Rigid diaphragms have infinite in-plane stiffness properties, and therefore they neither exhibit membrane deformation nor report the associated forces, whereas semi-rigid diaphragms simulate actual in-plane stiffness properties and behavior. For most reinforced-concrete slab systems, in which the slab is sufficiently thick and membrane deformation due to lateral loading is negligible, rigid diaphragms produce results nearly identical to those of semi-rigid diaphragms, while taking advantage of faster computation. Semi-rigid diaphragms should be modeled when significant in-plane deformation does occur, or when required by code.
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