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In recent years using steel plate shear wall system, because of its advantages in comparison with other earthquake resistant systems, has been a matter of attention. Some of its advantages relative to other systems include abundance advantage, high ductility, good hysteretic behavior and energy absorption capacity, high stiffness and economic advantages. Regarding that in Iran there is high seismicity risk and the need to strengthen old and unsafe urban textures and buildings, using this system as a lateral load resistant system seems appropriate and economical. In the present research strengthening of x-braced steel frames with steel plate shear walls is evaluated. Addition of bracing to unbraced frame spans, substituting braces with thin steel shear wall panels and adding thin steel shear wall panels to unbraced spans which do not have architectural requirements are considered as retrofitting strategies. The focus is on the methods in which retrofitting is only done by adding steel plate shear wall elements to braced frames. Some of these methods have many economic and practical advantages. Others are only proper for some special cases. In this study a number of x-braced steel frames designed by the first edition of Iranian code of practice for seismic resistant design of buildings (Iranian Standard No. 2800) are taken as basic frames which need to be retrofitted. These basic frames are retrofitted by adding steel panels with different methods. Then nonlinear static analysis (pushover analysis) with displacement control pattern has been done on both basic and retrofitted finite element frame models and the capacity curves (diagram of story displacements against base shear) of basic frames and retrofitted frames are compared. Considering the results of the pushover analysis of models in which seismic retrofitting is done by replacing x-bracing earthquake resistant system with steel plate shear walls and the results of other methods of strengthening, it is seen that seismic behavior of retrofitted frames is more desirable in terms of overstrength factor (Ω) and overall ductility of structure ( ). The failure and fracture mode in most of the medium-rise frames was ductile but in the short-rise frames the fracture was brittle. Thus, replacing the braces in short-rise structure with thin steel shear walls seems irrational and unjustified economically. But it is to be mentioned that strengthening and increasing the moment of inertia of the adjacent columns of steel shear wall panels in structures with brittle fracture mode could result the change from brittle to ductile fracture. The results of this research show that in the case of steel braced frames with regard to some scientific, technical and practical points; replacing concentric steel bracing earthquake resistant system by steel plate shear walls can be used as a suitable method for retrofitting a wide range of existing steel structures in Iran.

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Structure safety in the design of civil engineering projects has always been very important for engineers. One of the mechanisms that structure will fail and in recent years is much attentioned to it is progressive collapse. Progressive collapse in structures during earthquakes even in an explosion near the construction has become a major challenge and can create problems for structures and may even lead to the destruction of the entire structure. Currently the most available structures is only designing against the gravity loads and lateral loads (wind and earthquake).In fact a resistant structure against the earthquake is not resistant against the progressive collapse necessarily. Therefore designing the new and special structures against the progressive collapse is necessary. Progressive collapse is defined as extension of primary local failure from element to other element that finally collapsed all part of the structure or big part of it. Potential hazards that cause progressive collapse are fires, gas explosions, make a mistake in design of structure, accidents, bomb and even an unprincipled excavation that cause sudden removal one or more elements of structure and etc. The purpose of this paper is to investigate progressive collapse in steel structures with eccentric braced frames that also the influence of parameters such as height, bracing arrangement and type of structural system is examined In this study it is analyzed the progressive collapse due to column removal in steel eccentric braced frames that are designed seismically according to Iranians guidelines(seismic regulations of Iranian 2800 code) with using of alternate path method and nonlinear dynamic analysis. Also in the continuation of research it is analyzed the progressive collapse due to column and brace removal simultaneously in steel eccentric braced frames and analysis the progressive collapse in moment frames and comparison of it with eccentric braced frames. Also it is evaluated the influence of parameters like number of floors, location of braces and type of connections. For this intent two structures with five and ten stories with braces in middle spans, and also two structures with five and ten stories with braces in lateral spans, one structure with five story with system of moment frames and one structure with combinatorial system of moment frames with eccentric brace which is five story in ETABS program were analyzed. Then one of outside frames for analysis of progressive collapse modeled in SAP2000 program. Results showed that remove a single column only when there is not any brace beside the removed column and simultaneous removal of columns and braces only in the last floor causing progressive collapse to the structure. Results showed that the probability of progressive collapse with simultaneous removal of columns and braces will increase when the height of the frames that middle spans is braced increases and will decrease when the height of the frames that lateral spans is braced increases. Also with comparison of eccentric braced frames and moment frames is resulted that eccentric braced frames is stronger than the moment frames against the progressive collapse. But combinatorial system of moment frames and eccentric brace in comparison with the other analyzed systems is completely resistant system.

