Welcome to our Structural Engineering Blog! I’m Paul McEntee, Engineering R&D Manager at Simpson Strong-Tie. We’ll cover a variety of structural engineering topics here that I hope interest you and help with your projects and work. Social media is “uncharted territory” for a lot of us (me included!), but we here at Simpson Strong-Tie think this is a good way to connect and even start useful discussions among our peers in a way that’s easy to use and doesn’t take up too much of your time. Continue reading
There are products used in every building not referenced by the codes or standards.
These products can impact safety, public health and general welfare through their effect on structural strength, stability, fire resistance and other building performance attributes. I-code Section 104.11 (Alternative materials, design, and methods of construction and equipment) provides guidance on how these products are approved for use in the built environment and identifies the Building Official as the decision-maker. This is similar to a referee determining a player’s compliance with the rules.
Building Officials see submittals for a wide variety of alternative building products ranging from the simple to the very complex. The amount of data included in these submittals and their relevance and completeness varies significantly from insufficient and minimal to complete and very thorough. In the absence of publicly developed and majority-approved provisions, the Building Official is tasked to ensure the data provided is appropriate and adequately proves the alternative product meets code intent to protect public safety, no matter the product type or complexity. This is compared to the robust code and standards process in which committees with balanced representation publicly develop and deliberate on provisions in order to protect public safety. The question arises whether the 104.11 requirement implies that a similar robust process be used in the development of test and evaluation requirements for alternative products as is used for the development of code and standard provisions where there is public debate, resolution of negative opinions and a majority approval of the requirements. Requiring a similar code development process for alternative products would seems to make sense. Otherwise, a less rigorous process might be employed by those seeking to avoid a more robust code and standard process so as to achieve quicker and less stringent approval for their alternative products.
Some may argue that having to use a “code-like” evaluation process for alternative products would add too much of a burden in time and cost, and that it’s not necessary since individual registered design professionals and building officials have enough time, resources and expertise to determine acceptability. But this begs the question of why a similar public majority-approval process should not be required for new products as it is required for code-referenced products. Another question that comes up is ongoing acceptance of an alternative product, as their manufacture may have changed since their approval. Additionally, different jurisdictions have different expertise and resources and this can lead to different standards for approval for alternative products, leading to inconsistency.
Is there a solution which balances providing innovative and cost-effective alternative building product solutions to the industry in a timely manner with providing a thorough product assessment using a process similar to the codes and standards to better ensure consistency and public safety? Accredited building product certification companies, or evaluation service companies, that use a publicly developed and majority-approved acceptance or evaluation criteria and publish an evaluation report with the product’s description, design and installation requirements and limitations provide such a solution. These evaluation service companies are a third-party resource for building officials to assist in their determination of whether an alternative product meets code intent and should be approved for use in their jurisdiction.
The number of evaluation service companies has been increasing. The ICC Evaluation Service and the IAPMO Uniform Evaluation Service, two of the better-known such companies, are both ANSI accredited to ISO/IEC 17065 (Conformity Assessment – Requirements for bodies certifying products, processes, and services) to provide building-code product certifications (ICC-ES, IAPMO UES). However, accreditation by itself mainly verifies a certain process is implemented to ensure consistency and confidentiality. Both companies also have a public acceptance or evaluation criteria process. This process includes an evaluation committee made up of building enforcement officials. These officials evaluate the proposed criteria, listen to expert and industry input and only approve the criteria by a majority vote if products evaluated to those criteria will meet code intent. This is similar to how the codes and standards are developed — a transparent public process and a majority approval of requirements and not just an opinion of one or a couple of individuals.
The alternative building product review process for ICC-ES and IAPMO UES is similar and has the following important components.
- CRITERIA: The accredited product evaluation service develops an acceptance or evaluation criteria, with the manufacturer’s and public’s input, that is publicly debated, revised and ultimately approved by a majority vote of a committee of building enforcement officials.
- TESTING: The manufacturer contracts out to an accredited independent third-party test laboratory to either perform or witness the product testing in accordance with the criteria.
