Ultimate strain of stiffened steel box sections under bending moment and axial force fluctuations

The influence of axial force fluctuations on the ductility of short steel cylinders has been previously reported by the authors, together with a proposal for a design formula that takes this influence into account. The present paper describes a similar investigation for stiffened short steel box columns. Elastoplastic large-displacement analysis of parametric short steel box columns is carried out and the bending behavior under constant and fluctuating axial forces is compared. The maximum value of axial force fluctuation is taken to be the same as the axial force magnitude in the constant axial force case. It is found that ductile capacity corresponding to the post-peak region of the bending behavior is improved to different degrees depending on the magnitude and amount of fluctuation of the axial force, a finding that is similar to that for short steel cylinders. Design formulae for failure strain taking this improvement into account are proposed for two different limit states and their validity is demonstrated through numerical analysis.