Performance enhancement of eight bolt extended end-plate moment connections under simulated seismic loading

Extended end-plate (EEP) moment resisting connections provide the advantage of eliminating field welding and by virtue of this, facilitate fast field erection of building frames. The eight bolt stiffened (8ES) EEP connection is one of the prequalified moment connections in the AISC 358 standard for special moment frames (SMFs) in seismic regions. In this connection, a stiffener plate is welded between the end plate and the beam flanges to strengthen the extended portion of the end plate. This stiffener reduces prying action and more uniformly distributes flange forces among the bolt group. In experimental studies, the 8ES connection has shown ductile response to simulated seismic loading with test specimens typically failing due to beam buckling and gradual strength degradation. However, cracks initiating at the toe of the stiffener leading to brittle fracture of the beam flange has also been observed due to the high stress concentration in this region. The study reported herein proposes an eight-bolt EEP connection in which the end plate stiffener is removed and the bolt arrangement is modified to promote uniform distribution of flange forces among the bolt group. The proposed connection was developed through detailed finite element analysis in which various bolt arrangements for stiffened and unstiffened eight-bolt EEP connections were considered. The proposed connection displayed reduced beam flange stress and strain concentrations, delayed or reduced rate of strength degradation from local buckling and more uniform distribution of bolt forces when compared to the alternatives. Furthermore, when compared to the currently prequalified 8ES connection, despite requiring thicker end plates, the proposed connection is anticipated to result in cost savings from the removal of the end plate stiffener. Future analytical and experimental needs for further development of the proposed connection are discussed.