Ground Motion Time Histories Research Articles

An optimization method for the generation of ground motions compatible with multi-damping design spectra

For some crucial engineering projects, a compatible time history of ground motions is required as a loading input for seismic dynamic analysis. The compatibility with multi-damping design spectra is critical to make sure the high reliability of analysis results. In previous methods, ground motions were hard to be tuned highly compatible with multi-damping design spectra.Hence we propose a method to optimize the compatibility of a seed ground motion history, which is either obtained from actual seismic records or generated from previous methods, by superimposing adjusted ground motions in the time domain through the L∞ norm minimization. The optimization process makes the influences of the superimposed ground motions to the response spectra be under control by constraining objective variables. The high precision of the method is demonstrated by a concrete example, in which the max difference between the response spectra of the optimized ground motions and the design spectra for RG1.60 data [1] is 5.06% under five different damping ratios, and all response spectra envelope the design spectra.

Adding Fling Effects to Processed Ground-Motion Time Histories

Fling is the engineering term for the effects of the permanent tectonic offset, caused by a rupturing fault in the recorded ground motions near the fault. It is expressed by a one‐sided pulse in ground velocity and a nonzero final displacement at the end of shaking. Standard processing of earthquake time histories removes some of the fling effects that may be required for engineering applications. A method to parameterize the fling‐step time history and to superimpose it onto traditionally processed time histories has been developed by Abrahamson (2002). In this paper, we first present an update to the Abrahamson (2002) fling‐step models, in which the fling step is parameterized as a single cycle of a sine wave. Parametric models are presented for the sine‐wave amplitude ( D site) and period ( T f ). The expressions for D site and T f are derived from an extensive set of finite‐fault simulations conducted on the Southern California Earthquake Center broadband platform (see Data and Resources). The simulations were run with the Graves and Pitarka (2010) hybrid simulation method and included strike‐slip and reverse scenarios for magnitudes of 6.0–8.2 and dips of 30 through 90. Next, an improved approach for developing design ground motions with fling effects is presented, which deals with the problem of double‐counting intermediate period components that were not removed by the standard ground‐motion processing. Finally, the results are validated against a set of 84 empirical recordings containing fling.

Earthquake Fragility Assessment of the Underground Tunnel Using an Efficient SSI Analysis Approach

This paper represents a simplified seismic fragility analysis approach of the underground tunnel structure in consideration of the soil-structure interaction (SSI) effect. SSI effect founds to be essential in the estimation of dynamic analysis of underground structures like tunnels and thus needs to be considered. The ground response acceleration method for buried structures (GRAMBS) known to be a very efficient quasi-static method that can consider SSI effect is used in the proposed approach to evaluate seismic structural responses without sacrificing much accuracy. Seismic fragility curves are then developed by applying the maximum likelihood estimates (MLE) to responses of a large set of artificial ground motion time histories generated for multiple different levels of earthquake intensity. It is also assumed in this paper that the seismic fragility curve can be represented by a two-parameter lognormal distribution function with median and log-standard deviation that need to be defined using MLE.

Fractal characterization and frequency properties of near-fault ground motions

This study explores the irregularity and complexity of strong earthquake ground motions from the perspective of fractal geometry, and constructs a relation with the frequency content of the ground motions. The box-counting fractal dimensions and five representative period parameters of near-fault ground motions from the Chi-Chi and Northridge earthquakes are calculated and compared. Numerical results indicate that the acceleration and velocity time histories of ground motions present the statistical fractal property, and the dominant pulses of near-fault ground motions have a significant influence on their box dimensions and periods. Further, the average box dimension of near-fault impulsive ground motions is smaller, and their irregular degree of wave forms is lower. Moreover, the box dimensions of ground motions reflect their frequency properties to a large extent, and can be regarded as an alternative indicator to represent their frequency content. Finally, the box dimension D of the acceleration histories shows a considerably negative correlation with the mean period Tm. Meanwhile, the box dimension of the velocity histories Dvel is negatively correlated with the characteristic period Tc and improved characteristic period Tgi.

