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
The Greek philosopher Heraclitusis credited with saying “The only thing that is constant is change.”
If that applies to building codes, then it applies doubly to wind design using the 2012 International Building Code® (IBC).
The wind load requirements in Section 1609 of the IBC are based on ASCE 7 and refer to this document for most design information. In the 2012 IBC, the referenced version of ASCE 7 changed from the 2005 edition to the 2010 edition. In ASCE 7-10, the wind design requirements have been completely revised, including a complete design philosophy change.
Wind design has changed from an allowable strength-based philosophy with a load factor of 1 in the ASD load combination to an ultimate strength design philosophy with a load factor of 1 in the strength design load combination. This means wind design has a similar basis as seismic design. So the new load combinations for wind look like this:
Strength Design: 0.9D + 1.0W
Allowable Stress Design: 0.6D + 0.6W
Because of the change in load factor and philosophy, the basic wind speed map had to be altered. In the past, one map was provided and the design return period was increased for certain occupancies by multiplying the load by an importance factor. In ASCE 7-10 there are three maps provided so now an importance factor is no longer needed. The return period of the map depends on the risk to human life, health and welfare that would result from the failure of that type of building. This was previously called the Occupancy Category, but it is now called the Risk Category.
Risk Category III and IV buildings use a basic wind speed map based on a 1,700-year return period. Risk Category II buildings use a basic wind speed map based on a 700-year return period. And Risk Category I buildings use a basic wind speed map based on a 300-year return period. Because of the higher return period, the mapped design wind speed will be much higher than when using previous maps. However, with the lower load factors, actual design loads will be the same or in many areas lower due to other changes in the way the map was developed.
Another change to ASCE 7-10 for wind design is that Exposure D is no longer excluded from hurricane prone regions; so buildings exposed to large bodies of water in hurricane prone regions will have to be designed for Exposure D.
Because of the change in wind speeds, there is a change in the definitions of windborne debris regions. Due to the different wind speed design maps, the windborne debris region will be different depending on the Risk Category of the building being built. The windborne debris region is now defined as areas within hurricane-prone regions that are either within 1 mile of the coastal mean high water line where the ultimate design wind speed is 130 mph or greater; or any areas where the ultimate design wind speed is 140 mph or greater; or Hawaii. Risk Category II buildings and structures and Risk Category III buildings and structures (except health care facilities), use the 700-year Risk Category II map to define wind speeds for the purpose of determining windborne debris regions. Risk Category IV buildings and structures and Risk Category III health care facilities use the 1700-year return Category III/IV wind speed map to define wind speeds for the purpose of determining windborne debris regions.
Finally, a new simplified method for determining wind loading on ENCLOSED SIMPLE DIAPHRAGM BUILDINGS WITH h ≤ 160 ft has been added to ASCE 7-10. This is different from the simplified all heights method in the IBC, so it will be interesting to see which method becomes more widely used. Which method do you prefer? Let us know in the comments below.
On Saturday evening, Barclay Simpson passed away peacefully in his sleep, surrounded by his family. He was 93 years old. With Barc’s passing, Simpson Strong-Tie has lost a beloved and inspirational leader. Our country has lost a generous philanthropist, visionary and great American entrepreneur. Those of us who were fortunate enough to know and work with Barc have lost a dear friend, champion and guide.
Barc’s contributions to the construction industry, non-profit community and our employees are immeasurable. He instilled the core values — our “Secret Sauce” — that have made Simpson Strong-Tie a unique and inspiring place to work and have built our reputation as a quality, trusted manufacturer and solid corporate citizen.
The first time I met Barc was less than a week after I started working in the R&D department. I was meeting with a product manager and Barc was walking by, so he stopped in to say hello. We introduced ourselves and chatted for a few minutes. I told him about my work experience, where I went to school, what I was working on, and he even asked where I grew up. He was genuinely interested in getting to know me, which made me feel welcome.
I later noticed that Barc usually parked at the end of the building furthest from his office. He would take a different path through the building – sometimes through engineering, other times he would walk through marketing, accounting, or even the connector test lab. Barc cared deeply for all of his employees, and the intentionally long walk gave him the opportunity to talk with folks.
He firmly believed that everybody in the company is important, and he took every opportunity to remind us. In the video, Barclay Simpson’s Nine Principles of Doing Business, Barc speaks quite passionately about dignifying the contribution of every individual at every level.
In the end of a previous blog post, I mentioned Barc’s 1974 list of Rambling Thoughts on Making One’s Fleeting Moment on This Planet a Pleasant One. In the context of that post, the thoughts of “Attitude Conquers All” and “Keep it light. It really isn’t that important” were appropriate.
Thinking of Barc and his legacy, I prefer this rambling thought from his list:
Strive to have a POSITIVE EFFECT upon those lives touched by your own.
