buckling restrained brace

In the seismic active areas, strong ground motions usually consist of the numerous successive shocks (Foreshock-mainshock or mainshock-aftershock), which have the significant potential to increase the structural response and cumulative damage. This phenomenon (as called seismic sequence) can affect on the behavior of structures, control the seismic performace of buildings. Multiple earthquakes which have been recorded in all parts of the world are proven that the structures located in the mentioned areas are not only experienced a single event, but also they withstand a series of shocks. Due to the high importance of consecutive earthquakes, application of buckling restrained braces (BRB) and shape memory alloy (SMA) materials as smart materials in engineering sciences in the past decades, this paper tries to evaluate the seismic performance of steel frames equipped with buckling restrained brace by determination of the optimal percentage of shape memory alloy under successive earthquakes. Because SMA has unique advantages and characteristics such as no need to replace after an earthquake, high resistance to corrosion and fatigue, the ability to absorb high energy, the ability to return to the original state by applying temperature, tolerating strain up to about 10% without leaving residual strain after an earthquake, and tolerating multiple cycles of loading and unloading, various applications can be found separately and combined in controlling the behavior of structures. It should be noted that despite the high damage potential of successive earthquakes, they are neglected in the seismic codes and design earthquake is still proposed without successive shocks. Hence, the acceptance of new methods for improving the seismic performance of structures under consecutive shocks seems necessary by the engineering community. Therefore, in this regard, 4 and 7 story steel frames with diagonal buckling restrained braces representing short and mid rise structures were designed based on Iranian codes in ETABS software and then implemented in OpenSees software. After selecting the reference model, the performance of the studied models is verified for the linear and non-linear region through comparison of periods and pushover curve of reference and implemented model. In the following, different percentages of shape memory alloys including 20, 40, 60, 80 and 100% for the 4 story steel frames and 5, 10, 15, 20 and 25% for 7 story steel structure has been considered. The studied models are analyzed with/without shape memory alloys under seismic scenarios with and without seismic sequence in Opensees software. For this purpose, critical successive shocks are selected based on effective peak acceleration (EPA) from PEER center. For compatibility aspects between the seismic analysis and seismic design, the selected records should be scaled by designing spectrum for each fundamental period of studied structure in order to have identical spectral acceleration. The results of nonlinear dynamic analysis show that with the increase in the percentage of shape memory alloy in the 4 story steel frame, the response ratio of steel frames under single and consecutive earthquakes increased, but in the 7 story steel frame, it almost decreased, and this reduction is better felt in the higher stories under the single earthquake. Finally, the optimal percentage of shape memory alloy among the selected percentages in the present study is suggested to be 20% for 4 story steel frame and 15% for 7 story steel frame.

Choosing the appropriate seismic resisting system is a necessity in the design of buildings in seismic-prone areas. Seismic design-related requirements are controlled by considering parameters such as displacement amplification, over-strength, and response factors. Due to the lack of these coefficients for a dual system of buckling-restrained braced intermediate reinforced concrete (RC) moment frame in the Iranian seismic code (2800 standard), the aim of this research is to investigate the seismic performance and calculate the mentioned parameters of 4, 10, and 15-story RC structures with the same bay length and with a mentioned dual system and to compare them with the intermediate RC moment frame system. Nonlinear modeling of the structures is done in OpenSees software. Results of the pushover analysis show an increase in the capacity of 4, 10, and 15-story structures with a double system by 10, 5, and 11%, respectively, compared to structures with a moment frame system. Results of the time history analysis indicate the reduction of average maximum and residual drift of the structures with the double system by 37 and 27%, respectively, compared to the structures with the moment frame system. The results of the incremental dynamic analysis also showed that the average acceleration bearing of the structures with the double system is 35% more than the structures with the moment frame system. Also, by examining the fragility curves for collapse damage state, it is clear that this type of bracing reduces the probability of failure of the structures. The obtained seismic parameters for the structures indicate that the ductility, over-strength, and response factors of the structures with the double system are 7, 25 and 36% respectively more than the structures with the moment frame system, and on average the response factor of 5 has been suggested for the buckling-restrained braced intermediate RC moment frames.

