Effects Of Near-fault Ground Motions Research Articles

Unified framework for stochastic dynamic responses and system reliability analysis of long-span cable-stayed bridges under near-fault ground motions

Long-span cable-stayed bridges are complex systems with numerous components, and prone to failures of multiple components under near-fault ground motions with long-period velocity pulses. The evaluation of dynamic system reliability of cable-stayed bridges is an important challenging issue. In this work, a unified framework for simultaneously determining both the stochastic dynamic responses and system reliabilities of long-span cable-stayed bridges under near-fault stochastic ground motions is proposed based on direct probability integral method (DPIM) with adaptive strategy. Firstly, the 3-dimensional finite element model of typical cable-stayed bridge and the stochastic synthesis model of near-fault ground motions are established. Based on probability density integral equation, new formulas in input probability space of time-variant moments of response are derived. Then, DPIM with adaptive strategy is suggested to calculate the mean value and standard deviation of response via new formulas. Dynamic system reliability of cable-stayed bridge under near-fault stochastic ground motions is evaluated by establishing failure criteria of extreme value mappings of multiple performance functions. Finally, numerical results indicate that the long-period velocity pulse of near-fault motions imposes an important effect, resulting in large mean responses and failure of cables. Failure probability of cables is significantly increased with the occurrence of velocity pulses, which causes the decrease of system reliability of cable-stayed bridge. Thus, the randomness and velocity pulse effect of near-fault ground motions should be seriously considered for reliability-based design and safety assessment of cable-stayed bridges.

Seismic performance assessment of high arch dams considering the pulse-like and directionality effects of near-fault ground motions

This paper investigates the pulse-like and directionality effects of near-fault horizontal ground-motion records on the seismic performance of high arch dams. A 3-dimensional arch dam-reservoir-foundation finite-element model is built. Two near-fault bidirectional ground-motion suites (i.e. ordinary and pulse-like suite) are collected. The fault-normal and fault-parallel orientations of the horizontal ground-motion records are rotated to reflect the directionality effect. Nonlinear dynamic analyses are further implemented for the high arch dam excited by both near-fault ground motions with different incidence angles. The results show that the pulse-containing ground-motion records can yield more notable dynamic response and damage for high arch dams as compared to the ordinary records. Moreover, with changing the incidence angles, both the dam dynamic response and damage metrics exhibit periodic variation and constant trends for the pulse-containing and ordinary ground-motion cases, respectively. Therefore, the directionality effect has a significant influence on the seismic performance of high arch dams excited by pulse-containing ground-motion records. Compared with the near-fault ordinary ground motions, the pulse-like ground motions will induce a 40 %-increase in the concrete damage volume of high arch dams. In addition, peak ground velocity could be regarded as a preferable intensity measure for assessing the seismic damage of high arch dams.

Running Safety Assessment of a High-Speed Train on Bridges During Braking Under Near-Fault Ground Motions

This study aims to investigate the adverse effects of near-fault ground motions on the long-term development of high-speed railways. In this paper, the ground motions are analyzed based on the Train–Track–Bridge Coupling Braking System (TTBCBS) which has been validated during earthquakes. The effects of earthquakes on the whole system are discussed in depth, focusing on the random nature of earthquakes, the impulsive character of near-fault earthquakes and the effects of different initial speeds on the system behavior. The results show that under random earthquakes, the probability of train derailment gradually increases with the increase of peak ground acceleration (PGA) of earthquakes. When the PGA exceeds 0.2[Formula: see text]g, the train is highly susceptible to derailment and the bridge itself may incur damage. When analyzing the nature of pulses of near-fault ground motions, it was found that the responses induced by class B and class C pulses are significantly similar. Meanwhile, the calculated values of derailment coefficients are basically the same when the PGA of class A pulse wave is at 0[Formula: see text]g and 0.1[Formula: see text]g. This suggests that train braking somewhat mitigates the response induced by near-fault ground motions. Furthermore, the value of improved spectral intensity (SI) for the running safety during earthquakes indicates that the increase in seismic intensity is detrimental to the system. In terms of the effect of train initial speeds on the system during earthquakes, it is observed that the system response reaches the lowest point when the train speed is 250[Formula: see text]km/h, which is more favorable to the smooth deceleration of the train. When the train speed is 400[Formula: see text]km/h, the system response reaches its maximum value. These research findings provide crucial insights and guidance for seismic safety design and management of high-speed railways.

