Large Velocity Pulses Research Articles

Effect of ground motion filtering on the dynamic response of a seismically isolated bridge with and without fault crossing considerations

High-pass filtering not only removes the low-frequency noise from the near-fault ground motion records, but also eliminates the permanent ground displacement and reduces the dynamic ground displacement. This may considerably influence the calculated seismic response of a spatially extended engineering structure crossing a fault rupture zone. To demonstrate the importance of incorporating permanent ground displacements in the analysis and design of extended structures under specific fault crossing conditions, the dynamic response of a seismically isolated bridge located in the vicinity of a surface fault rupture (“Case A”) or crossing a fault rupture zone (“Case B”) is calculated by utilizing a near-fault ground motion record processed with and without a displacement offset. The seismically isolated bridge considered in this study is a 10-span continuous structure supported by 11 piers, resembling a typical segment of the 2.3km long Bolu Viaduct 1 located in west-central Turkey. The Lucerne Valley record from the 1992 Mw 7.2 Landers earthquake, which preserves a permanent ground displacement in the fault-parallel direction and exhibits a large velocity pulse in the fault-normal direction, is used as the basis for investigating the effect of high-pass filtering on the dynamic response of the bridge. For the seismically isolated bridge located in the vicinity of the surface fault rupture (“Case A”), the utilization of the high-pass filtered ground motion leads to underestimating the demands of pier top, pier bottom and deck displacements. However, the demands of isolation displacement, isolation permanent displacement and pier drift are almost identical for both the unfiltered and filtered versions of the ground motion record. On the other hand, for the seismically isolated bridge traversed by a fault rupture zone (“Case B”), all response quantities are significantly underestimated when the high-pass filtered ground motion is used. These results, though limited to a single bridge structure and a single ground motion input, clearly indicate the importance of permanent ground displacement on the dynamic response of spatially extended engineering structures crossing fault rupture zones.

Effect of Supplemental Damping on the Seismic Performance of Triple Pendulum Bearing Isolators under Near-Fault Ground Motions <i></i>

The isolating system absorbs part of the earthquake energy before transferring it to the structure, by shifting the natural period of the isolated structure. This period shift results in a reduction in the inertial forces. It is clear that the effects of near-fault (NF) ground motions with large velocity pulses can bring the seismic isolation devices to critical working conditions. In this study, two three-dimensional RC buildings with the heights of 9.0m and 21.0m which are supported by Triple Friction Pendulum Bearing (TFPB) isolators are idealized. Various TFPB configurations are selected for isolation systems. There are also viscous dampers to limit the excess deformation of isolators. Nonlinear time history analyses were performed by using OpenSees to study the influence of supplemental dampers on structural responses such as isolator displacements and maximum drifts under ten near-fault ground motion records. The results show noticeable reduction in isolator displacement when using dampers. However, maximum drift rises considerablely. Moreover by increasing the period range or reducing the damping ratio of isolation system, maximum driftreduces but the displacement of isolator increases.

Analytical Assessment of CFRP Retrofitted Reinforced-Concrete Buildings Subjected to Near-Fault Ground Motions

AbstractApplication of externally bonded fiber-reinforced polymers (FRPs) in the form of composite sheets has received a great deal of attention in the last two decades. However, their efficiency in improving the seismic performance of reinforced concrete (RC) structures subjected to near-fault ground motions has not yet been scrutinized in detail. Seismic motions near the fault rupture can be characterized by a large long-period velocity pulse at the beginning of the earthquake, causing severe damage or even collapse of code-compliant buildings. Although seismic design codes cannot adequately address the effect of these impulsive ground motions on structures, retrofitting of many important buildings located in regions near the active faults is high priority. Towards this, a numerical investigation is conducted in this study on the seismic retrofitting of existing RC building structures under near-fault ground motions using carbon FRP (CFRP) sheets. As the case study, an 8-story RC moment resisting frame ...

