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.

Primary differences

• Formulation – The infinite in-plane stiffness components of a rigid diaphragm allows the stiffness matrix to condense, decreasing computational time.

• Eccentricity – For rigid diaphragms, the accidental eccentricity associated with seismic loading is concentrated and applied at the center of mass, whereas for semi-rigid diaphragms, accidental eccentricity is applied to every node. If no diaphragm is assigned eccentricity will not be applied to any node.

• Reporting forces – In-plane chord, shear, and collector forces are only reported when using semi-rigid diaphragms.

See Also

• Diaphragm forces article

• Accidental eccentricity design article