1. Select edge joints of a frame corresponding to different storey.
Display>Show Tables>Model definitions>Analysis Results>Joint output>joint Displacement. OK
2. Sort the required displacement (Say U1) in descending order.
3. Export it to excel & calculate difference in displacement of adjacent storey, which is the drift you want.Computation of CM.
Before you can calculate the drift of CM. you have to locate it. Again, i haven't yet discovered a way to find it directly in SAP. so calculate it yourself. Data required are. 1. Joint Co-ordinates 2. Joint load. Both can be tabulated under "Model definitions"
Once you've done that. you may calculate the storey drift of all 2-d frames (say along X dirn.) You can linearly interpolate the drift of CM based on its co-ordinate.