جستجوی مقالات مرتبط با کلیدواژه "بهسازی لرزه ای" در نشریات گروه "عمران"

One of the methods to reduce the earthquake- induced responses in seismic retrofitting of buildings is using dampers. Among different types of dampers, viscoelastic dampers have been proposed as one of the methods of seismic retrofitting of existing structures or seismic design of new buildings in the last two decades. In this research, 3 steel frames of 3, 6 and 10 floors with lateral bearing system of simple braced frame in x direction and flexural frame in y direction were modeled based on edition 4 of standard 2008 in 2000 SAP software and analyzed dynamically and non- linearly based on three earthquake records. These dampers were added to Concentrated Braced Frames and Eccentrically Braced Frame, and the effect of their use in reducing the base shear force was compared. The results showed that viscoelastic dampers in Concentrated Braces had more base shear reduction than in Eccentrically Braces. The results of this study indicate that the reduction of base shear in mid-rise buildings (6 floors) is high and was 66 and 18 and 49 percent respectively in the earthquake records related to Northridge, Kobe and Imperial Valley. On the other hand, the highest energy dissipation of the dampers in 3, 6 and 10 story buildings are related to Kobe earthquake and is 40.78, 60.62 and 38.08%, respectively.

Unreinforced masonry buildings are quite popular in many countries such as Iran, even though they are prone to significant damage against even moderate seismic excitations. In 2003, 26000 people lost their lives and 30000 were injured in southeastern Iran during Bam earthquak. The regional investigations showed an almost total overall collapse of all adobe and masonry buildings. Even to this day, notable number of buildings in Iran is masonry buildings due to low costs, especially in rural areas. Hence, the popularity despite the poor performance has sought researchers to study efficient ways to improve the URM buildings. Generally speaking, masonry buildings are classifed into unreinforced and reinforced; the latter usually beneft from horizontal and vertical steel bars which signifcantly improve strength, ductility and energy dissipation capacity of such walls. Masonry buildings can also be categorized as confned and unconfned. Using horizontal and/or vertical ties usually in the form of lightly reinforced concrete members at the perimeter of walls, their intersection with perpendicular walls and if necessary, around large openings can considerably enhance their ductility and in some cases their strength. Unlike other masonry construction types, the behavior of Confned unreinforced Masonry (referred to CM hereafter) buildings has not yet been fully formulated. This is mainly because of more sophisticated behavioral characteristics of CM walls compared to unconfned Unreinforced Masonry (referred to URM hereafter) buildings. CM is the only masonry system that has been allowed practicing by the Iranian Code of Practice for Seismic Resistant Design of Buildings (Standard 2800) in seismic-prone areas. When it comes to seismic retrofitting of masonry buildings, there are a rather wide variety of available options, e.g. shotcrete, adding steel or FRP sheets on the wall, adding Reinforced Concrete (RC) ties, changing the arrangement and size of the openings, and using Near Surface Mounted (NSM) rods on the walls. Considering the significant number of two-story URM school ‎‎buildings in Iran, there is a need to provide instructions for ‎the ‎type of improvement plan of these buildings. In response ‎to this ‎need, the Organization of Renovation, Development ‎and ‎Equipping of Schools of Iran developed a guideline for ‎the ‎purpose of typifying the improvement plan of these ‎buildings ‎with the shotcrete method in 2021.‎ In this research, based on the results obtained from five two-‎‎story URM schools, the accuracy of this guideline is ‎evaluated. ‎For this purpose, the investigated buildings in the ‎two cases of ‎flexible roof without ties and rigid roof with ties ‎in ETABS ‎software using shell elements and spectral dynamic ‎analysis ‎method, once based on the requirements of the ‎guideline and ‎once based on the Code 360 was evaluated and ‎retrofitted using ‎Shotcrete.‎ The results of this research show that the guideline ‎in the case ‎‎of flexible roofs leads to ‎minimum vulnerability of the walls of ‎‎the studied buildings. In the case of a rigid roof, the ‎results ‎‎show that according to this guideline, some of improved and ‎‎unimproved walls are ‎vulnerable in both direction; In such a ‎‎way that the ‎average percentage of vulnerable walls in terms ‎of ‎‎relative length for the first floor is 63% in the ‎longitudinal ‎‎direction and 38% in the transverse ‎direction, and for the ‎‎ground floor in the ‎longitudinal direction is 56% and in the ‎‎transverse ‎direction is 17%.‎

