Fatigue performance of hybrid DSTBs with GFRP reinforcement under variable-amplitude loading

Hybrid fiber reinforced polymer (FRP)-concrete-steel double-skin tubular beams (DSTBs), featuring an outer FRP tube and an inner steel tube with concrete filling the interlayer space, have been utilized as girders and primary structural components in bridge applications where fatigue resistance is critical. To improve flexural stiffness and crack control, this study incorporates glass FRP (GFRP) bars into DSTBs and evaluates their fatigue behavior under realistic variable-amplitude (VA) traffic loading. Two sets of DSTBs, with and without GFRP bars, were tested under an overload ladder VA spectrum derived from actual vehicular loads. The results show that GFRP bars increased load-bearing capacity by 28%, enhanced effective bending stiffness by 60.2%, and reduced maximum crack widths by 54–75% across tested load levels. Specimens with GFRP bars also exhibited 9–51% longer fatigue lives compared with the unreinforced counterparts, demonstrating the ability of GFRP reinforcement to delay crack propagation and slow damage accumulation. Furthermore, VA loading produced fatigue degradation trends comparable to constant-amplitude (CA) loading within the same stress range. These findings provide essential data for fatigue-resistant design and detailing of hybrid DSTB components in bridge and precast construction. • GFRP bars reduced maximum fatigue crack widths by up to 75% under variable-amplitude overload ladder loading. • B-F specimens showed up to 51% longer fatigue life compared to unreinforced specimens. • Residual deflection and stiffness degradation progressed more slowly in GFRP-reinforced DSTBs. • VA loading induced similar fatigue life trends to CA loading.