Experimental and numerical studies of S700 high strength cold-formed steel built-up box section columns

Cold-formed high strength steel structural members are considered effective structural design solutions, owing primarily to their excellent strength-to-weight ratios. Built-up box sections, formed by interconnecting two channel sections face-to-face with self-tapping screws, provide geometry flexibility of high strength cold-formed steel members and further improve the member strength. In this paper, experimental and numerical studies were conducted to explore the load-bearing capacity and flexural buckling behaviour of S700 high strength cold-formed steel built-up box section columns. The testing program included material tests, measurements of initial geometric imperfections, and ten pin-ended column tests. The numerical modelling program involved establishing and validating finite element models, followed by parametric analyses using the validated numerical models to generate a total of 202 numerical datasets with varying cross-sectional dimensions and member lengths. These data were employed to assess the applicability of the existing European and American design standards for S700 high strength cold-formed steel built-up box section columns. The evaluation results revealed that both design standards led to conservative ultimate capacity predictions for intermediate columns, with the American design standard yielding more accurate predictions. Regarding slender columns, both design standards resulted in some unsafe ultimate capacity predictions. To address these limitations, an improved design approach was proposed by modifying the provisions set out in the American standard. The proposed design approach was shown to be capable of accurately predicting the ultimate capacity of S700 high strength cold-formed steel built-up box section columns than existing design codes, with its reliability verified.