TECH TIPS: Rigid vs. Semi-rigid Diaphragms in ETABS

In this video, we will discuss the differences between rigid vs. semi-rigid diaphragms in ETABS. 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. Floor diaphragms can only be assigned to elements in the same X-Y (i.e. Horizontal) plane.