Seismic response of RC bridges under near-fault ground motions: A parametric investigation

Natural disasters such as earthquakes can potentially harm bridges and incur significant socioeconomic losses. This study examines the effect of near-fault ground motions on reinforced concrete bridges, specifically the variation in the seismic performance with the height, aspect ratio, and longitudinal reinforcement. A suite of 332 near-fault ground motions is used to evaluate the performance of 60 bridge models in OpenSees, and the seismic responses are measured in terms of pier curvature ductility, maximum bearing deformation, and deck displacement. Linear regression analysis is used to generate probabilistic seismic demand models and fragility curves corresponding to different damage states. Expected behaviors are displayed within the models grouped with respect to height classes. Since the column diameters are based on aspect ratios, the effect of taller columns reduces the exceedance probabilities. This brought forward a need to define the damage states for tall columns in a manner different from short or medium columns. Moreover, this study underscores the need to update HAZUS fragility median values for near-field seismic response of RC bridges.

Near-fault ground motion effect on self-centering modular bracing panels considering soil-structure interaction

This study investigates the nonlinear seismic response behavior of self-centering modular steel bracing panel (SCMBP) systems subjected to near-fault ground motion records, considering the soil-structure interaction (SSI) effect. Analytical formulas for the load-displacement relationship of an SCMBP were validated through nonlinear 3D continuum finite element analysis while a simplified 2D wireframe model utilizing a constitutive spring model was verified by the 2D refined finite element model and subsequently adopted for time history analysis of the SCMBP structures with SSI effect. Based on the nonlinear time history analysis of a 6-story prototype building subjected to a suite of near-fault ground motions, the prototype structure is found to be able to re-center itself even under near-fault ground motion scaled to the design basis earthquake (DBE) level while frame members remain undamaged, except for fuse devices. Additionally, SSI reduces inter-story drift ratios under both far-field and near-fault ground motions. The findings provide insights into the promising performance of using SCMBP systems as a seismic strengthening system for sites with potential SSI effects and near-fault earthquake impact. The findings of this study can be insightful to other self-centering systems with flag-shaped story shear hysteresis.

The Effects of Near-fault Ground Motions on Optimized Triple Friction Pendulum in Asymmetric Superstructures

The effect of near-fault ground motions containing translational and rotational components is investigated by considering the analytic seismic response of a stiff single-story base-isolated asymmetric superstructure with triple friction pendulum bearing (TFPB). Firstly, six asymmetric base-isolated structures instrumented by optimum TFPBs were exposed to translational components of near-fault excitations with wide ranges of pulsive periods. Next, four single objective functions as target responses (including top floor absolute acceleration, maximum inter-story drift, base shear of structure, and isolator displacement), as well as a combined fitness function as a performance function, were determined to consider all the seismic responses simultaneously. Hence, the optimum design variables of TFPBs (including curvature radii, displacement capacity, and friction coefficient) were calculated using the Genetic Algorithm (GA) to minimize the fitness and performance functions separately for each defined eccentricity. Finally, the structural behavior in the presence of mass eccentricity on the optimum selected TFPB under transitional and transitional-rotational components was explored for a more careful comparison. Also, fifteen combinations of translational-rotational components were employed for a comprehensive investigation. The results demonstrate that roof acceleration could be increased by about 7.5% when mass eccentricity and rotational components are present at the same time.

