Portal Frames
Portal frames are generally low-rise structures, comprising columns and horizontal or pitched rafters, connected by moment-resisting connections. Resistance to lateral and vertical actions is provided by the rigidity of the connections and the bending stiffness of the members, which is increased by a suitable haunch or deepening of the rafter sections. This form of continuous frame structure is stable in its plane and provides a clear span that is unobstructed by bracing. They are very efficient for enclosing large volumes; therefore they are often used for industrial, storage, retail and commercial applications as well as for agricultural purposes.
A portal frame building comprises a series of transverse frames braced longitudinally. The primary steelwork consists of columns and rafters, which form portal frames, and bracing. The end frame (gable frame) can be either a portal frame or a braced arrangement of columns and rafters.
The light gauge secondary steelwork consists of side rails for walls and purlins for the roof. The secondary steelwork supports the building envelope, but also plays an important role in restraining the primary steelwork.
The roof and wall cladding separate the enclosed space from the external environment as well as providing thermal and acoustic insulation. The structural role of the cladding is to transfer loads to secondary steelwork and also to restrain the flange of the purlin or rail to which it is attached.

TYPES OF PORTAL FRAMES
Many different forms of portal frames may be constructed. Frame types described below give an overview of types of portal construction with typical features illustrated. This information only provides typical details and is not meant to dictate any limits on the use of any particular structural form.

Pitched roof symmetric portal frame
Generally fabricated from UKB sections with a substantial eaves haunch section, which may be cut from a rolled section or fabricated from plate. 25 to 35 m are the most efficient spans. Portal frame with internal mezzanine floor
Office accommodation is often provided within a portal frame structure using a partial width mezzanine floor.
The assessment of frame stability must include the effect of the mezzanine; guidance is given in SCI P292.

Crane portal frame with column brackets
Where a travelling crane of relatively low capacity (up to say 20 tonnes) is required, brackets can be fixed to the columns to support the crane rails. Use of a tie member or rigid column bases may be necessary to reduce the eaves deflection.
The spread of the frame at crane rail level may be of critical importance to the functioning of the crane; requirements should be agreed with the client and with the crane manufacturer. Tied portal frame
In a tied portal frame the horizontal movement of the eaves and the bending moments in the columns and rafters are reduced. A tie may be useful to limit spread in a crane-supporting structure.
The high axial forces introduced in the frame when a tie is used necessitate the use of second-order software when analysing this form of frame. Mono-pitch portal frame
A mono pitch portal frame is usually chosen for small spans or because of its proximity to other buildings. It is a simple variation of the pitched roof portal frame, and tends to be used for smaller buildings (up to 15 m span). Propped portal frame
Where the span of a portal frame is large and there is no requirement to provide a clear span, a propped portal frame can be used to reduce the rafter size and also the horizontal shear at the foundations. Mansard portal frame
A mansard portal frame may be used where a large clear height at mid-span is required but the eaves height of the building has to be minimised. Curved rafter portal frame
Portal frames may be constructed using curved rafters, mainly for architectural reasons. Because of transport limitations rafters longer than 20 m may require splices, which should be carefully detailed for architectural reasons.
The curved member is often modelled for analysis as a series of straight elements. Guidance on the stability of curved rafters in portal frames is given in SCI P281.
Alternatively, the rafter can be fabricated as a series of straight elements. It will be necessary to provide purlin cleats of varying height to achieve the curved external profile. Cellular beam portal frame
Rafters may be fabricated from cellular beams for aesthetic reasons or when providing long spans. Where transport limitations impose requirement for splices, they should be carefully detailed, to preserve the architectural features.
The sections used cannot develop plastic hinges at a cross-section, so only elastic design is used.

Permanent actions
Permanent actions are the self weight of the structure, secondary steelwork and cladding. Where possible, unit weights of materials should be obtained from manufacturers’ data.
Service loads
Service loads will vary greatly depending on the use of the building. In portal frames heavy point loads may occur from suspended walkways, air handling units etc. It is necessary to consider carefully where additional provision is needed, as particular items of plant must be treated individually.
Depending on the use of the building and whether sprinklers are required, it is normal to assume a service loading of 0.1–0.25 kN/m2 on plan over the whole roof area.
Variable actions
Imposed roof loads
Roof slope, α qk (kN/m²) α < 30° 0.6
30° < α < 60° 0.6[60 - α)/30] α > 60° 0
Imposed loads on roofs
Imposed loads on roofs depend on the roof slope. A point load, Qk is given, which is used for local checking of roof materials and fixings, and a uniformly distributed load, qk, to be applied vertically. The loading for roofs not accessible except for normal maintenance and repair is given in the table on the right.
It should be noted that imposed loads on roofs should not be combined with either snow or wind.

