A structural weld symbol is read from the reference line out: the arrow points to the joint, the shape on the line tells you the weld type, a symbol below the line means the arrow side and above means the other side, and the numbers give size, length and spacing. A flag means the weld is made in the field, and a circle means weld all around. Once you know those few rules, most of the symbols on a steel drawing become quick to read.
This guide walks through each part of the welding symbol, with examples of what you will see on Canadian structural and shop drawings. It goes deeper than the weld section of our general guide on how to read structural steel drawings.
The parts of a welding symbol
The system used on North American drawings comes from AWS A2.4, the standard for welding and nondestructive examination symbols. Strictly speaking, the weld symbol is the small shape (a triangle, a V, a half circle) and the welding symbol is the whole assembly around it. The assembly has up to eight parts:
| Part | Where it sits | What it tells you |
|---|---|---|
| Reference line | The horizontal line | The base everything else hangs from |
| Arrow | From one end of the reference line to the joint | Which joint, and which side is the "arrow side" |
| Weld symbol | On, above or below the reference line | Type of weld: fillet, groove, plug and so on |
| Dimensions | Left and right of the weld symbol | Size, length, pitch, root opening, groove angle |
| Supplementary symbols | At the arrow junction or on the weld symbol | Field weld, weld all around, contour, backing |
| Finish designation | Above or below a contour symbol | How the weld face is finished, for example ground |
| Tail | The fork at the far end of the reference line | Process, procedure or a note reference |
| Specification or note | Inside the tail | The detail it refers to, such as a WPDS or a typical note |
The tail is optional. If there is nothing to add, it is left off.
Arrow side, other side and both sides
This is the rule that trips people up most often, so it is worth stating plainly:
- Weld symbol below the reference line: weld on the arrow side of the joint, the side the arrow touches.
- Weld symbol above the reference line: weld on the other side of the joint.
- Weld symbols on both sides: weld both sides.
The rule does not change if the arrow points left, right, up or down, or if the reference line is drawn at the other end of the arrow. Below is always the arrow side.
For some groove welds only one member is prepared, for example a bevel cut on one plate. When it matters which member gets the bevel, the arrow has a break (a dog leg) and points at the member to be prepared. If the arrow is straight, the choice is left to the fabricator, which on structural work is usually settled on the shop drawings.
Fillet weld symbols
Fillet welds are the most common welds in building steel: clip angles, shear tabs, base plates, stiffeners and lintel plates are mostly fillet welded. Our post on structural weld types and sizes explains when designers choose fillet or groove welds.
The fillet symbol is a right triangle with its vertical leg always drawn on the left, whatever the joint looks like. The dimensions read like this:
- Size, to the left of the triangle. On metric drawings this is the leg size in millimetres, so a 6 to the left of the triangle means a 6 mm fillet. Unequal leg fillets show both legs (for example 6 x 10), and the drawing shows which leg goes on which member.
- Length, to the right of the triangle. If there is no length, the weld runs the full length of the joint, unless a note says otherwise.
- Pitch, after the length with a hyphen. For intermittent welds, 50-200 means 50 mm segments spaced 200 mm centre to centre.
Intermittent welds on both sides come in two patterns. Chain intermittent welds have the triangles directly opposite each other, so the segments line up on both sides. Staggered intermittent welds have the triangles offset, so the segments alternate.
Example
A triangle below the line, a 6 to its left, nothing to its right, a circle at the arrow junction and a flag: a 6 mm fillet on the arrow side, all around the joint, made in the field. That is a common symbol for a post welded to a base plate or embed on site.
Groove weld symbols
Groove welds fill a prepared gap between members and are used for splices, moment connections and other joints that need to transfer the full strength of the part. The symbol shapes reflect the joint preparation:
| Symbol | Joint preparation |
|---|---|
| Two parallel lines | Square groove (no bevel) |
| V | Both members bevelled |
| Bevel (one sloped leg, one vertical) | One member bevelled |
| U | Both members with a rounded groove |
| J | One member with a rounded groove |
| Flare V and flare bevel | Welds into the curved edge of round bar, bent plate or the corners of tube |
For bevel and J symbols, the vertical leg is drawn on the left, the same convention as the fillet triangle.
The dimensions on a groove weld carry more information:
- Depth of preparation and weld size, to the left of the symbol. Where both are shown, the weld size (effective throat) appears in brackets after the depth, for example 12 (10) for a 12 mm deep preparation with a 10 mm weld size.
- Root opening, inside the symbol: the gap between the members at the root.
