Application of Microwave Remote Sensing to Dynamic Testing of Stay-Cables

Recent advances in radar techniques and systems have favoured the development of microwave interferometers, suitable for the non-contact vibration monitoring of large structures. The paper addresses the application of microwave remote sensing to the measurement of the vibration response in the stay-cables of cable-stayed bridges. The reliability and accuracy of the proposed technique were investigated by comparing the natural frequencies (and the cable tensions predicted from natural frequencies) identified from radar data and the corresponding quantities obtained using more conventional techniques. The investigation, carried out on the cables of two different cable-stayed bridges, clearly highlights: (a) the accuracy of the results provided by the microwave remote sensin

Passive Wireless Vibration Sensing for Measuring Aerospace Structural Flutter

To reduce energy consumption, emissions, and noise, NASA is exploring the use of high aspect ratio wings on subsonic aircraft. Because high aspect ratio wings are susceptible to flutter events, NASA is also investigating methods of flutter detection and suppression. In support of that work a new remote, non-contact method for measuring flutter-induced vibrations has been developed. The new sensing scheme utilizes a microwave reflectometer to monitor the reflected response from an aeroelastic structure to ultimately characterize structural vibrations. To demonstrate the ability of microwaves to detect flutter vibrations, a carbon fiber-reinforced polymer (CFRP) composite panel was vibrated at various frequencies from 1Hz to 130Hz. The reflectometer response was found to closely resemble the sinusoidal response as measured with an accelerometer up to 100 Hz. The data presented demonstrate that microwaves can be used to measure flutter-induced aircraft vibrations

LOCAL POSITIONING SYSTEMS VERSUS STRUCTURAL MONITORING: A REVIEW

SUMMARY Structural monitoring and structural health monitoring could take advantage from different devices to record the static or dynamic response of a structure. A positioning system provides displacement information on the location of moving objects, which is assumed to be the basic support to calibrate any structural mechanics model. The global positioning system could provide satisfactory accuracy in absolute displacement measurements. But the requirements of an open area position for the antennas and a roofed room for its data storage and power supply limit its flexibility and its applications. Several efforts are done to extend its field of application. The alternative is local positioning system. Non-contact sensors can be easily installed on existing infrastructure in different locations without changing their properties: several technological approaches have been exploited: laser-based, radar-based, vision-based, etc. In this paper, a number of existing options, together with their performances, are reviewed. Copyright © 2014 John Wiley & Sons, Ltd

Two-probe implementation of microwave interferometry for motion sensing and complex reflection coefficient measurement

This paper presents the results of the investigations into microwave probe measurements conducted at the Department for Functional Elements of Control Systems of the Institute of Technical Mechanics of the National Academy of Sciences of Ukraine and the State Space Agency of Ukraine over the past five years. These investigations resulted in a two-probe implementation of microwave interferometry that allows one to measure both the displacement of a mechanical object and the complex reflection coefficient of a material specimen.В данной статье приведены результаты исследований по микроволновым зондовым измерениям, которые проводились в отделе функциональных элементов систем управления Института технической механики Национальной академии наук Украины и Государственного космического агентства Украины в течение последних пяти лет. В результате этих исследований разработан двухзондовый вариант сверхвысокочастотной интерферометрии, который позволяет измерять как перемещение механического объекта, так и комплексный коэффициент отражения образца материала.У статті наведено результати досліджень з мікрохвильових зондових вимірювань, що проводилися у відділі функціональних елементів систем керування Інституту технічної механіки Національної академії наук України і Державного космічного агентства України на протязі останніх п’яти років. У результаті цих досліджень розроблено двозондовий варіант надвисокочастотної інтерферометрії, який дозволяє вимірювати як переміщення механічного об’єкта, так і комплексний коефіцієнт зразка матеріалу

Determining and Investigating the Variability of Bridges’ Natural Frequencies with Ground-Based Radar

Assessing the condition of bridge infrastructure requires estimating damage-sensitive features from reliable sensor data. This study proposes to estimate natural frequencies from displacement measurements of a ground-based interferometric radar (GBR). These frequencies are determined from the damped vibration after each vehicle crossing with least squares and compared to a Frequency Domain Decomposition result. We successfully applied the approach in an exemplary measurement campaign at a bridge near Coburg (Germany) with an additional comparison to commonly used strain sensors. Since temperature greatly influences natural frequencies, linear regression is used to correct this influence. A simulation shows that GBR, combined with the least squares approach, achieves the lowest uncertainty and variation in the linear regression, indicating better damage detection results. However, the success of the damage detection highly depends on correctly determining the temperature influence, which might vary throughout the structure. Future work should further investigate the biases and variability of this influence

