One of the drawbacks of the mentioned connections after Northridge and kobe earthquakes was the destruction of BEAM after earthquake, difficulty and non economical displacement of the BEAM. Several studies are being carried out on a variety of materials and systems that dissipate the seismic loading effects in order to improve the seismic performance of steel frames. By placing dampers in the BEAM column connections, damage to the BEAMs and columns, which are very difficult to replace after an earthquake, was prevented during an earthquake. During the design process of dampers, in order to concentrate the earthquake loads on the damper, it was manufactured intentionally with a lower strength than the BEAMs and columns. Through this, BEAMs and columns are protected from damage by utilizing the energy absorption capacity of the damper. Slit damper is a plate or a standard SECTION with a number of slits in the web. The damper is attached to the BEAM by bolting a plate on the damper under the flange of the BEAM. In this study first compared slit steel damper connection and connection with the REDUCED BEAM SECTION (RBS) under cyclic loading. Some suggestions were provided for improvement of the performance of slit steel damper connection and for making its behavior similar to that of REDUCED BEAM SECTION connection and transforming it to a connection which can be used in the special bending frame; and these suggestions include increase in the thickness and number of stripe and decrease in the height. the results and simulations showed that increasing in the number of strips, the increase in thickness and decrease in the height of the SSD damper affects significantly the performance of SSD connection.

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 crossSECTION, 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.

Concentrated major stresses in area of BEAM-to-column welded connections of steel moment frames have been proposed in the high density demands as a critical factor in connection vulnerabilities. Reducing the stress concentration in the connections can be accomplished with the intentionally weakening of the cross SECTION of the BEAM connected to the column. By this method, the plastic hing is formed in the weakened SECTION and the demand is included on the connecting components. In this study, the effect of the panel rigidity on the behavior of the BEAM connection with the REDUCED bending strength SECTION with the tube has been investigated. Eight connections of various thicknesses have been investigated. Connections are analyzed using finite element modeling under static loading. In this research, it is observed that with increasing the thickness of the web panel zone, the loss of the strength curve decreases and the connection experiences more drift. Also by increasing the thickness the von mises stress in the panel zone decreased. The values of stress and displacement were observed in the tube web in larger thicknesses are bigger. The failure of the column and the connection panel zone in the connections decreased with the increase of the connection panel zone thickness. The lowest energy dissipation is related to the weakest panel zone connection. By increasing the thickness of the panel zone connection, it would be expected that more energy would be lost from the connection. In general, according to the results obtained in this study, application of thickness less than the calculated value in seismic design is not recommended.

Studies on damaged connections in recent earthquakes have shown that damage to some of the structures cannot be justified by the use of common failure mechanisms. This is due to the extremely low cyclic fatigue created by the critical cycle on the connections, which leads to a failure in the examined connections. In recent years, a lot of research efforts have been made to create a method for predicting a failure due to extremely low cycle fatigue to interpret the failure mechanism in steel materials and structural components. It can be said that the failure is due to the extremely low cycle fatigue state governing the steel structures in severe earthquakes. On the other hand, after the Northridge earthquake, the use of moment connection with REDUCED BEAM SECTION, which showed good ductility in laboratory studies, expanded rapidly. But studies on this kind of connection have focused on capacity, and the extremely low cycle fatigue and failure mechanisms in this type of connection have not been considered. The present study focuses on the effect of extremely low cyclic fatigue on damaged connections with REDUCED BEAM SECTION BEAMs in moment steel frames. In this regard, the effects of extremely low cyclic fatigue were studied in a 6-story steel moment frame complex with REDUCED BEAM SECTIONs. Comparison of the indices of start cracking by extremely low cycle fatigue in different floor showed that this index is far from the beginning of the failure. In other words, the connections with REDUCED BEAM SECTIONs in this structure will not damage by extremely low cycle fatigue.

Structural design for huge seismic events must explicitly consider the influences of response after the elastic range. The special moment frame (SMF) steel structures are designed such that the frame BEAM–column joists are able to absorb substantial energy through large rotational deformation. In this way, a major contribution occurs in the displacement ductility capacity of the system. One of the connections which is designed based on the concept of weakening the BEAM is the REDUCED BEAM SECTION (RBS) connection. The BEAM is weakened near its end, by trimming some parts of the flanges near the column face. In this way, the formation of the plastic hinge forcefully occurs in this region, because the RBS area acts as a fuse. Recent experimental results on RBS steel moment connections revealed that these connections tend to perform poorly by the early brittle fracture of the BEAM flange at the weld access hole. The measured strain data imply that the higher probability of base metal fracture in bolted web joints is -partly- related to the increased demand on the BEAM flanges. This demand is created due to the slippage of web bolt and the actual load transfer mechanism which is different from the one expected in connection design. Improvement methods include: using a better welding material and controlled welding process, using haunches at the BEAM-column interface (primarily intended for retrofitting of damaged frames), using cover plates on the BEAM flanges at the BEAM-column interface, and using a REDUCED BEAM SECTION (RBS) at a prescribed distance from the column face. The RBS appears to be the most economic method and is already being used by structural engineers for welded SMRF structures in seismic zones. In this study, cyclic performance of RBS connection is studied in the numerical environment of ABAQUS. The investigated connection is a half-scale single-sided BEAM-to-column assembly. The cyclic load is applied at the tip of the BEAM. Pinned boundary condition is applied at the top and bottom of the column and is restricted out of plane displacement of the BEAM. The loading protocol proposed by AISC is used for cyclic loading. In parametric study of this connection, the effects of changing dimensions of the REDUCED area are investigated. Although the model gives reasonable predictions for the material deformation in the RBS, the designer must consider the material limit states, as this model does not predict local buckling or fracture in the RBS. Results reveal that the moment capacity of RBS connection is less than the moment capacity of a corresponding intact SECTION connection. However, no plastic hinge is formed in intact SECTION connection.

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.

In the Eccentric braced frames (EBFs), which one end of the link BEAM is connected to a column, the safety of the link-to-column connection is a requirement for the ductile and safe performance of the EBF. But among of the tests that have been implemented on this connection yet, because of the intensity of forces on vulnerable region of the link-to-column connection, the tested specimens were confronted with brittle and sudden failures. So it seems that the REDUCED BEAM SECTION (RBS) can be a suitable solution for raising this problem, by concentrating flexural stresses at a location away from the connection, in flexural yielding link BEAMs. Therefore in this paper, the possibility of keeping the plastic hinge away from the location of link-to-column connection, in the meanwhile achieving the required plastic rotation of link BEAM, by using the RBS connection, were investigated. This evaluation was done by using a finite element program ETABS and with nonlinear static analysis (pushover) for a dual system of special moment frame and special eccentric braced frame. According to the present work, the models with RBS by earlier developing the hinge at the RBS region, delay yielding occurrence of link at the column face. So the yielding does not occur at the column face, at least prior to achieving the moment at the location of RBS region to 1.1 times of its expected plastic moment capacity.

Brittle failure can prevent structural connections from reaching their peak performance. It is therefore considered as one of the most destructive forms of failure. The prevalence of different failure in rigid connections of steel frames in the aftermath of the Northridge and Kobe earthquakes brought the performance of these connections under question. Research into rigid connections with complete penetrating welding revealed that it is highly probable for the welds to undergo premature brittle failure at low drifts. To address this problem, the use of REDUCED BEAM SECTIONs (RBS) was recommended by the scientific community after the Northridge earthquake. In this connection, the BEAM’ s flanges are cut (REDUCED) so that it can take on the form of a fuse, making it possible for the plastic hinge to be driven toward the inside of the BEAM, thereby preventing the panel zone from failing. RBSs, which are categorized as “ prequalified connections” , have been the subject of extensive investigations and have suitable energy absorption and ductility under cyclic loadings. They are not, nonetheless, without flaws and are accompanied by problems such as the need for replacement after average or severe earthquakes due to severe inelastic deformations in the REDUCED area. This problem is compounded by the connection of secondary BEAMs to primary BEAMs in the ceiling of the structures in which they are used. The objective of this investigation is the numerical evaluation of RBS connections with replaceable fuses. Numerical simulations on three models – namely, a conventional REDUCED BEAM SECTION connection (RBS), a REDUCEDflange connection with a replaceable fuse (RBS-F), and a REDUCED-web connection with a replaceable fuse (RWS-F) – were carried out using ABAQUS, with material and geometric nonlinearities having been considered. Also, the materials of the columns, BEAMs, and plates, stiffeners, doubler and continuity plates, seat plates, and bolts have been defined based precisely on experimental data. Loading and support conditions of the numerical models were the same as those of the experimental samples. In the numerical models, the bolts were first pre-stressed to a sufficient degree. Then, lateral cyclic loading was applied to the BEAM of each model. The hysteretic curves of the numerical models are in good agreement with those of the experimental samples, indicating that the numerical models can reliably be used for the evaluation of other SECTIONs. Seven different profiles were selected from IPB SECTIONs (IPB140 to IPB340) for the BEAM. Suitable columns and endplates were designed for every BEAM size. For every set, three RBS, RBS-F, RDS-F, and RWS-FR models were constructed, bring the total analyzed models to 28. The dimensions of the RWS model were selected so that its plastic SECTION modulus would be the same as that of the RBS sample. Similar to the tests, the analyses continued until a draft of 8% and the hysteretic moment-rotation diagram of each sample was obtained. Since in tall buildings BEAMs and columns with variable dimensions are used in the experiment was carried out for BEAMs and columns with one size, performing extensive numerical analyses can offer a better comparison of the performance REDUCED-depth SECTIONs and REDUCED-flange SECTIONs. The results of more than 28 numerical analyses showed that in the RBS and RBS-F models, increasing the size of the BEAM reduces ductility. However, for the RWS-F sample, not only does increasing the size of the BEAM maintains the BEAM’ s ductility, it also keeps it, noticeably, above those of the other two samples. The ultimate strength of the sample, however, is less than the other two samples. By increasing the web’ s thickness and its plastic SECTION modulus, an ultimate strength on par with those of the other samples can be achieved. Therefore, the modified RWS-F sample can be a suitable replacement for RBS connections.

