System Identification of a Steel Arch Bridge Using Ambient Vibration Tests, Video-Motion Analysis Technique, and Modal Response Analysis

This chapter describes how the modal properties of bridges can be identified by ambient vibration tests and modal analysis. For this purpose, a typical steel arch highway bridge in the Province of British Columbia, Canada, was selected and subjected to a series of ambient vibration measurements. Modal response analysis was then performed to identify the dynamic properties of the structure, including predominant natural frequencies and the corresponding mode shapes to support seismic assessment and upgrading of the bridge. The bridge was constructed in 1960 and consists of a 72.3 m long single span. The superstructure consists of three tied arches, with a steel ladder deck system comprised of transverse floor beams (located at the arch hanger points) and longitudinal stringers, supporting a cast-in-place concrete deck. The ambient vibration testing method was implemented using sophisticated methods of modal analysis. Vibration tests were conducted at the bridge in order to determine the dynamic modal properties (modal frequencies and mode shapes) of the bridge. The testing program consisted of static tests, speed test, and ambient vibration tests including multiple measurement setups. Tromino® velocity/acceleration wireless sensors were used for these measurements which were placed on predetermined locations. The computer program ARTeMIS was used to perform the system identification of the structure. The software allows to develop a 3D model of the structure and test points; the resulting mode shapes are displayed using this geometry. Two different techniques were used for modal identification: Enhanced Frequency Domain Decomposition (EFDD), and Stochastic Subspace Identification (SSI). These two modal identification techniques were used to cross-validate the results. The joint analysis of the signals measured in various strategic points of the structure made it possible to identify the modal configurations and the corresponding natural frequencies. As result, a total of 11 modes were identified in the 0–20 Hz range. Modal frequencies, modal damping, and mode shape were identified for each of the 11 modes. The modes associated with torsional response of the deck showed that the bridge supports are flexible. Video-motion analysis of videos obtained during the load tests were also used to estimate the vertical deformation of the arch near its center. The results were consistent with those obtained from a topographical survey during the load tests.