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1.
渤海海域软土层土对场地设计地震动参数取值具有显著影响。选取渤海中部钻孔剖面作为计算场地模型基础,分别构建软土和硬土场地模型,并通过改变软土层厚度,构造新的场地模型。采用等效线性化方法（EL法）和非线性计算方法（NL法）分别对场地模型进行地震反应分析,分析了海底软土层土对地震动参数的影响。研究结果表明:海底软土层土对地震动峰值加速度的影响显著,随着地震动输入增加,软土层放大效应减弱,减震作用逐渐增强;EL法中,软土层土对基岩反应谱的高频部分具有明显滤波作用,而NL法中,滤波作用较弱,海底面反应谱随地震动输入的增大先放大后减小;软土层土会降低设计地震动地震最大影响系数,增大特征周期。对于海域工程,特别是深基础工程抗震设计地震动参数的确定,从保守角度考虑,建立场地模型时建议删除软土层。 相似文献
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The purpose of this paper is to present a stochastical approach, which analyse the torsional ground motion, induced by the
spatial variability of seismic motions. For this purpose, a torsional ground motion analytical model is proposed and a normalised
differential motion parameter is introduced. The approach regards the seismic motion as the combination of a travelling wave
on the site (coherent component) and a zero mean randomizing factor that introduces a loss of correlation effect. The soil
parameters as fundamental frequency and damping coefficient are integrating by modeling the coherent component with the commonly
used Kanai-Tajimi power spectral density. The parametric analysis of the model shows an increase of the induced torsion with
both the soil frequency and the motion scattering parameter, and a decrease with the separation distance, the apparent wave
velocity and the correlation length. Finally, in order to test the proposed torsional ground motion model prediction, it is
compared to the experimental results recorded by the EPRI LSST array in Lotung, Taiwan (Laouami and Labbé, 2002). The comparison
leads to the identification of the model parameters for the Lotung soft site. 相似文献
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This paper presents a non-linear finite element study to back-interpret the free field seismic response recorded at the Lotung Large-Scale Seismic Test site. The study is carried out in the time domain by the Finite Element (FE) code PLAXIS 3D, considering the vertical wave propagation of both the horizontal components of motion. The non-linear soil behaviour is simulated through a constitutive model, the Hardening Soil model with Small-Strain Stiffness (HSsmall), capable of describing the cyclic response of the material at different strain levels. In the paper, the constitutive response of the HSsmall model is firstly investigated through numerical simulations of strain-controlled cyclic shear tests under single and multi-directional conditions at low strain levels. Then, it is adopted to back-analyse the recorded free field seismic response, comparing the FE numerical results to the in-situ down-hole and surface signals recorded during two earthquakes occurred on May 20th and July 17th 1986, characterized by different peak ground accelerations. 相似文献
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Observations from many recent strong motion events have shown the importance of local soil conditions and non-linear soil behaviour on the seismic ground response (site effects). As demonstrated by previous seismic microzoning studies (Lebrun et al.) [1]), as well as by at least three historical major earthquakes, Pointe-à-Pitre is prone to strong site effects, due to the particular geology of the area. In this paper, we present a comparison between the strong-motion data available from the stations operating on the swampy site of Pointe-à-Pitre airport and the ground motions derived from 1D non-linear finite element simulations.Results show that, for moderate to strong ground motions, 1D simulations reproduce the main characteristics of site response in terms of duration, energy distribution, amplitude and frequency content. It also shows the importance of very superficial soft layers as peat or saturated mud in low frequency site effects simulations. This point is important for further engineering studies since such very soft formations overlain by stiffer landfills are commonly expected in the Antilles context. Our work also shows that Anderson's criteria, used to quantify the goodness-of-fit of simulated ground motions to the observed ones, appear to be an interesting diagnostic tool for testing the quality of numerical simulations from an engineering point of view. 相似文献
