Introducing the New and Improved Simpson Strong-Tie Strong-Wall® Bracing Selector

This week’s post was written by Caleb Knudson, R&D Engineer at Simpson Strong-Tie.

It’s been said that the World Wide Web is the wave of the future. Okay, maybe this is slightly outdated news, as it’s been 25 years since Bill Gates penned his internet tidal-wave memorandum, but it’s a good lead-in to this week’s blog topic – web apps. More specifically, those apps that have been developed to address the wall-bracing requirements defined in the International Residential Code® (IRC). Designers and engineers have no doubt noticed that over the last several code cycles, the wall-bracing provisions in the IRC have become increasingly complex. To help navigate these requirements and calculate the required bracing length for a given wall line, Simpson Strong-Tie introduced the Wall-Bracing-Length Calculator (WBLC) a few years back, as discussed in an earlier blog post. I’ll also mention that the WBLC has since been updated to the 2015 IRC.

Those familiar with the wall-bracing provisions in the IRC know that there are twelve intermittent wall-bracing methods and four continuous-sheathing methods to address wall-bracing requirements. Each of these methods may be used in most applications, and, while some provide advantages over others, the code-based methods provide Designers with quite a bit of flexibility. However, there may be cases where the site-specific conditions are beyond the scope of the IRC, or there just isn’t enough available full-height wall space to accommodate the required wall-bracing length. These cases are most likely to occur at large window openings or at garage fronts.

Let’s take the following example of a house on Lake Washington – assuming the house is being designed in accordance with the IRC. Presumably, one might prefer to have unobstructed lake views, which of course means lots of large picture windows and not much room left for braced wall panels. Let’s also suppose you’ve got a brand-new Chris Craft that you’d like to protect against the weather when it’s not in the water – this means wide garage doors and, again, not much room for conventional wall bracing.

So what do we do now?

Thankfully, the International Residential Code provides some guidance. Section R301.1.3 states that when a building, or portion thereof, is outside the scope of the IRC, the element(s) may be designed in accordance with accepted engineering practice. The code goes on to state that the extent of the design shall be such that the engineered element(s) are compatible with the performance of conventional methods prescribed in the code. That creates some additional options for our tool box. We could design a site-built shearwall; however, due to aspect-ratio limitations defined in the Special Design Provisions for Wind and Seismic (SDPWS), we still may not be able to get the lake views and wide garage we want. The next option, and one we’ll focus on here, is the code-approved prefabricated Simpson Strong-Tie® Strong-Wall® shearwall.

In an earlier blog post, as previously mentioned, we introduced the Strong-Wall Bracing Selector (SWBS) and defined just how we determine equivalence to conventional bracing methods. We further described the benefit of using the selector in conjunction with the Wall-Bracing-Length Calculator (WBLC). To refresh your memory, when Designers start with the WBLC to determine required wall-bracing-lengths for up to seven parallel wall lines, they can export those bracing lengths as well as project and jobsite information directly to the SWBS with the click of a button. The SWBS will then provide a list of Strong-Wall panels that provide an equivalent bracing length, evaluate their anchorage requirements, and return a list of pre-engineered anchor solutions for a variety of foundation types.

On to the present: We just launched the Strong-Wall Bracing Selector web app version 2.0, and there are a few new features worth noting.

First, I’ll mention that all Strong-Wall solutions have been evaluated according to the 2015 I-Codes. Next, and hopefully this doesn’t come as too much of a surprise, the original wood Strong-Wall shearwall (SW) is being phased out with guaranteed availability through December 31, 2018. In light of this planned obsolescence, we have removed the SW solutions from the latest version of the bracing selector.

Here’s the good news – and this is big: We’ve now added the new Strong-Wall wood shearwall (WSW) to the app and recommend this as a replacement for the SW in all applications. In the interim, while the original wall is still available, version 1.0 of the bracing selector app may be used if an SW bracing solution is required.

