Flexural behavior of hybrid GFRP-reinforced concrete-steel double-skin tubular beams

The hybrid GFRP-concrete-steel double-skin tubular beams (DSTBs) are composed of the outer GFRP tube, the inner steel tube and the sandwich concrete between both tubes. They have attracted the attention of researchers owing to the high ductility, lightweight, corrosion resistance and convenient construction. However, there are few studies on the reinforced DSTBs under bending load compared with the unreinforced DSTBs. Eight circular cross-sectional composite beams are tested in this paper to investigate the flexural behavior, including five reinforced DSTBs, two unreinforced DSTBs and one solid reinforced concrete GFRP tubular beam. The main parameters include the longitudinal steel reinforcement ratio, the inner steel tube diameter, the GFRP tube thickness, and the concrete strength. The results show that all the composite beams exhibit ductile flexural failure, and the average displacement ductility factor of the reinforced DSTBs is 3.85. The arrangement of longitudinal steel bars significantly enhances the bearing capacity and reduces the mid-span deflection of the composite beam, and it is recommended to use a reasonable low steel ratio for the excessive steel reinforcement is adverse to the ductility. Increase of the inner steel tube diameter greatly improves the flexural capacity on premise of ensuring the stiffness and ductility. The outer GFRP tube provides moderate hoop confinement on the compressive concrete in the pure bending members, which limits the increasing extent of the flexural capacity. Reasonable confinement stiffness is suggested for the reinforced DSTBs to avoid insufficient or excessive constraint. Besides, concrete with sufficient compressive strength is recommended to be used in the hybrid DSTBs for it increases both the flexural capacity and the initial stiffness of the beam.