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The most critical phase in the building of Wembley Stadium is about to begin. In six weeks, the landmark arch will straddle the home of English football, ready to support the largest roof structure in the world.

At present, the 450 m long steel-truss arch, the primary structural element in the design by Foster and Partners and HOK, is nearly completed and lying on its side along the perimeter of the stadium bowl. A ring of concrete core structures for the stand, each with its accompanying tower cranes, stand guard over it. In its prone position, the 7.4 m wide arch is restricting work on the stadium's stands, so it needs to be swiftly lifted into place to allow that part of the programme to begin in earnest.

The lifting process is certain to be nerve-wracking. The arch, a helical steel latticework tube, will be pulled upright using a technique similar to that employed to raise the London Eye. That task, you will no doubt remember, almost ended in disaster when a lifting cables parted, causing the operation to be temporarily abandoned.

For the lifting of the Wembley arch, Multiplex, the main contractor, has employed the same lifting specialist that worked with the London Eye team. However, according to Mike Richardson, construction director of client Wembley National Stadium Ltd, the company is not taking any chances. "The temporary structure has been designed to be much stronger than that used on the London Eye," he emphasises (see "How the arch will be lifted", page 22). More than 1800 tonnes of temporary steelwork will be used for the lift – a huge amount considering that the arch structure itself weighs 1650 tonnes.

The contractor will bring the arch into position in stages. The first will raise the arch 300 mm off its temporary supports to allow a final coat of paint to be applied to the underside of the arch truss. During this stage, the lateral forces on the structure will be far greater than anything it will experience supporting the roof. These forces will make this "the most critical stage of the lift", according to Richardson.

When the painting is complete, the arch will be lifted to a height of 1 m. At this point, Multiplex will check the geometry of the arch using sophisticated survey equipment to ensure that it is behaving as calculated. The arch has been designed to flex by up to 400 mm during the lift. Richardson says the big concern at this point is that it may develop "a twist".

The temporary structure has been designed to be much stronger than that used on the london eye

Mike Richardson, Wembley National Stadium

Assuming the structure behaves as planned, the lift will continue in 1 m stages until the arch reaches an angle of 20° to the ground.

The forestay and backstay cables will then be attached, as will the cables that will eventually connect arch and roof.

The lift will then continue, a metre at a time, until the arch has moved past the vertical position to reach its final resting place at an angle of 22° from the vertical. Here, its centre point will be 133 m above the pitch. The contractor will then spend another three months connecting all the other supporting cables and stays while the roof structure is built beneath it.

On site, work is under way in preparation for the lift. Five temporary pivots, one for each of the temporary lifting trusses, stand in a line across the stadium bowl. Already some of the temporary lifting trusses have been connected to these pivots ready for the lift, and erection of the remaining temporary trusses is well under way.

The other focus of work on site is finishing the two massive pivots that will hold the ends of the arch in place. The steel support bases are in position and a team is installing the web of giant reinforcing bars that will tie them into the huge concrete foundation structure.

The arch has to be lifted over the weekend so that if something goes horribly wrong, nobody will get hurt

Meanwhile, the final three steelwork sections of the arch are being assembled. The lattice is constructed from pre-assembled sections about 30 m long. Each comprises three circular members linked by criss-crossing tubular steelwork to form the circular truss. The pre-assembled sections are joined to form the arch using what Richardson terms a "hit-and-miss" method of assembly. This means that the pre-assembled arch sections are positioned adjacent to one other so that the circular structural members are equally spaced. The "missing" criss-crossing tubular steelwork, which Richardson calls "straws", is then welded into position to stitch all the pre-assembled bits together.

The arch sections are being pre-assembled on site in four, 28 m long welding sheds in the middle of the pitch by steelwork contractor Cleveland Bridge. The sheds are mounted on rails, allowing them to be rolled away. This exposes the pre-assembled sections, which the cranes can then lift into place. The final two arch sections for the eastern end of the truss, and the one remaining section for the western end, are almost complete and ready to be lifted. Once the main arch structure is complete, the two tapered end sections, or "pencil ends" as Richardson knows them, will be added to complete the arch assembly.

The lift is planned to take place over four weekends. "We have to lift the arch over the weekend for safety reasons, when there are no people working on site," Richardson says. Or, to express it more bluntly, to ensure that if the lift goes horribly wrong nobody will get hurt.

Multiplex is happy to crow about its achievements in pulling the construction five weeks ahead of programme, but is rather less forthcoming on the actual date that the lift will begin.