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1.
A comprehensive numerical analysis of the seismic response and site period of curved alluvial valleys was performed by taking into account the characteristics of sedimentary materials. This study presents a criterion as a combination of the three following geometrical and geotechnical characteristics of curved valleys in order to provide a simple method for code implementation of complex site effects: depth ratio, filling ratio and impedance ratio. The parametric studies were performed by a HYBRID program combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The amplification patterns under above-mentioned characteristics were determined at the central point of valleys. The results are shown in the form of response spectra. Different impedance coefficients of materials were considered to evaluate effects resulting from combination with filling ratio and geometrical parameters. Finally, a criterion is proposed in terms of engineering applications to assess the spectral response at the surface of curved alluvial valleys. 相似文献
2.
An extensive numerical analysis on the seismic site effects due to local topographical and geotechnical characteristics is carried out. 2D configurations under incidence of vertically propagating SV waves is modeled with the aid of HYBRID program, combining finite elements in the near field and boundary elements in the far field. The filling ratio and the impedance ratio effects on the modification of the seismic response of alluvial valleys are underlined. Parametric analysis is done on the central point of alluvial valleys where the critical point of response under existence of sediments stands. Specifying the amplification pattern under filling ratio effects, effective geometrical parameters are introduced. Subsequently, to assess the effect of the type of sediments on the seismic response, mechanical properties of materials are changed and impedance contrast coefficient is considered in combination with geometrical parameters. Finally, practical curves are presented for engineering applications. 相似文献
3.
This study assesses the 3D amplification effects in shallow basins and quantifies the effects of site‐city interaction (SCI) on high‐rise buildings. A regional‐scale 3D spectral element simulation is conducted on the Tuen Mun‐Yuen Long basin, which contains multiple subbasins with heterogeneous and nonlinear soil profiles, while 3D city models with various building layouts are fully integrated into the basin model for our SCI study. We found a good correlation between spectral amplification factors and soil depths. Site response is significantly amplified at basin edges and centers due to surface waves generated at basin edges and the focusing effects stemming from 3D basin geometry. Transfer functions of 3D basins can be up to fourfold at fundamental frequencies as compared to 1D response, and further amplifications occur at high frequencies due to surface waves. In the SCI simulations, we observe wave trapping in the open space amid buildings resulting in energy concentration and up to twofold PGA amplifications. The wave trapping effect diminishes as the space between buildings increase beyond their range of influence (～100 m). The SCI analyses show that destructive kinetic energy in superstructures increases 28% in one horizontal direction but decreases 22% in the other. Our study concluded that, 1D site response analysis can significantly underestimate the seismic demand in shallow basins. Site‐city interaction of high‐rise buildings increases the short‐period spectra of ground motions, leading to an increase in their story accelerations by up to 50% and to a substantial decrease in the seismic safety of short structures in their vicinity. 相似文献
4.
采用有限元模拟方法建立了建筑群-沉积谷地二维模型,并在土体截断边界上施加粘弹性人工边界,在频域与时域中对比分析此体系和单独沉积谷地的地震反应,观察地震时沉积谷地与建筑群之间的动力相互作用规律。分析表明,沉积谷地中建筑群对谷地本身的地震反应具有显著影响。入射波频率较低时,由于共振效应的存在,在部分区域处建筑群-沉积谷地体系的地表位移响应幅值会大于单独沉积谷地,但随着入射波频率的增加,建筑群的存在又会对地震反应产生明显的减弱效果;建筑群对谷地的影响还与建筑高度和建筑间距有关,且不同位置处的响应也存在很大差异。计算结果可为沉积谷地中设防烈度的设置以及工程抗震设计提供部分理论依据。 相似文献
5.
An engineering approach is proposed for representing both site effects and soil-structure interaction in extended alluvial valleys, by using the one-dimensional model of shear were propagation corrected empirically to account for lateral heterogeneities and generated surface waves. The peak structural response is expressed by means of spectral contours that are a function of the predominant period of the site and the fundamental period of the structure. Variations of the peak spectral ordinates with the prevailing site period can be deduced from these contours. A number of events of firm ground, representative of the most dangerous earthquakes expected in Mexico City, are assumed as design earthquakes. Making use of the resulting spectral contours, the provisions for site effects recommended in the Mexican seismic code are evaluated. Also, considering as control motion the 1985 Michoacan earthquake recorded at a representative firm site, spectral contours with soil-structure interaction are obtained which allow one to identify the significant interaction effects originating in the Valley of Mexico for medium- and long-period structures. The influence and relative importance of the critical parameters involved are examined within practical ranges of interest. 相似文献
6.
