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Re: Rigid Diaphragm

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"More rigid???"  The rigid diaphragm analyses I have done by hand have
assumed that the diaphragm is "infinitely" rigid; i.e., there were not
corrections for flexural and shear displacements in the plane of the
diaphragm, and the forces were distributed to shear walls in proportion to
their rigidity, not equally.

Even if equally rigid shear walls were symmetrically placed about the center
of gravity of the diaphragm (which would theoretically result in an equal
force distribution), the code requires that the center of gravity be offset
a minimum of 5% of the diaphragm dimension to account for accidental
torsional eccentricities.

Bottom line: even if it were possible, the code wouldn't permit it.  (Of
course you could do a finite element analysis.)

Good luck explaning this to the owners.

Conrad Guymon, P.E.

> I have a situation where an existing slab is 4" thick. Owners would like
> make it "more" rigid", to distribute seismic loads to new shear walls
> equally.
> Is this possible?? If so, how??
> jim k