جستجوی مقالات مرتبط با کلیدواژه "cyclic loading" در نشریات گروه "عمران"

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.

In this research, he experimental findings of the behavior of self-centered coupled shear wall system, in which TADAS metal dampers are used at the beam-to-wall connection area, are evaluated and confirmed using OpenSees software. In order to prevent the possible failure of the beam, as well as to maintain the self-centering capability of the system and to minimize the residual deformation of the structure under seismic load, steel beams and PT strands have been used in the construction of subassemblies. To carry out the laboratory work, three subassemblies were made of the self-centered coupled wall system and subjected to quasi-static cyclic loading. The investigated samples included a subassembly without energy dissipation devices and two subassemblies equipped with TADAS dampers at the beam-to-wall connection area. The subassemblies were modeled using OpenSees software and the results showed a good agreement with the laboratory results. In the samples with TADAS dampers, the effect of the thickness of the triangular plates of the damper, as well as the effect of the initial tension of the pullback strands was investigated. The experimental and analytical studies of the subassemblies showed that the use of TADAS dampers at the region where the beam and wall are connected, while significantly reducing permanent deformations, led to an increase of 71% to 96% in the carrying capacity of the system and a significant increase in the energy dissipation capacity. Also, by increasing the thickness of the damper blades from 12 to 15 mm, the loading capacity increased by about 15%, while less residual deformation was observed in the subassemblies.

The shear wall is one of the most important systems to resist lateral loads in the building. In addition to controlling the lateral displacement of the structure and dealing with the lateral force, this system significantly increases the stiffness of the structure. The purpose of this article is to investigate the nonlinear behavior of semi-supported steel composite shear walls at the edges under monotonic and cyclic loading near- and far-fault. In this article, after verification, firstly, the semi-supported steel shear wall is modeled and analyzed using ETABS software to select the boundary members and the critical opening. Then, the base model of the semi-supported steel shear wall is converted to a semi-supported composite shear wall, and finally, it is modeled and checked using ABAQUS software. Among the investigated variables are reducing the thickness of the concrete coating on both sides of the steel plate of the wall, using concrete coating on one side of the steel plate of the wall, and increasing the thickness of the steel plate of the wall. The results showed that the addition of concrete to the SSSW model (converting the model to SSCSW) increases the initial in-plane hardness by 350%. Also, when concrete was added to the SSSW model, the ductility increased by 150% in two states near and far from the fault. Comparing the ultimate strength (peak of the cyclic diagram) also showed that regardless of the type of cyclic loading pattern, the calculated value for SSCSW is 28% higher than SSSW.

The lateral-torsional buckling of beam is a disadvantage for connections with reduced beam section (RBS). One effective method for preventing such buckling is to reduce web section (RWS) instead of flange. This, not only eliminates the disadvantage of RBS, but also, provides better behavior and more ductility. This paper investigates the effect of longitudinal and transverse web/flange slits on the behavior of bolted steel moment frames. The investigated frames include: frame with full beam section and frames with beams reduced by: radial cut in flange, longitudinal slits in flange, transverse slits in flange, longitudinal slits in web, transverse slits in web, mixed web slits, and, cruciform web slits. These one-story one-bay frames are analyzed using nonlinear finite-elements method with shell elements of type S4R and solid elements of type C3D8R in ABAQUS. The quasi-static cyclic loading according to Supplemental Access Control protocol is applied to the frames. The results show that by using RWS beams, the location of plastic hinge can move from column face to an appropriate distance at the reduction region. According to the results, the maximum rupture index (RI) occurs in frame with full (not reduced) section; while, the minimum RI belongs to RWS frame with longitudinal web slits. The larger the RI, the greater is the probability of rupture at low drifts. The hysteresis curves of frames show that the strength of frames with longitudinal or transverse flange slits decreases at final step of loading, i.e. at the last two cycles with greatest rotation angles. The frame with longitudinal web slits with 31.8% more load-carrying capacity and 30.9% more energy dissipation, shows better performance compared to the other RWS or RBS frames.

