GNGTS 2013 - Atti del 32° Convegno Nazionale

(1) where λ is the wavelength of the signal (Taylor, 2001). The main advantages of the interferometric radar for structural health monitoring of civil infrastructures are high accuracy of measurements (sensitivity of 0.01-0.02 mm) and high spatial resolution, simultaneous monitoring of several targets within the sensor applicable distance, independence from daylight and weather conditions for short recordings, portability, and quick set-up time. The sensor module is provided with two horn antennas (-3dB beamwidth of 17° and 15° in the horizontal and vertical plane) and transmits the electromagnetic signals in Ku frequency band from 17.1 to 17.3 GHz, receiving the echoes from several targets placed inside the radar scenario. The acquired time history s(t) is the displacement of the target projected along the sensor- target line of sight (LOS). The LOS displacement s(t) is linked to the displacements component V(t) and H(t) , in vertical and in horizontal plane respectively, through the following Eq. (2): (2) with β being the LOS angle computed from the horizontal axis. The interferometric survey may thus be considered a powerful tool to evaluate the deformation related to dynamic applied loads and to evaluate the stiffness of structures. Description and method. The test site was chosen in order to analyze a strongly damaged but not totally crushed bell tower, for which remote sensing approach was the only compatible one with safety conditions. The experimental settings were designed with the task to have comparable results from two main acquisition distances from the target and two different direction projection of measured displacement. The tower: a brief historical overview and description of the damage. The Emilia earthquake has caused numerous damages to the structures. The buildings most affected by the earthquake are industrial structures (factories and storage magazines) and cultural heritage buildings (churches, bell towers, civic towers, etc..), whose geometric and constructive features have contributed to increasing their vulnerability in case of earthquakes. In this work we present the study performed for the bell tower of San Giacomo Roncole. The actual structure was built from 1771 to 1774 with the same materials of the previous Carmelite Convent of Santa Maria delle Grazie della Galeazza (Andreoli, 1987). The building is located few metres away from the singular nave of the church. The structure replaced an earlier tower, adjacent to the body of the church and demolished by the damage caused by the vibrations of the bells. The new tower is a masonry structure located at South-west of the church. The tower reaches a height of about 36 metres with a square base (5 metres per side). After the earthquakes of 20 th and 29 th May 2012 (magnitude M l =5.9 and M l =5.8, respectively) the building sustained serious damage to the base, highlighted by displacements of about 2 cm toward the South-West side and of 1 cm on the South-East side. In other words, after the earthquake the structure was shifted at the base, assuming an eccentric position. Also there are various fractures on the first section of the tower horizontally and diagonally crossing its perimeter walls, where a rigid motion on the order of centimetres was measured. The top of the structure collapsed at about 9 metres from the base toward the South-West side. Sensor installation and data processing. The tower of San Giacomo Roncole is a slim structure with a shape characterized by several natural corners (see Fig. 1). Consequently, the measurements are performed without artificial corner reflectors installed on the body surface of the structure. This is an essential condition because the structure is seriously damaged. Dynamic surveys are carried out using three different configurations. In the first one, the IBIS-S system was placed in front of the tower in the South-East direction in order to monitor the dynamic behaviour of the full body of the structure with 30° LOS angle. The second radar 102 GNGTS 2013 S essione 3.2