وحید صابری

In this study, comparison of the performance of beam-to-column bolted end plate connections and T-connection under fire is considered. To achieve this objective, samples are selected in three types of bolt number, the thickness of end-plate, and T-connection. Then, they are investigated and nonlinearly analyzed in ABAQUS finite element software and the results such as displacement-time curves, counters of stress and displacement are studied. According to the nonlinear finite element analyses results obtained in the first and second types, the beam-to-column connections have been investigated by changing the number of bolts and increasing the thickness of the end plate. The results indicated that the highest and lowest values of displacement-time curves are related to the samples have a 25 mm thickness end-plate with 4 bolts and a 10 mm thickness end-plate with 4 bolts, which affect the thickness of the end-plate with 36% and 17% fire resistance to rupture, respectively. It can also be seen that among the samples, connections with weak end-plates and fewer bolts in the final moment of loading at the end of fire analysis due to the high stress of plastic hinges can be seen. In the third type, the T-connection is examined with increasing thickness. The results showed that the maximum and minimum values of displacement-time curves are related to samples with a thickness of 17.5 mm and 9 mm with 25% and 11% fire resistance up to rupture, respectively. By comparison of the results of beam-to-column connection by changing the number of bolts and increasing the thickness of the end-plate and T-connection, the best sample has 25 mm thickness end plate with 8 bolts.

Nowadays, the use of innovative methods such as cross-sectional reduction method in designing moment connections of steel structures has become widespread among designers. The method of reducing the cross section by deliberately weakening the beam in a part close to the connection of the beam and column reduces the plastic strains of the joint and the formation of a plastic hinge in the beam. In this paper, the effect of reduced beam section (RBS) on joint strength by removing the plastic hinge from the column and also the optimal performance of various reduced beam section shapes through different indicators such as hysteresis curve, pushover diagram, energy dissipation of connection and … are checked. In this research, modelling and nonlinear analysis of 10 examples of reduced beam section with and without beam section reduction using ABAQUS software has been investigated under cyclic loading. The results showed that all models with reduced beam section have the ability to transfer the place of formation of plastic strains or the same plastic hinge into the beam and away from the column. Among all the methods of reducing the cross-section, the method of reducing flange beam section with uneven holes and enlarging inward to the beam has shown a better performance than other samples. The energy dissipation of this method is increased 68% versus the reference sample without reduced section. The flexural capacities of sample with reducing flange beam section with uneven holes and enlarging inward to the beam has shown better cyclic performance. The values of flexural capacity in the mentioned sample is decreased 33% versus the reference sample without reduced section. This subject causes plasticization faster and as a result, more energy dissipation is obtained versus other samples.

Passive control method, which is one of the solutions for strengthening and designing structures under earthquake loadings, has different sub-branches. One of these methods is to improve the existing weak buildings by using different types of damper. The seismic behaviour of the strengthened structure with different type of dampers is one of the main issues in choosing the strengthening method. Due to the energy absorption property, dampers have many applications in both construction and retrofitting. During an earthquake, these devices are activated and convert the earthquake input energy into heat energy, in other words, it absorbs the incoming energy. In previous studies, the use of this damper in various structures has been evaluated, but there is no specific category to investigate its effect on various types of steel structures in terms of height and type of damper. In this paper, the performance of this type of dampers in steel structures with different heights is discussed. The effect of using this type of dampers on the response of the structure such as the story drifts and the base shear is investigated and its performance is compared in steel structures with different heights. In the study of displacement, base shear and energy absorption parameters for three structures of 3, 6 and 12 floors, it was observed that viscous dampers had the best response to friction dampers and ADAS.

