Structural / welding

Fillet weld strength

A fillet weld carries load on its throat, not its leg: the throat is 0.707 times the leg, and the nominal strength is 0.60 FEXX on that area. The plate it is attached to has to be checked separately, because a thin plate tears before a well-sized weld fails.

GovernsWeld metalSize checkWithin rangeMin size0.188 in

Design strength

55.7 kip

φ = 0.75 on 74.2 kip nominal

Utilisation

72%

40.0 kip applied

Throat

0.1767 in

0.707 × 0.250 in leg

Length required

7.18 in

for 40.0 kip at this leg

Weld and base-metal check

The weld is sized on its throat; the plate is checked separately, because a thin plate fails before the weld does.

56 kip

Weld and base-metal check
Quantity	Value	Note
Fillet leg	0.250 in	4.0/16 in
Effective throat	0.1767 in	0.707 × leg
Effective length	10.00 in
Electrode	E70	FEXX = 70 ksi
Weld nominal strength	74.2 kip	0.60 FEXX × throat × length
Base-metal nominal	174.0 kip	0.60 × 58 ksi × 0.500 in × length
Governing nominal	74.2 kip	weld metal controls
Design strength	55.7 kip	0.75 × nominal
Applied load	40.0 kip
Utilisation	72%	within capacity
Minimum size	0.188 in	AISC Table J2.4 by thinner part
Maximum size	0.438 in	AISC J2.2b along an edge
Required length	7.18 in	at this leg size

The minimum size exists to prevent cracking from rapid cooling on thick plate; the maximum exists because a weld larger than the plate edge cannot develop its full throat. Both are code requirements, not rules of thumb.

Capacity against leg size

Linear in the leg, because the throat is proportional to it.

Capacity against weld length

Linear in length — the cheapest way to add capacity is usually more length, not a bigger leg.

Canonical equations

The exact relationships implemented by this engine.

t_e = 0.707 w · For a 45° fillet.

Rn = 0.60 FEXX · t_e · l · AISC J2.4.

Rn = 0.60 Fu · t · l · Checked separately.

φRn or Rn/Ω · φ = 0.75 LRFD, Ω = 2.0 ASD.

Show model notation and computationOpen the symbolic form, the variable definitions, and the step-by-step derivation when you need the detail.

A clear path from inputs to results.

Step-by-step methodWorked exampleAssumptions explained

Worked example

1/4 inch fillet, 10 inches long, E70 on A36 plate

Inputs

1/4 inch fillet leg, 10 inches of effective length.
E70 electrode on A36 plate (Fu = 58 ksi).
1/2 inch plate, 40 kip applied load, LRFD.

Calculation path

1
The effective throat is 0.707 × 0.25 = 0.1767 in.
2
Weld nominal strength is 0.60 × 70 × 0.1767 × 10 = 74.2 kip.
3
Base-metal nominal strength is 0.60 × 58 × 0.5 × 10 = 174.0 kip, so the weld governs.
4
The design strength is 0.75 × 74.2 = 55.7 kip.
5
The 40 kip load uses 72% of it.
6
At this leg the weld would need 7.18 in of length to carry the load, so 10 in is comfortable.

Result

55.7 kip design strength · 72% utilised · weld metal governs.

Interpreting the result

Which limit governs is a property of the plate, not the weld. A 1/4 inch fillet on 1/2 inch plate is weld-controlled; the same fillet on 1/4 inch plate is base-metal-controlled, and adding weld does nothing. The minimum and maximum size rules then bound the leg from both sides, so on thin plate there is sometimes no legal fillet at all.

Assumptions & limitations

The weld is a 45° fillet loaded in shear through its throat.

The effective length excludes any end return or crater.

The base-metal check uses the thinner connected part and a uniform stress distribution.

Block shear, prying and the Whitmore section are not modelled.

Weld metal toughness, hydrogen control and preheat are material requirements outside this calculation.

Frequently asked questions

Why 0.707 times the leg?

Because the throat of a 45° fillet is the perpendicular distance from the root to the face, which is the leg times cos 45°, or 0.707. The leg is what you measure; the throat is what carries the load.

Why is there a maximum size?

Along an edge thinner than 1/4 inch the weld cannot exceed the plate thickness, and on thicker plate it is limited to the thickness minus 1/16 inch. A weld bigger than the edge cannot develop its full throat and just wastes metal.

Does weld length have a limit?

Yes. AISC requires a fillet weld to be at least four times its leg in length, and end-loaded welds longer than 100 times the leg are subject to a reduction for non-uniform stress distribution.

What is the minimum fillet size for?

It prevents cracking from rapid cooling on thick plate. A small weld on a thick plate cools too fast and can crack, so Table J2.4 sets a minimum leg that increases with the thickness of the thinner part.

When does the base metal govern?

When the plate is thin relative to the weld. The base-metal strength scales with plate thickness, so a 1/4 inch fillet on 1/4 inch plate is limited by the plate — and making the weld bigger changes nothing.