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Abstract Due to various deficiencies, inadequate lateral stiffness, weak design and weak construction, many reinforced concrete buildings are highly damaged during the major earthquakes. One way to retrofit these buildings is to improve the behavior and to prevent the total collapse of the reinforce masonry infilled walls in order to make them behave such as structural walls with composite materials. In this study, the concept of equivalent strut is investigated by finite element modeling and behavior of the compression strut and tension tie is presented to model unretrofit and retrofit masonry infilled walls. Analytical pushover results show great accordance with experimental results.

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It is well known that the interaction between adjacent buildings with limited separation distance or in the other ‎words earthquake-induced structural pounding has considerable effect on seismic performance of buildings. Earthquake induced vibrations may cause impact between two adjacent buildings with inadequate separation distance. Pounding hazard is considerable particularly in populated residential regions because of the limited separation distance due to limitation of lands. This ‎phenomenon may result in substantial damage or even contributes to total structural collapse of structures. Major seismic events during the past decade such as those that have occurred in Northridge, Imperial Valley (May 18,1940), California (1994), Kobe, Japan (1995), Turkey (1999), Taiwan (1999) and Bhuj, Central Western India (2001) have continued to demonstrate the destructive power of earthquakes, with destruction of engineered buildings, bridges, industrial and port facilities as well as giving rise to great economic losses. Among the possible structural damages, seismic induced pounding has been commonly observed in several earthquakes. As the cost of land in cities increases, the need to build multistory buildings in close proximity to each other also increases. Sometimes, construction materials, other objects and any projections from a building may also decrease the spacing provided between the buildings. This leads to the problem of pounding of these closely placed buildings when responding to earthquake ground motion. The main purpose of this paper is the modeling of adjacent structures in order to study the effects of pounding. Among the existing models, nonlinear visco-elastic model has been selected for numerical simulation of pounding. To study the effect of pounding on seismic performance of reinforced concrete structures, two RC frames with 4 and 6 stories are selected and their seismic performance under pounding effects are numerically studied. The effect of storey and total height of structures, size of separation distance and mass of buildings in series on the impact has been investigated in adjacent RC frames. The selected frames have been designed according to direct displacement based design method in order to investigate the effect of impact on ductility demand and obtain a desired maximum induced displacement at a considered hazard level. Then by putting the structural models with different dynamic characteristics close together, the effect of altitude, size of gap, and story height on performance of two adjacent structures has been studied. The nonlinear time history analysis and incremental dynamic analysis (IDA) has been done to predict the seismic collapse capacity of systems. The results of analysis show that the effect of pounding severely depends on the phase difference of vibrations of adjacent structures. The phase difference itself, depends on mass, stiffness and seismic capacity of adjacent systems and the value of imposed plastic deformation as well. The effect of pounding in structural systems with the same height with little difference in initial periods is negligible, whereas the local effect of pounding especially in the case of floor to column impact is considerable.

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Based on the seismic design, energy absorption by plastic deformation is necessary to prevent structures from collapsing during a severe earthquake. Therefore, estimating the behavior of structures to understand their response to earthquakes is particularly important. Seismic loads applied to structures are more significant than forces applied during design. This reduction in design applied loads is accomplished using a behavior factor. It is necessary to employ a behavior factor when evaluating the behavior of structures using linear analysis.
The behavior coefficient depends on ductility coefficient, structural damping coefficient, soil characteristics, earthquake characteristics, over strength coefficient, and design reliability coefficient. While in seismic code, this coefficient is entirely dependent on the type of lateral strength system used. At the same time, the behavior coefficient depends on the structural geometric properties which are investigated in this paper. Since nonlinear analysis is required to determine the effect of earthquake forces during design and nonlinear dynamic analysis is time-consuming, designers typically use nonlinear static analysis. Nonlinear static analysis is one of the nonlinear analysis methods that use the lateral load to represent the earthquake load on the structure statically and increasingly.
Estimating the behavior factor before starting the design process is a vital aid to designers. In this paper, we have examined the behavior factor of the reinforced concrete (RC) frame using gene expression programming. Gene expression programming is highly effective in this instance. Its effectiveness largely determines the success of the method. Gene expression programming is a class of genetic algorithms that utilizes a population of individuals, selects them based on their fit, and introduces genetic changes via one or more genetic operators.
Numerous inputs are required for this purpose, including the number of stories, the span length, the seismicity of the construction site, and the ratio of the compressive strength of concrete to the yield stress of longitudinal reinforcements. Afterward, 168 RC frames were designed via SAP2000 software, and the behavior factor value was obtained using nonlinear static analysis for each frame and subsequently transferred to the GeneXpro Tools software. The sixth and ninth national building regulations, Iran's seismic code, with the American Concrete Institute Code (ACI318-14), were used to analyze and design the structures examined. In the designed frames, the number of stories is 2, 4, 6, 8, 10, 12, and 15, and the ratio of span length to story height is 1, 15, 2, and 2.5, respectively.
The design base accelerations were 0.35, 0.3, and 0.25 in this study, and the longitudinal reinforcements' yield stress was initially set to 340 MPa and then increased to 400 MPa. The obtained results demonstrate that employing the gene expression programming method makes it possible to estimate the reinforced concrete frame's behavior factor with an acceptable degree of accuracy before initiating the design process.  Finally, the results show that the variations of the span length and the number of the stories significantly affect behavior factor. Furthermore, as the number of stories increases, the behavior factor decreases initially and then increases. Moreover, the impact of parameters, such as design base acceleration and yield stress of longitudinal reinforcements, is negligible in calculating the behavior coefficient.

