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 the elastic flexural buckling stress prediction of CFS built-up columns. The existing MSR applies to sections in which the screw fastener will experience shear, and hence this recommendation excludes the use of nested built-up columns. Thus, an improved slenderness ratio (ISR) has been 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 based computational tool CoSFSM has been used as the basis. The CoSFSM has been 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 performed in this paper. For this, a finite element model is developed and validated with the test results of authors and those available in the literature on CFS built-up nested long 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). First time in literature, the exact buckling stress solutions are used to evaluate the NAS (2016) recommendation for the nested built-up columns subject to global buckling. The proposed ISR will help the design engineers to obtain the accurate strength of CFS built-up nested columns.

Learning Objectives:
The modified slenderness ratio (MSR) of NAS (2016) can only be applied to specific types of cold-formed steel built-up section columns in which the screw fastener will experience shear during flexural deformations.

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