosed Building and 7-10 Enclosed Building.

It can be concluded that due to the higher internal pressure coefficient, all the cities see an increase.

Case Study A case study in the city of North Las Vegas demonstrates the comparison of wind uplift pressure for roof elements, including joists and joist girders. The results shown are wind-net-uplift pressures using a load combination equal to 0.6D-0.6W. The dead load is taken as 8.5 psf for a joist and 10.5 psf for a joist girder considering

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JULY 2020

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Figure 7. Joist net uplift – ASCE 7-16 enclosed building. Joist net uplift – ASCE 7-10 enclosed building.

Figure 8. Joist net uplift – ASCE 7-16 partially enclosed building. Joist net uplift – ASCE 7-10 enclosed building.

Figure 9. Joist girder net uplift – ASCE 7-16 enclosed building. Joist girder net uplift – ASCE 7-10 enclosed building.

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Figure 10. Joist girder net uplift – ASCE 7-16 partially enclosed building. Joist girder net uplift ASCE 7-10 enclosed building.

The reduction of net wind uplift on roofs could reduce the joist and joist girder cost by cutting down the bottom chord size and joist brace spacing. For the case study project in the city of North Las Vegas, comparing ASCE 7-16 Enclosed Building type with 7-10 Enclosed Building type, the roof joist net-uplift in the major center area is reduced from -14.4 psf to -6.2 psf. The bottom chord of the joists will see significantly less compression forces. The joist bottom chord size can be reduced from LL 2 x 0.203 to LL 2 x 0.145, and joist brace spacing can be relaxed to 12 feet from 10 feet. The reduction in the brace quantity will reduce the cost for the braces from the manufacturer and the install cost due to fewer braces.

The steel takeoff and cost reduction will depend on project location, the nature of the project, and contractors.

The numbers above are the estimated impacts for the case study project, applicable in this article only.

Conclusion As noted earlier, ASCE 7-16 basic wind speeds are updated for risk category II buildings. Wind speeds have remained the same or have been lowered for the major U.S. cities studied in this article.

It is also noted that, within the center area of the roof, the external uplift pressure coefficients from ASCE 7-16 are reduced significantly, by up to 60%, for the major center zone, as shown in Figure 2. The reduction of the external uplift pressure coefficient, combined with a reduction in basic wind speed, yields a significant reduction in roof wind uplift for many U.S. major cities. Results of the roof net uplift case study in the city of North Las Vegas illustrate the potential for significant savings from wind load reductions for both joists and joist girders and, as a result, lower overall construction material cost.■

The authors are with HSA & Associates, West Covina, CA. Ralph Gerges is a Principal with the firm, Guangle (Tyler) Xu is a Senior Project Engineer, and Mohan Cheng is a Senior Structural Designer.

32 STRUCTURE magazine

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CASE business practices My Project is in a Flood Zone… What Do I Do? Business Practice Tips for the Structural Engineer

By Kevin H. Chamberlain, P.E.

L

ong before Cheryl’s “she-shed” was struck by lightning... my shed was destroyed by a rotted oak tree blown over during Hurricane Irene. My home and office were without power for several days. All the while, calls were coming in from clients to evaluate the damage to their buildings.

Although I had been engineering structures in flood zones for many years by then, when you are personally and professionally affected by storm damage, it makes a lasting impact.

Most structural engineers will, at some point, take on a project that involves designing a building located in a flood zone. Such projects require an understanding of the technical engineering issues involved with the site topography, geology, sources of flooding, and how wind and water will affect the proposed building structures and foundations. Beyond that, structural engineers need to understand the business practice issues associated with such projects, including client evaluation, assessing the risk/reward curve, and knowing when to ask for help or pull the plug.

Moreover, an emergency management plan for your firm is time well spent to make sure your clients are taken care of following a disaster, and your business can operate.

• Pull up a flood map for every project.

Flood prone areas for a community are delineated on a series of Flood Insurance Rate Maps (FIRMs) published by FEMA. The current FIRM for a given property can be accessed on FEMA’s Map Service Center website, free of charge, either using the street address or latitude/longitude (https://msc.fema.gov/portal/home). It is easy to print an 8½ x 11 excerpt of the flood map showing the property you are interested in – called a “FIRMette.” The printout is free and only takes a few minutes once you have the property location.

