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Application of one-dimensional geotechnical modeling for site response predictions
Kwok, On Lei University of California, Los Angeles 2007 해외박사(DDOD)
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Ground motion attenuation relationships are used in seismic hazard analyses to provide a probabilistic distribution of ground motions. Estimates from such relationships represent averaged values across a wide range of site conditions, while actual site response can be influenced by sediment response to upwardly propagating body waves (ground response effects), basin effects, and surface topography. This study focuses on 1D models that can be used for site-specific evaluations of ground response. A set of theoretical site factors derived from one-dimensional ground response analyses for specific geologic categories in the Los Angeles area and the San Francisco Bay Area are reviewed. Analysis of residuals between the data and model predictions indicates that these theoretical site factors are able to capture the average effects of sediment nonlinearity. However, the short-period spectral accelerations are under-predicted and the long period spectral accelerations are over-predicted. The under-prediction at short periods may result from overestimation of a soft-to-firm reference rock correction factor that is needed to apply the site factors. The over-prediction bias at long periods varies with depth, suggesting that the depth term in the site factors may not fully account for the effect of basin geometry at long periods. Exact solutions for body wave propagation through an elastic medium are used to establish guidelines for two issues of nonlinear ground response analysis models. The full outcropping (rock) motion with an elastic base is recommended based on this work. Moreover, the use of full Rayleigh damping with target damping ratio set at the small-strain material damping is recommended. The parameter selection protocols are tested by comparing predictions to data from vertical arrays and looking for trends in the results. It is found that site amplification is under-predicted for all three sites across the full range of periods investigated except for the elastic site period. At the site period, the models predict strong site amplification that is not present in the data and which causes local over-prediction. The bias likely results from a combination of errors in velocity profiles (possible for two of the three sites) and potential over-damping (which would affect all three sites).