The Scalpay Bridge, officially opened two weeks ago by Prime Minister Tony Blair, is a triumph of modern engineering.

Crossing the Sound of Scalpay to provide a direct link with North Harris, the elegant bridge was built in weather conditions which were often severe. The site was shut down for three months at one stage.

The need for the bridge had long been clear, since the 430 inhabitants of Scalpay were served only by a bow-loading ferry crossing the Sound infrequently. Consulting engineers Halcrow Crouch, who designed the contract for Western Isles Council after winning a competitive tendering process, had

to face numerous challenges, according to Jim Johnston, the company's director for bridges.

''We had to consider the natural beauty of the location,'' he explained, ''plus the complex geology, the environmental impact, climactic conditions, the lack of existing infrastructure and the risk of overruns of cost and time during construction.''

The actual design was created by Iain Salisbury, Halcrow Crouch's principal bridge engineer. ''Because of low traffic flows we decided on a single lane on the bridge,'' he said. ''We adopted a 4.8m carriageway and a 2m footpath to enable traffic to pass broken down or stationary vehicles while maintaining economy of design.

''Since this left the structure very narrow in relation to its main span, we undertook studies, in conjunction with Glasgow University, to prove the

stability of the bridge in gusting winds. We consulted 24 different bodies during the design process. One of these bodies, the Royal Fine Art Commission for Scotland, has officially

commended the quality of the design.''

Environmental issues also came into play in the design

of the approach roads. This involved constructing an embankment over 10m depth of peat on the Scalpay side, plus the excavation of 12,000 cubic metres of rock and the placement of 68,000 cubic metres of rock infill, mainly at the Harris end. The treatment of these embankments was carefully planned to ensure that they would match the landscape.

A #700,000 contract for advanced earthworks was carried out by R J McLeod, which has bases in Glasgow and Dingwall. The main #6m contract for construction of the bridge itself was awarded to Edmund Nuttall Ltd, on whose recommendation the decision was taken to erect a structure of only three sections, thus maximising off-site fabrication and easing problems of on-site construction.

Work then began on the complex process of building the first long-span steel box girder bridge in the UK since Foyle Bridge in Londonderry in

1983. It was to consist of a 170m central span, with back spans of 52m and 70m on the Scalpay and Harris sides respectively. The integral box girder legs, inclined and tapered, would be set upon reinforced concrete foundations bearing directly on to the base rock below the structure. ''Though the geology was complex, we preferred to set the bridge in the natural rock,'' said Johnston. ''With water, you really don't know what lies under it.''

The first stage of the intricate operation, complicated by the often hostile weather, lasted until Christmas 1996, by which time the four concrete substructures were substantially completed. Since the nearest ready-mix plant was at Stornoway on the Isle of Lewis, Nuttall had decided to site-batch 1200 cubic metres of concrete, using self-loading mixers. In similar fashion, in 1997, about 60% of the deck concrete was site-batched. During the period of construction, access between Harris and Scalpay was achieved by using motorised uniflotes provided by Nuttall's Kilsyth division. This proved to be a flexible alternative to the local ferry.

One of Nuttall's sister companies, HSM (Hollandsche Staalbouw Maatschappij) took charge of the fabrication and painting of the steel box girders in its yard at Schiedam in Holland.

In May 1997 the three steel sections, weighing more than 1600 tonnes in total, were loaded on to a barge and set sail for Scalpay via the Cromarty Firth.

In June they were lifted into place by a floating crane barge. This involved locating each back span, complete with pier leg, then installing the central span to complete the structure of the bridge. ''Using a construction sequence which we devised at Halcrow Crouch,'' said Salisbury, ''they were able to carry out this demanding operation in only seven days.'' HSM then remained on location during June and July to finish the welded site splices and painting works. The external surfaces of the steelwork are coated with a high-build paint system 1mm thick, which was developed for use in the offshore oil industry and was chosen to offer a life of at least

25 years.

Nuttall then placed the 200mm thick reinforced concrete deck, which includes a 1450mm cantilever section on both sides of the steel box girder. Thereafter, during late summer and early autumn, a phased programme of operations was carried out - waterproofing, kerbing, drainage, electrical work, parapets and surfacing works, both on the bridge itself and on the approach roads. The completion of the contract was achieved in December 1997 and the bridge has been in use since then.

Western Isles Council is delighted with both the quality of the Scalpay Bridge and its contribution to community life. ''The bridge will do so much to improve the quality of life of the local people,'' said Donald M Mackay, convener of the council. ''They will now be able to access the mainland of Lewis and Harris whenever they wish. We hope the bridge will also create further opportunities for social and economic development in the future.''