Effects of edge support and reinforcement ratios on slab panel failure in fire
The advancement in structural fire engineering towards more cost-effective solutions has necessitated the increasing use of performance-based approaches to the design of multistorey composite buildings. These methods consider the real behaviour of structures and provide economic solutions which optimise fire protection usage. Optimising structures to use tensile membrane action requires the structural use of slab panels. These are vertically supported lightly-reinforced composite floor systems, allowing biaxial bending at elevated temperatures. Vertical support is achieved, in practice, by protecting a panel’s perimeter beams to achieve temperatures of no more than 620°C at the required fire resistance time. The Bailey-BRE design method, which incorporates tensile membrane action, uses these vertically supported panels to establish composite slab capacities in fire. The slab panel resistance is determined by a combination of the residual composite beam strength and the large-deflection enhanced slab resistance. The simple calculations of the Bailey-BRE method imply improved performance with higher reinforcement ratios. However, proportional increases have not been observed in the modelling work reported here. The discrepancy may be due to the geometry, composition or support conditions of the slab panels. Also, with exposure to fire, a panel’s ‘vertical’ support can be lost. This will in turn affect the tensile membrane capacity, pre-empting a structural failure of the floor system. This paper presents the results of a finite element investigation into the effects of reinforcements and vertical support on slab panel failure. The study examines the effect of various degrees of protection on the development of the tensile membrane action mechanism. It examines the development and failure of this mechanism, considering various degrees of edge-beam protection, and makes comparisons with the predictions of the Bailey-BRE method and various design acceptance criteria.