Evaluation of Site Response Analysis for Vertical Ground Motions in LS-DYNA Using Geotechnical Downhole

This study was motivated by the desire to conduct site-specific vertical site response analysis. The main body of knowledge in this field is limited to horizontal shaking, as such a goal of this study was to extend that knowledge to vertical shaking. Current practice for predicting vertical ground motion is to use the V/H method, which is not site specific. This method utilized measured vertical to horizontal (noise or earthquake) spectral ratios to convert ground motion prediction equation outputs to the vertical direction. The final goal of this study was to validate the methods used in order to justify the extension of the method to conduct soil structure interaction under vertical shaking, which is one of a research priority outlined in the NCHRP Synthesis 428. In this study, three arrays with observed vertical amplifications were selected for validation. Utilizing the LS-DYNA finite element analysis engine, the arrays were modeled as soil columns. The analysis were run under infield and outcrop boundary conditions, and under total and effective stress regimes. Initial Poisson’s ratio was estimated using correlations to small strain shear and primary wave velocities, and an estimate from literature for Poisson’s ratio is made. The predictions from these estimates are compared. Finally the outcrop-effective stress vertical site response analysis is extended to three directional Site response analysis and results are presented (this is not the main focus of the study). The residual of amplification factor is defined as the difference between the observed and predicted amplifications, the values calculated from the predictions in this study are within the limits of other studies. This study is limited to only modeling vertically propagating compression waves, all other wave forms are not captured due to modeling limitations