Weld Size⚠ unverified
Mechanical / Joints · Compute the required fillet weld leg size
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| F | F | N | 1.0 | Applied load on the weld |
| L | L | m | 1.0 | Total length of the weld |
| tau_allow | τallow | Pa | 1.0 | Allowable shear stress of the weld |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | h | m | Required weld leg size, in metres (m). Returns 0.0 when the product ``L * tau_allow`` is not positive |
The science & history
Understanding the Parameters
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Applied load $F$ — the load transmitted by the weld; direction is ignored in shear-on-throat design (conservative). Scale $h$ linearly with it.
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Weld length $L$ — the effective length: sum every weld line resisting the load, doubling for two-sided welds. Adding length is usually cheaper than adding leg.
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Allowable shear $\tau_{\text{allow}}$ — $0.30\,E_{XX}$ times the code resistance factor for the electrode class.
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Leg size $h$ — the fabrication dimension, specified in standard increments and bounded by code minimum (fusion) and maximum (edge) limits. See Throat Thickness for the $0.707$ leg→throat conversion.
Derivation (Approaching a Proof)
An equal-leg fillet has effective throat $t = 0.707\,h$ (the altitude of the $45^\circ$ triangle, Throat Thickness). The failure surface is the throat plane over the weld length, area $A = 0.707\,h\,L$. Assuming the weld fails in uniform shear on that plane, $\tau = F/A$; setting $\tau = \tau_{\text{allow}}$ and solving for the leg:
$$\tau_{\text{allow}} = \frac{F}{0.707\,h\,L} \;\Longrightarrow\; h = \frac{F}{0.707\,L\,\tau_{\text{allow}}}.$$
Dimensional check. $h = \dfrac{F}{0.707\,L\,\tau_{\text{allow}}} = \dfrac{\text{N}}{\text{m}\cdot\text{Pa}} = \dfrac{\text{N}}{\text{m}\cdot(\text{N}/\text{m}^2)} = \text{m}$ — a length, as required.
History and Development
The shear-on-throat convention and $0.707$ throat factor were codified by AWS (D1.1) and AISC in the early- to-mid 20th century, giving a single conservative rule for sizing fillet welds regardless of load direction. That this identical relation appears twice in the catalog — here as Weld Size and again as Fillet Weld Size — reflects the registry's origin from overlapping calculator sets, not two different methods.
Related Concepts: Fillet Weld Size, Fillet Weld Capacity, Throat Thickness, Weld Throat Stress, Weld Throat, Shear Stress
Notes: Duplicate of Fillet Weld Size (Welds category). Shear-on-throat basis (direction- independent). $L$ is the effective throat length. Effective throat $= 0.707\,h$ (Throat Thickness).