- Page 33 of 50 - News, Notes and Discussion from the Simpson Strong-Tie Engineering Department

BRACE FOR IMPACT! Bracing Design for Cold-Formed Steel Studs

While consideration of bracing is important for any structural element, this is especially true for thin, singly symmetric cold-formed steel (CFS) framing members such as wall studs. Without proper consideration of bracing, excessive buckling or even failure could occur. Bracing is required to resist buckling due to axial or out-of-plane lateral loads or a combination of the two.

There are two methods for bracing CFS studs as prescribed by the American Iron and Steel Institute (AISI) Committee on Framing Standards (COFS) S211 “North American Standard for Cold-Formed Steel Framing – Wall Stud Design” Section B1. One is sheathing braced design and the other is steel braced design.

Sheathing braced design has limitations, but it is a cost effective method of bracing studs since sheathing is typically attached to wall studs. This design method is based on an assumption that the sheathing connections to the stud are the bracing points and so it’s limited by the strength of the sheathing fastener to stud connection. Due to this limitation, the Designer has to use a steel braced design for most practical situations. AISI S211 prescribes a maximum nominal stud axial load for gypsum board sheathing with fasteners spaced no more than 12 inches on center. AISI S211 Section B1 and the Commentary discuss the design method and assumptions and demonstrate how to determine the sheathing bracing strength.

CFS Curtain Wall Stud Steel Clip and Bridging Bracing

Sheathing braced design requires that identical sheathing is used on each side of the wall stud, except the new AISI S240 standard Section B1.2.2.3 clarifies that for curtain wall studs it is permissible to have sheathing on one side and discrete bracing for the other flange not spaced further than 8 feet on center. The wall stud is connected to the top and bottom tracks or supporting members to provide lateral and torsional support and the construction drawings should note that the sheathing is a structural element. When the sheathing on either side is not identical, the Designer must assume the weaker of the two sheathings is attached to each side. In addition, the Designer is required to design the wall studs without the sheathing for the load combination 1.2D + (0.5L or 0.2S) + 0.2W as a consideration for construction loads of removed or ineffective sheathing. The Designer should neglect the rotational restraint of the sheathing when determining the wall stud flexural strength and is limited by the AISI S100 Section C5.1 interaction equations for designing a wall stud under combined axial and flexural loading.

Steel braced design may use the design methodology shown in AISI S211 or in AISI Committee on Specifications (COS) S100 “North American Specification for the Design of Cold-Formed Steel Structural Members.”

AISI S211 Table B1-1 Maximum Axial Nominal Load Limited by Gypsum Sheathing-to-Wall Stud Connection Capacity

Steel braced design is typically either non-proprietary or proprietary “clip and bridging” bracing, or “flat strap and blocking” bracing periodically spaced along the height of the wall stud.

CFS Wall Stud Steel U-Channel Bridging Bracing

CFS Wall Stud Steel Flat-Strap Bracing and Blocking Bracing

Proprietary wall bracing and wall stud design solutions can expedite design with load and stiffness tables and software as well as offer efficient, tested and code-listed solutions such as Simpson Strong-Tie wall stud bridging connectors.

Steel braced design is a more practical bracing method for several reasons. First, during construction, wall studs go unsheathed for many months, but are subjected to significant construction loads.This is especially true for load-bearing, mid-rise structures. Second, some sheathing products, including gypsum wallboard, can be easily damaged and rendered ineffective if subjected to water or moisture. Third, much higher bracing loads can be achieved using mechanical bracing. IBC Section 2211.4 permits Designers to design steel bracing for axially loaded studs using AISI S100 or S211. However, S100-07 requires the brace to be designed to resist not only 1% of the stud nominal axial compressive strength (S100-12 changes this to 1% of the required compressive axial strength), but also requires a certain brace stiffness. S211 requires the Designer to design the bracing for 2% of the stud design compression force, and it does not have a stiffness requirement. . AISI S100 is silent regarding combined loading, but S211 provides guidance. S211 requires that, for combined loading, the Designer designs for the combined brace force determined using S100 Section D3.2.1 for the flexural load in the stud and either S100 or S211 for the axial load. In addition, the bracing force for stud bracing is accumulative as stated by S211 Commentary section B3. As a result, the periodic anchorage of the bracing to the structure such as strongbacks or diagonal strap bracing is required.