Generic Radar Processing Methods for Monitoring Tasks on Bridge Infrastructure

Kritische Verkehrsinfrastrukturen, wie z. B. Brücken, können nur dann sicher betrieben werden, wenn ihr Zustand regelmäßig bewertet wird. Neben visuellen Inspektionen umfasst die Bewertung auch Messungen des Brückenverhaltens auf statische oder dynamische Lasten. Diese Messungen werden in der Regel mit einer Vielzahl von Sensoren durchgeführt, die direkt an der Brücke befestigt sind. Zunehmend werden jedoch auch Fernerkundungssensoren eingesetzt, wie z.B. das bodenbasierte interferometrische Radar (engl.: ground-based interferometric radar - GBR). GBR können aus der Ferne Verschiebungen mit einer Genauigkeit im Submillimeterbereich messen, indem sie eine elektromagnetische Welle aussenden, die von Strukturen an der Unterseite der Brücke reflektiert wird. Im Vergleich zu direkt befestigten Sensoren wird die Installationszeit verkürzt und der normale Betrieb der Brücke wird nicht beeinträchtigt. Vergleichbare Messunsicherheiten lassen sich jedoch nur erreichen, wenn bei der Prozessierung der Messungen bestimmte Herausforderungen berücksichtigt werden. Dabei geht es vor allem um die Entfernung externer Einflüsse wie Störungen des Signals oder Veränderungen atmosphärischer Parameter. Die Messungen werden außerdem durch statischen Clutter und Projektionsfehler beeinflusst, die zu systematischen Abweichungen führen. Statischer Clutter wird mit einer angepassten Kreisschätzung bestimmt, während Projektionsfehler durch die Verwendung mehrerer Sensoren zur Schätzung separater Verschiebungskomponenten vermindert werden. Mit diesen zusätzlichen Prozessierungsschritten erreicht GBR eine ähnliche Unsicherheit wie andere Fernerkundungssensoren, was durch Vergleiche mit Referenzsensoren validiert wird. Verbleibende Unterschiede zu diesen Referenzsensoren lassen sich durch Unsicherheiten bei der Schätzung von Clutter und durch die begrenzte Auflösung einzelner Reflexionen erklären. Die resultierenden Verschiebungsmessungen werden dann zur Schätzung schadensempfindlicher Merkmale wie Eigenfrequenzen und Eigenformen verwendet. Eigenfrequenzen werden bestimmt, indem ein Modell einer gedämpften Sinuskurve für die Schwingung nach einer Fahrzeugüberfahrt geschätzt wird. Mit diesem Ansatz wird jede Fahrzeugüberfahrt separat analysiert, was eine Unterscheidung zwischen verschiedenen Fahrzeugmassen ermöglicht. Außerdem erlaubt die große Anzahl von Frequenzschätzungen eine zuverlässigere Bestimmung des Temperatureinflusses auf die Eigenfrequenzen. Für die Bestimmung der Eigenformen wird ein alternativer Messaufbau erarbeitet. Dieser Aufbau nutzt die flache Unterseite einer Brücke, um das ausgesendete Signal auf einen Reflektor auf dem Boden zu spiegeln. Eine permanente Installation von Reflektoren an der Brückenunterseite ist daher nicht erforderlich, wodurch die Anwendung von GBR auf eine große Anzahl von Brücken erweitert wird. Darüber hinaus kann die Messung nicht durch andere Verschiebungskomponenten beeinflusst werden, was das Auftreten von systematischen Abweichungen verringert. Folglich sind die Eigenformen empfindlicher gegenüber Schäden, da die Unsicherheiten reduziert werden. Das zugrunde liegende Prinzip dieses alternativen Messaufbaus wird wiederum durch Vergleiche mit Referenzsensoren validiert

