This paper focuses on the behaviour of I-girder web panels subjected to combined bending and shear and stiffened by longitudinal trapezoidal stiffeners. This implies to account for the coupling between local panel buckling between stiffeners and global buckling of the whole stiffened panel. The study is restricted to panels with one or two longitudinal stiffeners, for which the well-known orthotropic plate approach may not be relevant. For such configurations, the most recent design rules available may either not address this specific problem or be too conservative. This contribution provides information on the behaviour of such girders based on numerical simulations. Results of extensive non-linear FEM calculations are presented and discussed. The effects of aspect ratio, web slenderness and position of stiffeners are studied. Numerical results are compared to design loads obtained according to Eurocode 3 Part 1.5. The parameter study shows that several aspects of the code may be improved and proposals in this respect are given and validated through the numerical results. In particular, the beneficial effect of closed longitudinal stiffeners is highlighted. It is shown that the high level of torsional rigidity exhibited by welded trapezoidal stiffeners may significantly increase the overall resistance of the web panel.