Multi-sensor measurement of dynamic deflections and structural health monitoring of flexible and stiff bridges

We investigated the response of bridges of different types to controlled and to wind and traffic-induced excitations; the emphasis was on deflections,derived from recordings of geodetic sensors and accelerometers (output-only analysis). Our focus was to push the limits of the existing experimental techniques, in order to cover not only flexible, but also stiff structures, and to present independently validated results. Our study focused on a 700m long, thin-deck cable-stayed bridge, a stiff steel pedestrian bridge, a historic composite (masonry/steel) train bridge and a 30m long, gradually decaying, currently swaying pedestrian timber bridge. Our basic strategy was first to develop data measurement and processing techniques using controlled (supervised learning) experiments, and then, (1) use collocated, redundant and distributed geodetic sensors (GPS/GNSS and Robotic Total Stations, RTS), as well as accelerometers, in order to record bridge excitations, especially con-trolled excitations leading to free attenuating oscillations;(2) develop techniques to denoise recordings of various sensors based on structural/logical constraints and sensor fusion, compensating for the weaknesses inherent in each type of sensor), validate results and avoid pitfalls;(3) monitor the episodic and gradual decay of a pedestrian bridge, through repeated surveys under similar loading and environmental conditions and using similar instrumentation.The output of our studies is to confirm the potential of modern sensors to measure, under certain conditions, reliable mm-level dynamic deflections even of stiff structures (3-6Hz dominant frequencies) and to provide firm constraints for structural analysis, including evidence for changes of first modal frequencies produced by structural decay, even to identify dynamic effects such as foundations response to dynamic loading

Measuring sub-mm structural displacements using QDaedalus: a digital clip-on measuring system developed for total stations

The monitoring of rigid structures of modal frequencies greater than 5 Hz and sub-mm displacement is mainly based so far on relative quantities from accelerometers, strain gauges etc. Additionally geodetic techniques such as GPS and Robotic Total Stations (RTS) are constrained by their low accuracy (few mm) and their low sampling rates. In this study the application of QDaedalus is presented, which constitutes a measuring system developed at the Geodesy and Geodynamics Lab, ETH Zurich and consists of a small CCD camera and Total Station, for the monitoring of the oscillations of a rigid structure. In collaboration with the Institute of Structural Engineering of ETH Zurich and EMPA, the QDaedalus system was used for monitoring of the sub-mm displacement of a rigid prototype beam and the estimation of its modal frequencies up to 30 Hz. The results of the QDaedalus data analysis were compared to those of accelerometers and proved to hold sufficient accuracy and suitably supplementing the existing monitoring techniques

Detection of GNSS antenna oscillatory motion and multipath conditions via exploitation of multipath-induced SNR variations

The multipath effect is well known as one of the dominant error sources in most high-precision GNSS applications, as its site-dependent and fast-changing nature render it challenging to model and mitigate using differencing techniques. However, in this study, we present a novel approach of using the multipath effect of GNSS signal in the case of oscillatory motion of a GNSS antenna to determine the characteristics of the oscillatory motion by utilizing the signal-to-noise (SNR) ratio measurements of the GNSS signals and without applying the positioning solution or the use of other sensors. The proposed method is based on the hypothesis that for a short time period, while the multipath conditions remain constant and the satellites are approximately at the same position, an oscillation-type motion of the antenna should generate changes in multipath, expressed as oscillatory pattern variations in the SNR measurements of the satellites. This approach can be used to identify the multipath parameters and extract characteristics of the oscillatory motion of the GNSS antenna. The SNR-based modeling of cm-level antenna oscillations was demonstrated in simulated and field experiments. The benefits of this method include immunity to poor satellite constellation geometry, adaptability to changing multipath geometry and fast processing time. This approach could potentially, under conditions, contribute to GNSS reflectometry (GNSS-R) applications, where the oscillatory motion of the GNSS antenna generates the same effect as the satellite motion-induced multipath, limiting simultaneously the observation periods and the amount of data to be processed

« Subsidence and evolution of the Thessaloniki Plain, Greece, based on historical levelling and GPS data ».

International audienc

Feasibility and Assessment of Real Time Monitoring Systems for Smart Structural Control of Wind Turbines

A semi-active (SA) control strategy has been proposed by the authors in the recent years to mitigate structural demand to high wind turbines against strong wind loads. Numerical as well as experimental analyses shown it is really promising and potentially useful to owners who want to optimize costs of realization and installation of such huge structures. The application to real cases of this technique, since based on the use of variable dissipative devices, requires the tower is instrumented for real time monitoring of structural response of the tower. This allows the control algorithm to make the decision about the optimal calibration, moment by moment, of the variable devices. Making reliable high frequency measurements of the horizontal displacement of points placed at a height of tens of meters can be not so trivial. With the aim of evaluating the efficiency and feasibility of Global Navigation Satellite System (GNSS)-based systems for structural control of wind turbines, this paper try to obtain insight into the characteristics (receiver type, type of observables, sampling data rate) and data processing techniques that can make the GNSS useful for such application. Finally, numerical investigations referred to a case study allow to discuss how the features of the measurement system may affect the performance of the proposed SA technique in reducing structural demand due to wind induced vibrations