Axial compressive performance and load‐carrying capacity of hybrid reinforced concrete columns with a built‐in spiral stirrup core

Abstract To enhance the durability of reinforced concrete columns, a new hybrid reinforced concrete column with outer GFRP (Glass Fiber Reinforced Polymer) and inner steel reinforcements is proposed in this paper. To investigate the axial compression performance of the proposed column structure, 12 specimens were designed, fabricated, and tested to evaluate their failure modes, load–displacement, and load‐strain responses. The test results show that the ultimate load‐carrying capacity and confined concrete strength of the hybrid‐reinforced columns have increased by 22.84% and 16.36%, respectively, compared to the corresponding GFRP reinforced columns. This is due to the interior concrete core being doubly confined by the outer ties and inner spiral stirrups. Even after reaching their ultimate load‐carrying capacity, the hybrid reinforced columns retain a residual load‐carrying capacity equivalent to 47.69% of their ultimate capacity. The finite element model of the hybrid reinforced columns was developed and validated by the experimental results. Further parametric analyses showed that the axial compressive load‐carrying capacity of the hybrid columns can be significantly improved by increasing the concrete strength and the inner concrete core area. The confining effect over the cross‐section area is analyzed and compared between circular and square columns. The residual load‐carrying capacity of hybrid reinforced columns can be improved by reducing the spacing of the spiral reinforcement. The longitudinal reinforcement ratio of the column core has a relatively lesser effect on the axial load‐carrying capacity of hybrid columns. Based on the double confinement mechanism of the hybrid reinforcement, the column cross‐section is divided into different confinement areas. Formulas for calculating the ultimate and residual axial compressive load‐carrying capacity of hybrid reinforced columns are proposed by considering the effective confining stresses exerted on different parts of the cross section.