- REVIEW: Registered design professionals with the accredited product evaluation service evaluate the testing and analyses performed and sealed by registered design professionals with the manufacturers or their representatives. The product evaluation service then publishes the evaluation report to their website, and the report typically contains the product description, design and installation requirements andlimitations.
- CONTINUOUS COMPLIANCE: The manufacturer’s quality system is inspected at least annually by the product evaluation service or an accredited third-party
inspection agency to ensure that the product currently being manufactured is the same as that which was evaluated.
While the term “product evaluation” is sometimes used, it is often “product certification” or “product conformity assessment.” ISO/IEC Guide 2:2004 defines “conformity assessment” as “Any activity concerned with determining directly or indirectly that relevant requirements are fulfilled. Some “product certification” companies also provide “product listing” services for when testing and evaluation requirements for the product are already in code-referenced consensus standards, making the development of acceptance criteria unnecessary, thus simplifying the process.
A couple of previous blog posts on evaluation or code reports that you may find informative discuss steps to obtain an evaluation or code report and provide a checklist to determine adequacy of a report.
A mechanism is available to the building industry to provide innovative and cost-effective alternative building products in a timely manner that implements a public and majority product acceptance criteria process, similar to the codes and standards development process. This solution involves the Building Official referencing building product evaluation service reports, based on acceptance criteria, offering a robust evaluation better ensuring that an alternative product meets code intent, thus protecting the public. In fact, several jurisdictions do require evaluation service reports for alternative products.
Should there be an easier path to approval for alternative products than for code-referenced products? What is a reasonable path to product approval? What basis do you use in reviewing evaluation or code reports to determine whether an alternative product is “in or out of -bounds”? We’d love to hear your thoughts.
Five Simpson Strong-Tie employees had the opportunity to participate in a week-long Habitat for Humanity build in the small town of Amarante, Portugal, in late April. The group was originally scheduled to work on a Habitat project in Nepal late last year as part of Habitat’s Jimmy and Rosalynn Carter Work Project (CWP), but following the signing of a new constitution and civil unrest in the country, the project was canceled.
The company decided to allocate the funds for the CWP to Habitat’s Global Village program, allowing these employees to help renovate and remodel the older home of a widowed mother (Doña Margarida Ribiero) and daughter (Sonia) living in the Portuguese countryside.
The group, along with five other volunteers from the U.S., ranging from 29 to 76 years in age, was the first to start work on the 30-plus-year-old home. Alan Hanson, one of the Simpson Strong-Tie participants, was asked to share his thoughts about the experience.
My journey to Portugal began with a one-week vacation in the country with my wife, Holli. We traveled from Lisbon to Sintra, and then to Porto, the capital of port wine. It was a wonderful way to get to know the country. We met a number of friendly, unreserved people throughout the area. Language wasn’t a real barrier, since many locals spoke English. We toured cities, beaches, castles, palaces and other points of interest.
Holli flew out Saturday morning, so fellow Simpson Strong-Tie employee Rick Reid and I explored Porto for the rest of the day. We took a tour of the city, tasted some port wine, and had great meals. We met the other employees from Simpson Strong-Tie (Desiree Aquino, Phil Taylor, and Doug Melcolm) that night and had a seafood dinner near the water.
On Sunday morning, we met the rest of the volunteers from the U.S. as well as Florbela, our Habitat for Humanity representative. We took the 45-minute trip to Amarante, the city where the build would take place and had the afternoon free to explore. We were all excited about getting started on the build!
On Monday morning, we were taken to a rural part of Amarante where we met Doña Margarida, the homeowner, and Rogerio, the Habitat for Humanity superintendent. The house was very old and in need of many renovations. It had been added onto several times and was not very functional. Our work would entail remodeling rooms (a bedroom would become a living room), creating a hallway where none existed, and creating more space throughout the home.
We hit the ground running, cutting two new doorways into the granite and block, leveling out the irregular floors, filling in doorways that could no longer be used, patching various holes and openings, digging a ditch for the waste lines, removing paint and concrete from the granite interiors, and making many other improvements to the home throughout the build week. As a thank you at the end of each day, Doña Margarida served us homemade red and “green” wine (the vino verde is a lightly carbonated white wine — delicious) with smoked ham and sausage. Despite the language barrier (she didn’t speak English), we could see that she was very grateful for our hard work, and she many times worked alongside us.