The cyclic behavior of steel shear walls has been the subject of numerous experimental and numerical investigations in recent years, most of which have focused on unidirectional loading. It should be noted When significant damage associated with bidirectional loading is anticipated, the test should be conducted under bidirectional deformation reversals. The present study was conducted with the aim of investigating the seismic performance of L-shaped steel shear wall under unidirectional and bidirectional cyclic loading. Accordingly, the L-shaped configuration was adopted to better simulate the real response of this system under bidirectional loading. Before creating the studied models in ABAQUS finite element software, verification was done. Then 14 numerical models, under two unidirectional and bidirectional cyclic loading patterns, considering variables such as plate thickness change, vertical boundary elements cross-section change, the use of high-strength steel in the boundary elements, use of simple connections and reduction of cross-section in elements It was created and subjected to nonlinear static analysis. ATC-24 code was used to apply unidirectional loading and FEMA 461 was used for bidirectional loading, and seismic parameters were evaluated. The results show that the finite element models under the bidirectional loading pattern had an increase in resistance of about 40% and hardness of about 20% compared to the average results of the models under unidirectional loading, but the dissipated energy was almost 70% lower than the model under unidirectional loading.

So far, it has been proven that concrete structures have good resistance against incoming forces, but the rebar inside the concrete may be damaged and fail prematurely due to execution or calculation errors. Therefore, it is necessary to take measures after construction to strengthen concrete buildings. It is customary to increase the capacity and durability of the column by planting composite rebar in concrete (NSM method). Therefore, it is important to investigate the effect of simultaneous reinforcement of composite sheet and rebar and to optimize its installation method on concrete columns. In this research, 12 concrete columns were strengthened by rebars and composite sheets with different numbers and modes of installation, which were analyzed in three separate groups and their results were compared. The initial model was adjusted according to the reference article using the finite element method in Abaqus software. Then this model was reinforced with GFRP rebars by NSM method and enclosure with FRP sheet. In this research, the important parameters of retrofitting such as load capacity and seismic durability, energy consumption rate, ductility ratio, hardness reduction rate, behavior coefficient, failure analysis and economic index were investigated. The results of this research showed that, on average, using the NSM method along with the FRP sheet wrap can increase the seismic durability of the column by 30%. Also, the use of 8 NSM rebars significantly increases the seismic durability of the column, but it will cause the behavior of the column to become brittle and cause unexpected and sudden fractures.

Beam-to-column joints in reinforced concrete structures play an essential role in the overall behavior of the structure. Therefore, it seems necessary to strengthen joints that have been damaged over time under various loads or designed only based on gravity loads. Nowadays, shear retrofitting of joints with fiber-reinforced-polymers (FRP) has attracted the attention of researchers. Since the shear force applied to the joint is transferred by the truss mechanism formed by horizontal and vertical bars and also the compressive strength of the concrete, so the concrete of the joint plays the main role in the shear capacity before retrofitting by FRP materials. In this study, 7 reinforced concrete external joint specimens were designed and constructed on a scale of 2/3 and in two concrete compressive strength groups of 30 and 42 MPa. Seismic criteria were not considered in these joints. Two specimens were damaged up to 1.5% drift and the other two specimens up to 3% drift, and then retrofitted with FRP materials by Near Surface Mounted (NSM) method. Of the other three specimens, one with and two without considering seismic criteria as control specimens were subjected to cyclic loading at two levels of concrete strength. The purpose of considering two different levels of concrete strength was to investigate the effect of increasing concrete strength within the normal range of engineering on the performance of the NSM-retrofitted joint. As the results show, damaged retrofitted specimens with concrete compressive strength of 42 MPa were able to increase the bearing capacity of the joint by 12% compared to the seismic control specimen with concrete compressive strength of 30 MPa. Also, the increase in concrete strength by 12 MPa caused a 20, 40, and 65 percent increase in the bearing capacity, hardness, and energy dissipation ability of the NSM retrofitted specimens, respectively.

