Design of cold-formed steel built-up closed sections with intermediate stiffeners

A series of column tests on cold-formed steel built-up closed sections with intermediate stiffeners is presented in this paper. The test specimens were first brake pressed from structural steel sheets to form open sections with intermediate web stiffeners, then two of the open sections were connected at their flanges using self-tapping screws to form the built-up closed sections. The high strength structural steel sheets had the measured 0.2% proof stress up to 586 MPa. Initial and overall geometric imperfections as well as material properties and residual stresses of the test specimens were measured. Tests were performed over a range of lengths such that column curves could be obtained. The test strengths are compared with the design strengths calculated using the direct strength method in the North American Specification and Australian/New Zealand Standard for cold-formed steel structures. Three different methods were used to obtain the local and distortional buckling stresses for the calculation of the direct strength method. Reliability analysis was performed to assess the reliability of the direct strength method on cold-formed steel built-up closed section columns. It is shown that the direct strength method using single section to obtain the buckling stresses is generally conservative.