Structural welding is welding that joins load-carrying steel, so that the weld itself carries part of the load in a building or structure. If a structural weld fails, something moves, drops or loses strength. Because of that, structural welds in Canada are designed and made to CSA W59, the structural welding code, by companies certified by the Canadian Welding Bureau (CWB) under CSA W47.1, and they are inspected against defined acceptance criteria.
This post explains what makes a weld structural, how it differs from non-structural welding, the welds and processes involved, and who is allowed to do the work. If you are hiring structural welding, our guide to structural welding services covers shop vs field work and what drives the cost.
What makes a weld structural
The test is simple: is the weld part of a load path? Follow the load from the floor or roof down to the foundation. Any weld it passes through is structural.
Common structural welds on Ontario buildings:
- Shear tabs and end plates that connect beams to columns or to other beams.
- Base plates and cap plates on columns and steel posts, including basement posts in houses.
- Moment connections, where a beam's flanges are welded so the joint resists bending.
- Built-up members, such as plate girders or two shapes welded together to make a stronger section.
- Bracing gussets that carry wind and earthquake forces.
- Lintels with welded plates carrying masonry over an opening.
- Reinforcement of existing steel, where plates are welded to a beam to increase its capacity.
The same piece of steel can carry both kinds of weld. On a column, the weld holding the base plate is structural; a small tab welded on to hang a sign may not be. The engineer and the drawings decide, not the welder.
Structural vs non-structural welding
Non-structural welding is welding where failure would not affect the strength or stability of the structure. That is not the same as unimportant, and it is not the same as unregulated.
| Structural welding | Non-structural welding | |
|---|---|---|
| Purpose | Transfer load between members | Hold parts together for shape, appearance or convenience |
| Design | Weld type and size set by an engineer | Usually set by the fabricator or designer |
| Code | CSA W59 on structural steel | Workmanship standard set by the job or the customer |
| Company | CWB certified to CSA W47.1 | Certification not usually required |
| Procedures | Approved welding procedures | Shop practice |
| Inspection | Visual, plus testing where specified | Visual, often for appearance |
| Examples | Base plates, beam connections, moment frames | Trim, decorative panels, enclosures, art |
Two cautions apply. First, some items that look ornamental do carry load. A guard or handrail must resist the loads the building code sets for guards, so its welds matter even if the design is decorative. Second, welding on boilers, pressure vessels and pressure piping is a separate regulated field entirely, governed by its own codes and, in Ontario, overseen by the Technical Standards and Safety Authority (TSSA). It is neither "structural" in the building sense nor unregulated.
The welds used in structural work
Structural welds come in a few basic types, each shown on drawings with a standard weld symbol:
- Fillet welds: a triangular weld in the corner between two pieces, such as a plate welded to a beam flange. By far the most common structural weld.
- Groove welds: made in a prepared joint between the edges of two pieces. A complete joint penetration (CJP) groove weld fuses the full thickness; a partial joint penetration (PJP) groove weld does not.
- Plug and slot welds: made through a hole or slot in one piece into the piece below.
- Stud welds: headed shear studs welded to beams so a concrete slab acts compositely with the steel.
How each type is sized and why one is chosen over another is covered in structural weld types and sizes, and the symbols themselves in how to read structural weld symbols.
The welding processes
Structural welds are made with arc welding processes. The ones you will see in Canadian steel shops and on site:
- Shielded metal arc welding (SMAW), or stick welding. Portable and forgiving of wind, so it is common for field welds and repairs.
- Flux-cored arc welding (FCAW). High deposition, widely used for structural work in the shop and the field.
- Gas metal arc welding (GMAW), or MIG. Fast and clean in the shop, but the shielding gas is easily disturbed outdoors.
- Submerged arc welding (SAW). Automated, used for long, heavy welds such as plate girders.
Each process needs its own approved procedure and qualified welders. For a comparison of when each is used, see structural welding processes compared.
What sets structural welding apart
Anyone with a machine can join two pieces of steel. Structural welding is defined less by the arc than by the system around it:
- The design. The engineer sets the weld type, size and length for the forces it carries. Where connection design is delegated to the fabricator, an engineer retained by the fabricator does this.
- The code. The 2024 Ontario Building Code requires structural steel to conform to CSA S16 (Article 4.3.4.1.). S16 in turn calls up CSA W59 for welded construction and requires fabricators and erectors who weld to be certified. Our guide to CSA W59 explains the chain.
- The company. The fabricator or erector is certified by the CWB to CSA W47.1, which audits its procedures, its welding supervision and its welders. See how to check CWB certification.
- The procedure. Each weld is made to a written, approved welding procedure that fixes the process, consumables, joint preparation and settings. See welding procedures (WPS and WPDS).
- The welder. Welders pass CWB qualification tests for the processes and positions they weld in.
- The inspection. Welds are visually inspected, and where the specification calls for it, tested by non-destructive methods. Welds outside the acceptance criteria are repaired and re-inspected; see structural weld defects and inspection.
Take away any one of those and the weld may look the same but is no longer a structural weld you can rely on, or defend to a building inspector.
Structural welding on houses and renovations
Structural welding is not only for commercial buildings. On houses it shows up most often in:
- Basement beams and posts, with cap plates and base plates welded to the posts.
- Lintels and flitch plates over widened openings.
- Reinforcing an existing beam when loads change in a renovation.
Two points for homeowners and renovators. Structural changes need a qualified designer or engineer and usually a building permit, and the steel and its welds should come from the drawings, not from a guess on site. And welding on old steel needs care: the grade may be unknown, and heat can damage the member or start a fire in surrounding wood framing. Before anyone welds to an existing beam, read can you cut or weld steel beams?
Wherever possible, structural welds on residential steel are best made in the shop, where the piece can be positioned for good welds and checked before it leaves. Posts and beams then arrive ready to bolt in place.
CWB certified welding at Procut
Procut Steel's shop welding is certified by the Canadian Welding Bureau. We fabricate beams, posts, lintels, plates and connections with the welds made in our Erin shop, prime or galvanize to suit, and deliver across the GTA and Ontario, with installation available for structural steel. Send us your structural or architectural drawings and we will prepare a free take-off and quote.
Request a quote, or see our fabrication and welding services.
Frequently asked questions
What is considered structural welding?
Any weld that carries load in a building or structure, so that if it failed, a member could move, drop or lose strength. Examples include beam connection plates, column base and cap plates, moment connections, bracing gussets, built-up members and reinforcement of existing steel.
What is non-structural welding?
Welding where the weld does not form part of a load path, such as ornamental trim, decorative panels, enclosures, furniture or art. It still needs good workmanship, but it is not designed and inspected the way structural welds are under CSA W59.
Who can do structural welding in Ontario?
On structural steel designed to CSA S16, welding is done by companies certified by the Canadian Welding Bureau to CSA W47.1, using welders who have passed CWB qualification tests, working to approved welding procedures and CSA W59.
What is a structural weld?
A structural weld is a weld designed to transfer force from one steel member to another. Its type, size, length and quality level are set by the design and the welding code, and it is made to an approved procedure and inspected.
