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Part of the hysteretic behavior involves local buckling.
This and the next 2 images are from:
Qusay A. Al-Kaseasbeh, “Hysteretic behavior of thin-walled steel tubular columns under constant axial fore and cyclic lateral loading”, PhD Dissertation, University of North Dakota, May 2019
ABSTRACT: Thin-walled steel tubular bridge piers (column refers to “bridge pier” in the subsequent text) with either circular and square box cross sections are becoming an increasingly attractive choice as cantilever bridge piers in severe earthquake regions due to their architectural, structural and constructional advantages. However, thin-walled steel tubular columns are vulnerable to local buckling, global buckling or interaction between both under extreme loading events such as strong earthquakes. This buckling results in a significant strength and ductility degradation, which eventually leads to an early and full collapse of the thin-walled steel tubular columns. The work presented in this dissertation investigates the inelastic structural behavior of uniform and newly proposed graded-thickness thin-walled steel tubular circular and square box columns under a constant axial force as a superstructure dead load and uni/ bidirectional cyclic lateral loading. First of all, the adopted finite element model (FEM) in ABAQUS/Standard version 6.14, which takes into account the effect of both material and geometric nonlinearities, is verified with the experimental results reported in the literature and employed for the analysis. Second, the newly proposed graded-thickness column, with size and volume of material equivalent to the BB column, is evaluated under a constant axial force and uni/bidirectional cyclic lateral loading. The proposed graded-thickness column is proved to have significant improvements in the overall hysteretic behavior compared to its counterpart conventional uniform column. Then, the deterioration of the circular bidirectional cyclic loading path over the unidirectional path is emphasized. Finally, a comprehensive parametric study is carried out to investigate the effect of key design parameters including: radius-to-thickness ratio parameter (Rt), width-to-thickness ratio parameter (Rf), column slenderness ratio parameter (λ), magnitude of axial load (P/Py), and number of loading cycles (N) on the overall hysteretic behavior of uniform and graded-thickness columns under a constant axial force and uni/bidirectional cyclic lateral loading. Subsequently, design formulae have been derived to predict the ultimate strength and ductility of both uniform and proposed graded-thickness columns.
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