When you model your building in RISAFloor, make sure that you define your slab edge as Flexible. That will ensure that the entire contents of the diaphragm will use flexible behavior.

The typical application of lateral loads on Rigid diaphragms occurs at the center of mass for that diaphragm. This requires that joints are created at this location and lateral loads are applied at these joints.

RISA-3D does not inherently have the capability to define a semi-rigid diaphragm. If you’re dealing with a complex roof system then you probably need to be able to model something that behaves accurately though.