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
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.
The 6.0 magnitude earthquake that struck Napa, CA, in August caused more than 200 injuries and structural damage to many homes and businesses throughout the area. The quake was the largest to hit the San Francisco Bay Area since the Loma Prieta earthquake (6.9 magnitude) in 1989, prompting the governor to declare a state of emergency.
I have done several posts about San Francisco’s Soft-Story Retrofit Ordinance and some of NEES-Soft testing related to soft-story retrofits. The soft-story ordinance only addresses multi-unit residential units and does not require retrofit of single-family homes. Cities are reluctant to mandate seismic evaluation and retrofit of single-family homes for a number of reasons that I won’t discuss here. The draft Earthquake Safety Implementation Program (ESIP) for San Francisco will not recommend mandatory retrofit of single-family homes until 2030.
The good news is homeowners can retrofit their homes without waiting for the government. A couple years ago in this post, I discussed some of the tools available to retrofit existing buildings.
One of these tools is the 2012 International Existing Building Code (IEBC). The IEBC has provisions for repair, alteration, addition or change of occupancy in existing buildings and for strengthening existing buildings. For alterations, these provisions may not comply with current IBC requirements, but they are intended to maintain basic levels of fire and structural life safety. The IEBC also provides prescriptive provisions for strengthening existing buildings against earthquake damage, which include strengthening residential houses on raised or cripple wall foundations.
Cripple wall failures are a common type of damage observed in older homes, caused by inadequate shear strength in the cripple wall. An additional failure point is the attachment of the wood sill plate to the foundation. Having a strong connection between the wood structure and the concrete foundation is critical in an earthquake. Since the work required to strengthen these connections is typically performed in a crawlspace or unfinished basement, it is a relatively low-cost upgrade that is extremely beneficial to structural performance.
Our website has information for retrofitting your home. The Seismic Retrofit Guide has information about how earthquakes affect a home and the steps to take to reinforce the structural frame of a house. The Seismic Retrofit Detail Sheet is intended to help building departments, contractors and homeowners with seismic retrofitting. It includes common retrofit solutions for reinforcing cripple walls and foundation connections.
One business owner in Napa chose to retrofit her building when she purchased it. You can see her video narrative here.
The transition from one building code to the next always begs the question, “how is the newer code different?” There are several changes between the 2009 IBC and 2012 IBC that will change the way designers approach seismic design. This blog post is a broad overview of some of the changes. Since it’s not practical to cover all the changes between the previous and new codes in detail in one post, the discussion will be mainly on 2012 IBC and the corresponding ASCE7-10 reference standard.
The seismic ground motion maps have been updated to match ASCE7-10. The titles of the maps in IBC were revised from “Maximum Considered Earthquake Ground Motion” to “Risk-Targeted Maximum Considered Earthquake (MCER) Ground Motion Response Accelerations” in order to reflect the titles in 2009 NEHRP and ASCE 7-10. As in previous editions, some areas will prove difficult to read due to the contour lines, so the USGS site and GPS coordinates are recommended (http://earthquake.usgs.gov). Additional information about changes made for 2009 NEHRP is available at www.nibs.org or www.bssconline.org.
The term “occupancy category” was replaced with “risk category” in the 2012 IBC for consistency with the term used in ASCE 7-10. This change was made because it was decided that the use of the word “occupancy” implied the category was directly tied to occupancy classifications in the code, while the word “risk” more accurately communicates that the category is based on acceptable risk of failure.
ASCE7-10 revised the way designers use the corresponding Drift amplification, Cd, and Overstrength factor, Ωo, of the Response modification factor, R. In ASCE7-05, when there is a vertical combination of different R-values, the Cd, and Ωo cannot decrease as you go down each level of a building. In ASCE7-10 (220.127.116.11), the Cd and Ωo always correspond to the R-Value as you go down. The adjacent figure illustrates the new provision to use the corresponding Cd, and Ωo with the R-value at each level.
ASCE7-10 (18.104.22.168) added a clarification for out-of-plane anchorage forces where the redundancy factor, p = 1.0. The intent of the redundancy factor was to ensure the vertical seismic-resisting system with insufficient redundancy had adequate strength. The design forces for out-of-plane wall loading are not redundancy requirements. ASCE7-10 (12.11.12) revised the out-of-plane wall anchorage force equation where the anchorage forces are reduced for shorter diaphragm spans.
