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Structural Steel Frames 309
Types of weld
Two types of weld are used, the fillet weld and the butt weld.
Fillet weld
This weld takes the form of a fillet of weld metal deposited at the junction of two parent
metal membranes to be joined at an angle, the angle usually being a right angle in structural
steelwork. The surfaces of the members to be joined are cleaned and the members fixed in
position. The parent metals to be joined are connected to one electrode of the supply and the
filler rod to the other. When the filler rod electrode is brought up to the join, the resulting
arc causes the weld metal to run in to form the typical fillet weld illustrated in Figure 5.32.
The strength of a fillet weld is determined by the throat thickness multiplied by the length
of the weld to give the cross-sectional area of the weld, the strength of which is taken as
115 N/mm 2 . The throat thickness is used to determine the strength of the weld, as it is along
a line bisecting the angle of the join that a weld usually fails. The throat thickness does not
extend to the convex surface of the weld over the reinforcement weld metal because this
reinforcement metal contains the slag of minerals other than iron that form on the surface
of the molten weld metal, which are of uncertain strength. The dotted lines in Figure 5.32
represent the depth of penetration of the weld metal into the parent metal and enclose that
part of the parent metal that becomes molten during welding and fuses with the molten
weld metal.
The leg lengths of fillet weld used in structural steelwork are 3, 4, 5, 6, 8, 10, 12, 15, 18,
20, 22 and 25 mm (Figure 5.32). Throat thickness is the leg length multiplied by 0.7 mm.
Fillet welds 5–22 mm are those most commonly used in structural steelwork, the larger
Penetration
Leg
length
Fusion face
Weld face
Reinforcement
Fusion face
Root
Leg length
Fillet weld
Throat thickness
Fillet weld
formed in
three runs
3
1 2
Figure 5.32 Fillet weld.