Ductile hybrid FRP-steel-concrete coupling beams: Concept and behaviour

Coupled shear walls are widely adopted as lateral force-resisting systems in high-rise structures, requiring coupling beams to possess high flexural stiffness and shear strength for effective wall interaction, alongside superior ductility for seismic energy dissipation. Conventional steel bar-reinforced concrete (RC) or steel-concrete composite coupling beams generally suffer from common failure modes such as concrete spalling and steel buckling, which limit their ductility. This paper proposes a novel form of ductile coupling beam, fiber-reinforced polymer (FRP)-Steel-Concrete Coupling Beam (FSC-CB), which consists of an FRP confining tube and a concrete-encased steel I-section. The FSC-CBs feature an innovative truncated elliptical cross-section, which leads to not only effective confinement to the concrete, but also sufficiently large flexural stiffness and shear strength. An experimental program, including axial compression tests, four-point bending tests, and a reversed cyclic loading test, was executed to demonstrate the rationale of this proposed concept. The results confirmed the effectiveness of the proposed sectional form in both column and beam configurations, exhibiting significantly enhanced ductility and load-carrying capacity. Specifically, the FSC-CB specimens demonstrated excellent hysteretic behavior, high flexural and shear capacities, and stability, with common failure modes of existing coupling beams, including concrete spalling, steel buckling, and shear failure, largely suppressed due to the dual confinement mechanism provided by the FRP tube and the steel section. The present study verifies the structural advantages of the novel configuration and provides essential data for the design of comprehensive future experimental and analytical investigations into the seismic behavior of these hybrid coupling beams.