Investigations on buckling behavior of intermittently fastened cold-formed steel built-up columns using spline finite strip method

In load-bearing cold-formed steel (CFS) framed wall systems, built-up columns are preferred over single CFS sections for their improved structural performance. In this study, a numerical formulation is presented to compute the elastic stability of isolated and compound wall studs, braced intermittently to the sheathing. These bracings can alter the buckling behavior of the compound columns altogether, especially in the distortional and global buckling modes. The present study is towards improving the design provisions of built-up columns presented in AISI-S100 (2016) which suggests adopting a modified global slenderness ratio. Although, this is intended to accommodate the loss of shear rigidity due to discrete fastener spacing, no guidance is provided to accommodate this effect on other buckling modes. In the literature, approximate methodologies have been reported, but comprehension of the composite behavior of built-up sections is still missing. In this paper, a numerical methodology using compound spline finite strip method is developed to compute the elastic buckling stress of built-up steel columns braced with and without sheathing. A compound model is generated by adding the stiffness matrix of fasteners into system global stiffness matrix where stiffness matrix of the fasteners is computed by adopting a three-dimensional beam model with adjustable geometrical properties and stiffness of wall system is added to the model with the help of translational and rotational springs. All the results are compared with FE based software ABAQUS and results are found to be in good agreement. A clarity is brought out in this paper between the effects of restraints provided by the presence of wall sheathing on the overall performance of built-up wall studs in comparison to the unsheathed ones.