Stochastic strong ground motion simulations in sparsely-monitored regions: A validation and sensitivity study on the 13 March 1992 Erzincan (Turkey) earthquake

Stochastic simulations have recently become quite popular for estimating synthetic ground motion time histories. For seismically active regions that are not well-monitored or studied extensively, input parameters of the simulations should be carefully selected as the reliability of the simulation results directly depends on the accuracy of the input parameters. In the first part of this study, 13 March 1992 Erzincan (eastern Turkey) earthquake (Mw=6.6), which is recorded at only three strong ground motion stations, is simulated using the stochastic finite-fault method. The source and regional path parameters for this event are adopted from previously validated studies whereas the local site parameters are derived herein. In the second part of the paper, sensitivity of the simulation results with respect to small changes in selected input seismic parameters is investigated. The parameters for which sensitivities are computed include stress drop, crustal shear-wave quality factor and kappa operator. A change of 20% in stress drop value results in 14% change in PGA, whereas a 20% difference in the Q0 value causes 17% change in PGA, and a 20% variation in kappa leads to 15% difference in PGA. Numerical experiments presented in this study prove that the ground motion simulations are prone to trade-off between the source, path and site filters. Hence, input models must be implemented carefully for reliable synthetic ground motions.

Selecting and scaling ground motion time histories according to Eurocode 8 and ASCE 7-05

Linear and nonlinear time history analyses have been becoming more common in seismic analysis and design of structures with advances in computer technology and earthquake engineering. One of the most important issues for such analyses is the selection of appropriate acceleration time histories and matching these histories to a code design acceleration spectrum. In literature, there are three sources of acceleration time histories: artificial records,synthetic records obtained from seismological models and accelerograms recorded in real earthquakes. Because of the increase of the number of strong ground motion database, using and scaling real earthquake records for seismic analysis has been becoming one of the most popular research issues in earthquake engineering. In general, two methods are used for scaling actual earthquake records: scaling in time domain and frequency domain. The objective of this study is twofold: the first is to discuss and summarize basic methodologies and criteria for selecting and scaling ground motion time histories. The second is to analyze scaling results of time domain method according to ASCE 7-05 and Eurocode 8 (1998-1:2004) criteria. Differences between time domain method and frequency domain method are mentioned briefly. The time domain scaling procedure is utilized to scale the available real records obtained from near fault motions and far fault motions to match the proposed elastic design acceleration spectrum given in the Eurocode 8. Why the time domain method is preferred in this study is stated. The best fitted ground motion time histories are selected and these histories are analyzed according to Eurocode 8 (1998-1:2004) and ASCE 7-05 criteria. Also, characteristics of both near fault ground motions and far fault ground motions are presented by the help of figures. Hence, we can compare the effects of near fault ground motions on structures with far fault ground motions\' effects.

Strong motion simulation at Abu Zenima city, Gulf of Suez, Egypt

Earthquake hazard assessments are an important task for the design of earthquake resistant structures and insurance industry. Such assessments get more importance when the site of interest is located near an active earthquake zone. Such situation is present for the location of Abu Zenima city. The city is characterized by the presence of industrial and Maritime platform in addition to other Oil production facilities. These industrial facilities motivated the present work.The simulated earthquake ground motion time histories are conducted using stochastic technique. The magnitude used for simulation is obtained using both probabilistic and deterministic approaches. An analysis using both approaches shows that moderate earthquakes in the vicinity of the site could have the largest effects on the area. Thus an earthquake of magnitude 4.5 at a distance of 21km is chosen as design earthquake.The simulated ground motions are presented in terms of acceleration, velocity, and displacement time histories. In addition the response spectra are also presented that may be used for engineering purposes.

Probabilistic seismic response analysis of a 3-D reinforced concrete building

This paper presents the probabilistic seismic demand analysis with respect to seismic input uncertainty only of a 3-D reinforced concrete building model subjected to three-component earthquake ground motion excitation. Response history analyses are carried out on a nonlinear frame model. Probability distributions are assumed for the ground motion Intensity Measure (IM) taken as the linear 5% damped elastic spectral acceleration at the fundamental period of the structure. Part of the framework of the so-called Performance Based Earthquake Engineering (PBEE) methodology developed by the Pacific Earthquake Engineering Research (PEER) Center is used in this study. This paper has two main objectives. The first objective is to perform a probabilistic demand analysis of an existing building conditional on the ground motion IM. The second objective is to use the results obtained from this existing testbed, with real-world complexities, to demonstrate the deficiency of the PEER PBEE methodology when using a scalar ground motion IM for 3-D structural models. This last objective shows the need for improving the definition of the seismic IM in the PBEE methodology for the general case of 3-D structures subjected to multi-directional input ground motions. To this effect, an ensemble of natural ground motion records is used to represent the inherent randomness in ground motion time-histories (i.e., record-to-record variability). The statistical correlation of different Engineering Demand Parameters (EDPs) with a set of IMs, taken as the 5% damped spectral accelerations at different periods for two horizontal ground motion components, is investigated in order to assess the dispersion in the EDPs due to different ground motion records. Some statistical correlation coefficients are found to be high, indicating that the dispersion of the EDPs is heavily influenced by the spectral content at periods different from the fundamental period. This result points to the need for using vector-valued ground motion Intensity Measures in the PBEE methodology currently proposed by PEER.

