Input Ground Motion Research Articles

Seismic response of large-span spatial structures under multi-support and multidimensional excitations including rotational components

To achieve rational and precise seismic response predictions of large span spatial structures (LSSSs), the inherent non-uniformity and multidimensionality characteristics of earthquake ground motions should be properly taken into consideration. However, due to the limitations of available earthquake stations to record seismic rotational components, the effects of rocking and torsional earthquake components are commonly neglected in the seismic analyses of LSSSs. In this study, a newly developed method to extract the rocking and torsion components at any point along the area of a deployed dense array from the translational earthquake recordings is applied to obtain the rotational seismic inputs for a LSSS. The numerical model of an actual LSSS, the Dalian International Conference Center (DICC), is developed to study the influences of multi-support and multidimensional excitations on the seismic responses of LSSSs. The numerical results reveal that the non-uniformity and multidimensionality of ground motion input can considerably affect the dynamic response of the DICC. The specific degree of influence on the overall and local structural displacements, deformations and forces are comprehensively investigated and discussed.

Structural dynamic response of underground structure with vertical ground motion input

Since many cases of structural damage in past earthquakes have been attributed to strong vertical ground motion, our understanding of vertical seismic load effects and their influence on seismic performance of subway station structure is limited. In this study, the Daikai subway station is taken as a typical example. A two-dimensional finite element model of both soil and structure was established using finite element software ABAQUS. Two input approaches of ground motion are considered, including the horizontal component alone and the vertical and horizontal motions simultaneously. Four groups of ground motion records are selected according to the site type of this station and scaled to the strong intensity which can make the station damage. Results show that the vertical seismic load increases the axial force of the column component apparently, while horizontal seismic load has little effect on axial compression ratio.

Considering SSI by an equivalent linear method for nonlinear analysis of concrete moment frames

This paper evaluates the effects of Soil-Structure Interaction (SSI) on the seismic response of multi-story concrete frames subjected to earthquake ground motions. To calculate the structural response, an iterative Wave-Propagation (WP) based method has been used. The exact solution of SSI problems, in addition to high computational costs, due to general complexity of finite element analysis, require an advanced computer system. According to the method proposed herein, buildings were modelled as an extension of the layered soil media by considering each story as a layer in the WP path. The motion in each layer could be determined from the motion in any other layer. This permitted to use an operation called deconvolution, which could be considered to investigate the effect of substructure soil layers on the structural response. One of the capabilities of WP method is to facilitate the computation of bedrock motion from a known free surface motion and consequently determination of correct foundation input motion. While most of the current methods assume constant values for basic parameters such as; stiffness and damping, in the present method in order to investigate the nonlinear behaviour, stiffness and damping are proportional to the displacement level of soil layers or structural floors. The computational speed and accuracy of the proposed method are also verified by a benchmark frame analysis. Also, based on the Park-Ang Damage Index, this paper evaluated the measures of damage (the relationship between earthquake characteristics (PGA) and damage level) which govern structural degradation and/or collapse obtained from different well-known destructive earthquakes.

A computationally efficient framework for rotation dependent and rotation independent intensity measures

AbstractOrientation of a structure in a site is generally known but not the direction of maximum shaking during a future seismic event. Two different types of intensity measures (IMs) are usually used to approximately account for this directionality effect, namely, the rotation dependent such as RotDxx and GMRotDxx and the rotation independent such as RotIxx and GMRotIxx. Rotation dependent IMs are presently constructed by performing time history analysis for all possible orientation (usually @ 1 degree) of the input ground motion set followed by picking the xx‐percentile spectral ordinate. In other words, the construction of RotDxx spectrum requires a set of 180 time history analysis of an oscillator per spectral ordinate. Similarly, the construction of GMRotDxx requires time history analysis of an oscillator against 90 pairs of orthogonal components per spectral ordinate. This paper presents a framework that enables the construction of rotation dependent IMs by performing time history analysis against a pair of as‐recorded components with some nominal supplemental processing. This reduces the computational cost more than 90% when compared with the state of the art. Rotation independent IMs are defined through finding out the rotation that minimizes the error (often termed as the penalty function) with respect to the target spectra of associated rotation dependent IM as the benchmark. Resulting rotation independent IMs show somewhat sensitivity on the maximum time period used in spectral representation. This paper presents an alternate definition (involving scaling and rotation) for rotation independent IMs that nearly eliminates such sensitivity.

