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Sight for sore eyes

Record floods and gales threaten to delay this week's planned positioning of a landmark footbridge across the River Tyne. David Hayward reports from Gateshead.

It began as a scribbled doodle, faxed four years ago from consulting engineer to architect. It should end this week, being positioned across the River Tyne, transformed from brainstormed sketch into arguably the most innovative footbridge yet designed.

'Elegant, simple but unique, ' is how John Johnson, director of engineering for client Gateshead Metropolitan Borough Council, describes the £22M, world-first tilting structure. It has already been dubbed the eyelid bridge and is now about to span the Tyne 600m downstream from Newcastle's famous cluster of city centre bridges.

En route from concept to 800t of eye-catching steel, Gateshead's Millennium footbridge has won through a 50-strong international design competition, exhaustive wind tunnel tests and, in the words of Gordon Clark, director with consultant Gifford & Partners: 'Its design has endured horrendous three dimensional structural gymnastics where every member is a different shape.'

With design complete, the challenges continued as client and river authorities 'debated' the need for mid-river ship protection. A resulting eight months pre-contract delay threatened crucial Millennium Commission funding. But a novel project management site team, geared towards instant decisions, has delivered the crossing on time with only the severe weather threatening last minute delays.

The climate was more tranquil four years ago as Gateshead launched a worldwide competition for a combined pedestrian and cycle bridge to link Newcastle's new office and leisure based northern quayside with Gateshead's south bank cultural development.

'The bridge was a key structure but it had to be stunning, as Millennium Commission funding was dependent on us providing a landmark crossing, ' Johnson recalls.

The 'stunning' brief was less of a problem than the need to provide 25m mid river headroom for Tyne shipping. 'Technical innovation was essential although we needed confidence it could actually be built, ' says Johnson.

Of the half dozen shortlisted designs, only the suggestion from Gifford, teamed up with architect Wilkinson Eyre, fulfilled both criteria and was, claims Johnson; 'the clear winner'. Described as a hybrid design, with a tied arch supporting a cable stayed deck, the 105m span bridge is basically a pair of arches braced against each other. The horizontally curved deck is supported by 18 stressed cables sloping down from an inclined box beam arch which mirrors the profile of the distinctive Tyne Bridge just upstream.

Arches converge above boat shaped hollow concrete piers close to each riverbank and concealing hydraulic rams that perform the clever bit. To accommodate shipping the whole structure must tilt upwards about its bearings like an opening eyelid.

The sloping cables end up horizontal, with the two now symmetrical arches alongside each other. Once fully open there is 25m headroom above the river's 30m wide navigation channel.

Innovation did not end with design as the client reckoned the tight construction timescale could only be achieved by bringing in a contractor early, when design drawings were little more than conceptual.

They also sought a partnering arrangement that really did rule out confrontation.

In January 1998, Harbour & General emerged as 'preferred contractor' - a competitive choice based on a priced bill and intense partnering scrutiny but no guarantee of an eventual contract. Johnson's comment, 'Harbour & General was the most committed to our new style of working', was to be fully tested when this planned eight month preferred status doubled, and the delay pushed project cancellation high up the risk agenda.

'Around 20 key personnel were committed full time with only direct costs being paid and a real prospect of no final contract, ' recalls Harbour & General's project manager Steve Aspinall.

By the time a £16M target cost contract was awarded in spring last year, the site's novel 'project manager' was already mitigating the fendering problem. This contractual project manager is not one person but four - directors from client, consultant, architect and contractor - meeting monthly to smooth progress.

'It meant crucial decisions were made across the desk in minutes rather than weeks' says Aspinall. 'Without our unusual project manager, we estimate the challenges would have extended the contract by 44 weeks.'

Those challenges included a late switch from sea dumping to onshore landfill when 10,000m 3of riverbed excavation proved to be too contaminated with mercury for disposal offshore. And the project manager also smoothed a change from early plans to construct the bridge deck insitu across the river to the time saving option of assembling the lot in Amec's oil module fabrication yard at Wallsend, 8km downriver.

Here, subcontractor Watson Steel, assembled and welded together the two dozen bridge sections it had fabricated in its Bolton factory. A conceptual 600t structure grew to 800t when Gifford realised the stress complexity and need for extra stiffening plates.

Fully pre-assembled, with cables stressed up to 200t each, the bridge was then further stabilised for its journey upriver by adding a temporary 110t trussed strut spanning between end bearings.

This would prevent the two arches contracting as they were lifted on four hooks by the world's second largest floating crane, Smit International's 3,200t capacity Asian Hercules II.

During its two hour trip, the bridge will be aligned fore and aft to reduce its sail effect and help negotiate tight river bends.

Swung laterally at the bridge site, the structure will be positioned to 50mm accuracy onto plate bearings in each pier.

Loads will then be gradually transferred from crane to foundations as final 3mm alignment takes place.

Earlier this week however, engineers were more concerned about the accuracy of weather forecasts, as river trip and positioning windows came and went.

Four hours, with less than a force four wind, was needed for the hook-up at Amec's yard.

Two more hours with no higher winds must be guaranteed for the 8km river journey while a more relaxed Force 7 limit was allowed for on site positioning.

'A rather delicate river trip with just 3m clearance either side followed by challenging positioning tolerances' was Smit contracts manager Roger Wilson's summary of the task earlier this week as he waited anxiously for the latest weather forecast.

Award winner 'Innovative, creative and powerful; yet simple in concept and providing a dramatic image of strength and grace.'

These were the words of ICE past president, professor Tony Ridley, as he declared the Gifford/Wilkinson design the competition winner in 1996. A tied arch and a cable stayed deck are not in themselves unique. Nor is the tilting mechanism of hydraulic rams - hidden within the piers - first pushing, then restraining, the rising bridge as its centre of gravity passes over end trunnion bearings. But what is a world first, claim the bridge's promoters, is the combination of all these factors in a single structure.

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