Responses of the odd couple Carquinez, CA, suspension bridge during the Mw6.0 south Napa earthquake of August 24, 2014

The behavior of the suspension bridge in Carquinez, CA, during the M_w6.0 24 August 2014 South Napa, CA earthquake is studied. Utilizing data from an extensive array of accelerometers that recorded the earthquake-excited motions, dynamic characteristics such as 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 (0.17 Hz) due to coupling. Higher frequencies for transverse and vertical coupled modes are also the same at 0.46 Hz and 0.98 Hz. Tower translational frequencies are 0.39 Hz in the transverse direction and 0.46 Hz in the longitudinal direction, and are also coupled with those of the deck. Coupling of torsional modes of the tower and deck is also identified. A beating effect is observed, particularly for torsional motions.