To mitigate the input energy of the earthquake, numerous vibration control systems have been broadly proposed. The vibration control system can be categorized as passive, active, semi-active, and hybrid. The metallic dampers as a passive vibration control system because of their simple construction and configuration, low cost, availability, high efficiency, rate-independent, resistance to ambient temperature, and require no external energy, are an appropriate and economical control system in structures to mitigate the input energy of the earthquake. Metallic dampers can control the system and absorb the input energy of the earthquake through a shear, flexural, axial, and torsional capacity of the metal element. Different configurations of the metallic damper have been proposed. One of the energy-absorbing mechanisms of the metallic damper is using axial capacity. This paper proposed a new type of buckling restrained brace. Energy Dissipating elements of this damper withstand applied force through the compressive strength of the core plates in each cycle. The proposed damper consists of four comb-teeth elements instead of one consistent element. Each comb-teeth element with a rectangular section of 20×8 mm consists of three narrow straps with different lengths of 390, 386 and 382 mm to make a jump in the hysteric response curve by making lag in the formation of the buckling mode shapes. These comb-teeth elements were welded to two positions on both sides of the middle plate. The middle plate’s height, width, and thickness are 500, 400 and 30 mm, respectively. The proposed damper carried out the axial load through the displacement of this plate. Twelve restraining elements with a section of 30×20 mm and a length of 830 mm were used to prevent global buckling of each strap of the comb-teeth damper. For this purpose, twelve grooves have been created on the middle plate and the restraining members passed through it. Also, to prevent the residual displacement of the system, six parallel springs with an equivalent stiffness of 2316 N/mm were used as a self-centering system on each side of the middle plate. By applying force to the middle plate, the pre-compressive springs provide self-centering force and bring the middle plate to the initial position. Finally, two end plates as reaction plates with a section of 300×200 mm and a thickness of 20 mm were bolted to the cross-section of the restraining members by using 10 mm high-strength grade 12.9 bolts. The springs are placed inside the pipes and between the middle plate and end plate. Both ends of the springs stay free. Also, to prevent the lateral displacement of the restraining members perpendicular to the length of the straps, 48 batten plates were used. These batten plates were bolted by using 8 mm high-strength grade 12.9 bolts at two positions from each end of the restraining members. The whole length of the proposed damper is 870 mm. The 1000kN-capacity servo-controlled hydraulic jack was used to experimentally determine the hysteretic behavior of the proposed damper. Experimental results indicate that the proposed system experienced the maximum axial load of 348.6 kN and -347.6 kN in the back-and-forth direction of applied load at the same axial displacement of 35 mm. The results show that the damper experienced a residual displacement of 10.9 and -11.4 mm at the end of the test, so using springs eliminates the residual displacement up to 68.8% and 67.4%, respectively. Also, to evaluate the performance of the proposed system, energy absorbing capacity and the equivalent viscous damping coefficient were calculated 35.25 kJ and 46% based on the load-displacement curve, respectively. As expected, the failure modes of the core plates of the proposed damper concentrated on buckling modes in the weak and strong axes. It is worth noting that all the systems exhibited satisfactory performance without any instability during the test.