This week’s blog post was written by Aram Khachadourian, R&D Engineer for Fastening Systems. Since joining Simpson Strong-Tie 14 years ago, he has designed and tested holdowns, hangers, truss connectors, and anchor bolts. He has drafted numerous acceptance criteria as well as quality standards. His current focus is the development, testing, and code approval of structural fasteners. Prior to his work at Simpson he spent his time designing steel buildings including strip malls, wineries, and airplane hangars. Aram graduated from the University of California at Davis with a Civil Engineering degree, and is a registered professional engineer in California.
In the past several years, there has been an increase in the use of screws in applications that have traditionally been reserved for bolts and lag screws. Greater innovation in the wood screw market has caused this shift. Proprietary wood screws now offer many more benefits than commodity bolts and lag screws. Today, this post will discuss some of those benefits.
Two of the obvious drawbacks of installing bolts are preboring or predrilling a hole through the wood and ensuring that both sides of the connection are accessible. The drilled hole must be aligned properly in the wood, which is especially important for groups of bolts. When bolting a steel connector on each side of the wood, it takes a skilled hand to guide the drill from one side and hit the steel hole on the other side of the wood. Using proprietary wood screws instead of bolts can relieve the installer of this hassle. Because there is no predrilling, an installer can step up and drive in the screw. They don’t have to worry about lining up the drilled hole with the steel hole because the screws are driven into the connector on each side. This is a real benefit in many applications, such as installing ledgers and steel column cap connectors. Bolts also require washers between the head and the wood and between the nut and the wood. In an all-wood connection, the bolted connection requires a bolt, nut, and two washers or steel plates. Sometimes access from both sides is not possible or is not safe from the drilling position. Usually both the nut and the head of the bolt require a tool to tighten them, and that means using both hands where one may be blind. Proprietary wood screws are typically much faster to install and thus can reduce labor costs for the project.
Proprietary screws also are used as alternatives to lag screws in wood construction. Lag screw installation is included in the NDS, section 11.1.4. Lag screws greater than 3/8-in. diameter require a pre-drilled hole whether loaded in withdrawal or by lateral force. The required hole is a two-step hole. The hole for the shank is supposed to match the diameter and length of the shank and the part of the hole for the threaded shank depends on the specific gravity of the wood member and the relative diameter of the screw.
When comparing proprietary wood screws and commodity wood screws, it’s important to note that in the NDS, commodity wood screws have predrilling requirements that depend on the specific gravity of the wood. An installer cannot just grab a screw and drive it in. In some cases, there are two different diameters that must be predrilled, one for the shank and one for the threads, similar to lag screws.
When our engineers design a Simpson Strong-Tie screw, they go to great lengths so that the installer almost never has to predrill the wood. This is achieved by adding special drill tips, optimizing thread designs, and utilizing knurls or reamers that prepare the wood to receive the shank of the screw. For structural screws that require evaluation reports, qualification testing is performed with no predrilled holes so that the qualified loads are based on the installation instructions that require no predrilled holes.
The other factor our engineering team considers is the performance of screws in wood. Often, using a greater number of screws in place of larger diameter bolts or lag screws can increase the ductility of the failure mode, which is advantageous in certain applications, such as for seismic holdowns. The special features and manufacturing processes of proprietary wood screws can often result in allowable loads that are comparable to larger diameter bolts. These loads are typically determined through the testing and load rating requirements of ICC-ES AC 233.
As more types of screws are developed and more conditions are tested, proprietary screws will continue to replace bolts and lag screws in applications, including ledger connections, pile construction, girder truss and beam connections, steel connector installations, and many more.
Have you made the switch from commodity bolts and lag screws to proprietary wood screws? Let us know how you are using them or tell us how we can support a new application in your market.
One of the questions I am asked most frequently is “Who is responsible for the truss-to-(fill in the blank) connection? One such example is the truss-to-wall connection. To answer this question, it helps to recognize there are two types of connections: a truss-to-truss connection and a “truss-to-everything-else-except-a-truss” connection. The Truss Designer is responsible for the former, and the Building Designer is responsible for the latter. Pretty simple, right? So why all the questions?
Some people incorrectly assume the Truss Designer is responsible for connecting the truss to everything the truss touches. Then, when the Truss Designer doesn’t specify a connection to something the truss touches (such as a wall), it prompts the question, “Hey, who is responsible for that connection? I thought the Truss Designer was!” In other cases, the person asking the question is actually challenging the answer, such as “Shouldn’t the Truss Designer be specifying the truss-to-wall connection? Why don’t they?” And finally, the question may be prompted at times when the project doesn’t have a Project Engineer (aka the Building Designer), so the question becomes, “Now who is going to specify that connection? It must be the Truss Designer, right?”
