Correlating measured and simulated dynamic responses of a tall building to long‐distance earthquakes

For almost a decade, a 66‐storey, 280m tall building in Singapore has been instrumented to monitor its dynamic responses to wind and seismic excitations. The dynamic characteristics of the tall building have been investigated via both the finite element method and the experimental modal analysis. The properties of the finite element model have been shown to correlate well with those derived from the data recorded during the ambient vibration tests. During the study period, 21 sets of earthquake ground motions have been recorded at the building site. The basement motions may be divided into three categories based on their predominant frequency components with respect to the building's fundamental frequency. The calibrated three‐dimensional finite element model is employed to simulate the seismic response of the tall building. Correlation analysis of the time histories between the recorded data and the simulated results has been carried out. The correlation analysis results show that the simulated dynamic response time histories match well with those of the recorded dynamic responses at the roof level. The results also show that the simulated maximum response at the roof level is close to the peak response recorded during the earthquakes. Copyright © 2004 John Wiley & Sons, Ltd.     

Displacement reducer fuses for improving seismic performance of caisson quay walls

This research is to study the efficiency of displacement reducer fuses, installed behind the caisson quay walls for controlling the dynamic backfill thrust and minimizing the displacement, settlement and tilting of the walls. For this purpose, two types of fuses, Displacement Reducer Panels (DRP) and Displacement Reducer Elements (DRE), were constructed and installed behind the wall. The DRPs were constructed by hollow Polypropylene sheets to reproduce elastoplastic and plastic mechanical behaviors. The DREs were cylindrical stainless-steel dampers, working upon friction mechanism that can reproduce perfect plastic behavior. Here, two series of shaking table 1-g tests were performed with DRP and DRE applications. In this regard, different mechanical behaviors and capacities were considered for fuses against demand thrusts of backfill in order to compare the mitigation tests with no-mitigation cases. Harmonic base motions with constant amplitude and constant frequency were used in the model test. The foundation soil and the backfill soil were constructed with the relative densities of 85 and 25%, respectively, to reproduce non-liquefiable base layer and loose backfill situations in the model, respectively. The results showed remarkable reduction in kinetic energy, dynamic backfill thrust and consequently seaward movement, settlement and inclination of the caisson quay wall in case of using fuses with plastic behaviors behind the wall.     

不同厚度饱和砂土中群桩结构动力响应试验研究

液化土中桩基础动力响应规律一直是工程抗震领域关注的热点问题。本文基于非液化砂土和不同厚度饱和砂土中的2×2群桩结构模型振动台试验,通过输入一定峰值加速度和频率的正弦波,对群桩在非液化土层和两种不同厚度饱和砂土层中的横向动力响应特性进行振动台试验研究。研究结果表明:在正弦波输入情况下,非液化砂土中群桩承台加速度和位移时程与台面输入时程相比,波形变化规律与峰值大小均相差不大;而对两种不同厚度饱和砂土中承台加速度和位移峰值放大较多,在相对较薄的饱和砂土中群桩承台加速度峰值较台面输入放大了1.83倍,较台面输出位移峰值放大了1.58倍;在相对较厚的饱和砂土中承台加速度和位移峰值则分别放大了2.18倍和1.91倍,说明在相同输入条件下,较厚的饱和砂土在发生液化后群桩承台的动力响应更加显著。     

Functional relationship to describe temporal statistics of soil moisture averaged over different depths

Detailed simulation studies, highly resolved in space and time, show that a physical relationship exists among instantaneous soil-moisture values integrated over different soil depths. This dynamic relationship evolves in time as a function of the hydrologic inputs and soil and vegetation characteristics. When depth-averaged soil moisture is sampled at a low temporal frequency, the structure of the relationship breaks down and becomes undetectable. Statistical measures can overcome the limitation of sampling frequency, and predictions of mean and variance for soil moisture can be defined over any soil averaging depth d. For a water-limited ecosystem, a detailed simulation model is used to compute the mean and variance of soil moisture for different averaging depths over a number of growing seasons. We present a framework that predicts the mean of soil moisture as a function of averaging depth given soil moisture over a shallow d and the average daily rainfall reaching the soil.     

