Near-fault Ground Motions Research Articles

Duration Effects of Near-Fault Ground Motions on Structural Seismic Responses

ABSTRACT This study reveals the duration effects of near-fault ground motions with distinct long-period pulse and without pulse on structural seismic responses and damage. To this end, several velocity-based duration measures (DMs) are proposed according to the characteristic of abundant low-frequency content in near-fault records, and non-structural compound intensity measures (IMs) incorporating DMs are suggested. Moreover, the correlations between the IMs and DMs of near-fault ground motions, as well as those between the IMs and DMs of ground motions and the seismic responses (i.e. maximum displacement D max, input energy E I and hysteretic energy E H) of bilinear single-degree-of-freedom (SDOF) systems, are scrutinized. It is shown that the acceleration-based absolute uniform durations present strong correlation with the acceleration-related IMs, while the velocity-based absolute uniform durations highly correlate with the velocity- and displacement-related IMs. In the short-period region, the acceleration-based absolute uniform duration U 0.10 g is well correlated with the seismic responses, while in the medium-, medium-long- and long-period regions, the velocity-based absolute uniform duration U v10 exhibits strong correlation. Additionally, the compound IMs combining with uniform durations (PGA•U 0.10 g and PGV•U v10) are the competent indices to quantify duration effects of near-fault records. The similar observations for duration effects are validated by four representative frame buildings considering Bouc-Wen hysteretic behavior without or with degradation of strength and stiffness.

Shaking table test of a half-scale three-story non-ductile RC building subjected to near-fault ground motions: Experimental and numerical modeling

This paper presents the shake-table tests of a half-scale, three-story, reinforced concrete (RC) vertically irregular non-ductile structure conducted at the newly constructed NCREE Tainan laboratory. The test specimen's structural components represent the configuration of an actual 17th-story building that collapsed during the 2016 Meinong earthquake. The collapsed structure was vertically irregular and had a non-ductile design. Existing old R.C. structures are vulnerable to significant earthquake events, as the columns in those structures have insufficient reinforcement and confinement, often leading to great seismic demands. Therefore, this study aims to understand the seismic behavior of vertically irregular non-ductile R.C. structures subjected to an increasing intensity of near-fault ground motion from the 1999 Chi-Chi earthquake. The structure remained stable throughout the tests, as only the columns at the first story sustained maximum damage. Non-linear time history analyses were performed to verify existing modeling parameters, i.e., ASCE 41-17 and the TEASPA model. The results from the analysis provided valuable insight into the ability of the existing modeling parameters to capture local and global responses of vertically irregular non-ductile structures. Furthermore, it was observed from the analysis results that using the TEASPA model provided better correlations between analytical and experimental results than the ASCE 41-17 model.

Structural and nonstructural damage assessment of steel buildings equipped with self-centering energy-absorbing rocking core systems: A comparative study

Self-centering energy-absorbing rocking core (SCENARIO) systems are emerging aseismic structural systems developed for obtaining excellent self-centering and collapse-resistant performance with negligible residual inter-story drifts in steel buildings under strong earthquakes. Despite its demonstrated superior structural performance, the nonstructural performance of SCENARIO systems, which is another essential consideration in seismic resilience evaluation of building structures, has not been well studied. For filling this research gap, this paper aims to make the original contributions by systematically assessing the structural and nonstructural damage of steel building structures equipped with SCENARIO systems under earthquakes and finding the efficient solution for enhancing the structural and nonstructural damage-control performance of SCENARIO systems. Two types of SCENARIO systems, including the SCENARIO system with friction spring dampers (denoted as SCENARIO-F system) and the SCENARIO system with hybrid dampers (denoted as SCENARIO-H system), were considered in this research. Three prototype steel buildings with SCENARIO systems were designed to achieve the same target displacement under design basis earthquakes (DBE). Incremental dynamic analyses (IDAs) were performed to study the seismic responses of the designed buildings subjected to far-field and near-fault ground motions with different intensities. The collapse-resistant performance, structural damage, post-earthquake repairability, and nonstructural damage of the designed buildings were investigated via seismic fragility analyses based on IDA results. Seismic fragility analysis results indicate that the SCENARIO-H systems show better performance than the SCENARIO-F systems in controlling structural and nonstructural damage under both far-field and near-fault earthquakes with high intensities. Benefiting from excellent self-centering capacity, the buildings equipped with SCENARIO-F and SCENARIO-H systems can show excellent post-earthquake repairability under strong earthquakes.

