Ph.D. Theses
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Browsing Ph.D. Theses by Subject "Earthquake engineering."
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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 A proposed ground motion selection and scaling procedure for nonlinear response history analysis(Thesis (Ph.D.)-Bogazici University.Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Zengin, Esra.; Akkar, Sinan.With the advancement in performance-based earthquake engineering, nonlinear response history analysis of structures has become more common in recent years. The selection and scaling of ground motions for use in nonlinear response history analysis is one of the most critical steps in performance-based seismic assessment procedures. This study presents ground motion selection and scaling procedure that addresses the uncertainty in the spectral demand with the preserved dispersion within the ground motion set. The candidate ground motion sets are constructed based on dispersion statistics about the target spectral demand. The optimum ground-motion set is linearly scaled by using an optimization algorithm that minimizes the error between scaled median and target spectra. The scaling stage ensures that the median record spectrum provides a reasonable match to target median in a previously defined period interval. This procedure allows performing further modification on each scaled ground motion in order to match the target variance of the scenario-based spectrum. In this study, a novel probabilistic framework is presented to propagate the uncertainties in both ground motion intensity and the structural response on fragility curve estimations. To investigate the effects of uncertainties on seismic damage estimations, the results of this study are compared with those obtained by the conventional fragility curve approach.Item Characterization of response spectra for near field conditions by earthquake ground motion simulation(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2007., 2007.) Şeşetyan, Karin.; Erdik, Mustafa.; Madariaga, Raul.Strong ground motion estimation tools such as empirical ground motion prediction equations rely mostly on recorded data. Good quality strong motion data became available mostly in recent years and are still sparse for near field conditions. Strong ground motion records on the other hand contain them several characteristics such as source, path and especially local site effects. These features can sometimes be hardly differentiated when one is willing to study only one of them. As such, realistic simulation of the 3D wavefield generated by a fault rupture in a heterogeneous medium becomes critical for the study of strong ground motion. Particularly features such as intra-event and inter-event spatial and temporal variability of ground motion can be studied in detail. With increasing computational power and newly emerged methods such simulations become increasingly feasible. In this study I use kinematic simulation with 3D Staggered Grid Finite Difference method to study the characteristics of near source effects of strong ground motion. I also aim to prove that kinematic simulation with 3D Staggered Grid Finite Difference method (3DFD) is a useful tool to analyze and estimate the characteristics of long period strong ground motion. For this purpose first a simulation model has been set for the 2004 Mw=6.0 Parkfield earthquake. Making use of the available 3D crustal velocity structure of the region and several slip models of the earthquake over 40 near source recordings produced by the event have been simulated. Encouraged by the very good agreement between observed and simulated waveforms, 3DFD has been used for the characterization of directivity effects in the near source region. Eleven earthquakes covering a magnitude range from 6.0 to 7.4 have been modeled using one or more slip models for each of them. Eight of these events had a strike-slip mechanism whereas the remaining ones were dip-slip events. Resulting spatial distributions of fault parallel, fault normal and average response spectral accelerations at longer periods have been analyzed to derive general formulations for the modification of the response spectral values found from earthquake hazard analysis to include near source directivity effects. With recently emerging structural design methods, engineering studies concentrate more and more on the long period characteristics of strong ground motion. The present study validates kinematic simulation tools, particularly the 3DFD method used herein as a useful method for the simulation of low frequency strong ground motion in realistic earth media for engineering purposes.Item Correlations between ground motion intensity measures and structural response parameters through nonlinear dynamic analyses(Thesis (Ph.D.)-Bogazici University.Kandilli Observatory and Earthquake Research Institute, 2009., 2009.) Hancılar, Ufuk.; Durukal, Eser.Item Dynamic shear amplification in seismic response of structural wall systems(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2008., 2008.) Celep, Umut Utku.; Aydınoğlu, M. Nuray.Previous research indicates that, shear force demand in yielding walls are not proportional to the design moments calculated by code procedures and higher shear force demands develop along the wall with respect to code predictions as a consequence of the higher mode effects after the plastic hinge formation at the base of the wall. 2007 version of the Turkish Seismic Design Code takes the dynamic shear amplification phenomena into account with a constant base shear amplification factor of 1.5 regardless of the first mode period and ductility level of the wall. However, results obtained from extensive non-linear time history analyses performed on generic walls in this study indicate that dynamic shear amplifications increase with increasing first mode period, Strength Reduction Factor (R) and ground motion intensity. A dynamic base shear amplification relationship as a function of the first mode period and strength reduction factor has been proposed for the Turkish Seismic Design Code (2007), based on the regression analysis of the non-linear time history analysis. A story shear force profile has been suggested for the Turkish Seismic Design Code (2007), which is intended for not only preventing shear failures at the base but also along the height of the wall. As a side product of the nonlinear time history analyses, a moment profile has also been proposed for use in the Turkish Seismic Design Code (2007). A modal decomposition technique is presented in this study for demonstrating the effects of the higher modes on the dynamic shear amplification phenomenon.Item Spatial variation of strong ground motion(Thesis (Ph.D.)-Bogazici University.Kandilli Observatory and Earthquake Research Institute, 2009., 2009.) Harmandar, Ebru.; Durukal, Eser.; Erdik, Mustafa.Earthquake ground accelerograms measured at different locations along a large engineered structure could be significantly different. This has led to considerable research in the last decade on the modeling of the spatially varying earthquake ground motion. The spatial variability of strong ground motion incorporates the effects of wave propagation, amplitude variability and phase variability, as well as the local site effects on the motion. This variation of ground motion could have the possibility to cause important effect on the response of linear lifelines such as long bridges, pipelines, communication systems, and should preferably be accounted for in their design. The objective is to evaluate and improve existing spatial variation quantification relationships by studying data available from different networks; investigate the possibility of employing functional forms for the characterization of spatial variation of ground motion in the assessment of strong ground motion distribution. This thesis focuses on studying on the spatial variability of ground motion using strong ground motion measurements. A rational and rigorous methodology for the interpolation of measured ground motion from discrete array stations to be used in the bias adjustment of the theoretical shake map assessments with the empirical ground motion measurements is developed. The generation of the estimated maps of shaking after an earthquake is often influenced by the limited number of sensors and/or difficulty of monitoring at inaccessible locations that impacts the collection of desired information. This gap in information can be filled through the estimation of missing information conditional upon the measured records. Methodology is presented for estimating properly-correlated earthquake ground motion parameters; herein peak ground acceleration (PGA), at an arbitrary set of closely-spaced points, compatible with known or prescribed ground motion parameters (PGA) at other locations. The variation of ground strain due to wave propagation, site response and loss of coherence is investigated. This study concentrates on the stochastic description of the spatial variation, and focuses on spatial coherency. The estimation of coherency from recorded data and its interpretation are presented. Coherency model for Istanbul for the assessment of simulation of spatially variable ground motion needed for the design of extended structures is derived. In addition to the realistic characterization of spatial variation, simulation of spatially variable earthquake ground motion is another essential part of the examination of the effects of spatial variation, especially for extended lifeline structures. This thesis concludes with the generation of earthquake ground motion compatible with prescribed target-response spectrum and their coherencies are consistent with a given spatial coherency function for a finite array of ground surface stations.