Abstract: The regular polygon stiffened shell is widely used for the outward shell of a large-scale gasholder. In order to get the practical design formula for the outward shell under gas pressure, the section parameters of a developable surface are converged on axes by introduction of appropriate assumptions. Four structure systems are set up. Reasonable thin-walled bar theories can be respectively used to deduct the deformation and internal force formula of them. In this paper, a wide-flange vertical bar is consisted by the partial wall plate in effective breath and column by introducing Kármán’s theory. Based on the theory of elastic foundation beams, the deformation and internal force formula of the vertical bar are deduced. The values of formula are well matched with ABAQUS results. Hence, the present theory, computation model and formula can be referred in the design and construction of relating specifications.