Design of cold-formed steel built-up nested columns subject to flexural buckling

Cold-formed steel (CFS) built-up columns are widely used nowadays. The North American design standard (NAS 2016) suggests using a modified slenderness ratio (MSR) for CFS built-up columns undergoing elastic flexural buckling. The existing MSR applies to sections in which the screw fasteners experience shear, and hence this recommendation excludes the use of nested built-up columns. An improved slenderness ratio (ISR) is proposed in this study for the nested built-up columns made of two channels. For this, the exact buckling stress results of a compound spline finite strip method (CoSFSM) based computational tool has been used as the basis. The CoSFSM tool was developed to account for the effect of screw fasteners on the buckling behaviour of built-up section columns. The proposed ISR equation is developed by generating a suitable curve-fitting equation based on a numerical study. For this purpose, a finite element model is developed and validated using the test results available in the literature on long CFS built-up nested channel columns. The obtained ultimate strength results are used to investigate the NAS (2016) based design predictions with the buckling stress results of CoSFSM, and the existing MSR suggested in NAS (2016). This is the first time the exact buckling stress solutions are used to evaluate the NAS (2016) recommendation for the nested built-up columns subject to flexural buckling. The proposed ISR will help the design engineers obtain accurate flexural buckling strengths of CFS built-up nested columns.