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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. Source

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. Source

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. Source

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. Source

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. Source

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