Performance limits of seismically isolated buildings under near-field earthquakes

The mission of seismic isolation is two folds: (i) protecting the integrity and (ii) protecting the contents of a structure by reducing floor accelerations to target limits and concurrently keeping base displacements below practical and economical limits. To this end, while seismic isolation has proven to be successful under far-field earthquakes, its success in case of near-field earthquakes is being questioned for over a decade now; the main reason being the threat of excessive base displacements due to the presence of long period large velocity pulses. Lowering isolation period and increasing isolation damping aiming to reduce base displacements unfortunately may result in a reduced seismic performance in terms of floor accelerations particularly under far-field earthquakes. And the success level of such a precaution under near-field earthquakes depends both on the fault-distance and the pulse period to the isolation period ratio. Thus, the aim of this study is to evaluate the performance limits of buildings equipped with seismic isolation systems of different characteristics when subjected to near-field ground motions at different fault distances with different velocity pulse periods. Accordingly, a methodology for assessing the performance limit of a seismic isolation system design that explicitly considers the seismic actions on the contents of the superstructure in the near-field region is introduced. Benchmark buildings with base isolation systems of different isolation periods and characteristic force ratios are subjected to synthetically developed near-field earthquake records at different fault-distances with different velocity pulse periods and their seismic performances are reported.

Seismic assessment of the rigid sliding displacements caused by pulse motions

This paper presents a comprehensive study on the rigid block sliding displacement of slopes subjected to ground motions with large velocity pulses. A comparison of the performance of various existing empirical displacement models is provided through analyses of the displacement residuals of slopes subject to pulse-like motions. Except for the PGA- and PGV-based Saygili and Rathje model (2008, referred to as SR08), positive medians of residuals are observed for selected models, indicating an under-estimation. There is a negative constant shift in the total residuals for the SR08 model, which can be easily fixed by changing the constant term in the predictive equation. The residuals from the SR08 model also have the smallest standard deviation compared to the other models. A modified SR08 model is developed for predicting rigid block sliding displacements for pulse-like motions. The modified predictive model is used in probabilistic seismic displacement analyses of slopes in a hypothetical near-fault region.

Response of Seismically Isolated Steel Frame Buildings with Sustainable Lead-Rubber Bearing (LRB) Isolator Devices Subjected to Near-Fault (NF) Ground Motions

Base isolation has been used as one of the most wildly accepted seismic protection systems that should substantially dissociate a superstructure from its substructure resting on a shaking ground, thereby sustainably preserving entire structures against earthquake forces as well as inside non-structural integrities. Base isolation devices can operate very effectively against near-fault (NF) ground motions with large velocity pulses and permanent ground displacements. In this study, comparative advantages for using lead-rubber bearing (LRB) isolation systems are mainly investigated by performing nonlinear dynamic time-history analyses with NF ground motions. The seismic responses with respects to base shears and inter-story drifts are compared according to the installation of LRB isolation systems in the frame building. The main function of the base LRB isolator is to extend the period of structural vibration by increasing lateral flexibility in the frame structure, and thus ground accelerations transferred into the superstructure can dramatically decrease. Therefore, these base isolation systems are able to achieve notable mitigation in the base shear. In addition, they make a significant contribution to reducing inter-story drifts distributed over the upper floors. Finally, the fact that seismic performance can be improved by installing isolation devices in the frame structure is emphasized herein through the results of nonlinear dynamic analyses.