Multi-pulse decomposition for nonlinear seismic analysis of structural systems

Large sympathetic, resonance-like, structural behaviour to earthquake excitations with analogous frequency content often plays a critical role in determining its maximum seismic response. Earthquake excitation typically contains a broad spectrum of non-stationary frequency content, wave packets, which are difficult to observe from the recorded time series. Therefore, identifying the root cause of large responses (which act sympathically with the input but do not achieve full-resonance) of a structure is problematic. Hence, this paper proposes a new multi-pulse decomposition method of ground motions, through which components of a ground motion within a specific period range are determined. In this method, a ground motion is approximated with a Gauss-Fourier wave packet series. The decomposed components, wave packets, contain information about its time-position, frequency, amplitude, pulse width and phase angle. Unlike the Ricker (Morlet) wavelet the Gauss-Fourier wave packet is not limited to symmetrical pulses. One ensemble of 40 near-fault ground motions and one ensemble of 44 far-fault ground motions are used to demonstrate the application and efficiency of the proposed method. The method is shown to be precise in reconstructing the original ground motion using its decomposed components. It is also concluded that the method is accurate in replication of elastic and inelastic response spectra of ground motions within a specific period range. It is demonstrated that for some structure/ground motion combinations, only a few Gauss-Fourier components are required to faithfully describe response behaviour. This highlights that, for these systems, most of the recorded earthquake time series acts like noise on a much simpler wave-packet signal.

Improvement of Stochastic Green’s Function Method for 3D Broadband Ground-Motion Simulation

Abstract Inputting reasonable ground motion is very significant in the seismic design of engineering structures under near-fault earthquakes. At present, the stochastic Green’s function method has been successfully applied to the simulation of moderate to high-frequency ground motions, but its accuracy is poor for low-frequency ground-motion simulation. In this study, an improved stochastic Green’s function method that is used to simulate broadband ground motion is established by considering the variation of the correlation of phase spectra among small earthquakes in different subfaults with the frequency and distance as well as the variation of the radiation pattern with the frequency and distance. Taking the 1994 Northridge earthquake in America and the 2013 Lushan earthquake in China as examples, the simulation results by the improved stochastic Green’s function method are compared with observed ground-motion records. The results show that considering the influence of near-field and intermediate-field terms has a little effect on the accuracy of ground-motion simulation. The directivity effect of near-fault ground motion can be reflected to a certain extent by considering the variation of the correlation of phase spectra among small earthquakes in different subfaults with the frequency and distance. Considering both the variation of the correlation of phase spectra among small earthquakes in different subfaults with the frequency and distance and the variation of the radiation pattern with the frequency and distance, the simulated acceleration response spectra generally show good agreement with the observed records. Therefore, the improved stochastic Green’s function method proposed in this study can simulate the broadband ground motion effectively.

Probabilistic seismic demand model of UBPRC columns conditioned on Pulse-Structure parameters

Unbonded prestressed reinforced concrete (UBPRC) column has attracted the attention of many researchers in bridge seismic design because of its small residual displacement. However, the research on the mechanism and quantitative method of its seismic response under near-fault impulse ground motions is still insufficient. On this basis, a quantitative probabilistic mathematical model was established to evaluate the seismic demand of UBPRC columns with different structural parameters subjected to earthquakes with varying pulse parameters. Firstly, in order to describe the seismic response of UBPRC columns under pulse-like ground motions accurately and conveniently, a new parameter (xT) composed by fundamental period (T1) and pulse period (Tp) was proposed. It is found that Gaussian function can well describe the relationship between the newly defined parameter and seismic demand, based on a set of seismic response databases of UBPRC columns with different heights under impulse ground motions. Secondly, considering that the peak pulse velocity (Vp) has a significant impact on the seismic response of bridge structures, the influence of Vp on the parameters of Gaussian function was studied, and the functional relationship between these parameters and Vp was established. Finally, a novel probabilistic seismic demand model (PSDM) considering structure parameters and pulse parameters was established, and its accuracy is verified. The quantitative mathematical model with multi pulse-structure coupled parameters can be adopted to conduct the seismic fragility analysis of UBPRC columns. The verification and application indicate that the proposed PSDM model can quickly and accurately estimate the seismic demand of UBPRC column under different near-fault impulse ground motions.

