Deflection Measurement of Bridge Using Images Captured Under the Bridge by Sampling Moiré Method

Measuring the deflection of bridges in service is crucial for monitoring their structural health conditions. Recently, the image measurement method has been developed to measure the bridge deflection, wherein the camera for taking images is installed at a distant place where the target bridge can be seen. In this study, we developed a simple but effective optical method to measure the out-of-plane displacement of a bridge, which could not be done before, from the images captured with a camera installed under the bridge. The proposed method converts the virtual in-plane displacement evaluated from the sampling moiré measurement into the out-of-plane displacement under the assumption that the target bridge and the reference point move in the in-plane direction as a rigid-body deformation, or the in-plane displacement is absent or negligible. In field experiments to measure the deflection of a viaduct during the passage of a high-speed train (Japanese Shinkansen) with a speed of 320 km/h, we successfully measured the bridge deflection less than 1 mm from images recorded with a camera installed under the bridge floor. When comparing the difference in deflections measured from the proposed method and a conventional Laser Doppler vibrometer, the average absolute difference was less than 0.05 mm. Our developed method is simple and cost-effective, making this technique very efficient in structural health monitoring of elevated railway or road bridges.