Target Response Spectrum Research Articles

Effects of Significant Duration of Ground Motions on Seismic Responses of Base-Isolated Nuclear Power Plants

The purpose of this study is to investigate the effects of the significant duration of ground motions on responses of base-isolated nuclear power plants (NPPs). Two sets of ground motion records with short duration (SD) and long duration (LD) motions, scaled to match the target response spectrum, are used to perform time-history analyses. The reactor containment building in the Advanced Power Reactor 1400 (APR1400) NPP is numerically modeled using lumped-mass stick elements in SAP2000. Seismic responses of the base-isolated NPP are monitored in forms of lateral displacements, shear forces, floor response spectra of the containment building, and hysteretic energy of the lead rubber bearing (LRB). Fragility curves for different limit states, which are defined based on the shear deformation of the base isolator, are developed. The numerical results reveal that the average seismic responses of base-isolated NPP under SD and LD motion sets were shown to be mostly identical. For PGA larger than 0.4g, the mean deformation of LRB for LD motions was bigger than that for SD ones due to a higher hysteretic energy of LRB produced in LD shakings. Under LD motions, median parameters of fragility functions for three limit states were reduced by 12% to 15% compared to that due to SD motions. This clearly indicates that it is important to select ground motions with both SD and LD proportionally in the seismic evaluation of NPP structures.

The effect of amplitude scaling limits on conditional spectrum‐based ground motion selection

SummaryAmplitude scaling is commonly used to select ground motions matching a target response spectrum. In this paper, the effect of scaling limits on ground motion selection, based on the conditional spectrum framework, is investigated. Target spectra are computed for four probabilistic seismic hazard cases in Western United States, and 16 ground motion suites are selected using different scaling limits (ie, 2, 5, 10, and 15). Comparison of spectral acceleration distributions of the selected ground motion suites demonstrates that the use of a scaling limit of 2 yields a relatively poor representation of the target spectra, because of the small limit leading to an insufficient number of available ground motions. It is also shown that increasing scaling limit results in selected ground motions with generally increased distributions of Arias intensity and significant duration Ds5‐75, implying that scaling limit consideration can significantly influence the cumulative and duration characteristics of selected ground motions. The ground motion suites selected are then used as input for slope displacement and structural dynamic analyses. Comparative results demonstrate that the consideration of scaling limits in ground motion selection has a notable influence on the distribution of the engineering demand parameters calculated (ie, slope displacement and interstory drift ratio). Finally, based on extensive analyses, a scaling limit range of 3 to 5 is recommended for general use when selecting ground motion records from the NGA‐West2 database.

A Multi-Objective Ground Motion Selection Approach Matching the Acceleration and Displacement Response Spectra

For seismic resilience-based design (RBD), a selection of recorded time histories for dynamic structural analysis is usually required. In order to make individual structures and communities regain their target functions as promptly as possible, uncertainty of the structural response estimates is in great need of reduction. The ground motion (GM) selection based on a single target response spectrum, such as acceleration or displacement response spectrum, would bias structural response estimates leading significant uncertainty, even though response spectrum variance is taken into account. In addition, resilience of an individual structure is not governed by its own performance, but depends severely on the performance of other systems in the same community. Thus, evaluation of resilience of a community using records matching target spectrum at whole periods would be reasonable because the fundamental periods of systems in the community may be varied. This paper presents a GM selection approach based on a probabilistic framework to find an optimal set of records to match multiple target spectra, including acceleration and displacement response spectra. Two major steps are included in that framework. Generation of multiple sub-spectra from target displacement response spectrum for selecting sets of GMs was proposed as the first step. Likewise, the process as genetic algorithm (GA), evolvement of individuals previously generated, is the second step, rather than using crossover and mutation techniques. A novel technique improving the match between acceleration response spectra of samples and targets is proposed as the second evolvement step. It is proved computationally efficient for the proposed algorithm by comparing with two developed GM selection algorithms. Finally, the proposed algorithm is applied to select GM records according to seismic codes for analysis of four archetype reinforced concrete (RC) frames aiming to evaluate the influence of GM selection considering two design response spectra on structural responses. The implications of design response spectra especially the displacement response spectrum and GM selection algorithm are summarized.

