Axial Compressive Behavior of Square Spiral-Confined High-Strength Concrete-Filled Steel-Tube Columns

A new type of concrete-filled steel plate (CFSP) composite shear wall that employs high-strength concrete (HSC) and square spiral-confined concrete-filled steel tube (SCCFST) boundary elements was proposed to reduce the core wall thickness of skyscrapers. Since the axial compressive behavior of the SCCFST boundary elements is essential to the overall seismic performance of the proposed composite shear wall, studies on square SCCFST members with HSC were first conducted to lay a foundation for further studies on the composite shear walls. Twelve SCCFST specimens and two conventional concrete-filled steel tube (CFST) specimens with the concrete compressive strength of 112 MPa were tested. The test variables included the amount and yield strength of the spiral and the ratio of the spiral-confined area to the total concrete area. The axial load capacities of the SCCFST specimens were essentially not increased, whereas the postpeak performances of the SCCFST specimens were significantly improved in comparison with the corresponding CFST specimen. The spirals in all the SCCFST specimens ruptured several times during the test, and each spiral rupture induced a considerable drop in the axial load of the specimens with high-strength spirals. The trend of the ductility of the concrete in SCCFST columns can be well captured by the spiral confinement index which reflects the effects of both the amount and strength of the spiral. Design expressions were derived based on the test results to determine the amount of spirals required for the boundary elements of the proposed composite shear wall using 110 MPa concrete. (C) 2020 American Society of Civil Engineers.