Local Buckling Restraining Behavior of Thin-Walled Circular CFT Columns under Seismic Loads

The local buckling restraining behavior of thin-walled circular concrete-filled tubular (CFT) columns is examined under seismic loads by conducting a bidirectional cyclic loading and a bidirectional shaking table test. According to the responses of the deformations and strains measured in the bidirectional cyclic loading test, the axial compressive force acting on the buckled part of a steel tube is reduced with the increase in its local buckling deformation because most of the compressive axial force is transferred from the steel tube to the in-filled concrete. The reduction in the compressive axial force in the steel tube slows the progress of the buckling deformation. In addition, under a cyclic load applied after the occurrence of local buckling, the opening and closing of major horizontal cracks and dilation occur in the in-filled concrete. As a result, a predominant tensile axial force acts repeatedly on the buckled part of the outer steel tube. This tensile force restrains or restores the local buckling deformations by stretching them. The magnitude of the tensile force is enhanced further if a diaphragm is installed on the steel tube at the upper surface of the in-filled concrete. The shaking table test confirms that the local buckling restraining behavior is similar under seismic accelerations. The shaking table test together with the numerical analysis illustrates that the ratio between the residual sway displacement ${\delta }_r$ and the maximum response sway displacement ${\delta }_m$, defined as ${\delta }_r/{\delta }_m$ for CFT columns, is generally much smaller than that for hollow columns because of the enhanced strength and ductility of CFT columns.