Circular concrete-encased concrete-filled steel tube (CFST) stub columns subjected to axial compression

Circular concrete-encased concrete-filled steel tube (CFST) consists of the circular inner CFST and circular outer reinforced concrete. In this paper, the axial compressive behaviour of the composite column is experimentally and analytically investigated. Ten concrete-encased CFST specimens, four reference CFST specimens and two reference reinforced concrete specimens were tested; the parameters include the ratio of the steel tube diameter to the column diameter and the concrete strength. The influence of these parameters on the axial load in relation to axial strain, ultimate compressive strength and ductility is discussed. A finite-element model is established, where a stress against strain model is suggested for the concrete confined by circular stirrups located outside the steel tube. The predictions are verified against the test results of the stub columns. Analysis is then performed to investigate the internal force distribution and stress development during the loading process. Parametric analysis is conducted to discover the influence of critical parameters. Finally, simplified superposition methods are suggested to predict the ultimate compressive strength.