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North v South

WEMBLEY

THE 4.4 ha roof is divided into three areas: two fixed cantilevered sections over the north and south stands and retractable panels able to enclose most of the central third, though always leaving the area over the pitch open.

The 5,000 sections of beam, truss or rafter will all be erected from the ground 60 m beneath using crawler cranes.

But initial support for the two fixed areas is very different. The northern section is hung immediately from the arch directly above it - through cables fed down to an integral catenary wire - and needs only three temporary support towers. By contrast the southern section demands a forest of 20 falsework props.

Crucial to both roof sections is their front edge truss, spanning the full 260 m across the stadium. Onto this large beam will be bolted roof rafters running back to the stadium's perimeter truss.

On the northern side, this edge beam is formed from a row of seven pyramid trusses, hanging by their top corners from the catenary cable above.

Kentledge, trailing down to the ground on long wires, holds these 71 tonne trusses stable until the front faces of the pyramids can be joined to create a continuous 13 m-deep truss across the stadium. To bolt it all together, three 60 m-tall towers, linked by temporary walkway beams, are erected beneath the roof to provide constant access.

But all roof loads are immediately transferred up through the catenary cable to the arch towering above.

Not so on the more distant southern roof where support towers are needed.

Roof loading can only be transferred across to the northern side and up to the arch when the four 150 m-long side trusses are in place.