The FIRMette is also a rough doublecheck that the civil engineer is showing any flood boundaries and flood elevations accurately on the site plan. If there is a discrepancy, ask.

• Are you practicing in your area of competence? If you have never designed a foundation subject to scour or had to calculate wave forces, are you comfortable going it alone? What about a submerged basement design; what should the design water table be based on, flooding or 36 STRUCTURE magazine

groundwater? The project’s geotechnical engineer can provide crucial guidance on erodibility, liquefaction potential, and soil permeability and its effect on the design water table for a flood event. Do your foundations constitute an impermissible obstruction in a V zone? (“V” stands for velocity. V zones are Hurricane Irene barrels towards coastal buildings in Fairfield, CT, August 2011.

DO design the basement as an “inverted Coastal High Hazard Areas – essentially, bathtub” (like a bathtub but with the they are areas of high energy flooding with water on the outside) for the full effects of wave action.) A coastal engineer expert hydrostatic pressure and buoyancy. This in modeling storm events, wave runup, will mean using a mat slab foundation scour depth, and calculation of pressures for the basement floor instead of a thin may also be a team member you will want. slab on grade. ALWAYS have a fail-safe in Speak up about what you are knowledgecase water levels are higher than anticiable about and what areas of the project pated. For low-rise light-frame buildings, warrant additional consultants. the building and foundation together • Do not over-promise. As with earthwill not weigh enough to resist buoyquakes, you are designing to minimum ancy. Thickening concrete to add mass is code requirements for structural integrity usually counterproductive. Anchor the and life-safety. That does not mean a foundation walls and mat slab into the structure will not sustain damage. Some ground using rock or soil anchors. Or, damage is all but guaranteed. For instance, provide hydrostatic relief through opena building supported on deep foundations ings in the slab with raised rims that will will still be standing after a flood scours allow floodwaters to overtop the rim to the soils from underneath it, but then the relieve pressure once the maximum design crater left behind will need to be filled in. water table height is exceeded. The last Breakaway walls and slabs on grade will thing you want is for the foundation to breakaway and need to be replaced. There heave and fail from hydrostatic pressure, will be a loss of landscaping and hardscapbecause such a catastrophe may prove ing. Roofing and siding materials may be fatal for the building – and your liveliblown off the building. Water and moishood. When you explain to the client all ture can damage finishes. And, incoming the weak links involved in submerging a utilities may be severed. Flood insurance basement, the proposed basement will go will cover some but not all damage. Be sure away – if you are lucky. to temper the Owner’s expectations with a • Higher is better – if you can. An owner dose of reality. and their designer can choose to elevate a • When the client insists on having a building higher than necessary. Often, in basement . . . even when it is a really bad coastal communities, the design team is idea. When a basement is permitted in a simultaneously working to fit in the maxiflood zone, do not count on pumping to mum number of stories possible under avoid designing for hydrostatic pressure the zoning height limit, so you may not and buoyancy. Pumps will eventually fail, get much higher than the minimum. That as can backup power sources. Although also means you will be pressed to minibasements in limited circumstances are mize structural depth to fit everything in. permitted in flood zones, they are expen• Know the FEMA Technical Bulletins. sive, risky, and usually a bad idea. When Although not law, FEMA publishes a the flood regulations allow a basement, and the project must have one, NEVER series of Technical Bulletins to explain count on sump pumps to depress the various common design and construcwater table and keep the basement dry. tion issues for buildings in flood zones.

The bulletins are written by FEMA’s staff and consultants and are important guidance on how to interpret and implement FEMA’s model regulations adopted by local jurisdictions. Perhaps more importantly, the Technical Bulletins are a window into how the jurisdiction, and FEMA itself, will judge your design.

• “Yes, but so-and-so said we could do it,” doesn’t take you off the hook. Have you received a schematic design from an architect that shows something noncompliant? Like a solid foundation in a V zone? An addition that is set below the BFE to match the existing building?

(BFE = Base Flood Elevation, which is typically found on the FIRM). Are you told that deep foundations are not required when you know your building sits on erodible beach sand? Raise the issue. Most communities that participate in the National Flood Insurance Program (NFIP) are on top of their game and are the gatekeepers preventing noncompliant buildings from getting built by owners inclined to push the limits.