Equivalent linear computation of response spectra for liquefiable sites: The spectral interpolation method

A simplified methodology is presented for the computation of elastic response spectra at liquefiable sites, based on the widely used equivalent linear site response analysis method. The target response spectrum is obtained by frequency dependent interpolation between the response spectra for “non-liquefied” and for “liquefied” ground, the former computed for the native soil properties, without excess pore pressure buildup, and the latter for the properties of the liquefied soil. Apart from the structural frequency, the interpolation coefficient is also related to the factor of safety against liquefaction. In this way, the proposed methodology takes indirectly into account the initial segment of the seismic excitation time-history, until the onset of liquefaction, which is of cornerstone importance for the overall ground response at sites with low and moderate liquefaction potential. Results of parametric nonlinear numerical analyses and three field case studies are used to calibrate the methodology and evaluate its accuracy. In addition, the potential of the proposed methodology for defining code-oriented design spectra for liquefiable sites is examined.

Impact of the type of the target response spectrum for ground motion selection and of the number of ground motions on the pushover-based seismic performance assessment of buildings

The impact of the type of target response spectrum and the number of ground motions on the response of buildings is investigated. The parametric study involves the selection of ground motions based on the Eurocode 8 spectrum, a conditional spectrum using the official seismotectonic model of Slovenia, a uniform hazard spectrum based on the seismotectonic model of the SHARE project, and the corresponding conditional spectrum. In addition to the variation of the target response spectrum, the number of selected ground motions was varied from 7 to 60. Selected sets of ground motions were used to investigate the seismic response of eight reinforced concrete buildings with fundamental vibration periods from 0.15 s to 1.76 s. The aim of the study was to analyse the variation of target displacements obtained by a pushover-based method, i.e. the median displacement corresponding to a seismic intensity with a return period of 475 or 2475 years, and the median spectral acceleration causing collapse, which was estimated by incremental dynamic analysis. It was found that the target spectrum and the number of ground motions have a limited impact on the target displacement, especially if it corresponds to seismic intensities with a return period of 475 years. Additionally, the impact of the number of ground motions on the median spectral acceleration causing collapse is much lower than the impact of the target response spectrum. When the conditional spectrum was used as the target spectrum for ground motion selection instead of the design response spectrum prescribed by Eurocode 8, the resulting median spectral acceleration causing collapse increased by a factor of between 1.2 and 2.3.

An approach for predicting multi-support seismic underground motions in layered saturated soil under surface water

Simulation of multi-support (i.e. spatially variable) seismic underground motions in sea areas plays a significant role in the seismic analysis of cross-sea structures such as cross-sea bridges or subsea tunnels. However, existing approaches for predicting multi-support seismic motions mainly focus on the dry site soils without overlying surface water. This paper proposes an approach for predicting multi-support seismic underground motions in layered saturated half space under surface water, subjected to oblique incident P waves. The transfer function in saturated soil under surface water, as the theoretical basis of the subsequent numerical simulation, is first derived based on wave propagation theory and the calculated reflection coefficients of P wave–induced P1, P2, SV waves in saturated soils. The derived transfer function is further employed to deduce and obtain the underground (sub-seabed) power spectral density function and response spectrum function. The two obtained functions, combined with the additional cross-coherence function, are subsequently employed to construct the cross power spectral density matrix and thus to simulate multi-support seismic underground motions. The solutions are validated against the target power spectral density, target response spectrum and target cross-coherence functions. A parametric analysis is presented where the effects of the soil thickness, the incident angle and the overlying water depth are investigated. Results show that the soil thickness, incident angle and overlying water depth have significant influences on the amplitude of transfer functions, which further affect the ratios between seismic ground and underground motions.

