Fatigue Performance of Hybrid FRP–Concrete–Steel Double-Skin Tubular Beams

Hybrid double-skin tubular beams (DSTBs) containing an outer fiber-reinforced polymer (FRP) tube, annular concrete, and an inner steel tube have been used as girders or main elements of arch bridges. The fatigue behavior of DSTBs is of particular significance when they are employed as members for bridge construction, while the fatigue behavior of DSTBs has never been investigated. This study presents an experimental study on the static and fatigue behavior of hybrid DSTBs with different degrees of shear connection (i.e., configurations without shear connectors, with connectors at both ends and with connectors distributed along the entire span). Static tests demonstrated that the bearing capacity of hybrid DSTB specimens enhanced with an increasing degree of shear connection. Specimens with shear connectors spanning the entire beam effectively delayed the onset of cracks, postponed the yielding of the steel tube, and suppressed slips between the concrete and the steel tube. In fatigue tests, a gradual decrease in the bending stiffness of hybrid DSTBs as the number of loading cycles increased. A higher degree of shear connection was found to significantly suppress the slip between the concrete and the steel tube during fatigue loading. However, it was associated with a decrease in fatigue life. The higher upper limit load resulted in more accumulated damage in the hybrid DSTBs over the same loading cycles, demonstrating to be a crucial factor influencing their fatigue life.