Frequency Content Of Ground Motion Research Articles

Simulation of Cumulative Absolute Velocity Consistent Endurance Time Excitations

ABSTRACT Endurance time method is a dynamic analysis in which structures are subjected to increasing excitations. These excitations are generated to be compatible with ground motions. Conventional excitation generating processes take into consideration the dynamic characteristics related to intensity and frequency contents of ground motions; however, duration-related characteristics have never been considered directly. In this study, the existing methodology is extended to include Cumulative Absolute Velocity as a duration-related parameter in the generating process. These newly generated excitations are then employed to estimate nonlinear responses of three concrete structures as case studies. Results show sound consistency with incremental dynamic analysis.

Insights on the Japanese Subduction Megathrust Properties From Depth and Lateral Variability of Observed Ground Motions

AbstractTwo ground motion prediction equation models for subduction zones have been tested using a public ground motion database of the KiK‐net records obtained by automated processing protocols (Dawood et al., 2016, https://doi.org/10.1193/071214EQS106). The database contains records of more than 700 interface earthquakes that occurred on the Japan subduction between 1998 and 2012. The Zhao et al. (2006, https://doi.org/10.1785/0120050122) ground motion prediction equation was shown to be the best suited model for the region. It was then used as backbone to analyze the variability of ground motion records. The residuals between observed and predicted ground motions have been analyzed to study the spatial variation of the earthquakes' ground motion frequency content on the Japan megathrust. This analysis revealed a depth dependency of generated ground motions consistent with the downdip segmentation proposed for subduction interfaces (Lay et al., 2012, https://doi.org/10.1029/2011JB009133), a regional ground motion dependency that may be related with lateral variations of the mechanical properties of the subduction interface and a high‐frequency radiations drop in the earthquake sequence that preceded the Tohoku‐Oki earthquake Mw 9.0. The regional ground motion dependency suggests the existence of different domains along trench of the Japan subduction megathrust that control the ground motions and the wave radiation patterns of interface earthquakes. The location of their boundaries is consistent with the extension of the rupture of the 2011 Tohoku‐Oki earthquake, with pre‐Tohoku interseismic coupling, and with the free air gravity anomalies.

Permanent seismic drifts in steel moment frames

This paper examines residual drift demands in steel moment-resisting frames incorporating the influence of degradation and ground motion frequency content. Detailed assessments are carried out using 54 multi-storey framed buildings, with a wide range of structural characteristics, which are designed according to the provisions of Eurocode 8. In order to identify the influence of cyclic and in-cycle degradation effects, the analysis is carried out with and without degradation modelling. Incremental dynamic analysis is employed in order to achieve various limits of lateral strength demand, using a suite of 56 ground motion records. It is shown that residual drifts are markedly higher in degrading models in comparison with non-degrading models, with the differences being more pronounced in relatively short period ranges, when higher rates of cyclic deterioration are employed, and for comparatively high lateral strength demand levels. The residual drift demand is also shown to increase with the increase in number of stories, and is often concentrated in the lower levels when degrading models are used. Overall, significant residual drift demands are observed in the structural systems considered, with a high likelihood of exceeding a 0.5% residual drift limit in most cases. Based on the results, two simplified prediction relationships are proposed to estimate the permanent drifts of multi-storey steel moment framed systems. The first is concerned with the design stage based on the results of elastic analysis, whilst the second is associated with post-earthquake structural assessment based on actual measurements of residual drifts.

Identification of transient vibration characteristics of pile-group models during liquefaction using wavelet transform

A time–frequency approach based on the wavelet transform is used to examine the transient vibration characteristics of two 2 × 2 pile-group models tested in a shake table. The models are subjected to three different records consisting of white noise input and two differently scaled records from the 2011 Christchurch Earthquake. In contrast to conventional Fourier analysis, the proposed method has the advantage of enabling the visualisation of the temporal variation in structural frequencies and frequency content of ground motion due to liquefaction in an effective way. It is found that liquefaction causes a decrease in structural frequency, whose reduction depends on the rate of excess pore pressure build-up, whereby high rates (“fast liquefaction”) lead to greater reduction, ie, up to 51%. Liquefaction is also responsible for the elongation of the predominant period of the ground motion and narrowing of its overall frequency bandwidth. The combined effect of reduction in structural frequency and filtering of high frequency components of the ground motion may lead to moving resonance condition, resulting in amplification of structural response. After the onset of liquefaction, there is a redistribution of maximum bending moment toward deeper elevations, indicating that kinematic soil-structure interaction dominates the overall seismic response.

