Effects of FRP fiber orientations on four-point bending behaviour of FRP-concrete-steel tubular beams: Experimental study and modeling

FRP-concrete-steel double-skin tubular beams (DSTBs), comprising an inner steel tube, an outer FRP tube, and an intermediate concrete layer, are increasingly used in bridge structures. Previous research has mainly concentrated on DSTBs with FRP tubes featuring fibers oriented in or near the hoop direction. However, such orientations can result in cracking of the FRP tube under early loading or normal service conditions due to inadequate longitudinal tensile strength. This cracking compromises the corrosion resistance and long-term serviceability of DSTBs. To address this issue, this study systematically investigates the effects of different fiber orientations ( ± 80°, ± 60°, and ± 45° relative to the longitudinal direction) on the four-point bending performance of DSTBs. Key experimental and theoretical findings include: (1) All DSTBs demonstrated excellent ductility under four-point bending, regardless of the FRP fiber orientations. (2) ± 60° and ± 80° fiber-wound FRP tubes exhibited significant tensile-side cracking, with cracks propagating along the fiber winding direction. Conversely, ± 45° fiber-wound FRP tubes showed superior cracking resistance and provided adequate longitudinal tensile capacity on the tensile side. (3) The bending capacity was highest in specimens with ± 45° fiber-wound FRP tubes, followed by those with ± 60° tubes, and lowest for those with ± 80° tubes. (4) The inclusion of shear studs could effectively mitigate the relative slippage between the concrete and steel tube. (5) The bending performance of DSTBs was simulated using OpenSees, with the constitutive model of the FRP tubes carefully accounting for the stress states associated with different fiber orientations during failure. The developed numerical model accurately predicted the load-deflection curves of DSTBs, but featuring with a conservative trend. The findings of this study confirm that optimizing fiber orientation is crucial for enhancing the performance and durability of DSTBs in practical applications.