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

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

Geophysical techniques for urban environment monitoring

The research activities conducted in this thesis contributes, through the application of geophysical techniques, to the mitigation of seismic risk with the twofold objective of studying the interaction between the urban subsoil and the overlying-built heritage and carrying out a modal characterisation of a strategic infrastructure. The former objective was pursued by producing a map of the double soil-structure resonance levels of the Matera urban area, while the latter was achieved by setting up and applying an innovative multi-methodological geophysical approach on the Gravina Bridge. As part of the first study, I performed 230 single-station ambient seismic noise measurements on the main lithologies (134) and on the main typology of buildings (96) in reinforced concrete (RC) and unreinforced load-bearing masonry buildings (URM) of the Matera urban area. The ambient seismic noise recorded on the soil 12 min time duration and on buildings 14 min time duration was recorded with a compact digital seismometer and processed using a non-reference site method, the Horizontal-to-vertical noise spectral ratio technique, HVNSR. The measurements taken on the ground and buildings allowed the resonance frequencies and relative amplitudes of the fundamental peaks of the soil and the first elastic frequency of vibration of the buildings to be estimated. A deterministic interpolator (Inverse Distance Weight, IDW) was used in GIS environment to derive the iso-frequency and iso-amplitude maps of the urban area by using as variables the resonance frequencies and amplitudes of the soil HV ratios. A linear period-to-height relationship for the buildings was derived from the experimental results, allowing the fundamental elastic frequency to be estimated for all buildings in the study area. An intersection approach between soil and building frequency bands was used for the first time to derive a map of double soil-structure resonance levels in the linear elastic domain for the whole urban area. Matera represents an important case study since the elastic frequency of vibration for most of the buildings is quite close to that of the foundation soils. In the study area, 21% of the buildings show a high susceptibility to the effect of double soil-building resonance, 63% of the buildings could be characterised by a medium level of double resonance, while 16% could exhibit a zero or very low resonance level. The proposed approach also makes it possible to locate the areas of the city characterised by these different levels of double resonance. Therefore, the first part of the thesis work provided a contribution in assessing the soil – structure interaction effect (SSI, influence of built structures in modifying the ground motion during earthquake shaking) between urban soil and all the overlying buildings in the city of Matera by characterising all the foundation soils of the urban area and all the overlying buildings. A geo-database, the CLARA WebGIS portal (available at this link: https://smartcities-matera-clara.imaa.cnr.it/), for storing and sharing the data and results collected during my PhD activity has been implemented with 488 pre-existing geological, geotechnical, geophysical data. CLARA WebGIS is the first useful tool for predicting which and how many buildings could suffer higher damage due to the double soil-building resonance effect and is the first open geo-platform that shares the results of the double soil-building resonance from experimental data for an entire urban area. CLARA WebGIS addresses a wide range of end-users (local administrations, engineers, geologists, etc.) as support for the implementation of seismic risk mitigation strategies in terms of urban planning, seismic retrofit, and post-earthquake crisis management. The knowledge of the spatial distribution of the site effects (modifications of the ground motions due to changes in the shallow geological layers) in terms of amplification effect, the primary characteristics of buildings, and of soil-building resonance levels estimations, a three-part objective have been achieved: (i) through CLARA's WebGIS every citizen is aware of the characteristics of buildings and foundation soils, so this knowledge makes each individual citizen more resilient to the effects of a seismic event; (ii) preventing the potential losses in economic and social terms; (iii) reducing recovering phase time to facilitate the return of the urban system to equilibrium pre-existing conditions. A deepening of this first study was made by specialising the linear period-height relationship derived from the experimental results as a function of the construction typology and foundation soil for unreinforced load-bearing masonry buildings (URM) founded on rigid soil (Gravina calcarenite characterised by flat HVNSR curves). This relationship is more representative of the condition of a fixed-base masonry building. Variations in the dynamic response of masonry buildings due to soil-foundation-structure interaction at urban scale can be evaluated by simplified analytical approaches based on the traditional compliant-base oscillator model and on simplified assumptions about the geometry and mechanical properties of the soil and foundations. The experimental period-height relationship for URM buildings founded on Gravina calcarenite were integrated in a simplified analytical procedure extended to complex and more realistic stratified soils and irregular foundation geometry. The modified simplified procedure were applied at an urban scale to predict the fundamental period of seven masonry buildings studied in the historic centre of Matera, for which all soil and structural data necessary for the analytical model were available. The comparison of the fundamental periods obtained with the three approaches, traditional, simplified-modified, and experimental, shown that the adoption of the simplified-modified approach significantly improved the agreement between the experimental and analytical periods. This part of the thesis work therefore appears promising to encourage an extended application of the analytical and experimental techniques to other historic urban area characterised by similar characteristics of the built heritage and soil stratification. In the second study of the thesis, has been implemented a multi-methodological approach that allowed to estimate the main modal parameters of the Gravina bridge by analysing short duration ambient noise signals (less than two hours) recorded by low-cost and non-invasive sensors and by performing dynamic tests. The Gravina is an arch bridge located on outcropping limestone in the city of Matera and spans 144 m along a steel-concrete deck suspended by two tubular steel arches. Ambient seismic noise was recorded using two acquisition configurations on the deck and inside the arch. The noise signal data were processed by applying: the standard spectral analysis (FFT), to examine frequencies and energy content distribution, a spectral ratio method with reference station, the Standard Spectral Ratio (SSR) technique, to check and validate eigenfrequencies, the Operational Modal Analysis (OMA) technique, i.e., the Frequency Domain Decomposition (FDD) method, to derive eigenfrequencies and mode shapes, and a seismic interferometric method, the Ambient Noise Deconvolution Interferometry (ANDI), to derive the propagation velocity of ambient noise in the infrastructure. Six eigenfrequencies have been estimated on the deck. The examination of the energy content distribution played a key role for the interpretation of the mode shapes. The variation of the eigenfrequencies of the infrastructure with the seasons as a function of temperature (°C) were monitored: the frequency variations are less than 5% and the behaviour of the structure do not exhibit degradation since the Gravina Bridge is a newly constructed road infrastructure. Deconvolution interferometry has been applied on the ambient noise signals recorded on the deck deriving the wave propagation velocity on the infrastructure. The results presented showed that the ANDI method is sensitive to the distribution of infrastructure stiffness. The multi-methodological approach used in this part of the thesis is promising for (i) evaluating the behaviour of standard structure like buildings and critical infrastructure like a bridge at different scales (global and local), (ii) examining variation of eigenfrequencies, mode shapes and ambient noise waves propagation velocities as a result of aging, degradation, and/or occurrence of potential damage, (iii) controlling and validating outcomes comparing the results obtained from different techniques, (iv) supporting at an early stage as a quick, non-invasive, low-cost tool applied without either diverting, blocking the traffic flow, or stopping the infrastructure service