3D Collapse Simulation of Concrete-Filled Steel Tube Columns through Multi-Axis Cyclic and Hybrid Simulation

Concrete-filled steel tube (CFT) columns have been widely used in low and high seismic regions. These elements combine the high strength and ductility of steel with the ability of concrete to efficiently carry compressive loads. Numerous experimental studies have been performed to examine the behavior of CFT columns under pure axial or combined axial-lateral loads. Due to testing difficulties, however, limited data is available on the response of CFT columns under complex time-varying six-degrees-of-freedom (6-DOF) boundary forces during seismic events. In this paper, experimental studies are conducted to investigate the three-dimensional seismic response of CFT columns from the onset of damage to the state of complete collapse. The experiments include a series of large-scale quasi-static cyclic and hybrid simulation (HS) tests carried out on square and circular CFT columns. In the quasi-static (QS) tests, the specimens are subjected to bidirectional lateral deformation reversals that follows the hexagonal orbital pattern suggested in FEMA 461, combined with varying axial load. Hybrid simulation (HS) is then used to provide more insight into the response of these elements under realistic scenarios of seismic events. In the hybrid model, each test specimen serves as the first story column of a symmetrical 5x5 bay 5-story framed building that is subjected to sequential biaxial ground motions with increasing intensities to collapse. The hysteretic response behaviors obtained from the QS and HS tests are then used for calibrating the analytical models employed in a comparative fragility analysis.