Theoretical and experimental study on axial compression concrete-filled tubes with different confinements

In order to investigate the mechanical properties of concrete-filled tube columns with a wider range of confinement factors under axial compression, 14 specimens, including plain concrete columns and concrete-filled PVC, plexiglass and steel tube columns are subjected to axial compression. The axial load-displacement curves and damage process of the columns are analyzed, and the results indicate that the ultimate axial compressive strength of the concrete-filled tubes increases as the confinement factor ascends. The stiffness and deformation capacity of columns with lower hoop restraint increase as the confinement factor ascends. The axial load-displacement curve of concrete-filled steel tube (CFST) columns with higher confinement factors can be divided into five stages, and the secondary strengthening effect of CFST columns is discovered and explained. The nominal Poisson's ratio of the steel tube under triaxial stress is derived by the elastic property and the geometric parameters of the concrete and tube. The local initial yield of the steel tube is discovered based on the plastic yield theory, which precedes the entire yield of the CFST columns. Theoretical formulas of the ultimate bearing capacity for the CFST columns based on the four yield criteria are presented and the present results are compared with the values calculated by the typical codes and the experimental results. The value calculated by Tresca yield criterion is more conservative and safer, so it is an appropriate reference for the lower limit design of axial compressive capacity of CFST columns. [Display omitted] •The secondary strengthening effect of concrete-filled steel tube columns is discovers and explain with higher confinement factors.•The nominal Poisson''s ratio of the steel tube under triaxial stress is derived.•The theoretical formulas of the ultimate bearing capacity for the CFST columns based on the four yield criteria are proposed.