Vibration-Based Monitoring (VBM) from modal strains is a promising alternative to traditional VBM methods, which are primarily based on exclusively monitoring natural frequencies. The advantages of modal strains are that they offer a lower sensitivity to temperature and a higher sensitivity to small-scale damage, when compared to natural frequencies, rendering them ideal for detecting damage at an early stage. Furthermore, modal strains can be obtained in a dense grid with a relatively low cost, when fiber-optic sensors, such as fiber-Bragg gratings (FBG) are employed, which enables the monitoring of several critical locations and increases the possibility of early damage detection and localization. Modal strains can be obtained with high accuracy and precision thanks to a recently developed methodology that makes use of FBG sensors and of a high resolution acquisition system. The temperature sensitivity of modal strains has been investigated in the field but their damage sensitivity has previously only been investigated through laboratory experiments and numerical simulations. In this study, a 110-years-old steel railway bridge is monitored with 80 FBGs. Its modal strains and natural frequencies are automatically identified with the use of hierarchical clustering from ambient and operational dynamic strains. Before the scheduled replacement of the bridge, artificial damage is introduced by means of local cuts, simulating fatigue cracks, in order to investigate the damage detection and localization capabilities of modal strains on a full-size civil structure for the first time. The influence of temperature and operational factors is also investigated and compared with that of damage.
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