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Overarching ambition

Cover story Wembley Stadium

English football will soon have a new symbolic landmark. Alan Sparks met the engineers charged with making this icon a reality.

Construction of the £757M new English national stadium at Wembley is finally under way. Debate has raged over the past four years over the project's validity.

But now its future is secure, attention has shifted to the magnificent steel arch that will span 315m, the longest single span roof structure in the world.

Obstructed views are anathema to the new stadium's design.

Originally this was going to be avoided by supporting the roof from large pylons on the perimeter. 'But to replace the world famous twin towers a real icon was needed - and I think with this arch we have achieved just that, ' enthuses Connell Mott MacDonald director Mike Barker.

Every one of the 90,000 fans who will fill the stadium will be protected from the elements by a total roof area of 4.4ha, 1.6ha of which are retractable. The public is generally aghast at the total project cost, but only £352M of this is down to the basic shell, and for this money the client is arguably getting the most avantgarde stadium on the planet.

Main contractor is Multiplex.

The pylon version of the design was able to site the structure right up to its boundary with neighbouring land belonging to property company Wembley plc.

In the original deal to buy the land any left over following construction of the stadium would need to be handed back to Wembley plc. So it was decided to position the stadium as close to this line as possible to restrict the amount of land that would eventually be handed back.

However, when the team shifted to the arch solution, the extra foundations that are needed compared to the pylon version meant that this could not fit on the available land using the logical foundation solution. At a late date Wembley plc refused to allow the stadium's foundations to stretch beneath any of its land - forcing the engineers to come up with an alternative.

On the east arch base there are no such problems and the chosen solution features a circular 20m diameter pile cap, 2.75m thick, supporting 10m high, 500mm thick walls. This structure was arranged so that the arch reaction passed through the centre of the pile group beneath, which has 19 bored concrete piles of 1.5m diameter and up to 30m long. These foundations are as deep as the existing towers are tall.

But on the west arch base, this was a non-starter due to the small patch of land available.

The challenge was to restrain arch loads of 23MN horizontally 34MN vertically on a foundation which has to stretch back under the main stand structure and thus carry additional eccentric vertical loads from the stands.

Accurate analysis was essential as a lateral deflection limit of 70mm was placed on the arch bearing.

Simple plate analysis was not enough. The huge couple applied to the chosen 25m by 45m by 2.75m deep foundation meant that a 3D finite element analysis of the whole base was ordered.

So involved was the design that 3D analysis of the pile spring stiffness had to be undertaken.

The design package used was unable to do this and so a spreadsheet was developed to calculate deflections. In an iterative process the extra forces resulting from these deflections could be inputted back through the program until the final forces and deflections were found.

The pedestal below the west arch bearing is 'H' shaped in plan and 10m tall. This helps transfer the horizontal component of the reaction into the large base - which has 60 bored concrete piles of 1.5m diameter and up to 30m length.

Assembled at ground level, the 1.650t arch must be hoisted from horizontal to vertical with the help of a derrick atop the north stand. This process alone will exert a 15MN horizontal load on the bases. To help the western foundations cope with this one-off hit, a large temporary concrete strut will spread part of this load into foundations below the stand nearer to the pitch area. 'This whole problem is about spreading the load as much as possible, ' explains Barker.

An engineer's natural reaction to such an arch would be to tie it, at or close to ground level. 'We looked at this, but when you consider its length, the elastic expansion of any tie would be too great. When considering all the other works that need to be built underneath the arch, we decided that we had to find restraint another way, ' says Barker.

At its ends, the arch narrows to pencil points. These then connect to the bases via knuckle joint connections in strengthened steel housings, the details of which are being ironed out by Cleveland Bridge, which must meet a 100 year design life brief twice that of the stadium.

But to raise the arch these joints are fixed and the arch will rotate about lower secondary hinges, which will eventually be fully encased in concrete.

The scale of the project is difficult to explain. 'It took quite a while to get used to working in MN all the time - so massive are the forces we are manipulating here, ' says Connell Mott MacDonald associate, David Hooper.

The arch will support the north stand roof edge catenary entirely and 60% of the south via 130mm thick steel cables.

Catenary trusses with tapered tubes and cable string are as thin as possible as no shadows were to be cast on the hallowed turf. The bowl's lateral forces are gathered via a perimeter prismatic truss.

Two levels of concrete frame are then topped with the steel framed bowl, which uses 10m long seating planks for restraint and a little bit of composite action. To meet planning height limits, the new stadium had to be deeper than the old one - with the pitch level now 4m lower.

*****

There s only one Wembley Stadium

New Wembley will have:

lMore toilets than any other building in the world.

lAn arch that will be tall enough in its final position for the London Eye to be able to roll beneath it.

l90,000 seats that will be totally covered.

lRoom for 25,000 London buses to be fitted within the bowl.

l23,000t of steel l7,000t roof l4,500km of re-bar l400m of escalators l138,000m 3of cut which is 25,000 lorry loads . . . and the trussed arch will be wide enough at 7.4m for a channel tunnel train to run through its centre.

*****

Wembley programme

Now to March 2003 Wembley's twin towers are currently being torn down by demolition contractor Griffiths McGee, which is calling on the largest demolition machine of its kind in the UK - dubbed Alan the Shearer.

November 2002 As demolition moves on, installation of 3,700 concrete piles, up to 35m long and 1.5m diameter, by Stent foundations begins.

Late 2003 Superstructure gets under way with the steelwork bowl built by Cleveland Bridge above the two storey concrete frame by PC Harrington. The arch is built at ground level within a temporary work shed.

Mid 2004 Once welded together, the 1,650t steel arch will be raised to the vertical via a series of pulleys.

Late 2004 Fit out begins.

Early 2005 Once the roof structure is complete the arch can then be dropped to its eventual position at 68infinity from horizontal.

Mid 2005 Seats and pitch installed.

Early 2006 Kick off.

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