In the recent past, the application of various types of passive dampers (e.g., yielding metallic dampers) has become a common practice for improving seismic performance of the under-construction buildings and rehabilitation of existing constructions. In this study, a new elliptical metallic damper was introduced to improve the seismic behavior of existing steel structures. Among other parameters, geometrical characteristics are known to affect the seismic performance of the constructions rehabilitated with the proposed elliptical damper. Accordingly, the performance of the proposed damper was investigated through accurate numerical studies on various types of dampers considering various damper dimensions, ellipse major and minor axes length-to-plate thickness ratios, and placements of elliptical shear diaphragm. To study the proposed elliptical damper in terms of its effect on the seismic behavior of rehabilitated buildings, three benchmark structures with 3, 9, and 20 stories were used. Further, far-field and near-field earthquake records were used to undertake nonlinear dynamic analyses. In this work, the proposed elliptical damper was verified by the Abaqus finite-element software, and nonlinear time-history analyses were conducted in the SAP2000 software to check for seismic performance of rehabilitated structural frames with the considered damper. Results of the nonlinear dynamic analyses indicated the appropriate performance of the proposed elliptical damper in terms of reducing the seismic responses of the rehabilitated structures and suitable behavior of the proposed elliptical damper in dissipating the imposed earthquake energy to the structures. Based on these results, upon rehabilitation with the proposed damper, the 3-, 9-, and 20-story structures exhibited smaller maximum lateral roof displacements by 66, 64, and 31%, respectively.

Many of the existing reinforced concrete structures have inappropriate details in the reinforcement, which causes problems such as low lateral displacement capacity, low energy consumption, strength deterioration, and occurrence of mechanisms in undesirable places in the structure. One of these problems is the weakness in the shear resistance of the joint core due to the lack of transverse reinforcement in the joint core. In this research, the effect of using the method of enlarging the joint core in improving the behavior of joints without transverse reinforcements in the joint core has been investigated. For this purpose, after conducting experimental studies on the control and improved connections and validating their numerical model, the control and improved concrete frames of 4 and 8 stories have been modeled in OpenSees software, and their lateral behavior has been analyzed using non-linear static analyzes under Uniform lateral loading and incremental cyclic loading were compared and evaluated. The results of the studies indicate the use of the studied improvement method in connections stirrups in the core increases the bearing capacity, reduces pinching, increases energy disipation and forms a flexural hinge in the beam.

Use of passive control systems causes reduction in seismic demand and prevents damage to the main components of the structure. Yielding dampers are among the common tools used for dissipation of earthquake energy entering the structure. The main goal of the present research is introduction of a novel pseudo elliptical yielding damper for improvement of the seismic performance of the existing steel structures. Considering the impact of geometric parameters of the pseudo elliptical damper on its stiffness and strength, a precise numerical study was performed on the ratio of diameter to thickness and other dimensions of the damper in this research.  For evaluating the impact of proposed damper on the seismic performance of existing steel buildings, use was made of three benchmark structures with 3, 9 and 12 stories which were retrofitted by this type of dampers. Also, the nonlinear time history analysis has been performed using the near field and far filed accelerometers to evaluate the analyses results at different seismic states. The results of numerical analyses indicated the good performance of propose pseudo elliptical yielding damper in terms of dissipating the earthquake energy entering the structure and reduction in the seismic responses of retrofitted benchmark buildings by this type of dampers. It was found that on average, the maximum inter-story drift in the 3, 9 and 20 story benchmark buildings were reduced by 66, 69 and 67%, respectively.

