The moment beam-to-column connections include those connections that can transfer the moment between the beam and the column. Bolted unstiffened and stiffened extended end-plate moment connections are fromthis category. This type of connection, which was investigated after severedamage to connections in the Northridge earthquake, Due to properbehavior under cyclic loading, was introduced as one of the pre-qualifiedconnections of the AISC code and subsequently by the Iranian Code. Sincelow cyclic fatigue is another cause of failure in connections under the Northridge earthquake, it is necessary to study the fatigue behavior of thisype of connection under the effect of low cycle fatigue. So far little studieshave been done on the effect of low cycle fatigue on various types ofconnections, including end plate connections. In the present study, the S-N fatigue chart (Effective stress to the number of load cycles) that was usedto estimate the effect of high cycle fatigue was developed to the lowfatigue region for the end plate connections. Then, with using the S-N chart, the effect of low cycle fatigue on a 12-story structure under the influence of various earthquake records was studied to investigate the effect of low-cycle fatigue on structures with end plate momentconnections. The analysis of steel structures showed that in end plate moment connections at the plastic hinge position, the effect of the cumulative cyclic fatigue before the plasticization of the connection undersevere earthquakes is influential. Since the effects of fatigue are cumulative, it will be significant and destructive for end plate connections of buildings that have experienced several medium and large earthquakes.

One of the connections that are less use by designers due to the difficult implementation is welded unreinforced flange welded web connection. Almost can be said that this type of connection can be use without design. Studies on this connections indicated presence of plastic strains in near the beam to column connection causes decreased levels of confidence this connections. In this article studied on I shape beam to column moment connections with welded unreinforced flange-welded web (WUF-W)، welded top and bottom flange plate (WFP) and reduced beam section (RBS). For this propose for each specimen design connection with ST37 material properties according to Iranian national Cone No-10 (Design Steel Structures specifications). With the Abaqus finite element analysis software studied and compares hysteresis curves and performance of each connection under cyclic loading. Finally recommended details for Increase safety level of WUF-W moment connection. This study has shown that energy dissipate of WFP 20 and 52 present respectively was more than WUF-W and RBS moment connection. Load capacity of WFP 19 and 54 present respectively was more than WUF-W and RBS moment connection. Reinforce WUF-W with haunch had best results in energy dissipate، rigidity and load capacity that increase respectively 10. 5، 18. 5 and 8 present parameters.

There are various factors in the analysis and design of steel frames that uncertainty of them can have a significant effect on the structural safety. Accordingly, study of these factors to understand the actual behavior of the structure are important. Meanwhile, the initial stiffness of beam-to-column connection, can be considered as one of these variable.Therefore, in this paper, the effects of uncertainty in the initial stiffness of BUEEP, WFP and BSEEP connections, from rigid connections pre-qualified of Iran steel codes, on drift of simple steel frame are evaluated. In the firstly, the initial stiffness of mentioned connections based on the mechanical method of the European Code (EC3) and numerical method based on the finite element method by using of the commercial software ABAQUS obtained. By evaluating the responses of the two methods with experimental findings, stiffness obtained from finite element method selected as appropriate stiffness. Then, the probabilistic analysis performed by using of the Monte Carlo simulation method. It should be added, for stiffness of any connections, uniform distribution, normal distribution and lognormal distribution with coefficient of variation from 5 up to 20 percent are considered and drift of frames for thousands simulation are calculated. Results that exceed the allowable drift of Iran code, indicating count of failure of structure. Accordingly, the structure analysis performed 72000 times and probability of the failure will be evaluated. For all connections, the probability of failure for different distributions and coefficients of variation graphically displayed. In the following, reliability of the frame obtained from graphs of cumulative distribution function (CDF). Moreover, for any of the connections and in all investigated cases, a reduction coefficient to achieve 100% reliability and coordination of lateral displacement of with allowable drift with Iran codes becomes available.

Welding of Continuity plates in box columns are not easily possible, therefore some researches has been done for substitution of external continuity plates. In this study first effects of continuity plate in I beam to Box column with top and bottom plate (WFP) and welded unreinforced flange-welded web connection (WUF-W) and reduced beam section connection (RBS) was discussed. Then, usage of triangular plates in connection of beam to box column as continuity plates under cyclic loading was studied. Studies have shown that existence of continuity plates in connections mentioned above have averagely increased loading capacity, rigidity and energy absorption 63, 86 and 75 percent respectively. The results also showed that using of triangular plates as continuity plates of box columns causing plastic strain in column flange in the area that concentration of materials is not much in triangular plates and increased the probability of failure in weld of plates to the column flange. Also using of triangular plates as continuity plates have not affected plastic hinge location.

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%.

