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
Previously published on January 30, 2014.
Drywall. Wall board. Sheetrock. Sackett Board?
A product called Sackett Board was invented in the 1890s, which was made by plastering within wool felt paper. United States Gypsum Corporation refined Sackett Board for several years until 1916, when they developed a new method of producing boards with a single layer of plaster and paper. This innovation was eventually branded SHEETROCK®. More details about the history of USG can be found here.
No matter what you call it, gypsum board is found in almost every type of construction. Architects use it for sound and fire ratings, while structural engineers need to account for its weight in our load calculations. A common technical support question we receive is for allowable fastener loads for ledgers to wood framing over gypsum board.
One method to evaluate a fastener spanning across gypsum board is to treat the gypsum material as an air gap. Technical Report 12, General Dowel Equations for Calculating Lateral Connection Values, is published by the American Wood Council.
TR12 has yield limit equations that allow a designer to account for a gap between the main member and side member of a connection. With a gap of zero (g=0), the TR12 equations provide the same results as the NDS yield limit equations.The equations are fairly complex, but it should be intuitive that the calculated fastener capacity decreases with increasing gap. Engineers are often surprised to see a 40, 50, even 60% drop in fastener capacity with one layer of 5/8” gypsum board. So what else can you do?
Testing, of course! In So, What’s Behind a Screw’s Allowable Load? I discussed the methods used to load rate a proprietary fastener such as the Simpson Strong-Tie® Strong-Drive® SDS or SDW screws. To recap, ICC-ES Acceptance Criteria for Alternate Dowel Type Fasteners, AC233, allows you to calculate and do verification tests, or load rate based on testing alone. We develop our allowable loads primarily by testing, as the performance enhancing features and material optimizations in our fasteners are not addressed by NDS equations.
So to determine the performance of a fastener installed through gypsum board, we tested the fastener through gypsum board. This is easier to do if you happen to have a test lab with a lot of wood and fasteners in it. We did have to run down to the local hardware store to pick up gypsum board for the testing.
A full set of allowable loads for Strong-Drive SDWH and SDWS are available on strongtie.com. The information is given as single fastener shear values for engineered design, and also screw spacing tables for common ledger configurations. As much fun as writing spreadsheets to do the Technical Report 12 calculations is, having tabulated values based on testing is much easier.
In the fastener marketplace, Simpson Strong-Tie stands apart from the rest. Quality and reliability is our top priority.
Experiential learning — has it happened to you? Certainly it has, because experiential learning is learning derived from experience. It happens in everyday life, in engineering and in product development, too. For example, experience has taught us that after a product is launched, our customers will find applications for the product that were never expected or listed in the product brief. Also, experience has shown us that larger fasteners tend to be placed in applications that have greater structural and safety demands.
When the larger Deck-Drive™ DWP screws were manufactured, we decided that they should be marketed as “load-rated” screws because they were big enough to support physically large parts and would be expected to provide structural load resistance.
So what is a “load-rated” screw? To Simpson Strong-Tie, a load-rated screw is a threaded fastener that has controlled dimensions and physical properties, as well as validated connection properties. Load-rated fasteners are also subject to the same quality inspection that would occur if they were undergoing an evaluation report.
The products in the focus of this blog are Deck-Drive DWP Wood stainless-steel tapping screws. They are made from stainless steel (Types 305 and 316) and are particularly interesting because they have a box thread design feature. What is a box thread and what are its benefits? A box thread is a thread that is square rather than round. It is formed by rolling (not a trivial tooling challenge) like a standard thread. The box thread is preferred for some applications in part because of the low torque required to install the screw; that is, the installation demand is low relative to standard threads of the same pitch (number of threads per inch). You can easily see the box thread by looking from the point of the screw toward the head. The square corners of the box thread rotate at a defined angle, giving the threaded length a twisted appearance. The box thread is also used on the Timber-Hex SS screws. See Figure 1 for an illustration.
When we load rate a fastener, ICC-ES AC233 (Acceptance Criteria for Alternate Dowel-type Threaded Fasteners, 2015) is the guiding document. Essentially, we do everything that would be done if the product was going into an evaluation report. The testing uses representative products and is witnessed by a third party, and every test report is reviewed and stamped by a professional engineer. The DWP screws that are fully load rated are No. 12 and No. 14 that are three to six inches long. This means that we have evaluated by test the shear and tensile strengths, bending yield strength, head pull-through resistance, withdrawal resistance and certain logical lateral shear configurations of these models. The connection properties are developed in at least three species combinations of wood representing a range of specific gravities. Each cell in the connection load matrix is a reference allowable value based on a mean of at least 15 tests that is subject to a precision of five percent at a 75-percent confidence level. Table 1 is snipped from the prepublication spreadsheets.
While we were working on the No. 12 and No. 14 screws, we also realized that No. 10 DWP screws often require withdrawal loads because they are used in decks and docks to fasten the decking to the structural frame. You can see in Table 1 that the withdrawal loads were included for No. 10 DWP screws and the related properties, because uplift resistance is often engineered for those applications.
What is the test method for each property in the load table? See Table 2 for the test method used for each property and the related data for that property. The reference allowable shear loads shown in Table 1 represent more than 1,200 individual tests, and each test includes wood specific gravity, moisture content and continuous load-displacement data from start of test to past ultimate load.
Load rating screws is a big job, and it creates an elevated continuous quality-monitoring obligation. However, our experience has taught us that the engineering community needs information and reference properties that can be relied on when specifying, and thus working with load-rated screws makes it possible to put your stamp on a design with confidence.
We look forward to hearing from you about load-rated fasteners, because we learn from you every time you contact us.
