PSI - Issue 44

Elena Michelini et al. / Procedia Structural Integrity 44 (2023) 1530–1537 Elena Michelini et al / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction Preservation of historic heritage represents an important cultural and economic issue for European countries, and especially for Italy. During time, cultural heritage experiences the occurrence of different damages, related to material deterioration, differential settlements, static and seismic actions, which may significantly alter the structural response, especially when dealing with unreinforced masonry buildings. In this perspective, a good management of cultural heritage requires the planning of a conservation strategy, and the possible execution of targeted retrofit interventions, especially in high seismicity areas. A periodic control of the “state of health” of hi storic buildings is a primary tool for their preservation, as also suggested by the “Guidelines on the Assessment and Mitigation of Seismic Risk of the Cultural Heritage” (Moro, 2007). One of the strategies that can be pursued in this sense is represented by Structural Health Monitoring (SHM), which allows to collect a large amount of information on building condition and damage evolution with time (Farrar and Worden, 2007; Ramos et al., 2010). In particular, methods based on the measurement of freely available ambient vibrations have gained popularity, due to their very limited invasiveness and interference with the normal use of the building. Ambient Vibration Testing (AVT) exploits indeed natural sources, like wind, and do not require a specific external excitation equipment (Gentile et al., 2015; De Stefano et al., 2016; Pierdicca et al., 2016; Clementi et al., 2017; Fiorini et al., 2021). The data collected through AV measurements allow the identification of the dynamic properties of the monitored structures, through Operational Modal Analysis (OMA) techniques (Peeters and De Roeck, 2001; Brincker et al., 2001). The dependency of dynamic data (modal shapes, frequencies, and damping ratios) on the health state of buildings has been discussed and highlighted in several studies from the literature (i.e., Salawu, 1997; Sohn, 2007; Shimpi et al., 2019). Finite element (FE) models are one of the most accurate tools to be used in the vulnerability and seismic risk assessment of existing structures; however, when dealing with historic and monumental buildings, an appropriate calibration of the model requires a deep knowledge of the case study, together with a high level of expertise in modelling techniques (Asteris et al., 2014). Indeed, FE modelling of existing historic buildings is strongly affected by some unavoidable uncertainties, related to boundary conditions, as well as to spatial distribution material properties and crack patterns induced by previous damaging events. On one side, the calibration of mechanical properties in numerical models usually requires the execution of destructive or partially-destructive tests, which are not always feasible for heritage structures. On the other side, a well-recognized correlation between data coming from non-destructive tests (such as thermography, sonic or radar investigations, etc.) and mechanical parameters has not been found yet (Binda et al., 2000; Gentile et al., 2015). Therefore, a possible strategy for obtaining reliable linear FE models is based on the so-called model updating, which consists in appropriately modifying mechanical parameters of the FE model to obtain the best match between numerical and experimental modal parameters (Miraglia et al., 2019, Miraglia et al., 2020). This work discusses the main phases of the installation of a SHM system on the Preatorian Palace in Borgo Val di Taro (Italy), hosting the Town Hall, for its inclusion within the Italian national network of the Seismic Observatory of Structures (OSS), managed by the technical staff of the Seismic Risk Service of Italian Civil Protection Department. The OSS network allows the monitoring of the structural behavior under earthquake of about 160 public properties of strategic interest, belonging to different structural typologies (mainly reinforced concrete and unreinforced masonry buildings, but also bridges and dams). Along with the installation of a limited number of permanent sensors for long time monitoring, short-time AVT with a larger number of removable sensors was performed, so to identify the dynamic behavior of the building. The obtained parameters were then used to update a FE model of the structure, by modifying some uncertain material parameters, and in particular the elastic modulus, so to have a good match between numerical and experimental mode shapes and frequencies. 2. The Historical Town Hall of Borgo Val di Taro The historical Town-Hall building, also known as Praetorian Palace, is one of the most important examples of neoclassical civic architecture in Borgo Val di Taro (Fig. 1). A proper dating of the Palace is not simple, because the available documentation is scarce and fragmented. According to some documents stored in the municipal historic archive, the first structure dates back to the 15 th century. At the beginning of the 19 th century, the Palace was donated to the “Opera Pia Manara” , founded as a provision in the will of Abbot Domenico Manara, who was later remembered