The connection with REDUCED BEAM SECTION was proposed after the 1994 Northridge earthquake. Until then, it was generally believed that connections with complete groove welding can withstand large plastic deformations. However, the cracks and brittle failures taken place in connections revealed that the actual ductility in these connections might be lower than what was predicted by design codes. By forming a plastic hinge outside the joint, this connection reduces the damage inflicted upon the panel zone. It has to be mentioned, however, that due to the concentration of damage in the REDUCED area, the entire BEAM has to be replaced after average earthquakes that is practically impossible. The aim of this study is to experimentally investigate the use of the REDUCED SECTION in a replaceable fuse. The column and the BEAM were chosen to be made of SECTIONs equivalent to IPE 240 and IPB 180 wide flange profiles and the cyclic quasi-static load was applied until a drift of about 9 percent. The hysteresis moment-drift diagram was drawn. The first sample was a REDUCED BEAM SECTION with end plate and stiffeners (RBS). Under loading, this sample satisfied the criteria for the ductility of special moment resisting frame. However, due to the fact that after an average or strong earthquake damage concentrates in the BEAM and replacing it after earthquake is either extremely difficult or not possible at all, it was tried to use a short replaceable fuse at the end of the BEAM in the second and third samples. The second sample incorporated a fuse with the length of 35. 5 cm and a BEAM with a REDUCED flange (RBS-F). Since the ratio of the width of the flange to the height of the BEAM is directly correlated to its resistance against lateral-torsional buckling, cutting the BEAM in RBS connections causes different types of buckling to occur faster. To overcome this problem, in the third sample, only the height of the BEAM was decreased and the dimensions of the flange were not altered. Therefore, the third sample included a 35. 5 cm long fuse and a BEAM with a REDUCED web (RWS-F). All of the samples satisfied the required drift for the rigid connection special moment resisting frames and using different types of RBS connections reduces the damage inflicted upon the column and the panel zone. The results showed that in addition to having very suitable ductility, the RBS-F and RWS-F samples can be very good post-earthquake replacements for conventional RBS connections.

Unexpected brittle failures in steel rigid frame connections by concentrated tensions on the welded area and connection vulnerability in high ductility demands as occurred during the Northridge (1994) and Kobe (1995), have challenged ductility properties and ability to absorb seismic energy in this type of structural system. So in designing this type of structural system, creating ductility and ability to withstand cyclic large deformations is important. On this basis, in the present study a new BEAM-to-column moment connection with REDUCED BEAM SECTION using Tubular Accordion Web called TAW-RBS is analytically studied. The results show that TAW-RBS creates a ductile fuse away from BEAM-to-column connection components, through removing web from BEAM flexural capacity. Reduction of BEAM flexural strength in the plastic hinge is calculated based on the dimensions of BEAM and tube. So bending strength of the remaining tubular web would be proportional to squared amount of the thickness of tube and inversely related to the thickness of flat web and the diameter of tube. The optimal location of the tubular web is determined due to the length of BEAM and the plastic modulus ratio at the plastic hinge to the total cross SECTION. Also shear control at tubular web according to local and global shear buckling are other points discussed in this study. The results show that TAW-RBS reduces flexural stiffness about 14 to 19% in the plastic hinge locations that is equal with the RBS20%. Also the proposed connection, depending on the size of the BEAM, reduces bending strength between 22 to 28% in the plastic hinge location that is intermediate between RBS20% and RBS50%.