Next month, the 133m tall signature arch of the new Wembley Stadium will be 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 will rise 133m above ground level to become the largest single span roof structure in the world.
But the 1,850t total capacity lift in February 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 will be engineers from Cleveland Bridge.
The Darlington outfit is taking on the entire arch-raising challenge, which will be 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 (News last week).
Without the struts, the jacks would merely compress the arch, not raise it.
The towers are designed as turning struts, which will rotate 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.
First off, strand jacks will take up the slack and raise the arch from its temporary supports. At this point, a menagerie of surveying and monitoring equipment will be checked to confirm all is well. Temporary supports will be removed, paintwork touched up and lighting finished off. This will take around a week.
Second lift will raise the arch to 30infinity. A further week will be needed to fit the catenary cable that will eventually support the roof.
The next lift takes the arch to 65degrees. At this point, restraint cables will be tensioned from jacks on the opposite side of the arch to those pulling it up. These will enable the arch to be pulled beyond the vertical, eventually taking its weight. This final stage will carry 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.'
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, there will be significant lateral and eccentric forces applied to the arch. To beef it up, sections of the lattice have been thickened - increasing the arch's weight by 10%.
The jacks will be applying 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 £60M contract for the £353M 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 are built on giant hinge bases and have pins at their top around which the cables are looped. They will be raised to 45infinity by crawler cranes before the initial lift.
'When they lifted the London Eye (NCE 16 December 1999), they used a similar technique using one static strut. But here we will be using five - and this time they will move, ' says Muldoon.
As the arch is raised, the struts will pivot, falling backwards in the same direction as the arch.
The lines between the struts and the arch will remain a constant length. Jacks positioned on the ground will pull in the cables between themselves and the struts only. Once the arch is at its final position, the cables will be used to lower the struts to the ground.
There will be four cables running 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 will run 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.
How the roof works
The arch is not an architectural folly - it plays a key structural role, holding up 60% of the roof. On the north stand, towards which the arch will permanently lean, it picks up a perimeter truss at the rear and a truss at the front.
This holds up four trusses spanning the pitch from the south stand, needed to support the moving roof when the stands have to be covered over. Loads from the front truss are transferred via cables to the arch. Total weight to be supported by the arch will be 7,000t.
Deflection of 600mm is expected when the arch is fully loaded.
Trusses spanning the pitch have been designed to minimise shadow effects on the pitch such as the 1986 World Cup 'spider' shadows. The moveable roof will not enclose the whole stadium, just the fans - the players deserve to get wet.
Putting the package together
Piece by piece, the whole of Wembley arch will have travelled down the M1 from Darlington. The 7.2m diameter tubular steel structure is made up of 21m long modules weighing up to 100t. Each module comprises three circular diaphragms and the tubular straws of the giant lattice tube.
They are joined together on site in large sheds in the centre of the pitch. 'Once the latest module is completed, these sheds can slide out of the way so that tower cranes can come in and pick up each section and place it on temporary supports ready to be butt-welded to the rest of the arch, ' says Multiplex project manager Dan Scanlon.
Following the main arch lift, a perimeter truss must be lifted around the outside of the stands.
This will be craned into position in 8m and 16m units and welded insitu. Finally, the modular turning struts will be redeployed to support roof elements during assembly.
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
Demolition and groundworks: Griffiths McGee
Piling: Stent Foundations