Almost every structural steel connection uses one of two weld types: fillet welds, the triangular welds laid in the corner between two pieces, and groove welds, which fill a prepared gap between pieces and can be complete joint penetration (CJP) or partial joint penetration (PJP). A fillet weld is sized by its leg length, a groove weld by how deep it penetrates the joint. In Canada the engineer designs weld sizes to CSA S16, and CSA W59 sets the rules for how they are made, including minimum sizes.
This post explains each structural weld type, how size is measured, and what decides the size you see on a drawing. If you are trying to decode the symbols themselves, see our guide on how to read structural weld symbols.
The structural weld types at a glance
| Weld type | What it looks like | Typical structural uses |
|---|---|---|
| Fillet weld | Triangular bead in a corner, no edge preparation | Shear tabs, clip angles, base plates, stiffeners, lintel plates, bracing gussets |
| CJP groove weld | Fills the full thickness of a prepared joint | Moment connection flanges, column splices, butt splices of plate |
| PJP groove weld | Fills part of the thickness of a prepared joint | Column splices in compression, built-up members, joints where full strength is not needed |
| Flare groove weld | Fills the groove formed by a rounded edge | Rounded corners of HSS against plate, bar against plate |
| Plug or slot weld | Fills a round hole or slot in one plate | Joining overlapping plates where edges are not available |
| Stud weld | Stud welded on end in a single shot | Shear studs on beams under composite slabs, headed anchors on embeds |
Fillet welds are by far the most common because they need no edge preparation and are fast to make. Groove welds take more preparation, more weld metal and usually more inspection, so engineers use them where a fillet weld cannot do the job.
Fillet welds: how size is measured
A fillet weld joins two surfaces at roughly a right angle, in a T joint, lap joint or corner joint. Its size is described by its leg length, the distance from the root of the joint out to the toe of the weld on each piece. An "8 mm fillet" has 8 mm legs.
The leg is what you measure with a fillet gauge, but the strength comes from the throat: the shortest distance from the root to the face of the weld. For a standard equal-leg fillet with a flat face, the throat is the leg multiplied by about 0.707 (the sine of 45°).
| Fillet leg size | Approximate throat |
|---|---|
| 5 mm | 3.5 mm |
| 6 mm | 4.2 mm |
| 8 mm | 5.7 mm |
| 10 mm | 7.1 mm |
| 12 mm | 8.5 mm |
A few practical points about fillet welds:
- Weld metal grows fast with size. The cross section of a fillet weld increases with the square of the leg, so a 12 mm fillet holds roughly four times the weld metal of a 6 mm fillet. Oversizing adds cost, heat and distortion.
- Single pass or multi pass. Small fillets can often be made in one pass. Larger ones need several passes, which takes more time and more heat input.
- Continuous or intermittent. Intermittent (stitch) welds are shown with a length and pitch, such as 50 mm long at 300 mm centres. They save welding where full length is not needed.
- Profile. W59 has workmanship rules for weld profile. Excess convexity, undercut and underfill are defects, covered in our post on structural weld defects and inspection.
Groove welds: CJP and PJP
A groove weld is made in a gap between two pieces, usually after one or both edges have been prepared. Common preparations include square, single bevel, single V, J and U grooves, and double-sided versions of each for thicker material. The prep, the root opening and the angle all come from the welding procedure.
Complete joint penetration (CJP). The weld fuses through the full thickness of the joint. A properly made CJP weld develops the strength of the connected part. To get complete fusion at the root, the joint is either welded from one side onto a steel backing bar, or welded from both sides with the root back gouged to sound metal before the second side is welded.
Partial joint penetration (PJP). The weld goes only part way through the thickness. It is sized by its effective throat, which depends on the depth of preparation and the process and position. PJP welds are useful where the load does not need the full section, such as column splices that carry mainly compression.
Groove welds are where inspection becomes most important. Engineers often call for ultrasonic testing on CJP welds in critical connections, and the acceptance criteria come from CSA W59. Because a groove weld's quality is inside the joint, visual inspection alone cannot confirm it.
Plug, slot, flare and stud welds
Plug and slot welds fill a hole or slot in one plate to join it to the plate behind. They are used where two pieces overlap and there is no edge to fillet weld, or to stop two plates from separating. They have their own size, spacing and depth rules in W59.
Flare groove welds fill the groove formed against a rounded surface. You see them where the rounded corner of an HSS sits against a plate, or where round bar is welded to plate. The effective throat is smaller than it looks, which is why engineers specify them carefully.
