Simulation of screw connected built-up cold-formed steel back-to-back lipped channels under axial compression

Abstract

The objective of this paper is to validate finite element (FE) modeling protocol for screw connected, back-to-back built-up cold-formed steel (CFS) columns. The protocol is developed and validated using results from previously conducted experiments. The effort is motivated by two applications: (1) to augment experimental findings on built-up CFS columns, particularly for fastener demands and (2) to provide a simulation path for modeling the built-up CFS columns that are used as shear wall chord studs. Shell FE-based models were created in ABAQUS and include monotonic loading, nonlinear geometric and material behavior, geometric imperfections based on laser scanned measurements of tested specimens and a contact model that includes friction. Additionally, the screw fasteners were integrated into the modeling protocol using user-defined element subroutines capable of reproducing strength and stiffness deterioration under monotonic load as well as the pinching that occurs when screw fasteners are subjected to cyclic loads. Monotonic, concentric compression tests on 17 back-to-back CFS columns using two cross-section sizes and varying fastener layouts with sheathing conditions were simulated. Deformations, strength, and collapse mechanisms obtained by the models were in close agreement with the experimental results. An assessment of the loading demand on screw fasteners reveals the conservatism in built-up column fastener layout and design as required by the North American Specification for the Design of Cold-Formed Steel Structural Members (AISI S100-16 Section I1.2). Also, under the tested semi-rigid column end conditions, there is a little boost in axial capacity with the addition of member end fastener groups (EFGs) at the top and bottom of the columns. Numerical models are also used to assess the cyclic performance of axially-loaded columns so that chord stud buckling limit states could be captured in seismic simulations of CFS-framed shear walls in future work.