The Impact of Flood-Induced Scour on Seismic Fragility Characteristics of Bridges

Earthquake in the presence of flood-induced scour is a critical multihazard scenario for bridges located in seismically-active, flood-prone regions. The present article evaluates seismic performance of four example reinforced concrete bridges when they are pre-exposed to regional flood hazards. Nonlinear time history analyses of the example bridges are performed for a suite of ground motion time histories in the presence and absence of scour expected from different intensity flood events. Fragility analysis is performed to develop seismic fragility curves of the example bridges for various scour depths. Results show nonlinear increase in bridge seismic fragility with increase in scour depth.

Prediction of Broadband Ground-Motion Time Histories: The Case of Tehran, Iran

The northern Tehran fault (NTF) is potentially capable of causing large earth-quakes (Mmax~ 7.2) in a very densely populated area of northern Tehran, Iran. Due to the lack of recorded strong motion data for earthquakes on the fault, a hybrid simulation method is used to calculate broadband (0.1–20 Hz) ground-motion time histories at bedrock level for deterministic earthquake scenarios on the NTF. Low-frequency components of motion (0.1–1.0 Hz) are calculated using a deterministic approach and the discrete wave number-finite element method in a regional one-dimensional (1-D) velocity model. High frequencies (1.0–20.0 Hz) are calculated by the stochastic finite fault method based on dynamic corner frequency. The results were validated by comparing the simulated peak values and response spectra with the empirical ground motion models available for the area and the Modified Mercalli intensity (MMI) observations from historical earthquakes of the region.

Seismic response of embankment dams under near-fault and far-field ground motion excitation

The characteristics of near-fault and far-field earthquakes differ. Records suggest that a near-fault ground motion is characterized by a large high-energy pulse and a distinctive pulse shape for the velocity time history. The present study compared near-fault and far-field ground motion effects and investigates the consequence of well-known characteristics of near-fault ground motion on the nonlinear dynamic response of an embankment dam. It was shown that median maximum demands, such as crest settlement and critical slip surface displacement of the embankment dam, were higher for near-fault ground motion than far-field motion. For pulse-type near-fault input, the maximum demand was a function of spectral shape and the ratio of the pulse period to the fundamental period of the embankment dam. Near-fault ground motion with a period pulse 1.5 to 4 times greater than the natural period of the dam created a larger seismic demand than when the pulse period was equal to or smaller than the natural period of dam. This difference increased up to 1.85 times as the ground motion intensity increased. Velocity-related ground motion intensity indices, such as peak ground velocity, were much more effective than peak ground acceleration to correlate with seismic demand. This was because shear stress level has a direct relation with the velocity time history of ground motion.

Synthetic earthquake ground motions on an array

Linear and especially non-linear analyses of spatially extended structures, such as pipelines and bridges, often requires specification of time histories of ground motion at an array of closely spaced points. As the number of dense accelerograph arrays worldwide is small, and the number of earthquake observations is limited, synthetic motions with desired characteristics become necessary. This paper presents a method for synthesizing such motions, which is an extension of the SYNACC method, developed first in the early 1970s for synthetic accelerations, velocities and displacements at a point, and later extended to synthetic near surface strains, rotations and curvatures of ground motion at a point. It consists of unfolding in time a site specific Fourier amplitude spectrum of ground acceleration, obtained by an empirical scaling model, by representing the ground motion as a superposition of traveling wavelets of Love and Rayleigh waves and body waves, which propagate with phase and group velocities consistent with the dispersion characteristic of the site geology, approximated by parallel layers. Uniform hazard Fourier spectra or any specified target Fourier spectrum can also be used. Derivations of the point strains, rotations and curvatures are also presented. The method is illustrated for scenario M6.5 and M7.5 earthquakes and three dispersion models.

스펙트럼 적합 입력지반운동에 의한 면진구조의 응답 특성

Structures in a nuclear power system are designed to be elastic even under an earthquake excitation. However a structural component such as an isolator shows inelastic behavior inherently. For the seismic assessment of nonlinear structures, response history analysis should be performed. In this study, the response of base isolation system was analyzed by response history analysis for the seismic performance assessment. Firstly, several seismic assessment criteria for a nuclear power plant structure were reviewed for the nonlinear response history analysis. Based on these criteria, the spectrum matched ground motion generation method modifying a seed earthquake ground motion time history was adjusted. Using these spectrum matched accelerograms, the distribution of displacement responses of the simplified base isolation system was evaluated. The resulting seismic responses excited by the modified ground motion time histories and the synthesized time history generated by stochastic approach were compared. And the response analysis of the base isolation system considering the different intensities in each orthogonal direction was performed.

