Reading Weld Symbols on Structural Drawings: CWI Guide

Weld symbols are a compressed language. A single callout on a structural drawing conveys joint type, weld size, length, location, and whether backing, weld-all-around, or field welding applies — all without written text. The language works when the sender (the engineer or detailer who drew it) and the receiver (the welder, CWI, and fabricator) are reading from the same dictionary.

The dictionary is AWS A2.4. When it is not read correctly, the result is a weld made to the wrong geometry: the wrong size, the wrong side, or the wrong joint type. Any of those errors can mean a weld that is out of compliance with the WPS, the connection design, or both.

The reference line: the backbone of every weld symbol

Every weld symbol in AWS A2.4 is built around a horizontal reference line with an arrow at one end and an optional tail at the other.

         Weld symbol (above = other side)
         ─────────────────────────────────── (reference line)
         Weld symbol (below = arrow side)
    ↙ arrow (pointing to the joint)          tail ──▷ (WPS note, etc.)

The arrow points to the joint to be welded. Where the arrow meets the joint matters:

• For groove welds, the arrow points to the member that will be prepared (beveled, grooved, or J-prepared). If the arrow has a kink in it (a "bent arrow"), the kinked side is the member to be prepared.
• For fillet welds, the arrow points to the intersection of the two members being joined.

The reference line is the organizing axis. Symbol position relative to the reference line — above, below, or both — determines which side of the joint receives the weld.

Arrow side vs. other side

This distinction is the one that causes the most fabrication errors among welders who read symbols casually:

• Symbol below the reference line = weld is on the arrow side (the side of the joint you can see from where the arrow is pointing)
• Symbol above the reference line = weld is on the other side (the opposite face)
• Symbols both above and below = weld on both sides (a double-sided groove or back-to-back fillets)

On a drawing of a T-joint between a web plate and a flange, an arrow pointing to the toe of the web with the fillet weld symbol below the line means: fillet weld on the visible side. Symbol above the line means: fillet weld on the back side, which may require rolling the member or welding from the opposite face.

Getting this wrong means the welder puts the weld on the accessible face rather than the specified face. If the design requires a fillet on the loaded tension face and the welder makes it on the compression face, the structural capacity of the joint may be different from design intent — even if the weld itself is a perfectly executed fillet.

Fillet weld symbols: size and length

The most common weld symbol in structural steel fabrication is the fillet weld triangle. Its parameters:

• Size: a number to the left of the weld symbol (in front of the triangle). This is the leg size of the fillet in inches (US) or mm (metric). A fillet weld without a size specified is drawn to the minimum size for the thinnest plate joined, per the code.
• Length: a number to the right of the weld symbol. Absent a length, the weld runs the full length of the joint.
• Pitch: when intermittent fillet welds are specified, the callout reads as "length–pitch" to the right of the symbol (e.g., 2–6 means 2-inch weld segments at 6-inch on-center pitch).
• Contour symbol: a flat, concave, or convex arc drawn on the weld symbol indicates the required contour of the finished bead face. A flat line means flush; a concave arc means a concave face is required; a convex arc means convex. Finish method (G = grind, C = chip, M = machine) may be added.

The fillet weld size on the drawing must fall within the range the applicable WPS is qualified to produce. If the design calls for a 5/16-in fillet but the WPS was qualified with a 1/4-in test plate size, that is a WPS coverage question to resolve before production.

See: Fillet weld size requirements under AWS D1.1

Groove weld symbols: joint type and weld designation

Groove welds have individual symbols for each joint geometry:

For CJP groove welds, the weld size may be omitted from the symbol when complete fusion through the full joint thickness is implied by the joint designation. CJP welds on prequalified joints are called out by the joint detail reference letter (e.g., "TC-U4a" from Annex B of AWS D1.1:2025) rather than by a bevel dimension.

For PJP groove welds, the effective throat is specified to the left of the weld symbol, in parentheses, to distinguish it from the weld size. A weld symbol reading "(3/8)" with the groove symbol means 3/8-in effective throat — which requires a larger actual groove depth depending on the included angle of the joint.

See: CJP vs PJP groove weld: WPS and qualification differences

Supplementary symbols: all-around, field weld, backing

All-around weld: a small open circle at the reference line/arrow junction. The weld runs around the complete perimeter of the joint — common on column base plate connections, HSS connections, and handrail posts. A WPS covering an all-around weld on a tube must be qualified for all positions likely to be encountered going around the joint.

