# On Site Investigation and Health Monitoring of a Historic Tower in Mantua, Italy
## 1 Introduction  
The Italian seismic sequence of May-June 2012 [1] highlighted the high vulnerability of the historical architectures, especially in the southern part of the province of Mantua and in the neighboring Emilia-Romagna region, where several brittle collapses of towers, fortification walls, and castles occurred [2, 3], despite the supposed low seismicity of the area.  
After the earthquake, an extensive research program was carried out to assess the structural condition of the tallest historic tower in Mantua: the _Gabbiia_ tower [4, 5]. The tower (Figure 1), about 54.0 m high, is a symbol of cultural heritage in Mantua so that the fall of masonry pieces from its upper part, reported during the earthquake occurred on 29 May 2012 [1], provided strong motivations for deeply investigating the structural condition of the building. The post-earthquake survey, described in [6], included: (a) historic and documentary research; (b) an on-site survey and visual inspection of the load-bearing walls and structural discontinuities; (c) non-destructive and minor-destructive tests of materials on site; and (d) dynamic tests in operational conditions.  
As shown in Figure 2, the available historic pictures indicate modifications and successive additions at the top of the tower. Visual inspection [6] of the upper load-bearing walls revealed traces of past structures on all fronts, and ineffective links between the added parts and extended masonry decay. In addition, one local mode--involving the top of the building--was clearly detected in the dynamic tests [6]; the presence of such a local mode highlights the remarkable structural effects of the change in the morphology and quality of masonry observed on top of the tower and, along with the poor structural arrangement of the upper region, suggests the need for repair interventions to be carried out.  
While waiting for the retrofit design and funding, a dynamic monitoring system (consisting of three highly-sensitive accelerometers and one temperature sensor) was installed in the tower [7] for structural health monitoring (SHM) purposes. The continuous dynamic monitoring was mainly aimed at: (a) evaluating the response of the tower to the expected sequence of far-field seismic events; (b) assessing the impact of the environmental (_i.e._, temperature) effects on the natural frequencies of the building [7; 8; 9; 10; 11; 12; 13]; and (c) detecting the occurrence of any abnormal change or anomaly in the structural behavior. Further potential objectives are providing the retrofit design with additional information and evaluating the effects of the future strengthening.  
Approximately six months after the beginning of the continuous monitoring, the tower was subjected to a far-field earthquake [14] and the maximum measured acceleration exceeded about 50 times the highest response that was observed under normal ambient excitation; hence, the monitoring project provided with a challenging opportunity to identify the occurrence of seismic-induced damage under a changing environment.  
Figure 1: The _Gabbia_ tower in Mantua, Italy: (**a**) view from the 17 th century [4]; (**b**) recent view from the east; and (**c**) section of the tower (dimensions in m).  
Figure 2: Views of the _Gabbia_ tower dating back to: (**a**) 17 th century; (**b**) 1830; (**c**) 1852; and (**d**) present days.  
After a brief description of the investigated tower and the post-earthquake survey, the paper presents the results of the first 15 months of dynamic monitoring and full details are given on the following main tasks: (1) installation of the continuous dynamic monitoring system with remote control and data transmission via the Internet; (2) development of efficient tools [15] to process the raw data regularly received and to automatically identify the modal parameters [16]; (3) tracking the evolution in time of natural frequencies and modeling the temperature effects on those features; and (4) identification of structural performance anomalies.