A brief theory and computing of seismic ground rotations for structural analyses

The seismic ground rotations are important with respect to spatial structural models, which are sensitive to the wave propagation. The rotational ground motion can lead to significant increasing of structural response, instability and unusual damages of buildings. Currently, the seismic analyses often take into account the rocking and torsion motions separately using artificial accelerograms. We present an exact analytical method, proposed by Nazarov [15] for computing of three rotational accelerograms simultaneously from given translational records. The method is based on spectral representation in the form of Fourier amplitude spectra of seismic waves, corresponding to the given three-component translational accelerogram. The composition, directions and properties of seismic waves are previously determined in the form of a generalized wave model of ground motion. It is supposed that seismic ground motion can be composed by superposition of P, SV, SH- and surface waves. As an example, the dynamic response analysis of 25-story building is presented. Here recorded (low-frequency) and artificial (high-frequency) accelerograms were used; each of them includes three translational and three rotational components. In this structural analysis, we have clarified primarily conditions under which rotational ground motion should be taken into account. Next, we have calculated three rotational components of seismic ground motion. Then they were taken as additional seismic loads components for further seismic analysis of the building. Note, soil–structure interaction (SSI) is not considered in this study. For computing, we use the special software for structural analyses and accelerogram processing (FEA Software STARK ES and Odyssey software, Eurosoft Co., Russia). It was developed and is used in engineering practice in the Central Research Institute of Building Constructions (TsNIISK, Moscow, Russia).     

Simulation and generation of spectrum-compatible ground motions based on wavelet packet method

This paper deals with the problem of generating spectrum-compatible artificial accelerograms for seismic dynamic analysis of engineering projects. A wavelet-packet-based, two-step procedure for the issue is proposed. The first step is to generate acceleration time history that could account for temporal and frequency non-stationarities of recorded ground motions. The second step is to decompose it into a desired number of wavelet packet vectors with high frequency resolution and non-overlapping frequency contents. Then each wavelet packet vector is scaled suitably and iteratively for the response spectrum of the simulated accelerogram to fit a specified design spectrum. The advantages of this procedure are that it can simulate user-specified acceleration time history with only 6 input parameters and the adjusted accelerogram has similar characteristics to the recorded one. The proposed procedure has been illustrated by simulating and modifying acceleration time history that are compatible with two different design spectrums for nuclear power plants. In addition, iterative efficiency of the method is investigated by simulating and adjusting acceleration time history for 100 successive times. The maximum relative error of the 76 period control points can reach 6% or below. Results show that the proposed method is effective and practical to generate and find spectrum-compatible ground motions with both stochastic and deterministic aspects.     

Selection of spectrum- and seismo-compatible accelerograms for the Tuscany region in Central Italy

This article illustrates the results of a study aimed at developing a methodology for the automatic identification of the seismic input at outcropping rock sites and flat topographic conditions necessary to carry out non-linear dynamic analysis of structures and geotechnical systems. The seismic input is provided in terms of a set of 7 natural accelerograms recorded on outcropping rock and satisfying the average spectral compatibility requirements prescribed by the Italian seismic code (NTC08).The study focuses on the territory encompassing Tuscany region in Central Italy and it has been carried out for six return periods, which are 50, 75, 101, 475, 712 and 949 years. The procedure involved four main steps: (1) grouping of the response spectra with similar features; (2) definition of the reference response spectrum for each group; (3) selection of spectrum-compatible accelerograms using the reference response spectrum of each group; and (4) linear scaling of the accelerograms to satisfy the compatibility requirement with respect to other response spectra of the group. The last step is implemented through an interactive, user-friendly program named SCALCONA 2.0, which provides the seismic input in agreement with the site location and return period specified by the user. The program is freely available at the following web site: http://www.rete.toscana.it/sett/pta/sismica/01informazione/banchedati/input_sismici/progettazione/index.htm.     