Estimation of damping ratio of soil sites using microtremor

It is widely known that the seismic response characteristics of a soil site depends heavily on several key dynamic properties of the soil stratum, such as predominant frequency and damping ratio. A widely used method for estimating the predominant frequency of a soil site by using microtremor records, proposed by Nakamura, is investigated to determine its effectiveness in estimating the damping ratio. The authors conducted some microtremor measurements of soil sites in Hong Kong and found that Nakamura’s method might also be used to estimate the damping ratio of a soil site. Damping ratio data from several typical soil sites were obtained from both Nakamura’s ratio curves using the half power point method and resonant column tests. Regression analysis indicates that there is a strong correlation between the damping ratios derived from these two different approaches. Supported by: Research project (PolyU 5076/97E), of the Department of Civil and Structural Engineering, The Hong Kong Polytechnic University     

地下RC结构物地震响应特征土工离心试验的模拟

地下结构物抗震稳定性的研究具有重要实际意义。基于动态土工离心试验,应用有效应力法分析了不同水平地震作用下土层和地下RC结构物的最大水平位移、不同埋深的土层加速度和超孔隙水压力变化规律,以及结构物的破坏发展特征,并进一步与试验结果进行了相互验证和对比,取得了较好效果,为重要地下结构物的抗震设计提供了依据和参考。     

Experimental study on the aseismic capacity of a wooden house using shaking table

The aseismic capacity of a typical Korean wooden house built using traditional construction methods is quantitatively estimated. Tenon joints were used in wooden frames. Two 1:4 scale models were tested for rock and soil foundation conditions. Scaled real earthquake time histories were input for the tests. The natural frequency and modal damping ratio of the wooden house in the elastic range were 1·66 Hz and 7 per cent, respectively. The Peak Ground Acceleration (PGA) at the collapse of the house at the soil site was 0·25g, whereas PGA for moderate damage at the rock site was 0·6g. A significant reduction in acceleration response and increase in displacement response was observed for rock and soil foundation conditions, respectively. The wooden house studied is much more vulnerable at soil sites than at rock sites due to the rich low-frequency contents of the input motion and the flexible characteristics of the wooden house. Non-linear dynamic analyses using the modified Double-Target model were compared with test results. The modified Double-Target model appropriately simulates the non-linear inelastic behaviour of a wooden house with tenon joints. Copyright © 1999 John Wiley & Sons, Ltd.     

Mechanics and performance of a tied-back wall under seismic loads

In this paper the stability of a tied-back wall subjected to seismic loads is analysed for a predetermined mode of failure (rotation about the top of the wall) and the analysis is compared with data from tests on this type of wall using the seismic simulator at the State University of New York at Buffalo. We carried out a pseudo-static analysis of the problem using the Mononobe-Okabe earth pressure coefficients, wherein the dynamic effects due to the seismic loading are converted into equivalent static loads. The acceleration ratio at which the wall fails by rotation about the top was obtained by considering the moments due to the various lateral earth pressure resultants and the inertial forces induced in the soil due to the seismic loading. We found that the presence of wall friction on the passive side significantly enhances the stability of the flexible retaining wall under seismic loads. Thus, flexible retaining walls supporting dry cohesionless soil can be very efficient during earthquakes. Under moderate earthquakes, an increase in the depth of embedment increases the dynamic factor of safety significantly. However, beyond a certain acceleration ratio for a soil with a particular value of ø, any increase in the depth of emdedment has no effect in impeding failure, irrespective of any change in the geometry of the system. Seismic design charts are presented to evaluate the stability of, and to design, flexible retaining walls embedded in dry cohesionless soils under seismic loading.     

近断层强地震动场预测

以1997年4月11日新疆伽师地震（M_w6.1）为例,详细介绍了近断层强地震动场的预测方法.首先,用有限断层震源建模方法建立了该次地震的震源模型;然后,基于动力学拐角频率的地震动随机模拟方法,模拟了该次地震仅有主震加速度记录、且位于巨厚土层上的三个台站的加速度时程,并用实际地震记录进行了验证.在此基础上,基于预测的近断层77个节点的加速度时程的峰值绘制了该次地震的加速度场.结果表明,上述方法模拟的加速度时程在0.5 Hz以上的高频段是可行的、实用的;预测的近断层加速度场具有非常明显的上盘效应.地表最大加速度的范围与断层面上最大凹凸体位置相对应,说明与断层面上凹凸体相对应的地面上的建（构）筑物将会遭受到较为严重的震害.     

Physical modeling and visualization of soil liquefaction under high confining stress

The mechanisms of seismically-induced liquefaction of granular soils under high confining stresses are still not fully understood. Evaluation of these mechanisms is generally based on extrapolation of observed behavior at shallow depths. Three centrifuge model tests were conducted at RPI‘s experimental facility to investigate the effects of confining stresses on the dynamic response of a deep horizontal deposit of saturated sand. Liquefaction was observed at high confining stresses in each of the tests. A system identification procedure was used to estimate the associated shear strain and stress time histories. These histories revealed a response marked by shear strength degradation and dilative patterns. The recorded accelerations and pore pressures were employed to generate visual animations of the models. These visualizations revealed a liquefaction front traveling downward and leading to large shear strains and isolation of upper soil layers.     

