The application of composite spiral stirrups confined reinforced concrete columns can improve the core concrete strength and realize high ductility

which can be utilized to design and construct important structures in seismic zones of high intensity. A total of 6 concrete columns confined by composite spiral stirrups were tested under eccentric loading with different stirrup spacing and axial force eccentricities

and the results were compared with other two conventional composite stirrups confined columns under the same condition. The relationships of axial force with lateral deformation

stirrup strain

and compressive strain at the edge of the compression zone were obtained. Failure modes of longitudinal reinforcement yield and concrete crushing of the columns occurred successively under different spacing and eccentricities. Test results indicate that when the spacing is smaller than 80 mm

the composite spiral stirrups confined columns exhibited higher bearing capacity than conventional columns

the strength of the confined concrete is increased

and the defined displacement ductility is improved. Furthermore

the dual-confinement to the concrete consisted of core confinement by spiral stirrups and surrounding confinement by rectangular stirrups had significant effects on failure modes. The failure of the specimen was mainly caused by crushing and spalling of the concrete after buckling of longitudinal reinforcement. It was observed in the tests that the rectangular stirrups continuously provided a superior confinement effect on concrete even after the peak load while the spiral stirrups lost its efficacy. Based on the failure mechanism and the contribution of two different transverse reinforcement

spiral stirrups and rectangular stirrups

a calculation formula for eccentric bearing capacity of composite spiral stirrups confined concrete columns was proposed.