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
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.
We wish you a happy and safe Fourth of July from all of us here at Simpson Strong-Tie.
Check back next week for our regular blog posts.
For many, the first day of summer means it is time to cinch up your favorite hip-hugging bathing suit and enjoy the warm weather. For the truss industry, it’s time to keep those hip-hugging bathing suits in the closet and take advantage of the favorable weather months by bidding and building as many jobs as possible. During the bid and build frenzy, there will be several hip end jobs leaving truss yards across the country, but what exactly is a hip end and what are the different styles?
The Structural Building Components Association website (SBCA) defines a hip roof as a “Roof system in which the slope of the roof at the end walls of the building is perpendicular to the slope of the roof along the sides of the building.” While framing terms differ by region, most trussed hip end systems will include hip trusses, jack trusses (end and side) and a rafter or corner girder truss. Hip end style and setback (distance from side or end walls to the hip girder truss) may also vary by building design and region.
In the western part of the country, a California Hip system is typically seen in many trussed structures. In this hip system, the hip truss flat top chord is dropped by the plumb cut of the jack top chord at the roof pitch. By doing this, the top chords of the end jack trusses can pass over and bear on the dropped flat top chords. As the height of the hip end roof plane increases, the height of the flat top chord also increases, though the interval at which the flat top chord height increases may vary by building design and region.
East of the Rocky Mountains, the California Hip is rare and a Step-Down Hip system is more popular. Differing from the California Hip, a Step-Down Hip system is one where every truss under the hip end plane decreases in height, or “steps down” from the apex until it reaches the hip girder, which is placed at a pre-determined setback.
Less regional and more situational depending on the building design, are the Lay-In Gable, Dutch and Terminal Hip systems. The Lay-In Gable Hip system is one with many regional names and shares similarities with the California and Step Hip systems. Like the Step Hip, every truss steps down moving from the apex to the setback. Like the California, every truss flat top chord has a drop. However, the flat top chord is dropped by the plumb cut of a 1.5” member at the roof pitch, as the gable frame lays flat.
In a Dutch Hip system, the hip end roof plane does not converge with the side planes to form an apex. Instead, the hip end plane pitches directly into the girder truss that is placed at a predetermined setback. Jack trusses then connect to the hip girder truss or to a ledger attached to the hip girder truss. This hip system is also referred to as a Dutch Gable.
Assuming like roof pitches and heel heights, a Terminal Hip system is one where the hip girder truss setback is half of the main truss span or building width. If pitches and heel heights vary, the girder truss is placed at the apex of the three converging roof planes, which could be more or less than half of the main truss span or building width.
While these are some common hip end styles in the truss industry, there are definitely others. Each style has its own advantages and disadvantages, and a discussion of those will be the topic of a future post.
What other types of hip end styles are you familiar with? Let us know in the comment section below.
I’ll admit that I’m biased, but structural engineers have the best job in the world. We’re needed to create safe sound structures while factoring in the effects of environmental forces using a combination of physics and experience. It takes a really well rounded individual to do all of that.
In my opinion, the key to being a well rounded professional is to never stop learning or seeking out new resources in your industry. I thought I’d share with you some resources that may be helpful to you as a structural engineer, from my own experience:
Continuing Education Webinars
Attending webinars online is a great way to get Continuing Education credits you need. Webinars enable you to stay sharp on topics that are continually changing and that you may need to adapt to in our industry.
Some of the resources engineers at Simpson Strong-Tie go to for webinars and CECs include:
AWC – American Wood Council
Keeping in touch with fellow structural engineers means that you can talk shop and get some great advice about issues you face on the job. Some associations you can look into:
Simpson Strong-Tie also offers great software resources for structural engineers and other building industry professionals. What resources do you recommend? Let us know in the comments below.
The IRC® contains several different narrow bracing methods that are made up of portal frames. One method that is useful if you are using intermittent wall bracing is the Method PFH Portal Frame with Holdowns. This method relies on low-deflection holdown anchorage at the bottom, and substantial nailing at the overlap of the sheathing and the header at the top to prevent overturning of the narrow panel. An identical method for use as wall bracing is in the Conventional Construction section in Chapter 23 of the IBC®. These portal frames were first included in the 2006 IBC and IRC.