Stud welds attach headed shear studs or anchors to steel in one quick shot with a stud gun. Shear studs on beams make the beam act together with a concrete slab, and headed studs anchor embedded plates in concrete. W59 includes stud welding requirements, including simple production checks such as bending a stud to confirm the weld holds.
Welding rebar is a separate subject with its own standard (CSA W186). See weldable rebar in Canada before anyone welds reinforcing bar.
What decides the weld size
The size on the drawing is the result of a few separate requirements:
- Strength. The engineer works out the force the weld has to carry and sizes the weld so its resistance under CSA S16 is enough. The calculation uses the throat area, the electrode strength (E49XX electrodes, the metric counterpart of E70XX, are typical) and the direction of load relative to the weld. A fillet loaded across its length resists more than one loaded along its length, and S16 accounts for this.
- Minimum size. CSA W59 sets a minimum fillet weld size tied to the thickness of the material joined. A small weld on thick plate cools very quickly, which can cause cracking or lack of fusion, so even a lightly loaded joint between thick parts needs a weld above a minimum size.
- Maximum size along an edge. A fillet along the edge of a plate cannot usefully be larger than the plate is thick, and the code limits the size so the edge is not melted away and the weld can be measured.
- Effective length. Very short fillet welds are not counted at full value, and end returns are sometimes required. These limits are in W59 and S16.
- Base metal capacity. A weld cannot carry more than the steel it is attached to. For thin plate or HSS walls the base metal often governs, not the weld.
We are not giving the code values for minimum sizes, lengths or resistance here on purpose. They depend on the edition of CSA S16 and W59 called up for your project, and they are applied by the engineer and built into the fabricator's welding procedures. If you need to know whether a weld on your job is adequate, ask the engineer of record.
Structural welding design: who does what
| Step | Who is responsible |
|---|---|
| Decide the connection type and weld sizes | Engineer of record (or a connection designer the engineer delegates to) |
| Show welds on shop drawings with correct symbols | Steel detailer |
| Choose joint preparation, process and parameters | Fabricator's welding engineer, through approved procedures |
| Make the weld | Qualified welder at a CWB-certified company |
| Inspect and test | Fabricator's quality control and, where specified, an independent inspection company |
In Canada, a CWB-certified fabricator welds to written procedures reviewed by the CWB. The procedure sets the joint preparation, consumable, position, preheat and settings for each type of weld. Our post on welding procedures (WPS and WPDS) explains how that system works, and our overview of CSA W59 explains how the code ties into the building code.
Shop welds vs field welds
The same weld type can be made in the shop or in the field, and the choice affects cost and quality. In the shop, pieces can be turned so welds are made flat or horizontal, under cover, with easy access for inspection. In the field, welds are often overhead or vertical, need weather protection and need a certified welder on site. That is why most designs keep welding in the shop and use bolted field connections where they can.
Getting welded steel made right
Our shop welding is certified by the Canadian Welding Bureau, and we fit, weld, grind and mark every piece to the approved shop drawings, from base plates and posts to beams with end plates and full frames. Send us your structural drawings for a free take-off and quote, or read more about our fabrication and welding services.
Frequently asked questions
What are the main types of structural welds?
Fillet welds and groove welds do almost all of the work. Groove welds are either complete joint penetration (CJP) or partial joint penetration (PJP). Plug, slot, flare groove and stud welds cover more specialized connections.
How is a fillet weld size measured?
By its leg length, the distance from the root of the joint to the toe of the weld on each plate. The strength is based on the throat, the shortest distance from the root to the face, which is about 0.7 times the leg for a standard equal-leg fillet.
Is there a minimum structural weld size?
Yes. CSA W59 sets minimum fillet weld sizes that depend on the thickness of the parts joined, because a small weld on thick steel cools too quickly and can crack. The engineer and the fabricator's welding procedures apply the exact values for the steel on your job.
Is a bigger weld always stronger and better?
Not in practice. Weld metal volume grows with the square of the leg size, so oversized welds add cost, heat and distortion without helping a connection that is already adequate. The engineer sizes welds for the load, and the shop welds to that size.
When is a full penetration weld needed?
A complete joint penetration groove weld is used where the full strength of the connected part has to be developed through the joint, such as some moment connections, column splices and butt splices of plate. The engineer decides, and these welds usually carry inspection or testing requirements.
