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Overarching achievement

Wembley Stadium - After 18 months waiting for the rest of the stadium to catch up, the iconic Wembley arch is back in the limelight. Ruby Kitching went to see.

When the mighty Wembley Stadium roof was first erected in June last year, it grabbed the world's attention - the home of English football had been crowned with a glorious 315m wide, 133m high tubular steel trussed arch.

Since then the arch has been held by five giant temporary restraint cables and has had to wait while the rest of the stadium bowl has matured.

But the arch is back in action now as, in the run-up to Christmas, it undergoes the final stages of depropping, allowing the 7,000t roof to ease into its final position.

For once, what is arguably the most famous structure under construction in the UK has escaped notice during the last six weeks of arch depropping.

Probably this is because transferring the arch load from temporary cables to a permanent cablenet structure has involved the tiniest of tweaks, tilts and rotations.

Only an eager eye would have spotted that the arch has rotated between just 0.3- and 2.6- since October.

'So much of what goes on at Wembley goes unnoticed, ' laments Riccardo Petaccia, structural steel site manager for main contractor Multiplex.

The beauty of the Wembley Stadium arch is that it supports 70% of the overall roof weight, removing the need for columns in the terracing. This ensures all spectators have an unobstructed view of the pitch.

Wembley's roof is made up of six components: the north roof, south roof, retractable south roof, four main north-south trusses, the prismatic perimeter truss (PPT) and the arch (see diagram). A cablenet with 16 backstays and 16 forestays connect the arch with the north roof, which in turn supports the north-south trusses and part of the south roof.

The original steelwork contractor, Cleveland Bridge, was replaced by Hollandia after a dispute with Multiplex.

The arch was first constructed in the stadium pitch area and raised into position using turning struts. As it passed through 90five restraint cables kicked in to fix the angle at 112- (NCE 10 June 2004).

With the arch in position, the PPT was constructed around the top of the stadium seating structure. The north and south steel roof structures were then built off the PPT and supported off temporary towers. These roof canopies are made up of tubular steel truss rafters where the bottom boom is a stressed cable.

Petaccia has been in charge of the structural steelwork since February 2003 and supervised every piece of steelwork in the roof.

'It's not that we've got hundreds of pieces to erect, it's a case of going through a very definite sequence to make sure all the deflections are as predicted, ' he says.

For Petaccia, the most stressful period was connecting the main cablenet element, CT17, to the arch and PPT last month (see diagrams). Rotating the arch from 109.5- to 111.1ensured that CT17 was stressed as it stretched, allowing the weight of the arch to transfer from the temporary restraint cables to the cablenet.

Stressing CT17 also caused the PPT to contract up to 60mm at its eastern and western extremities.

'The initial survey to check that the nodes were in the right place was very tense, ' he admits. More than 70 people joined in with Petaccia's concerns by surveying around 247 monitoring points on the arch and stadium bowl.

With the arch, PPT, roof canopies and cables now in their final positions, work in the New Year will concentrate on adding roof cladding and the retractable southern roof. These extra loads will mean that the whole structure will continue to redistribute loads until the last panels are installed.

The nal crucial moment for Petaccia and the Wembley roof will take place in January when the north roof rafters are jacked up to 1m before the temporary towers are dismantled.

Multiplex aims to hand over the site to client Wembley National Stadium Ltd on 31 March 2006, and Petaccia is confident this will happen. Steelwork erection is on programme and pitch construction well under way.

Drainage and underground heating has been installed and the sub base is being laid.

However, speculation is still rife over whether the stadium will be ready for the May FA Cup final - the Cardiff Millennium Stadium has been booked in case the Wembley deadline is missed.

Last week the final back stays were being stressed, and by Christmas the five temporary restraint cables and roof support towers will have been removed to allow the entire north roof and 60% of the south roof to be supported by the arch.

'It's been hectic, but tremendously rewarding seeing the roof go up. When it's finished, it'll be the best stadium in the world, ' Petaccia concludes.

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