dingel:
You don't mention which version of ArchiCAD you are using, but if you are using version 10 or 11 you can create the profile of the turn down slab you want with the Profile Manager, set it to be used with either the Wall or Beam Tool, and draw the edge where needed as a wall or beam. If the Section fill of the concrete slab and edge beam match, then they will automatically clean up in section.
HTH
David