Spatial Variability Of Ground Motion Research Articles

A Study on Effects of Subsurface Soil Properties on the Spatial Variability of Ground Motions

A Study on Effects of Subsurface Soil Properties on the Spatial Variability of Ground Motions

The effect of foundation soil behavior on seismic response of long bridges

In this paper, a comprehensive investigation of the dynamic response of a long-bridge subjected to spatially varying earthquake ground motions (SVEGM) is performed based on a proposed analytical model which includes the effect of soil-structure interaction (SSI). The spatial variability of ground motions is simulated by the powerful record generator, SIMQKE II. Modeling of the SSI in the system is simplified by replacing the pile foundations and soil with sets of independent equivalent linear springs and dashpots along the pile groups. One of the most fundamental objectives of this study is to examine how well the proposed model simulates the dynamic response of a bridge system. For this purpose, the baseline data required for the evaluation process is derived from analyzing a 3D numerical model of the bridge system which is validated in this paper. To emphasize the importance of the SVEGM and SSI, bridge responses are also determined for the uniform ground motion and fixed base cases. This study proposing a compatible analytical model concerns the relative importance of the SSI and SVEGM and shows that these effects cannot be neglected in the seismic analysis of long-bridges.

Evaluation of Magnitude, Hypocenter Distance and Subsurface Soil Effect to Spatial Variability of Ground Motions between Adjacent Sites

千葉アレイ観測施設における観測記録およびK-NET，KiK-net観測点でごく隣接する観測点ペアの観測記録を分析することにより，地震規模，震源距離，表層の地盤構造が隣接地点間における地震動の空間変動に与える影響について評価した．地震観測記録のコヒーレンスおよびフーリエスペクトル比の標準偏差を分析した結果，地震規模，震源距離，表層の地盤構造はいずれも地震動の空間変動に影響する傾向が見られた．特に表層の地盤構造が空間変動に与える影響は大きく，地震動のS波速度が小さい地盤ほど変動が大きくなることを確認した．また観測点間の離間距離が大きくなるにしたがい，より深い地盤構造が地震動の空間変動に影響を与えることを確認した．

The indirect boundary integral equation method for the broadband scattering of plane P, SV and Rayleigh waves by a hill topography

The indirect boundary integral equation method (IBIEM) is developed to solve the broadband scattering of plane P, SV and Rayleigh waves by a hill topography, incorporated with the technique of domain-partition construction. It illustrates that the semi-analytical IBIEM can accurately and steadily solve the wave scattering around a steep or a gentle hill both for low and high frequency waves. Numerical results indicate that the seismic response characteristics of a hill strongly depend on the incident angle, wave frequency, the medium stiffness and height-width ratio of the hill. In general, the steep hill has more significant amplification effect on the surface displacement amplitude of the hill, and the displacement peak usually located at the top of the hill for vertically incident SV waves but can also appear near the hill bottom for high-frequency P waves. Multiple standing wave points can be observed for high-frequency wave and steep hill due to the interface effect of scattered waves. Moreover, the high hill has significant shielding effect on the Rayleigh waves. In practice, the notable amplification effect and obvious spatial variation of ground motion must be taken into account in the anti-seismic design of engineering structures in mountainous area.

Risk assessment for a long-span cable-stayed bridge subjected to multiple support excitations

Current probabilistic seismic risk assessment framework does not fully address the impact of the spatial variability of ground motions on long bridges. However, recent studies have highlighted that long-span bridges are more vulnerable to damage due to spatial varying ground motions. To achieve this increased vulnerability, this research presents a reliable risk assessment tool for long-span bridges subjected to spatial varying ground motions. The proposed method includes the following tasks: (i) the simulation of spatially variable ground motions, (ii) creation of numerical bridge model, (iii) generation of probabilistic seismic demand models, (iv) development of component and system fragility curves, (v) estimation of annual exceedance of damage through site-compatible hazard curves, and (vi) computation of repair time. As an illustration, a cable-stayed bridge is selected to highlight the effect of spatial variability of ground motions on the seismic risk and repair time of individual bridge components and bridge system in comparison to uniform excitations. Additionally, a sensitivity analysis is performed to examine the effect of spatial variability parameters and hazard curve-related parameters on the seismic risk of the bridge.

