Vibration Monitoring and Live Load Tests of Civil Infrastructure with Interferometric Radar

With advances in technology, interferometric radar is expanding the possibilities for monitoring vibrations and performing live load tests of bridges and other structures. This paper presents an instrument (IBIS-S) that utilizes interferometric radar to measure displacement as a function of time. Displacement of most structural elements in the line of sight can be measured to a precision of 0.01 mm at a sampling rate of 200 Hz in a noncontact manner. Traditional instruments used to monitor vibrations often require contact with the members, or noncontact measurements can only be done over a limited range. Further, traditional instruments can be time consuming to set up and costly to maintain. With interferometric radar technology, data typically are obtained in hours, and the monitoring is done from a remote point, precluding the need to access the structure or disrupt the use. It will be shown that the IBIS-S instrument can be deployed quickly, without interrupting the use of the structure, allowing the user to begin collecting vibration data under live loads within minutes. Included in this paper is a literature review of interferometric radar use on buildings, bridges, and wind turbines. Uses include fundamental frequency and mode shape determination of buildings, bridges, and other structures, as well as live load tests. Interferometric radar can be used successfully as part of a structural health monitoring program, wherein baseline vibration and displacement data are obtained and then the measurement process is repeated periodically. Depending on the structure type, changes in fundamental frequency or mode shape can reveal a loss of stiffness, which may be an indication of critical deterioration. Also discussed are the results of vibration monitoring for a cable-stayed pedestrian bridge. It will be shown that interferometric radar technology can be used to determine the global natural frequency of a cable-stayed bridge and the local natural frequency of individual elements (towers, cables, railing, etc.). Limitations of the technology are also discussed.