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软土具有高灵敏度、低强度等特性,在地震过程中极易产生震陷。基于OpenSees数值模拟方法对软土场地的震陷反应进行非线性动力有限元分析,通过改变地震动峰值加速度、频谱特性、输入方式来研究其对软土震陷的影响。结果表明,地震动峰值加速度对地基土的不均匀震陷有显著影响,地震动峰值加速度越大,震陷量显著增大,震陷影响深度更大,对水平地表造成的破坏范围也更大;地震动频谱特性对软土震陷有重要影响,当地震动卓越频率与场地自振频率相近时,其幅值越大,产生的震陷越严重;水平、竖向同时输入地震动的方式能更好地反映土体的振动及震陷响应。该研究成果对探索软土震陷的机理有一定的指导意义。 相似文献
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Preloading is a temporary loading, usually an embankment, applied to improve subsurface soils by densification. This paper studies the effect of preloading on the amplification characteristics of soft sites with an elaborate parametric analysis. The soil type, the depth of the bedrock, the water table depth, the level of preloading, the applied earthquake, the shear wave velocity of the bedrock and the shear modulus and damping versus shear strain relations were varied in a systematic manner. The analysis was performed by the commonly used one-dimensional equivalent-linear dynamic method. The shear wave velocity versus depth and the effect of preloading on shear velocity are computed with well-established soil mechanics equations. The results illustrated that the seismic response at the top of the profile generally decreases as a result of preloading. A more detailed analysis of results shows that the effect of preloading on the seismic response depends on the soil type and the depth of the bedrock. Based on these results, a method is proposed by which a practicing engineer involved with improvement of soft ground can simulate the effect of preloading on the seismic motion. 相似文献
8.
将软土地层中地铁车站结构与周围土体地震时产生的水平加速度,以静态的水平加速度代替,使两者作用下结构内力最大值相等、出现部位相同,实现将动力问题转化为静力问题。结果表明,该方法能反映软土地层中两层三跨地铁车站的在地震时的动力响应,是适用于工程设计的简单、实用的数值方法。 相似文献
9.
以计算结果可靠的土层反应结果为基础数据,基于随机森林算法（Random Forest,简称“RF”）,建立地震动、场地土层参数特征量与土层变形特征的关系,提出一种依据常规地震动和场地土层参数为特征量的地震荷载下土层变形等级预测新方法,给出了典型场地土体发生大应变的地震动阈值。结果表明:基于RF模型对土体的变形程度预测结果较为理想,各项评价指标始终保持一个较高水平;对于软土、中硬土及硬土场地,输入地震动分别大于0.05 g、0.15 g及0.19 g时,将产生大应变。同时,本文方法可在不进行土层地震反应数值模拟的情况下,得到场地各土层的剪应变等级,为强地震作用下土体实际力学状态的界定和认识提供新的分析手段,而且也可为地下结构抗震设计问题提供指导和参考。 相似文献
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The seismic response of pile foundations is a very complex process involving inertial interaction between structure and pile foundation, kinematic interaction between piles and soils, seismically induced pore-water pressures (PWP) and the non-linear response of soils to strong earthquake motions. In contrast, very simple pseudo-static methods are used in engineering practice to determine response parameters for design. These methods neglect several of the factors cited above that can strongly affect pile response. Also soil–pile interaction is modelled using either linear or non-linear springs in a Winkler computational model for pile response. The reliability of this constitutive model has been questioned. In the case of pile groups, the Winkler model for analysis of a single pile is adjusted in various ways by empirical factors to yield a computational model for group response. Can the results of such a simplified analysis be adequate for design in all situations?The lecture will present a critical evaluation of general engineering practice for estimating the response of pile foundations in liquefiable and non-liquefiable soils during earthquakes. The evaluation is part of a major research study on the seismic design of pile foundations sponsored by a Japanese construction company with interests in performance based design and the seismic response of piles in reclaimed land. The evaluation of practice is based on results from field tests, centrifuge tests on model piles and comprehensive non-linear dynamic analyses of pile foundations consisting of both single piles and pile groups. Studies of particular aspects of pile–soil interaction were made. Piles