Experimental and numerical investigation on axial behavior of circular and square slender reinforced columns

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To investigate the axial performance of slender concrete-filled steel tube (CFST) columns reinforced by outer steel tube and sandwiched concrete jackets, an axial compression test was conducted on 20 circular and square slender reinforced columns. The study aimed to examine the influence of the cross-sectional shape, steel content of the outer tube, the strength of sandwiched concrete, and slenderness ratio on the failure mode, load-displacement curves, and ultimate load. A comparative test on six slender CFST columns was also conducted. The test results showed that the local buckling degree of the outer steel tube in square reinforced columns was greater than that in circular reinforced columns, while the overall bending deformation was smaller. The influence of the sandwiched concrete strength on the load-displacement curve was weaker compared to the influence of the outer tube thickness and slenderness ratio. As the slenderness ratio increased, both the longitudinal and lateral strains of the specimens tended to decrease. The difference in strain between the tension and compression sides was more pronounced in circular reinforced columns than in square reinforced columns. The average SI values of circular and square reinforced specimens were 1.05 and 1.00, respectively, but the gap in SI values between the two types of reinforced columns is reduced as the slenderness ratio increases. Then, a numerical model on the slender reinforced columns was established by the finite element software ABAQUS. A good agreement was obtained between test results and simulation results. The mechanical mechanism of typical slender reinforced columns was further analyzed, including longitudinal stress contour map of concrete, the development of longitudinal stress of steel tube, as well as distribution of transverse confinement stress. A formula was subsequently proposed based on a design code to predict the load-bearing capacity of the retrofitted columns, the prediction results agreed well with the experimental data.