After the Northridge earthquake, a set of prequalified connections were introduced by international design codes. Thease connections include reduced beam secrion (RBS) moment connection, bolted unstiffended and stiffended extended end-plate moment connection, bolted flange plate (BFP) moment connection, welded unreinforced flange-welded web (WUF-W) moment connection, kaiser bolted bracket (KBB) moment connection, conxtech conxl moment connection, sideplate moment connection, simpson strong tie strong frame moment connection, double tee moment connection, slottedweb (SW) moment connection. After ensuring the seismic performance of this set during earthquakes, concerns surfaced that forming plastic hinges in beam elements would result in either making repairs impossible or incredibly expensive in the event of a moderate or severe earthquake. Hence, a type of replaceable connections was introduced wherein plastic hinges would be placed in pre-determined elements. Their intuitive replaceability feature would make repairs and reutilization of the structure a much easier task. In this study, the experimental investigations of 4 full-scale samples of a replaceable rigid connection under cyclic loading were carried out. The results of the experiments demonstrated that in the proposed connection, the plastic hinge is formed in the fuse element while the beam and the column maintain their elasticity, allowing the connection to be replaced. Also, taking into account the early buckling of the fuse plates installed on the beam flanges, the moment capacity of the connection is decreased by 22 percent compared to the moment capacity of the fuse. According to the results obtained from the backbone diagrams, stiffness of the connection after replacing the fuse plates in P12 and P15 samples has decreased by 8.61% and 6.14%, respectively; this could be due to slight changes on the holes on the beam web and flanges as well as changes in the pre-tensioning forces of the bolts. Investigations have revealed that, the 20% increase in the moment capacity of the fuse (using 15 mm-steel plates instead of 12 mm in the fuse plates of the beam flanges) has increased the cumulative energy dissipation of the connection by 12%.

In steel structures, joints play an important role in the behavior of the structure. In this paper, using numerical modeling with Abacus finite element software, several different models of connection to steel columns, modeling and placement cases have been taken. This paper investigates the effect of beam-to-column connection with two types of t-bar and corner joints. Therefore, 20 models with different conditions of different thickness and different types of hardeners, as well as 6 models with columns have been studied. Abacus finite element software has been used for modeling. All models are modeled in this software. Used to load cyclic loads.The results show that the model with t-bar hardener has a higher and has 12 to 28% higher bending capacity in the joint than the corner hardener. With increasing the thickness of the corner or spray, the connection of the beam to the column has increased to about 12 to 25% of the capacity, in the thickness of 18 mm, the anchor capacity has increased by 12% and in the thickness of 20 mm, it has increased by 25%. Is. The model of columns with concrete is about 15 to 25% different from the similar model without concrete, and the model of columns with concrete has more bearing capacity.

