Uni-axial behaviour of externally confined UHSCFST columns

Thanks to the larger and more continuous confining stress provided by hollow steel tube, ultra-high-strength concrete-filled-steel-tube (UHSCFST) columns are advocated rather than conventional transverse steel confined ultra-high-strength concrete. However, during elastic stage, de-bonding occurs as the Poisson's ratio of steel is larger than that of concrete, whereas in post-elastic stage, confining stress, strength and ductility of concrete decrease since the steel tube would buckle. Thus, the composite action cannot be fully developed. To overcome these problems, external confinement in the form of tie bars, steel rings and steel spirals are proposed in this paper to increase the confinement effect of UHSCFST columns. A series of uni-axial compression test has been conducted to study the behaviour of externally confined UHSCFST columns. Results showed that the proposed confining schemes can effectively improve the strength, elastic stiffness and ductility of UHSCFST columns. Moreover, rings and spirals are more effective than tie bars. Lastly, a modified theoretical model based on that developed previously by the authors was used to predict the ultimate load and axial load-strain behaviour of the tested columns, in which excellent agreement between the theoretical and experimental results has been obtained.