But the Truss Designer isn’t responsible for the truss-to-wall connection – and here’s why. Unless the scope of work has been expanded by contract, the Truss Designer is responsible for designing an individual component. The truss gets designed for a given set of specified loads, environmental conditions, serviceability criteria and support locations, all which are specified by the person responsible for the overall building: the Building Designer. Once designed, the truss will have a maximum download reaction and uplift reaction (if applicable) at each support location. Is that enough information to specify a truss-to-wall connection? No, it is not. First, the Truss Designer may not know what the truss is even sitting on; he or she may only know that the bearing is SPF material and 3 ½” wide. Is it a single top plate or double top plate? Is there a stud below the truss that can be connected to, or is the stud offset? Or, is the truss sitting on a header spanning across a wide window?
Second, even if the Truss Designer had all of the information regarding the bearing conditions, there is another problem. The Truss Designer has the reactions resulting from the loads applied to the truss. What about the reaction at the top of the wall (perpendicular to the wall) resulting from the lateral loads applied to that wall? And the shear loads acting parallel to the wall as a result of lateral loads applied to the end wall? These loads also need to be resisted by the truss-to-wall connection (hence, the F1 and F2 allowable loads that are published for hurricane ties), so the Truss Designer cannot select an adequate truss-to-wall connection based on the truss reactions alone.
Finally, there’s one more scenario to consider. Say a Building Department requires that truss-to-wall connections must be specified by the Truss Designer on projects that have no Engineer of Record. It wants to ensure trusses are adequately secured to the walls, and the Truss Designer may seem best equipped to determine those connections (this has actually happened in some places). The Truss Designer can find out what exactly the truss is sitting on, and can even calculate some approximate reactions for the top of the wall to conservatively take into account during the selection of the connection. Problem solved? Not entirely. That takes care of the top of the wall, but the load doesn’t stop there. So requiring the Truss Designer to specify the truss-to-wall connection only transfers the problem to the bottom of the wall. Who is going to address those connections?
While most people don’t think of the Truss Designer as being the person responsible for the connections at the bottom of the wall, many do think the Truss Designer should be responsible for the connections at the top of the wall. But because someone – namely, the Building Designer – still needs to ensure that a continuous load path has been satisfied by the connections in the building, does it really help to increase the scope of work of the Truss Designer to specify the truss-to-wall connection?
Let us know your thoughts in the comments below.
As an engineer, it makes things easy when the buildings being designed are rectangular. This tends to make the connections occur between nice perpendicular members, and standard connectors and joist hangers can be used.
But buildings are not always rectangular and connections are not always between perpendicular members. Non-perpendicular members can have a skewed connection, where the supported member is moved side to side from perpendicular; or a sloped connection, where the supported member slopes up or down from a standard horizontal orientation; or a combination of the two.
To help with these situations, Simpson Strong-Tie offers a couple of options. The option chosen may depend on the timeframe in which the hanger is needed, the load demands on the hanger or the cost of the hanger.
If the demand load is low and an immediate solution is desired, Simpson Strong-Tie offers several adjustable hangers that can be skewed, sloped or both in the field.
A common adjustable joist hanger is the LSU/LSSU series, which can be sloped up or down and skewed right or left up to 45 degrees.
Remember that these hangers must be installed to the carried member prior to installation of the supported joist.
Other series of hangers are only adjustable for skew or slope. For example, the THASR/L series is designed to accommodate connections skewed from 22½ to 75 degrees. Conversely, the new LRU ridge hanger is designed to support rafters at ridge beams with roof slopes of 0:12 to 14:12. Finally, the SUR/SUL/HSUR/HSUL series is not adjustable, but is manufactured with a skew of 45 degrees either right or left in several sizes.
If none of these pre-manufactured solutions fits your specific need, there are still options. This entails a custom-manufactured hanger. Many, but not all, joist hangers can be custom-made for specific slopes, skews, combinations of slopes and skews, and even alternate widths and alternate top flange configurations.
If this type of hanger is needed, a good place to start is the Hanger Options Matrix at the back of the Simpson Strong-Tie® Wood Construction Connectors Catalog. It is also available at strongtie.com. An excerpt is shown below. This chart identifies which hangers can be modified, how they can be modified and to what extent they can be modified. There are two tables – one for top flange hangers and one for face mount hangers.
Once the user has found a hanger that can be modified to fit the actual situation, the next step is to calculate any load reductions, if applicable. The column at the far right gives the Wood Construction Connectors Catalog page number that lists any load reductions for the various options. If multiple options with reductions are specified, only the most restrictive load reduction needs to be applied, not all the reductions.
As an example, let’s say we need to hang a heavily loaded double LVL hip member from the end of an LVL ridge beam. We would look at a GLTV top flange hanger, skewed 45 degrees to the right, sloped down 45 degrees, with its top flange offset to the left. We see from the table above that all these options are permitted. If we go to page 220 (or strongtie.com), we can see what the load reductions would be for these options. The reductions are as follows:
- Sloped and skewed configuration for the GLTV has a maximum down load of 5,500 pounds.
