AbstractCellular steel beams are widely used in modern constructions to achieve long spans and to provide passage for service installations such as ducts and cables. The present paper is concerned with the use of stainless steel in cellular beams. In fact, available design methods to calculate the shear strength of web‐post are applicable only to carbon steel that exhibits a linearly‐elastic behaviour with a clear yield point followed by plastic deformation with little amount of strain hardening. Conversely, stainless steel exhibits a non‐linear stress‐strain relationship with high level of strain hardening. Consequently, new design provisions for the web‐post buckling of cellular beams made from stainless steel capable of exploiting the material strain hardening are highly desirable. Since no experimental investigations are available in literature for cellular beams made from stainless steel, finite‐element models have been developed and validated against tests conducted for carbon steel cellular beams. The structural behaviour of stainless steel cellular beams with various geometries and material characteristics is investigated through a large number of finite element analyses and a simple mechanical model for the Web‐Post Buckling (WPB) failure is developed and calibrated against the numerical results to provide the basis of a reliable design method.