Enrichment of seismic ground motion databases using Karhunen–Loève expansion

Best estimate seismic analysis are generally based on time‐domain simulations of structural responses. The seismic load is then modeled by a stochastic process representing ground motion. For this purpose, the analyst can use recorded accelerograms or work with synthetically generated ones. The number of ground motion time‐histories available for a given scenario and site condition is limited and generally not sufficient for carrying out more advanced probabilistic structural response analysis. It is then necessary to have at our disposal methods that allow for generating synthetic accelerograms that realistically characterize earthquake ground motions. However, most of the methods proposed in literature for generating synthetic accelerograms do not accurately reproduce the natural variability of ground motion parameters (such as PGA, cumulative absolute velocity, and Arias intensity) observed for recorded time histories. In this paper, we introduce a new method for generating synthetic ground motion, based on Karhunen‐Loève decomposition and a non‐Gaussian stochastic model. The proposed method enables the structural analyst to simulate ground motion time histories featuring the properties mentioned above. To demonstrate its capability, we study the influence of the simulation method on different ground motion parameters and on soil response spectra. We finally compute fragility curves to illustrate the practical application of the proposed method. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis of accelerograms compatible with the Chinese GB 50011-2001 design spectrum via harmonic wavelets： artificial and historic records

A versatile approach is employed to generate artificial accelerograms which satisfy the compatibility criteria prescribed by the Chinese aseismic code provisions GB 50011-2001. In particular, a frequency dependent peak factor derived by means of appropriate Monte Carlo analyses is introduced to relate the GB 50011 -2001 design spectrum to a parametrically defined evolutionary power spectrum (EPS). Special attention is given to the definition of the frequency content of the EPS in order to accommodate the mathematical form of the aforementioned design spectrum. Further, a one-to-one relationship is established between the parameter controlling the time-varying intensity of the EPS and the effective strong ground motion duration. Subsequently, an efficient auto-regressive moving-average (ARMA) filtering technique is utilized to generate ensembles of non-stationary artificial accelerograms whose average response spectrum is in a close agreement with the considered design spectrum. Furthermore, a harmonic wavelet based iterative scheme is adopted to modify these artificial signals so that a close matching of the signals' response spectra with the GB 50011-2001 design spectrum is achieved on an individual basis. This is also done for field recorded accelerograms pertaining to the May, 2008 Wenchuan seismic event. In the process, zero-phase high-pass filtering is performed to accomplish proper baseline correction of the acquired spectrum compatible artificial and field accelerograms. Numerical results are given in a tabulated format to expedite their use in practice.     

A method for generating fully non-stationary and spectrum-compatible ground motion vector processes

Earthquake ground motion spatial variability can influence significantly the response of certain structures. In order to accurately evaluate probabilistic characteristics of the seismic response of structures, the Monte Carlo simulation technique is still the only universal method of analysis when strong nonlinearities and input uncertainties are involved. Consequently, realizations of ground motion time histories taking into account both time and spatial variability need to be generated. Furthermore, for some design applications, the generated time histories must also satisfy the provision imposed by certain seismic codes stating that they have to be also response-spectrum-compatible. For these purposes, a spectral-representation-based methodology for generating fully non-stationary and spectrum-compatible ground motion vector processes at a number of locations on the ground surface is proposed in this paper. The simulated time histories do not require any iterations on the individual generated sample functions so that Gaussianity and prescribed coherence are suitably preserved. The methodology has also the advantage of providing the fully non-stationary and spectrum-compatible cross-spectral density matrix of the ground motion time-histories that can be used for reliability studies in an analytic stochastic fashion.     

Numerical simulation of ground acceleration spectra and accelerograms for engineering application

A numerical approach to the earthquake ground motion analysis is proposed for regions where no accelerograms are available. Using Haskell matrix techniques, the response spectra of a layered substratum for SV waves were calculated and then multiplied by the spectra corresponding to Brune's type pulses. The ground acceleration spectra were obtained for different angles of pulse incidence at the substratum base. The spectrum shape depends upon the substratum response and the pulse shape, while its level was related to the maximum ground acceleration corresponding to the expected maximum intensity. Transformation of the ground spectra into the time domain produced numerical accelerograms for horizontal and vertical components and for different angles of pulse incidence. Finally, a standard statistical procedure was applied to obtain the design response spectra used in engineering applications.     

