EARTHQUAKE GROUND MOTION FOR DESIGN OF HOOVER DAM BYPASS BRIDGE (US HIGHWAY 93)

Maximum magnitudes and peak horizontal accelerations were determined for earthquakes on 17 active faults within 169 km of the Hoover Dam Bypass Bridge. A 332-m-long Composite Concrete Deck Arch Bridge was designed on the basis of a nonlinear dynamic analysis using three-component seismograms at each skewback. A 1-s spectral acceleration of 0.139 g was selected as the target ground motion on which to anchor design earthquakes and response spectra. The target acceleration would be produced by a magnitude 6.2 earthquake on the Mead Slop fault at a hypocentral distance of 16km, or by a magnitude 7.0 earthquake on the California Wash fault at 36 km. The smaller magnitude earthquake produced the maximum high-frequency motion, whereas the larger magnitude earthquake produced the maximum low-frequency motion. The design response spectrum for the river bridge was the maximum of the two motions. Synthetic, three-component seismograms at each skewback are produced with a Composite Source Model for use in a nonlinear dynamic analysis of the arch bridge. Input parameters for the model included fault locations, fault-rupture parameters (length, width, displacement, rake and rupture velocity), and seismological source parameters (seismic moment, stress drop, and Green's functions). Acceleration time histories were adjusted to bring their acceleration response spectra into close agreement with the design spectrum.