IS1893 (Part-1)- 2016 standard has provided the following guideline –
STAAD.Pro CE can consider this provision in equivalent Static load cases in the presence of Rigid Diaphragm in the model. There are two parameters to be considered in the presence of Rigid Diaphragms in the model-
DEC- Multiplying factor for natural torsion –arising due to static eccentricity which is the difference between center of mass and center of rigidity of a rigid floor diaphragm– to be used to multiply the seismic horizontal torsion load.
ACC- Multiplying factor for accidental torsion, to be used to multiply the seismic accidental torsion load. Negative values are allowed.
Let us consider X direction IS1893 Seismic load case, we need to have following load cases to cater the requirements-
LOAD 1 LOADTYPE Seismic-H TITLE SLX +VE
1893 LOAD X 1 DEC 1.5 ACC 0.05
LOAD 2 LOADTYPE Seismic-H TITLE SLX -VE
1893 LOAD X 1 DEC 1 ACC -0.05
Please have a look into DEC and ACC parameter values. In one set, DEC will be 1.5 and ACC 0.05. In another set DEC will be 1 and ACC -0.05.
This process should be repeated in Z directional seismic load case. There is no requirement to consider torsional provision in Y direction.