Snow loads, or Precipitation loads
Snow loads may sometimes be the dominant gravity loading. Any drift condition must be allowed for not only in the design of the frame itself, but also in the design of the purlins that support the roof cladding. The intensity of loading at the position of maximum drift often exceeds the basic minimum uniform snow load. The calculation of drift loading and associated purlin design has been made easier by the major purlin manufacturers, most of whom offer free software to facilitate rapid design.
Wind actions
Wind actions are inherently complex and likely to influence the final design of most buildings. The designer needs to make a careful choice between a fully rigorous, complex assessment of wind actions and the use of simplifications which ease the design process but make the loads more conservative. Free software for establishing wind pressures is available from purlin manufacturers.
Crane actions Gantry girders carrying an overhead travelling crane
The most common form of crane is the overhead type running on beams supported by the columns. The beams are carried on cantilever brackets or, in heavier cases, by providing dual columns.
In addition to the self weight of the cranes and their loads, the effects of acceleration and deceleration have to be considered. For simple cranes, this is by a quasi-static approach with amplified loads
For heavy, high-speed or multiple cranes the allowances should be specially calculated with reference to the manufacturer.
Accidental actions
The common design situations which are treated as accidental design situations are:
 Weather related or climate related circumstances.
 The opening of a dominant opening which was assumed to be shut.

Each project should be individually assessed whether any other accidental actions are likely to act on the structure.
Robustness
Robustness requirements are designed to ensure that any structural collapse is not disproportionate to the cause.
Fire

Collapse mechanism of a portal with a lean-to under fire, boundary condition on gridlines 2 and 3.
Structural steel in single storey buildings does not normally require fire resistance. The most common situation in which it is required to fire protect the structural steelwork is where prevention of fire spread to adjacent buildings, a boundary condition, is required. There are a small number of other, rare, instances, for example when demanded by an insurance provider, where structural fire protection may be required.
When a portal frame is close to the boundary, there are several requirements aimed at stopping fire spread by keeping the boundary intact:
 The use of fire resistant cladding
 Application of fire protection of the steel up to the underside of the haunch
 The provision of a moment resisting base (as it is assumed that in the fire condition rafters go into catenary)

Frame dimensions Dimensions used for analysis and clear internal dimensions
A critical decision at the conceptual design stage is the overall height and width of the frame, to give adequate clear internal dimensions and adequate clearance for the internal functions of the building.

Clear span and height
The clear span and height required by the client are key to determining the dimensions to be used in the design, and should be established early in the design process. The client requirement is likely to be the clear distance between the flanges of the two columns – the span will therefore be larger, by the section depth. Any requirement for brickwork or blockwork around the columns should be established as this may affect the design span.
Where a clear internal height is specified, this will usually be measured from the finished floor level to the underside of the haunch or suspended ceiling if present.
Main frame
The main (portal) frames are generally fabricated from UKB sections with a substantial eaves haunch section, which may be cut from a rolled section or fabricated from plate. A typical frame is characterised by:
 A span between 15 and 50 m
 An clear height (from the top of the floor to the underside of the haunch) between 5 and 12 m
 A roof pitch between 5° and 10° (6° is commonly adopted)
 A frame spacing between 6 and 8 m
 Haunches in the rafters at the eaves and apex
 A stiffness ratio between the column and rafter section of approximately 1.5
 Light gauge purlins and side rails
 Light gauge diagonal ties from some purlins and side rails to restrain the inside flange of the frame at certain locations.

Haunch dimensions Typical haunch with restraints
The use of a haunch at the eaves reduces the required depth of rafter by increasing the moment resistance of the member where the applied moments are highest. The haunch also adds stiffness to the frame, reducing deflections, and facilitates an efficient bolted moment connection.
The eaves haunch is typically cut from the same size rolled section as the rafter, or one slightly larger, and is welded to the underside of the rafter. The length of the eaves haunch is generally 10% of the frame span. The haunch length generally means that the hogging moment at the end of the haunch is approximately equal to the largest sagging moment close to the apex. The depth from the rafter axis to the underside of the haunch is approximately 2% of the span.
The apex haunch may be cut from a rolled section – often from the same size as the rafter, or fabricated from plate. The apex haunch is not usually modelled in the frame analysis and is only used to facilitate a bolted connection.

Positions of restraints General arrangement of restraints to the inside flange
During initial design the rafter members are normally selected according to their cross sectional resistance to bending moment and axial force. In later design stages stability against buckling needs to be verified and restraints positioned judiciously.
The buckling resistance is likely to be more significant in the selection of a column size, as there is usually less freedom to position rails to suit the design requirements; rail position may be dictated by doors or windows in the elevation.
If introducing intermediate lateral restraints to the column is not possible, the buckling resistance will determine the initial section size selection. It is therefore essential to recognise at this early stage if the side rails may be used to provide restraint to the columns. Only continuous side rails are effective in providing restraint. Side rails interrupted by (for example) roller shutter doors, cannot be relied on as providing adequate restraint.
Where the compression flange of the rafter or column is not restrained by purlins and side rails, restraint can be provided at specified locations by column and rafter stays.