Evaluation of RC building strengthened with column jacketing method with consideration of soft-story

The assessment and strengthening method of existing reinforced concrete buildings in terms of seismic responses have been highly important for the past decades especially buildings with soft-story mechanism which are found to be more vulnerable. Several strengthening techniques have been proposed including Fibre Reinforced Polymer and steel jacket of RC column elements. This paper aims to evaluate the effectiveness of two strengthening techniques with variability of soft-story under severe earthquake excitations. Five-story RC building is investigated and non-linear dynamic analysis is performed to capture the inelastic responses of the structure. Four sets of ground motion are selected and matched with target response spectrum for Aceh earthquake. Seismic responses of the considered models and strengthening techniques are compared.

Seismic responses comparison of RC building in consideration of plastic hinge models and properties

Assessment of reinforced concrete building capacity under seismic excitation has been recognized to be highly important for the past decades especially in Indonesia. Previous studies showed failures of RC building occurred in columns with bad confinement. The confinement in RC column has significant role in providing ductility and shear capacity for seismic loads. For this aim, a five-story RC building having different confinement on column members are investigated under seismic excitation. Three set of ground motions are selected and matched with target response spectrum of Indonesian code. Nonlinear analysis is performed by utilizing OpenSEES finite element software. To account the inelastic behaviour of beam and column elements, two types of plastic hinge are applied for both elements, namely finite length plastic hinges (plastic zone) and distributed hinges along the length of elements. The results show that there is a noticeable difference in seismic responses in correlation with the building model.

A procedure to select ground-motion time histories for deterministic seismic hazard analysis from the Next Generation Attenuation (NGA) database

Abstract. In performance-based seismic design, ground-motion time histories are needed for analyzing dynamic responses of nonlinear structural systems. However, the number of ground-motion data at design level is often limited. In order to analyze seismic performance of structures, ground-motion time histories need to be either selected from recorded strong-motion database or numerically simulated using stochastic approaches. In this paper, a detailed procedure to select proper acceleration time histories from the Next Generation Attenuation (NGA) database for several cities in Taiwan is presented. Target response spectra are initially determined based on a local ground-motion prediction equation under representative deterministic seismic hazard analyses. Then several suites of ground motions are selected for these cities using the Design Ground Motion Library (DGML), a recently proposed interactive ground-motion selection tool. The selected time histories are representatives of the regional seismic hazard and should be beneficial to earthquake studies when comprehensive seismic hazard assessments and site investigations are unavailable. Note that this method is also applicable to site-specific motion selections with the target spectra near the ground surface considering the site effect.

Seismic risk assessment of cold-formed steel shear wall systems

This paper presents the probabilistic seismic performance and risk assessment of cold-formed steel (CFS) sheathed shear wall panel (SWP) structures adopting conventional steel moment-resisting frame (MRF) systems as a benchmark with the aim of exploring the viability of using CFS-SWP as a new structural solution in seismic prone regions. A set of 12 building structures of both systems, with 2-, 4- and 5-storey, have been designed for two seismic intensity levels. To simulate their nonlinear behaviour, the structures were modelled adopting recently developed deteriorating hysteresis models. Based on probabilistic seismic hazard analyses (PSHA), a site-specific selection of ground motion records for Incremental Dynamic Analyses (IDA) has been carried out adopting the Conditional Mean Spectrum (CMS) as a more realistic target response spectrum. Subsequently, the seismic risk was evaluated over the structure lifetime (i.e., 50years) in terms of the annual probability of exceeding the Damage Limitation, No-Local Collapse and Near Collapse limit states. The importance and usefulness of the risk metrics are highlighted and adopted as an indicator to explore the behavioural features of both structural systems. Overall, the assessment procedure showed that both systems present an acceptable seismic performance and therefore the CFS-SWP can be seen as a reliable structural solution to achieve performance-based objectives in seismic regions.

