Numerical and operational modal analyses of a masonry bell-tower inserted in a building aggregate

Historical structures are a key asset of the cultural heritage for the fundamental role they play in preserving social identity. Therefore, any conservation techniques should respect their historical and cultural values. In this respect, structural health monitoring through in situ dynamic tests are becoming very popular to define the maintenance condition of a structure and to give insight into its overall behavior. Especially for historical structures, Operational Modal Analysis (OMA) is a powerful approach since it avoids to shake the structure artificially. This paper discusses the assessment of the dynamic behavior of the Sant'Agostino bell-tower in Benevento (Italy). The selected case study represents a typical example of a masonry tower inserted into a building aggregate. Based on experimental results obtained through an ambient vibration test, the frequency domain decomposition (FDD) and Stochastic Subspace Identification (SSI) techniques are employed to evaluate frequencies and mode shapes. These modal parameters are subsequently used to calibrate a 3D finite element (FE) model of the bell-tower by changing material properties and boundary conditions through the application of a systematic manual tuning. A very good match is reached between analytical and experimental modal parameters and some important structural parameters are identified by the model updating. The results confirms that the OMA technique is able to detect effective information on the dynamic behavior of historical structures that can be useful to update a reliable numerical model for structural analysis.