Structure and support · Frames & Load-Bearing Walls
Portal Frame System
This entry explains why stability across a portal frame building comes from the stiffness of its own joints while stability along the building comes from bracing, and why the purlins, rails and sheeting are structural participants rather than supports for the covering.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What portal frame is
A portal frame is a rigid frame: a column and a rafter joined so that the connection between them transmits bending rather than simply passing a reaction. Frames of that kind are repeated along the length of a building and linked by secondary members, so the structure is a row of stiff planes tied to one another rather than a grid of posts and beams. The usual result is a clear internal volume with no intermediate floor and few or no columns inside it.
The stiffness the frame depends on is concentrated at the knee, where column meets rafter, and at the apex where the rafters meet. Those connections are commonly deepened by a haunch so that the joint is stiffer than the members it joins, which is why a portal frame appears thickened exactly where a simply supported beam would look at its slenderest. What happens at the foot of the column matters just as much: whether the base is held against rotation changes how the whole frame leans under load.
The nearest sibling is the structural steel frame system, and the knee joint settles which one is in front of you. In a portal frame the column-to-rafter connection is stiff and is itself the mechanism resisting sway across the building; in a general steel frame that connection is normally simple, and sway is resisted by a braced bay or a core elsewhere. Standing inside, ask whether the joints are visibly haunched and continuous, or whether something else in the plan is doing that work.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- steel portal frame
- rigid frame building
- single-storey portal frame
- pitched portal frame
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Enclose a large clear internal volume without intermediate columns and without a floor above.
- Resist leaning across the building through the stiffness of the frame's own joints rather than through added bracing.
- Carry the roof and wall covering on secondary members that run between frames.
- Repeat one structural arrangement along a building so that the same connection detail recurs throughout.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportColumns
The upright members of each frame. They carry vertical load from the rafters and, because the knee is stiff, they also take bending from anything that pushes the building sideways, which is why they are rarely uniform over their height.
- Primary supportRafters
The inclined members running from knee to apex. They carry the roof loads brought to them by the purlins and pass bending into the knee, so the rafter and the connection at each end are designed as one thing rather than separately.
- AttachmentHaunched knee and apex connections
The bolted joints that make the frame rigid. A haunch deepens the member locally so the connection is stiffer than what it joins, and it is these joints, not the members alone, that give the frame its resistance to leaning.
- Edge and terminationBase connections
Where each column meets its foundation. Whether the base is arranged to hold the column against rotation or to let it turn changes how the frame above behaves, so the base detail belongs to the frame rather than to the groundworks.
- Primary supportPurlins and side rails
Light members running between frames. They carry the covering, but by connecting to rafters and columns along their length they also restrain those members from moving sideways, which is part of how the frame works.
- Primary supportBracing planes
Diagonal arrangements in the roof plane and in the walls that hold the row of frames upright along the building. Without them the frames are stiff in their own plane and have nothing keeping them from folding along the length.
- SubstrateSheeting plane
The surface presented by the secondary members for the roof and wall covering. Where the covering is fixed directly to purlins and rails it also stiffens them, so the envelope and the structure are not entirely separable here.
- Edge and terminationLarge door and opening trimming
Members added where a wide door or a wall opening interrupts the side rails and possibly a braced bay. The trimming carries the covering around the hole and reinstates whatever the opening displaced.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
Stability here is directional, and the two directions are held by entirely different means. Across the building the frame stands because the knee and apex joints keep the angle between column and rafter; nothing else is doing that work. Along the building the frames would fold like a row of cards without the bracing planes and the members tying them together. Treating the two as one is how a straightforward alteration turns into a stability problem.
Purlins and side rails are not there only to hold the covering. By connecting to the rafter and the column along their length they restrain those members from buckling sideways, and the frame is arranged on the assumption that restraint is present. Taking out a run of purlins to form an opening, or leaving a bay unsheeted for access, therefore changes the member as well as the envelope, and neither can be considered on its own.
What happens at the foot of the column travels through the whole frame. A base arranged to resist rotation attracts bending into the foundation and holds the eaves steadier; a base free to turn hands that work back to the knee. The frame, the base detail and the foundation are therefore a single decision, and a base altered on site because it was awkward changes the frame it belongs to.
Anything hung from the structure joins the same load path. A hoist, an inserted platform, plant sitting on the rafters or a wide door cut into a braced bay all deliver their effects into members already working over their full length. Because the arrangement repeats, a change at one frame is seldom local: the bracing that served it may sit in another bay altogether.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Structural steel is the family most often encountered in frames arranged this way, and engineered timber is encountered in the same geometry. What any member has to be follows from the engineer's design for that frame rather than from the family it belongs to.
