Drop, Cover, and Hold On – Becoming Earthquake-Smart in the 2017 Great ShakeOut

This week’s post was written by Jacob McAuley, Associate Regional Marketing Manager at Simpson Strong-Tie.

Every October, millions of people across the globe participate in earthquake drills as part of an event called the Great ShakeOut in order to improve their earthquake preparedness. This year, the Great ShakeOut took place on October 19 and involved more than 60 countries. In addition to the earthquake drill, participants in the event often take part in other activities such as seminars, Q&As and more. At Simpson Strong-Tie, we practiced earthquake drills at each of our major branches, and, in our Pacific Northwest region, we were part of a Reddit Ask Me Anything event (an online live Q&A) to talk about earthquake safety and answer people’s questions. Below, I discuss our participation in both of these activities.

If you’ve ever participated in an earthquake drill, you know that they can be a bit surreal. Sure, at certain points jokes are made and people laugh, because after all, it is pretend. But at other points during the drill, you will probably recall the very real threat that an earthquake presents, and you will wonder, “Would I really be prepared?” That’s why these drills are so important and why we as a company participate every year because in order to react quickly, you need to be well-practiced. Here’s a little about the drill we did.

The Great ShakeOut earthquake drill lasts only about 20 minutes and is fairly easy to prepare for and participate in (learn about how to participate here). At our Pleasanton office, where I work, at 10:19 am we heard a recording over the intercom that announced the earthquake drill was now under way. Each of us immediately did what the announcement instructed us to do: Drop, cover and hold on. The website of the ShakeOut expounds the three steps as follows: “Drop to the ground, take Cover under a table or desk, and Hold On to it as if a major earthquake were happening (stay down for at least 60 seconds).” They also recommend that during the drill, while you’re under the table, you should look around and imagine what would happen in a major earthquake and consider various potential hazards around you (e.g., items that might fall, break or cause a fire, such as lighting fixtures, unsecured bookcases or other furniture, computer or television screens, unstrapped water heaters) and how you might make them safer. Once the drill was over, another announcement indicated that we exit the building and go to a predesignated meeting area, where selected trained employees were responsible for taking roll of their assigned groups. Once everyone was accounted for, we went back in and resumed our usual workday.

Then, continuing in the spirit of preparedness, at 12:00 noon (PT), our very own Emory Montague, R&D engineering manager, joined Pacific Northwest earthquake-preparedness experts for a lively two-hour Reddit Ask Me Anything forum. The conversation is still live so be sure to check it out. The group of experts included a seismologist, two geologic hazards managers, a structural engineer (Emory), two geologists and an earthquake and volcano coordinator. Questions could be asked by anyone, and topics ranged from building safety to predictions about earthquakes to necessary earthquake supplies and more. Overall, the Ask Me Anything event was a success and helped make this a sobering and very informative Great ShakeOut day.

5 Steps to a Successful Soft-Story Retrofit

Last year, I gave a presentation at the annual National Council of Structural Engineers Associations (NCSEA) Summit in Orlando, Florida, titled “Becoming a Trusted Advisor: Communication and Selling Skills for Structural Engineers.” As this was a summit for the leaders of the structural engineers associations from across the country, I wasn’t sure how many people would find it valuable to spend their time learning about a very nontechnical topic. To my surprise and delight, the seminar ended up being standing-room only, and I was able to field some great questions from the audience about how they could improve their selling and communication skills. In the many conversations I had with the conference attendees after my presentation, the common theme was that engineers felt they needed more soft-skills training in order to better serve their clients. The problem, however, was finding the time to do so when faced with the daily grind of design work.

Structural Engineers In a Training for Seismic Retrofits

Presenting at the NCSEA Summit, I’m the tiny person in upper left hand corner.

When I started my first job as a design engineer at a structural engineering consulting firm straight out of school, I was very focused on improving and expanding my technical expertise. Whenever possible, I would attend building-code seminars, design reviews and new product solution presentations, all in an effort to learn more about structural engineering. What I found as I progressed through my career, however, was that no matter how much I learned or how hardworking I was, it didn’t really matter if I couldn’t successfully convey my knowledge or ideas to the person who really mattered most: the client.

Contractors discussing building plans with an engineer.

Contractors discussing building plans with an engineer.

How can an engineer be most effective in explaining a proposed action or solution to a client? You have to be able to effectively sell your idea by understanding the needs of your client as well as any reasons for hesitation. The importance of effective communication and persuasion is probably intuitive to anyone who’s been on the sales side of the business, but not something that occurs naturally to data-driven folks like engineers. As a result of recent legislation in California, however, structural engineers are starting to be inundated with questions from a group of folks who have suddenly found themselves responsible for seismically upgrading their properties: apartment building owners in San Francisco and Los Angeles.

Imagine for a moment that you are a building owner who has received a soft-story retrofit notice under the City of Los Angeles’ Ordinance 183893; you have zero knowledge of structural engineering or what this term “soft-story” even means. Who will be your trusted advisor to help you sort it out? The City of Los Angeles Department of Building and Safety (LADBS) has put together a helpful mandatory ordinance website that explains the programs and also offers an FAQ for building owners that lets them know the first step in the process: hire an engineer or architect licensed in the state of California to evaluate the building.

Simpson Strong-Tie Structural Engineer Annie Kao at a jobsite.

Checking out some soft story buildings in Los Angeles. The Los Angeles Times has a great map tool.

I’ve had the opportunity to be the first point of contact for a building owner after they received a mandatory notice, because it turns out some relatives own an apartment building with soft-story tuck-under parking. Panicked by the notice, they called me looking to understand why they were being forced to retrofit a building that “never had any problems in the past.” They were worried they would lose rent money due to tenants needing to relocate, worried about how to meet the requirements of the ordinance and, most importantly, worried about how much it was going to cost them. What they really wanted was a simple, straightforward answer to their questions, and I did my best to explain the necessity behind retrofitting these vulnerable buildings and give an estimated time frame and cost that I had learned from attending the first Los Angeles Retrofit Resource Fair in April 2016. With close to 18,000 buildings in the cities of San Francisco and Los Angeles alone that have been classified as “soft-story,” this equates to quite a number of building owners who will have similar questions and be searching for answers.

To help provide an additional resource, Simpson Strong-Tie will be hosting a webinar for building owners in the Los Angeles area who have received a mandatory soft-story retrofit notice. Jeff Ellis and I will be covering “5 Steps to a Successful Retrofit” and helping to set a clear project path for building owners. The five steps that Simpson Strong-Tie will be recommending are:

  1. Understanding the Seismic Retrofit Mandate
  2. Partnering with Design Professionals
  3. Submitting Building Plans with the Right Retrofit Product Solutions
  4. Communicating with Your Building Tenants
  5. Completing Your Soft-Story Retrofit

We encourage you to invite any clients or potential clients to attend this informative webinar, which will lay the foundation for great communication between the two of you. As part of the webinar, we will be asking the building owners for their comments, questions and feedback so we can better understand what information they need to make informed decisions, and we will be sure to share these with the structural engineering community in a future post. By working together to support better communication and understanding among all stakeholders in retrofit projects, we will be well on our way to creating stronger and more resilient communities!

For additional information or articles of interest, there are several resources available:

SEAOSC Safer Cities Survey Results: How Are We Building Strength and Transparency in Our Communities?

Back in January, employees at Simpson were given the opportunity to learn more about the 401K retirement and investment plan. The big takeaways from my training session were a) save as much as you can as early as you can in life and b) use asset allocation to diversify your portfolio and avoid too much risk. Now, I’m not a big risk taker in general, so I dutifully picked a good blend of stocks and bonds with a range of low to high risk. It seems like a pretty sound strategy and it made me think of all the other ways I tend to minimize risk in my life. When I head to a restaurant, for example, I almost instinctively look for the county health grade sign in the window. When my husband and I went to go buy a new family car a couple years ago, I remember searching the National Highway Traffic Safety Administration (NHTSA) website for crash test ratings. Even when I’m doing something as mundane as having a snack, I will invariably flip over the Twinkie package to see just how many grams of fat are lurking inside (almost 5 per serving!). For all the rankings and information available to the general public for restaurants, cars and snacks, there isn’t much, if any, information to help us know if we’re minimizing our risk for one of the most common activities we do almost every day: walking into a building.

