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Failure Analysis of Welded Steel Moment Frames Damaged in the Northridge Earthquake.

pdf icon Failure Analysis of Welded Steel Moment Frames Damaged in the Northridge Earthquake. (21663 K)
Kaufmann, E. J.; Fisher, J. W.; DiJulio, R. M., Jr.; Gross, J. L.

NISTIR 5944; 175 p. January 1997.

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earthquakes; brittle failure; building technology; connections; earthquake damage; failures; fracture analysis; frames; steels; structural failures; welded joints; material properties


A study was performed to characterize the origin of fracture and material properties of welded steel moment frame (WSMF) connections damaged in the Northridge earthquake. Sixteen connection fractures were obtained from five different buildings in the Los Angeles area which suffered damage in the earthquake. These fractures represented a variety of the types of fractures observed in post-earthquake building inspections. The mechanical and physical properties of the connection members and weld metal were determined including composition, strength, and fracture toughness. A fractographic examination of the fracture surfaces was performed to locate and characterize the fracture origin and determine the fracture mechanism. A fracture analysis was performed using linear elastic fracture mechanics. The analysis indicated that in all cases fracture resulted from crack instability which developed within the weld metal at the weld root at an incomplete fusion flaw contiguous with the notch introduced by the weld backing. The weld metal in all cases was determined to be E70T-4 weld metal and was found to have very poor fracture toughness. The fracture toughness of the weld metal was estimated to be 44 MPa m1/2 to 65 MPa m1/2. A fracture mechanics analysis of the defect condition based upon the measured material properties and flaw sizes indicated that the cleavage fracture initiation observed in all the connections would occur without significant yielding in the beam flange and in some cases would occur under elastic stresses. Estimates of stress levels at the sample connections experienced during the earthquake were determined using simulated ground motion spectra for each building site and compared to the fracture analysis model. In all cases the range of estimated stress exceeded the fracture stress predicted by the fracture model.