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Skarlatoudis A.A.,Aristotle University of Thessaloniki | Papazachos C.B.,Aristotle University of Thessaloniki | Theodoulidis N.,Institute of Engineering Seismology and Earthquake Engineering
Geophysical Journal International | Year: 2011

The detailed wave propagation characteristics and the spatial distribution of site effects in the metropolitan area of Thessaloniki are studied using a 3-D finite-difference method. Fourier amplitude spectra (FAS) and standard spectra ratios (SSR) are computed for various scenarios and their spatial distribution is examined throughout the Thessaloniki metropolitan area, in order to study the spatial variability of site-response. The variability of the contribution of different model layers in site amplification at different frequency windows and the identification of high amplification areas due to the presence of trapped waves in the surficial layers of the model identified along selected cross-sections, verify the impact of the complex 3-D wave propagation on the computed synthetics. Moreover, examination of the spatial distribution of the fundamental period (TSSR 0) from synthetic SSR and comparisons with results from ambient noise measurements show that for (thicker soil formations) the average H/V results for the fundamental period from ambient noise measurements, TNoise H/V, tend to overestimate (by roughly 30-35 per cent) the average fundamental periods from 3-D synthetics, The characteristics of the computed time-series and the type and properties of the dominating seismic waves are also examined along the same typical cross-sections spanning the study area, revealing the selective propagation of Love surface waves for various seismic scenarios. The previous results, as well as the strong spatial and interscenario variability of various measures of seismic motion such as Fourier spectra, peak ground velocity, cumulative energy and Housner intensity show a complicated 3-D wave propagation pattern, affecting the final site-effect distribution, both in the time and frequency domain. In most cases, even for areas with relatively simple shallow structure, the final site-response strongly depends on the seismic source characteristics, indicating the necessity of specific earthquake scenario 3-D synthetics for the study of complex 3-D geometry sedimentary basins, such as the broader Thessaloniki region. © 2011 The Authors Geophysical Journal International © 2011 RAS.

Mitolidis G.J.,Aristotle University of Thessaloniki | Salonikios T.N.,Institute of Engineering Seismology and Earthquake Engineering | Kappos A.J.,Aristotle University of Thessaloniki
Composites Part B: Engineering | Year: 2012

The paper reports tests on three groups of reinforced concrete (R/C) beam full-scale specimens, strengthened in flexural or shear using Steel Reinforced Polymers (SRP) and Carbon Fibre Reinforced Polymers (CFRP). The first group of five specimens represents the middle part of the span of a continuous beam and specimens are flexurally strengthened. The second group represents the support region of a continuous beam and its four specimens are strengthened in flexure. The third group also represents the support region of a continuous beam and its four specimens are strengthened in shear. Four specimens in total are tested unstrengthened to allow comparisons with the response of strengthened specimens. In addition to the different part of the beam that each specimen represented and the shear or flexural strengthening, the main parameters that varied among the specimens were: the type of polymer (SRP of two different types, or CFRP), the type of steel bars (ribbed or smooth, the latter being representative of older R/C members), the type of anchorage used for the polymers, and the way loading is applied to the specimens. Low strength concrete grade is used for the specimens, again to simulate older R/C members. The recorded response of the specimens is presented and discussed, and the experimentally measured strengths of the specimens are estimated analytically on the basis of the measured deformations of the specimens. Finally conclusions are drawn regarding the relative performance and merits of SRPs and CFRPs as strengthening materials for R/C beams. © 2011 Elsevier Ltd. All rights reserved.

