# On Site Investigation and Health Monitoring of a Historic Tower in Mantua, Italy
## 6 Conclusions  
The paper focuses on the post-earthquake assessment of a historic masonry tower and summarizes the results of visual inspection, ambient vibration tests, and long-term dynamic monitoring of the building.  
Visual inspection and the stratigraphic sequencing of the load-bearing walls clearly indicated that the upper part of the tower is characterized by the presence of several discontinuities due to the historic evolution of the building, local lack of connection, and extensive masonry decay. The poor state of preservation of the same region was confirmed by the observed dynamic characteristics, as one local mode, involving the upper part of the tower, was clearly identified from ambient vibration data.  
The results of the SHM exercise performed through the continuous dynamic monitoring of the tower at study highlighted that a limited number of accelerometers and temperature sensors installed  
Figure 16: Change in the cleaned natural frequency (Auto-Regressive model with eXogenous input) induced by the seismic event of 21/06/2013: (**a**) Mode B\({}_{1}\); (**b**) Mode B\({}_{2}\); and (**c**) Mode T\({}_{1}\).  
in the structure are sufficient to provide meaningful information for the preventive conservation and/or SHM of historic towers. More specifically, the following conclusions can be drawn:  
* The application of state-of-art tools for automated operational modal analysis allows tracking of natural frequencies;
* The temperature turned out to significantly affect the (daily) variation of the natural frequencies in masonry towers, with the frequencies of global modes increasing with increased temperature;
* Dynamic regression models are suitable to represent the environmental effects on modal frequencies, even when only one (outdoor) temperature sensor is installed in the structure;
* The application of automated operational modal analysis and dynamic regression models turned out to be effective in the identification of seismic-induced structural anomalies under changing environment;
* ARX models are suitable to completely remove the environmental effects from modal frequencies, but exhibit no damage detection skills (_i.e.,_ the frequency drops associated to slight abnormal structural changes are not distinguished from similar variations induced by temperature or other unobserved factors);
* The application of automated operational modal analysis highlighted the possible progress of a damage mechanism, involving the upper part of the tower, remarked by the significant fluctuations of the natural frequency of the identified local mode.  
As a concluding remark, this full scale experience demonstrates that low-cost dynamic monitoring systems, combined with state-of-art tools for automated operational modal analysis and statistical tools for the minimization of the environmental effects, are effective and sustainable means for the preservation of historic masonry towers.  
The research was partially supported by the Mantua Municipality. [PERSON], [PERSON] (VibLab, Politecnico di Milano) and [PERSON], PhD are gratefully acknowledged for the assistance during the visual inspection, the field tests, the installation and maintenance of the dynamic monitoring system.  
[PERSON] supervised the research; [PERSON] and [PERSON] conceived and designed the experimental tests as well as the monitoring system; [PERSON] mainly performed the data analysis.  
The authors declare no conflict of interest.