Portal frame
Portal frames were first developed during the Second World War and became popular in the 1960s. They are now commonly used to create wide-span enclosures such as; warehouses, agricultural buildings, hangars, entertainment and sport venues, factories, large retail units, and so on, where a clear space is required uninterrupted by intermediary columns.
They were originally used because of their structural efficiency, meaning that large spaces could be enclosed with little use of materials and for a low cost. Tata Steel suggest that today, 50% of constructional steel used in the UK is in portal frame construction.
(Ref. SteelConstruction.info, Portal Frames.)
Portal frames are a type of structural frame, that, in their simplest form, are characterised by a beam (or rafter) supported at either end by columns, however, the joints between the beam and columns are 'rigid' so that the bending moment in the beam is transferred to the columns. This means that the beam can be reduced in sectional size and can span large distances. Typically, the joint between the beam and the columns is made 'rigid' by the addition of a haunch, bracket, or by a deepening of the section at the joints
Portal frames are generally fabricated from steel, reinforced precast concrete, or laminated timber sometimes referred to as 'glulam'.
Where a pitch is required, portal frames can have a mono pitch, or can have a double pitch with a rigid joint at the apex. Other forms include; tied portal frames, propped portal frames and multi-span portal frames which can cover very large areas. Where the portal frame includes a pitch, the wider the span of the frame, the higher the apex.
To reduce the overall height, a curved rafter might be adopted, or a mansard form. This can also help with water runoff, which can be significant on large roofs. A curved, or mansard form increases the pitch of the roof towards the eaves, where the runoff is likely to be at its greatest.
Generally, a building structure will be formed by a series of parallel portal frames running down the length of the buildings, typically 6 - 8 m apart. A secondary framework of purlins fixed to the rafters and rails fixed to the columns provides support for cladding. Portal frame structures are often clad with prefabricated composite metal panels, incorporating insulation. Masonry cladding may be provided at low level to give greater resilience and security.
They are a deceptively simple structural form, but care must be taken, particularly in the design of joints to ensure that appropriate restraint is provided to prevent buckling. Bracing is also required to give the row of parallel frames lateral stability, and this may be provided in an end bay or intermediary bay between the frames.
Portal frames can be a simple and rapid form of structure to erect, creating a wide, clear-span, weather-proof enclosure at low cost, with little material. They tend to be lightweight and can be fabricated off-site, then bolted to a substructure. The portal frames themselves may be left exposed to the internal space, and if carefully designed can be very beautiful.
[edit] Related articles on Designing Buildings Wiki
- Arches.
- Barrel vault.
- Braced frame.
- Concrete.
- Conoid shell.
- Folded plate construction.
- Hyperbolic paraboloid.
- Long span roof.
- Pendentive dome.
- Rafter.
- Shell roof.
- Skeleton frame.
- Steel frame.
- Structural engineer.
- Structural steelwork.
- Superstructure.
- Substructure.
- Timber.
- Truss.
- Types of frame.
[edit] External references
Featured articles and news
Professional practical experience for Architects in training
The long process to transform the nature of education and professional practical experience in the Architecture profession following recent reports.
A people-first approach to retrofit
Moving away from the destructive paradigm of fabric-first.
International Electrician Day, 10 June 2025
Celebrating the role of electrical engineers from André-Marie Amperè, today and for the future.
New guide for clients launched at Houses of Parliament
'There has never been a more important time for clients to step up and ...ask the right questions'
The impact of recycled slate tiles
Innovation across the decades.
EPC changes for existing buildings
Changes and their context as the new RdSAP methodology comes into use from 15 June.
Skills England publishes Sector skills needs assessments
Priority areas relating to the built environment highlighted and described in brief.
BSRIA HVAC Market Watch - May 2025 Edition
Heat Pump Market Outlook: Policy, Performance & Refrigerant Trends for 2025–2028.
Committing to EDI in construction with CIOB
Built Environment professional bodies deepen commitment to EDI with two new signatories: CIAT and CICES.
Government Grenfell progress report at a glance
Line by line recomendation overview, with links to more details.
An engaging and lively review of his professional life.
Sustainable heating for listed buildings
A problem that needs to be approached intelligently.
50th Golden anniversary ECA Edmundson apprentice award
Deadline for entries has been extended to Friday 27 June, so don't miss out!
CIAT at the London Festival of Architecture
Designing for Everyone: Breaking Barriers in Inclusive Architecture.
Mixed reactions to apprenticeship and skills reform 2025
A 'welcome shift' for some and a 'backwards step' for others.
Comments