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FIGURE 37.12 A continuous slab exposed to fire. Load Bearing Capacity of One-Way Spanning Composite Slabs

When calculating the load bearing resistance of a composite slab in fire, it is often assumed that it is one-way spanning, being effective only in the direction of the concrete rib. For a continuous composite slab, the plastic design method may be used, in which plastic hinges are assumed to form at the supports and locations of the maximum bending moment in the span of the slab. For example, for the interior span of a continuous slab under uniformly distributed load, as shown in Figure 37.12,

where M+,fi and M_>fi are the sagging and hogging bending moment resistance of the slab, respectively, and Mfi,max is the maximum free bending moment in the slab under fire conditions. For the end span of a continuous slab with uniformly distributed load, as shown in Figure 37.12,


or approximately

In order to determine the slab load carrying capacity in fire, the sagging bending moment capacity M+,fi and hogging bending moment capacity M_>fi should be evaluated.

When calculating the sagging bending moment capacity of a slab, the reinforcement near the fire side is in tension and the compressive concrete is on the unexposed side of the slab. Since the temperature rise on the unexposed side of the slab is required to be below 140°C to fulfill the insulation requirement, the concrete in compression can be assumed to be cold and its cold strength may be used when calculating the sagging bending moment capacity of the slab. Contributions from the steel decking are usually ignored because the decking will be unprotected and may debond because it is under direct fire attack. Figure 37.13 shows the calculation procedure. In Figure 37.13, fc is the design strength of concrete in bending at ambient temperature, Ar, py,r, and ky,r(T) are the area, design strength at ambient temperature, and strength retention factor of the reinforcement at temperature T, and ky,r(T) may be obtained from Figure 37.14.

Under a hogging bending moment, the compression face of a composite slab is exposed to fire where there is a very steep temperature gradient. When calculating the hogging bending moment capacity, the composite slab, including the concrete in the ribs, should be divided into a number of layers each of approximately constant temperature. The contribution of each layer should be evaluated separately and then integrated to give the total slab resistance.

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