CFS Wall Stud Diagonal Strap Steel Bracing Anchorage

Some benefits and challenges of steel clip and bridging bracing include:

Proprietary solutions, such as the Simpson Strong-Tie SUBH bridging connector, can significantly reduce installed cost since many situations require only one screw at each connection.
Unlike strap bracing, u-channel bracing can be installed from one side of the wall.
U-channel bracing does not create build-up that can make drywall finishing more difficult.
Extra coordination may be required to ensure that u-channel bridging does not interfere with plumbing and electrical services that run vertically in the stud bay.
Bracing for axial loaded studs requires periodic anchorage to the structure, such as using strongbacks or diagonal strap bracing.
Bracing of laterally loaded studs does not require periodic anchorage since the system is in equilibrium as torsion in the stud is resisted by bridging (e.g., U-channel) bending.

Some benefits and challenges of steel flat strap and blocking bracing include:

May be installed at other locations than stud punchout.
Required to be installed on both sides of wall.
Bumps out sheathing.
Bracing for axial loaded studs requires periodic anchorage to structure, such as using strongbacks or diagonal strap bracing (same load direction in stud flanges).
Bracing for laterally loaded studs requires design of periodic blocking or periodic anchorage to the structure (opposite load direction in stud flanges).

There are several good examples Designers may reference when designing CFS wall stud bracing. They include AISI D110 Cold-Formed Steel Framing Design Guide that may be purchased from www.cfsei.org, SEAOC Structural/Seismic Design Manual Volume 2 Example 3 that may be purchased from www.seaoc.org, and the Simpson Strong-Tie wall stud steel bracing design example on page 60 of the C-CFS-15 CFS catalog.

AISI S110 Cold-Formed Steel Framing Design Guide

SEAOC 2012 IBC Structural/Seismic Design Manual Volume 2

Cold-formed steel framing is a versatile construction material, but Designers need to carefully consider the bracing requirements of the AISI specification and wall stud design standard. What cold-formed steel wall bracing challenges have you encountered and what were your solutions?

Report Back from Nepal – Assessing Seismic Damage from April/May Earthquakes

As soon as news spread that 7.8-magnitude and 7.3-magnitude earthquakes struck Nepal in April and May of this year, earthquake structural engineering experts from our firm, Miyamoto International, hopped on planes from three countries to offer assistance. We do this in hopes that our expertise and technical advice might help stricken communities recover; help them to build better and ultimately help save lives.Continue Reading

Florida Product Approvals Made Simple

This year, the new 5^th Edition of the Florida Building Code was released and is now in effect statewide. First printed in 2002, the Florida Building Code was developed as part of Florida’s response to the destruction caused by Hurricane Andrew and other hurricanes in the state.

Another component, which I would like to take a closer look at in today’s post, is a separate Florida Product Approval system designed to be a single source for approval of construction products for manufacturers, Designers and code enforcers. This single system streamlines the previous approach of different procedures for product approval in different jurisdictions. While statewide approval is not required, many jurisdictions, manufacturers and specifiers prefer using the statewide system to the alternative, which is called local product approval. To ensure uniformity of the state system, Florida law compels local jurisdictions to accept state-approved products without requiring further testing and evaluation of other evidence, as long as the product is being used consistent with the conditions of its approval.

The rules of the Florida Product Approval system are in Florida Rule 61G20-3. Here is some basic information about Florida Product Approval.

The Florida Product Approval system is only available for “approval of products and systems, which comprise the building envelope and structural frame, for compliance with the structural requirements of the Florida Building Code.” So users will only find certain types of products approved there. However, if you work in areas where design for wind resistance is required, the Florida system can be a gold mine of information for tested, rated and evaluated products. Not only will you find products like Simpson Strong-Tie connectors with our ICC-ES and IAPMO UES evaluation reports, but thousands of other tested and rated windows, doors, shutters, roof covering materials and other products that don’t typically get evaluation reports from national entities. The specific categories of products covered under the Florida system are exterior doors, impact protective systems, panel walls, roofing, shutters, skylights, structural components and windows.

To protect consumers, a recent law passed in Florida states that a product may not be advertised, sold or marketed as offering protection from hurricanes, windstorms or wind-borne debris unless it has either State Product Approval or local product approval. Selling unapproved products in this way is considered a violation of the Florida Deceptive and Unfair Trade Practices Act.

