The rigid floor diaphragm assumes infinite in-plane stiffness of floors and therefore reduce the size of stiffness matrix and therefore the program performs a faster analysis. The semi-rigid diaphragm uses the actual in-plane stiffness of the slab elements and does not condense the stiffness matrix. For most concrete structures where slab is sufficiently thick and you expect negligible in-plane deformation due to lateral loads , results for semi-rigid diaphragm will be very similar to those for rigid diaphragm. However, if you are expecting 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 it allows you to assign the accidental ECC due to Wind or Seismic load cases. At the same time, wind loads can be applied at the center of masses and you do not need to use Area Exposure method to define Wind load case for semi-rigid diaphragm cases. Additionally if in plane forces on a slab (i.e.chord, shear, collector diaphragm forces) need to be retrieved a semi-rigid diaphragm must be used.
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