A welding symbol is the most information-dense mark on a structural steel drawing. It states the weld type, its size, which side of the joint it goes on, how long it runs, how it is finished and whether it is made in the shop or on site — in a notation compact enough to sit on a leader line. Get it right and the fabricator needs no further instruction. Get it wrong and the error is not caught until steel is on the ground.
Two Australian standards govern it. AS 1101.3 defines the graphical symbols themselves. AS/NZS 1554 defines the welding — procedures, categories and acceptance criteria. A drawing has to be right under both: the symbol says what to make, the 1554 reference says to what standard it is made and inspected.
Anatomy of the Symbol
Every welding symbol is built from the same parts, and each position on it carries fixed meaning:
- Reference line — the horizontal line everything attaches to.
- Arrow — points at the joint. The side it touches is the arrow side.
- Weld symbol — the shape (fillet, butt, plug) placed below or above the reference line.
- Dimensions — size to the left of the symbol, length (and pitch, if intermittent) to the right.
- Tail — the fork at the end, carrying the specification, procedure or process reference.
- Supplementary symbols — flag for a site weld, circle for a weld all round, and contour or finish marks.
Below the line, above the line
This is the convention most often misread, and it is worth stating plainly. A weld symbol placed below the reference line means the weld is on the arrow side of the joint. Placed above the line, it is on the other side. Symbols on both sides mean welds on both sides.
Note that the ISO convention used by AS 1101.3 is the reverse of the American AWS A2.4 arrangement. Imported details, overseas supplier drawings and some software defaults follow AWS. A drawing that mixes the two produces welds on the wrong face of a connection, and it is not obvious on inspection of the drawing alone. If a project takes detail from an overseas source, the convention must be stated on the drawing and checked.
The Symbols You Will Actually Use
| Weld | Symbol | Sized by | Typical use |
|---|---|---|---|
| Fillet | Right triangle | Leg length, mm | Cleats, stiffeners, most connections |
| Square butt | Two parallel lines | Root gap | Thin plate, full penetration |
| Single-V butt | V | Depth of preparation and angle | Full-penetration plate and flange splices |
| Single-bevel butt | Half V | Depth and angle | T-joints needing penetration |
| Plug / slot | Rectangle | Hole size and spacing | Lapped plate, packers |
| Fillet, both sides | Triangle above and below | Leg length each side | Double-sided cleats, gussets |
A fillet weld is dimensioned by leg length, not throat, unless the drawing states otherwise. Where design throat thickness governs, say so explicitly — the assumption differs between design offices, and a silent drawing invites the fabricator’s assumption rather than the engineer’s.
Intermittent welds
Written as length–pitch to the right of the symbol: 6 50-150 is a 6 mm fillet, 50 mm long, repeating on a 150 mm pitch. Staggered intermittent welds are shown with the symbols offset above and below the line. Intermittent welding saves weld metal and distortion but is not permitted where fatigue or sealing against corrosion governs — a common and expensive substitution on site.
AS/NZS 1554: What the Tail Reference Means
AS/NZS 1554 is a multi-part standard and the part cited changes what the fabricator must do. The parts a structural drawing normally references:
- Part 1 — welding of steel structures. The default for general structural work.
- Part 2 — stud welding.
- Part 3 — welding of reinforcing steel.
- Part 4 — welding of high-strength quenched and tempered steels.
- Part 5 — welding of steel structures subject to high levels of fatigue loading.
- Part 6 — welding of stainless steels for structural purposes.
- Part 7 — welding of sheet steel structures.
Weld category: SP or GP
Part 1 defines two weld categories. GP (general purpose) and SP (structural purpose) differ in permitted imperfections, inspection requirements and the strength that may be relied on in design. SP is the more demanding and the more expensive. Where fatigue governs, Part 5 applies and category FP is used.
The category must appear on the drawing. A drawing that shows weld sizes but never states a category leaves the fabricator to choose, and a design that assumed SP capacity delivered as GP is a structural deficiency that no one on site will see. The usual and correct place for it is a general note — for example, “All welds category SP to AS/NZS 1554.1 unless noted otherwise” — with the exceptions marked individually in the tail.
Six Mistakes That Reach the Shop
- No weld category anywhere on the drawing set. The single most common defect, and the one with the largest consequence.
- Arrow-side convention mixed with AWS details. Produces welds on the wrong face.
- Leg versus throat left ambiguous. A 6 mm throat and a 6 mm leg are not the same weld.
- Site welds not flagged. The flag drives sequencing, access, scaffold and inspection — omitting it changes the erection plan.
- “Weld all round” used on a section that is not continuous. The circle means exactly what it says; on an interrupted profile it cannot be executed as drawn.
- Full-penetration butt welds with no preparation detail. The symbol states the intent; the fabricator still needs the included angle, root gap and root face, or backing where used.
Getting the Notes Right
Most welding information belongs in the general notes rather than repeated on every leader. A workable set states the standard and category, the default fillet size, the consumable class, the treatment of site welds, the inspection regime and the surface treatment after welding. Individual symbols then carry only what departs from the default, which keeps the drawing readable and the exceptions visible.
The current editions of AS 1101.3 and the AS/NZS 1554 parts are published by Standards Australia, and the part and year cited on a drawing should match the edition the project is contracted under rather than whatever a title block template carried over from the last job.
Frequently Asked Questions
Does the symbol go above or below the reference line?
Below the line for the arrow side, above for the other side, under AS 1101.3. The American AWS convention is reversed, so the source of any imported detail matters.
What is the difference between SP and GP welds?
Permitted imperfections, inspection requirements and the design capacity that may be relied on. SP is the higher category. If the design assumed SP, the drawing must say SP.
Is a fillet weld sized by leg or throat?
Leg length by default. If the design is governed by throat thickness, state it on the drawing rather than relying on the reader to infer it.
How is a site weld shown?
A flag at the junction of the arrow and the reference line. It affects erection sequence, access and inspection, so it should never be left off a weld genuinely made on site.
Detailing to the Standard
Consistent welding annotation is a drafting discipline as much as an engineering one — the categories, defaults and exceptions have to be applied the same way across every sheet in a set. Our structural steel detailing work is produced to AS 1101.3 and AS/NZS 1554, and if you have a set you want reviewed before it goes to the fabricator, send it through.
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