Structural behavior of built-up I-shaped CFS beams

The use of back-to-back built-up C-section beams is becoming increasingly common in CFS construction due to their cost-effectiveness and enhanced load-carrying capacity, making them suitable for longer beam spans and convenient for transportation. These built-up sections are utilized in wall studs, truss components, and floor joists, with intermediate screw fasteners placed at specific intervals to prevent the separate bowing of channels. This study reveals a ratio of 1.003 between experimental findings and finite element analysis results, and 1.002 between experimental findings and direct strength method results, indicating a strong correlation between experimental data from nonlinear finite element analysis and predictions based on the American Iron and Steel Institute and Australian and New Zealand Standards, particularly in predicting the flexural buckling strength of beam specimens. Furthermore, ongoing research is investigating the impact of screw spacing on flexural strength. This study presents results from 175 finite element tests, evaluating seven distinct cross-sections with twelve unique screw spacings. These spacings correspond to the half wavelength of local, distortional, and global buckling, divided by values from one to four. It was found that screw spacing based on half the local buckling half-wavelength along the centerline of the webs increased the critical global buckling moment capacity and the nominal flexural strength by 56 % and 27 %, respectively. For double-lane screws with the same spacing, these increases were even more substantial, reaching 65 % and 31 %, respectively. Economically, the recommended spacing for single-lane screws is half the local buckling half-wavelength.