Seismic Fragility of a Highway Bridge in Quebec

The seismic vulnerability of the Chemin des Dalles Bridge over Highway 55 located in Trois-Rivieres (Quebec, Canada) was evaluated through fragility analysis using field- and laboratory-validated models. This approach offers an effective means to capture the uncertainties in ground-motion realizations, the demands placed on key structural components, and the capacity of the components to resist various levels of seismic excitation. A series of 180 synthetic ground-motion time histories (GMTHs) compatible with eastern Canada was used to capture the uncertainties related to the hazard. Nonlinear time history analyses were performed with these GMTHs and statistically analyzed to define the probabilistic seismic demand model for the abutments, bearings, and columns, which are the critical components. Data from the literature, coupled with sectional and damage mechanics analyses, were used to define the limit states (LSs) for these components. Bridge component and system fragility curves were used to evaluate the likely failure modes of the bridge and potential targets for retrofit, while accounting for key sources of uncertainty in the performance assessment. The results reveal the seismic vulnerability of this specific bridge and even offer insight into the seismic vulnerability of a typical multispan girder-type concrete bridge in Quebec.

Wave passage and incoherency effects on seismic response of high arch dams

The effects of incoherency and wave-passage on the nonlinear responses of concrete arch dams are investigated in this study. A double curvature arch dam is selected as a numerical example. The reservoir is modeled as a compressible material and the foundation is modeled as a massless medium. Ground motion time-histories are artificially generated using the Monte Carlo simulation approach. Four different finite element models (FEM) are considered: uniform excitation; incoherence effect; wave passage effect; and both incoherence and wave passage effects. It was revealed that modeling multiple-supports excitation could have a significant impact on the structural response of the dam by inducing a pseudo-static effect. Also, it was concluded that the coherency effect overshadows the wave passage effect and the results obtained from non-uniform excitation of FEM, including the wave passage effect, is close to the results of the FEM when it is uniformly excited.

On the monitoring of historic Anime Sante church damaged by earthquake in L'Aquila

The paper shows the activity of a 2-year static and dynamic monitoring activity to check the structural response and the actual level of damage of a historic church hit by earthquake in L'Aquila (Italy) in 2009. Structural health monitoring was carried out with accelerometers and transducers that recorded time histories of the main earthquake-induced ground motions and ambient vibrations. The paper focuses particularly on residual performance and assessment of the ancient church after six earthquake response data set—because of the seismic swarm—via a modal identification of output-only systems by using frequency domain decomposition. In this way, fundamental frequency, mode shapes, damping ratios and displacements can be estimated without knowing the input that has excited the system. The dynamic monitoring provides also a way to check possible drops of tension in the cables and relaxation in the FRP belts used for safety. Efficiency and reliability of modal identification of basilica-type churches, via the dynamic response of macro-elements only in comparison with the whole analysis, are finally proposed. Copyright © 2012 John Wiley & Sons, Ltd.

Scaling ground motions using optimal feedback control for testing structures by shake table

Shake tables are widely used for testing structural models subjected to earthquakes. Time histories of real ground motions are reproduced by a moving shake table platform, controlled using an appropriate algorithm, considering the limitations in the platform's maximum displacement. As the real peak ground displacement is usually higher, compared with the platform's stroke, the earthquake records are scaled. Scaling yields distortion in the ground motion spectrum and consequently changes the influence of the earthquake on the tested structure. A method for minimization of the undesired effects, on the basis of linear models of a tested structure, was developed previously. Closeness of power spectral density to response spectra was chosen as the main criterion of scaling efficiency. However, strong earthquakes lead to nonlinear effects in structural response. Supplemental active and passive devices are widely applied for enhancing structural seismic response. Active and semi-active devices are activated according to structural dynamic behavior, measured during the earthquake. But if the accelerations in the measured feedback include high distortions, control will not be effective as desired. An earthquake record scaling method, proposed in this study, is based on criteria that are more consistent with the tested structure. A mathematical formulation of the problem is developed. Effectiveness of the proposed method is demonstrated by comparing the total earthquake energy and response spectra of real and scaled earthquakes as well as by comparative evaluation of the acceleration distortions of their time histories. Response of two multistory structural models to real and scaled seismic records is compared to demonstrate the proposed method's efficiency, accuracy and convenience. Copyright © 2012 John Wiley & Sons, Ltd.

