Design of flexural buckling resistance of lightweight concrete-filled cold-formed built-up box section (CFBBS) columns under axial compression

Built-up section columns composed of two or more cold-formed channel sections offer a practical solution when single channel sections fall short in compression capacity. However, their thin-walled nature makes them susceptible to local and local—global interaction buckling. To address this, infilling lightweight concrete into nested built-up sections has emerged as a promising strategy in light gauge steel framing and modular construction to enhance structural stability without relying heavily on lateral bracing. The need for such bracing and closely spaced supports can limit the use of these columns in applications requiring large open spaces. Despite their potential, current design standards lack specific provisions for lightweight concrete-filled cold-formed built-up box section (CFBBS) columns. This study proposes a streamlined and robust design methodology to predict the flexural buckling resistance of these members. Validated finite element models were developed to expand the test database and investigate the parametric influence on the member slenderness reduction factor, concrete contribution, and mechanical behaviours. The proposed method demonstrated exhibited consistently superior accuracy compared to existing design provisions. An optimisation study was also conducted to assess strength efficiency, cost-effectiveness, and weight efficiency, offering practical guidance for structural design. • Finite element modelling of lightweight concrete-filled CFBBS columns. • Study on parametric effects on member slenderness reduction and concrete contribution. • Reliable design method and modified buckling curve for flexural buckling. • Optimal design recommendations for strength efficiency, cost, and weight savings.