Characteristics Of Near-fault Ground Motions Research Articles

Simulation of Near-Fault Pulse-Like Ground Motion and Investigation of Seismic Response Characteristics in Subway Stations

ABSTRACT The availability of reasonable input ground motions is a prerequisite for investigating the seismic response of near-fault structures. Currently, realistic near-fault pulse-like seismic records are lacking internationally. Moreover, the existing records exhibit distinct regional characteristics, which hardly meet the demand for seismic analysis of various engineering structures. Collecting and analyzing the pulse-like seismic records, a linear attenuation expression is proposed in this study to describe the time-varying frequency amplitude decay over duration. On this basis, a new model compatible with horizontal and vertical components is developed for near-fault ground motions. The model parameters are estimated for 100 near-fault records and regressed against the prediction equations. The variability and correlation of the ground motion in two directions are analyzed through the residual correlation matrix. Finally, taking a subway station as the engineering background, the effects of velocity pulse and vertical seismic motions on the underground structures are addressed through simulated motions and realistic records. Compared with existing approaches, the proposed method not only aligns with the time-frequency distribution characteristics of near-fault ground motions but also considers the correlation between horizontal and vertical components. In addition, consistent with the results from recorded ground motions, the synthetic motions yield enlarged seismic responses of the underground structure, thereby guaranteeing analysis accuracy for the engineered structures subjected to pulse-like ground motions. The inclusion of vertical excitation prominently amplifies the vertical displacement of the slab under pulse-like ground motions.

Single-pulse-like and double-pulse-like characteristics of near-fault ground motions

An approach is proposed for quantitatively identifying the number of one-cycle pulses of ground motions. The one-cycle pulse signals are derived from the original velocity time series using the low-pass acausal fourth-order Butterworth filter and the zero-point method, and the adaptive cut-off frequency of the filter is determined by the number of extreme points in the extracted pulse signals. Then, according to the relative energy and the comprehensive evaluation index of the extracted pulse signals, ground motions are classified into four classes: nonpulse-like, single-pulse-like, double-pulse-like, and ambiguous. The statistical relationships between pulse characteristics and source parameters, including moment magnitudes, site conditions, and rupture distances, are discussed. In addition, the relationship between the first-arriving-pulse signal and the last-arriving-pulse signal in double-pulse-like ground motion is investigated. Finally, this approach analyzes the distribution of double-pulse-like and single-pulse-like ground motions recorded during the 1999 Chi-Chi earthquake.

Scaling of earthquake ground motions for tri-directional nonlinear response history analyses of reinforced concrete rectangular bridge piers

In performance-based earthquake design (PBED), both the capacity of the structure and the demand from earthquake ground shaking are required to establish the desired performance levels. A range of possible demands can be estimated through nonlinear response history analyses (NRHA), which requires a suite of earthquake ground motions appropriately scaled to the target earthquake intensity for which the structure's performance is to be evaluated. The scaling is usually done for a particular earthquake intensity measure (IM) to get the associated dispersion in the engineering demand parameter (EDP). Several scaling strategies are available for uni-directional (Uni-NRHA) and bi-directional nonlinear response history analyses (Bi-NRHA). However, for tri-directional nonlinear response history analysis (Tri-NRHA), one literature suggests using the same scale factor (SF) for the vertical component, which is estimated for the Bi-NRHA. Therefore, in this study, three well-known scaling strategies, which are usually used for Uni-NRHA, are employed with 16 different methods for application in Tri-NRHA of a reinforced concrete (RC) single-column bridge pier modelled as a single-mass three-degree-of-freedom (3DoF) system. The absolute maximum drift is taken as the EDP, and a total of 1920 Tri-NRHA of the pier are performed using two suites of 20 far-fault and 20 near-fault ground motions, which are scaled to three levels of target earthquake intensity. The sufficiency and efficiency of all the scaling methods are checked statistically under all three target intensities. The study reports the effects of (i) vertical components in calculating the SF, (ii) earthquake intensity and structural nonlinearity, and (iii) far-fault and near-fault ground motion characteristics on the dispersion of the EDP. Out of 16 scaling methods, one is found to be acceptably efficient than the others for application in Tri-NRHA under both far-fault and near-fault ground motions at all target intensities.

