جستجوی مقالات مرتبط با کلیدواژه « near-field record » در نشریات گروه « فنی و مهندسی »

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.

In this research, a review was conducted on the characteristics of strong ground motions in near-fault zones, as well as the analysis of its effects on seismic behavior of mid-rise structures with two types of earthquake resistant skeletons. The studied structural models are mid-rise 10 story buildings with bundled tube frame and 3d moment frame. Also, the analytical capability of the modal pushover analysis (MPA) method has been studied on the estimation of structural response parameters. The maximum inter-story drift, axial force, roof maximum displacement of the studied structures was determined by conducting nonlinear dynamic time history (NTH) analyses and MPA method under the effects of an ensemble of lateral loads. An analytical comparison was accomplished among the obtained results.

This paper presents a computational approach to the analytical performance of modal pushover method (MPA) in predicting nonlinear response parameters of tall buildings compriding hybrid framed tube with large scale zipper elements. The accuracy of the results based on MPA is evaluated by comparing the benchmark responses obtained through conducting two sets of nonlinear time history analyses (NLRHA). Also, the effect of higher modes on the structural response parameters is measured by considering three computational vectors of the ordered lateral loading prepared according to the participation of the basic mode as well as the first 3 and 5 transitional modes, separately. In this study, the determination of the target displacement in MPA was set based on the results of NLRHA under two groups of near and far field records. The variation range of response parameters of the three high-rise 30storey studied structures was evaluated based on conducting a series of MPA as well as NLRHA analyses. The computational outputs of the MPA are compared with the results of the NLRHA (as exact values) and the standard error percentage is estimated. Evaluation of the results represented in this study demonstrates the relatively desirable computational capability of the MPA method in predicting the behavior characteristics of tall building structures with symmetric and regular rigid skeleton at plan and height. Moreover, it was observed that the presence of large-scale zipper elements in resistant system can reduce the seismic response parameters and also relatively increases the overall dynamic stability of high-rise structural skeleton.

In this study, the nonlinear behavior of a studied 20story steel bundled tube frame structure is evaluated under applying the assumed incidence angles to the imposed directions of near-field records. Additionally, the related effects on the stability of the studied structure subjected to the probable forming of progressive collapse, have been studied too. The results of this research were obtained based on conducting nonlinear dynamic time history analyses. Four assumed incidence angle case studies were considered to an ensemble of three-component earthquake records. Moreover, three-member removal situations were also assumed for the selective column elements of the studied structure subjected to all chosen ground motions.The results of this study show that the structural response parameters have a relatively larger amplitude under removing of the corner column at the first story, than the other two cases of column removal which to be assumed at 10th story and also for the middle column of the outer frame. Furthermore, the structural responses did not change significantly with the increase of the incidence angle from 0° to 45°. The obtained results show that the formation of plastic hinges and inelastic zones in the structure would indicate high deformation demands at the middle stories and also the induced demand variation decreases along upward of the height. Additionally, removing of the corner column would result in the formation of more plastic hinges, which cause relatively greater structural damages and gradually reduces the stability of the studied structure. It is worth noting that the structural response parameters under each element removal and subjected to strong near-field records, remains in a relatively stable domain. Furthermore, the variation of response parameters did not exceed the life safety limit.

The study includes an analytical approach to the issue of improving the seismic performance of the frame tube resistant skeleton. In this regard, the use of large-scale zipper elements in resisting skeleton of structures is investigated. Add a configuration of these elements only on the lower floors of structures can be a positive effect on reducing the parameters of the dynamic response of combined frame tube structures. Reduction of inter story drift and permanent drift, Creating downward trend in absolute acceleration and relative velocity graphs of the center of mass, Reducing non-linear rotation of panel zones, Represents the better performance in the skeletal configuration of a prototype tall frame-tube structure. Three 30-story building of frame tube skeleton and two configurations in combination with large-scale zipper elements selected and designed. A set of nonlinear analysis for side loading of these structures are done based on a choice of five powerful earthquake records. The presence of high-energy pulses and extensive acceleration spike with a combination of high speed pulses in selected time history records, is characterized by the modelling of near-fault ground motions. The skeleton of selected structures is designed based on the sixth and tenth National Building Regulations and the 2800 (Fourth Edition). Evaluation of the results showed that presence of large-scale configuration Zipper-Elements in the lower decks, Create an integrated behavior mechanism and the continuity of the seismic performance in tall building rigid skeleton. Showing the stable structural dynamic behavior of structures, particularly in large velocity pulse range, Along with reducing drift parameter, are the most important characteristics of such infrastructure in resistant skeleton tall buildings.