Improved endurance time analysis for seismic responses of concrete dam under near-fault pulse-like ground motions

This study aimed to modify the Endurance Time Analysis (ETA) method using a novel prediction model to evaluate the seismic demands of concrete dams subjected to near-fault pulse-like excitations. To this end, a two-dimensional concrete gravity dam–reservoir–foundation system was employed in the numerical model. Twenty as-recorded pulse-like (original) ground motions were decomposed into representative low- and high-frequency components using the continuous wavelet transform method. The contributions of these components toward the dynamic responses under different intensity measures were investigated, based on an Incremental Dynamic Analysis (IDA). The effect of Peak Ground Acceleration (PGA) on the responses was considered, and the response prediction model was further modified, including some dimensionless parameters. Coupled with the response results under residual motion obtained via the ETA method, the estimated response under pulse-like motion can be derived using the proposed ETA prediction method. Based on a comparison of the average results of the IDA and improved ETA methods under pulse-like motion, the average error was found to be approximately 10%. Furthermore, 10 as-recorded earthquakes were used to verify the proposed procedure. The results revealed that the ETA method can be applied to pulse-like motions, which reduces the calculation time and contains pulse properties.

Experimental study on effects of U-shape dampers on earthquake responses of a base-isolated LNG inner tank

Base-isolated liquefied natural gas (LNG) storage tanks are prone to violent liquid sloshing and large isolator displacements when subjected to near-fault earthquakes, which are characterized by long-period velocity pulses and large-amplitude displacements. Employing supplemental damping into isolation systems is an effective method to control the displacement of isolators. This paper aims to investigate effects of U-shape dampers on earthquake responses of an inner steel tank isolated by multiple friction pendulum bearings (MFPB). Two materials were selected to manufacture the U-shape dampers, i.e., Mg-Al alloy and mild steel. Hysteretic behaviors of MFPBs with and without U-shape dampers were first evaluated by quasi-static cyclic tests. Afterward, shaking table tests were conducted on a 1:25 scaled steel tank with different isolation systems. Results showed that the U-shape dampers could be used to effectively control the displacement of the isolators. Under the excitation of near-fault ground motions, the peak displacement of isolation bearings with Mg-Al alloy dampers decreased by 16 % ∼ 26 % and that with mild steel dampers decreased by 20 % ∼ 22 %. Meanwhile, adoption of U-shape dampers exhibited excellent reduction effects in both acceleration and hydrodynamic pressure of the tank. In comparison with MFPB, isolation bearings with U-shape dampers exhibited a more plumper hysteresis curve and higher energy dissipation capacity provided by sufficient slides of the bearings.

Comparative vulnerability analysis of decomposed signal for the LRB base-isolated structure under pulse-like ground motions

The pulse signal is the most significant component in the pulse-like ground motion. The investigation of the pulse signal in the pulse-like ground motion has become a focus in earthquake engineering. In this study, the pulse signal is extracted from the near-fault pulse-like ground motion by wavelet packet transform, and the residual signal of the original signal is also retained. The near-fault pulse-like ground motions are decomposed into 2N groups of wavelet coefficients by wavelet packet transform. Several groups of wavelet coefficients, which cumulative energy ratio is more than 70%, are used to reconstruct the pulse signal. The vulnerability performance for the lead rubber bearing (LRB) base-isolated structure under the original signal, pulse signal, and residual signal are compared, respectively. The vulnerability performance analysis of the LRB base-isolated structure under the above three signals is completed by the incremental dynamical analysis (IDA) method, respectively. The numerical results indicate that the pulse signals in the pulse-like ground motions will amplify the responses of the LRB base-isolated structure, and the quantitative analysis results are also given. The influence of the residual signals on the LRB base-isolated structure is relevantly small, which is similar to the influence of the non-pulse ground motions on the same LRB base-isolated structure.

