Buckling Strength and Ductility Evaluation of Thin-Walled Steel Tubular Columns with Uniform and Graded Thickness under Cyclic Loading

Thin-walled steel tubular columns with circular cross sections are widely used as cantilever piers in bridges due to their excellent structural and constructional advantages. However, local buckling, global buckling, or the interaction between both is usually the main reason for the significant strength reductions in these columns, which eventually leads to their collapse. This article investigates the behavior of thin-walled steel tubular columns with conventional uniform circular sections (Cs) and newly proposed graded-thickness circular sections (GCs) under combined constant axial and cyclic lateral loading. The analysis is carried out using a finite-element model (FEM) that takes into account both material and geometric nonlinearities. First, the accuracy of the employed FEM is substantiated using the experimental data available in the literature. Then, the GC column with a size and volume of material equivalent to the C column is introduced. The proposed GC column is proved to have significant improvements in strength, ductility, and postbuckling behavior compared with its counterpart C column. As part of the investigation, an extensive parametric study is carried out to investigate the effects of various important parameters, such as radius-to-thickness ratio parameter (R_t), column slenderness ratio parameter (λ), magnitude of axial load (P/P_y), and number of loading cycles (N), on the strength and ductility of the columns. Finally, design formulas for strength and ductility evaluation of C and GC columns are proposed.