Slot Weld Strength⚠ unverified
Mechanical / Welds · Compute the strength of a slot weld
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| F | F | N | 1.0 | Applied load on the weld, in newtons (N). Present for interface consistency; not used in the strength computation |
| L | L | m | 1.0 | Length of the slot |
| w | w | m | 1.0 | Width of the slot |
| tau_allow | τallow | Pa | 1.0 | Allowable shear stress of the weld |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | P | N | Allowable load on the slot weld, in newtons (N) |
The science & history
Understanding the Parameters
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Slot length $L$ and width $w$ — set the footprint. Codes bound both: length limited relative to thickness, width to roughly $2$–$2.5\times$ plate thickness, with rounded ends, so the slot fills fully.
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Perimeter term $(L + 2w)$ — the model treats the load as carried by a fillet running the two long sides plus the two ends; $(L + 2w)$ is its approximate developed length. This differs from the standard plan-area treatment ($A = L\,w$), which is why the page flags the formula as heuristic.
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Throat $0.707\,w$ — assumes a fillet whose leg equals the slot width, giving the usual $45^\circ$ throat $0.707\,w$. A truly filled slot instead shears on its full plan area.
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Allowable shear $\tau_{\text{allow}}$ — as for other welds, $0.30\,E_{XX}$ times the code resistance factor.
Derivation (Approaching a Proof)
Model the slot weld as a fillet weld deposited around the slot rather than a fully filled plug. Two ingredients combine:
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Effective throat. Taking the fillet leg as the slot width $w$, the $45^\circ$ throat is $t = 0.707\,w$ (see Throat Thickness).
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Effective length. The load-carrying weld runs the slot perimeter; the model approximates that developed length as $L + 2w$ (the length plus the two end contributions).
The shear area is throat $\times$ length, and multiplying by the allowable shear gives the capacity:
$$A = (L + 2w)(0.707\,w), \qquad P = \tau_{\text{allow}}\,A = (L + 2w)(0.707\,w)\,\tau_{\text{allow}}.$$
The standard code alternative — effective shear area equal to the slot plan area $L\,w$ — will in general give a different (often larger) capacity; use it for code-compliant design.
Dimensional check. $(L + 2w)(0.707\,w)\,\tau_{\text{allow}} = \text{m}\cdot\text{m}\cdot\text{Pa} = \text{m}^2\cdot(\text{N}/\text{m}^2) = \text{N}$ — a force, as required.
History and Development
Slot welds, like plug welds, are the welded descendants of riveted lap connections: where a round plug is too small to carry the shear, an elongated slot spreads the fused metal over a longer line while still reaching the interior of a wide overlap. AWS D1.1 and AISC specify slot geometry (length, width, end radius, spacing) and take the effective area as the plan area of the slot — the reference against which this calculator's fillet-perimeter heuristic should be checked.
Related Concepts: Plug Weld Shear, Fillet Weld Capacity, Butt Weld Strength, Throat Thickness, Shear Stress
Notes: Heuristic fillet-in-slot model — differs from the AISC plan-area ($L\,w$) method; verify for design. Input $F$ unused (reference only). Observe code limits on slot length/width/end radius.