Failure Analysis of Welded Steel Moment Frames Damaged in the Northridge Earthquake.
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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Keywords:
earthquakes; brittle failure; building technology;
connections; earthquake damage; failures; fracture
analysis; frames; steels; structural failures; welded
joints; material properties
Abstract:
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.
