A little info on how they do this. In the framing of the home, the contractor places steel I-beams over the basement openings. On top of that they lay plywood to support the wet concrete. After the concrete has dried, the forms and plywood are removed from the bottom. One of the major concerns I have about this is the supports at the slabs edges. Steve's is absolutely right. Do not attemt any of this on your own. Please let us know what is decided. Ron J.