Near-fault ground motion characteristics and its effects on a collapsed reinforced concrete structure in Hatay during the February 6, 2023 Mw7.8 Kahramanmaraş earthquake

Ground motion in the vicinity of fault rupture produces pulse-type loading that may impose severe lateral displacement demands on structures due to forward-directivity effects. The Mw7.8 Kahramanmaraş Earthquake on February 6, 2023 created very high amplitude ground motions in the city of Hatay with distinct near-fault characteristics. In the city of 400,000 inhabitants, 33 % of the building stock was severely damaged or collapsed. Among the collapsed buildings, it was observed that there were new buildings built according to the latest seismic regulations. The seismic characteristics of the ground motion in Hatay were examined, the fault rupture, ground conditions and earthquake source characteristics affecting the engineering demand parameters were determined. Then, the seismic behavior and the reasons for the collapse of a 13-story reinforced concrete building, which is thought to be earthquake resistant with its structural system and material quality, were investigated using acceleration time-series recorded at a station very close to the building location. It has been understood that in shear wall systems, wall position and seismic detailing can impart significant damage (total destruction) on these elements under the effect of large displacement and high axial load demand by limiting the required ductility. Wall boundaries require more stringent confinement detailing than stated by the code under near-fault effects.

Seismic Fragility Analysis of Container Crane Considering Far-Fault and Near-Fault Ground Motion Characteristics

Seismic Fragility Analysis of Container Crane Considering Far-Fault and Near-Fault Ground Motion Characteristics

Sensitivity of structural responses to the processing of near-fault ground motion records containing fling-step

Fling-step is one of the important characteristics of near-fault ground motions that is caused by a permanent static offset of the ground and appears as a one-sided velocity pulse and a large residual displacement at the end of the motion. Due to baseline errors in many ground motion records, they are processed (processing is a combination of baseline correction and filtering) before adding accelerograms to some earthquake databases whereby the residual displacements ( i.e., the permanent static offsets) in the fling records are typically eliminated. It is not clear to what extent the record processing can affect the nonlinear responses of structures subjected to fling-step records. To address this issue, the current study is conducted to investigate the sensitivity of the nonlinear structural responses to record processing. To achieve this goal, the seismic responses of three code-conforming reinforced concrete (RC) special moment-resisting frames with different heights subjected to two versions of fling-step ground motions including a processed version that static offsets are removed, and a baseline-corrected version with static offsets preserved are evaluated. The findings of this assessment indicate that even though the processing procedure does not preserve the static offsets of the fling-step ground motions, the seismic responses of the structures to the processed records may resemble those obtained under only the baseline-corrected ground motions (with static offsets preserved).

Application of BRB to Seismic Mitigation of Steel Truss Arch Bridge Subjected to Near-Fault Ground Motions

In this paper, the seismic response of a steel truss arch bridge subjected to near-fault ground motions is studied. Then, the idea of applying buckling restrained braces (BRBs) to a steel truss arch bridge in near-fault areas is proposed and validated. Firstly, the basic characteristics of near-fault ground motions are identified and distinguished. Furthermore, the seismic response of a long span steel truss arch bridge in the near fault area is analyzed by elastic-plastic time analysis. Finally, the braces prone to buckling failure are replaced by BRBs to reduce the seismic response of the arch rib through their energy dissipation properties. Four BRB schemes were proposed with different yield strengths, but the same initial stiffness. The basic period of the structure remains the same. The results show that near-fault ground motion will not only obviously increase the displacement and internal force response of the bridge, but also cause more braces to buckle. By replacing a portion of the normal bars with BRBs, the internal forces and displacements of the arch ribs can be reduced to some extent, which is more prominent under the action of pulsed ground motion. There is a clear correlation between the damping effect and the parameters of BRB, so an optimized solution should be obtained by comparison and calculation.