Primary support
Cold-formed galvanised sections are commonly encountered as purlins and side rails. Because these members restrain the frame as well as carrying the covering, what may stand in for them is a structural question rather than a supply one.
Protection
Coating families are commonly encountered on exposed steelwork. What any coating achieves depends on exposure, on the preparation beneath it and on the manufacturer's documentation, and no performance of any kind is stated here.
Finish surface
Sheeting and panel families are commonly encountered on the secondary members. Where a covering is fixed directly to purlins and rails it interacts with them structurally, which makes the choice a matter for the designer as well as for appearance.
Substrate
Concrete families are commonly encountered at the bases, where the frame hands over its load and, in some arrangements, bending as well. What the foundation has to do follows from the frame's design and is settled by the engineer.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Column bases and foundations
Each column arrives at the ground as a concentrated point, sometimes with bending and outward push as well as weight. The foundation arrangement therefore follows from the base detail chosen for the frame, and the two are settled together.
Read about Pad Foundation →Internal floor slab
The floor is usually laid independently of the frame and around the bases rather than onto them. Where the slab is relied on to tie the feet of opposing columns against outward push, it stops being a surface and becomes part of the structure.
Read about Ground-Bearing Slab →Roof build-up on the purlins
A layered roof assembled on the purlins is carried entirely by them, and its fixings reach the same members that restrain the rafters. Where the assembly stands off the purlins on spacers, that restraint has to be reconsidered rather than assumed.
Read about Twin-Skin Metal Roof →Inserted platform within the frame
A platform put into the clear volume either stands on its own supports or leans on the frame. If it is tied to the columns it shares the building's stability arrangement, which is a change to a structure designed without it.
Read about Mezzanine Structure →Bracing along the building
The braced bays and the roof plane bracing are the only things holding the frames upright lengthways. Removing a diagonal to make room for a door or a duct is an alteration to the building's stability, not to a wall.
Read about Lateral Stability →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Durability
- Steelwork in a building of this kind is often visible and reachable, so its condition can be seen, but the base of a column sitting in a wet or salted floor area is the part that is least visible and most exposed. Protection there is a design decision rather than a maintenance one.
- Movement
- A long building expands and contracts along its length, and the frames, the sheeting and the floor beneath do so at different rates. Where that movement is allowed to occur, and what is expected to slide rather than crack, is arranged by the designer.
- Thermal
- Every fixing that passes from the covering into a purlin or rail crosses the insulating layers, and there are a great many of them. How that is handled in a given build-up is assessed for the whole assembly by a qualified professional, and no thermal outcome is stated here.
- Fire
- Fire performance is a property of a tested arrangement, and none is claimed in this entry. What is relevant at this level is that the frame, its connections and any applied treatment are considered together rather than member by member.
- Interfaces
- The demanding places are the base, the eaves, the braced bays and any wide opening. Each is a point where the frame, the envelope and something else in the building have to be resolved as one detail rather than in sequence by different trades.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Is the column base arranged to resist rotation, and does the foundation reflect that assumption?
- Which bays are braced, and are the doors and openings kept clear of them?
- What is the covering fixed to, and is that member relied on to restrain the frame?
- Is the floor slab expected to tie the column feet against outward push, or is it independent of the frame?
- Where might a platform, a hoist or plant be added later, and has the frame been arranged for that possibility?
- How is movement along the length of the building accommodated, and what is expected to move?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Purlins and side rails are treated as cladding supports, when they also restrain the members they are fixed to.
- A frame is assumed to be stable in every direction, when its own joints only hold it across the building.
- The floor slab is read as a finish, although in some arrangements it is holding the feet of the columns apart.
- A wide door is thought of as an opening in cladding, when it may interrupt the only braced bay on that side.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is the frame relying on for stability along the length of the building?
- Which members are restrained by the purlins and rails, and what happens if a run is removed?
- Is the base intended to be held against rotation, and how is that achieved at the foundation?
- Can anything be hung from the rafters, and if so where and under what limitations?
- Where can a wide opening be formed without disturbing the bracing arrangement?
- How is the slab related to the frame, and does it carry any structural duty?
What this page does not do
- No span, capacity, frame arrangement or connection design is stated here; those belong to a qualified engineer for the specific building.
- Removing purlins, sheeting or bracing changes how the frame behaves and is not an envelope alteration.
- Fire and thermal outcomes depend on complete tested assemblies and their fixings, and nothing of that kind is asserted in this entry.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Primary Structural Frames and Load-Bearing Wall Systems
Complete primary load paths from roof to foundation in masonry, timber, steel and concrete, together with the stability system without which none of them stands up.
Browse all frames & load-bearing walls entries →