Behavior of axially loaded back-to-back cold-formed steel built-up channel sections

Abstract

Back-to-back built-up cold-formed steel channel columns are being increasingly used by cold form steel structures, such as trusses, wall frames and portal frames. In such an arrangement, independent buckling of channel-sections can be prevented by using intermediate fasteners, at discrete points along the length. Current guidelines in the North American Standard (NAS) for such back-to-back built-up cold-formed steel channel sections requires the use of a modified slenderness approach in order to take into account the spacing of the screws. This method, however, is well-known to be conservative. This paper presents the results of 60 tests performed on back-to-back built-up cold-formed steel channel sections. Test results are then compared against the design strengths calculated in accordance with AISI and the Australian and New Zealand standard. It is shown that the AISI and the Australian and New Zealand Standards are un -conservative for columns sections failed by local buckling whereas standard under estimate the strength of intermediate and long columns which were failed mainly by overall member buckling for the both the section BU 75 and BU 90 considered in the investigations.