Numerical Study on the Behavior and Design of Screw Connected Built-up CFS Chord Studs

This paper presents the development and validation of a finite element (FE) modeling protocol for screw connected, back-to-back built-up cold-formed steel (CFS) columns using results from experiments conducted at Johns Hopkins University. The shell finite element-based models have been constructed in the ABAQUS FE software and include non-linear geometric, material, and contact behavior. A unique feature of the model is the inclusion of a User Element subroutine (UEL) for the screw-fastener connections. This UEL can reproduce strength and stiffness deterioration under monotonic load as well as the pinching effect that occurs in the shear behavior of steel-to-steel and steel-to-wood connections when subjected to cyclic loading. Sixteen monotonic, concentric compression tests on two different built-up CFS cross-section sizes with varying fastener layouts and sheathing conditions were simulated. Good agreement is achieved between experimental and numerical results in terms of the prediction of strength and limit states. Based on a parametric study, the results indicate that under the tested end boundary conditions there is no significant boost in axial capacity with the addition of member end fastener groups at the top and bottom of the columns. Furthermore, the assessment of the loading demand on screw-fastened connections reveals the conservatism in built-up column fastener layout and design as required by AISI S100 (2016) section I1.2. The goal of this study is to examine the buckling, peak and post-peak behavior of built-up CFS columns, with both an experimental and a numerical approach to improve or augment existing design guidelines in which all relevant failure modes are considered in the design of built-up CFS columns. In addition, the characterization of monotonic and cyclic behavior is sought such that chord stud buckling limit states could be captured in seismic simulation of CFS-framed shear walls.

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