Post-buckling of composite I-sections. Part 1: Theory

A theoretical investigation of initial local buckling and post-buckling beh avior of composite I-sections is presented. The equilibrium equation for in itial buckling is solved both exactly and with an approximate method, namel y, Galerkin's method. In Galerkin's method, the out-of-plane deflection of each plate element is approximated by a weighted sum of polynomial function s. The post-buckling response is studied as an extension of the approximate analysis with Galerkin's method where now both equilibrium and compatibili ty equations must be solved. The bending deflection in the post-buckling re gime is assumed to be a magnification of the deflection function used in in itial buckling analysis. No mode-shape change is thus allowed in the post-b uckling region. A polynomial type of function is also adopted for stress di stribution in order to take into account the deviation from uniform in-plan e load distribution in the plate elements following the onset of local buck ling. The paper provides an efficient and accurate method for predicting th e post-buckling behavior of composite structural sections composed of plate elements. Galerkin's method was previously applied to isotropic flat plate s only. The present approach is tested against a commercial code, STAGS, wi th a very good agreement in results and a very large saving in computer tim e for post-buckling analysis of an I-section.