Risk level knob positioned on medium position, white background and orange light. 3D illustration concept for business security management.

 

Now before you accuse me of being overly dramatic about such a trivial activity, here’s some food for thought: research has shown that Americans spend approximately 90% of their day inside a building. That’s over 21 hours a day! Have you ever once thought to yourself, “I wonder if this building is safe? Would this building be able to withstand an earthquake or high wind event?” Or how about even taking a step back and asking, “Are there any buildings that are already known to be potentially vulnerable or unsafe, and has my city done anything to identify them?” Unfortunately, that kind of information about a city’s building stock is not usually readily available, but some in the community, including structural engineers, are working to change that.

Los Angeles skyline on a partly cloudy day with a row of palm trees in the foreground.

 

The charge is being led in California, a.k.a. Earthquake Country, where structural engineers are teaming up with cities to help identify buildings with known seismic vulnerabilities and provide input on seismic retrofit ordinances. Structural engineers have learned quite a bit about how buildings behave through observing building performance after major earthquakes, and building codes have been revised to address issues accordingly. However, according to the US Green Building Council, “…the annual replacement rate of buildings (the percent of the total building stock newly constructed or majorly renovated each year) has historically been about 2%, and during the economic recession and subsequent years, it’s been much lower.” This means that there are a lot of older buildings out there that have not been built to current building codes and were not designed with modern engineering knowledge.

Several cities in California have enacted mandatory seismic retrofit ordinances that require the strengthening of some types of known vulnerable buildings, but no state or nation-wide program currently exists. The Structural Engineers Association of Southern California (SEAOSC) recently decided to launch a study of which jurisdictions in the southern California region have started to take the steps necessary to enact critical building ordinances. According to SEAOSC President Jeff Ellis, S.E., “In order to develop an effective strategy to improve the safety and resilience of our communities, it is critical to benchmark building performance policies currently in place. For southern California, this benchmarking includes recognizing which building types are most vulnerable to collapse in earthquakes, and understanding whether or not there are programs in place to decrease risk and improve recovery time.” These results were presented in SEAOSC’s Safer Cities Survey, in partnership with the Dr. Lucy Jones Center for Science and Society and sponsored by Simpson Strong-Tie.

safer-cities-ca

This groundbreaking report is the first comprehensive look at what critical policies have been implemented in the region of the United States with the highest risk of earthquake damage. According to the Los Angeles Times, the survey “found that most local governments in the region have done nothing to mandate retrofits of important building types known to be at risk, such as concrete and wooden apartment buildings.”

The Safer Cities Survey highlights how the high population density of the SoCal region coupled with the numerous earthquake faults and aging buildings is an issue that needs to be addressed by all jurisdictions as soon as possible. An excerpt from the survey covers in detail why this issue is so important:

No building code is retroactive; a building is as strong as the building code that was in place when the building was built. When an earthquake in one location exposes a weakness in a type of building, the code is changed to prevent further construction of buildings with that weakness, but it does not make those buildings in other locations disappear. For example, in Los Angeles, the strongest earthquake shaking has only been experienced in the northern parts of the San Fernando Valley in 1971 and 1994 (Jones, 2015). In San Bernardino, a city near the intersection of the two most active faults in southern California where some of the strongest shaking is expected, the last time strong shaking was experienced was in 1899. Most buildings in southern California have only experienced relatively low levels of shaking and many hidden (and not so hidden) vulnerabilities await discovery in the next earthquake.

 The prevalence of the older, seismically vulnerable buildings varies across southern California. Some new communities, incorporated in the last twenty years, may have no vulnerable buildings at all. Much of Los Angeles County and the central areas of the other counties may have very old buildings in their original downtown that could be very dangerous in an earthquake, surrounded by other seismically vulnerable buildings constructed in the building booms of the 1950s and 1960s. Building codes do have provisions to require upgrading of the building structure when a building undergoes a significant alteration or when the use of it changes significantly (e.g., a warehouse gets converted to office or living space). Seismic upgrades can require changes to the fundamental structure of the building. Significantly for a city, many buildings never undergo a change that would trigger an upgrade. Consequently, known vulnerable buildings exist in many cities, waiting to kill or injure citizens, pose risks to neighboring buildings, and increase recovery time when a nearby earthquake strikes.

1994-northridge

The survey also serves as a valuable reference in being able to identify and understand what the known vulnerable buildings types are:

  1. Unreinforced masonry buildings: brick or masonry block buildings with no internal steel reinforcement — susceptible to collapse
  2. Wood-frame buildings with raised foundations: single-family homes not properly anchored to the foundation and/or built with a crawl space under the first floor — possible collapse of crawl space cripple walls or sliding off foundation
  3. Tilt-up concrete buildings: concrete walls connected to a wood roof — possible roof-to-wall connection failures leading to roof collapse
  4. Non-ductile reinforced concrete buildings: concrete buildings with insufficient steel reinforcement — susceptible to cracking and damage
  5. Soft first-story buildings: buildings with large openings in the first floor walls, typically for a garage — susceptible to collapse of the first story
  6. Pre-1994 steel moment frame buildings: steel frame buildings built before the 1994 Northridge earthquake with connections — susceptible to cracking leading to potential collapse

1933-earthquake-shot

Along with the comprehensive list of potentially dangerous buildings, the survey also offers key recommendations on how cities can directly address these hazards and reduce potential risks due to earthquakes. As a good starting point, the survey recommends having “…an active or planned program to assess the building inventory to gauge the number and locations of potentially vulnerable buildings…is one of the first steps in developing appropriate and prioritized risk mitigation and resilience strategies.

Economic costs can be substantial for businesses whose buildings have been affected by an earthquake. After a major seismic event, a structure needs to be cleared by the building department as safe before it can be reoccupied, and it will generally receive a green (safe), yellow (moderately damaged) or red (dangerous) tag.  A typical yellow-tagged building could take up to two months to be inspected, repaired and then cleared, meaning an enormous absence of income for businesses. The survey offers a strategy for getting businesses up and running quickly after an earthquake, in order to minimize such losses. The Safer Cities Survey recommends that cities adopt a “Back-to-Business” or “Building Re-Occupancy” program, which would “create partnerships between private parties and the City to allow rapid review of buildings in concert with City safety assessments…Back-to-Business programs…[allow] private parties to activate pre-qualified assessment teams, who became familiar with specific buildings to shorten evaluation time [and] support city inspections.

oes-inspectors-program

Basically, a program like this would allow a property owner to work with a structural engineer before an earthquake occurs. This way, the engineer is familiar with the building’s layout and potential risks, and can plan for addressing any potential damage. Having a program like this in place can dramatically shorten the recovery time for a business, from two months down to perhaps two weeks. Several cities have already adopted these types of programs, including San Francisco and Glendale, and it showed up as a component of Los Angeles’ Resilience by Design report.

Ultimately, the survey found that only a handful of cities have adopted any retrofit ordinance, but many cities indicated they were interested in learning more about how they could get started on the process. As a result, SEAOSC has launched a Safer Cities Advisory Program, which offers expert technical advice for any city looking to enact building retrofit ordinances and programs. This collaboration will hopefully help increase the momentum of strengthening southern California so that it can rebound more quickly from the next “Big One.”

We all want to minimize the risk in our lives, so let’s support our local structural engineering associations and building departments in exploring and enacting seismic building ordinances that benefit the entire community.

For additional information or articles of interest, please visit:

Great ShakeOut Earthquake Drill 2016

The Great ShakeOut Earthquake Drill is an annual opportunity for people in homes, schools and organizations to practice what to do during earthquakes and improve their preparedness. In a post I wrote last October about the Great ShakeOut, I reminisced about the first earthquake I had to stop, drop and cover for – the Livermore earthquake in January, 1980. This year got me thinking about how our evacuation drills work.

At Simpson Strong-Tie, we use the annual Great ShakeOut drill to practice our building evacuation procedures. Evacuation drills are simple in concept – alarms go off and you exit the building. We have volunteer safety wardens in different departments who confirm that everyone actually leaves their offices. There are always a few people who want to stay inside and finish up a blog post. Once the building is empty and we have all met up in the designated meeting area, we do a roll call and wait for the all-clear to get back to work.