地震动频谱非平稳性对结构非线性反应的影响

应用均匀调制随机过程模型和演变的随机过程模型合成了人工地震加速度过程。以此为输入，计算了一个三跨十层呆结构的地震反应，比较结果表明，考虑地震动的频谱非平稳稳征后，所计算结构的层位移，梁端塑性转角，柱的弯矩和剪力显著增大，结构在相对较小的地震动峰值下发生倒塌。     

A new energy-based ground motion selection and modification method limiting the dynamic response dispersion and preserving the median demand

A novel ground motion selection and modifications method to perform response history analysis of structures is presented in this paper. Currently, the accessibility of ground motion information permits the analysis of structures using real ground motion data. Predicting the dynamic behavior of structures is a primary objective; therefore, the selection of a set of ground motions that shows a reduction in the variability of the structural response and accuracy in preserving the median demand is a challenging task. The new selection and scaling procedure emerges from comparing a set of horizontal ground motions at various ranges of frequency. In this study, the conditional mean spectrum and the design response spectrum are used as target spectra, and the records that give an applicable and compelling contribution to the hazard are considered. It is possible to obtain a set of ground motions with similar seismic severity by matching the target spectrum at the period of interest T _ref , where the scaled spectrum should have an equivalent Housner intensity in the period range 0.2T _ref –2T _ref . The horizontal components for every band of frequency is obtained using a specific index that depends on the energy-frequency trend’s shape as well as on its scattering degree around the mean value. This allows obtaining a set of spectrum-compatible records with almost identical severity and low dispersion of the structural response parameters. The methodology has been tested showing a significant effectiveness in terms of low variability of parameters and accuracy in preserving the median demand for a given hazard scenario.     

设计反应谱长周期区段的研究

本文利用近20年国内外大地震时获得的数字强震仪记录分析强震动的长周期分量特性,给出了不同场地上的平均加速度反应谱及其拟合曲线。结果表明,现行抗震设计规范中设计谱的特征周期和长周期谱值明显偏小。在此基础上提出了长周期设计反应谱的修正建议。文中还根据统计分析提出了不同阻尼比的反应谱修正公式。     

An approach to digitize analog form of accelerograms recorded at Tarbela—Pakistan

Pakistan has a long history of seismological activity. The devastation caused by Kashmir earthquake has been administered all over the world. In Pakistan first seismological network, consisting of analog accelerograms and seismographs, was setup in early 1969 at Tarbela, where the largest water reservoir of the country is located. An approach to convert analog ground motion records into digital form is explained using a simple technique. The digitized data was compared with the original analog record and found in good agreement. The data has been used to plot response spectra. The digitized data will be available for seismic response analysis of structures and seismic risk analysis of the region.     

基于小波变换的拟合规范反应谱多维地震动模拟

本文提出一种基于小波变换的拟合规范反应谱的多维地震动模拟算法。首先将规范反应谱推广到三维相关设计反应谱,然后将已有的三维地震动加速度时间历程曲线分解为一系列不同频段上的地震动分量,调整每一个地震动分量的幅值使其在相应的频率范围内拟合设计反应谱,最后经过调整后的地震动分量进行重构得到更新的地震动时间历程曲线。将该时间历程曲线的反应谱与目标反应谱进行比较,重复该过程直到误差位于特定的范围内。该方法可以保留原始地震动的局部时-频特性,为多维地震动的模拟提供了一种新的方法。     

Response spectral attenuation relations for in-slab earthquakes in Indo-Burmese subduction zone

From an analysis of a limited number of strong-motion data recorded in northeast Indian region, the in-slab earthquakes along the Indo-Burmese subduction zone are found to be characterized by much larger ground motion amplitudes than that for the earthquakes along other subduction zones around the world. Specific type of source, propagation path and site geologic condition may perhaps simultaneously be responsible for such anomalous behaviour. The empirical attenuation relations developed by Atkinson and Boore [3] using a global database for subduction zone earthquakes have been therefore suitably modified to be more appropriate for the northeast India. The modified relationships are developed by combining the data for both horizontal and vertical components of motion. The response spectra of the accelerograms recorded in northeast India from earthquakes with widely varying magnitude and distance are, in general, found to match very well with the predictions from the modified attenuation model. The proposed model can thus be used to obtain more realistic estimate of the contribution of in-slab subduction zone earthquakes to seismic hazard in the northeast Indian region.     

