Membrane action of unrestrained lightly reinforced concrete slabs at large displacements

Following full-scale fire tests on a steel-framed building, together with observations from real fires, it has been shown that membrane action, at large displacements, of composite floors comprising steel deck, concrete, and anti-crack mesh, is extremely beneficial to the survival of the building. It was therefore decided to review previous research conducted on unrestrained concrete slabs, under large displacements, at normal temperatures. It was found that the assumptions used to develop previous theoretical predictions for the load-carrying capacity, for a given vertical displacement, are only valid for square slabs and do not conform to test observations for rectangular slabs. A new theoretical approach is therefore presented which is valid for both square and rectangular slabs and conforms to the mode of behaviour observed in tests. The design method is shown to give excellent correlation with published test data. A prediction for ultimate collapse of the slab due to fracture of the reinforcement is also presented, which limits the allowable mechanical strain in the reinforcement. Comparison with available test data shows that this prediction is always conservative.