The fixing device is an important component in the China Railway Track System (CRTS) II slab track, through which the longitudinal force of the track can be transferred to bridges and piers. The stiffness of the fixing device significantly affects the mechanical properties of the track. However, it is difficult to evaluate stiffness using conventional detection methods, because the fixing device is located under the base plate. In this study, the fixing-device stiffness of simply supported bridges was identified using long-term monitoring data and finite element (FE) analysis. First, a monitoring scheme was proposed based on the principle of track–bridge interaction. The strain in the base plate on both sides of the fixing device, as well as the temperature of the base plate and bridge, were obtained from 2015 to 2018. Then, the stiffness of the fixing device was proved to be closely related to the strain difference of the base plate on both sides of the fixing device, and the corresponding nonlinear mapping relationship was established through FE analysis. Finally, the fixing-device stiffness over 4 years was identified using long-term monitoring data and the established mapping function, and the influence of the stiffness degradation on the mechanical property of the track was discussed. This study can provide a reference for the design and maintenance of CRTS II slab tracks on multi-span simply supported bridges.
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