The seismic site response of alluvial valleys with limited width is evaluated using three engineering models. The models are based on the one-dimensional, two-dimensional and the frame model approaches. The objective is to analyse the effects of the main parameters governing surface motions and provide engineering guidance for predicting them. The limitations on the use of the one-dimensional model in site response evaluation in valleys are pointed out. The frame model, which accounts for the limited width of valley, gives response results that are in good agreement with the two-dimensional model results. It is found that the effect of the two-dimensional amplification is significant over a distance from the valley edges beyond which the response may be adequately represented by one-dimensional analysis. The soil amplification varies depending on the soil type, site location relative to the valley and the dominant period and amplitude of input rock record. © 1997 by John Wiley & Sons, Ltd. 相似文献
7.
Seismic site effects for shallow and deep alluvial basins: in-depth motion and focusing effect 总被引：1，自引：0，他引：1
J. F. Semblat P. Dangla M. Kham A. M. Duval 《Soil Dynamics and Earthquake Engineering》2002,22(9-12):849-854
The main purpose of the paper is the analysis of seismic site effects in various alluvial basins. The analysis is performed considering a numerical approach (boundary element method). Two main cases are considered: a shallow deposit in the centre of Nice (France) [Soil Dyn. Earthquake Engng 19 (2000) 345] and a deep irregular basin in Caracas (Venezuela) [Comput. Geotech. 29 (2002) 573].
The amplification of seismic motion is analysed in terms of level, occuring frequency and location. For both sites, the amplification factor is found to reach maximum values of 20 (weak motion). Site effects nevertheless have very different features concerning the frequency dependence and the location of maximum amplification. For the shallow deposit in Nice, the amplification factor is very small for low frequencies and fastly increases above 1.0 Hz. The irregular Caracas basin gives a much different frequency dependence with many different peaks at various frequencies. The model for Caracas deep alluvial basin also includes a part of the local topography such as the nearest mountain. One can estimate seismic site effects due to both velocity contrast (between the basin and the bedrock) and local topography of the site.
Furthermore, the maximum amplification is located on the surface for Nice, whereas some strong amplification areas also appear inside the basin itself in the case of Caracas. One investigates the influence of this focusing effect on the motion versus depth dependence. This is of great interest for the analysis of seismic response of underground structures. The form and the depth of alluvial deposits are then found to have a great influence on the location of maximum amplification on the surface but also inside the deposit for deep irregular basins. It is essential for the analysis of the seismic response of both surface and underground structures. 相似文献
8.
The so called “valley effect” relates to the typical seismic response of basin shaped bedrock filled by quaternary sediments. It is an aspect of the renown “local seismic effect” that shall be taken into account when dealing with microzoning studies. Several experimental surveys and numerical simulations performed worldwide over the last 40 years, confirmed that valley responses under seismic excitations show common features in various geological contexts as far as the sedimentary valleys (e.g. alluvial and lacustrine plains), the intermountain valleys (e.g. alpine valleys) and graben shaped basins. Such features mainly depend on the basin geometry, referred to as the shape ratio SR, and the sediment and basin impedance contrast IC. Although researchers agree on the prominent role of local seismic effects for interpreting erratic damages caused by seismic shaking in urbanized areas, no fully shared strategies have been identified for taking into account valley effect within microzoning studies. In this paper, a numerical simulations on three models of trapezoidal shaped basins have been performed. These valley models relate to sediments and basins detected within the Tuscany Region territory during the VEL project. Results, in terms of the amplification index $\text{ F }_{\mathrm{A}}$ F A have been provided. Three “valley effect charts” for various SR and IC values have been propose for taking into account the local seismic effects due to the basin amplifications within microzoning maps. 相似文献
9.
This paper deals with the evaluation of seismic site effects due to the local topographical and geotechnical characteristics. The amplification of surface motions is calculated by a numerical method combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The numerical technique is improved by time truncation. In the first part of this article, the accuracy and the relevance of this optimized method are presented. Moreover, parametric studies are done on slopes, ridges and canyons to characterize topographical site effects. The second part deals with sedimentary valleys. The complexity of the combination of geometrical and sedimentary effects is underlined. Extensive parametrical studies are done to discriminate the topographical and geotechnical effects on seismic ground movement amplifications in two-dimensional irregular configurations. Characteristic coefficients are defined to predict the amplifications of horizontal displacements. The accuracy of this quantitative evaluation technique is tested and discussed. 相似文献
10.
Seismic wave amplification: Basin geometry vs soil layering 总被引：1，自引：0，他引：1
J.F. Semblat M. Kham E. Parara P.Y. Bard K. Pitilakis K. Makra D. Raptakis 《Soil Dynamics and Earthquake Engineering》2005,25(7-10):529-538
11.
