The buckling behaviour of panels may be determined according to EN 1993-1-5 [1]. Most of the design rules relating to stiffened panels in EN 1993-1-5 were derived on the basis of opensection stiffeners. Several recent investigations have shown that the application of the design rules according to EN 1993-1-5 considering the torsional stiffness of the stiffeners may overestimate the resistance of the panels. Therefore, the recent Amendment A2 to EN 1993-1-5 states that the torsional stiffness of stiffeners should generally be neglected in determining critical plate buckling stresses. In addition, prEN 1993-1-5 [2] provides rules for considering the torsional stiffness of stiffeners. However, in this article it is shown that even the rules of prEN 1993-1-5 are not sufficient to overcome the safety deficiencies. The article focuses on the investigation of the buckling behaviour of stiffened panels with closed-section stiffeners subjected to constant longitudinal compression stresses. Improved rules have been developed that allow to consider the torsional stiffness of the stiffeners. Based on an extensive numeri cal parametric study, a new interpolation equation be tween column-and plate-like behaviour is proposed. In comparison to [3], the investigations have been extended to the effec tive width method. They show that the proposal provides a safe and economic solution for the reduced stress method and the effective width method when considering the torsional stiffness of stiffeners by calculating the critical plate buckling stresses.Keywords bridge design; plate-like behaviour; column-like behaviour; stiffened panel; longitudinal; reduced stress method; effective width method; torsional stiffness of stiffenerThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.