Open web steel joists support a reinforced concrete slab--typically 2 1/2 to 3 1/2 inches thick--while it is hardening; then the two act together to resist live loads on the structure. Construction is simplified because shoring is usually not required and loose plywood sheets are readily stripped for reuse. The composite structural action of the patented system helps reduce dead load on the foundation, and in some cases permits a floor system 2 to 4 inches shallower than others of comparable span and load capacity.
HOW IS THE SLAB FORMED?
To get ready for concrete placement, bundles of joists are lifted into position on the supporting walls, beams, or other bearing surfaces, and spread out roughly 4 feet apart. After the joists are in place, secondary members called roll bars are put in place. These flat steel bars with two handles for rotating and handling are designed to support the plywood forms, the dead weight of the concrete, and a construction load of 40 pounds per square foot. Plywood sheets are laid across the roll bars, with the long dimension parallel to the joists. The plywood should be coated with a release agent to ease stripping and prolong its useful life.
Required concrete strength is at least 3000 psi, and aggregate should be no larger than 3/4 inch. A slump in the 3 1/2- to 4 1/2-inch range is recommended as easy to work and finish, but stiff enough to prevent excessive leakage and costly cleanup.