Effect of supplemental damping on seismic response of base isolated frames under near & far field accelerations

The seismic isolation, as a passive control method of structures and an innovative approach in resistant seismic design, significantly reduces the response of structural systems induced by strong ground motions. The performance of these systems in Near Fault (NF) ground motions with large pulses is different from that of Far Fault (FF) ground motions. It is indicated that the effects of near field earthquake with large velocity pulses can bring the seismic isolation devices to critical working conditions. To overcome this problem, the application of viscous damper is represented as an approach to improve the performance of these systems in near field earthquakes. In the present paper, through modeling six base isolated structures, the seismic responses were studied under near field and far field earthquakes. Nonlinear time history analyses were performed, applying the finite element software of ABAQUS to study the influence of various values of supplemental damping (5% ∼ 40%) on the responses of base isolated structures. The results demonstrated that, generally, by increasing supplemental damping ratio under near field earthquakes, the base displacement decreases; however, the stories relative displacements and accelerations increase. It is observed that by increasing supplemental damping ratio under far field earthquakes, the base displacement and the stories relative displacements decreases; however, the stories accelerations increase. Based on the analyses, these variations proved to be more prevalent under near field earthquakes and taller structures.

Near-fault isolation of cable-stayed bridges using RNC isolator

This paper investigates the near-fault (NF) seismic performance of a recent isolation system, referred to as Roll-N-Cage (RNC) isolator, considering the cable-stayed Bill Emerson Memorial Bridge in Missouri, USA. Under NF ground motion, the seismic isolation devices might perform poorly because of large isolator displacements caused by long-period large velocity and displacement pulses associated with such strong motion. The RNC isolator is designed to achieve a balance in controlling isolator displacement demands and structural accelerations. It provides in a single unit all the necessary functions of rigid support, horizontal flexibility with enhanced stability and energy dissipation characteristics. Moreover, it is distinguished from other isolation devices by two unique features: (1) it has a built-in energy absorbing buffer to limit the isolated deck displacement under severe seismic excitations to a preset value and (2) it has a built-in linear recentering mechanism that prevents residual displacement after earthquakes. The seismic response of the RNC-isolated cable-stayed bridge is investigated using nonlinear time history analysis under three recorded NF earthquakes and three synthetic ground accelerations that capture many of the kinematic characteristics of recorded NF ground motions. The results show that the RNC isolator is a convenient isolation system in protecting cable-stayed bridges against NF earthquakes.

Seismic Response of a Viaduct under near-Fault Earthquakes

Abundant long-periodic components endue near-fault earthquakes with large velocity pulses while the viaducts owe long periodic fundamental period. To explore the seismic response patterns of viaducts under near-fault earthquakes, the paper analyzed the time history response of a viaduct under near-fault seismograms including TCU 1063, TCU 065, El centro and Artificial wave via FEM software Civil. The analysis shows a significant boost in near-fault seismic response as the value of PGV/PGA increased accordingly, indicates that the near-fault pulse affects strongly on the structural dynamic response of the viaducts.

The effect of near‐fault directivity on building seismic collapse risk

SUMMARYForward directivity may cause large velocity pulses in ground motion time histories that are damaging to buildings at sites close to faults, potentially increasing seismic collapse risk. This study quantifies the effects of forward directivity on collapse risk through incremental dynamic analysis of building simulation models that are capable of capturing the key aspects of strength and stiffness degradation associated with structural collapse. The paper also describes a method for incorporating the effects of near‐fault directivity in probabilistic assessment of seismic collapse risk. The analysis is based on a suite of RC frame models that represent both past and present building code provisions, subjected to a database of near‐fault, pulse‐like ground motions with varying pulse periods. Results show that the predicted collapse capacity is strongly influenced by variations in pulse period and building ductility; pulse periods that are longer than the first‐mode elastic building period tend to be the most damaging. A detailed assessment of seismic collapse risk shows that the predicted probability of collapse in 50 years for modern concrete buildings at a representative near‐fault site is approximately 6%, which is significantly higher than the 1% probability in the far‐field region targeted by current seismic design maps in the US. Copyright © 2012 John Wiley & Sons, Ltd.