The advantages of structures equipped with bracing systems such as high lateral stiffness, light weight of the skeleton compared to flexural frames plus the weakness of flexural frame connections as well as high risk of the performance of these connections during earthquakes always place the bracing lateral stiffness system at equal level of the other seismic-resistant systems in the minds of designers. However, Experiences gained from the concentric brace frames performance during earthquakes showed the undesirable function and hysteresis loops of the bracing system. The early buckling of the brace members causes joint failure, instability and unpredictable seismic behavior of the frame. Researchers tried to avoid the buckling phenomenon by making some changes in brace members structure. Using energy dissipation systems like buckling restrained braces, viscose dampers, friction dampers and yielding dampers were the methods which have been investigated during years. yielding dampers due to their benefits like economic aspects, easy construction, available material, flexible design, durability and significant impact on the seismic responses were one of the devices which has been considered. Many researchers worked on different types of yielding dampers. they used parallel plates yielding dampers on top of the chevron braces and slit dampers along diagonal brace. despite of many researches has been done, but it seems more efficient projects can be achieved in this field. the yielding dampers constructed so far have several considerable problems: i) the existence of one or two-level behavior against earthquakes, ii) the implementation of the welding process in energy-absorbing parts causing premature rupture of steel, and iii) the lack of support system in the event of severe earthquakes or unusual loads outside the design leading to frame instability. This research tries to design a two-level yielding damper with parallel fuse plates using finite element sensitivity analyses on an effective component of these types of dampers. After that to assessment of the damper function, an OpenSees code developed to analyze the nonlinear time history of the seven far-field selected ground motions. All the ground motions selected according to the FEMA P-695 suggested ground motions with the site class of C and the base shear, roof acceleration, story velocity and drift nonlinear time history responses of a three-story braced frame compared with and without damper. To prevent buckling of the brace members, dampers with the capacity of 90% of brace members capacity designed to use at any story brace and the maximum displacement capacity of dampers adjusted to the maximum allowable drift of the building stories. Results showed that, there are some effective and less effective parameters whose variation such as geometrical parameters can seriously change the total energy absorption level and improve the damper hysteresis loops. Also, According to the flexible design of the presented damper, if it needs to be designed with a force bearing capacity and energy absorption in accordance with the seismic design of the desired frame, it is possible to achieve the desired capacity by making changes in the overall dimensions and number of energy absorbing plates. time history responses assessment showed that using the new damper has a significant decreasing effect on the seismic responses of the building.

This paper introduces arched steel haunches (ASHs) as a novel technique in the seismic retrofitting of RC frames. In this regard, parameters such as ASH initial eccentricity and width-to-thickness ratio are evaluated as two factors affecting on their cyclic behavior. A series of cyclic loading tests were performed on four specimens with single and double rectangular cross-sections and with the same nominal area and length, but with different eccentricities of 0.1 and 0.2 nominal length. Experimental results showed that the slenderness and width-to-thickness ratios play a significant role on the cyclic performance in compression and even tension, and by reducing the buckling potential and the cross-section reaching to the fully plastic state, a more desirable hysteretic behavior is achieved. Therefore, with 50% reduction of these ratios simultaneously, the maximum compressive and tensile strength enhanced up to 59% and 27%, respectively, and the dissipated energy and the maximum viscosity damping ratio increased up to 152% and 14%, respectively. Also, the arched haunches showed different behavior in tension and compression for ultimate strength and plastic stiffness, which with decreasing the initial eccentricity became more apparent. With increasing the initial eccentricity, the cross-sectional area effect on increase of compressive strength and especially maximum tensile strength decreased. In addition, by reducing it by 50% and despite 59% reduction in cross-sectional area, the ultimate tensile plastic strength and stiffness increased up to 1.31 and 3.5 times, respectively. In addition, the obtained results will be used for further research on experimental behavior of RC beam-column joints.

