Fluctuant bearing capacity of shallow foundations during earthquakes

A limit equilibrium method to compute both dynamic bearing capacity and earthquake-induced settlements of foundations on soils using a log spiral potential failure surface is presented in this paper. The method is limited to soil materials that conform to the Mohr–Coulomb criterion. Time-varying inertia forces can be applied directly to the building. The procedure is capable of estimating vertical displacements and foundation tilting by integrating the differential equation developed from equilibrium of acting and resisting moments. The reliability of the method is assessed by comparing calculated and observed permanent movements of foundations for the September 1985 Earthquake in the soft ground area of Mexico City. The position of the failure surface is obtained by a minimization process at every time increment during the shaking. Results show that when an earthquake occurs the potential failure surface, obtained from static load, moves upwards and its length shortens as the seismic accelerations increase, reducing the bearing capacity and thus increasing its seismic risk. In this paper, the influence of earthquake acceleration on the time-varying positions of the potential failure is shown graphically.     

Static and seismic bearing capacity of shallow strip footings

In this study, the evaluation of static and seismic bearing capacity factors for a shallow strip footing was carried out by using the method of characteristics, which was extended to the seismic condition by means of the pseudo-static approach. The results, for both smooth and rough foundations, were checked against those obtained through finite element analyses.Under seismic conditions the three bearing capacity problems for N_c, N_q and N_γ were solved independently and the seismic bearing capacity factors were evaluated accounting separately for the effect of horizontal and vertical inertia forces arising in the soil, in the lateral surcharge and in the superstructure.Empirical formulae approximating the extensive numerical results are proposed to compute the static values of N_γ and the corrective coefficients that can be introduced in the well-known Terzaghi׳s formula of the bearing capacity to extend its applicability to seismic design of foundations.     

Numerical analysis of liquefaction-induced bearing capacity degradation of shallow foundations on a two-layered soil profile

Earthquake-induced excess pore pressure build-up and the associated shear strength degradation of liquefiable soils may result in bearing capacity degradation and seismic settlement accumulation of shallow foundations, two detrimental effects which need to be taken into account in order to ensure a viable performance-based design. This paper focuses on the first effect, in the case of strip and rectangle footings, resting on a deep liquefiable soil layer overlaid by a thinner non-liquefiable clay crust. A simplified analytical methodology is presented, based on the Meyerhof and Hanna [14] composite failure mechanism and the use of a reduced friction angle for the liquefied sand. The methodology is verified and evaluated against parametric numerical analyses with the Finite Difference Method, applying an advanced bounding surface constitutive model to account for the liquefied sand response. In addition, the existence of a critical clay crust thickness is explored, beyond which subsoil liquefaction does not affect the bearing capacity of the foundation.     

Conventional vs. modified pseudo-dynamic seismic analyses in the shallow strip footing bearing capacity problem

The conventional pseudo-dynamic(CPD) and modified pseudo-dynamic(MPD) methods are invoked to obtain the seismic bearing capacity of strip foundations using the limit equilibrium method, with a two-wedge failure mechanism.A spectral version of the conventional pseudo-dynamic method(SPD) is also invoked by considering the ground motion amplification factor, to be a function of the non-dimensional frequency λ/B and soil damping. Numeric analyses show that bearing capacity results obtained by the MPD and SPD methods are generally consistent. Both experience the same general reduction in bearing capacity with the increase of λ/B, with successive ups and downs corresponding to soil′s natural frequencies. For 5λ/B10, SPD and MPD results fluctuated between falling above and below CPD results. For λ/B2.5, SPD and MPD results were consistent with attenuation of the shear wave, while for 10λ/B, amplification was exhibited. Results obtained by the CPD method monotonically decrease, due to the fact that CPD fails to inherently consider site effects and damping, and instead and relies on a single factor to consider the ground motion amplification.     