Seismic Response Analysis of a Curved Bridge under Near-Fault and Far-Field Ground Motions

To study the influence of near-fault and far-field (FF) ground motions on a curved girder bridge, a 1:10 scale curved bridge shaking table test model was constructed. Three types of ground motions, namely, near-fault pulse (NP), near-fault non-pulse (NN), and FF ground motions, were selected and employed in the test model. The test results indicated that the seismic response of the curved bridge in the excitation of NP ground motion is significantly higher than that in the excitation of NN and FF ground motions, and is destructive to the curved bridge. The displacement of the curved bridge has spatial characteristics in the excitation of NP ground motions. The rotation generated by the curved bridge is easy to cause bearing shedding or girder falling at the low pier. The amplification of pier height increases the displacement response at the high pier.

Residual drift risk of self-centering steel MRFs with novel steel column bases in near-fault regions

This paper evaluates the potential of novel steel column bases to reduce the residual drift risk of steel buildings located at near-fault regions when installed to post-tensioned self-centering moment-resisting frames (SC-MRFs). To this end, a prototype steel building is designed that consists of either conventional moment-resisting frames (MRFs) or SC-MRFs or SC-MRFs equipped with the novel steel column base (SC-MRF-CBs). The MRFs and SC-MRFs are used as benchmark frames. The frames are modelled in OpenSees where material and geometrical non-linearities are considered along with stiffness and strength degradation. A set of 91 near-fault ground motions with different pulse periods is used to perform incremental dynamic analysis (IDA), in which each ground motion is scaled appropriately until different residual storey drift limits are exceeded. The probability of exceedance of these limits is then computed as a function of the ground motion intensity and the period of the velocity pulse. Finally, the results of IDA are combined with probabilistic seismic hazard analysis models that account for near-fault directivity to evaluate and compare the residual drift risk of the frames used in this study. Results show that the predicted residual drift performance of the frames is influenced by the pulse period of the near-fault ground motions. The use of the novel steel column base significantly reduces the residual drift risk of the frames and the SC-MRF-CB exhibits the best residual drift performance. Finally, the paper highlights the effectiveness of combining post-tensioned beam-column connections with the novel steel column base, by showing that the SC-MRF-CB improves the residual drift performance of the MRF and SC-MRF by 80% and 50%, respectively.

Response Prediction of VFPI Through Equivalent Long Period Wavelet of Near-Fault Ground Motion

Past research has revealed that a Variable Frequency Pendulum Isolator (VFPI) effectively controls the structural responses such as base shear and structural acceleration under far and near fault earthquakes and the VFPI may show excessive sliding displacement under some earthquakes having 4-6 s time-period long waves. But, the impact of the amplitude of the long period wave on the sliding displacement of VFPI is missing in past research due to difficulty in finding out the amplitude of long period wave of an earthquake. Therefore, the present study considered the amplitude of the long period wave of an earthquake to analyse the behaviour of the structure isolated by VFPI. For this, the most dangerous long period wave of earthquakes has been extracted in the form of noise free wavelets by using Mavroeidis and Papageorgiou proposed numerical approach. The results indicate that the noise free long period wavelet effectively represent the low frequency earthquake for the structure isolated by VFPI. It is also found out that, the VFPI shows the excessive sliding displacement for only those earthquakes which contains peak ground displacement of long period wave more than 0.40 m. The variation in the base shear and structural acceleration of structure isolated by VFPI under various value of Frequency Variation Factor (FVF) obey the exponential and cubic form respectively. According to this, the present research provides empirical formulas, chart, and tables to predict the structural and isolator responses by using the peak ground velocity (PGV) and dominating low frequency (fd) of a near fault earthquake.