Structural control of cable-stayed bridges under traveling earthquake wave excitation

Post-earthquake damages investigation in past and recent earthquakes has illustrated that the ground motion spatial variation plays an important role in the structural response of long span bridges. For the structural control of seismic-induced vibrations of cable-stayed bridges, it is extremely important to include the effects of the ground motion spatial variation in the analysis for design of an effective control system. The feasibility and efficiency of different vibration control strategies for the cable-stayed bridge under multiple support excitations have been examined to enhance a structure

Investigation of Soil Layers Stochasticity Effects on the Spatially Varying Seismic Response Spectra

The present study investigates the contribution of the stochasticity of a layered soil deposit on the seismic ground motion spatial variability via the random vibration theory. The seismic response evaluation is based on the 1-D shear wave propagation through random media. The soil layers stochasticity is introduced as spatial random fields of mass density and shear modulus of the soil deposit. The stochastic variation of these two parameters results in random natural frequency and damping ratio of an equivalent single degree of freedom oscillator modelling the soil deposit. Under the assumption of stationary Gaussian random process of the incident motion, the ground surface response spectra are obtained from the cross power spectral density of the total ground surface motion. The efficiency and the validity of the present approach are conducted via some applications. Numerical results showed that, the shape of the global coherency is controlled by that of the incident motion. The stochastic aspect of the soil layers causes disturbances in the surface motion and affects the coherency and the variability of the ground surface response spectra.

Quantification of the amplitude variability of the ground motion in Argostoli, Greece. Variability of linear and non-linear structural response of a single degree of freedom system.

The term “spatial variability of seismic ground motions” denotes the differences in the amplitude and phase content of seismic motions. The effect of such spatial variability on the structural response is still an open issue. In-situ experiments may be helpful in order to answer the questions regarding both the quantification of the spatial variability of the ground motion within the dimensions of a structure as well as the effect on its dynamic response. The goal of the present study is to quantify the variability of the seismic ground motion accelerations in the shallow sedimentary basin of Argostoli, Greece, and thereafter to identify its effect on the linear and non-linear elasto-plastic response of a single degree of freedom system in terms of spectral displacements. Around 400 earthquakes are used, recorded by the 21-element very dense seismological array deployed in Argostoli with inter-station spacing ranging from 5 to 160 meters. The seismic motion variability, evaluated in terms of spectral accelerations, is found to be significant and to increase with inter-station distance and frequency. Thereafter, the amplitude variability in terms of spectral displacements, which is indeed the linear response of a single degree of freedom (SDOF) system with various fundamental periods, is compared with the amplitude variability of a SDOF with non-linear elasto-plastic response. The variability of the maximum top displacement of the linear single degree of freedom system is estimated to be on average 12% with larger variabilities to be observed within two narrow frequency ranges (between 1.5 and 1.7 Hz and between 3 and 4 Hz). Such high variabilities are caused by locally edge-generated diffracted surface waves. The non-linear perfectly elasto-platic structural response of the SDOF system shows that although the variability has the same trends as in the case of linear response, it is almost constantly increased by 5%.

Effect of spatial variability of ground motion on non-linear dynamic behavior of cable stayed bridges

This present paper summarizes the main results of incoherence of Spatial Variability of Ground Motion (SVGM) component on the non-linear dynamic behavior of a Mila cable stayed bridge. The Hindy and Novack coherence model is developed for the present study in order to examine the SVGM on bridge responses, Nonlinear bridge responses are investigated in terms of transverse displacements and bending moments along the superstructure and substructure of the study bridge, as well as temporal variations of rotational ductility demands at the bridge piers ends under the incoherence SVGM component. The results are systematically compared with those obtained assuming uniform ground motion. As a general trend, it may be concluded that incoherence component of SVGM should be considered for the earthquake response assessments of cable-stayed bridges.

Effect of spatial variability of ground motion on non-linear dynamic behavior of cable stayed bridges

This present paper summarizes the main results of incoherence of Spatial Variability of Ground Motion (SVGM) component on the non-linear dynamic behavior of a Mila cable stayed bridge. The Hindy and Novack coherence model is developed for the present study in order to examine the SVGM on bridge responses, Nonlinear bridge responses are investigated in terms of transverse displacements and bending moments along the superstructure and substructure of the study bridge, as well as temporal variations of rotational ductility demands at the bridge piers ends under the incoherence SVGM component. The results are systematically compared with those obtained assuming uniform ground motion. As a general trend, it may be concluded that incoherence component of SVGM should be considered for the earthquake response assessments of cable-stayed bridges.

