Simplifications to finite element models are the subject of this article. These simplifications form part of a new method for the fire design of concrete and composite slab systems with partly unprotected secondary steel beams, taking into account tensile membrane action. Internal forces can be determined with the simplified model and cross-section design procedures can be applied to calculate the required reinforcement amount equivalent to ambient temperature. The FE model is simplified by replacing the thermal analysis with a substitute thermal loading. Non-linear material behaviour is taken into account via a reduced model stiffness, which allows an efficient linear elastic calculation. The new method presented therefore enables the simple and efficient design of slab systems for fire.