Monitoring of Structures by a Laser Pointer: Dynamic Measurement of Displacements of a Bridge

Vertical displacements are among the most important technical parameters to evaluate the health status of a bridge structure and to verify its bearing capacity. Several methods, both conventional and innovative, are used for structural displacement monitoring; no one of these does allow, at the same time, precision, automation, long term and dynamic monitoring without using high cost instrumentation. The proposed system makes use of a common laser pointer and image processing. The measurement of the lowering is obtained by analyzing the single frames of a HD video of the laser beam imprint projected on a flat target. For the processing of images, a code was developed in Matlab® that provides the instantaneous displacement of a bridge, charged by a mobile load. An important feature is the synchronization of the load positioning, obtained by GNSS receiver or by a video. After the calibration procedures, a test was carried out during the movements of a heavy truck maneuvering on a bridge. Data acquisition synchronization allowed to relate the position of the truck on the deck to the displacements. The results show high accuracy and demonstrate that the method is suitable for dynamic load tests, ever more adopted for the bridge controls and monitoring.