Light-frame construction structures are no longer exempt from amplification of accidental torsion in ASCE7-10 (22.214.171.124). There are many structures vulnerable to torsional effects including some “tuck under” parking buildings that are often light-frame structures. See posts titled Soft-Story Retrofits and City of San Francisco Implements Soft-Story Retrofit Ordinance for more discussion of soft-story, light-frame buildings.
This is just a brief summary of changes related to seismic design found in the 2012 IBC. What are other changes that will modify your approach to seismic design?
Although Simpson Strong-Tie is best known for our structural products: engineered structural connectors, lateral systems, fasteners and fastening systems, anchoring products and most recently, concrete repair, protection and strengthening (RPS) systems, we are continually developing new and exciting software solutions. As we’ve discussed in prior blog posts, Simpson Strong-Tie has numerous software programs and web and mobile apps available for download or online use at www.strongtie.com/software. Today, I’d like to review our recently launched web app, the Steel Deck Diaphragm Calculator. The calculator is accessible from any web browser and doesn’t require downloading or installing special software.
While the method of designing and specifying a steel deck and its attachment can vary by region, most designers are familiar with the Steel Deck Institute (SDI) and its Diaphragm Design Manual, 3rd Edition (DDM03). DDM03 presents diaphragm shear strength and stiffness equations for various steel deck profiles and commonly used attachment types (welds, power-actuated fasteners, or screws). The calculations can be quite tedious, so the SDI has developed numerous tables using these equations and placed them at the back of DDM03 for easy reference.
Since the tables in DDM03 are based solely on the fasteners and deck profiles included, determining diaphragm capacities utilizing any other proprietary fastener or deck profile fall on the designer or the proprietary product’s manufacturer. Enter Simpson Strong-Tie.
Our Steel Deck Diaphragm Calculator enables users to produce custom diaphragm tables similar to those in DDM03, generate detailed calculations using SDI equations based on project-specific inputs, as well as optimize deck fastening systems to ensure the most cost-effective design is utilized. The calculator incorporates our X-series steel decking screws, including the recently launched Strong-Drive® XL Large-Head Metal Screw, which has one of the highest capacities in the industry and in most cases, can be used as a 1-for-1 replacement of pins or 5/8 diameter puddle welds. (For additional information comparing Simpson Strong-Tie X-series and XL screws to pins or welds, review F-Q- STLDECK14.)
The app can be used with minimal required input to generate tables and project-specific calculations. A more detailed analysis can be performed by inputting parameters for up to five unique zones, including overall dimensions, diaphragm shear, joist spacing, uplift and more.
One unfortunate aspect of many web apps is that your work is typically lost once you close your web browser. I’m happy to report that the folks here in our app development group have added the ability to save and upload project files. The calculator also provides a clean PDF printout of your results while giving you the option to generate a submittal package with supporting documentation, such as code reports, product approvals and installation recommendations.
Try the revised Steel Deck Diaphragm Calculator yourself and let us know what you think. We always appreciate the feedback!
This week’s blog post is written by Jason Oakley. Jason is a California registered professional engineer who graduated from UCSD in 1997 with a degree in structural engineering and recently earned his MBA from Cal. State Fullerton. Before joining Simpson Strong-Tie in 2002, he was a design engineer for 5 years working on subterranean parking lots, movie sets, offshore drilling platforms, nuclear power plants, oil refineries, blast-resistant structures, fall protection, dry-dock supports for large seafaring ships including vibration analysis of components inside ships. He has amassed almost 20,000+ hours experience as an anchor systems field engineer for Simpson Strong-Tie. His territory includes Southern California, Hawaii and Guam.
For the first time, ACI 318 – 11 includes a design provision for adhesive anchors in concrete. Previously, adhesive anchors were designed according to provisions found in both ICC Evaluation Service (ICC-ES) AC308 and ACI 318 – 08. A relatively new standard, ACI 355.4, must be used to qualify adhesive anchors in concrete. This new standard, along with ACI 318 – 11, contains important changes that will affect anchor systems designed to the 2012 IBC. Not all changes are discussed here. I will only focus on what you – the engineer – should be aware of.
ACI 355.4 requires that adhesive anchors in concrete be evaluated using a bond strength (measured in terms of psi and used with the surface area of the embedded portion of the anchor) that corresponds to a long-term temperature (LTT) of 110 degrees F to account for potential elevated temperature exposure conditions. This wasn’t necessarily the case previously where, for example, the engineer could elect to use a temperature category that listed bond strength values based on a LTT of 75 degrees F. The issue here is creep.
