AbstractWeb‐post buckling next to Vierendeel effect, is one of the main failure modes for cellular beams. Several solutions are available to avoid web‐post buckling, such as ring stiffener or infill opening. A new cost‐effective solution is to weld a vertical plate stiffener on the web‐post. This plate stiffener should bring enough inertia to prevent buckling. It is not necessary that the plate stiffener is welded to the flanges, so it does not require as accurate cutting as a ring stiffener or an infill opening. An analytical method is proposed to calculate the bearing capacity of such a stiffened cellular beam. This consists in an adaptation of the existing design method presented in prEN 1993‐1‐13 for steel beams and in EN 1994‐1‐1: Eurocode 4 Annex Composite beams with Web Openings for composite beams. The analytical method has been compared to a numerical parametrical study using Finite Element software SAFIR®. Influence of various parameters has been studied. Results show that depending on the geometry, vertical stiffeners can increase the bearing capacity of the beam up to 35%. In all modelled cases, the stiffener avoids the web‐post buckling. Two main webpost failure modes were identified, buckling of the first unstiffened web‐post (generally when only one web‐post is stiffened) and bending of the first stiffened web‐post. For the latter, a verification has been added in the analytical method. Additionally, a cost analysis has been performed to compare different solution to increase the bearing capacity of the beam.