Effective stiffness modelling of composite concrete slabs in fire

In this paper a modified layered slab element is developed based on the layered procedure previously developed for the modelling of composite slabs in fire. In the development reported here, the ribs forming the lower part of any slab cast onto metal decking are included in the slab modelling. The basic idea is to use the nominal thickness of the composite slab as the thickness of the slab element, and to use an effective-stiffness factor to modify the material stiffness matrices of plain concrete in order to take into account the orthotropic properties of the slab. A maximum-strain failure criterion is used in the modelling of concrete. Three standard fire tests on metal-deck composite slabs and one full-scale natural fire test on the Cardington composite building frame are modelled for validation. The validations indicate that the model proposed is clearly capable of predicting the fire resistance of this type of composite slab in fire with reasonable accuracy. It is evident from this study that the influence of the ribs across the bottom of the slabs is significant and should be accounted for. The calculation of effective stiffness factors, which are based on the theory of elastic beam bending, is adequate and efficient, and the maximum-strain failure criterion is simple and suitable for such problems.