Buckling Behavior of Built-up Cold-Formed Steel un-lipped rectangular Channel column Sections Under Axial compression

Abstract

Cold-formed steel structures can be used extensively in the building industry, either as complete structures in low to mid-rise construction, or in combinations with hot-rolled or fabricated steel framing. Cold formed steel (CFS) has been used as the primary structure for flexural and compression load-bearing members such as bridges, roof trusses, transmission towers, and other multi-storied buildings due to varieties of advantages such as high strength to weight ratio, high corrosion resistance, and ease of fabrication. Dual-channel sections, built-up of CFS into new member such as back to back without gap, back to back with gap, laced, battened or perforated plates with higher strength can be produced efficiently by attaching the CFS to each other at few places along the lengths using connectors. Built-up cold-formed steel sections commonly used as compression members to carry heavier loads and over longer spans when a single individual section is insufficient. These built-up columns improve the lateral stiffness when two individual channels shapes connected together thus preventing the structural member from wobbling during lifting and installation. Un-lipped rectangular CFS channel sections with constant size has been used to produce built-up, back to back (BTB), and face to face box-up (BU) with 6(0.6 to 3meter) varieties of length. The double built-up back to back connected on the web and face to face sections connected at flanges were built-up from two identical un-lipped rectangular channel-sections attached with surface to surface tie constraint. The connector spacing is constant along the length of the column at 400 mm centre to centre with 5x5mm square surface area contact size. Concentrated centric unit load was applied at the centre of gravity in the reference point at the column top end. In total of 18 columns, 6 single, 6 back to back and 6 face to face built-up CFS un-lipped rectangular channel sections of size 200x80x4-section were modeled and analyzed on the axial capacity and mode of failures, and reported herein. The ultimate loads were compared to the experimental test results. FEA simulation was conducted in ABAQUS 6.13-1/ABAQUS CEA software for different values of slenderness ratios covering from short to long columns on the axial capacity of single and built-up CFS channel sections. Initial imperfections were not considered for both single and built-up CFS sections. Load-axial capacity, failure modes, and deformed shapes at failure were discussed for all CFS columns. Because of the low thickness to width ratio, the members buckled at stresses that are lower than the yield stress when compressive forces are applied. Analysis results show that all short columns failed through local buckling. However, for long columns global buckling was observed. The structural response of the built-up CFS columns composed of two identical un- lipped channels in this study was significantly affected by the member’s slenderness ratio and channels orientation. The orientation of the column also substantially impacts the ultimate load and buckling mode shapes of the dual built-up CFS columns