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Post-earthquake Assessment of Nevada Bridges using ShakeMap/ShakeCast
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Post-earthquake capacity of Nevada highway bridges is examined through a combination of engineering study and scenario earthquake evaluation. The study was undertaken by the University of Nevada Reno Department of Civil and Environmental Engineering with the collaboration of the Nevada Seismological Laboratory under project funding from the Nevada Department of Transportation, with technical engagement through the NDOT Bridge Division. The vulnerability of Nevada bridges relative to earthquake hazard is evaluated using two different methods. First, a distributed set of 112 realistic earthquake scenarios were processed with USGS program ShakeMap, and USGS program ShakeCast was used to extract site-specific ground motion levels for the 1831 Nevada bridges in the Federal Highway Administration National Bridge Inventory. Second, using hazard curves underlying the 2014 USGS National Seismic Hazard Map (NSHM), return periods for earthquakes causing extensive damage to bridges were extracted and compared to the 1000-year design level adopted by the AASHTO. For both methods, demand vs. capacity comparisons were restricted to spectral acceleration at 1 Hz. Initial bridge capacities for ShakeCast were adopted from corresponding HAZUS estimates on the basis of bridge design type. HAZUS capacities were found to be too high for five bridge design types: HWB10-205 and HWB11-205, Continuous Concrete Box Girder, non-seismically and seismically designed, respectively; HWB15-402, Continuous Steel Bridge; and HWB22-605 and HWB23-605, non-seismic and seismic Continuous Prestressed Concrete Box Girder. HAZUS capacities for these bridge types considered only columns and not the broader suite of potential failure modes recognized in subsequent studies. Revised capacity values were proposed in consultation with NDOT engineers using a combination of literature review, Nevada bridge plans, and Nevada bridge design spectra. The two evaluation approaches provide complimentary views of bridge performance. Scenarios from ShakeMap provide points in a deterministic seismic hazard approach. The occurrence of the earthquake is assumed, and bridge response is interpreted independent of the probability of the demand. The probability aspect of strong ground shaking enters the analysis through the NSHM hazard curves, which are based on a probabilistic approach. A graphical method is presented to unite the two approaches. A list of potentially vulnerable bridges was developed for use by the Nevada Department of Transportation (NDOT) in bridge retrofit planning. As a continuing benefit, ShakeCast now operates in Nevada to provide near-real-time inspection priorities in the event of a serious earthquake. In additional a damage assessment and repair manual has been developed, presented in an appendix. The manual initially describes the typical damage that would be observed as a result of an earthquake and then describes methods for repairing this damage.
Report No. CCEER-16-04