ABSTRACT Currently, in the EN1993‐1‐5 the ultimate strength of a longitudinally stiffened webs is calculated based on a design procedure which considers the ratio between the yield resistance and the critical buckling load. In the available formulation which is based on numerical modelling, the critical load increases with the relative position of the longitudinal stiffener. This behaviour contrasts with that observed at ultimate limit state, several experimental results have shown that the ultimate strength increases when the longitudinal stiffener is placed closer to the loaded flange. To solve this problem, new hypotheses of boundaries restrictions at the stiffener location are presented to model the elastic buckling behaviour of webs subjected to patch loading. A comprehensive finite element model is presented in this study to evaluate theses hypotheses by varying the web and longitudinal stiffener dimensions. Finally, a methodology to model the buckling behaviour of stiffened webs is presented.