Probabilistic Nonlinear Displacement Ratio Prediction of Self-centering Energy-absorbing Dual Rocking Core System under Near-fault Ground Motions Using Machine Learning

ABSTRACT Near-fault pulse-like ground motions can lead to significant seismic demand on building structures due to velocity pulses. The self-centering energy-absorbing dual rocking core (SEDRC) system is a newly developed seismic resilient structural system. This paper investigates the seismic demand of SEDRC systems subjected to near-fault pulse-like ground motions by determining their nonlinear displacement ratios. Two hundred and five near-fault pulse-like ground motion records are used to consider the uncertainties of seismic events. The influences of design hysteretic parameters and near-fault ground motion characteristics on the nonlinear displacement ratio of the SEDRC system are investigated through parametric dynamic analysis of single-degree-of-freedom (SDOF) systems. The dynamic analyses results indicate that the stiffness hardening ratio α and energy-absorbing ratio β of the SEDRC system, predominant period, pulse period of ground motions, and earthquake magnitude show obvious effects on the nonlinear displacement ratio responses of SDOF systems, while the unloading stiffness ratio ε, site condition, and source-to-site distance show limited or negligible influence on that. The past studies mainly used the mean or median responses of single-degree-of-freedom systems to predict the nonlinear displacement ratio of structures, which may not be enough to guide the design of buildings with great importance (e.g., hospital and fire station). This paper proposes an innovative framework for predicting the nonlinear displacement ratio of structures underground motions using a probabilistic estimation method and machine learning technique. Based on the dynamic analysis results of SDOF systems, a probabilistic model for predicting the nonlinear displacement ratio of the SEDRC system under near-fault pulse-like ground motions is developed through the proposed framework. The proposed framework is also applicable to estimate the seismic demand of other structures under near-fault or far-field ground motions to facilitate the development of the performance-based seismic design.

Response of inelastic SDOF systems subjected to dynamic rupture simulations involving directivity and fling step

Directivity is a phenomenon perceived during fault ruptures wherein the ground motion and spectral response in the direction of the rupture is more significant than in any other direction. The objective of this study is to evaluate the variability in characteristics of near-fault ground motions as well as the ductility demand on structures due to directivity based on simulations of strike-slip earthquake events by employing SPECFEM3D, a physics-based ground motion simulation code. To understand the finite fault propagation effect, a source effect, the vertical strike-slip fault is considered to be embedded in an elastic half-space, to prevent the influence of path and site effects. Two scenarios are designed based on the positioning of nucleation asperity (NA): (1) in the middle of the fault face to simulate bilateral (BL) rupture and (2) shifted to one end for the case of unilateral (UL) rupture. The ground motions at near-fault stations, located in a racetrack configuration around the surface trace of the fault, are analyzed. In addition to a high spectral content in the forward directivity stations as a result of UL rupture, directivity velocity pulses identified in the fault-normal components are higher than the fling step velocity pulses in the fault-parallel component for the racetrack stations considered. Furthermore, the study examines the correlation between ductility demands computed based on elastoplastic rheology with direct point parameter and ground motion intensity measures for directivity and fling step stations.

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 isolation design for simply-supported beam bridges based on the energy balance method under near-fault ground motions

The seismically isolated simply-supported beam bridge (SISBB) with the seismic isolation bearing (SIB) is converted into an equivalent bilinear single-degree-of-freedom system. Then, by inputting the 139 near-fault ground motion (GM) records into the equivalent system, the parametric study on the near-fault seismic input energy spectra (SEI) of the SISBB is conducted by considering the equivalent system parameters and the near-fault GM characteristics. So, it can be found that: the increase of post-yield stiffness ratio (ηe) can slightly increase the peak values of the near-fault SEI, and ηe = 0.2 can be used to calculate the near-fault SEI design spectra of the SISBB; with the increase of the damping ratio (ξe) (or ductility ratio (μe)), the spectral values of the near-fault SEI increases, the hysteretic energy dissipation capacity of the SIB decreases, and the damping energy dissipation capacity of the equivalent system increases; when μe ≥ 10, the effect of μe on the near-fault SEI can be ignored; the softer the site soil or the smaller the Joyner-Boore distance to rupture plane (Rjb) is, the greater the near-fault SEI is; if the near-fault SEI with a standard peak ground acceleration (PGA) is known, the near-fault SEI with other PGAs can be obtained by multiplying the (PGAOther/PGAStandard)2 by the spectral values of the near-fault SEI with a standard PGA. Finally, the near-fault SEI design spectra of the SISBB are obtained, and the seismic isolation design for SISBB based on the energy balance method is proposed, the feasibility of which is verified by a case study.

