جستجوی مقالات مرتبط با کلیدواژه "velocity pulse" در نشریات گروه "مهندسی زلزله"

This study aims to investigate the analytical scheme of both the bi-normalized acceleration response spectrum and the bi-normalized tripartite response spectrum according to an ensemble of near-field strong ground motion records in the forward directivity conditions. For this purpose, the horizontal components of the ground acceleration time history were considered and a group of 16 ground motion pairs were selected, which have been recorded during two great earthquakes occurred in California, namely the Northridge (1994) and the Imperial Valley (1979). Also, the analytical scheme of the normalized acceleration response spectrum and the normalized tripartite response spectrum corresponding to a symbolic single-degree-of-freedom system have been compared with the related bi-normalized acceleration response spectra. It should be noted that the identification of near-field strong ground motion records is usually recognized by their physical characteristics such as peak ground acceleration (PGA), peak ground velocity (PGV), and the relatively short period process of energy release. Many strong ground motions have been recorded in less than 20 km from the fault rupture surface during intensive powerful earthquakes. In this case, the most famous earthquake tremors are the Imperial Valley (1979), Loma Prieta (1989), Landers (1992), and Northridge (1994) in California, plus the Kobe (1995) in Japan, Erzincan (1992), as well as Kocaeli (1999) in Turkey. Moreover, there are two very strong records, which were related to the Tabas (1978) and Bam (2003) earthquakes in Iran. The ensemble of the selected earthquake records in this research includes near-field ones that contain various tectonic occurrences. The main physical characteristics of the chosen records cover a wide range of the frequency content and strong ground motions duration as well as various high seismological amplitudes. The related values of the peak ground acceleration and velocity are numerically high. Generally, the peak ground velocity (PGV) is often viewed as a better indicator of the earthquake record damage potential than the peak ground acceleration (PGA). The large velocity pulses evident in the related plots can be viewed as damaging features. Moreover, the earthquake record damage potential also depends on how much dynamic ground displacement occurs during these velocity pulses. Referred to apparent physical influences caused by strong faulting mechanisms, the presence of high-amplitude velocity pulses is one of the most important characteristics of near-field records, which can be registered in a time-history domain. In general, these pulses appear as wave-shaped features with high amplitudes and long periods, which have a compound and continuous shape. Distinct powerful velocity pulses are resulted corresponding to high-amplitude acceleration pulses and spikes usually with a less than 1.5 second time-domain and also high-amplitude and short-time spikes with 0.2 to 0.3 second time-step in the spectral windows of the horizontal parallel and perpendicular components with respect to the fault rupture couture. These processes would essentially cause an enormous amount of the kinetic energy of strong ground motions to get released in the time-range of compound coherent and long-term velocity pulses in the related time history. In order to investigate these effects on the configuration of the aimed response spectrum, the velocity pulse time step, the ratio of corresponding kinematic energy of both two horizontal components, peak ground acceleration (PGA), and peak ground velocity (PGV) were considered.According to the performed computational assessments in this research, it was resulted that the bi-normalized response spectrum by the basic criterion of predominant period has a monotonous configuration and would get fewer effects due to variation of the notified spectral parameters.

A special class of ground motions near the fault regions has distinct characteristics that are different from far-field ground motions. One of the critical features of these ground motions is the existence of a velocity pulse in the ground motion record in the direction perpendicular to the fault rupture. These pulse-like ground motions generally occur when the fault rupture propagates towards a site located near the fault. The accumulation of energy in the seismic wave front results in a velocity pulse with a relatively long period in the direction perpendicular to the fault line. This phenomenon could adversely affect the seismic performance of tall buildings with relatively long fundamental periods.In this paper the effect of velocity pulse on seismic response of a 42-story reinforced concrete building with a central core wall structural system is evaluated. The building has already been studied in the TBI project of the Pacific Earthquake Engineering Research Center (PEER). The structural model of the building is developed based on the information contained in the PEER report. The model is first verified by conducting a nonlinear response analysis using one of the ground motion records in the PEER report and comparing the results with that report. After verification, the model is subjected to three earthquake records each containing a velocity pulse (i.e., Northridge 1994, Cape Mendocino 1992, and Chuetsu-Oki, Japan 2007). Subsequently, the velocity pulse of each record is removed using a recently proposed wavelet-based signal processing approach and the building is analyzed again to determine the impact of the velocity pulse. The results of the analyses show that the velocity pulse significantly affects the seismic response of the building. By removing the velocity pulse, the lateral drift and rotation of the coupling beams decrease by about 50% and 60%, respectively. Also, the forces in the structure (shear and bending moment) are reduced by about 40%. The dominant effect of the velocity pulse on the seismic response of the building is due to the high energy content in the frequency range of the pulse velocity. However, due to the different frequency content of each record, the effect of the pulse period cannot be accurately assessed. Therefore, the study of buildings with the same lateral bearing system and different natural periods (i.e.,