An Efficient Method for Predicting the Residual Displacement of UBPRC Columns Under Near-Fault Ground Motions

ABSTRACT As an effective measure to decrease the residual displacement under earthquakes, unbonded prestressed concrete (UBPRC) columns have been carefully investigated in previous research, while the influence of the complex characteristics of near-fault excitations on the residual displacement of UBPRC is less mentioned. This paper, therefore, focuses on the influence mechanism of the pulse effect and vertical effect of near-fault ground motions on the residual displacement of UBPRC columns. The residual displacement, at first, is formulated as a function of the coupled parameter T 1-p (the ratio of the structural period to the pulse period). Then, the effect of near-fault vertical ground motion on the residual displacement is qualified based on the established mathematical model. Further, the model has been applied to investigate the effect of structural properties (dead load axial compression ratio, longitudinal reinforcement ratio, prestressing axial compression ratio, prestressing reinforcement ratio) on the residual displacement of UBPRC columns subjected to the joint action of horizontal and vertical ground motions. This research provides a quantitative methodology to quickly obtain the residual displacement spectra of the UBPRC columns under varying conditions combined with ground motion and structural parameters.

Effects of pulse-like ground motions on tunnels in saturated poroelastic soil for obliquely incident seismic waves

It has been widely recognised that near-fault ground motions can be distinctly different from far-fault ground motions, in terms of their amplitudes and spectral characteristics, and there are a number of studies investigating the effects of near-fault ground motions on the structural response. However, only a few studies focus on underground tunnels, and most of them consider vertically incident seismic waves, neglecting the wave passage effect which is critical for long tunnels. This paper investigates the consequences of near-fault pulse-like ground motions on the seismic tunnel response, with an example of a tunnel embedded in saturated poroelastic soil. The input motions are represented by obliquely incident P1 waves, and the wave passage effect along the longitudinal direction is considered by a 2.5D modelling technique. The seismic tunnel response for near-fault pulse-like and far-fault ground motions is compared. Additionally, artificial ground motions, which have consistent acceleration response spectra with the near-fault pulse-like ground motions, but without velocity pulses, are generated to gain further insight into the contribution of velocity pulses. It is shown that the near-fault pulse-like ground motions can noticeably increase the tunnel internal forces, due to large seismic energy associated with the velocity pulses. For pulse-like ground motions with similar acceleration response spectra, the fling-step velocity pulse can be more detrimental to the tunnel than the forward-directivity velocity pulse. Moreover, the amplification effect of the near-fault pulse-like ground motions on the tunnel internal forces tends to be more prominent for large vertical angles of incidence, highlighting the significance of considering the oblique incidence case for seismic design.

Vertical effects of near-fault ground motions and the optimal IMs for seismic response of continuous girder bridges with FPB isolators

The characteristics of vertical components in the near-fault region were investigated based on 121 pairs of typical near-fault ground motions. A weak positive correlation between the vertical and horizontal peak ground acceleration (PGA) and peak ground velocity (PGV) is shown. Basically, the ratios of vertical to horizontal peak ground acceleration and velocity of the near-fault ground motions comply with the lognormal distribution, and the vertical ground motions are significantly stronger at lower periods. Fifteen scaler horizontal and nine scaler vertical IMs are compared to select the optimal scaler IMs of horizontal and vertical seismic demands of an isolated girder bridge with friction pendulum bearings (FPB isolators). It is found that acceleration-related scalar IMs perform better for vertical seismic demands, while velocity-related scaler IMs perform better for horizontal seismic demands. The optimal vector-valued IM, consisting of scalar horizontal and vertical IMs in the forms of IMh,IMv, for horizontal seismic demands were proposed. The optimal vector-valued IMs show an apparent improvement in efficiency compared with the corresponding scaler IMs when vertical components are considered. Furthermore, the probabilistic seismic demand models (PSDMs) for horizontal seismic demands with optimal vector-valued IM were adopted to assess the seismic demands under a given intensity of vertical and horizontal ground motions. It is identified that the vertical effects significantly influence the shear force of piers while slightly influencing the curvature of piers and bearing displacements.

Effects of Near-Fault Ground Motions on Civil Infrastructure

Near-fault earthquakes (NFEs), characterized by high peak ground velocity (PGV) and long period pulses, show different properties from far-field ones [...]

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.

