Kandilli Rasathanesi ve Deprem Araştırma Enstitüsü
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Browsing Kandilli Rasathanesi ve Deprem Araştırma Enstitüsü 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 Evaluating site effects for estimation of seismic ground response: a practical approach(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2013., 2013.) Çetiner, Barbaros.; Şafak, Erdal.; Ansal, Atilla.Expansion of scientific methodologies to estimate the probable extents of ground motion effects in earthquake-prone areas is crucial for proper planning, investigation, and design of engineering structures. In this regard, accurate estimation of site effects instigated by the terminal geological structure at a considered location, in the presence of a seismic tremor, plays a particularly significant role. The key emphasis of this document is to develop a practical ground response analysis procedure comprised of a line of objective, well-outlined and simple-but-adept individual processes to address this aspect of the problem. In this study, to achieve that goal, a total of 1188 ground response analyses were performed for a combination of ground motion selection/scaling and equivalent-linear site response analysis techniques – all of which by itself constitute the current agreed practice within their parts – at 10 accelerometer sites. Each location evaluated herein was selected so that the recording station is a part of the Turkish Strong Ground Motion Network, with readily accessible, detailed geotechnical and geophysical reports; and at least a single earthquake event record, that correspond to the 10 per cent exceedance level in 50 years, existing. This allowed for a direct comparison of the result patterns obtained using each analysis component with the actual earthquake response; thus a truthful appraisal of how changing different parts of the overall procedure is reflected in the attained approximation level. For each station, the main output of the proposed method is a uniform hazard spectrum (UHS), fashioned in a similar way as that in major seismic codes. Therefore, as an auxiliary outcome of this assessment, a case study presenting the insufficiency of the currently adopted site factors for faithful estimation of site effects, and a vital application regarding how the suggested approach may play a role in improving the existing seismic code practice is as well provided.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.Item Microzonation with respect to rainfall - induced landslides(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2015., 2015.) İlhan, Okan.; Şafak, Erdal.; Ansal, Atilla.Expansion of scientific methodologies to quantify the rainfall – induced hazard paves the way for constructing proper investigation of potential landslide in interested areas. In this context, a simple, concise but sufficiently detailed to incorporate all the phenomena in itself microzonation methodology is proposed which merges two different approaches (1) Iverson (2000), which bears on the exact solution of Richards’ Equation and (2) uniform seepage condition, dependent on topographical features rather than solution to water movement equation in soil continuum. Such a straightforward concept enables utilizers to perform factor of safety calculations not exhaustive but elaborative on GIS system. In this study, hypothetical topography is at first produced to portray the efficacy of recommended algorithm with an intention that different soil conditions and fluctuations in rainfall intensities on soil mass are able to be evaluated. In addition to this, Tekirdağ City Center is selected to examine the pros and cons of proposed routine but all the required parameters for the analysis are specified by means of plenty of empirical correlations, which may lead to unexpected results. Thus, two additional methodology, Montgomery and Dietrich (1994), Mora and Vahrson (1994) are also, employed for the justification of below – stated method since the former one operates on similar parameters and the latter one sketches the potential hazardous locations in relevant area. The outcome of comparison between these three algorithms is able to be interpreted that suggested technique can fairly be presumed as one simulating the pore – pressure acumulations and corresponding factor of safety degradations properly.