Advances in Mechanical Behaviour of Cold-Formed Thin-Walled Steel Built-Up Sections

This paper presents a comprehensive review of recent advances in the mechanical behaviour of cold-formed thin-walled steel (CFTWS) built-up section members. Experimental, theoretical and numerical milestones for built-up beams and columns are critically examined. The evolution is traced from early load-capacity validation tests to the integration of the direct strength method (DSM) and effective-width method (EWM) for coupled local-global buckling. More recently, the focus has shifted to performance-based investigations covering high-strength steel, stainless steel, multi-material hybrids and post-fire conditions. A closed test-theory-code loop has thereby been established. Synthesis shows that the built-up effect markedly increases strength and stiffness; nevertheless, interface slip, shear lag and interactive buckling must be accommodated through calibrated modification factors or curved design surfaces. Future work should develop a unified mechanical model, expand multi-failure-mode databases and create AI-driven optimisation platforms, enabling probability-based limit-state design and transforming built-up members from empirically tuned systems into computable, optimisable, high-performance structural products. The review furnishes an engineering-ready design roadmap for green modular and high-rise cold-formed steel (CFS) construction.