Condition Assessment of the Splicing Joint in Widened Bridges Using Vehicle-Induced Quasi-Static Displacement Transfer Ratios: Theory and Experiments

The splicing joint is the key load-transfer path for existing and newly added girders in widened bridges, yet its condition is difficult to quantify directly in practice. This study proposes the use of vehicle-induced quasi-static displacement transfer ratios to predict stiffness and local damage in the splicing joint. The widened bridge is modeled as dual girders of the Bernoulli–Euler type connected by distributed springs representing the splicing joint. Closed-form solutions are derived for the coupled system response under moving vehicle loads. The analysis shows that the displacement transfer ratio is governed by joint stiffness, which enables the joint stiffness to be directly identified from measured displacements. Low-pass filtering combined with time integration yields a stable and physically interpretable index for quantifying the joint condition, classified as weak, transitional, and rigid coupling. Furthermore, by using multiple measurement points together with a moving time window, the framework is extended to interval-based damage localization in the splicing joint. Numerical and scaled test results verify the effectiveness of the proposed method. The global index is highly sensitive to stiffness degradation, and the identified stiffness remains valid for varying vehicle masses and speeds, with deviations <5%.