Seismic collapse of buildings is the level of structural performance in which the amount of damage reaches its maximum, so this event can be the worst happening in the construction industry. Due to the seismicity of Iran, more accurate assessment of the collapse of structures under earthquakes is one of the important challenges of structural engineering. In this paper, the effect of the number of bays on the seismic response of three sets of short steel frames is investigated. In all three sets, moment frame, an eccentric braced frame and the concentric braced frame are considered in the regular state. Then, the number of bays in all three sets of frames are 2, 3 and finally 4, respectively. Nonlinear static analysis is conducted based on base shear and roof displacement to determine the possible failure mechanisms of these frames and incremental nonlinear dynamic analysis is performed with intensity parameter (IM) corresponding to maximum relative displacement between stories and damage measure parameter (DM) corresponding to the spectral acceleration of the first mode Sa (T1, 5%) were considered. In addition, the collapse-preventing performance level of CP was analyzed. The results of the fragility curves in the limit state (CP) indicate that with increasing the number of bays, the probability of failure of ten percent for all studied frames has increased and consequently their vulnerability has decreased.

In this study, RBS connections behavior reinforced by horizontal and vertical stiffeners are assessed under cyclic loading and the effect of stiffeners is studied in order to improve the cyclic behavior of connections. The finite element ABAQUS software has been used to verify and simulate reduced beam section (RBS) connection behavior. Parametric study have been conducted by changing the stiffeners number, material and arrangement in connection area. The finite element analyses results showed that when using a combination of two horizontal and two vertical stiffeners, the initial and ultimate strength of the connection have been increased by 6.2% and 26.1%, respectively, compared to that of the connections without stiffeners. When using horizontal and vertical stiffeners in a rectangular arrangement with a length equals to 75% of the section reduction area length and its height equals to 75% beam height, the initial and ultimate strength of the connection has been increased by 6% and 20%, respectively in comparison the state without stiffeners. By comparing the use of ST37 and ST52 in stiffeners, it has been observed that both used steels have the same effect on the connection strength. Meanwhile, the effect of steel material of stiffeners could be ignored on improving the connection strength.

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.

In recent years, Coupled Steel Plate Shear Wall system has been used in mid and high-rise buildings because it has advantages such as providing large spaces for creating various uses. So this was brought into the attention of the designers. The lateral structural response is exactly dependent on the shear walls behavior; therefor these elements must response well under different loading situation. So this paper aims to study the effects of near and far fault earthquakes on the two series of mid-rise frame with the steel plate shear wall with the coupling system. In both series, the coupled steel plate shear wall will be evaluated by variation of opening dimensions, one in length of 1 and 2 m and the width of 0.6 m and 0.9 m with regularity in the plan. Firstly, structures are modeled in ETABS software three dimensionally. Then, the axis frame is analysed in OpenSees two dimensionally. In this paper, the nonlinear static pushover and dynamic time history analyses are carried out. The results of nonlinear analyses show that by increasing the dimensions of the openings, the drift and absolute displacement of the stories are enhanced and also by decreasing the dimensions of the openings, the amount of stories shear and base shear are decreased, which indicates the great effect of the dimensions of the openings on the seismic behavior of coupled steel plate shear wall with considering the performance levels of the frame.

Increasing demands for the use of lightweight concrete as well as blast-induced hazards in all over the world have led to performing extensive studies to achieve a better understanding of the behavior of the impact and heat-resistant lightweight concrete. In fact, the plastic concrete is of high ductility and low permeability whose compressive strength is much lower than that of the ordinary concrete. This type of concrete is obtained by mixing cement, sand, water and bentonite. Bentonite in this mixture is a type of clay that promotes its formation and stability. In this paper, the effect of steel, polypropylene and glass fibers on the post-heat strength (both tensile and compressive) of the plastic lightweight concrete has been investigated. To do so, after the process of heating on the specimens (exposure temperatures of 25, 100, 250, 500 and 700 °C), the compressive and tensile strength tests were conducted on the specimens. Accordingly, the results indicate that the addition of the steel fibers greatly affects the concrete strengths such that in some cases, the post-heat strengths of the concrete was enhanced by more than 40%. However, it was found that in contrast to the steel fibers, polypropylene and glass fibers leave insignificant effects on the post-heat performance, which is attributed to their physical and visual characteristics.

