Forced Vibration Testing of the I-15 South Temple Bridge Phase 1 Final Report

16. Abstract The objective of this research program is to investigate the potential for using system identification as a non-destructive evaluation technique. This research examines the feasibility of performing system identification on a large, multi-degree of freedom structure and a simple, single span structure. The testing consisted of performing sine sweeps over a range of excitation frequencies, with the excitation induced in the horizontal direction by an eccentric mass shaker. The response of the two bridge structures was recorded with accelerometers. The simple span structure was tested in seven condition states that included post-damage testing. In the case of the nine-span bridge the lowest five response modes and frequencies were determined, demonstrating that system identification of large bridge structures is possible. For the simple span structure the lowest three mode shapes and frequencies were determined for each condition state. The change in the natural frequencies for each condition state demonstrated the new condition of the bridge, whether it was post-damage or post-repair. This indicates that system identification (modal analysis) has potential as a non-destructive evaluation method for determining structural integrity. Executive Summary The objective of this research is to investigate the potential for using system identification, the determination of a structural system's response characteristics through dynamic testing procedures, as a non-destructive evaluation technique to assess damage. This project consisted of two phases. The first examined the feasibility of performing a system identification for a large, multi-degree of freedom (MDOF) structure in the form of a multi-span bridge. The second phase consisted of a forced vibration study of a simple bridge span, the remnant of the multi-span bridge after demolition, in various states of damage and repair. The bridge selected for this research, made available through the I-15 reconstruction, was the northbound I-15 bridge over South Temple Street. This was a nine-span, skewed bridge with a considerable amount of open area underneath that making the physical testing of the bridge possible. Previous studies on system identification have focused on single-degree of freedom (SDOF) systems and reveal the benefits of, and methods for, system identification. System identification becomes much more complicated when applied to large, MDOF systems. This project reveals the difficulties in testing large, in-situ structures and the modifications of typical SDOF testing methods required to accommodate large MDOF structures. The results of this research in terms of frequencies and mode shapes for the nine-span bridge are shown as well as …