Flexural behaviour of back-to-back built-up cold-formed steel channel beams: Experiments and finite element modelling

In cold-formed steel (CFS) structures, the application of back-to-back built-up channel beams is increasing steadily. The increasing demand of CFS sections as a cost-effective design solution has led to the development of CFS built-up sections. These sections take advantage of higher load-carrying capacities and may be used for longer beam spans. Applications include beam members in CFS wall studs, truss members and floor joists. In such an arrangement, the intermediate screw fasteners at discrete points along the length prevent the individual channel sections from buckling independently. However, limited experimental or finite element (FE) analyses have been reported in the literature for back-to-back built-up channel sections under four-point bending. Furthermore, there is limited investigation of the effect of screw spacing for flexural strength. This paper addressees this issue with 12 experimental test results. Two different cross-sections, referred to as BU75 and BU90, were considered with different screw spacings of 1900 mm, 950 mm and 475 mm. Initial geometric imperfections were measured prior to the bending tests. A nonlinear elasto-plastic FE model was then developed, the results of which showed good agreement against the experimental test results. Using the validated FE model, an extensive parametric study was conducted to investigate the effect of screw spacing on flexural strength of such back-to-back built-up CFS channel beams. In total, the results of 108 parametric analyses are reported. Both the experimental and FE results were compared against the design strengths calculated in accordance with the American Iron and Steel Institute (AISI) and Australian and New Zealand Standards (AS/NZS).