On Thursday we had our R&R day. We traveled to Guimaraes, about 45 minutes away. We had the opportunity to tour the “birthplace of Portugal” castle and palace and learn a lot of early Portuguese history. Friday came very early and we were off to Doña Margarida’s house again. We tore out another wall, finished fixing a few more openings, patched various holes in walls and leveled another floor.
Our last day at Doña Margarida’s house was actually only half a day on Saturday. We laid block in the door we removed earlier, filled in the floor where we tore out the wall, and made finishing touches to the patching on the other doors we filled in, as well as the hole from the wood stove. We accomplished a TON of work in 4½ days! We were told that we had finished ahead of schedule and completed more projects than were expected. We all had lunch together, including the family. In true Simpson Strong-Tie fashion, we had gifts for the family and our superintendent. We gave Rogerio a Simpson Strong-Tie-branded knife and sweatshirt and Doña Margarida a comforter and a wooden bowl that Phil made. He is quite a craftsman and did a wonderful job on it! Tears were shed, and we loaded into the van for the last time in Amarante. I took a nap when we got in because I was exhausted!
On Sunday, we left Amarante, heading to Porto. We attended a port wine tasting and took in a tour of the city. April 25 is “Freedom Day” in Portugal and marks the Carnation Revolution, when the military dictatorship was overthrown in 1974 with very little bloodshed, so there were fireworks at midnight and we had an incredible view. What a great way to end our trip to Portugal!
P.S. The complete renovation of the house is expected to be done in July, and we all can’t wait to see the home finished.
What do a chicken house, a water treatment plant and a raised wood floor system all have in common? Very likely, they all involve preservative-treated lumber. They’re also all examples of common environments in which preservative-treated, metal-plate- connected (MPC) wood trusses may be specified.
Although trusses are successfully used in a variety of environments that require treated lumber, the first mention of “treated lumber” usually sends up a red flag in a truss design office. While the corrosion protection of truss plates is no different from the corrosion protection of any other steel fastener or hanger that comes in contact with treated lumber, there are a few more considerations that come into play whenever treated lumber is going to be used in a truss application.
When fire-retardant-treated lumber or preservative-treated lumber is specified, the first (and easiest) step is to determine whether standard G60 truss plates are acceptable for use with the treated lumber, or whether the chemical treatment requires additional protection of the plates. Recent blog posts have discussed how fasteners are evaluated for corrosion resistance and how the Corrosion Resistance Classifications in our catalog help facilitate selection of hardware and fasteners for different types of treated wood and environmental conditions. Similar guidelines are also available for determining the proper metal connector plate for different wood treatments. For example, when using the sodium borate–based preservatives and fire retardants, standard G60 galvanized metal connector plates are acceptable. However, ammoniacal/alkaline/amine copper quaternary preservative types require more protection, such as G185, ASTM A153 galvanized- or stainless-steel truss plates. The complete guidelines – Quick Guide for Alternative Preservative Treatments with Metal Connector Plates – are available from the SBCA website.
When trusses are used in particularly corrosive environments such as coastal environments or salt storage buildings, the ANSI/TPI 1 standard lists coatings that will provide increased corrosion protection for the plates (see insert, below).
The paint coating systems listed in (a) and (b) have been specified in the TPI standard since 1985. These paint coatings, which are applied to the truss plates after the trusses are manufactured, provide alternatives to the double-dipped galvanized or stainless-steel plates used in coastal high hazard areas. In fact, the ANSI/TPI 1 Commentary states that one study – SSPC Report 87-08, Evaluation of Coatings for Metal Connector Plates – concluded that the paint coating systems over standard galvanized plates would be expected to outperform the double-galvanized metal connector plates in field use.
