Page 243 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

228 Chapter Three

second joint, a larger fillet weld has been specified. Since the residual
stresses are assumed to be of the order of the yield point of the weaker
material in the joint, the first example would have residual stresses in
the weld metal and the base metal of approximately 100 ksi. In the sec-
ond example, the residual stresses in the base material would be
approximately 60 ksi, since the filler metal has the lower yield point.
These lower residual stresses will reduce cracking tendencies.
In situations where the weld size is controlled by the minimum
permitted size, undermatching is a particularly desirable option. If a
1
⁄ 4 -in fillet weld is required because of the minimum fillet weld size, it
may be made of undermatching weld material without increasing the
weld size due to the undermatching requirement.
Overmatching weld metal should be discouraged. It offers no advan-
tages, and will increase residual stresses and distortion. Higher yield-
strength weld metal generally is less ductile and more crack sensi-
tive. Exceptions to this guideline are the filler materials used to join
A588 weathering steel. In the process of adding alloys for atmospher-
ic corrosion resistance, most filler metals for weathering steel will
deposit 80-ksi tensile strength weld metal. Compared to the 70-ksi
tensile strength weathering steel, this is an overmatch. The combina-
tion, however, performs well and because of the limited alternatives,
this slight overmatch is permitted.
Caution must be exercised when overmatching filler metal is deliber-
ately used. The strength of fillet and PJP groove welds is controlled by
the throat dimension, weld length, and capacity of the weld metal. In
theory, overmatching filler metal would enable smaller weld sizes to be
employed and yet create a weld of equal strength. However, the strength
of a connection is dependent not only on the weld strength but also on
the strength of the fusion zone. As weld sizes are reduced, the fusion
zone is similarly decreased in size. The capacity of the base metal is not
affected by the selection of filler metal, so it remains unchanged. The
reduction in weld size may result in an overstressing of the base metal.
Consider three tee joints containing PJP groove welds and illustrated
in Fig. 3.22. A load is applied parallel to the weld, that is, the weld is
subject to shear. The allowable stress on the groove weld is 30% of the
nominal strength of the weld metal, that is, the “E” number (for example,
E60, E70, etc.). Allowable stress on the base metal shall not exceed 40%
of the yield strength of the base metal. The first combination employs a
very close match of weld metal to base metal, namely, A572 GR50 weld-
ed with E70 electrode. The second example examines the same steel
welded with undermatching E60 electrode, and the final illustration
shows an example of overmatching with E80 electrode.
The weld capacity, in kips per inch, has been determined by multi-
plying the weld throat by the allowable stress. In the undermatching

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