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Re: Wood Fire Rated Wall Assembly F'c reduction

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The reduction factors for load-bearing studs are in the UBC and in Underwriters Laboratories specifications to "calibrate" current design methodology per 1997 NDS back to originally-tested Fc-perp values (385 psi for Douglas fir). This was done so that old designs would not have to be retested.

AF&PA has recently addressed this issue with new fire tests on load-bearing wood framed walls (2x6) with Fc-perp value of 625 psi.  It requires longer gypsum wallboard fasteners (2-1/4" Type S screws spaced 7" oc were used in the tests), so that the fastener penetrates into uncharred wood at the end of the 1-hr test to retain the gypsum wallboard attachment.  The tested walls had 5/8" Type X gypsum wallboard on both sides (symmetrical construction).There are plans to test with 2x4 framing, and also non-symmetrical construction with wood structural panel sheathing on the unexposed side of the wall.

John Rose/APA - The Engineered Wood Assn. (Tacoma, WA)

Jeff Albert wrote:

 The Uniform Building Code requires that the design stress for studs be reduced to 78 percent of allowable F'c for all 1-hour rated walls.  This appears in a footnote #18 for Table 7-B. First, my question is whether other engineers in California, and other UBC states are applying this provision to their wood design.  Also, how have building officials responded / reacted to designs not using the reduction.  I will note that it is my belief that a reduction is unnecessary, and that designs done without using the reduced values are safe.  But this brings me to my second question. If this provision is being waived away, what are the liability concerns.  I am particularly interested in projects that are condominiums.  With the state of condo associations these days, it seems to me that not adhering to a specific provision in the code leaves the design open for problems if reviewed at a later date. Finally, is anyone aware of any new studies, or fire assembly testing, etc. that is currently being done that might address this issue.  My understanding of the reason for this provision is due to a change in allowable perpendicular to grain design stress values which were not reflected in the original fire assemblies tested.  Therefore, a reduction factor was applied which basically brought you back to where you were before the increased values. I look forward to reading your responses. Jeff AlbertKPFF Consulting EngineersSeattle WA