Maybe the community is a small town with part-time zoning and building staff not in tune with flood-resistant design.

If you are told “shut up and stamp it”

– do not do it. FEMA takes violations of the NFIP very seriously, and they do not hesitate to bring enforcement actions against participating communities. If your Owner has to retrofit their new building to appease FEMA (or their flood insurance provider), that Owner may come looking for a pocket to reach into; avoid letting it be your pocket.

• Non-residential projects have more leeway. There are a few reasons for this.

Commercial or institutional projects typically engage a full design team, with a project architect, structural engineer, and other consultants. Single-family and townhome-style residential projects may not have a structural engineer or even an architect. That is one reason, for example, why a basement below the BFE is not permitted for a residential project but is permitted for a mixed-use or non-residential project.

Another factor is the risk associated with the building’s use. If an office building or store is flooded, people cannot shop or go to work; if residences are flooded, people may be displaced from their homes.

• What about your business? If you practice in a region of the country subject to flooding, it is not just your projects you have to worry about. Is your office located

in a flood zone? How about your employees’ homes? When a disaster strikes, can they get to work? Will you be able to respond to your clients? Power may be out for some time; does your office need backup power? Are your files kept in a basement? Being prepared for a flood could be crucial to your business’ survival.

Plan ahead.

• When in doubt, take a pass. There is no harm in declining a project that is a no-win situation. The same criteria you use to select clients can be applied to projects in flood zones. Has the existing building been damaged before? Will the proposed project make conditions better or worse? Are you building on an erodible site? Does the client insist on spread footings instead of piles, when you know the soils are prone to scour? If you know the design will not comply with the flood regulations – walk away. It is not worth the risk. Some of your best projects may be the ones you never take in the first place.■ Kevin Chamberlain is the CEO and Principal of DeStefano & Chamberlain, Inc. in Fairfield, CT, and the Chair of the CASE Guidelines Committee. (kevinc@dcstructural.com)

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NCSEA News NCSEA

News from the National Council of Structural Engineers Associations

National Council of Structural Engineers Associations

Call to Action | A Joint Effort from CASE, NCSEA, and SEI

On Friday, June 12, the NCSEA Board of Directors, in conjunction with CASE and SEI, issued the following Call to Action.

As leading professional institutions of the structural engineering profession, we unequivocally denounce the senseless death of George Floyd.

This and other tragic events of recent weeks have brought forth numerous stories from our members who have suffered discrimination, injustice, or intimidation, or have feared for their safety. We must identify and eradicate behaviors that perpetuate racism and inequality within our profession. We recognize change is everyone’s responsibility and that we will only succeed through sustained, collective action at individual, organizational, company, and societal levels. To this end CASE, NCSEA, and SEI will: • work in unity and with our parent, member, and local organizations to engage our collective power to bring about real and lasting change; • collaborate with other organizations outside our profession who are committed to ending racism and to promoting equity, diversity, and inclusion; • urge, support, and empower our members to speak out and act with conviction and courage; and • ensure that we have open communication channels that allow all members of our structural engineering community to be informed and to be heard.

Over the coming weeks and months, we will strengthen our existing programs for equity, diversity, and inclusion to accelerate progress to a just and safe environment for all members of our profession.

Following the Joint Call to Action, the NCSEA Board of Directors issued its own action list.

The NCSEA Board of Directors is committed to this joint Call to Action. In partnership with the state SEAs, NCSEA will: • partner with CASE and SEI to form a joint committee to collaborate and coordinate on actions to improve equity and opportunity in our profession; • encourage SEAs to endorse this message and adopt similar plans of action; • compile and share resources for our members on the topics of racism, discrimination, and equality specific to the AEC industry, the design professions, and engineering; • offer sensitivity and unconscious bias training to NCSEA and SEA membership; • host an upcoming session about racism, discrimination, social justice, and structural engineering; • examine and strive to increase the diversity of individuals involved in NCSEA and SEA leadership positions and committee activities; • work with the NCSEA SE3 committee to report on additional dimensions of diversity in the profession and consider programs to commit to greater engagement and equity accordingly; • fund a scholarship program for disadvantaged students; • focus the NCSEA Students and Educators Subcommittee on advocacy efforts for disadvantaged individuals in the middle school and high school levels; • develop a mentoring program for disadvantaged professionals; • work with SEAs to engage with student chapters at the university level to increase the diversity of engagement in programs that lead to careers in structural engineering.