Drift and rotation demands in steel frames incorporating degradation effects

This paper is concerned with the assessment of seismic drift demands in steel moment frames designed to comply with Eurocode 8 provisions, with due account for cyclic and in-cycle degradation. In addition to degradation effects, the main parameters examined include the ground motion frequency content as well as the level of constant relative strength or inelasticity. To represent a wide range of structural characteristics, a set of 54 multi-storey frames are considered, in which the number of stories, steel profiles, seismic hazard and compliance criteria are varied. Detailed incremental dynamic analyses are performed on the full set of frames using a suite of 56 far-field ground motion records, which are scaled appropriately to achieve different levels of inelastic demand or equivalent behaviour factors. The seismic performance is evaluated in terms of maximum global and local drifts as well as beam chord rotations. Characteristic results show that maximum response in terms of global deformations and inter-storey drifts is significantly affected by degradation phenomena, along with the ground motion frequency content and the level of inelastic demand. For medium rise typologies subjected to earthquakes with relatively high frequency content, concentration of seismic demand in terms of inter-storey drift is captured by the degrading models and leads to early development of plastic mechanisms. The seismic demand scenarios used in this study, through spectral acceleration-based scaling of ground motions, indicate that the influence of degradation can be significant not only at collapse levels but also at those associated with typical design situations. Finally, based on the extensive analyses carried out in this investigation, expressions for predicting the global and local demands are proposed and discussed.

Influences of the duration and frequency content of ground motions on the seismic performance of high-rise intake towers

Abstract The main purpose of this study is to investigate the influences of the duration and frequency content of ground motions on the seismic performance of high-rise intake tower considering structures-reservoir-foundation interactions. Thirty earthquake records matched with the standard design response spectrum according to Chinses code for seismic design of hydraulic structures of hydropower project were selected as ground motion input. The nonlinear seismic responses were classified according to the duration to investigate the effects of ground motion duration; while the nonlinear dynamic response under other fourteen earthquake records matched with previous standard design response spectrum with different response spectrum curves and characteristic periods were compared to study the effects of frequency content. The results show that the duration and frequency content in the seismic input can have significant effects on the seismic performance of high-rise intake towers.

Reliability-based fragility analysis of nonlinear structures under the actions of random earthquake loads

This study presents the reliability-based analysis of nonlinear structures using the analytical fragility curves excited by random earthquake loads. The stochastic method of ground motion simulation is combined with the random vibration theory to compute structural failure probability. The formulation of structural failure probability using random vibration theory, based on only the frequency information of the excitation, provides an important basis for structural analysis in places where there is a lack of sufficient recorded ground motions. The importance of frequency content of ground motions on probability of structural failure is studied for different levels of the nonlinear behavior of structures. The set of simulated ground motion for this study is based on the results of probabilistic seismic hazard analysis. It is demonstrated that the scenario events identified by the seismic risk differ from those obtained by the disaggregation of seismic hazard. The validity of the presented procedure is evaluated by Monte-Carlo simulation.

Study on frequency content of provided strong ground motion in West Azerbaijan and East Azerbaijan

<p>This study aims to better reveal the characteristics of the assessed ground motion in west and east Azerbaijan. Due to existence of happened great earthquakes and large number of potential seismic sources in North-West of Iran which is located in junction of Alborz and Zagros seismotectonic provinces, it is an interesting area for seismologists. Considering to population and existence of large cities like Tabriz, Ardabil and Orumiyeh which play crucial role in industry and economy of Iran, authors decided to focus on study of frequency content of strong ground motion to achieve ground acceleration in different frequencies indicate critical frequencies in the studied area. in this study have been applied is professional industrial software which has been written in 2009 and provided by authors; Because This applied software can cover previous software weak points very well. Obtained hazard maps illustrate that maximum accelerations will be experienced in North West to South East direction which increased by frequency reduction from 100 Hz to 10 Hz then decreased by frequency reduce (to 0.25 Hz). Maximum acceleration will be occurred in the basement in 10 Hz.</p>