Once the necessary corrosion protection of the plates has been addressed, the next consideration is the effect of certain lumber treatments on the truss plates’ lateral resistance, or tooth-holding capacity. Fire-retardant treatments generally require strength reductions to be applied to both the lumber and metal connector plate design values. The proprietary treatment manufacturer specifies these design reductions. As soon as the specific treatment is known, the appropriate design reductions can be easily applied by the truss design software and noted on the truss design drawing accordingly.
Besides lumber treatment, there may be other reasons for plate design reductions whenever extra galvanization or special coatings are required. While extra galvanization itself does not necessarily require a reduction in plate values, if the treated lumber’s moisture content (MC) exceeds 19% at the time of truss fabrication, then a 20% reduction to the tooth-holding values is required. The same 20% reduction applies if the environment for the intended end use of the trusses is expected to result in wood moisture content exceeding 19%.
Special Considerations and Red Flags
One corrosive environment that requires special consideration is an enclosed swimming pool. ANSI/TPI 1 requires that trusses be separated from the pool environment by a vapor barrier and be separately ventilated from the pool environment. The exception to this requirement is if the truss plates are made with a stainless steel that is not susceptible to stress corrosion cracking (SCC), i.e., not Types 304 and 316. Since truss plates made with SCC-resistant stainless steel are not readily available (if at all), a vapor barrier is basically required anytime trusses are used over enclosed swimming pools.
Another important consideration in roof truss applications involving treated lumber is the effect of elevated temperatures. For example, when FRT lumber is going to be used in an environment where high moisture content will exist, an FRT formulated for exterior use may be specified. However, if the exterior FRT has not been tested with elevated temperatures as specified in TPI 1 Section 184.108.40.206, it should not be used in a roof application.
But the biggest concern when treated lumber is specified for use in metal-plate-connected wood trusses has nothing to do with corrosion at all. When a truss Designer gets a job that calls for a preservative treatment for exterior use or an exterior FRT, the very first question will be why is an exterior treatment required/what is the application? Although trusses can be adequately designed for many types of environments, there is one environment that does not mix well with metal-plate- connected wood trusses – exposed exterior applications. The TPI/WTCA Guidelines for Use of Alternative Preservative Treatments with Metal Connector Plates concludes with the following statement:
When trusses are exposed to repeated wetting and drying, the corresponding swelling/shrinkage of the wood causes what is commonly referred to as truss plate “back out”. Since the ability of a truss plate to provide lateral resistance depends on the teeth having adequate embedment into the wood members, any plate “back out” or withdrawal from the lumber due to weathering has an adverse effect on the load capacity of the truss plate.
For this reason, MPC wood trusses must be protected from the elements, from the time they are built and stored through the extent of their life in service. High moisture content that is consistently high can be accounted for; but if the trusses will be exposed to moisture cycling, then it is time to consider something other than a metal-plate-connected wood truss.
What are your experiences with treated lumber and/or corrosive environments and wood trusses? Let us know in the comments section below.
On Thursday, May 5, 2016, Washington State University at Pullman, state dignitaries, construction leaders, WSU construction alumni, PACCAR management, Simpson Strong-Tie management and the press celebrated the grand opening and dedication of the PACCAR Environmental Technology Building (PETB) and the Simpson Strong-Tie Research and Testing Laboratory.
The Simpson Strong-Tie team comprised senior leadership, engineering and marketing representatives, led by our CEO, Karen Colonias. In her speech at the opening ceremony, Karen Colonias highlighted the leadership of Simpson Strong-Tie in the engineering and construction materials industry in the U.S. and the world. She emphasized the longstanding partnership between WSU and Simpson Strong-Tie, which spans over twenty years of collaboration in various testing and code development programs, and communicated our excitement at the opportunity to collaborate more closely with WSU’s highly respected engineering department on testing and engineering programs.
The Paccar Environmental Technology Building (PETB) is 96,000 square feet and houses the Composite Materials and Engineering Center (CMEC) – a highly integrated hub of interdisciplinary research and education in the areas of renewable materials, sustainable design, water quality, and atmospheric research. The shared space in this new building will foster the synergy needed to find new solutions to complex industry problems, such as creating human environments that are at once safe, economical and resilient.