In the engineering community, it is very important to study the seismic behavior and the different damage states of structures under far/near earthquakes, and safe design of structures seems necessary in the seismic active zones. Special Truss Moment Frame (STMF), as a lateral force resisting system, is similar to other common moment frames; however, the difference is in the existence of truss beams instead of solid-section beams. Since the depth of truss beams is usually greater, they have more stiffness and are more resistant against bending moments than other beams. As a result, they are suitable for long spans. To prevent the formation of plastic hinges in the columns, an area is determined in the middle of the truss beams, which is called a special segment. The role of the members in the special segment is to yield under lateral loads and prevent the creation and expansion of plastic hinges to other structural members. In other words, the special segment acts like a fuse and prevents the damage in other structural members. Therefore, this paper compares the seismic performance of special truss moment frames equipped with energy dissipation devices such as Buckling Restraint Braces (BRBs), Viscous Dampers (VDs), and Friction Dampers (FDs) under near and far field earthquakes. In this regard, the desired structures have been implemented in OpenSEES software, considering the non-linear behavior of materials, and subjected to incremental dynamic analysis (IDA) considering seven far-field and seven near-field earthquakes with peak ground accelerations of 0.1g to 1.5g with a fixed incremental step of 0.1g. In order to perform the nonlinear dynamic time history analysis, four ten-story models have been used, which differ in the implementation of their special segment. In this way, one of the cases has Vierendeel special segments (without braces or dampers), and in the rest of the cases, buckling restraint braces, viscous dampers and friction dampers were installed inside the special segments. For this purpose, dampers and braces are placed diagonally inside the special segments. The results of IDA indicate that the near-field earthquakes have more destructive effect on structures than far-field earthquakes. Moreover, the structures equipped with viscous dampers have a greater ability to absorb and dissipate the earthquake energy than other investigated systems. In the following, the comparison of the fragility curves for the studied structures has shown that the probability of complete failure of the structure equipped with buckling restraint braces near-field earthquakes is 13.33% more than far-field earthquakes. This value is equal to 12.5% and 23.5%, considering the friction and viscous dampers, respectively. Furthermore, the fragility curves indicate that damages in the structures increase strongly in the partial range with small changes in acceleration, because this range of damage depends on the intensity of the earthquake, and it occurs with small changes in the acceleration of the earthquake. The effect of viscous dampers in reducing the response of the structure at the moment of the earthquake pulse, in near-field earthquakes, is much higher than the friction dampers and buckling restraint braces.

BRBs are a new type of seismic resistance system that is being used extensively nowadays due to their enhanced seismic performance than conventional braces. In BRB braces, because the buckling of the steel core is prevented, the structure shows more stable behavior. In this type of bracing, the hysteresis performance of the bracing is similar to the hysteresis performance of the core material. Another feature of these braces is that the ductility of the steel material occurs over a considerable length of the brace. Although BRB braces are capable of dissipating large amounts of energy, they are unable to eliminate their residual strains. In other words, they do not have the property of self-centering. This leads to the non-return of the structure and to its original configuration after the seismic excitations; in the absence of a return mechanism. There may arise many permanent deformations in the structure during an earthquake. To overcome these permanent deformations, various innovative solutions have been developed in the construction of steel frames, including the use of shape memory alloys (SMA) that have two prominent features of shape memory and superelastic behavior and can return to their original position after subjected to the various loadings condition. In recent years, beside the Nitinol shape memory alloy (NiTi), Iron-based shape memory alloys (SMA-Fe), which have many advantages over previous SMAs and particularly due to their lower cost, have been introduced and being used in many construction projects. In this research, the seismic behavior of structures braced with BRB, and iron-based shape memory alloy and Nitinol shape memory alloys has been investigated. Seismostruct finite element software has been used to model these systems. Incremental dynamic analysis (IDA) has been performed on seven story structures equipped with X braces with different materials. The results of this study show that braced structures with iron-base shape memory alloys undergo less maximum displacement and permanent displacement compared to nitinol-braced structures. However, these structures experience more maximum displacement than BRB braced structure. The more the structures enter the nonlinear stage (in the maximum values of the relative inter-floor displacement demand) the more the dispersion of the results increases and the structure is more affected by the input accelerometers. The structure with buckling bracing will reach instability later than the two structures with shape memory alloy bracing.It is also observed that the elastic stiffness (slope of the linear behavior region) in all 3 braced frames is equal to each other. And finally, the IDA curve of the BRB structure is higher than the two shape memory alloy structures, and at equal acceleration, it is clear that the displacement of the shape memory alloy structures is more than the buckling structure, and it can also be seen that the iron-based shape memory alloy brace has a favorable performance and its curve is slightly higher than the NiTi shape memory alloy. Also, two shape memory alloy structures move almost together and reach instability at one point. According to the curves, it seems that the braced structures with shape memory alloys have performed well and until these structures reach instability.

