Near-fault Ground Motions Research Articles

Finite Element Analysis of Reinforced Concrete Single-Column Bridge Bent with Flexural Failure Under Near-Fault Ground Motion

Finite Element Analysis of Reinforced Concrete Single-Column Bridge Bent with Flexural Failure Under Near-Fault Ground Motion

Seismic performance of frame‐bent structures subjected to near‐fault ground motions

Reinforced concrete frame-bent structures are one of the structural forms commonly used in industrial factory buildings. In order to study the seismic behaviour of frame-bent structures located near fault areas, eight pulse-like, eight non-pulse near-fault motions and eight far-fault ground motions have been selected in this paper. The finite element model of a reinforced concrete frame-bent structure was established to study seismic responses to near-fault and far-field ground motions. Numerical analysis results indicate that the maximum base shear of the structure under the action of near-fault ground motion was 5.10 times that of far-fault and 1.47 times that of non-pulsed near-fault ground motion; the maximum story drift angle under pulse-like near-fault ground motion was 3.8 times that of non-pulse and 19.7 times that of far-fault ground motion. Therefore, engineering designers should pay more attention to the seismic performance of frame-bent structures under the action of near-fault ground vibration.

Dynamic Rupture Study of Near-Field Velocity Pulses during the 2016 Kumamoto Earthquake, Japan

ABSTRACT Simulating the ground motions of future earthquakes requires a proper understanding and modeling of source, path, and site effects. Ground motions recorded during recent earthquakes very close to their ruptured faults provide new evidence of the importance of source effects and suggest that physics-based rupture modeling is critical to account for them. Here, we develop dynamic rupture models to simulate the near-fault ground motions generated by the 2016 Kumamoto, Japan, earthquake (Mw 7.0) at Nishihara village, which feature a large-amplitude velocity pulse. Comparison of mainshock and foreshock waveforms suggests that the source of the velocity pulse is on the Futagawa fault segment located very close to the site. Our dynamic models use the spectral element method and are built upon a previous kinematic description of the event via a so-called “characterized source model,” with three strong-motion generation areas (SMGAs) on the assumed fault plane. We first develop a reference model that reproduces the main features of the rupture process in agreement with previous results of kinematic source inversion. We then examine the sensitivity of the simulated near-fault ground motions to the frictional parameters (critical slip-weakening distance and stress drop) in the shallow part of the fault and to the geometrical properties of the shallow SMGA. Even assuming drastically different frictional properties in the shallow part of the fault, the amplitude of the simulated ground motions was affected little. On the other hand, changes of geometrical properties of the shallow SMGA generated large differences in simulated ground motions. The results indicate that geometrical features of the shallow SMGA played a more important role in generating near-fault ground motions with velocity pulses as observed at Nishihara village during the 2016 Kumamoto earthquake.

Fragility analysis of steel MRFs: Effects of frequency-content components of near-fault pulse-like ground motions and setbacks

Near-fault (NF) pulse-like ground motions may contain long-period velocity pulses and high-frequency acceleration records, which impose significant damage on structures. This paper attempts to investigate the effects of forward directivity pulses and the high-frequency (residual) components of such records on the seismic performance of regular and setback structures. To this end, a group of ten-story steel moment-resisting frames with regular and irregular configuration were subjected to 55 NF records and their corresponding components. The records were classified into four groups to evaluate the effects of the ratio of the pulse period to the fundamental period of the structure, TP/T1, which is known to have critical effects on structural responses. Incremental dynamic analyses were carried out to develop fragility curves. The results revealed that pulse records with TP/T1 ≠ 1 can be the representatives of original NF pulse-like records to develop fragility curves for the regular structures. However, the fragility curves for setback frames should be developed under the original NF earthquake records, when TP/T1 ≤ 1. The residual part is the predominant component for the regular structure only when TP/T1 ≈ 1. However, in some setback frames, the effects of the residual part in exceeding the performance level are considerable even when TP/T1 > 1. Meanwhile, pulse records with TP/T1 ≈ 0.5 are the most damaging directivity pulses in the IO limit state of regular and setback structures, while the pulse records with TP/T1 ≈ 2 and TP/T1 ≥ 2 are the most critical pulses in the LS and CP performance levels of regular and setback frames, respectively.

