NOT ANYMORE. Rigid diaphragms is a technique introduced in the begginning of the computer analysis era in order ro reduce the time it takes a computer to perform such analysis. The catch, fortunately of little importance, is that since the floor beams are embedded in the diaphragm it was not possible to retrieve the values of axial loads acting on the beams. Since computer speed has progressed exponentially over the years, many analysis programs, including ETABS, have recognized this fact and now do not require a rigid diapragm definition. The time it will save is usually insignificant and the results will be, say, less informative.