in layered liquefiable soils were analysed in detail as case histories show that these conditions increase the seismic demand on pile foundations. These studies demonstrate the importance of kinematic interaction, usually neglected in simple pseudo-static methods. Recent developments in designing piles to resist lateral spreading of the ground after liquefaction are presented. A comprehensive study of the evaluation of pile cap stiffness coefficients was undertaken and a reliable method of selecting the single value stiffnesses demanded by mainstream commercial structural software was developed. Some other important findings from the study are: the relative effects of inertial and kinematic interactions between foundation and soil on acceleration and displacement spectra of the super-structure; a method for estimating whether inertial interaction is likely to be important or not in a given situation and so when a structure may be treated as a fixed based structure for estimating inertial loads; the occurrence of large kinematic moments when a liquefied layer or naturally occurring soft layer is sandwiched between two hard layers; and the role of rotational stiffness in controlling pile head displacements, especially in liquefiable soils. The lecture concludes with some recommendations for practice that recognize that design, especially preliminary design, will always be based on simplified procedures. 相似文献
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The seismic response of the Mexico City Cathedral built of very soft soil deposits is evaluated by using motions recorded in various parts of the structure during several moderate earthquakes. This unique set of records provides significant insight into the seismic response of this and other similar historic stone masonry structures. Free‐field ground motions are carefully compared in time and frequency domains with motions recorded at building basement. The dynamic characteristics of the structure are inferred from the earthquake records by using system identification techniques. Variation of seismic response for different seismic intensities is discussed. It is shown that, due to the soil–structure interaction, due to large differences between dominant frequencies of earthquake ground motions at the site and modal frequencies of vibration of the structure, and due to a particularly high viscous damping, seismic amplifications of ground motion in this and similar historic buildings erected on soft soil deposits are much smaller than that induced in most modern constructions. Nevertheless, earthquake records and analytical results show that several components of the structure such as its central dome and the bell towers may be subjected to local vibrations that significantly amplify ground motions. Overall, results indicate that in its present state the structure has an acceptable level of seismic safety. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
14.
利用天津滨海地区丰富的地震地质钻孔资料及测试数据,建立了相应的地震反应模型.选取美国加州1992年Landers地震的远场记录P0841作为基底输入,采用工程上常用的等效线性化方法进行了土层地震反应分析,以期对天津滨海软土场地的大震远场作用得到一些定性的认识．结果显示,天津滨海软土场地对远场大震地震动峰值加速度的放大作用比较显著,但不同场地条件放大作用有明显差异,Ⅳ类场地的放大效应明显减弱.对基岩与地表反应谱比的分析显示, 滨海场地对基岩地震动的不同频率分量的放大作用具有明显的不同,对短周期分量甚至出现了缩减,但当滨海软土场地受到与场地卓越周期相吻合的地震动影响时,可能会产生很可观的放大作用,这对建在其上的高层、超高层建筑可能会构成较大的威胁． 相似文献
15.
三种土层结构反应谱平台值的统计分析 总被引：4，自引：1，他引：3
本文以建筑抗震设计规范规定的反应谱为目标谱,通过调整加速度峰值和特征周期来人工合成数百条加速度时程曲线,并将其作为土层地震反应分析的地震动输入。在若干有工程意义的场地剖面中,选取和构造了部分软弱土层分别在底部、中部和顶部的三种土层剖面,利用土层地震反应分析的一维等效线性化波动方法,计算了不同土层剖面在不同地震动输入下的地表加速度反应谱的平台值。在统计分析的基础上,给出了不同场地三种土层结构的反应谱平台值的平均值。通过与正常剖面的反应谱平台值比较,给出了三种土层结构的反应谱平台值的影响系数。本文的研究获得了一些有意义的成果。 相似文献
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根据特定震源机制、震级、断层距和场地条件选取69条地震动记录并进行分组,利用Nspectra软件计算隔震结构的弹塑性位移反应谱,分析断层距、场地条件、震级、阻尼比对弹塑性位移谱的影响,探讨隔震层的力学参数对地震能量耗散的影响。研究结果表明:相较于远场,处于近场的隔震结构最为不利,隔震层位移谱值受场地条件、地震加速度和速度大小影响较大;随着断层距的增大,位移谱值衰减较快,且在软土场地中隔震层的位移谱值衰减幅度大于硬土场地;地震震级大小对位移谱形状的影响不明显,但能够使隔震层的位移谱值产生整体缩放效应;阻尼比在小于0.4的范围内,隔震层在不同地震动特性作用下位移谱值差别较大,但在大于0.4以后,位移谱值及谱形基本趋于一致;屈服力较小(恢复力/重力小于等于1)的隔震层随自振周期增大其耗能性能更加突出。 相似文献
17.
