جستجوی مقالات مرتبط با کلیدواژه « زلزله نزدیک گسل » در نشریات گروه « مهندسی زلزله »

Performance-based design is a new approach to topics of the seismic design of structures, which unlike the traditional methods of force-based design, is based on changing the location of the structure. The use of this approach in the process of structure design results in the access to structures with proper performance and an acceptable level of reliability. The main goal of this contribution is to investigate the impact of near- and far- field earthquakes on the collapse capacity and fragility of performance based optimization of RC moment frames using the center of mass meta-heuristic algorithm. Push over analysis has been utilized in the optimization process to control the responses of the studied frames at functional levels and incremental dynamic analysis has been used to evaluate the fragility of the obtained optimal frames. According to the results for the collapse margin ratio and the adjusted collapse margin ratio for the 3-, 6-, and 12-story frames, it is indicated that the collapse margin ratio and therefore the seismic safety under far-field earthquakes are 7%, 16%, and 8% higher than those of the near-field earthquakes, respectively. In other words, the optimized frames in this study against near-field earthquakes have low seismic safety and more fragility than far-field earthquakes.

In the present research, the seismic fragility and collapse capacity of concrete moment frames have been investigated by considering different ratios for the weak beam-strong column rule in the optimization process in the performance-based design framework. In order to implement performance-based optimization, the center of mass metaheuristic algorithm has been applied in this research. The philosophy of design approach based on performance and even traditional design methods allows the structure to suffer damage facing strong and relatively strong earthquakes. Therefore, in order to estimate the level of safety of the structure against earthquakes, it seems necessary to use quantitative indicators of seismic safety and the collapse capacity of the structure. In order to predict the collapse capacity of each optimal structure, using incremental dynamic analysis, the modified collapse safety margin ratio under far and near fault earthquakes has been calculated. Two examples, 3-span three and six floor frames have been studied in this research, which are designed in the performance-based optimization framework and considering the coefficients of 0.8, 1.2 and 1.6 to control the weak beam-strong column rule in the optimization process. The results indicate that increasing the rigidity of the column compared to the beam in this research actually affects the ductility of the structure, and by choosing structures with greater rigidity of the column compared to the beam, it leads to an increase in the collapse capacity and a decrease in the fragility of the structure.

In recent years, near-fault earthquakes have received more attention due to their variable frequency nature, pulsing motion, and causing incremental damage. On the other hand, due to the abundance of concrete structures in Iran, more than any other structures, they are at risk of damages caused by near-fault earthquakes. This issue clearly indicates the necessity of using new lateral bracing systems in RC structures. One of the lateral bracing systems is the combination of Chevron braces with zipper elements. Owing to this issue, the main purpose of this study is to investigate the seismic performance of RC frames equipped with zipper braced elements under near-fault earthquakes. Therefore, seven far-fault and seven near-fault earthquake records and three RC frames of 3, 6, and 9 floors with and without considering zipper elements were selected. Then, RC frames were designed using linear static analysis, after which all frames were analyzed by dynamic nonlinear time history analysis. After performing the analysis, the results such as total displacement, relative displacement, story shear, and plastic hinge formation were compared. The results of this study present that the zip-shaped elements have significantly succeeded in transferring asymmetric loads of the lower story braces to the upper stories and have prevented the creation of a soft story in the first height level of the structure. Finally, the comparison of the results ehibits the significant effect of the distance from the fault on the overall and relative deformations of the structures with increasing the height of the stories

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.

Near-fault seismic events have been popular research topics in earthquake engineering, especially, in last two decades. A variety of research programs have been conducted to characterize the unique nature of near-fault events from both structural and seismological point of views. In this paper, after the introduction of S-Transform as a modern timefrequency representation which can measure local characteristics of a signal such as amplitude, frequency, and phase at any time instant, a new robust filtering method is proposed and applied to extract the pulse-like part of nearfault ground motion records. Seven pulse-like near-fault recorded ground motions have been selected from literature such that the comparison of results with similar available methods can be possible. After the extraction of pulse-like parts, the nonlinear dynamic behavior of two 3-D reinforced concrete moment frames with 6 and 9 stories is studied by using Incremental Dynamic Analysis (IDA) under support excitation by original ground motions, extracted pulses and residual ground motions. To investigate the sensitivity of results to the pulse period, two extra steel moment frames with 3 and 12 stories are modeled. The results deeply questions the reliability of the application of low-frequency extracted pulses as the representative of original ground motions in dynamic time history analysis, especially, when the nonlinear behavior is considered.

The present paper is a study of the (Behavior of the) elastomeric bearing isolated bridges located near the faults. The earthquakes near the faults have high frequency and narrow bandwidth. Such characteristics cause the strikingly high energy released due to production acceleration happening at the initial stage of the earthquakes to be imposed on the system. The present research undertakes to study the effects of the forces resulted by such type of earthquakes on the bridges. For this purpose, a three span highway bridge being of 60 meters length, for which were applied three types of lead rubber isolators in pursuant to the regulation envisaged in ASHTO standard, was designed. Using SAP2000, a 10.1 version was designed and the same was modeled in base-isolated and non-isolated modes. Considering the effect of three seismic recorders near the fault along longitudinal and latitudinal directions as well as in (YZ and XZ) directions, a nonlinear time history analysis was performed. The study of the behavior and load shift curves of the lead rubber isolators showed that initial strains and yield strength of such isolators cause significant reduction of base shear and bending moment in the bridge under study. Furthermore, simultaneous considering of the vertical and horizontal component of the earthquake in X and Y directions in the analysis showed increase compress or tension forces in column. These results show the shortcomings of the provisions recommended by, as instance, NEHRP and IBC about the isolated bridges in not considering the vertical effect of the earthquake. Such an effect should be taken into consideration as is the case with the non-isolated bridges.