Component fragility assessment of a long, curved multi-frame bridge: Uniform excitation versus spatially correlated ground motions

This paper presents the results of an assessment of the seismic fragility of a long, curved multi-frame bridge under multi-support earthquake excitations. To achieve this aim, the numerical model of columns retrofitted with elliptical steel jackets was developed and validated using existing experimental results. A detailed nonlinear numerical model of the bridge that can capture the inelastic response of various components was then created. Using nonlinear time-history analyses for a set of stochastically generated spatially variable ground motions, component demands were derived and then convolved with new capacity-based limit state models to obtain seismic fragility curves. The comparison of failure probabilities obtained from uniform and multi-support excitation analyses revealed that the consideration of spatial variability significantly reduced the median value of fragility curves for most components except for the abutments. This observation indicates that the assumption of uniform motions may considerably underestimate seismic demands. Moreover, the spatial correlation of ground motions resulted in reduced dispersion of demand models that consequently decreased the dispersion of fragility curves for all components. Therefore, the spatial variability of ground motions needs to be considered for reliable assessment of the seismic performance of long multi-frame bridge structures.

An Investigation on the Parameters Influencing the Pounding in Highway Bridges

The present aim of this study is to investigate the effect of different parameters influencing pounding in highway bridges. Pounding is the result of a collision between two parts of the deck and/or the deck and abutments at the separation distance during the earthquake. In the present study, the period ratio of the adjacent frames, ground motion spatial variation, and soil-structure interaction were considered as the significant parameters influencing the pounding. Accordingly, 144 different models of bridge were generated by changing characteristics of the piers and spans length, and were subjected to non-linear dynamic analysis. The results indicated that ignoring the effects of soil-structure interaction and ground motion spatial variation led to calculate unrealistic responses in the bridges. Finally, it is found that designing bridges including frames with similar or close period is not regarded as an appropriate solution to reduce the pounding effects in the bridges.

Single-Station Sigma for the Iranian Strong Motion Stations

In development of ground motion prediction equations (GMPEs), the residuals are assumed to have a log-normal distribution with a zero mean and a standard deviation, designated as sigma. Sigma has significant effect on evaluation of seismic hazard for designing important infrastructures such as nuclear power plants and dams. Both aleatory and epistemic uncertainties are involved in the sigma parameter. However, ground-motion observations over long time periods are not available at specific sites and the GMPEs have been derived using observed data from multiple sites for a small number of well-recorded earthquakes. Therefore, sigma is dominantly related to the statistics of the spatial variability of ground motion instead of temporal variability at a single point (ergodic assumption). The main purpose of this study is to reduce the variability of the residuals so as to handle it as epistemic uncertainty. In this regard, it is tried to partially apply the non-ergodic assumption by removing repeatable site effects from total variability of six GMPEs driven from the local, Europe–Middle East and worldwide data. For this purpose, we used 1837 acceleration time histories from 374 shallow earthquakes with moment magnitudes ranging from M w 4.0 to 7.3 recorded at 370 stations with at least two recordings per station. According to estimated single-station sigma for the Iranian strong motion stations, the ratio of event-corrected single-station standard deviation (Φ ss) to within-event standard deviation (Φ) is about 0.75. In other words, removing the ergodic assumption on site response resulted in 25% reduction of the within-event standard deviation that reduced the total standard deviation by about 15%.