Buckling restrained braces have been shown to exhibit favorable energy dissipating characteristics in steel structures during an earthquake and are therefore widely used in passive control of structures. However, they face the problem that they do not return to their original shape upon unloading, and consequently, the structure experiences large permanent deformations after the earthquake which usually makes the structure impossible or uneconomical to repair. In recent years, to solve this problem, researchers have used Iron-based shape memory alloys which have two essential properties of superelasticity and shape memory behavior. These alloys have been considered due to their high energy dissipation capacity, their ability to withstand large strains, recentering and not leaving permanent deformations upon unloading, and their much lower cost than other shape memory alloys. In this research, the seismic behavior of three-story structures braced with buckling restrained braces, iron-based shape memory alloy, and nitinol shape memory alloy, which is one of the most famous shape memory alloys, is investigated. Modeling and Nonlinear dynamic analysis for these structures have been performed in Seismostruct software. Maximum displacements, residual strains, and force-displacement diagrams of these structures have been studied due to the application of different accelerometers of large earthquakes of recent years with different intensities. The results of this study show that braced structures with Iron-based shape memory alloys experience more maximum displacements than Buckling restrained braces, although unlike BRBF they do not leave any permanent displacement. Also, these structures undergo fewer maximum displacements and permanent displacements compared to nitinol-braced structures, and in general, show better performance.

Buckling-Restrained Braces (BRBs) are one of the new seismic resistant systems. The cross-sectional area and length of the BRB brace is one of the most important characteristics of these braces that directly affects the cost of BRB frames. Since beams, columns, and connections are designed for the maximum forces developed in BRB, the decrease in cross-sectional area of the BRBs decreases the steel consumption in the whole structure.The main purpose of this study is to optimize the weight of the structure, BRBs weight while uniforming the drift profile by changing the cross-sectional area and the length of the BRBs using genetic algorithms and other multi-objective optimization algorithms. Optimization is based on the results of nonlinear time history analysis under seven earthquake records using OpenSEES software. For this purpose, the objective function and constraints were defined in the genetic algorithm NSGA_II, MOPSO, MOEA_D, PESA_II, SPEA_II, and the initial population produced was entered as the initial cross-sectional area and length of the braces in the OpenSEES software. The optimization results show that for all three objective functions, the optimization values with high percentages of structural performance were optimized in such a way that the weight of BRB can be decreased up to about 50%.

In areas with high seismicity, structures need a resistant and load-bearing system against lateral forces caused by earthquakes. In addition to high stiffness and resistance to displacement caused by earthquakes, these systems must have good ductility and energy dissipation ability. Therefore, apply of the buckling-resistant braces (BRB) with reduced beam section (RBS) connections is recommended instead of the previous conventional braces. In this research, the cyclic behavior of the recommended system is investigated. The finite element method and ABAQUS software have been utilized for modeling and study of the numerical models. The investigated parameters include stress contour, cyclic response (force-displacement), stress in column, and beam. The results of numerical models indicate that by creating RBS with a length of 0.85d to a shear radius of 0.25bf compared to the 0.2bf mode, the amount of stress in the beam increases by about 24%.

