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Real building fires have shown that composite steel framed structures have the ability to carry load even when they have no structural fire protection applied on the beams. This was confirmed by the full-scale fire tests carried out at the BRE Cardington facility and a subsequent numerical modelling programme. It was observed that in a fire the load-carrying mechanism in the fire-affected floors of the building changes from flexure at ambient to tensile membrane action, allowing floors to continue sustaining loads even at very high temperatures. If this additional capacity could be quantified, it could be used to provide reliable fire resistance as part of a performance based design process without the need for protecting some of the secondary steel beams. This paper presents a new method for determining the membrane load capacity of a laterally restrained composite floor slab in fire. The method was developed from first principles and consists of three stages. Initially the type of fire and the subsequent temperature distribution that this produces through the slab depth are calculated. Secondly the deflection and stress-strain distribution due to the thermally induced strains are determined. Finally an energy method is used to calculate the membrane load capacity of the slab based on an assumed failure criterion. In the fire scenario use of geometrically based limits is unsuitable as, due to thermal straining, large deflections do not necessarily mean that the mechanical strains in the structure are large. A more suitable approach used here is to look at the mechanical strains that develop in the reinforcement and define a limiting value based on the ductility of the steel. N. J. K. Cameron, BEng, PhD Whitby Bird, 60 Newman Street, London, W1T 3DZ A. S. Usmani, BE, MS, PhD, CEng, MIStructE School of Engineering and Electronics, University of Edinburgh, Edinburgh, EH9 3JN