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.Portal frames are very common, in fact 50 of constructional steel used in the UK is in portal frame construction.
![]() 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. This article describes the anatomy and various types of portal frame and key design considerations. 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 secondary steelwork supports the building envelope, but also plays an important role in restraining the primary steelwork. 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. 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. Use of a tie member or rigid column bases may be necessary to reduce the eaves deflection. A tie may be useful to limit spread in a crane-supporting structure. ![]() Because of transport limitations rafters longer than 20 m may require splices, which should be carefully detailed for architectural reasons. Guidance on the stability of curved rafters in portal frames is given in SCI P281. Portal Frame Example Series Of StraightAlternatively, 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. Where transport limitations impose requirement for splices, they should be carefully detailed, to preserve the architectural features. ![]() 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. The haunch also adds stiffness to the frame, reducing deflections, and facilitates an efficient bolted moment connection. The length of the eaves haunch is generally 10 of the frame span.
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