Page 234 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 219

would occur in the base material. Poor weld quality, however, may
adversely affect weld strength. Porosity, slag inclusions, and lack of
fusion may decrease the capacity of a complete joint penetration (CJP)
groove weld.
A connection that duplicates the base metal capacity is not always
necessary and when unwarranted, its specification unnecessarily
increases fabrication costs. In the absence of design information, it is
possible to design welds that have strengths equivalent to the base
material capacity. Assuming the base plate has been properly selected, a
weld sized around the base plate will be adequate as well. This, however,
is a very costly approach. Economical welded structures cannot be
designed on this basis. Unfortunately, the overuse of the CJP detail and
the requirement of “matching filler metal” serves as evidence that this
is often the case.

3.6.2 Variables affecting welded
connection strength

The strength of a welded connection is dependent on the weld metal
strength and the area of weld that resists the load. Weld metal
strength is a measure of the capacity of the deposited weld metal itself,
measured in units such as ksi (kips per square inch). The connection
strength reflects the combination of weld metal strength and cross-
sectional area, and would be expressed as a unit of force, such as kips.
If the product of area times the weld metal strength exceeds the load
applied, the weld should not fail in static service.
The area of weld metal that resists fracture is the product of the
theoretical throat multiplied by the length. The theoretical weld
throat is defined as the minimum distance from the root of the weld
to its theoretical face. For a CJP groove weld, the theoretical throat is
assumed to be equal to the thickness of the plate it joins. Theoretical
throat dimensions of several types of welds are shown in Fig. 3.18.
For fillet welds or partial joint penetration groove welds, using filler
metal with strength levels equal to or less than the base metal, the
theoretical failure plane is through the weld throat. When the same
weld is made using filler metal with a strength level greater than that
of the base metal, the failure plane may shift into the fusion boundary
or heat-affected zone. From a design perspective, this is an undesir-
able condition and may lead to performance problems.
Complete joint penetration groove welds that utilize weld metal
with strength levels exactly equal to the base metal will theoretically
fail in either the weld or the base metal. Since the weld metal is gen-
erally slightly higher in strength than the base metal, the theoretical
failure plane for transversely loaded connections is assumed to be in
the base metal.

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