Improved calculation method for insulation-based fire resistance of composite slabs

Floor slabs play a critical role in the fire resistance of buildings, not only by maintaining structural stability and integrity, but also by providing thermal insulation to limit the rise in temperature of floors above a fire. Composite slabs, consisting of concrete topping on steel decking, are common in steel building construction, but the profiled geometry of the decking makes the analysis of heat transfer in composite slabs more complex than for flat slabs. A method for calculating the insulation-based fire resistance of composite slabs with profiled steel decking is provided in Annex D of Eurocode 4 (EC4). However, the applicability of the EC4 calculation method is limited to a range of commonly used slab geometries from the 1990s, which is narrower than the range used in current practice. In addition, the EC4 calculation method assumes a specific value of moisture content for the concrete, and different values of moisture content can significantly affect the fire resistance, as shown in this study. This paper proposes an improved algebraic expression for estimation of the insulation-based fire resistance of composite slabs that explicitly accounts for moisture content and is applicable to an extended range of slab geometries. The proposed expression is developed based on computed values of fire resistance obtained from a validated finite element modeling approach. A set of 54 composite slab configurations are selected for analysis using a sequential experimental design. The accuracy of the proposed method is verified against numerical results for an additional set of 32 slab configurations and is also validated against experimental data. Comparisons of the proposed calculation method with the results of the verification analyses show deviations of less than 15 min in all cases for the insulation-based fire resistance of the composite slabs.