The effect of fatigue on the behavior of steel structures has long been the focus of researchers. However, with the widespread damage of steel structures after the Northridge earthquake, it is important to investigate the issue of low cycle fatigue. The bylaws attempted to address the weaknesses of the connections and introduced prequalified connections. In the present study, the behaviors of two prequalified welded unreinforced flange-welded webs and reduced beam section under low cycle fatigue were investigated. The behavior of high cyclic fatigue was the focus of many researchers, while there is only a limited scope of experimental data available for LCF. In this study, the S-N-curve was used to obtain the Nastar method and available experimental data. This curve was developed using the available experimental results for the low cycle region. Four buildings with different heights were analyzed using linear time history analysis and their behavior under low cycle fatigue was considered. Then, the cumulative fatigue damage was investigated by the Palmgren-Miner fatigue analysis method for the above two connections. Rainflow method was also used for counting cycles. By defining the cumulative fatigue index, for the critical beam of structures, the maximum value of this index was observed at the WUF-W, which was obtained for 0. 059 3-storey structure and was increased by 0. 24 with the increasing structural height. The index for the critical column in the WUF-W junction in the 15-storey structure was 0. 042, while in the other structures, the maximum value was up to 0. 197. Also, the fatigue index values for WUF-W binding were higher than those obtained for RBS, indicating that the first connection performance was weaker. Maximum rate of cumulative fatigue index in beams on WUF-W to RBS was 1. 29 and for columns, this rate was 1. 34. The results indicated the need for greater attention to the effect of low cycle fatigue on connections of steel moment frames, especially on the WUF-W.

When a primary structural element fails due to sudden load such as explosion, the building undergoes progressive collapse. The method for design of moment connections during progressive collapse is different to seismic design of moment connections. Because in this case, the axial force on the connections makes it behave differently. The purpose of this paper is to evaluate the performance of a variety of moment connections in preventing progressive collapse in steel moment frames. To achieve this goal, three prequalified moment connections (BSEEP, BFP and WUP-W) were designed according to seismic codes. These moment connections were analysed numerically using ABAQUS software for progressive collapse. The results show that the BFP connection (bolted flange plate) has capacity much more than other connections because of the use of plates at the junction of beam-column.

Based on the past experience of engineers, welded connections usually do not perform well during earthquakes. Nevertheless, welding is used in all prequalified moment connections in chapter 10 of the National Building Code of Iran and as a result, there is a need for a fully-bolted prequalified moment connection for use in intermediate moment frames (IMF) and special moment frames (SMF). Thus, in this paper, a novel type of fully-bolted connection was investigated using T-shaped sections and web angles. First, the finite element model of the connection was developed in Abaqus software and validated with respect to experimental results. Next, using the concepts of Eurocode 3, section 1-8, a method for calculating the stiffness and strength of the connection was presented and was validated with the help of several experimental tests and finite element modeling. The results confirmed the proper performance of this method in predicting the stiffness of the connection. Since finite element modeling and experimental tests require a lot of cost and time, the proposed method can be a good option for engineers. Next, with the aim of parametric study on the connection, a large number of connections were designed based on capacity. Based on the component method, the connections stiffness was calculated using supervised machine learning techniques and multiple linear regression learning algorithm. Next, using the data obtained from the design and component method, a parametric study was performed on the connection and a simple formula was presented to calculate the initial stiffness of the connection. The results of this study show excellent regression performance in predicting connection stiffness. In addition, according to the obtained formula, the depth of the beam and diameter of the bolts connecting the T-shape flanges to the column flange have the most impact on the connection stiffness. Finally, the studied connection was classified according to its rigidity. The results showed that the studied connection acted as a fully-restrained connection and can be used in IMF and SMF systems.

Soil-structure interaction expresses the difference of structural responses between the actual and theoretical rigid based conditions, and depends on the stiffness, mass and damping of soil and structure systems. Nowadays, moment-resisting frame is one the most applicated structural systems. Ductility of these frames is due to the flexural yielding of beams, columns and the shear yielding of panel zone of columns. The influence of modeling beam-column connections and soil-foundation-structure interaction on the seismic responses of 10-story intermediate steel moment-resisting frames that located on the soil type IV with various shear wave velocities (150 and 80 m/s) and soil type II (with shear wave velocities 56 m/s) is studied in this paper. Prequalified welded flange plate connections (WFP) are used in these buildings. For this purpose, several 2D finite element models are developed using OpenSees software by assuming three conditions such as models with considering soil and beam-column connections effects, models with considering soil and without connections, models with fixed based and without considering connections. The maximum responses of the studied frames are calculated and compared with nonlinear time-history dynamic analyses. The numerical results show that in the models located on soil type IV, considered connections, SFSI, or just soil, the maximum lateral displacement and maximum inter-story drift are more, compare to models without connections and with fixed based conditions. In the models rested on soil type II, some of these parameters are reduced. The maximum base shear of structures is reduced in the mentioned models.