Nowadays, according to the performance of prefabricated friction dampers, which are expanding rapidly, they can increase the resistance of structural systems against earthquakes and depreciate the energies created by earthquakes in the structure. To control the vibrations of the structure at one level, it is very important to use passive control systems. But in the design at different levels, they cannot depreciate the incoming energy from the earthquake. Friction dampers focus on displacement variable and are mostly used in steel structures.The friction damper works according to the rules of a Coulomb damper or a friction brake that converts kinetic energy into heat through friction. In this article, a new type of friction damper called a flat cylindrical friction damper has been designed using brake pads in the diagonal brace, and the performance and seismic resistance of this system as well as the amount of energy loss have been investigated. The configuration of the damper is designed in such a way that grooved bolt connections and twin steel cables of different sizes and thicknesses are used.And the way the internal and external components are placed in the damper is such that innovation in tension and pressure has been created.Also, the movement of the cylindrical element in the damper has increased the amount of friction due to the presence of two types of brake pads with friction coefficients of 0.11 and 0.16 and bolt connections.  It shows the performance of the damper by different sliding force at different friction levels. Geometric dimensions, thickness of brake pad, number of bolts, size of bolts, diameter of cables, sliding force, location of damper are among the variables investigated in this article. The role of brake pads, steel cables and bolt connections is very important and economically very affordable. The seismic performance of the intended frame has been investigated by 80 different modeling of the damper and the placement of the research variables. The desired optimal models were modeled in Abaqus software and analyzed and designed. The aim of this system is to reduce the relative horizontal displacement of floors and increase the amount of energy absorption. The increase in the axial force created in multiple loading cycles has always caused damage to the damper components and frames. In this article, it has been tried to use a special multi-level geometry that, in addition to reducing the axial force created in the damper, reduces the relative displacement of the frame, damages in the elements and increases the ductility.The results show that the friction surfaces of steel plates and brake pads is very high due to the displacement and damping of the cables And with the consumption of energy and its absorption by the damper in cyclic loads, displacement control is easily done. It also shows the seismic response of structures in terms of frame and damper displacement, base shear forces, energy absorption. Numerical study confirms the intended damper as an independent seismic resistant member in critical building structures when high seismic performance or seismic resilience in moderate and strong earthquakes is desirable.

In this research, the effect of the elliptical web reduction, the effect of the radial flange reduction and also the effect of the use of stiffener, on the behavior of beam, on the behavior of connection and on the performance of steel moment frame is studied. By using Finite-Elements Method and 3D elements in ABAQUS, the nonlinear-static analyses of the frame under cyclic loading are carried out and the Von-Mises criterion is checked. The results show that in the beam with no reductions, maximum stresses and strains occur in the web. By reducing the web section of the beam, maximum stresses and strains are transferred to the location of the reduced section. The ratio of maximum strength to the strength at the final step of loading, and also, the value of dissipated energy, show that, in comparison with the connections with radial-shaped reductions, the connections with elliptical-shaped reductions have less strength degradation and more ductility.

This paper describes the development of a ductile fuse system to reduce Seismic demand in steel frames with knee element connections. In these types of structures, connections often require reinforcement to withstand the tensile capacity of the brace to comply with the capacity design process.To overcome this problem, it is necessary to think of a solution to prevent the premature failure of the connection.For this purpose, in this research, different models of ductile fuses consisting of a reduced cross-sectional area are placed on the brace of the knee element in a braced frame.The fuses are designed to reduce the tensile capacity of the knee element braces to the capacity of the joints. The results show that the braced frame with fuse can be used to reduce the seismic load demand to the connections sufficiently, to prevent the strengthening of the connection caused by the application of capacity design principles. It was also observed that the properly designed fuse system in braced frames shows a stable hysteretic response under cyclic loading and maintains sufficient ductility with a reasonable reduction in the compressive strength of the braced members. Also, the results showed that the failure of all samples occurs in the fuse, and as a result, by using the fuse, it is possible to use the full capacity of the connection and brace. Finally, based on the results of the study, the best fuse models that creates both Sufficient ductility and compressive strength to an acceptable level were identified for design applications.

The presence of holes in the sensitive areas of the beam-to-column connection can cause unwanted damage. In this article, using the connection truss model and time history loading, the effect of the mentioned holes in changing the functional levels of the connection has been investigated. Validation was done by modeling and numerical analysis of a healthy lateral connection whose cyclic test results are available up to the collapse threshold. Four functional levels were defined based on the crack width and the effect of the presence of the cavity on the change of the crack width in the connection was investigated. It was found that due to the creation of an air cavity, the bearing capacity of the connection is reduced and it enters the failure range earlier, but the presence of the cavity can change the way of damage distribution in the connection so that some cracks are more open and others are closed. Also, the sliding of rebars increases due to the presence of holes in compression cycles, these changes increase the need for connection ductility. So that in the recent study, this slip shows an increase of up to 91%. By dividing the connection into 40 equal parts and placing the hole in different parts, the holes created at the joint of beam and column can be more destructive. On the other hand, the sensitivity is less than the central cavities. Also, the holes placed in the vicinity of the longitudinal bars, even with a small volume, can accelerate the destruction of the connection due to the intensification of the slip. It was found that the presence of a cavity can increase energy consumption, so that a cavity with a volume of 2.5% near the column can improve the average amount of energy loss by 1.6%.

