Abstract
This paper discusses the performance of a terrestrial radar interferometer for the structural monitoring of ancient masonry towers. High-speed radar interferometry is an innovative and powerful remote sensing technique for the dynamic monitoring of large structures since it is contactless, non-destructive, and able to measure fast displacements on the order of tenths of millimeters. This methodology was tested on a masonry tower of great historical interest, the Saint Prospero bell tower (Northern Italy). To evaluate the quality of the results, data collected from the interferometer were compared and validated with those provided by two types of accelerometer-based measuring systems directly installed on the tower. Dynamic tests were conducted in operational conditions as well as during a bell concert. The first aimed at characterizing the dynamic behavior of the tower, while the second allowed to evaluate the bell swinging effects. Results showed a good agreement among the different measuring systems and demonstrated the potential of the radar interferometry for the dynamic monitoring of structures, with special focus on the need for an accurate design of the geometric aspects of the surveys.
Highlights
The measurement of structural dynamic responses to ambient and induced excitations contributes to the health monitoring of historic masonry towers and toward the identification of possible damages due to natural or anthropogenic sources and material degradation [1,2]
For a proper comparison, the power spectral density (PSD) functions of the accelerometer-based systems were calculated with reference to the same time interval
The experimentation carried out on the Saint Prospero tower allowed to identify the modal parameters of the structure and the displacements induced by playing the major bell, weighing 2.4 tons, with 360-degree oscillations
Summary
The measurement of structural dynamic responses to ambient and induced excitations contributes to the health monitoring of historic masonry towers and toward the identification of possible damages due to natural or anthropogenic sources and material degradation [1,2]. The available literature has proven in a few case studies the reliability of TInRAR techniques to measure masonry tower oscillations caused by natural excitations or bell ringing, and by setting the instrumentation 1-km away from the monitored structures [18]. Monitoring data collected from older TInRAR radar sensors has been validated with dynamic data provided by traditional sensors (such as velocimeters or accelerometers) under natural or induced excitations [23,24] Such a validation procedure needs an accurate design of the experimental tests, a proper definition of the sensor positions in order to allow for a reliable comparison of results, and suitable integration operations when comparing displacements with quantities measured from other types of sensors.
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