Welded Steel Moment Connections: Old and New

Fully restrained steel moment connections have been traditionally designed using a prescriptive design procedure. This procedure was invalidated by brittle fractures in these connections discovered after the 1994 Northridge earthquake. Until today, a widely accepted new design that satisfies the requirements for earthquake resistance has not been developed. This paper presents a summary of the test results on a post-Northridge steel moment connection design proposed by the SAC Joint Venture. This connection design deploys some fracture mitigation measures, such as the use of notch-tough weld metal and improved welding practices. This connection is capable of achieving a mean plastic rotation of 1.5% radian, averaged over a wide range of connection parameters. The results of these tests show that the use of fracture mitigation measures alone can not produce a good connection design. An independent study at the University of Michigan demonstrated the effect of the boundary conditions on the stress flow in the connection and the importance of considering the resulting over-stress in the flanges. A new, earthquake-resistance connection design must be based on a combination of fracture and over-stress mitigation measures. Several candidate designs implement such combinations of mitigation measures and are close to achieving satisfactory seismic behavior.