Welded Connections: A Practical Series for SEs

The Structural Engineers Association of Illinois (SEAOI) and NCSEA have teamed up to deliver a brand new Web-Based Seminar on welding connections. This Welding Seminar will be delivered over four weeks in four, Series Schedule: 2-hour webinars by one of the industry’s best and brightest, Duane Miller, P.E. July 8 – Welding Processes and Welding Details Welded connections are an essential part of today’s steel structures and July 15 – Principles of Welded Connection Design yet many Structural Engineers have received little or no training on this July 22 – Special Welding Applications and Problems and Fixes important topic. This 4-week seminar will serve as a primer on welded July 29 – Welded Connections for Seismic Service connections, covering topics from the fundamentals to advanced subjects. Including welding processes, applicable codes, basics of welded connections, principles of connection design, special welding applications, seismic considerations, and more.

Duane K. Miller, Sc.D., P.E., Lincoln Electric, is a recognized authority on welded connections. He has authored and co-authored chapters of many texts, including the AISC Design Guide on Welding and the Mark’s Handbook of Engineering, 12th Edition.

Dr. Miller is the former Chair of the AWS D1 Structural Welding Committee and was the first Chair of the Seismic and Bridge Welding subcommittees. He is a Professional Engineer, and formerly a Certified Welding Inspector and Qualified Welder.

NCSEA Webinars

Register by visiting www.ncsea.com

July 7, 2020

July 23, 2020

August 11, 2020

Kevin O’Connell, S.E. and Daniel Zepeda, S.E.

Lori Koch, P.E.

Randy Kissell, P.E.

Existing Buildings and the "10% Rule" – Are We in Agreement?

2018 National Design Specification (NDS) Updates

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J U L Y 2 02 0

39

SEI Update News of the Structural Engineering Institute of ASCE Don’t Miss Monthly:

#SEILive Chats on SEI Instagram: Wednesday, July 8, 12:30pm ET – SEI Director Laura Champion and G-I Member Dr. Menzer Pehlivan discuss non-traditional engineering and women in engineering.

ASCE SE Exam Review: • Step-by-step guidance to solve problems • Practice problems with solutions • Practical and concise reference materials • On-demand recordings www.asce.org/live-exam-review-courses

Summer Engineering – Fun Activities for Parents and Kids Check out Everyday Engineering: STEM@Home activities at https://www.asce.org/pre-college_outreach/ and how-to videos at https://bit.ly/2MPuP9j including: • Windy City Tower • Daylight in a Bottle • Build an Earthquake Resistant Structure • Build a Better Bubble Blower • Foil Boats • Colorful Chemistry

For fun viewing, check out these short webisodes: • Dream Big – Holding Sway: Wind Engineering https://youtu.be/nxnrtqd9Duc • Dream Big – Virtual Modeling: Engineering the Future https://youtu.be/vQJ5rPu2rls • Dream Big – Lessons from the Great Wall: Reverse Engineering https://youtu.be/BefKbn6LisI Dream Big: Engineering Our Future is available on Netflix, for purchase on Amazon, and via https://vimeo.com/ondemand/dreambigfilm.

Also, visit the discovere.org/dreambig Activities for Build a Straw Bridge, Critical Load, Marble Run, and more.

NEW ASCE/SEI 48-19

Design of Steel Transmission Pole Structures

Provides a uniform basis for the design, detailing, fabrication, testing, assembly, and erection of steel tubular structures for electrical transmission poles. These guidelines apply to cold-formed single- and multi-pole tubular steel structures that support overhead transmission lines. The design parameters apply to guyed and self-supporting structures using a variety of foundations, including concrete caissons, steel piling, and direct embedment. Learn more at ascelibrary.org.

Vote in SEI Online Election for SEI Board Members by July 31

The SEI Board of Governors consists of two representatives from each of the five SEI Divisions (Business & Professional, Codes & Standards, Global, Local, and Technical Activities), one appointee from ASCE, the SEI President, SEI Past President, and the SEI Director as a nonvoting member. The Division representatives each serve a four-year term. In accordance with the SEI Bylaws, this year, SEI is conducting an election for one Business & Professional Activities Division representative and one Codes & Standards Activities Division representative to the Board of Governors, terms effective October 1.