Lastly, we’ve provided the Designer with a bit more flexibility and control over the Strong-Wall bracing solutions provided by the app. If you recall, version 1.0 provided a solution using the minimum possible number of Strong-Wall panels to satisfy the bracing length requirement. We’ve changed that in version 2.0; Designers may still select a solution using the minimum number of panels, but they may also select the exact number of Strong-Wall panels to satisfy their wall-bracing-length requirements. Typically, it’s desirable to address the bracing requirement with the minimum number of Strong-Wall shearwall panels possible. Sometimes, however, it may be advantageous to increase the number of panels used, in order to decrease the Strong-Wall panel width used for a solution or to reduce anchorage requirements, i.e., lesser footing dimensions and anchor embedment depths. Stated a little differently, we’re providing the option to find the right balance between the braced wall panel design and the anchorage design – i.e., the Goldilocks zone for prescriptive wall bracing.

So now that we’ve reviewed just why a Designer may need to specify a Strong-Wall shearwall in prescriptive applications and how the Wall-Bracing-Length Calculator and Strong-Wall Bracing Selector web apps help to navigate this process, we’re interested to see what you think. Is there any additional functionality that you’d like to see in the future, or are these apps just right for your design needs? Let us know in the comments below.

Shrinkage Compensation Devices

Over the weekend, I had the pleasure of watching my daughter in her cheer competition. I was amazed at all the intricate detail they had to remember and practice. The entire team had to move in sync to create a routine filed with jumps, tumbles, flyers and kicks. This attention to detail reminded me of the new ratcheting take-up device (RTUD) that Simpson Strong-Tie has just developed to accommodate 5/8″ and ¾” diameter rods. The synchronized movement of the internal inserts allows the rod to move smoothly through the device as it ratchets. The new RTUDs are cost effective and allow unlimited movement to mitigate wood shrinkage in a multi-story wood- framed building. When designing such a building, the Designer needs to consider the effect of shrinkage and how to properly mitigate it.

Our SE blog post on Continuous Rod Restraint Systems for Multi-Story Wood Structures explained the importance of load path and  the effects of wood shrinkage. This week’s blog post will focus on the importance of mitigating the shrinkage that typically occurs in multi-story light-frame buildings.

Shrinkage is natural in a wood member. As moisture reaches its equilibrium in a built environment, the volume of a wood member decreases. The decrease in moisture causes a wood-framed building to shrink.

The IBC allows construction of light-framed buildings up to 5 and 6 stories in the United States and Canada respectively. Based on the type of floor framing system, the incremental shrinkage can be up to ¼” or more per floor. In a 5-story building, that can add up to 1-¼” or more and possibly double that when construction settlement is included.

rods1

Typical Example of gap forming between nut and plate when wood shrinkage at top level occurs without shrinkage device.

The Simpson Strong-Tie Wood Shrinkage Calculator is a perfect tool to determine the total shrinkage your building can experience.

Wood Shrinkage Calculator

Wood Shrinkage Calculator

In order to accommodate the shrinkage that occurs in a multi-story wood-framed building, Simpson Strong-Tie offers several shrinkage compensating devices. These devices have been tested per ICC-ES Acceptance Criteria 316 (AC316) and are listed under ICC-ES ESR-2320 (currently being updated for the new RTUD5, RTUD6, and ATUD9-3).

AC316 limits the rod elongation and device displacement to 0.2 inches between restraints in shearwalls. This deflection limit is to be used in calculating the total lateral drift of a light-framed wood shearwall.

rod3

3 Part Shearwall Drift Equation

The 0.2-inch allowable limit prescribed in AC316 is important to a shearwall’s structural ability to transfer the necessary lateral loads through the structure below to the foundation level. This limit assures that the structural integrity of the nails and sill plates used to transfer the lateral loads through the shearwalls is not compromised during a seismic or wind event. Testing has shown that sill plates can crack when excessive deformation is observed in a shearwalls. Nails have also been observed to pull out during testing.  Additional information on this can be found here.