Modeling of non-ductile RC structure under near-fault ground motions: A nonlinear finite element analysis

The old existing reinforced concrete (RC) structures in Taiwan are susceptible to severe damage under earthquakes because of the soft-story mechanism. Moreover, the effects of near-fault ground motions often contain a long period of velocity pulse and permanent ground displacement. This study is based on the tri-axial shaking table test conducted at the National Center for Research on Earthquake Engineering in Taiwan and a series of seismic performance evaluations of a non-ductile RC structure in Taiwan. Finite element analysis (FEA) will be conducted to simulate the linear and nonlinear behavior of the seven-story building. This accommodates the damage plasticity model for concrete and the elastic-perfectly plastic model for reinforcement. Comparison of the results between the experimental test and numerical model showed that the similarity of the vibration period has a great influence on the simulation results. The findings showed that the data of acceleration and displacement behavior corresponded with the experimental results in a satisfactory margin. Also, the damage mode is very similar to the shaking table test results. The study found that using FEA can satisfactorily simulate the seismic performance of mid-rise buildings under a near-fault earthquake.

Probabilistic seismic assessment of steel gabled frames with web-tapered members under near-fault ground motions along strike-normal and strike-parallel components

The seismic assessment of steel gabled frames (SGFs) is of great importance and a key problem in any high-seismicity region, given the enormous costs of industrial equipment and the remarkable number of individuals working in such structures. Near-fault ground motions, especially their strike-normal component that usually contain a long-period pulse in the velocity time-history, can lead to a significant demand in these structures in comparison to the far-fault ground motions. Although numerous studies have confirmed the destructive effects of near-fault ground motions on concrete and steel structures, no research has been devoted to the impact of this ground motions on steel gabled structures yet. Hence, the findings of this research can unveil novel dimensions of such structures. In this regard, in the present paper, an incremental dynamic analysis (IDA) was conducted for the first time on four SGFs with the spans of 20 m and 60 m and heights of 6 m and 12 m using far-fault ground motions (OR set), as well as near-fault ground motions along the strike-normal and strike-parallel components (SN and SP sets, respectively). The results were presented in the form of multi-record IDA curves, summarized IDA curves, probabilistic seismic demand models (PSDMs) and probabilistic seismic demand analysis (PSDA) curves. The outcomes indicated that compared to far-fault ground motions, near-fault ground motions (especially pulse-like ones) produced significant changes on the seismic behavior of long-period SGFs, resulting in raises the changes of stiffness and demand sensitivity, reduces the dynamic capacity, enhances the data dispersion and uncertainty, and increases the mean annual frequencies (MAFs) in such structures. However, the results of PSDA analysis showed that under any type of ground motion (OR, SN and SP), short-period SGFs are more vulnerable than long-period SGFs and should be prioritized for retrofitting. Finally, the importance of combining the hazard curve of the study region with the results of IDA analysis of the structure in evaluating the seismic behavior of SGFs is highlighted.

Empirical models of bridge seismic fragility surface considering the vertical effect of near-fault ground motions

Near-fault vertical ground motions (VGMs) caused great damage to bridge structures, but investigations of the effect on probabilistic seismic performance and structural vulnerability are insufficient. For this purpose, a novel model of bridge seismic fragility surface conditioned on the horizontal peak ground velocity (Vx) and the αVH (ratio of vertical to horizontal ground motion) is proposed. The quantitative formulation for probabilistic seismic demand model (PSDM) and probabilistic seismic capacity model (PSCM) of bridge piers considering the effect of VGM are established, and then the empirical model of fragility surface with the parameters of Vx and αVH is obtained by convolution. A simply supported bridge model is simulated in OpenSees as an illustrated example to establish the seismic fragility surface under the combined actions of horizontal ground motion and VGM. Furthermore, the damage probability (Fr) influenced by VGM is analyzed compared with the damage probability (Fr0) without VGM, and the sensitivity intervals of the difference between Fr and Fr0 for varying damage states are identified. The mechanism of fragility variation under VGM is explored, indicating that the variation of seismic demand caused by VGM is the main reason. Finally, the seismic fragility surface model considering VGM is applied to a set of variable piers, and the sensitivity interval associated with the fundamental period is investigated under different Vx.