Local site effect of a clay site in Shiraz based on seismic hazard of Shiraz Plain

Propagation of seismic waves through soil layers would drastically change the frequency content and amplitude-based features of ground motions at the surface. These alterations are known as seismic site effects. Computation of site effects of high-populated areas such as large cities is of great importance (e.g., it is used in development of seismic microzonation of a region). Shiraz is one of the most populous cities of Iran and is located in a high seismic hazardous region. A representative clay site in this city is selected to assess local site effects. The time series and random vibration theory procedure in the frequency domain are implemented to analyze the aforementioned site. Furthermore, the nonlinear dynamic soil behavior is simulated by the equivalent linear method and the nonlinear method via DEEPSOIL program. Three types of soil column uncertainties such as shear wave velocity, modulus reduction, and damping ratio of soil layers as well as depth of underlying rock half-space (D bed) are considered herein. The mean amplification and standard deviation of natural logarithm of amplification factors are computed for a variety of analysis types. The results of the current study show that the computed mean and standard deviation of amplification factor in ln units by considering only V S uncertainty are in good agreement with the corresponding ones by considering V S and modulus reduction and damping ratio variabilities simultaneously for the studied site. Furthermore, it seems that the effect of bedrock depth in definition of spectral shapes of the Iranian seismic building code should be taken into account.

Effects of ground motion frequency content on performance of isolated bridges with SSI

The present study considers a multi-span continuous bridge, isolated by lead rubber bearing (LRB). Dynamic soil-structure interaction (SSI) is modelled with the help of a simplified, sway-rocking model for different types of soil. It is well understood from the literature that SSI influences the structural responses and the isolator performance. However, the above-mentioned effect of SSI also depends on the earthquake ground motion properties. It is very important to understand how the interaction between soil and structure varies with the earthquake ground motion characteristics but, as far as the knowledge of the authors go, no study has been carried out to investigate this effect. Therefore, the objectives of the present study are to investigate the influence of earthquake ground motion characteristics on: (a) the responses of a multi span bridge (isolated and non-isolated), (b) the performance of the isolator and, most importantly, (c) the soil-structure interaction. Statistical analyses are conducted by considering 14 earthquakes which are selected in such a way that they can be categorized into three frequency content groups according to their peak ground acceleration to peak ground velocity (PGA/PGV) ratio. Lumped mass model of the bridge is developed and time history analyses are carried out by solving the governing equations of motion in the state space form. The performance of the isolator is studied by comparing the responses of the bridge with those of the corresponding uncontrolled bridge (i.e., non-isolated bridge). On studying the effect of earthquake motions, it is observed that the earthquake ground motion characteristics affect the interaction between soil and structure in such a way that the responses decrease with increase in frequency content of the earthquake for all the types of soil considered. The reverse phenomenon is observed in case of the isolator performance where the control efficiencies increase with frequency content of earthquake.

Influence of deterioration modelling on the seismic response of steel moment frames designed to Eurocode 8

SummaryThis paper assesses the influence of cyclic and in‐cycle degradation on seismic drift demands in moment‐resisting steel frames (MRF) designed to Eurocode 8. The structural characteristics, ground motion frequency content, and level of inelasticity are the primary parameters considered. A set of single‐degree‐of‐freedom (SDOF) systems, subjected to varying levels of inelastic demands, is initially investigated followed by an extensive study on multi‐storey frames. The latter comprises a large number of incremental dynamic analyses (IDA) on 12 frames modelled with or without consideration of degradation effects. A suite of 56 far‐field ground motion records, appropriately scaled to simulate 4 levels of inelastic demand, is employed for the IDA. Characteristic results from a detailed parametric investigation show that maximum response in terms of global and inter‐storey drifts is notably affected by degradation phenomena, in addition to the earthquake frequency content and the scaled inelastic demands. Consistently, both SDOF and frame systems with fundamental periods shorter than the mean period of ground motion can experience higher lateral strength demands and seismic drifts than those of non‐degrading counterparts in the same period range. Also, degrading multi‐storey frames can exhibit distinctly different plastic mechanisms with concentration of drifts at lower levels. Importantly, degrading systems might reach a “near‐collapse” limit state at ductility demand levels comparable to or lower than the assumed design behaviour factor, a result with direct consequences on optimised design situations where over‐strength would be minimal. Finally, the implications of the findings with respect to design‐level limit states are discussed.

