Radar-based multipoint displacement measurements of a 1200-m-long suspension bridge

https://doi.org/10.1016/j.isprsjprs.2020.06.017Get rights and content

Abstract

Structural displacement monitoring remains a challenging problem, especially for long-span bridges. A ground-based microwave interferometric radar system was developed and used to obtain multipoint displacement measurements of a suspension bridge with a main span of 1200 m. First, a microwave interferometric radar system with the merits of low price, straightforward usage, and good stability and robustness was developed. The phase was obtained by performing fast Fourier transform (FFT) of the in-phase/quadrature signals and calculating the displacement according to the phase interference. Second, the radar system was used in field tests of a long-span suspension bridge, including static, ambient vibration, and moving load tests. Multipoint displacements of the bridge girder, tower, and cables were obtained, and simultaneous measurements of the multipoint displacement influence lines of the bridge girder were performed. A method that uses three reference points to measure the bridge girder displacement was proposed. Good agreements between the results from the developed radar system and those from traditional sensors, including connecting pipes and total stations, demonstrated the excellent performance of the proposed method for remote multipoint displacement measurements of long-span bridges.

Introduction

Bridge displacement due to different environmental conditions and traffic loads is a key indicator of structural safety (Khuc and Catbas, 2017); however, obtaining measurements poses a challenge, especially for long-span bridges (Cho et al., 2018, Liu et al., 2019, Sekiya et al., 2017, Vicente et al., 2018, Ye et al., 2019), due to the limitations of traditional sensors. For example, a linear variable differential transformer (LVDT) usually requires a stationary platform near the measuring point as a reference point, which is often difficult or impossible to achieve. Similarly, a prism has to be used as the observation mark at the monitoring point when using a total station. It is well known that installing a prism on a high bridge tower is a dangerous job. Although a global position system (GPS) sensor is generally easier to install, many studies have shown that its measurement accuracy is low, with an error between 5 and 10 mm (Fukuda et al., 2013, Fukuda et al., 2010, Ribeiro et al., 2014). A laser vibrometer is relatively accurate, but the limited measurement distance limits its application for bridge displacement measurements because long-distance measurements require a high-intensity laser beam that would endanger the workers (Gentile and Bernardini, 2010, Nassif et al., 2005). In addition, all of the above-mentioned sensors can only monitor the displacement of a single measuring point.

Non-contact displacement measurement technologies have been developed to overcome these limitations; these methods include radar interferometry technology and computer vision-based technology. Due to the recent development of advanced optical sensors and high-performance lenses, various vision-based sensors have been widely employed in the field displacement measurement of bridges (Xu et al., 2018, Busca et al., 2014, Ye et al., 2015). Lee and Shinozuka (Lee and Shinozuka, 2006) introduced a vision-based system using digital image processing techniques to measure the dynamic displacement of a bridge remotely and in real-time; the system had the advantages of cost-effectiveness, remote sensing, and real-time measurements. Fukuda et al. (2013) developed a vision-based displacement monitoring system for the remote measurement of the displacement of large-size structures, bridges, and buildings. The authors proposed an object search algorithm to track existing features on structures without requiring a traditional target to be attached to the structures. Feng et al. (2015) presented a vision-based sensor for the remote measurement of the displacement of structures and proposed an advanced template-matching algorithm, which was embedded in the software for displacement extraction from images. The measurement accuracy could be improved by simply adjusting the upsampling factor. Luo et al. (2018) developed a video image processing technique and a gradient-based template-matching algorithm to overcome low lighting conditions in a field test. The displacement of multiple points could be measured simultaneously, even if camera vibration occurred. However, improper lighting or weather conditions have a significant impact on the efficiency of vision-based systems.

In addition to the above-mentioned vision-based techniques, radar interferometry has made significant progress in the past two decades. The method is not affected by weather conditions and has been widely used for real-time displacement measurement of bridges. A microwave interferometer, which was developed based on a commercial Gunn diode transceiver, was first applied to measure the displacement response of a bridge remotely in a forced vibration test. The equipment has the advantages of quick set-up time, low cost, and wide frequency range of response, but does not provide any resolution range because the equipment cannot detect different targets in the field of view of the radar beam (Farrar et al., 1999). To overcome this problem, researchers developed an improved coherent radar and used it to monitor the static displacements of various bridges and buildings (Pieraccini et al., 2004). However, this radar, which was based on a phase-locked loop (PLL) microwave synthesizer, was too slow to capture the dynamic displacement response. Subsequently, through cooperation with the University of Florence, the IDS company developed a high-speed coherent radar called IBIS-S (Image By Interferometric Survey of Structures), which is suitable for dynamic displacement monitoring of structures (Pieraccini et al., 2004). For over a decade, the IBIS-S radar has been widely used for displacement monitoring and analysis in civil engineering construction to measure bridges (Pieraccini and Miccinesi, 2019;11., Gentile and Bernardini, 2008), historical architecture (Liu et al., 2018, Diaferio et al., 2015), and a telecommunications tower (Luzi et al., 2015). Pieraccini et al. (Pieraccini et al., 2005, Pieraccini et al., 2013) used the IBIS-S radar to measure the displacement of the Leaning Tower of Pisa, the bell tower of Giotto, and other famous monuments of the world. The authors addressed the instability of the microwave interferometric radar by installing accelerometers on the back of the radar equipment and using the integral method to reduce the influence of the radar head movement over 30 dB. Gentile and Bernardini (Gentile and Bernardini, 2010) performed a systematic study of the application of the IBIS-S radar for monitoring the vibration of a bridge. The authors extracted the natural vibration frequency and mode of the bridge using radar data and demonstrated the application value of the instrument for bridge modal analysis by comparing the results with those obtained from traditional accelerometers and other sensors. More recently, Rice et al., 2012, Guan et al., 2017 proposed a portable continuous wave (CW) radar sensor with active transponders and used it to monitor in real-time the dynamic displacement of a 50-m long bridge excited by a person jumping on it; the results illustrated the potential of the flexible displacement sensor. The CW radar sensor is suitable for the measurement of small and medium bridges, but for the measurement of long-span bridges, many sensors would be required, and the layout of the active transponders would prove difficult. In addition, researchers have conducted laboratory tests to measure the displacement of objects using a linear frequency-modulated continuous-wave (LFMCW) radar; the sensor was used successfully for displacement measurements (Amies et al., 2019, Xiong et al., 2018).

