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Going up in the world

Structures Wembley arch

The 133m tall signature arch of the UK's new Wembley Stadium is being raised high into the London skyline.

Alan Sparks finds out how.

At eventual pitch level, where the good, the bad and the ugly of world sport will do battle from early 2006, the final steel sections for Wembley's 315m span arch are coming together. This tubular lattice structure is set to rise 133m above ground level to become the largest single span roof structure in the world.

But the 1,850t total capacity lift, due to start as NCEI went to press, will be much more than one quick tug. The whole process could take over six weeks. Principal contractor Multiplex project director Ashley Muldoon explains: 'Because the site must be cleared of labour during the lifting procedure, we can only lift in certain periods, probably weekends.'

Taking to the field are engineers from Cleveland Bridge.

The Darlington, UK, outfit is taking on the entire arch-raising challenge, which is being tackled in four distinct stages.

The pencil ended bases of the arch lattice are connected to pin joints, which will enable the structure to rotate as it is raised.

It will be hauled aloft by cables running over temporary towers, which will provide the angle needed to generate the initial upward force. Without the struts, the jacks would merely compress the arch, not raise it.

The towers are designed as turning struts, rotating in the direction of the lift, helping to maximise the performance of the jacks. Towers closest to the ends of the arch are 46m tall, two are 82m tall, and the central tower is 102m tall.

In the first stage, strand jacks take up the slack and raise the arch from its temporary supports. At this point, an array of surveying and monitoring equipment check to confirm all is well.

Temporary supports are removed, paintwork touched up and lighting finished off. This should take around a week.

The second lift takes the arch to 30infinity with a further week needed to fit the catenary cable that will eventually support the roof.

At the next lift to 65infinity, restraint cables will be tensioned from jacks on the opposite side of the arch to those pulling it up. These enable the arch to be pulled beyond the vertical, eventually taking its weight. This final stage carries the arch over to 112infinity, at which stage the turning struts can be removed.

'Here it will rest for six months supported purely by the restraint cables from one side, waiting until the steel structure of the upper stands and roof elements catch up, ' says Multiplex site engineer Riccardo Petaccia.

'But should the weekend lifting slots be missed due to wind or poor visibility, the process could be knocked back, ' he says.

On the other hand, if ideal conditions prevail, the nerve-wracking exercise could be polished off in as little as three weeks.

During the lift significant lateral and eccentric forces are applied to the arch. To beef it up, sections of the lattice have been thickened, increasing the arch's weight by 10%.

The jacks apply up to 10,000t to the arch during the lift. 'We will be keeping a close eye on the stroke length of each jack and how much they are pulling, ' says Petaccia.

'Although there is a good deal of flex in the arch, there are effectively five restraining points along its length. And we can't allow any jack to gather more than 450mm more cable than the jacking point next to it.'

Cleveland Bridge has a $100M contract for the $600M stadium.

In addition to the mammoth arch itself, this includes knitting over 26,000t of steel around the stand's concrete cores.

Most striking of the extensive temporary works are the turning struts. These square sectioned, trussed modular towers on giant hinge bases have pins at their top around which the cables are looped. They are raised to 45infinity by crawler cranes before the initial lift.

'When they lifted the London Eye (the capital's giant Ferris wheel), they used a similar technique using one static strut. But here we are using five - and this time they move, ' says Muldoon.

As the arch is raised, the struts pivot, falling backwards in the same direction as the arch. The lines between the struts and the arch remain a constant length.

Jacks positioned on the ground pull in the cables between themselves and the struts only. Once the arch is at its final position, the cables will lower the struts to the ground.

Four cables run from every jacking point, each with four jacks. Each cable is made up of 25 strands 18mm thick and each jack exerts a maximum 600t of pull when the arch is flat.

From the top of each strut, a pair of cables runs down to the arch, which has five single lifting points. Ensuring there is enough rotation in these connections is key to managing the lift. It was this lack of movement which led to lifting delays on the London Eye.

Who's who

Client: Wembley National Stadium Limited (a subsidiary of the English Football Association) Principal contractor: Multiplex Structural and M&E engineer:

Connell Mott MacDonald and Sinclair Knight Merz Steelwork: Cleveland Bridge Concrete: PC Harrington

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