Comparison of Bre Simple Design Method for Composite Floor Slabs in Fire with Non-linear Fe Modelling

On the basis of test evidence a new design method has been recently developed by one of the authors, for calculating the performance of composite flooring systems subject to fire. The method models simply the influence of tensile membrane action in the composite floor slabs. The objective of this paper is to show some detailed comparisons between the simple design method and finite element modelling using the computer program Vulcan, which has been developed at the University of Sheffield, in order to check the applicability and inherent conservatism of the method. Initially a 9m x 9m square ribbed concrete slab, for which all four edges are vertically supported, is analysed. Different temperature distribution patterns across the thickness of the slab are used to investigate the influence of thermal curvature on the structural behaviour. The effect of changing the edge support conditions is also analysed. As part of this study a large generic composite flooring system with a footprint of 36m x 36m has been designed. The frame is based on a regular 9m x 9m column grid. A series of analyses has been performed, based on different patterns of fire protection to the downstand steel beams. The influence of the proportion of steel reinforcement on the structural behaviour has been investigated, and it is evident that the presence or absence of tensile membrane action in the concrete slabs is a major influence on the ultimate integrity of the flooring system at high distortions. The ability of the slab reinforcement to sustain the tensile stresses caused at high temperatures and deflections is clearly a key factor in ensuring that fracture of slabs does not occur. From both the Vulcan modelling and the simplified design method it is shown that tensile membrane action can be important in carrying the loads applied to the slabs at high temperatures and deflections. However it is apparent that the simple design method predicts a greater contribution to load-carrying capacity due to tensile membrane action than does the Vulcan modelling, especially for high reinforcement ratios, and that further work needs to be done to resolve this discrepancy.