The method was originally tested with straps clamped to a steel test bed to simulate the embedded holdown straps. The straps were nailed to the wood with enough nails to mimic a 4,200 lb. strap anchor. The original test report is APA T2002-70. At that time, the Simpson Strong-Tie® STHD14 had a published allowable load in excess of 4,200 lbs. based on then-current Acceptance Criteria, so hardware was available to construct this frame throughout the country. However, in 2008, ICC Evaluation Service developed a new acceptance criteria for embedded connectors, AC398, Acceptance Criteria for Cast-in-place Cold-formed Steel Connectors in Concrete for Light-frame Construction. This was in response to the changes in ACI 318 for anchors in concrete. When re-tested and evaluated using the new Acceptance Criteria, the allowable load for STHD14 was reduced below 4,200 lbs. for use in buildings designed for Seismic Design Categories C through F. The same thing happened to other manufacturers’ embedded holdown allowable loads. That made it impossible to properly construct this bracing method in those areas. In response to this, Simpson Strong-Tie worked with APA, the Engineered Wood Association, to design a new test that would determine if a lower capacity holdown could be used with this portal frame method. APA performed the testing at their Tacoma, Washington testing lab. Since the initial testing of the portal frames with the 4,200 lb. holdown was performed using the outdated SEAOSC protocol with an older testing rig that used a stiff beam above the wall, both the old tests with a simulated 4,200 lb. holdown and new tests with a simulated 3,500 lb. holdown were rerun in accordance with the current ASTM E2126 test method using the CUREe protocol. The test was published as Test Report T2012L-24. The tests showed little to no effect of reducing the holdown from 4,200 lbs. to 3,500 lbs. allowable load. Here is one of the graphs of the backbone curves comparing the two assemblies for a 16-inch wide, 10-foot tall portal frame.
With the testing complete, APA prepared and submitted code changes to both the 2012 International Building Code® and 2012 International Residential Code®. The IBC proposal is S291-12, and can be found on page 605 of the 2012 Proposed Changes to the International Building Code – Structural. The IRC proposal is RB311-13, and can be found on page 613 of the 2013 Proposed Changes to the International Residential Code-Building. With support from Simpson Strong-Tie, both of the proposals were approved. So in the 2015 IRC, bracing method PFH will require an embedded strap-type holdown with a minimum capacity of 3,500 lbs. instead of 4,200 lbs. The same will hold true for the Alternate Braced Wall Panel Adjacent to a Door or Window Opening bracing method in the 2015 IBC. APA also re-tested the portal frames with only two sill plates instead of three. This will allow the use of a 5/8” by 8” Titen HD® anchor as a retrofit anchor bolt. What are your thoughts? Let us know in the comments below.
When the opportunity presents itself, glance up at the ceiling. Do you ever wonder who the responsible parties were for the design and construction of the roof above? If you’re involved in the truss industry, there is no doubt you have. If not, it never hurts to be in the know. Since we spend a significant portion of our life under a roof, it helps to know a few facts about what’s over our heads.
Roofs built from prefabricated wood trusses used in light-frame and residential construction will be the focus of this blog post.
The current national design standard for metal plate connected wood truss construction is ANSI/TPI 1-2007, which is the referenced standard in the 2009 and 2012 IBC and IRC. So what are design responsibilities for wood trusses and why are they important? They are a series of responsibilities required by key parties for applications of trusses in the construction of a building. These key parties (Owner, Building Designer, Registered Design Professional, etc.) are important because each is required to produce pertinent information about the truss and truss system from its inception to erection and long in-service life.
As wood trusses have evolved, so have publications about their construction, quality and use. The first standard was published in 1960, with subsequent standards published periodically. (see http://www.tpinst.org/publication_archives.html).
