Analysis and design of slender square concrete-filled double-skin stainless steel tubular columns

This study investigates the structural behaviour and ultimate strength of slender square concrete-filled double-skin tubular (CFDST) columns comprising stainless steel outer tubes and carbon steel inner tubes. A validated finite element model was used to conduct an extensive parametric study examining the influence of slenderness, material properties, hollow ratio, and interfacial contact behaviour. The results highlight slenderness as the dominant factor affecting buckling resistance, with two slenderness thresholds proposed to delineate short, intermediate-length, and long columns. The slenderness ratio of the outer tube showed negligible effect on normalised strength and lateral deflection behaviour, confirming slenderness as the principal parameter controlling failure. Concrete strength notably influenced the post-peak response of intermediate-length columns but had minimal effect on long columns. The hollow ratio had a limited impact on axial strength but significantly improved the strength-to-weight ratio due to reduced column weight. Contact pressure analyses revealed maximum values at the section corners, decreasing with increased slenderness and reducing to zero at the mid-span of the sides. Among existing design provisions, AISC 360–16 demonstrated the greatest accuracy, with predictions typically within ±10 % of the finite element results. Existing design methods were evaluated, and two new design models are proposed, validated, and shown to provide improved accuracy and practical applicability for slender square CFDST columns. The proposed design models are presented in a simplified and practical format to facilitate ease of application in engineering practice, while providing a robust and reliable framework for structural design. • A validated finite element model was developed to assess the behaviour of slender square CFDST columns with stainless steel outer tubes. • Slenderness ratio was identified as the most critical parameter influencing axial strength, failure mode, and contact interaction. • Two new design models were proposed to predict the ultimate strength across plastic, inelastic, and elastic regimes with high accuracy. • The proposed design curves showed close agreement with FE results and outperform or match current code predictions, including AISC 360–16.