- Offset top flange for the GLTV requires a reduction factor of 0.50 of the table roof load.
- BUT, skewed and offset top flange hangers have a maximum allowable load of 3,500 pounds.
- Offset top flange results in zero uplift load.
So the allowable load of our skewed, sloped, offset top flange GLTV would be 3,500 pounds downward and 0 pounds uplift. In this case, it was clear what the reduction was for our combination of modifications. If it is not listed specifically and you have multiple modifications with multiple reduction factors, use only the factor that results in the biggest reduction, not all of the listed reduction factors.
The next thing to do is to call out the desired hanger properly so that Simpson Strong-Tie can manufacture it to your needs. This is typically done by taking the regular product name, adding an X, and then calling out the modifications individually at the end.
For our hanger, assuming the hip is 3-1/2″ by 11-7/8″, the standard hanger would be a GLTV3.511, and the modified hanger would be called out as a GLTV3.511X, Skew R 45, Slope D 45, TF offset L.
There is one final consideration when hangers are both sloped and skewed. In this case, the top of the supported member (joist) will not be horizontal when it is cut, one side will be higher than the other. The user must decide and specify where he or she wants the upper side of the joist to fall. There are three options: high-side flush, center flush or low-side flush. We see that often users will want to specify high-side flush so that the joist ends up flush with the top of the supporting member, but that would be up to the user. This specification is added to the end of the callout name listed above. These cases are illustrated below.
A related matter occurs when the top flange of a hanger is sloped up or down. In this case the user also has to specify whether the joist is to be low-side flush, center flush, or high-side flush. But, in this case, the side is in reference to the top flange, not the joist. Specifying low-side flush will result in the top of the joist being flush with the lower side of the sloped top flange, not the low side of the joist.
If all of this seems confusing and somewhat difficult, it can be. Fortunately, Simpson Strong-Tie has developed a new web application – the Joist Hanger Selector – which automates this entire process. This app is located on strongtie.com/software.
Once you agree to the terms and conditions, choose the type of hanger you want to specify, then select the types of members being connected. This is what it would look like for our example.
This is where the user specifies any modifications required. Required loads can also be entered at this point. This is what it would look like for our example.
Then, just click “CALCULATE” and the possible options will be shown. And here we see our GLTV3.511X, SK R 45, SL DN 45, TF Offset L, with a load of 3,500 pounds, just as we thought! I love it when a plan comes together.
Hopefully, this web app will help you specify custom hangers with ease. Are there any other applications we could develop that would make specifying connectors easier? Let us know.
The 22nd International Specialty Conference on Cold-Formed Steel Structures is coming up Nov. 5-6 at the Hilton Ballpark Hotel in St. Louis, MO. It is sponsored by the Wei-Wen Yu Center for Cold-Formed Steel Structures at the Missouri University of Science and Technology.
A biannual event, this conference brings together leading scientists, researchers, educators and engineers in the field of research and design of cold-formed steel structures to discuss recent research findings and design considerations. This year’s conference features 12 technical sessions covering a wide variety of topics. For more details, visit the conference website.
This week’s blog post was written by Ryan Vuletic, Manager of Engineering/R&D for Anchor Systems. Prior to joining Simpson Strong-Tie in 2001, he was a structural engineering consultant on projects such as state highways and bridges, Las Vegas hotels and casinos, Disney entertainment buildings and military facilities. Ryan received his B.S. in civil engineering from the University of California Irvine and MBA from the University of Southern California. He is a registered professional engineer in California.
Over the last decade, special types of cast-in-place inserts such as the Simpson Strong-Tie® Blue Banger Hanger® threaded insert have become very popular for anchoring suspended mechanical, electrical and plumbing equipment, piping and conduit.These types of anchors are quickly fastened to wood formwork, or through corrugated metal deck before concrete is poured.
Up until now, design engineers who wanted to specify these anchors had to address the question of code-compliance since these anchors are not specifically included within ACI 318 Appendix D or the International Building Code (IBC).
This issue has now been resolved through the code report process. On October 1, 2014, Simpson Strong-Tie received the first code report (ICC-ES ESR-3707) for this type of anchor under ICC-ES AC 446. The code report:
- Covers all sizes of the Blue Banger Hanger Wood Form Inserts (BBWF) and Metal Deck Inserts (BBMD)
- Addresses both cracked and uncracked normal-weight concrete and sand-lightweight concrete
- Addresses static, wind and seismic loads
AC 446 (Acceptance Criteria for Headed Cast-In Specialty Inserts in Concrete) is a relatively new approval criteria that was developed by Simpson Strong-Tie and several other anchor manufacturers. It was adopted by ICC Evaluation Service in June 2013. Anchors that are approved under these criteria will utilize ACI 318 Appendix D and are deemed to conform to Sections 1908 and 1909 of the 2012 IBC.
