**Rigid diaphragms** have infinite in-plane stiffness prope=
rties, and therefore they neither exhibit membrane deformation nor report t=
he associated forces, whereas **semi-rigid diaphragms** simula=
te actual in-plane stiffness properties and behavior. For most reinforced-c=
oncrete 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 ad=
vantage of faster computation. Semi-rigid diaphragms should be modeled when=
significant in-plane deformation does occur, or when required by code.

**Formulation**=E2=80=93 The infinite in-plane stiffness = components of a rigid diaphragm allows the stiffness matrix to condense, de= creasing computational time.

**Eccentricity**=E2=80=93 For rigid diaphragms, the accid= ental eccentricity associated with auto seismic loading is concentrated and= applied at the center of mass, whereas for semi-rigid diaphragms, accident= al eccentricity is applied to every node for auto seismic loads. If no diap= hragm is assigned eccentricity will not be applied to any node. For a= uto wind load cases and rigid diaphragm , loading is applied at geometric c= entroid, in case of semi-rigid diaphragm auto wind loads are distributed in= 10 nodes, so that the summation of these forces with respect to centroid w= ill be equivalent to lateral and torsional wind cases.

**Reporting forces**=E2=80=93 In-plane chord, shear, and = collector forces are only reported when using semi-rigid diaphragms.