The present study investigates the influence of the number, material and configuration of bolts on rigid bolted connections under fire. In this study, 6 steel rigid bolted connection specimens with end-plate were modeled as A490 and A300 in ABAQUS software and they are assessed under fire by nonlinear dynamic analysis. By examining the results, the highest stress in the bolt is occurred with material A300. This is due to the lower yield stress of the A300 bolts than to the A490 because the A300 bolts yield at lower stresses and, as they enter the plastic deformations, cause the steel plates to become more involved and the values Increase the stress, especially around the bolt holes. Sample stresses CON.M22 8B A490 and CON.M18 12B A490 T2 are different about 4%. This displacement value for specimens with A300 bolt materials increses to 240 MPa which is equivalent to the yield stress of material A300. The strain with material A490 for CON.M22 8B A490 and CON.M18 12B A490 T2 samples is about 59% and for CON.M18 12B A490 and CON.M22 8B A490 T2 samples is about 67%. The strain with materials A490 for CON.M22 8B A490 and CON.M18 12B A490 T2 samples is about 38% and for CON.M22 8B A490 and CON.M18 12B A490 samples is about 20%. Specimen displacement for bolts with materials A490 whereas this value for specimens with A300 bolts for CON.M22 8B A300 and CON.M18 12B A300 T2 specimens had the highest and lowest vertical displacements, respectively, which is about 25% difference. In the CON.M22 8B, CON.M18 12B and CON.M18 12B T2 samples with different bolt materials, the percentages are 15, 24 and 44%, respectively. The use of materials with lower yield and rupture stresses caused yielding in the lower temperature and reducing the resilance of structure under fire.

Since unreinforced structures have the highest potential for earthquake damage and have multiple failure mechanisms, due to the lack of many uncertainties in the parameters of this type of structures, their vulnerability should be studied with proper knowledge of this and considering the parameters and the most probable modes for its failure. Also, most of the structures that are of historical importance are made of building materials and bricks. These types of structures usually have major weaknesses against earthquakes. Recognition of these weaknesses is a prelude to choosing the appropriate method to strengthen them. In recent decades, researchers have used various methods to enhance the seismic behavior of brick walls. Common methods for this work are coating and reinforcement with plaster and sprayed concrete, which is the most common method of using sprayed concrete. In this study, the micro method is used for finite element modeling in ABAQUS software. In modeling, three methods of reinforcement are used, which include reinforcement with steel sheet, FRP sheet and mortar. The results of this study showed that the sequential arrangement of FRP strips, compared to other methods, had a greater effect on improving the flexibility and the diagonal arrangement of the mentioned strips had a tangible effect on increasing the final strength of masonry walls.

Application of dual SMRF-BRBF system can be considered as an appropriate choice for resisting seismic loads in high seismicity regions. Taking advantage of having buckling resistant braces which can resist high axial compressive forces without local and global buckling, buckling restrained bracing frame (BRBF) system can be used as an efficient alternative for conventional bracing system. Response modification factor (R factor) is required for seismic design of structures. Therefore, in this research, performance-based R factor is evaluated for dual SMRF-BRBF structures. Considering the fact that in addition to structural properties the seismic load characteristics effect on the load-carrying capacity and R factor, R factor is evaluated specifically for near fault ground motions. To consider the effect of performance level, the R factor is calculated distinctly for Life Safety (LS) and Collapse Prevention (CP) limit states. Nonlinear time history and pushover analysis results are employed for determination of ductility, over-strength and R factor of the sample structures. According to the obtained results, the average of R factor coincident to LS and CP performance levels is 6.1 and 7.9, respectively. The comparison of results obtained for near fault records with those obtained for far fault motions reveals that the R factor is averagely 25% lower for near fault ground motions.