Mitolidis G.J.,Anelixis Techniki | Mitolidis G.J.,Aristotle University of Thessaloniki | Salonikios T.N.,Institute of Engineering Seismology and Earthquake Engineering | Kappos A.J.,Aristotle University of Thessaloniki
Journal of Composites for Construction | Year: 2012

The main objective of the experimental work reported herein is the comparative evaluation of steel-reinforced polymers (SRPs) and carbon-reinforced polymers (CFRPs) used as externally-bonded reinforcement in strengthening of reinforced-concrete (RC) members. Tensile stress strain as well as bond constitutive laws for these materials were first derived from 16 tests and are summarized here. Results are then reported from four-point bending tests of five full-scale RC beams strengthened at their span using SRP and CFRP strips. The bond tests have shown that by providing a bond length greater than the effective one, neither the bond strength nor the deformation capacity are increased, whereas by increasing the width of the strip the bond strength is increased. From the bending tests of beams it was found that the use of both SRP and CFRP strips resulted in a significant increase in strength (up to 92%) with respect to the strength of the initial specimen. The experimentally measured strengths were estimated analytically using both the experimental measurements of the specimen deformations and the pertinent provisions of standards from the American Concrete Institute and the European Committee for Standardization. © 2012 American Society of Civil Engineers.

Manolis G.D.,Aristotle University of Thessaloniki | Makra K.,Institute of Engineering Seismology and Earthquake Engineering | Dineva P.S.,Bulgarian Academy of Science | Rangelov T.V.,Bulgarian Academy of Science
Earthquake and Structures | Year: 2013

We study seismically induced, anti-plane strain wave motion in a non-homogeneous geological region containing tunnels. Two different scenarios are considered: (a) The first models two tunnels in a finite geological region embedded within a laterally inhomogeneous, layered geological profile containing a seismic source. For this case, labelled as the first boundary-value problem (BVP 1), an efficient hybrid technique comprising the finite difference method (FDM) and the boundary element method (BEM) is developed and applied. Since the later method is based on the frequency-dependent fundamental solution of elastodynamics, the hybrid technique is defined in the frequency domain. Then, an inverse fast Fourier transformation (FFT) is used to recover time histories; (b) The second models a finite region with two tunnels, is embedded in a homogeneous half-plane, and is subjected to incident, time-harmonic SH-waves. This case, labelled as the second boundary-value problem (BVP 2), considers complex soil properties such as anisotropy, continuous inhomogeneity and poroelasticity. The computational approach is now the BEM alone, since solution of the surrounding half plane by the FDM is unnecessary. In sum, the hybrid FDM-BEM technique is able to quantify dependence of the signals that develop at the free surface to the following key parameters: seismic source properties and heterogeneous structure of the wave path (the FDM component) and near-surface geological deposits containing discontinuities in the form of tunnels (the BEM component). Finally, the hybrid technique is used for evaluating the seismic wave field that develops within a key geological cross-section of the Metro construction project in Thessaloniki, Greece, which includes the important Roman-era historical monument of Rotunda dating from the 3rd century A.D. © 2013 Techno-Press, Ltd.

Raptakis D.,Aristotle University of Thessaloniki | Makra K.,Institute of Engineering Seismology and Earthquake Engineering
Soil Dynamics and Earthquake Engineering | Year: 2010

The aim of the paper is the definition of the material parameters of sediments in the western part of Thessaloniki useful in the prediction of strong ground motion. The region of interest is elongated in the E-W direction and is confined between the Kalochori suburb and the harbor of Thessaloniki, while in the N-S direction it is extended between the coastline and the region of sediments and outcrop rock boundary. Many geological and seismotectonic studies, as well as geophysical surveys with electrical soundings and geotechnical boreholes, contribute to our understanding of the general sedimentary structure. Nonetheless, these studies could not provide any information regarding the material stiffness in terms of shear wave velocity; the most useful parameter in site response studies. In the present paper, alternative SPAC method is used to analyze array microtremor data to determine the shear wave Vs velocity distribution in two areas at the northwestern part of the urban area of Thessaloniki. At the beginning, the differences between alternative and standard SPAC are stressed and the reliability of the first one is shown. The Rayleigh wave phase velocity dispersion curve is determined using vertical component and their inversion lead to the determination of deep Vs profiles. The existence of conventional seismic measurements (Cross Hole, Down Hole) in combination with deep Vs profiles from microtremor data are used to verify Vs values for the characteristic deposits of the area. Moreover, the correlation between theoretical transfer functions based on the determined Vs profiles with the empirical ones from ambient noise and earthquakes recordings are used to validate the Vs profiles and to have a first idea of their site response. Finally, the thickness of the Quaternary sediments in the area of interest is found to increase from the east (city) to the west and from north to south where they attain their maximum thickness (almost 500 m) in consistency with other independent estimations. These sediments are distinguished in 4 horizons according to their Vs velocity, which from top to bottom are as follows: a) a layer of 80-150 m/s present only at the offshore zone, b) a layer of 200-250 m/s, very thin at the northern zone and until 20-55 m depth at the coastal sites c) a layer of 350-450 m/s down to 10-30 m and 70-140 m depth at the northern and offshore zones, respectively, and d) a layer of 550-650 m/s until 60-200 m at the north and 150-485 m depth at offshore zone. © 2009 Elsevier Ltd. All rights reserved.

