Steel girder web panels have been subjected in recent decades, to a number of experimental and numerical studies but the mechanisms that regulate the behaviour of the panels composed by two subpanels with different thickness were not deeply studied. Furthermore specific design rules regarding the estimation of the buckling coefficient for panels with variable thickness are not included in the codes even if this is a common situation particularly for steel bridge girders with beams having significant height. In this framework,this work aims to investigate buckling behaviour of steel beams with webs composed of panels with different thicknesses subjected to both in-plane axial compression and bending moment and gives some simplified equations for the estimation of the buckling coefficient.