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Re: Plywood Shear Wall Deflection

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Bob-
If you search the archives you'll find some lengthy discussions on this topic.
IMO the author was attempting to point out that in estimating tiedown deflection there is more to da than simply the deflection of the tie down bracket.  Simpson's catalog lists some 10 items that contribute to the total deflection (they also omit assembly slop related to stud length and squareness of cuts).  Unfortunately, there is a serious lack of basic test data from which to estimate da.  With ICBO complicity, the manufacturers continue to hide their test results.
Short shearwalls magnify tie down deflection due to their poor aspect ratio.  Mixing short walls(4' and 6') with long walls  with "loose" tie down systems can create relative rigidity problems.  That's when rigid diaphragm analysis can become important.  It can be an important issue to consider, even if the exact deflection/stiffness calculation is somewhat problematic.  FWIW, in my deflection spreadsheets da is treated as a spring constant that is unique to each type and size of tie down--if and when if I think there may be a significant problem in this area.
Chuck Utzman,P.E.

Bob Chou wrote:

In calculating the deflection and the rigidity of plywood shear walls, the sample shown in Volume Two of the Seismic Design Manual includes the shrinkage effect of the top/sole plate and the floor assembly in the da calculation.  Within a building structure the loss of moisture content of wood members should be uniform and thus no differential shrinkage should occur, especially across a short shear wall (say a 6 feet wall).  It is my opinion that the wood shrinkage effect need not be considered.  Only the deformation caused by the externally applied loads should be included in the deflection calculation. Please respond or comment on this topic.  Bob ChouVCA Engineers(714) 978-9780