Renalier F.,Laboratoire Of Geophysique Interne Et Tectonophysique | Jongmans D.,Laboratoire Of Geophysique Interne Et Tectonophysique | Savvaidis A.,Institute of Engineering Seismology and Earthquake Engineering | Wathelet M.,Laboratoire Of Geophysique Interne Et Tectonophysique | And 2 more authors.
Geophysics | Year: 2010

Inversion of the fundamental mode of the Rayleigh wave dispersion curve does not provide a unique solution and the choice of the parameterization (number of layers, range of velocity, and thickness values for the layers) is of prime importance for obtaining reliable results. We analyzed shear-wave velocity profiles derived from borehole tests at 10 sites where soil layers overlay bedrock in various geologic contexts. One to three seismic layers with linear velocity laws could model all of them. Three synthetic models defined from this preliminary study were used to understand the influence of parameterization on the dispersion curve inversion. This analysis resulted in the definition of a two-step inversion procedure for sites exhibiting a strong impedance con-trast. In the first step, the dispersion curve is inverted with an increasing number of layers over half space. The evolution of the minimum misfit and bedrock depth with number of layers allows the estimation of the true bedrock depth range. In the second step, this information is introduced in inversions with linear velocity laws. Synthetic tests showed that applying this procedure requires the dispersion curve over a frequency range from F0 to 10F0, where F0 is the site resonance frequency. The strategy was tested on two real cases for which Rayleigh wave dispersion curves were measured over this frequency range using passive and active seismic methods. The strategy was successful at the first site, while the bedrock depth was overestimated by 15% at the second site, probably resulting from the existence of a higher mode affecting the dispersion curve at low frequency. © 2010 Society of Exploration Geophysicists.

Makarios T.K.,Institute of Engineering Seismology and Earthquake Engineering
Structural Engineering and Mechanics | Year: 2012

All contemporary seismic Codes have adopted smooth design acceleration response spectra, which have derived by statistical analysis of many elastic response spectra of natural accelerograms. The above smooth design spectra are characterized by two main branches, an horizontal branch that is 2.5 times higher than the peak ground acceleration, and a declining parabolic branch. According to Eurocode EN/1998, the period range of the horizontal, flat branch is extended from 0.1 s, for rock soils, up to 0.8 s for softer ones. However, from many natural recorded accelerograms of important earthquakes, the real spectral amplification factor appears to be much higher than 2.5 and this means that the spectrum leads to an unsafe seismic design of the structures. This point is an issue open to question and it is the object of the present study. In the present paper, the spectral amplification factor of the smooth design acceleration spectra is re-calculated on the grounds of a known "reliability index" for a desired probability of exceedance. As a pilot scheme, the seismic area of Greece is chosen, as it is the most seismically hazardous area in Europe. The accelerograms of the 82 most important earthquakes, which have occurred in Greece during the last 38 years, are used. The soil categories are taken into account according to EN/ 1998. The results that have been concluded from these data are compared with the results obtained from other strong earthquakes reported in the World literature. Copyright © 2012 Techno Press.