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Sill Plates Cracked due to excessive uplift at ends of shearwall.

rods5

Nails pull out due to excessive uplift at ends of shearwall.

In AC316, 3 types of devices are listed.

  • Compression-Controlled Shrinkage Compensating Device (CCSCD): This type of device is controlled by compression loading, where the rod passes uninterrupted through the device. Simpson Strong-Tie has several screw-type take-up devices, such as the Aluminum Take-Up Device (ATUD) and the Steel Take-Up Device (TUD), of this type.
rods6

ATUD (CCSCD)

  • Tension-Controlled Shrinkage Compensating Device (TCSCD): This type of device is controlled by tension loading, where the rod is attached or engaged by the device and allows the rod to ratchet through as the wood shrinks. The Simpson Strong-Tie Ratcheting Take-Up Device (RTUD) is of this type.

rod7

RTUD (TCSCD)

  • Tension-controlled Shrinkage Compensating Coupling Device (TCSCCD): This type of device is controlled by tension loading that connects rods or anchors together. The Simpson Strong-Tie Coupling Take-Up Device (CTUD) is of this type.
CTUD (TCSCCD)

CTUD (TCSCCD)

Each device type has unique features that are important in achieving the best performance for different conditions and loads. The following table is a summary of each device.

rods9The most cost-effective Simpson Strong-Tie shrinkage compensation device is the RTUD. This device has the smallest number of components and allows the rod unlimited travel through the device. It is ideal at the top level of a rod system run or where small rod diameters are used. Simpson Strong-Tie RTUDs can now accommodate 5/8″ (RTUD5) and ¾” (RTUD6) diameter rods.

How do you choose the best device for your projects? A Designer will have to consider the following during their design.

rod10

RTUD Assembly

Rod Tension (Overturning) Check:

  • Rods at each level designed to meet the cumulative overturning tension force per level
  • Standard and high-strength steel rods designed not to exceed tensile capacity as defined in AISC specification
    • Standard threaded rod based on 36 / 58 ksi (Fy/Fu)
    • High-strength Strong-Rod based on 92 / 120 ksi (Fy/Fu
    • H150 Strong-Rod based on 130 / 150 ksi (Fy/Fu)
  • Rod elongation (see below)

 Bearing Plate Check

  • Bearing plates designed to transfer incremental overturning force per level into the rod
  • Bearing stress on wood member limited in accordance with the NDS to provide proper bearing capacity and limit wood crushing
  • Bearing plate thickness has been sized to limit plate bending in order to provide full bearing on wood member

 Shrinkage Take-Up Device Check

  • Shrinkage take-up device is selected to accommodate estimated wood shrinkage to eliminate gaps in the system load path
  • Load capacity of the take-up device compared with incremental overturning force to ensure that load is transferred into rod
  • Shrinkage compensation device deflection is included in system displacement

 Movement/Deflection Check

  • System deformation is an integral design component impacting the selection of rods, bearing plates and shrinkage take-up devices
  • Rod elongation plus take-up device displacement is limited to a maximum of 0.2″ per level or as further limited by the requirements of the engineer or jurisdiction
  • Total system deformation reported for use in Δa term (total vertical elongation of wall anchorage system per NDS equation) when calculating shearwall deflection
  • Both seating increment (ΔR) and deflection at allowable load (ΔA) are included in the overall system movement. These are listed in the evaluation report ICC-ES ESR-2320 for take-up devices

 Optional Compression Post Design

  • Compression post design can be performed upon request along with the Strong-Rod System
  • Compression post design limited to buckling or bearing perpendicular to grain on wood plate
  • Anchorage design tools are available
  • Anchorage design information conforms to AC 318 anchorage provisions and Simpson Strong-Tie testing

In order to properly design a continuous rod tie-down system for your shearwall overturning restraint, all of the factors listed above will need to be taken into consideration.

