PSI - Issue 84

Francesco Nigro et al. / Procedia Structural Integrity 84 (2026) 183–190

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1. Introduction Ensuring the long-term performance and structural integrity of ageing bridge networks is currently a top priority for infrastructure managers globally. As Farrar and Worden (2007) emphasised, many existing facilities were designed according to outdated codes and are now facing environmental degradation and traffic volumes far beyond their original specifications. While periodic visual inspections remain the standard practice for condition assessment, Moore et al. (2001) demonstrated their inherent limitations, as they noticed that their outcome is often subjective in nature and may overlook critical internal deterioration or subsurface defects. To address these shortcomings, Structural Health Monitoring (SHM) has evolved into a robust framework for continuous, data-driven diagnostic assessment (Lynch et al., 2016). Within the wide spectrum of currently available SHM techniques, Brincker and Zhang (2015) advocated for the use of Operational Modal Analysis (OMA) as a non destructive tool for evaluating in-service bridges. Their research highlighted how OMA can accurately extract modal parameters by only leveraging ambient excitation, thus eliminating the need for traffic closures or costly forced vibration tests. Advancements in hardware have also played a crucial role in the widespread adoption of these methods. Sabato et al. (2017) proved that high-density sensor arrays using Micro-Electro-Mechanical Systems (MEMS) technology can deliver high-fidelity data at a significantly lower cost compared to traditional piezoelectric instrumentation. To handle the huge amount of data produced by sensors over time, García-Macías et al. (2022) introduced the “P3P” software environment, which streamlines the transition from raw vibration signals to autonomous modal tracking and health indicators. The value of persistent, long-term monitoring strategies has been further validated by recent large-scale applications. For instance, Tomassini et al. (2025) recently analysed a landmark steel arch bridge, showing that long term monitoring is essential to characterise structural response under fluctuating operational and climatic conditions. Their study underlines the need for automated identification routines to distinguish between natural frequency shifts caused by thermal variations and those indicating actual structural degradation phenomena. Despite this progress, certain structural typologies, such as bridges with Gerber-type half-joints, present unique monitoring challenges as these joints are particularly susceptible to localised damage and are notoriously difficult to be inspected and detected, as highlighted by Granata et al. (2022), Tomassini et al. (2024) and Meoni et al. (2024). The present paper discusses the implementation of a permanent vibration-based SHM system on a multi-span bridge in central Italy designed in 1970s and characterised by a Gerber beam system. The research focuses on the dynamic characterisation of the structure, which has undergone static and seismic strengthening interventions (especially on half joints), to evaluate the effectiveness of OMA in capturing the global effects of the occurred structural modifications. To this end, Section 2 briefly outlines the history of the bridge and the structural interventions, Section 3 highlights the results of the OMA and of the preliminary frequency tracking, whereas Section 4 reports some concluding remarks. The structure under consideration is a four-span road viaduct located in central Italy, completed in the late 1970s. Characterised by a straight horizontal alignment in a flat topographic area, the bridge has a total length of 150 m with individual span lengths of 29, 46, 46 and 29 meters. The 19 m wide deck, covering a total area of approximately 2850 m 2 , supports two separate dual-lane carriageways. The viaduct is designed as a Gerber beam system consisting of three cast-in-place, post-tensioned cantilever units supporting four prefabricated drop-in spans. Each cantilever unit is designed as a symmetric multi-cell box-girder extending approximately 15 m from the pier axis to the Gerber-type half-joints. The 19 m wide deck incorporates five 30 cm thick webs, with an overall depth varying linearly from 2.5 m at the piers to 1.19 m at the cantilever ends. The drop-in spans consist of 16 prefabricated, pre-tensioned I-beams (80 cm in height) with an integrated cast-in-place slab, resting directly on the half-joints, in the original bridge configuration. Figures 1, 2 and 3 depict the bridge and its original structural configuration. 2. The bridge and the monitoring system 2.1. The structure of the bridge decking