PSI - Issue 84

Ana Avramova et al. / Procedia Structural Integrity 84 (2026) 560–568

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technological advances, allowing relatively cheap installation of monitoring systems, fully computer-based operation, quick transmission and processing of the collected data; (d) the possibility of using OMA for assessing the structural health. On the other hand, most of the existing applications have focused on arch bridges, cable-stayed and suspension bridges (see, e.g., Peeters et al., 2009; Magalhães & Cunha, 2011; Cross et al., 2013; Cunha et al., 2013; Gentile & Saisi, 2015), while limited attention has been paid to the dynamic characteristics of multi-span continuous viaducts with steel–concrete composite decks (Gentile et al. 2023). The present study presents the results collected during about 16 months of dynamic monitoring of two similar bridges, both crossing the Olona River in the province of Varese. The investigated bridges, denoted to as Olona viaducts, are multi-span bridges with steel–concrete composite deck supported by reinforced concrete piers and abutments; both bridges consist of 4 spans but are characterized by different length of the central spans and different cross-sections. After a concise description of the investigated bridges and the related monitoring systems, details are given on the software tools (Gentile et al., 2024) adopted to process the continuously acquired data. Subsequently the paper focuses on the dynamic characteristics of the viaducts as well as on the OMA results obtained over more than 1 year of continuous observation, with particular emphasis being given to the effects of environmental variability on the time evolution of natural frequencies. Finally, a comparative analysis of the dynamic behavior of the two bridges is presented, highlighting the potential of such comparison in SHM perspective. 2. Olona viaducts: description of the bridges and the monitoring systems As part of a project funded by the Italian Government that began in 2022, dynamic monitoring systems have been installed on selected steel-concrete composite structures along the Autostrada Pedemontana Lombarda (APL), a regional motorway connecting various provinces in Lombardy. Among these infrastructures, APL has selected the Olona bridges (Fig. 1), referred to as Olona East (VI03) and Olona West (VI04). The two viaducts at study are crossing the Olona River in the province of Varese (Lombardy region Northern Italy). As previously stated, each bridge includes 4 spans: The VI03 bridge exhibits a span sequence of 55 m + 62 m + 62 m + 55 m, for a total length of 234 m; the VI04 bridge extends for 242 m with span of 55 m + 66 m + 66 m + 55 m. Both bridges include twin longitudinal steel girders, with each girder consisting of welded double-T section, that are composite with a reinforced concrete slab, 31 cm thick. The girders of VI03 bridge are equally spaced at 9.05 m, whereas VI04 is characterized by girder spacing from 8.75 m to 11.17 m. As shown in Fig. 2, a series of lattice diaphragms connect the girders, with lower and upper bracing providing torsional rigidity to the beam bridge. The steel-concrete composite decks are respectively supported by two concrete abutments (S1 on the Solbiate side and S2 on the Mozzate side) and three piers (P1-P3). All the bearing devices are elastomeric seismic isolators, characterized by (a) appropriate dissipative capacities, (b) low horizontal stiffness to guarantee the decoupling between the horizontal motion of deck and piers/abutments and (c) high vertical rigidity to support vertical loads.

Fig. 1 View of the Olona viaducts (VI04 bridge in the foreground and VI03 bridge in the background).