Current SEI members above the grade of Student will receive a notice July 1 via ASCE Collaborate on how to verify and submit your secure ballot online. Ballots are due no later than 11:59 pm US ET, July 31, 2020.

Sponsor/Exhibit to reach industry professionals. Contact Sean Scully at sscully@asce.org. www.structurescongress.org #Structures21

ETS Call for Abstracts – Due September 2.

Sponsor/Exhibit to reach industry professionals. Contact Sean Scully sscully@asce.org. www.etsconference.org #ETSC21

SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle Errata 40 STRUCTURE magazine

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

If you would like to submit errata, contact Jon Esslinger at jesslinger@asce.org.

CASE in Point News of the Coalition of American Structural Engineers Did you know?

CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need!

CASE has several tools available for firms to use for managing projects and training staff: CASE 962-H: Tool 3-2 Tool 3-4 Tool 3-5 Tool 4-1 Tool 4-2

National Practice Guideline on Project and Business Risk Management

Staffing and Revenue Projection Project Work Plan Templates Staffing Schedule Suite Status Report Template Project Kick-off Meeting Agenda

Tool 4-5 Tool 4-6 Tool 5-5 Tool 6-3

Project Communications Matrix Project Team Coordination Project Management Training Guide Project Scoping Tool

NEW CASE Publications Released!

CASE 962-I: Structural Engineer’s Guide to Working with a Geotechnical Engineer Practicing Structural Engineers are faced with educating their clients about services needed from the Geotechnical Engineer, how to retain the right Geotechnical Engineering firm for the project, and how best to implement the recommendations of the Geotechnical Report on their project. This Guide was created to discuss structural engineering business practice aspects of working with a Geotechnical Engineer.

Tool 6-3: Project Scoping Tool Detailed project scope development is essential to project success. It is crucial to reducing your firm’s risk and avoiding disputes of what is and is not included with your fee. One significant facet of project proposal development is developing a comprehensive description of the project scope by phase. CASE Tool 6-3 provides a simple checklistbased tool for defining your project scope, which can be included as part of your proposal or as an appendix to your contract.

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

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

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

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

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

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

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

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

To apply, your firm should: • Be a current member of ACEC • Be a member of the Coalition of American Structural Engineers (CASE); or be willing to join the Coalition • Be able to attend the groups’ usual face-to-face meetings each year: August, February (hotel, travel partially reimbursable) • Be available to engage with the committees via email and video/conference call • Have some specific experience and/or expertise to contribute to the group Please submit the following information to Heather Talbert, Coalitions Director (htalbert@acec.org): • Letter of interest indicating which committee • Brief bio (no more than a page)

Thank you for your interest in contributing to advancing the structural engineering profession!

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

JULY 2020

41

INSIGHTS The January 2020 Puerto Rico Earthquake The Reality Not Covered in the Media

By Dr. Kit Miyamoto, S.E.

P

once is a beautiful coastal city that was once the capital of colonial Puerto Rico. Its magnificent houses and historic buildings, often exceeding 10,000 square feet, survived for centuries but were damaged by the January 2020 magnitude-6.4 earthquake. In some locations, ground acceleration exceeded 50 percent of gravity – something we expect in highly seismic regions like the Western U.S. – and many buildings are now red-tagged, labeled too dangerous to enter. The island’s economy has suffered from depopulation for decades, with an estimated one million people leaving in the last decade alone. Hurricane Maria in 2017 accelerated this trend, so many of the buildings impacted by the earthquake were not occupied nor maintained.

The earthquake damaged about 10 percent of the building stock in the impacted towns.

A few hundred collapsed. Most structures are of concrete frame construction with infill of unreinforced masonry units. Puerto Rico uses the International Building Code (IBC) and a licensing system similar to the mainland U.S., but only half of all residential buildings receive valid construction permits.

Many newer houses sit on slender columns without proper ductile details. The open area below is commonly used for BBQs or parking, but this soft story construction is a major collapse hazard. There are still more than 100,000 of these dangerous buildings standing on this island.

