Applying seismic loads to structures with rigid diaphragms can be done automatically in RISA-3D. In the image below we have a structure that has rigid diaphragms and the seismic load has already been applied at each diaphragm level.
To do this, go to Insert > Seismic Loads and input the code and the necessary parameters to calculate the seismic force in the structure. Note that we also need the Seismic Weight defined in a load combination that we choose here.
Once we hit the Calc Loads button the program calculates the forces at each diaphragm level in each orthogonal direction and gives the parameters used to calculate these forces. For the ASCE 7-05 code the program uses the Equivalent Lateral Force Procedure.
If we then hit OK, these loads are added to your Basic Load Cases automatically. RISA-3D also automatically provides accidental eccentricity Basic Load Cases prescribed for rigid diaphragms.