How can I model a slab in STAAD.Pro such that it sits on top of the supporting beams and not at the center of these members ?
By default STAAD.Pro will model the beams and slabs at the center line elevation of the respective entities. In other words half of the depth for the beam or slab will be on either side of the elevation at which the nodes, which belong to these entities, are modeled.
To make the slab sit on top of the supporting beams, you need to model both the slab and the beams such that the nodes for these entities are at the true centerline elevations as in the structure. The slab needs to be modeled using a mesh of plate elements and at each node location for the plate mesh, the beams directly underneath should be split into the same number of divisions such that the plate mesh nodes directly line up with the beam nodes below it. Subsequently one can define master slave connections to connect the plate nodes with the corresponding beam nodes as shown in the picture below
A sample model is also uploaded here for reference