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Vassilios Zervas

Bio: Vassilios Zervas is an academic researcher. The author has an hindex of 1, co-authored 1 publications receiving 184 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors address the topic of the spatial variation of seismic ground motions as evaluated from data recorded at dense instrument arrays, focusing on spatial coherency and its interpretation.
Abstract: This study addresses the topic of the spatial variation of seismic ground motions as evaluated from data recorded at dense instrument arrays. It 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. Some empirical and semi-empirical coherency models are described, and their validity and limitations in terms of physical causes discussed. An alternative approach that views the spatial variation of seismic motions as deviations in amplitudes and phases of the recorded data around a coherent approximation of the seismic motions is described. Simulation techniques for the generation of artificial spatially variable seismic ground motions are also presented and compared. The effect of coherency on the seismic response of extended structures is highlighted. This review article includes 133 references. @DOI: 10.1115/1.1458013#

213 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, the authors used ground motions observed during seven past earthquakes to estimate correlations between spatially distributed spectral accelerations at various spectral periods, which is required for the joint prediction of ground-motion intensities at multiple sites.
Abstract: Risk assessment of spatially distributed building portfolios or infrastructure systems requires quantification of the joint occurrence of ground-motion intensities at several sites, during the same earthquake. The ground-motion models that are used for site-specific hazard analysis do not provide information on the spatial correlation between ground-motion intensities, which is required for the joint prediction of intensities at multiple sites. Moreover, researchers who have previously computed these correlations using observed ground-motion recordings differ in their estimates of spatial correlation. In this paper, ground motions observed during seven past earthquakes are used to estimate correlations between spatially distributed spectral accelerations at various spectral periods. Geostatistical tools are used to quantify and express the observed correlations in a standard format. The estimated correlation model is also compared with previously published results, and apparent discrepancies among the previous results are explained. The analysis shows that the spatial correlation reduces with increasing separation between the sites of interest. The rate of decay of correlation typically decreases with increasing spectral acceleration period. At periods longer than 2 s, the correlations were similar for all the earthquake ground motions considered. At shorter periods, however, the correlations were found to be related to the local-site conditions (as indicated by site V s 30 values) at the ground-motion recording stations. The research work also investigates the assumption of isotropy used in developing the spatial correlation models. It is seen using the Northridge and Chi-Chi earthquake time histories that the isotropy assumption is reasonable at both long and short periods. Based on the factors identified as influencing the spatial correlation, a model is developed that can be used to select appropriate correlation estimates for use in practical risk assessment problems.

372 citations

Journal ArticleDOI
TL;DR: In this article, an approximate method to model and simulate spatially varying ground motions on the surface of an uneven site with non-uniform conditions at different locations in two steps is presented.

153 citations

Book ChapterDOI
01 Jan 2014
TL;DR: In this paper, the authors present a short but comprehensive review of the available seismic design methods, denoting the crucial issues and the problems that an engineer could face during the seismic analysis, and present the most recent developments on the evaluation of adequate fragility curves for shallow tunnels.
Abstract: Underground structures, tunnels, subways, metro stations and parking lots, are crucial components of the build environment and transportation networks. Considering their importance for life save and economy, appropriate seismic design is of prior significance. Their seismic performance during past earthquakes is generally better than aboveground structures. However several cases of severe damage to total collapse have been reported in the literature, with that of the Daikai metro station in Kobe during the Hyogoken-Nambu earthquake (1995) being one of the most characteristic. These recent damages revealed some important weaknesses in the current seismic design practices. The aim of this chapter is not to make another general presentation of the methods used for the seismic design of underground structures, but rather to discuss and highlight the most important needs for an improved seismic performance and design. In that respect it is important to consider that the specific geometric and conceptual features of underground structures make their seismic behavior and performance very distinct from the behavior of aboveground structures, as they are subjected to strong seismic ground deformations and distortions, rather than inertial loads. Several methods are available, from simplified analytical elastic solutions, to sophisticated and in principal more accurate, full dynamic numerical models. Most of them have noticeable weaknesses on the description of the physical phenomenon, the design assumptions and principles and the evaluation of the parameters they need. The chapter presents a short but comprehensive review of the available design methods, denoting the crucial issues and the problems that an engineer could face during the seismic analysis. The main issues discussed herein cover the following topics: (i) force based design against displacement based design, (ii) deformation modes of rectangular underground structures under seismic excitation, (iii) seismic earth pressures on underground structures, (iv) seismic shear stresses distribution on the perimeter of the structure, (v) appropriateness of the presently used impedance functions to model the inertial and the kinematic soil-structure interaction effects, (vi) design seismic input motion, accounting of the incoherence effects and the spatial variation of the motion and (vii) effect of the build environment (i.e. city-effects) on the seismic response of underground structures. The discussion is based on detailed numerical analysis of specific cases and recent experimental results in centrifuge tests. Other important issues like the design of submerged tunnels to liquefaction risk, or the complexity to evaluate the response of the joints of submerged tunnels are also shortly addressed. Finally we present the most recent developments on the evaluation of adequate fragility curves for shallow tunnels.

133 citations

Journal ArticleDOI
TL;DR: In this article, a semi-empirical estimation of the correlation coefficient, as a function of intersite separation distance, between residuals with respect to ground motion prediction equations (GMPEs) of horizontal peak ground acceleration (PGA) and peak ground velocity (PGV).
Abstract: Spatial modeling of ground motion intensity measures (IMs) is required for risk assessment of spatially distributed engineering systems. For example, when a lifeline system is of concern, classical site-specific hazard tools, which treat IMs at different locations independently, may not be adequate to accurately assess the seismic risk. In fact, in this case, modeling of ground motion as a random field is required; it basically consists of assigning a correlation structure to the IM of interest. This work focuses on semiempirical estimation of the correlation coefficient, as a function of intersite separation distance, between residuals with respect to ground motion prediction equations (GMPEs) of horizontal peak ground acceleration (PGA) and peak ground velocity (PGV). In particular, subsets of the European Strong-Motion Database (ESD) and the Italian Accelerometric Archive (ITACA) were employed to evaluate the intraevent residual correlation based on multiple earthquakes, considering different GMPEs fitted to the same records. The analyses were carried out through geostatistical tools, which enabled results to be found that are generally consistent between the two datasets. Correlation for PGV appears to attenuate more gradually with respect to PGA. In order to better understand the dependency of the results on the adopted estimation approach and dataset, some aspects related to the working hypotheses are critically discussed. Finally, estimated correlation models are used to develop illustrative applications of regional probabilistic seismic-hazard analysis.

112 citations

Journal ArticleDOI
TL;DR: In this article, the seismic performance of a sea-crossing cable-stayed bridge is comprehensively evaluated based on the fragility function methodology, and the effect of seawater on the bridge seismic responses is modeled using the hydrodynamic added mass method.

95 citations