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Re: wood panel wall aspect ratios: shear transfer around openings

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Scott-
The recent SEAONC wood design seminar included an excellent presentation on this subject by Doug Thompson. The analytical method is Ed Diekmann's and is published in the Fahey/Williamson book. Kelly was sort of Ed's protégé and is no doubt well versed in the the calculation. The intent of this method is to provide a rational method for analyzing the shear flow around an opening in a shearwall with 2 tiedowns( and coincidentally allows a way around the 2:1 aspect ratio problem for some walls).

Ben's understanding of the method is incorrect. His suggestion (and yours) creates a statically indeterminant system in which the relative deflections of tiedowns, shearpanels, and strapping would determine the internal load distribution--something well beyond our meager design capabilities. At low load levels the forces would certainly head for the stiffest elements--your 2 "extra", interior tiedowns. Depending on the wall geometry and relative stiffness these tiedowns could be quickly overwhelmed. No question about it though, the analysis is a PITA. One of this days I'll try to do a spreadsheet.

The perforated analysis by Dolan/Sugiyama/BOCA etc. is intended for unreinforced openings and is now limited to long walls with 4' panels at each end. The reason for Ed's analysis is the fact that the shear in the panels above and below the window can easily exceed the "pier" shear and unless you do the calc's there is really no way to design the sheathing/nailing schedule. That said, I'll bet most engineers just eyeball it unless they get caught by a plan checker who really knows his stuff. I occasionally just use the empirical "perforated shearwall" method and add some eyeballed strapping above and below the window--quick and safe (but conservative).
Chuck Utzman, P.E.




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