Over 25% of the impacted schools were damaged, and one collapsed. Most of the damaged schools are concrete with non-ductile construction and built prior to 1987 when the building

42 STRUCTURE magazine

code was updated with seismic provisions. It was fortunate that the earthquake happened during a holiday; otherwise, Puerto Rico would have lost a few hundred students.

The island has not experienced an earthquake disaster of this magnitude in over 100 years, so the public was not quite ready for it. Unlike hurricanes, earthquakes have no warning, nor a beginning or end. Aftershocks are still rumbling and causing anxiety. Roof or window damage from hurricanes is easy to see, but cracks caused by earthquakes are mysterious. It can be hard to tell if superficial cracks indicate more serious structural damage. One of the ways to provide confidence for the public has been for structural engineers to conduct Rapid Damage Assessments house by house.

There were more than 8,000 displaced people in tent cities and more than 10,000 buildings damaged in January. When Miyamoto International arrived with a team of disaster response specialists, the government was just establishing a rapid damage assessment system and training engineers.

Eight weeks later, 80 percent of buildings had been assessed using the ATC 20 rapid assessment procedure. The displacement camps are reduced to about 600 people, thanks to Rental Housing Assistance vouchers. Almost all roads damaged by landslides have been repaired, and 100 percent of the electrical service has been restored.

While the response phase winds down, the most challenging component starts – recovery.

Is this going to accelerate depopulation and more empty houses, or can this trigger economic expansion? The answer depends on policy funding. After

Hurricane Maria, billions of dollars were allocated by Congress, but only a fraction of it was spent. Why? There was a 10% match required by the federal government. But Puerto Rico was already bankrupt before Maria, so there was simply no money for matching. This time, the matching requirement for FEMA public funding is 25%. This reality needs to be resolved rapidly.

The quick solution after an earthquake is to demolish damaged buildings as fast as possible. However, once a building is taken down, it takes a lot of money and time to reconstruct. Empty lots deter private investment, and it can take years to build back a city.

As the team assessed damaged buildings, it was estimated that more than 90% of yellow tagged buildings and more than 50% of red-tagged buildings could be repaired at a fraction of the cost and time of demolition and reconstruction. Between 10,000 to 20,000 residential and heritage buildings need to be examined for repairability. This can be done swiftly. Engineering technology can help avoid empty lots and protect this beautiful city’s history, while construction can encourage economic expansion and investments in the future.■ Dr. Kit Miyamoto is a world-leading expert in disaster resiliency, response, and reconstruction.

He provides expert engineering and policy consultation to the World Bank, USAID, UN agencies, governments, and the private sector.

He is a California Seismic Safety Commissioner and Global CEO of Miyamoto International.

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STRUCTURE JULY 2020

Bonus Content

TECHNOLOGY The Embodied Carbon in Construction Calculator Tool (EC3) Why Now and How Do You Use It?

By Donald Davies, P.E., S.E., and Dirk Kestner, P.E., LEED AP BD+C

T

he building and construction sectors play a vital role in minimizing our future carbon footprint. Each year, the built environment contributes almost 40% of global greenhouse (GHG) emissions. The industry’s focus on operational carbon

reductions – the energy used to heat, cool, and power our buildings – has led to many successes. However, the attention to embodied carbon, the emissions associated with material production and construction processes, has been lagging.

Between now and 2050, embodied carbon will be responsible for an ever-increasing part of the total building and construction carbon emissions due to the rapidly improving carbon footprint of our energy grid. Not only will embodied carbon exceed the operational carbon footprints for new construction between now and 2050, the carbon being emitted at the start of a building’s life cycle is where the time value of carbon is often most important.

As part of a growing commitment to action on these issues, already demonstrated by the structural engineers SE2050 Challenge issued by the Carbon Leadership Forum (CLF) and endorsed by SEI’s Board of Governors, the Embodied Carbon in Construction Calculator (EC3) tool was created with input from a coalition of more than 50 forward-looking and innovative building industry leaders.

This article presents a strategy for including the EC3 tool in today’s design, procurement, and construction practices. Like design and construction today, there is more than one option for how the EC3 tool can be used and it is adaptable to a variety of use cases.

