Monitoring the structural health of the “Due Torri” in Bologna

Located in the heart of Bologna, the two medioeval towers (called “Due Torri”) represent the main symbol of the city. These towers, the Asinelli tower and the Garisenda tower, both built during the 12th century, are characterized by a height of about 97m and 48m respectively. Nowadays, they represent two of the few surviving examples of the more than 100 tall medieval towers which characterized the city in ancient times. They are part of the Italian cultural and monumental heritage and their safeguard is an extremely important issue for both the city of Bologna and the entire country. In order to preserve these monuments and to monitor their health, an exhaustive monitoring system has been recently installed on both towers. This monitoring system, originally designed by the “Studio Ceccoli e Associati” in 1997, has started to be operative (four data acquisitions per hours have been planned) in the beginning of 2011. The system spoils a number of capabilities, allowing to monitor the time evolution of: (i) the state of deformation in selected critical regions located at the bottom of the towers (where the effects of the inclination are high); (ii) the tensional state of the steel ties placed over the years to confine the walls (the first ties inserted are those added in the Asinelli Tower in 1700’s; the last ones are those added in the Garisenda Tower in 2010), (iii) the (eventual) movements across selected cracks; (iv) the relative distances between fixed reference points (distance between Garisenda and Asinelli as well as distance between these towers and selected buildings in the area); (v) the inclination of the towers. Also, a complete weather station allows to register the environmental conditions (i.e. temperature, pressure, wind direction and velocity, rain intensity). The monitoring system also allows to acquire accelerometric measures to provide information regarding the dynamic behavior of the towers (i.e. the response of forced vibration caused by wind, earthquake, traffic vehicles ...). This paper aims at presenting the rationale, the characteristics and the capabilities of the monitoring system as well as selected preliminary results obtained from the first months of data acquisition