In 1995, the Truss Plate Institute (TPI) published ANSI/TPI 1-1995, which served as the first ANSI consensus-based national design standard for metal-plate connected wood truss construction. One of many new chapters established in ANSI/TPI 1-1995 was chapter 2, identifying design responsibilities. While early versions of ANSI/TPI 1 introduced design responsibilities, chapter 2 of ANSI/TPI -2007 has clarified and added areas of responsibility that are vital for today’s component industry. In addition, the 2007 edition defined responsibilities regarding temporary and permanent restraint and bracing, and special inspection requirements to long span trusses (any truss with a span of 60 feet or greater). These are just a few, yet critical additions to the standard.
Without clear definitions of responsibility, how would the industry know who specifies truss connections, or who provides bracing locations necessary to a roof assembly and its duration of service? Additionally, who determines if a project requires a truss submittal package, or the type of information it must provide? While these questions and more are answered in ANSI/TPI 1-2007, any provisions of the TPI 1 Design Responsibilities can be changed in the contract documents for a given project, so long as all parties are made aware of and agree to the revisions.
Ensuring all parties’ know and follow the design standard can help ensure a properly designed, manufactured and erected truss that will lead to a safe roof system. If you’re a component manufacturer, knowing what you’re responsible for and required to produce can get you out of a jam or better yet, help you avoid one altogether. Communication is key to the industry. The Commentary and Appendices of ANSI/TPI 1-2007 is available for web review: http://www.tpinst.org/TPI1-2007_Commentary-Appendices_web.pdf
Do you know or want to know the answers to the above questions? Or perhaps think there are responsibilities that need to be clarified or added to future publications of ANSI/TPI 1? Let us know in the comment section below.
This week’s blog was written by David Finkenbinder, P.E., who is a regional engineer working out of the Simpson Strong-Tie Ohio branch which services 24 states through the Northeast, Midwest, and Mid-Atlantic. He graduated from Penn State with a B.S. in Agricultural and Biological Engineering in 2004 and earned his M.S. in Civil Engineering with a focus on Structural Engineering from Virginia Tech in 2007. His master’s thesis investigated the splitting strength of bolted connections in solid-sawn lumber and structural composite lumber. Since joining Simpson Strong-Tie in 2007, David has shown a passion for deck safety and has served on committees developing prescriptive information and building code provisions for decks. Here is David’s post.
“Decks cause more injuries and loss of life than any other part of the home structure. Except for hurricanes and tornadoes, more injuries may be connected to deck failures than all other wood building components and loading cases combined.”
This quote, taken from Washington State University’s magazine article Making Decks Safer, underscores the critical importance of proper deck design, construction, and maintenance. An engineer who is encountering their first deck may be surprised that the deck design resources available are not as plentiful as he/she might have expected. The following resources can be helpful start:
For decks built to the IRC, the book Deck Construction Based on the 2009 International Residential Code provides a review of applicable code provisions and related commentary. The book gives background on important durability considerations such as flashing at points where the deck connects to an adjacent structure. The book also briefly discusses variations with IBC provisions, which can be significant for examples such as minimum guard height and live loads.
The American Wood Council (AWC) has several tools available in addition to using the NDS for wood member and connection design. Calculators for evaluating simple span joists and single fastener connections are available in both web-based and mobile app format. Technical Report 12, which was the topic of our May blog post, provides the ability to design connections with a gap between members, or with members having a hollow cross section. AWC’s DCA6 – Prescriptive Residential Wood Deck Construction Guide presents information for common deck details and a commentary covering important considerations for alternate designs. While the guide is helpful, please note that it is limited in scope to single level residential decks and does not address wind or seismic design.
Researchers at Virginia Tech and Washington State University conducted laboratory testing and published information to help in several common topics needing attention. An article in the May 2008 issue of Structure Magazine featured test performance of ledger-to-band joist connections using bolts or lag screws – this information has since been adopted into the IRC.
For lateral design there has been some uncertainty regarding lateral loads that can be generated by occupants, and if the magnitude of such is significant in comparison with wind and seismic forces calculated from ASCE 7. Tests were conducted of occupants performing several types of movement on a deck floor configuration. Separate articles summarizing results for each load type were published in the Summer 2013 issue of Wood Design Focus, along with a fourth article on the lateral performance of IRC ledger attachments (online copies of the articles courtesy of Professional Deck Builder magazine: Wind Loads; Seismic Loads; Occupant Loads).
