HSS (hollow structural sections) in Canada are most often specified as CSA G40.21 grade 350W, in Class C or Class H, with a minimum yield strength of 350 MPa. ASTM A500 Grade C tube, with a minimum yield of 345 MPa for square and rectangular shapes, is also common, and so is tube certified to more than one standard. The grade matters for more than strength: it changes the design wall thickness, the column resistance an engineer can use, and what the mill certificate has to show.
For tube dimensions and weights, see our HSS size chart. This article is about the material.
HSS steel grades at a glance
| Specification | Grade or class | Min. yield, square and rectangular | Min. yield, round | Notes |
|---|---|---|---|---|
| CSA G40.21 | 350W Class C | 350 MPa (50 ksi) | 350 MPa | Cold-formed; the everyday Canadian HSS grade |
| CSA G40.21 | 350W Class H | 350 MPa (50 ksi) | 350 MPa | Hot-formed or stress-relieved; higher column resistance under CSA S16 |
| ASTM A500 | Grade C | 345 MPa (50 ksi) | 315 MPa (46 ksi) | Common North American tube grade |
| ASTM A500 | Grade B | 315 MPa (46 ksi) | 290 MPa (42 ksi) | Older, lower-strength grade |
| ASTM A1085 | Single grade | Per the standard | Per the standard | Tighter tolerances, capped yield, mandatory Charpy toughness |
Atlas Tube, a Canadian HSS producer, lists the CSA G40.21 350W requirements as a minimum yield of 350 MPa, a tensile strength of 450 to 620 MPa and 22% minimum elongation in 50 mm, identical for Class C and Class H.
About A36: searches for "HSS A36" are common, but ASTM A36 covers structural plates, bars and shapes, not tube. HSS isn't made to A36. Pipe made to ASTM A53 is also a different product from HSS, with its own requirements, even when it's used structurally.
CSA G40.21 350W: Class C vs Class H
CSA G40.21 defines two classes of hollow sections:
- Class C: cold-formed. The tube is rolled from flat strip, welded into a round and then shaped into a square or rectangle, and left that way. Cold forming leaves residual stresses, concentrated in the corners and along the weld.
- Class H: either hot-formed to final shape, or cold-formed and then stress-relieved by heating to at least 450°C and cooling in air. The heat treatment relieves the residual stresses without changing the minimum strength.
Why it matters: residual stresses make a column buckle sooner. CSA S16's column formula uses an exponent n, and Class H HSS is assigned n = 2.24 instead of the n = 1.34 used for most other members, including Class C. Atlas Tube cites this (Clause 13.3.1 in CSA S16-14) and notes that Class H is available only under CSA G40.21, not under ASTM A500.
To see the effect, here's an illustration with an HSS 152x152x6.4 column (3,610 mm², r = 59.2 mm), pinned at both ends, 350 MPa, using the S16 formula:
| Length | Class C (n = 1.34) | Class H (n = 2.24) | Increase |
|---|---|---|---|
| 3.0 m | about 910 kN | about 1,060 kN | about 17% |
| 4.0 m | about 750 kN | about 920 kN | about 22% |
| 5.0 m | about 600 kN | about 730 kN | about 22% |
These are factored resistances for comparison only, not design values. In practice, Class H can let an engineer drop to a lighter column, but it's not stocked as widely as Class C and may cost more per tonne. It only makes sense when columns are a significant part of the steel and the supply works for the schedule.
ASTM A500 and the design wall thickness
ASTM A500 is the most common US specification for cold-formed welded and seamless structural tubing, and plenty of A500 tube is used in Canada. Grade C is the normal choice today; the CISC Handbook of Steel Construction (12th edition) bases its member tables on steel grades including A500 Grade C and CSA G40.21 350W.