SelEQ: An advanced ground motion record selection and scaling framework

The consensual agreement that ground motion record selection plays an important role in the non-linear dynamic structural response has contributed to numerous research studies seeking the definition of accurate ground motion record selection techniques. However, most of the available tools only allow for record selection based on spectral compatibility between the mean response spectrum of a record suite and a target response spectrum. This paper presents SelEQ, a fully integrated framework that implements state-of-the art procedures for ground motion record selection and scaling. In addition to typical record selection procedures, SelEQ allows obtaining the Conditional Mean Spectrum (CMS) for the European territory, the latter making use of the open source platform OpenQuake and the recently proposed SHARE seismic hazard model. This important feature allows state-of-the-art record selection for probabilistic-based assessment and risk analysis. SelEQ incorporates a number of procedures available in the literature that facilitate preliminary record selection (e.g. disaggregation for a specific site) and that allow advanced selection criteria (e.g. control of mismatch of individual ground motion records). The framework makes use of the Adaptive Harmony Search meta-heuristic optimization algorithm in order to significantly minimize computational cost and analysis time, whilst still meeting the imposed selection constraints. Application examples of the framework indicate that it can accurately select suites of ground motion records for code-based and probabilistic-based seismic assessment.

Generating Spectrum‐Compatible Time Histories Using Eigenfunctions

In this article, the causes of drift in the velocity and the displacement time history are investigated. It is found that, in addition to numerical error, drift is caused by overdeterminacy in the constants of integration. Because there are six independent prescribed at‐rest conditions (three initial and three terminal), the eigenfunctions from an eigenvalue problem, which is described by a sixth‐order ordinary differential equation satisfying the six initial and terminal at‐rest conditions, are chosen as a basis of expansion. The eigenfunctions form a dense and complete set and span a vector space. The eigenfunctions are used to expand an accelerogram. Drift‐free consistent velocity and displacement time histories are then obtained, also in terms of the eigenfunctions, without direct integration and baseline correction. A method is also proposed to modify a real recorded accelerogram using the eigenfunctions to generate time histories compatible with the target response spectra without drift.

Simplified approaches for Arias Intensity correction of synthetic accelerograms

For the generation of synthetic accelerograms, spectrum compatibility is usually emphasized. However, it is well known that the correct evaluation of the seismic response depends on suitable seismic inputs. For example, Arias Intensity, that measures the energy of an earthquake and which attracts more and more attention in probabilistic seismic analysis, cannot be ignored. Thus, simplified methods which could generate both spectrum-compatible and energy-compatible accelerograms are required. This study focuses on the correction of Arias Intensity when generating artificial synthetic accelerograms for given specific earthquake records. Two simple and efficient approaches are proposed. The first approach introduces an energy-compatible algorithm to the spectrum-compatible model, which enables the generated accelerograms to match both the target response spectrum in the frequency domain and Arias Intensity in the time domain. The second approach refers to an empirical way in which empirical envelope shape functions are directly defined based on the energy distribution profile of given earthquake records. The two approaches are validated using various earthquake records, their performance is proven satisfactory and their application is straightforward in the relative fields of earthquake engineering.

