Abstract
The concrete-filled steel tube (CFT), especially in case of a circular section, reportedly has a large confining effect on filled-concrete by circular steel tube. In this paper the interactive mechanism between concrete and steel tube under axial force is discussed considering the diagonal sliding failure with its localization in concrete. CFT is classified as a conventional "Bond" and the recently proposed as "Unbond". It is shown that the failure mechanism of "Bond" can be well explained by analytical model of "Unbond". This analytical model of concrete is characterized by the ultimate strength based on Mohr-Coulomb criterion and the falling branch due to the decreasing friction coefficient in rigid-plastic model.