Creep, in the world of adhesive anchors, looks at how well the anchor can resist load without too much axial displacement over a period of not minutes, not hours, not even years but decades. As a general rule, it’s no surprise that creep worsens as the temperature rises for almost any material. In our case, the bond strength is effectively reduced. Most adhesives, if not all, currently list bond strength values that correspond to a LTT of 110 degrees F. Make sure to select the temperature category that meets this minimum requirement. Some adhesives will experience a reduction in bond strength at an LTT of 110 degrees F, some won’t.
What about applications involving short-term-only loading? Is creep still relevant? Generally, you’ll find that adhesive anchors negatively impacted by the higher LTT requirement will gain back much of their load for seismic/wind-only load applications. So creep becomes irrelevant.
While adhesive anchors used solely for the purpose of resisting short-term loads will remain largely unaffected by this code change, significant changes have been made to the design and installation of adhesive anchors when used for sustained loading applications (e.g. dead load, live load, etc.).
First, the bond strength must be reduced by a factor of 0.55 as compared to 0.75 under the previous code (following ICC-ES AC308). New to the code, section D.9.2.2 of ACI 318 App. D requires that adhesive anchors used for resisting sustained loads be installed by someone who has taken the Adhesive Anchor Installation Certification (AAIC) program. The installer must show proof that he/she is certified by passing both a written and performance examination. Installing adhesive overhead requires some skill. So it’s no surprise that the installer must satisfactorily demonstrate proficiency by blindly installing adhesive overhead into an inverted test tube that will later be cut in half and graded for the presence of voids. Figure 1 shows no voids, so the installer passed.However, exceptions do exist. If you’re working on a hospital or school in California, the 2013 CBC (Table 1705A.3 footnote c) requires that all horizontal and overhead adhesives anchors – irrespective of load condition – be installed by a Certified Adhesive Anchor Installer (CAAI). This deviates from ACI 318 D.9.2.2.
Arguably, with AAIC, there’s an added cost to using adhesives for anchorage designed for sustained loading. However, for sustained loading applications best suited for adhesive anchors it should come as peace of mind to the engineer, owner, contractor and other parties involved with the construction project that a certified installer has been employed to ensure that the adhesive anchor has been installed in accordance with the manufacturer’s printed installation instructions.
While the engineer should be aware of the above limitations placed on adhesive anchors, by no means should it hamper their design. There are several options available to the engineer. Table 1 compares the tensile design strength of three common types of anchors – two adhesives, two mechanical anchors (one screw and one expansion type) – determined using the new design provision ACI 318 -11. While the creep test results show a reduced capacity for adhesive A, it does show a significant increase in load for seismic-only applications because , as we discussed earlier, creep is no longer an issue. Some adhesives, like adhesive B, will do well under the creep test (at an elevated LTT of 110 degrees F), so any capacity increase for seismic-only applications will be small.
What three important points can we glean from Table 1? First, all things being equal, mechanical anchors will typically achieve higher “code values” for sustained loading applications relative to adhesives. Second, mechanical anchors are easier to install overhead. Third, AAIC is not required for mechanical anchors. While these reasons support using mechanical anchors for overhead anchorage, doing so is nothing new. The bulk of overhead attachments have almost always been made with mechanical anchors mainly because it’s just easier to do it that way.
Perhaps up to 95% of adhesives are used to secure rebar to concrete – we’ll call them rebar dowels. Like any anchor, rebar dowels can be used to resist seismic and/or sustained loads. While the exact breakdown is hard to determine, arguably, the bulk of rebar dowels in the west coast are found in seismic retrofits and renovations used to thicken walls, tie-in new concrete shear walls, connect new drag struts, strengthen existing concrete elements, etc., all for the purpose of strengthening the lateral capacity of the existing structure to withstand greater earthquake and/or wind loads. These typically won’t require a CAAI, but it might if it’s a school or hospital project that requires overhead or horizontal anchors. Some rebar dowels are used for enlarging footings to withstand greater dead and live loads, so these would require a CAAI. Remember: the bond strength can be lower than expected for sustained loading applications, so you may want to use an adhesive that does well at a LTT of 110 degrees F if that’s what your design requires.