Testing in accordance with AC446 establishes the strength of headed cast-in specialty inserts based on the strength design provisions of ACI 318. Although the shear testing requirements of this criteria are similar to those found in AC 193/ACI 355.2, the tension tests required are performed in a unique manner. The tension tests and the seismic tension tests are conducted in a steel jig.
The tests are conducted in this fashion to validate that the specialty insert has a strength that exceeds the calculated concrete breakout strength when f’c is set at 10,000 psi (maximum compressive strength permitted per ACI 318). The test program determines the following parameters for the specialty insert:
- Nominal tension strength
- Nominal seismic tension strength
- Nominal steel shear strength
- Nominal steel shear strength for seismic loading
- Nominal steel shear strength in the soffit of concrete on metal deck
- Nominal steel shear strength for seismic loading in the soffit of concrete on metal deck
This new qualification procedure and code report will give designers increased confidence that they can now properly design and specify the Simpson Strong-Tie Blue Banger Hanger and comply with the requirements of the 2012 IBC. What are your thoughts? Let us know in the comments below.
Imagine that it’s 4:30 a.m. and suddenly you’re awakened by strong shaking in your home. Half asleep, you hang on to your bed hoping that the shaking will stop soon. All of a sudden, the floor gives away and you fall. You think, “What just happened? How could this have possibly occurred? Am I alive?”
These could have been the thoughts of Southern California residents living in one of the many apartment buildings, which collapsed on January 17, 1994, during a 6.7 magnitude earthquake. The Northridge Earthquake brought awareness to buildings in our communities with a structural weakness known as a soft story, a condition that exists where a lower level of a multi-story structure has 20% or less strength than the floor above it. This condition is prevalent in buildings with tuck-under parking and is found in multistory structures throughout San Francisco, Los Angeles and other cities (see Figure 1). These structures are highly susceptible to major damage or collapse during a large seismic event (see Figure 2).
Soft story retrofits help to strengthen our communities and make them more resilient to major disasters. There are several resources available to structural engineers that need to retrofit weak-story buildings. Some of these resources are mentioned in our September 18 blog post.
During the 2014 SEAOC Convention held in Indian Wells on September 10-13, speakers discussed different methods, analysis and research that address the behavior of various materials and construction types during seismic events along with approaches to retrofit historically poor performing structures. This information can be viewed from the convention’s proceedings available at www.seaoc.org.
On October 20, 2014, the Structural Engineers Association of Southern California (SEAOSC) will be hosting their 4th annual Strengthening Our Cities BAR Summit in downtown Los Angeles. This event brings together many different stakeholders in our built environment, including public officials, building owners and managers, business owners, insurance industry representatives, emergency managers and first responders, and design professionals.
Many prestigious thought leaders, including USGS Seismologist Dr. Lucy Jones will be speaking at the summit, discussing such topics as tools and analysis methods for retrofitting vulnerable buildings and the Building Occupancy Resumption Program (BORP).
Expect a great day full of useful information about ways to strengthen our communities and prepare for major earthquakes as well as opportunities to network with like-minded peers. For additional information and to register, visit www.barsummit.org. We also hope you’ll visit our booth. We look forward to speaking with you there.
Steve Pryor, S.E., has been with Simpson Strong-Tie for 17 years and currently serves as the International Director of Building Systems. Prior to joining the company, Mr. Pryor was a practicing structural engineer in California. While at Simpson Strong-Tie, he developed the Tyrell T. Gilb Research Laboratory, one of the world’s premier large-scale structural systems test facilities. The lab has the capability to simulate both wind and seismic effects on light-frame structures via both quasi static/cyclic and dynamic test machines that can apply vertical and lateral loading simultaneously. A recognized expert in the structural response, analysis and testing of light-frame buildings, Mr. Pryor participates on several state and national building code committees. He was the primary industry technical consultant for the highly successful NEESWood Capstone seismic testing in Miki, Japan, which tested a full-scale seven-story mixed-use steel/wood structure, the largest building ever tested on a shake table.
We all know that earthquakes physically shape the landscape here in California, but they shape careers as well. Earthquakes I felt while growing up in California’s southern San Joaquin Valley got me thinking about engineering as a career while in high school. When the Loma Prieta earthquake struck on October 17, 1989, like many of you I was watching the World Series live on television and thus got to see the earthquake live as well. I was in my senior year of college at the time, studying Civil Engineering with a structural emphasis. This earthquake cemented the direction I would take in my career. I wanted to be a structural engineer, and I wanted to design buildings that would not fall down in earthquakes.
