An objection often raised to the use of concrete floor systems in multistory office buildings is the difficulty of installing a flexible wiring system. Owners and tenants need and demand a wiring system that will allow easy relocation of electrical outlets and telephones as the office needs of the tenants change. To provide this flexibility in concrete floors, an underfloor duct system can be used and at a cost reported to be competitive with cellular steel decks.
Underfloor ducts are incorporated in the structural slab or in a concrete topping placed over the slab. Trench ducts or feeder ducts lead out from the main telephone and electrical closets, and distribution ducts branch off from the trench ducts. Since location of the trench ducts may affect the structural design of the floor, it should be considered early in the design process.
A two-level duct system has been found generally to be the most compatible with reinforced concrete slab construction as well as the most practical and economical. If a two-level system is used, and if the ducts are incorporated in the structural slab, the slab must be at least 7 inches thick. If the ducts are incorporated in an overlay, the overlay should be at least 4 inches thick, unless the ducts are depressed into the slab. The slab itself should be thick enough to provide adequate bending capacity with a reasonable amount of steel and adequate shear capacity with a minimum number of stirrups. To save on forming costs, joints should be made as deep as the beams.