This study investigates the structural behavior of an innovative cold-formed steel (CFS) modular built-up section featuring an interlocking, self-stiffening closed geometry. The proposed built-up section consists of four S-shaped elements oriented, assembled, and screwed to form a unified built-up cross section without the need for external jigs or fixtures. A total of 30 specimens, varying in thickness, slenderness ratio, and screw spacing, were fabricated, scanned for geometric imperfections, and tested under axial compression. Additionally, 3D digital image correlation (3D-DIC) was employed during testing to accurately characterize deformation patterns and failure modes. The experimental program revealed four distinct local buckling patterns influenced by the unique cross-sectional closed geometry, width-to-thickness ratio, and screw spacings. Test results demonstrated a delayed local buckling and a substantial increase in both load-carrying capacity and stiffness, attributed to the inherent self-stiffening behavior of the section. Comparison with AISI S100 predictions showed that for local slenderness ratios λl1.5, this ratio exceeded unity for the majority of specimens (16 out of 19). The subsequent reliability analysis showed notable deviations, with only six test results within 0.95<PT/Pd<1.05 and an overall coefficient of variation of 0.15, indicating significant scatter and a maximum deviation of 48%, thereby highlighting the need for refinement of current provisions to more accurately capture the structural behavior of innovative closed sections used in modular construction. Based on the observations, it is recommended that DSM equations be extended to explicitly account for the effects of element overlap due to screw spacing and midpanel stiffening on local and global buckling behavior.
Sivakumar et al. (Fri,) studied this question.
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