This paper presents and clarifies the treatments included in EN 1993-1-1 [1] relating to checking the resistance of steel cross-sections under torsion and its interaction with other internal forces. Specifically, the origin of the formulations for shear-torsion interaction, which has not been found in the literature, is presented. Furthermore, a very simple formulation based on the expressions used for shear-torsion interaction is developed in order to take into account bending-warping torsion interaction for symmetrical I-sections (IP and HE steel profiles). Such a formulation overcomes the overly conservative approach given in EN 1993-1-1 [1] for class 1 and class 2 cross-sections (plastic and compact cross-sections). Finally, a rigorous method for determining the bending resistance of cross-sections is proposed considering the interaction with shear and torsion. The proposal is well suited to the concept of cross-section class and is perfectly consistent with the approaches set out to consider bending-shear and bending-warping torsion interactions.