Stability and low-cycle fatigue limits of moment connection rotation capacity

A fully restrained connection in a steel moment-resisting frame designed in US after the 1994 Northridge earthquake is expected to sustain a rotation demand of 0.04 rad. However, satisfying this criterion guarantees only that the connection will fail at a larger rotation level, but it does not guaranty that the eventual failure mode of the connection or of the frame will be ductile. Failure modes of post-Northridge US connections can be classified into three categories: local beam buckling; lateral-torsional buckling of the connection sub-assembly; and low-cycle connection fatigue. While such failures are not brittle, they have a varying degree of ductility. Failure modes of post-Northridge connections are investigated herein to develop ductile design recommendations regarding beam slenderness, lateral bracing and connection configuration.