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Re: Rigid plywood diaphragms

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Our office has designed a number of three story wood framed hotel buildings
which have a width to length ratio of 3.5:1.  In an attempt to be as accurate
as we could we performed the lateral analysis assuming a semirigid diaphragm
and the more conventional tributary area method in distributing the lateral
forces to the shear walls.  We found that there was about a 15% difference in
the forces distributed to the shear walls.

While in discussion with the plan reviewer, who is a California S.E., we came
to the conclusion that either method was representative of "good engineering
practice".  I feel that this is a case in which good engineering judgment is
of the essence.  The method one uses to distribute these forces very much
depends on the geometry of the building (i.e., how the shear walls are
distributed along the diaphragm boundary, and the "relative stiffness" of the
shear walls along the same line, similar to a rigidity analysis as commonly
used in the design of masonry shear walls).  I would recommend further
discussion with your colleagues to determine the method in which you are most
comfortable.  I would also suggest a quick check using both methods to
determine if one method yields grossly different results compared to the other
method.

Dave Puskas