Noncontact operational modal analysis of light poles by vision-based motion-magnification method

Abstract

Light and sign poles on elevated highway bridge are secondary structural elements that support functionality of a highway system. Over the time, they may suffer from fatigue damage due to traffic, wind, and earthquake-induced vibration in a direct or indirect manner. As a secondary structural component, vibrations of light and sign poles are influenced by the bridge vibration, the primary structure. Structural condition assessment of the light and sign poles are normally conducted by visual inspection and/or vibration measurement using contact or noncontact method. In this paper, a vision-based vibration measurement of the light poles on elevated highway bridges using motion magnification and dynamic mode decomposition methods is proposed. Using the method, vibrations of the light poles are captured by video camera and the phase-based motion magnification method is implemented to magnify micro-vibration of the structure. By employing discretized centroid searching method, spatial displacements of the light poles are extracted. Modal parameters of the structure are then identified from the displacement responses by dynamic mode decomposition method. The study includes laboratory experiments for verification, and the full-scale implementation on existing light poles on elevated highway bridges. The vision-based results are compared with the noncontact vibration measurement using Laser-Doppler vibrometers. Results of experiments and full-scale tests demonstrate the capability of the proposed method in extracting multi-mode vibration characteristics of the light poles accurately in a noncontact long-distance measurement.