A detailed numerical simulation of the ground motion and a site response analysis for two towns in the Marche Region (Treia
and Cagli) is carried out on the basis of structural models deduced from available geological and geophysical data. In both
cases, the reference event is an M = 5.7 earthquake associated with a normal fault located beneath each town. The ground motion is computed using the 2D spectral
element method (SPEM 2D). The method solves the propagation of the seismic field through complex geological structures and
enables an estimate of the effects of deep crustal structure, superficial geology, and topography on ground motion. Numerical
simulations of the seismic field are performed along 2D vertical planes containing the seismic source. Strong ground motion
has not been yet recorded in the two towns; therefore, the numerical simulation of ground motion represents a way to overcome
the lack of instrumental data. The simulations carried out for Treia show that ground motion is influenced by both source
mechanism and effects due to propagation through the geological structure, while ground motion in Cagli features strong local
effects, caused by the presence of alluvial deposits under a large area of the town. 相似文献
12.
当参考场地震源距小于研究场地震源距时,传统谱比法低估了场地放大效应,且参考场地与研究场地震源距相差越大,低估程度越大;当参考场地震源距大于研究场地震源距时,传统谱比法高估了场地放大效应,且参考场地与研究场地震源距相差越大,高估程度越大,计算结果依赖参考场地的选择。上述问题存在的原因是传统谱比法估计场地放大效应时未考虑地震波传播过程中介质的吸收衰减作用,故传统谱比法适用于参考场地与研究场地地质构造类似且震源距相差较小的情况,为此本文通过考虑介质对地震波的吸收衰减作用,提出改进谱比法,可适用于地质构造类似、任意震源距的情况。本文分别采用未改进方法和改进方法估计汶川地震中渭河盆地多个台站场地反应,证实了传统谱比法存在的上述问题。采用改进谱比法估计的场地放大倍数均在2 Hz以下出现1个峰值,且这些峰值多为最大值;采用未改进谱比法估计的场地放大倍数峰值出现的频率多大于2 Hz;改进谱比法可信度较高。 相似文献
13.
Three-dimensional scattering of seismic waves by a cylindrical alluvial valley embedded in a layered half-space is investigated by using the combination of the boundary integral representation and the finite element method. The surface displacements due to incident plane harmonic body waves (P, SV and SH) propagating at an arbitrary angle to the axis of the cylindrical valley are evaluated numerically for two semi-elliptical alluvial valleys. The presence of the layer is found to have a strong effect on the amplification of the surface displacements in some cases. The three-dimensional motion seems to be quite critical and may cause large amplification. The surface ground motion becomes significant when compared with corresponding free-field motion as the wavelengths become comparable to the characteristic length of the valley. The maximum amplification always occurs atop the valley. Numerical results show that the amplitude and the amplification pattern of the surface displacement strongly depend upon the frequency, the angle and the type of the incident waves. 相似文献
14.
Azimuth dependent wave amplification in alluvial valleys 总被引：1，自引：0，他引：1
An extension of the indirect boundary element method (IBEM) to three-dimensional scattering by two-dimensional alluvial valleys is presented. While the valley is two-dimensional, the incident plane waves can arrive outside the 2D plane so the scattering is three-dimensional with coupling of P---SV---SH waves. Such a method makes it possible to take earthquake location into account in the estimation of site effects in alluvial valleys. The method is validated by transparency tests, by comparison with 2D simulations, and by comparison with results of other authors. The advantage of the method is that is combines high accuracy with cost-efficiency in terms of computer-time. It is applied to theoretically estimate site effects across a simplified model of an alluvial valley in the French Alps where azimuth dependence of local amplification has been observed. A parametric study with simulations for a range of azimuths and incidence angles shows that (1) the local amplification depends strongly on both azimuth and incidence of the incoming waves, (2) the global pattern of amplification across the valley is very complex for all azimuths, and (3) it is not possible to predict the 3D response of the valley from 2D modeling. Theoretical spectral ratios are in approximate agreement with observed ones for a station in the center of the valley where the local structure justifies use of a simplified model for the comparison. 相似文献
15.
基于黏弹性人工边界的显式动力有限元方法,研究了突出平台状山谷地形场地的地形效应,分析了山谷地形对地表地震动的影响。结果表明:(1)沿着地震动输入方向地表反应较其他方向上的反应强烈;(2)与单一凸起山体地形对地表地震动的放大效应相比,相邻地形的存在对地表地震动位移傅里叶谱谱比曲线的形状影响不大,但是对PGD有着较大的影响,随着相邻凸起山体之间距离的增大,该影响效应逐渐减弱。 相似文献
16.