Comparative seismic fragility assessment of mid-rise steel buildings with non-buckling (BRB and SMA) braced frames and self-centering energy-absorbing dual rocking core system

During the past two decades, many emerging smart lateral force-resisting systems have been proposed for achieving better post-earthquake recoverability than the conventional ones in steel buildings. The Self-centering Energy-absorbing Dual Rocking Core (SEDRC) system has been developed recently by the authors for achieving excellent self-centering behavior with large deformability in steel buildings. This paper intends to investigate the benefit of steel buildings using the SEDRC system compared to that using other emerging smart lateral force-resisting system (e.g., shape memory alloy braced frame (SMABF)) and conventional one (e.g., buckling restrained braced frame (BRBF)). For this purpose, first, three six-story steel buildings with BRBFs, SMABFs, and SEDRC systems, were designed to obtain similar inter-story drifts subjected to the design basis earthquakes. Second, nonlinear static and dynamic analyses were conducted to preliminarily study the seismic performance of the designed buildings. Then, incremental dynamic analyses (IDAs) with far-field and near-fault ground motion inputs were conducted to study the seismic responses of the designed buildings under different seismic hazard levels. Finally, the results from the IDAs were analyzed in a probability framework. The inter-story drifts, residual inter-story drifts, and floor acceleration responses of the considered systems were analyzed to investigate the seismic fragility of the structural and nonstructural components in the designed buildings. The analysis results indicate that the SEDRC system can achieve the best structural collapse-resistant capacity under both far-field and near-fault ground motions because of the excellent deformability. The SMABF and SEDRC system show better seismic resilience performance by controlling the residual inter-story drift responses than BRBF. Moreover, the SEDRC system can obtain a smaller exceedance probability of damage states associated with nonstructural components depending on floor acceleration responses than SMABF because of uniform inter-story drift responses.

On the assessment of seismic performance of bridge piers on caisson foundations subjected to strong ground motions

AbstractSubstructure method is widely used to evaluate the seismic performance of caisson foundations supporting bridge piers subjected to strong ground motions, mainly because of its simplicity. However, the strongly‐simplifying assumption of linear viscous‐elastic behaviour for the foundation soil limits its applicability to flexible systems subjected to low‐intensity earthquakes, for which irreversible strains and pore water pressure build‐up are not anticipated. Furthermore, lumped‐parameter models are typically adopted in calculations in which soil‐foundation compliance is reproduced via dynamic impedance functions, whose dependency on frequency of excitation is often neglected. Modification of free‐field motion leading to foundation input motion (FIM), due to the presence of caisson embedment, is also mostly ignored. The influence of these simplifying assumptions on the seismic performance of bridge piers on caisson foundations is assessed in this paper through a parametric study, where soil‐caisson‐bridge pier‐deck systems differing in geometric and mechanical properties are subjected to real seismic records. Dynamic analyses were carried out in the time domain with the finite element method, using a 3D continuum and a lumped‐parameter model for the foundation soil. In the 3D model both the linear viscous‐elastic and the nonlinear soil behaviour were assumed, while linear viscous‐elastic behaviour was assumed in the lumped‐parameter model. The influence of inelastic soil behaviour was assessed by comparing the seismic performance of the systems obtained with the 3D model, while the role of FIM was evaluated by comparing the results of the dynamic analyses computed assimilating the soil to a linear elastic medium.

Seismic wave input method for high earth dams considering the transmission amplification effect of the bedrock–overburden interface