Several years ago the alarms went off. While waiting for the drill to end, we were concerned to see fire fighters arrive and rush into the building. Realizing this was not a drill, there were some tense moments of waiting. The fire chief and our president eventually walked out of the building and our president was yelling for one of our engineers. Turns out the engineer (who shall remain nameless) was cooking a chicken for lunch. Yes, a whole chicken. The chicken didn’t make it – I’m not sure what the guilty engineer had for lunch afterwards. At least we received extra evacuation practice that year. We aren’t allowed to cook whole chickens in the kitchen anymore.

Simpson Strong-Tie is helping increase awareness about earthquake safety and encouraging our customers to participate in the Great ShakeOut, which takes place next Thursday on October 20. It’s the largest earthquake drill in the world. More than 43 million people around the world have already registered on the site.

On October 20, from noon to 2:00 p.m. (PST), earthquake preparedness experts from the Washington Emergency Management Division and FEMA will join scientists with the Washington Department of Natural Resources and the Pacific Northwest Seismic Network for a Reddit Ask Me Anything – an online Q&A. Our very own Emory Montague will be answering questions. The public is invited to ask questions here. (Just remember that this thread opens the day before the event and not sooner.)

Emory Montague from Simpson Strong-Tie

Emory, ready to answer some seismic-related questions.

We’re also providing resources on how to retrofit homes and buildings, and have information for engineers here and for homeowners here.

Earthquake risk is not just a California issue. According to the USGS, structures in 42 of 50 states are at risk for seismic damage. As many of you know, we have done a considerable amount of earthquake research, and are committed to helping our customers build safer, stronger homes and buildings. We continue to conduct extensive testing at our state-of-the-art Tye Gilb lab in Stockton, California. We have also worked with the City of San Francisco to offer education and retrofit solutions to address their mandatory soft-story building retrofit ordinance and have created a section on our website to give building owners and engineers information to help them meet the requirements of the ordinance.

Last year, Tim Kaucher, our Southwestern regional Engineering Manager, wrote about the City of Los Angeles’s Seismic Safety Plan in this post. Since that time, the City of Los Angeles has put that plan into action by adopting mandatory retrofit ordinances for both soft-story buildings and non-ductile concrete buildings. Fortunately, California has not had a damaging earthquake for some time now. As a structural engineer, I find it encouraging to see government policy makers resist complacency and enact laws to promote public safety.

Participating in the Great ShakeOut Earthquake Drill is a small thing we can all do to make ourselves more prepared for an earthquake. If your office hasn’t signed up for the Great ShakeOut Earthquake Drill, we encourage you to visit shakeout.org and do so now.

Simpson Strong-Tie® Strong-Wall® Wood Shearwall – The Latest in Our Prefabricated Shearwall Panel Line Part 2

In last week’s blog post, we introduced the Simpson Strong-Tie® Strong-Wall® Wood Shearwall. Let’s now take a step back and understand how we evaluate a prefabricated shear panel to begin with.

First, we start with the International Building Code (IBC) or applicable state or regional building code. We would be directed to ASCE7 to determine wind and seismic design requirements as applicable. In particular, this would entail determination of the seismic design coefficients, including the response modification factor, R, overstrength factor, Ωo, and deflection amplification factor, Cd, for the applicable seismic-force-resisting system. Then back to the IBC for the applicable building material: Chapter 23 covers Wood. Here, we would be referred to AWC’s Special Design Provisions for Wind and Seismic (SDPWS) if we’re designing a lateral-force-resisting system to resist wind and seismic forces using traditional site-built methods.

Design Documents: IBC, ASCE7 and SDPWS

Design Documents: IBC, ASCE7 and SDPWS

These methods are tried and true and have been shown to perform very well in light-frame construction during wind or seismic events. But over the years, many people have come to enjoy things like lots of natural light in our homes, great rooms with tall ceilings and off-street secure parking.

prefab2Due to Shearwall aspect ratio limitations defined in SDPWS as well as the strength and stiffness limitations of these traditional materials – including wood structural panel sheathing, plywood siding and structural fiberboard sheathing, to name a few – we’re left looking for alternative solutions. Thankfully, the IBC has left room for the use of innovative solutions beyond what’s explicitly stated in the code. Section 104.11 of the 2015 IBC provides the following provision:

104.11 Alternative material, design and methods of construction and equipment

The provisions of this code are not intended to prevent the installation of any material or prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method, or work offered is, for the purpose intended, not less than the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety…

104.11.1 Research Reports. Supporting data, where necessary to assist in the approval of materials or assemblies not specifically provided for in this code, shall consist of valid research reports from approved sources.

104.11.2 Tests. Whenever there is insufficient evidence of compliance with the provisions of this code […] the building official shall have the authority to require tests as evidence of compliance…

Research Reports

The route we at Simpson Strong-Tie typically take is to obtain a research report from an approved source, i.e., the ICC Evaluation Service or the IAPMO Uniform Evaluation Service. Each of these evaluation service agencies publishes acceptance criteria that have gone through a public review process and contain evaluation procedures. The evaluation procedures might contain referenced codes and test methods, analysis procedures and requirements for compatibility with code-prescribed systems.

Prefabricated Panel Evaluation

Let’s once again take a step back and consider the function of our Strong-Wall® shearwalls. They’re prefabricated panels intended to provide lateral and vertical load-carrying capacity to a light-framed wood structure where traditional methods are not applicable or are insufficient. We need to provide a complete lateral load path, which ensures that the load continues through the top connection into the panel and then into the foundation through the bottom connection. To evaluate the panel’s ability to do what we’re asking of it, we use a combination of testing and calculations with considerations for concrete bearing, fastener shear, combined member loading, tension and shear anchorage, panel strength and stiffness, etc.

I could write a five-thousand-word feature story for the New York Times discussing the calculations in great detail, but let’s focus on the more exciting part – testing! Simpson Strong-Tie has several accredited facilities across the country where all of this testing takes place; click here for more info.

Testing Acceptance Criteria

Now to pull back the curtain a bit on the criteria we follow in our testing: We test our panels in accordance with the criteria provided in ICC-ES AC130 – Acceptance Criteria for Prefabricated Wood Shear Panels or ICC-ES AC322 – Acceptance Criteria for Prefabricated, Cold-Formed, Steel Lateral-Force-Resisting Vertical Assemblies, as applicable. These criteria reference the applicable ASTM Standard, ASTM E2126-11, which illustrates test set-up requirements and defines the loading protocol among other things. If you’re interested, the work done by the folks involved with the CUREE-Caltech Woodframe Project, which is the basis for the testing protocol we use today, makes for an excellent read. The CUREE protocol, as it’s known, is a displacement-controlled cyclic loading history that defines how to load a panel. A reference displacement, Δ, is determined from monotonic testing, and the cyclic loading protocol, which is a series of increasing displacements whose amplitudes are functions of Δ, is developed. I’ve provided a graphic depicting the protocol below.

CUREE Loading Protocol (Excerpt from ASTM E2126-11)

CUREE Loading Protocol (Excerpt from ASTM E2126-11)

When prefabricated shear panels are subjected to the loading protocol shown above, a load-displacement response is generated; we call this a hysteresis loop or curve.

Hysteresis Curve (Excerpt from ASTM E2126-11)

Hysteresis Curve (Excerpt from ASTM E2126-11)

We then use this curve to generate an average envelope (backbone) curve that will be used for analysis in accordance with the procedures defined in AC130 or AC322 as applicable.

Average Envelope Curve (Except from ASTM E2126-11)

Average Envelope Curve (Except from ASTM E2126-11)

Returning to the acceptance criteria, there are different points of interest on the average envelope curve depending upon whether we’re establishing allowable test-based values for wind-governed designs or for seismic-governed designs. I should also note that both wind and seismic designs consider both drift and strength limits when determining allowable design values.

Wind is fairly straightforward, so let’s start there. While the building code does not explicitly define a story drift limit for wind design, the acceptance criteria do. The allowable wind drift, Δwind, shall be taken as H/180, where H is the story height. The allowable ASD in-plane shear value, Vwind, is taken as the load corresponding to Δwind. I mentioned a strength limit as well; this is simply taken as the ultimate test load divided by a safety factor of 2.0.

