Numerical analysis and design of concrete-filled double skin CHS with 316L+Q355 stainless-clad bimetallic steel outer tubes under axial compression

Concrete-filled double skin steel tubular (CFDST) stub columns with stainless-clad bimetallic steel outer tubes combine the cost-effectiveness and high corrosion resistance of stainless-clad steel with the superior strength-toweight ratio inherent in traditional CFDST structures, making them well-suited for applications in demanding marine environments. This paper presents a comprehensive numerical study on CFDST stub columns with stainless-clad bimetallic steel circular hollow section (CHS) outer tubes under axial compression. Finite element (FE) models were established and validated against existing experimental results from the literature, focusing on load-average axial strain relationships, ultimate strength, and failure modes. Employing the validated FE models, an extensive parametric analysis comprising 320 models was undertaken to investigate the influence of clad ratios and wall thicknesses of stainless-clad bimetallic steel outer tubes, as well as concrete strength on the compressive behaviour of the investigated CFDST stub columns. The ultimate loads from the parametric analysis were used to assess the suitability of existing design codes, including the European Standard EN 1994-1-1, the American National Standard ANSI/AISC 360-22, and the Chinese Code T/CCES 7-2020, with respect to the studied stub columns. It was observed that the EN 1994-1-1 can be most consistent but unconservative in their capacity predictions, while the ANSI/AISC 360-22 tends to yield over-conservative capacity predictions. The T/ CCES 7-2020 offers the most accurate capacity predictions. Finally, a modified approach based on the EN 1994-1-1 method was proposed, which features the utilisation of a revised concrete confinement parameter eta c* that more accurately reflects the concrete confinement effects in CFDST sections, which has been shown to enhance prediction accuracy for the ultimate resistances of CFDST stub columns with stainless-clad bimetallic steel outer tubes.