By default, STAAD.Pro will model 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 check the same you can either switch on "Section Outline" from View >> Structure Diagram or 3D rendered view from View >> 3D Rendered View.
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Hope, this helps.
Thanks.
