Portals and arches

Arches, which can take a variety of forms, are efficient structures for long span roofs.

For the large single clear span, the need for internal columns can be eliminated by the use of arched vaults or portals. This traditional arched form of construction provided the first long span railway structures and airship hangars mentioned earlier.

The heavy fixed arch of St Pancras Station would have been difficult to determine in terms of loading calculations and equally difficult to erect; however, these problems were simplified in later arched roofs with the introduction of 'pinned joints'.

The parabolic arched structure of the airship hangars at Akron, Sunnyvale and Weeksville, constructed between the wars in the USA, provided some of the most elegant long span steel structures ever built

An interesting recent example of this form of structure can be seen in the International Rail Terminal at Waterloo by Nicholas Grimshaw and Partners with the engineers YRM/Anthony Hunt Associates.

Here the more modest span of 55m is achieved with bow-string arches pin jointed at the base and with an eccentrically positioned third pin joint to one side where the structure changes form. These arches are much flatter than their Victorian counterparts in order to reduce the volume of space enclosed. The steel sections of the trusses are tapered to lighten the appearance and steel castings are used for the connections.

At the Ponds Forge International Sports Centre the architects FaulknerBrown, with Ove Arup and Partners, chose a vaulted structure of cross arches to enclose the main swimming pool. This most interesting structure of tubular steel sections forming three pinned arches with nodes of steel castings achieves a clear span of 54m by 84m with a total steel weight of 400 tonnes.

Both the International Rail Terminal at Waterloo and Ponds Forge clearly demonstrate the successful union of architecture and engineering, that is to say they both have clear innovative steel structures designed for their specific purpose which is an integral part of the architecture. For this to work successfully, the structural solution must be entirely appropriate, refined and clearly expressed. The other building elements must then be co-ordinated with the structure to enhance and strengthen the concept.

There are many alternative 'off the peg' solutions available, of which the steel portal frame is perhaps the most common.

Portal frame structures, where standard beam sections are bolted together with a range of joint types, are commonly used for routine applications requiring a long span single storey building. Different spans and clear heights are achieved from a set of standard components with the advantage of speed and economy.

Despite their simplicity and efficiency, providing good workable enclosures for manufacturing, storage and a whole range of other activities, the standard portal frame does not have to be uninteresting.

In 1972 Foster Associates used a low cost pitched portal for their Modern Art Glass office and warehouse at Thamesmead and created an award winning piece of industrial architecture. The shallow pitched frame, painted bright yellow, was clad with bright blue profiled sheeting which wrapped around the eaves and ridge with gentle rolling bends. The showroom was placed at one end with the office above, set behind an elegant glazed wall which offered a full view of the building in section. The clarity in expression of its form, precision of detailing and use of colour transformed this simple form of shed into architecture.