Once a manufacturer understands the process for achieving a statewide approval, it is not difficult to achieve, but it can be expensive. The manufacturer must apply on the State of Florida Building Code Information System (BCIS) website at www.floridabuilding.org. To prove compliance with the code, the manufacturer must upload either a test report, a product certification from an approved certification entity, an evaluation report from a Florida Professional Engineer or Architect, or an evaluation report from an approved evaluation entity (ICC-ES, IAPMU UES, or Miami-Dade County Product Control). Then, the manufacturer must hire an independent validator to review the application to ensure it complies with the Product Approval Rule and that there are no clerical errors. Finally, once the validation is complete, staff from the Department of Business and Professional Regulation reviews the application. Depending on the method used to indicate code compliance, the application may be approved at that time or it may have to go through additional review by the Florida Building Commission.

Here are several ways to find out if a product is approved.

For Simpson Strong-Tie products, we maintain a page on www.strongtie.com that lists our Florida Product Approvals.
The Florida Department of Business and Professional Regulation maintains a page where users can search Product Approvals by categories such as manufacturer, category of product, product name, or other attributes such as impact resistance or design pressure.
A third-party group we work with has created a website called www.ApprovalZoom.com that lists various product evaluations and product approvals. In addition to listing Florida Product Approvals, they also list ICC-ES evaluation reports, Miami-Dade County Notices of Acceptance, Texas Department of Insurance Approvals, Los Angeles Department of Building Safety Approvals, AAMA certifications and Keystone certifications among others.

florida3 — Florida Department of Business and Professional Regulation Product Approvals search

The process for searching for approved products on the Florida BCIS is fairly simple.

Go to www.floridabuilding.org
On the menu on the left side of the page, click on Product Approval. Or, click this link to go directly to the search page.
On the Product Approval Menu, click on Find a Product or Application. Note that at this location you can also search for approved organizations such as certification agencies, evaluation entities, quality assurance entities, testing laboratories and validation entities.
Ensure the proper Code Version is shown. The current 2014 Florida Code is based on the 2012 International Codes.
At this point, several options can be searched. You can search for all approvals by a specific product manufacturer or a certain type of building component by searching Category and Subcategory, or if searching for a specific product, by entering the manufacturer’s name and the product name.

florida2 — Select the option highlighted in red

I hope you find the information contained in the Florida Product Approval system useful. Do you have other needs to find approved products?

Kids + Structural Engineering = The Tech Challenge!

This week’s post comes from Marlou Rodriguez who is an R&D Engineer at our home office. Prior to joining Simpson Strong-Tie, Marlou worked as a consulting engineer. His experience includes commercial, multi-family residential, curtain wall systems and the design of seismic bracing for non-structural components. Marlou is a licensed professional Civil and Structural Engineer in California, and too many other states to list. He received his bachelor’s degree in Architectural Engineering from Cal Poly San Luis Obispo. Here is Marlou’s post.Continue Reading

Design Examples for Steel Deck Diaphragm Calculator Web App

This week’s blog post was written by Neelima Tapata, R&D Engineer for Fastening Systems. She works in the development, testing and code approval of fasteners. She joined Simpson Strong-Tie in 2011, bringing 10 years of design experience in multi- and single-family residential structures in cold-formed steel and wood, curtain wall framing design, steel structures and concrete design. Neelima earned her bachelor’s degree in Civil Engineering from J.N.T.U in India and M.S. in Civil Engineering with a focus on Structural Engineering from Lamar University. She is a registered Professional Engineer in the State of California.

Like most engineers, you are probably often working against tight deadlines, on multiple projects and within short delivery times. If you have ever wished for a design tool that would make your work easier, we have an app for that. It’s a simple, quick and easy-to-use tool called the “Steel Deck Diaphragm Calculator” for designing steel deck diaphragms. This tool is so user friendly you can start using it in minutes without spending hours in training. This app can be found on our website, and you don’t need to install anything.

The Steel Deck Diaphragm Calculator has two parts to it: “Optimized Solutions” and “Diaphragm Capacity Tables.” Optimized Solutions is a Designer’s tool and it offers optimized design solutions based on cost and labor for a given shear and uplift. The app provides multiple solutions starting with the lowest cost option using different Simpson Strong-Tie® structural and side-lap fasteners. Calculations can be generated for any of the solutions and a submittal package can be created with the code reports, Factory Mutual Approval reports, fastener information, corrosion information, available fliers, and SDI DDM03 Appendix VII and Appendix IX that includes Simpson Strong-Tie fasteners. Currently, this tool can be used for designing with only Simpson Strong-Tie fasteners. We will be including weld options in this calculator very soon. Stay tuned!

