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Concrete-filled tube (CFT) columns consists of a steel tube filled with concrete. The concrete core adds stiffness and compressive strength to the tubular column and reduces the potential for inward local buckling. Conversely, the steel tube acts as longitudinal and lateral reinforcement for the concrete core helping it to resist tension, bending moment and shear and providing confinement for the concrete.
Due to the benefit of composite action of the two materials, the CFT columns provide excellent seismic event resistant structural properties such as high strength, high ductility and large energy absorption capacity. Also, circular hollow sections possess many advantages over open sections, including aesthetic appearance and economy in terms of material costs.
Due to the complexity of connections between steel beams and circular hollow sections, their use in structural steelwork is limited. This is because the use of standard bolting is not feasible and costly unpopular welded connections are the normal solution.
Concrete-filled steel tube (CFT) columns combine the advantages of ductility, generally associated with steel structures, with the stiffness of a concrete structural system.
The advantages of the concrete-filled steel tube column over other composite systems include:
- The steel tube provides formwork for the concrete,
- The concrete prolongs local buckling of the steel tube wall,
- The tube prohibits excessive concrete spalling, and
- Composite columns add significant stiffness to a frame compared to more traditional steel frame construction.
While many advantages exist, the use of Concrete filled Tubes in building construction has been limited, in part, to a lack of construction experience, a lack of understanding of the design provisions and the complexity of connection detailing. Consequently, a joint is needed that could utilize the favourable strength and stiffness characteristics of the concrete-filled tube column yet be constructible.