Ambient vibration measurements for non-destructive evaluation of structures by means of seismic methods and ground-based microwave interferometry

This thesis collects the results of the researches carried out during the Ph.D. course in “Technologies for the Conservation of Architectural and Environmental Heritage” into the School of Civil Engineering and Architecture of the University of Cagliari. The main topics of this research are focused on the experimental dynamic analysis of the structures with prevalent, but not exclusive, interest for the analyses of historic structures. The research deals with several issues concerning both the experimental measurements and the relationships between vibration properties and structural features and materials. The study starts with an introduction, developed into the first chapter, where the general principles of the experimental dynamic analysis methods of structures are presented and summarised, especially for the passive techniques based only on ambient vibrations records. The next chapter presents and describes the main features of the Ground-based Radar Interferometry to perform remote measurements of vibrations, using the phase difference between reflected signals coming from the surface of the same object inside the radar scenario. This technique has been developed in relatively recent years and has seen a considerable spread thanks to the short time need for the measurements and for the capability to retrieve reliable time series of displacement without any contact sensors above the structures. Furthermore the vibration data, acquired with both conventional systems (such as seismic sensors, velocimeters, accelerometers, etc.) and the microwave interferometer IBIS-S (Image By Intereferometry Survey), have been compared. Different case studies have been examined and critically discussed in the following chapters. In particular, chapter three is focused on the analysis of the vibration properties of an earthquake damaged bell tower located near the epicenter of the Emilia earthquake. Both ex ante and ex post conditions respect to the seismic induced damage have been compared. For this purpose a non-contact dynamic survey has been carried out by means of the radar interferometry method. The campaign of measurements has been conducted after the earthquake to describe the dynamic behaviour of the structure with open fractures pattern and with significant structural damages. Finally, a Finite Element model of the structure has been done in order to compare the actual dynamic response of the tower with that one of the undamaged structure. Chapter four looks at the influence of the vibration artificially induced by the coordinated movement of twenty people to improve main dynamic properties identification of the structure. In this case, the measurements have been carried out using the radar sensor by means of four stations located around the examined structure, the Leaning Tower of Pisa. The measurements have been performed in both operational mode using only wind induced vibrations and also with the artificial human forcing, applied at the top floor of the tower. Chapter five describes both the experimental measurements and the numerical modelling carried out in order to derive the dynamic features of two similar bell towers. The comparison between the dynamic behaviour of the towers is aimed at studying the influence of the mechanical properties of different construction materials. In fact, the towers are symmetrically built on both sides of the main façade of a church but the two structures are made using different materials and with different construction techniques. The oldest tower is a stone masonry building and the second one is a Reinforced Concrete structure. In this case, the analyses have been carried out using vibration data acquired by means of both available systems, i.e. the IBIS-S radar interferometer for the 2 measurements related to the upper parts of the structure (not easily accessible) and some seismic transducers for the stations located inside of the building. Chapter six finally presents the vibration measurements performed on a double curvature arch dam with different reservoir water level in order to analyse the variation of the linear dynamic response of the structure related to the water level height on the upstream side of the dam. The experimental surveys are described and the comparison with a numerical modelling is shown.