THE STRONG MOTION RECORDS OF THEMs8.0 WENCHUAN EARTHQUAKE BY THE DIGITAL STRONG EARTHQUAKE NETWORK IN SICHUAN AND THE NEIGHBORING REGION

The Ms8.0 Wenchuan Earthquake of May 12, 2008 resulted in oblique dextral-thrust motion in the Longmen Shan tectonic belt resulted. During this earthquake event, 133 sets of three-component acceleration records were collected by the digital earthquake network in Sichuan Province. By using these records and some strong motion records from the networks in Shanxi and Gansu provinces, contours of peak ground acceleration were determined. These contours are elliptically shaped with the major axis oriented in a northeast direction. The peak acceleration decayed more gradually toward the northeast, parallel to the rupture propagation than toward the southwest, indicating a directivity effect. The peak acceleration also decayed more gradually toward the northwest, on the hanging wall, than toward the southeast on the footwall. A relatively high rate of attenuation in the peak acceleration was also evident on the Yingxiu-Beichuan section of the fault; this can be attributed to the seismic source fracture pattern and rupture progress. The measured peak vertical and horizontal ground acceleration components were far larger than the design values prescribed by the Code for Seismic Design of Buildings (GB50011-2001). As distance from the rupture increased, the acceleration response spectrum gradually became dominated by long-period motions. A large velocity pulse was also measured at a distance of about 80–100 km from the fault.

Seismic Response of Base-Isolated Structures with LRB and FPS under near Fault Ground Motions

Seismic response of structures in the vicinity of causative earthquake faults can be significantly different than those observed further away from the seismic source. In the near fault zone, ground motions are significantly influenced by the rupture mechanism and slip direction relative to the site and by the permanent ground displacement at the site resulting from tectonic movement. Forward directivity and fling effects have been identified by the seismologists as the primary characteristics of near fault ground motions. Because of the unique characteristics of the near-fault ground motions and their potential to cause severe damage to structures designed to comply with the criteria mostly based on far-field earthquakes, the estimation of seismic response of base-isolated structures for a project site close to an active fault should account for these special aspects of near fault ground motions. This paper investigates the seismic response of base-isolated structures with LRB and FPS isolators under near fault ground motions. A seismic evaluation of the building, isolated with the LRB and FPS, is performed using a nonlinear three-dimensional analytical model. The parametric study is concentrated on base shear, accelerations and displacements of isolated models. Large displacement and velocity pulses in records of near fault ground motions can significantly change the results of seismic response of base-isolated structures.

Response spectra for nuclear structures on rock sites considering the near-fault directivity effect

Near-fault ground motions, potentially with large amplitude and typical velocity pulses, may significantly impact the performance of a wide range of structures. The current study is aimed at evaluating the safety implications of the near-fault effect on nuclear power plant facilities designed according to the Chinese code. To this end, a set of nearfault ground motions at rock sites with typical forward-directivity effect is examined with special emphasis on several key parameters and response spectra. Spectral comparison of the selected records with the Chinese and other code design spectra was conducted. The bi-normalized response spectra in terms of different corner periods are utilized to derive nuclear design spectra. It is concluded that nuclear design spectra on rock sites derived from typical rupture directivity records are significantly influenced both by the earthquake magnitude and the rupture distance. The nuclear design spectra specified in the code needs to be adjusted to reflect the near-fault directivity effect of large earthquakes.

Study on elastic response of structures to near-fault ground motions through record decomposition

Accelerograms recorded near active faults have some important characteristics that make them different from those recorded in far-fault regions. High-frequency components in acceleration records and long-period velocity pulses are among notable specifications of such ground motions. In this paper, a moving average filtering with appropriate cut-off frequency has been used to decompose the near-fault ground motions into two components having different frequency contents: first, Pulse-Type Record (PTR) that possesses long-period pulses; second, the relatively high-frequency BackGround Record (BGR), which does not include large velocity pulses. Comparing the results with those extracted through wavelet analysis shows that moving average filter is an appropriate and efficient tool for near-fault records decomposition. The method is applied to decompose a suite of 91 selected near-fault records and the elastic response of structures is examined through their response to the decomposed parts. The results emphasizes that in contrast with ordinary far-fault earthquake records, response spectra of near-fault ground motions typically have two distinct local peaks, which are representatives of the high- and low-frequency components, i.e., BGR and PTR, respectively. Moreover, a threshold period is identified below which the response of structures is dominated by BGR while PTR controls the response of structures with periods longer than this period.

