Simplified equivalent design for novel hybrid steel-FRP composite bars and closed-type FRP stirrups reinforced seawater sea-sand concrete columns

Due to steel corrosion and shortage of freshwater and river sand for marine infrastructure, combing fiber-reinforced polymer (FRP) bars and seawater sea-sand concrete (SSC) has garnered increasing attention. Nevertheless, applying this combination in column is prevented by brittle failure and inferior compressive performance of FRP bars, and inferior confinement efficiency of conventional pultruded FRP stirrups. Here, a novel hybrid reinforcing system composed of longitudinal steel-FRP composite bars (SFCBs) and closed-type FRP stirrups was adopted for SSC column to obtain comparable mechanical behavior to steel bars reinforced concrete column. To overcome the inapplicability of existing design method, a simplified equivalent design method was explored and verified for hybrid-RC column. Results demonstrate that, in comparison with longitudinal steel bars, equal-stiffness longitudinal SFCBs offer comparable contribution for column under compression. Linear elastic properties and high strength of closed-type FRP stirrups result in excellent confinement, significantly enhancing the ductility by 58–163%. The desirable mechanical contribution of longitudinal SFCBs and the excellent confinement provided by FRP stirrups provide the foundation for establishing the equivalent design method. Equivalent-designed hybrid-RC column exhibits equivalent or even better compression behavior than steel-RC column. This equivalent method offers simple and safe design strategies for SSC column with hybrid reinforcing system.