The paper illustrates how the dynamic identification techniques can be used to justify significant vibration phenomena observed in modern bell tower during the swinging motion of the bells. First, the modal model of the tower is extracted from the structure response to ambient excitation, using different output-only identification procedures working in the frequency (EFDD) and the time domain (SSI). Then the bell excitation is characterized from the steady-state forced response to the bell swinging motion. The resonance between the fundamental tower frequency and the third harmonic of the horizontal dynamic force applied to the structure by the swinging bells is clearly recognized as source of large oscillations. A finite element model is developed and updated to properly reproduce the tower spectral properties. Aiming to mitigate the identified phenomenon, the model allows the design of a stiffening intervention to properly shift the tower resonant frequency. The effectiveness of the designed solution is successfully verified by a post-intervention dynamic test campaign.
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