A steel beam's unbraced length is the distance between points where its compression flange is held against moving sideways or the beam is held against twisting. The longer that distance, the more likely the beam is to buckle sideways and twist (lateral-torsional buckling) before it reaches its full bending strength. So two identical beams on the same span can have very different capacities depending on how they're braced. Bracing comes from joists or deck fastened to the compression flange, or from dedicated braces such as kickers, knee braces and struts.
This post explains what unbraced length means, how bracing works and which details actually brace a beam. It's background for builders and owners; the bracing for a real beam is part of the engineer's design. For the effect on span in Ontario houses, see how far a steel beam can span without support.
Why I-shaped beams need bracing
A wide-flange (W) beam is built to bend about its strong axis: deep web, flanges far apart. Sideways, it is much weaker. A W200x27, for example, is roughly eight times stiffer in normal bending than sideways.
When the beam carries load, one flange is squeezed (the compression flange) and the other is stretched. The squeezed flange behaves like a slender column lying on its side. If it is free to move sideways, it can buckle, and because it's attached to the rest of the beam, the whole section twists as it goes. That's lateral-torsional buckling.
How likely it is depends on:
- Unbraced length. The longer the free length of compression flange, the lower the buckling load.
- Section shape. Wider flanges and heavier sections resist it better. Closed shapes such as rectangular HSS are far stiffer in torsion and rarely governed by it; see using HSS as a beam.
- Moment along the length. A beam whose bending moment varies between brace points does better than one under uniform moment. CSA S16 accounts for this with a moment gradient factor.
- Where the load is applied. A load hanging off the top flange with nothing holding that flange tends to make buckling worse than the same load applied at mid-depth or through the bottom flange.
CSA S16 reduces the beam's factored moment resistance as unbraced length increases. Our post on how much weight a steel beam can hold shows where that reduction sits in a capacity check.
Which flange needs bracing?
Bracing is about the compression flange, and which flange is in compression depends on how the beam is supported:
| Beam situation | Compression flange | Notes |
|---|---|---|
| Simple span under gravity load | Top | The usual case for floor beams |
| Cantilever | Bottom, near the support | Bottom flange bracing near the support is often critical |
| Continuous beam over an interior post | Bottom, over the post | Bottom flange is in compression for some distance each side |
| Roof beam under wind uplift | Bottom | A beam braced by roof deck on top can be unbraced for uplift |
That's why a beam that looks well braced (decking screwed down every few hundred millimetres) can still need kickers. The deck holds the top flange, but if the bottom flange is the one in compression, the deck alone may not stop it moving.
What counts as a brace
A brace has to stop the compression flange moving sideways, or stop the section rotating, at that point. Common ways:
- Joists or deck on the top flange. Floor joists, roof joists or steel deck fastened to the top flange brace it continuously or at close intervals. This is the most common bracing in houses and buildings.
- Joists framing into the beam. Joists or beams connected to the web with hangers, clip angles or shear tabs can provide bracing points, depending on the connection depth and stiffness. The designer decides whether they count. Our post on joist hangers for steel beams covers the residential connections.
- Kickers. Short diagonal angles from the bottom flange up to the floor or roof framing. They restrain the bottom flange and stop twisting.
- Knee braces. Diagonals from a column to the underside of a beam near the connection. They brace the beam and stiffen the frame.
- Struts and cross frames. Members between adjacent beams that hold them against moving sideways together, common in bridges, mezzanines and equipment platforms.
- Stiffeners and end connections. At supports, the beam has to be held against rolling over. End connections, bearing stiffeners and connections to columns provide that.
For frame bracing (X-braces, chevrons and portal frames that resist wind and seismic loads on the whole building), see our post on structural steel bracing types. That's a related but separate subject.
A brace has to be anchored to something
A common mistake is to "brace" two beams to each other, with nothing tying either of them to a stiff support. If both beams want to buckle the same way, the brace just goes along with them. A working brace connects back to something that can resist the force: a floor diaphragm, a braced frame, a wall or a heavily braced member.
Braces also need enough strength and stiffness. CSA S16 sets requirements for both. A light angle tacked in place may stop small movements but not the force a buckling flange generates. Size, connection and spacing of bracing are part of the engineer's design, not something to improvise on site.
Unbraced beams in houses
Ontario's Part 9 steel beam tables apply to beams with laterally supported top flanges. Sentence 9.23.4.3.(3) of the Building Code accepts a beam as laterally supported if wood joists bear on its top flange at intervals of 600 mm or less over its entire length, the load comes 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.
Situations that commonly fall outside that condition:
- Flush beams with joists hung from the sides in hangers rather than sitting on top
- Beams carrying a post load from above, with no joists attached near it
- Headers in open walls or garage door openings with nothing connected along the top flange
- Beams with a stretch of nothing attached, such as at a stair opening or a skylight
- Cantilevered beams supporting a deck or overhang
None of these are unusual, but they need a designer to check the unbraced condition. Sometimes the answer is a heavier or wider-flanged section, sometimes added blocking or kickers, sometimes a rectangular HSS instead of a W-shape.
Bracing during construction
A beam is often at its least braced while it's being installed. Set on its supports before the floor or deck is attached, it's unbraced over the full span, and it may carry construction loads, stacked materials or workers. Erectors use temporary bracing, guys and an installation sequence that ties beams into the structure as soon as possible. Our post on the steel erection sequence explains how that is planned on larger jobs.
In a house, the same idea applies at a smaller scale: get the beam fastened to its posts and the joists bearing and secured as part of the install, not days later.
Braces and beams fabricated together
Kickers, knee braces and strut connections are easiest to install when the clips and holes are fabricated in the shop. When your engineer's drawings show bracing, send them through our quote form: we prepare shop drawings in-house, fabricate beams and their bracing with CWB certified welding, prime or galvanize to suit, and supply and install structural steel across the GTA and Ontario. See our steel products for what we make.
Frequently asked questions
What is the unbraced length of a steel beam?
It is the distance between points where the beam's compression flange is held against moving sideways, or the beam is held against twisting. A beam with joists on its top flange every 600 mm has a short unbraced length; a beam with nothing attached between its supports is unbraced over its full span.
Why does an unbraced steel beam carry less load?
Under load, the compression flange of an I-shaped beam acts like a column. If nothing holds it sideways, it can buckle sideways and twist the beam at a load well below its full bending strength. This is lateral-torsional buckling, and CSA S16 reduces a beam's design resistance as its unbraced length grows.
What is a kicker on a steel beam?
A kicker is a short diagonal brace, usually an angle, running from the beam's bottom flange up to the floor or roof framing. It stops the bottom flange moving sideways and the beam twisting, which matters where the bottom flange is in compression or where the top flange alone doesn't stop rotation.
Does a steel beam need bracing in a house?
A beam sized from the Ontario Part 9 tables must be laterally supported. The code accepts wood joists bearing on its top flange at 600 mm or less, with the load coming through the joists and wood strips nailed along both sides of the flange. A beam that doesn't meet those conditions needs a designer's check.