People are always innovating new things! There are always new tools, software, apps or, more recently, digital assistants to help us organize our life! Here’s something I want to share with you. Recently my family bought Google Home, and both my boys (ages 8 and 5) are constantly exploring it and testing its capabilities: “Hey, Google, play this music” or “Hey, Google, what time is it?” or “Hey, Google, repeat ‘Nathan is bad.’” While Google Home helps them with the former requests, it simply says, “I am still learning,” in response to commands like “repeat ‘Nathan is bad.’” It’s funny to see them experiment and come up with creative ideas to use the tool. Many of us appreciate tools that help us be more organized or increase our efficiency or that are simply fun to use. Our new revised diaphragm calculator for designing metal decks is our attempt to help the engineering community get more done in less time.
So What Are the Updates and Revisions?
We have updated our design software to design per Canadian Standards like CSA136 and to design per Limit States Design. The app is so easy to use that you can design a steel deck diaphragm in minutes! The software designs steel decks for both shear and uplift forces acting on the deck and provides tables with diaphragm shear capacities for a given deck span using Simpson Strong-Tie deck fasteners that conform to Canadian codes and standards. These fasteners have an evaluation report, IAPMO UES ER-326, are recognized in SDI (Steel Deck Institute) DDM03 Appendix VII and IX and the CSSBI (Canadian Sheet Steel Building Institute) Design Manual and have FM approvals.
Overview of the App
When you open our diaphragm design software, Steel Deck Diaphragm Calculator, there is an option to “Select your Country.” You can choose to design for US standards, in which case you select the USA option, or you can select Canada Imperial or Canada Metric, which are new additions. The app has three sections: (1) Optimized Solutions, (2) Diaphragm Capacity Tables and (3) Other Diaphragm Tables. All three options are available for the USA option. The Optimized Solutions help you to design a deck for any given shear and uplift. You can refer to our previous blog, Design Examples for Steel Deck Diaphragm Calculator Web App, for some examples on how to design steel decks using the Optimized Solutions selection. Diaphragm Capacity Tables are available to the USA and both Canada selections. Other Diaphragm Tables is available only to the USA selection.
Metal Deck Diaphragm Design Using Limit States Design (LSD)
When you select Canada for the country, you will have the option to select Diaphragm Capacity Tables as shown in the screen shot below. You can generate diaphragm shear tables by entering:
- Steel Deck Information: In this section, you select the type of the deck, the design method, the load type you would like the tables to be generated in and the deck thickness. You can enter uplift if you would like to design the deck for combined shear and tension, or leave the net uplift as zero if you are generating shear-only tables.
- Quik Drive Fastener Information: In this section, you input information about the structural and side-lap fasteners.
Click the Calculate button to generate the tables.
A PDF copy of the tables can be generated in either English or French.
This easy-to-use design software can be used by the designers, specifiers or erectors to generate the tables required. More information about our X series of screws (including XL and XM), tools and the required industry approvals for designing the profiled deck diaphragms can be found on our website at strongtie.com.
Please try out the app and let us know your comments and feedback so we can continue to improve our software to better serve your needs!
This week’s post is written by Kevin Davenport, who is the Southeast US Field Engineering Manager for Simpson Strong-Tie. Kevin is also responsible for providing technical support on Simpson Strong-Tie products for Infrastructure, Commercial and Industrial market segments within his own Southeastern territory. He is a registered professional engineer in Georgia and received his B.S. (’97) and M.S. (’98) from Clemson University. Kevin is a member of ICRI, ACI and various local chapters of SEA.
The primary benefit of fiber-reinforced polymer (FRP) systems as compared with traditional retrofit methods is that significant flexural, axial or shear strength gains can be realized using an easy-to-apply composite that does not add significant weight or section to the structure. Many times it is the most economical choice given the reduced preparation and labor costs and may be installed without taking the structure out of service. However, it is important to make sure the composite is properly designed following industry standards in order to ensure that it is the right product for the application.
To provide you with a better understanding of the topic, it’s important to dispel some common myths and misconceptions that you might hear about FRP:
1. “FRP can solve all my retrofit and strengthening problems”
Composite strengthening systems are another tool for your toolbox, providing a possible solution to certain specific retrofit problems. However, they can’t do everything, and there are times when they may not be able to meet the project requirements. Simpson Strong-Tie’s design team will work with you to prepare a feasibility study to ensure suitable solutions for your application. One very important check when strengthening a structure is to verify that the existing, unstrengthened capacity is capable of resisting a certain percent of the newly applied loads. The following equations are strengthening limit checks that should be considered. These checks will sometimes determine how much additional strength the FRP composites are capable of providing to the existing structure.
- ACI 440.2R-08
(φRn)existing ≥ (1.1SDL + 0.75SLL)new (9-1)
- ACI 562-13
Uex ≥ 1.2D + 0.5L + Ak + 0.2S (5.5.1)
2. “FRP is 10 times stronger than steel”
Although the ultimate tensile strength of some FRP dry fibers can exceed the yield strength of mild reinforcing bars (60 ksi) by up to 10 times, there are two main reasons an engineer should not assume that using FRP will provide 10 times the capacity of steel. First, the cured composite properties, not the dry fiber properties, are more relevant when designing with FRP composites. The ultimate tensile strength of cured composites will be more on the magnitude of two to three times stronger than 60 ksi (not 10 times stronger). Second, the ultimate tensile strengths of FRP systems occur at ultimate strain. When full design calculations are performed, the FRP design strain and resulting FRP strength will often be much lower after accounting for all possible failure modes and all recommended reductions based on durability testing and/or environmental reduction factors. Code limits often govern design over published ultimate strength properties.
