Analytical Assessment of Rigid Connection with External Diaphragm Plate and Side Plate in Hollow Box Columns through Seismic Loading

The huge number of failures and collapsed structures in the 1994 Northridge earthquake is always referred to as a turning point in the design of moment-resisting connections because it influenced the design of this type of beam-to-column connections. The occurrence of brittle failures in the welding of the joints, which led to the rupture of the connection plate, beam or column flanges, was one of the major damages that occurred in this type of connection in the mentioned enormous ground motion. After the Northridge earthquake, the attitude of researchers about designing moment-resisting connections completely changed. Many researchers tried to propose novel methods and ideas to improve the performance of rigid beam-to-column connections. Some of these methods are focused on different areas of the connections such as preventing the creation of three-dimensional stresses at the location of beam-to-column connection, reducing the shear deformation of the panel zone of connections, and removing the double or continuing plates of them. Furthermore, the proper seismic performance of each method was assessed based on numerous experimental and analytical studies. One of the most applicable rigid connections that we can refer to in this case is the connection with Welded Flange Plates (WFP). This connection is designed in such a way that the plastic hinge occurs at the end of the connecting plates. This type of connection is often used with hollow box columns. In this connection, due to the creation of concentrated forces in the column, which are created in the alignment of the connection plates, the flange or the web of the column may be damaged. For this reason, connection plates are used inside the column (continuing plates), which are generally installed by going through multiple steps and making additional cuts in the column. On the other hand, the occurrence of significant shear forces in the panel zone of connection may cause damage to it and require the implementation of double plates. The implementation of these types of steel plates can also cause difficulties in implementing the whole connection. For this reason, finding a solution that makes the implementation of rigid connections with welded flange plates unnecessary from continuing plates or double plates can be an important solution to facilitate the implementation of such connections. Rigid connections such as connection with the side plate and connection with the diaphragm plate (external stiffener) are options that have provided modifications to remove the connection continuing and double plates. The side plates prevent three-dimensional stresses and assist in strengthening the panel zone of connection. On the other hand, by removing the connection plates and preventing the discontinuity of the column, the diaphragm plate makes the panel zone stiffer and improves its seismic performance. In this article, a novel idea for rigid connection is proposed, which is a combination of connection with side plate, connection with diaphragm plate and rigid connection with welded flange plates, prequalified in steel structures standard of Iran. The new method has been tried to use the advantages of three rigid connection methods (WFP, connection with side and diaphragm plate) in hollow square (box) columns. In other words, the main purpose of this study is to integrate the advantages of the three mentioned methods and help to improve the seismic performance of the rigid connection. To assess the seismic performance of this connection, its 3D model has been created in finite element software and then analyzed with a nonlinear method. To perform the analysis, the models are analyzed under dynamic loading and their results, including the distribution of plastic strains, the location of failures, and the moment-rotation and shear force-deformation diagrams of the connection panel zone are calculated and evaluated. To show the damage in the components of this steel connection, the Birth and Death method is used in the analysis.
The results show that the proposed connection has significant advantages. In addition to removing the internal connection plates, this connection has significantly strengthened the panel zone of the connection, and by removing the direct connection of the beam to the column, it has reduced the deformation of the column flange and, as a result, eliminated the three-dimensional stresses at this location.