Erratum to: Implications of non-synchronous excitation induced by nonlinear site amplification and of soil-structure interaction on the seismic response of multi-span bridges founded on piles

This work investigates the effects of soil-structure interaction and spatial variability of seismic motion due to nonlinear site amplification on the seismic behaviour of long multi-span bridges founded on piles. An analysis framework able to include the spatial variation of ground motion induced by specific geological and geomorphological scenarios in the seismic soil-structure interaction analysis of long bridges is adopted, exploiting advantages of the substructure approach. The methodology is applied to a case study constituted by a pile-supported multi-span bridge founded in a soft clay deposit overlaying a stiff bedrock with three different configurations: horizontal, inclined and wedge-shaped. The reference input motion at the outcropping bedrock is represented by a set of real accelerograms and different seismic response models are used to compute site amplification effects, discussing the contribution to the free-field ground motion of both the two-dimensional configuration of the deposit and the nonlinear soil behaviour. The ground motions obtained from the different models are then used for computing the foundation input motion accounting for the pile–soil kinematic interaction; thereafter, inertial interaction analyses are performed on structural models with either fixed or compliant base, considering the non-synchronous seismic actions at the piers foundation. The results, compared in terms of piers head displacements, ductility demand and deck transverse bending moments, finally show the relative importance of bedrock morphology, soil nonlinearity and soil-structure interaction on the structural response.

Partially Nonergodic Empirical Ground‐Motion Models for Predicting Horizontal and Vertical PGV, PGA, and 5% Damped Linear Acceleration Response Spectra Using Data from the Iranian Plateau

We present new ground‐motion prediction equations (GMPEs) to estimate horizontal and vertical strong ground motion intensity measures (GMIMs) generated by shallow active crustal earthquakes occurring within the Iranian plateau. To this end, a dataset containing 688 records from 152 earthquakes with moment magnitudes ranging from 4.7 to 7.4 and Joyner–Boore distances up to 250 km has been used. The effects of the local site condition are taken into account using the time‐averaged shear‐wave velocities in the upper 30 m ( V S 30). We decided not to include the style‐of‐faulting term in the final functional form because the total standard deviation is reduced 10% by removing this term from the functional form. We used a nonlinear mixed‐effects regression to determine the coefficients of the functional form and to separate out the between‐event and between‐station standard deviations from the total standard deviation. Significant standard deviation of site‐to‐site variability demonstrates that the ergodic assumption is not able to account for the spatial variability of ground motions. We introduced random‐effects coefficients to capture regional variations between different tectonic regions of the Iranian plateau, such as Alborz and Zagros, in the regression analysis to investigate the effects of regionalization on GMPEs. The results showed that, although the effects of regional variations for considered regions are negligible at close distances, they are significant at longer distances. The complexity and performance of the final functional form is justified by comparing Akaike and Bayesian information criteria values over many trial functional forms. Moreover, the distribution of between‐event, site‐to‐site, and event‐station corrected residuals demonstrates that no trends are evident, implying satisfactory performance of the proposed GMPEs. Therefore, the derived GMPEs can be employed to predict GMIMs and to do seismic‐hazard assessments within the Iranian plateau. [Electronic Supplement:][1] Lists of the events and stations considered in the final dataset and derived coefficients of the functional form. [1]: http://www.bssaonline.org/lookup/suppl/doi:10.1785/0120160205/-/DC1

地盤不均質性に基づく地震動の空間変動の評価に関する検討

地盤不均質性に基づく地震動の空間変動の評価に関する検討

Stochastic response of suspension bridges for various spatial variability models

The purpose of this paper is to compare the structural responses obtained from the stochastic analysis of a suspension bridge subjected to uniform and partially correlated seismic ground motions, using different spatial correlation functions commonly used in the earthquake engineering. The spatial correlation function employed in this study consists of a term that characterizes the loss of coherency. To account for the spatial variability of ground motions, the widely used four loss of coherency models in the literature has been taken into account in this study. Because each of these models has its own characteristics, it is intended to determine the sensitivity of a suspension bridge due to these losses of coherency models which represent the spatial variability of ground motions. Bosporus Suspension Bridge connects Europe to Asia in Istanbul is selected as a numerical example. The bridge has steel towers that are flexible, inclined hangers and a steel box-deck of 1074 m main span, with side spans of 231 and 255 m on the European and Asian sides, respectively. For the ground motion the filtered white noise model is considered and applied in the vertical direction, the intensity parameter of this model is obtained by using the S16E component of Pacoima Dam record of 1971 San Fernando earthquake. An analytically simple model called as filtered white noise ground motion model is chosen to represent the earthquake ground motion. When compared with the uniform ground motion case, the results obtained from the spatial variability models with partial correlation outline the necessity to include the spatial variability of ground motions in the stochastic dynamic analysis of suspension bridges. It is observed that while the largest response values are obtained for the model proposed by Harichandran and Vanmarcke, the model proposed by Uscinski produces the smallest responses among the considered partially correlated ground motion models. The response values obtained from the uniform ground motion case are usually smaller than those of the responses obtained from the partially correlated ground motion cases. While the response values at the flexible parts of the bridge are totally dominated by the dynamic component, the pseudo-static component also has significant contributions for the response values at the rigid parts of the bridge. The results also show the consistency of the spatial variability models, which have different characteristics, considered in this study.

