Membrane action in ground-bearing concrete slabs

Abstract

Loading a ground-bearing slab at a location distant from the slab edge will inevitably incur membrane action. This membrane action can result in considerable enhancement of maximum load-carrying capacities. Although the enhancement may not be as great as for suspended slabs laterally restrained by their supports, it is nevertheless still substantial. Now that slabs are often continuously cast over large areas there may be considerable benefit from taking advantage of this load-carrying enhancement. The present paper presents a theoretical study of the effects of membrane action in these circumstances. It suggests that the inability of previous theoretical models to reproduce the failure loads from tests on slabs or to account for the considerable scatter in results may be the result of ignoring membrane action. An upper-bound plastic analysis is used to shed new light on the evolution of mechanisms and shows that, depending on stiffness, slabs on grade reach collapse loads with one of two modes. Either collapse takes place in flexure at a peak of the non-linear load–deflection behaviour or by punching through provoked by a through-depth crack at the perimeter of the loaded area. With pure flexure plastic methods, past analyses have failed to provide a non-trivial estimate of the size of the failing mechanism. Inclusion of membrane action is shown to predict the radius of the probable preferred collapse mechanism. A parametric study is provided, but further research work providing experimental scrutiny and convenient design routines is needed before this is of immediate benefit to the practitioner.