Failure Assessment of Simply Supported Floor Slabs under Elevated Temperature

It is well established that current codes dealing with the behaviour of structures under elevated temperature can be over-conservative, partly due to the neglect of the reserve capacity that beams and slabs can develop as a result of tensile membrane action. In this respect, the fire resistance of composite steel-concrete floor slabs has received considerable attention, since it was established that such slabs can continue to carry the imposed load well after the supporting secondary steel beams have lost strength at elevated temperatures. The level of the deformations suggests that the slab is under tensile membrane action, with slab failure occurring by rupture of the reinforcement along cracks whose location coincides with the predictions of yield line theory. However, current methods fail to address this, as they are based on semi-empirical equations of average strain. In this paper, the behaviour of slabs simply supported on all four sides, with no planar restraint along the edges, is represented by means of a simple, yet realistic, mechanical model that accounts for failure by rupture of the reinforcement. The model accounts for the important influence of bond between the steel reinforcement and concrete, the in-plane movement of the slab segments and the increase in the crack widths with deflection up to the point where the strain concentration in the steel reinforcement leads to rupture. Two forms of the kinematic model are presented, corresponding to full depth cracks at midspan and at the intersection of the diagonal yield lines, respectively. It is shown that the proposed model compares favourably against experiments and nonlinear finite element analysis, thus paving the way for the incorporation of rational failure criteria into the fire resistance design of floor slabs.