A rigid floor diaphragm is assigned infinite in-plane stiffness components such that the size of the stiffness matrix may condense, decreasing computational time. A semi-rigid diaphragm uses actual in-plane stiffness properties, in which case the stiffness matrix may not condense. In addition to these differences between formulation, in-plane chord, shear, and collector forces may be retrieved only for semi-rigid diaphragms. Further, accidental eccentricity due to seismic load cases may only be assigned to semi-rigid diaphragms.
For most reinforced-concrete slab systems, where the slab is sufficiently thick and membrane deformation due to lateral loading is negligible, rigid-diaphragms produce results nearly identical to that of semi-rigid diaphragms, while taking advantage of faster computation. When significant in-plane deformation does occur, or when required by code, semi-rigid diaphragms should be modeled.
