A thirteen span bridge was recently constructed along the I-15 corridor in Salt Lake City, Utah. The S-shaped steel plate girder bridge continuously spans twelve concrete bents over a total length of 670 meters. Testing of this bridge provided an opportunity to analyze the bridge's natural frequencies and mode shapes from both forced and ambient vibration. An eccentric mass-shaking machine placed at the middle bent induced the forced vibration. Frequency sweeps were conducted in the horizontal directions. Local traffic, wind, and other sources produced the ambient vibrations. This paper focuses on the forced vibration results. Data was collected with a permanent array of eighteen accelerometers wired to two digital recorders at an adjacent free field site as well as arrays of temporary accelerometers and velocity transducers recorded by a separate system. The instruments were connected to the underside of the deck and the bent surfaces along four linear bridge spans. The analyzed data provides a baseline for comparing possible post-earthquake testing to determine deterioration or damage of the bridge. This paper presents the natural frequencies determined for the structure and compares them for three different testing setups. The testing duration spanned several weeks.
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