Duration effect of spectrally matched ground motion records on collapse resistance capacity evaluation of RC frame structures

SummaryAs one of the most important characteristics of strong ground motion, duration is intuitively expected to influence the seismic responses of structures. Up to now, the prescriptive provisions on ground motion duration are not clear in current seismic design codes around the world. For collapse resistance capacity evaluation of the structures under severe earthquake, it is more urgent to investigate the influence of ground motion duration and appropriate consideration for evaluation. Because the factors to be considered in the modeling and dynamic analysis are not very comprehensive, previous studies on the effect of ground motion duration on structural response have not got a consistent conclusion. In this study, a spectral matching approach is used to decouple the influence of the frequency content of ground motion on structural seismic response. And the effect of the infill wall, the strength and stiffness degradation of the structural components, and the axial‐shear‐flexure interaction are also considered. First, the 60 ground motion records with different duration and peak ground acceleration were selected from PEER strong ground motion database and matched spectrally. Then, the three‐dimensional numerical model for an infilled RC frame structure was built via OpenSees, and the incremental dynamic analysis was carried out under the matched ground motion records. The geometric mean incremental dynamic analysis curves of the structure for 30 long‐duration ground motion records and 30 short‐duration ground motion records were obtained. And the correlation between maximum residual roof displacement and maximum residual inter‐story drift ratio with the significant duration was further investigated. Finally, the correlation between collapse resistance capacity in terms of the spectral acceleration at the fundamental period of the structure Sa(T1) and significant duration of all ground motion records was discussed, and collapse probability under different Sa(T1) were obtained. The analytical results show that with the same intensity measure, ground motion records with longer duration may induce larger inter‐story drift ratio, larger residual displacement, and higher collapse probability for the structure. It is also shown that the collapse resistance capacity of the structure tends to decrease with the increase of the significant duration of ground motion.

Effects of frequency contents of aftershock ground motions on reinforced concrete (RC) bridge columns

This study focuses on exploring effects of frequency contents of aftershock ground motions on seismic responses of reinforced concrete (RC) bridge columns. It has been well recognized that RC columns damaged by a sizeable main shock event become more vulnerable to following aftershocks. Therefore, it is essential to use proper main shock-aftershock sequential ground motions in seismic analyses to ensure the safety and integrity of infrastructure. Using frequency –invariant scaling factors, conventional methods were developed in the past. These methods combine sequential ground motions when performing a time history nonlinear analysis. However, these conventional methods neglect frequency contents of aftershock ground motions, which are usually different from those of main shock ground motions.This research demonstrates the importance of properly representing aftershock ground motions in estimating seismic responses of RC columns, and presents the differences in frequency contents between aftershock ground motions and the corresponding main shock ground motions. Time history seismic analyses using a finite element analysis program OpenSees are carried out. First, main shock motions recorded during the 1994 Northridge, California, the United States of America, the 1997 Umbria-Mache, Italy and the 1999 Chi-Chi, Taiwan earthquakes are used in this study. Then, the corresponding aftershock motions are selected or obtained: (1) from recordings during the seismic events, (2) by scaling main shock (which represents a traditional method), or (3) by spectrally matching main shock motions to the aftershock motions. The peak displacements and residual displacements of the RC columns using the spectrally matched motions are closer to those results using real aftershock motion records, as opposed to using the scaled motions. This demonstrates that the frequency contents of aftershock ground motions have significant impacts on the seismic responses of RC columns.

