Study on circular CFST stub columns with double inner square steel tubes

Abstract

This paper presented axial compression tests on the behavior of the circular concrete filled steel tube stub columns with double inner square steel tubes (DIS-CFST). A total of 27 DIS-CFST stub columns with different eccentricity ratio (e), hollow area ratio (ψ) and concrete compressive strength (C) were tested. The study mainly investigated the failure modes, ultimate bearing capacity, load-strain curves, load-displacement curves, initial stiffness at elastic stage (ki) and ductility of the DIS-CFST stub column. The failure mode of DIS-CFST stub columns observed from the tests was local buckling and no crack was observed on the surface of the specimens. It was found that concrete compressive strength had remarkable influence on ductility and ultimate bearing capacity of DIS-CFST stub columns. However, eccentricity ratio had little influence on the behavior of DIS-CFST stub columns. Experimental results showed that when the hollow ratio was small, it had little effect on the ultimate load-carrying capacity of DIS-CFST stub columns. However, the ultimate bearing capacity of specimens significantly decreased when hollow area ratio was large. The eccentricity ratio, hollow area ratio and the concrete compressive strength had slight influence on initial stiffness at elastic stage. Furthermore, the ductility of DIS-CFST stub columns increased with the decrease of hollow area ratio. A simplified formula was proposed to calculate the ultimate bearing capacity of DIS-CFST stub columns. The predicted ultimate bearing capacities were in good agreement with DIS-CFST stub column test results. By comparing with the existing strength calculation formula, it can be concluded that the ACI code, EC4 and Han's method were more satisfied with the requirements of the design.