Contrary to wind design, the building code does define a story drift limit for seismic design. ASCE7 Table 12.12-1 defines the allowable story drift, δx, as 0.025H for our purposes, where H is the story height. The strength design level response displacement, δxe, is now determined using ASCE7 Equation 12.8-15 as referenced in AC130 and AC322 as follows:

formula1

Where:

  • δxe = LRFD strength design level response displacement
  • δx = Allowable story drift = 0.025H for Risk Category I/II Buildings (ASCE7 Table 12.12-1)
  • Ie = Seismic importance factor = 1.0 for Risk Category I Buildings (ASCE7 Table 1.5-2)
  • Cd = Deflection amplification factor = 4.0 for bearing wall systems consisting of light-frame wood walls sheathed with wood structural panels rated for shear resistance (ASCE7 Table 12.2-1)

We then consider the shear load corresponding to the strength level response displacement, VLRFD, and multiply this value by 0.7 to determine the allowable ASD shear based on the seismic drift limit, VASD. Lastly, the seismic strength limit is taken as the ultimate test load divided by a safety factor of 2.5.

Compatibility with Code-Prescribed Methods

We’ve gone through the steps to evaluate the allowable design values for our panels, but we’re not done yet. AC130 and AC322 define a series of criteria to ensure that the seismic response is compatible with code-defined methods with respect to strength, ductility and deformation capacity. Once we verify that these compatibility parameters have been satisfied, we may then apply the response modification factor, R, overstrength factor, Ωo, and deflection amplification factor, Cd, defined in ASCE7 for bearing wall systems consisting of light-frame wood or cold-formed steel walls sheathed with wood structural panels or steel sheets. This enables the prefabricated shearwalls to be used in light-frame wood or cold-formed steel construction. I’ve very briefly covered an important topic in seismic compatibility, but there has been plenty published on the issue; I recommend perusing the article here for more details.

We’ve now followed the path from building code to acceptance criteria to evaluation report. More importantly, we understand why Strong-Wall® shearwall panels are required and the basics of how they’re evaluated. If there are items that you’d like to see covered in more detail or if you have questions, let us know in the comments below.

 

Temblor Insights: Is the San Andreas “locked, loaded, and ready to go?”

Editor’s Note: This is a republished blog post with an introduction by Jeff Ellis.

This is definitely an attention-grabbing headline! At the National Earthquake Conference in Long Beach on May 4, 2016, Dr. Thomas Jordan of the Southern California Earthquake Center gave a talk which ended with a summary statement that the San Andreas Fault is “locked, loaded and ready to go.”

The LA Times and other publications have followed up with articles based on that statement. Temblor is a mobile-friendly web app recently developed to inform homeowners of the likelihood of seismic shaking and damage based on their location and home construction. The app’s creators also offer a blog that provides insights into earthquakes and have writtene a post titled “Is the San Andreas ‘locked, loaded, and ready to go’?” This blog post delves a bit deeper to ascertain whether the San Andreas may indeed be poised for the “next great quake” and is certainly a compelling read. Drop, cover and hold on!

Volkan and I presented and exhibited Temblor at the National Earthquake Conference in Long Beach last week. Prof. Thomas Jordan, USC University Professor, William M. Keck Foundation Chair in Geological Sciences, and Director of the Southern California Earthquake Center (SCEC), gave the keynote address. Tom has not only led SCEC through fifteen years of sustained growth and achievement, but he’s also launched countless initiatives critical to earthquake science, such as the Uniform California Earthquake Rupture Forecasts (UCERF), and the international Collaboratory for Scientific Earthquake Predictability (CSEP), a rigorous independent protocol for testing earthquake forecasts and prediction hypotheses.

In his speech, Tom argued that to understand the full range and likelihood of future earthquakes and their associated shaking, we must make thousands if not millions of 3D simulations. To do this we need to use theTom Jordan portrait next generation of super-computers—because the current generation is too slow! The shaking can be dramatically amplified in sedimentary basins and when seismic waves bounce off deep layers, features absent or muted in current methods. This matters, because these probabilistic hazard assessments form the basis for building construction codes, mandatory retrofit ordinances, and quake insurance premiums. The recent Uniform California Earthquake Rupture Forecast Ver. 3 (Field et al., 2014) makes some strides in this direction. And coming on strong are earthquake simulators such as RSQsim (Dieterich and Richards-Dinger, 2010) that generate thousands of ruptures from a set of physical laws rather than assumed slip and rupture propagation. Equally important are CyberShake models (Graves et al., 2011) of individual scenario earthquakes with realistic basins and layers.

But what really caught the attention of the mediaand the public—was just one slide

Tom closed by making the argument that the San Andreas is, in his words, “locked, loaded, and ready to go.” That got our attention. And he made this case by showing one slide. Here it is, photographed by the LA Times and included in a Times article by Rong-Gong Lin II that quickly went viral.

Source: http://www.latimes.com/local/lanow/la-me-ln-san-andreas-fault-earthquake-20160504-story.html
Source: http://www.latimes.com/local/lanow/la-me-ln-san-andreas-fault-earthquake-20160504-story.html

Believe it or not, Tom was not suggesting there is a gun pointed at our heads. ’Locked’ in seismic parlance means a fault is not freely slipping; ‘loaded’ means that sufficient stress has been reached to overcome the friction that keeps it locked. Tom argued that the San Andreas system accommodates 50 mm/yr (2 in/yr) of plate motion, and so with about 5 m (16 ft) of average slip in great quakes, the fault should produce about one such event a century. Despite that, the time since the last great quake (“open intervals” in the slide) along the 1,000 km-long (600 mi) fault are all longer, and one is three times longer. This is what he means by “ready to go.” Of course, a Mw=7.7 San Andreas event did strike a little over a century ago in 1906, but Tom seemed to be arguing that we should get one quake per century along every section, or at least on the San Andreas.

Could it be this simple?

Now, if things were so obvious, we wouldn’t need supercomputers to forecast quakes. In a sense, Tom’s wake-up call contradicted—or at least short-circuited—the case he so eloquently made in the body of his talk for building a vast inventory of plausible quakes in order to divine the future. But putting that aside, is he right about the San Andreas being ready to go?

Because many misaligned, discontinuous, and bent faults accommodate the broad North America-Pacific plate boundary, the slip rate of the San Andreas is generally about half of the plate rate. Where the San Andreas is isolated and parallel to the plate motion, its slip rate is about 2/3 the plate rate, or 34 mm/yr, but where there are nearby parallel faults, such as the Hayward fault in the Bay Area or the San Jacinto in SoCal, its rate drops to about 1/3 the plate rate, or 17 mm/yr. This means that the time needed to store enough stress to trigger the next quake should not—and perhaps cannot—be uniform. So, here’s how things look to me:

The San Andreas (blue) is only the most prominent element of the 350 km (200 mi) wide plate boundary. Because ruptures do not repeat—either in their slip or their inter-event time—it’s essential to emphasize that these assessments are crude. Further, the uncertainties shown here reflect only the variation in slip rate along the fault. The rates are from Parsons et al. (2014), the 1857 and 1906 average slip are from Sieh (1978) and Song et al. (2008) respectively. The 1812 slip is a model by Lozos (2016), and the 1690 slip is simply a default estimate.
The San Andreas (blue) is only the most prominent element of the 350 km (200 mi) wide plate boundary. Because ruptures do not repeat—either in their slip or their inter-event time—it’s essential to emphasize that these assessments are crude. Further, the uncertainties shown here reflect only the variation in slip rate along the fault. The rates are from Parsons et al. (2014), the 1857 and 1906 average slip are from Sieh (1978) and Song et al. (2008) respectively. The 1812 slip is a model by Lozos (2016), and the 1690 slip is simply a default estimate.

So, how about ‘locked, generally loaded, with some sections perhaps ready to go’

When I repeat Tom’s assessment in the accompanying map and table, I get a more nuanced answer. Even though the time since the last great quake along the southernmost San Andreas is longest, the slip rate there is lowest, and so this section may or may not have accumulated sufficient stress to rupture. And if it were ready to go, why didn’t it rupture in 2010, when the surface waves of the Mw=7.2 El Major-Cucapah quake just across the Mexican border enveloped and jostled that section? The strongest case can be made for a large quakeNicolas Ambraseysoverlapping the site of the Great 1857 Mw=7.8 Ft. Teton quake, largely because of the uniformly high San Andreas slip rate there. But this section undergoes a 40° bend (near the ‘1857’ in the map), which means that the stresses cannot be everywhere optimally aligned for failure: it is “locked” not just by friction but by geometry.