One new benefit of ACI 318 is that the engineer can now design adhesive anchors to go into lightweight concrete using the factors found in section D.3.6.
One significant change engineers should include in their specification is that the concrete must be aged at least 21 days before installing an adhesive. Previously, the industry standard was to wait seven days. For additional information regarding adhesives installed into younger normal-weight concrete, read the following Simpson Strong-Tie engineering letter: http://www.strongtie.com/ftp/letters/generic/L-A-ADHGRNCON14.pdf
What are you experiencing in the design of your anchors in your jurisdictions? Leave a comment down below because we would like to know.
Early this summer a package arrived at my office that I knew right away was either a copy of a new building code or design standard. Some codes or standards are more exciting than others to open up and see what’s new and different. As it turns out, this package was the just-published 2015 International Residential Code (IRC). With my interest in wood decks, I have to admit that this was new information that I was happy to see.
Why? Similar to my blog post in May mentioning the limited design resources currently available to engineers, the IRC itself is also a work in progress when it comes to the prescriptive details included for decks. Performance requirements for the framing and guards has always been included in Chapter 3, but it wasn’t until the 2009 and 2012 editions that prescriptive information for attaching a deck ledger to a wood band joist with lag screws or bolts, and a detail for transferring lateral loads to a support structure, were included. Key improvements for the 2015 IRC include provisions for composite materials, clarification of the prescriptive ledger information, and prescriptive information for decking, joist and beam allowable spans, post heights and foundations.
Lateral load connections at the support structure were a significant topic during the development of the 2015 IRC. The permitted method already in the code involves constructing the Figure 507.2.3(1) detail with 1,500 pound hold-downs, in two or more locations per deck. The detail transfers the lateral load by bypassing the joist hanger and ledger connections, and ultimately transfers it into the floor diaphragm of the support structure. The concentrated nailing on the floor joist and the need to have access from below to the install the hold-down can cause undesirable complications for builders with existing conditions. A number of common conditions also differ significantly from the detail, such as the floor joists running parallel to the deck ledger and alternate floor joist types, including i-joists or trusses. In response to frequently-asked-questions from the industry, our technical bulletin T-DECKLATLOAD provides commentary to consider for these situations. The technical bulletin also offers an alternate floor joist-to-sheathing connection that may save the builder from removing a finished floor in an existing condition or from adding additional sheathing nailing from above.
In order to provide greater flexibility, a second option is now included in the 2015 IRC: constructing Figure R507.2.3(2) with 750 pound hold-downs in four locations per deck. This detail also transfers the lateral load in bypassing the joist hanger and ledger connections, but transfers the load to the wall plates, studs, or wall header by means of a screw anchoring the hold-down. In some cases, builders will hope this detail can save removing interior portions of an existing structure, but close attention will be required in terms of the deck joist elevation with respect to components of the wall and ensuring that hold-down anchor has proper penetration into the wall framing.
There are still a number of scenarios where a residential deck builder may need or want to consider hiring a structural engineer. Prescriptive details for guards and stairs are still not included in the code, as well as lateral considerations such as the deck diaphragm or the stability of a freestanding deck. Alternate loading conditions, such as the future presence of a hot tub, are also outside the scope of the current code. The allowance for alternative means and methods permitted by Chapter 3 of the 2015 IRC, is also something to keep in mind when the prescriptive options do not fit well with the project conditions. For example, the IRC ledger fastening table applies for connections to a band joist only and not to wall studs or other members of the adjacent support structure.
Have you been involved with any residential deck projects? Let us know in the comments section below.
Designing buildings and dealing with construction has always been a satisfying career for me. It is challenging to design a complete structural system, coordinate with the other consultants and create a clear set of construction documents for the contractor. Throughout my career, I’ve occasionally had a few panicked “Uh oh!” moments. I hope I’m not alone in admitting those happen. These typically occur far away from work when something prompts me to think about a project. I might see concrete being placed, then question whether I remembered to change the reinforcing callout on a mat slab I had just designed. I can’t stop thinking about it until I get back in the office to check.
I had an “Uh oh!” moment a few days after I started work at Simpson Strong-Tie. We have a training plan I call Catalog 101 where new engineers meet with each engineer who is an expert for a given product line. After I had met with our experts on holdowns, concrete anchors and engineered wood products, it was on to top-flange hangers (and my “Uh oh!” moment).