After Loma Prieta hit, I was relieved when I finally got through to my family and realized everyone was okay. If an earthquake like that happened again today, I would get an alert on my cell phone and know within minutes exactly where it happened, how big it was, and how deep it was. I would also be able to look at the USGS ShakeMap online to get a feel of ground-level damage potential and locations. But in 1989, none of this information was available so over the course of the next few days along with the public, I learned about what had happened in the Bay Area.
After graduating in 1990, several great mentors guided me as I pursued the art of earthquake-resistant structural engineering. I began to realize that earthquakes are like a great predator of the built environment: occasionally they take down healthy buildings in their prime, but they particularly focus on the old and the weak amongst our building stock; buildings with known (and sometimes unknown) deficiencies that if improperly designed or left unretrofitted cause them to fall prey to the earthquake. After several years of practicing structural engineering, and after obtaining my P.E. and S.E., it was with some irony that I came to work at Simpson Strong-Tie in 1997. I became part of a team of dedicated people working to provide structural solutions to new and existing buildings in an effort to keep them from falling prey to the next earthquake. I was now a Bay Area resident, living in the shadows of the same seismic hazards that had manifest themselves on October 17, 1989, and which had shaped my career choice.
While there were many different types of structural weakness on display as a result of the Loma Prieta earthquake, soft- or weak-story wood-frame structures commanded much of the attention. These multi-story wood light-frame structures have an inherent weakness at the ground floor because the area open for parking also cuts down on the area available for shear walls, and thus the available lateral strength. Without the requisite lateral strength in these weak stories, many buildings suffered heavy damage and even collapse. And all of this from an earthquake that was centered about 56 miles south of San Francisco. One can only imagine what would happen if a similar earthquake occurred much closer.
In response to this threat, the City of San Francisco has embarked on a groundbreaking mandatory retrofit ordinance that will hopefully allow many of the city’s residents who live in these structures to “shelter in place” after the next “big one.” What buildings are affected by this ordinance? Wood-frame buildings built or permitted prior to January 1, 1978 with two or more stories over a soft- or weak-story that contain five or more dwelling units.
There are many questions that automatically pop up in response to this. How well does my building have to perform in order to enable me to shelter in place? Could I possibly shelter in place with a yellow tag on my building, or does it have to be green tagged? There is debate on this issue. What constitutes the “big one” – is it the “expected” more frequent earthquake, or is it the extremely rare, very large earthquake? The engineering criteria of the retrofit ordinance points toward it being the “expected” earthquake. Can these retrofits be done (economically) and will they make a difference? Absolutely. How can you say that? We’ve tested them.
Simpson Strong-Tie was a proud sponsor and contributor to an ambitious project known as NEES-Soft. Led by Dr. John W. van de Lindt of Colorado State University, the project culminated with shake table testing of a full-scale four-story weak-story building on the outdoor shake table at the University of California San Diego in the summer of 2013. Constructed to be like a real San Francisco weak-story structure, the building offered a fantastic platform to test various retrofit technologies. One of the retrofit technologies tested was the Simpson Strong-Tie Strong Frame® special moment frame. Designed according to one of the engineering approaches (FEMA P-807) permitted in the City of San Francisco’s retrofit ordinance, the Strong Frame special moment frame performed extremely well, and we were able to conclusively demonstrate the improved performance of the building. Check out this link to understand some of the unique features that make this frame especially well suited for seismic retrofits of wood-frame structures. And don’t forget that any retrofit frame or wall is only one part of the complete load path needed for a successful retrofit!
The Strong Frame special moment frame did not materialize overnight in response to the new retrofit ordinance. The work on it and the other lateral force resisting products we manufacture began years earlier. It turns out that there are a whole bunch of people at our company, professional engineer or not, who are constantly thinking about this and share the same desire that developed in me back in 1989: let’s make products that help buildings not fall down in earthquakes. How can we help you on your project? Let us know – we’d love to hear from you.
This week’s post was written by Bob Leichti, Manager of Engineering for Fastening Systems. Prior to joining Simpson Strong-Tie in 2012, Bob was an Engineering Manager covering structural fasteners, hand tools, regulatory compliance and code reports for a major manufacturer of power tools and equipment. Prior to that, Bob was a Professor in the Department of Wood Science and Engineering at Oregon State University. He received his B.S. and M.S. from the University of Illinois, and his M.S. and Ph.D. from Auburn University.
Structures and connections can be designed either using Allowable Strength Design (ASD) method or Load and Resistance Factor Design (LRFD) method. In the ASD method, the allowable strength is calculated by dividing the nominal strength by a safety factor. In the LRFD method, the design strength is calculated by multiplying the nominal strength by the resistance factor. In design, the adjusted ASD design value is compared to a calculated load or stress. As long as the adjusted ASD design value exceeds the calculated load of stress, then the ASD design value is judged safe. In LRFD design, the nominal strength is equated to factored loads. If the factored strength is greater than the factored loads, then the design can be accepted. ASD is the more common method adopted in the professional world.