A reality check from Turkey

Sometimes simplicity is a tantalizing mirage, so it’s useful to look at the San Andreas’ twin sister in Turkey: the North Anatolian fault. Both right-lateral faults have about the same slip rate, length, straightness, and range of quake sizes; they both even have a creeping section near their midpoint. But the masterful work of Nicolas Ambraseys, who devoured contemporary historical accounts along the spice and trade routes of Anatolia to glean the record of great quakes (Nick could read 14 languages!) affords us a much longer look than we have of the San Andreas.

The idea that the duration of the open interval can foretell what will happen next loses its luster on the North Anatolian fault because it’s inter-event times, as well as the quake sizes and locations, are so variable. If this 50% variability applied to the San Andreas, no sections could be fairly described as ‘overdue’ today. Tom did not use this term, but others have. We should, then, reserve ‘overdue’ for an open interval more than twice the expected inter-event time.

This figure of North Anatolian fault quakes is from Stein et al. (1997), updated for the 1999 Mw=7.6 Izmit quake, with the white arrows giving the direction of cascading quakes. Even though 1939-1999 saw nearly the entire 1,000 km long fault rupture in a largely western falling-domino sequence, the earlier record is quite different. When we examined the inter-event times (the time between quakes at each point along the fault), we found it to be 450±220 years. Not only was the variation great—50% of the time between quakes—but the propagation direction was also variable.
This figure of North Anatolian fault quakes is from Stein et al. (1997), updated for the 1999 Mw=7.6 Izmit quake, with the white arrows giving the direction of cascading quakes. Even though 1939-1999 saw nearly the entire 1,000 km long fault rupture in a largely western falling-domino sequence, the earlier record is quite different. When we examined the inter-event times (the time between quakes at each point along the fault), we found it to be 450±220 years. Not only was the variation great—50% of the time between quakes—but the propagation direction was also variable.

However, another San Andreas look-alike, the Alpine Fault in New Zealand, has a record of more regular earthquakes, with an inter-event variability of 33% for the past 24 prehistoric quakes (Berryman et al., 2012). But the Alpine fault is straighter and more isolated than the San Andreas and North Anatolian faults, and so earthquakes on adjacent faults do not add or subtract stress from it. And even though the 31 mm/yr slip rate on the southern Alpine Fault is similar to the San Andreas, the mean inter-event time on the Alpine is longer than any of the San Andreas’ open intervals: 330 years. So, while it’s fascinating that there is a ‘metronome fault’ out there, the Alpine is probably not a good guidepost for the San Andreas.

If Tom’s slide is too simple, and mine is too equivocal, what’s the right answer?

I believe the best available answer is furnished by the latest California rupture model, UCERF3. Rather than looking only at the four San Andreas events, the team created hundreds of thousands of physically plausible ruptures on all 2,000 or so known faults. They found that the mean time between Mw≥7.7 shocks in California is about 106 years (they report an annual frequency of 9.4 x 10^-3 in Table 13 of Field et al., 2014; Mw=7.7 is about the size of the 1906 quake; 1857 was probably a Mw=7.8, and 1812 was probably Mw=7.5). In fact, this 106-year interval might even be the origin of Tom’s ‘once per century’ expectation since he is a UCERF3 author.

But these large events need not strike on the San Andreas, let alone on specific San Andreas sections, and there are a dozen faults capable of firing off quakes of this size in the state. While the probability is higher on the San Andreas than off, in 1872 we had a Mw=7.5-7.7 on the Owen’s Valley fault (Beanland and Clark, 1994). In the 200 years of historic records, the state has experienced up to three Mw≥7.7 events, in southern (1857) and eastern (1872), and northern (1906) California. This rate is consistent with, or perhaps even a little higher than, the long-term model average.

So, what’s the message

While the southern San Andreas is a likely candidate for the next great quake, ‘overdue’ would be over-reach, and there are many other fault sections that could rupture. But since the mean time between Mw≥7.7 California shocks is about 106 years, and we are 110 years downstream from the last one, we should all be prepared—even if we cannot be forewarned.

Ross Stein (ross@temblor.net), Temblor

You can check your home’s seismic risk at Temblor

References cited:

Sarah Beanland and Malcolm M. Clark (1994), The Owens Valley fault zone, eastern California, and surface faulting associated with the 1872 earthquake, U.S. Geol. Surv. Bulletin 1982, 29 p.

Kelvin R. Berryman, Ursula A. Cochran, Kate J. Clark, Glenn P. Biasi, Robert M. Langridge, Pilar Villamor (2012), Major Earthquakes Occur Regularly on an Isolated Plate Boundary Fault, Science, 336, 1690-1693, DOI: 10.1126/science.1218959

James H. Dietrich and Keith Richards-Dinger (2010), Earthquake recurrence in simulated fault systems, Pure Appl. Geophysics, 167, 1087-1104, DOI: 10.1007/s00024-010-0094-0.

Edward H. (Ned) Field, R. J. Arrowsmith, G. P. Biasi, P. Bird, T. E. Dawson, K. R., Felzer, D. D. Jackson, J. M. Johnson, T. H. Jordan, C. Madden, et al.(2014). Uniform California earthquake rupture forecast, version 3 (UCERF3)—The time-independent model, Bull. Seismol. Soc. Am. 104, 1122–1180, doi: 10.1785/0120130164.

Robert Graves, Thomas H. Jordan, Scott Callaghan, Ewa Deelman, Edward Field, Gideon Juve, Carl Kesselman, Philip Maechling, Gaurang Mehta, Kevin Milner, David Okaya, Patrick Small, Karan Vahi (2011), CyberShake: A Physics-Based Seismic Hazard Model for Southern California, Pure Appl. Geophysics, 168, 367-381, DOI: 10.1007/s00024-010-0161-6.

Julian C. Lozos (2016), A case for historical joint rupture of the San Andreas and San Jacinto faults, Science Advances, 2, doi: 10.1126/sciadv.1500621.

Tom Parsons, K. M. Johnson, P. Bird, J.M. Bormann, T.E. Dawson, E.H. Field, W.C. Hammond, T.A. Herring, R. McCarey, Z.-K. Shen, W.R. Thatcher, R.J. Weldon II, and Y. Zeng, Appendix C—Deformation models for UCERF3, USGS Open-File Rep. 2013–1165, 66 pp.

Seok Goo Song, Gregory C. Beroza and Paul Segall (2008), A Unified Source Model for the 1906 San Francisco Earthquake, Bull. Seismol. Soc. Amer., 98, 823-831, doi: 10.1785/0120060402

Kerry E. Sieh (1978), Slip along the San Andreas fault associated with the great 1857 earthquake, Bull. Seismol. Soc. Am., 68, 1421-1448.

Ross S. Stein, Aykut A. Barka, and James H. Dieterich (1997), Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering, Geophys. J. Int., 128, 594-604, 1997, 10.1111/j.1365-246X.1997.tb05321.x

Simpson Strong-Tie® Strong-Wall® Wood Shearwall – The Latest in Our Prefabricated Shearwall Panel Line Part 1

calebphoto1This week’s post comes from Caleb Knudson, an R&D Engineer at our home office. Since joining Simpson Strong-Tie in 2005, he has been involved with engineered wood products and has more recently focused his efforts on our line of prefabricated Strong-Wall Shearwall panels. Caleb earned both his Bachelor’s and Master’s degrees in Civil Engineering with an emphasis on Structures from Washington State University. Upon completion of his graduate work, which focused on the performance of bolted timber connections, Caleb began his career at Simpson and is a licensed professional engineer in the state of California.

Some contractors and framers have large hands, which can pose a challenge for them when they’re trying to install the holdown nuts used to attach our Strong-Wall® SB (SWSB) Shearwall product to the foundation. Couple that challenge with the fact that anchorage attachment can only be achieved from the edges of the SWSB panel, and variable site-built framing conditions can limit access depending upon the installation sequence. To alleviate anchorage accessibility issues, we’ve required a gap between the existing adjacent framing and SWSB panel equal to the width of a 2x stud to provide access so the holdown nut can be tightened. Even so, try telling a framer an inch and a half is plenty of room in which to install the nut!

SWSB Edge Access

SWSB Edge Access

2x Gap for SWSB Installation

2x Gap for SWSB Installation

 

 

 

 

 

 

 

 

 

While the SWSB is a fantastic product with many great features and benefits from its field adjustability to its versatility with different applications and some of the highest allowable values in the industry, the installation challenges were real.

