Dynamic Evaluation of a Bridge Substructure with Experimental Modal Analysis

According to FHWA, more than 36,000 bridges over waterways (riverine and tidal) are identified as having unknown foundations. Unknown bridge foundations pose a significant problem to the assessment of hydraulic and seismic vulnerability and post-hazard evaluation of bridges. In addition, bridge foundations reuse could become a viable option in the replacement of structurally deficient superstructures, potentially providing technical, economic, time and environmental benefits. Characterization and evaluation of existing bridge substructures has become an essential issue for the safety and serviceability of bridges. In recent years experimental modal analysis has become an effective tool for bridge condition assessment and system identification. However, many studies tend to emphasize dynamic testing and analysis of bridge superstructures with little consideration given to bridge substructure and their foundations. In this paper, a 3 non-continuous spans bridge with concrete deck and steel girders is used as a case study to investigate the possibility of characterization and condition evaluation of the substructure system leveraging dynamic testing methods. MIMO impact testing and operating modal analysis was operated on one of the piers of this bridge. Corresponding modal identification algorithms including CMIF and Bayesian operational modal analysis method is discussed. Modal parameters, including modal frequencies and mode shapes obtained from two experimental modal analysis methods, are presented and compared.