Modal analysis and tension estimation of stay cables using noncontact vision‐based motion magnification method

Summary This paper describes a study on modal analysis and tension estimation of stay cables by vision‐based measurement under ambient condition. Microvibration of stay cable is captured by video camera, and the amplitude is amplified using phase‐based video motion magnification method. From the sequences of cable images, spatial displacements of the cable are extracted via discretized centroid searching method. Modal parameters of the cable, namely, natural frequency, damping ratio, and mode shape, are identified by dynamic mode decomposition method from cable displacement responses. Furthermore, tension of the stay cable is estimated based on the identified natural frequencies by minimizing an error function of an approximate relationship between frequency and tension iteratively. The technique is verified in the laboratory‐scale experiments and implemented to full‐scale measurement of medium size and long‐span cable‐stayed bridges. To compare the accuracy and efficacy of the vision‐based method, noncontact vibration measurement using laser Doppler vibrometer was also conducted. The results demonstrate that the proposed vision‐based vibration measurement techniques can estimate modal parameters and tension of the stay cables accurately in a noncontact and distant measurement under ambient condition. The proposed method offers an alternative of effective and accurate vibration measurement of stay cables using only motion of the cables recorded by video camera.