Primary Framing
Flange Braces in Metal Building Frames
Definition: A flange brace is a short steel angle connecting the inner flange of a rafter or column to a purlin or girt. It restrains the compression flange against lateral-torsional buckling, allowing slender frame members to reach their design strength.
Key facts
- Typically a small angle bolted between the frame flange and a purlin or girt.
- Positions are set by the frame design and shown on erection drawings.
- Often installed on both sides of the frame web.
- Missing flange braces can significantly reduce frame capacity.
- They work only if the connected purlin or girt is itself adequately restrained.
Role in the structure
Under gravity load, the bottom flange of a rafter near the knee is in compression, and under wind uplift long lengths of it can be. Without restraint, that flange can buckle sideways. The flange brace ties it diagonally to the purlin or girt, which is itself held by the roof or wall system and bracing, giving the flange a series of braced points.
Why do frames need flange braces?
Built-up frame members are deep and slender. Their strength in bending depends on keeping the compression flange from moving sideways and twisting. The outer flange is restrained by purlins and girts; the inner flange is not, unless flange braces are provided.
By bracing the inner flange at intervals, the engineer can use lighter frame sections safely. The frame design assumes those braces are present.
Where are flange braces located?
Flange braces are concentrated near knees and interior supports, where moments put the inner flange in compression. Additional braces along the rafter control the uplift case.
Locations are tied to purlin and girt positions, so any change to secondary framing layout must be checked for its effect on flange bracing.
What happens if flange braces are left out?
Omitting braces increases the unbraced length of the compression flange, which can greatly reduce the member capacity. This may not be obvious until the building sees significant snow, wind or crane load.
Inspectors should therefore treat flange braces as structural members, verify every one against the drawings and never allow them to be removed for services or racking without engineering review.
Materials & coatings
- Hot-rolled or cold-formed steel angles.
- Bolts matching the secondary framing connections.
- Galvanized or primed finish to match framing.
Design considerations
- Compression flange regions under gravity and uplift.
- Required brace strength and stiffness to the governing code.
- Coordination with purlin and girt spacing.
- Clearance for services, cranes and doors.
- Restraint capacity of the connected secondary member.
Fabrication
- Cut angles to length and punch holes.
- Punch matching holes in frame flanges and purlins or girts.
- Bundle and mark braces by frame line.
Erection
- Install flange braces as soon as purlins and girts are fixed.
- Check each location against the erection drawings.
- Do not cut or remove braces to fit services.
Inspection & quality
- Count and check every flange brace position.
- Verify bolt installation at both ends.
- Report any missing or bent braces to the engineer.
What to include in your RFQ
- Planned services that may conflict with brace locations.
- Liner or ceiling systems that affect bracing access.
- Coating requirement for braces in exposed conditions.
3D model & IFC of this component
Rotate the model, isolate the component and download its IFC4 file for your BIM coordination model.
IFC entity: IfcMember (BRACE). Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
Can a flange brace be moved to fit a duct?
Only with engineering approval. The engineer may relocate it or provide an alternative restraint.
Are flange braces needed on columns as well as rafters?
Usually yes, where the inner column flange is in compression, using the girts as restraint points.
Why are some braces on one side only?
The design determines whether one or two braces are needed at a point, based on forces and member geometry.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
