# On Site Investigation and Health Monitoring of a Historic Tower in Mantua, Italy
## 5 Monitoring Results and Data Analysis  
This section presents the main results of the continuous dynamic monitoring for a period of 15 months, from 17/12/2012 to 17/03/2014. During the examined monitoring period, more than 10000 1-h datasets were collected, with the ambient excitation being mainly provided by micro-tremos and wind, and automated modal identification has been performed. Figure 7 shows the RMS value of the acceleration measured at points 2 and 3 (Figure 6a), computed considering time windows of 1 h. The inspection of Figure 7 highlights that: (a) the ambient excitation is generally very low and the RMS acceleration does not exceed 0.05-0.06 mg (with the corresponding peak accelerations being lower than 0.4 mg); (b) windy days are easily identified through \"alignments\" of RMS acceleration points in the range 0.06-0.20 mg. Furthermore, the information provided by the weather station in Mantua and the Italian Institute of Geophysics and Volcanology (INGV) allowed the classification of the isolated points exceeding the amplitude of 0.06 mg as generally associated to far-field seismic events or large micro-tremors. It should be noticed that, until June 2013, the tower's response to different far-field earthquakes was recorded by the monitoring system. The strongest seismic event--corresponding to the earthquake (5.2 Richter magnitude) that occurred in the Garfagnana region (Tuscany) on 21/06/2013 [14] and strongly felt in many regions of Northern Italy--is marked in red in Figure 7 and was characterized by a measured peak acceleration of about 20 mg, exceeding about 40-50 times the highest amplitude of normally observed ambient vibrations. Figure 8 shows the evolution in time of the outdoor temperature (S-W front) and reveals yearly variations, ranging between \(-2\)\({}^{\circ}\)C and 45\({}^{\circ}\)C, and daily fluctuations of about 30 \({}^{\circ}\)C in sunny days. The corresponding variations of the modal frequencies _vs._ time (Figure 9a) and _vs._ temperature (Figure 9b) indicate that the natural frequencies of the global modes (B\({}_{1}\)-B\({}_{3}\) and T\({}_{1}\), Figure 6b) are related to the temperature. More specifically, Figure 9b shows that the frequency of global modes tends to increase with increased temperature. This behavior, observed also in other studies of masonry towers [9; 10; 11; 12; 13], is conceivably related to the materials thermal expansion that, in turn, gives rise to the  
Figure 7: The root mean square (RMS) value of the acceleration measured on the tower at points 2 (**a**) and 3 (**b**) from 17/12/2012 to 17/03/2014 (the response to the seismic event of 21/06/2013 is marked in red). closure of minor masonry discontinuities, superficial cracks, and mortar gaps. Hence, the \"compacting\" of the materials induces a temporary increase of stiffness and modal frequencies, as well. The statistics of the natural frequencies identified from 17/12/2012 to 17/03/2014 are summarized in Table 1. This table includes the mean value (\(f_{\text{mean}}\)), the standard deviation (\(\sigma_{\beta}\)), and the extreme values (\(f_{\text{min}},f_{\text{max}}\)) of each modal frequency. It should be noted that standard deviations are larger  
Figure 8: Time evolution of the temperature measured on the S-W front of the tower from 17/12/2012 to 17/03/2014. Figure 9: Natural frequencies, automatically identified from 17/12/2012 to 17/03/2014, plotted (**a**) _vs._ time and (**b**) _vs._ temperature. than 0.03 Hz for all global modes and especially significant for the local mode. Indeed, the frequency evolution of mode L\({}_{1}\) looks very different from the others (Figure 9) and characterized by remarkable variations, as the natural frequency ranges between 8.33 Hz and 10.33 Hz over the examined time period.