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Rigid vs Flexible Diaphragms - Some Comm

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These comments are based only on a visualization approach.

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?)

This is because the diaphragm does not even start to behave as a rigid 
element *until* the sheathing panels come in contact with each other.  Since 
the panels are/should be installed with 1/8 inch spacing between them, a lot 
of movement can occur before the diaphragm starts to act as a rigid diaphragm.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona