Efficient risk assessment of lifeline networks under spatially correlated ground motions using selective recursive decomposition algorithm

For effective hazard mitigation planning and prompt-but-prudent post-disaster responses, it is essential to evaluate the reliability of infrastructure networks accurately and efficiently. A nonsimulation-based algorithm, termed as a recursive decomposition algorithm (RDA), was recently proposed to identify disjoint cut sets and link sets and to compute the network reliability. This paper introduces a ‘selective’ RDA, which preferentially identifies critical disjoint cut sets and link sets to calculate the probabilities of network disconnection events with a significantly reduced number of identified sets. To this end, the original RDA is improved by replacing the shortest path algorithm with an algorithm that identifies the most reliable path, and by using a graph decomposition scheme based on the probabilities associated with the subgraphs. The critical sets identified by the algorithm are also used to compute conditional probability-based importance measures that quantify the relative importance of network components by their contributions to network disconnection events. This paper also introduces a risk assessment framework for lifeline networks based on the use of the selective RDA, which can consider both interevent and intraevent uncertainties of spatially correlated ground motions. The risk assessment framework and the selective RDA are demonstrated by a hypothetical network example, and the gas and water transmission networks of Shelby County in Tennessee, USA. The examples show that the proposed framework and the selective RDA greatly improve efficiency of risk assessment of complex lifeline networks, which are characterized by a large number of components, complex network topology, and statistical dependence between component failures. Copyright © 2012 John Wiley & Sons, Ltd.     

Simulation of spatially correlated earthquake ground motions for engineering purposes

A new model to simulate spatially correlated earthquake ground motions is developed. In the model, the main factors that characterize three distinct effects of spatial variability, namely, the incoherency effect, the wave-passage effect and the site-response effect, are taken into account, and corresponding terms/parameters are incorporated into the well known model of uniform ground motions. Some of these terms/parameters can be determined by the root operation, and others can be calculated directly. The proposed model is fi rst verif ied theoretically, and examples of ground motion simulations are provided as a further illustration. It is proven that the ensemble expected value and the ensemble auto-/cross-spectral density functions of the simulated ground motions are identical to the target spectral density functions. The proposed model can also be used to simulate other correlated stochastic processes, such as wave and wind loads.     

Conditional simulation of spatially variable seismic ground motions based on evolutionary spectra

This paper proposes a computational procedure for the conditional simulation of spatially variable seismic ground motions for long span bridges with multiple supports. The seismic ground motions, with part of their time histories measured at some supports, are regarded as zero‐mean nonstationary random processes characterized by predefined evolutionary power spectral density. To conditionally simulate unknown seismic ground motion time histories at other supports, the Kriging method is first described briefly for the conditional simulation of a random vector comprised of zero‐mean Gaussian variables. The multivariate oscillatory processes characterized by the evolutionary power spectral density matrix are then introduced, and the Fourier coefficients of the oscillatory processes and their covariance matrix are derived. By applying the Kriging method to the random vector of the Fourier coefficients and using the inverse Fourier transform, unknown nonstationary seismic ground motion time histories can be simulated. A numerical example is selected to demonstrate capabilities of the proposed simulation procedure, and the results show that the procedure can ensure unbiased time‐varying correlation functions, especially the cross correlation between known and unknown time histories. The procedure is finally applied to the Tsing Ma suspension bridge in Hong Kong to generate ground accelerations at its multiple supports using limited seismic records. Copyright © 2012 John Wiley & Sons, Ltd.     

相位演化零点时间与地震动模拟研究

提出了多频分量相位演化零点时间的概念.对于复杂非平稳地震动过程,为降低求解多频分量相位演化零点时间的工作量,引入地震波波群演化的思想,在地震波相位谱敏感性分析的基础上,考察了典型实测地震动时程的模拟.结果表明,拟合地震动过程与实测非平稳地震动吻合较好,本文提出的相位谱重构方法具有重要的实践意义.同时,这一方法也为非平稳...     

相关多点地震动合成HOP方法的分析与改进

解释了相关多点地震动合成的HOP方法具有局部场地收敛性,分析了其生成各点地震动幅值相差较大的原因。除第一点外,HOP方法生成的各点地震动自功率谱围绕真实值波动,导致某些频率的能量过大而某些频率变小。由于结构频谱的离散性,如果恰好与能量大的频率吻合,则会激发结构强烈的振动响应,反之结构的响应值偏小,这与实际不符;进行非线性响应计算时或因为某些频率能量过大而使结构进入非线性阶段,则响应的期望值也是错误的。通过对实际地震波传播机制的模拟,提出了改进的HOP的方法。算例表明,此方法具有局部场地收敛性,各点自功率谱吻合给定的目标值,进而保证了各点幅值的一致性,可以用于结构抗震非线性响应分析。     

Linear analysis of concrete arch dams including dam–water–foundation rock interaction considering spatially varying ground motions

The available substructure method and computer program for earthquake response analysis of arch dams, including the effects of dam–water–foundation rock interaction and recognizing the semi‐unbounded size of the foundation rock and fluid domains, are extended to consider spatial variations in ground motions around the canyon. The response of Mauvoisin Dam in Switzerland to spatially varying ground motion recorded during a small earthquake is analyzed to illustrate the results from this analysis procedure. Copyright © 2009 John Wiley & Sons, Ltd.     

Simulation of broadband seismic ground motions at dam canyons by using a deterministic numerical approach

As a deterministic numerical approach for simulation of earthquake ground motions, the spectral element method (SEM) is applied to generate a broadband acceleration array for dam-canyons instead of the traditional empirical or stochastic methods. Specifically, the SEM analysis model with an extra fine mesh is used for the Pacoima Canyon to simulate the entire path starting from earthquake source rupture via the propagation medium to the local site. The source and the 3D earth model (velocity structure) are validated through the modeling of the Newhall earthquake on 28 October 2012 at a frequency of up to 8 Hz. Subsequently, the San Fernando earthquake records on 13 January 2001 are further used to study the effects of propagation path in simulation. Finally, the spatially varying ground motions at the Pacoima Canyon are obtained for different source mechanisms. The results show that the source mechanism and the local site topography significantly affect the distribution of the peak accelerations along the canyon.     

Dominant pulse simulation of near fault ground motions

In this study, a new mathematical model is developed composed of two parts, including harmonic and polynomial expressions for simulating the dominant velocity pulse of near fault ground motions. Based on a proposed velocity function, the corresponding expressions for the ground acceleration and displacement time histories are also derived. The proposed model is then fitted using some selected pulse-like near fault ground motions in the Next Generation Attenuation (NGA) project library. The new model is not only simple in form but also simulates the long-period portion of actual velocity near fault records with a high level of precision. It is shown that the proposed model-based elastic response spectra are compatible with the near fault records in the neighborhood of the prevailing frequency of the pulse. The results indicate that the proposed model adequately simulates the components of the time histories. Finally, the energy of the proposed pulse was compared with the energy of the actual record to confirm the compatibility.     

The effect of spatially varying earthquake ground motions on the stochastic response of bridges isolated with friction pendulum systems

In this paper, a comprehensive investigation of the effect of spatially varying earthquake ground motions on the stochastic response of bridges isolated with friction pendulum systems is performed. The spatially varying earthquake ground motions are considered with incoherence, wave-passage and site-response effects. The importance of the site-response effect, which arises from the difference in the local soil conditions at different support points of the isolated bridge, is investigated particularly. Mean of maximum and variance response values obtained from the spatially varying earthquake ground motions are compared with those of the specialised cases of the ground motion model. It is shown that site-response component of the spatially varying earthquake ground motion model has important effects on the stochastic response of the isolated bridges. Therefore, to be more realistic in calculating the isolated bridge responses, the spatially varying earthquake ground motions should be incorporated in the analysis.     

Stochastic model for simulation of near‐fault ground motions

A parameterized stochastic model of near‐fault ground motion in two orthogonal horizontal directions is developed. The major characteristics of recorded near‐fault ground motions are represented. These include near‐fault effects of directivity and fling step; temporal and spectral non‐stationarity; intensity, duration, and frequency content characteristics; directionality of components; and the natural variability of ground motions. Not all near‐fault ground motions contain a forward directivity pulse, even when the conditions for such a pulse are favorable. The proposed model accounts for both pulse‐like and non‐pulse‐like cases. The model is fitted to recorded near‐fault ground motions by matching important characteristics, thus generating an ‘observed’ set of model parameters for different earthquake source and site characteristics. A method to generate and post‐process synthetic motions for specified model parameters is also presented. Synthetic ground motion time series are generated using fitted parameter values. They are compared with corresponding recorded motions to validate the proposed model and simulation procedure. The use of synthetic motions in addition to or in place of recorded motions is desirable in performance‐based earthquake engineering applications, particularly when recorded motions are scarce or when they are unavailable for a specified design scenario. Copyright © 2016 John Wiley & Sons, Ltd.     

