Fire resistance of concrete slabs acting in compressive membrane action

In building renovation, the real behaviour of reinforced concrete slabs cannot always be explained by the bending theory according to classical structural mechanics. Indeed, the bearing capacity, as assessed for instance by a loading test, sometimes appears to be much higher than what would be expected. This phenomenon may be caused by the activation of an arch-effect or so-called compressive membrane action (CMA) which can develop even with small vertical deformations. For a slab which is completely restrained, the presence of reinforcement becomes of lesser importance when this phenomenon is activated (except for end fields). Hence, for fire resistance purposes, it can be discussed whether reinforcement and concrete cover has a smaller influence on the bearing capacity for slabs subjected to fire which exhibit a significant concrete compressive membrane behaviour. This paper presents a loading test performed on a real concrete building which highlighted the development of CMA as the load bearing mode. It then proposes a strategy to evaluate the behaviour resulting from the development of CMA in reinforced concrete slabs at ambient and at elevated temperature based on numerical modelling. The numerical analyses are performed with the finite element software SAFIR® using a strip of layered shell elements. A plastic-damage constitutive model with an explicit transient creep formulation is used to capture the concrete behaviour at elevated temperature.