Current radar interferometry technology has the following limitations. A stepped-frequency continuous-wave (SFCW) radar interferometer is costly, and the operators require considerable expertise because the SFCW waveform signal is complex and difficult to process. Although radar interferometry has been used to measure many structures, its successful use for the measurement of long-span bridges remains rare, and often, corner reflectors are required, resulting in the same difficulties as contact measurements. The objective of this study is to develop ground-based microwave interferometric radar equipment with the merits of low price, straightforward usage, and good stability and robustness. The radar equipment is used without corner reflectors to measure the multipoint displacements of a 1200-m-long suspension bridge using static, vibration, and moving load tests.

The rest of this paper is organized as follows: Section 2 introduces the details of the bridge, including the load cases. Section 3 describes the development of the microwave interferometric radar and the laboratory test results. Section 4 gives the field test results of the measurement of the long-span bridge. Conclusions are drawn in Section 5.

Section snippets

Description of the bridge and the field test

The bridge investigated in this study is a single-span suspension bridge with two towers and a main span of 1200 m. The bridge has 2 main cables and 184 additional cables; the distance between the main cables is 42.1 m, and the distance between the adjacent two cables is 12.8 m. The bridge has steel box girders with a height of 4 m and a width of 44.7 m, and there is an inspection car track at the bottom of the girder. The bridge tower is a portal tower with a height of 193.1 m, and there are 2

Methodology

As shown in Fig. 3, in a test, the radar is usually placed on a pier platform to receive strong echo signals. We assume that the pitch angle is α, the beam angle is θ, and the distance between the radar and the range bin in the direction of the centerline of the beam angle is Rmid. If these parameter values are known, the range of the bridge girder, which is from L1 to LS and is covered by the main lobe, can be calculated, and the distance between the range bin and the radar in the direction of

Static load test (case 1: bridge tower displacement measurement)

The objective of this test was to measure the displacement of the Dongguan Tower in the longitudinal direction. A total of 80 trucks were equally divided into 4 groups to obtain four load levels. The first two groups of trucks were placed on the downstream lane, and the last two groups were placed on the upstream lane [see Fig. 2(a)]. As shown in Fig. 7, the radar was mounted on the beam-column joint of the bridge approach to avoid base point movement. Since we needed to obtain echo signals with

Conclusion

Displacement measurements are challenging to obtain, and radar interferometric technology provides unique advantages. We developed a ground-based interferometric radar system based on the LFMCW and used it to measure the multipoint displacement of a 1200- m-long suspension bridge.

The following conclusions were drawn:

  • 1.

    The proposed ground-based microwave interferometric radar equipment has the merits of low price, straightforward usage, good stability, and robustness. The LFMCW is easily modulated

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This research was financially supported by the National Key R&D Program of China (No.: 2018YFC0705601) and the National Natural Science Foundation of China (Grant No.: 51778134, 51578139).

References (36)

  • G. Busca et al.

    Vibration monitoring of multiple bridge points by means of a unique vision-based measuring system

    Exp. Mech.

    (2014)
  • Z.-W. Chen et al.

    Damage quantification of beam structures using deflection influence lines

    Struct. Control Health Monit.

    (2018)
  • Z. Chen et al.

    A systematic method from influence line identification to damage detection: application to RC bridges

    Comput. Concr.

    (2017)
  • S. Cho et al.

    Comparative study on displacement measurement sensors for high-speed railroad bridge

    Smart Struct. Syst.

    (2018)
  • Diaferio, M., Foti, D., Gentile, C., Giannoccaro, N.I., Saisi, A., 2015. Dynamic testing of a historical slender...
  • D. Feng et al.

    A vision-based sensor for noncontact structural displacement measurement

    Sensors

    (2015)
  • Y. Fukuda et al.

    Cost-effective vision-based system for monitoring dynamic response of civil engineering structures

    Struct. Control Health Monit.

    (2010)
  • Y. Fukuda et al.

    Vision-based displacement sensor for monitoring dynamic response using robust object search algorithm

    IEEE Sens. J.

    (2013)
  • Cited by (0)

    View full text