I have defined a rigid floor diaphragm in STAAD.Pro using the master slave and the axial forces in all the beams are coming out as zero. Why ?
When you define a rigid floor diaphram using the master slave or using the floor diaphragm option in STAAD.Pro, the entire floor is considered as a rigid body with no relative displacements being possible in between the various nodes at that floor. The whole group of nodes can translate in X or Z or rotate about the vertical Y axis as a rigid body. With no relative displacement happening between the end nodes, the beams cannot develop an axial force. So it is the idealization of the diaphragm that results in this structural behavior. In your real structure too if you have a truly rigid diaphragm, the axial force will be taken up by the rigid body as a whole and simply passed on to the supporting lateral framing system. If on the other hand you would want the axial forces in the beams, the rigid body idealization would not work and you would have to define the diaphragm using a mesh of plates.