Sensitivity of ground-motion scenarios to earthquake source parameters in the Tehran metropolitan area, Iran

The northern Tehran fault (NTF) is a principal active fault of the Alborz mountain belt in the northern Iran. The fault is located north of the highly populated Metropolitan Area of Tehran. Historical records and paleoseismological studies have shown that the NTF poses a high seismic risk for the Tehran region and the surrounding cities (e.g. Karaj). A series of ground-motion simulations are carried out using a hybrid kinematic-stochastic model to calculate broadband (0.1–20Hz) ground-motion time histories for deterministic earthquake scenarios (M7.2) on the NTF. We will describe the source characteristics of the target event to develop a list of scenario earthquakes that are probably similar to a large earthquake on the NTF. The effect of varying different rupture parameters such as rupture velocity and rise time on the resulting broadband strong motions has been investigated to evaluate the range of uncertainty in seismic scenarios. The most significant parameters in terms of ground-shaking level are the rise time and the value of the rupture velocity. For the worst-case scenario, the maximum expected horizontal acceleration, and velocity at rock sites in Tehran range between 128 and 1315cm/s/s and 11–191cm/s, respectively. For the lowest scenario, the corresponding values range between 102 and 776cm/s/s and 12 to 81cm/s. Nonlinear soil effects may change these results but are not accounted for in this study. The largest variability of ground motion is observed in neighborhood of asperity and also in the direction of rupture propagation. The calculated standard deviation of all ground-motion scenarios is less than 30% of the mean. The capability of the simulation method to synthesize expected ground motions and the appropriateness of the key parameters used in the simulations are confirmed by comparing the synthetic peak ground motions (PGA, PGV and response spectra) with empirical ground-motion prediction equations.

DAMAGE EVOLUTION SPECTRA FOR SEISMIC ANALYSIS OF HIGH-RISE BUILDINGS

Seismic response spectra are amongst one of the most important tools for characterizing earthquake ground motions. In design practice, the response spectra are presented without including any load history, hence the nonlinear analysis of structures based solely on conventional earthquake response spectra is theoretically unsound, particularly for long-period or vertically irregular high-rise buildings. In this paper, a concept of seismic damage evolution is introduced and the method of analysis for characterizing the process of seismic damage to structures under earthquakes is presented. Seismic damage evolution spectra for analysis and design of high-rise buildings are then developed as an effective means of describing and simplifying earthquake ground motions. These spectra are shown to be very useful in selecting the ground motion-time history and, particularly, validating the equivalent static-load analysis and design of high-rise buildings under near-fault pulse-like ground motions. Case studies of the seismic inelastic performance of two vertically irregular, tall buildings are presented considering the seismic damage evolution spectra.

Calibration of the Specific Barrier Model to the NGA Dataset

The seismic design of structures sometimes necessitates the use of synthetic strong ground motion time histories (European Committee for Standardization 2003; International Code Council 2000). Toward this end, the stochastic method with a point-source representation of the seismic source is a fast and efficient way to generate synthetic time histories ( e.g. , Boore 2003). While other modeling procedures may be more refined and physically realistic ( e.g. , full finite-fault simulations), they generally require a larger number of input parameters for which calibration relations do not exist (see Douglas and Aochi 2008, and references therein). Even for well-recorded earthquakes, these parameters are difficult to develop calibration relations for because of (1) the observed variability in the results from earthquake source process inversion and its dependence on available data ( e.g. , Custodio et al. 2005) and (2) the overall lack of confidence in even the best estimates of the parameters controlling fault rupture (Monelli and Mai 2008; e.g. , Monelli et al. 2009). The simple seismological models also have advantages over empirical ground motion predictive equations (GMPE) (Abrahamson and Shedlock 1997; Power et al. 2008), although the forms of the source, path, and site functions in the simple seismological models may be somewhat less flexible than empirical GMPEs when trying to fit existing data. Seismological models can provide information on the physical nature of the parameters controlling the strong motion while the GMPEs cannot ( e.g. , Boore 2003; Olafsson et al. 2001; R. Sigbjornsson and Ambraseys 2003). Moreover, despite the increase in strong motion recordings over the last couple of decades, and presumably the corresponding increase in knowledge, there are still considerable differences among the median ground motions estimated using the various GMPEs. These differences are a measure of epistemic uncertainty and have not been …