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Accidental fire can be a catastrophe for engineering constructions, especially in building structures. Among structures made of various engineering materials exposed to fire, the reinforced concrete (RC) structures show better performance against fire, due to lower relative thermal conductivity, higher specific heat capacity of concrete, and slower reduction of concrete mechanical characteristics compared with other types of the structure materials. However, in case of severe fire exposure, the RC structures may experience serious structural damage due to the explosive concrete spalling resulting in a high-temperature rise in the reinforcing rebars and relatively large deformation with very limited residual bearing capacity. Although the explosive spalling and significant loss of the cross-sectional area of RC structural elements is a sign of severe damage to these elements, the reduction of mechanical properties of the materials and the performance level of the structure due to chemical reactions such as C-S-H gel dehydration caused by penetration of high temperature in the interior layers of the element cross-section may not be easily visible and evaluated.
A building that has experienced a fire, cannot be exploited for immediate reuse, even when the fire is completely extinguished until load bearing capacity of its members is determined. Therefore, it is necessary to determine the residual capacity of structural elements through logical engineering methods to facilitate the re-operation or development of strengthening methods in the fired RC structures.
Due to the importance of recognizing the behavior and residual seismic capacity of the structures exposed to fire, in this paper, a numerical study based on the nonlinear finite element method has been performed on RC frames. In the first step of the research, the process of heat distribution in the frames located in the furnace based on the previous experimental study is simulated by heat transfer analysis. All three modes of heat transfer including convection, radiation, and conduction were considered in this analysis and the effect of moisture content and emissivity coefficient was evaluated. In the second step, using the residual mechanical properties of materials (reinforcing steel rebar and concrete) based on the maximum heat experienced in the previous step, the seismic behavior of RC frames is evaluated using the pushover analysis. The experimental RC specimens used to validate the proposed numerical model consist of two frames with various beam/column bending capacity ratios in two cases, at room temperature and after being exposed to fire. Due to the different relationships available to determine the residual compressive strength of concrete, the seismic response of the frame was investigated based on three common relations Shi, Lie, and Schneider. The results showed that the proposed numerical analysis method has good accuracy in both steps of analysis and different models for estimating the residual compressive strength, despite some differences, have the ability to predict the post-fire performance of RC frames. It was also shown that for the RC frame specimen with the strong beam-weak column, the ratio of reduced post-fire load bearing capacity and energy absorption is higher.
 

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Steel bracing is known as one of the most effective systems resistant to lateral loads, and its use has been the subject of numerous studies to improve the lateral deformation tolerance of existing reinforced concrete frames. In this study, the seismic performance of steel bracing in the concentric plane in order to strengthen the existing reinforced concrete structures has been numerically investigated. A scaled reinforced concrete frame was modeled by finite element method by simple cross bracing. In the retrofitting of damaged reinforced concrete structures, attention should be paid to the continuity of service of the structure in structures of high importance. The important point in buildings of special importance such as hospitals and government buildings is that in such buildings the maintenance of the structure must be maintained at all hours. In addition, the implementation of in-plane bracing causes the destruction of intermediate frame components and can reduce the effective role of intermediate frame components in the seismic load of reinforced concrete frames. The interaction between the frame and the frame in seismic loading is an important issue that has been extensively focused on by various researches. Another important point is to pay attention to architectural issues and match the retrofit method with the aesthetic aspects of the structure. If there is an opening in the damaged frame, using the internal reinforcement method may cause problems in the opening space in the desired frame.
According to the mentioned points, in order to continue the service of the structure during the retrofit operation and to reduce the destruction operation in the intermediate frame components, the reinforcement member can be externally connected to the damaged frame. Therefore, in this study, in order to achieve the mentioned goals, the implementation of steel bracing outside the plane was also investigated using the numerical method and its Possibility was verified. The studied sample was subjected to lateral load by displacement control method by ABAQUS software and analyzed by quasi-static method. This enables a better understanding of the performance of frames strengthened with in-plane and out-of-plane steel braces and the evaluation of the proposed method. In this study, the models were examined in terms of deformation and cracking characteristics, hysteresis, lateral stiffness reduction and energy absorption capability.The results of this study showed that after strengthening with braces, there was no local rupture due to the application of lateral load in the place of the plastic joints of the frames. As a result of the application of lateral load, the normal moment frame specimen showed a more fragile hysteretic behavior. The maximum resistance value of the reinforced concrete frame in a certain displacement after strengthening increased to 2.5 times of its original sample and resulted in less stress concentration in the boundary elements compared to in-plane bracing. The amount of hardness created in the sample increased to 1.35 times of the original sample and the amount of energy absorption increased to 2.25 times of the original sample. The results obtained in hysteresis, stiffness reduction and energy absorption sections indicate the effective performance of the proposed method in strengthening damaged reinforced concrete structures.