Effect of the first local buckling mode on the behaviour of steel I-section beam-columns

In this paper, the effect of the first local buckling mode on the strength and post-buckling behavior of steel I-section beam-columns is investigated. The first local buckling mode type and the corresponding critical load of the studied I-beam-columns are determined through bifurcation analysis, using incremental nonlinear finite element approach in the standard eigen problem form. In this approach the elements of the diagonal matrix D, resulting from linear factorization of the overall tangent stiffness matrix Kt, are monitored at each load increment, instead of performing the expensive eigenvalue analysis. At the critical load, where one of the diagonal elements becomes zero or very small positive value, eigenvalue analysis is carried out to obtain the local buckling mode. The ultimate strength and post-buckling behavior of the I-section beam-columns are determined by the finite element method considering both geometric and material nonlinearities. The effect of first local buckling mode type is investigated through a parametric study, where several values of web width-thickness ratio and flange width-thickness ratio, are considered. Moreover, the influence of the axial force ratio with the different geometric parameters is studied.