Weight for weight, an I-beam (a W-shape in Canada) is the better beam. A C-channel of similar depth and mass has close to the same strong-axis stiffness and bending resistance when it's fully braced, but it's much weaker sideways, and because its shear centre sits behind the web, a load applied through the web tries to twist it. That's why I-beams carry floors and channels usually do framing, stringers and edge members. The numbers below compare real Canadian sections side by side.
What we mean by "I-beam" and "C-channel"
The words get used loosely, so first the shapes:
- I-beam. In Canada this almost always means a W-shape (wide-flange), named like W200x15: 200 mm nominal depth, 15 kg/m. The older S-shape (American Standard beam) with sloped flanges is the "true" I-beam but is much less common. See I-beam vs W-beam vs H-beam.
- C-channel. A hot-rolled standard channel, named like C200x17: about 203 mm deep, 17 kg/m. It has one flat back (the web) and two flanges on the same side. MC channels are a related family. Our C-channel size chart lists them all.
- U-channel, PFC and "I-channel". U-channel is an informal name for a channel, and often for light cold-formed sections. PFC (parallel flange channel) is a UK and Australian shape, not a standard Canadian one. "I-channel" isn't a shape; people searching it usually mean an I-beam.
Side-by-side section properties
Here are two pairs of sections with similar depth and weight. Properties for W-shapes are from the CISC structural section tables; channel properties are from the AISC Shapes Database metric columns for the equivalent imperial channels.
| Property | C200x17 | W200x15 | W200x19 |
|---|---|---|---|
| Mass (kg/m) | 17.1 | 15.0 | 19.5 |
| Strong-axis I, Ix (10⁶ mm⁴) | 13.5 | 12.7 | 16.6 |
| Strong-axis section modulus, Sx (10³ mm³) | 133 | 127 | 163 |
| Strong-axis plastic modulus, Zx (10³ mm³) | 158 | 145 | 187 |
| Weak-axis I, Iy (10⁶ mm⁴) | 0.545 | 0.869 | 1.15 |
| Weak-axis radius of gyration, ry (mm) | 15.8 | 21.4 | 21.6 |
| Property | C250x23 | W250x22 |
|---|---|---|
| Mass (kg/m) | 22.8 | 22.4 |
| Ix (10⁶ mm⁴) | 28.0 | 28.9 |
| Sx (10³ mm³) | 221 | 227 |
| Iy (10⁶ mm⁴) | 0.945 | 1.23 |
| ry (mm) | 18.1 | 20.7 |
Three things stand out:
- Strong-axis stiffness per kilogram is close. C250x23 and W250x22 are nearly identical in mass and Ix. If deflection under a well-braced, centred load were the only check, they'd perform about the same.
- Weak-axis stiffness is not close. The channel has 23% to 37% less weak-axis inertia than the equal or lighter W-shape it's paired with, and a smaller ry. That's the property that controls lateral-torsional buckling of an unbraced beam and buckling of a column.
- Weight for weight the W-shape edges ahead in bending. The W200x19 is 14% heavier than the C200x17 but has about 23% more Ix and Sx.
Why channels twist
Every section has a shear centre, the point a load must pass through to bend the member without twisting it. For a symmetric I-beam, it's at the middle of the web. For a channel, it's outside the section, behind the back of the web; for a C200x17, the AISC database puts it about 18 mm behind the web.
Load a channel through its web or flanges, which is how loads normally arrive, and the offset creates a twisting moment along the whole span. The channel rolls, its flanges move sideways, and its effective resistance drops. Engineers deal with this by:
- Bracing the channel at close intervals, or connecting it continuously to a deck, wall or slab.
- Loading it in line with the shear centre, for example with a connection on the back of the web.
- Pairing channels back to back, or toes together closed with plate, which gives a symmetric section with no offset.
An I-beam doesn't have this problem, which is the main reason it's the default choice for beams that carry floors. Its weak-axis stiffness is also higher for the same weight, so it needs less bracing to reach its full bending resistance. For a beam with a long unbraced length, that difference can matter more than anything in the strong-axis rows of the tables above.
Spans: why there's no simple C-channel span table
You'll find "C-section beam span tables" online, but check what they're for. Many are for cold-formed C-sections (light-gauge purlins and joists), which are a different product designed under a different standard. Others are UK or Australian PFC tables based on their own codes and shapes.
In Ontario, the Part 9 steel beam span tables (Table 9.23.4.3 and Tables 9.23.4.3.-A to -J) list W-shapes only, and require steel meeting at least Grade 350W of CSA G40.21. There's no prescriptive span table for a channel. A channel used as a beam has to be designed, which in practice means an engineer working to CSA S16 who accounts for bracing and twisting.
If you're sizing a house beam, start with the steel beam span tables for Ontario homes, and use a W-shape.
When a C-channel is the better choice
Channels are not weaker steel; they're a different shape for different jobs. They're often the right call when:
- You need a flat back. Stair stringers, ledgers bolted to concrete or masonry, trimmers and headers framing floor openings, and edge members all benefit from one flat face.
- The member is braced along its length. A channel fastened to a wall or slab, or carrying a deck, can't roll, so the twisting issue largely goes away.
- Loads are light. Girts, struts, equipment frames and small supports rarely need the efficiency of a W-shape.
- You want a built-up section. Two channels back to back, or boxed toes-in, make a stiff, symmetric member with flat outer faces that's easy to connect to.
- Clearance matters. The open side of a channel can face away from finishes or nest against other framing.
One practical note: standard C-channels have sloped inner flange faces, so bolts through the flanges need bevelled washers. Connections through the flat web are simpler.
C-channel or I-beam: quick guide
| Situation | Usually better |
|---|---|
| Floor beam carrying joists over a long span | I-beam (W-shape) |
| Beam with little or no lateral bracing | I-beam |
| Column or post | I-beam or HSS, not a single channel |
| Stair stringer | Channel |
| Ledger or edge member against a wall or slab | Channel |
| Framing an opening between beams | Channel or light W, per design |
| Built-up member with flat faces | Paired channels |
For posts and frames where twisting matters, a tube is often better than either. See C-channel vs square tube.
Getting channels and beams fabricated
Whichever shape the drawings call for, we supply it cut to length and fabricated with holes, copes and connection plates, along with the posts and plates it connects to. If you're weighing a substitution, ask your engineer first; we build to the stamped drawings.
Send your drawings for a quote, or browse our steel products, including beams, C-channels, posts and plates.
Frequently asked questions
Is a C-channel stronger than an I-beam?
Not as a beam, weight for weight. A C-channel and a W-shape of similar depth and weight have similar strong-axis stiffness and bending resistance when fully braced, but the channel is weaker sideways and twists under load because its shear centre sits outside the web. Unbraced, the I-beam is the better beam.
Can I use a C-channel instead of an I-beam?
Only if a designer checks it. A channel that matches the I-beam's depth and weight may work when it is braced and loaded in line with its shear centre, but it may need a heavier section, bracing or a pair of channels. Never substitute one for the other on a structural drawing without the engineer's approval.
Are there span tables for C-channel beams in Ontario?
No. The span tables in Part 9 of the 2024 Ontario Building Code list W-shapes only. A channel used as a beam has to be designed, usually by an engineer under CSA S16.
What is a PFC or U-channel?
PFC means parallel flange channel, a channel family used mainly in the UK and Australia. In Canada, hot-rolled channels are standard C and MC shapes with sloped inner flange faces. U-channel is an informal name for the same profile, and often for light cold-formed channels.
