Efficient arrangement of reinforcement for membrane behaviour of composite floor slabs in fire conditions

Abstract

Abstract At large displacements concrete floor slabs can support, by membrane action, a vertical applied load which is significantly larger than that calculated assuming simple flexural behaviour. In an accidental limit state, such as a building fire, membrane action at large displacements can be beneficial to the survival of composite floor slabs used in steel-framed buildings. By utilising membrane action significant cost savings can be achieved by identifying a large number of steel beams, which would have required applied fire protection using previous design methods, to be left exposed to the high temperatures experienced during a fire. This paper extends a previous derivation of a design method, which predicted the membrane load-carrying capacity of composite floor slabs in fire, but was limited to isotropic reinforcement. The extension to the method allows the specification of orthotropic reinforcement, presenting the designer with the tools to specify the most economical arrangement of reinforcement in the floor slab. It is shown, in the fire design example presented in this paper, that for a given area of reinforcement the membrane load-carrying capacity of a rectangular floor slab with an aspect ratio of 2 can be increased by 23% by placing more reinforcement in the longer span.