提高浅层地震反射静校正精度的一种方法与数值模拟

浅层地震反射法是一种常用的勘探方法.在浅层地震资料处理中,静校正的精度直接影响速度反演的结果和叠加剖面的质量,在地形平缓时,固定基准面静校正可以满足勘探精度的要求,但在复杂地形条件下,其存在较大误差,即使采用浮动基准面,仍会由于地表一致性假设而残余静校正量,不能消除地形起伏引起的影响,为了提高浅层地震反射静校正的精度必须在常规静校正后进行一次剩余静校正,本文给出起伏地形条件下,滑动基准面(过共中心点的水平面即为该共中心点的滑动基准面)的剩余静校正量,该校正量与炮检距、反射层埋深、地层波速以及炮点和接收点高程有关,适用于单一介质和层状介质情况,本文通过对典型地形起伏的3个水平均匀层状介质理论地质模型的速度谱计算和分析,阐明在复杂地形条件下,应用本文提出的剩余静校正方法可以消除地形起伏的影响,提高静校正精度,在此基础上做动校正可以得到高质量的水平叠加剖面.     

A numerical study on seismic damage of masonry fortresses

Studies oriented to restoration and conservation of historical monumental buildings have recourse to structural analysis as a way to investigate the genuine structural features of the construction, to better understand its present condition and actual causes of existing damage, to estimate its safety conditions and to determine necessary remedial measures. Based on this background, this paper discusses on the seismic vulnerability of masonry fortresses by means of an analysis methodology based on three different analytical procedures, according to an increased knowledge of the structure. As a relevant case study the Albornoz fortress, a 14th stone masonry construction located in central Italy, was selected. Initially, the strategy proposed to perform this task was aimed at testing and developing an expeditious and non-destructive procedure to evaluate both the seismic vulnerability and the main mechanical properties of the different masonry typologies. The macroscale structural behavior of the fortress was then evaluated through a nonlinear static analysis (pushover) and a more simple approach based on the kinematic theorems of the limit analysis. From this point of view, by comparing the capacity of the construction to withstand lateral loads with the expected demands resulting from seismic actions, these methods provided a highly effective means of verifying the safety of the masonry structure and its vulnerability to extensive damage and collapse.     

基于位移的抗震设计中结构设计承载力的取值研究

合理的设计承载力取值能确保结构满足各性能水平的承载力需要。本文将多自由度结构等效为相应的单自由度体系,并假定单自由度体系为理想弹塑性体;根据结构各性能水平的目标位移,由能力谱法确定相应的实际承载力需求,取其中最大值作为结构实际承载力需求;引入超强系数定义结构实际承载力与设计承载力的比例关系,进而由实际承载力需求除以超强系数得到结构设计承载力,以此为依据对结构构件进行截面承载力设计,确保了结构能够满足性能目标要求。通过例题说明了本文方法的计算过程,并由静力弹塑性分析验证了其可行性。     

Seismic behaviour of shallow foundations: Shaking table experiments vs numerical modelling

The capability of a simplified approach to model the behaviour of shallow foundations during earthquakes is explored by numerical simulation of a series of shaking table tests performed at the Public Works Research Institute, Tsukuba, Japan. After a summary of the experimental work, the numerical model is introduced, where the whole soil–foundation system is represented by a multi‐degrees‐of‐freedom elasto‐plastic macro‐element, supporting a single degree‐of‐freedom superstructure. In spite of its simplicity and of the large intensity of the excitation involving a high degree of nonlinearity in the foundation response, the proposed approach is found to provide very satisfactory results in predicting the rocking behaviour of the system and the seismic actions transmitted to the superstructure. The agreement is further improved by introducing a simple degradation rule of the foundation stiffness parameters, suitable to capture even some minor details of the observed rocking response. On the other hand, the performance of the model is not fully satisfactory in predicting vertical settlements. Copyright © 2007 John Wiley & Sons, Ltd.     

浅层地震勘探中有限元数值模拟方法的应用研究

针对目前广泛用于解决工程地质问题的浅层地震勘探特点,通过对浅层小尺度介质进行地震波数值模拟,揭示地震波在不同浅层介质模型中的波场特征和传播规律.为浅层地震勘探野外数据采集前合理选择观测系统提供指导性的方案,并能对地震勘探成果解释的合理性给予客观评价.在分析各种数值模拟方法特点的基础上,选择具有精度高、易模拟复杂结构的有...     