One of the most important issues in buildings is irregularity in the plan or height of the structure, which causes increasing the seismic damages. In irregularity in height, sudden changes in the dynamic properties of the structure including mass, lateral stiffness and strength of the structure occur. Therefore, the stage of making decision for determining the building configuration has a fundamental importance Irregular buildings must be able to withstand the lateral forces of earthquakes and winds. One of the most common earthquake-resistant systems in steel structures is the bracing system, which has been considered by engineers due to its good performance in previous earthquakes and reduced drift of buildings. However, some architectural constraints may cause the braces to be moved to the story level, which is subjected to irregularity of cutting the lateral bearing system, or the lateral braces may be weakened or removed due to poor supervision in some stories, in which case the story may be irregular. There is very soft story in the building. In this study, 7 scenarios in three states such as regular, very soft irregularity, and irregularity of cutting the lateral bearing system is considered for 5-story frame. These frames are designed based on seismic guidelines of standard 2800, and then, the performance of them has been evaluated by SAP2000 software using analyses methods such as equivalent static, spectral dynamics and time history dynamics under near-fault earthquakes. The aim of this study is to investigate the accuracy of these methods. The results showed that time history dynamic analysis is more accurate than other methods. For example, the error rate of the maximum displacement response of frame with irregularity of cutting the lateral bearing system in equivalent static analysis compared to spectral and time history dynamic analyses are 15% and 77%, respectively.

The most common disadvantage in moment connections is the brittle fracture of the welded area during an earthquake. One creative way to fix such disadvantages is to use Side Plates to connect the beam to the column. Previous studies have focused more on the performance of connections with side plates and comparison of these connections with other types of moment connections. In this study, the effect of material type, thickness, and perforation of the side plates on the cyclic performance is investigated. For this purpose, in addition to using side plates of soft steel (ST37) and high strength structural steel (ST52), nickel-titanium-shaped memory alloy (SMA-Ni-Ti) was also used to investigate the superelastic effect of this alloy on the connection performance. Modeling and analysis were performed in ABAQUS finite element software under cyclic loading. The results showed that the increased capacity and ductility of the side plate connections with shape memory alloy. Also, the findings revealed that optimal thicknesses can be obtained for a side plate to create the maximum possible ductility at the connection and preventing the formation of plastic hinges. According to the results obtained by changing the configuration and cutting in connection and in general, the capacity of the connection decreased by 0.04 radian (moment frame acceptance limit) and stress concentration in the cutting corners had the greatest effect on the failure of the side plates.

Due to several reasons as the low resistance of constructed concrete and also change in codes or application of structures, some concrete frames need to be retrofitted. By adding the steel prop and curb to the reinforced concrete, many parameters such as resistance, ductility, stiffness and the resistance reduction coefficient change. This study numerically investigates the impact of adding the prop and curb, slit damper, gusset plate and also prop with a ductile ring on resistance, stiffness, ductility and energy dissipation of RC frames. For this purpose, the effect of the aforementioned methods on the linear and nonlinear moment frame behavior of reinforced concrete under monotonic loads have been numerically investigated using the ABAQUS software. Twelve samples of RC frames with one span and one story with the same characteristics were constructed and retrofitted by different methods. All frames were subjected to the monotonic loading, and pushover-displacement graphs of them were plotted. The novelty of the work was using such props and slit damper in RC frames .The results obtained from the modeling showed that although the frame retrofitted by the steel prop and curb showed a better performance in terms of resistance and stiffness, but the retrofitted frame with a ring, slit damper and gusset plate also showed a better behavior in terms of resistance and stiffness compared to the RC frame and the sample with slit damper and prop with a ductile ring as well as compared to the sample with the prop and curb showed more ductility and energy dissipation Also the optimal pattern of reinforcement is in terms of ductility, resistance and stiffness of the slit dampers.

