Modeling Membrane Action of Concrete Slabs in Composite Buildings in Fire. II: Validations

A companion paper has documented a geometrically nonlinear layered procedure for modeling of the membrane actions in concrete slabs subject to very high deflections and in fire. The model is based on a layered procedure developed previously by the writers in which only material nonlinearities were taken into account. In this paper two solid reinforced concrete slabs with simply supported edges, tested by other writers at ambient temperature under uniform loading, are modeled. This is followed by a simulation of a full-scale fire test on a solid reinforced concrete slab floor. Finally very detailed simulations of two full-scale fire tests on the composite frame at the Cardington Laboratory are conducted. It is evident that the proposed model can predict structural behavior of reinforced concrete slabs and their influence on composite steel-framed buildings in fire with good accuracy, although the multiplicity of parameters which can affect a test result cannot always be controlled or measured. In all cases the development of membrane actions is demonstrated, and the structural behavior differs compared with the geometrically linear case. These studies provide evidence that while exposed steel temperatures in composite buildings remain below 400°C, the much cooler concrete slab plays little part in the load-carrying mechanism, other than in generating thermal curvature of composite beams. For steel temperatures higher than about 500°C the significance of the slab progressively increases, and it becomes very important to model concrete slabs correctly. At very high temperatures the floor slab becomes the main load-bearing element, and the floor loads above the fire compartment are carried largely by tensile membrane forces developed mainly in the steel anticracking mesh or reinforcing bars.