For this reason, it is not good practice to size the required area of FRP using:
AFRP = (Arebar x fy rebar) / ffu FRP
It may be possible to achieve higher increases depending on member properties, but the following are some good rules of thumb when estimating the amount of strengthening that can be provided by FRP: flexural = 40%, shear = 20%, axial = 20%. Design is usually governed by the existing strength check, the FRP debonding strain (can’t develop infinite tension capacity through the bond line), or a ductility check (flexural φ factor based on strain in rebar at section failure).
4. “Stamped calculations and drawings were submitted, so it must have been designed properly”
Often, the FRP design engineer may make various assumptions in the design calculations, and the EOR (reviewer) should ensure that the FRP is designed “correctly” and verify that any assumptions made by the FRP engineer are accurate. Note that Simpson Strong-Tie calculations have an “Assumptions” section to make it is very easy for the EOR to identify where we took educated guesses.
Simpson Strong-Tie Can Help
We recognize that specifying Simpson Strong-Tie® Composite Strengthening Systems™ is unlike choosing any other product we offer. Leverage our expertise to help with your FRP strengthening designs. Our experienced technical representatives and licensed professional engineers provide complimentary design services and support – serving as your partner throughout the entire project cycle. Since no two buildings are alike, each project is optimally designed to the Designer’s individual specifications. Our pledge is to address your specific condition with a complete strengthening plan tailored to your needs, while minimizing downtime or loss of use, at the lowest possible installed cost.
Your Partner During the Project Design Phase
During the Designer’s initial evaluation or preparation of the construction documents, Simpson Strong-Tie can be contacted to help create the most cost-effective customized solution. Simpson Strong-Tie Engineering Services will work closely with the Design Engineer to provide all the necessary information required to design the system. The solution we deliver will include detailed design calculations for each strengthening requirement and design drawings with all the necessary details to install the CSS.
Why Use Simpson Strong-Tie Design Services?
- To assess feasibility studies that will help ensure suitable solutions to your application
- To receive customized FRP strengthening solutions
- To work with our trained contractor partners to provide rough-order-of-magnitude (ROM) budget estimates
- To collaborate during the project design phase
- To receive a full set of drawings and calculations to add to your submittal
- To maintain the flexibility to provide the most cost-effective solution for your project
- To gain trusted technical expertise in critical FRP design considerations
The Design Fundamentals of FRP Strengthening
If you would like to view the recorded webinar, you can find it at our website at https://www.strongtie.com/css.
The Design Fundamentals of FRP Strengthening
Learn the best practices for FRP strengthening design during this innovative webinar presented by Simpson Strong-Tie.
If you’d like more information about FRP design, you can learn the best practices for fiber-reinforced polymer (FRP) strengthening design from a recorded webinar offered by Simpson Strong-Tie professional engineers. We look at FRP components, applications and installation. We also take you behind the scenes to share the evaluation process informing a flexural beam-strengthening design example and talk about the assistance and support Simpson Strong-Tie Engineering Services offers from initial project assessment through installation.
For complete information regarding specific products suitable to your unique situation or condition, please visit strongtie.com/css or call your local Simpson Strong-Tie RPS specialist at (800) 999-5099.
For a structural engineer working on multiple projects in various stages of design and construction, it can be challenging to keep up to date on the latest industry trends. However, many of us in the construction industry enjoy learning about new construction techniques and unique projects. Being educated about new technology and design tools can also increase efficiency in the office.
To make it easier to catch up on pertinent industry news, we are sharing our top five tips and shortcuts.
1. Make Time to Stay Informed
Make sure you block off some time on your calendar each week to read up on construction news. Pick a consistent day and time (if possible) that is usually a little slower and less likely to be booked with meetings. At our office, Monday mornings and Friday afternoons tend to be the best times.
2. Subscribe to Industry Newsletters
After you block off time on your calendar, the next step is to subscribe to a few construction industry newsletters. Depending on the newsletter, you can sign up for a hard copy or have them delivered electronically to your inbox. Here are some great construction industry newsletters to get you started:
- Structural Engineers Association Newsletters: If you haven’t signed up for your local city or state SEA newsletter, you should start here. Many structural engineering association chapters have newsletters. For example, the Structural Engineers Association of Northern California has a monthly online newsletter. The state of Texas offers an online quarterly journal, and a few local chapters, including Austin, Dallas, Fort Worth and Houston, have their own newsletters. With a quick Google search, you can find one in your area.
- ICC eNews: Subscribe to the International Code Council’s weekly digital newsletter for ICC news, programs and industry events.
- Civil + Structural Engineer e-News: Sign up on the home page of their website.
- Hanley Wood newsletters: You can choose from more than 30 different online industry newsletters focused on residential construction and remodeling, or commercial design and construction.
- Structural Report® newsletter: Subscribe to this quarterly print and online newsletter for structural engineers and architects that provides industry and building safety news and Simpson Strong-Tie product information.
- Strong-Tie News: For a quick read, sign up for our monthly company newsletter sent via email. The e-news features new products and software, literature, videos, industry news and training events.
- Concrete News: If you are involved in concrete construction and repair, this triannual print and digital newsletter has articles on the latest code changes, industry news and Simpson Strong-Tie product solutions.
3. Attend a Technical Webinar
Webinars are an easy way to stay connected to your profession and the construction industry while learning new things. As an added bonus, some webinars offer CEUs or PDH credits so you can stay current with professional development requirements. Click here to find out our top three reasons why you should attend webinars.
Here is a list of organizations that offer webinars that many of our engineers attend:
- ACI – American Concrete Institute
- AISC – American Institute of Steel Construction
- ASCE – American Society of Civil Engineers
- AWC – American Wood Council
- CFSEI – Cold-Formed Steel Engineers Institute
- NCSEA – National Council of Structural Engineers Association
- SEAOSC – Structural Engineers Association of Southern California
4. Get Out to a Live Training Event
There are many courses devoted to improving building standards and the overall safety of structures. . We provide hundreds of classes to engineers, architects, builders and code officials each year, so make sure to sign up for a workshop in your area or to try one of our online courses.