近断层效应使得沉积盆地对地震动放大效应更为复杂。本文针对逆断层发震下三维层状沉积盆地地震反应,基于波动谱元法,采用有限断层动力学模型,模拟断层动力破裂、地壳层地震波传播和层状沉积盆地对地震波散射全过程。在此基础上,对比分析了层状和均质沉积盆地对近断层地震动放大效应的影响,讨论了不同断层倾角下层状沉积盆地地震动加速度特性。结果表明：层状沉积盆地PGA空间分布与均质沉积盆地存在较大差异,由于近断层效应和盆地效应,层状沉积盆地地表局部范围竖向PGA大于水平向PGA;90°断层倾角下层状沉积盆地地表地震动放大范围与60°断层倾角结果明显不同,主要集中在盆地中心区域和断层附近,且幅值远小于60°断层倾角下结果;沿断层走向,盆地内地表地震动加速度峰值对应时刻较盆地外延后。 相似文献
17.
A boundary integral formulation is presented and applied to model the ground motion on alluvial valleys under incident P, S and Rayleigh waves. It is based on integral representations for the diffracted and the refracted elastic waves using single-layer boundary sources. This approach is called indirect BEM in the literature as the sources' strengths should be obtained as an intermediate step. Boundary conditions lead to a system of integral equations for boundary sources. A discretization scheme based on the numerical and analytical integration of exact Green's functions for displacements and tractions is used. Various examples are given for two-dimensional problems of diffraction of elastic waves by soft elastic inclusion models of alluvial deposits in an elastic half-space. Results are displayed in both frequency and time domains. These results show the significant influence of locally generated surface waves in seismic response and suggest approximations of practical interest. For shallow alluvial valleys the response and its resonant frequencies are controlled by a coupling mechanism that involves both the simple one-dimensional shear beam model and the propagation of surface waves. 相似文献
18.
The phenomenon of localized amplification of seismic waves due to subsurface irregularities has attracted the attention of seismologists and engineers for many years. In this paper we present a hybrid numerical method to investigate the ground motion amplification due to sedimentary basins. This method combines the integral representation and the finite element to solve the scattering problem due to an arbitrary cylindrical scatterer. A study is done on two shapes of valleys to show the effect of the angle of incidence, the incident wave type and the layering on the amplification of the surface displacement components by different valleys. 相似文献
19.
沉积盆地能够加重震害,在抗震设防中需要定量考虑盆地效应。本文叙述了盆地效应的主要表现形式以及对震害的影响;详细介绍了盆地效应的研究方法以及取得的研究成果,同时也指出了各个方法的优缺点;讨论了抗震设防中对盆地效应考虑的必要性;着重回顾和总结了当前在抗震设防中考虑盆地效应的两种方法:（1）在场地地震动预测模型中加入盆地基底埋深项。（2）求取盆地模型相对一维层状模型的附加放大因子,提出针对当前抗震设防规范中地震动参数的校正系数。最后指出开展盆地抗震设防研究面临的问题及工作展望。 相似文献
20.
Dino Bindi Lucia Luzi Francesca Pacor Roberto Paolucci 《Bulletin of Earthquake Engineering》2011,9(6):1921-1939
In this work, we analyze the records of the Italian strong motion database (ITACA,
) with the aim of identifying stations affected by site effects that are not captured by standard seismic classification schemes.
In particular, we consider four different site classifications, two of them based on geological/geophysical characteristics
and two driven by data. For each classification we develop a ground motion prediction equation using a random effect approach
to isolate the between-station and within-station distribution of errors. The site coefficients obtained for the different
classes confirm that site amplification effects are significant for both the horizontal and vertical components. The between-station
error normalized to the standard deviation of the between-station error distribution is then used to identify stations characterized
by large errors, attributable to site effects not accounted for by the classification schemes. The results show that large
errors can affect the predictions when the site effects are not uniquely related to the reduction of the seismic impedance
in the uppermost layers. For example, amplifications of ground motion over the long period range are observed for stations
installed within alluvial closed-shape basins, as consequence of locally generated surface waves. For these stations, classifications
based on the horizontal to vertical response spectra ratio are not reliable, since amplifications are also affecting the vertical
component. Another interesting feature which emerges from the analysis is the significant de-amplification of short period
spectral ordinates that seems to be related to stations typically set in at the foundation level of massive structures. To
increase the usefulness of the data set, the most important distinctive features of the strong motion stations are documented
in the ITACA database reports containing the instrument information and the available geological-geotechnical data. 相似文献