The method for determining the input ground motion of concrete and earth dams on bedrock is not applicable to earth dams on overburden soil due to the transmission amplification effect of the seismic waves at the bedrock–overburden interface. Moreover, the free-field and artificial boundary parameters of the overburden foundation cannot be calculated using analytical methods because of the nonlinear characteristics of the soil under strong earthquakes. Therefore, the nonlinear variations in the artificial boundary parameters and the equivalent load with the dynamic shear strain of the soil must be considered. First, the influencing factors of the transmission amplification effect under different types of waves are analyzed. Second, a method for determining the input ground motion with consideration of the transmission amplification effect is developed. Then, the free-field response at the truncation boundary of the overburden soil is obtained on the basis of the shear box model. The equivalent nonlinear viscoelastic artificial boundary element attached to the boundary soil element of the overburden foundation is generated; the stiffness and damping matrix of the viscoelastic artificial boundary element change with the shear strain of the boundary soil element. Subsequently, a nonlinear wave input method suitable for overburden foundations is established. Finally, the error degrees of the seismic response of the overburden foundation–dam system with respect to the far-field artificial boundary are compared using the methods of nonlinear wave input, fixed and free boundaries, and the viscoelastic boundary combined with the equivalent inertial force input. The efficiency and accuracy of the nonlinear wave input are high. The influence degree of the ground motion considering the transmission amplification effect on the response of the earth dam is also analyzed. Moreover, the influence mechanisms of the dynamic parameters of the overburden soil and bedrock on the transmission amplification effect and the seismic response of the overburden–dam system are discussed.

Response evaluation and analysis using impulse of base‐isolated buildings during a collision with retaining wall

AbstractThe performance of base‐isolated buildings during collision with a retaining wall is examined via shaking table experiments, and a simplified numerical simulation method is proposed to predict structural response. Input ground motions are scaled to various amplitudes to examine the pounding effect of the superstructure under different collision velocities. The influence of wall rigidity on the superstructure response is examined by replacing rubber member attachments on the retaining wall with pads that have different rubber hardness values. Factors influencing the response of the superstructure during collision are identified from experimental results. A numerical simulation method that uses impulse forcing function to replace the collision simulation is evaluated. Time history analysis of a lumped‐mass model considering an impulse at the expected time and location of the collision are shown to efficiently predict the maximum floor acceleration and story shear force of the superstructure during collision without employing a collision analysis model. This research contributes towards developing simplified procedures for estimating the effects of collision in base isolated structures.

Dynamic Soil–Structure–Equipment Interaction (II): Closed-form Analytical Solution for Incident Plane SH-wave Based on Flexible Foundation Model

ABSTRACT We study a two-dimensional structure–equipment model standing on a semi-circular flexible foundation embedded in the homogeneous half-space subjected with plane SH-waves, as a companion to the previous study of dynamic soil–structure–equipment interaction based on rigid-foundation model. We obtain the analytical solution of the system response that demonstrates the influence of foundation stiffness on the response of the soil–structure–equipment interaction system. The results show that, after considering the influence of the foundation stiffness, the effective foundation input motion is amplified, and the maximum amplification percentage can reach to 32.8%. The peak of the floor relative response is amplified by 18.5% and the frequency corresponding to the peak shifts to the lower range with the maximum shift percentage 16.1%. The peak of the equipment relative response can be amplified by 76.7%. There is significant relative deformation in the foundation when the wave is obliquely incident. The equipment-structure frequency ratio and mass ratio are important parameters that reflect the dynamic characteristics of the soil-structure–equipment system. When the equipment-structure frequency ratio is between 0.5 and 1.0, the dynamic characteristics of the structure–equipment system change. We propose a parameter (Ψ) to measure the extent to which the equipment participates in the soil–structure interaction.

A Method to Find an Appropriate Input Motion Using a Given Motion on Ground Surface

Some seismic design codes require to determine earthquake motions at ground surface, although we need sometimes a motion at engineering basement of ground. For example, it is necessary for optimal design of structures to introduce effects of soil-structure interaction and to input ground motions to engineering basement, which should satisfy the motions at surface required by a design code. This is simplified as a problem to find a possible input signal of a known non-linear-single-input and single-output (SISO) system with a given output signal. This study proposes a very simple algorithm to find a possible input signal for a known SISO system satisfying a given output signal from the system. The proposed algorithm searches for a possible input signal, whose system response is similar in shape to the given output signal, without any system identifications. Thus, it is not an inversion technique; it is an algorithm used to search for one of the possible input signals. Herein, the proposed algorithm is described and its performance is demonstrated using numerical examples.

