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Rigid vs. Flexible Diaphragms

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Is it possible for an 1.5" Wide Rib - 20 Gage Roof deck that is welded to steel joists to act like a rigid diaphragm?  I realize that the engineer is supposed to check the deflection of the diaphragm and the deflection of the lateral load resisting elements to determine if the diaphragm is 'flexible' or 'rigid', but how many engineers out there actually run this calculation?  I generally use a wood truss/plywood roof AND metal deck/steel joist roof as a 'flexible' diaphragm, but have seen a reputable engineering firm use metal deck/steel joist roof as a 'ridid' diaphragm.  

The example building was approximately 100' wide x 200' long.  The only lateral load resisting system was masonry shearwalls at the stairway centered on the right side of the building and an elevator shaft centered on the front wall of the building.  This building has a 2nd floor and a roof, which I'm sure the 2nd floor was a rigid diaphragm, but I don't see how a frame was not required in the back left corners of the building to provide resistance along all four sides of the diaphragm.

Any help is greatly appreciated.


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