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 responses and damage assessment of a mid-rise cold-formed steel building under far-fault and near-fault ground motions

The responses and damages of cold-formed steel (CFS) structures under near-fault ground motions are not clear currently, which are important for seismic design of CFS structures. This paper investigates the effects of near-fault ground motions on a 6-story CFS building. Three groups of records were chosen from the same events and different stations. The mean damage index of the first-story wall increases by 6.9∼18.7% and 34.3∼53.4% under the near-fault (no-pulse) and near-fault (pulse-like) records, respectively. The effects of near-fault earthquakes should be considered, and the Park-Ang seismic damage index is more appropriate for the seismic design of CFS buildings.

Pulse-like near-fault ground motion effects on controlled rocking steel cores

Controlled rocking steel cores (CRSCs) with reliable self-centering capability have been known as a low-damage seismic force-resisting system. The flag-shaped hysteresis response of CRSCs can be provided using unbonded post-tensioning tendons and replaceable energy dissipation devices. This paper examines the seismic performance of CRSCs subjected to a suite of pulse-like near-field and far-field ground motions. Non-linear dynamic response analysis is performed for 3-, 6-, 9-, and 12-story archetypes with single and coupled configurations. Results are quantified for different seismic responses, and the influences of earthquake type and structural configuration are compared from various aspects. The findings indicate remarkable changes in the performance of CRSCs due to modeling and ground motion properties. The outcomes reveal that rocking-core archetypes are efficient for mitigating residual damage and restricting peak displacements under both far- and near-field motions.

A simplified iterative approach for testing the pulse derailment of light rail vehicles across a viaduct to near-fault earthquake scenarios

Near-fault (NF) earthquakes cause severe bridge damage, particularly urban bridges subjected to light rail transit (LRT), which could affect the safety of the light rail transit vehicle (“light rail vehicle” or “LRV” for short). Now when a variety of studies on the fault fracture effect on the working protection of LRVs are available for the study of cars subjected to far-reaching soil motion (FFGMs), further examination is appropriate. For the first time, this paper introduced the LRV derailment mechanism caused by pulse-type near-fault ground motions (NFGMs), suggesting the concept of pulse derailment. The effects of near-fault ground motions (NFGMs) are included in an available numerical process developed for the LRV analysis of the VBI system. A simplified iterative algorithm is proposed to assess the stability and nonlinear seismic response of an LRV-reinforced concrete (RC) viaduct (LRVBRCV) system to a long-period NFGMs using the dynamic substructure method (DSM). Furthermore, a computer simulation software was developed to compute the nonlinear seismic responses of the VBI system to pulse-type NFGMs, non-pulse-type NFGMs, and FFGMs named Dynamic Interaction Analysis for Light-Rail-Vehicle Bridge System (DIALRVBS). The nonlinear bridge seismic reaction determines the impact of pulses on lateral peak earth acceleration (Ap) and lateral peak land (Vp) ratios. The analysis results quantify the effects of pulse-type NFGMs seismic responses on the LRV operations' safety. In contrast with the pulse-type non-pulse NFGMs and FFGMs, this article's research shows that pulse-type NFGM derail trains primarily via the transverse velocity pulse effect. Hence, this study's results and the proposed method can improve the LRT bridges' seismic designs.

Assessment of Near-Fault Ground Motion Effects on the Fragility Curves of Tall Steel Moment Resisting Frames

Nowadays it is common to use the fragility curves in probabilistic methods to determine the collapse probability resulting from an earthquake. The uncertainties exist in intensity and frequency content of the earthquake records are considered as the most effective parameters in developing the fragility curves. The pulse-type records reported in the near-fault regions might lead to the major damages in the structures having moderate and long periods since response spectra of near-fault ground motions within the long period range are different from those of the far-fault ground motions. In the present study, the influence of this type of earthquake records on the fragility curves of the steel special moment resisting frames, SMRFs, was examined. The results indicated that the median value of the collapse capacity (i.e.ŜCt Parameter, which defines the earthquake intensity leading to the collapse of the structure in half-set of the chosen records) due to near-fault ground motions was 76% that of the far-fault records for the ten-story example SMRF.

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.