The behaviour of restrained steel columns in fire is analysed by quantifying the evolution of the mechanical and thermal energies from the onset of fire till the column’s collapse. In Structural Engineering, the temperature at which columns collapse is of paramount interest, since it defines the column’s resistance in such conditions. To this end, an appropriate structural model is adopted to account for both the restraining effect provoked by the surrounding structure and the serviceability axial load applied before fire. This model is analysed here by means of the voxels-based Rayleigh-Ritz method [1], which accounts for both the spread of plasticity and the degradation of the steel’s mechanical properties with heating. The column’s behaviour in fire consists of a buckling problem followed by plastic collapse [2]. Due to the First Principle of Thermodynamics, the heat given to the column shall be equal to the sum of the strain and thermal energies stored by the column itself, plus the energy transferred from the column to the neighbouring areas minus the work performed by the external loading. However, even in experimental fire tests at the laboratory, the full application of this principle is practically impossible because it is impossible to quantify accurately all energy forms and transfers during a fire. Consequently, we focus on the evolution of the types of energy we can quantify exactly during a fire test at each stage of heating, namely the thermal energy and the strain energy absorbed by the column, and the work performed by the external loading as the column deforms. It is observed that these quantities follow typical patterns from the onset of fire until the column’s collapse. This strategy creates a new link between the thermal and the mechanical problems and opens new perspectives for the derivation of innovative energy based criteria that surely will improve columns’ fire resistance design.