A Designer can also contact Simpson Strong-Tie by going to www.strongtie.com/srs and filling out the online “Contact Us” page to have Simpson Strong-Tie design the continuous rod tie-down system for you. This design service does not cost you a dime. A few items will be required from the Designer in order for Simpson Strong-Tie to create a cost-effective rod run (it is recommended that on the Designer specify these in the construction documents):

  • There is a maximum system displacement of 0.2″ per level, which includes rod elongation and shrinkage compensation device deflection. Some jurisdictions may impose a smaller deflection limit.
  • Bearing plates and shrinkage compensation devices are required at every level.
  • Cumulative and incremental forces must be listed at each level in Allowable Stress Design (ASD) force levels.
  • Construction documents must include drawings and calculations proving that design requirements have been met. These drawings and calculations should be submitted to the Designer for review and the Authority Having Jurisdiction for approval.

More information can be obtained from our website at www.strongtie.com/srs, where a new design guide for the U.S., F-L-SRS15, and a new catalog for Canada, C-L-SRSCAN16, are available for download.

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US Design Guide F-L-SRS15 and Canadian Catalog C-L-SRSCAN16

Anchor Reinforcement for Concrete Podium Slabs

Guest blogger Scott Fischer, R&D Engineer

Guest blogger Scott Fischer, R&D Engineer

This week’s post comes from Scott Fischer who is an R&D Engineer at our home office. Since joining Simpson Strong-Tie in 2006, Scott has worked on cast-in-place connectors to concrete. He helped develop the current testing criteria for cast-in-place concrete products and also performed the testing and code report requirements needed for these product lines. Prior to joining Simpson Strong-Tie, Scott worked for nine years as a consulting engineer. His experience includes the design and analysis of concrete structures, including post-tensioned slab design, concrete lateral systems and foundations. Scott is a licensed professional engineer in the state of California and received his bachelor’s degree in Architectural Engineering from Cal Poly San Luis Obispo.

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 really 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.

Anchor Reinforcement Testing and Research Program

Simpson Strong-Tie has been studying cast-in-place tension anchorage and anchor reinforcement concepts extensively over the past several years. Designing an anchor solution in a thin concrete slab for a high anchor demand load while meeting the ductility provisions of ACI 318-11, D.3.3.4.3 is extremely challenging. A strong industry-need for a safe, logical, yet economical design solution, led to a cooperative research program between Structural Engineers Association of Northern California (SEAONC) members and Simpson Strong-Tie. Testing was initiated by a Special Project Initiative grant from SEAONC to members Andy Fennell, P.E. (principal at SCL) and Gary Mochizuki, S.E. (principal at Structural Solutions at the time, now with Simpson Strong-Tie). We completed the program with continued involvement of SEAONC members. This is the second time we have partnered with SEAONC on non-proprietary anchor bolt testing. The first partnership, on sill plate anchor bolts in shear, resulted in successful code change provisions (led by SEAONC) restoring the capacity of these connections to pre-ACI 318 Appendix D values.

This current research program has focused on non-proprietary anchor reinforcement detailing that increases the nominal breakout capacity of concrete slabs. The anchor design satisfies the seismic ductility requirements of ACI 318-11 Appendix D and also significantly increases design capacity for wind applications. The project goal was to provide a design solution for the industry with independently witnessed proof testing of the anchor reinforcement detailing and application of ACI 318-11 Appendix D design procedures. (Note: Appendix D is moved into a new Chapter 17 in ACI 318-14.)