Makarios T.K.,Institute of Engineering Seismology and Earthquake Engineering
Structural Design of Tall and Special Buildings | Year: 2012

In the present article, a new method is presented which attempts to identify the dynamic characteristics (eigen frequencies, eigen periods, mode shapes and modal damping ratios) of spatial asymmetric tall multi-storey buildings through measured seismic responses (accelerations). This new method is entitled 'method of modal time-histories', because its main target is to identify the modal time-histories of accelerations that are obtained by accelerograms recorded on the points of buildings where suitable accelerometers have been installed. In the case of an earthquake, the multi-channel local network of accelerometers records the time-histories of the accelerations of the building. In addition, in order to have a successful outcome, the instrumentation form of the multi-channel local network on the building can potentially play the most important role. This paper, first, presents a relevant mathematical analysis that is adapted to the instrumentation form applied to the multi-storey buildings and, second, outlines the new method, which consists of nine steps. Finally, in order to illustrate the theory, a suitable numerical example of an instrumented asymmetric five-storey r/c building that has been oscillated by a weak earthquake is also provided. On the one hand, the identification of the dynamic characteristics of spatial asymmetric buildings contributes to the removal of the uncertainties of building models in order to perform advanced non-linear analyses about inherent building seismic capacity. On the other hand, this method supports the simple monitoring of a building's 'structural integrity'. Copyright © 2010 John Wiley & Sons, Ltd. Copyright © 2010 John Wiley & Sons, Ltd.

Makarios T.K.,Institute of Engineering Seismology and Earthquake Engineering
Structural Design of Tall and Special Buildings | Year: 2012

In order to obtain the seismic demands of spatial asymmetric multi-storey reinforced concrete (r/c) buildings, a new seismic nonlinear static (pushover) procedure that uses inelastic response acceleration spectra is presented in this paper. The latter makes use of the optimum equivalent nonlinear single degree of freedom system, which is used to represent the general spatial asymmetric multi-storey r/c building. For each asymmetric multi-storey building, a total of 12 suitable nonlinear static analyses are needed according to the new proposed procedure, whereas at least 96 suitable nonlinear dynamic analyses are required in the case of nonlinear response history analysis (NLRHA), respectively. In addition, the present paper provides answers to a series of further questions with reference to the spatial action of the two horizontal seismic components in the static nonlinear (pushover) analyses, as well as to the documented calculation of the available behaviour factor of the asymmetric multi-storey r/c building. According to the paper, this new proposed seismic nonlinear static procedure is a natural extension of the documented equivalent seismic static linear (simplified spectral) method that is recommended by the established contemporary seismic codes, with reference to torsional provisions. Finally, through a restricted parametric analysis carried out in this paper, a relevant numerical example of a two-storey r/c building is presented for illustration purposes, where the seismic demand floor inelastic displacements are compared with the respective displacements obtained by the NLRHA. Consequently, the new proposed seismic nonlinear static procedure, which uses inelastic response acceleration spectra, can reliably evaluate the extreme values of floor inelastic displacements (on the flexible and stiff side of the building), as is shown by the above comparisons. Copyright © 2010 John Wiley & Sons, Ltd. Copyright © 2010 John Wiley & Sons, Ltd.

Makarios T.K.,Institute of Engineering Seismology and Earthquake Engineering
Journal of Earthquake Engineering | Year: 2013

In order to design a structure according to Eurocode EN 1990, a uniform 'reliability index' must be defined for all design parameters (strengths, loadings, etc.) that present an 'uncertainty.' However, Design Earthquakes constitute exception of the above parameters; thus, the present article covers this gap. For various values of reliability index and using an ideal, fictitious, Standard Normal Probability Density Function, new Peak Ground Acceleration functions of the mean return period of the design earthquakes are proposed. As a pilot scheme, we chose the 82 most important seismic records of the last 40 years for the seismic area of Greece. Copyright © A. S. Elnashai and N. N. Ambraseys.

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