Critical Analysis of Nonlinear Base-Isolated Building Considering Soil–Structure Interaction under Impulsive and Long-Duration Ground Motions

Critical responses are investigated for nonlinear base-isolated buildings considering soil–structure interaction under near-fault ground motions and long-duration ground motions. A double impulse and a multi impulse are employed to simulate the nonlinear critical responses of the models under such ground motions. The base-isolation story is assumed to consist of lead rubber bearings and to have a bilinear force–deformation relation. Two types of critical timings for a MDOF building model supported by a swaying-rocking spring-dashpot system are derived: (1) the timing that maximizes the total input energy to the whole system and (2) the timing that maximizes the instantaneous input energy to the base-isolated building excluding the swaying-rocking system. These two types of critical timings are compared through numerical examples. Finally, time-history response analyses were conducted under the critical double impulse, the corresponding one-cycle sine wave, and the critical multi impulse. The effect of the soil–structure interaction on the maximum responses of the nonlinear base-isolated building is clarified.

What Is Fling Step? Its Theory, Simulation Method, and Applications to Strong Ground Motion near Surface Fault Ruptures

ABSTRACT We present the theory of the fling step and a theoretical method for simulating accurately the near-fault strong motions, and apply it to reproduce various strong-motion records near surface faults. Theoretically, the fling step is the contribution of the static Green’s function in the representation theorem (Hisada and Bielak, 2003), and we show that this theory holds for any seismic velocity structure. We first demonstrate the validity of this theory using theoretical solutions of a circular fault model in a homogeneous full-space. Next, we apply the theory to layered half-spaces, present a theoretical method based on the wavenumber integration method, and introduce various techniques to simulate the near-fault ground motions including fling steps with high accuracy. Finally, we demonstrate the effectiveness of the method by reproducing various strong-motion records near surface fault ruptures and discuss the characteristics of near-fault strong motions including the fling step and the forward directivity pulse. We made all of the software and data used in this article available on the internet.

Effects of near-fault ground motions on dynamic response of slopes based on shaking table model tests

Slope instability and failure frequently occur in areas near faults. A series of shaking table model tests were conducted to examine dynamic response of slopes to near-fault ground motion in the time and frequency domains. Effects of the intensity and pulse characteristics of near-fault ground motions on slope response were examined. Test results show that peak ground acceleration (PGA) induced by near-fault ground motion is amplified by elevation. When the amplitude of seismic loading exceeds a critical value (which is between 0.2 g and 0.5 g), intensity of near-fault ground motion increases, amplification of PGA by elevation weakens, and the high-frequency filtering effect of the slope is enhanced. Amplification of PGA by elevation and interfacial reflections are more prominent under pulse-like than non-pulse-like waves. The PGA amplification factor of the slope crest under pulse-like wave is 1.2 times of that triggered by non-pulse-like wave. Due to the interfacial reflections, vertical accelerations can be monitored simultaneously in cases which have only horizontal seismic loadings. The vertical PGA is 72.4% of horizontal PGA when the intensity of pulse-like waves is 1.1 g. These findings can provide a basis for designing earthquake-resistant of slopes near faults.

Seismic Response of Torsionally Coupled Building Isolated with Multiple-Variable Frequency Pendulum Isolator

The behavior of torsionally coupled structures isolated with a Multiple-variable frequency pendulum system (MVFPI) is presented. Friction pendulum system (FPS) has limitations due to constant isolator period and restoring force characteristics, while this MVFPI overcomes this limitation. The sliding surface of MVFPI was predefined as a continuous piecewise function to combine the seismic performance of MVFPI with different seismic intensities. The governing equations of motion of the building-isolation system are derived and solved by New mark's step-by-step method assuming a linear variation of acceleration over a small-time interval. Six pairs of near-fault ground motions are selected as input ground motions in this study. The coupled lateral-torsional response is obtained under different parameter variations for selected earthquake ground motions. Moreover, some parametric study of structure isolated with MVFPI has been compared with FPS. The parametric study's numerical results help to understand the torsional behavior of the structure isolated with MVFPI. It is found that the MVFPI behaves remarkably better than FPS to reduces the base shear and base torque responses.