Back to the Drawing Board

Our goal was to develop a new holdown for the SWSB that would allow for face access of the anchor bolts, making the panel compatible with any framing condition, while maintaining equivalent performance. All we needed to do is cut a large hole in each face of the holdown without compromising strength or stiffness — piece of cake, right? Well, that’s exactly what we did. In the process, we addressed the needs of the architect, the engineer and the builder — and for bonus points, anchorage inspection is now much easier, which should make the building official happy too.

Introducing the Simpson Strong-Tie® Strong-Wall® Wood Shearwall

Simpson Strong-Tie® has just launched the Strong-Wall® Wood Shearwall (WSW) panel, which replaces the SWSB. The new panel provides the same features and benefits, and addresses the same applications as the SWSB; however, now it also features face-access holdowns distinguished by their Simpson Strong-Tie orange color.

Strong-Wall Wood Shearwall

Strong-Wall Wood Shearwall

We’ve also updated the top connection, which now provides two options based on installer preference. The standard installation uses the two shear plates shipped with the panel which are installed on each side of the panel by means of nails. As an alternative, the builder can install a single shear plate from either side of the panel using a combination of Strong-Drive® SD Connector screws and Strong-Drive® SDS Heavy-Duty Connector screws.

woodshear4

Allowable In-Plane Lateral Shear Loads

I mentioned that one of our primary development requirements was to meet the existing allowable design values of the SWSB. Not only did we meet our target values, but we exceeded them by as much as 25% for standard and balloon framing application panels and up to 50% for portal application panels. I’ve included a table below showing the most commonly specified standard and portal application SWSB models and how the allowable wind and seismic shear values compare to those of the corresponding WSW model.

woodshear11

Grade-Beam Anchorage Solutions

I’d be remiss if I didn’t point out the grade-beam anchorage solutions we’ve developed for use with the Strong-Wall Wood Shearwall. The solutions have been calculated to conform to ACI 318-14, and testing at the Simpson Strong-Tie Tyrell Gilb Research Laboratory confirmed the need to comply with ACI 318 requirements to prevent plastic hinging at anchor locations for seismic loading. The testing consisted of 1) control specimens without anchor reinforcement, 2) specimens with closed-tie anchor reinforcement, and 3) specimens with non-closed u-stirrups. Flexural and shear reinforcement were designed to resist amplified anchorage forces and compared to test beams designed for non-amplified strength-level forces.

Significant Findings from Testing

We found that grade-beam flexural and shear capacity is critical to anchor performance and must be designed to exceed the demands created by the attached structure. In wind load applications, this includes the factored demand from the WSW. In seismic applications, testing and analysis have shown that in order to achieve the anchor performance expected by ACI 318 Anchorage design methodologies, the concrete member design strength needs to resist the amplified anchor design demand from ACI 318-14 Section 17.2.3.4. To help Designers achieve this, Simpson Strong-Tie recommends applying the seismic design moment listed below at the WSW location.

woodshear7

We also found that closed-tie anchor reinforcement is critical to maintain the integrity of the reinforced core where the anchor is located. Testing with u-stirrups that did not include complete closed ties showed premature splitting failure of the grade beam. In a previous blog post, we discussed our grade-beam test program in much greater detail as it applies to our Steel Strong-Wall panels.

Strong-Wall® Wood Shearwall

To support the Strong-Wall Wood Shearwall, Simpson Strong-Tie has published a 52-page catalog with design information and installation details. We’ve also received code listing from ICC-ES; the evaluation report may be found here. Now that you’re all familiar with the WSW, be sure to check out next week’s blog post where we’ll cover the basics of prefabricated shear panel testing and evaluation. In addition, to help Designers understand all of the development and testing as well as design examples using prefabricated shearwalls, Simpson Strong-Tie will be offering a Prefabricated Wood Shearwall Webinar on June 21, 2016, covering:

  • The different types of prefabricated shearwalls and why they were developed.
  • The engineering and testing behind prefabricated shearwalls.
  • Best practices and design examples for designing to withstand seismic and wind events.
  • Code reports on shearwall applications.
  • Introduction of the latest Simpson Strong-Tie prefabricated shearwall.

You can register for the webinar here.

Last but not least, we always appreciate hearing from you, whether you’re an engineer specifying our panels or in the field handling the installation. If there are applications that we haven’t addressed or additional resources that would be beneficial, please let us know in the comments below.

Understanding and Meeting the ACI 318 – 11 App. D Ductility Requirements – A Design Example

If you’re one of the many engineers still confused by the ACI 318 – 11 Appendix D design provisions, this blog will help explain what’s required to achieve a ductile performing anchorage. Most building codes currently reference ACI 318 – 11 Appendix D as the required provision for designing a wide variety of anchor types that include expansion, undercut, adhesive and cast-in-place anchors in concrete base materials. This blog post will focus on section D.3.3.4.3(a) for an anchor located in a high seismic region. We’ll go over what these requirements are with a simple design example.

Ductility is a benefit in seismic design. A ductile anchor system is one that exhibits a meaningful degree of deformation before failure occurs. However, ductility is distinct from an equally important dimension called strength. Add strength, and a ductile steel element like the one shown in Figure 1 can now exhibit toughness. During a serious earthquake, a structural system with appreciable toughness (i.e., one that possesses both strength and ductility in sufficient degree) can be expected to absorb a tremendous amount of energy as the material plastically deforms and increases the likelihood that an outright failure won’t occur. Any visible deformations could help determine if repair is necessary.

Figure 1 – ½" mild steel threaded rod tensilely loaded to failure (starting stretch length = 8d)

Figure 1 – ½” mild steel threaded rod tensilely loaded to failure (starting stretch length = 8d)

Let’s start off with a simple example that will cover the essential requirements for achieving ductility and applies to any type of structural anchor used in concrete. We’ll arbitrarily choose a post-installed adhesive anchor. This type of anchor is very common in concrete construction and is used for making structural and nonstructural connections that include anchorage of sill plates and holdowns for shear walls, equipment, racks, architectural/mechanical/electrical components and, very frequently, rebar dowels for making section enlargements. We’ll assume the anchor is limited to resisting earthquake loading in tension only and is in seismic design category C – F. Section D.3.3.4.2 requires that if the strength-level earthquake force exceeds 20% of the total factored load, that the anchor be designed in accordance with section D.3.3.4.3 and D.3.3.4.4. We will focus on achieving the ductility option, (a), of D.3.3.4.3.

To understand anchor ductility we need to first identify the possible failure modes of an anchor. Figure 2 shows the three types of failure modes we can expect for an adhesive anchor located away from a free edge. These three failure modes generically apply to virtually any type of anchor (expansion, screw, cast-in-place or undercut). Breakout (Nb) and pullout (Na) are not considered ductile failure modes. Breakout failure (Nb) can occur very suddenly and behaves mostly linear elastic and consequently absorbs a relatively small amount of energy. After pullout failure (Na) has been initiated, the load/displacement behavior of the anchor can be unpredictable, and furthermore, no reliable mechanism exists for plastic deformation to take place. So we’re left with steel (Nsa). To achieve ductility, not only does the steel need to be made of a ductile material but the steel must govern out of the three failure modes. Additionally, the anchor system must be designed so that steel failure governs by a comfortable margin. Breakout and pullout can never control while the steel yields and plastically deforms. This is what is meant by meeting the ductility requirements of Appendix D.

Figure 2 –Three possible failure modes for an adhesive anchor loaded in tension

Figure 2 – Three possible failure modes for an adhesive anchor loaded in tension

Getting back to our design example, we have a single post-installed 5/8” diameter ASTM F1554 Gr. 36 threaded rod that’s embedded 12” deep, in a dry hole, in a concrete element that has a compressive strength of 2,500 psi. The concrete is 18” thick and we assume that the edge distance is large enough to be irrelevant. For this size anchor, the published characteristic bond strength is 743 psi. Anchor software calculations will produce the following information:

ductility4

The governing design strength is compared to a demand or load combination that’s defined elsewhere in the code.

Here’s the question: Before proceeding with the remainder of this blog, judging by the design strength values shown above, should we consider this anchorage ductile? Your intuition might tell you that it’s not ductile. Why? Pullout clearly governs (i.e., steel does not). So it might come as a surprise to learn that this adhesive anchor actually is ductile!

