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Rigid vs. Flexible Diaphragm

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The IBC code gives a way to check if a diaphragm is "rigid" or
"flexible" based on the deflection of the diaphragm relative to the
sidesway of the lateral bracing system below the level of the diaphragm.
Does this check for "rigid" or "flexible" need to be made in both
orthogonal directions?  Depending on the building configuration, it may
be possible to classify a diaphragm as flexible for loading from one
direction, but rigid for the other.  This may be the case for a building
that is very long, but not very wide.

IMO, this gets messy where you have a long, narrow building with one of
the long sides nearly free of shear walls... such as for a strip mall
type building.  If I check it with wind or seismic force against the
long dimension, the diaphragm could be flexible.  For load from the
other direction, I need the diaphragm to be rigid, since one side of the
building is free of shear walls.  

Also, how do I really apply the check for "flexible" vs. "rigid"?  Do I
assume either "flexible" or "rigid" and design the lateral force
resisting system, then check my assumption?   

Bruce D. Holcomb, PE
Butler, Rosenbury & Partners
300 S. Jefferson, Suite 505
Springfield, MO 65806
ph. 417-865-6100
fax 417-865-6102
Architecture, Engineering, Interior Design, Planning & Development
Your Vision.  Our Focus.

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