Intermittent Welds and Pitch — How to Read the Symbol
An intermittent weld is a row of short weld segments instead of one continuous seam. It saves weld metal, heat input and distortion where full strength is not needed. The symbol carries three numbers: weld size, segment length and pitch.
How the symbol is built
ISO 2553 writes the size in front of the triangle and the segment data behind it.
- a4 5 × 50 (100) — throat 4 mm, 5 segments, each 50 mm long, pitch 100 mm.
- Pitch is centre-to-centre, not the gap. A 50 mm segment at 100 mm pitch leaves a 50 mm gap.
- AWS A2.4 writes length–pitch after the symbol: 1/4 2-4 means 2 in long segments on 4 in centres.
- Number of segments is optional in ISO and rarely used in AWS.
Chain vs staggered
With welds on both sides of the joint, the segments are either aligned or offset.
| Type | Symbol | Use |
|---|---|---|
| Chain intermittent | Triangles aligned on both sides of the reference line | Symmetric loading, best stiffness |
| Staggered intermittent | Triangles offset (Z-shape) across the reference line | Lower distortion, thin webs, long girders |
Design rules
Intermittent welds are a fatigue and corrosion compromise. Use them where that is acceptable.
- Never use intermittent welds where the joint must be watertight, airtight or pressure-bearing.
- Avoid them in corrosive environments — the unwelded gap traps moisture.
- Avoid them in fatigue-loaded joints; every start and stop is a crack initiation point.
- Segment length is normally at least 4 × the throat and at least 25 mm.
- Pitch is normally 2–4 × the segment length; beyond that the joint behaves as separate welds.
- Always weld to the end of the joint at both ends, not into empty space.
Worked example
A 600 mm long stiffener needs a4 continuous by strength, but the load is only 40 % of capacity. Using a4 50 (150) gives 50 mm welded out of every 150 mm — one third of the weld metal, about one third of the heat input, and clearly less distortion, while keeping enough capacity with margin.
Frequently asked questions
What does 50 (100) mean on a weld symbol?
It means 50 mm long weld segments with a 100 mm pitch, measured centre to centre. That leaves a 50 mm unwelded gap between segments.
Is pitch the gap between welds?
No. Pitch is the centre-to-centre distance between the start of one segment and the start of the next. The gap is the pitch minus the segment length.
What is the difference between chain and staggered intermittent welds?
Chain welds are aligned on both sides of the joint; staggered welds are offset. Staggered welds distort less and are common on long thin webs.
When should I not use intermittent welds?
Never on leak-tight, pressure-bearing, corrosive or fatigue-loaded joints. Each stop and start is a weak point and a moisture trap.
Related pages
Fillet weld: a-dimension vs z-dimension
Throat or leg? Conversion table, sizing rules and the mistakes that cost the most.
ISO 2553 System A vs System B
The two systems inside one standard, and how to tell which a drawing uses.
ISO 2553 vs AWS A2.4
Full comparison of the European and US welding symbol conventions.
Threads and tapped holes
Metric sizes, pitch, drill diameters, minimum engagement and wall thickness.