Seismic technique to determine the allowable bearing pressure for shallow foundations in soils and rocks

Based on a variety of case histories of site investigations, including extensive bore-hole data, laboratory testing and geophysical prospecting, an empirical formulation is proposed for the rapid determination of allowable bearing capacity of shallow foundations. The proposed expression consistently corroborates the results of the classical theory and is proven to be rapid and reliable. It consists of only two soil parameters, namely, the in situ measured shear wave velocity, and the unit weight. The unit weight may be also determined with sufficient accuracy by means of another empirical expression using the P-wave velocity. It is indicated that once the shear and P-wave velocities are measured in situ by an appropriate geophysical survey, the allowable bearing capacity as well as the coefficient of subgrade reaction and many other elasticity parameters may be determined rapidly and reliably through a single step operation, not only for soils, but also for rock formations. Such an innovative approach, using the seismic wave velocities only, is considerably cost- and time-saving in practice.     

RPC面层加固约束空斗墙参数分析和抗震承载力研究

为弥补空斗墙抗震性能差和发挥活性粉末混凝土(RPC)的超高韧性,对1片未加固处理的空斗墙和2片由RPC面层加固处理的空斗墙分别进行拟静力试验,并结合有限元对三组试验进行参数分析。得出影响墙体抗震性能的主要因素有:墙体构造柱尺寸、轴压比、RPC单双面加固以及面层加固厚度。根据试验和有限元所得到的破坏特征,结合圈梁-构造柱对墙体的约束作用,引入等效受压斜撑模型,并量化圈梁-构造柱对墙体约束效应。基于圈梁-构造柱与墙体、RPC面层与被约束的墙体的协同受力特性,建立约束墙体及面层加固体墙的抗剪承载力的计算式。     

基于Zhou-Nowak数值积分法的结构整体概率抗震能力分析

结构整体概率抗震能力分析是新一代基于性能的地震工程中的一个重要模块,以往的研究主要采用平均值一次二阶矩法或Monte Carlo模拟法,分析了这2种方法存在的问题。为了有效地考虑结构系统随机性对结构整体抗震能力的影响,将Zhou-Nowak数值积分法与结构非线性静力推覆分析相结合,提出了基于Zhou-Nowak数值积分法的结构整体概率抗震能力分析方法,以新一代地震工程模拟仿真软件OpenSees为计算平台,以最大层间位移角作为结构整体抗震能力参数,对钢筋混凝土框架结构的整体概率抗震能力进行分析,并用Monte Carlo模拟法结果进行验证,从而建立了钢筋混凝土框架结构相应于不同性能水准的整体概率抗震能力模型。算例分析表明,所提方法是一种具有较高效率和较好精度的结构整体概率抗震能力分析方法。     

Macroseismic foundations of seismic microzoning

The Russian seismic safety regulations are based on macroseismic evaluations in terms of seismic intensity values. According to the seismic scale, intensity value determines both the degree of destruction of buildings and engineering structures and such parameters of seismic impacts as peak ground accelerations and response spectra. Application of macroseismic approach in various aspects of seismic microzoning has led to achievements in earthquake engineering; these aspects are discussed. The advantages and disadvantages of the macroseismic approach are considered.     

地震海啸传播过程数值模拟研究

运用数值模拟的方法,对常水深4 000 m,8.0级纯倾滑直立断层触发的地震海啸,在大洋中传播的前2 000 s各时刻的形态以及峰值进行研究.结果表明,地震海啸产生后的前1 000 s左右的时间内,存在数值震荡,幅值先快速衰减,再逐渐增大,此后,幅值随着时间的推移缓慢衰减.     

