Tests of cold-formed steel built-up open section members under eccentric compressive load

An experimental study was performed on cold-formed steel built-up open section members under eccentric compressive load. The specimens with member lengths varied from 300 to 1500 mm were manufactured by composing two identical channels using self-tapping screws. The channels with longitudinal stiffeners were fabricated from the steel grades G500 and G550 zinc-coated sheets with nominal 0.2% proof stresses of 500 and 550 MPa, respectively. A total of thirty-three combined minor axis bending and compression tests were carried out under pin-ended supports to explore the buckling behaviour of the newly designed built-up section members. Six test series with a relatively wide range of axial load-to-moment ratio were included in this study to examine the interaction relationship of the built-up section beam-columns. The test loading capacities, full-history responses and failure modes were obtained for all the test specimens. Since the novel built-up open sections are not covered in the current cold-formed steel design standards, the comparisons of experimental loading capacities with nominal strengths predicted by the North American Specification, Australian/New Zealand Standard, European Code and American Specification were conducted to assess the appropriateness of the existing design rules. The applicability of axial load-moment interactive formulae specified in the aforementioned design standards with the nominal pure compressive resistance and nominal pure bending resistance determined by direct strength method was evaluated for the built-up section beam-columns. It is found that the axial load-moment interactive formulae generally underestimate the strengths of the cold-formed steel built-up open section beam-columns.