What is the semi rigid diaphragm option under Assign > Shell/Area > Diaphragms > Add Diaphragm?
The semi-rigid diaphragm feature uses actual finite element stiffness (flexible diaphragm) of area and frame elements instead of a rigid diaphragm. Because the program needs to know the extents of each diaphragm in order to determine and apply auto-wind loads, users need to first assign a semi rigid diaphragm if they want ETABS to automatically determine and apply auto-wind loadings on their flexible diaphragms, including torsion cases.