After learning a lot of things I didn’t know about hangers, we moved on to available options for some of our top-flange hangers – sloped, skewed, sloped and skewed, sloped top-flange, and offset top-flange. I learned that some hanger options get full load, some have small reductions and others large reductions. For example, the GLT with an offset top-flange gets 50% of the table load.
I had recently designed a project and specified a bunch of GLT hangers with offset top-flanges. I hadn’t noticed there was a reduction for this modification; I just thought it was really cool that Simpson Strong-Tie had a hanger that worked at the end of a beam. Minor panic set in until I could check my calculations. Fortunately, the beams at the framing conditions that required offset hangers had half the load of the typical beams, so the hanger was okay even with the load reduction.
The Wood Construction Connectors Catalog has a Hanger Options Matrix that makes it relatively simple to see which options – sloped, skewed, concealed, welded – are available for each hanger. The pages following the options matrix have more detailed information about size restrictions and load reductions associated with each option. It can be somewhat tedious to sift through all of the options and apply the reduction factors, so I always recommend using the Simpson Strong-Tie Connector Selector® software to do the work for you.
Connector Selector software allows you to input you geometry and loads and returns a list of connectors that meet those requirements, including any reductions due to modifications. Connector Selector is a desktop application, which needs to be downloaded and installed on your PC. Engineers have indicated they like the functionality of Connector Selector, but wished the input was more intuitive and preferred it as a web application.
I’m happy to say we listen, and the new Simpson Strong-Tie Joist Hanger Selector web app is available now. The easy-to-use interface enables users to quickly select the connection details and print out results. You can access the app from any web browser without having to download or install special software. The allowable loads are automatically calculated to reflect reductions associated with modifications – no more “Uh oh!” moments for me (at least with hangers).
Give the new Joist Hanger Selector web app a try and let us know what you think. We always appreciate your feedback!
In my former life working as a consulting engineer, I reviewed many truss submittal packages. I remember during my reviews wondering how it was possible to get so much information on to an 8½ inch by 11 inch piece of paper. I also remember how a lot of what was being reported was difficult to understand without some help interpreting the information.
As many of you may know, Simpson Strong-Tie has ventured into the truss industry and we are now offering truss connector plates and software to component manufacturers around the country. So given my past experiences, I figure some of you might appreciate some insight into the engineering that goes on behind those truss submittal packages. So I have asked one of our great truss engineers, Kelly Sias, to put together some blog posts on the topic that we can share our knowledge with you. Kelly has worked in the truss industry for years and spent time as the Technical Director at the Truss Plate Institute. I am sure her blog posts are going to help all of us have a better appreciation for trusses.
Have you ever been involved in a discussion with someone on a project that ended with “but that’s the way we’ve always done it!”? I heard those words spoken by a contractor in my first engineering job when I tried explaining why his single stud would not work at a particular location. When he said something about his grandfather having always done it that way, I knew I could explain the calculations all day and it wouldn’t do much good.
Fast forward several years to the present. The topic and audience are different, but the issue is still the same – it’s difficult to change the way something has always been done. Take snow loading on trusses as an example. Historically, snow load has been lumped in as part of the top chord live loading on a truss. A long-standing practice in many areas has been to take the ground snow load and simply enter it into the Top Chord Live Load (TCLL) box in the truss design software. Even the truss design standard, ANSI/TPI 1, and the IRC/IBC codes have included snow load as part of the top chord live load in the list of required design loads to be included on the truss design drawing:
The only problem is that snow load is not a live load, and no additional snow load considerations, such as unbalanced snow loads, are taken into account when it is applied as a live load in the design program.
This may in fact be acceptable at times, particularly when the full ground snow is used as the top chord live load. After all, this is in-line with the prescriptive approach taken in the IRC, as specified in section R301.6 Roof Load:
where Table R301.2.(1) is based on the local ground snow load. In many jurisdictions, the use of the full ground snow load for the balanced snow case is considered adequate to address any other snow-related effects including unbalanced snow loads.