A.-W. Elgamal M. Zeghal E. Parra R. Gunturi H.T. Tang J.C. Stepp 《Soil Dynamics and Earthquake Engineering》1996,15(8):499-522
Seismic downhole-array data provide a unique source of information on actual soil (and overall site) behavior over a wide range of loading conditions that are not readily covered by in-situ or laboratory experimentation procedures. In this paper, free-field downhole-array seismic records are employed to identify and model the recorded response at the Lotung (Taiwan) and Treasure Island (California) sites. At Lotung, a five-accelerometer array recorded the site response during 18 earthquakes (1985–1986). The Treasure Island site was instrumented in 1992 with an array of six accelerometers that recorded a low amplitude earthquake in 1993. Using this downhole data, correlation and spectral analyses are performed to evaluate shear wave propagation characteristics, variation of shear wave velocity with depth and site resonant frequencies and model configurations. In addition, the actual seismic shear stress-strain histories are directly evaluated from the recorded downhole accelerations. These histories provide valuable insight into the mechanisms of site amplification, damping and pore-pressure build-up. Computational simulations of these case histories are performed based on the identified mechanisms of site response. In a companion paper, two additional case histories of site liquefaction are analyzed using records of downhole seismic response. 相似文献
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基于土层结构的场地分类方法 总被引：5，自引：0，他引：5
本文在详细研究了不同土层结构在不同地震动输入下的地表反应谱的平台值和特征周期的基础上,提出了基于土层结构的场地分类的初步方案。本方案给出了在现行建筑抗震设计规范四类场地划分的基础上,进一步考虑软弱夹层在底部、中部和上部3种情况的场地分类方法。本文的结果对进一步推动场地分类的研究具有一定的意义。 相似文献
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Fragility curves constitute the cornerstone in seismic risk evaluations and performance-based earthquake engineering. They describe the probability of a structure to experience a certain damage level for a given earthquake intensity measure, providing a relationship between seismic hazard and vulnerability. In this paper a numerical approach is applied to derive fragility curves for tunnel shafts built in clays, a component that is found in several critical infrastructure such as urban metro networks, airport facilities or water and waste water projects. The seismic response of a representative tunnel shaft is assessed using tridimensional finite difference non-linear analyses carried out with the program FLAC3D, under increasing levels of seismic intensity. A hysteretic model is used to simulate the soil non-linear behavior during the seismic event. The effect of soil conditions and ground motion characteristics on the soil-structure system response is accounted for in the analyses. The damage is defined based on the exceedance of the concrete wall shaft capacity due to the developed seismic forces. The fragility curves are estimated in terms of peak ground acceleration at a rock or stiff soil outcrop, based on the evolution of damage with increasing earthquake intensity. The proposed fragility models allows the characterization of the seismic risk of a representative tunnel shaft typology and soil conditions considering the associated uncertainties, and partially fill the gap of data required in performing a risk analysis assessment of tunnels shafts. 相似文献
20.
Takaji Kokusho Ryuichi Motoyama Hiroshi Motoyama 《Soil Dynamics and Earthquake Engineering》2007,27(4):354-366
Seismic wave energy in surface layers is calculated based on vertical array records at four sites during the 1995 Hyogo-ken Nambu earthquake by assuming vertical propagation of SH waves. The upward energy generally tends to decrease as it goes up from the base layer to the ground surface particularly in soft soil sites. Theoretical study on 1D multi-layers model to investigate the basic energy flow mechanism indicates that the energy at the ground surface can be smaller on softer soils due to high soil damping during strong shaking even if resonance effect is considered. A simple calculation for a shear-vibrating structure resting on foundation ground shows that induced strain in the structure is directly related to the energy or the energy flux of surface layers. Hence, a general perception that soft soil sites tend to suffer heavier damage than stiff sites should be explained not by greater incident energy but by other reasons such as degree of resonance. Furthermore, it is recommended that not only acceleration or velocity but also S-wave velocity should be specified at a layer where a design seismic motion is given, so that the seismic wave energy can clearly be quantified in seismic design practice. 相似文献