Inelastic dynamic analysis and damage assessment of a hydraulic arched tunnel under near-fault SV waves with arbitrary incoming angles

The incoming directions and incoming angles of near-fault (NF) seismic waves may play important roles in potential seismic-induced damage to hydraulic arched tunnels due to the uncertainty and randomness of NF ground motions. Moreover, the correlation between the seismological characteristics of NF ground motions and the seismic behaviour of underground structures, especially hydraulic arched tunnels, is not well understood. However, the current seismic dynamic analysis procedures and seismic designs of hydraulic arched tunnels generally ignore the abovementioned key factors. For this reason, the focus of this paper is to evaluate the seismic behaviour and damage degree of a hydraulic arched tunnel under NF SV waves with arbitrary incoming angles. Based on the elasticity and wave theory, an obliquely incoming method of SV waves that considers the incoming angle and direction is first developed by the equivalent nodal force method and verified through two examples. Ten as-recorded NF seismic events that display ground motion with a high frequency content are selected to reflect the seismological characteristics of NF ground motions. In addition, a hydraulic arched tunnel is built using the commercial software ABAQUS, and it considers fluid-structure-rock interaction systems based on the coupled Eulerian-Lagrangian method to approximate the environmental conditions experienced by hydraulic arched tunnels when a real earthquake event occurs. Subsequently, an inelastic dynamic analysis of hydraulic ached tunnels under NF SV waves with arbitrary incoming angles are conducted. The numerical results indicated that the propagation directions and incoming angles of SV waves have notably different impacts on the inelastic dynamic responses and damage degrees of hydraulic arched tunnels. In addition, by comparison with our team’s previous work, it can be found that obliquely incident SV waves may be more likely to cause severe damage to hydraulic ached tunnels compared to obliquely incident P waves. On the other hand, significant durations and velocity-related intensity measures present strong correlations with the global tensile damage index of the linings of hydraulic arched tunnels. Therefore, both the incoming angles of SV waves and the seismological characteristics should be considered when evaluating the seismic performance and design of hydraulic arched tunnels.

Multi-pulse characteristics of near-fault ground motions

Multi-pulse characteristics of near-fault ground motions, such as the number of inherent pulses, pulse periods, and amplitudes, have notable influences on the response of structures. To investigate these important parameters, an automatic detection procedure, which is conducted on the rough pulse signal that is extracted by the HHT method, is proposed in this work. This procedure can localize all inherent pulses in the time domain independently and discontinuously. Important parameters can be automatically obtained at the same time. Then, statistical relationships between these multi-pulse parameters and earthquake parameters, including moment magnitudes, site conditions (Vs30), rupture distances and types of faults, are investigated comprehensively. With an increasing number of pulses, the multi-pulse ground motions are more likely to be recorded in relatively limited areas. All pulse periods in a velocity record are similar to each other and can be represented by the period of the pulse with the largest energy (TPE). TPEs are almost identical to periods of the first pulse in the time domain (TP1). They are related not only to magnitudes but also to fault-types and site conditions. New empirical models are proposed in this work according to fault-types that can predict most TPEs across all magnitudes (from 5.7 to 7.6). For amplitudes, all PGVs of inherent pulses can be expressed by the PGV of the pulse with the largest energy (PGVE) in a linear attenuation relationship. PGVEs are related to rupture distances, site conditions, and fault-types. New empirical models are also developed in this work.