- Groove angle, just outside the symbol, on the side away from the reference line.
- CJP or PJP in the tail. Complete joint penetration (CJP) means the weld goes through the full thickness. Partial joint penetration (PJP) means it stops short, by design.
When a joint needs a backing bar, a backing symbol appears on the opposite side of the reference line, with a note if the backing has to be removed after welding. A back weld or backing weld, made on the opposite side of a groove weld, uses a half circle.
Supplementary symbols and finish
These small marks change how a weld is made or finished:
- Flag at the arrow junction: field weld. Look for these early. A field weld needs access, a certified welder on site, suitable weather protection and often inspection, all of which affect the erection schedule.
- Circle at the arrow junction: weld all around the joint.
- Contour symbols on the weld symbol: a straight line for flush or flat, a curve bulging outward for convex, and a curve dipping inward for concave.
- Finish letters with the contour symbol, such as G for grinding or M for machining. A flush contour with a G means grind the weld flush with the surface, which is common on exposed architectural steel and where something has to sit flat over the weld.
- Melt-through symbol, a filled half circle on the opposite side, where visible penetration through the root is required.
Multiple reference lines
Sometimes two or three reference lines are stacked on one arrow. They show a sequence of operations: the line nearest the arrow is the first operation, and each line further away is the next. A common example is a groove weld on one line and a back weld or test requirement on the next.
Weld maps: tracking every weld
"Structural weld map" turns up in searches because fabricators and inspectors use the term for a different document. A weld map is a drawing or marked-up shop drawing where each weld is given an identifier and linked to the information needed to make and check it:
- the welding procedure data sheet (WPDS) or procedure used, covered in our post on welding procedures
- the welder or operator
- inspection results and any repairs
Weld maps are not a code-mandated format on every job. They are a quality-control tool used on complex or heavily inspected work: bridges, critical connections, pressure parts and projects where the specification asks for full weld traceability. On routine building steel, the same traceability usually lives in the shop's quality records. Our post on structural weld defects and inspection covers what inspectors check.
Reading the symbols on your drawings
On Canadian projects the design drawings may show only typical weld details, with a note that welds are to be made to CSA W59 by a company certified by the CWB. The full set of symbols then appears on the shop drawings, where the fabricator's detailer shows every weld on every piece for the engineer to review. A few practical checks:
- Read the general notes first. They often set a minimum fillet size and say that unspecified welds are to be a certain size, continuous or all around.
- Check units. Metric drawings give weld sizes in millimetres. Imperial drawings use fractions of an inch. Do not mix them.
- Count the field flags. Every field weld is site work, equipment and inspection, so it is worth knowing how many there are before pricing erection.
- Watch the arrow side on unequal joints. On a tee or corner joint, getting the arrow side backwards puts the weld on the wrong face.
- Ask, do not guess. If a symbol is ambiguous, raise it as a question on the shop drawings. Fixing a misread weld in the field costs far more than an RFI. For other marks on the sheets, see steel shop drawing symbols and abbreviations.
If you are looking for a structural welding symbols PDF to keep on hand, quick-reference charts from welding training providers are fine as a memory aid. The authoritative source is AWS A2.4 itself, and the project drawings and specification always govern.
Get your welded steel detailed and fabricated
We prepare shop drawings in-house, so every weld on your beams, posts, plates and connections is shown clearly before anything is cut, and our shop welding is CWB certified. Send your structural drawings through our quote request for a free take-off, or see our fabrication and installation services.
Frequently asked questions
Which side of the reference line is the arrow side?
The bottom. A weld symbol drawn below the reference line is made on the arrow side of the joint, a symbol above the line is made on the other side, and symbols on both sides mean weld both sides.
What does the flag on a weld symbol mean?
A flag at the junction of the arrow and the reference line means a field weld, made on site during erection rather than in the fabrication shop.
What does the circle on a weld symbol mean?
A circle at the junction of the arrow and reference line means weld all around: the weld continues around the full perimeter of the joint, such as all the way around a post where it meets a base plate.
What do the numbers on a fillet weld symbol mean?
The number to the left of the triangle is the weld size (the leg size, in millimetres on Canadian metric drawings). Numbers to the right give the weld length and, for intermittent welds, the pitch, which is the centre-to-centre spacing of the segments.
Is there an official structural weld symbols chart?
The welding symbol system used on North American drawings is defined in AWS A2.4, Standard Symbols for Welding, Brazing and Nondestructive Examination. Quick-reference charts are summaries of it; if a symbol on your drawings is unclear, ask the engineer rather than guessing.
