Flexural behavior of nonposttensioned and posttensioned concrete-filled circular steel tubes

Concrete-filled steel tubes (CFTs) have recently been used for columns and bridge piers, especially in seismic regions, for their enhanced strength and ductility over the conventional steel and concrete construction. This enhancement is due to the composite actions of the concrete infill and the steel tube: The concrete compressive strength is significantly increased by the lateral confinement provided by the steel; and local buckling of the steel tube is delayed by the concrete infill. Similar enhancement has also been observed in flexural strength of CFT. A novel concept of posttensioning the concrete core inside circular steel tubes to further enhance the synergistic interactions has been developed, and numerical analyses have shown significant increase in their flexural strength over nonprestressed CFT. A numerical algorithm has been developed for predicting the moment capacities of circular CFT. The increased concrete strength and ductility due to confinement were taken into account. The analytical predictions compared well with flexural test data. The validated algorithm was further developed to include the posttensioning effects. Numerical results suggest that moment capacity of a CFT can be significantly increased by prestressing the concrete core with only moderate posttensioning forces.