Don’t forget to attend technical conferences, too. The Structural Engineering Institute (part of ASCE) has multiple conferences throughout the year that help you earn CEU and PDH credits. The American Wood Council has an event calendar with live trainings and webinars on hot topics in the industry, also.
5. Talk with Other Structural Engineers
It’s so easy to take this tip for granted. We sometimes forget that the greatest asset and resource we have are our colleagues. At Simpson Strong-Tie, we offer “lunch and learn” sessions where different departments share initiatives that affect the business. If you work in an engineering firm with different specialties, a lunch-and-learn session is an easy way for everyone to find out about a new project or design challenge.
Another great way to connect with fellow structural engineers is to take part in networking events with structural engineering organizations. Here are some to look into:
- SEAINT – Structural Engineers Association – International
- NCSEA – National Council of Structural Engineers Associations
- SEAOC – Structural Engineers Association of California
- SEAOSC – Structural Engineers Association of Southern California
- SEAOCC – Structural Engineers Association of Central California
- SEAOSD – Structural Engineers Association of San Diego
- SEAU – Structural Engineers Association of Utah
- ASCE – American Society Of Civil Engineers
- ACI – American Concrete Institute
- AISC – American Institute of Steel Construction
- PCA – Portland Cement Association
- PCI – Precast/Prestressed Concrete Institute
- CRSI – Concrete Reinforcing Steel Institute
- AISI – American Iron and Steel Institute
There are also several professional LinkedIn groups, like this one, that provide not only educational content, but also a way for you to ask questions and hear the thoughts and opinions of your peers.
These are a few tips to get you started, but there are myriad resources to help you stay informed, including traditional trade magazines, industry blogs and social media sites. Simpson Strong-Tie is always here to help, as well. Make sure to follow us on Facebook, LinkedIn and Twitter to learn about industry news and our latest products and resources.
Written by Brandon Chi, Engineering Manager, Lateral Systems at Simpson Strong-Tie.
Wood shearwalls are typically used as a lateral-force-resisting system to counter the effects of lateral loads. Wood shearwalls need to be designed for shear forces (using sheathing and nailing), overturning (using holdowns), sliding (using anchorage to concrete) and drift, to list some of the main dangers. The Simpson Site-Built Shearwall Designer (SBSD) web app is a quick and easy tool to design a wood shearwall based on demand load, wall geometry and design parameters.
The web application provides two options for generating an engineered shearwall solution: (1) Solid Walls; and (2) Walls with Opening using the force-transfer-around opening (FTAO) method. Both options generate solutions that offer different combinations of sheathing, nailing, holdowns, end studs and number/type of shear anchors. The app can generate a PDF output for each of the possible solutions. Design files can be saved and reused for future projects.
Figure 1 shows the input screens for the “Solid Walls” and “Walls with Opening” designs with common wall parameters that are applicable to both design options. The user interface uses quick drop-down menu and input fields for the designer to select the different options and parameters. Unless otherwise noted, all the input loads are to be nominal (un-factored) design loads. The application will apply load combinations to determine the maximum demand forces for the shearwall design.
Figure 2 shows the allowable stress design (ASD) load combinations used for calculating the demand loads for the different components of the wood shearwall (i.e., holdown, compression post, sheathing and nailing design, etc.).
In addition to the lateral loads (wind and seismic) applied at the top of the wall and the wall’s own weight, uniform loads on top of the wall and concentrated point loads at the end posts can also be modeled. (See Figure 3.)
Embedded anchor or embedded strap holdowns can be modeled by the app. (See Figure 4.) For the embedded strap option, additional input parameters are required since they will affect the allowable load of the selected strap holdown.
The Designer has the option to include additional sources of vertical displacement for drift calculation. (See Figure 5.)
For hand-calculated design when the demand forces are determined, the holdown size and shear anchorage can be selected from tabulated values. Design for the sheathing/nailing and compression post is relatively straightforward as well; however, the shearwall drift calculation may take a bit more work. This is where the SBSD app comes in handy. Below are two sections on the shearwall drift and strap force calculations and assumptions used in the SBSD application. If you are interested, please contact Simpson Strong-Tie for other design assumptions used in designing the SBSD app.
Shearwall Deflection Calculations:
Equation 1 shows the shearwall deflection equation from the 2008 Edition of Wind & Seismic Special Design Provisions for Wind and Seismic (SDPWS).
The Δa value from the third term of the equation is the total vertical elongation of the wall holdown system from the applied shear in the shearwall. The third term accounts for the additional displacement from holdown displacement. For holdown deflection, the deflection value depends on the post size used with the holdown size. When hand-calculating shearwall drift, Designers may have to perform a couple of iterations to come to the final post and holdown size. The SBSD app accounts for the holdown displacement and the post size used for overturning force calculation.
For shearwall-with-opening deflection calculation, EQ-2 is used in the SBSD app.
The solid wall, ∆solid wall, term is calculated using EQ-1 above. For the window strip and wall pier deflection terms, the height “h” used in EQ-1 is taken as the height of the window opening. ∆a is the deflection from nail slip in the shearwall. For more information regarding shearwall deflection with opening, please refer to Example 1 in Volume 2 of the 2015 IBC SEAOC Structural/Seismic Design Manual.
Strap Force Calculations:
For the Wall with Opening design option, there are several methods (Drag Strut, Cantilever Beam, SEAOC/Tompson, Diekmann) to calculate the force transfer around the opening. In the SBSD app, the Diekmann technique is used to calculate the pier forces in the shearwall and the strap forces around the opening. When calculating the strap forces, the SBSD app assumes they are the same at the top and bottom of the opening. In addition, contribution of the gravity load only affects the overturning forces in the holdown and post design but not the wall pier forces or strap forces.
