There is no single weight a steel beam can hold. A beam's capacity depends on its span, how the load is applied, whether its top flange is braced, the deflection limit and the steel grade. Span matters most: a W250x33 (W10x22) that resists a factored load of about 119 kN per metre over 3 m manages only about 30 kN/m over 6 m, and at 6 m it reaches its deflection limit long before that. The tables below show how the numbers are worked out, using real Canadian section properties.
Everything in this article is an illustration of the method. A beam for an actual building has to be sized by a designer or engineer, and structural work in Ontario usually needs a building permit.
Why there's no single answer
People ask "how much weight can an I-beam take?" expecting a number like a shelf rating. A beam doesn't work that way. Five things set what it can carry:
- Span. The distance between supports. For a uniform load, bending capacity falls with the square of the span and stiffness falls with the cube.
- How the load arrives. A load spread evenly along the beam (joists from a floor) is easier on it than the same load concentrated at mid-span (a post from the floor above). Point loads near a support raise shear instead.
- Lateral support. A beam whose top (compression) flange is held sideways by joists or a deck can develop its full bending strength. An unbraced beam can buckle sideways and twist at a much lower load.
- Deflection limit. A beam can be strong enough and still sag too much for the finishes under it. For house floor beams, the Ontario Building Code's own span tables are based on a limit of span/360.
- Grade. Most W-shapes in Canada are CSA G40.21 350W (350 MPa minimum yield), often dual-certified to ASTM A992. Grade changes strength, not stiffness.
That's why the honest answer to "how much can a W8 hold?" is "over what span, carrying what, braced how?"
How a beam's capacity is worked out
In Canada, steel beams are designed to CSA S16 using limit states design. The steps, in plain terms:
- Factor the loads. The building code multiplies the loads to cover uncertainty. For a floor carrying dead and live load, the 2024 Ontario Building Code's main combination (Table 4.1.3.2.-A, case 2) is 1.25 x dead load + 1.5 x live load.
- Check bending. For a beam with a laterally supported compression flange and a Class 1 or 2 section (most W-shapes in 350W qualify), the factored moment resistance is Mr = φ x Zx x Fy, where φ = 0.90, Zx is the plastic section modulus and Fy is the yield strength.
- Check lateral-torsional buckling. If the compression flange isn't braced, Mr drops as the unbraced length grows. Long, unbraced beams are often governed by this, not by the simple formula above.
- Check shear. Rarely governs for long beams with spread-out loads, but it can for short, heavily loaded beams or big point loads near a support.
- Check deflection. Using unfactored (specified) loads, the designer compares the sag with the limit. For a simply supported beam under uniform load, deflection = 5wL⁴ / (384EI), with E = 200,000 MPa.
- Check bearing and the supports. The beam's end has to bear on enough wall or post, and the post and footing have to carry the reaction.
Capacity vs span: one beam, five spans
Here's what happens to a single W250x33 (350W, Zx = 424 x 10³ mm³, Ix = 48.9 x 10⁶ mm⁴) as the span grows. It's simply supported, uniformly loaded and fully braced along its top flange.
| Span | Bending: maximum factored uniform load | Specified uniform load that causes span/360 deflection |
|---|---|---|
| 3 m | 119 kN/m | 77 kN/m |
| 4 m | 67 kN/m | 33 kN/m |
| 5 m | 43 kN/m | 17 kN/m |
| 6 m | 30 kN/m | 9.7 kN/m |
| 7 m | 22 kN/m | 6.1 kN/m |
To get a feel for the units: 1 kN is roughly the weight of 102 kg, so 10 kN/m is about a tonne of load on every metre of beam.
Two lessons jump out:
- Doubling the span from 3 m to 6 m cuts bending capacity to a quarter and the deflection-limited load to an eighth. A few extra feet of span has a big effect.
- Deflection takes over as spans grow. The two columns aren't directly comparable (one is factored, one is specified), but the deflection column shrinks much faster. On longer spans the beam is usually chosen for stiffness, not strength.
Bending resistance of common house beams
A "steel beam load chart" in its most useful form lists each section's factored moment resistance. These values use the CISC section properties, 350 MPa and φ = 0.90, and assume a laterally supported Class 1 or 2 section.
| Beam | Imperial | Mass (kg/m) | Zx (10³ mm³) | Mr (kN·m) |
|---|---|---|---|---|
| W200x27 | W8x18 | 26.6 | 279 | 88 |
| W250x33 | W10x22 | 32.9 | 424 | 134 |
| W310x39 | W12x26 | 38.8 | 610 | 192 |
| W360x45 | W14x30 | 44.8 | 778 | 245 |
To turn Mr into a load for a simple span with uniform load, the maximum factored load is 8 x Mr / L². For a single point load at mid-span it's 4 x Mr / L. Then the deflection check still has to pass, using Ix. Our Canadian W-shape chart gives the dimensions of these and other sizes.
