Dynamic Investigation on a masonry bell tower in Mola di Bari

This article shows the 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 in case of a structure with a cultural-historical value. 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. The non-linear behavior of masonry structures is due to some characteristics, such as asymmetry and inhomogeneity of the masonry itself. This leads to many difficulties to provide a real and actual behavior of the whole structure that is possible only if we know the history and the technical characteristics and geometry of the structure, in order to get accurate results. In the present paper, the original bell tower is dated back to the first half of the XVII century and due to a lightning strike it was destroyed and immediately rebuilt at the end of the same century. It was built entirely in tuff stone. The only damages present are minor cracks on the keystones of the openings and on the North and South sides below the openings. The tower has a rectangular plan and it is connected to the church for about ½ of its total height, so that its total free height is about 34 m. The tower is rigidly joined on the South, West and partly the North sides, to the sidewalls of the church. In order to monitor its mechanical and dynamic properties, the tower has been instrumented with accelerometers and subjected to environmental vibrations in the N-S and E-W directions. To analyze the data (FFT) obtained from the monitoring, the Peak Picking method has been utilized, in order to identify a value of the effective natural frequencies and damping factors of the structure. Obtained the main frequencies and the modal classification at some measurable locations, these responses are then extrapolated and extended to the entire tower through a 3D Finite Element Model. In this way, knowing the modes of vibration, it will be possible to get an idea of the overall dynamic behavior of the structure.