The Simpson Strong-Tie® Research and Testing Lab at Washington State University (WSU) is a versatile laboratory designed specifically for the structural testing and prototyping of tall timber buildings, post frame buildings, concrete durability, building repair and retrofit and deck safety, as well as seismic and wind mitigation.
The lab includes a high-capacity reaction 28′ x 46′ concrete floor area with tie-downs, 75-kip capacity at two foot centers through the floor area; a high-capacity wall 28′ long by 2’thick by 18′ tall strong wall that is capable of withstanding a 200-kip reaction in any direction; a central 90-gallon-per-minute hydraulic pump, overhead crate and concrete mixing station. The laboratory is a dynamic space to test new material and design concepts developed in the PETB. This is one of the most visible spaces in the PETB and includes capabilities for mock-ups of new building systems, structural testing and advanced digital manufacturing. Adjoining the lab is an outdoor 32′ by 52′ reaction slab that allows for project display (e.g., Solar Decathlon competition), for developing taller and or larger structures than would be possible on the interior strong floor and for natural weather exposure testing.
The lab is part of the Composite Materials and Engineering Center (CMEC), which has been a leader in the development of wood composite materials for more than 65 years. It is an International Code Council–accredited testing facility. The laboratory highlights engineered wood composites and is constructed of cross-laminated timber, glulam, Parallam and, of course, Simpson Strong-Tie® No- Equal connectors.
Simpson Strong-Tie and WSU, as Karen Colonias mentioned in her speech, have a longstanding and productive partnership going back over 20 years. The two institutions have worked together in a number of areas, including new product testing, deck safety and seismic risk mitigation.
This year, Simpson Strong-Tie made a significant commitment and established the Simpson Strong-Tie Excellence Fund at the Voiland College of Engineering and Architecture at Washington State University (WSU). The fund provides an annual gift of $100,000 per year over the next eight years to support the new Simpson Strong-Tie® Research and Testing Lab in the PACCAR Environmental Technology Building (PETB). In addition to the lab, the Excellence Fund will support fellowships for professors and graduate students to present research findings, brainstorm about future research and conduct continuing education training.
The faculty of the Composite Materials and Engineering Center is committed to addressing the challenge of restoring and improving the U.S. civil infrastructure and offering an integrated approach linking material discovery, manufacturing innovation, product development, and customized design methodologies that will lead to high-performing, cost-effective solutions for the built environment. The core faculty possess diverse expertise that spans materials science (polymers, wood, cement, steel), durability and corrosion protection, manufacturing and sustainable design. The faculty also has a long history of involvement in developing building codes, standards and product acceptance criteria.
This year, the WSU Voiland College of Engineering and Architecture has more than 1,050 students enrolled in civil engineering, architecture and construction management programs. The alumni from these programs are founders of and senior executives in America’s top construction and design firms. The Wall Street Journal ranked WSU among the 25 universities whose graduates are top-rated by industry recruiters, and the Civil Engineering program is the 13th largest in the nation.
On October 29, 2016, and in line with this partnership, Simpson Strong-Tie is conducting its first annual engineering symposium at Washington State University Pullman. In this symposium, Simpson Strong-Tie engineers will share with the engineering and construction management students the various career opportunities that are available in the industry upon their graduation and introduce them to the exciting history of research and innovation at Simpson Strong-Tie. The Symposium will also include testing in the new lab of our No-Equal structural connectors and solutions.
At Simpson Strong-Tie, we are excited to be strengthening the partnership and increasing the collaboration with WSU faculty and students. We are looking forward to an extended and outstanding relationship that drives research and innovations and introduces new methods to design and construct safer, more resilient, sustainable and economical structures.
Onward and Upward!
Louay Shamroukh, P.E., S.E.
Engineering Manager, Northwestern U.S.
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.
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.
Due 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…
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.
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.
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.
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:
- 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.
This week will see the ultimate combination of events intended to raise public awareness of the necessity for disaster-resistant construction: It is week three of ICC’s Building Safety Month; National Hurricane Preparedness Week, as proclaimed by the U.S. president; the NOAA Hurricane Awareness Tour of the Gulf Coast; and the kickoff of the new HurricaneStrong program.