Reliability-based stochastic optimal control of frame building under near-fault ground motions

Most of stochastic optimal control methods were developed on the basis of Itô stochastic differential equation, which assumes that the external excitation is white Gaussian noise or filtered white Gaussian noise. However, this assumption is far from the real excitations, which hinders the applications of stochastic optimal control theory. In this paper, the reliability-based stochastic optimal control via direct probability integral method (DPIM) for building structure is proposed, which is applicable to performance-based design of general control systems of linear structures under non-stationary and non-white random excitations. Firstly, the DPIM is utilized to accurately and efficiently compute dynamic reliability of the controlled system. Then, the reliability-based objective function is suggested to optimize the parameters of the placed control devices of building structure, and reliability-based maximum story controllability index is established to determine the optimal placement of control devices, thus fulfilling the stochastic optimal control of structure. Finally, a 10-story shear frame building controlled by active tendons under random excitations of near-fault earthquake ground motions verifies the effectiveness of the proposed optimal control method. Moreover, the optimal control schemes of frame structure under near-fault non-pulse and impulsive ground motions are compared, indicating that the velocity pulses of near-fault ground motions impose significant effect on structural responses, and the more control energy is needed for guaranteeing structural safety.

Smooth Crustal Velocity Models Cause a Depletion of High-Frequency Ground Motions on Soil in 2D Dynamic Rupture Simulations

ABSTRACTA depletion of high-frequency ground motions on soil sites has been observed in recent large earthquakes and is often attributed to a nonlinear soil response. Here, I show that the reduced amplitudes of high-frequency horizontal-to-vertical spectral ratios (HVSRs) on soil can also be caused by a smooth crustal velocity model with low shear-wave velocities underneath soil sites. I calculate near-fault ground motions using both 2D dynamic rupture simulations and point-source models for both rock and soil sites. The 1D velocity models used in the simulations are derived from empirical relationships between seismic wave velocities and depths in northern California. The simulations for soil sites feature lower shear-wave velocities and thus larger Poisson’s ratios at shallow depths than those for rock sites. The lower shear-wave velocities cause slower shallow rupture and smaller shallow slip, but both soil and rock simulations have similar rupture speeds and slip for the rest of the fault. However, the simulated near-fault ground motions on soil and rock sites have distinct features. Compared to ground motions on rock, horizontal ground acceleration on soil is only amplified at low frequencies, whereas vertical ground acceleration is deamplified for the whole frequency range. Thus, the HVSRs on soil exhibit a depletion of high-frequency energy. The comparison between smooth and layered velocity models demonstrates that the smoothness of the velocity model plays a critical role in the contrasting behaviors of HVSRs on soil and rock for different rupture styles and velocity profiles. The results reveal the significant role of shallow crustal velocity structure in the generation of high-frequency ground motions on soil sites.

Effects of different empirical ground motion models on seismic hazard maps for North Iceland

This study builds on previous site-specific hazard studies for North Iceland, specifically regarding delineation of seismic sources and seismicity parameters. Using a Monte Carlo approach to generate synthetic earthquake catalogues for North Iceland, and multiple ground motion models (GMMs) that have been proposed and used for probabilistic seismic hazard analysis (PSHA) in Iceland in the past, the variability of the resulting hazard estimates is presented in a map-form. The variability in the hazard estimates is quite large, which is a direct result of the inconsistency in the GMMs used in previous studies. We show how this is caused by the inability of these models to capture the characteristic amplitude attenuation of Icelandic earthquake ground motion with distance, thus casting doubt on the validity of the resulting PSHA of past studies. In contrast, we re-evaluated the variability of PSHA for North Iceland based on new empirical Bayesian GMMs that not only satisfy all the conditions required for use in PSHA, but also fully capture the characteristics of the existing Icelandic ground motion dataset and in addition contain elements that account for the saturation of near-fault peak ground motions at large magnitudes. The results quantify how the variability in the GMMs, contribute to the range of spatial distribution of PSHA amplitudes and uncertainties. The results show that the confidence in the PSHA values is significantly increased using the new models vs. the older ones. The confidence of the PSHA values is quantified through the coefficient of variation. The confidence is shown to be largest over distance ranges where data is most abundant. On the other hand, the confidence decreases considerably at near-fault and far-field distances, primarily because of lack of data for those distances. The findings highlight the importance of using appropriate GMMs for PSHA in Iceland and give a spatial sense of the relative levels of confidence of hazard estimates. They moreover highlight the need for a revision of the PSHA using not only the new GMMs, but also physics-based earthquake source models.

