Membrane action in isotropic patch-loaded unrestrained slabs

The increased strength of concrete slabs resulting from the compressive membrane action produced by in-plane restraint at the slab perimeter is very sensitive to the stiffness of that restraint. Even when there is no perimeter restraint the benefits are worth considering. This is especially so where complete failure mechanisms occur well within the slab perimeter, as is the case for interior bays of suspended slabs and for ground-bearing slabs. An upper bound plastic yield line analysis is used in this paper to identify the mechanics of an isotropic slab yielding under an interior (non-edge) concentrated load. Monitoring the neutral axis variation illustrates that at low stiffnesses there is a qualitative change in the mode of failure from collapse at a peak value of load to punching at the transition to catenary action. The pattern of load–deflection behaviour is described for mechanisms having restraint stiffnesses ranging from zero to infinite.