Once all design parameters are entered and calculated, a list of possible solutions (where available) will be shown. (See Figure 6.) Common parameters such as sheathing material and type, wood species, minimum lumber grade, etc., are shown first, followed by other design parameters. The user can filter the solutions by seismic drift or wind drift.
The Designer can select the PDF button next to the desired solution to see a PDF design file on a separate screen. (See Figure 7.) The PDF design file contains the detailed design criteria input by the Designer, calculated demand loads, shearwall material summary, and a design summary for holdown, sheathing, and compression post design. A detail summary for shearwall deflection is also shown, with each term of the shearwall deflection equation (EQ-1) separated. Shear anchorage and design assumption notes follow the design summary section. This PDF file can be saved and printed by the Designer.
I hope you find the SBSD web app helpful for your day-to-day wood shearwall design needs. If you have any questions or comments, please leave them in the comments section below.
I was driving under a concrete bridge one nice clear day in Chicago, and I happened to look up to see rusted rebar exposed below a concrete bridge. My beautiful wife, who is not a structural engineer, turned to me and asked, “What happened to that bridge?” I explained that there are many reasons why spalling occurs below a bridge. One common reason is the expansion of steel when it rusts or corrodes.
This week’s blog will briefly explain the corrosion process and why concrete spalls when the embedded metals corrode. Corrosion may be defined as the degradation of a material as a reaction to its environment.1. As described in our previous SE Blog post, “Corrosion: The Issues, Code Requirements, Research and Solutions” dated January 3, 2013, corrosion of metallic surfaces is an electrochemical process. Because of moisture evaporation, concrete is a porous material. Water and oxygen molecules enter the pores of the concrete, and an electrochemical process occurs with the carbon-steel bar. The iron in the steel is oxidized, which then produces rust. A buildup of rust products at the surface of the carbon-steel bar exerts an expansive force on the concrete. Based on the amount of oxidation, the rust products of steel can occupy more than six times the volume of the original steel.2 Over time, further rust occurs and surface cracks will form. Eventually spalling will occur, exposing the rusted carbon steel bar. (See figure 1.)
Just as with reinforcing bars below a concrete bridge, cracking and spalling can occur when a carbon-steel anchor is used adjacent to a concrete edge. Simpson Strong-Tie® has many anchorage products that can be used in these conditions to prevent cracking. One specific product is the new stainless-steel Titen HD® screw anchor. This new innovative screw anchor is made up of Type 316 stainless steel. As seen in Figure 3, Type 316 stainless steel has a high level of resistance. This makes the stainless-steel Titen HD an excellent choice when it comes to an anchorage solution in corrosive environments. These environments include wastewater treatment plants, exterior handrails, exterior ledger attachments, stadium seating, central utility plants, and kitchens just to name a few.
Unlike expansion anchors, screw anchors require the leading threads to cut into predrilled holes. This can be easily achieved with hardened carbon-steel cutting threads. Stainless steel is not hard enough to cut into concrete. The new innovative stainless-steel Titen HD solves the problem by brazing heat-treated carbon-steel cutting threads to the surface of the stainless-steel tips of the screw anchor. (See figure 4.) These carbon-steel threads are hard enough to cut grooves into the surface of a predrilled hole, allowing the anchor to be installed with ease. The volume of the carbon-steel cutting threads is less than 1% of the stainless steel, reducing the buildup of rust that eventually spalls the concrete edge. Other stainless-steel screw anchor manufacturers in the market have a bi-metal product that attaches a full carbon-steel tip. This bi-metal screw anchors contain up to 18% carbon steel. Such a large amount of carbon steel can expand up to six times its volume when it corrodes and can spall the concrete when used adjacent to an edge.
When designing an anchorage solution for your next job in a corrosive environment, the stainless-steel Titen HD will provide the best resistance for corrosion, and also give the ability to drive these anchors into the concrete with ease. More information about the product can be obtained by visiting strongtie.com/thdss.
- Corrosion Technology Laboratory (https://corrosion.ksc.nasa.gov/corr_fundamentals.htm).
- Galvanized Rebar (http://www.concreteconstruction.net/how-to/repair/galvanized-rebar_o).
Stainless-Steel Titen HD®
The Next Era of Stainless-Steel Screw Anchor For Concrete and Masonry.
This blog post was written by Charlie Roesset, Director of Training for Simpson Strong-Tie.
When it comes to training, there are many well-researched principles about what makes an environment conducive to improved adult learning.
While we try to hold all training events in facilities that meet most of these principles, (even when traveling to our customers or users means we have to conduct events in hotel meeting rooms) we prefer to host you at our own locations.
To this end, we invest a tremendous amount of time and resources to build and offer dedicated training facilities across the country. These facilities meet all the basic requirements for improved adult learning, but much more as well.
By having our own dedicated training facilities, we can provide learners with a much richer experience and contextually relevant displays.
These displays include partially deconstructed wall segments, foundations and roof systems that give learners a bigger picture of the applications being studied.
Many displays allow for hands-on installations and exercises that allow for improved comprehension of the product use and limitations. Even for the engineering community, who typically are limited to images from a catalog, the hands-on activities add great value. It’s always interesting to see the reaction that engineers have to actually seeing a system approach and having an opportunity to participate in learning that goes way beyond sitting and listening to a lecture.
Sometimes learners just need to see, feel or hold something in order to really understand a concept or product application. We make every effort to bring legitimate educational content to our workshops, supported by products that we hope will furnish solutions to your needs.
Many of our facilities include a plant tour and/or testing-facility tour as well. While these components don’t always align directly with the learning objectives, they do offer a chance for our guests to raise their energy levels and get a better understanding of that scale, capabilities, and commitment to quality that we bring to bear in our endeavor to help people build safer structures.