The conventional bracing frame (CBF) systems show a limited drift capacity before buckling subjected to seismic loads. So, the induced damage in the structure reduces the strength and stiffness. In the last two decades, self-centering (SC) systems have been developed to resolve the deficiencies of the conventional seismic-resistant systems. In SC systems, the structural damage and residual drift are negligible, while they provide sufficient strength and stiffness. In these systems, prestressed elements are used to provide the initial stiffness. On the other hand, the steel plate shear wall, bracing, beam connection to the column provide energy dissipation mechanism. These elements are used as replaceable fuses after sever earthquakes. When the force applied to the structure is greater than the initial prestressing force, the gap created in the structure causes the energy dissipating elements to work. The main feature of SC systems is that they return to zero deformation after each load cycle. So, the post tensioned elements must remain elastic to be able to reduce the residual displacement. This property of the systems represents flag-shaped hysteresis lateral load-deformation curves. Among the engineering community, three methods of equivalent lateral forces (ELF), dynamic spectral analysis and dynamic time history analysis are commonly used for seismic analysis of structures. The endurance time (ET) method is a new method for seismic analysis and also for performance-based design of structures. In this method, the structure is subjected to special ET accelerations in which the dynamic response of the structure increases with time. The time needed for the structural failure index (such as the maximum drift of stories) to reach a certain level of performance or failure is defined as the structural ET. As a result, a structure that has a longer ET, has better performance against earthquakes. The main advantages of the ET method include: 1) by providing a suitable estimate of the structural response in each time history analysis, saves a lot of computational time for seismic evaluation, 2) the nonlinear properties of the structures may be considered which can be used for a variety of structures and complex behavior, 3) this method has a simple concept and principles for engineering applications, and 4) this method has a high capability for experimental work with a shake table. In the current research, the self-centering buckling restraining column braced frame (SC-BRC-BF) system was examined. This system not only increases the drift capacity, it also reduces damage and residual drift in the system. The SC-BRC-BF system consists of two rigid cores connected by buckling resistance columns (BRC) between tha adjacent floors. The BRCs are used as replaceable fuses to dissipate the input energy and to reduce the seismic responses. Vertical post tensioned cable is used to restore the system. For this purpose, a preliminary design approach was introduced for SC-BRC-BF systems with 3, 6 and 9 stories via SAP 2000 software. The simulation of structures under time history analysis and ET method was done via OpenSees software fin a 2D framework. Different seismic responses were investigated including: 1) roof drift, 2) the maximum strain of core elements, 3) drift concentration factor (DCF), and 4) Inter-story drift. The response of structures was examined at both DBE (Design Base Earthquake) and MCE (Maximum Considered Earthquake) levels. Comparing the responses from ET method and the conventional time-history method, the error rate does not exceed 10 and 15 % at the DBE and the MCE levels, respectively. The results obtained from seismic evaluation using the two mentioned approaches, corroborated the high efficiency of ET method with a few number of time history analyzes.

In this article, a new type of all-steel buckling brace is introduced. The proposed bracing member consists of two tubular members. The inner tube is intended to act as the structural core such that energy can be suitably dissipated by its yielding under cyclic loading. The outer tube is supposed to act as a lateral restraint for the inner one without interfering in the axial load-carrying inner tube, that is, using a gap separating the two tubes, the inner tube should be free to slide inside the outer one under the application of cyclic loading. Hence, axial loads are resisted by the inner core tube and the outer tube may only interfere in axial force resistance through friction at contact points after the core deforms laterally. In this study, while accurately introducing the proposed system, its behavior under cyclic loading has been studied using both experimental tests and numerical modeling. The test specimen was designed based on the parametric study previously done by the authors. In order to investigate the cyclic performance of the proposed BRB member, the displacement loading protocol proposed by the AISC was applied. Numerical modeling was performed using ABAQUS software, taking into account all nonlinear effects, including nonlinear behavior of materials, geometric nonlinearity, and contact. Based on the experimental and numerical results, it was demonstrated that the proposed BRB - if well designed - would be quite competent in accomplishing the intended tasks as a buckling restrained bracing member. Properly designed TiTBRBs can exhibit stable cyclic behavior and satisfactory cumulative plastic ductility capacity so that they can serve as effective hysteretic dampers. At the same time, such all-steel TiTBRBs concreting was eliminated; hence, much lighter members were obtained. This is also associated with ease and speed of fabrication, erection, inspection, replacement, and a more economical and environmentally friendly design.