The Diaphragm Capacity Tables calculator can be used to develop a table of diaphragm capacities based on the effects of combined shear and tension.

When “Optimized Solutions” is selected, the following input is requested:

Step 1: Building Information ̶ Enter general information about the project, like the project name, the length and width of the building to be designed along with spacing between the support members such as joist spacing, is entered.

Step 2: Steel Deck Information ̶ Select the type of the steel deck along with the fill type. You can select the panel width from the options or select “Any panel width” option for the program to design the panel width. Choose the deck thickness or select the “Optimize” option for the program to design the optimum deck thickness. You also have an option of editing the steel deck properties to accommodate proprietary decks that are within the limitations of SDI DDM03 Section 1.2. Select the joist steel (support) thickness that the deck material will be attached to. For some fasteners, the shear strength of the fastener is dependent on this support thickness.

Step 3: Load Information ̶ Enter the shear and uplift demand and select the load type as either “wind” or “seismic” and the design method as “ASD” or “LRFD.”

Step 4: Fastener Information ̶ This is the last step of input before designing. In the fastener information section, you have the option to choose a structural and side-lap fastener or let the program design the most cost-effective structural and side-lap options. This can be done by checking the “Provide optimized solutions” option. The default options in the program are usually the best choice. However, you can change or modify as needed for your project. You can also set the side-lap fastener range or leave it to the default of 0 to 12 fasteners.

Now let’s work on an example:

Design a roof deck for a length of L = 500 ft. and a width b = 300 ft. The roof deck is a WR (wide rib) type panel, with a panel width of 36″. The roof deck is supported by joists that are ¼” thick and spaced at 5 ft. on center. Design the diaphragm for wind loading using Allowable Stress Design method. The diaphragm should be designed for a diaphragm shear of 1200 plf. and a net uplift of 30 psf. The steel deck is ASTM A653 SS Grade 33 deck with F_u = 45 ksi.

This information is entered in the web app, as seen below.

After inputting all the information, click on the Calculate button. You will see the five best solutions sorted by lowest cost and least amount of labor. Then click on the Submittal Generator button. Upon pressing this button, a new column called “Solution” is added with an option button for each solution. You can select any of the solutions. Below the Submittal Generator button, you can select various Code Reports and Approvals and Notes and Information selections that you want included in the submittal. After selecting these items, click on the Generate Submittal button. Now a pdf package will be generated with all of your selections.

Below is the screen shot of the first page containing Table of Contents from the PDF copy generated. The PDF copy contains the solutions generated by the program, then the detailed calculations for the solution that is selected. In this case, as you can see in the screen shot above, detailed calculations for solution #1 are included with XLQ114T1224 structural screws; XU34S1016 side-lap screws; 36/9 structural pattern and with (10) side-lap fasteners; diaphragm shear strength of 1205 plf. and diaphragm shear stiffness of 91.786 kip/in. The detailed calculations are followed by IAPMO UES ER-326 code report and FM Approval report #3050714.

Below is another example of a roof deck to be designed for multiple zones.

Design a roof diaphragm that will be zoned into three different areas. Zoning is a good way to optimize the economy of the roof diaphragm. Below are the required diaphragm shears and uplift in the three zones.

Zone 1: Diaphragm shear = 1200 plf.; Net uplift = 30 psf.; Length and width of zone 1 = 300 ft. x 200 ft.
Joist spacing = 5 ft.

Zone 2: Diaphragm shear = 1400 plf.; Net uplift = 0 psf.; Length and width of zone 2 = 500 ft. x 200 ft.
Joist spacing = 5.5 ft.

Zone 3: Diaphragm shear = 1000 plf.; Net uplift = 25 psf.; Length and width of zone 3 = 300 ft. x 200 ft.
Joist spacing = 4.75 ft.

Refer to the example above for all other information not given.

