PSI - Issue 78

Daniele Storni et al. / Procedia Structural Integrity 78 (2026) 237–244

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Keywords: SHM, Heritage, Assisi Basilica, Masonry, Bell Tower.

1. Introduction The preservation of cultural heritage is a key topic in the structural engineering of existing structures. Bell towers represent a challenging aspect in terms of analysis and mitigation due to their historical and artistic value that have to be preserved. In this light, the Assisi Basilica represents an excellent example of the unique architectural heritage that distinguishes the Italian peninsula. The Basilica is located in one of the most seismic prone areas of the entire Mediterranean zone and was struck by significant seismic events. The last one heavily damaged the church on 26th of September 1997. During that event, the vaults of the nave partially collapsed. The event is a warning about the weaknesses of such inimitable architectures. Due to the relevant role of bell towers in Italian culture, several researchers investigated their dynamic behavior by means of ambient vibration campaigns (Cacciola et al. 2022, Chisari et al. 2023, Sun et al. 2023, Li Rosi et al. 2023). All the authors identified the vibration-based Structural Health Monitoring (SHM) systems as a powerful tool for assessing remotely the structural conditions of the bell tower in operational conditions or in case of hazardous events. Although additional investigations are needed to evaluate the nonlinear mechanical behavior of the structure, Operational Modal Analysis (OMA) and SHM procedures can help identify the stiffness properties at a macroscopic level. In the case of numerical simulations, the modal properties obtained by the vibration monitoring can be implemented in numerical updating frameworks that help to calibrate FE models useful for seismic or robustness assessment (Chisari et al. 2023, Lollobrigida et al. 2025). According to other researchers, the monitoring system plays a crucial role in detecting and quantifying the structural response to seismic events, enabling the assessment of dynamic behavior, damage potential, and long-term performance under repeated shaking. SHM provides essential data for evaluating the effects of seismic loading on historic buildings and supports the development of timely maintenance, reinforcement, and risk mitigation strategies. This is particularly relevant in the Umbria – Marche Apennines, where Assisi is located — one of the most seismically active regions in Italy — characterized by relatively frequent small-to-moderate earthquakes and less frequent but potentially damaging larger events with longer recurrence intervals (e.g., Chiaraluce et al., 2003). The paper presents the results of more than two years of monitoring and the response of the bell tower during the main recent earthquakes 2. The Assisi Basilica case study The bell tower of the Upper Basilica of St. Francis in Assisi (Figure 1.a), whose construction began around 1239, is a structure that incorporates elements of the Romanesque and Gothic styles and is a clear example of the structural sobriety typical of the religious architecture of Central Italy at the beginning of the 13th century. The Romanesque style bell tower was built on a 11m×11m square base with a total height of 52 m. According to a geometric survey the four walls have slightly different thicknesses that decrease along the structure. This aspect assumes a relevant role in the dynamic behaviour of the tower. A clever and robust stair system was designed, resulting in fly arches and corner masonry pillars (Figure 1.b). Following the vertical development of the bell tower, it culminates in the bell chamber.

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Fig. 1. View of the a) San Francesco d’Assisi monumental complex (Peter K Burian, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=60507234) and b) inner side of the bell tower