Revisiting the composite action in axially loaded circular CFST columns through direct measurement of load components

Abstract

The composite action in circular concrete-filled steel tube (CFST) columns under axial compression is one of the most fundamental problems in the research area of steel-concrete composite structures. However, it cannot be easily investigated through experiments as the load components of the steel tube and concrete infill cannot be measured in a conventional axial loading test. In this study, an innovative testing method was devised to directly measure the load components of circular CFST columns under axial compression. Six innovative specimens with varying concrete strength and diameter-to-thickness (D/t) ratio and their conventional counterparts were tested. Although some difference existed between the load-deformation responses of the innovative and corresponding conventional specimens, it was essentially insignificant. The development of hoop stresses in the steel tube tends to reduce the corresponding axial stresses, but this effect can be offset by the strain hardening of the steel tube. Strain hardening of the steel tube should be considered when evaluating the confinement effect at the ultimate load, especially for columns with low concrete strength and small D/t ratio. The average values of the coefficients related to the axial and hoop stresses of the steel tube at the ultimate load, computed using available tests results, are 0.92 and −0.14, respectively. The proposed axial load capacity formula with these two values can reasonably predict the ultimate loads of circular CFST columns under axial compression.