07.07: Numerical study of screw fasteners in built‐up CFS chord studs

Abstract

ABSTRACTThis paper presents the development of finite element (FE) models for advanced analyses of screw connected and axially‐loaded back‐to‐back, built‐up cold‐formed steel (CFS) chord studs. Results are validated using column test data from displacement‐controlled experiments conducted at Johns Hopkins University. Shell finite element‐based models have been assembled in ABAQUS and include non‐linear geometric, material, and contact behavior. A unique feature of the developed models is the simulation of screw‐fastener connections using a user‐defined element (UEL), which can reproduce strength and stiffness deterioration under a monotonic load as well as the pinched response that occurs in the shear behavior of these connections when subjected to cyclic loading. Connection stiffness and strength data from pure shear tests of steel‐to‐steel and steel‐to‐oriented strand board (OSB) connections are used in the formulation of the subroutine and is discussed herein. Seventeen monotonic and concentric compression tests on two different built‐up CFS cross‐section sizes with varying fastener layouts and sheathing conditions were simulated. The buckling, peak, and post‐peak behavior of the built‐up columns is examined. Good agreement was reached among experimental and numerical results in terms of load‐deformation histories, as well as buckling and failure modes. 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 built‐up columns in common sheathed configurations; further, the response of screw fasteners remains elastic up until the post‐peak regime, revealing conservatism in the built‐up column fastener layout currently required by AISI S100‐16 section I1.2. The goal of this study is to improve 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 of columns is sought so that chord stud buckling limit states are captured in simulations of shear walls.