Portalframe structures

Portal-frame type structures are examples of rigid frames that can take a number of forms. They were first developed in the 1960s, and have now become the most common form of enclosure for spans of 20 to 60 m. Portal frames are generally fabricated from hot-rolled sections, although they may be formed from lattice or fabricated girders. They are braced conventionally in the orthogonal direction.

In general, portal-frame structures are used in single-storey industrial type buildings where the main requirement is to achieve a large open area at ground level and, as such, these structures may not be of architectural significance. However, the basic principles can be used in a number of more interesting architectural applications, as illustrated in Figures 3.2 and 3.7. Also, portal frames can be used in other applications, such as in roof structures for multi-storey buildings, long-span exhibition halls, and atrium structures.

The frame members normally comprise rafters and columns with rigid connections between them. Tapered haunches are introduced to strengthen the rafters at the eaves and to form moment-resisting connections. Either pinned or fixed bases may be used. Roof and wall bracing is essential for the overall stability of the structure, especially

Glas Architecture Frame Structure Corner

3.7 Portal frame expressed internally behind a glazed-end elevation of a building for Modern Art Glass (architect: Foster and Partners)

during erection. Typical examples of portal-frame structures using hot-rolled sections, fabricated sections and lattice trusses are illustrated in Figure 3.8. Portal frames generally provide little opportunity for expression but, with care, the chosen details can enlighten the appearance of these relatively commonplace structures.

Other applications of portalised structures are illustrated in Figures 3.9 and 3.10. The articulated lattice structure using tubular elements was used to great effect in the Sainsbury Centre, Norwich. An arch or mansard shape can be created from linear members, as in Figure 3.11.

In tied portals, the horizontal forces on the columns may be restrained by a tie at, or close to, the top of the column. Ties are usually not preferred because they can interfere with the headroom

Steel Construction Architecture
(a) Standard portal (typical span 15 m to 45 m, typical pitch 6")
Tubular Column Portal Frame
(b) 3 pinned lattice portal (spans up to 80 m)

(c) Mansard portal (spans up to 60 m)

3.8 Typical portal structures using a variety of members

(c) Mansard portal (spans up to 60 m)

Tapered Portal Frames
(d) Tapered portal fabricated from plate (spans up to 60 m)

3.9 Articulated lattice portal structure (often using tubular sections)

Tapered Portal Frames
and arched rafter

3.10 Arched portal using tubular sections

Richard Roger Steel Architecture

3.11 Long-span portal frame used to create an arch structure

3.12 Tied portal frame used at Clatterbridge Hospital (architect: Austin-Smith: Lord)

Richard Rogers Renzo Piano Hospital

3.14 Example of continuity achieved through a series of pinned connections, Centre Pompidou, Paris (architect: Renzo Piano and Richard Rogers)

of the space. Long ties also require intermediate suspension support to prevent sag. However, ties can be detailed effectively, as illustrated in Clatterbridge Hospital in Figure 3.12.

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