Fillet Weld Capacity⚠ unverified
Mechanical / Welds · Load capacity of a fillet weld
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| F | F | N | 10000.0 | Applied load (reference) |
| L | L | m | 0.1 | Weld length |
| h | h | m | 0.006 | Weld leg size |
| tau_allow | τa | Pa | 100000000.0 | Allowable shear stress |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| cap | Fcap | N | Weld capacity |
The science & history
Understanding the Parameters
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Leg size $h$ — the specified fabrication dimension; capacity scales linearly with it (through the throat), so doubling the leg doubles the strength but also roughly quadruples the deposited metal and heat input, so oversizing is costly and can distort the part.
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Weld length $L$ — the effective throat length; the single cheapest way to add capacity is usually to lengthen the weld (or add a return) rather than to thicken it.
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Allowable shear stress $\tau_{\text{allow}}$ — set by the electrode class and the code resistance factor, $0.30\,E_{XX}$ typically. Matching filler to base metal ("matching electrode") lets the weld develop the base-metal strength.
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Throat area $0.707\,h\,L$ — the governed failure surface. Everything in fillet-weld design reduces to this area times an allowable shear.
Derivation (Approaching a Proof)
The effective throat of an equal-leg fillet is $t = 0.707\,h$ (see Throat Thickness), so the shear failure plane running the length of the weld has area
$$A_{\text{throat}} = t\,L = 0.707\,h\,L.$$
By the shear-on-throat design assumption, the weld can develop a uniform shear $\tau_{\text{allow}}$ over this area before failure, so the force it resists is stress $\times$ area:
$$F_{\text{cap}} = \tau_{\text{allow}}\,A_{\text{throat}} = 0.707\,h\,L\,\tau_{\text{allow}}.$$
This is exactly the inverse of the sizing relation $h = F/(0.707\,L\,\tau_{\text{allow}})$: the same physical statement, rearranged.
Dimensional check. $0.707\,h\,L\,\tau_{\text{allow}} = \text{m}\cdot\text{m}\cdot\text{Pa} = \text{m}^2\cdot(\text{N}/\text{m}^2) = \text{N}$ — a force, as required.
History and Development
The capacity form dates from the same AWS/AISC codification that gave fillet welds their $0.707$ throat and shear-on-throat rule (early-to-mid 20th century). Expressing weld strength as allowable force per unit length ($0.707\,h\,\tau_{\text{allow}}$, N/mm) became the basis of tabulated design aids: engineers read a strength-per-millimetre value for a given leg and electrode, then multiply by weld length — the practical shortcut this calculator automates.
Related Concepts: Fillet Weld Size, Throat Thickness, Weld Throat, Weld Throat Stress, Slot Weld Strength, Shear Stress
Notes: Input $F$ is unused (reference only) — compare $F_{\text{cap}}$ to your load by hand. Shear-on-throat basis; conservative for transverse loading. Apply the code resistance/safety factor to $\tau_{\text{allow}}$.