Seismic performance of two classes of earth dams

AbstractIn the last decades, Italy has experienced several strong earthquakes, triggering the attention on the high seismic risk associated with the potential instability of large existing earth dams. Under intense seismic events, dam response depends on various factors including the geometry and the type of water retention scheme. This paper explores the influence of these factors, comparing the seismic performance of two idealized earth dams: a homogeneous dam and a zoned dam. The simplified models adopted in the analyses are representative of these two classes of earth dams, often adopted in the sixties to design various dams located in southern Italy. The idealized models, here examined in detail, have comparable heights and foundation soil conditions, differing in terms of water retention scheme. The seismic response of the dams was evaluated through time‐domain nonlinear dynamic analyses in which the same input ground motion, represented by real‐time histories, was applied at the base of the models. It is shown that for highly inelastic systems, such as those at hand, duration of input motion should be accounted for in addition to compatibility criteria with a design elastic response spectrum. Also, different slopes of dam flanks, shear strength mobilization at the end of dam construction and high stiffness contrast at the shells‐core contacts are shown to result in different deformation patterns at the end of earthquake loading. Some peculiarities in the behavior of these classes of earth dams are highlighted in the paper, providing a guidance for the rational assessment of their seismic performance.

Probabilistic solution for the seismic sliding displacement of slopes in Greater Vancouver

Greater Vancouver has the highest seismic risk in Canada with significant potential to cause earthquake-induced hazards including seismic slope failures. The available methods to predict seismic slope failures in practice are using seismic displacement prediction models (SDPMs) based on Newmark sliding displacement analogy. Seismically induced permanent displacements are calculated using earthquake hazard parameters and soil strength parameters represented by yield acceleration of the slope (ky). The primary source of uncertainty in assessing the likely performance of a slope during an earthquake is the input ground motion parameters along with the uncertainties associated with various SDPMs available. Considering the seismicity sources in Greater Vancouver and the latest updates in SDPMs, in this paper a probabilistic solution for the seismic sliding displacement of slopes for the region is presented. The uncertainties in input seismic parameters and SDPMs are both taken into account and probabilistic displacements are determined for different values of ky. Displacement seismic hazard curves are an output from the probabilistic evaluation of the problem, which are additionally presented as uniform displacement hazard spectrum (UDHS) for a 2% probability of exceedance in 50 years hazard level. The current procedure for seismic slope evaluation in Greater Vancouver is reviewed and critiqued with an illustrative example and the alternative probabilistic method is recommended for engineering practice.

Spectral Correlation Between the Non-uniform Input Ground Motion and the Dynamic Response of the Surrounding Rocks of a Long Tunnel

To generate several different time-history curves of ground motion, which have the ability to reflect local site effects, such as the traveling-wave effect and attenuation effect, for long circular tunnels, a random method for the synthesis of ground motion is employed. The dynamic response of the surrounding rocks of a long tunnel under the uniform and non-uniform input ground motions is analyzed using the vertical input method. Based on this, the spectral correlation between the input ground motion and the dynamic response of the surrounding rocks is investigated through the coherence function. Unlike the results under the uniform input ground motion, the transmission coefficient of the surrounding rock monitoring points of each section along the axial direction of the tunnel under the non-uniform input ground motion varies in the main frequency band. In addition, at the same frequency, the transmission coefficient of the monitoring points gradually increases along the tunnel’s axial direction, but the increasing magnitude is variable. The correlation function of each monitoring point along the tunnel axis under the non-uniform input ground motion generally decreases with frequency. Moreover, the further the distance from the tunnel entrance, the faster the attenuation of correlation. The maximum coherence coefficient and the axial distance of the tunnel are basically in conformity with the quadratic polynomial relationship. However, the maximum coherence coefficient of the non-uniform input ground motion is considerably greater than that of the uniform input ground motion. The gap increases with the distance, and the maximum gap is about 0.15.