The ICC says that “Building Safety Month is a public awareness campaign to help individuals, families and businesses understand what it takes to create safe and sustainable structures. The campaign reinforces the need for adoption of modern, model building codes, a strong and efficient system of code enforcement and a well-trained, professional workforce to maintain the system.” Building Safety Month has a different focus each week for four weeks. Week One is “Building Solutions for All Ages.” Week Two is “The Science Behind the Codes.” Week Three is “Learn from the Past, Build for Tomorrow.” Finally, Week Four is “Building Codes, A Smart Investment.” Simpson Strong-Tie is proud to be a major sponsor of Week Three of Building Safety Month.
National Hurricane Preparedness Week is recognized each year to raise awareness of the threat posed to Americans by hurricanes. A Presidential Proclamation urged Americans to visit www.Ready.gov and www.Hurricanes.gov/prepare to learn ways to prepare for dangerous hurricanes before they strike. Each day of the week has a different theme. The themes are:
⦁ Determine your risk; develop an evacuation plan
⦁ Secure an insurance check-up; assemble disaster supplies
⦁ Strengthen your home
⦁ Identify your trusted sources of information for a hurricane event
⦁ Complete your written hurricane plan.
This week also marks the NOAA Hurricane Awareness Tour, where NOAA hurricane experts will fly with two of their hurricane research aircraft to five Gulf Coast Cities. Members of the public are invited to come tour the planes and meet the Hurricane Center staff along with representatives of partner agencies. The goal of the tour is to raise awareness about the importance of preparing for the upcoming hurricane season. The aircraft on the tour are an Air Force WC-130J and a NOAA G-IV. These “hurricane hunters” are flown in and around hurricanes to gather data that aids in forecasting the future of the storm. As with Hurricane Preparedness Week, each day of the tour features a different theme. Simpson Strong-Tie is pleased to be a sponsor for Thursday, when the theme is Strengthen Your Home. Representatives from Simpson Strong-Tie will be attending the event on Thursday to help educate homeowners on ways to make their homes safer.
Finally, this week is the official kickoff of a new hurricane resilience initiative, HurricaneStrong. Organized by FLASH, the Federal Alliance for Safe Homes and in partnership with FEMA, NOAA and other partners, the program aims to increase safety and reduce economic losses through collaboration with the most recognized public and private organizations in the disaster safety movement. HurricaneStrong is intended to become an annual effort, with activities starting prior to hurricane season and continuing through the end of the hurricane season on November 30. To learn more, visit www.hurricanestrong.org.
Experts consider these public education efforts to be more important every year, as it becomes longer since landfall of a major hurricane and as more and more people move to coastal areas. The public complacency bred from a lull in major storms has even been given a name: Hurricane Amnesia.
All these efforts may be coming at a good time, assuming one of the hurricane season forecasts is correct. A forecast from North Carolina State predicts an above-average Atlantic Basin hurricane season. On the other hand, forecasters at the Department of Atmospheric Science at Colorado State University are predicting an approximately average year.
Are you prepared for the natural hazards to which your geographic area is vulnerable? If not, do you know where to get the information you need?
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 the 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 media—and 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.
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:
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 quakeoverlapping 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.
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 (firstname.lastname@example.org), Temblor
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
This 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!
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.
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.
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.
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 220.127.116.11. To help Designers achieve this, Simpson Strong-Tie recommends applying the seismic design moment listed below at the WSW location.
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.
This week’s post was written by Kevin Gobble of Habitat for Humanity. Kevin is the Program Manager for Habitat for Humanity’s new Habitat Strong initiative. Kevin has spent over 22 years in residential construction building energy-efficient, high-performing home, and has consulted with several sustainable building programs on ways to develop their own best practices. As a third-generation builder, he has knowledge in the field of residential building science and has furthered his education to include many industry certifications — NARI Certified Remodeler, NAHB Certified Green Professional, RESNET Certified Green Rater, BPI Building Analyst, FORTIFIED evaluator, and Level 1 Infrared Thermography — while working directly with industry partners to focus on cost-effective construction solutions. Kevin has built and remodeled numerous homes to high-performance standards as certified by various building programs, including his latest project for himself: converting a condemned historic property in Atlanta to EarthCraft House Platinum.