The EC3 tool is intended to supplement and improve, not to replace, current Whole Building Life Cycle Analysis (WBLCA) efforts. It targets improving the data considered at a critical stage within many WBLCA efforts when supplier procurement decisions are made. It makes material

Figure 1. The EC3 tool focus.

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Environmental Product Declarations (EPDs) more accessible, easier to compare, and with their uncertainties better defined.

For the LCA experts reading this, the EC3 tool focuses on the Product Category LCA stages A1 to A3, or a cradle-to-gate material Environmental Product Declaration (EPD) embodied carbon evaluation. EPDs, for those wondering, are similar to “nutrition labels” that would come with a material, reporting environmental impact data on that specific material. While the EC3 tool does not attempt to address the other whole building LCA stages A4 through D, these other stages are each fundamentally important. Holistically evaluating all aspects of the building’s life cycle is critical to understanding the full implications of our choices (Figure 1).

Another long-term goal of this effort is to encourage this next generation database of EPDs, which includes statistical uncertainty evaluations of the EPD data, to be integrated into the current and future generation WBLCA tools being used. As a result, the EC3 tool is a free, open-access tool.

The EC3 tool initially focuses on these materials: • Structure: Concrete, Steel, Timber • Enclosure: Aluminum, Glass, Insulation • Finishes: Carpet, Ceiling Tiles, Gypsum Wall Board

The EC3 tool’s information helps carry forward from what is estimated and measured by the design team into the contractor’s budgeting, procurement, and construction efforts, to support a double bottom line accounting process to analyze embodied carbon and cost.

Remember, though, that the EC3 tool is not conceived of as the final decision-making tool when deciding how to manage the carbon budget or procurement process. Instead, it makes embodied carbon information available at significant decision-making points of a project. It is only one part of a larger design and construction decision-making process.

The EC3 Tool Structural Use Case The flow diagram shown in Figure 2 Figure 2. The EC3 tool flow chart. describes how the EC3 tool integrates into the current design, procurement, construction, and close-out processes. A further breakdown of this process can be found at www.buildingtransparency.org/en. With this use case, there are several points to highlight.

For starters, make system and material decisions based upon the best value and materially efficient use as a starting point. Consider the use of lower carbon footprint materials wherever possible, but maintain an awareness of the best-value optimized design that meets the owner’s program and goals.

Why the focus on optimization and not carbon first? Considering the holistic impacts of where the material of a project comes from, how it is made or harvested, and if the design concepts are an optimized material use can often create a more significant swing in a project’s final carbon footprint than the choice of the initial material only. Picking what is perceived to be the lower carbon material is a good starting point, but it does not guarantee a low carbon design, especially when the material use is not optimized or not in an appropriate use case.

Figure 3. EC3 tool showing EPD data evaluation.

Also, when BIM modeling is started, decide if it is to be used for structural quantity material reporting (the best approach for the future), or if you plan to track quantities through a spreadsheet or rely on the contractor quantity estimates. However quantities are tracked, be consistent. The EC3 tool can accept direct imports from Revit through BIM 360 to simplify the importing of data, but you should always check any direct imports with a verifying sample hand calculation as well to know your Revit model and the EC3 tool are in alignment with how the data is being handled.

The EC3 tool includes both industry average EPD data, for an early reference building definition prior to the actual material suppliers being known, as well as North American vendor-specific EPDs to the extent they are known and published. This includes the National Ready-Mix Concrete Association’s (NRMCA) regionally averaged EPD data sets, the American Institute of Steel Construction’s (AISC) national average EPD’s for steel, and the American Wood Council (AWC) national average EPD’s for lumber. (Figure 3).

For any pre-determined, wholeproject carbon budget targets, conservative to aggressive targets can be set from within the EC3 tool. A key feature of the tool is that EPD uncertainty and material variability is reported.

Note that when considering total variability reporting within a project, when materials are combined but only based upon industry average EPD data, it usually shows that definitive early project decisions comparing between one material or another are inconclusive or inappropriate based upon the material choice alone. Trend data is credibly shown, but the JULY 2020 BO N US CO N T EN T

Figure 4. EC3 tool Sankey Diagram of carbon within a reference building.

industry average uncertainty bars will often overlap. Therefore, making initial material decisions based upon the best-value optimized use is a sounder starting point than trying to compare non-compatible industry average LCA data and getting faulty findings because of it.

