ATC-24 Cumulative Damage Tests and Fracture Analyses of Bolted-Welded Seismic Moment Frame Connections

There was extensive damage to steel beam-to-column moment connections in several hundred buildings caused by the M6.8 earthquake in Northridge, California on 17 January 1994. Subsequent inspection of identical connections in San Francisco Bay Area steel structures indicated similar connection damage caused by the 1989 Loma Prieta earthquake. The damaged Moment Resisting Frames (MRF) were fabricated with the beam flanges attached to the column flanges by full penetration welds and with the beam webs bolted to single plate shear tabs. This design is based upon the concept "the flanges resist the moment and the web resists the shear." As a result of the observed connection fracture modes, it was concluded that the field-welded, field-bolted connection, which has become known as the "pre-Northridge" connection, is fundamentally flawed and should not be used in new seismic moment frames." This conclusion, which is stated in the Structural Engineers Association of California Seismic Structural Design Blue Book (1996), was made after post-earthquake surveys of connection damage and their modes of fracture and a review of literature of the historic laboratory tests that led to the design rationale of the "pre-Northridge" connection. The majority of these laboratory tests were made at the University of California at Berkeley during the time period of late 1960 through 1990. Additional testing of this connection was made at Lehigh University during the 1980s and at the University of Texas during the 1990s. This literature review indicated that unexplained sudden and premature connection failures occurred in a significant numbers of these tests. This observation was consistent with the fractures observed in recently (1994–1998) performed full-scale. ATC-24 Multiple-Specimen Multiple Step (1992) tests performed at the University of Texas and at the University of Michigan that failed prematurely. Presented herein are ATC-24 Multiple-Specimen Cumulative Damage tests performed on cantilever beam/column pre- and post Northridge connections. This paper relates the state of stress in these connections to the dominant failure mechanism, which is low-cycle fatigue.