Automated operational modal analysis of bell towers subjected to narrowband input

Vibration-based Structural Health Monitoring (SHM) systems are very attractive to remotely assess the health state of structures of the historical urban landscape, such as bell towers, in operational conditions or after hazardous events, because of their limited invasiveness and the opportunity to enhance the knowledge about their mechanical behavior. The present study focuses on towers because, in spite of their relatively simple structural scheme, aging of materials as well as earthquake-induced damage resulted in a number of structural collapses occurred worldwide over the years. In this context, automated Operational Modal Analysis (OMA) techniques are key elements for the development of vibration-based SHM systems. After recognizing the critical aspects in the development of effective automated OMA procedures able to deal with the operational vibration response of bell towers, the present paper describes a strategy for automated OMA based on power spectral density transmissibility (PSDT) functions. The validation of the procedure is documented with reference to the dynamic response of a bell tower located in a minor center of the Italian Inner Areas, highlighting its effectiveness in identifying the fundamental modes from operational vibrations, and its robustness to the narrowband input associated with swinging bells.