Responses of the Carquinez, California suspension bridge during the MW6.0 South Napa earthquake of August 24, 2014

The behavior of the suspension bridge in Carquinez, CA, during the Mw6.0 24 August 2014 South Napa, CA earthquake is studied using data recorded by an extensive array of accelerometers. Modes, corresponding frequencies and damping are identified and compared with previous studies that used ambient data of the deck only plus mathematical models. Data are systematically analyzed for vertical, transverse and torsional motions of the deck, and transverse, longitudinal and torsional motions of the towers. The transverse and vertical fundamental mode frequencies of the deck are the same (0.17Hz) due to coupling. Higher frequencies for transverse and vertical coupled modes are the same at 0.46Hz and 0.98Hz. Tower translational frequencies are 0.39Hz in the transverse direction and 0.46 Hz in the longitudinal direction, and are coupled with those of the deck. Coupling of torsional modes of the tower and deck are identified. A beating effect is observed, particularly for torsional motions.