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How the roof works

WEMBLEY

SIMPLY PUT, the roof of the new Wembley is a lattice of criss-crossing roof trusses held up from above by the giant arch and from below by a perimeter truss that rings the entire stadium bowl.

Starting on the south stand, the roof cladding is fixed to lightweight rafters that run from the perimeter truss to a 260 m-long roof truss spanning the full length of the stadium.

This is supported at either end by the perimeter truss and along its length by four massive lateral trusses - two 150 m span and two 130 m span - that cross the pitch.

These trusses have one end fixed to the perimeter truss and the other end suspended from the arch.

'They get lost with all the talk about the arch but these 150 m spans are serious structures, ' says David Satchell, the engineer in charge of the roof and arch package for Connell Mott MacDonald.

'They are up to 15 m deep.'

The square sectioned top chord of these main trusses is fabricated from 80 mm-thick steel plates and is 800 mm wide by 700 mm deep. Rails for the moving roof run on top. The bottom tension chord is a bundle of four cables.

'The bottom chord could have been a tube, ' says Mr Satchell. 'But it was important to keep the shadows to a minimum. So that at 3 pm on Cup Final day, there are as few shadows as possible.'

Because of the slenderness of the trusses, the engineer had to carry out a very detailed buckling analysis to ensure they were adequate. The trusses are braced with a slender high-tension cable system.

'We had to do a lot of analysis on the trusses. Nearly as much as on the arch, ' says Mr Satchell.

The leading edge of the north stand roof is suspended directly from the arch.

Between the cables holding up the roof is a catenary cable. This is crucial in the early stages of the arch erection and provides permanent stability to the arch.

Once the arch is erected (see feature, pages 18-19), it will be held in place by temporary stays while the catenary is installed. When the two structures are connected, the arch will then be lowered to its final position.

At this point the entire arch/catenary structure is stable, and construction of the remainder of the bowl and roof can continue.

The next members to be fitted will be the two 150 m lateral trusses, which impose massive point loads on the arch, pulling it forward. At this point the back stays will stop the arch falling. These are tied to the perimeter prismatic truss (PPT) that rings the entire stadium bowl.

'The PPT is a longitudinal truss that has a certain amount of ring action. The catenary cables put massive tension into the PPT, which distributes the load to the cores, ' explains Mr Satchell.

'The PPT has just about every load from every direction. Because it is the perimeter point of support for the roof and the bowl, it is probably the most complex interface.'

The PPT was designed by the London office of Sinclair Knight Merz, which is part of the Mott Stadium Consortium.

'It's not just a case of us saying what the roof loads are into the PPT and job done, ' says Mr Satchell.

'The PPT has its own stiffness. How stiff it is affects the roof loads. Then the roof loads affect the stiffness and so on.

It becomes an iterative process. We had to do half a dozen iterations.'