Stay cables are critical structural components in cable-stayed bridges and are continuously subjected to cyclic stresses induced by traffic and environmental actions, which may lead to premature fatigue damage, especially when combined with corrosion. This paper presents an experimental framework for in-service monitoring and fatigue evaluation of stay-cable tension based on strain measurements calibrated through vibration-based identification. The methodology is applied to two stay cables of the Dovalí Jaime Bridge in Mexico, instrumented with bonded strain gauges and accelerometers. An in-situ calibration factor relating microstrain to axial tension is obtained from the correlation between measured strains, identified natural frequencies, and direct cable weighing performed during a static load test. Once calibrated, the framework enables the extraction of live-load-induced tension histories under normal traffic operation. These histories are processed using the Rainflow counting method to characterize stress cycles and derive equivalent stress ranges for fatigue assessment. The results show that the monitored stay cables exhibit a composite axial behavior, in which the protective tube and concrete infill contribute significantly to global stiffness, leading to low effective stress ranges in the steel strands. The proposed approach provides a practical and reliable solution for continuous, in-service monitoring of stay-cable tension and fatigue demand, integrating strain-based measurements with vibration-based verification and directly supporting fatigue evaluation from measured data.