Test picture of anchor reinforcement

Figure 1: Test picture of anchor reinforcement

Anchor reinforcement detail at mid-slab

Figure 2: Anchor reinforcement detail at mid-slab

Significant Test Findings and Design Concepts

Anchoring to relatively thin concrete slabs introduces many unique challenges, so testing was bound to reveal some unique findings. The goal was to increase concrete breakout capacities and also satisfy the ACI 318 anchor ductility requirements with anchor reinforcement detailing. Here are some of the significant findings:

  • Relatively thin concrete slabs do not allow the placement of anchor reinforcement to drag the load down into a larger mass of concrete as shown in RD.5.2.9. Modified anchor reinforcement was required (Figure 2). The required area of anchor reinforcement is based on D.3.3.4.3(a) where the required area of anchor reinforcement exceeds the anchor steel strength, or 1.2Nsa < (nAsfy x 0.707). The 0.707 is for the 45 degree slope of the bars. The proof testing showed the horizontal leg development outside the cone and continuity through the cone adequately developed the anchor reinforcement.
  • ACI 318-11, D.4.2.1 states that when anchor reinforcement is provided, calculation of concrete breakout strength is not required. You know we love load path discussions, so where does the load go once it gets into the anchor reinforcement? The tests indicated that when the anchor reinforcement is provided, the concrete breakout area increases. This limit state is an extended breakout area past the anchor reinforcement bends that will form when that reinforcement is properly quantified and configured. The extended breakout is similar to multiple anchors loading the slab at each bottom bend of the anchor reinforcement. We have applied this concept to the calculations to evaluate extended breakout past the anchor reinforcement bends.
Initial breakout surface from head of anchor

Figure 3: Initial breakout surface from head of anchor

Breakout surface from bends in anchor reinforcement

Figure 4: Breakout surface from bends in anchor reinforcement

  • Let’s follow the load path some more. Once the anchor is connected to the slab, what is the slab bending capacity? The testing showed that to achieve the anchorage capacity, the slab must have an adequate amount of flexural reinforcement with anchorage forces corresponding to ACI 318-11, Section D.3.3.4.3 applied to the slab. Guidance from this section says to apply the anchor tension loads obtained from either design load combinations that include E, with E increased by Omega, or 1.2 x Nominal steel strength of the anchor (Nsa). If the anchors are not oversized, designing for 1.2Nsa should be the most economical solution. For wind applications, the slab Designer should consider the project specified design loads.
  • A vertical concrete block shear forming at the anchor bearing plate is possible if the anchor embedment is shallow and the anchor reinforcement is working to resist the initial anchor bolt breakout area. Our testing showed that this block shear is separate from Appendix D Pullout and is dependent on embedment depth, perimeter of the bearing surface and concrete strength.
  • For anchors with shallow embedment that have a double nut and washer, the concrete breakout can begin from the top nut, thereby reducing the effective embedment depth. To address this, we eliminated the top nut from our specified anchor assembly kit to insure the breakout begins from the top of the fixed-in-place plate washer.
  • The testing and modeling also allowed us to re-examine the appropriate bearing area for the plate washer, Abrg. The flat top surface of a nut is typically circular due to the chamfer at the points, so the resulting bearing area of the plate washer is circular extending out the thickness of the plate from the flat-to-flat dimension of the nut. For near-edge conditions, the side-face blowout capacity can be the controlling limit state and where the plate washer bearing area becomes more important.
  • Edge testing with anchor reinforcement details showed that the breakout area will spread out and begin from the anchor reinforcement bends, like the mid-slab. In a mid-slab condition, the breakout slope follows the 1.5:1 or 35 degree slope used in Appendix D. However for the near edge, we found that 1.5 x effective embedment (hef) from anchor reinforcement bends only holds true parallel to the edge. Due to eccentricities, the breakout angle from the anchor reinforcement bends into the slab (perpendicular to the edge) is steeper and a steeper 45 degree slope should be used.
Edge test with anchor reinforcement

Figure 5: Edge test with anchor reinforcement

Edge anchor reinforcement configuration

Figure 6: Edge anchor reinforcement configuration

  • The testing showed that even though we had cracks intersecting the anchor as it was loaded, the capacities exceeded uncracked assumptions. This is likely due to the flexural reinforcement running through the breakout cone providing continuity across the cracks. We’re still studying the effect of the flexural reinforcement, so for now we recommend assuming cracked concrete and providing a minimum of 4 – #5 flexural bars each way at anchor locations that require anchor reinforcement. Your slab design may require more flexural bars or they may already be there to meet other slab design requirements.

