I have specified Rigid Diaphragms in my model. But when I run the STAAD file, it stops with the following warning.
** ERROR: NO REFERENCE LOAD CASE OF TYPE MASS/GRIVITY/DEAD/LIVE IS DEFINED FOR MASS MODELING. CENTRE OF MASS CALCULATION IS NOT POSSIBLE.
If you are specifying Rigid Diaphragms, it is a must that you specify the Mass reference Load Case. One of the important functions of specifying the Rigid Diaphragms is to calculate the torsion on the floor due to seismic loads and the program needs to identify the center of mass for the same. This is done with the help of the mass data defined in the mass reference load case.