Field weld: a solid flag pointing up from the reference line/arrow junction. The weld is to be made in the field (on-site erection) rather than in the shop. This has WPS implications: field welds may be made at different ambient temperatures, positions, and with different equipment than shop welds. Some shops maintain separate field WPS documents with more conservative preheat requirements.

Backing symbol: an open rectangle below (or above) the weld symbol indicates that backing is required. When the backing symbol appears, the detail should also call out the backing type — steel backing bar, ceramic backing, or flux backing for SAW. Steel backing changes the root geometry of the joint and typically requires a different qualified WPS than the same joint made without backing, because removing backing for back-gouging is a separate operation.

Weld-all-around + field weld: when both symbols appear together, the weld is made in the field and runs all around the joint — common for moment frame column splices or base plate anchor welds at erection.

The tail: WPS traceability on the drawing

The tail is the forked end of the reference line, opposite the arrow. It is used to record supplementary information that doesn't fit in the standard symbol geometry. In structural fabrication, the tail commonly contains:

• The WPS number applicable to this weld (e.g., "WPS-012")
• A note to see a separate detail drawing for joint geometry
• A special process requirement not capturable in the standard symbol

When the tail specifies a WPS number directly on the drawing, the weld symbol becomes a self-contained traceability record — the drawing callout links the weld location to the qualifying procedure without a separate weld map document. This approach works well for projects with a limited number of distinct WPS documents.

See: Weld maps and WPS traceability on fabrication drawings

Connecting weld symbols to WPS selection

The weld symbol tells the CWI what joint is required. The WPS is what must exist before that joint can legally be welded. The CWI's job is to verify both:

1. The symbol correctly specifies the required joint — dimensionally feasible, consistent with the structural design intent, and unambiguous. An incorrectly drawn weld symbol is a drawing error, not a field decision.
2. A qualified WPS covers the joint — process, base metal, thickness, position, joint type, and filler all fall within the qualified ranges documented by the supporting PQR or prequalified joint designation.

The most common WPS coverage gap discovered from weld symbol review:

• A drawing calls out a 3G position groove weld, but the WPS is qualified in 1G and 2G only
• A detail calls for a PJP groove weld with a 45° included angle, but the WPS was qualified at 60° — a different effective throat and a different prequalified category
• The fillet weld size specified exceeds the maximum single-pass size the WPS allows

See: WPS essential variables vs nonessential variables: which changes trigger requalification

Common weld symbol callout errors in the field

Missing size on a fillet weld symbol: the welder defaults to the minimum size for the thinner plate joined. If the design required a larger fillet, the connection is understrength. This is a drawing deficiency that requires engineering clarification before welding.

CJP symbol on a joint that can't achieve CJP: a double-bevel CJP groove weld on an inaccessible joint (column web-to-web, for example) physically cannot be welded as a CJP. The symbol is aspirational; the joint as designed either needs access from both sides or needs to be redesigned as a PJP.

Weld both sides when only one side is accessible: a double-sided fillet symbol on a box column splice where one side is closed is a constructability error. The symbol requires field welding from inside the box — or the connection needs to be reconsidered before fabrication.

Intermittent fillet where continuous is required: AWS D1.1:2025 restricts intermittent fillet welds in specific environments (exposed to weathering, corrosion-prone conditions) and in certain seismic applications. An intermittent fillet symbol on a drawing for a weathering steel structure in a corrosion exposure category may conflict with code requirements regardless of whether the weld itself can be made.

Building a weld symbol review checklist

A CWI reviewing a fabrication drawing before production starts should check every weld symbol for:

• Arrow side / other side correctly identified
• Weld size specified (fillet) or joint designation given (groove)
• Position implied by the joint geometry — does the shop have a WPS for that position?
• Supplementary symbols (all-around, field weld, backing) present and consistent with the fabrication sequence
• Tail reference present where required by the project quality plan

Any ambiguous symbol is a request for clarification to the engineer of record — not a judgment call by the shop floor.

When the drawing review is complete and WPS assignments are confirmed for every weld, that confirmation — together with the weld map and weld schedule — becomes the front page of the project documentation package that auditors review. WPS Welding links weld procedure records to project documentation so the traceability chain is complete from drawing callout to signed PQR.

Rule library based on AWS D1.1:2025; verify against your governing edition and the applicable revision of AWS A2.4.