Failure of unrestrained lightly reinforced concrete slabs under fire, Part I: Analytical models

Novel models are developed in this paper for the assessment of the failure load of lightly reinforced concrete slabs under fire conditions, considering simply supported rectangular slabs without planar edge restraints. In the limit, this load corresponds to the failure load of composite slabs under fire, since fire tests have demonstrated that the steel deck de-bonds leaving a lightly reinforced concrete slab. The developed models account for the temperature effect on the geometric and material properties, and they consider the tensile membrane action developed at large deflections. The deflected shape, used as the basis of model formulation, was observed experimentally to match the failure mode described by yield line theory, and in the developed models it is assumed that cracks forming along the yield lines penetrate through the slab depth. The strain concentration in the reinforcement along these cracks is established by considering the bond–slip characteristics, and the failure load is determined as that corresponding to a specific rupture mechanical reinforcement strain. Comparisons against the nonlinear finite element analysis program ADAPTIC are presented. Simplified versions of the proposed models are also presented to assess the failure of composite slabs under elevated temperatures which are shown to correlate favourably with the complete formulations.