Effect of Near-Fault Pulsed Ground Motions on Seismic Response and Seismic Performance to Tunnel Structures

Seismic analysis of tunnels close to or crossing seismogenic faults is a complex problem, which is often neglected at the design stage for the lack of specific codes or guidelines and also because underground structures are considered less vulnerable than that of the corresponding above-ground facilities. Near-fault ground motions are generally assumed to providing more powerful energy to tunnel structures. Therefore, a recently developed velocity pulse equivalent model is proposed to synthesize the artificial near-fault pulsed ground motion for the seismic response behavior of the tunnel structure. A newly proposed nonlinear dynamic time history methodology, the incremental dynamic analysis method, is introduced into the analysis of seismic performance and fragility for tunnel structures. This study takes the Zheduoshan tunnel as a case study to illustrate the effects of velocity pulse on the seismic response behavior and seismic performance. The applicability of different seismic intensity measures is preliminarily discussed, and the vulnerability of the tunnel structure at different characteristic locations is analyzed. Afterward, the seismic vulnerability probabilities of the tunnel structure under the action of the near-fault pulsed ground motions and the far-field ground motions are presented, and then, the failure probabilities of the tunnel structure under the three-level support requirements are obtained. Research results provide an objective assessment of the velocity pulse effects and acts as a reference for the likely seismic damage assessment of tunnel structures.

Dimensionless analysis of pulse-like effects on the seismic behavior of a dam based on wavelet-decomposed near-fault ground motions

In this paper, the inherent self-similarity between seismic characteristics and the dynamic behavior of a concrete gravity dam under decomposed near-fault ground motions is investigated. The representative low- and high-frequency components were decomposed using the continuous wavelet transform method to explore their effects on the dam response. The Koyna concrete gravity dam system was used as the basis for a numerical model. A series of dam dynamic responses under near-fault motion with and without pulse properties were obtained by numerical analyses. The relationship between the proposed dimensionless indices (intensity measure ratio (ΦIM), pulse period ratio (T1/Tp)), and response ratio (ΨEDP, m) was established to study the inherent self-similarity in the dynamic behavior of dams, which has an explicit physical significance with smaller dispersion. The response proportions of two main frequency components in the original near-fault record were revealed. Furthermore, a series of new threshold values were calculated based on the intersection of the pulse and high-frequency ratios, and the range of influence of the low- and high-frequency components was quantitatively divided. Finally, a predictive model was proposed for the dam responses to near-fault ground motions containing the proposed dimensionless indices that exhibited reasonable patterns. The pulse-like seismic characteristics and seismic responses to near-fault ground motions were analyzed and selected in the context of sensitivity, inherent self-similarity, threshold point, and regression error. Results show that the self-similarity between ΦSa(T1), ΦASI and ΨDVR,pulse,ΨDVRn,pulse is superior to that of other parameter ratios, indicating that more attention should be given to the role of intensity and pulse parameters in practical concrete dam seismic design.

Control performance of sloped rolling-type isolators designed with stepwise variable parameters

With the same horizontal acceleration control performance, the horizontal displacement control performances of sloped rolling-type seismic isolators passively provided with stepwise variable parameters, as well as constant ones, are numerically investigated in this study. The first design possesses a smaller sloping angle with larger damping force at smaller horizontal isolation displacement and a larger sloping angle with smaller damping force at larger horizontal isolation displacement. In other words, this design has stepwise increased sloping angles and stepwise decreased damping force with increasing horizontal isolation displacement. The second design has an opposite design philosophy to the first one, i.e., it has stepwise decreased sloping angles and stepwise increased damping force with increasing horizontal isolation displacement. A series of numerical results present that for sloped rolling-type seismic isolators designed with a constant sloping angle and damping force, in general, the larger the damping force (in other words, the smaller the sloping angle), the smaller and the larger the horizontal maximum and residual displacement responses presented, respectively. The first and second designs with stepwise variable parameters each have its advantage for suppressing horizontal isolation displacement under far-field and pulse-like near-fault ground motions because of their larger energy dissipation capabilities designed at different stages. When the horizontal isolation displacement responses at the end of ground motions are still within the first slope rolling range with a larger sloping angle of the second design, as expected, adopting the second design can exhibit a better re-centering performance than adopting the first design. To have acceptable displacement control performances and without scarifying acceleration control performances under diverse seismic demands, compared with adopting the designs with constant parameters and the first design, adopting the second design could be an alternative solution and better choice.

