Experimental Assessment of Reduced Beam Section Connection in Moment Frames using I-Shaped Replaceable Links

In 1994, Northridge, California, an earthquake created brittle fractures and cracks near the beam-to-column connection welds in more than 170 steel MRF buildings. In many cases, these fractures and cracks were observed in the beam without any signs of plastic deformation. Since connections are one of the most important components of a building frame, a good understanding of the structural behavior of connections and sufficient knowledge of how it is transmitted is a prerequisite for designing a safe connection. Steel moment frame structures to resist the earthquakes are designed based on the assumption that they can resist yield or plastic deformation without reducing strength. The plastic deformation involves the plastic rotations of the beams at their connection to the columns and theoretically resulting in the loss of seismic energy applied to the structure. All the modifications proposed in recent years for the moment connections are aimed at shifting the maximum plastic capacity or ductility demand from the end of the beam or connection zone and transferring the ductility demand to an area within the beam and away from the column. On the other hand, the details of seismic moment connections provided in FEMA351 can be classified into two groups: a- Connection zone strengthening methods; b- Reduced beam methods. There are different types of the connections presented with the idea of ​​strengthening the connection zone; for example, we can refer to the connection with welded cover plates to the flange, the connection with the side plate, the connection with the screwed seat, the connection with the welding muscle, etc. However, the connections provided and used with the idea of ​​reducing the beam are more limited, such as RBS or reduced section beam connection, free-flange beam connection, sheared beam-to-web connection, and reduced beam-to-web connection. RBS connections form a plastic hinge outside the connections site and reduce force and moment at the connection, and is one of the pre-confirmed rigid connections after the 1994 Northridge earthquake. One method of building RBS connections is to use a connection of beam with a reduced section by replaceable links. The use of these replaceable links is one of the methods to increase the ductility and transfer the plastic hinge into the reduced area of ​​the beam. In these connections, the goal is to displace the plastic hinge into the reduced area of ​​the beam at a certain distance from the column, so in these connections, moving the plastic hinge away from the column reduces the concentration of strain in the welded zone. As a result, it reduces the weld cracking rate and thus reduces brittle failure in the connections.
Connection of the reduced beam section was proposed after damage to structures in the 1994 Northridge earthquake. This connection reduces the damage caused to the panel zone by forming a plastic hinge outside the beam-to-column connection area. In this research, we investigate in a laboratory a new type of beam section connection, which in recent years has been named as replaceable links. Among the links designed in the first laboratory sample, the semicircular cutting method on the flange of the replaceable beam was used to reduce the finding, and in the second sample, circular holes in the flange of the sample were used to reduce the beam section. Studies have shown that high stress concentration occurs in the reduced area and other parts of the beam and column elements will remain without significant stress, which makes the beam replaceable after an earthquake. Analytical results show that the link with reduction by creating a hole in the flange has more energy absorption and ductility than the link with reduction by cutting a semicircle in the flange of the link beam. Also, according to the maximum 
amount of time recorded in the samples and referring to the existing regulations in the design of structures, it can be stated that both samples are among the allowable connections in a special moment frame.