Centrifuge study into the effect of liquefaction extent on permanent settlement and seismic response of shallow foundations

Centrifuge experiments were conducted to investigate how the liquefaction extent affects the seismic and post-seismic settlement of shallow foundations resting on saturated sand. Two rigid foundations with different bearing pressures were placed on the ground surface in a model container. Multiple input motions were applied to achieve different extents of soil liquefaction. The results indicate that foundation settlement can be divided into three distinct phases: (I) during shaking, (II) during the time period after shaking has ceased and before soil reconsolidation in the shallowest layers has taken place, and (III) during soil reconsolidation. Contrary to the free-field ground, most of the total settlement of the foundations occurred before soil reconsolidation, i.e., during Phases I and II. The volumetric strain during these phases was not significant as opposed to the shear strain produced by the foundation surcharge. It was demonstrated that foundation settlement is not necessarily proportional to the liquefied depth of the sand. The extent of the liquefaction in the sand medium mostly affected the post-seismic settlement of the foundations, while the co-seismic settlement was relatively the same for both foundations. The response of the foundations was significantly influenced by the liquefaction extent, whereas the foundations did not experience large accelerations when the soil profile was entirely liquefied. However, the foundations tolerated large settlement under severe liquefaction conditions. The results of this study highlight the role of the liquefaction extent on co-seismic and post-seismic settlement as well as the seismic response of shallow foundations.     

Refinement of the design procedure for shallow strip foundations

Suggestions regarding the accounting of various factors in the design of shallow strip foundations on heaving soils are examined. An example of a design developed in accordance with the proposed procedure is cited.     

基于性能抗震设计与常规抗震设计比较研究

指出了传统抗震设计的目标是尽量减少结构破坏和生命的损失,基于性能的抗震设计则追求建筑结构性能、安全与经济三者的优化平衡,对常规抗震设计与基于性能抗震设计的理论及设计方法进行了比较,并着重阐述了基于性能抗震设计的理论与方法。     

Influence of site conditions on seismic design parameters for foundations as determined via nonlinear site response analysis

Site conditions, including geotechnical properties and the geological setting, influence the near-surface response of strata subjected to seismic excitation. The geotechnical parameters required for the design of foundations include mass density (ρ), damping ratio (β_s), shear wave velocity (V_s), and soil shear modulus (G_s). The values of the last three parameters are sensitive to the level of nonlinear strain induced in the strata due to seismic ground motion. In this study, the effect of variations in soil properties, such as plasticity index (PI), effective stress (σ′), over consolidation ratio (OCR), impedance contrast ratio (ICR) between the bedrock and the overlying strata, and depth of soil strata over bedrock (H), on seismic design parameters (β_s, V_s, and G_s) was investigated for National Earthquake Hazards Reduction Program (NEHRP) site classes C and D, through 1D nonlinear seismic site response analysis. The Morris one-at-a-time (OAT) sensitivity analysis indicated that β_s, V_s, and G_s were significantly influenced by variations in PI, while ICR affected β_s more than it affected V_s and G_s. However, the influence of H on these parameters was less significant. It was also found that variations in soil properties influenced seismic design parameters in soil type D more significantly than in soil type C. Predictive relationships for β_s, V_s, and G_s were derived based on the 1D seismic site response analysis and sensitivity analysis results. The β_s, V_s, and G_s values obtained from the analysis were compared with the corresponding values in NEHRP to determine the similarities and differences between the two sets of values. The need to incorporate PI and ICR in the metrics for determining β_s, V_s, and G_s for the seismic design of foundations was highlighted.     

#br# 基于永久位移的边坡地震稳定性安全评价方法研究

基于室内动三轴试验方法研究了最大动孔隙水压力随最大偏应力和平均有效应力的变化规律,提出了砂质边坡最大动孔隙水的简化计算方法,将其代入永久位移的计算式中提出了可以考虑动孔隙水压力影响的边坡永久位移简便计算方法|通过开展室内振动台试验验证了永久位移简便计算方法的准确性,并进一步研究了地下水位对砂质边坡破坏模式的影响规律,最后提出了边坡基于性能的地震稳定性安全评价方法,并形成了一套结合永久位移的边坡地震稳定性评价新思路。研究表明,无地下水时,边坡的破坏开始于坡顶,表现出了明显的“鞭梢效应”|在较高水位时边坡的破坏开始于坡脚,且边坡顶部有明显的震陷现象|通过将本文建立的边坡动孔隙水压力的简化关系式和永久位移简便计算方法与室内试验值进行了对比,验证了其准确性。最后建立了基于安全系数和永久位移的边坡地震稳定性评价体系为坡体稳定性判识提供参考。     

性能设计中设防标准与设计参数的确定

讨论了在基于结构性能的抗震设计理论中结构抗震设防标准的确定方法,分析了抗震设防的依据和需要考虑的因素,提出了方便使用的抗震设计参数,可供结构设计人员参考借鉴。     

Use of shallow slotted foundations

Experience acquired with the design and installation of slotted foundations during the construction of buildings in Moscow and the Moscow Oblast is codified. Examples are cited for use of these foundations on construction projects.     

Analysis of shallow double-slot foundations

An analysis is developed on the basis of interaction between double-slot foundations and the soil bed for broader use of data on foundation structures in construction practice. __________ Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 11–14, January–February, 2006.     

