Abstract This paper aims to investigate the ultimate strength of steel-concrete composite cellular beams, considering the web post buckling. The numerical model is calibrated via experimental tests consolidated in the literature by geometrical non-linear analyses. Asymmetric and symmetric sections are investigated. For each section, the ratio of the web-post width to opening diameter, and opening diameter to the depth of the parent section are varied. The results are compared with analytical procedures. In the models, web post buckling and the Vierendeel mechanism were observed. The end post width affected the ultimate behavior of the concrete slab. The calculation procedures proved to be conservative.