There are several methods for structural analysis and design, one of which is equivalent to static analysis. There are limitations. 4 models are examined in this research, the general shape of the plan of a five-story frame, in the first sample is a regular frame, the second sample is a mass irregularity in the first floor, the third sample is a mass irregularity in the third floor and the fourth sample is a mass irregularity in the fifth floor and Its seismic loading has been studied using the standard design spectrum of 2800 fourth edition. Mass irregularities in the first floor with a difference of 45% were higher than other classes and mass irregularities in the third floor were 15% less than the regular sample. Dynamic time history analysis was performed using different near and far fault earthquakes. Responses of lateral and relative displacement were calculated for different stories and compared with the allowable values of the regulations. As a deductive result of this study, we can point to the conservatism of the equivalent static method in estimating the incision and the inability of this method to consider the shape of the load. The results show a decrease in the seismic capacity of structures with mass irregularities in height and therefore it is recommended to avoid creating mass irregularities in height as much as possible.

This study focuses on the effect of viscos dampers arrangement over the height on the control of seismic behavior of tall steel moment-resisting frame structures by considering nonlinear dynamic analyses. To this end, 2D steel moment-resisting frame with 15-story is selected as sample structure with special ductility. The seismic demand parameters (i.e. inter-story drift ratio and base shear) of the sample frame is evaluated under two sets of near and far fault ground motions, each of which including 3 records. In next step, the sample structure is retrofitted by viscous dampers in different arrangements over the height and the effect of each arrangement on the seismic response of the sample structure is studied.  The nonlinear dynamic time history analysis is utilized to derive the seismic responses of the case study structure. The results of the study demonstrate that the application of dampers leads to reduction of inter-story drift response and increase in base shear demand. Also, it was found that the arrangement of the dampers in top one-third height of the structure or uniform distribution of the dampers over the height gives more efficient result than other arrangement patterns, in terms of inter-story and base-shear demand control.   Finally, it is observed that the insertion of dampers in all of the stories, although reduces the drift demand more than other patterns, but increases the base shear demand by up to 40%.

This study examines the type, thickness, and the way of gridding and placement of FRP sheets with concrete slabs. Shear and bending failures are two major failures in concrete members. One of the ways to reinforce structural members is to use polymer plates that are used due to their low thickness, good effect and balanced price. In this study, eight samples of reinforced concrete reinforced slab reinforced single, double and different location of modeling and results were investigated, of which 2 samples were single FRP, 3 samples were double FRP and 3 were grid mode of Reinforced concrete slab. Reinforced concrete slabs with dimensions of 3400 mm in length, 2500 mm in width and 160 mm in thickness. Longitudinal and transverse rebars with a score of 12 and at a distance of 20 mm from the surface of the slab. After reviewing, it was found that in general, slabs reinforced with CFRP fibers performed better in terms of loading and breaking, and by doubling these plates, a better effect is observed. In the discussion of FRP placement, whatever the regular grid mode and the placement of the force, has a great impact.

In this research, it will be paid to evaluate the effects of the link beam length on seismic behavior of eccentric braced frames under near and far-fault earthquakes. For this purpose, four frame structures with different number of stories such as 3, 6, 9 and 12 have been used. For each of these frames, three different lengths of link beam are considered. After designing structures and preparation of non-linear models, these models are analyzed under a suite of 10 and 3 near and far-fault records, respectively. Nonlinear dynamic time history analyses are performed using scaled records for these structures, and the response values of the stories displacement, base shear and the rotation of link beam are evaluated for them. Based on the results of this study, it is specified that increasing the length of link beam and change the mode of the link beam from the shear to bending, the values of stories drift are considerably increased, but the values of base shear are decreased relatively. This result is similar for near and far-fault records. Therefore, it seems that the use of a short link beam would offer a better behavior to the braced frame if the architectural conditions allow. The results showed that in 0.8 m as an optimal length, the displacement response of frames under near-fault records up to 15 percent larger than the far-fault records. This difference is lower for base shear response.

In this study, the effects of adding copper and plastic fibres and stone powder on flow parameters and hardening properties of self-compacting concrete have been investigated. The basic disadvantages of concrete include high specific gravity, low tensile strength compared to its compressive strength, low durability, and increasing tensile strength. By The use of fibres is expected to improve tensile strength, as well as reducing stiffness and, on the other hand, increasing the concrete plasticity. Today, self-compacting concrete solves one of the major problems in the implementation of concrete works in urban environments, which includes the noise pollution caused by the use of a vibrator to replacement concrete. Self-compacting concrete is a high-performance modern concrete that distinguishes its characteristics such as the lack need to internal or external density and the cross of dense reinforcement networks from conventional concrete. Another feature of self-compacting concrete is its high viscosity and stability as a result of the addition of fillers and the use of cement materials. But increasing the amount of cement materials and fillers in self-compacting concrete increases the brittleness of the concrete matrix, resulting in a decrease in deformability. Considering the successful experience of using fiber in concrete over the past years, in order to increase the deformability of ordinary, lightweight and high strength concrete, the use of fibres is an appropriate proposal to enhance of deformability of the self-compacting of concrete. Fiber concrete also has high energy absorption capacity and is not easily disassembled under impact loads.

In this research, the co-operation of steel and concrete in composite columns is considered. Using numerical modeling to study the behavior of these sections, a new type of composite sections named concrete-filled double-skin steel column is introduced. The factors affecting numerical simulation, which bring this simulation closer to the laboratory conditions, determine the axial resistance of the samples by changing the dimensions and geometry as well as the properties of the materials. The purpose of this thesis is to investigate the effect of concrete compressive strength on the axial performance of concrete-filled double-skin steel column CFDST concrete. A total of 20 samples with different parameters such as effects of concrete, loading capacity and width to thickness ratio on the strength of columns with square and round cross section in inner and outer walls were investigated by ABAQUS software. The results of the study showed that the loading capacity of CFDST columns under axial pressure increased by about 4% with increasing compressive strength of concrete in the inner wall. Also, studies showed that with increasing cross-section in the inner wall, the loading capacity in square columns increased by about 7% and decreased by about 3% in circular columns.

Steel shear walls are one of the most commonly used systems for resistance and stability of structures to lateral loads, which are used to rehabilitate and improve the seismicity of structures in seismic zones.Among the advantages of this lateral load resistance system, similar to high-performance systems, high ductility, high absorption capacity and energy depreciation, high initial hardness, light weight relative to similar concrete, saving materials, reduced time and the cost of conruction pointed out.Recent research has shown that the buckling of these plates before the submission of environmental elements would produce a better performance of this kind of lateral load system.This leads to the expansion of the use of thin-walled plates that lead to emerging executive problems and affect the economic benefits of the plan.In order to overcome the dilemmas and accelerate the buckling of these sheets, the idea of using a perforated steel plate was introduced.In this paper, the behavior of steel shear walls with perforated plates is investigated.To perform a parametric study of the effects of holes with different geometric shapes on the shear wall, the method of laying and placing holes in the shear wall on structural performance, in terms of energy absorption, the total load bearing capacity of the system was investigated.For modeling the samples, Abaqus finite element software was used.Parametric results on 60 finite element models under cyclic loading of displacement type showed that if using the same hole percentage, the maximum bearing capacity was attributed to the shear wall model with holes with circular geometry It shows the best seismic behavior.

The damage distribution of the steel moment-resisting frame (SMRF) structures is greatly a function of the pulse period of earthquake. In addition of frequency content of earthquake, the deformation demand distribution pattern depends on the seismic intensity more. In this study, the effect of pulse period of near fault earthquakes on the damage index as inter-story drift demand distribution of SMRFs with special ductility is investigated for different performance levels, including Immediate Occupancy (IO), Life Safety (LS), and Collapse Prevention (CP) limit states. For this purpose, the seismic response parameters of three SMRFs with special ductility and 3, 9 and 15-story under the effect of 30 near fault earthquake records with different frequency characteristics are evaluated through the implementation of Incremental Dynamic Analysis (IDA). To study the effect of pulse period, the pulse-like near fault records are categorized into three groups of short, medium, and long-period records. Moreover, two numerical variables are defined to quantitatively describe the seismic demand distribution patterns over the height of the sample frames. Based on the results of this study, it is found that the concentration of the damage distribution in the SMRFs for highly nonlinear state of the seismic response, transfers to the lower stories. However, for elastic and near-elastic zones, the short-period pulse-like excitations transmit the seismic demands toward the upper and mid stories.

