Experimental investigation on the axial compression behavior of cold-formed steel triple-limbs built-up columns with half open section

Cold-formed steel triple-limbs built-up columns with half open sections, which are generally used in shear wall chords, corner column of exterior walls, door and window opening frame jambs, and other parts that need to be strengthened, are becoming increasingly popular. At present, no systematic research is available for such section type. Therefore, this paper reports on a comprehensive experimental investigation addressing the buckling behavior of triple-limbs built-up columns affected by pure global, distortional, and local buckling modes. In total, the results from 54 experimental tests are reported. Different screw spacings were designed for each specimen with the same cross-sectional dimension and length in order to study its effect. In addition, the isolated shear performance of the screws used in the built-up columns was investigated through single shear screwed connection tests. It was observed that the G type, D type, and L type triple-limbs built-up columns were failed by pure global, distortional, and local buckling, respectively, and the longitudinal spacing of the screws did not seem to significantly affect the occurrence of the failure mechanism, but can affect the ultimate strength of the specimen. Finally, the ultimate strengths obtained in the test were compared against the predicted strengths calculated in accordance with the current direct strength method, which is provided in AS/NZS 4600 (2018) and AISI S100 (2016). The comparisons show that the direct strength method is accurate and reliable for the design calculation of pure buckling mode of triple-limbs built-up columns.