In this study, the shear-lag effects are evaluated on the performance of L-shaped reinforced concrete (RC) shear walls. At first, 42 L-shaped RC shear walls are simulated employing the FlashLab program, and then, the numerical analyses are performed due to the cyclic loading applied within the framework of ABAQUS software. Based on the results of the numerical studies and taking into account the shear-lag effects, axial strain as well as axial deformation distribution are obtained for L-shaped RC shear walls. Thereafter, on the strength of the evolutionary polynomial regression (EPR) and genetic algorithm, new expressions have been developed and introduced for L-shaped sections which allow quick estimation of the axial strain and deformation distributions. The reliability of the proposed expressions has then been examined based on the coefficient of determination, of which the average values for the axial strain and deformation are 80 and 72 percent, respectively. Also, the results of the proposed expression are compared with those of the previous studies for non-rectangular RC shear walls. Finally, on account of the Mean Absolute Relative Error (MARE) index attained for the proposed expressions to that of the previous studies available in the literature, the uniform distribution exhibits the best performance in the response prediction of L-shaped RC walls. Furthermore, calculation of the MARE values indicates that the established formulations can respectively predict the axial strain and deformation of L-shaped RC shear walls with average error of 65 and 54 percent of the MARE values of the available expressions addressed in the literature.

Permanent deformation after an earthquake increases the cost of the retrofit and even the implementation of the rehabilitation plan. A resisting system to control the lateral forces due to earthquakes is to use shear walls with good energy dissipation capability. Permanent deformation and cracking in the border and critical areas of the wall after relatively strong earthquakes leads to a decrease in the seismic performance of the wall. In this research, the responses and cyclic behavior of concrete shear walls under the cyclic loading protocol are investigated. For this purpose, using OPENSEES software platform, concrete shear wall modeling based on SFI-ΜVLEM method or wall modeling using multiple vertical elements based on the macro fiber method has been studied. The main models analyzed in this study include 2 concrete walls with steel rebars and memory alloy (SMA) with different dimensions and arrangement of reinforcements and innovative concrete wall with separate border elements in the form of concrete columns made of engineered concrete (ECC) and SMA rebars.  Validation of the models was based on previous studies. After validation of the initial models, the parametric study of the models was performed in order to investigate the effects of the dimensions of the boundary areas, the type and arrangement of the reinforcements and the amount of gravitational forces on the shear walls in the models. The results obtained from the outputs show that the use of SMA materials in the boundary areas of the wall has a significant effect on the self-centering behavior of the wall and the energy dissipation of the shear wall is reduced by using SMA materials.