Wavelet-based generation of spectrum-compatible time-histories

This paper deals with the well-known problem of generating spectrum-compatible synthetic accelerograms for the linear and non-linear time-history analyses of structural systems. A wavelet-based procedure has been used to decompose a recorded accelerogram into a desired number of time-histories with non-overlapping frequency contents, and then each of the time-histories has been suitably scaled for matching of the response spectrum of the revised accelerogram with a specified design spectrum. The key idea behind this iterative procedure is to modify a recorded accelerogram such that the temporal variations in its frequency content are retained in the synthesized accelerogram. The proposed procedure has been illustrated by modifying five recorded accelerograms of widely different characteristics such that those are compatible with the same USNRC design spectrum.     

Ground motion characterization and seismic hazard assessment in Spain: context,problems and recent developments

This article points out some particular features conditioning seismic hazard assessments (SHA) in Spain, a region with low–moderate seismicity. Although sized earthquakes occurred in the past, as evidenced by historical documents and neotectonic studies, no large events occurred during the last decades. The absence of strong motion records corresponding to earthquakes with magnitude larger than 5.5 is an important obstacle for the development of ground motion models constrained by local data, with the consequent difficulty in SHA studies. In this paper, some recent developments aiming at providing solutions to these difficulties are presented. Specifically, a strong motion databank containing a massive collection of accelerograms and response spectra from different configurations source-path-site corresponding to earthquakes all over the world is introduced, together with software utilities for its management. A first application of this databank is the development of specific ground motion models for Spain and for the Mediterranean region that predict peak ground accelerations as a function of several definitions of magnitude, distance and soil class. The predictive power of these ground motion models is tested by contrasting their estimates with recently recorded ground motions. The comparison between our ground-motion models with others proposed in the literature for other areas reveals a regular overestimation of the expected ground motions at Spanish sites by the non-local models. Consequently, SHA studies based in external models may overestimate the predicted hazard at the Iberian sites. In the last part of the paper a method for checking whether the response spectra proposed in the Spanish Building Code (NCSE-02) are consistent with actual accelerometric data from recent low magnitude earthquakes is applied. The spectral shapes of the Spanish Building Code NCSE-02 are compared with the response spectral shapes deduced from the available accelerograms by normalising the response spectra with the recorded PGA. It is appreciated that the NCSE-02 spectral shapes are exceeded by a large number of actual spectral shapes for short periods (around 0.2 s), a result to be taken into account in further revisions of the NCSE-02 code. The issues tackled in this work constitute not only an improvement for ground-motion characterisation in Spain, but also provide guidelines of general interest for potential applications in other regions with similar seismicity.     

DISCUSSION ON THE SPECTRA SHAPE OF SEISMIC MARGIN EARTHQUAKE OF NUCLEAR POWER PLANT IN CHINA

Fukushima nuclear accident caused widespread concern of earthquake initiated severe accident. Under this background, China nuclear utilities carried out research and application of seismic margin assessment(SMA)approach to evaluate the seismic margin of the existing nuclear power plants(NPP)by different spectra shape of seismic margin earthquake(SME). By reviewing the method used to determine SME of operational NPP in central and eastern United States(CEUS), this paper analyzed the seismic hazard characteristic of China NPP sites, contrasted the design basis ground motion between NPP in CEUS and China, and suggested giving priority to evaluating the seismic margin of operational NPP that adopted the improved second generation technology for the urgency and importance of assessment on the actual seismic capacity of NPP. Comparing RG1.60 spectrum to normalized site-specific SL-2 level acceleration spectra, we found that some normalized spectra overtook the RG1.60's in high frequency range, so it is not always adequate to scale RG1.60 spectrum to evaluate the seismic margin for sites of the improved second generation NPP. We selected a sample site whose site-specific SL-2 level ground motion is close to the standard design of improved second generation NPP(0.2g scaled RG1.60 spectrum)to determine the seismic margin earthquake by probabilistic seismic hazard analysis method of the sample site. Compared to the given PGA(0.3g)scaled scenario earthquake ground motions and the uniform hazard response spectrum(UHRS), whose PGA is 0.3g to PGA(0.3g)scaled standard spectra(median NUREG/CR0098 spectrum and RG1.60 spectrum), the results demonstrated that uniform hazard response spectrum and scaled scenario earthquake ground motions are both significantly higher than the PGA scaled median NUREG/CR0098 spectrum, and all the three spectra are enveloped by PGA scaled RG1.60 spectrum. Then, this paper suggests adopting the uniform hazard response spectrum or scenario earthquake ground motions to evaluate the seismic margin of improved second generation NPP beyond site SL-2 ground motion; and to evaluate the seismic margin of improved second generation NPP beyond standard design, we recommend to use PGA scaled RG1.60 spectrum.     