Additionally, we offer our facilities to customers, associations and industry organizations to use for their own meetings and training events. If you haven’t been to one of our workshops or visited one of our facilities, I highly encourage you to join the 35,000 plus who have over the last four years. You can find a complete list of workshops on our training home page. I expect that you’ll find it an educational and highly engaging experience that helps you build safer structures as well.
How would a six-story light-frame wood building perform in a large earthquake? Back in 2009, Simpson Strong-Tie was a partner in the World’s Largest Earthquake Test, a collaboration of the NEESWood project, to answer that question. This was a full-scale test which subjected the building to 180% of the Northridge earthquake ground motions (approximately a M7.5). Within the building, Simpson Strong-Tie connectors and Strong-Frame SMF were used, with the Strong-Rod™ anchor tiedown system (ATS) serving as holdown for each shearwall.
The NEESWood building was designed under Performance-Based Design methodology, and the test was conducted as validation for the approach. Buildings of similar size to the NEESWood building are built to current codes using similar products. Mid-rise light-frame wood structures continue to be a popular form of construction in various densely populated cities across the country. As part of the lateral-force-resisting system, continuous rod systems are used as the holdown for the shearwall overturning restraints. Simpson Strong-Tie has been involved with continuous rod systems since the early 2000s when we launched the Strong-Rod anchor tiedown system.
Today, rod manufacturers design the continuous rod systems with design requirements (loading, geometry, etc.) Supporting documents (e.g., installation details, layouts, RFI/markups and calculations) are submitted for each unique project. Over the years, engineers have asked many questions related to the design of these systems. In this week’s blog, we will explore Frequently Asked Questions pertaining to Strong-Rod ATS systems used as shearwall overturning restraints (holdowns).
Is there a code report for the system?
The majority of these components are designed in accordance with the building code and reference standards (e.g., NDS, AISC). A project-specific calculation package is submitted for each job that addresses the evaluation of these elements. Therefore, these elements are not listed in evaluation reports.
Shrinkage compensation devices, on the other hand, are proprietary components which are not addressed by the building code or reference standards. Therefore, they are tested in accordance with ICC-ES acceptance criteria AC316 and are listed in ICC-ES ESR-2320.
What is the material specification of the rods used above concrete?
The specified rod materials are shown in Table 1.
Can threaded rods or couplers be welded to steel beams?
Simpson Strong-Tie generally does not recommend this practice. Of the materials listed in Table 1, ASTM A307 material is the only specification that contains supplementary requirements for welding. When standard strength rod is supplied to the job, it is not guaranteed that this will be the material provided.
ASTM A449 and A193-B7 high-strength rods develop strength and ductility characteristics through controlled quenching and tempering treatments. Quenching is the rapid cooling of metal (usually by water or oil) to increase toughness and strength. This process often increases brittleness. Tempering is a controlled reheating of the metal which increases ductility after the quenching process. Precise timing in the application of temperature during the tempering process is critical to achieving a material with well-balanced mechanical properties. It is unlikely that field welding will satisfy the requirements of quenching and tempering.
Coupler nuts are generally fabricated from material exhibiting characteristics similar to high-strength rods. Thus, it is not recommended to weld coupler nuts to steel beams due to the potential for embrittlement.
Simpson Strong-Tie specifies a weldable cage which is fabricated from ASTM A36 material for such applications.
How do you calculate the Maximum ASD Tension Capacity provided in the job summary?
Simpson Strong-Tie provides a comprehensive design package for continuous rod systems used as holdowns for multi-story stacked shearwalls. The individual run calculations, as shown in Figure 1, provide the Maximum Tension Capacity, which correlates to the maximum force the system can deliver. Plan check often requests justification on how these values are derived at each level. These values are calculated, and the process explained below may be used on any Simpson Strong-Tie ATS Job Summary as justification.
The maximum tension capacity published within the Job Summary and the Installation Details is derived using the following procedure:
- Step 1: Evaluate the top-most level. Compare the published capacities of the rod in tension, plate in bearing and the take-up device. The lowest of these three will govern and becomes the Maximum Tension Capacity for this level.
- Step 2: Evaluate the next level down. (a) Sum the Maximum Tension Capacity from Step 1 and the published capacity of the take-up device from this level. (b) Sum the Maximum Tension Capacity from Step 1 and the published capacity of the plate in bearing from this level. (c) Compare derived values from (a) and (b) to the published capacity of rod in tension. The lowest of these three values will govern and becomes the Maximum Tension Capacity of this level.
- Step 3: Repeat Step 2 as necessary until the bottom-most level is reached.
Applying this procedure to the sample run, SW9, will wield the following result:
- Step 1: Evaluate capacities published at Level 4
- Plate in bearing (PBRTUD5-6A) = 7.06 kips governs
- Take-up device (RTUD6) = 20.83 kips
- Rod in tension (ATS-R6) = 9.61 kips
- The lowest value in Step 1 is the plate in bearing, hence 7.06 kips is the maximum load that can be delivered at Level 4 and is the Maximum Tension Capacity.
- Step 2: Evaluate capacities at Level 3
- Maximum Tension Capacity from Level 4 = 7.06 kips (See Step 1)
- Maximum Tension Capacity from Level 4 + take-up device (ATS-ATUD9-2) = 7.06 + 12.79 = 19.85 kips
- Maximum Tension Capacity from Level 4 + plate in bearing (PL9-3×5.5) = 7.06 + 10.03 = 17.09 kips
- Rod in tension (ATS-R7) = 13.08 kips governs
- The lowest value in Step 2 is the rod in tension, hence 13.08 kips is the maximum load that can be delivered at Level 3 and is the Maximum Tension Capacity.
