A portal frame structure uses columns and rafters connected by moment-resisting joints to form a stable frame. It is widely used in warehouses, workshops, factories and other single-storey buildings requiring large clear spaces.
Main components
- Columns transfer frame forces to foundations.
- Rafters span between columns and support the roof.
- Knee connections join columns and rafters, often with haunches.
- Ridge connections join rafters near the roof apex.
- Purlins and side rails support cladding.
- Roof and wall bracing provide longitudinal stability.
Types of portal frames
Single-span
Provides clear space between two column lines and is common in workshops and warehouses.
Multi-span
Uses internal columns to support multiple roof spans. Column positions must suit circulation and operations.
Mono-pitch
Has a roof sloping in one direction and may suit extensions or buildings with drainage constraints.
Crane-supporting
Must be designed for crane vertical, horizontal and dynamic actions and their effects on columns and foundations.
Load path and design workflow
Cladding transfers loads to purlins and side rails, then to rafters and columns. Rigid joints develop bending and shear. Longitudinal stability is commonly provided by roof and wall bracing.
- Set building width, eaves height, roof pitch and bay spacing.
- Determine dead, roof imposed, wind, crane and other loads.
- Analyse governing combinations and second-order effects where required.
- Design rafters and columns for axial force, bending, shear and stability.
- Design knee, apex, splice and base connections.
- Design purlins, side rails, bracing, anchors and foundations.
- Check deflection, drift, drainage, fire and corrosion protection.
Construction and erection checks
Verify foundation levels and anchor-bolt positions. Follow the approved lifting sequence and use temporary bracing. Inspect bolts, welds, alignment and permanent bracing before removing temporary supports. Never modify members or connections without written approval.
Related reading and references
See beam types and column design. Consult the BIS standards catalogue for current Indian steel design standards.
FAQs
Where are portal frames used?
Warehouses, factories, workshops, hangars and other single-storey buildings requiring open space.
What is a haunch?
A haunch increases member depth near a rigid joint where bending moments are often high.