The braced tube structure is one of the most efficient lateral load resistant skeletal systems under seismic loads. This high-rise mega frame has desirable aspects in terms of seismic response. Also, this system increases the resistance potential of high-rise structures in comparison with three-dimensional steel rigid frames. The present study denotes the seismic response properties of four mega braced tube structural systems of 20-story with different bracing configurations, which are the same in plan and loading arrangements, subjected to strong near-field records. The two studied structures have skeletal configuration of centralized braced panels. Moreover, the two other studied models were designed based on mega braced tube system considered as the resistant structure. In order to assess dynamic response parameters of the studied structures, several nonlinear time history analyses were performed. The selected earthquake records contain a variety of wave-like characteristics such as long period pulses and high amplitude spikes in the ground velocity and acceleration time histories. The analytical evaluation of seismic response parameters indicates the desirable performance of mega braced tube structural systems under powerful near-fault records. The aforementioned seismic performances were investigated in terms of maximum lateral displacement, inter-story drift as well as absolute acceleration, relative velocity of floors and total base shear along with the integrated formations of skeletal nonlinear hinges. It was obtained that the concentrated configuration of a limited number of aligned braced panels in high-rise framed skeleton is not relatively suitable. Also, dynamic response parameters of the 20-story models with the configuration of centralized braced panels are influenced more while being subjected to near-field ground motions in comparison to the mega braced tube systems. The application of centralized braced panels would lead to the more intensive variation in seismic response parameters. It should be noted that the application of single elements as well as concentric aligned braced panels cannot be effective in controlling the maximum lateral displacement and drift demands. The structural vulnerability assessment indicated that mega braced tube structures have better seismic performance than the other type of resistant skeletons under influencing of large coherent velocity pulses which displayed in the time history of strong earthquake records.

The purpose of this research is to generate a pulse-like acceleration spectrum from the horizontal component of a non-pulse-like acceleration spectrum. Three different methods have been used for this purpose. Therefore, 63 accelerograms were selected from 450 recorded accelerograms in the near-field and according to the pulse-like and near-
fault acceleroram criteria. The geological location of the stations is based on the data published by BHRC (Building and Housing Research Center). Required corrections have been applied in the frequency and time range of the used accelerograms, their values have been scaled to 1 g, and the spectrum of each of the accelerograms is calculated. Computing and modeling of pulse-like coefficients of the accelerograms have been done using three statistical methods as follows:1) Representing the average for ratios of acceleration spectrum of horizontal components, which are perpendicular along the fault, to the non-pulse-like acceleration spectrum of horizontal components for the same accelerogram, for all of the rock and soil type data, individually. 2) Representing the average for the ratios of acceleration spectrum for horizontal components of non-pulse-like accelerograms, which are perpendicular along the fault, to the acceleration spectrum horizontal components of non-pulse-like accelerogram that is recorded in another station. 3) The average for the ratio of acceleration spectrum for vertical component, to the acceleration spectrum of the horizontal component perpendicular along the fault, for the same pulse-like accelerogram.
Due to the scattering of data and their shortage, especially in sites of groups 3 and 4, the outputs of these two groups are combined and represented as the group of soil, and the outputs recorded in the sites of groups 1 and 2 are also combined and represented as a rock group representative. Then, a comparison was made between the correction coefficients of the obtained spectra and the coefficient N introduced in the 2800 buildings standard in Iran. Errors in each method have been determined and appropriate mathematical models are presented for the 50% balance in each of the three methods. Then, the coefficients obtained from the methods are applied to the non-pulse-like spectrums and are compared with the recorded pulse-like spectrums. The following results have been obtained from these studies:- The proposed mathematical models for modification of the design spectrum in each of the methods have a fairly high accuracy and reliability; however, these methods have considerable differences compared to the Iran's 2800 standard of buildings.
- The basis and criterion for obtaining the N coefficient in Iran's 2800 standard is not known, whereas by applying this coefficient it can be said that the effects of the near-field have been applied. It is suggested that this issue is addressed in revision of Iran's 2800 standard.
- The natural period of the structures designed by the regulations (short and medium order construction structures) is usually less than 2 seconds. In this interval, the proposed coefficient of the regulations and the proposed coefficient models presented in this study have the largest differences in relation to each other.
- The N coefficient always increases after a period of 0.5 seconds for up to 4 seconds, after which it remains constant, which is very conservative and irrational. While in the first and second methods suggested in this study, the slope of the curve is descending from a certain periodic range. This part confirms the justification of the applied procedures too.
- Error values are presented separately for each method. In the model provided for the soil, the error value is slightly higher than the rock model.
- The magnitude of the error is mostly positive in many periods, which means that the domains of the proposed models of the N coefficient are less.
- The absolute and relative errors in the first method are about 0.33 and 5%, and for the second method, it is equal to 0.05% and 15% respectively.
- Among the results of the three methods carried out at 50% level, the first and second methods have a much better and more reliable consistency than the third method, and also compared to the 2800 standard.
- As one of the most important results of this research, it can be stated that the results of the third method presented in these studies has no logical connection with the pulse-like accelerograms to be used to understand the N coefficient.
- According to the comparisons, from the first and second methods, the results of the first method are found more countable. The reason for this is the insignificant difference between the pulse-like modeled spectrum by the application of this coefficient (N) in the proposed model and the pulse-like spectrum of the real records obtained from the earthquakes.