A MULTI-EPOCHHSTSTUDY OF THE HERBIG-HARO FLOW FROM XZ TAURI

We present nine epochs of Hubble Space Telescope optical imaging of the bipolar outflow from the pre–mainsequence binary XZ Tauri. Our data monitor the system from 1995 to 2005 and include emission line images of the flow. The northern lobe appears to be a succession of bubbles, the outermost of which expanded ballistically from 1995 to 1999 but in 2000 began to deform and decelerate along its forward edge. It reached an extent of 6 �� from the binary in 2005. A larger and fainter southern counterbubble was detected for the first time in deep Advanced Camera for Surveys images from 2004. Traces of shocked emission are seen as far as 20 �� south of the binary. The bubble emission nebulosity has a low excitation overall, as traced by the [Sii]/Hα line ratio, requiring a nearly comoving surrounding medium that has been accelerated by previous ejections or stellar winds. Within the broad bubbles there are compact emission knots whose alignments and proper motions indicate that collimated jets are ejected from each binary component. The jet from the southern component, XZ Tau A, is aligned with the outflow axis of the bubbles and has tangential knot velocities of 70–200 km s −1 . Knots in the northern flow are seen to slow and brighten as they approach the forward edge of the outermost bubble. The knots in the jet from the other star, XZ Tau B, have lower velocities of ∼100 km s −1 . To explain the observations of the outer bubble, we propose that the XZ Tau A stellar jet underwent a large velocity pulse circa 1980. This ejection quickly overtook older, slower-moving ejecta very near the star, producing a ∼70 km s −1 shock in a hot (T ∼ 80,000 K), compact “fireball.” The initial thermal pressure of this gas parcel drove the expansion of a spherical bubble. Subsequent cooling caused the bubble to transition to ballistic expansion, followed by slowing of its forward edge by mass loading from the pre-shock medium. Repeated pulses may explain the multiple bubbles seen in the data. Collimated jets continue to flow through the bubble’s interior, and with the fading of the original fireball they are becoming the primary energizing mechanism for the emission line structures. Future evolution of the flow should see the outer bubble structures fade from view, and the emergence of a more typical Herbig–Haro jet/bow shock morphology. We present a preliminary numerical model of a pulsed jet to illustrate this scenario.

Analytical Prediction of the Seismic Performance of Masonry Infilled Reinforced Concrete Frames Subjected to Near-Field Earthquakes

In the recent past, a number of ground motions with large velocity pulses have been recorded in the near-field regions of earthquakes. A ground motion recorded in the near-field region of a strike-slip earthquake (i.e., near the fault rupture of the earthquake) exhibits distinctive long-period velocity pulses that are characteristic of near-field earthquakes and are not found in far-field ground motion records. The present study is based on analytical evaluation of the effect of distribution of masonry infill panels over the elevation of planar masonry-infilled RC frames on the seismic response of the frame subjected to near-field earthquakes implementing rational seismic analysis methods within the performance-based framework such as nonlinear dynamic time-history analysis and nonlinear static force-displacement (push-over) analysis using realistic and efficient computational models. Despite intense worldwide research in the area of masonry infilled frame structures during the past few decades, displacement-based nonlinear analyses of masonry infilled frames with explicit consideration of infill panels as structural elements is far from common practice, mostly due to the analytical complexity in modeling the infill panel realistically. The displacements are of particular interest from the viewpoint of performance-based design (PBD), the emerging model for the next generation of standard codes of practice for earthquake-resistant design. One of the objectives of the study is to demonstrate rational displacement-based analysis methodologies within the performance-based framework for accurately and efficiently predicting the global and local seismic demands and related intensity of seismic damage in practical masonry infilled multistory RC framed construction with seismically undesirable vertical layouts of masonry panels under the influence of near-field earthquakes.

