IS1893 (Part-1)- 2016 standard has provided the following guideline –
STAAD.Pro CE can consider this provision in Response Spectrum load cases in the presence of Rigid Diaphragm in the model. There are two parameters to be considered-
DEC- It is a factor which when multiplied with static eccentricity (i.e., eccentricity between center of mass and center of rigidity) gives dynamic eccentricity. Since the applied load is acting at the center of mass, the effect of inherent torsion arising due to static eccentricity is included in the analysis.
When ECC > 0, DEC defaults to 1.5
When ECC < 0, DEC defaults to 1.0
ECC- It is a factor which indicates the extent of accidental eccentricity. For all buildings this factor is to be provided as 0.05 or -0.05. 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 6 LOADTYPE Seismic-H TITLE RSX +VE
SPECTRUM SRSS IS1893 2016 TOR DEC 1.5 ECC 0.05 X 0.036 DAMP 0.05
SOIL TYPE 1
LOAD 7 LOADTYPE Seismic-H TITLE RSX -VE
SPECTRUM SRSS IS1893 2016 TOR DEC 1 ECC -0.05 X 0.036 DAMP 0.05
Please have a look into DEC and ECC parameter values. In one set, DEC will be 1.5 and ECC 0.05. In another set DEC will be 1 and ECC -0.05.
This process should be repeated in Z directional seismic load case. There is no requirement to consider torsional provision in Y direction.