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August 2023 #15289 Page #108
Why Fire-Rated Hangers Are Required in Type III Wood-Frame Buildings
By Paul McEntee
ne of the first mixed-use designs I worked on as a consulting structural engineer was a four-story wood-frame building over two levels of parking. Designing the main lateralforce-resisting system with plywood shearwalls was a challenge for this project that required new details to meet the high design loads. The high overturning forces were resisted using the Simpson Strong-Tie® Strong-Rod™ anchor tiedown system, which incorporates high-strength rods, bearing plates, and shrinkage compensation devices.
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At the time, these construction details using Strong-Rod systems and high-load shearwall diaphragms were new, innovative concepts. However, this method of construction rapidly became commonplace as intense demand for housing fueled the trend toward denser, mixed-use developments in downtown areas. I discussed the trend toward taller, denser developments in this post. A more recent trend in wood-frame construction has been the shift to Type III wood-frame construction, which allows designs up to five stories. To help educate designers on some of the nuances of Type III wood-frame construction and provide guidance on meeting the associated code requirements, we reached out to Bruce Lindsey, the South Atlantic Regional Director for WoodWorks. Bruce wrote a two-part article entitled Fire Protection Considerations with Five-Story Wood-Frame Buildings – Part 1 and Part 2. This article will go into more detail on connecting the floor system to the two-hour fire-rated exterior walls and discuss our new DGT series joist hangers that are specially designed for this application. As a consulting engineer, I was aware of fire requirements because I needed to account for the weight of fire sprinklers, added layers of gypsum board, fire-proofing on steel, or concrete slab thickness in my design. While the increased loads affect the vertical- and lateral-force-resisting systems, I seldom needed to change the details and connections in my designs. The exterior walls in Type III wood-frame construction require fire-retardant-treated (FRT) lumber with two layers of gypsum board to provide a two-hour fire rating. There are many established fire-rated floor and wall assemblies available. The challenge, as discussed in Part 2 of Mr. Lindsey’s post, is detailing the intersections between the floor and wall systems. Connecting the floor framing to the exterior walls in Type III construction requires careful detailing to transfer the vertical loads without compromising the two-hour fire rating of the wall assembly.
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