Our January 2013 post, Corrosion: The Issues, Code Requirements, Research, and Solutions, touches on the corrosion considerations that are significant for most projects as well.
Have you found any other resources that have been helpful in your designs? Let us know by posting a comment.
This January I wrote a blog post, Spanning the Gap, which discussed two methods for establishing allowable loads for fasteners installed through gypsum board – testing or calculations using American Wood Council’s Technical Report 12. AWC recently published a new version of TR12 and this week’s guest blogger, Lori Koch, Project engineer with AWC, authored this post to explain some of the new features of TR12.
Lori Koch graduated from Penn State University with a BS in Civil Engineering, and from Clemson University with an MS in Civil Engineering. After graduating from Clemson, Lori worked as a forensic structural engineer doing field inspections, job site monitoring for compliance with project specifications and structural analysis on existing structures. She then enrolled at Virginia Tech pursuing a Master of Forestry degree in the Department of Wood Science and Forest Products (now called the Department of Sustainable Biomaterials). Her research at Virginia Tech involved connections for fall-protection harnesses for residential roofers and construction workers. After graduation in 2012, Lori started working with the American Wood Council as a Project Engineer. Her work at the AWC ranges from assisting in codes and standards development, answering HelpDesk inquiries, outreach and educational opportunities and just about anything that can help promote the use of wood in safe and sustainable buildings.
The American Wood Council’s Technical Report 12 – General Dowel Equations for Calculating Lateral Connection Values (TR12) was recently updated. TR12 provides background and derivation of the mechanics-based approach for calculating lateral connection capacity used in the National Design Specification® (NDS®) for Wood Construction for connections using dowel-type fasteners including bolts, lag screws, wood screws, nails, spikes, and drift pins. It also provides additional flexibility and broader applicability to the NDS provisions, including design provisions for connections with gaps. The 2014 version of TR12 provides new information on design of wood members attached to hollow members and design of driven-fasteners with tapered tips.
The previous version of TR12 presented mechanics-based derivations of lateral yield equations for solid members joined with a dowel-type fastener (Figure 1). Following the same approach, yield equations were derived for connections between solid members and members with hollow cross sections (Figure 2). These new equations are presented in tabular form for connections with a solid main member and hollow side member(s) and connections with a hollow main member and solid side member(s). Derivations of these yield equations are also presented in the report.
The 2012 NDS section 220.127.116.11 adopted new provisions for driven fasteners with tapered tips. For a driven fastener where the penetration length includes the length of the tapered tip, the dowel bearing length is taken as the length of penetration minus one half of the length of the tip. TR12 provides derivation of yield equations that account for the full penetration length, including the reduced bearing capacity at the tip. Design values using these yield equations are then compared against the simplified approach in the 2012 NDS Results of that comparison are contained in a new example included in TR12, and show excellent agreement between the simplified and exact models.
There are many applications where TR12 can be used by engineers to expand upon the NDS connection provisions. Previous versions of TR12 have provided designers with the ability to design connections with a gap between the members. The new provisions in TR12 can be used to calculate the connection capacity of a hollow steel tube connected to solid lumber, where the tube can be either the main member or side member(s).
The recent updates to TR12 will provide increased flexibility for designers while providing additional background information on the derivation of the connection equations. The report is available for free download on the AWC’s webpage at http://www.awc.org/publications/TR/index.php.
What are your thoughts on these updates to TR12? Let us know in the comments below.
Designing wood structures properly requires a broad knowledge base of a variety of materials and how they go together. However, it can often be difficult to find educational opportunities for designers to learn about wood design or keep up with new technologies on wood construction.
Fortunately, there are some unique chances this summer to increase your knowledge about wood as a construction material.
