Compressive behavior of a novel cold-formed steel built-up box section: Tests, modelling and design

• A total of 48 compression tests were conducted on novel CFS built-up box section columns. • The numerical simulations were used to assess the effect of screw spacing on the ultimate strength of the CFS built-up columns. • The EWM and the DSM were evaluated for determining the ultimate strength of the CFS built-up columns. This paper investigates a novel built-up box section constructed from single cold-formed steel (CFS) C- and U-sections, which are connected using self-drilling screws along their extended flanges. Both experimental and numerical analyses were conducted to study the buckling behavior and ultimate strength of these pin-ended, axially loaded CFS built-up box section columns. The study focuses on key parameters such as thicknesses (0.95 mm and 1.15 mm), screw spacing (300 mm and 600 mm), and slenderness ratios (42, 72, 102, 132). A total of 48 specimens were tested under axial compression, exhibiting significant local-distortional and local-flexural interactive buckling modes. Finite element models were developed using ABAQUS software and validated based on the experimental results. The numerical simulations were used to assess the effects of screw spacing and thickness on the ultimate strength of the built-up box columns. The ultimate strengths obtained from both the experiments and the finite element analysis were then compared to the predictions made using the Effective Width Method (EWM) and the Direct Strength Method (DSM), as outlined in AISI S100 (2016) and AS/NZS 4600 (2018). The comparison shows that DSM overestimated the axial capacity of these novel built-up box section columns by an average of 15 %, while EWM was overly conservative, with mean experimental-to-design strength ratios exceeding 2.5.