鉄筋コンクリート造多層平面骨組の地震応答と柱梁接合部の耐震設計

Non-linear time history earthquake response of reinforced concrete multi-story moment resisting frame structures are calculated with a model of beam-column joints considering their restoring force characteristics which represents the interaction of moment, axial force and shear (Figure 1). The influence of the failure mechanism and strength degradation of beam-column joint are investigated by the comparison of the simulation of twenty-seven model frames (Table 1) satisfying the current seismic provisions in Japan, the model parameters for which are combination of column-to-beam strength ratio of 1.2, 1.5, and 2.0 and joint hoop reinforcement ratio of around 0.2% to 0.6% which are neglected as design parameters affecting the seismic safety in design practice in Japan. Statically cyclic response analyses of the model frames subjected to lateral load distribution pattern of Ai by displacement control at the top floor are calculated to classify the type of hysteresis loop and strength degradation (Figure 3). Then dynamic analyses of the model frames are carried out subjected to five artificial base ground motions conformed to maximum considered intensity design (MCD) spectrum and five observed strong motions (Figure 4). It is found that there is significant difference in the story drift response of the structures with different beam-column joint attributes. Particularly, in some cases, the story drift ratio increased significantly due to concentration of lateral deflection to several stories and large residual drift remains to one direction (Figure 5). It is confirmed that the differences are due to softening of stiffness and strength degradation of the beam-column joints. It is caused by the mechanism identified as joint hinging (ref. 1) which features the yielding of longitudinal reinforcement within the beam-column joint and repetition of diagonal crack open and close. The state-of-the-practice analytical model for frames does not consider the joint hinging so it significantly underestimates the story drift responses by far than seismic design criteria (Figure 8). The response of frames with beam-column joint failing in joint hinging is very sensitive to base ground motions which were synthesized with same target response spectrum. So the prediction of the response is difficult by non-linear static procedure nor non-linear time history procedure the model of which consider plastic deformation only by non-linear rotational springs at beam-ends considering hysteresis model with slip behavior. The results of incremental dynamic analysis (IDA) shown in Figure 9 reveals that the amplification less than 50% of MCD yields little difference in the response of the models whilst the amplification of 100% of MCD causes near collapse to four-story model and concentration of story drift exceeding 6% to the models failing in joint hinging mechanism, which is far beyond seismic design criteria for MCD. The current Japanese seismic design practice for beam-column joint obviously lacks the necessary reinforcing detailing provision such as minimum column-to-beam strength ratio and design of required joint hoop ratio which is much behind the seismic provisions found in current NZ3101 and ACI 318 building codes.

An approach for synthesizing tri-component ground motions compatible with hazard-consistent target spectrum - Italian aseismic code application

A versatile approach for synthesizing non-stationary multicomponent ground motions compatible with a given target response spectrum is presented. In conjunction with performance-based structural design, the proposed approach combines a scenario-based procedure to select the target spectra of the three components of the motion, and a stochastic scheme, with an iterative wavelet-based method to generate the artificial time histories. In this context, the selection of an appropriate target spectrum is mandatory to synthesizing accelerograms with proper frequency content, input-energy, effective duration, and inelastic demand. In the scenario-based assessments, the target spectrum is derived directly from an earthquake scenario defined by seismic hazard disaggregation and strong ground-motion attenuation relationships. Firstly, the design spectrum specified by aseismic code provisions is replaced by a set of conditional mean spectra (CMS) for each of the ground motion components. Subsequently, an inverse stochastic dynamics problem, defined by a set of parametric evolutionary power spectra (EPS), is formulated and solved in a point-wise format for each CMS and for the three components of the motion. To improve the matching between the response spectrum of the simulated records and the CMS, an iterative procedure involving the family of harmonic wavelets is then employed. Post-processing for a proper baseline correction of the records is performed to synthesize ground-motions yielding realistic velocity and displacement traces. To illustrate the effectiveness of the approach, extensive numerical results pertaining to the Italian aseismic code provisions are included in this paper. In this regard, Monte Carlo simulations are undertaken to relate the CMSs to the evolutionary power spectrum. In particular, non-constant peak factor expressions via regression analysis are derived for various ground-motion durations. Finally, the nonlinear responses of elasto-plastic SDOF oscillators with different values of the yield strength reduction factor are considered. The oscillators subjected to artificially generated records and a set of natural unscaled and spectrum-compatible accelerograms, are examined in terms of ductility demand and equivalent number of yield cycles.