Effects of nonlinear soil–structure-interaction on seismic damage of 3D buildings on cohesive and frictional soils

The present paper investigates the impact of nonlinear soil–foundation–structure-interaction (NLSFSI) on the damage response of 3D R/C buildings supported on different soil types with varying flexibility. The main goal of the paper is to present a first approach towards the possible quantitative differentiation, with respect to fixed base conditions, on the overall damage level when NLSFSI effects are accounted for. To accomplish this purpose nine buildings with various heights and structural systems are studied. For the foundation of the buildings two different soil types are considered, namely cohesive and frictional ones. Moreover, the soil flexibility is taken into account by using two different values of the soil’s shear wave velocity. The buildings are subjected to 65 bidirectional earthquake records, for which nonlinear time history analyses are conducted. The accelerograms of each record are scaled to two different seismic intensities corresponding to certain performance levels using appropriate scaling factors. The damage state of the buildings is expressed through the maximum interstorey drift ratio. The assessment of the results revealed that the role of soil–foundation–structure-interaction (SFSI) is not necessarily beneficial. In order to assess the SFSI effects a number of parameters are important as the frequency content of the earthquake ground motion, the building’s structural system, the foundation soil flexibility as well as the earthquake intensity.

A probabilistic study of ground motion simulation for Bangkok soil

Due to the soft soil condition, it has been found that buildings in Bangkok locating 200 km away from epicentral of an earthquake can be damaged as a result of high ground motion (e.g. earthquakes of magnitudes 5.3–5.9 in 1983). Because of rapid urban expansion and population growth in cities with soft soil condition, such as Bangkok, the assessment of seismic vulnerability of building structures becomes necessary. The purpose of this study is to quantify variability and develop attenuation and amplification models of ground motions for Bangkok sites. First, by analysing soil profile of Bangkok using Latin Hypercube sampling technique, critical attenuation and amplification characteristics, such as peak ground acceleration, ground motion intensity, frequency content and significant ground duration, were obtained. Then, the statistical information on the attenuation and amplification models of these characteristics was established and used to conduct a series of non-linear seismic analysis of a typical four storey commercial building in Bangkok. The research outcomes demonstrate that the developed models are capable of predicting the damage indices of buildings in Bangkok under different earthquake intensities and epicentral distances.

A methodology for defining seismic scenario‐structure‐based limit state criteria for rc high‐rise wall buildings using net drift

SummaryAs a result of population growth and consequent urbanization, the number of high‐rise buildings is rapidly growing worldwide resulting in increased exposure to multiple‐scenario earthquakes and associated risk. The wide range in frequency contents of possible strong ground motions can have an impact on the seismic response, vulnerability and limit states definitions of RC high‐rise wall structures. Motivated by the pressing need to derive more accurate fragility relations to be used in seismic risk assessment and mitigation of such structures, a methodology is proposed to obtain reliable, Seismic Scenario‐Structure‐Based (SSSB) definitions of limit state criteria. A 30‐story wall building, located in a multi‐seismic scenario study region, is utilized to illustrate the methodology. The building is designed following modern codes and then modeled using nonlinear fiber‐based approach. Uncertainty in ground motions is accounted for by the selection of forty real earthquake records representing two seismic scenarios. Seismic scenario‐based building local response at increasing earthquake intensities is mapped using Multi‐Record Incremental Dynamic Analyses (MRIDAs) with a new scalar intensity measure. Net Inter‐Story Drift (NISD) is selected as a global damage measure based on a parametric study involving seven buildings ranging from 20 to 50 stories. This damage measure is used to link local damage events, including shear, to global response under different seismic scenarios. While the study concludes by proposing SSSB limit state criteria for the sample building, the proposed methodology arrives at a reliable definition of limit state criteria for an inventory of RC high‐rise wall buildings under multiple earthquake scenarios. Copyright © 2016 John Wiley & Sons, Ltd.