There is a short course titled Advanced Design Topics in Wood Construction Engineering, being held May 21 and 22 at Virginia Tech University in Blacksburg, VA. It is intended for designers, inspection professionals and builders that want to expand their general knowledge of wood as a building material and their knowledge of building design beyond the introductory level. The agenda includes sessions on Decay Processes, Design for Durability, and Insects that Attack Wood; Wood Shrinkage Issues in Construction; Lumber Grading Methods and Design Values; Design of Built-Up Beams and Columns; Glulam Beam Design; Evaluating Structural Capacity of Fire-Exposed Timber Beams and Columns; Multiple-Bolt Wood Connection Design; Basics of Diaphragm and Shear Wall Design; Post-Frame Building Design and Diaphragm/ Shear Wall Tests; Creep of Solid-Sawn Joists, I-Joists, and Floor Trusses; Design Considerations for Preventing Flat Roof Failures from Gravity Loads or Sustained Live Loads; Wood Truss Design Responsibilities; Wood Truss Repair Design Techniques; Permanent Truss Bracing Design Basics; and Lateral Design of Decks.
You can find more information about the Virginia Tech Short Course here. Web registration ended May 14, 2014; you can register by calling the Conference Registrar (540) 231–5182 up to the first day of the course.
If you feel like travelling, the World Conference on Timber Engineering (WCTE) will be held in Quebec City on August 10-14. WCTE is an international biannual event focusing on timber engineering, engineered wood products and design of timber structures. The conference theme is “Renaissance in Timber Construction.” Information on the conference can be found here.
But you don’t have to necessarily travel far to get quality training on wood design.
WoodWorks is a cooperative venture of major North American wood associations, research organizations and government agencies that aim to encourage and assist architects, engineers and others in the use of wood in non-residential and multi-family buildings. WoodWorks deliver knowledge to designers in three main ways: webinars, short 2-3 hour seminars and Wood Solutions Fairs. Upcoming webinars include Mixed Use Podium Design, Changes to Wood Design Standards and Healthy Buildings. Seminars scheduled for June focus on Cross Laminated Timber in California, Pennsylvania, Texas, and Washington. Finally, Wood Solutions Fairs are excellent all-day events where attendees can choose from more than 15 classes in six sessions throughout the day. The Fairs also include exhibits to allow for networking with building product manufacturers. Upcoming Wood Solutions Fairs are May 22 in Chicago, August 27 in Washington, DC, October 23 in Portland, Oregon, and November 12 in Arlington, Texas. Here is a full schedule of WoodWorks events.
If you just can’t get out of the office, or you don’t like to travel, there are still ways to keep up with the wood industry. Several groups offer webinars or self-study classes on various subjects.
WoodWorks, mentioned above, is a good resource. The American Wood Council (AWC) is the voice of North American traditional and engineered wood products, representing more than 75% of the industry. AWC’s engineers, technologists, scientists, and building code experts develop state-of-the-art engineering data, technology, and standards on structural wood products for use by design professionals, building officials, and wood products manufacturers to assure the safe and efficient design and use of wood structural components. AWC also provides technical, legal, and economic information about wood design, green building and manufacturing environmental regulations advocating for balanced government policies that sustain the wood products industry. AWC has begun offering regular webinars on various subjects with complimentary registration. Upcoming webinars include the AWC Prescriptive Residential Wood Deck Construction Guide on May 22, AWC Web-based Calculators and Other Resources on June 24, and Prescriptive and Engineering Design per the 2012 WFCM will be offered some time in the fall. Also, AWC has a comprehensive library of e-courses on their website as well as a helpdesk via email, email@example.com.
In addition, the International Code Council offers a variety of online training classes as part of their ICC Campus Online. Most have a nominal fee, but several are available free of charge. They have a Catalog of Classes on their website.
And finally, don’t forget about resources available from Simpson Strong-Tie. These resources range from full and half-day workshops offered at various locations throughout the country to online courses you can take from the comfort of your own office. Many of these courses come with CEU credits and some also offer AIA credits. And if you would like a personal visit, such as a lunch-and-learn, contact your local sales rep, or one of our regional offices and ask to speak with the training manager.
Do you know of any other good events coming up? Keep the conversation going.