Effect of Bolted Splice Design Method on Seismic Performance and Prequalification of Column-Tree Connections

Moment resisting steel frames (MRSFs) are widely used as a lateral load resisting system in steel structures in very high seismic regions. The seismic performance of this system depends mainly on the behavior of beam-to-column moment connections such that trivial damages in the connections may lead to the collapse of the whole structure or at least post-earthquake demolition of the structure. In the 1994 Northridge earthquake, beam-to-column connections in the MRSFs damaged noticeably and unexpectedly. Several researchers then proposed various suggestions to improve the seismic performance of beam to column connections. One of these suggestions was using a column-tree system to avoid the low-quality field-welded moment connections. These days, column-tree connections are widely used in the special moment resisting frames (SMRFs) buildings due to their well-known ease of installation and inspection of welding zone, especially in the high seismic areas. However, the prequalification and seismic behavior of these connections had not been evaluated well prior to this research; hence the structural designers usually consider these connections as the prequalified connection for using in the SMRFs without following a robust and validated approach. Therefore, in this study, the effect bolted splice design method on the prequalification and cyclic response of the column-tree connections were investigated using experimentally validated finite element analysis in ABAQUS FEA software. The column-tree connection must be designed such that the ductile failure modes occur prior to the brittle failure modes. This may be achieved through an appropriate design approach. Based on the bolted joint type in the AISC specification (i.e., pre-tensioned joint and slip-critical joint) and removing plastic hinge from the column edge (weakened bolted splice), there are three bolted splice design methods available. These are bolted splice design methods based on the slip critical joint, pre-tensioned joint, and weak splice plates. This research studied three samples to evaluate the effects of the bolted splice design methods on the prequalification and seismic behavior of the connection. The results show that the column-tree connection prequalification depends on the bolted splice design methods; moreover, the bolted splice design method influences the monotonic and cyclic behavior, strength, stiffness, fracture tendency, ductility, and energy dissipation characteristics of the connection. Also, it is observed that based on the moment strength and rotational stiffness of the bolted splice, the column-tree connection is classified as a rigid or semi-rigid moment connection. This is a significant point that needs to be taken into account in the column-tree moment frame design. The structural designers should consider these effects in their design approach for the column-tree connections. The column-tree connection with its bolted splice designed based on the pre-tensioned joint exhibits a reduction in the fracture tendency and increase in the ductility of the connection and also a smaller number of required bolts. For these reasons, it is recommended that the pre-tensioned joint method be implemented in designing a bolted splice instead of the slip-critical joint method.