Ground‐Motion Prediction Equations for Subduction Slab Earthquakes in Japan Using Site Class and Simple Geometric Attenuation Functions

The frequency content of strong ground motions from subduction slab earthquakes differs significantly from that of ground motions produced by other categories (tectonic locations: shallow crustal, upper mantle, and subduction interface) of earthquakes in subduction zones. In the last two decades, a large number of records from subduction slab events have been obtained in Japan. We present a ground‐motion prediction equation (GMPE) for this category of earthquakes. We used a large dataset from reliably identified slab events up to the end of 2012. The GMPEs were based on a set of simple geometric attenuation functions, site classes were used as site terms, and nonlinear site amplification ratios were adopted. A bilinear magnitude‐scaling function was adopted for large earthquakes with moment magnitude M w≥7.1, with the scaling rates for large events being much smaller than for the smaller events. A magnitude‐squared term was used for events with M w<7.1 as well as the bilinear magnitude‐scaling function. We also modeled the effect of volcanic zones using an anelastic attenuation coefficient applied to a horizontal portion of the seismic‐wave travel distance within possible volcanic zones. We found that excluding the records from sites with inferred site classes improved the model goodness of fit. The within‐event residuals were approximately separated into within‐site and between‐site residuals, and the corresponding standard deviations were calculated using a random effects model. The separation of within‐event residuals into within‐site and between‐site components allows for the possibility of adopting different standard deviations for different site classes in a probabilistic seismic‐hazard analysis if desired. Online Material: Figures showing the distribution of between‐event residuals with respect to magnitude and fault‐top depth and the distribution of within‐event residuals with respect to magnitude and source distance.

Ground‐Motion Prediction Equations for Subduction Interface Earthquakes in Japan Using Site Class and Simple Geometric Attenuation Functions

The frequency content of strong ground motions from subduction slab earthquakes differs significantly from that of ground motions produced by other categories (tectonic locations: shallow crustal, upper mantle, and subduction interface) of earthquakes in subduction zones. In the last two decades, a large number of records from subduction slab events have been obtained in Japan. We present a ground‐motion prediction equation (GMPE) for this category of earthquakes. We used a large dataset from reliably identified slab events up to the end of 2012. The GMPEs were based on a set of simple geometric attenuation functions, site classes were used as site terms, and nonlinear site amplification ratios were adopted. A bilinear magnitude‐scaling function was adopted for large earthquakes with moment magnitude M w≥7.1, with the scaling rates for large events being much smaller than for the smaller events. A magnitude‐squared term was used for events with M w<7.1 as well as the bilinear magnitude‐scaling function. We also modeled the effect of volcanic zones using an anelastic attenuation coefficient applied to a horizontal portion of the seismic‐wave travel distance within possible volcanic zones. We found that excluding the records from sites with inferred site classes improved the model goodness of fit. The within‐event residuals were approximately separated into within‐site and between‐site residuals, and the corresponding standard deviations were calculated using a random effects model. The separation of within‐event residuals into within‐site and between‐site components allows for the possibility of adopting different standard deviations for different site classes in a probabilistic seismic‐hazard analysis if desired. Online Material: Figures showing the distribution of between‐event residuals with respect to magnitude and fault‐top depth and the distribution of within‐event residuals with respect to magnitude and source distance.

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

SummaryEarthquake 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 &amp; Sons, Ltd.

Damping Reduction Factors for Crustal, Inslab, and Interface Earthquakes Characterizing Seismic Hazard in Southwestern British Columbia, Canada

High-damping displacement spectra and corresponding damping reduction factors ( η) are important ingredients in seismic design and analysis of structures equipped with seismic protection systems, as well as in displacement-based design methodologies. In this study, we investigated η factors for three types of earthquake characterizing seismic hazard in southwestern British Columbia, Canada: shallow crustal, deep inslab, and interface subduction. We used a large and comprehensive database including records from recent relevant earthquakes, such as the 2011 Tohoku event. Our key observations were as follows: (1) there is negligible dependence of η on soil class; (2) there is significant dependence of η on the frequency content and duration of ground motions that characterize the different record types, and (3) η is dependent on period, particularly for inslab events. Period-dependent equations were proposed to predict η for damping ratios between 5% and 30% corresponding to the three event types.