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Rigid Diaphram and shearwall stiffness analysis in residential Construction

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Dennis Wish wrote:

<John Lawsons examples justifying diaphragm rigidity if I recall
correctly) came from examples of tilt-up diaphragm failures rather than
smaller residential models. <snip>

If I left that impression, I was not clear during my presentation.  My
point was that rigid diaphragm analysis for wood diaphragms will be
limited almost only to residential (plywood shear walls with small
diaphragm spans).  Tilt-up, masonry, and braced frames will primarily be
always flexible by UBC definition.  Small commercial projects with
plywood shear walls and large diaphragm spans or steel moment frame with
plywood diaphragms could end up being flexible or rigid depending on the
analysis.  I think most of us will get a quick feel for what were up
against with a few under our belts.  There was no evidence that I am
aware of that any tilt-up failures led to the formation of this code
provision.

Remember, the UBC deflection equations are for "blocked" plywood
diaphragms.  Unfortunately the lack of information regarding plywood
diaphragm deflections for unblocked or glued diaphragms is worrisome. 
(and who says there's no place for engineering judgment in today's code?
;^)

John Lawson SE
Kramer & Associates Structural Engineers, Inc.
jlawson(--nospam--at)kramerinc.com