Seismic responses and resilience of novel SMA-based self-centring eccentrically braced frames under near-fault ground motions

Seismic responses and resilience of novel SMA-based self-centring eccentrically braced frames under near-fault ground motions

Uncertainties in Prediction of Near-Fault Long-Period Ground Motion: An Application to the 1970 Tonghai Earthquake (Ms 7.8)

Uncertainties in Prediction of Near-Fault Long-Period Ground Motion: An Application to the 1970 Tonghai Earthquake (Ms 7.8)

Performance of clutched inerter damper for base-isolated structures under near-fault motions

The performance of the supplemental clutched inerter damper (CID) for the base-isolated multi-story structures subjected to near-fault earthquakes is investigated. The isolation system is considered as lead-rubber bearings with bi-linear characteristics and viscous damping. The resisting force of the CID is proportional to the relative acceleration between two terminals under the attached condition and zero when detached. The governing equations of motion of base-isolated structure and the CID are derived and solved using numerical techniques under seven near-fault ground motions data. The variation of peak bearing displacement, top floor absolute acceleration, total base shear, and the CID force is plotted against the inertance mass ratio of the CID. The above peak responses were also analyzed for different values of damping, period of isolation, yield strength of LRB, and superstructure stories. Application of the CID is observed to effectively facilitate the reduction in bearing displacement while the combined effect of isolation and the CID prevents the top floor acceleration to shoot up. The optimum value of inertance mass ratio is also determined by minimizing the total base shear which is the measure of equivalent lateral force on the structure. The optimum inertance lies in the range of 35%–45% of the total mass of the isolated building under near-fault motions. In addition, the performance of the CID base-isolated structure subjected to cycloidal pulses is also investigated. It is observed that the CID is quite effective in controlling the displacement of the isolation system under cycloidal pulses.

Endurance time analysis of seismic performances of long-span continuous rigid-frame bridges with corrugated steel webs

The seismic performances of continuous rigid-frame bridges must be evaluated efficiently and accurately using simplified methods during their design and strengthening. As a simplified method to evaluate the seismic performances of bridges, the endurance time method can be used to simulate the complete process of the collapse of an intact bridge via nonlinear time history analysis. The endurance time method is a novel approach of generating seismic acceleration time steps, which allow engineers and researchers to obtain maximal information from the states of the structures with the lowest computational cost. The Fei-Long Bridge—a long-span continuous rigid-frame bridge with corrugated steel webs—has the largest span of this kind in the world and is situated in a medium-risk seismic zone. In this study, we investigated the applicability and accuracy of the endurance time method to estimate the seismic performance of the bridge. Three endurance time accelerograms were generated as seismic inputs based on the design response spectrum provided by the Chinese specifications for the seismic design of highway bridges. In addition, 16 natural far-field and 16 natural near-fault ground motions were selected for incremental dynamic analysis. The seismic response characteristics of the bridge obtained by the endurance time method and incremental dynamic analysis were compared. The results of the endurance time method are observed to be within the envelope of the incremental dynamic analysis results, with an average error lower than 5%, and the median value of the endurance time method agrees well with the mean curve of the incremental dynamic analysis results. Therefore, the endurance time method can be used as a novel simplified and efficient method to evaluate the seismic performances of continuous rigid-frame bridges with corrugated steel webs.

Effect of an Innovative Friction Damper on Seismic Responses of a Continuous Girder Bridge under Near-Fault Excitations

Continuous girder bridges have been extensively constructed in China over the past 30 years, and these bridges tend to experience severe damage under ground motions with velocity pulses. In the current research, an innovative linear friction damper (LFD) is proposed to mitigate the seismic damages of continuous girder bridges subjected to near-fault ground motions. The OpenSees platform is adopted to establish the numerical model of a continuous girder bridge in the near-fault region. Sixteen ground motions with velocity pulses are selected from the PEER ground motion database. The wavelet method is used to extract the maximum velocity pulse from the two orthogonal components of a ground motion. The effects of the initial gap, the coefficient of friction, and the spring stiffness of LFD on the seismic responses of the bridge are investigated by the response surface method (RSM). The seismic responses of the bridge for the original system (Non-isolated), LFD system (Isolated-LFD), and lead rubber bearing (LRB) system (Isolated-LRB), such as force–displacement relationship, bearing displacement, and pier curvature, are obtained after conducting a series of nonlinear time history analyses. The numerical results reveal that this innovative device (LFD) can effectively control the relative displacements between the superstructure and substructure of the bridge. Meanwhile, the seismic responses of the piers can be significantly decreased compared with the non-isolated system.

