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Structural Steel Frames 311
Weld face
Reinforcement
Fusion face
60°
Throat
thickness
Figure 5.34 Single V butt weld.
Root face
Sealing run
Root
Gap
Single U butt weld
Single J butt weld
Root
Double V butt weld
Gap
Double U butt weld
Figure 5.35 Butt welds.
Figure 5.35 is an illustration of a double V and a single U weld. The U-shaped weld section
provides room to manipulate the filler rod in the root of the weld but uses less of the
expensive weld metal than would a single V weld of similar depth. It is more costly to form
the edges of plates to the U-shaped weld than it is to form the V-shaped weld, and the
U-shaped weld uses less weld metal than does a V weld of similar depth. Here the designer
has to choose the weld that will be the cheapest. Plates over 24 mm thick are joined with
a double U weld, as illustrated in Figure 5.35. Butt welds between plates of dissimilar thickness
are illustrated in Figure 5.35.
The throat thickness of a butt weld is equal to the thickness of the thinnest plate joined
by the weld, and the strength of the weld is determined by the throat thickness multiplied
by the length of the weld to give the cross-sectional area of throat. The size of a butt weld
is specified by the throat thickness, i.e. the thickness of the thinnest plate joined by the
weld. The shape of the weld may be described in words as, for example, a double V butt
weld or by symbols.