To design for multiple zones first select the Multi-Zone Input button, which is below the Fastener Information section as shown below:

When you click on the Multi-Zone Input button, you can see a toggle button appearing above a few selections as shown below. The default for the toggle button is globalbutton , which means that this selection is same for all the zones. You can click on the toggle button to change to zonebutton . Then the selection below changes to a label and reads Zone Variable. After all the selections that need to be zone variables are selected, click the Add Zone button. Keep adding zones as needed. A maximum of five zones can be added. After creating the zones, add the information for each zone and click the Calculate button.

When the Calculate button is clicked, the results for each zone are listed. The five best solutions are listed for each of the zones as shown below.

Similar to previous example, select the Generate Submittal button to select the solutions to be included in the submittal generator. Select one solution for each zone and then check the items like the code reports or notes to be included in the submittal. Click Generate Submittal to create the submittal package.

See the screen shot below for the steps.

Now that you know how easy it is to design using our web app, use this app for your future projects. We welcome your feedback on features you find useful as well as your input on how we could make this program more useful to suit your needs. Let us know in the comments below.

Truss Repair Information: The Never-Ending Search

Truss repair is one of the most frequently asked about truss topics. Not surprisingly, when we asked for suggested truss topics in a truss blog earlier this year, truss repair information made the list. Because the summer months bring about a peak in new construction – and plenty of truss repairs to go along with it – the beginning of June is the perfect time to visit this topic.

From trusses that get dropped or cut/drilled/notched at the jobsite, to homeowners who want to modify their existing trusses to add a skylight or create attic space to fire-damaged trusses, a multitude of scenarios fall under the broad topic of truss repair. Today’s post focuses on various references and resources that can provide some assistance. But first it helps to break down the broad “truss repair” topic into more manageable-sized categories.

New Construction vs. Recent Construction vs. Old Construction

By far, the easiest type of truss repair is new construction, when the trusses either haven’t been installed yet or are still in the process of being installed. Whether the repair is relatively simple (e.g. a broken web) or a little more complicated (e.g. the trusses need to be stubbed), the beauty of new truss construction is that the truss manufacturer – and truss Designer – can be contacted and help with the repair. The truss Designer can easily open up the truss designs in the truss design software, quickly evaluate the trusses for the appropriate field conditions and issue a repair.

A good reference related to truss repairs for new truss construction is the Building Component Safety Information (BCSI) booklet jointly produced by SBCA and TPI. Section B5 of the BCSI booklet, which is also available as a stand-alone summary sheet, covers Truss Damage, Jobsite Modifications & Installation Errors. This field-guide document describes the steps to take when a truss at the jobsite is damaged, altered or improperly installed, common repair techniques, and the information to provide to the truss manufacturer when a truss is damaged, which will assist in the repair process.

The next easiest truss type to repair is recent construction, where the trusses were constructed recently enough that: a) the truss plates are easy to identify, and b) the truss design drawings may even still be available. In these cases, design professionals other than the original truss Designer may be contacted to repair the trusses. For some types of repairs, the design professional can work off the truss design drawing to design the repair. Other times it might be necessary to model and analyze the truss using structural design software; alternatively, a truss manufacturer can be contacted to model the truss in their truss design software for a fee.

Often, the design professional wants to know the design values for the truss plates that were used to construct the truss. If there are truss design drawings available, they will indicate which truss plates were used in the design, and then the truss plate manufacturer can be contacted for more information. It is also easy to search for the truss plate code reports online (for instance, check icc-es.org). If no truss design drawing is available, there is still a way to identify the truss plates. Currently, there are only five major truss plate manufacturers in the United States, and they are listed on the Truss Plate Institute website. That makes identification of the truss plates used in recently constructed trusses easier because all of the current manufacturers’ plates will have markings that are described in their code reports. (Note that there are also a couple of truss manufacturers in the U.S. that manufacture their own truss plates.)

Finally, the most challenging type of trusses for truss repairs are those found in older buildings. Design professionals involved in these types of repair often aren’t sure where to start. Truss design drawings are often not available, and the act of trying to identify the truss plate manufacturer is challenging at best, unsuccessful at worst. As a point of reference, there were 14 truss plate manufacturers that were TPI members in 1987 (see image below), and only one of those companies is still in the current list of five companies. Therefore, the truss plates found in a truss built around 1987 will be difficult to identify. One option is to contact TPI and see if they can point you in the right direction.