Nonlinear seismic responses of the soft rock site for nuclear power plants

With the development of the nuclear power industry, high-quality bedrock nuclear power stations have become scarce. Thus, it is inevitable that nuclear power stations will be built on soft rock sites. This involves the issue of the seismic safety of soft rocks as sites for nuclear power plants. Based on the soft rock site involved in the site selection for nuclear power plants, this paper uses ground motions with different spectral characteristics. Using the DEEPSOIL software for soil layer analysis and the Abaqus finite element software, one-dimensional equivalent linear and nonlinear methods are employed to compare and analyze the characteristics of the nonlinear seismic response of the soft rock site for nuclear power plants. The soft rock site has a significant amplification effect on the input ground motion. The equivalent linear and nonlinear methods have different levels of influence on the acceleration response of the soft rock site. Under the action of far-field earthquakes, surface acceleration and its response spectrum, which were calculated using the abovementioned methods, are relatively close. As the input ground motion intensity increases, the nonlinearity of the soft rock increases, and the amplification effect of the soft rock site weakens. However, the peak ground acceleration of the surface obtained using the two methods gradually differs. Moreover, the nonlinearity is greater than the equivalent linear. Therefore, the nonlinear characteristics of soft rocks should be considered in the site selection and seismic analysis of nuclear power plants.

Nonlinear Seismic Response Characteristics of CAP1400 Nuclear Island Structure on Soft Rock Sites

CAP1400 nuclear island structure is an advanced and novel nuclear power plant structure. In order to explore the seismic response characteristics of CAP1400 nuclear island structure on soft rock sites, a three-dimensional refined nonlinear seismic response analysis model was established for a soft rock foundation-nuclear island structure system using ABAQUS software. The influences of the input ground motion intensity and the frequency spectrum characteristics on the acceleration, relative displacement, and floor response spectrum, as well as the critical shear wave velocity of nonbedrock sites for CAP1400 nuclear island structure, were proposed. The results suggested that the increasing amplitude of the peak acceleration and relative displacement of nuclear island structure decreased as the soft rock site entered a nonlinear state, and the high-frequency components of the input ground motion became more abundant. Specifically, the earthquake response was the largest at the cooling water tank on the top of the shield building, which was the focus of the seismic research on nuclear island structure. Due to the influence of the ground motion frequency spectrum characteristics and the nonbedrock site effect, the peak acceleration, peak relative displacement, and acceleration response spectrum of the nuclear island structure showed different changing trends for the near-field and far-field ground motions. Based on the influence of the site shear wave velocity on the seismic response of nuclear island structure, it was recommended that the critical shear wave velocity of nonbedrock sites for CAP1400 nuclear island structure should be 1250 m/s, and the nuclear island structure-foundation dynamic interaction could be ignored at this time. The research conclusions could provide some technical support and theoretical basis for the construction and seismic performance research of CAP1400 and other nuclear power plants.

A coupled numerical approach to simulate the effect of earthquake frequency content on seismic behavior of submarine tunnel

Roughly 90% of all natural vibrations have epicenters in offshore zones and may cause destruction of submarine and floating structures. Such excitations can influence the safe performance of facilities set up on the seabed, like tunnels, jacket legs and subsea oil pipelines. Some researches on this theme have been carried out to demonstrate the importance of seaquake analyses and their effects have been underlined. The present study intends to numerically simulate a two-dimensional fluid-structure interaction (FSI) problem in order to examine the dynamic response of submarine tunnel under real horizontal earthquakes. Pressure is considered as independent nodal variables to represent the fluid flow effects and the induced time-dependent acceleration in porous medium equation is incorporated in the analysis and the tunnel shell is considered as flexible. This work highlights the importance of the input ground motion frequency content that governs the development of the induced seismic stress/strain around the lining of the tunnel. The results demonstrate that for deep sea the increment rate of the circumferential stress caused by surface gravity waves is below 7% when compared to the no-wave interface condition. Moreover, it is confirmed that long-period record may amplify the overall response of the system (up to 60%) specially the lateral and vertical displacements, as well as the principal stress to a lesser extent. The developed numerical model can attend to further analysis of tunnels embedded in a half-space in conjunction with fluid undergoing the severe long-period earthquakes.

A Winkler-based model for inelastic response of soil–structure systems with embedded foundation considering kinematic and inertial interaction effects

In this paper, a parametric study is carried out on the ductility and displacement demands of structures with embedded foundation considering nonlinear soil–structure interaction (SSI). The soil–embedded foundation interaction is simulated using the beam on Winkler foundation concept. The bilinear behavior is also assumed for the equivalent single-degree-of-freedom (SDOF) structure. Effect of key parameters including fundamental period, level of inelasticity, soil flexibility, embedment ratio, and slenderness ratio on the seismic demands is examined. A family of soil–structure systems are analyzed subjected to a group of 33 ground motions recorded on alluvium deposits. Both sway and rocking components of foundation input motion are applied to the system. Results reveal that foundation embedment generally increases the ductility demand of the SDOF system. Moreover, the kinematic interaction (KI) effect on ductility demands is more prominent for structures with deep embedment ratios. As the aspect ratio increases, the role of rocking input motion on ductility and displacement demands becomes more noticeable. It is also concluded that the nonlinear SSI can increase the displacement demand of interacting systems as compared to the equivalent linear soil model. Based on the obtained results, a simplified expression is proposed to estimate the displacement ratio for soil–structure systems with embedded foundation considering KI effects.

