Generally, we do not model floor slabs in STAAD.Pro. Floor slabs can effectively be modelled using plate elements. But, having plates in the models induces a lot of other problems. Discretization of larger elements into the smaller parts are also required in this process the adjoining beams are also divided. The analysis time gets increased as you have higher numbers of elements present in the model. The interpretation of analysis and design results for the broken beams becomes difficult. It can be understood that if we don’t model slab using plates we are simply ignoring the stiffness of the slabs. There is another thing that we need to consider if modelling. There are two distinct load path in a structure- vertical load path and lateral load path. The following picture explains this.
There is no major problem in applying Floor loads in the model. It gets transferred to floor beams and then, it is transferred to the columns. Columns transfer that to the footings. It can be seen that the vertical load path is being maintained. In case of lateral load path, the slabs play the role of diaphragm and gets displaced against the lateral loads. Slab- moment resisting frame connections gets the forces from the diaphragm. We can somehow maintain the vertical load path but lateral load path gets hampered if we don’t model any diaphragm in the model. Generally, the internal force i.e, the seismic loads are applied at the center of mass and if there is any difference in between the center of mass and center of rigidity, the floor will experience torsional forces which simply effects the corner columns. In order to have a realistic model, we suggest you to model diaphragm- in STAAD.Pro we have the option of Rigid Diaphragm and on using this the requirement is being followed by the program.