- Step 3: Evaluate capacities at Level 2
- Maximum Tension Capacity from Level 3 = 13.08 kips (See Step 2)
- Maximum Tension Capacity from Level 3 + take-up device (ATS-ATUD9-2) = 13.08 + 15.56 = 28.64 kips
- Maximum Tension Capacity from Level 3 + plate in bearing (PL9-3×5.5) = 13.08 + 10.03 = 23.11 kips
- Rod in tension (ATS-R7) = 13.08 kips governs
- The lowest value in Step 3 is the rod in tension, hence 13.08 kips is the maximum load that can be delivered at Level 2 and is the Maximum Tension Capacity.
- Step 4: Evaluate capacities at Level 1
- Maximum Tension Capacity from Level 2 = 13.08 kips (See Step 3)
- Maximum Tension Capacity from Level 2 + take-up device (ATS-ATUD14) = 13.08 + 24.39 = 37.47 kips
- Maximum Tension Capacity from Level 2 + plate in bearing (PL14-3×8.5) = 13.08 + 13.98 = 27.05 kips governs
- Rod in tension (ATS-R11) = 32.30 kips
- The lowest value in Step 4 is due to the plate in bearing, hence 27.05 kips is the maximum load that can be delivered at Level 1 and is the Maximum Tension Capacity.
In the System Deflection Summary page(s) of the Job Summary, is the Total System Deflection provided at Allowable or Strength levels?
Immediately following the individual run calculations in each job summary, Simpson Strong-Tie provides a summary of deflection of the rod system similar to what is shown in Figure 2. This breaks down the deformation of all components being considered. In the example below, the rod elongation and deflection of the take-up device are summed to provide the total deflection.
The calculated system deflection is presented at ASD level. See section below for how to use these system deflections for your drift calculation.
What system deflection limit do you typically design to, and what does that include?
Unless otherwise specified on the plans or required by the building jurisdiction, Simpson Strong-Tie will design the continuous rod system to satisfy the deformation limits set forth in ICC-ES Acceptance Criteria (AC316). In some instances, the Designer may need a more restrictive deformation due to project specific conditions (e.g., tight building separations) and will require rod manufacturers to design for a lower deformation. Some jurisdictions (e.g., City of San Diego, City of San Francisco) may also have specific design requirements that continuous rod systems must conform to. The minimum recommended per-floor deformation limit set forth in AC316 is:
(Rod Elongation) + (Shrinkage Compensation Device Deflection) ≤ 0.2” (ASD),
Or (PDL/AE) + [ΔR + ΔA(PD/PA)] ≤ 0.2” (ASD)
PD = ASD demand cumulative tension load (kips)
L = length of the rod between restraints – i.e., floor-to-floor (in.)
A = net tensile area of the rod (in.2)
E = Young’s Modulus of Elasticity (29,000 ksi)
ΔR = seating increment of the shrinkage compensation device (as published in ICC-ES evaluation report)
ΔA = deflection of the shrinkage compensation device at the allowable load (as published in ICC-ES evaluation report)
PA = Allowable capacity (kips)
Should deformation limits be specified in the construction documents?
Simpson Strong-Tie strongly recommends this information be included in the construction documents. Along with the cumulative tension and compression forces, the required deformation limits for the holdown are important to ensure that rod manufacturers are designing the holdown to satisfy the desired shearwall performance.
How do I use the system deformation limit?
The System Deflection is the total deformation of the holdown system from floor to floor (refer to the last two columns in Figure 2). This information represents the total ASD holdown deformation term, Δa, for each level and is to be used in the shearwall drift equation from the Special Design Provisions for Wind and Seismic (2015 SDPWS 4.3-1).
ASCE 12.8.6 requires that shearwall drift be calculated at strength level. Therefore, the information provided within the System Deflection Summary page needs to be converted from ASD to Strength Level. The conversion factors in Table 2 can be used to convert the ASD deformations to strength level. For discussions and methodology in converting bearing plate deformation to strength level, please refer to the WoodWorks Design Example of a Five-Story Wood Frame Structure over Podium Slab found here.
Can rod systems be used in Type III construction?
Yes! 2015 IBC §2303.2.5 requires that Fire Retardant-Treated Wood (FRTW) design values be adjusted based on the type of treatment used on the project. Adjustment factors vary for each FRTW manufacturer; refer to the ICC-ES evaluation report of the specified FRTW manufacturer for the unique adjustment values. Rod manufacturers need to know what treatment is being used so this information can be taken into consideration when designing compression posts and incremental bearing (bearing plates).
For more information and previous discussions on fire protection in mid-rise construction, see our previous posts: Fire Protection Considerations with Five-Story Wood-Frame Buildings Part 1 and Part 2, and Connectors and Fasteners in Fire-Retardant-Treated Wood.
What are Simpson Strong-Tie’s guidelines for fire caulking material?
While there are many options for fire-rated caulking, these products can be used in conjunction with the Simpson Strong-Tie ATS system. Below is a list of considerations when selecting and specifying a material for use where the rods penetrate the top and sole plates:
- The fire-rated caulking shall not be corrosive to metal when used in contact with ATS components.
- Direct contact with shrinkage compensating devices (e.g., TUD, ATUD, RTUD) shall be avoided. Shrinkage compensating devices have moving components and may not function properly with debris interference.
- Indirect contact with shrinkage compensating devices shall also be avoided. Shrinkage compensation accumulates up the building and therefore the largest shrinkage occurs at the top of the building. As such, when the building shrinks, remnants of the material may still be stuck to the threads of the rod and may be detrimental to the performance of some shrinkage compensating devices (e.g., an RTUD). It is recommended to detail the installation with shrinkage taken into consideration.
- The fire-rated caulking should be pliable to accommodate wood shrinkage and the building moving down during this process.