In a previous blog post, we discussed the background of the Habitat Strong program. Habitat Strong promotes the building of resilient homes that are better equipped to withstand natural disasters in every region of the country. This program uses IBHS FORTIFIED Home™ standards and works well within Habitat’s model of building affordable, volunteer-friendly homes.
Project Spotlight – Habitat for Humanity New Haven, CT
Habitat for Humanity New Haven’s innovative approach to building a traditional New England– style home with modern improvements began with a design from an historic home they rehabilitated years ago. The original was an old Winchester factory worker home, but the style was adapted to fit the narrow lots and surviving character of New Haven. Along with the design shift, the plans were standardized to incorporate FORTIFIED Gold techniques and practices for hurricanes.
New Haven has fully embraced FORTIFIED building practices following Superstorm Sandy. They have completed eight FORTIFIED Gold homes to date with three more under construction, perfecting their techniques as they go. An example to other Habitat affiliates, they have provided a model for using affordable construction methods and volunteers. They have also created a positive impact on their community by sharing their knowledge with other builders in the area.
“Improving the roof is a no-brainer, and it makes sense to tape the plywood seams,” noted construction manager Antoine Claiborne. Habitat has gone one step further by using the ZIP system on the roof for safety and durability improvements. This eliminates the need to nail down the underlayment every six inches o.c. along the edge and in the field, which can prove difficult for volunteers.
In addition to employing these roof techniques, New Haven uses Simpson Strong-Tie connectors (after re-engineering plans) to meet these new guidelines and to create a continuous load path. To promote ease of use for new volunteers, an advanced framing center is set up onsite, using diagrams and videos to demonstrate how the process works and what to expect. Documentation is another key to the FORTIFIED process – in New Haven’s case, the onsite construction manager documents all the FORTIFIED elements.
For opening protection, New Haven uses pressure-rated doors with Hurricane Fabric for impact protection as well as impact-rated windows. It was recently discovered that impact windows can shatter in a small area if hit there while otherwise remaining intact. Thus there’s a need to use caution when mowing grass where there are small rocks near the home.
Larger beams are often built up out of smaller 2x or 1¾” members. This can be done for several different reasons: for the convenience of handling smaller members on the jobsite, or because solid 4x, 6x or glulam material is not readily available, or for reasons of cost. Engineered wood such as laminated veneer lumber (LVL) is often used for its high load capacity and multiple 1¾” plies are built up to get the required capacity for the application.
When a built-up beam is loaded concentrically as in the test setup shown, fastening the members is not critical since that giant steel plate will load each ply of the beam. In the field, built-up beams or girders commonly support joists or beams framing into their side. The built-up members must be connected to transfer load from the loaded ply into the other plies.
Page 303 of our Fastening Systems catalog, C-F-14 provides allowable uniform load tables for side-loaded multi-ply assemblies using LVL, PSL or LSL material. The calculation for the allowable load applied to the outside ply of a multi-ply beam is:
While uniform loads are very common, Designers often request additional information to design multi-ply beam connections to transfer concentrated loads. Simpson Strong-Tie has created a new engineering letter, L-F-SDWMLTPLY16, which complements the information in the Fastening Systems catalog by providing allowable loads in a single fastener format. Designers can use the information to calculate the number of fasteners required for a given point load.
In order to ensure load transfer, the SDW screws need to be located relatively close to the connection. At first glance, it may appear challenging to fit enough fasteners while meeting the non-staggered row-spacing requirements. However, we have found that most loads can be managed by taking advantage of the ⅝” stagger allowance.
If you are curious what happened in that HHGU14 test, the screws pulled out of the header with a load slightly exceeding 101,000 pounds. Failure photo 2 shows a close-up of the pullout failure. The tested load was very close to the maximum calculated capacity for the SDS screws in the connector, so it was a great test result. What are your thoughts? Let us know in the comments below.