Effect of axial compression ratio on seismic behavior of hybrid FRP-steel reinforced concrete columns

Concrete columns reinforced with hybrid longitudinal steel-fiber reinforced polymer (FRP) composite bars (SFCBs) and FRP stirrups (hybrid-RC columns) are attractive for marine concrete structures due to exceptional corrosion resistance and mechanical properties. While the axial compression ratio can significantly affect the seismic performance and failure mechanism of conventional RC columns, its effect on hybrid-RC columns remains insufficiently explored. To address this gap, six full-scale specimens were tested under combined axial compression and cyclic lateral loadings to investigate the seismic performance of hybrid-RC columns. Fiber Bragg grating sensors embedded in SFCB were used to reveal failure modes and underlying mechanisms. Results indicate that hybrid-RC columns primarily exhibited small or large eccentric compression failures, depending on the axial compression ratio. Columns with large eccentric compression failure exhibited desirable seismic behavior in terms of ultimate displacement, ductility, energy dissipation, and post-damage repairability. To ensure this ductile failure, a calculation method for the boundary axial compression ratio was proposed through sectional analysis and validated against test data. Notably, the allowable axial compression ratio of hybrid-RC columns should be lower than that of conventional steel reinforced concrete columns. Our findings provide valuable experimental evidence and design recommendations for hybrid-RC columns.