Behaviour of concrete floor slabs at ambient and elevated temperatures

This paper presents the latest developments of a simple design method used to predict the membrane action of unrestrained concrete, or concrete and steel composite, floors under fire conditions. The developments include the refinement of the assumed in-plane stress pattern and a prediction of when concrete crushing occurs in the proximity of the corners of the slab. The design method is compared against 44 small-scale tests carried out at ambient and elevated temperatures on horizontally unrestrained slabs with an aspect ratio of 1.0 or 1.55. The slabs were reinforced with either mild steel or stainless-steel welded mesh of different grades, ductility, sizes and bar spacing. Both the ambient and fire tests highlighted the occurrence of membrane action, either supporting loads higher than the theoretical yield-line load in the case of the ambient tests, or reaching higher failure temperatures compared to those calculated based on yield-line theory. Comparison between the developed simple design approach and test results showed good correlation both at ambient and elevated temperatures.