Axial strength and ductility of square composite columns with two interlocking spirals

• Square composite columns confined by two interlocking spirals are proposed.
• Test verified specimens achieved excellent axial strength and ductility.
• Analytical approach confirmed concrete confinement effect.
• Two interlocking spirals in a square composite column appear to be very affirmative.

The axial compressive capacity and load–displacement behaviour of composite columns confined by two interlocking spirals were experimentally and analytically investigated. The innovative spiral cage used for a square column was fabricated by interlocking a circular spiral and a star-shaped spiral to enhance the confinement effect for the core concrete. Eight full-scale square composite columns were tested under monotonically increased axial compression. Experimental results demonstrated that, with significant savings of the transverse reinforcement, the composite columns confined by two interlocking spirals achieved excellent axial compressive strength and ductility capacity. Moreover, an analytical model was developed to take into account the concrete confinement due to the structural steel in addition to the transverse reinforcement and distributions of the longitudinal bars. The analytical results accurately predicted the axial compressive capacity and load–displacement behaviour of the specimens. Consequently, the application of the two interlocking spirals in a square composite column appears to be very affirmative.