Study on equivalent confinement coefficient of composite CFST column based on unified theory

This article introduces the Unified Theory of Concrete Filled Steel Tube (CFST) into counting strength of composite CFST columns subjected to axial compression. There were four types of composite CFST columns listed, and one equivalent confinement coefficient was proposed with consideration of different sections of confining materials, so the whole cross section can be taken as one new composite material to research axial compressive behaviors of the composite CFST column. One unified design formula was derived from the Unified Theory, which was suitable for solid or hollow, circular or non-circular various cross sections of the composite CFST columns. The ultimate compressive bearing capacity was calculated in comparison with corresponding test results. The results show that the equivalent confinement coefficient can be used to calculate the compressive strength of the composite CFST column, and it has a linear relation with the enhanced coefficient of composite strength. Thus, the equivalent confinement coefficient was one important parameter to analyze the mechanical behavior and it can clearly reflect the confinement between concrete and confining material. This method is one more convenient and simpler approach for design work of composite CFST structures.