In this study, firstly, the 12-story 3D structure with hybrid system of buckling restrained brace and moment frame with 6 states such as 1) whole stories with buckling restrained brace system, 2) the first 6 stories with buckling restrained brace system and the second one with moment frame, 3) the first 5 stories with buckling restrained brace and the 7 stories with moment frame, 4) the first 7 stories with buckling restrained brace and the 5 stories with moment frame, 5) the first 3 stories with buckling restrained brace and the 9 stories with moment frame, 6) the first 9 stories with buckling restrained brace and the 3 stories with moment frame. Then, using Opensees software, the side axle frame is modeled subjected to incremental nonlinear static analysis and incremental nonlinear dynamic analysis (IDA) with the intensity parameter (IM) corresponding to the maximum relative displacement and the DM response parameter corresponding to the first mode spectral acceleration Sa (T1, 5%)and the level of collapse prevention performance (CP) has been evaluated. In the following, fragility curves and modification factor are presented. By investigation of the results, it is observed that the frame with more braces, at a constant seismic intensity level, is less likely to fail than other frames. Therefore, increasing the percentage of brace use leads to a higher modifaction factor and less probability of failure than other cases, and by reducing the ductility of the frames, the probability of their failure also decreases.

Although Buckling-Restrained Braces (BRBs) can dissipate a large amount of the seismic input energy. However, they need to be repaired or replaced due to large permanent deformation after a severe earthquake. To overcome this issue, the use of Shape Memory Alloys (SMAs) in the braces has recently received attention. These alloys are able to return to their original state after loading. The present study aims to analyze the fragility curves and to investigate the sidesway collapse of the BRB frames equipped with SMA during near-field earthquakes in comparison with those given for the case without SMA. For the purposes, two 5 and 15-story BRB and BRB-SMA frames subjected to 7-pair of near-fault earthquake records are studied. Nonlinear Incremental Dynamic Analyses (IDAs) are carried out using OpenSees software. On average, the simulation results showed that collapse capacity and collapse duration of the BRB-SMA frames are about 30% and 35% more than those given for the BRB frames, respectively. For an instance, a collapse probability of 38% for the 5-story BRB-SMA frame and a collapse probability of 60% for the BRB frame is given for 3g spectral acceleration. Furthermore, at the performance level of 50% for the 15-story frame, the collapse duration of the BRB-SMA frame is obtained 25.6 seconds, while it is given about 10 seconds for the BRB frame. In addition, the use of a memory alloy for spectral accelerations of 1 to 4 g resulted in a reduction of 50% to reach the collapse performance level of the frames.

When a structure is exposed to an earthquake, depending on the severity of the earthquake, it may be completely destroyed or remain completely intact, or some of its members may become out of their stable state, and in any case, it may be damaged. The damage can be seen in decreasing the stiffness or increasing the softness and shifting the roof and by changing the drift of stories and hysteresis energy. In recent years, with advances in structural-earthquake engineering, advancing knowledge and experience about the seismic behavior of structures, new methods for evaluating the seismic behavior of structures have been proposed to express a damage amount and the quality of the damage requires the definition of the indicators of the damage. In this paper, the performance of a 10-story frame with three suitable lateral systems of moment frame (MF), concentric brace (CBF) and buckling restrained brace (BRBF) is investigated from the theoretical point of view of seismic damage index. Damage indices such as Drift, Park-Ang, Energy, Deformation, Roufaiel and Meyer and Banon Failure under near-fault earthquakes are calculated and compared based on nonlinear dynamic time history analysis using SeismoStruct 2018 software. Therefore, the seismic performance of these systems under near fault earthquakes can be properly investigated. The results showed that the values of the damage index BRBF were in the limited range and its performance is more appropriate compared to the other two systems.

