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The objective of this paper is to summarise the results of numerical modelling of six full-scale fire tests carried out in 1995–1996 on the composite frame constructed at the BRE laboratory at Cardington, representing cases in which different degrees of in-plane restraint to heated steel members were provided by the adjacent structure. The computer program Vulcan, developed at the University of Sheffield for three-dimensional modelling of the behaviour of composite buildings in fire has been used. The modelling is seen to represent the logged test results well in the cases in which the test conditions were well defined, with lower accuracy in one test in which a local column collapse occurred and another in which most structure temperatures were not recorded. In general terms, when the temperatures of exposed steel beams were below 300–400°C the influence of the concrete slabs on the structural behaviour of the composite building is shown to be small, other than as flanges of the composite beams, but when steel beam temperatures rose above 500°C the slabs became increasingly influential as part of the load-carrying mechanism. The influence of membrane tensions in the slab cannot be ignored, particularly when a fire compartment is subject to high horizontal restraint from surrounding cool, stiff structure, or when it is vertically supported around its perimeter at protected lines of support. When the double-curvature deflections of floor slabs become large the influence of tensile membrane action can become very important in supporting the slab loading. Whether this is capable of preventing final fracture of the slabs depends mainly on the properties of the steel reinforcement used, and the degree to which it is insulated from temperature rise by its concrete cover, even if this is cracked. It is clear that the tensile strength of concrete slabs can provide a considerable reserve of extra fire resistance in this type of construction, provided that a suitable protection strategy is adopted. Zhaohui Huang, BEng, PhD Department of Civil & Structural Engineering, University of Sheffield, S1 3JD, UK Ian W. Burgess, BA, PhD Department of Civil & Structural Engineering, University of Sheffield, S1 3JD, UK Roger J. Plank, BSc, PhD, CEng, MIStructE, MICE School of Architectural Studies, University of Sheffield, S10 2TN, UK