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RE: Rigid vs Flexible Diaphragms - Some

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Roger Turk wrote:
 
"If the supports were infinitely stiff, I can't see how that would affect
whether or not the diaphragm is flexible or rigid.  A flexible diaphragm
would impart one set of forces into the infinitely stiff supports and a rigid
diaphragm would impart a different set of forces into the infinitely stiff
supports.  The deflection of the diaphragm at midspan would be one distance
if the diaphragm was flexible and another distance if the diaphragm was
rigid.  For a diaphragm supported on flexible supports, the total movement of
the midspan of the diaphragm would be a rigid body displacement, due to the
deflection of the supports (shear walls), plus the deflection of the
diaphragm at midspan.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona"
 
You're right, of course.  But I guess the intention of the code is to compare the diaphragm deflection as if it was on infinitely stiff supports with the deflection of the REAL supports (acting as a cantilever).  That is the way to get  relative rigidities.
 
Don't get me wrong.  I'm not that familiar with wood framing construction so as to have a valid point on wether it is or not correct to enforce a rigid diaphragm analysis.  I'm just saying what I know about this type of analysis (as applied to masonry and concrete construction).
 
Rodrigo Lema.