A nonlinear layered finite element procedure for predicting the structural response of reinforced concrete slabs subjected to fire is described. The proposed procedure is based on Mindlin/Reissner (thick plate) theory, and both geometric and material nonlinearities are taken into account. The complications of structural behavior in fire conditions, such as thermal expansion, cracking or crushing, and change of material properties with temperature are modeled. In this study a total Lagrangian approach is adopted throughout, in which displacements are referred to the original configuration and small strains are assumed. A numerical example, in which a rectangular reinforced concrete slab is modeled at elevated temperatures, is presented. The influences of different thermal expansion characteristics, tensile membrane action, and differential temperature distributions across the thickness of the slab are investigated. It is evident that the nonlinear layered procedure proposed in this paper can properly model the membrane action of concrete slabs in fire conditions. More extensive comparisons with experimental results obtained by previous researchers, both at ambient temperature and in fire conditions, are presented in the companion paper.