Long-Term Performance and Durability Assessment of FRP-Strengthened Rib-to-Deck Welds in Orthotropic Steel Bridges: Six-Year Field Monitoring Study

This study investigated the long-term fatigue performance of rib-to-deck welds in the Runyang Suspension Bridge (RSB)’s orthotropic steel deck (OSD), strengthened with L-shaped fiber-reinforced polymer (L-FRP) plates. A six-year field monitoring program was conducted. While microcracks were observed during early service, no significantly visible progression was observed over the subsequent five years, indicating stable interfacial performance under in-service conditions. Furthermore, stress analysis demonstrated significant fatigue mitigation by L-FRP strengthening, with equivalent stress amplitude (Seq) at the unstrengthened midspan weld measured at 10.94 MPa, reduced to 3.58 MPa at the L-FRP C-strengthened weld. An Interface Health index was proposed to quantify bond degradation. In addition, to evaluate the long-term evolution of Interface Health, a two-stage model combining exponential decay and linear decline was developed. Using a critical threshold of 80%, the effective service life of L-FRP C was estimated at 17.4 years (midspan) and 39.6 years (quarter-span). Results confirm that larger FRP plates offer better fatigue resistance and durability. The proposed model provided a reliable tool for evaluating long-term performance of FRP-steel bonded joints in orthotropic bridge decks.