Influence of frequency‐dependent soil–structure interaction on the fragility of R/C bridges

Bridge performance under earthquake loading can be significantly influenced by the interaction between the structure and the supporting soil. Even though the frequency dependence of the interaction mentioned in this study has long been documented, the simplifying assumption that the dynamic stiffness is dominated by the mean or predominant excitation frequency is still commonly made, primarily as a result of the associated numerical difficulties when the analysis has to be performed in the time domain. This study makes use of the advanced lumped parameter models recently developed 1 in order to quantify the impact of the assumption on the predicted fragility of bridges mentioned in this study. This is achieved by comparing the predicted vulnerability for the case of a reference, well studied, actual bridge using both conventional, frequency‐independent, Kelvin–Voigt models and the aforementioned lumped parameter formulation. Analysis results demonstrate that the more refined consideration of frequency dependence of soil–structure interaction at the piers and the abutments of a bridge not only leads to different probabilities of failure for given intensity measures but also leads to different hierarchy and distribution of damage within the structure for the same set of earthquake ground motions even if the overall probability of exceeding a given damage state is the same. The paper concludes with the comparative assessment of the effect for different soil conditions, foundation configurations, and ground motion characteristics mentioned in this study along with the relevant analysis and design recommendations. Copyright © 2016 John Wiley & Sons, Ltd.     

Field observations of instantaneous water slopes and horizontal pressure gradients in the swash-zone

Field observations of instantaneous water surface slopes in the swash zone are presented. For free-surface flows with a hydrostatic pressure distribution the surface slope is equivalent to the horizontal pressure gradient. Observations were made using a novel technique which in its simplest form consists of a horizontal stringline extending seaward from the beach face. Visual observation, still photography or video photography is then sufficient to determine the surface slope where the free-surface cuts the line or between reference points in the image. The method resolves the mean surface gradient over a cross-shore distance of 5 m or more to within ±0.001, or 1/20th −1/100th of typical beach gradients. In addition, at selected points and at any instant in time during the swash cycle, the water surface slope can be determined exactly to be dipping either seaward or landward. Close to the location of bore collapse landward dipping water surface slopes of order 0.05–0.1 occur over a very small region (order 0.5 m) at the blunt or convex leading edge of the swash. In the middle and upper swash the water surface slope at this leading edge is usually very close to horizontal or slightly seaward. Behind the leading edge, the water surface slope was observed to be very close to horizontal or dipping seaward at all times throughout the swash uprush. During the backwash the water surface slope was observed to be always dipping seaward, approaching the beach slope, and remained seaward until a new uprush edge or incident bore passed any particular cross-shore location of interest. The observations strongly suggest that the swash boundary layer is subject to an adverse pressure gradient during uprush and a favourable pressure gradient during the backwash. Furthermore, assuming Euler's equations are a good approximation in the swash, the observations also show that the total fluid acceleration is negative (offshore) for almost the whole of the uprush and for the entire backwash. The observations are contrary to recent work suggesting significant shoreward directed accelerations and pressure gradients occur in the swash (i.e., ∂u/∂t>0∼∂p/∂x<0), but consistent with analytical and numerical solutions for swash uprush and backwash. The results have important implications for sediment transport modelling in the swash zone.     

Seismic active pressure distribution history behind rigid retaining walls

Evaluating the seismic active earth pressure on retaining walls is currently based on pseudo-static method in practices. In this method, however, it is not simple, choosing an appropriate value for earthquake coefficient, which should fully reflect the dynamic characteristics of both soil and loading is an important problem. On the other hand, by using only two extra dynamic parameters that are shear wave velocity of soil and predominant frequency of probable earthquake, one can benefit from another more accurate tool called pseudo-dynamic method to solve the problem of earth pressure.In this study in the framework of limit equilibrium analysis, pseudo-dynamic method has been applied into horizontal slice method of analysis to account for the effect of earthquake on lateral earth pressure history behind rigid retaining walls. The pressure history resulted from a number of analyses shows that before and after reaching the peak resultant force, different pressure distributions occur behind a wall that put more local pressure than the same at peak. This method would be a tool to control this phenomenon in wall design.     

Dynamic instability of clays: an energy approach

The dynamic instability of any soil is based on the ability to accumulate from cycle to cycle a part of the energy to be dissipated. Using the general thermodynamic approach in combination with high resolution experiments it is possible to analyze the transformations of energy in soil under dynamic loading. Undisturbed specimens of two Norwegian clays, sufficiently different in their engineering properties, were used in a number of undrained cyclic triaxial tests to study the influence of confining stress, coefficient of earth pressure at rest (K₀), dynamic stress amplitude and frequency of loading in the development of instability in the clays. This paper discusses the unit dissipated energy and the unit activation energy as possible energy criteria for evaluation of dynamic instability in soils.     

