The paper addresses the numerical modelling of concrete-filled steel tubular columns in fire. Both the thermal and the mechanical problems are analysed, and these problems are supposedly separable, in the sense that the displacements observed in the latter do not interfere in the temperature distributions obtained from the former. Consequently, the thermal problem generates the temperature distributions that are later input into the mechanical modelling. An adequate structural model is adopted for the mechanical problem, to reproduce the interaction between the analysed column and the surrounding frame. It comprises an axial serviceability load and an axial spring, to oppose to the column’s thermal expansion. For each problem, adequate numerical models are developed in ABAQUS. These models are firstly calibrated against the results that arise from experimental data. They account for all nonlinearities involved in the problem: thermal buckling, plasticity, the degradation of the steel and concrete properties with heating and the evolution of the concrete to steel adhesion during fire. The mechanical problem consists in a sequence of two nonlinear phenomena: at first buckling occurs, and afterwards a plastic mechanism is formed, which leads the column to collapse [1]. The collapse state determines the collapse temperature and is defined when the compressive restriction force, applied by the axial spring to the column, returns to zero. It is proposed that the resistance in fire of composite columns shall be described by the amount of thermal energy the column stores at the collapse state [2]. In fact, the thermal energy stored by composite columns of reinforced concrete and steel is much higher than the energy stored by columns made up only in steel, even if these columns collapse at similar temperature. Consequently, the heat storage capacity of a column at collapse assumes great relevance, and new fire resistance criteria shall be developed to include such effects. REFERENCES [1] Simão P.D., Rodrigues, J.P.C., Fernandes, H.D., A voxels-based Rayleigh-Ritz method for the post-buckling elasto-plastic analysis of restrained steel columns in fire, Journal of Constructional Steel Research, Vol. 201, paper n. 107736, 2023. [2] Simão P.D., Rodrigues, J.P.C., Consistent Thermodynamics analysis of restrained steel columns in fire – part 2: Application to the voxels-based Rayleigh-Ritz method, submitted to Elsevier, 2025.