Hysteresis performance and restoring-force model of RC columns jacketed by octagonal and spiral stirrups

The RC columns jacketed by an octagonal stirrup at the center and four spiral stirrups at the corners exhibited excellent seismic performance. The purpose of this study was to explore the hysteresis performance and establish the restoring-force model of RC columns jacketed by octagonal and spiral stirrups. Six concrete columns jacketed by octagonal and spiral stirrups were tested under lateral cyclic loads, and one-hundred eighty finite element analysis columns under lateral cyclic loads were carried out after verified by the test results. Based on the results, a design-oriented restoring-force model which consisted of skeleton curve models and hysteretic rules was proposed, which can evaluate the seismic load-bearing capacity and deformation conventionally. The characteristic points in the proposed model of the skeleton curve, which include the yield load, the peak load, the elastic stiffness, the elastic-plastic stiffness and the strength descent stiffness, were calculated by a set of equations by considering the influence of the axial compression ratio of the existing column, the shear span ratio of the jacketed column, the reinforced configuration of the jacket, the compression strength of concrete in the jacket, and the area ratio between existing column and the jacket. Furthermore, the hysteretic rules were developed for the unloading and reloading stages by the unloading stiffness formula, ignoring the strength degradation. The comparison of the experimental and calculated results illustrated that the proposed restoring-force model can predict hysteretic curves accurately.