FLEXIBLE DIAPHRAGM: A diaphragm is flexible for the purpose of distribution of story shear and torsional movement when lateral deformation of the diaphragm is more than two times the average story drift of the associated story, determined by comparing the calculated maximum in-plane deflection of the diaphragm itself under lateral load with the story drift of adjoining vertical-resisting elements under equivalent load.
A ridgid diaphragm does not conform to the defintion of a flexible diaphragm.
