Flexural behaviour of aluminium–concrete–steel double-skin (SHS outer and CHS inner) tubes

Aluminium-concrete-steel double-skin tubular (ACSDST) members, featuring an outer aluminium alloy tube and an inner steel tube, have emerged as a promising structural solution. This configuration offers excellent corrosion resistance, making it well-suited for demanding applications such as coastal buildings and offshore platforms. However, their flexural behaviour remains insufficiently studied. This study investigates ACSDST members with square hollow section (SHS) aluminium outer and circular hollow section (CHS) steel inner through experiments and numerical analysis. Six specimens were tested to obtain the failure modes, bending moment versus curvature responses, and the longitudinal strain development of aluminium tubes. The experimental results show that the curvature ductility coefficient is at least 3.2. A total of 192 specimens were analysed using the validated finite element (FE) model, revealing that the hollow ratio and width-to-thickness ratio of the outer tube most significantly impact the flexural strength. Evaluations demonstrate that flexural strength predictions from T/CCES 7-2020 (2020) and EC4 (2004) exhibit mean deviations of 1.5 % from test and FE results. Similarly, the newly proposed formulas for section flexural stiffness achieve a mean deviation of less than 1.0 %. These findings establish a theoretical and practical basis for calculating the flexural strength and stiffness of ACSDST members in design applications.