# On Site Investigation and Health Monitoring of a Historic Tower in Mantua, Italy
## 5 Monitoring Results and Data Analysis
A close inspection of the frequency tracking of mode L\({}_{1}\) allows the recognition of three different phases of the frequency evolution in time: (a) in a first phase (between 17/12/2012 and 14/08/2013) the modal frequency clearly decreases in time, from an initial value of about 10.0 Hz to a final value of about 8.5 Hz; (b) after the summer period (from 19/09/2013 to 19/01/2014) the frequency increases again, even if the new maximum values do not reach those identified one year before in similar environmental conditions; and (c) finally, the natural frequency decreases again from 02/02/2014 to the end of the monitoring period. Furthermore, each of those three phases is characterized by the presence of some discontinuities (drops or increases), which define the general trend of the frequency tracking. For example, considering the first time period, three drops can be detected at the beginning of February 2013, at mid-March 2013, and at mid-April 2013 [7]. These drops divide the examined time span in four parts, which can be easily identified in the temperature-frequency plot of Figure 10. This figure highlights that the populations of temperature-frequency points, corresponding to the four different periods, exhibiting similar slope of the best fit line, whereas the average frequency value significantly decreases. The subsequent time periods are characterized by similar trends, with the frequency increasing and decreasing, respectively. It should be noted that the changes exhibited by the frequency of the local mode, illustrated in Figures 9 and 10, can hardly be explained by moisture. In fact, the figures show that the natural frequency of the local mode tends to decrease as: (a) the average daily temperature increases and (b) the temperature daily range includes higher temperatures. On the contrary, it has to be expected [9] that the modal frequency decreases with increased water absorption (_i.e._, the moisture inside the walls) at the beginning of the raining season and not at the beginning and during the hot seasons, as it happens in the present case (Figures 9a and 10). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mode** & \(f_{\text{mean}}\) **(Hz)** & \(\sigma_{f}\) **(Hz)** & \(f_{\text{min}}\) **(Hz)** & \(f_{\text{max}}\) **(Hz)** \\ \hline
1 (B\({}_{1}\)) & 0.977 & 0.035 & 0.903 & 1.102 \\  
The observed behavior suggests the progress of a possible damage mechanism, conceivably related to the effect exerted by the wooden roof with increased temperature, and confirms the poor structural condition of the upper part of the tower, as well as the need of preservation actions. This conclusion seems to be confirmed also by the frequency loss detected after one year of monitoring, with the natural frequency of the local mode being unable to reach the maximum values identified one year before in similar environmental conditions. As previously pointed out, the earthquake occurring in the Garfagnana region on 21/06/2013 [14] determined accelerations on top of the _Gabbia_ tower that were significantly higher (Figure 7) than the usual ambient vibration responses. Zooming in the time evolution of the natural frequencies of modes B\({}_{1}\), B\({}_{2}\), and T\({}_{1}\)--considering three weeks before and three weeks after the earthquake--is shown in Figures 11a, 12a and 13a; the figures show a clear decrease of all modal frequencies on 21/06/2013, corresponding to the occurrence of the seismic event. Furthermore, the temperature-frequency relationships (Figures 11b, 12b and 13b) highlight that the regression lines of all modes exhibited clear variations after the earthquake, with the temperature range being almost unchanged. It is worth mentioning that this trend is confirmed by:  
1. Estimating the distribution of the modal frequencies before and after the seismic event.