Dynamic investigation of an ancient masonry bell tower with operational modal analysis a non-destructive experimental technique to obtain the dynamic characteristics of a structure

This paper shows the results of an experimental analysis on the bell tower of “Chiesa della Maddalena” (Mola di Bari, Italy), to understand the structural behavior of a slender masonry structure. The research aims, through Operational Modal Analysis (OMA), to calibrate a numerical model, which assumed realistic conclusions about the dynamic behavior of the structure. The choice of using an OMA derives from the necessity to know the modal parameters of a structure with a non-destructive testing, especially in case of cultural-historical value structures. Therefore through an easy and accurate process, it is possible to acquire in-situ environmental vibrations. The data collected are very important to estimate, with true boundary conditions, the mode shapes, the natural frequencies and the damping ratios of the structure. To analyze the data obtained from the monitoring, the Peak Picking has been method applied to the Fast Fourier Transforms (FFT) of the signals, in order to identify the values of the effective natural frequencies and damping factors of the structure. Obtained the main frequencies and the damping ratios have been determined from measures at some location;, the responses have been then extrapolated and extended to the entire tower through a 3-D Finite Element Model. In this way, knowing the modes of vibration, it has been possible to understand the overall dynamic behavior of the structure.