Ph.D. Theses
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Browsing Ph.D. Theses by Author "Ansal, Atilla."
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Item A parametric study for the characterization of site amplification(Thesis (Ph.D.)-Bogazici University.Kandilli Observatory and Earthquake Research Institute, 2020., 2020.) Fercan, Nazife Özge.; Şafak, Erdal.; Ansal, Atilla.In earthquake engineering, the approximation of site amplification by using practical ways has been an important issue. Various site parameters were proposed and applied in the engineering practice. Among these, time averaged shear wave velocity for the top 30 m, Vs30, and fundamental frequency, f0, have been used widely. In this study, we investigated the reliability of Vs30 parameter, and the performance of alternative time averaged shear wave velocities (e.g., Vs40, Vs50, etc.) and shear wave travel times (Ttz) at various depths for the estimation of site amplification. For the same bedrock depth, we considered 17 shear wave velocity profiles, changing from convex (i.e., the velocities changing faster near the surface and slower near the bedrock) to concave (i.e., the velocities changing slower near the surface and faster near the bedrock). We divided the soil media, first into layers with equal thickness, and then into layers with equal wave travel times. For each layering type and soil profile, we calculated the site amplification factors and fundamental frequencies, and studied their correlations with time averaged shear wave velocities (Vsz) and wave travel times (Ttz) for different depths, z. We have also investigated the correlation of site amplification factors, surface PGAs (Peak Ground Accelerations), and fundamental soil frequencies (f0) for each case. We have identified the optimal averaging depths for the averaged shear wave velocity and the wave travel time to characterize site amplification. The study showed that there is a sharp change in the correlations when switching from convex to concave profiles. By gradually increasing the bedrock acceleration levels, we have also studied the nonlinear soil response and its correlations with linear soil response. We presented guidelines to estimate nonlinear soil amplification factors and fundamental frequency from the linear ones. Considering that the linear fundamental frequency and amplification can easily be calculated from field tests (e.g., ambient noise measurements for f0 detection), these guidelines provide a useful tool to estimate nonlinear ones.Item Factors affecting site response analysis(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2009., 2009.) Tönük, Gökçe.; Ansal, Atilla.The objective of a site response analysis is to estimate free-field ground shaking characteristics during an earthquake for a specific hazard level and set of site conditions. The mandatory components for a site response analysis are: one or more design earthquake records with representative acceleration time histories, an idealization of the soil-rock system at the site of interest, and a scheme to generate response solutions to simplified assumed wave fields in other words appropriate modeling of the soil behavior. Normally, the free-field ground response is presented in terms of either response spectra or the variation of acceleration or velocity with time. The study aims to review and improve different components of site response analyses in order to achieve a robust methodology for more comprehensive and realistic assessment. The effects of input acceleration time histories, the applied numerical methodology, stress and frequency dependence and nonlinear site response analysis were reviewed and methodologies were suggested based on case studies. Site response of layered soil deposits was analyzed using equivalent linear and modified equivalent linear schemes. The developed methodology would be utilized to estimate earthquake characteristics on the ground for site specific investigations based on probabilistic earthquake hazard assessment. Within this perspective, site response analysis was studied with respect to (a) the determination of different scaling parameters including derivation of attenuation relationships for these parameters, (b) the evaluation of scaling parameters with respect to magnitude and distance ranges, (c) the methodology of selection and scaling of input acceleration time histories for site response analyses, (d) the methodology for selection of ground motion parameters from site response analysis as design or damage parameters for various earthquake engineering analysis such as liquefaction susceptibility, microzonation, vulnerability assessments for buildings and pipeline networks, (e) the methodology for confining stress and frequency dependence of modulus reduction and damping in equivalent linear site response analysis, (f) the review concerning the available equivalent linear site response analysis models and software, (g) formulation of modified version of Shake91 to account for stress and frequency dependency, (h) comparison of results with modified Shake91 based on selected borings, and (i) the review concerning nonlinear models for site response analysis.