Liquefaction performance and deformation of slightly sloping site in floodplains of the lower reaches of Yangtze River

Serious seismic damage is expected owing to the occurrence of lateral displacement following soil liquefaction after a strong earthquake. Currently, there is a need for increased research on the lateral deformation occurring inside a liquefied foundation and its spatial distribution characteristics. Considering the initial geostatic stress of a slightly sloping site, a finite-element model that reflects the liquefaction-induced slippage of the liquefiable site with a slightly inclined ground was designed. According to the results, the steeper the ground slope is, the weaker the soil liquefaction in the slope body will be. As the ground inclination and the intensity of input ground motion vary, the lateral slippage of the liquefiable soil layer transforms from the liquefaction-induced slipping along a single sliding surface to that along multiple sliding surfaces. The dominant lateral displacement of liquefiable soil layers in the slope body changes as well, from liquefaction-induced slippage of soil to largely large single-direction accumulated shear deformation of softening soil, as a result of the drastically reduced intensity of the liquefiable soil. In addition, a similar linear relationship is found among the maximum lateral displacement, the ground settlement near the slope crest, and the ground uplift near the slope foot.

Influence of frequency content of ground motions on seismic fragility of equipment in nuclear power plant

The objective of this study is to investigate the influence of frequency content of ground motions on the seismic fragility of nuclear power plant (NPP) equipment. Transfer functions of a steam generator, a reactor vessel and an auxiliary building of a Korean APR1400 type NPP are extracted from finite element models. The transfer functions are used to investigate the influence of frequency contents of input ground motion on the seismic fragilities of the equipment. A methodology is proposed to transform fragility functions that are defined in terms of peak ground acceleration (PGA) of the design response spectrum to those that are defined in terms of realistic response parameters. The method is applied to a three-dimensional finite element model of the auxiliary building as a case study. The results show that all equipment has a higher probability of failure when the auxiliary building is subjected to ground motions with high-frequency content.

Influence of trapped soft/stiff soil layer in seismic site response analysis

As the ground response analysis serves an integral part of site specific seismic hazard study to obtain design ground motion, a proper and accurate estimation should be of prime importance. The paper presents a study on the effect of trapped soft and stiff soil layer on equivalent linear ground response analysis implemented in computer program STRATA. For this purpose, the concept of normally stiff and inversely stiff soil profiles have been introduced. The study clearly indicates the higher impact of a trapped soft soil layer profile, i.e., inversely stiff soil profile with soft layer, in comparison with trapped stiff layer profile, i.e., inversely stiff profile with stiff layer. For low to moderate ground motions, as the depth of the trapped soft layer increases, peak amplification and peak frequency reduces, and for high intensity input ground motion, significant reduction only in the peak frequencies is observed. On the other hand, as the depth of trapped stiff soil layer increases the outcome of ground response analysis remain quite similar. Peak transfer function, peak frequency, peak spectral ratio and peak spectral acceleration are found to exhibit a COV ≤ 60 to 100% for all combinations of IS-Soft profiles, whereas, IS-Stiff profiles exhibit a maximum COV ≤ 15 to 25% for all the considered input ground motions. Computed normalized-root mean square error (Norm-RMSE) values also clearly indicate the higher deviations in the ground response analysis for different combinations of trapped soft soil profiles, from that of normally stiff profile, whereas, trapped stiff layer profiles show lower deviations in the Norm-RMSE values. The profiles with trapped soft layer exhibit a Norm-RMSE value ≤0.8, whereas, trapped stiff layer profiles exhibit a Norm-RMSE value ≤0.2, which clearly demonstrate the large deviations in the outcome of ground response analysis in case of trapped soft layer profiles.