There is no single maximum span for a steel beam. How far it can go without support depends on the load it carries, its depth and weight, how well it's braced sideways and how much it's allowed to deflect. In Ontario houses, the Part 9 span table for steel floor beams tops out at 11.4 m (about 37 ft), for a W310x39 carrying one storey with a 2.4 m supported joist length. Engineered beams in commercial buildings span further, and heavier loads shorten every one of those numbers.
"Without support" also means two different things, and mixing them up leads to bad decisions. This guide covers both.
Two meanings of "without support"
When people ask how far a steel beam can span without support, they usually mean one of these:
- No posts or columns in between. This is the clear span: the distance between the supports at each end. Longer clear span means more bending and more deflection.
- No bracing along its length. This is the unbraced length: how far the beam's compression flange runs without anything stopping it from moving sideways. A wide-flange beam with a long unbraced length can twist and buckle sideways at a load well below what the same beam carries when braced.
A beam can have one problem without the other. A beam carrying a joisted floor may have a long clear span but be braced at close intervals by the joists sitting on it. A beam used as a header in an open wall, or a beam carrying a point load from a post above with nothing attached to it, can have a modest span but no bracing at all.
How far steel beams span in Ontario houses
For houses under Part 9 of the 2024 Ontario Building Code, Table 9.23.4.3 sets maximum spans for W-shape beams supporting floors. It assumes simple spans, a laterally supported top flange, 350 MPa steel and a deflection limit of span/360.
The range across the table:
| Situation | Shortest listed span | Longest listed span |
|---|---|---|
| W150x22 (smallest section) | 3.2 m (two storeys, 6.0 m joist length) | 5.5 m (one storey, 2.4 m joist length) |
| W310x39 (deepest section) | 6.7 m (two storeys, 6.0 m joist length) | 11.4 m (one storey, 2.4 m joist length) |
Source: Table 9.23.4.3, 2024 Ontario Building Code (O. Reg. 163/24).
Notice how much the span drops as the load goes up. The same W310x39 goes from 11.4 m to 6.7 m between the lightest and heaviest cases in the table. The question is never just "how far can this beam span", but "how far can it span carrying this". Our guide to steel beam span tables for Ontario homes explains the supported joist length and how to read the full table.
For spans past the table, such as 40 or 50 ft, see can a steel beam span 30, 40 or 50 feet.
What limits a clear span
Several checks set the maximum span, and on a given beam any one of them can govern:
- Bending strength. Bending moment grows with the square of the span. Past a certain length, the beam can't resist the moment from its load.
- Deflection. Sag grows with the fourth power of the span. For floor beams, deflection limits often govern before strength, especially where finishes or walls sit on the beam.
- Vibration. Long, light floors can feel bouncy even when they're strong and stiff enough on paper. Designers check long-span floors for it.
- Shear and bearing. At the ends, the web must carry the shear, and the beam must bear on enough length of masonry, concrete or steel. See steel beam bearing length on concrete and masonry.
- Lateral bracing. Covered in the next section, and the one most often overlooked.
- Practical limits. Depth available for the beam, weight to lift and the length that can be delivered and moved into place.
Unbraced length: why bracing matters as much as span
A wide-flange beam is very strong about its deep axis and much weaker about its narrow one. Under load, the top flange is squeezed like a column. If nothing holds it sideways, it can buckle sideways and drag the beam into a twist. Engineers call this lateral-torsional buckling. The longer the unbraced length, the lower the bending resistance the beam can actually reach, and CSA S16 reduces the design resistance accordingly.
In practice:
- Part 9 tables assume the beam is braced. Under Sentence 9.23.4.3.(3), a beam can be considered laterally supported if wood joists bear on its top flange at intervals of 600 mm or less over its entire length, the load is applied through those joists, and 19 mm by 38 mm wood strips in contact with the top flange are nailed on both sides of the beam to the bottom of the joists. The code's explanatory note says other positive methods of lateral support are acceptable.
- A beam that doesn't meet that condition is outside the table. Flush beams with joists in hangers, beams carrying point loads, headers in open walls and beams with nothing attached for part of their length all need a check of the unbraced condition.
- There are several fixes. Adding bracing points, choosing a section with wider flanges, using a heavier section, or switching to a closed shape like rectangular HSS, which is far less prone to twisting. See can HSS be used as a beam.
Bracing also matters during construction. A beam set on its supports before the floor is attached is unbraced along its whole length. Erectors install temporary bracing until the permanent framing is in.
What about a beam supported at only one end?
A cantilever, a beam that projects past its support with nothing under the end, is a different case. The span tables cover simple spans only. Cantilevered beams for balconies, canopies and overhangs deflect far more than simple spans of the same length, need a firm back span or fixed connection to resist the overturning, and are always engineered.
When a post is the better answer
If a beam is getting very deep, very heavy or hard to brace, the cheapest solution is often a post. Splitting one span into two cuts the bending moment in each beam to a quarter of the original. In a basement or a garage, a single well-placed steel column on a proper footing may let you use a much lighter beam. Our homeowner's steel beam sizing guide walks through those trade-offs.
Whichever way it goes, beams outside the Part 9 tables, and beams that don't meet the table's bracing conditions, need an engineer's design and usually a building permit.
Beams and posts fabricated to your drawings
We supply wide-flange beams, HSS beams and steel posts cut to length and fabricated with plates, holes and connections to your shop drawings, with CWB certified welding and delivery across the GTA and Ontario. We also install basement beams and posts. If you need help with the design, engineering services are available for projects that need them.
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Frequently asked questions
How far can a steel beam span without support?
There is no fixed maximum. In Ontario houses, the Part 9 table for steel floor beams goes up to 11.4 m (about 37 ft) for a W310x39 carrying one storey with a short supported joist length. Engineered beams in commercial buildings span much further. The limit for any beam is set by its load, depth, bracing and deflection.
What is the maximum span of a steel beam without a column?
It depends on the section and the load. A deeper, heavier beam spans further, and the span tables show how quickly the allowable span drops as the load on the beam increases. Beyond the tables, an engineer sizes the beam to CSA S16.
What does lateral support mean for a steel beam?
It means the top (compression) flange is held so it cannot move sideways. Without it, a wide-flange beam can twist and buckle sideways before reaching its full strength. In Ontario houses, wood joists bearing on the top flange at 600 mm or less, with wood strips nailed alongside the flange, count as lateral support.
How far can a steel I-beam span?
As far as its depth and the loads allow. In Part 9 house tables the listed W-shapes span from about 3.2 m to 11.4 m depending on section, storeys and joist length. Longer spans use deeper beams, joists or trusses, designed by an engineer.