Conditional Ground-Motion Model for Damaging Characteristics of Near-Fault Ground Motions Based on Instantaneous Power

ABSTRACT The velocity pulse in near-fault ground motions has been used as a key characteristic of damaging ground motions. Characterization of the velocity pulse involves three parameters: presence of the pulse, period of the pulse, and amplitude of the pulse. The basic concept behind the velocity pulse is that a large amount of seismic energy is packed into a short time, leading to larger demands on the structure. An intensity measure for near-fault ground motions, which is a direct measure of the amount of energy arriving in short time, called instantaneous power (IP (T1)), is defined as the maximum power of the bandpass-filtered velocity time series measured over a time interval of 0.5T1, in which T1 is the fundamental period of the structure. The records are bandpass filtered in the period band (0.2T1−3T1) to remove the frequencies that are not expected to excite the structure. Zengin and Abrahamson (2020) showed that the drift is better correlated with the IP (T1) than with the velocity pulse parameters for records scaled to the same spectral acceleration at T1. A conditional ground-motion model (GMM) for the IP is developed based on the 5%-damped spectral acceleration at T1, the earthquake magnitude, and the rupture distance. This conditional GMM can be used for record selection for near-fault ground motions that captures the key features of velocity pulses and can lead to a better representation of the median and variability of the maximum interstory drift. The conditional GMM can also be used in a vector hazard analysis for spectral acceleration (T1) and IP (T1) that can be used for more accurate estimation of drift hazard and seismic risk.

Reconnaissance and learning after the February 6, 2018, earthquake in Hualien, Taiwan

An earthquake with an epicenter offshore of Hualien City in eastern Taiwan occurred at midnight on February 6, 2018. The Richter magnitude (ML) of the earthquake was 6.26 and the seismic intensity ranged up to level VII, the strongest seismic intensity level regulated in Taiwan. Almost all the major damage resulting from this seismic event was occurred near both sides of the Milun Fault, where records from nearby strong motion stations displayed the characteristics of near-fault ground motions. The main seismic damage was the collapse of four buildings with soft bottom stories, one of which resulted in fourteen of the seventeen total fatalities. Comparing the acceleration response spectra with the design response spectra sheds light on the effects of near-fault ground motions on the collapsed buildings. Based on the eventual forms of collapsed buildings, building collapses that have generally led to major casualties in past seismic events around the world can be classified into sit-down, knee-down and lie-down types. In addition to the four collapsed buildings, seismic reconnaissance on other buildings, bridges, ports, and non-structural components have also been conducted. This study explores the issues and challenges arising from the reconnaissance results and thereby enhances learning from the seismic event.

Fragility analysis of a containment structure under far-fault and near-fault seismic sequences considering post-mainshock damage states

Due to the unique characteristics of near-fault ground motions with an intense pulse, near-fault seismic sequences have the potential to cause more severe damage to structures compared to far-fault seismic sequences. This study focuses on quantitatively comparing the seismic response and fragility of a containment structure subjected to near-fault and far-fault seismic sequences. For this purpose, both as-recorded and artificial seismic sequences are used as input to conduct the nonlinear dynamic analysis. The effect of fault types of aftershocks on a mainshock-damaged containment is investigated in terms of the global response and local damage, respectively. Seismic demands on secondary systems are also studied through floor response spectra considering post-mainshock damage states. To evaluate the influence of near-fault and far-fault seismic sequences on the fragility of a containment structure, a new methodology is used to generate fragility curves, which can effectively consider the effect of post-mainshock damage states when computing the exceedance probability. The results show that near-fault seismic sequences can cause greater dynamic responses of a containment than far-fault ground motions, and the fragility evaluation without considering near-fault seismic records can overestimate the safety margin of a containment structure.

Characterization of Near-Fault Effects on Potential Cumulative Damage of Reinforced Concrete Bridge Piers

The objective of this study is to characterize the potential damage of near-fault ground motions on reinforced concrete bridge piers, providing information for engineering practice. To achieve this objective, 200 real near-fault pulse-type records and a bridge pier were selected. The same number of real far-fault ground motions was also selected for comparison. Inelastic time history and damage analyses were performed for the pier subjected to the selected near-fault and far-fault ground motions. The obtained results showed that more than 91% near-fault motions caused the pier to collapse while the pier survived under almost all far-fault motions. The responses of the pier under near-fault motions were characterized by one or few large hysteretic cycles. The damage indices of the pier subjected to near-fault ground motions increased rapidly when the pier underwent pulses of near-fault motions, and the pier damage was mainly caused by pulses. Collapse of the pier subjected to near-fault motions occurred at the pulse time. The duration to collapse, which was defined as the duration from light damage to collapse, of the pier under near-fault motions was extremely short (approximately 2.3–2.5 s). These special and negative structural damage characteristics of near-fault ground motions should be considered in designing and mitigating seismic hazards for structures located in near-fault regions.