LRFD is relatively new to wood design. Prior to 2005, the National Design Specification for Wood Construction (NDS) was based on allowable stress design (ASD). In the 2005 edition, the American Wood Council incorporated Load and Resistance Factor Design (LRFD) into the NDS. To this day, most wood design in the US relies on ASD, but the use of LRFD is becoming more common. On the other hand, the steel design industry already uses the LRFD philosophy for design, and for that reason, design values for steel self-drilling tapping screws are offered in both ASD and LRFD.
The published design values for Simpson Strong-Tie wood fastener products are in ASD format and the allowable loads are generally shown at a load duration factor of CD = 1.0. The reference design loads listed shall be multiplied by all adjustment factors listed in Table 10.3.1 of NDS 2012 to determine adjusted design values. The load tables are listed in ASD format because ICC-ES acceptance criteria, such as AC233 (Alternate Dowel-type Threaded Fasteners) and AC120 (Wood Frame Horizontal Diaphragms, Vertical Shear Walls, and Braced Walls with Alternate Fasteners), that are used to qualify structural wood screws do not address the development of LRFD values for wood screws. However, one can establish the nominal strength values for fasteners from reference ASD design values for use in LRFD format by following the instructions of NDS (2012), Table 10.3.1. Reference design values shall be multiplied by the format conversion factor KF as specified in Table N1 of NDS 2012. Format conversion factors adjust reference ASD design values to LRFD reference resistances. They are also multiplied by the resistance factor, Φ as specified in Table N2 and Time Effect Factor, λ as specified in Table N3.
For e.g., the table below lists the ASD allowable shear loads for SDWS screw in Douglas Fir-Larch and Southern Pine Lumber:
For the SDWS22300DB screw with a wood side member thickness of 1.5 inches, the allowable shear load is 255 lbs. with a wood load duration factor of CD = 1.0. To convert this to an LRFD load, refer to table 10.3.1 of NDS 2012 and Appendix N, Tables N1, N2 and N3. Per Table 10.3.1 we need to multiply the reference load with format conversion factor KF, resistance factor, Φ and time effect factor, λ From the Table N1, the format conversion factor KF for connections is 2.16/Φ. From Table N2, for connections Φ=0.65. Let us assume a λ of 1.0 from Table N3. The LRFD load is calculated by multiplying the allowable shear load with the factors above.
LRFD load = Allowable shear load (at a load duration factor of CD = 1.0) x KF x Φ x λ
LRFD Load = 255 x (2.16/0.65) x 0.65 x 1.0 = 551 lbs.
For steel self-drilling, self-tapping screws, the omega and resistance factors used for calculating ASD and LRFD loads are based on American Iron and Steel Institute (AISI) standard S100. For Simpson Strong-Tie steel self-drilling, self-tapping screws the load tables are listed in both ASD and LRFD format.
If the screw connection capacities are calculated based on tests, the ASD values are calculated by dividing the tested nominal strength which is the average of the ultimate strength values from all the tests with the safety factor, Ω. For LRFD load, the tested nominal strength is multiplied by a resistance factor, Φ. When tests are performed for evaluating the connection capacities, the safety factor, Ω and the resistance factor, Φ are evaluated in accordance with Section F of AISI S100. Simpson Strong-Tie derives the LRFD values for steel self-drilling, self-tapping screws in LRFD format because this is part of ICC-ES AC118 (Tapping Screw Fasteners). See evaluation report ICC-ES ESR 3006 for examples of ASD and LRFD design values for the same fastener products.
If the screw capacities are determined based on the calculated nominal strength, the ASD loads and LRFD loads are determined based on Section E4 of AISI S100. For e.g., the table below lists the ASD loads and the LRFD loads based on calculations for #14 x 1” screw.
From the table, for 33 mil (20ga) steel to 33 mil (20ga) steel shear connection, the calculated nominal shear strength is 600 lbs.
Nominal Strength = 600 lbs.
From Section E4, the safety and resistance factors for connections are:
Ω = 3.0
Φ = 0.5
ASD Load = Nominal Strength/Ω = 600 lbs./3.0 = 200 lbs.
LRFD Load = Nominal Strength x Φ = 600 lbs.x 0.5 = 300 lbs.
Now that you know the basics of ASD and LRFD, make sure you choose the one best suited for your specific material and construction application. If you have ideas for which of our products you would like to see in ASD and LRFD loads, be sure to let us know!