The catch is wall thickness. Historically, A500 allowed tube walls to be thinner than nominal, so US design practice uses a reduced design wall thickness for A500 tube. You can see it in producer tables: Atlas Tube's A500 section property tables list a design wall of 0.93 times the nominal thickness (for example 0.233 in for a nominal 1/4 in wall), while its CSA G40.21 tables use the full nominal wall.
The result is that the same nominal tube has different design properties depending on the standard it's certified to:
| HSS 102x102x6.4 (4 x 4 x 1/4) | Area used in design |
|---|---|
| CSA G40.21 350W (CISC tables, nominal wall) | 2,320 mm² |
| ASTM A500 (AISC tables, design wall) | 2,170 mm² |
That's about 6% less area, and similar reductions in the other properties. Your engineer has to use the properties that match the material actually specified, and the fabricator has to supply that material.
ASTM A1085
ASTM A1085 is a newer tube specification aimed at more demanding applications. Atlas Tube describes it as offering tighter tolerances, a capped yield stress and mandatory Charpy toughness compared with A500, and suited to uses such as seismic or bridge work. It's specified by the engineer for a reason, so don't substitute A500 for A1085 or the other way round without approval.
Which grade should you specify?
That's the engineer's call, but here's how it usually plays out in Ontario:
- General building columns, posts and frames: CSA G40.21 350W Class C, or a tube certified to both 350W and A500 Grade C where the design allows it.
- Heavily loaded columns where mass savings count: consider 350W Class H, after checking availability.
- Seismic systems, bridges or toughness-critical work: whatever the specification requires, often A1085 or a G40.21 grade with impact testing.
- Basement posts in houses: the designer's drawing governs; Part 9 also has its own column rules, covered in basement column rules in the Ontario Building Code.
For how HSS stacks up against wide-flange shapes as beams and columns, see W-beam vs HSS.
Ordering HSS: what to put on the order
- Size, exactly as on the drawing, including wall thickness.
- Standard, grade and class, for example "CSA G40.21 350W Class C" or "ASTM A500 Grade C." If the drawing says only "HSS," ask the designer.
- Mill test certificates, if the project specification requires them. The certificate shows which standards the tube was certified to.
- Cut lengths, plates and holes, or the shop drawings.
- Finish. Galvanized tube needs vent and drain holes agreed between the designer and galvanizer.
Substitutions: swapping A500 tube for CSA 350W, or Class C for Class H, can change the design properties. Treat any grade change as a substitution that needs the engineer's written approval.
HSS fabricated to your drawings
We supply HSS posts and members cut to length and fabricated to shop drawings, with base and cap plates and CWB-certified shop welding. Tell us the grade and class on your drawings when you ask for a quote. For how the material, design and welding standards fit together, see Canadian structural steel codes.
Send your drawings for a quote, or browse our steel products, including posts, beams, plates and channels.
Frequently asked questions
What grade is HSS steel in Canada?
Most Canadian HSS is specified as CSA G40.21 350W, Class C (cold-formed) or Class H (heat-treated or hot-formed), with a minimum yield strength of 350 MPa. ASTM A500 Grade C tube is also widely used, and the CISC Handbook's member tables include both.
What is the yield strength (Fy) of HSS?
For CSA G40.21 350W HSS, the minimum yield strength is 350 MPa (50 ksi). For ASTM A500 Grade C it is 345 MPa (50 ksi) for square and rectangular tube and 315 MPa (46 ksi) for round tube. A500 Grade B is lower, at 315 MPa shaped and 290 MPa round.
Is HSS made from A36 steel?
No. ASTM A36 is a specification for structural plates, bars and rolled shapes, not for tube. HSS is made to tubing specifications such as CSA G40.21 (HSS classes), ASTM A500 or ASTM A1085.
What is the difference between Class C and Class H HSS?
Class C is cold-formed and left as formed. Class H is hot-formed, or cold-formed and then stress-relieved by heating. Both have the same minimum strength, but Class H has lower residual stresses, so CSA S16 gives Class H columns a higher compressive resistance.