Seismic vulnerability assessment of base isolated liquid storage tanks under near-fault ground motions

This paper aims to develop the fragility curves of base isolated liquid storage tanks under near-fault ground motions. Liquid storage tanks are important lifeline structures for industrial plants, but the damages from the past earthquakes showed their vulnerability. The seismic isolation technique is a promising approach to reduce the seismic demand in liquid storage tanks. It should be considered that near-fault ground motions can damage base-isolated liquid storage tanks due to their characteristics such as pulses in their time history velocity. Therefore, it is important to assess the vulnerability of the liquid storage tanks under near-fault ground motion. Fragility curves are useful tools for structural engineers to evaluate seismic performance of structures. In this study, two types of liquid storage tanks, i.e., broad and slender, are considered. The fragility curves for both non-isolated tanks and base-isolated tanks by Friction Pendulum System are developed and compared. Six failure criteria have been considered for the fragility curves, including vertical displacement of convective mass, elastic buckling of tank wall, yielding of anchorages, elephant foot buckling, pipe-line fracture and base isolation displacement. Furthermore, the effect of the isolation period and the coefficient of friction on the fragility curves are investigated. The results show that the pipeline fracture and the tank’s wall buckling are the two most important parameters, which govern the behaviour of liquid storage tanks.

Evolution criteria of overall damage of concrete gravity dam body and foundation under near-fault ground motion

Strong earthquake cases of concrete gravity dams show that the foundation damage has an important influence on the seismic response and damage characteristics of the dam body. To study the real dynamic damage characteristics of concrete gravity dams under the action of near-fault ground motions, this study establishes a dam-reservoir-foundation system model which can reasonably reflect the dynamic damage evolution process of dam concrete and foundation rock mass. Select three different types of near-fault ground motion records with rupture directional pulse, fling-step pulse and non-pulse, an improved dam damage index evaluation model is proposed, which can reasonably reflect the overall damage degree of the dam, and deep researchon the overall damage evolution law of concrete gravity dams. The results show that the foundation of the concretes gravity dam often get damaged before the dam body under the action of strong earthquakes. Compared with the near-fault non-pulse ground motion, the overall damage index of the dam under the action of the near-fault directivity pulse ground motion is significantly increased, and causes greater damage and displacement response to the dam body. The near-fault fling-step pulse ground motion has the least impact on the dynamic response of the gravity dam structure.

A new endurance time analysis method for damage evaluation of high arch dams under the oblique incidence of mainshock-aftershock seismic sequences by wavelet decomposition

Pulse and oblique incidence features that may be generated by near-fault seismic sequences are neglected in current earthquake-resistant design codes for arch dams. To solve the shortcomings of endurance time analysis method which cannot consider near-fault pulse-like ground motions, this paper attempted to develop a new endurance time analysis method by wavelet decomposition to assess the differences in damage mechanisms of arch dams under the cross-stream and stream oblique incidences of near-fault seismic sequences. This study decomposed a typical near-fault earthquake into high-frequency and low-frequency ground motion through wavelet decomposition, which is used to perform several nonlinear dynamic analyses under different seismic intensities and seismic sequence incidence angles. On the basis of dynamic calculations, the local and global damage indices of the dam were presented, with a focus on the comparison of the damage distribution and damage volume ratios of arch dams under the action of single and sequence earthquakes and by analysing the damage mechanisms of arch dams under two diagonal incidence conditions at different peak accelerations. Then, the effects of different frequency components of ground motions on dam damage indices were compared, and finally, additional damage to the dam after the mainshock damage was quantified for various aftershocks. The proposed novel prediction model was used to estimate aftershock damage by considering mainshock damage. The numerical results indicate that the dam damage development is highly complex during original mainshock cross-stream and stream oblique incidences. This finding differs from dam damage that spread from the downstream face to the upstream face during the vertical incidence. The dam damage degree is smaller for the original mainshock stream oblique incidence than for the original mainshock cross-stream oblique incidence. The original aftershock induces stronger damage requirements on the damaged dam during the mainshock than during the high-frequency aftershock. Original and high-frequency seismic sequences with cross-stream and stream oblique incidences do not contribute to variations in dam damage patterns during aftershocks. Therefore, the influence of near-fault pulse impacts and the oblique incidence angle of seismic sequences need to be considered comprehensively in the seismic calculations and reinforcement studies of arch dams.