Engineering and Construction

Slab depths are normally in the range of 110–150mm for shallow profiles. Clearly, the minimum depth for structural adequacy is usually selected, but the slab depth is affected by insulation requirements in a fire (see Section 20.6.3) and serviceability criteria as influenced by the ratio of slab span to depth. As a general rule, span:depth ratios (considering the overall, slab depth) of continuous composite slabs should be less than 30 for LWC and 35 for NWC. Longer spans may suffer from excessive deflections and vibration of the supporting beams.

In designing the composite slab it is difficult to develop the full flexural resistance of the slab determined by the area of the deck acting as effective tensile reinforcement. Failure is normally one of shear-bond rupture resulting in slip between the deck and the concrete.Nevertheless, load capacities of composite slabs with the above proportions are normally adequate for most imposed loads up to 10kN/m2.The shear-bond resistance of composite slabs is determined from tests; this resistance may be enhanced by end-anchorage provided by shear-connectors attached to the support beams.

The concrete slab also acts as the principal compressive element in the design of the composite beams.This interaction between the flexural behaviour of the slab and the beams is only important where both the slab and beam span in the same direction.