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

This contribution presents an integrated static–dynamic monitoring study for civil applications using long-gauge Fiber Bragg Grating (FBG) strain sensors. An experimental campaign was conducted on prestressed concrete beams with a total length of 8 m, simply supported over a clear span of 7 m, which are representative of highway bridge girders. Two dedicated inspection windows were opened along the beams to expose the steel tendons, which were progressively cut to simulate distinct structural defect scenarios. A bonded prototype was instrumented with four long-gauge FBG sensors sensing four different zones along the beam span, each measuring the average strain over a 0.60 m length. Zones 1 and 3 were located in undamaged regions and served as references, while zones 2 and 4 were positioned in regions affected by the progressive tendon cuts within the inspection windows. In particular, zone 2 was placed at approximately L/3 and included two inspection windows, while zone 4 was centered at mid-span with a single inspection window. This layout enabled tracking the evolution of longitudinal strain in both damaged and undamaged parts of the beam, and investigating how local prestress loss redistributes strain under service and ultimate loads. 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 field along each 60 cm gauge zone. The results revealed clear strain concentrations and a gradual increase in tensile strain in zones 2 and 4 as strands are cut, while zones 1 and 3 exhibited nearly elastic behavior. These static measurements allowed quantification of local stiffness and prestress losses and helped distinguish between global deflection and highly localized damage in the tendon region. Simultaneously, the same FBG sensors were used as dynamic transducers to perform Operational Modal Analysis (OMA) directly based on strain response. Ambient and shaker forced-vibration tests were carried out after each damage step, and standard OMA techniques (such as frequency and time domain methods implemented in a Python toolbox) were applied to the FBG strain time series to study the evolution of natural frequencies, damping ratios, and strain mode shapes with increasing damage levels. A detailed finite element model of the prestressed beam, including tendon layout and strand cuts, was developed to compute numerical strain mode shapes along the FBG gauge lines. Experimental and numerical strain mode shapes were compared using standard correlation metrics, allowing for a quantitative model validation and demonstrating the sensitivity of FBG-based OMA to tendon damage. The combined use of long-gauge FBG sensors for static and dynamic monitoring confirmed that these transducers can offer a powerful and compact sensing strategy for detecting and localizing strand-level damage in prestressed concrete bridge components.