Equivalent geometrical imperfections for local and global interaction buckling of welded square box section columns

The demand for welded box-sections is increasing in the construction industry due to their easy fabrication and limited stability issues compared to other open sections. The instabilities of welded box-section can be mainly categorized into three types: global buckling, local buckling, and interaction buckling. Nowadays, more attention is paid to interaction buckling, as designers tend to use lighter sections to save on weight and cost. Nevertheless, there is no suitable method to consider the nonlinear effect of interaction between the global and the local buckling, which can be directly incorporated using FEM-based design. Moreover, the currently adopted equivalent geometrical imperfections for flexural buckling in the Eurocode were developed based on geometric nonlinear imperfect analysis (GNIA), and it is inappropriate to be used in geometrically and materially nonlinear analysis (GMNIA). Therefore, the current research investigates the accurate application of the imperfections and imperfection combinations for welded box-sections using the GMNIA technique to determine the accurate buckling resistance by the FEM-based design approach. The investigation starts with developing a numerical model and validating it against test results available in the literature. The validated numerical model is used to conduct a parametric study to find the accurate buckling resistance using previously developed combinations of geometrical imperfections and residual stresses. Then, additional parametric studies are executed to back-calculate the necessary equivalent global and local imperfections and find a suitable rule for combining the global and local imperfections based on the accurate buckling resistance. The proposed equivalent geometric imperfection magnitudes can be applied in the numerical model to aid in the FEM-based design approach.