Efficiency of Intensity Measures Considering Near- and Far-Fault Ground Motion Records

This paper focuses on the identification of high-efficiency intensity measures to predict the seismic response of buildings affected by near- and far-fault ground motion records. Near-fault ground motion has received special attention, as it tends to increase the expected damage to civil structures compared to that from ruptures originating further afield. In order to verify this tendency, the nonlinear dynamic response of 3D multi-degree-of-freedom models is estimated by using a subset of records whose distance to the epicenter is lower than 10 km. In addition, to quantify how much the expected demand may increase because of the proximity to the fault, another subset of records, whose distance to the epicenter is in the range between 10 and 30 km, has been analyzed. Then, spectral and energy-based intensity measures as well as those obtained from specific computations of the ground motion record are calculated and correlated to several engineering demand parameters. From these analyses, fragility curves are derived and compared for both subsets of records. It has been observed that the subset of records nearer to the fault tends to produce fragility functions with higher probabilities of exceedance than the ones derived for far-fault records. Results also show that the efficiency of the intensity measures is similar for both subsets of records, but it varies depending on the engineering demand parameter to be predicted.

Velocity pulse effects of near-fault earthquakes on a high-speed railway vehicle-ballastless track-benchmark bridge system

The near fault (NF) line waves send out signal envelopes that oscillate over lengthy periods of time with periodic impulses. Like train bridges, train tracks demonstrate comparable track-bridge (TB) motion dynamics. Using these coupling dynamics, are the high-speed train-track-bridge (HSTTB) system designs sensitive to those parameters? This research incorporates a Finite Element Analysis (FEA) technique developed for simulating the dynamic reactions of the coupled TB system when faced with simultaneous NF lateral and vertical ground motions (GMs). For the first time, data from the pre-commissioning field testing of the Beijing-Shanghai high-speed train are utilized to validate the Train-Track-Bridge Dynamic Analysis (TTBDA) test. As a matter of fact, the current research has concentrated on the running safety of the high-speed train's operations, as well as the possible derailment mechanism of the high-speed train, in light of the far-field (FF) earthquakes. This analysis reveals that the NF GMs in the bridge structure's seismic reactivity are considerable. Many high-speed train derailments are due to frequent wheel displacement, elevated wheels, and significant lateral motion. The data discovered in the field may give engineers vital information for calculating relevant situations and railroad engineering projects.

Questionnaire surveys on damages at high-rise buildings and synthesized near-fault long-period strong ground motions of the 2018 Hualien, Taiwan, earthquake

Long-period pulse-like strong ground motions were recorded near the Milun fault and caused serious building damages in the Hualien City during the main shock of the 2018 Hualien, Taiwan, earthquake. In this paper, questionnaire survey on feeling of shaking, indoor damages, structural, and nonstructural damages to high-rise reinforced concrete residential buildings were analyzed by three building groups according to the positions to the Milun fault and different site conditions. Relationships between the results of questionnaire survey on building damages and characteristics of strong ground motions were studied. Furthermore, a characterized seismic source model was proposed to elucidate the generation mechanism of near-fault long-period strong ground motions. In this model, a shallow strong motion generation area (SMGA7) in the south part of Milun fault with rise-time of 2.6 s contributed to the long-period pulse-like strong ground motions. Besides, a deep SMGA (SMGA8) under the SMGA7 with rise-time of 1.2 s also contributed to the strong ground motions. Using this model, strong ground motions at the sites of buildings for questionnaire survey were synthesized. Because of the directivity effects, amplitudes of ground motions were increased from north to south along the Milun fault, which can explain the serious building damages at the buildings located at the southwest to the Milun fault. The reasons of building damages were also examined from the aspect of predominant periods of ground sites and fundamental periods of buildings, which were extracted from microtremor measurement. Damages at the buildings located on the central part of the hanging wall with relatively good site condition were the lightest for almost all the surveyed items. Pulses with period in the range of 1 s–2 s which were generated by the SMGA8 and amplified by the near surface layers are considered to be responsible for building damages.

