Page 94 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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CONNECTIONS

3.26 CHAPTER THREE

TABLE 3.8 Minimum Plate Thickness for Fillet Welds
b
Minimum plate thickness for fillet
welds on each side of plate, in
a
b
Size of fillet weld, in Maximum plate thickness, in Steel: F y = 36 ksi Steel: F y = 50 ksi
1 c 1
/ 8 / 4
3 1 0.32 0.29
/ 16 / 2
1 3 0.43 0.38
/ 4 / 4
5 3 0.53 0.48
/ 16 Over / 4
a
Weld size need not exceed the thickness of the thinner part joined, but the AISC requires that care be taken to provide suf-
ficient preheat to ensure weld soundness.
b
Plate thickness is the thickness of the thinner part joined.
c 3
Minimum weld size for structures subjected to dynamic loads is / 16 in.
A limitation is also placed on the maximum size of fillet welds along edges. One reason is that
edges of rolled shapes are rounded, and weld thickness consequently is less than the nominal
thickness of the part. Another reason is that if weld size and plate thickness are nearly equal, the
plate comer may melt into the weld, reducing the length of weld leg and the throat. Hence, the
1
AISC Specification (Sec. J2.2b) requires the following: “Along edges of material less than / 4 in
1
thick, maximum size of fillet weld may equal material thickness. But along edges of material / 4 in
1
or more thick, the maximum size should be / 16 in less than the material thickness.” Weld size may
exceed this, however, if drawings definitely show that the weld is to be built out to obtain full
throat thickness.
AWS D1.1 requires that the minimum effective length of a fillet weld be at least four times the
nominal size, or else the weld must be considered not to exceed 25% of the effective length. Subject
to the preceding requirements, intermittent fillet welds maybe used in buildings to transfer calcu-
lated stress across a joint or faying surfaces when the required strength is less than that developed
by a continuous fillet weld of the smallest permitted size. Intermittent fillet welds also may be used
to join components of built-up members in buildings. Intermittent welds are advantageous with
light members where excessive welding can result in straightening costs greater than the cost of
welding. Intermittent welds often are sufficient and less costly than continuous welds. An exception
is girder web-to-flange fillet welds, where automatic welding equipment makes continuous welds
preferable.
Weld lengths specified on drawings are effective weld lengths. They include distances needed for
start and stop of welding. No reduction in effective length need be made in design calculations for
the start and stop weld craters. To avoid the adverse effects of starting or stopping a fillet weld at a
corner, welds extending to corners should be returned continuously around the corners in the same
plane for a distance of at least twice the weld size. This applies to side and top fillet welds connect-
ing brackets, beam seats, and similar connections, on the plane about which bending moments are
computed. End returns should be indicated on design and detail drawings.
Filet welds deposited on opposite sides of a common plane of contact between two parts must be
interrupted at a corner common to both welds. An exception to this requirement must be made when
seal-welding parts prior to hot-dipped galvanizing.
If longitudinal fillet welds are used alone in end connections of flat-bar tension members, the length
of each fillet weld should at least equal the perpendicular distance between the welds.
5
In material / 8 in or less thick, the thickness of plug or slot welds should be the same as the material
5
thickness. In material greater than / 8 in thick, the weld thickness should be at least half the mate-
5
rial thickness but not less than / 8 in.
5
The diameter of the hole for a plug weld should be at least equal to the depth of the hole plus / 16 in,
1
but the diameter should not exceed 2 / 4 times the thickness of the weld. Thus, the hole diameter
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