This week’s blog was written by Louay Shamroukh, P.E., S.E., who is a regional engineering manager working out of the Simpson Strong-Tie Stockton branch. Louay is a licensed Structural Engineer in the State of California. He started his career with Simpson Strong-Tie in 1999 as a R&D engineer responsible for testing, improving and developing products for the light frame construction industry and he holds several wood construction connector patents. Louay serves as the liaison between the engineering department, customers, sales and manufacturing. He supervises a department that is tasked with providing technical and application support for Simpson Strong-Tie products to sales, specifiers and building officials. He explores market opportunities for developing new products through interaction with customers in the field and at industry events. Here is Louay’s post.
We have written about San Francisco’s Soft-Story Retrofit Ordinance and Soft-Story Retrofits before on the blog. I wanted to discuss in more detail the issues with soft story buildings and FEMA’s new tool for addressing them. Under the San Francisco Ordinance, wood-framed residential structures that have two or more stories over a “soft” or “weak” story require seismic retrofit. So far, more than 6,000 property owners have been notified about complying with the mandate.
Multi-unit wood-frame buildings with more than 80% open area on one first story wall or more than 50% on two adjacent walls are considered weak story buildings. During the 1971 San Fernando earthquake, 1989 Loma Prieta quake and the 1994 Northridge earthquake, this type of building often sustained major damage or completely collapsed. One cause for this structural weakness is the mixed use of the buildings, which dictates an open space and less partition walls on the first story than the upper stories.
The lack of exterior walls or partition walls on the first story leads to a considerable difference in lateral strength, stiffness and stability between the first story and the upper stories. During an earthquake, this difference exposes the first story to a concentrated lateral deformation in lieu of distributing it over the height of the structure. In the presence of large openings in the exterior walls, the concentrated lateral deformation is superimposed with the building’s tendency to twist.
Buildings built prior to 1978 were constructed of materials and finishes that are archaic, non-ductile, with low displacement capabilities and poor detailing that can lead to earthquake damage, and in some cases, to building collapse. Some of these materials are stucco, diagonal sheathing, plaster on wood lath and plaster on gypsum lath that possess a maximum inter-story drift ratio of 2% or less.
The Federal Emergency Management Agency (FEMA) has developed the FEMA P-807 guideline, “Seismic Evaluation and Retrofit of Multi-Unit Wood-Frame Buildings with Weak First Stories.” FEMA P-807 provides procedures for the analysis and seismic retrofit of weak first story buildings built with structurally archaic material.
The guideline’s design philosophy is to provide a cost-effective seismic retrofit method limited to the first story without disrupting the occupancy of the upper stories. The guideline limits the retrofit to the first story by introducing sheathing materials or structural elements with high lateral displacement capacity. This is designed to improve seismic performance and reduce the risk of collapse without driving the earthquake forces into the upper stories and exposing them to the risk of damage or collapse.
FEMA’s Weak Story Tool, an electronic tool developed for FEMA P-807, tabulates the walls in a building graphically. Each wall in the building has its inherent material capacity to provide resistance during an earthquake. The tool applies the rules of the provisions and performs the analytical calculation to evaluate the building before and after the retrofit. Performing the analysis manually and iteratively requires a considerable amount of time and calculation. On the other hand, the tool is a convenient mean that aids in the analysis and keeps checking the input as the assemblies, special moment frames and walls are added for seismic retrofit. A report summarizing the data and formulas is available once the retrofit meets the provisions of FEMA P-807.
Recently, the Simpson Strong-Tie® Strong Frame® special moment frame was added to the Weak Story Tool. The Strong Frame® special moment frame is a 100% field bolted connection frame that does not require field welding for the retrofit of an existing building. It has a unique replaceable patented Yield-Link™ structural fuse that provides the ductile lateral resistance with high lateral displacement capacity. In close quarters of an existing building, such as a parking garage or commercial space, the Strong Frame footprint is considerably smaller than other retrofit assemblies. It also eliminates the need for beam bracing normally required for special moment frames, which was discussed in a previous post.
To use the Simpson Strong-Tie lateral system solution in the Weak Story Tool, go to the Assemblies Tab, where you can select Strong Frame special moment frame as a retrofit assembly. The frame is specified in the Simpson Strong-Tie screen functionality after inputting the frame’s dimensions and the ultimate target force. After selecting the frame, the functionality provides the initial stiffness, yield strength, ultimate strength and drift at ultimate strength for the tri-linear backbone curve, which are seamlessly inputted into the Weak Story Tool.
The Weak Story Tool is a convenient and powerful tool that can save the specifier several hours of mundane work and resources. Please try out the Weak Story Tool with the addition of the Simpson Strong-Tie® Strong Frame® special moment frame and let us know what you think. We always appreciate the feedback!
If you’re in the Bay Area, please join us for hands-on training on the use of the FEMA Weak Story Tool. Register here, bring your laptop, and join us in the Weak Story Tool workshop presented by Simpson Strong-Tie engineers on Wednesday, October 22 at Oakland City Hall, 1 Frank H. Ogawa Plaza, Oakland, California 94612.