The rigid floor diaphragm assumes Rigid diaphragms have infinite in-plane stiffness of floors properties, and therefore reduces the size of stiffness matrix as a result analysis is generally performed faster. The semi-rigid diaphragm uses the they neither exhibit membrane deformation nor report the associated forces, whereas semi-rigid diaphragms simulate actual in-plane stiffness of the slab elements and does not condense the stiffness matrixproperties and behavior. For most concrete structures where reinforced-concrete slab systems, in which the slab is sufficiently thick and you expect negligible in-plane membrane deformation due to lateral loads , results for semi-rigid diaphragm will be very similar to those for rigid diaphragm. However, if you are expecting significant slab in-plane deformations or when required by code, then modeling a semi-rigid diaphragm will be the correct way to handle such cases. A semi-rigid diaphragm is the same as assigning no diaphragm except that it allows you to assign accidental eccentricity due to Seismic load cases. Additionally if in plane forces on a slab or beams(i.e.chord, shear, collector diaphragm forces) need to be retrieved a semi-rigid diaphragm must be used.See also: https://wiki.csiberkeley.com/display/etabs/Accidental+eccentricity+in+ETABSloading 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.
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 auto seismic loading is concentrated and applied at the center of mass, whereas for semi-rigid diaphragms, accidental eccentricity is applied to every node for auto seismic loads. If no diaphragm is assigned eccentricity will not be applied to any node. For auto wind load cases and rigid diaphragm , loading is applied at geometric centroid, 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 will be equivalent to lateral and torsional wind cases.
- Reporting forces – In-plane chord, shear, and collector forces are only reported when using semi-rigid diaphragms.