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 aim of this research is to study the parameters of nonlinear dynamic behavior of steel tall buildings having a compound braced-tube resistant skeleton with belted trusses in higher levels. For this purpose, four 30-story study models have been designed in 3D. The first study model has a resistant skeleton without a belted truss. Yet, the remaining three structural models, each have a belted truss in higher levels with layouts of one, two and three stories, respectively. Numerical study results of this research are acquired according to a set of time-history nonlinear analyses on the mentioned study models. The chosen ensemble of three-component ground motions includes six highly powerful near-field records and one far-field record. Assessing study results reveal that applying belted trusses in tall braced-tube skeletons would lead in a remarkable increase in stiffness and a growth in the capability to absorb the earthquake’s kinetic energy. This issue also results in a large decrease in lateral displacement and dynamic drift of the structure and will therefore cause an increase in using the axial capacity of peripheral columns of the structure plan. Thus it becomes possible to introduce the application of resistant structures with belted trusses in structural skeletons of high-rise buildings, especially in areas with high seismicity as a suitable and efficient alternative plan compared to other structural systems.

Steel moment frame and bracing panels compound structures are suitable systems for medium-height resistant skeletons due to their desirable ductility along with high rigidity when facing large dynamic forces caused by earthquakes. In this research, the seismic behavior of two 10-story buildings with compound systems of moment frame/eccentrically braced frames as well as moment frame/eccentrically braced frame with zipper elements are evaluated. The evaluation and changes of the seismic response parameters of the studied models are based on performing nonlinear time history analyses. The studied models are designed according to the fourth edition of seismic design provisions in Code 2800, plus the 6th and 10th issues of the Iranian National Building Regulations. Following the results of this research, it has been recognized that the existence of large coherent pulses in the time history of powerful earthquakes affects the response parameters of the building to a great extent. The results of this research includes the analytical process in the graphs related to maximum push of the drift, floors acceleration and velocity of the studied structural models and also the time history of story’s drift and axial force of the columns. The results show that seismic responses and demands under near-field records have a bigger appearance than the corresponding effects obtained exposed to far-field records.

Study of the seismological aspects of major earthquakes occurred in California, Japan and New Zealand indicates that the structures located in regions with high level of seismicity, experience aftershocks with different intensities in addition to the mainshock. Multiple earthquakes create inelastic response in structures and lead to the accumulation of considerable damage in the structural and non-structural elements. The aim of this research is to determine the effect of aftershocks on the response parameters of a 10-story steel bundled tube frame structure. According to the analytical results of this study, the occurrence of severe aftershocks following the near-field earthquakes does not have a significant contribution to the changing maximum inter-story drift parameter. Additionally, by increasing the intensity of the aftershocks, the residual inter-story drift does not indicate a clear trend height-wise, obviously. Moreover, when the dominant period of the mainshock is close to the fundamental period of the structure, and the dominant period of the aftershock is close to the fundamental period of the damaged structure, then the occurrence of the aftershock increases the amplitude of the nonlinear response of structural elements. The response parameters studied in the current paper include maximum interstory drift, residual inter-story drift, plastic hinge mechanism and induced forces due to shear lag effect . It should be noted that the maximum inter-story drift in all stories of the studied structure subjected to the fault normal component of the Bam 2003 mainshock record has exceeded the allowable value prescribed by the Iranian seismic code 2800. This is due to the dominant period of the Bam record that is very close to the fundamental period of the studied structure. The findings of this study display that the occurrence of the aftershocks following the mainshock does not change the maximum inter-story drift considerably. Moreover, the strong aftershock (PGAas/PGAms=1.0) occurring after the Cape Mendocino 1992 mainshock i.e. PET record, increased the maximum inter-story drift at the middle and upper stories. Results apply that by changing the aftershock intensities, no clear trend in residual drift values is emerged. The reason could be attributed to the fact that the damaged structure may not have the more maximum displacement when it stops oscillating. However, the Bam mainshock record caused maximum residual drift equal to 0.024, which according to FEMA356 is beyond the Life Safety (LS) performance level.