结构抗震新技术在世博永久建筑工程中的应用

上海世博永久主体建筑均为大型公共建筑,具有规模宏大、功能复杂、重要性高以及工期紧迫等特点。以往工程设计中通过概念设计或增加结构刚度来满足抗震性能要求,本文通过世博工程中的实例阐述了消能减震技术在工程中的应用,提出了结构抗震设计的新思路。世博中心为上海2010世博会永久场馆之一,其结构设计时采用了基于性能的抗震设计理念,在结构关键部位设置防屈曲耗能支撑(BRB)等消能减震构件,使结构的抗震性能提高,降低了主体结构的用钢量。采用此项抗震新技术,既满足可持续发展和低碳建筑要求,又切合上海世博会主题——城市,让生活更美好。     

Calculations on shallow foundations from various standards

Foundation calculations are compared on the basis of the German standards, Eurocode 7, and the Russian SNiP ones. Proposals are made on revising the Russian standardization documents. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 2, pp. 18–21, March–April, 1999.     

建筑结构抗震的概念设计浅议

本文简要阐述了建筑结构抗震中概念设计的主要内容,经过我国历年地震中建筑物的损坏程度对比,强调了抗震概念设计的重要性,提出了对设计人员在进行建筑结构抗震设计时的灵活性、创新性要求。     

橡胶垫层间隔震结构设计参数研究

采用双线型模型,由弯矩转角关系判别状态刚度的方法,分析了隔震结构设计中橡胶垫的弹性模量与厚度变化情况下的隔震效果,旨在确定出层间隔震的设计参数,为层间隔震的合理应用提供理论参考。     

Influence of connection design parameters on the seismic performance of braced frames

Special concentrically braced frames (SCBFs) are commonly used lateral-load resisting systems in seismic design. In SCBFs, the braces are connected to the beams and columns by gusset plate connections, and inelastic deformation is developed through tensile yielding and inelastic post-buckling deformation of the brace. Recent experimental research has indicated that the seismic performance of SCBFs can be improved by designing the SCBF gusset plate connections with direct consideration of the seismic deformation demands and by permitting yielding in the gusset plate at select performance levels.Experimental research provides important information needed to improve SCBF behavior, but the high cost of experiments limits this benefit. To extend and better understand the experimental work, a companion analytical study was conducted. In an earlier paper, the inelastic finite element model and analysis procedure were developed and verified through detailed comparison to experimental results. In this paper, the model and analytical procedure extend the experimental results. A parametric study was conducted to examine the influence of the gusset plate and framing elements on the seismic performance of SCBFs and to calibrate and develop improved design models. The impact of the frame details, including the beam-to-column connections, the brace angles, and their inelastic deformation demands, was also explored. The results suggest that proper detailing of the connections can result in a large improvement in the frame performance.     

水平与竖向加速度-时程曲线叠加效应下边坡永久位移计算的极限上限分析

实际边坡动力稳定性受地震竖向与水平方向效应共同作用,传统边坡地震永久位移计算方法较少考虑竖向地震波影响,采用实际地震的竖向与水平方向加速度时程曲线共同效应更符合工程实际。基于极限分析上限法和Newmark刚塑性滑块模型,提出一种基于实际水平向与竖向地震加速度时程曲线的边坡永久位移计算改进方法,以3个工程边坡为例,探讨了两组具有代表性实测典型水平和竖向地震地面运动记录对边坡地震永久位移计算的影响。研究结果表明:不考虑竖向地震加速度时程曲线时,本文方法可蜕化为与前人方法兼容;不同地震波的竖向与水平地震动时程曲线的叠加效应不同,竖向地震对边坡永久位移的影响不可忽略。     

Life-cycle optimization in the establishment of performance-acceptance parameters for seismic design

A life-cycle formulation is presented for the determination of optimum values of the mechanical properties of a structural system exposed to seismic risk. The resulting values are intended for providing support for the establishment of performance-acceptance criteria and parameters for seismic design. A method is developed for the determination of expected damage functions in terms of simplified reference models of the complex nonlinear systems that are typical of engineering practice. The uncertainties associated with the use of the simplified model to estimate peak dynamic responses of the system of interest are accounted for by means of first-order second-moment probabilistic criteria. An illustrative application of the criteria proposed is presented, together with a discussion about the translation of the results of the optimization studies into engineering criteria and methods expressed in conventional design formats.     

成层地基一维土层地震反应解析解

１　前　　言　*对于土层地震反应,只有当模型简单时,才可以得到封闭解析解或半解析解。假定地震输入是竖直向上入射的平面剪切波,土层地震反应问题常简化为一维波动问题,对于变参数一维土层地震反应,可从两个方面求解解析解或半解析解：一是针对单层地基,假定模型参数随深度变化规律已知进行求解［１,２］;二是针对成层地基,每层土层取不同模型参数进行求解［３～５］。文献［３～５］采用的是等效线性化解法或传递矩阵解法,属于半解析解。本文针对成层地基,把基岩输入地震化作边界静止不动而在计算域各点上作用着惯性力的问题进行计算,建立了成层地基一维土层地震反应定解问题,并给出了成层地基在任意基岩地震输入作用下、时间域内     

Influence of fabric anisotropy on seismic responses of foundations

Earthquakes,as one of the well-known natural disasters,are highly destructive and unpredictable.Foundation failure due to liquefaction induced by earthquakes can cause casualties as well as signi fi cant damage to the building itself.Fabric anisotropy of soil grains is considered to be an important factor in dynamic soil response based on previous researches and laboratory tests.However,the limited availability of real physical data makes it less persuasive.In this study,a shake table installed on a geotechnical centrifuge is used to provide the designed seismic motions,and therefore,to simulate the realistic earthquake motion to foundations.Important parameters in the responses such as acceleration,excess pore pressure and deformation are evaluated to investigate the in fl uence.Implications for design are also discussed.