AD.2007-09.2.1 Print Center of Rigidity
The PRINT DIAPHRAGM CR command may be used to obtain a print-out of the center of rigidity and center of mass at each rigid diaphragm in the model.
The lateral force at each floor, as generated by earthquake and wind loading, acts at the center of rigidity of each floor which is modeled as rigid floor diaphragm. The center of mass of each floor is defined as the mean location of the mass system of each floor. The mass of the floor is assumed to be concentrated at this point when the floor is modeled as rigid diaphragm. The distance between these two is the lever arm for the natural torsion moment for seismic loads when that option is used.