Page 260 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 245

Ductile hinges in connections. The fundamental premise regarding the
special moment-resisting frame (SMRF) is that plastic hinges will form
in the beams, absorbing seismically induced energies by inelastically
stretching and deforming the steel. The connection is not expected to
break. Following the Northridge earthquake, there was little or no evi-
dence of hinge formation. Instead, the connections or portions of the
connection experienced brittle fracture, inconsistent with expected and
essential behavior. Most of the ductility data are obtained from smooth,
slowly loaded, uniaxially loaded tensile specimens that are free to neck
down. If a notch is placed in the specimen, perpendicular to the applied
load, the specimen will be unable to exhibit its normal ductility, usually
measured as elongation. The presence of notchlike conditions in the
Northridge connections decreased the ductile behavior.
Initial research on SMRF connections conducted in the summer of
1994 attempted to eliminate the issues of notchlike conditions in the
test specimens by removing weld backing and weld tabs, and controlling
weld soundness. Even with these changes, “brittle” fractures occurred
when the standard details were tested. The testing program then evalu-
ated several modified details with short cover plates, with better suc-
cess. The reason for these differences can be explained analytically.
Referring to Fig. 3.27, the material at point A, whether it be weld
metal or base metal, cannot exhibit the ductility of a simple tension
test. Ductility can take place only if the material can slip in shear
along numerous slip planes. That is, it must be free to neck down.

A

B
Restrained No Restraint

Figure 3.27 Regions to be analyzed relative to poten-
tial for ductile behavior. (Courtesy of The Lincoln
Electric Company.)

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