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RE: Rigid vs Flexible Diaphragms - Some

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Rodrigo Lema wrote:

. > Roger Turk wrote:

. > "It seems to me that the definition of what constitutes a rigid and 
. > flexible diaphragm is reversed.  It seems to me that a flexible diaphragm 
. > would be one whose "... maximum lateral deflection is *less* than ?? 
. > times the average story drift ... ."  (Actually, why is it even related 
. > to story drift?)"
. > I think the tie between the rigid/flexible diaphragm and the story drift 
. > is that, if you think of the diaphragm as a deep beam supported on lateral
. > shearwalls, the deflection at midspan compared to the story drift (that 
. > is, how much the supports move) is a good parameter to see if the 
. > diaphragm is rigid or not.  Just a thought.
. > Rodrigo Lema.

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