GNGTS 2013 - Atti del 32° Convegno Nazionale

station was placed in the SW direction (30° LOS angle). The last acquisition was performed with the IBIS-S system located under the tower (70° LOS angle). The acquisition geometry was selected in order to reduce the effects of all possible noise sources, such as suspended cables and other metal structures, or trees and vegetation inside the radar profile. Each acquisition had a duration of about 10 min with a sampling frequency of 200 Hz. The vibrations of the building were monitored in real time during field operations. The Fourier spectra of signals were provided by processing the acquired data with IBIS Dataviewer and Matlab software in order to assess the main frequencies of the structure. Before of the spectral analysis, the displacement data were detrended and tapered with Hamming function to reduce leakage effects. The knowledge of the deformation kinematics allows to evaluate main vibration properties of the structure by means of the Frequency Domain Decomposition technique FDD (Brincker et al. , 2001). This method is based on the Singular Values Decomposition of the output Power Spectral Density matrix G yy through the following Eq. (3): (3) where [U] i is a unitary matrix of the singular vectors u ij , S is a diagonal matrix of the scalar singular values s ij and [U] H is the hermitian matrix of [U] (complex conjugate transposed matrix). Assuming that each mode will be dominant around its corresponding frequency, the first singular vector represents an estimation of the mode shape and the corresponding singular value belongs to the auto-power spectral density function of the corresponding Single Degree Of Freedom system. By using this decomposition method, main modal parameters can be identified with high accuracy even in the case of strong noise contamination of the signals. Results and discussion. The range-bins with high signal-to-noise ratio (SNR) along the radar power profile are associated with different natural back-scatterers located inside the radar scenario. However, the signal-to-noise ratio of the radar response from this structure is high and the structure is isolated inside the radar scenario. Thus several range-bins may be selected to be analyzed. Five range-bins of the radar power profile are selected to estimate fundamental vibration properties of the structure by means of the FDD decomposition technique. The radar- bins are selected in function of the distance along the radar line of sight between each radar- bin and the IBIS-S sensor. These range-bins correspond to five reflective points placed on the bell tower facade. This operation is easily performed knowing the geometric characteristics of the tower (the height of several architectonic elements such as windows and other ledges), the horizontal distance of the radar station, and the inclination of the IBIS-S head. The displacements retrieved from radar data are shown in Fig. 2. These time series have been obtained observing the tower from a position to the base of the tower, at a distance of 23 metres Fig. 1 – a) Installation of the IBIS-S system in several acquisition stations placed at 25 m (St 01) and 23 m (St 02) from the base of the tower with LOS angle of 30° (central beam) and at 8 m (St 03) with LOS angle of 70°. b) Radar line of sight displacement and true displacement vector. c) Image of the IBIS-S system used during the experiments. 103 GNGTS 2013 S essione 3.2