Simulation of spatially varying ground motions including incoherence,wave‐passage and differential site‐response effects

A method is presented for simulating arrays of spatially varying ground motions, incorporating the effects of incoherence, wave passage, and differential site response. Non‐stationarity is accounted for by considering the motions as consisting of stationary segments. Two approaches are developed. In the first, simulated motions are consistent with the power spectral densities of a segmented recorded motion and are characterized by uniform variability at all locations. Uniform variability in the array of ground motions is essential when synthetic motions are used for statistical analysis of the response of multiply‐supported structures. In the second approach, simulated motions are conditioned on the segmented record itself and exhibit increasing variance with distance from the site of the observation. For both approaches, example simulated motions are presented for an existing bridge model employing two alternatives for modeling the local soil response: i) idealizing each soil‐column as a single‐degree‐of‐freedom oscillator, and ii) employing the theory of vertical wave propagation in a single soil layer over bedrock. The selection of parameters in the simulation procedure and their effects on the characteristics of the generated motions are discussed. The method is validated by comparing statistical characteristics of the synthetic motions with target theoretical models. Response spectra of the simulated motions at each support are also examined. Copyright © 2011 John Wiley & Sons, Ltd.     

Modeling of spatially correlated,site-reflected,and nonstationary ground motions compatible with response spectrum

Seismic risk analysis and mitigation of spatially extended structures require the synthesis of spatially varying ground motions in the response history analysis of these structures. These synthetic motions are usually desired to be spatially correlated, site reflected, nonstationary, and compatible with target design response spectra. In this paper, a method is presented for simulating spatially varying ground motions considering the nonstationarity, local site effects, and compatibility of response spectra. The scheme for generating spatially varying and response spectra compatible ground motions is first established for spatial locations on the ground surface with varying site conditions. The design response spectrum is introduced as the “power” spectrum at the base rock. The site amplification approach is then derived based on the deterministic wave propagation theory, by assuming that the base rock motions consist of out-of-plane SH wave or in-plane combined P and SV waves propagating into the site with assumed incident angles, from which tri-directional spatial ground motions can be generated. The phase difference spectrum is employed to model ground motions exhibiting nonstationarity in both frequency and time domains with different site conditions. The proposed scheme is demonstrated with numerical examples.     

Physically compliant,conditionally simulated spatially variable seismic ground motions for performance‐based design

The performance‐based design of lifeline systems requires spatially variable seismic excitations at the structures' supports that are consistent with prescribed seismic ground motion characteristics and an appropriate spatial variability model—such motions can be obtained through conditional simulation. This work revisits the concept of conditional simulation and critically examines the conformity of the generated motions with the characteristics of the target random field and observations from data recorded at dense instrument arrays. Baseline adjustment processing techniques for recorded earthquake accelerograms are extended to fit the requirements of simulated and conditionally simulated spatially variable ground motions. Emphasis is placed on the use of causal vs acausal filtering in the data processing. Acceleration, velocity and displacement time histories are evaluated in two example applications of the approach. The first application deals with a prescribed synthetic time history that incorporates nonstationarity in the amplitude and frequency content of the motions and depends on earthquake magnitude, source–site distance and local soil conditions; this example results in zero residual displacements. The second application considers as prescribed time history a recording in the vicinity of a fault and yields nonzero residual displacements. It is shown that the conditionally simulated time histories preserve the characteristics of the prescribed ones and are consistent with the target random field. The results of this analysis suggest that the presented methodology provides a useful tool for the generation of spatially variable ground motions to be used in the performance‐based design of lifeline systems. Copyright © 2006 John Wiley & Sons, Ltd.     

Study on elastic response of structures to near-fault ground motions through record decomposition

Accelerograms recorded near active faults have some important characteristics that make them different from those recorded in far-fault regions. High-frequency components in acceleration records and long-period velocity pulses are among notable specifications of such ground motions. In this paper, a moving average filtering with appropriate cut-off frequency has been used to decompose the near-fault ground motions into two components having different frequency contents: first, Pulse-Type Record (PTR) that possesses long-period pulses; second, the relatively high-frequency BackGround Record (BGR), which does not include large velocity pulses. Comparing the results with those extracted through wavelet analysis shows that moving average filter is an appropriate and efficient tool for near-fault records decomposition. The method is applied to decompose a suite of 91 selected near-fault records and the elastic response of structures is examined through their response to the decomposed parts. The results emphasizes that in contrast with ordinary far-fault earthquake records, response spectra of near-fault ground motions typically have two distinct local peaks, which are representatives of the high- and low-frequency components, i.e., BGR and PTR, respectively. Moreover, a threshold period is identified below which the response of structures is dominated by BGR while PTR controls the response of structures with periods longer than this period.     

