Of course you know about creating a continuous load path with either connectors or rod tiedown systems, but have you considered using fasteners instead? In this post, Bryan Wert follows up on our May 2 webinar, Drive a New Path: Resisting Uplift with Structural Fasteners, by answering some of the interesting questions raised by the attendees.
On May 2, Simpson Strong-Tie hosted an interactive webinar where we discussed different methods of creating a continuous load path for wind uplift resistance. Most of the hour-long webinar was devoted to the innovative structural screw system comprising our Strong-Drive®SDWC Truss screw and the SDWF Floor-to-Floor screw with TUW take-up washer. In addition to sharing load capacities, installation details and various benefits of this system, we included a design example with illustrative specification options. In case you weren’t able to join our discussion, you can watch the on-demand webinar and earn PDH and CEU credits here. Continue Reading
Structural screws are designed and tested to do hard work, but that doesn’t make them hard to use. In this post, Simpson Strong-Tie structural engineer Bryan Wert explains how the load-rated strength, versatility and easy installation of the code-listed Strong-Drive® SDWC Truss screw and SDWF Floor-to-Floor screw make it a cinch to create a continuous load path to resist wind uplift. Learn more during our May 2 webinar.
Winter’s finally shedding her blanket and unveiling springtime in Texas. There’s now a short window of picture-perfect weather where my purchases at Home Depot are no longer foam hose bib covers to protect outdoor faucets from freezing temperature, but aren’t quite yet tiki torches and floats for the pool for hot and humid summer days. I find myself in the garden center looking at the freshly delivered trees, shrubs and flowers, along with just about every other adult in my city. This year, my wife’s decided we need to surround our outdoor living space with hanging planters displaying perky red, purple, yellow and blue flowers. Continue Reading
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.
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.
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 AC316Acceptance 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 AC391Acceptance 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.
IBC Section 1604.9 requires structural members, systems, components and cladding be designed to resist forces due to earthquakes and wind, with consideration of overturning, sliding and uplift. It also states that a continuous load path be provided for transmitting these forces to the foundation. Seems obvious to engineers that a continuous load path is needed, but it’s still nice to have the code say so.
But what happens if your structure’s upper and lower story walls do not stack? How do you create the required continuous load path? As engineers, we try to steer the architect towards eliminating the offset, making things line up, and keeping construction simple. But architectural requirements cannot always accommodate simple, and non-stacking walls occur all the time.
Being from California, I had always bought into the common misperception that wood light-frame construction can’t be designed to resist tornadoes. While it is true that debris impact can’t be cost-effectively designed into residential structures, there is a lot that can be done to strengthen the structure and protect the occupants inside. Using the same technology common in hurricane-prone regions, these buildings can protect people for more than 95% of reported tornadoes.
The effect of tornadoes on wood light-frame structures has been extensively researched over the last few years, and researchers agree: A strong, continuous load path is essential to minimize destruction.