Beam-column joints are one of the most important members of the reinforced concrete structures that are responsible for transferring existing loads. These joints require some criteria such as the development length of the beam's and column's rebars for proper operation. Lack of sufficient development length reduces the capacity of the structure and thus increases its deformation. In this study, three specimens of external reinforced concrete beam-column joints were tested on a real scale with applying lateral loads. The dimensions of beams and columns are 300 and 300 mm. Samples include a control sample with insufficient development length of longitudinal rebars of the beam (at joint area), a sample with sufficient development length through end mechanical restraint, and another sample with insufficient development length and retrofitting with dowel anchor carbon FRP composites (to improve the beam-Column joint). For making FRP anchors, columns should be pierced and dowels pass through this hole and they connect to concrete elements surface with proper epoxy adhesive. In all samples, lateral displacement load cycle diagrams (hysteresis diagram), failure modes, ductility coefficient, and stiffness were extracted and compared with each other. The results of this study showed that lack of sufficient development length leads to severe damage in the joint and plastic hinge is not formed, and energy dissipation and crack formation is occurring in the joint; while in retrofitted and restrained sample improvement of seismic performance, formation of the plastic joint in the beam and effects of sufficient development length is observed and energy dissipates in that area. Pinching in the hysteresis diagram of the control specimen is observed and maximum load capacity is much less than the others. end restraint of the rebar and reinforcement of the specimen with FRP increases the lateral bearing capacity of the joint by 53% compared to the control specimen. Also, the residual capacity increases by about 84% and 73% respectively at the end of the test (corresponding to 6% drift). The ductility of the sample with end restraint and the sample reinforced with dowel anchor FRP composites are 34% and 19% higher than the control sample, respectively. Insufficient development length also reduces the stiffness of the control sample by up to 45% compared to the restrained and retrofitted sample. Another result of insufficient development length of rebars of the beam is the angle between beam and column. In the fixed connections, this angle should remain constant while in the control specimen, this angle changes dramatically. By retrofitting the connection with FRP composites or restraining the rebars, this angle varies much less. The results of three tested samples showed that strengthening the connection with anchor dowel FRPs can effectively fix the lack of development length of rebars in the beams and increase the load capacity, stiffness, and ductility of the connection.

Past seismic experience shows that many existing mid to high-rise reinforced concrete (RC) moment frame structures require seismic evaluation and possibly retrofitting. In this study, TADAS metallic yielding dampers were used for seismic retrofitting of reinforced concrete moment frames. For this purpose, three high-rise reinforced concrete frames of 12, 15 and 20 stories were designed using the old version of the Iranian seismic code and concrete regulations. Control of structures showed the need for seismic improvement in these structural models. Therefore, to satisfy the basic and enhanced performance objectives, metallic dampers were added to the structures and were designed using performance-based plastic design method. Validation was performed by selecting a valid experimental model and simulating the experiment with an accurate behavioral model of the TADAS damper in OpenSees software. Then, by performing nonlinear dynamic and static analyzes, the seismic behavior of the retrofitted structures was evaluated at two performance levels of life safety (LS) and collapse prevention (CP). The results show that TADAS metallic yielding dampers can increase the strength and lateral stiffness of RC frames by 30 to 60% with the least number of bracing bays. Also, in addition to uniformizing the distribution of maximum inter-story drift over the height, it reduces drift by more than 60%. Moreover, due to the improvement of structures, the ratio of plastic rotation angle (θ/θy) in the critical beam and column of the frames for different earthquake hazard levels is reduced by about 70%.

Uncertainties are inevitable in the estimation of structural engineering issues and increase the cost of retrofitting. In the presence of uncertainties, the results of seismic evaluation are incorrect and create conservatism in acceptance criteria at structural performance levels. Considering and quantification of uncertainties in design and rehabilitation of structures reduces the existing conservatism and can lead to the economic design and rehabilitation of structures. In the seismic rehabilitation of structures, uncertainties have been studied on existing structures and have been applied by coefficients in guidelines. Adding a secondary system to rehabilitate of existing structure can enter uncertainties into the computation and can be effective for results of reliability. Therefore, in this study, reliability of rehabilitated steel moment frame with steel shear wall has been discussed in order to quantify the uncertainty of the steel shear wall. The selected structure is a nine-storey steel moment frame of SAC project, which was rehabilitated by steel shear wall. The studied structures were analyzed pre- and post-rehabilitation, with probabilistic variables considered for steel shear wall by OpenSees software. Based on results of incremental dynamic analysis and obtained fragility curves, the values of the reliability index have been obtained for the rehabilitated structure in the presence of uncertainties. Results showed that considering of uncertainties and reducing them can reduce the existing conservatism and the cost of rehabilitation.

