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 ...view middle of the document...
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 are the self weight of the structure, secondary steelwork and cladding. Where possible, unit weights of materials should be obtained from manufacturers’ data.