Concrete-filled steel tube columns and the confinement effect

Concrete-filled steel tube (CFST) columns are one of the pioneering types of composite columns and are increasingly used in applications that require the support of large loads such as tall buildings, bridges, and offshore structures. CFST columns are used due to their high strength, high stiffness, high ductility, and high energy absorption capacities besides high speed of construction, relatively low economic cost, and compatibility with different types of connections. These advantages result due to the composite action between the steel and the concrete where the concrete and the steel act together to combine the best features of both the steel tube and concrete to yield efficient performance. The steel tube confines the infilled concrete and serves as a perpetual formwork. Also, it works as longitudinal and transverse reinforcement. The circular cross-section offers enhanced confinement to the concrete compared to other shapes. This thesis examines the conduct of circular CFST columns and the confinement effect with different types of strengths of concrete. In addition to classical CFST columns, a new configuration, concrete-filled double circular steel tube (CFDCST) stub columns were also studied, which provides an effective improvement in compression, ductility, and stiffness capacities relative to classical CFST columns of the corresponding size. Furthermore, the possibility of the CFDCST column concept for repairing deformed CFST columns was examined and gave good results. For assessment, test results were compared with the estimates of the AISC360-16, EC4, AS5100.6, and AIJ2001 design specifications.