Structural steel bracing is the set of diagonal members that stop a steel frame from swaying, racking or buckling sideways. The main types are braced frames (X-bracing, single diagonals, chevrons), knee braces at beam-to-column corners, kickers that hold a beam flange or wall in line, and horizontal bracing in roofs and floors. Which one a building uses is the engineer's call, based on wind, earthquake and stability loads.
This guide explains each type, how they behave, and what they mean for fabrication.
Why steel frames need bracing
Most steel beams are connected to columns with simple bolted connections: shear tabs, clip angles or end plates that carry vertical load but let the joint rotate. A grid of beams and columns joined this way is stable under gravity but behaves like a pin-jointed box sideways. Push on it and it racks into a parallelogram.
Something has to resist that push. Under Part 4 of the Ontario Building Code, buildings are designed for wind and earthquake loads as well as gravity, and every building needs a complete path that carries those horizontal forces down to the foundations. In steel, the main options are:
- Braced frames: diagonal members in selected bays, forming triangles that cannot rack.
- Moment frames: rigid beam-to-column connections, so the frame resists sway by bending. They leave bays open but use heavier members and more expensive connections.
- Walls: concrete, masonry or steel plate walls that take the lateral load, with the steel frame carrying gravity only.
So a "steel structure without bracing" is possible, but only when one of the other two systems is doing the job. Bracing is also needed during erection, before the permanent system is complete, which is why erectors install temporary guys and braces.
Types of braced frames
| Type | Layout | Notes |
|---|---|---|
| X-bracing (cross bracing) | Two diagonals crossing in one bay | Very common in warehouses and low-rise buildings; often tension-only |
| Single diagonal | One diagonal per bay | Simple; the diagonal must work in both tension and compression |
| Chevron (inverted V) | Two braces meeting at mid-span of the beam above | Leaves room for a door or corridor below the beam |
| V-bracing | Two braces meeting at mid-span of the beam below | Similar to chevron, upside down |
| Eccentric bracing | Braces offset from the joints, leaving a short "link" | Used in seismic design; the link yields to absorb energy |
| Buckling-restrained bracing | A steel core inside a casing that stops it buckling | Specialized seismic system with manufactured braces |
Braces are usually angles, double angles, HSS, W-shapes or steel rods, connected through gusset plates at the beam-column joints. Our post on base plates, end plates and gussets explains the gusset itself.
Tension-only vs tension-compression bracing
Tension-only bracing uses slender members, such as rods with turnbuckles, flat bar or single angles, that buckle easily in compression. They are installed as X-bracing so that whichever way the wind blows, one diagonal is pulling. The other one goes slack. It is light and simple to fabricate, and common in pre-engineered buildings and light industrial frames.
Tension-compression bracing uses stiffer members, typically HSS or W-shapes, that can push as well as pull. A single diagonal or chevron works this way.
The 2024 Ontario Building Code (Table 4.1.8.9.) lists both kinds separately among the steel seismic force resisting systems, with different limits on where each may be used, so the choice is a design decision, not a fabrication preference.
Seismic categories for braced frames
For earthquake design, the code recognizes several classes of steel braced frame, each designed and detailed to CSA S16. Table 4.1.8.9. of the 2024 OBC includes, among others:
- Moderately ductile concentrically braced frames
- Limited ductility concentrically braced frames
- Ductile eccentrically braced frames
- Ductile buckling-restrained braced frames
- Conventional construction of braced frames
The more ductile classes allow lower design forces but come with stricter detailing: specific brace slenderness, gusset geometry, and connections designed for the brace's capacity. Seismic demand depends on the site and the building, and the category is always set by the engineer and shown on the drawings. It directly affects how the gussets and connections are detailed and fabricated.
Knee braces
A knee brace is a short diagonal between a column and a beam, a short distance from the joint. It forms a small triangle in the corner. Knee braces show up in:
- Portal frames and canopies, where full-height bracing would block the opening.