In recent decades, the need for tall towers with different uses has increased day by day. The proper performance of these towers against earthquakes is of great importance due to the volume of investments and their residents. In high-rise structures, one of the problems that makes it difficult to use the structure is the high displacement values ​​of the floors, including the roof of the structure. Therefore, in this study, by calculating the values ​​of floor and roof displacement and comparing it with the allowable structural values, the need to use new methods such as long braces, truss belts and seismic isolation system is investigated. In this paper, first, using a selected laboratory sample from the frame, how to model and validate the results of nonlinear static analysis in the SAP software is discussed. Then, by modeling the 18, 22, 26 and 30 floor samples and performing nonlinear and dynamic static analysis of nonlinear time history, the seismic performance of the system has been investigated. And their behavioral curves are presented. Finally, the results of comparing seismic parameters and displacement values ​​in different samples are presented. The results indicate poor bending system performance in high-rise structures, making it difficult to use the building normally.

The presence of concrete slab in steel framing frame structures always makes retrofitting problems difficult. On the other hand, most of damages reported from Northridge earthquake (1994) has focused on the bottom flange of the beam. Therefore, in order to reduce the cost of retrofit, this operation can be done only at the bottom flange of the beam. In other words, by defining and applying a ductile fuse in the beam, by weakening the bottom flange of the bottom, it is possible to reduce the concentration of tension in the near-weld regions in connection and fracture failure in these areas. In this paper, a steel frame connection was analyzed numerically and after ensuring the modeling accuracy, it was under rehabilitation. Retrofit techniques have been made on the bottom flange of the beam. Two general "cut-off" and "heat" methods were considered to weaken the beam. In this research, incomplete penetration of the weld was placed in the bottom flange connection to the column as a deficiency in the joint. The modeling results showed that the connection with incomplete weld, underwent 4/4%, suffering a deflection in the weld area in the flange. In the specimens which were retrofitted asymmetrically, the possibility of brittle fracture in the weld region was decreased due to relocation of the plastic hinge in the weakened area of the beam. In the method of reducing the bottom flange, lateral buckling led to sudden decrease in flexural strength at 2.5 % inter story drift. In the heat-induction method accompanied with steel annealing, the out of buckling occurred at 5% drift. The retrofitted connection with this method showed a post elastic displacements of 3% and could be expected to be suitable for special moment frames.

In the present study, seismic analysis of concrete gravity dams strengthened by asphalt buttressing is presented for improving the seismic behavior of the Koyna dam in India subjected to Koyna ground motion. Fluid-Structure interaction is modeled including water compressibility and reservoir bottom absorption. The foundation is considered as rigid. A three-dimensional fixed smeared crack model is used to consider the nonlinear behavior of mass concrete. The analysis is carried out in the time domain by Newmark time integration scheme. Linear and nonlinear behavior of dam models subjected to horizontal and vertical components of selected record have been analysed. In order to investigate the effects of asphalt buttressing on the interface of dam and asphalt, the contact surface is defined using joint elements with a thickness of zero. The results of the analyzes confirm that the asphalt buttressing can improve the stability of the dam due to the pressure applied to the dam in counteracting the hydrostatic and hydrodynamic forces, Also the significant effect of asphalt Buttressing on the optimal distribution of stresses in the entire body of the dam as well as the prevention of stress concentration and reduction of fracture in the upper body near the dam crest show so that the crack at the lower section of the dam and at the interface of the dam and foundation is partially developed with a slower rate, and the cracking at the upper part near the crown of the dam does not spread to the upstream body of the dam and does not cause a total failure. Overall, it can be said that asphalt buttressing can improve the seismic stability of gravity dams by exerting pressure on the dam in opposition to hydrostatic and hydrodynamic loads.

