Membrane action of slab/beam composite floor systems in fire

Abstract

A structural performance-based design approach for steel beams supporting a composite floor system is presented allowing designers to specify fire protection to only a proportion of the steel beams within a given floor plate. The approach is a further development of a previous simplified design method where membrane action of the composite floor slab was included. The new method, presented in this paper, has been extended to incorporate the membrane action of the slab and beam system acting compositely, whereas the previous design method only considered the membrane action of the composite slab. In addition, the new method also includes the effect of the membrane action of the slab due to the variation in the deflected form, which is assumed to follow the changing yield-line patterns as the slab/beam system is heated in a fire. Previously, the membrane action of the composite slab was based on the lower-bound yield-line pattern assuming the slab and supporting beams acted independently. Comparison with the previous design approach shows that the simplifications, of ignoring the variation in the deflected form, and ignoring the contribution of the unprotected steel beams to the membrane load-carrying capacity of the system, were conservative.