Seismic Hazards in the Eastern U.S. and the Impact on Transportation Lifelinesby Klaus H. Jacob, Columbia Univ, Palisades, United States,
Document Type: Proceeding Paper
Part of: Lifeline Earthquake Engineering in the Central and Eastern U.S.
In the eastern US seismicity is low, hazard is moderate, and risk is quite high, especially in metropolitan regions with vulnerable transportation systems typically not designed to resist earthquake loads. Seismic load ratios for earthquakes with 2,500-year vs. 500-year average recurrence periods measure from 2 to 4; hence, they are about 2-times higher in the eastern than in the western US. To account for these higher load ratios we suggest that for the design and retrofit of ordinary bridges, a recurrence period of about 1,000 years be chosen. For essential and critical bridges recurrence periods of about 2,500 and 5,000 years, respectively, are proposed (subject to further review). Ideally, design strategies should be fully risk-based (i.e. based on societal loss/cost/benefit relations), rather than mostly hazard-based as is essentially the case now. Site or soil factors in current seismic bridge design codes (e.g. AASHTO) do not serve well to account for the higher ground motion amplification on soft soils, nor reduction on hard rock, typical for the eastern US and for periods ≥.3sec. The site factors urgently need revisions for which models exist as exemplified by the newly drafted New York City seismic code provisions. Quantitative methods need to be codified for screening soil profiles for their liquefaction potential. In the eastern US, soil liquefaction and slope instabilities have been observed to much larger distances than in the western US. In short: the seismic risk to bridges and transportation lifelines in the eastern US is higher than the low level of seismicity alone would suggest. This higher risk needs to be addressed for the design of new, and retrofit of existing systems. Risk-rather than purely hazard-based design/retrofit strategies could be cost-effectively applied to gauge the seismic safety requirements of essential and critical transportation systems which society expects to function even after significant earthquakes.
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