In designing engineering systems, achieving the desired goals with low cost is the ultimate aim. Therefore, minimizing cost and optimizing engineering designs have been considered in the engineering sciences for a long time. The optimization process is important in building concrete dams due to the large volume of concrete used in the dam body. Much of the dynamic force acting on the dam caused by the hydrodynamic force is generated through the interaction between the dam and reservoir, which increases the amount of seismic responses of the dam. Therefore, hydrodynamic dampers with different materials were used
for the upstream of the dam body to reduce the effect of the dam and reservoir interaction on seismic response. The use of hydrodynamic dampers, in addition to reducing the hydrodynamic pressure against the dam, the economic aspect of the design is also less expensive than other designs. In this paper, in order to increase the efficiency of hydrodynamic dampers, the optimized dimensions through Monte Carlo probability analysis as a modern method of optimizing structures, were studied. ANSYS software that is based on finite element method is used for modeling and analysis. Flexibility of foundation is considered in modeling and Sommerfeld boundary condition is used for far field reservoir boundary. The water is taken as an inviscid, compressible and irrotational fluid with small displacements. According to governing equations and geometrical shape of dam, the problem is considered two-dimensional, and seismic analysis is performed by using Newmark time integration scheme. The pine flat dams in California are selected as a case study, and EL Centro, San Fernando, and North Ridge earthquakes is applied for the model. According to the obtained results of the probabilistic analysis model, which shows the sensitivity of the dam seismic responses to the rubber damper dimensions, a suitable range of the rubber damper dimensions to the safe and optimum design has been proposed.

The old-type RC buildings constructed before 1970s are mostly designed according to gravity loads and lack seismic detailing. In these buildings, the strong column-weak beam concept is not usually observed, and due to the probability of soft-storey event, they are vulnerable to seismic loads. Therefore, the flexural strengthening of the columns is proposed as one of the top priorities in seismic retrofitting of these buildings. This paper experimentally studies the flexural strengthening of old-type RC columns with near surface mounted (NSM) technique. Six half-scale columns, including two control columns and four columns strengthened via NSM method with GFRP and steel bars, were tested under constant axial load and increasing lateral load; the effect of strengthening method and materials were also investigated. The mean flexural strength and energy dissipation capacity of the strengthened columns with GFRP bars increased respectively by 62% and 46% compared to corresponding control columns; but the equivalent hysteresis damping of these columns did not have a substantial increase. In addition, the mean flexural strength, energy dissipation capacity, and equivalent hysteresis damping of the strengthened columns with steel bars increased respectively by 86%, 197%, and 104% compared to the corresponding control columns. The results indicate that NSM technique remarkably increases the flexural strength and improves the seismic parameters of columns, especially when steel bar are used as NSM reinforcement.

Existing reinforced concrete (RC) frame buildings with non-ductile detailing represent a considerable hazard during earthquakes. These types of buildings suffered severe damages and were responsible for most of the loss of life during the major seismic events. Among seismic performance upgrading methods, several options are normally available, and one of them is to employ energy dissipation devices (Aniello, 2007). Energy input by a strong earthquake is expected to be greatly dissipated by these devices, (Ghobarah, 2001) and if they are damaged, the rehabilitation is quite easy after the earthquake since these devices are designed to be replaceable (Ramadan, 1995). In particular, this paper focuses on removable steel eccentric bracing systems (EBs).

The experience of past earthquakes has shown that most reinforced concrete structures have been damaged by earthquake-bending frame systems. Also, existing reinforced concrete structures that are designed in accordance with previous editions of seismic regulations are not retrofit in terms of seismic improvement instructions and current valid seismic design codes. Stiffness and less strength of the flexural frame compared to the bracing, shear wall and dual systems require that the increase of the strength and stiffness of the structure as a rehabilitation technical strategy with the strategy of adding steel bracing to the economic, administrative and administrative aspects of the use of the building during the upgrade. To improve the performance and to provide the desired performance level. Steel braces for internal, external and large scale external applications that are used extensively in high-rise buildings are of interest to researchers. In this research, the nonlinear behavior of reinforced concrete structures rehabilitated with steel braces in the form of internal, external and large scale with a number of floors of 4,8,12 and 15 floors (samples of short, medium and high rise buildings) through the pushover analysis has been studied. The results show a significant increase in the stiffness, absorption of the shear and ductility of reinforced concrete structures rehabilitated with large-scale bracing compared to reinforced concrete structures rehabilitated with internal and external braces and less relative displacement.

