Performance Evaluation of One-Dimensional Site Response Analysis: Insights from Physics-based 3D Earthquake Simulations
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Authors
Huang, Junfei
Issue Date
2024
Type
Dissertation
Language
en_US
Keywords
1D site response , earthquake simulation , ground motion , inclined seismic wave , sedimentary basin , soil-structure interaction analysis
Alternative Title
Abstract
Despite the complexity of real earthquake ground motions, one-dimensional site response analysis (1D SRA) is the current state of practice for considering the modification of incident seismic waves due to local soil deposits and predicting site-specific ground motions. In this idealized 1D approach, the local soils are assumed to be composed of stacked horizontal layers without lateral heterogeneity, and the horizontal and vertical ground shaking are assumed to be induced by vertically incident shear and compressional waves, respectively. This 1D idealization decouples the horizontal and vertical components of the actual seismic wavefield and greatly simplifies the reconstruction of the input excitation for practical engineering applications. However, recent site response studies have highlighted potentially significant deviation of the 1D predictions to actual recordings from earthquakes. Of particular concern is the estimation of site-specific vertical ground motions for critical infrastructures, where this 1D approach has routinely been observed to predict abnormal vertical site amplifications. These observations have collectively cast new doubts on the validity of this idealized 1D site response procedure for approximating real-world complex seismic wavefields.This doctoral study conducts a comprehensive performance evaluation of 1D SRA when applied to free-field site response prediction and soil-structure interaction (SSI) analysis of civil infrastructures. A simulation-based approach is adopted, and a progressive set of numerical studies based on realistic simulated ground motions from broadband physics-based three-dimensional (3D) earthquake simulations were carried out to gain physical insights into the observed discrepancies. The results of this study show that in actual seismic wavefields, the horizontal and vertical motions are governed by distinct site amplification mechanisms. While the horizontal motion exhibits a dominant shear wave propagation phenomenon, the vertical motion results from a combination of compressional and shear wave propagation. Two deficiencies inherent in this 1D approach, namely, the systematic overprediction of the vertical motion and excessively long-duration motions predicted with the in-column input motion, are identified, and corresponding physical explanations are provided. The vertical motion overprediction is associated with the limitation of the 1D approach in accounting for the refraction of large-amplitude inclined shear waves when propagating through the surficial soft layers. The excessively long-duration motions are caused by the enforced fixed boundary at the base of the 1D soil column that is unable to accommodate the outgoing waves, resulting in wave trapping in the soil column near the site periods. The observed spatial variability in the comparison results and sensitivity studies on the basin profile demonstrate that the accuracy of the 1D procedure is dependent on the wavefield composition of both the 1D input motions and the actual site response. Overall, in the horizontal direction, the free-field site amplification and subsequent dynamic filtering of the base motions within representative structural systems can be reasonably captured by the assumed vertically propagating shear waves. In contrast, vertical free-field seismic motions and structural demands are overpredicted in most cases when using idealized vertically propagating compressional waves. Special attention should be given to the potentially severe in-structure vertical floor accelerations predicted by the 1D approach due to the combined effects of fictitious free-field vertical site amplification and significant vertical dynamic amplification within the structure, as this can pose unrealistic challenges to seismic certification of secondary equipment systems necessary for structural and operational functionality and containment barrier design of critical infrastructures. It is also demonstrated that vertical SSI effects can be more significant than those in the horizontal direction due to the large vertical structural stiffness and should be considered in vertical floor acceleration assessments, especially for massive high-rise structures.