Experimental investigation of spatially varying effect of ground motions on bridge pounding

Pounding between adjacent bridge structures with insufficient separation distance has been identified as one of the primary causes of damage in many major earthquakes. It takes place because the closing relative movement is larger than the structural gap provided between the structures. This relative structural response is controlled not only by the dynamic properties of the participating structures but also by the characteristics of the ground excitations. The consequence of the spatial variation of ground motions has been studied by researchers; however, most of these studies were performed numerically. The objective of the present research is to experimentally evaluate the influence of spatial variation of ground motions on the pounding behaviour of three adjacent bridge segments. The investigation is performed using three shake tables. The input spatially varying ground excitations are simulated based on the New Zealand design spectra for soft soil, shallow soil and strong rock using an empirical coherency loss function. Results confirm that the spatially nonniform ground motions increase the relative displacement of adjacent bridge girders and pounding forces. Copyright © 2012 John Wiley & Sons, Ltd.     

断层附近强地震动半经验合成方法的研究

本文对利用经验格林函数法合成断层附近的强地面运动等问题做了初步研究。基于云南施甸5.9级主震和余震强震观测记录和本文提出的移动窗技术对断层附近的主震记录进行了分析识别，发现大震的Asperity(凸凹体）与小震在时程和Fourier幅值谱上存在一定程度的相似性。这种相似性从理论上，可以解释为这种小震视为大震Asperity的“种子”，大震的Asperity是由这些“种子”发育而成的。在此基础上合成了云南施甸距震中仅为1.1公里的主震记录，同时也提出以断层附近小震作为经验格林函数存在着一些缺陷。     

Spatial variability effects of ground motions on cable-stayed bridges

In this paper, stochastic analysis of a cable-stayed bridge subjected to spatially varying ground motions is performed. While the ground motion is described by power spectral density (PSD) function, the spatial variability of ground motions is taken into account with the incoherence and the wave-passage effects. The incoherence effect is examined by taking into account two extensively used models. As the effect of the wave-passage effect is investigated by using various wave velocities, the effect of local soil conditions where the bridge supports are constructed is outlined by using homogeneous firm, medium and soft soil conditions. Solutions obtained for the spatially varying ground motions are compared with those of the specialised cases of the ground motion model. Stationary as well as the transient response analyses are performed for the considered bridge model. It is concluded that spatial variability and propagation effects of ground motions have important effects on the dynamic behaviour of the bridge and the variability of the ground motions should be included in the stochastic analysis of cable-stayed bridges.     

海域地震动研究现状分析

海域地震动是海洋工程建设必须考虑的因素,近年来学者们对海域地震动做了大量研究,这些研究涵盖海域地震动的各方面。为总结对海域地震动特性的研究成果并分析存在的问题,分类阐述了不同学者对海域地震的研究结果。根据采用的研究方法可分为解析方法、数值模拟方法和统计方法。在此基础上,归纳了不同研究方法得出的海域地震动场地影响与传播规律的结论:解析方法与数值方法得出的结论侧重于分析海域地震动受海水、淤泥沉积层与地形等场地条件的影响;统计方法得出的结论集中在海域地震动与陆地地震动在地震动特性与传播规律方面的差异,这些结论可以为采用相同方法的研究提供参考。最后,对当前研究中存在的问题进行探讨,由于数据相对匮乏,导致缺少对海域地震动与陆域地震动差异的定量分析,也缺少对海域地震动不确定性的分析。     

液化条件对地表动力响应影响的有效应力动力分析

本文根据上海市区的典型地层分布及其力学参数,选取和构造了若干计算剖面,利用Biot动力固结方程和亚塑性边界面模型的排水有效应力动力分析方法,计算了地下水位、粉砂层厚度及其埋深等对地表峰值加速度和位移的影响,得到了一些有意义的结果。     

Influence of time‐varying frequency content in earthquake ground motions on seismic response of linear elastic systems

Earthquake ground motion records are nonstationary in both amplitude and frequency content. However, the latter nonstationarity is typically neglected mainly for the sake of mathematical simplicity. To study the stochastic effects of the time‐varying frequency content of earthquake ground motions on the seismic response of structural systems, a pair of closely related stochastic ground motion models is adopted here. The first model (referred to as ground motion model I) corresponds to a fully nonstationary stochastic earthquake ground motion model previously developed by the authors. The second model (referred to as ground motion model II) is nonstationary in amplitude only and is derived from the first model. Ground motion models I and II have the same mean‐square function and global frequency content but different features of time variation in the frequency content, in that no time variation of the frequency content exists in ground motion model II. New explicit closed‐form solutions are derived for the response of linear elastic SDOF and MDOF systems subjected to stochastic ground motion model II. New analytical solutions for the evolutionary cross‐correlation and cross‐PSD functions between the ground motion input and the structural response are also derived for linear systems subjected to ground motion model I. Comparative analytical results are presented to quantify the effects of the time‐varying frequency content of earthquake ground motions on the structural response of linear elastic systems. It is found that the time‐varying frequency content in the seismic input can have significant effects on the stochastic properties of system response. Copyright © 2016 John Wiley & Sons, Ltd.