To understand why, we need to look at the nominal strength (not the design strength) of the different anchor failure modes. But first let’s examine the equations used to determine the design strength values above:

ductility5

The above values incorporate the notation φ (“phi”) and a mandatory 0.75 reduction factor for nonductile failure modes (Ncb ,Na) for applications located in high seismic areas (seismic design category C–F). The φ factor is defined in section D.4. However, manufacturers will list factors specific to their adhesive based on anchor testing. The mandatory 0.75 reduction comes from section D.3.3.4.4 and is meant to account for any reduction associated with concrete damage during earthquake loading. The important thing to remember is that the nominal strength provides a better representation of the relative capacity of the different failure modes. Remove these reduction factors and we get the following:

ductility6

Now steel governs since it has the lowest strength. But we’re not done yet. Section D.3.3.4.3.(a).1 of Appendix D requires that the expected steel strength be used in design when checking for ductility. This is done by increasing the specified steel strength by 20%. This is to account for the fact that F1554 Gr. 36 threaded rod, for example, will probably have an ultimate tensile strength greater than the specified 58,000 psi. (Interestingly, the ultimate strength of the ½” threaded rod tested in Figure 1 is roughly 74 ksi, which is about 27% greater than 58,000 psi.) With this in mind, the next step would be to additionally meet section D.3.3.4.3.(a).2 such that the following is met:

ductility7

By increasing the steel strength by 20%, the nominal strength of the nonductile failure modes (Ncb ,Na) must be at least that much greater to help ensure that a ductile anchor system can be achieved. The values to compare finally become:

 

ductility8Now steel governs, but one more thing is required. As shown in Figure 3, Section D.3.3.4.3.(a).3 of Appendix D also requires that the rod be made of ductile steel and have a stretch length of at least eight times the insert diameter (8d). Appendix D defines a ductile steel element as exhibiting an elongation of at least 14% and a reduction in area of at least 30%. ASTM F1554 meets this requirement for all three grades of steel (Grade 36, 55 and 105) with the exception of Grade 55 for anchor nominal sizes greater than 2”. Research has shown that a sufficient stretch length helps ensure that an anchor can experience significant yielding and plastic deformation during tensile loading. The threaded rod shown in Figure 1 was tested using a stretch length of 4” (8d). Lastly, section D.3.3.4.3.(a).4 requires that the anchor be engineered to protect against buckling.

Figure 3 – Stretch length

Figure 3 – Stretch length

Appendix D doesn’t require that an anchor system behave ductilely. Three additional options exist for Designers in section D3.3.4.3. Option (b) allows for the design of an alternate failure mechanism that behaves ductilely. Designing a base plate (or support) that plastically hinges to exhibit ductile performance is one example. Option (c) involves a case where there’s a limit to how much load can be delivered to the anchor. Although option (c) under D.3.3.4.3 falls under the tensile loading section of Appendix D, the best example would apply to anchorage used to secure a wood sill plate or cold-formed steel track. We know from experiments that the wood crushes or the steel yields and locally buckles at a force less than the capacity of the concrete anchorage. Clearly energy is absorbed in the process. The most commonly used option is (d), which amplifies the earthquake load by Ωo. Ωo can be found in ASCE 7 – 10 for both structural and nonstructural components. The value of Ωo is typically taken to be equal to 2.5 (2.0 for storage racks) and is intended to make the anchor system behave linear elastically for the expected design-level earthquake demand.

These same options exist for shear loading cases. However, achieving system ductility through anchor steel is no longer an option for shear loading according to ACI 318 – 11, because the material probably won’t deform appreciably enough to be considered ductile.

While factors such as edge-distance and embedment-depth restrictions make achieving ductility difficult for post-installed anchors, it should come as some consolation that in many cases the Designer can achieve ductile performance for cast-in-place anchors loaded in tension through creative detailing of reinforcing steel (section D.5.2.9) to eliminate breakout as a possible failure mode. This has been explored in some detail in two previous Simpson Strong-Tie blogs titled “Anchor Reinforcement for Concrete Podium Slabs” and “Steel Strong Wall Footings Just Got a Little Slimmer.”

 

Seismic Bracing Requirements for Nonstructural Components

Have you ever been at home during an earthquake and the lights turned off due to a loss of power?  Imagine what it would be like to be in a hospital on an operating table during an earthquake or for a ceiling to fall on you while you are lying on your hospital bed.

One of the last things you want is to experience serious electrical, mechanical or plumbing failures during or after a seismic event. During the 1994 Northridge earthquake, 80%-90% of the damage to buildings was to nonstructural components. Ten key hospitals in the area were temporarily inoperable primarily because of water damage, broken glass, dangling light fixtures or lack of emergency power.

Complete loss of suspended ceilings and light fixtures in the 1994 Northridge Earthquake. (FEMA 74, 1994)

Complete loss of suspended ceilings and light fixtures in the 1994 Northridge Earthquake. (FEMA 74, 1994)

Broken sprinkler pipe at Olive View Hospital in Sylmar, California after the 1994 Northridggde, Earthquake. (FEMA 74, 1994)

Broken sprinkler pipe at Olive View Hospital in Sylmar, California after the 1994 Northridggde, Earthquake. (FEMA 74, 1994)

ASCE 7 has an entire chapter titled Seismic Design Requirement of Nonstructural Components (Chapter 13 of ASCE 7-10) that is devoted to provisions on seismic bracing of nonstructural components. Unfortunately, not a lot of Designers are aware of this part of the ASCE. This blog post will walk Designers through the ASCE 7 requirements.

Nonstructural components consist of architectural, mechanical, electrical and plumbing utilities. Chapter 13 of ASCE 7-10 establishes the minimum design criteria for nonstructural components permanently attached to structures. First, we need to introduce some of the terminology that is used in Chapter 13 of ASCE 7.

seismicbrace3

  • Component – the mechanical equipment or utility.
  • Support – the method to transfer the loads from the component to the structure.
  • Attachment – the method of actual attachment to the structure.
  • Importance Factor (Ip) – identifies which components are required to be fully functioning during and after a seismic event. This factor also identifies components that may contain toxic chemicals, explosive substances, or hazardous material in excess of certain quantities. This is typically determined by the Designer.

Section 13.2.1 of ASCE 7 requires architectural, mechanical and electrical components to be designed and anchored per criteria listed in Table 13.2-1 below.

seismicbrace4

Architectural components consist of furniture, interior partition walls, ceilings, lights, fans, exterior cladding, exterior walls, etc. This list may seem minor compared to structural components, but if these components are not properly secured, they can fall and hurt the occupants or prevent them from escaping a building during a seismic event. The risk of fire also increases during an earthquake, further endangering the occupants.

Failure of office partitions and ceilings during the Northridge 1994 Earthquake. (FEMA 74, 1994)

Failure of office partitions and ceilings during the Northridge 1994 Earthquake. (FEMA 74, 1994)

Damage to overloaded  storage racks during the 1994 Northridge Earthquake. (FEMA 74, 1994)

Damage to overloaded storage racks during the 1994 Northridge Earthquake. (FEMA 74, 1994)

Section 13.5 of ASCE 7-10 includes the necessary requirements for seismic bracing of architectural components. Table 13.5-1 provides various architectural components and the seismic coefficients required to determine the force level the attachments and supports are to be designed for.

Mechanical and electrical components consist of floor-mounted and suspended equipment. It also includes suspended distributed utilities such as ducts, pipes or conduits. These components are essential in providing the necessary functions of a building. In a hospital, these components are required to be fully functioning both during and after a seismic event. A disruption of these components can make an entire hospital building unusable. In order for hospitals to properly service the needs of the public after a seismic event, fully functioning equipment is essential.

Failed attachments to a chiller after the 1994 Northridge Earthquake. (FEMA 74, 1994)

Failed attachments to a chiller after the 1994 Northridge Earthquake. (FEMA 74, 1994)

Section 13.6 of ASCE 7-10 provides the requirements of seismic bracing for mechanical and electrical components. Table 13.6-1 provides a list of typical components and the coefficients required to determine the force level the attachments and supports are to be designed for.

