NH41A-1794
Natural vibration dynamics of Rainbow Bridge, Utah
Thursday, 17 December 2015
Poster Hall (Moscone South)
Jeffrey R Moore1, Michael Scott Thorne2, John R Wood3, Sarah Doyle4, Erik Stanfield4 and Benjamin White1, (1)University of Utah, Geology and Geophysics, Salt Lake City, UT, United States, (2)University of Utah, Salt Lake City, UT, United States, (3)National Park Service, Geologic Resources Division, Lakewood, CO, United States, (4)National Park Service, Glen Canyon National Recreation Area, Page, AZ, United States
Abstract:
We measured resonant frequencies of Rainbow Bridge, Utah, one of the world’s longest rock spans, during a field experiment recording ambient vibration data. Measurements were generated over 20 hours on March 23-24, 2015 using two broadband three-component seismometers placed on the bridge, and compared to concurrent data from nearby reference stations 20 and 220 m distant. We identified seven distinct modes of vibration for Rainbow Bridge between 1 and 6 Hz. Data for each resonant frequency was then analyzed to determine the frequency-dependent polarization vector in an attempt to clarify mode shapes; e.g. the fundamental mode represents out-of-plane horizontal flexure. We compared experimental data to results of 3D numerical modal analysis, using a new photogrammetric model of Rainbow Bridge generated in this study imported into COMSOL Multiphysics. Results compare well with measured data for seven of the first eight modeled modes, matching vibrational frequencies and polarization orientations generally within 10%. Only predicted mode 6 was not explicitly apparent in our experimental data. Large site-to-reference spectral ratios resolved from experimental data indicate high amplification on the bridge as compared to nearby bedrock.