Specification language to support the asking for and tracking of vendor-specific EPDs also should be incorporated at the earliest stages possible. The Carbon Leadership Forum provides sample specification language that can be used for this purpose. Similar language has also been posted at the newly formed BuildingTransparency 501(c)3 web site, https://bit.ly/3foW9rf, which has become the specific host of the EC3 tool as an outgrowth of the Carbon Leadership Forum's incubation of the EC3 tool.

Using performance targeted LCA specifications that ask for vendor-specific EPDs within the project materials often leads to detailed discussions on embodied carbon reduction strategies up and down the supply chain.

By asking for EPDs and letting this reporting tell the material embodied carbon data story, the vendor is open to select their own strategy for how they choose to compete on this issue. A successful strategy is to telegraph intentions to potential material suppliers well in advance of the actual ask, to allow them to organize around the topic before the time of the bid.

As the use of vendor-supplied EPDs grows, it will become easier to follow this process. It is currently the easiest to compare this data for West Coast U.S. cities, but the trend to make EPDs available is happening nationally. Being wise to the market variability of a project’s location and engaging the full project team early in the process is the key to success.

It is notable how fast the topic of embodied carbon EPD reporting is moving across the industry and the nation. When motivated large project development clients express an interest in this information, and they are requesting double bottom line accounting, regional material suppliers respond. From the time of the EC3 tool’s conception to its November 2019 launch, the number of available EPDs within the concrete industry alone grew to over 23,000 within the EC3 tool.

Structural sub-contractor bids containing material quantity assumptions, cost, and EPD embodied carbon information is where the EC3 tool becomes most valuable, and where things all come together (Figure 4).

The EC3 tool calculations can and should be updated based on the subcontractor’s data collected during the bidding process. Decisions between like materials can then be made, choosing where and when to pursue aggressive embodied carbon targets. This can happen with STRUCTURE magazine

due consideration of other project goals around schedule and cost, while still looking to stay within an overall embodied carbon project budget. Any schedule or cost premiums, if they exist, can be evaluated against the benefit of the embodied carbon reductions. This allows a more informed decisionmaking process to occur, lower embodied carbon material suppliers to be identified, and best value trade-offs to be considered in the award of that project scope.

It is typically a very competitive marketplace for material suppliers. The variables that are tracked and paid attention to at the time of bidding are something suppliers cannot help but look to optimize and differentiate around. This is one of the crucial ways that the EC3 tool is helping to move the industry to a lower embodied carbon future for construction.

The EC3 tool, within its data collection and reporting, has a portal for uploading project data, anonymizing the information to the level the user putting in the data desires, and publishing it as part of an ever-growing database of verified building embodied carbon reference buildings. As noted earlier, this reporting can also help support initiatives such as the SE 2050 Challenge (https://se2050.org). As the EC3 tool’s project database grows, primarily when populated with as-built project data, the collective ability to better define reference buildings and starting points for what is possible will result in mutual, shared benefits.

Conclusion We all have a stake in improving how the industry moves toward a lower carbon footprint of construction. The design and construction communities are in a unique position to define the process for evaluating embodied carbon and can utilize the supply chain process to deliver more low-carbon material options. The EC3 tool’s use encourages project teams to set a “carbon budget” during design and consistently manage it through material “quantity control” and procurement efforts that are later validated during the construction process. The authors invite fellow structural engineers to adopt this vital tool. The current use of the EC3 tool among team members and collaborators has already delivered demonstrated impacts on reducing the embodied carbon footprint in the building industry.■ The online version of this article contains a reference.

Please visit www.STRUCTUREmag.org.

Donald Davies is President of Magnusson Klemencic Associates (MKA), headquartered in Seattle. He frequently lectures on Embodied Carbon Life Cycle Analysis and is a founding member of the Carbon Leadership Forum, an academic and industry collaboration hosted at the University of Washington. (ddavies@mka.com) Dirk Kestner is a Principal and Director of Sustainable Design at Walter P Moore. He is a past Chair of the SEI Sustainability Committee and the current Chair of the USGBC Materials and Resources Technical Advisory Group. (dkestner@walterpmoore.com)