A procedure for matching the near-fault ground motions based on spectral accelerations and instantaneous power

Selection of ground motions for use in nonlinear dynamic analysis is one of the most critical steps for both code-based design and probabilistic seismic risk assessment of structures. In practice, time-domain spectrum-matching methods, which add wavelet functions to an initial acceleration time series, have been widely used to obtain a record whose response spectrum closely matches the desired target spectrum. Although the spectral shape is known to be a good predictor of structural response, it does not represent the critical aspects of the velocity pulses, such as pulse amplitude and pulse period for near-fault ground motions. The Instantaneous Power ( IP( T1)), defined as the maximum rate of change of energy of the bandpass-filtered velocity time series over a short time interval given by half of the structural period, has been shown to be an effective alternative parameter to capture effects of the presence of a velocity pulse and the pulse period in near-fault record selection. We introduce an approach to modify time series so as to simultaneously match a target response spectrum and IP spectrum over a specified period interval. We demonstrate that the records modified using the proposed approach produce results comparable to those obtained using unscaled records, and prevent potential bias in structural response, relative to results when matching is performed without consideration of IP.

Development of Performance Based Plastic Design of EBF Steel Structures Subjected to Forward Directivity Effect

Performance-based plastic design (PBPD) is a method which considers the inelastic behavior of a structure for the seismic design that follows a pattern in which a pre-selected yield mechanism is defined for each type of structural system. Despite the general adequacy of this method compared to the code-based method, the need to further improve this method against Forward-Directivity, the well-known characteristic of near-fault ground motions, is of high importance. On the other hand, Eccentrically Braced Frame (EBF) is considered to be an efficient conventional steel-framed system that combines the advantages of moment-resisting frame and concentrically braced frame. In this study, attempts have been made to improve the conventional PBPD method in order to consider the effect of forward-directivity. To this end, the energy modification factor (γ) was modified by recalculating ductility demand µ and ductility reduction factor Rµ of three 6-, 12-, and 18-story EBF structures. Using incremental dynamic analysis, these structures were subjected to 12 well-known near-fault ground motions, and accordingly, the modified energy factor (γ′) was obtained for each set. Then, an equation was proposed to calculate the γ′ based on the fundamental period of the structure. As expected, the code-based designed structures exceeded the allowable drift limits. Although the structures which were designed based on conventional PBPD followed the pre-selected yield mechanism and performed better, they exceeded the drift limits as well. Consequently, the structures which were re-designed using modified energy factor γ′ could well satisfy the acceptance criteria as well as achieving the desirable yield mechanism.

Risk-based design and optimization of shape memory alloy restrained sliding bearings for highway bridges under near-fault ground motions

The Shape Memory Alloy (SMA)-restrained bearing consists of a typical sliding bearing and SMA wires. The SMA wires are adopted to limit the excessive displacement caused by near-fault ground motions, which can improve the re-centering capacity and energy dissipation capability of sliding bearings. Such bearings should be carefully designed to balance the pier force and bearing displacement so that the bridge system can achieve better performance. The present paper proposed a novel seismic risk-based methodology for the optimization design of SMA-restrained sliding bearings for highway bridges subjected to near-fault ground motions. First, the seismic risk of the bridge system was served as the performance index in this methodology. Second, identical component fragility curves were derived in order to minimize the seismic risk of the whole bridge system. Then, the Particle Swarm Optimization (PSO) approach was implemented to obtain the optimal mechanical properties of SMA-restrained sliding bearings that yield the same fragility curves. Finally, a simplified optimization design method was proposed for searching optimal designs, which can significantly reduce the computational effort but maintain the accuracy of optimal results. The proposed methodology was demonstrated by using a two-span highway bridge as a case study. It is shown that the proposed methodology can be adopted to obtain the optimal designs of the SMA-restrained sliding bearings for highway bridges, which can minimize the seismic risk of the whole bridge system under near-fault ground motions.