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FIGURE 37.13 Calculation method for sagging bending moment capacity.

Temperature, °C

FIGURE 37.14 Retention factors of cold worked reinforcing steel (from European Committee for Standardisation (CEN), 2001, prEN 1994-1-2, Eurocode 4: Design of Composite steel and Concrete Structures, Part 1.2: Structural Fire Design (London: British Standards Institution)).

Temperature, °C

FIGURE 37.14 Retention factors of cold worked reinforcing steel (from European Committee for Standardisation (CEN), 2001, prEN 1994-1-2, Eurocode 4: Design of Composite steel and Concrete Structures, Part 1.2: Structural Fire Design (London: British Standards Institution)).

37.4.4.1.2 Load Bearing Capacity of Two-Way Spanning Slabs

The load carrying capacity of a composite slab in one-way spanning is usually sufficient under fire conditions. However, in some cases, it may be necessary to utilize the slab strength in two-way spanning. This is particularly the case when it is necessary to justify the elimination of fire protection from some of the slab-supporting steel beams. The yield line analysis (Johansen 1962) for reinforced concrete slabs may be used to give a safe estimate of the slab load carrying capacity in two-way spanning.

The real benefit of utilizing the strength of a slab in two-way spanning is the possibility of using tensile membrane action in the slab, under which the strength of the slab can be many times higher than that given by yield line analysis. Recently, a design method has been developed to use tensile membrane action in steel framed buildings to eliminate fire protection to some steel beams. For detailed design equations and examples, reference should be made to the two papers by Bailey and Moore (2000a, 2000b) and a publication by the United Kingdom's Steel Construction Institute (Newman et al. 2000).

37.4.4.2 Composite Beams

For a conventional composite beam with concrete slabs on top of the steel section, the sagging bending moment capacity of the composite cross-section may be calculated by the bending moment capacity method, similar to steel beams (Section 37.4.3.1). The concrete in compression maybe assumed to be cold and the steel temperatures may be calculated using Equations 37.16 or 37.17. If the steel section is protected, it may be assumed to have a uniform temperature distribution and its section factor Ap/V is that of the entire section, calculated according to Table 37.3. If the steel section is unprotected, the steel section will have a nonuniform temperature distribution. For temperature calculations, the steel section may approximately be divided into two parts: the upper flange and the lower flange plus the web.

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