Lateral Cyclic Performance of Initially Cracked Steel SHS Columns Strengthened with CFRP Wrapping Under Different Axial Load Ratios

Steel square hollow section (SHS) members are widely used in buildings, bridges, and industrial structures due to their high strength-to-weight ratio and torsional resistance. In steel SHS members, crack-like initial defects may occur due to environmental effects, especially corrosion, and these defects may adversely affect their performance. Although the cyclic behavior and stability of SHS members have been studied, previous investigations have mainly focused on uncracked members, while the response of initially cracked SHS columns under combined axial and cyclic lateral loading remains insufficiently understood. In this study, the lateral cyclic performance of steel SHS columns with initial crack defects was investigated, and a local retrofit strategy using CFRP wrapping was proposed. Eighteen specimens with wall thicknesses of t=2 and 4 mm were tested under cyclic lateral loading combined with nominal axial load ratios of 0, 0.15, and 0.30. Failure of the 2 mm cracked specimens was governed by local buckling, whereas the 4 mm cracked specimens exhibited fracture and combined fracture-local buckling. 2D-DIC analysis showed strain localization around the crack tips. Initial crack defects reduced the lateral load capacity by up to 5.4% and 13.0% for the 2 and 4 mm groups, respectively, whereas CFRP-wrapped cracked specimens exhibited capacities up to 25.6% and 5.4% higher than the corresponding uncracked specimens. This study provides a better understanding of the interaction among initial crack defects, section slenderness, axial load ratio, and local CFRP wrapping in the lateral cyclic behavior of steel SHS columns, thereby contributing to the further development of the theory of steel structures.