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The need to solve the complex, nonlinear, and variable problems grows with time. Conventional mathematical models perform linear and constant analysis effectively. Although techniques that work on a particular model, capable of analyzing complex nonlinear and time-varying problems, however, they also face some limitations. Combining these with other issues such as decision making, etc., has inspired the development of intelligent techniques such as fuzzy logic, neural networks, genetic algorithms, and expert systems. Intelligent systems mainly employ a combination of these techniques to solve very complex problems. Although both fuzzy logic and artificial neural networks have been very successful in solving time-varying nonlinear problems, each has its own limitations which reduces their use in solve of many of these problems. The roof global ductility, is a comprehensive reflection of various engineering demand parameters (EDP), such as story-drift, plastic rotation at member ends, roof displacement, etc. Careful estimation of this parameter will certainly lead to greater accuracy in the design of structural members. One of the methods which establish a good estimate of the nonlinear seismic response is the using of EDP parameters and measuring the seismic intensity index. The main purpose of this paper is to establish an accurate intelligent model related to the geometrical characteristics of the structure, performance level, the behavior factor and global ductility in eccentrically steel frames, under earthquakes near-fault. For this purpose, genetic algorithm is used. Initially a wide database consisting of 12960 data with 3-, 6-, 9-, 12-, 15- and 20- stories, 3 column stiffness types, and 3 brace slenderness types were designed, and analyzed under 20 pulse-type near-fault earthquakes for 4 different performance levels. To generate the proposed model, 6769 training data were used in the form of adaptive-neural fuzzy inference system(ANFIS). Subtractive clustering and FCM methods have been used to generate the purposed model. The results showed that Subtractive clustering provides more accurate results than the other FIS. To validate the proposed model, 2257 test data were used to calculate the mean squared error of the model. The proposed model is an intelligent model in the range of data used, and can be used to estimate the global roof ductility of EBFs. To evaluate the efficiency and performance of the model, correlation coefficient and common error calculation criteria including RMSE and MARE were used. The correlation coefficient for the Subtractive clustering method was 0.888, based on intelligent model in the test data. In the other hand, the developed intelligent model can be used as a precise alternative to prediction of (μR) for EBFs under near-earthquakes. To evaluate the model’s efficiently and accuracy, various error criteria including Error, Mean Error, RMSE, MARE% and R were used between model values and real values, in the test data. From the results of this study, it can be pointed out that, the developed intelligent model can be used as an accurate substitute method to predict the (μR) for EBF structures, under near-fault earthquakes. The results of correlation analysis of the proposed model show that the proposed intelligent model has high accuracy.

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In the present paper, the behavior of eccentric braced steel frames with thin infill plates is investigated. The main purpose is to provide a new form of eccentric bracing, which improves the seismic behavior by adding a thin steel plate under the link beam. In the proposed model, due to the increase in frame stiffness, flexural stiffness and shear of the beam in the bracing frame will not respond to the forces. Therefore, in order to provide the required stiffness and ductility of the frame, it is suggested to use a steel plate under the link beam connected to the bracing connection plates. In the proposed models, two groups of models with steel plate under the link beam (middle steel plate) have been studied. In the first and second groups, all analysis are of static type, taking into account the geometric nonlinear effects in an eccentric frame with infill plates, the height of the plate under the link beam (middle steel plate) is 460 and 742 mm, respectively. The studied parameters include the height of the middle plate, the thickness of the middle plate and the effect of the arrangement of stiffeners on the performance of the frame. For numerical analysis of the models, the finite element method using Abaqus finite element software with increasing load has been used. Extraction results in the models include force-displacement curve, stiffness decay, amount of wall out of plane displacement due to buckling, inelastic dissipation energy and stress distribution in the structure (stress contour). According to the results of numerical models, the middle steel plate is very important in providing ductility and increasing the strength of the structure. Also, with increasing the height of the middle plate, the development of the diagonal tensile field into the infill plate increases, so local buckling can be converted into general buckling in the infill plates. Among the arrangements of stiffeners, it was observed that the stiffener under the middle plate has the least effect on increasing the force-displacement response of the structure. By evaluation of models with a middle plate with a height of 460 mm and two vertical stiffeners, compared to the model with four stiffeners, the structural capacity (force-displacement) has increased by about 31%. By evaluating the models with a middle plate height of 760 mm, it can be found that the use of stiffeners with different geometric arrangements do not have a major effect on increasing the stiffness in the elastic and inelastic stages, prevent the sudden decrease in stiffness in models without stiffeners due to buckling observed at the junction of the brace to the middle plate. Also, the free edge stiffener of the middle plate has practically no effect on the sudden decrease in stiffness, but on the other hand, vertical stiffeners or a combination of horizontal and vertical stiffeners have performed well in terms of preventing a sudden decrease in structural stiffness. The thickness of the steel plate has a significant effect on increasing the strength and reducing the local buckling in the middle plate. Local buckling was observed at the junction of the middle plate to the brace, which is recommended to use stiffeners for the middle steel to reduce the effects of local buckling of the plate and to prevent a sudden decrease in strength and stiffness. In steel plates with shear behavior that do not allow the complete formation of diagonal tensile fields, stiffeners prevent a sudden decrease in strength but do not have a significant effect on increasing the overall stiffness of the structure.