Near-fault ground motions with prominent acceleration pulses: pulse characteristics and ductility demand

Major earthquakes of last 15 years (e.g., Northridge 1994, Kobe 1995 and Chi-Chi 1999) have shown that many near-fault ground motions possess prominent acceleration pulses. Some of the prominent ground acceleration pulses are related to large ground velocity pulses, others are caused by mechanisms that are totally different from those causing the velocity pulses or fling steps. Various efforts to model acceleration pulses have been reported in the literature. In this paper, research results from a recent study of acceleration pulse prominent ground motions and an analysis of structural damage induced by acceleration pulses are summarized. The main results of the study include: (1) temporal characteristics of acceleration pulses; (2) ductility demand spectrum of simple acceleration pulses with respect to equivalent classes of dynamic systems and pulse characteristic parameters; and (3) estimation of fundamental period change under the excitation of strong acceleration pulses. By using the acceleration pulse induced linear acceleration spectrum and the ductility demand spectrum, a simple procedure has been developed to estimate the ductility demand and the fundamental period change of a reinforced concrete (RC) structure under the impact of a strong acceleration pulse.

Effect of LRB isolators and supplemental viscous dampers on seismic isolated buildings under near-fault excitations

Base isolation is a quite sensible structural control strategic design in reducing the response of a structural system induced by strong ground motions. It is clear that the effects of near-fault (NF) ground motions with large velocity pulses can bring the seismic isolation devices to critical working conditions. In the present paper, nonlinear time history analyses were performed using a commercial structural analysis software package to study the influence of isolation damping on base and superstructure drift. Various lead-rubber bearing (LRB) isolation systems are systematically compared and discussed for aseismic performances of two actual reinforced concrete (RC) buildings. Parametric analysis of the buildings fitted with isolation devices is carried out to choose the appropriate design parameters. The efficiency of providing supplemental viscous damping for reducing the isolator displacements while keeping the substructure forces in reasonable ranges is also investigated.

Simulation method of near-fault pulse-type ground motion

The two characteristics of near-fault ground motions, i.e., the forward directivity effect and permanent displacement effect, result in long period and large velocity pulse in the velocity time history and large step pulse in the displacement time history. Considering the two effects, a simple expression of continuous function for equivalent velocity pulse time history is presented in this paper. The equivalent pulse model, in which the pulse period, peak velocity and pulse shape are described by five parameters, is highly advantageous to fit and simulate the pulse-type velocity time history. The equivalent pulse model comprises only one low-frequency component while the high-frequency component of a pulse-type earthquake record cannot be considered. Based on 28 records of 11 earthquakes, the pulse frequency of pulse-type records is generally less than 1 Hz. Therefore the low-frequency component and high-frequency component are simulated respectively and combined them together to generate a pulse-type ground motion.

Effect of source parameters on forward-directivity velocity pulse for vertical strike slip fault in half space

It has been found that the large velocity pulse is one of the most important characteristics of near-fault strong ground motions. Some statistical relationships between pulse period and the moment magnitude for near-fault strong ground motions have been established by Somerville (1998); Alavi and Krawinkler (2000); and Mavroeidis and Papageorgiou (2003), where no variety of rupture velocity, fault depth, and fault distance, etc. were considered. Since near-fault ground motions are significantly influenced by the rupture process and source parameters, the effects of some source parameters on the amplitude and the period of a forward-directivity velocity pulse in a half space are analyzed by the finite difference method combined with the kinematic source model in this paper. The study shows that the rupture velocity, fault depth, position of the initial rupture point and distribution of asperities are the most important parameters to the velocity pulse. Generally, the pulse period decreases and the pulse amplitude increases as the rupture velocity increases for shallow crustal earthquakes. In a definite region besides the fault trace, the pulse period increases as the fault depth increases. For a uniform strike slip fault, rupture initiating from one end of a fault and propagating to the other always generates a higher pulse amplitude and longer pulse period than in other cases.