Page 274 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 259

area expected to have the lowest toughness in the cross section of the
shape. Since columns are not designed as tension members under
most conditions, this requirement would not automatically be applied
for column applications. However, as an interim specification, it
seems to be a reasonable approach to ensure minimum levels of notch
toughness for heavy columns.
Weld metal properties. Four properties of interest typically are
applied to weld metal: yield strength, tensile strength, toughness, and
elongation. These properties are generally obtained from data on the
particular filler metal that will be employed to make the connection.
The American Welding Society (AWS) filler metal classification sys-
tem contains a “coding” that defines the minimum acceptable proper-
ties for the weld metal when deposited under very specific conditions.
Most 70 series electrodes (for example, E7018, E70T-1, E70T-6) have
a minimum specified yield strength of 58 ksi and a minimum tensile
strength of 70 ksi. As in the specifications for steel, there are no
upper limits on the yield strength. However, in welded design, it is
generally assumed that the weld metal properties will exceed those of
the base metal, and any yielding that would occur in the connection
should be concentrated in the base metal, not in the weld metal, since
the base metal is assumed to be more homogeneous and more likely
to be free of discontinuities that may be contained within the weld.
Most of the commercially available filler metals today have a 70 clas-
sification, which exceeds the minimum specified strength properties
of the commonly used A36 and A572 grade 50.
These weld metal properties are obtained under very specific test-
ing conditions that are prescribed by the AWS A5 filler metal specifi-
cations. Weld metal properties are a function of a variety of variables,
including preheat and interpass temperatures, welding parameters,
base metal composition, and joint design. Deviations in these condi-
tions from those obtained for the test welds may result in differences
in mechanical properties. Most of these changes will result in an
increase in yield and tensile strength, along with a corresponding
decrease in elongation and, in general, a decrease in toughness. When
weld metal properties exceed those of the base metal, and when the
connection is loaded into the inelastic range, plastic deformations
would be expected to occur in the base metal, not in the weld metal
itself. The increase in the strength of the weld metal compensates for
the loss in ductility. The general trend to strength levels higher than
those obtained under the testing conditions is generally of little con-
sequence in actual fabrication.
There are conditions that may result in lower levels of strength being
obtained, and the Northridge earthquake experience revealed that this
may be more commonplace and more significant than originally

Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com)
Copyright © 2009 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.