STAAD Pro modeling

Could anyone correct me on the way of modeling? I have a slab with 300 mm thickness and beams at the soffit of the slabs. Beam sizes are 300 mm x 600 mm. I attached the simple STAAD Pro model in order to get the idea.


My doubt is whether I need to move the beam coordinate, as you can see from the Figure 1. The centroid of the beam is similar to the centroid of the slab as well. Do I need to move the beam coordinate downwards? If yes, there will be a gap between the nodes or elements. They are not connected. In that case, how to deal with this?

Figure 1

Another doubt is when I create a plate; the plate is a flat element (without thickness) as shown in Figure 2.

Figure 2

When the thickness is added as shown in Figure 3, the surface of the slab will go above the zero at Y axis. The thickness is added 150 mm above and below Y axis. Do I need to take this into account? So, am I required to move the slab coordinate as well based on the thickness of the slab? It is because I will need to add beam at the soffit of the slab, I wonder if I need to take note of these issues. 

Figure 3