- The performance and the suitability of fire-rated caulking are outside the scope of Simpson Strong-Tie.
Why doesn’t your design include compression post design?
If the Engineer of Record has already specified compression posts to be used with a continuous rod system, Simpson Strong-Tie will not provide these on the holdown installation drawings. This is primarily done to prevent discrepancies between the specification in the contract documents and what is shown on the installation drawings.
What is the maximum spacing between compression posts?
For platform-framed structures, the maximum spacing between compression posts is 9″. The large majority of Simpson Strong-Tie bearing plates will fit within the 9″ spacing requirement, eliminating the need for notching compression posts. In some framing conditions, such as balloon framing or a top chord bearing truss, the maximum spacing will be reduced to 6″. This is due to the limited amount of space between the top of the compression posts transferring uplift (via bearing) into the point of restraint (e.g., bearing plate) at the level above. To ensure this load path is complete, the posts need to be spaced closer.
What is the nailing schedule for the bridge block to the king studs?
Simpson Strong-Tie doesn’t recommend nailing the bridge block to the cripple as the bridge block member will shrink. Locking the bridge block in place may result in a gap forming between the bottom of the bridge block member and the top of the cripple studs, which is not accounted for in the Total System Deflection.
Are there any published documents with design examples of continuous rod systems used in mid-rise construction?
There are two resources publicly available that provide discussion and examples. The first is a manual published by the Structural Engineers Association of California (SEAOC). Titled 2015 IBC SEAOC Structural/Seismic Design Manual Volume 2 – Examples for Light-Frame, Tilt-Up and Masonry Buildings, this document provides two examples – one for a four-story wood hotel building, and the other for a three-story cold-formed steel apartment building on concrete podium deck.
What questions do you have about the Strong-Rod ATS System? Leave them below.
This blog post continues our series on the final results of the 2016 ICC Group B Code Change Hearings. This post will focus on approved changes to the International Residential Code (IRC) that are of a structural nature. The changes outlined here will be contained in the 2018 IRC, which is expected to be published in the fall of this year.
In Chapter 3, the seismic design category / short-period design spectral response acceleration maps will be updated to match the new USGS/NEHRP Seismic Maps. These new maps are based on the worst case assumption for Site Class. Significantly, a new set of maps will be provided in Figure 301.2(3) “Alternate Seismic Design Categories”. These are permitted to be used when the “soil conditions are determined by the building official to be Site Class A, B or D.” See page 29 of the linked document for the new maps and a good explanation of the changes that will be occurring in various parts of the country. In addition, the ICC Building Code Action Committee authored a reorganization of the seismic provisions of Chapter 3 to try to reduce confusion.
Another change in Chapter 3 will clarify that guards are only required on those portions of walking surfaces that are located more than 30 inches above grade, not along the entire surface. To bring consistency with the IBC, another change will require that staples in treated wood be made of stainless steel.
A broad group of parties interested in deck safety, known as the Deck Code Coalition, submitted 17 different code changes with revisions to Section R507 on decks. Of those, 12 were approved, making significant changes to that section. The various approved changes included the following: a complete re-write of that section; new/clarified requirements for deck materials, including wood, fasteners and connectors; clarified requirements for vertical and lateral connections of the deck to the supporting structure; new requirements for sizes of deck footings and specification that deck footings must extend below the frost line, with certain exceptions; clarification for deck board material, including an allowance for alternative decking materials and fastening methods; adding new columns to the deck joist span table that show the maximum cantilever for joists; adding the allowance for 8×8 deck posts, to allow notching for the support of a three-ply beam; and clarification of the deck-post-to-footing connection.
In Chapter 6, a new table permitting 11ʹ- and 12ʹ- long studs was added. In the 2015 IRC, load-bearing studs were limited to 10 feet in length. A new high-capacity nail, the RSRS (Roof Sheathing Ring Shank) nail was added as an option for fastening roof sheathing. This nail will become more widely used once the higher roof component and cladding forces from ASCE 7-16 are adopted. The rim board header detail that was added for the 2015 IRC was corrected to show that hangers are required in all cases when the joists occur over the wall opening.
There were several changes made to the Wall Bracing Section, R602.10. The use of the 2.0 increase factor was clarified for use when the horizontal joints in braced wall panel sheathing are not blocked. Narrow methods were added to the column headings for the wind and seismic bracing amount tables, to make them consistent, and the methods for adding different bracing were clarified. When using bracing method PFH, the builder can omit the nailing of the sheathing to the framing behind the strap-type holdown. Finally, offering some relief for high-seismic areas where brick veneer is used, an allowance was added to permit a limited amount of brick veneer to be present on the second floor without triggering the use of the BV-WSP bracing method.
In Chapter 8, the requirements for a “stick-framed” roof system were completely re-written to make such systems easier to use.
A couple of significant changes were made to the prescriptive requirements for cold-formed steel framing. The requirements for the anchorage of cold-formed steel walls were revised, and the wind requirements for cold-formed steel framing were changed to match the new AISI S230 prescriptive standard.
Finally, it may be helpful to mention some of the proposed changes that were not adopted. While the new ASCE 7-16 was adopted as the IRC reference standard for loads as part of the Administrative changes, several changes to the IRC to make it consistent with ASCE 7-16 were not approved. A change to update the IRC wind speed maps, roof component and cladding pressures, component and cladding roof zones, and revise the remainder of wind-based requirements to match ASCE 7-16 was not approved. Similarly, a proposal to increase the live load on decks, from 1.0 to 1.5 times the occupancy served, was denied.
Once the IRC is published, it will be time to start a new code change cycle once again, with Group A code changes due January 8, 2018. The schedule for the next cycle is already posted here.
What changes would you like to see for the 2021 codes?