A numerical predicting method on monthly seismic tendency

IntroductionEarthquakepredictionisanundertakingofpublicwelfare,butearthquakecannotbesuccessfullypredictedatthepresentstageduetotechnologicalreasons.Howtoadaptthepresentsituationofearthquakepredictiontotheneedofsocietyisasubjectthatshouldbestudiedearnestly.Logicallyspeaking,themostimportantproblemindisasterpreventionisthemagnitudeofthecomingearthquake(becauseitconcernstherangeandinvestmentforprevention);Thesecondproblemisthemostdangerousseismiczone(becauseitconcernstheconcretearrangementforprev…     

空心砖保温夹心墙体的抗震承载能力计算与分析

在9片空心砖复合夹心墙体的低周期反复荷载试验的基础上，分析了夹心墙体的抗震承载能力，提出了夹心墙体开裂荷载与极限荷载计算的建议公式。     

A review of numerical experiments on seismic wave scattering

This paper reviews applications of the finite-difference and finite-element methods to the study of seismic wave scattering in both simple and complex velocity models. These numerical simulations have improved our understanding of seismic scattering in portions of the earth where there is significant lateral heterogeneity, such as the crust. The methods propagate complete seismic wavefields through highly complex media and include multiply scattered waves and converted phases (e.g.,P toSV, SV toP, body wave to surface wave). The numerical methods have been especially useful in cases of moderate and strong scattering in complex media where multiple scattering becomes important. Progress has been made with numerical methods in understanding how near-surface, low-velocity basin structures scatter surface waves and vertically-incident body waves. The numerical methods have proven useful in evaluating scattering of surface waves and body waves from topography of both the free surface and interfaces buried at depth. Numerical studies have demonstrated the importance of conversions from body waves to surface waves (andvice versa) when lateral heterogeneities and topographic relief are present in the uppermost crust. Recently, several investigations have applied numerical methods to study seismic wave propagation in velocity models which vary randomly in space. This stochastic approach seeks to understand the effects of small-scale complexity in the earth which cannot be resolved deterministically. These experiments have quantified the relationships between the statistical properties of the random heterogeneity and the measurable properties of high-frequency (1 Hz) seismograms. These simulations have been applied to the study of many features observed in actual high-frequency seismic waves, including: the amplitude and time decay of seismic coda, the apparent attenuation from scattering, the dispersion of waveforms, and the travel time and waveform variations across arrays of receivers.     

On the seismic response of under-designed caisson foundations

The seismic behaviour of caisson foundations supporting typical bridge piers is analysed with 3D finite elements, with due consideration to soil and interface nonlinearities. Single-degree-of freedom oscillators of varying mass and height, simulating heavily and lightly loaded bridge piers, founded on similar caissons are studied. Four different combinations of the static ( $\text{ FS }_\mathrm{V}$ FS V ) and seismic ( $\text{ FS }_\mathrm{E}$ FS E ) factors of safety are examined: (1) a lightly loaded ( $\text{ FS }_\mathrm{V}= 5$ FS V = 5 ) seismically under-designed ( $\text{ FS }_\mathrm{E} < 1$ FS E < 1 ) caisson, (2) a lightly loaded seismically over-designed ( $\text{ FS }_\mathrm{E} >1$ FS E > 1 ) caisson, (3) a heavily loaded ( $\text{ FS }_\mathrm{V} = 2.5$ FS V = 2.5 ) seismically under-designed ( $\text{ FS }_\mathrm{E} < 1$ FS E < 1 ) caisson and (4) a heavily loaded seismically over-designed caisson. The analysis is performed with use of seismic records appropriately modified so that the effective response periods (due to soil-structure-interaction effects) of the studied systems correspond to the same spectral acceleration, thus allowing their inelastic seismic performance to be compared on a fair basis. Key performance measures of the systems are then contrasted, such as: accelerations, displacements, rotations and settlements. It is shown that the performance of the lightly loaded seismically under-designed caisson is advantageous: not only does it reduce significantly the seismic load to the superstructure, but it also produces minimal residual displacements of the foundation. For heavily loaded foundations, however ( $\text{ FS }_{V} = 2.5$ FS V = 2.5 ), the performance of the two systems (over and under designed) is similar.