A New Method to Analyze the Membrane Action of Composite Floor Slabs in Fire Condition

During 1995 and 1996, a total of six localized fire tests were conducted on a full-scale, eight-storey, steel framed building at Building Research Establishment Laboratory at Cardington. The test results show that the fire performance of steel-concrete composite floor is better than that obtained from traditional design method, and the load capacity of composite floor slab in fire condition is usually higher than the predicted capacity without considering membrane action. In the past decade, researchers have done a series of analyses in this area, such as Bailey, Usmani, and Li. In this paper, a new method to calculate the load capacity of simply supported composite floor slabs with considering the membrane action is presented. The slab is divided into five parts at the limit state of load capacity, including a center-elliptic part and four rigid parts around. The deflection of the slab, the force of rebars in high temperature, and the force distribution between four rigid parts are reasonably assumed. According to force and moment equilibrium requirements on the slab, a series of equations are obtained to calculate the ultimate load capacity of floor slabs in fire condition. The effectiveness of this new method is validated through comparison with results from experiments and different theoretical simulations. The comparison shows that this new method is more reasonable in predicting the deflection and ultimate load capacity of floor slabs in fire condition than previous methods.