An analysis-based model for axially loaded circular CFST columns

Concrete-filled-steel-tube (CFST) columns have been increasingly adopted in many modern structures due to the beneficial composite action between steel tube and core concrete. Previous experimental and theoretical studies have proved this composite action can be further improved by providing external confinement in the form of steel rings, tie bars, spirals or jackets. A theoretical model developed by the authors previously based on (1) an accurate hoop strain equation; (2) an actively confined concrete model; (3) a three-dimensional steel model; (4) Interaction of core concrete, steel tube and external confinement was adopted in this paper to conduct a parametric study, which aims at investigating the effects of steel tube yield strength, concrete compressive strength, steel ratio and external confinement on the uni-axial behaviour of CFST columns. From the results in parametric study, two sets of critical steel ratios based on two different levels of ductility at different steel yield and concrete compressive strengths have been obtained. In addition, the maximum column strength obtained from the model and previous experimental studies was compared with the design strength calculated using different design codes. New design equations that can predict the axial strength more satisfactorily have also been proposed.