GNGTS 2013 - Atti del 32° Convegno Nazionale

10 Hz±2 Hz for typical working conditions. Then a low-pass filtering may be applied in order to assure that the measured vibrations are not imputable to the natural response of the tripod- head sensor system. All amplitude spectra are characterized by a clear harmonic component at 0.96 Hz (corresponding period of 1.04 s) whose amplitude increases with the height of the analyzed range-bin. This behaviour agrees with the expected amplitude variation associated at the first natural mode (simple bending). This periodicity cannot be directly retrieved from all displacement time series but only for the 50 th range-bin corresponding to the reflection from the height of 30 metres on the tower (see Fig. 3). The obtained value of 0.96 Hz could be influenced by the presence of structural damage caused by the earthquake. This value could be compared with the theoretical one achieved through numerical modelling of the building before the damage. However, even without a direct model, several empirical relationships available for masonry structures, like those proposed by the Italian Building Code NTC-2008, the Spanish National Code NSCE-02 and by literature studies (Rainieri and Fabbrocino, 2012), are used to assess the fundamental frequency of vibration. The obtained results range between 1.4 and 1.5 Hz. The significant difference (31- 36%) between the experimental value and the estimated values could be imputed to the damage caused by the earthquake. It is generally recognised that the period of vibration grows while increasing the mass of the vibrating system and while reducing the stiffness. The radar measurements allows the evaluation of the dynamic behaviour (amplitude of vibration) of several parts of the building located at different heights. In this case we have monitored the structure from different perspectives and we have compared the experimental mode shapes obtained for the IBIS-S configurations 1 and 2. The mode shape retrieved by the configuration 3 was not used for this analysis because the displacements measured in LOS direction are not comparable with the others. In fact, in this configuration the microwave sensor is more inclined (70°) and is placed at short distance from the tower (8 metres). Measured displacements along two directions are projected along the horizontal plane. The experimental mode shapes show a similar trend. We observe that maximum values are measured from St 02 IBIS-S station and are characterized by amplitude ranges from 0.05 to 0.2 mm. The fundamental mode of vibration is a bending mode with the same frequency of 0.96 Hz in both directions of measurement. Anomalous behaviour is detected at the point placed at 17 metres height with experimental measured displacements greater than those expected. This trend could be correlated with the structural damages of the building at the middle level. Moreover, the larger displacements obtained in the second acquisition are justified by the major shift recorded in this direction (about 2 cm) due to seismic action. Conclusions. This paper proposes the use of the ground-based microwave interferometry for remote sensing of vibrations of structures aimed at providing useful information for specialists about the damage state of structures in areas affected by earthquakes. For this reason, this work is the first application of ground-based radar interferometry to the study of an inaccessible damaged building. The authors report the results of the experimental dynamic test aimed at estimating the vibrations of an historical bell tower in an urban environment, performed through ambient vibration testing. Also, the basic principle of the interferometric technique has been briefly resumed and the main experimental data obtained for the vibration features of the structure have been critically discussed to identify anomalous dynamic behaviours linked to structural damages. In general, the proposed method can estimate displacements ranging from a few microns up to several millimetres for large structures, ranging from thin and tall structures to other types of buildings (towers, skyscrapers and bridges). The possibility of working remotely makes this approach suitable for the dynamic control of buildings that have reported structural damage after an earthquake, especially for civil structures of strategic interest during the emergency and for cultural heritage buildings which represent the history and the social identity of a community. 105 GNGTS 2013 S essione 3.2