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Cleveland's Christmas wish: to raise the roof at Wembley

WEMBLEY

Fabrication of the national stadium's 26,000 tonnes of structural steel starts this April in County Durham. David Hayward visited the Darlington base of steelwork contractor Cleveland Bridge to discuss the construction challenges MIKE McHugh is not really looking forward to Christmas - or at least he doesn't quite share the enthusiasm that his two young children have for the festive season. For it is around then that he must oversee a 10-day operation regarded as one of the most complex and conspicuous his company has ever undertaken.

As project manager for the new Wembley stadium's steelwork subcontractor Cleveland Bridge, Mr McHugh will be responsible for the high-profile task of raising the structure's 1,650 tonne arch from a ground level assembly area to its near vertical future home, towering 133 m above the construction site.

With the arch claimed to be the world's longest single span roof structure, and the stadium's landmark icon, there is little chance of Mr McHugh raising it on the quiet.

Football fans worldwide will be watching, with any perceived own goals analysed in dozens of languages.

But Mr McHugh exudes quiet confidence. Four months of detailed planning, plus countless brainstorming sessions dissecting every conceivable scenario, are aimed at giving those observers little to discuss but success.

The arch is only one of the steelwork contractor's tasks.

Included in its £60 million subcontract is fabrication and erection of the stadium bowl and roof.

Cleveland Bridge is no stranger to jacking up heavy bridge deck sections. But the sheer complexity, and congested interfaces, thrown up by knitting together Wembley's arch, bowl and roof, prompts managing director Alan Nightingale to comment: 'This project ranks among the most challenging and high profile we have yet undertaken.'

Mr McHugh suggests Wembley's roof erection will have his boss reaching for the aspirin. But right now the arch takes centre stage, and will remain the priority until after it is lifted into place around Christmas.

For him, the structure's architectural role - as a dramatic icon, replacing Wembley's famous twin towers - is secondary. 'People forget its prime function is to hold up the roof, ' he explains.

Once in position, angled at 22 deg off vertical, this open latticed tube will be supported by fore and back stay cables fixed to the stand's perimeter roof truss. But the arch itself is a prime supporter of that 7,000 tonne roof.

Cables hung from the arch are tied to a lower curved catenary wire, spanning across the stadium just above roof level. This cable in turn supports the roof 's two fixed end sections above the stands, plus sliding panels that can partially cover the central area.

The arch is designed as a slim truss, casting as few shadows as possible on the pitch beneath. It is formed as an open 7.4 m diameter cylindrical tube with a dozen steel pipes spiralling the length of the arch around circular braced diaphragms at 10 m centres.

Cleveland Bridge's engineers considered, briefly, erecting the arch in sections, using either all embracing falsework or tall individual towers from which to jack up preassembled lengths.

But the company had a strong desire to minimise working at height, so its planners asked to temporarily commandeer a large section of the pitch area on which to erect four 28 m-long work sheds. These will allow the whole 450 m-long arch to be fabricated piecemeal at Wembley.

Planners have also reserved a narrow curved swathe of ground right across the site where all 15 main arch sections will be laid out in their correct profile, supported on short angled trestles. The sheds are mounted on rails so they can be rolled back to allow the completed 20 m-long arch sections to be craned across the pitch to their correct place in the arch assembly.

At each end of the arch, permanent rocker bearings will link it to large concrete piled foundation blocks. But these bearings allow only a few degrees movement so during the lift, the ends of the arch will be pinned and fixed by a large rotating pin bearing, formed in a 68 tonne underground steel casting.

This casting also contains thrust blocks to counteract horizontal forces of up to 8 MN as the arch is 'rolled' upward to the correct angle.

A bank of computer controlled strand jacks will pull the arch vertically using an 800 m-long network of cables threaded across the tops of five temporary steel support towers positioned across the site.

Each cable and tower needs its own piled foundations to resist vertical forces up to 14 MN as the arch is swung upward in 100 mm jack strokes.

Though Mr McHugh reckons he could complete the entire operation in less than a week, he is allowing for the possibility of it taking twice as long. Weather will be his major worry as he needs two weeks of wind speeds less than 20 m/s - a tough demand for December.

Once temporarily positioned a few degrees above its permanent home, the arch will remain static for five months, anchored by cables, while the roof beneath is built.

As increasing roof loads are taken up by the cables, the arch flexes downward to its final angle.

The downside of this 'all-in-one' lift is that, until well airborne, the arch demands a lot of valuable ground space right across an already congested multi-disciplinary site.

Two of the stadium bowl's 40 m-high concrete sheer core towers will not be built until the arch is lifted. And a wide slot must be left in the centre of the stand frames to allow one of the lift cables to pass through.

In order to deal with an estimated 6,000 lorry loads of steel and concrete floor planks arriving through a narrow site entrance, to be met by 18 crawler cranes, Cleveland Bridge invested in a £40,000 computer program just to work out optimum crane positions. And a nearby lorry holding park will allow 'just in time' daily site deliveries.

Erection of the relatively conventional 13,500 tonne bolted steel frame, for the stadium's three-tier stands, also starts this summer. Construction of the 50 m-tall stands will be piecemeal. Some areas will be erected from the centre outwards, with top sections built first to allow an early start on the 1 km-long perimeter roof truss around the edge of the stadium.

During all this activity the roof 's 7000 tonne steel frame, arguably the most complex of all tasks (see box, below), will also have to be erected.

'We will be working on multiple construction fronts throughout our already tight 81-week site programme, ' says Mr McHugh. 'As well as being technically complex, this contract is likely to be a huge logistics exercise.'

But if the latest construction programme remains on schedule until winter, Mr McHugh should complete the arch erection just a few days before Christmas which, if nothing else, should keep his kids happy.

Steelwork details Steelwork subcontractor: Cleveland Bridge UK Package: £60 million Duration: October 2002 - March 2005 Steel tonnage: 26,000