Prediction of the median IDA curve by employing a limited number of ground motion records

A methodology has been proposed which can be used to reduce the number of ground motion records needed for the reliable prediction of the median seismic response of structures by means of incremental dynamic analysis (IDA). This methodology is presently limited to predictions of the median IDA curve only. The reduction in the number of ground motion records needed to predict the median IDA curve is achieved by introducing a precedence list of ground motion records. The determination of such a list is an optimization problem, which is solved in the paper by means of (1) a genetic algorithm and (2) a proposed simple procedure. The seismic response of a simple, computationally non‐demanding structural model has been used as input data for the optimization problem. The presented example is a three‐storey‐reinforced concrete building, subjected to two sets of ground motion records, one a free‐field set and the other a near‐field set. It is shown that the median IDA curves can be predicted with acceptable accuracy by employing only four ground motion records instead of the 24 or 30 records, which are the total number of ground motion records for the free‐field and near‐field sets, respectively. Copyright © 2007 John Wiley & Sons, Ltd.     

Stochastic finite fault modelling of M w 4.8 earthquake in Kachchh, Gujarat, India

The modified stochastic finite fault modelling technique based on dynamic corner frequency has been used to simulate the strong ground motions of M _w 4.8 earthquake in the Kachchh region of Gujarat, India. The accelerograms have been simulated for 14 strong motion accelerographs sites (11 sites in Kachchh and three sites in Saurashtra) where the earthquake has been recorded. The region-specific source, attenuation and generic site parameters, which are derived from recordings of small to moderate earthquakes, have been used for the simulations. The main characteristics of the simulated accelerograms, comprised of peak ground acceleration (pga), duration, Fourier and response spectra, predominant period, are in general in good agreement with those of observed ones at most of the sites. The rate of decay of simulated pga values with distance is found to be similar with that of observed values. The successful modelling of the empirical accelerograms indicates that the method can be used to prepare wide range of scenarios based on simulation which provide the information useful for evaluating and mitigating the seismic hazard in the region.     

Influence of intensity of motion on the seismic response of structures with asymmetric force-deformation curves

Asymmetrically yielding single-degree-of-freedom elastoplastic systems are subjected to simulated accelerograms based on El Centro-NS, 1940 ground motion (wide frequency band) to calculate the corresponding ductility demands. Results are compared with those corresponding to simulated accelerograms of the SCT-EW, 1985 Mexico earthquake (narrow frequency band) obtained in a previous work. Results obtained reveal that the characteristics of the excitation influence the response of asymmetrically yielding structures and that the differences found on responses corresponding to both earthquakes are due to frequency content, duration and/or intensity of motion. The effect of the latter concepts is studied using modulating sinusoidal excitations as well as accelerograms recorded on soft and hard soils of Mexico City. Two alternate expressions are proposed to evaluate the increase in ductility demands of structures with asymmetric force-displacement curves with respect to symmetric ones. Those expressions take into account motion intensity and duration, as well as seismic design coefficient.     

Site-dependent models of earthquake ground motion

Random vibration analyses of structural systems subjected to seismic loading are dependent upon the characterization of earthquake ground motion as a stochastic process. The response of structural systems to earthquakes is dependent strongly on the local geological conditions, which should be incorporated into seismological models of ground motion. In the study presented herein, three previously developed ground-motion models are adapted to incorporate site-dependent characteristics. Records obtained from two recording stations in California are used as a basis for the ground-motion models. Single-degree-of-freedom (SDOF) oscillators are subjected to ensembles of accelerograms generated from these models, and both elastic and inelastic response are considered. Response statistics are compared to those generated by the analysis of structural response to ensembles of recorded motion from the two sites. The important features of the ground motion for effective reproduction of response statistics are identified, and observations are made on the sensitivity of specific response parameters to site-dependent characteristics of the ground motion.