Limit capacity of laterally restrained reinforced concrete floor slabs in fire

Abstract

This paper presents a new concept for determining the limit capacity of rectangular reinforced concrete slab panels subjected to fire. The results from extensive previous work on computational modelling of composite steel frame structures in fire at the University of Edinburgh has shown that the response of such structures is dominated by the thermal strains induced in the structure as a result of heating. Naturally, as the limit state of collapse is approached, the conventional influences of loading and structural strength and stiffness (reduced by heating) will once again begin to dominate the response. However, in tests carried out on such structures and in many instances of real fires no collapse or failure has been observed and the exact mechanism of failure is still a matter for speculation. Because of this apparently considerable robustness of composite framed structures in fire, tests (such as the full scale fire tests at Cardington) have been carried out (with fire protection applied to the steel columns only while most steel beams being left unprotected), but again without any structural failure. A number of investigations, including the authors own, have attributed this robustness to the tensile membrane mechanism in the composite deck slab. The research group at Edinburgh has discovered that the development of tensile membrane mechanism in fire is much more reliable relative to ambient conditions. This is because the large amount of thermal strain allows composite floor systems to assume highly deflected shapes while limiting the magnitude of damaging tensile mechanical strains, thereby retaining the ability to carry loads for much longer. In the context of performance based design this additional capacity of composite decks or reinforced concrete slabs can be exploited for providing structural stability in fire given that a quantitative estimate of this capacity can be reliably made. Some previous work exists in this regard but none of this work attempts to determine the correct deflected geometry of the floors and therefore the membrane capacity is not properly estimated. This paper introduces a new three-step method that analyses the limit capacity of laterally restrained RC slabs in fire. First the temperature distribution over the depth of the slab is estimated for a given fire scenario. Then the deflected shape of the RC slab and its membrane stress state is determined using a rigorous analytical method. Finally an energy based method is used to determine the maximum load that the slab could carry based on the geometric form and stress state determined in the previous steps.