The alternative approach is to treat snow loads as snow loads and live loads as live loads, and actually design the truss for the input snow loads and corresponding snow load design criteria. This puts all of the relevant snow loading parameters right onto the truss design drawing. However, because of the historical precedence to treat snow loads as live loads, this method has actually caused confusion in some Building Departments. Some departments see both a snow load and a live load and get confused by the live load. Some want to see snow load, but only the ground snow load. Others say they want to see a TCLL on the drawing and that’s it. Interestingly, the IBC-09 actually modified its provision regarding truss design drawings to remove snow load from the top chord live load provision and list it separately as part of the environmental loads:
Being from snow country (and actually being a fan of the white stuff every year), I might be a bit biased, but I think the IBC change is a change for the better. Maybe it will help remind people that snow loads are not live loads. I’m not saying that ground snow shouldn’t ever be used as the roof design load; I’m just saying it should still be called (and reported as) a snow load. I think that’s an important first step to making sure everyone in the job is on the same page regarding what snow load considerations have (and have not) been included in the design.
What are your thoughts about snow loads being treated as live loads in the design of roof trusses? Let us know in the comments below.
Have you ever seen this famous sign? You may have seen it while riding the London Underground, to draw attention to the gap between the rail station platform and the train door. The warning phrase is so popular that you may also recognize it from souvenir T-shirts or coffee mugs.
In the connector world, the phrase comes to mind when thinking of the space, or “gap” between the end of the carried member and the face of the carrying member. Industry standards for testing require that a 1/8” gap be present when constructing the test setup (in order to prohibit testing with no gap, where friction between members could contribute significantly), so this is the gap size that is typically permitted for the joist hangers listed in our catalog.
Gaps exceeding 1/8” can affect hanger performance in several ways. A larger gap creates more rotation for the connector to resist by moving the downward force further from the header. Fasteners may also have reduced or no penetration into the carried member due to the gap. Testing confirms that these factors decrease hanger allowable loads for larger gaps.
What are my options then if the field conditions create a gap larger than 1/8”? We have performed testing to establish allowable loads for many common joist and truss hangers with gaps up to 3/8” (up to ½” for HTU hangers), as well as testing for possible field remedies and repair scenarios. Our technical bulletin T-HANGERGAPS10 provides this information, along with a design example, and general recommendations and guidelines for preventing gaps. Notes on shim details are also included – shim size, material, and attachment (independent of the hanger fasteners) are key design considerations that must be covered by the engineer or truss designer.
What is your experience dealing with hangers that exceed 1/8” gaps? Let us know in the comments below.
When designing a shearwall according to the International Building Code (IBC), a holdown connector is used to resist the overturning moment due to lateral loading. From a structural statics point of view, a shearwall without dead load or holdowns would have zero lateral-resisting capacity without any restraint to resist the overturning moment. Since the wall assembly still has the sill plate anchorage providing resistance to overturning, testing can measure the capacity of a wall assembly without holdowns.
We have performed multiple tests comparing the performance of a shearwall with and without holdowns. Diagrams of the test setups are provided in Figure 1 below.
The top of wall was attached to the actuating ram using a steel channel and fastened to the double top plates with 3” SDS screws. The ram pushed and pulled the top of wall according to the CUREE test protocol.
No Holdown Wall:
A wall assembly without holdowns can only rely on the wood sill plate members, sill plate anchorage and sheathing to resist the overturning force. The two limit states commonly observed in the test: 1) The sheathing fasteners prying the sill plate in cross-grain tension. (see Figure 2) 2) Fastener tearing the sheathing at the sill plate. (see Figure 3) Little damage was observed between the sheathing and end post along the height of the post. Figure 4 is the load vs. displacement graph showing the peak load, 928 lbs., at relatively small displacement, 1.57”.
Wall With Holdown:
The change in restraining the end posts increases wall stiffness, capacity and ductility of the assembly. The peak load was 2,907 lbs. at a displacement of 2.3”. (see Figure 5) The use of a holdown to restrain the post and engage additional sheathing fasteners minimized cross-grain tension on the sill plate compared with the test without holdowns. (see Figure 6) The increase in both strength and ductility comes from the additional number of fasteners engaged along the height of the post when the post is restrained. (see Figure 7) The assembly with holdowns was able to achieve approximately three times more strength compared with the same amount of material used without holdowns. Ductility also increased substantially, which can be observed from illustrating the hysteresis curves of both tested assemblies for comparison. (see Figure 5)
The comparison between the two walls is based on a 4 foot wide by 8 foot tall configuration. A wall with a different aspect ratio may change the performance, but walls with holdowns will achieve higher loads, and lower displacements, and more ductile performance.
What are your thoughts about shearwall assembly? Let us know in the comments below.