How a Portal Frame Works

A portal frame is a rigid frame made up of columns and rafters connected by moment-resisting joints at the eaves (where the column meets the rafter) and sometimes at the apex (where two rafters meet at the ridge). The rigid connections mean the frame acts as a single structural unit, transferring loads from the roof down through the rafters, through the eaves connection, into the columns, and into the foundations.

The key difference between a portal frame and a simple beam-and-column structure is the rigid eaves connection. In a simple structure, the beam sits on top of the column and the joint is pinned, meaning it can rotate freely. In a portal frame, the eaves joint is rigid, so the rafter and column bend together. This means the bending moment from the roof loads is shared between the rafter and the column rather than being concentrated at mid-span, which allows lighter, more efficient sections to be used.

At the eaves, where the bending moment is highest, the section is deepened by welding a tapered haunch to the underside of the rafter. This increases the moment capacity exactly where it is needed most, without making the entire rafter heavier.

Primary Structure: Columns, Rafters & Haunches

Columns

The vertical members supporting the roof. Portal frame columns are almost always Universal Beam (UB) sections rather than Universal Columns (UC), because UBs are more efficient in bending about their major axis, which is the primary loading direction. Column sizes typically range from 305x165 UB for small frames up to 762x267 UB for large-span or high-eaves buildings. The column height (eaves height) is measured from the finished floor level to the underside of the haunch at the column face.

Rafters

The inclined members forming the roof slope. Like columns, rafters are UB sections. The roof pitch is typically between 5 and 10 degrees, with 6 degrees being the most common in UK practice. The rafter size is driven primarily by the span, with bay spacing and loading as secondary factors. For a typical 25m span building, rafters are commonly in the range of 457x191 to 533x210 UB.

Eaves Haunch

The deepened section at the eaves connection, formed by cutting a tapered piece from a UB (usually the same section as the rafter) and welding it to the underside of the rafter. The haunch length is typically about 10% of the span, and the haunch depth at the column face is approximately twice the rafter depth, tapering back to rafter depth at the cut end. The haunch allows the eaves connection to resist the large bending moments at this critical location without needing an excessively heavy rafter section for the full length of the roof.

Apex Haunch

A much smaller haunch at the ridge, where the two rafters meet. The apex haunch is typically only about 50 to 150mm deeper than the rafter itself, and its purpose is mainly to provide space for the bolted apex connection rather than to increase structural capacity. Some apex details do not use a haunch at all, relying instead on a direct end plate connection between the two rafters.

Secondary Steelwork: Purlins, Side Rails & Eaves Beams

Purlins

Light cold-formed steel sections (Z-purlins or C-purlins) that span between rafters, supporting the roof cladding. Purlins are typically spaced at 1.5 to 2.0 metre centres, and their size depends on the span between frames (bay spacing), the cladding weight, and the wind and snow loading. Purlins also serve a critical structural role beyond just supporting the cladding: they provide lateral restraint to the compression flange of the rafter, preventing lateral-torsional buckling.

Side Rails (Cladding Rails)

The wall equivalent of purlins. Side rails span horizontally between columns, supporting the wall cladding. They are the same type of cold-formed section as purlins and provide lateral restraint to the column flanges.

Eaves Beam

A member running along the eaves between adjacent portal frames, connecting the tops of the columns. The eaves beam serves several functions: it provides lateral restraint to the column heads, supports the gutter, and acts as part of the roof bracing system. Eaves beams are typically cold-formed sections or light hot-rolled angles, depending on the loads and span.

Sag Rods and Anti-Sag Systems

Thin steel rods (typically 12mm diameter) connecting purlins at mid-span to prevent them from sagging under their own weight before the cladding is fixed. Once the cladding is in place, it provides diaphragm action that prevents purlin sag. Proprietary anti-sag systems (such as Metsec anti-sag bars) clip between purlins as an alternative to traditional sag rods.

Bracing & Stability

A portal frame is a 2D structure, meaning it resists loads in its own plane (gravity and wind on the side walls) but has no inherent stiffness along the length of the building. A separate bracing system is needed to resist longitudinal forces such as wind on the gable end and crane surge loads.

Vertical Bracing

Cross-bracing in the vertical plane of the side walls, typically using angle sections (60x60 to 100x100 equal angles) or circular hollow sections (42.4 to 76.1mm OD). Vertical bracing is required in at least one bay at each end of the building, and for buildings over about 60 metres long, at least one intermediate braced bay is also needed. The most common configuration is an X-pattern (cross-bracing), but K-bracing or a single diagonal can be used where the cross-bracing would obstruct doors or openings.

Plan Bracing (Roof Bracing)

Cross-bracing in the horizontal plane of the roof, at both ends of the building. Plan bracing connects the eaves level members and transfers wind loads on the gable down to the vertical bracing in the walls. The eaves strut, a member running along the eaves between frames in the braced bay, forms part of this system.

Portal Bracing

An alternative to cross-bracing where a small portal frame is used in the wall plane to provide longitudinal stability. This is used where cross-bracing would block large openings such as roller shutter doors.

Lateral Restraint and Fly Braces

The compression flanges of rafters and columns must be restrained against lateral-torsional buckling. The outer flange of the rafter (the top flange, facing the cladding) is restrained by the purlins. But the inner flange is in compression near the eaves and apex, and needs additional restraint from fly braces, which are short diagonal members connecting the purlin to the inner flange of the rafter. Every purlin within the haunch zone should typically have a fly brace to the inner flange.

Portal Frame Types

There are several variations of the basic portal frame, each suited to different building requirements.

Deflection Limits

Portal frames are relatively flexible structures, and deflection checks often govern the design rather than strength checks. The most critical deflection for a portal frame is eaves spread, which is the horizontal outward movement of the column tops under load. Excessive eaves spread can cause the cladding to distress and the gutters to lose their fall, leading to ponding.

Fire Design & Boundary Conditions

Most portal frame buildings do not need fire protection to the steelwork for life safety, because single-storey buildings can be evacuated quickly. However, when a building is close to a site boundary, Building Regulations Part B requires the external wall to maintain stability in fire to prevent it collapsing outward and spreading fire to adjacent properties.

The accepted approach for portal frames near a fire boundary is to allow the unprotected rafter to collapse in fire while keeping the column standing. As the rafter collapses, it pulls inward on the column top, and the column, its base plate, and its foundations must be designed to resist this overturning moment. The column and the eaves connection within the boundary zone are fire-protected (typically with intumescent paint), while the rafter is left unprotected.

This means the column fire protection, base plate capacity, and holding down bolt specification are all affected by fire boundary proximity. The critical temperature of structural steel (the temperature at which it loses enough strength to fail under the applied load) is typically around 550°C for beams and 520°C for columns, at which point the steel retains approximately 60% of its room-temperature yield strength.

Crane Buildings

Portal frames can support overhead travelling cranes, with the crane rails mounted on brackets from the columns or on separate crane beams. Crane buildings have additional design considerations beyond a standard portal frame: the horizontal surge loads from the crane accelerating and braking, fatigue from repeated crane loading cycles, and tight tolerances on crane rail alignment.

For light cranes (up to about 5 tonnes), the crane bracket can be supported directly from the portal column. For heavier cranes, separate crane columns are typically used alongside the portal columns to keep the crane loads independent of the portal frame action.

Crane beam sizes are indicative for a 6m bay spacing. Actual sizing depends on crane duty class, wheel loads, and the specific crane manufacturer's data.