Notice that deeper beams gain capacity much faster than they gain weight. The W360x45 weighs 68% more than the W200x27 but has nearly 2.8 times the bending resistance. Depth is the cheapest way to add strength and stiffness, when headroom allows.
Worked example: a basement beam
Given: a beam spanning 5.0 m between posts in a house basement, carrying floor joists that span 4.0 m on each side. The beam carries one floor only. We'll use the loads the Ontario code lists as the basis of its span tables: 1.5 kPa dead load and 1.9 kPa live load.
Tributary width. Each side contributes half its joist span: 2.0 m + 2.0 m = 4.0 m.
Loads on the beam:
- Dead: 1.5 kPa x 4.0 m = 6.0 kN/m, plus about 0.3 kN/m for the beam itself = 6.3 kN/m
- Live: 1.9 kPa x 4.0 m = 7.6 kN/m
- Factored: 1.25 x 6.3 + 1.5 x 7.6 = 19.3 kN/m
Bending: Mf = 19.3 x 5.0² / 8 = 60 kN·m.
| Check | W200x27 | W250x33 |
|---|---|---|
| Mr | 88 kN·m | 134 kN·m |
| Bending used | 68% | 45% |
| Live load deflection | 12.0 mm | 6.3 mm |
| Limit (5,000 / 360) | 13.9 mm | 13.9 mm |
| Result | Passes | Passes |
Both beams pass with full lateral support. The W200x27 is closer to its deflection limit than its strength limit, which is typical. For comparison, the OBC's one-storey table (Table 9.23.4.3) allows a W200x27 about 6.3 m at a 4.2 m supported joist length, so a 5 m span sits comfortably inside the code table.
What this example leaves out is exactly what a designer adds: a post or bearing wall from the floor above, a point load from a stair or girder truss, an unbraced length where joists are hung beside the beam rather than sitting on it, and the posts and footings below. Our homeowner sizing guide and the Ontario span tables post cover when a table applies and when it doesn't.
Why online beam load calculators can mislead
A steel beam load calculator gives a tidy number, but it only knows what you type in. Watch for these gaps:
- Assumed bracing. Most assume a fully braced top flange. If your beam is unbraced for part of its length, real capacity can be much lower.
- Uniform load only. A post landing on the beam can control the design, and many calculators can't model it.
- Imperial or US basis. Calculators built on US codes use different load factors and resistance factors than the Ontario code and CSA S16.
- No supports. The beam is only as good as its bearing, posts and footings. A beam that holds the load on a lally column that can't is still a failure.
Use a calculator to understand why a size was chosen, not to choose it.
The supports carry the same load
Whatever the beam holds, it hands to its supports. Each end reaction on a simple span with uniform load is half the total load, and that reaction goes into a post, wall or pocket, then a footing. In the example above, each end carries about 48 kN factored. Check the posts with how much weight a lally column can support and the beam ends with steel beam bearing length on concrete and masonry.
Getting a beam sized and made
If you have a beam size on stamped drawings or a design, we can supply and fabricate it with the connections, plates and holes it needs, along with the posts. If you don't have a design yet, tell us about the project; engineering services are available for jobs that need them, and we provide free take-offs and quotes from drawings.
Send your drawings or beam schedule for a quote, or browse our steel products, including beams, posts and plates.
Frequently asked questions
How much weight can an I-beam hold?
There is no single figure. The same beam carries far less as the span gets longer, and its capacity also depends on how the load is applied, whether the top flange is braced, the deflection limit and the steel grade. A designer works out the capacity for your span and loads.
How do you calculate the load capacity of a steel beam?
Under CSA S16, the designer factors the loads, compares the bending moment and shear with the beam's factored resistances, checks lateral stability, then checks deflection under the specified loads. For a laterally supported Class 1 or 2 section, the bending resistance is 0.90 x plastic section modulus x yield strength.
Does a longer beam hold less weight?
Yes, and quickly. For a uniform load, the load a beam can carry in bending drops with the square of the span, and the load that causes a given deflection limit drops with the cube. Doubling the span cuts bending capacity to a quarter and stiffness-limited capacity to an eighth.
Can I use an online steel beam load calculator for my house?
Use it to learn, not to size a beam for construction. Calculators usually assume a uniform load, a braced top flange and one deflection limit, and they ignore point loads, posts, bearing and footings. Structural work in Ontario usually needs a permit and a designer or engineer.
