Applications of the Vision-Based Monitoring System Using a Digital Image Correlation Technique to Long-Span Bridge Cables

Static or dynamic characteristics of civil infrastructures may generally differ from those of their initial design stage. Especially serviceability and stiffness degradation of structures, which are major factors used to verify structural conditions, could be identified by dynamic characteristics. As one of the most important measures to identify the dynamic characteristic of structures, displacement responses have been frequently measured for civil infrastructures even though they have some difficulties depending on the on-site condition of structures. In addition, the contact type displacement sensors may require temporary support frames to fix their sensor bodies or targets. As an alternative to reduce the installation expenses and enhance the reliability of measured data, a noncontact sensing method to measure the displacement responses in a remote distance without attaching sensors is needed. In this study, a method to measure multi-point displacement responses by using a digital image processing technique is suggested. Digital images are acquired by a commercial digital camcorder, which is the sensor to remotely measure displacement responses considering convenient and economical aspects for use. A digital image correlation (DIC) technique as one of digital image processing techniques is applied to develop the image processing algorithm and an image transform function (ITF) to correct the geometric distortion induced from the deformed images is used to increase the resolution of displacement responses. And, the shifting of image measurement equipment is also corrected to increase the resolution of displacement responses and modal frequencies.