Chapter 13 lists some typical requirements for which components are to be anchored and supported under specific conditions:

  1. Section 13.1.4 item 6c: Any component weighing more than 400 pounds.
  2. Section 13.1.4 item 6c: Any component where its center of gravity is more than 4 feet above the floor.
  3. Section 13.6.5.6 has specific electrical conduit size and weight requirements.
  4. Section 13.6.7 has specific size and weight requirements for suspended duct systems.
  5. Section 13.6.7 has specific size and weight requirements for suspended piping systems.

The chapter also has some general exceptions to the rules:

  1. 12 Inch Rule: When a distributed system such as conduit ducts or pipes are suspended from the structure with hangers less than 12 inches in length, seismic bracing is not required.
  2. If the support carrying multiple pipes or conduits weighs less than 10 pound/feet of lineal weight of the component, the seismic bracing of the support does not have to be considered.
Example of a support with multiple pipes and the hanger rod length. These exceptions do have limitations, which that are clearly listed in Ssections 13.6.5.6, 13.6.7 and 13.6.8.

Example of a support with multiple pipes and the hanger rod length.

These exceptions do have limitations that are clearly listed in Sections 13.6.5.6, 13.6.7 and 13.6.8.

These systems may not seem important in the structural systems of a building, but they are essential in allowing the building to function the way it was designed to serve the public. It is also important that occupants are able to escape a damaged building after a seismic event. Obstacles such as bookcases blocking exit doors or falling debris may prevent occupants from leaving a building after a seismic event.

It is important that Designers are aware of these code requirements and take the time to read and understand what is needed to provide a safe structure.

 

“You Cannot Escape Responsibility Tomorrow by Evading it Today”

While the contents of this blog are certainly not what Abraham Lincoln had in mind when he made the statement that I’m using to title this blog post, it does speak volumes to the pertinence of what will be discussed today. “Design by others” or some variation of this appears in many parts of Simpson Strong-Tie details.

Simpson Strong-Tie receives technical calls from contractors and plans examiners inquiring about information that requires input from the Designer. When these calls occur during construction there can be confusion and frustration in the field because the Designer is needed to evaluate and resolve the issue. Designers and engineers that identify these conditions ahead of time will reduce confusion and delays on their projects.

Strong-Wall® Shearwalls

The Strong-Wall® shearwall product line uses several iterations of “design by others” within its installation details. It is important to note that many details within the installation drawings may require input from the Designer when certain conditions exist.

For example, Detail 7 on SSW2 shows an alternate first-story installation where the Steel Strong-Wall shearwall bypasses the floor framing and bears directly onto concrete. A ledger may be attached to the shearwall to support perpendicular floor framing. The specification of the hanger and attachment is project specific and would require evaluation by the Designer.
framing1

This condition is also on the Strong-Wall SB shearwall installation details as seen in the following detail. The detail is generic and requires attachment information from the Designer.  When these conditions do not exist on the project, it may be beneficial to cross out the detail or delete and state “not used” to lessen confusion during plan check and construction.

framing2

The note “Foundation Dimensions are for Anchorage only. Foundation design (size and reinforcement) by others” can be found in multiple locations on the Wood Strong-Wall,  Steel Strong-Wall and Strong-Wall Shear Brace detail sheets.

framing3

Soil type and loading conditions not related to the product design vary from project to project and cannot be designed into a one-size-fits-all foundation solution. Thus, the information provided by Simpson Strong-Tie in the installation drawings only addresses the concrete anchorage requirements of ACI 318 (Section D5.2.9/ACI 318-11; Section 17.4.2.9/ACI318-14). These details assume no reinforcement in the footing, resulting in rather large foundations. Since design requirements vary with every project, it’s important for Designers to evaluate and verify each condition.

The new Steel Strong-Wall shearwall grade beam solutions reduce the size of the footings required for anchorage. However, the Designer must specify the grade beam reinforcement for proper performance. The details for the grade beam solutions (see SSW1.1 sheet) are based on ACI 318 and testing that was conducted by Simpson Strong-Tie.

For the grade beam solutions, Designers have two options:

(1) Design grade beam to resist the moment induced from amplified forces to the anchor, or

(2) The lesser of the tabulated moment or the amplified LRFD design moment for seismic (ASD Shear / 0.7) x Ω0 x (SSW Height).

Furthermore, the Designer is responsible for specifying the size and number of shear and flexural reinforcement throughout the grade beam beyond the anchor reinforcement depicted in the details.

Delivery of forces to the Strong-Wall shearwall (to the top of wall), including properly sizing the structural members, should be based on project specific requirements.

Strong Frame® Moment Frames

Simpson Strong-Tie Strong Frame® ordinary moment frames and special moment frames contain similar requirements for the Designer. Moment frames have been discussed many times in this blog: Special Moment Frame Installation: What Structural Engineers Should Watch For, Steel Moment Frame Beam Bracing and Breaking News:  Simpson Strong-Tie Strong Frame Special Moment Frame Testing Today.

The notes on SMF2 state “Footing/Grade beam size and reinforcing shall be specified by the Designer as required to resist the imposed loads, such as foundation shear and bending, soil bearing pressure, shear transfer, and frame stability/overturning.”

Moment frame foundation solutions are based on satisfying the minimum concrete anchorage requirements. Detailing can be a crucial area for this product line as it is common to find deeper footings at these locations, which should be reflected on your construction documents.

Like Strong Wall shearwalls, Designers must evaluate the project conditions and detail the load path of the forces to the resisting element (in this case the Strong Frame moment frame). Ensuring proper transfer of forces that are detailed within your construction documents will reduce headaches down the road.

Strong-RodTM Systems

Strong-Rod™ systems are continuous rod tiedown solutions for multi-story, light-frame wood construction. These systems include Anchor Tiedown Systems (ATS) for shearwall overturning restraint and Uplift Restraint Systems (URS) for roofs. Both systems utilize the same components (i.e. bearing plates, rods, couplers and shrinkage compensation devices), however the detailing, design and locations of these two systems differs.

For ATS, the Designer is responsible for the following:

  • Developing the cumulative tension/compression loads,
  • Determining the system displacement requirements as defined in ICC-ES Acceptance Criteria AC316 to satisfy code required drift equations (this specifically addresses the holdown part of the Equation 4.3-1 of the 2015 Special Design Provisions for Wind and Seismic)
  • The location of each holdown/shearwall.

A more elaborate description of the Designer’s responsibilities for ATS can be found on page 23 of our  Strong-Rod Systems design guide (F-L-SRS15). Simpson Strong-Tie incorporates this information into our design of the system and provides calculations and installation drawings to the Designer for review (a sample two-story run is shown below).

framing4

For URS, there are two different approaches to design that have different level of responsibilities. The Designer has fewer responsibilities when specifying a rod with a “system (CRTS)” evaluation report per AC391, but they are still responsible for developing the project’s wind uplift loads, specifying URS details and designing systems for shearwall overturning.

More requirements must be taken into account when designing and specifying a rod system using steel components with a “rod-run only (CRTR)” AC391 evaluation report, or no report at all. (A more elaborate description of the Designer’s responsibilities for URS can be found on page 43 of the  Strong-Rod Systems design guide).

Based on the project information provided, the rod manufacturer will design and detail the system and submit calculations and installation drawings to the Designer for review.

Podium Deck Anchorage

The newest details published by Simpson Strong-Tie on podium deck anchorage solutions were developed to reduce the impact of an industry-wide challenge; resolving large tension forces (upwards of 50 kips) from four- to five-story narrow shearwalls into thin (often 10-14 inch thick) concrete podium decks. These solutions (e.g., design tables installation drawings and sample calculations), which are on our website, rely on special anchor reinforcement details using standard construction rebar.

framing5

More information about these solutions can be found this blog post and in the shallow anchor page on our website. It is important to note that the Designer is responsible for selecting the best anchorage detail to satisfy the demand loads based on his/her concrete specification and specific project conditions. The Designer is also responsible for designing and detailing the flexural reinforcement within the slab to achieve the amplified forces.

Summary

Adding standard installation details to your construction documents saves significant design time. However, the responsibility does not end with the copy-and-paste. The installation details by Simpson Strong-Tie contain details of many common applications. Some may not apply to your project, while others may require additional input from you. Of course, the Designer is permitted to use alternate details and is not limited to what is shown on the installation drawings. Providing complete information will save time and frustration during plan check and construction. Simpson Strong-Tie is here to answer questions and help with your next project. Please reach out to us by calling 800-999-5099 or by clicking here.