Analysis of seismic collapse mechanism of concrete frame structures subjected to near-fault ground motions

The characteristics of near-fault ground motions are significantly different from far-field ground motions. There are also great differences in the damage of concrete structures caused by these two kinds of ground motions. Earthquake damage indicates that the damage of concrete frame structures subjected to near-fault ground motions is more serious and collapse mechanism is more complicated. In this paper, the traditional alternative path method (APM) in the analysis of collapse was improved. Collapse mechanism of reinforced concrete frame was studied by numerical simulation method. A typical ten-story reinforced concrete frame model was established. Three harmonic waves and three practical near-fault ground motions were selected as acceleration time history input. The results show that different damaged parts of structure caused by strong earthquake have little effect on the final collapse mode when the structure is under free vibration. The failure mode of columns subjected to different ground motions is almost the same. Simple harmonic excitation whose period is close to the predominant period of ground motion can approximately simulate the failure mode of structure caused by earthquake.

Evaluation of seismic performance of reinforced concrete (RC) buildings under near-field earthquakes

Near-field ground motions are significantly severely affected on seismic response of structure compared with far-field ground motions, and the reason is that the near-source forward directivity ground motions contain pulse-long periods. Therefore, the cumulative effects of far-fault records are minor. The damage and collapse of engineering structures observed in the last decades’ earthquakes show the potential of damage in existing structures under near-field ground motions. One important subject studied by earthquake engineers as part of a performance-based approach is the determination of demand and collapse capacity under near-field earthquake. Different methods for evaluating seismic structural performance have been suggested along with and as part of the development of performance-based earthquake engineering. This study investigated the results of illustrious characteristics of near-fault ground motions on the seismic response of reinforced concrete (RC) structures, by the use of Incremental Nonlinear Dynamic Analysis (IDA) method. Due to the fact that various ground motions result in different intensity-versus-response plots, this analysis is done again under various ground motions in order to achieve significant statistical averages. The OpenSees software was used to conduct nonlinear structural evaluations. Numerical modelling showed that near-source outcomes cause most of the seismic energy from the rupture to arrive in a single coherent long-period pulse of motion and permanent ground displacements. Finally, a vulnerability of RC building can be evaluated against pulse-like near-fault ground motions effects.

SEMI-ACTIVE FUZZY CONTROL OF STRUCTURES SUBJECTED TO NEAR-FAULT GROUND MOTIONS HAVING FORWARD DIRECTIVITY AND FLING STEP USING FRICTION DAMPING SYSTEM WITH AMPLIFYING BRACES (FDSAB)

In this paper, the consequences of well-known characteristics of near-fault ground motions, forward directivity and fling step, on the seismic response control is investigated. An integrated fuzzy rule-based control strategy for building structures incorporated with semi active friction damping system with amplifying braces (FDSAB) is developed. The membership functions and fuzzy rules of fuzzy controller were optimized by Genetic Algorithm (GA). The main purpose of employing a GA is to determine appropriate fuzzy control rules as well to adjust parameters of the membership functions. Numerical study is performed to assess the effects of near-fault ground motions on a building that is equipped with FDSABs. To demonstrate the effectiveness of the fuzzy logic algorithm, it is compared with that of a conventional linear quadratic regulator (LQR) controller, while the uncontrolled system response is used as the base line. Results reveal that the fuzzy logic controller with FDSAB is capable of improving the structural responses and is promising for reducing seismic responses during near-fault earthquakes. It is also shown that, the near-fault earthquakes require much more control force than the far-field earthquakes and result in less response mitigation.