Failure of composite steel-concrete slabs under elevated temperatures
The behaviour of composite steel-concrete slabs under fire has attracted considerable attention over recent years and has led researchers to develop performance based models capturing the phenomena observed during fires. However, while the limit load proposed defined corresponds to fracture of the reinforcing mesh, the criterion employed is semi-empirical ignoring fundamental issues such as the bond slip characteristics. A recent model has addressed this issue for lightly reinforced beams, considering the bond slip response of the reinforcement along with other salient problem characteristics, however, it becomes complex for practical application when extended to slabs. In the current work, novel models are developed for the assessment of the failure load of lightly reinforced concrete slabs under fire conditions, considering simply supported rectangular slabs with and 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 non-linear finite element analysis program ADAPTIC and experimental results are presented along with case studies highlighting the influence of various parameters. Simplified versions of the proposed models are also presented for direct use by designers to assess the failure of composite slabs under elevated temperatures.