Study on a Longitudinal Cooperative Performance Indicator for Simply Supported-Then-Continuous Beam Bridges

During service, wet joints at the top of middle piers in simply supported-then-continuous (SSC) beam bridges are structurally the weakest parts, prone to degradation of longitudinal cooperative performance. Such deterioration may cause the continuous system to degenerate into a purely simply supported bridge, thereby endangering safety. To address this issue, this study proposes the strain correlation coefficient of the bottom of adjacent span beams as a monitoring indicator for longitudinal cooperative performance. A typical mechanical model of an SSC bridge is established, and the analytical expression of the indicator under statistical steady state is derived. Theoretical results show that the indicator converges to a stable value; sensitivity analysis further demonstrates that this value is governed primarily by wet joint stiffness, while remaining largely insensitive to span stiffness and robust against measurement noise. Numerical simulations under statistical steady-state traffic flow confirm the applicability of the indicator and its effectiveness in representing longitudinal cooperative performance. Validation through both numerical simulations and long-term field monitoring of the Tongji Road Viaduct further demonstrates the indicator’s sensitivity, robustness, and stability. Beyond this case study, the proposed method offers a generalizable analytical framework for evaluating longitudinal cooperative performance in multispan precast beam bridges, providing both practical utility for structural health monitoring and broader theoretical advancement.