Characteristics of frequency content of near-fault ground motions during the Chi-Chi earthquake

Introduction The dynamic response of structural systems subjected to earthquake ground shaking is sig-nificantly affected by the frequency content of input ground motions. When the frequency content of a predominant earthquake ground motion closely matches the natural period of a structural sys-tem, the dynamic response is significantly enhanced and thus may cause severe damage (Chopra, 1995). Therefore, it is of great importance to evaluate the frequency content of ground motions. In recent …     

集集地震近断层地震动频谱特性

利用5个反映地震动频谱特征的周期(反应谱卓越周期Tp, 平滑化反应谱卓越周期To, 傅氏幅值谱平均周期Tm, 等效速度脉冲周期Tv和拟速度反应谱卓越周期Tpv), 对集集地震的近断层三分量地震动进行研究. 结果表明, 上盘地震动的频谱周期小于下盘地震动; Tp小于To和Tm, 且Tp反映的三分量之间的关系与To和Tm不同; 在地表断裂带的北端,Tv和Tpv所反映的近断层地震动长周期分量的频谱特征, 与走滑断层中方向性效应作用的规律相类似. 得出的定性或定量结论可以为近场抗震设计谱的建立与地震危险性区划研究提供参考.      

Recording-based identification of site liquefaction

Reconnaissance reports and pertinent research on seismic hazards show that liquefaction is one of the key sources of damage to geotechnical and structural engineering systems. Therefore, identifying site liquefaction conditions plays an important role in seismic hazard mitigation. One of the widely used approaches for detecting liquefaction is based on the time-frequency analysis of ground motion recordings, in which short-time Fourier transform is typically used. It is known that recordings at a site with liquefaction are the result of nonlinear responses of seismic waves propagating in the liquefied layers underneath the site. Moreover, Fourier transform is not effective in characterizing such dynamic features as time-dependent frequency of the recordings rooted in nonlinear responses. Therefore, the aforementioned approach may not be intrinsically effective in detecting liquefaction. An alternative to the Fourier-based approach is presented in this study, which proposes time-frequency analysis of earthquake ground motion recordings with the aid of the Hilbert-Huang transform （HHT）, and offers justification for the HHT in addressing the liquefaction features shown in the recordings. The paper then defines the predominant instantaneous frequency （PIF） and introduces the PiF-related motion features to identify liquefaction conditions at a given site. Analysis of 29 recorded data sets at different site conditions shows that the proposed approach is effective in detecting site liquefaction in comparison with other methods.     

随机地震荷载作用下黄土动本构关系的试验研究

本文利用动三轴试验系统,试验研究了随机地震荷载作用下,非饱和原状黄土的动本构关系。结果表明,其本构关系依然服从双曲线模型,但模型参数随所施加的随机地震荷载时程的不同而不同。在试验中还对不同随机地震荷载和正弦波等效荷载作用下的模型参数、剪切模量和阻尼比进行了对比研究,初步建立了动本构模型参数与所施加的随机地震荷载的卓越周期之间的定量关系。通过试验和计算,证实了曼辛准则对黄土的适用性。     

双向地震作用下锯末混合土场地动力特性分析

在北京工业大学振动台台阵系统上开展了一系列锯末混合土地基自由场振动台模型试验,试验中模型箱采用装配式连续体刚性模型箱,试验中输入地震动时程采用El Centro地震动记录、Taft地震动记录和天津地震动记录,地震动输入方向分为水平单向和水平双向。文中,重点考察了双向地震动输入下锯末混合土模型场地的动力特性及其变化规律,主要指标包括模型场地地震动反应的峰值加速度及其动力放大系数、加速度时程及其傅氏谱。试验结果表明:随着输入地震动强度的增大,同一测点反应的峰值加速度总体上在增大,而其加速度动力放大系数总体上呈现减小的趋势,反应的频谱组成从较高频率向较低频率移动;双向地震作用下锯末混合土模型场地的动力变化规律与单向地震作用下较为一致。     

Experimental study of near-shore pile-to-pile interaction

This paper presents the results of lateral impact load field tests carried out on a system of three steel pipe piles vibro-driven into soft clay in a near-shore marine environment, with the aim of evaluating the pile–soil–pile dynamic interaction. Piles are arranged in an “L” shaped horizontal layout and are instrumented with accelerometers at their free heads. The obtained results show the complex dynamic behaviour at very small strain of the vibrating soil–water–piles system. The role of different type of waves in the pile to pile interaction is investigated by analyzing the results in the time and frequency domains and by means of a time–frequency analysis. The effects of the pile spacing and input direction on these interaction mechanisms are also presented. Finally, important dynamic parameters of the soil, such as the velocities of the shear waves and surface waves (Scholte waves) of the upper soil are directly estimated from the time delays between signals recorded at the pile heads.