The seismic rehabilitation of exsisting buildings is a key issue in seismic zones. In fact, moderate to strong earthquakes are threatening the lives of many people and inflict severe damages on structures and infrastructures, so that they put a stop to daily life and cause the economy suffer from downturn in regional scale [1, 2]. Hence, extensive researches on the evaluation of seismic performance of the steel moment frames have been carried out in recent years [3]. Ghodrati et al. studied ordinary steel moment frame buildings designed by Code 2800 (second edition) [4] based on the Seismic Retrofitting Provisions [5] and came to the conclusion that buildings with moderate-to-high hieghts need retrofitting in the base upgrading level when using nonlinear static method to control [6, 7]. They also concluded that all buildings do not satisfy the base upgrading level when the spectral dynamic method is employed to control the accuracy of the design. In this study, it is intended to investigate and employ the most appropriate method in order to rehabilitate steel buildings in a way that not only it shows the real performance of the structure, but also it has the lowest cost.2.1. Studied models The plan for designated sample buildings is a 15*15m square with frames that are 5 meters away from each other. The studied buildings were selected in four types concerning the height: two, four, six and eight-story in which the height of each story was 3m. The buildings were assumed residential types with intermediate importance in the south of Tehran. The ground type was selected IV. In all buildings, the resistant system against the lateral loads in both directions has been steel ordinary moment frame. In order to bear the gravity load of stories, the oneway slab system (rib and block) has been used in ceiling. The connections are assumed to be rigid. Due to the type of selected plan and the direction of ribs, all of the frames in buildings have almost similar situations, and the required conclusions for the whole building can be reached through studying one frame. In all of these buildings, the frame being studied is the frame of axis-2 (a middle frame) that has more critical state due to the loading, especially in the columns. Fig. 1 shows the plan of buildings under the consideration and the frame being studied. Previous researches showed that ordinary steel moment frame buildings which have been designed according to the Code 2800 (2nd edition), in controlling through the some methods of Seismic Retrofitting Provisions were not accepted and they need to be improved [6, 7]. In the present study, it is intended to investigate and identify the most appropriate method in order to rehabilitate such buildings in a way that it shows the real performance of the structure, meanwhile, it has the lowest cost. To this end, after rehabilitating of the members using different analyses in Seismic Retrofitting Provisions, the weights of structures and new members are calculated and compared with their previous weights, and overweight percentage, due to rehabilitating, is obtained. Fig. 2 shows the overweight percentage diagram of rehabilitated buildings in different methods.

Abstract: Considering the seismicity of most of the areas in Iran, it is inevitable to confront the earthquake because of its major property and life damages. Therefore researchers put a great effort on designing and strengthening against earthquake. The researches conducted so far for improving the reinforced concrete structures using different types of steel braces and analytical studies and widespread experiments has been done for confronting destructive effects of earthquake on structures, results show the proper effectiveness of different mechanisms of inactive seismic control of structures as an efficient option for confronting the earthquake forces. Of different control methods, using friction damper could be counted as one of the best methods for improving the seismic behavior of current structures, as it uses a simple mechanism and doesn’t need specific materials or technology. By using friction dampers both the rigidity and the structure’s hysteresis energy loss ability is increased. These dampers by their inelastic behavior in different points of the structure cause the loss of incoming energy of earthquake. Also in high importance structures, by selecting the proper design parameters, it is possible to prevent the main structural members to enter the inelastic behavior limit which causes local damages to some parts of them or minimize that. This system was first introduced by Pall and March in 1982. The mechanism of this system is creating slippage friction surfaces at the intersection of braces. For building frames, these dampers could be used in crossed tension bracing, single diagonal bracing and Chevron bracing. The first model of pall friction damper was tested in chevron bracing against earthquake in Eaton Building in Canada. The purpose of this study is to investigate the role of Pall friction dampers in reducing structural response during the earthquake. Therefore, modeling of the damper is based on the model used by the Pall Corporation in Eaton Building. The functioning of this damper is by generating friction under lateral shear force which causes the movement of the damper and generation of slippage in it. Therefore, three concrete moment frames with 5, 8 and 10 stories, have been designed according to the Iranian National Codes. Using SAP2000, v14, several static nonlinear analyses were done to get the performance point of frames on the basis of the capacity spectrum method. Adding chevron braces to the mid span, the target displacement of frames were determined. Considering the fact that none of the braced moment frames satisfied the Life Safety criteria under Design Based Earthquake, Pall friction dampers have been added to the frames and static nonlinear analysis were done by several slip loads such as 1%, 10%, 25%, 50%, 75% and 100% of frame weights. Evaluation showed that in optimum slip Load, the performance level of the frames improves.