Conventional seismic resistant systems generally dissipate earthquake energy through the plastic deformation of structural elements. These systems withstand high residual displacements increasing the cost of repair or even necessitate reconstruction of the building. In recent years, researchers have focused on self-centering lateral load resisting systems due to earthquake energy absorption in replaceable elements and reduced residual displacement. One of the recent types of these systems is self-centering buckling restrained 2-story-X-brace system. In this study, the hysteretic performance of a 9-story structure with a dual steel moment resisting frame system equipped with a self-centering buckling restrained brace is investigated. At first, the parameters affecting the performance of the bracing were specified. These parameters included the yielding stress of the bracing core, the ratio of cable pre-stressing force to the cable yielding force, and the ratio of cable to core area. For the yielding stress of the core two values of 240 and 360 MPa, for the pre-stressing ratio five values from 0.1 to 0.5, and for the area ratio, seven values of 0, 0.25, 0.5, 1, 2.5, 5 and infinity were selected. So therefore, a total of 62 models of 9-story structures with dual steel moment resisting frame system equipped with self-centering buckling restrained brace were constructed. The models were analyzed using OpenSEES finite element software. Performing nonlinear analyses under cyclic loading, the hysteresis diagrams were obtained. The optimal design for these models was performed considering residual displacement and energy absorption as two main parameters. Finally, the performance of the optimized structure equipped with self-centering buckling restrained brace was compared with the structure equipped with ordinary buckling restrained brace. According to the results, the optimized self-centering structure may reduce 60% of the residual displacement while its energy absorption drops by only 37%. Generally, the results indicated the prominence of the optimized self-centering structure.

One of the most important deficiency of buckling restrained braced frames is the concentration of lateral drifts in some specific stories and the soft story mechanism, under moderate to severe earthquakes. In this paper, in order to prevent the development of soft story mechanism in buckling restrained braced frames, asymmetric configuration along with a zipper strut is proposed for chevron type ordinary buckling restrained braced frame.The zipper strut connects the midpoints of beams at the brace intersections in all stories and carry the unbalanced axial force developed at the intersection. In addition, it causes the BRBs to uniformly yield over the height of structure. For this purpose, 4, 10, and 14 story buckling restrained braced frames were designed according to Iranian seismic code, and the nonlinear time history analyses were performed in Opensees, subsequently. The analysis results showed that the asymmetric buckling restrained braced frames possess more uniform lateral drift demands in comparison to ordinary symmetric buckling restrained braced frames, is less prone to the formation of soft story mechanism. The zipper elements were found to carry a significant axial forces during analyses, and were responsible for distribution of unbalanced forces among BRBs over the height of braced frames.

In the present study, the use of buckling-restrained braces (BRBs) has been evaluated as a research innovation aimed at reducing the potential of progressive collapse in steel braced frames. These braces prevent the overall buckling of brace and provide much more energy absorption than conventional convertible bracing systems. For this purpose, three-dimensional finite element models of 4, 8, and 12-story buildings were simulated in two modes using ordinary and buckling braces and their response against progressive collapse was evaluated. The progressive collapse analysis was carried out using an alternative load path method and the response of the structures to column removal was investigated. Simulation of models was done using ABAQUS software. Also, the accuracy of the finite element method used in simulating the models was evaluated and a suitable agreement between the results was observed. The most significant results show that in steel frames that have BRBs, less stresses have been created compared with conventional steel braces. In all cases, BRBs with internal energy absorption of the structure have caused lower stresses to other structure member, thereby reducing the potential of progressive collapse. Also, the use of BRBs has reduced the rotation of the beam to column connection, compared to conventional bracing. This effect is especially noticeable in buildings with higher altitudes; the maximum amount of joint rotation corresponding to the 12-story building with BRBs has been reduced by 21% compared with the conventional braces. The reason for this is that, in buckling-restrained braces, the members are able to withstand a pressure above the tensile yield strength, and, unlike ordinary systems, the inherent ductility is due to the occurrence of yield in pressure before it begins to buckle.