Nowadays, with the advances in welding technology and increased use of steel structures in the construction of buildings, especially high-rises, the necessity of matching the construction procedure with the structure analysis procedure has become more evident. In steel structures with moment-resisting frame-concentrically braced frame (MRF-CBF) dual systems, as the moment-resisting frame can individually withstand gravity and seismic loads during construction, the braces can be installed on a story-by-story basis simultaneous with the installation of the moment frame or after its complete construction. This issue cannot be investigated and studied using ordinary analysis, where it is assumed that the entire structure is built at once, and then all the involved loads are applied to the completed structure. Accordingly, this research investigated the effects of staged construction on MRF-CBF dual systems with X brace. In this regard, 3D models with 5, 10, 15, 20, and 25 stories incorporating MRF-CBF dual system were first generated and subjected to pushover analysis under cyclic loading protocols. Subsequently, parameters such as column shortening and column axial forces under dead load, displacement and base shear corresponding to the first plastic hinge formation, effective lateral stiffness, ultimate lateral strength, and energy dissipation of the structure under cyclic loading protocols were investigated. From the obtained results, it was concluded that staged construction could lead to maximum increases of 17.9% in the axial forces in internal columns due to dead load, 3.8% in the displacement corresponding to the first plastic hinge formation, 8.3% in the base shear corresponding to the first plastic hinge formation, 18.4% in the ultimate lateral strength, and 4.3% in the effective lateral stiffness. Also, staged construction was not found to significantly affect the axial forces in corner and braced-bay columns due to dead load and the energy dissipation under cyclic loading protocols.

The Welded Unreinforced Flange-Welded Web (WUF-W) Moment Connection is one of the most common types of rigid connections in steel moment frames. However, due to continuity plates installing challenges in the box columns, developing the design equation of continuity plates and finding creative alternative solutions instead of installing inner continuity plates in box columns are the matter of interest to the researchers. Considering the new continuity plate design approach in the AISC 341-16 and the part 10 of the Iranian National Building Code and the ambiguities that have arisen, in this research 42 WUF-W connections have been modeled and numerically studied under monotonic and cyclic loading. The results show that the approach has been taken by the 5th editions of part 10 of the Iranian National Building Code based on mandatory installing the continuity plate in box columns is strict. It seems that the equation for design the minimum thickness of the continuity plates in 4th edition of part 10 of the Iranian National Building Code can be satisfy the seismic provisions of AISC 341-16 by the correction factor equal to 1.7.

During the construction of steel structures, the executive groups often fabricate some stories and tighten the connection bolts, defined as the snug-tightened bolt in this research. The lower stories, in which the connection bolts are snug-tightened, will be pre-tensioned at least to the level of preloading based on design codes, called pre-tensioned bolts, in this paper. The connections will be complete, while some upper stories will have snug-tightened bolts. As a result, the stiffness of the bolted connections varies throughout construction, and the structural characteristics change with time. So it is necessary to investigate the seismic behavior of bolted extended end-plate moment connections in both snug-tightened and pre-tensioned bolts while constructing high-rise structures. Bolted unstiffened end plate moment connections are one of the most usable connections used as prequalified connections in special steel moment frames. According to the AISC design code, this connection can be considered one of the most important parts of moment-resisting frames with enough bolt pre-tension levels. In this paper, using three full-scale bolted unstiffened end plate moment connections designed according to AISC, the effects of bolt pre-tension levels have been examined experimentally under SAC cyclic loading protocol. Bolt pre-tension level has been defined as α coefficient to show the pre-tensioning level in three specimens. The bolts of the first specimen are not pre-tensioned, called snug-tightened bolts, and is reference connection. The bolt pre-tension levels of the second and third specimens were created in accordance with AISC and Iranian National design code and more to Fu of bolts, called pre-tensioned and fully pre-tensioned, respectively. The bolts' moment capacity, total energy absorption, initial rotational stiffness, ductility of connection, and stress and strain variation are investigated. According to the results, an increase in bolt pre-tension level would significantly improve the cyclic behavior of connections. Further, an increase in bolt pre-tension led to the initiation of the inelastic deformation from a minor rotation, and the ductility of the connection improved. The results show that the increase in moment capacity and energy dissipation in the pre-tensioned compared to snug-tightened is 27 and 23%, respectively. However, In comparison with the pre-tensioned, the fully pre-tensioned specimen has increased by 11 and 9%, respectively. As a result, the connection with bolt pre-tension level, under design regulations in comparison with the reference connection, can be considered a connection of a special moment resisting frames. So bolt pre-tension level higher than the value mentioned in the design code is better but not needed.