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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
The companion paper to this one presented the method to determine the membrane capacity of composite floor slabs in fire. In this paper the method is applied to four of the Cardington fire tests. Two calculations were carried out for each test. Firstly, the design method was used to calculate the required deflection to carry the actual load on the floor. This would allow an assessment to be made of how accurately the method can predict the structual behaviour of a composite floor slab in fire. Secondly, the ultimate load capacity of the floor was calculated based on the chosen failure criterion of a limiting mechanical strain. This would show how close each test was to failure. Failure envelopes were also produced for each test showing the load capacity of the slab for a range of reinforcement temperatures. The results showed that the method can accurately predict the behaviour of composite floor slabs in fire with the predicted deflections agreeing well with the experimental results within the bounds of the assumptions made. It was also shown that the load capacity of the floors in all tests was significantly higher than the loads applied during the tests with the highest utilisation being 64% for the BRE large compartment test. 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