Concrete floors at all levels are squeak-free, unsusceptible to rotting and low in maintenance costs. Those above grade help isolate noise. Several ways of building concrete floors above grade are described here. A system called the "Integrated Distribution" (I/D) floor system is designed to incorporate heating, plumbing and electrical utilities economically. It makes use of permanent forms with light steel reinforcing trusses in the bases of thickened sections. The floors are 12 inch thick including ceiling and can be used for simple spans up to 24 feet. Low-cost fiberboard forms used in this system become a permanent part of the structure, thus eliminating labor needed to strip and clean them. Undersides of forms becomes the exposed ceilings. A system that provides complete flexibility in room layout is one in which I-beam capitals are supported on steel pipe columns. Six inch deep beams are designed to be concealed with in partition walls. The system, using 5 inch thick floors, makes spans up to 16 feet feasible. Concrete floor slabs are reinforced in accordance with the requirements of the American Concrete Institute Building Code, ACI 318-71. Site-precast floors, erected by crane, have also been used. For large projects it is sometimes efficient to precast solid panels of reinforced concrete on the site for both floor and roof requirements. Panels can easily be reinforced, pre-stressed or post-tensioned. It is most convenient to cast the panels on a slob on grade using simple edge forms to contain them and a bond breaker between the slab on grade and the concrete being cast. They can also be cast in a stack. The slabs are erected by crane and supported on concrete walls. Several types of factory precast units are also available for concrete floors, including solid panels similar to those above. Other units are concrete joints, cored units, and double tees.