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RE: Rigid vs. Flexible Diaphragm

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Mike –


Maybe I’m misunderstanding you, but, when you say “all gridlines in the same direction must have the same deflection”, you have no rotation? That doesn’t seem right, either. For example, say you have two (and only two) grid lines, A & B, in one direction. At grid line A, you have one 20 ft. long shear wall and at grid line B you have two four foot long shear walls. Are you saying that your program will distribute the load so that the deflection of grid line A is the same as grid line B? I understand that, if you have a 20 ft. shear wall (say SW Mark A.1) and a 4 ft. shear wall (say SW Mark A.2) at grid line A you need to distribute the load so that you have deflection compatibility between SW A.1 and SW A.2, but a rigid diaphragm will/should rotate and I don’t see the rational basis for making all the grid lines deflect the same. But, then again, maybe I am misunderstanding you.


T. William (Bill) Allen, S.E. (CA #2607)

San Juan Capistrano, CA


I do force deflection compatibility along a girdline, all walls along a gridline must have the same deflection (the forces are redistributed between walls along the gird line till the walls all have the same deflection), in fact, all girdlines in the same direction must have the same deflection, otherwise you don't have relative rigidities between gridlines that are meaningful.