TPI Member Listing from a 1987 Publication — TPI member listing from a 1987 publication

Simple vs. Complex Repairs

Another way to break down truss repairs is to divide them into easy and challenging repairs. People often ask for “standard” truss repair details. Unfortunately, standard details only address the simplest types of repair; and those usually aren’t the types of repair that are asked about. Details simply cannot cover the wide range of truss configurations and every type of repair situation.

Sample Repair Detail for a Simple Repair — Sample repair detail for a simple repair

With the exception of simple repairs, most truss repairs rely heavily on the judgment and experience of the design professional doing the repair. And because there are not entire textbooks devoted to truss repair (that I am aware of, anyway), Designers must pull from a variety of resources, both to learn more about truss repair and to design the repair. For repairs using plywood or OSB gussets, the APA Panel Design Specification is a must-have reference. Some people prefer to use dimension lumber scabs for their repairs, whenever possible, simply because they are more familiar with dimension lumber (and the NDS) than they are with Plywood/OSB or the APA Panel Design Specification.

Next, the fasteners for the repair must be selected and the allowable loads determined. For nail design values, I am a big fan of the American Wood Council’s Connection Calculator, which provides allowable nail shear values for just about any combination of main and side members that you can think of, including OSB and plywood side members – particularly handy for truss repairs. For more complex repairs, and especially repairs involving higher forces, an excellent fastener choice is a structural wood screw such as our Strong-Drive® SDS or SDW screws. When I worked in the R&D department at Simpson Strong-Tie, a frequently asked question was whether we had double-shear values for our SDS screws. The questions always seemed to come from Designers who wanted them for truss repairs. Fortunately, we do have double-shear values for our SDS screws.. You can find them on page 319 of our Fastening catalog.

Page 319 of the Fastening Systems Catalog (C-F-14)

The Strong-Drive SDW screw was developed after the SDS screw, and while there are currently no double-shear values for the SDW, it is still another good option for repairs.

Fire-Damaged Trusses and Truss Collapses

These situations are in a category by themselves because they go beyond even the most complex repairs involving a major modification to the truss. The biggest difference is that the latter case involves mostly known facts and perhaps some conservative assumptions, whereas damage due to fire or collapse includes many unknowns. Most of the truss Designers I have spoken to about truss damage due to fire or truss collapse often recommend replacement of the trusses rather than repair because it is usually too difficult to quantify the damage to the lumber and/or joints. In fires, there can be “hidden” damage due to the sustained high temperatures, while the truss appears to have no visible damage. Likewise, in a truss collapse, not only may there be too many breaks in the trusses involved in the collapse, but there may also be trusses that suffered severe stresses during the collapse and have damage that is not visible. To attempt a repair in either of these cases often requires an inspection at the jobsite, and the result may still end up being replacement of some or all of the trusses. Therefore, the cost of a full-blown inspection should be weighed against the cost of replacing the trusses.

The Structural Building Components Association website has a page with information pertaining to fire issues. It includes a couple of documents related to fire damage that are worth checking out.

Beyond the Blog: Where to Get More Truss Repair Information

The best bet for getting practical design information related to truss repairs is to keep an eye out for short courses, workshops or seminars. ASCE has hosted a Truss Repair Seminar (Evaluating Damage and Repairing Metal Plate Connected Wood Trusses) in the past and may very well offer something like it again. Virginia Tech recently hosted a short course on Advanced Design Topics in Wood Construction Engineering, which included a section on Wood Truss Repair Design Techniques.

What other references or resources for truss repair do you use? Are there any upcoming truss repair courses that you know of? Please let us know in the comments below!

Anchor Reinforcement for Concrete Podium Slabs

How often do you get the opportunity to high five a co-worker in the office? Maybe it’s when you just worked through a complex calculation, or finally figured out that tough detail. Whatever it might be, there are times when we should raise a hand and celebrate the hard work that we do. So when we recently relaunched the Simpson Strong-Tie Strong-Rod™ Systems website, which includes a link to our new Shallow Podium Anchorage Solutions, there were a few high fives going around the office. With that in mind, we want to share the latest developments and continue our anchorage-to-concrete blog discussions that began in May 2012, continued with a March 2014 post referencing the Structure magazine article on anchor testing, and more recently one discussing our release of anchor reinforcement solutions for Steel Strong-Wall® shearwall to grade beams.

Continuous Rod Restraint Systems for Multi-Story Wood Structures

This week was our new employee Sales and Product Orientation class. It reminded me of the post A Little Fun with Testing where we broke a bowling ball. Although breaking stuff is fun, my second favorite part of the class is teaching about the importance of a continuous load path. I think it is really the most important thing a Structural Engineer does. If we don’t pay attention to the loads, where they occur and create a path so they can get where they need to go, a building may not stand up. This week, we also released some new tools and information for our new Strong-Rod™ Systems, which are used to complete the load path for multi-story wood-framed shearwall overturning restraint and roof uplift restraint.

Two Load Paths

All wood-framed buildings need to be designed to resist shearwall overturning and roof-uplift forces. To transfer these tensile forces through the load path, connectors (hurricane ties, straps and holdowns) have been the traditional answer. Simpson Strong-Tie offers a few options there. With the growth in multi-story wood-framed structures, where the code requires shrinkage to be addressed and overturning and uplift forces are typically higher, rod systems have become an increasingly popular load restraint solution. Our Anchor Tiedown System (ATS) for shearwall overturning restraint has been around for many years. A new Strong-Rod Systems Design Guide and revamped web pages provide information on new design options, components and configurations.

Strong-Rod Systems Seismic and Wind Restraint Systems Guide

The guide and website focus more on the unique design considerations for rod systems, how you should specify the system and highlight the design services that we provide. They also provide more detail and design information for our relatively new Uplift Restraint System (URS) for roofs. Connectors are a common choice for transferring the net roof uplift forces from wind events down the structure. Although in some high-wind areas, rod systems are preferred.

ATS and URS Continuous Rod Tiedown Systems

I’ll touch on some of the design considerations for these types of systems below, but back to the load path. For shearwall overturning restraint using holdowns, the load path is fairly simple. Once the lateral load is in the shearwall, the sheathing and nailing lifts up on the post. The holdown connects to the post, holding it down and transferring the forces to the foundation or level below. A continuous rod tiedown system follows a little different path. The sheathing and nailing lifts up on the boundary posts and the posts push up on the framing above until the load is resisted in bearing by a bearing plate. The load is then transferred into the rod and down to the foundation. There has been a lot of testing and research on the effects of skipping restraint locations where a bearing plate restraint is installed at every other floor or only at the top level. Doing that will change the load path because the load has to continue to travel up until a restraint holds it down. It also negatively impacts the stiffness and drift of the shearwall stack, not to mention increases project cost because the boundary posts, rod and bearing must be sized to transfer the cumulative overturning forces from each level.

Wood Shrinkage, Take-up Devices and Displacement Limits

Shrinkage is not just a Seinfeld episode cult classic. It is also something that designers need to consider when designing wood structures. IBC Section 2304.3.3 requires that designers evaluate the impact of wood shrinkage on the building structure when bearing walls support more than two floors and a roof. The effects of wood shrinkage can impact many things in the structure from finishes to MEP systems to the continuous rod system. As the wood members lose moisture, the wood shrinks and the building settles. This can cause gaps at the bearing plate locations of continuous rod systems because the continuous steel rod doesn’t shrink. That is where the magic of take-up devices comes in. They allow the building to shrink but keep gaps from forming by filling the gap (expanding devices – can be screw style or ratcheting), ratcheting down the rod (ratcheting devices), or making the rod shrink as much as the wood (contracting coupling device).

In addition to keeping the rod system tight to insure the intended performance, it is important to consider the movement associated with the rod system when under wind or earthquake loading. The IBC requires shearwall displacements to be within story drift limits in moderate to high seismic regions. We highlighted some of the changes coming for the evaluation of shearwall deflection in the previous post discussing the New Treatment of Shear Wall Aspect Ratios in the 2015 SDPWS. For continuous rod systems, there are some additional limits. ICC-ES AC316 Acceptance Criteria for Shrinkage Compensating Devices requires designs to limit displacement between restraints to 0.20 inches (including rod elongation and device displacement) for shearwall restraint. The movement of the take-up device plays a big part in meeting this requirement and the rod diameter required. Screw-style devices have the lowest total movement. Ratcheting devices are appropriate in many cases as well such as the upper levels where loads are lower, but may require larger rod diameters to meet the displacement limit.

