A number of researchers have focused on the performance of reinforced concrete slabs exposed to fire. These studies have shown that the membrane forces and the redistribution of bending moments in the slabs considerably affected the behaviour of the slabs in fire conditions. Consequently, the fire resistance of the slabs can be enhanced if tensile membrane behaviour is mobilised. However, the behaviour of reinforced concrete slabs in fire conditions is not clearly known. This thesis uses a non-linear finite element program, SAFIR, developed at the University of Liege, Belgium, to model reinforced concrete flat slabs and one-way slabs at elevated temperatures. The slabs were modelled as 3-D shell elements in the numerical structural models. The research analyses nine-bay flat slabs exposed to an ISO 834 Standard fire beneath with and without a decay phase. The location of the fire is varied from beneath all the bays to beneath the middle strip bays and the middle bay of the flat slabs. The study also considers the surrounding thermal conditions of the flat slabs. In addition, the research also analyses one-way slabs exposed to an ISO 834 Standard fire with and without a decay phase; the restraint conditions on the supports are varied. Furthermore, the effect of the widths of oneway slabs on the distributions of membrane forces and bending moments is investigated. It was found that the location of the fire under flat slabs and the arrangement of reinforcing bars in flat slabs significantly affect the distribution of bending moments and membrane forces in flat slabs. It was also found that the worst scenario of fire exposure could be when the flat slabs are exposed to fire with a decay phase or after the fire was exhausted or extinguished. The fire resistance of flat slabs is significantly increased if tensile membrane action can be mobilised. However, the distribution of membrane forces in the slabs is nonlinear.