Confinement-based design model and design-code assessment for axially-loaded hexagonal CFDST short columns

Abstract

Hexagonal concrete-filled steel tube (CFST) and concrete-filled double-skin steel tube (CFDST) columns offer structural efficiency and practical advantages, but current design codes lack specific provisions for their axial resistance, leading to inconsistent and sometimes inaccurate strength estimates. To address this gap, this study presents a confinement-based design model that captures the restraining effects of the hexagonal steel tube and stiffeners on the sandwiched concrete. A comprehensive dataset of experimental and numerical results for axially-loaded hexagonal CFST/CFDST columns was compiled to evaluate the performance of existing international design codes and analytical approaches. The comparative assessment shows that design provisions for circular sections in Eurocode 4, GB-50936-2014, AS/NZS-2327-17, and AIJ-2008 provide reasonably accurate strength estimates, with mean estimation ratios between 1.02 and 1.05, while AISC-360-16 and CSA-S16–19 significantly underestimate axial capacity, and T/CCES-7-2020 and BS-5400 significantly overestimate it. For square and rectangular provisions, Eurocode 4 and AS/NZS-2327-17 remain slightly conservative, but AISC-360-16, CSA-S16–19, and T/CCES-7-2020 produce unreliable and generally unsafe estimations. Although several analytical models provide greater accuracy than the code-based design equations, the proposed confinement-based model achieves the highest predictive accuracy and a reliability index of 2.55. As a result, the proposed model is highly recommended for practical design applications of hexagonal CFST/CFDST columns.