- Steel decks, porches and carports, to stiffen post-and-beam frames.
- Pipe racks and equipment supports.
- Older industrial buildings and some mezzanines.
A knee brace stiffens the frame and shortens the beam's effective span, but it is not free. It pushes into the column partway up its height, so the column must be designed for that bending. In heavier buildings, a proper braced bay or moment connection is usually more efficient. Adding a knee brace to an existing structure needs an engineer's check of both members it connects to.
Kickers
A kicker is a small diagonal brace, often an angle, that holds something in line rather than carrying the main lateral load. Common uses:
- Bottom flange kickers from a beam's lower flange up to the deck or joist above, to stop the compression flange from buckling sideways where it is in compression, such as over an interior support.
- Wall kickers from the top of a masonry or partition wall to the structure above, to brace the wall out of plane.
- Top-of-steel kickers for lintels, parapets and roof screens.
Bottom flange bracing ties into the idea of unbraced length, which controls how much a beam can carry before it buckles sideways. Our post on lateral bracing and unbraced length of steel beams covers that in detail.
Horizontal bracing in roofs and floors
Wind hitting a building wall has to travel through the roof or floor to the braced frames. That job is done by a diaphragm: usually the steel roof deck or the concrete floor slab, fastened to the steel. Where the deck cannot do it (open roofs, large openings, crane buildings, roofs with light or no deck), the engineer adds horizontal bracing: X-bracing in the plane of the roof between the trusses or beams.
Other horizontal and secondary bracing includes:
- Bottom chord bracing and bridging for trusses and joists, which keeps long slender members straight.
- Flange braces from purlins to rafters in pre-engineered buildings.
- Sag rods in girts and purlins.
What bracing means for fabrication
For the fabricator, bracing is mostly about gussets and connections. The details to get right:
- Work points. Brace centrelines meet at defined points; the gusset is sized and shaped to suit. Shop drawings show these geometry points clearly.
- Connection forces. The engineer of record gives the brace forces or designs the connection, and in seismic frames the connection may be sized for the brace's capacity rather than the calculated load. See types of structural steel connections.
- Slotted holes and turnbuckles for rod bracing, so the braces can be tensioned and the frame plumbed.
- Fit-up tolerance. Braces are installed into an already standing frame, so the holes must line up after the columns are plumbed.
- Erection sequence. Braced bays usually go up first so the rest of the frame can be stabilized against them.
Bracing members are also among the first things people want to remove or relocate during renovations, to open up a wall or add a door. Never cut or remove a brace without an engineer's design for whatever replaces it.
Bracing steel built to your drawings
We fabricate bracing members, gusset plates, knee braces, kickers and the frames they belong to, cut, drilled and welded in our CWB certified shop to shop drawings we prepare in-house, primed or galvanized to suit. Send your structural drawings for a free takeoff and quote, or see our steel products.
Frequently asked questions
What is X-bracing in a steel structure?
X-bracing is a pair of diagonal members crossing in a bay between two columns. It stops the frame from racking sideways under wind or earthquake load. With slender rods or angles, one diagonal works in tension in each direction; with stiffer HSS or W-shapes, both diagonals can work.
What is a knee brace in steel construction?
A short diagonal member between a column and a beam, near their joint. It stiffens the corner so the frame resists sideways load, and it shortens the beam's effective span. It also bends the column below the brace, which the engineer has to design for.
What is a kicker on a steel beam?
A kicker is a small diagonal brace, often an angle, running from a beam's bottom flange or from the top of a wall up to the structure above or beside it. It keeps the flange or wall from moving sideways or buckling. Its size and connections come from the engineer.
Can a steel structure stand without bracing?
Yes, if it uses another lateral system: rigid moment connections, concrete or masonry shear walls, or a steel plate wall. A steel frame with simple bolted connections and no bracing or walls has almost no resistance to sideways load and can collapse during erection or in a storm.
