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Bridges: Amazing grace

The 0resund fixed link was formally inaugurated by Queen Margrethe of Denmark and King Carl Gustav of Sweden this week. The 27km road and rail link between the two countries is expected to form the keystone of an entire new economic region. 0resund's comp

If British consultants were to be involved in a major Scandinavian project, then Ove Arup had to be among them. The firm was founded by an expatriate Dane, and probably to a greater extent than in any consultancy of similar size, the founder's spirit still permeates its style.

Angus Low has been closely involved with the 0resund project since the beginning, leading a number of the teams set up to work on the design of the bridges and supplying input to others. Arup formed part of the ASO Group which emerged as the winner of the competition to provide the concept design for the crossing in 1993.

The brief had been unusual in that the client had simply outlined the concept and the parameters it had to fulfill, giving the designers free rein to use their expertise to the full. Low describes 0resundkonsortiet as in some ways an ideal client, having a clear idea of what it wanted and the knowledge to judge whether it was getting it.

'It was quite refreshing, ' he says. 'They behaved in the way that, as a designer, you hope a client would behave. They took an intelligent interest and there was a high-powered in-house team who could assess everything'.

Among the factors in the design brief for the 8km long bridge was resistance to ship impact - it spans a busy channel - and also to aircraft impact on the towers of the cable stayed high bridge.

Low recalls hours spent wrestling with the numerical studies to achieve the optimum balance between efficiency and aesthetics. Colleagues suggested that a cross member linking the pairs of pylons would provide a simple answer, but he was clear in his mind that he wanted them to stand free, as this he felt would be a major factor in designing a truly elegant structure. And, he adds, there was a sound engineering reason since in the case of aircraft impact, there is greater flexibility in the free standing tower so the impact on the foundations would be reduced.

But making the figures work took many hours, he admits.

'The stability of very tall towers increases with the taper, so it was a question of working this to the point where a cross member would have no benefit.' On the completed high bridge the four towers now rise 204m above sea level. The cables are arranged in the classic harp pattern and are anchored to the superstructure truss at 20m intervals. The cast insitu concrete upper deck carries the motorway and the lower steel box the rail tracks.

0resundkonsortiet describes the high bridge as the landmark element of the crossing and speaks of its grandeur and simplicity. The towers mark the highest point of the crossing and the border between Denmark and Sweden. The cable arrangement is echoed in the geometry of the bridge girder truss which has been modified so that every diagonal follows the same angle as the cables. In certain weather conditions the cables seem to vanish, leaving the towers the most outstanding element.

The piers on the approach bridges are prefabricated units with prefabricated caisson foundations. The pylons are supported on caissons but were cast insitu. There was an anxious moment when a problem was discovered in an area of concrete on one leg. This was compounded by the fact that the next lift had already been cast.

Initially the contractor, which was working under a self-certification regime, proposed a repair.

But after consideration, and after learning that the client - had he been in the same position - would have broken out the concrete and replaced it, it was decided to recast the section.

The tools available to today's designers mean that visual images are quickly available, so the bridge soon becomes familiar as a virtual structure. But there is nothing like the real thing, as Low discovered.

Bridge designers from Arup's offices around the world meet each year and in 1999 the Copenhagen office was host. Low recalls the excitement of seeing his concept real and complete.

'It was wonderful to see it as originally conceived, ' he admits, adding that the thrill of looking up at the slender towers against the blue sky more than justified the hours of effort in the design.

Surprisingly, the 0resund bridge is the longest-span Arupdesigned structure to have been built. This is set to change, however, and Low is part of the team working on the Metsovitikos bridge in Greece. 'It's longer, and it's different, being a suspension bridge, but some of the things we learned on Oresund will come into play, ' he says.

www.oresundskonsortiet.com

Project team

Client: 0resundkonsortiet

Contractors Dredging and reclamation: 0resund Marine JV - Per Aarsleff/ Ballast Nedam Dredging/Great Lakes Dock and Dredge company/ consultant Carl Bro Tunnel: 0resund Tunnel Contractors - NCC International/ Dimez GTM/John Laing/ E Pihl & son/Boskalis Westminster Dredging/consultant Symonds Travers Morgan Bridges: Sundlink Contractors - Skanska/Hojaard & Schultz/ Monberg & Thorsen/Hochtief/ consultant CV Joint Venture: COWIconsult/VBB Anlaggning Consultants Tunnel, dredging and reclamation: 0resund Link Consultants - Ramboll/ Scandiaconsult/Sir William Halcrow & Partners/Tunnel Engineering Consultants/Dissing + Weitling Bridges: ASO Group: Ove Arup & Ptns Danmark/SETEC/Gimsing & Madsen/ISC Final cost of project at 2000 prices £2.4bn

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