This is another type of floor deck that is used with steel-framed construction (Fig. 8.7). The plank is prefabricated in standard widths, usually ranging between 4 and 8 ft, and is normally prestressed with high-strength steel tendons. Shear keys formed at the edges of the plank are subsequently grouted, to allow loads to be distributed between adjacent planks. Voids are usually placed within the thickness of the plank to reduce the deadweight without causing significant in plank strength. The inherent fire resistance of the precast concrete plank obviates the need for supplementary fire protection.
Topped versus Untopped Planks. Precast planks can be structurally designed to sustain required loadings without need for a cast-in-place concrete topping. However, in many cases, it is advantageous to utilize a topping to eliminate differences in camber and elevation between adjacent planks at the joints and thus provide a smooth slab top surface. When a topping is used, the top surface of the plank may be intentionally roughened to achieve composite action between topping and plank. Thereby, the topping also serves as a structural component of the floor-deck system.
A cast-in-place concrete topping can also be used for embedment of conduits and outlets that supply electricity and communication services. Voids within the planks can also be used as part of the distribution system. When the topping is designed to act compositely with the plank, however, careful consideration must be given to the effects of these embedded items.
Dead-Load Deflection of Concrete Plank. In design of prestressed-concrete planks, the prestressing load balances a substantial portion of the dead load. As a result, relatively small dead-load deflections occur. For planks subjected to significant superimposed dead-load conditions of a sustained nature, for example, perimeter plank supporting an exterior masonry wall, additional prestressing to compensate for the added dead load, or some other stiffening method, is required to prevent large initial and creep deflections of the plank.
Diaphragm Action of Concrete-Plank Systems. The diaphragm action of a floor deck composed of precast-concrete planks can be enhanced by making field-welded connections between steel embedments located intermittently along the shear keys of adjacent planks.
(See also Art. 8.1.)
Attachments of Concrete Plank to Framing. Precast-concrete planks are attached to and provide lateral bracing for supporting steel framing. A typical method of attachment is a field-welded connection between the supporting steel and steel embedments in the precast planks.