Buckling behaviour of S700 high strength cold-formed steel built-up I-section columns

High strength cold-formed steel (CFS) built-up sections have been widely used in the construction sector, mainly due to high load-bearing capacities and flexible cross-section shapes. To further broaden the knowledge and develop efficient design provisions for high strength CFS built-up sections, experiments and numerical simulations were conducted to investigate the local–flexural interactive buckling behaviour and resistances of S700 high strength CFS built-up I-section columns. In the physical experiments, two identical unlipped channel sections were adopted to assemble each built-up I-section in a back-touching configuration with self-tapping screws. Material testing and measurements of initial geometric imperfections were first conducted; nine column specimens of two cross-sections – 90 × 70 × 2.9 and 160 × 100 × 2.9 were employed and then tested under pin-ended boundary conditions. In the numerical simulations, finite element (FE) models were developed and observed to precisely capture experimental observations. A parametric study was then conducted to derive additional numerical data across a wider range of geometric parameters and member slenderness. After that, the effective width method (EWM) in the European code and American specification, and the direct strength method (DSM) in the American specification were assessed, according to the experimental and numerical results, for their applicability to S700 high strength CFS built-up I-section columns failing by minor-axis local–flexural interactive buckling. The assessment results showed that the EWM yielded scattered and conservative results, while the DSM resulted in relatively accurate but still overall conservative resistance predictions. Therefore, a revised AISI design approach was proposed to improve design consistency and accuracy, with its reliability also verified.