Page 275 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

260 Chapter Three

thought. The interpass temperature is the temperature of the steel
when the arc is initiated for subsequent welding. There are two aspects
to the interpass temperature: the minimum level, which should always
be the minimum preheat temperature, and the maximum level, beyond
which welding should not be performed. Because of the relatively short
length of beam-to-column flange welds, it is possible for a welder to
continue welding at a pace that will allow the temperature of the steel
at the connection to increase to unacceptably high levels. After one or
two weld passes, this temperature may approach the 1000°F range.
Under these conditions, a marked decrease in the strength of the weld
deposit may occur.
Although it would be unexpected to see the strength drop below the
minimum specified property for A572 grade 50 steel, it may fall below
the typical strength of the weld deposit made under more controlled
conditions. The restraint associated with the geometry at the beam-
to-column junction does not encourage yielding, so the decrease in
uniaxial yield strength may have less significance than the decrease
in tensile capacity.
Much emphasis has been placed on elongation of materials, but as
discussed under sections “Demands on Structural Systems” and
“Demand for Ductility,” geometric constraints on ductility would gen-
erally preclude welds from being able to deform, regardless of their
uniaxial elongation properties.
Weld metal toughness is an area of particular interest in the post-
Northridge specifications. Previous specifications did not have any
requirement for minimum notch toughness levels in the weld
deposits, allowing for the use of filler metals that have no minimum
specified requirements. For connections that are subject to inelastic
loading, it seems apparent that minimum levels of notch toughness
must be specified. The actual limits on notch toughness have not been
experimentally determined. With the AWS filler metal classifications
in effect in 1996, they are either classified as having no minimum
specified notch toughness, or with properties of 20 ft lb at a tempera-
ture of 0°F or lower. As an interim specification, 20 ft lb at 0°F or
lower has been recommended. However, this has been based upon
availability, not on an analysis of actual requirements. It is expected
that actual requirements will be less demanding, and once these
requirements are determined, new filler metals will be developed that
will meet the appropriate requirements. It should be recognized that
the more demanding notch toughness requirements impose several
undesirable consequences upon fabrication, including increased cost
of materials, lower rates of fabrication (deposition rates), less opera-
tor appeal, and greater difficulty in obtaining sound weld deposits.
Therefore, ultraconservative requirements imposed “just to be safe”

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