On selection and scaling of ground motions for analysis of seismically isolated structures

A broader consensus on the number of ground motions to be used and the method of scaling to be adopted for nonlinear response history analysis (RHA) of structures is yet to be reached. Therefore, in this study, the effects of selection and scaling of ground motions on the response of seismically isolated structures, which are routinely designed using nonlinear RHA, are investigated. For this purpose, isolation systems with a range of properties subjected to bidirectional excitation are considered. Benchmark response of the isolation systems is established using large sets of unscaled ground motions systematically categorized into pulse-like, non-pulse-like, and mixed set of motions. Different subsets of seven to 14 ground motions are selected from these large sets using (a) random selection and (b) selection based on the best match of the shape of the response spectrum of ground motions to the target spectrum. Consequences of weighted scaling (also commonly referred to as amplitude scaling or linear scaling) as well as spectral matching are investigated. The ground motion selection and scaling procedures are evaluated from the viewpoint of their accuracy, efficiency, and consistency in predicting the benchmark response. It is confirmed that seven time histories are sufficient for a reliable prediction of isolation system displacement demands, for all ground motion subsets, selection and scaling procedures, and isolation systems considered. If ground motions are selected based on their best match to the shape of the target response spectrum (which should be preferred over randomly selected motions), weighted scaling should be used if pulse-like motions are considered, either of weighted scaling or spectral matching can be used if non-pulse-like motions are considered, and an average of responses from weighted-scaled and spectrum-matched ground motions should be used for a mixed set of motions. On the other hand, the importance of randomly selected motions in representing inherent variability of response is recognized and it is found that weighted scaling is more appropriate for such motions.

An Accurate and Efficient Method for Selecting and Scaling Ground Motions Considering Target Response Spectrum Mean and Variance

본 논문은 2015년도 정부 (미래창조과학부)의 재원으로 한국연구재단 (2014R1A2A1A11049488, 2015R1A2A1A15055248)의 지원을 받아 수행되었습니다.

A Method for Selecting Ground Motions that Considers Target Response Spectrum Mean and Variance as Well as Correlation Structure

This study proposes a method for selecting ground motions from a ground motion library with response spectra that match the target response spectrum mean, variance, and correlation structures. The proposed method is conceptually simple and straightforward. In this method, a desired number of ground motions are sequentially selected from first to last. The accuracy and consistency of the proposed method are verified through comparisons of the ground motions selected using the proposed method with those selected using conventional methods. This study shows that the seismic responses of the frames vary according to ground motion selection and correlation structures.

An efficient method for selecting and scaling ground motions matching target response spectrum mean and variance

SummaryThe purpose of this study is to propose an accurate and efficient method for selecting and scaling ground motions matching target response spectrum mean and variance, which does not require excessive computation and simulation. In the proposed method, a desired number of ground motions are sequentially scaled and selected from a ground motion library without iterations. Copyright © 2016 John Wiley & Sons, Ltd.

Input ground motions for earthquake site response analyses: deep sediment column with cementation

Earthquake ground motions (GMs) are developed for input to structural response analyses of a highway bridge in Las Vegas, Nevada (southwestern US), an area of arid climate and moderate seismicity. A target response spectrum for rock-outcrop (‘bedrock’) motions at the site is produced from a probabilistic seismic hazard analysis and GMs are found that are compatible with the target. Three approaches to generate input GMs are tested: a set of unscaled real (natural) GMs, a set of scaled real GMs, and a single, spectrally-matched GM. The input motions are filtered through a representative sediment column, nearly 400 m deep, to produce surface GMs. Both nonlinear and equivalent-linear analyses are conducted. The sediment column tended to deamplify short-period motions while amplifying long-period motions and shifting peak response to longer period, as expected. For nonlinear analysis, all three approaches to generating the input GMs yielded acceptable results. The single, spectrally-matched GM yielded the highest spectral accelerations. To explore effects of a heavily cemented layer (caliche) on site response, an alternative profile is tested. The 30-m depth-averaged shear wave velocity is unchanged, but a thick caliche layer is inserted just beneath it. In a third test, the caliche profile is truncated at the stiff inclusion. Only the nonlinear analyses showed effects of the caliche. The shallow profile predicted the highest amplification by far, demonstrating how consideration of only the upper 30 m of a sediment column can significantly overpredict short-period response.