Portal frame

A portal frame is a structural frame of columns rigidly connected to rafters that resists load through moment connections. It is the standard system for sheds, garages, and wide clear-span spaces, and is designed to AS 4100 for steel, with stability and sway checks.

The defining feature is the rigid (moment) connection at the knee, where the column meets the rafter. Because that joint transfers bending, not just shear, the frame acts as one continuous member: load on the rafters is carried around the knee and down the columns to the footings, with no internal columns needed. That is what gives a portal frame its big clear span, ideal for a shed, workshop, or warehouse where you want an open floor.

The trade-off is that the rigid joints push outward (and the frame can sway sideways), so the design has to handle:

• Knee and apex connections: bolted moment joints (often with haunches) that must be made exactly as detailed, the bolt category and tightening matter (some are snug-tight, some fully tensioned).
• Footings: which can see uplift and outward thrust, not just downward load, so they are often larger than they look like they should be.
• Bracing: roof and wall bracing to resist wind and keep the frame stable out of plane.

For a builder or erector the practical points are to build the connections to the engineering detail (haunches, bolt grade, tightening, base-plate fixing), to set out the footings and holding-down bolts precisely (a portal frame is unforgiving of footing position errors), and to install all the temporary and permanent bracing, because a portal frame relies on its bracing for stability and is vulnerable until it is all in.

Also known as: Portal, rigid frame, knee-braced frame.

Category: Steel / Structure.

Related

• AS 4100 steel structures
• Snug-tight

See also

• Dead load

References

• AS 4100 Steel structures, Standards Australia (verified 2026-06-01)

---

Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.