What Solutions Are Now Being Offered and How Do I Get Them?

With our newly launched Strong-Rod Systems webpages, you now go to the Shallow Podium Anchor link to find anchorage solutions. On the website, you will find anchor reinforcement detail drawings and design load tables with slab design recommendations. We’ll also be adding sample calculations, a guideline for selecting your anchor solutions, 3-D anchor reinforcement graphics and guidelines for addressing your condition if your installation is outside the scope of the current solutions that we offer. The anchor reinforcement is non-proprietary and is fabricated by the rebar supplier, but the configuration and placement is described in the details. In addition, you will see detailed information about the Simpson Strong-Tie Shallow Anchor Rod and Anchor Bolt Locator that is specified as a kit in the load tables. Note the absence of the top nut for reasons described above.

Shallow Anchor Kit

Figure 7: Shallow Anchor Kit

How Do We Specify It and Use It?

So now you know what is on the website but how do you put all these pieces together and apply them to a specific design? As a design professional, you will drive the bus on applying these details and design tables onto your drawings. Similar to specifying Strong-Wall® shearwalls or Strong Frame® moment frames, it just takes a little upfront coordination on your drawings. Typically, you’ll start with a slab key plan that shows the anchor bolt locations. You will need to know the design uplift forces from the light-frame structure above the slab and some basic project variables like specified concrete strength, slab thickness(es) and whether the structure is in high seismic or wind-controlled areas. Now you can choose the necessary design tables from the website by clicking on the individual design tables tab. Your key variables will help you select your specific table based on slab thickness, concrete strength, near edge, etc., and of course wind or seismic.

Sample Shallow Anchor design table

Figure 8: Sample Shallow Anchor design table

Once you have selected your design table, just match your project demand loads or your project-specified anchor bolt with the tabulated ASD or LRFD capacity to select the appropriate shallow anchor callout and reference detail that you can identify on the key plan. The detail callout from the table will send you to sheet SA1 where you will find the anchor reinforcement details and Shallow Anchor Kit recommended for your condition. As mentioned previously, the anchor reinforcement would be fabricated and bent by the rebar guys, but would follow these details. You can download the details shown on sheet SA1, place them on your construction documents and then coordinate them with your plans or schedules similar to how you might provide a shear wall schedule. The footnotes that accompany the tables provide important slab design information and other design and installation recommendations. You’ll soon be able to download the sample design calculations, use them as a tool to help follow the design procedure of the recommended details and submit with your project.

What if My Situation Does Not Fit the Details on the Website?

Though a great number of installations will be covered by these details and tables, there will be conditions that currently cannot be addressed with anchor reinforcement solutions. Installs that may be outside of the current scope could include: a demand uplift that’s too large, a slab that’s too thin, lower concrete strength, corner installs, double wood frame shear walls or two close anchors in tension. To address these conditions, we suggest alternatives like slight adjustment or reconfiguration of the shear walls, thickening slab edges, adding downturn concrete beams or extending the anchorage from above down into a cast-in-place wall or further extended down into the footing at ground level.

The joint SEAONC-Simpson Strong-Tie testing project has shown that anchor reinforcement details can greatly increase the breakout strength of concrete to support cast-in-place anchor bolts in concrete slabs. The testing also showed that the design provisions in ACI 318-11 Appendix D can be rationally applied to these anchor reinforcement details. The testing and Appendix D calculation approach are the basis of the details, load tables, graphics and application guides that can be found on our new Strong-Rod Systems webpages. We’re updating these pages as we create more content, so check back frequently. We look forward to working with you on your anchorage installation challenges and hope that some of these solutions will help you with projects you are working on today. How about a high five?

What do you think about these new anchorage solutions? Let us know by posting a comment below.

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

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

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.

ATS load path

ATS load path

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.

figure5ICC-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.