ICC-ES AC391 Acceptance Criteria for Continuous Rod Tie-down Runs and Continuous Rod Tie-down Systems Used to Resist Wind Uplift covers continuous rod systems for roof uplift restraint. The displacement limit for the Continuous Rod Tie-down Run (just the rod system components) is limited to 0.18 inches of rod elongation for the total length of rod. The Strong-Rod URS evaluates the Continuous Rod Tie-down System (the whole load path). Displacement limits for the system are L/240 for the top plate bending and 0.25 inches total deflection at the top plate between tie-down runs (including top plate bending, rod elongation, wood bearing deformation and take-up device displacement). The differences between the rod run and rod system analysis as well as other design considerations are explained in more detail in the design guide and on our website.

I always end my continuous load path presentation during orientation class with the same questions and if they were paying attention I get the response I want.

“What is the most important thing a Structural Engineer does?”

“Designs a continuous load path for the building!”

“What does Simpson Strong-Tie do?”

“Provides product and system solutions to help engineers do their job!”

Take a look at the new Strong-Rod Systems tools and information and let us know how we can help you with your next multi-story wood-framed project.

What related blog topics would you like to discuss? Let us know in the comments below.

Holdown Anchorage Solutions

A common question we get from specifiers is “What anchor do I use with each holdown?” Prior to the adoption of ACI 318 Appendix D, this was somewhat simple to do. We had a very small table near the holdown section of our catalog that listed which SSTB anchor worked with each holdown.

The good old days! (Don’t use this today.)

During the good old days, anchor bolts had one capacity and concrete wasn’t cracked. ACI 318 Appendix D gives us reduced capacities in many situations, different design loads for seismic or wind and reductions for cracked concrete. These changes have combined to make anchor bolt design more challenging than it was under the 1997 Uniform Building Code.

This blog has had several posts related to holdowns. So, What’s Behind a Structural Connector’s Allowable Load? (Holdown Edition) explained how holdowns are tested and load rated in accordance with ICC-ES Acceptance Criteria. Damon Ho did a post, Use of Holdowns During Shearwall Assembly, which discussed the performance differences of shearwalls with and without holdowns, and Shane Vilasineekul did a Wood Shearwall Design Example. So I won’t get in to how to pick a holdown.

Once you have determined your uplift requirements and selected a post size and holdown, it is necessary to provide an anchor to the foundation. To help Designers select an anchor that works for a given holdown, we have created different tables that provide anchorage solutions for Simpson Strong-Tie holdowns.

There is one Engineering letter that addresses slab-on-grade foundations and another version that covers stemwall foundations. The tables are separated by wood species (DF/SP and SPF/HF) to give the most economical anchor design for each post material. The preferred anchor solutions are SSTB or SB anchors, as these proprietary anchor bolts are tested and will require the least amount of concrete. When SSTB or SB anchors do not have adequate capacity, we have tabulated solutions for the PAB anchors, which are pre-assembled anchors that are calculated in accordance with ACI 318 Appendix D.

The solutions in the letters are designed to match the capacity of the holdowns, which allows the contractor to select an anchor bolt if the engineer doesn’t specify one. They are primarily used by engineers who don’t want to design an anchor or select one from our catalog tables. We received some feedback from customers who were frustrated that some of our heavier holdowns required such a large footing for the PAB anchors, whereas a slightly smaller holdown worked with an SB or SSTB anchor in a standard 12″ footing with a 1½” pop out.

To achieve smaller footings using our SB1x30 anchor bolts, we reviewed our original testing and created finite element (FEA) models to determine what modifications to the slab-on-grade foundation details would meet our target loads. Of course, we ran physical tests to confirm the FEA models. With a 6″ pop out, we were able to achieve design loads for HD12, HDU14 and HHDQ14.

HD12, HDU14 & HHDQ14 Solutions — HD12, HDU14 and HHDQ14 Solutions

The revised footing solutions for the heavier holdowns require less excavation and less concrete than the previous Appendix D calculated solutions, reducing costs on the installation.

What has been your experience with holdown anchorage? Tell us in the comments below.

An Introduction to the Helical Wall Tie

What do you do when brickwork is in bad condition? Depending on what state the brickwork is in, a tear-down may be called for. However, often brickwork can be restored and strengthened using helical ties such as Simpson Strong-Tie® Heli-Tie™ wall ties and stitching ties. This post introduces these two types of helical ties, which might be just what you need for your next brick restoration project.