Carmelo Gentile

An interferometric radar for non-contact measurement of deflections on civil engineering structures: laboratory and full-scale tests

Recent progress in radar techniques and systems has led to the development of a microwave interferometer, potentially suitable for non-contact vibration monitoring of civil engineering structures. The main characteristic of the new radar system, named IBIS-S, is the possibility of simultaneously measuring the (static or dynamic) displacement at several points of a structure with high sensitivity. The paper first describes the technical characteristics of the microwave interferometer and the results of laboratory tests carried out to evaluate the actual displacement sensitivity of the equipment. Subsequently, the application of the sensor to the measurement of ambient vibration response on a concrete bridge is presented and discussed.

Dynamic investigation of a repaired cable-stayed bridge

The theoretical and experimental investigation of a cable-stayed bridge after major repair is described in this paper. Strengthening mainly involved the suspension system (originally with prestressed concrete stays) which was retrofitted by means of external tendons. Full-scale tests were conducted to measure the dynamic response of the repaired system; the experimental program included both traffic-induced and free vibration measurements. A total of 16 vertical frequencies and mode shapes were identified in the frequency range of 0–10 Hz. In the theoretical study, vibration modes involving deck, towers and cables were determined by using finite element models which accounted for the strengthening effects. Two- and three-dimensional models were used so that the importance of three-dimensional modes was estimated as well. The experimental results were compared to natural frequencies and mode shapes computed using theoretical models. For most modes the measured and predicted modal parameters compare well, especially for the vertical modes involving in-phase motion of the stays.

Structural Identification of a Masonry Tower Based on Operational Modal Analysis

A wide program of structural assessment has been carried out by Politecnico di Milano on the historic bell tower of the church Chiesa Collegiata in Arcisate (Varese, northern Italy). Within this context, the first part of the article summarizes the results obtained from the application of the stochastic subspace identification method to ambient response data collected in two dynamic tests, carried out in June 2007 and June 2008. Next presented is the vibration-based methodology developed for the calibration of a three-dimensional finite element model of the tower, consisting in the successive application of systematic manual tuning, sensitivity analysis, and simple system identification algorithm.

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 sensing; (b) the simplicity of use of the radar technique (especially when compared with conventional approaches) and its effectiveness to simultaneously measuring the dynamic response of all the stay-cables of an array.

A Software Tool for Processing the Displacement Time Series Extracted from Raw Radar Data

The application of high‐resolution radar waveform and interferometric principles recently led to the development of a microwave interferometer, suitable to simultaneously measuring the (static or dynamic) deflection of several points on a large structure. From the technical standpoint, the sensor is a Stepped Frequency Continuous Wave (SF‐CW), coherent radar, operating in the Ku frequency band.In the paper, the main procedures adopted to extract the deflection time series from raw radar data and to assess the quality of data are addressed, and the MATLAB toolbox developed is described. Subsequently, other functions implemented in the software tool (e.g. evaluation of the spectral matrix of the deflection time‐histories, identification of natural frequencies and operational mode shapes evaluation) are described and the application to data recorded on full‐scale bridges is exemplified.

Dynamic assessment of a curved cable-stayed bridge at the Malpensa 2000 Airport, Milan, Italy

Theoretical and experimental investigations of a curved cable-stayed bridge are described in this paper. Full-scale tests were conducted to measure the dynamic response of the bridge, and two excitation techniques were considered: traffic-induced ambient vibrations and free vibrations. Since the modal behaviour identified from the two types of tests correlated very well, the validity of the ambient vibration survey is assessed in view of future monitoring. In the theoretical study, vibration modes were determined by means of a three-dimensional finite-element model of the bridge and the information obtained from the field tests provided a detailed assessment and validation of the model formulation and accuracy.

Fusion of wireless and non-contact technologies for the dynamic testing of a historic RC bridge

In this paper, a dynamic testing and corresponding signal processing methodology is presented for condition assessment of bridge structures, via use of a diverse and potentially dense grid of low-cost and easily deployable monitoring technologies. In particular, wireless and non-contact sensors are simultaneously deployed on a historic reinforced concrete bridge in order to record acceleration and dynamic displacement response, under operational loading conditions. An innovative monitoring approach is proposed on both the hardware (sensors) and software (algorithmic) front, in which an effective data fusion procedure is adopted for fusing these alternative technologies for vibration-based monitoring in terms of both acceleration and displacement information. The demonstrated efficacy of the fusion procedure on the case-study of an actual operating system, the historic Brivio bridge, reveals the potential of this approach within the context of structural monitoring, where acquisition of heterogeneous information certainly proves advantageous.

Radar-based measurement of deflections on bridges and large structures

ABSTRACT Recent progresses in radar techniques and systems have favoured the development of microwave interferometers, suitable to the non-contact measurement of deflections on large structures. The main characteristic of the new radar systems is the possibility of simultaneously measuring the static or dynamic deflection of several points on a large structure with high accuracy. The objectives of the paper are: a) to discuss some radar techniques implemented in the microwave interferometer, in order to highlight advantages and potential issues of the radar-based measurement; b) to illustrate the application of the radar technique in live-load static and ambient vibration tests performed on a full-scale bridge.

Dynamic Assessment of the Iron Bridge at Paderno d’Adda (1889)

The paper presents the experimental modal analysis recently carried out on the historic iron bridge at Paderno d’Adda (1889). The dynamic tests were performed in operational conditions (i.e. under traffic and wind-induced excitation) between June and October 2009 and different output-only identification techniques were used to extract the modal parameters from ambient vibration data. The described tests represent the first experimental investigation carried out on the global characteristics of the bridge, since the load reception tests of 1889 and 1892.

Ambient vibration testing of bridges by non-contact microwave interferometer

Recent progress in radar techniques and systems led to the development of an interferometric radar, suitable to the application in dynamic testing and vibration monitoring of civil engineering structures. The main characteristic of the new radar system is the possibility of simultaneously measuring the (static or dynamic) displacement of several points of a structure with high sensitivity. This describes the major characteristics of the microwave interferometer and its application to ambient vibration testing of full-scale bridges.