Targetless precision monitoring of road and rail bridges using video cameras

Video cameras have been used within the research community for a number of years for assessing the static and dynamic displacements, including vibration frequencies, of bridges (decks and cables). Researchers have had considerable success at generating measurements on bridges that correlate well with traditional sensors, demonstrating the potential of a technique that could offer substantial cost savings over an installed network of strain gauges, accelerometers and displacement sensors. As there is often no local reference point by a bridge for making direct displacement measurements, video techniques enable readings to be taken that could not be obtained otherwise, as in the case of the static load test considered here. This paper looks at a number of case studies using a recently developed system based on Digital Image Correlation (DIC) principles, where accuracy has been improved by an order of magnitude over other published algorithms. DIC uses pattern recognition techniques, and then compares these patterns frame by frame to measure displacements (which can be converted into strain or rotation). These case studies demonstrate real-time measurements at over 100Hz, and compare measurements against potentiometer readings, a laser tracker and traditional DIC. Measurements differ by less than 30micrometres on 1mm displacements for relatively close (10m) range and field of view of around 4m2. Rotations are measured to a resolution of 0.001degrees. These are all without the addition of any targets to the bridges in question, enabling truly remote monitoring. The structures investigated include a post-tensioned concrete road bridge with a drop-in centre span, a riveted steel girder rail bridge and a steel highway bridge. Case studies include focused examinations for precision investigations, plus views of whole structures so that movement can be characterised using a single camera.