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Fillet Weld Capacity from Throat Area

Also known as weld strength · weld capacity · allowable weld load · fillet weld allowable · throat area strength · how much can my weld hold · weld shear capacity

F=τ AtF = \tau \, A_t

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See also
Fillet Weld Effective Throat — Where the throat comes from, and why 0.707 times the leg is an upper bound rather than a measurement.Fillet Weld Size for a Load per Unit Length — The same check driven straight from load and weld length to a leg size.Average Shear Stress (τ = V/A) — The general form of the same relation: force over the area resisting it.

Learning zone

The capacity of a fillet weld is the allowable stress on the weld metal multiplied by the area of throat resisting the load. That is the whole equation, and its simplicity is deceptive, because each of the two terms hides a decision.

Start with the convention it rests on. A fillet weld is checked in shear on its throat regardless of which direction the load pulls. That is a design convention, not a description of the stress field inside a real fillet weld, which is a genuinely messy three-dimensional thing with stress concentrations at the root and the toe. The convention survives because it has been calibrated against a very large number of destructive tests, and because it is conservative for loads transverse to the weld axis — a transversely loaded fillet is measurably stronger than a longitudinally loaded one of the same size, and some codes allow a directional strength increase to recover part of that margin. Take the increase only if your code offers it and you have satisfied its conditions.

This page asks for the throat AREA rather than deriving it from a leg size, deliberately, because the throat is the term that is actually uncertain. Multiply the effective throat by the effective length and enter what you can defend. On a concave fillet that is less than 0.707 times the leg. Overstating the throat overstates the capacity in exact proportion, and no amount of care with the allowable stress compensates for it.

The allowable stress belongs to the weld metal, not to the plate. It is set by the electrode classification and by the code you are working to, it already carries that code's safety factor, and it is a table lookup rather than something to remember. Do not apply a second safety factor on top without knowing exactly what the first one was.

Three things this product does not know about. Effective length is not always the length you measure: codes discount craters, treat welds returned around corners separately, and require a minimum length below which a fillet is not credited at all. Long welds are not fully effective — past a length of roughly a hundred times the leg size, the load does not distribute evenly along the weld, the ends take more than their share, and codes apply a reduction factor. And the base metal has to be checked separately, both at the fusion face and through the plate behind the weld, including block shear and lamellar tearing on through-thickness loading. A weld stronger than the plate it sits on has not made the joint stronger; it has moved the failure.

One design habit worth forming. Throat and length multiply the same way in this equation but they are not interchangeable in practice, because weld metal grows as the square of the leg size while length grows in proportion. Doubling the leg quadruples the metal, the arc time, the heat and the distortion — for twice the capacity. Doubling the length doubles the capacity for double the metal. If the joint has room, take the length.

Fillet Weld Capacity from Throat Area
F=τ AtF = \tau \, A_t
FAtτ
Where
  • FF= Weld capacity (load) (kN)
  • τ\tau= Allowable shear stress on the throat (MPa)
  • AtA_t= Effective throat area (mm²)
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