With increasing the earthquake records over the past decades, it has been determined that ground motions in the vicinity of the causative fault (up to 15 km from the fault) can be significantly different from the motions away from the fault. These movements often include high displacement and velocity pulses with a significant structural damage potential and impose considerable seismic demand on the structure. Due to the devastating effects of such earthquakes, many engineers and seismologists have focused on the quantitative identification of pulse bearing records and simulation of the near-fault ground motions. Many simulation models extract the prevalent velocity pulse of the near-fault motions through fitting the displacement, velocity, acceleration time histories, and the corresponding elastic-response spectra obtained from the model and the actual record. So far, determination of the analytical models parameters has been accompanied by a manual trial and error process. Such trial-and-error-based process limits the ability of engineers to apply these relationships and investigate their effects on research and practical applications. In this study, a new approach is proposed for identifying and extracting the prevalent velocity pulse from earthquake records by optimization algorithms and the mentioned models. Particle Swarm Optimization (PSO) algorithm is used to simultaneously minimize the difference between the time history and the corresponding elastic-response spectra of the simulation model and those of actual, by defining a suitable objective function.
The objective function in the optimization process is as a constrained function where the root-mean-square difference between the values of the pseudo-velocity elastic-response spectrum obtained from the pulse simulation model and its actual record is as the target function, and the root-mean-square difference between time histories of the corresponding velocity is as the constraint. With the objective function defined, the physical parameters of the simulation models are determined without the need for manual trial and error process. The optimization algorithm converts the manual trial and error in process of pulse extraction into the systematic trial and errors with the minimum analytic intervention and judgment. Then, by the proposed method and Hosseini Vaez mathematical model, a set of near-fault records have been simulated and stated in mathematical expression. The mentioned model includes harmonic and polynomial expressions, which is capable to simulate various pulses with a simpler form. Although this model simulates the long-period portion of near fault records, the parameters of model are determined based on a try and error method. The proposed method has been used to extract and simulate the prevalent pulses of near-fault records of Iran for the Tabas and Bam earthquakes.
Comparing the results with other studies represents the efficiency of the proposed method for extracting the prevalent velocity pulse from near-fault records and expressing them as closed mathematical equations. The generated pulse history can be used to structural analysis and investigation of structures response to near-fault ground motions. Besides, since the synthetic near-fault ground motions are a combination of long-period-dependent velocity pulse and high-frequency independent seismic wave, the proposed approach can be used to generate time history of the long-period dependent velocity pulse.