Design Ground Motions for Cooper River Bridge, Charleston, South Carolina

Abstract

Design horizontal and vertical earthquake response spectra for the replacement Cooper River Bridge in Charleston, South Carolina were developed based on site-specific probabilistic and deterministic ground motion studies. Two basic analyses were completed to develop horizontal response spectra for soft rock conditions. The first was a probabilistic seismic hazard analysis (PSHA) for hard rock conditions. The information used to conduct the PSHA includes the characterization of ground motion attenuation relationships applicable to hard rock in the eastern United States. The second basic analysis conducted for the study was an analysis of crustal response to the hard rock motions. For this analysis, the response of soft rock extending from the top of hard rock at 800 m depth to the base of Cooper Marl at 100m depth was evaluated. The presence of a thick, relatively low velocity soft rock layer having a strong velocity (impedance) contrast with the underlying hard rock resulted in a substantial amplification of long-period ground motions in the analysis. Two additional supporting analyses that were completed include development of deterministic response spectra and vertical response spectra.