Integrated Static and Dynamic Monitoring of Prestressed Concrete Beams Using Long-Gauge FBG Strain Sensors and OMA Based Strain Mode Shapes

This paper presents an integrated static-dynamic monitoring study for civil applications based on long-gauge Fiber Bragg Grating (FBG) strain sensors. An experimental campaign was conducted on a prestressed concrete beam with a total length of 8 m, simply supported over a clear span of 7 m, representative of highway bridge girders. Three dedicated inspection windows were opened along the beam to expose the steel tendons, which were progressively cut to simulate different structural damage scenarios. Quasi-static tests included repeated service-level load cycles and loading up to ultimate conditions. Under static conditions, the FBG data provided continuous measurements of the average strain along four 0.60 m gauge zones. The results showed clear strain concentrations and a progressive increase in tensile strain in zones affected by structural damage, while the other zones exhibited nearly elastic behaviour. Simultaneously, the same FBG sensors were used as dynamic transducers to perform Operational Modal Analysis (OMA) directly from the strain response. Ambient vibration and shaker-forced vibration tests were conducted after each damage step, and standard OMA techniques were applied to the FBG strain time series to investigate the evolution of natural frequencies, damping ratios, and strain mode shapes as damage increased. A detailed finite element model of the prestressed beam, including the tendons' layout and the progressive strand cuts, was also developed to compute numerical strain mode shapes along the FBG gauge lines. The combined use of long-gauge FBG sensors for static and dynamic monitoring confirmed that these transducers can provide an effective and compact sensing strategy for the detection and the localization of strand-level damages in prestressed concrete bridge components.