Shake table tests of steel towers supporting extremely long-span electricity transmission lines under spatially correlated ground motions

Transmission lines from electricity transmission systems crossing wide rivers or canyons can have much longer spans (e.g. longer than 1000m) compared with components from conventional civil structures. Steel towers supporting such extremely long-span transmission lines are unavoidably subjected to spatially correlated ground motions together with the coupling action between supporting towers and transmission lines. However, the influence of spatial variation of ground motions is not considered in current seismic design of the towers supporting extremely long-span transmission lines. This research was focused on an electricity transmission system crossing the 2nd longest river in China (which is also the 5th longest river in the world) to address the influence of spatial variation of ground motions on seismic response of the towers supporting extremely long-span transmission lines. A reduced-scale experimental model of the prototype was tested using shake tables. Spatially correlated ground motions generated taking into account the wave passage effect, the effect of coherency loss and the effect of local site conditions were used as inputs of the shake table tests. It was found that the spatially correlated ground motions can significantly amplify tower responses and such an effect should not be neglected in seismic analysis and design. Based on the test database, an empirical model was proposed to modify the acceleration response, member stress response and top displacement responses of the towers supporting extremely long-span transmission lines from uniform ground motions as conservative estimates of responses of the system under spatially correlated ground motions with the same magnitude.

Computing spatial correlation of ground motion intensities for ShakeMap

Modeling the spatial correlation of ground motion residuals, caused by coherent contributions from source, path, and site, can provide valuable loss and hazard information, as well as a more realistic depiction of ground motion intensities. The U.S. Geological Survey (USGS) software package, ShakeMap, utilizes a deterministic empirical approach to estimate median ground shaking in conjunction with observed seismic data. ShakeMap-based shaking estimates are used in concert with loss estimation algorithms to estimate fatalities and economic losses after significant seismic events around the globe. Incorporating the spatial correlation of ground motion residuals has been shown to improve seismic loss estimates. In particular, Park, Bazzuro, and Baker (Applications of Statistics and Probability in Civil Engineering, 2007) investigated computing spatially correlated random fields of residuals. However, for large scale ShakeMap grids, computational requirements of the method are prohibitive. In this work, a memory efficient algorithm is developed to compute the random fields and implemented using the ShakeMap framework. This new, iterative parallel algorithm is based on decay properties of an associated ground motion correlation function and is shown to significantly reduce computational requirements associated with adding spatial variability to the ShakeMap ground motion estimates. Further, we demonstrate and quantify the impact of adding peak ground motion spatial variability on resulting earthquake loss estimates.

Influence of spatial variation of ground motions on dynamic responses of supporting towers of overhead electricity transmission systems: An experimental study

An overhead electricity transmission system typically consists of a series of steel lattice towers supporting transmission lines (referred to herein as the tower-line system). Recent major earthquakes suggest that the tower-line system constructed based upon existing knowledge can be vulnerable. A tower-line system typically extends over a large region and it may be subjected to spatially varying ground motions during an earthquake event. The influence of spatial variation of ground motions is not explicitly considered in current design of the system and very limited research (particularly, experimental research) has been completed in this regard. A two-phase experimental program was conducted using an array of three shake tables to quantitatively assess the respective impact of three factors causing non-uniform excitations (namely, the wave passage effect, the ground motion coherency loss effect, and the local site effect) on dynamic response of the supporting towers. Test results show that neglecting the wave passage effect may result in underestimates of peak dynamic responses of the towers. Additionally, it is found that non-uniform ground motions with a higher degree of coherency loss tend to amplify tower dynamic responses. Furthermore, test results reveal that when the soil at the bases of adjacent towers becomes more flexible, the resulting spatially varying ground motions cause larger tower dynamic responses.