Axial and lateral stress-strain model for circular concrete-filled steel tubes with external steel confinement

In a concrete-filled steel tube, delamination often occurs at the steel tube-concrete core interface due to the larger Poisson's ratio of the steel tube. For resolving this problem, it has been advocated to install external restraints in the form of steel rings or spirals so as to restrain the lateral expansion of the steel tube. This also provides additional confinement to the concrete core. To study the effectiveness of such external steel confinement, a theoretical model for evaluating the confining stress and axial load in a concrete-filled steel tube with external steel confinement up to the post-peak stage is developed. The theoretical model is first verified by analyzing a total of 98 specimens tested by previous researchers and comparing the measured and predicted lateral strain-axial strain curves and axial load-strain curves of the specimens. It is then used to perform a parametric study to evaluate the required equivalent thickness/diameter ratios of the external steel confinement for eliminating the delamination effect and for achieving Level I ductility.