Noncontact displacement monitoring of stay cables using videogrammetric method

For large-scale civil engineering structures, such as cable-stayed bridges, it is difficult to find fixed reference point to measure the vibration displacement, an important parameter for structure safety evaluation in construction and operation. Therefore, it is favorable and necessary to develop a remote noncontact displacement monitoring method for civil engineering structures. Videogrammetric method is an important non-contact method using video-photos to get space information. This paper investigates the digital image processing technology to detect the vibration displacement of structures. We collect videos on target points on the monitored structure, convert them into a series of digital image frames, calibrate the pixel-distance sensitivity, select a speckle on the image frame, locate the speckle using digital speckle correlation method (DSCM) in each image frame to get the coordinate data of the speckle, and finally obtain the vibration displacement of the target points. In this way, we can carry out the continuous monitoring of dynamic displacement and modal analysis of the structures. After introducing the principle of this method, the one-dimensional vibration displacement of a target object excited by a vibration platform is monitored using the videogrammetric method, and verified by comparing the results with a displacement meter. A stay cable scaled from the longest cable-stayed Sutong Bridge is setup for further test. The cable vibration displacement is monitored using the videogrammetric method. The vibration frequencies are obtained from the FFT transformation of the displacement monitoring data, agree well with the theoretical results. It is proved that the present videogrammetric method is effective and feasible for noncontact displacement monitoring of stay cables and other civil engineering structures.