Two-layer composite columns are a family of single-layer composite columns consisting of two circular steel tubes, which are assembled in a center and are filled with concrete. In this type of column, the important effect of concrete is that it delays local buckling of the steel skin, and that the concrete in its confined state can tolerate higher strains and tensions than the non-confined state. The advantages of two-layer composite columns, in comparison to single-layer columns, can be attributed to the lower weight of two-layer composite columns, more ductility and greater axial force strengthening. The behavior of these columns under pure axial load is of interest. Therefore, the study of the behavior of two-layer composite columns under the influence of axial load is of particular importance due to the behavior close to the reality in these columns. In this paper, the capacity of double-skin steel columns filled with concrete has been investigated using ABAQUS finite element, under the influence of axial stress. in this paper, sections were compared with changing the thickness of the layers and the diameter of the inner core and the existence and absence of concrete core. the results showed that the diameter of the tube increases with increasing the core diameter and the effective parameter in this issue is the ratio of thickness to diameter (D/t). In the discussion of the existence and absence of concrete in the core, the confinement effect has an important result and has increased the load strength capacity of the columns.

Nowadays, using FRP materials as reinforcing rebars in concrete structures, bridge decks and roads is common to prevent rusting of steel rebars. The purpose of this study was to strengthen the flexural strength of the concrete beams with FRP rebars and also to improve the weak beams that were reinforced with these rebars and reinforced by attaching the CFRP Laminates in the lower part of the beam along its length. For this purpose, two main parameters of ductility and flexural strength of sections reinforced with FRP rebars, designed in 12 models and subjected to nonlinear static analysis (pushover) in ABAQUS software and then the GFRP sample is also subjected to cyclic loading. The results show that to reinforce the beams if these rebars are used in the absence of steel bars, it is better to use the AFRP rebar because it provides flexural strength of beam sections about 8 to 18% more than the sections reinforced with CFRP and GFRP rebars and if the ductility is considered, It is advisable to use GFRP rebar which has about 7-15% more ductility than AFRP and CFRP sections, and if the weak section is strengthened with CFRP Laminates under cyclic loading, the flexural capacity can be more than 2.28 times to the nonlinear static analysis (pushover).

Today, the used method by most researchers to design new structures and retrofitting the existing structures is to reduce the movements and forces resulting from the earthquake in the structure. Accordingly, control of the structural behavior through the application of controlling forces on the structure by means of appropriate tools and separating the structure from the ground motion has been formed. The passive energy dissipation devices are grouped by seismic protection, which are quickly categorized into two groups of dependent velocity and non-dependent velocity devices. In structural control systems, damping has a crucial role in controlling seismic responses. On the other hand, nonlinear behavior of structures and the amount of hysteresis energy that is known as the structural hysteresis damping has a very important effect on the amount of structural damage and controls the collapse prevention state of the structure. By varying the damping values of the structure, the nonlinear behavior of the resistant elements of structure changes and the amount of hysteresis energy is affected. In this study, to investigate this topic, the structures with a number of different stories have been modelled in SeismoStruct software and dampers have been applied to the structure. By observing the results, the damping causes decreasing the seismic response of the structures, but the relationship between the increasing the damping and lowering the responses is not linear. But in general, the amount of damping value reduces displacement, drift, and base shear and increasing the performance level of the structures. The highest internal force reduction for the damping ratio 10, 20 and 30% has been 35, 39 and 43%, respectively. Also, the maximum reduction of the base shear for the damping ratio 10, 20 and 30 is 55, 59 and 64%, respectively.