Building components · Frame Members
Portal Frame Haunch
Explains the haunch as the knee of a rigid frame, and why it exists for structural rather than fabrication reasons.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a haunch is
A portal frame works as a single rigid shape rather than as a rafter sitting on a pair of legs. Because the corner is rigid, bending does not stop there — it passes around the knee, and the knee is where the bending in the whole frame is at its greatest. The haunch is the position where the member is deepened to take it, commonly by welding a cut length of section beneath the rafter so that the frame is deepest exactly where it is worked hardest. It is a discrete fabricated piece that makes the corner continuous — drawn, cut and welded as an item of its own even though what it does is behave as frame.
The haunch is also where the frame is bolted together. Deepening the member gives a face for an end plate and room for the bolts that join the rafter to the leg, so the position serves both the structural demand and the practical one of getting a frame made in a shop and assembled on site. Its presence shapes the inside of the building at the eaves, which is why it governs where linings, services and the usable envelope of the space begin.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a support bracket. A bracket is a fixed piece that carries something back into a structure it is attached to; the haunch is part of the frame itself, formed from the same members it joins, and the frame's continuity through the corner is the whole point of it.
- It is not a structural bearing. A bearing is the interface through which one member is supported by another and its load passes downward; a haunched eaves connection is arranged to carry bending around the corner as well, so the leg and the rafter act as one continuous frame rather than one resting on the other.
- It is not a column base plate. Both are connection positions in a portal frame, but the base plate sits at the foot and spreads load into the foundation, while the haunch sits at the knee and takes the bending that a rigid corner generates.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- haunch
- eaves haunch
- haunched knee
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Whether the frame is designed to be rigid at that corner, because a connection arranged to transfer bending and one arranged only to transfer a reaction are different things entirely.
- The end plate and bolt arrangement, whose stiffness is part of the frame's behaviour rather than a detail beneath it.
- Restraint to the part of the section that is in compression along the haunch, which is commonly provided by tying back to the purlins or side rails.
- The fact that it is made in a fabrication shop, so what exists on site is what was drawn and cutting into it is not a site decision.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Bending peaks at the knee of a rigid frame, so the connection there carries the frame's heaviest demand rather than a simple end reaction.
The haunch is bolted up at height during erection, and access to make and later to inspect that connection is a distinct consideration from the connection itself.
A bolted end plate connection depends on the two faces meeting as drawn, and fabrication and erection tolerances meet each other at this position.
What a portal frame is required to do in fire, and what protection applies at the eaves connection, are design and regulatory matters rather than properties that can be read from the haunch itself.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Clarifications
Commonly misunderstood
- That the haunch is there to make the connection easier to bolt. It is there because the bending in a rigid frame peaks at the eaves; the room it gives for bolts follows from that rather than causing it.
- That a haunch can be trimmed back to gain headroom or clearance inside a building. It is part of the frame's structural section at the point where the frame is worked hardest.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is this frame designed as rigid at the eaves, and what does the haunch connection transfer?
- What restrains the haunch along its length, and does any proposed work disturb that restraint?
- Have the bolted connections at the knee been made as drawn, and what record exists of that?
- What protection applies at this connection, and is it continuous around it?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry describes a connection position. It states no section, depth, bolt arrangement, capacity, span or fire rating, and none can be inferred from it.
- Cutting, drilling or modifying a haunch changes the stiffness of the corner and therefore the bending everywhere else in the frame, which is why it is qualified design work rather than a local modification.
Structural Frame Members
The columns, beams, joists, rafters, plates and bracing that make up a load-carrying frame, identified by the position each occupies.
Browse all frame members entries →