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Structures - Test of time

Melbourne Cricket Ground - On of the world's most famous arenas is being redeveloped in time for the 2006 Commonwealth Games.Ruby Kitching reports from Australia.

Melbourne Cricket Ground (MCG) has experienced a slow metamorphosis over the past two years - so slow, in fact, that the change will have gone largely unnoticed by TV audiences tuning in on this side of the world.

But for cricketers and MCG regulars, the shift from a fusty 1950s venue to a state of the art ground with conference rooms, shopping and dining is welcomed with open arms.

Main contractor Grocon and a joint venture of Connell Mott MacDonald and Arup (CMMA) are responsible for completing the £186M project just in time for the 2006 Commonwealth Games.

Asking any of the engineers to describe the project draws a crazed expression and a stammered 'Where do I start-' Although the southern half of the stadium was rebuilt in 1992 to give unrestricted views, the Olympic Stand was a legacy of the 1956 Melbourne Games and the Members' Pavilion was trapped in the first half of the 20th century.

The project involves knocking down and rebuilding the northern half of the stands including a new roof canopy and the pitch itself.

Essentially this includes erecting a 55,500 capacity stand and tucking a seven storey structure beneath it.

'The southern stand is a very enclosed building compared to the northern stand, which has more of a shopping mall feel with views out to the city and Yarra Park, ' says architect Brian Felgate of MCG-5 Sports Architects, a joint venture of five practices formed for the job.

With the new building, light and space are being brought to MCG for the first time. A spectacular glazed wall and three atria front the new MCG with bars, restaurants and a new museum.

Rebuilding work on the Olympic Stand and associated buildings is well under way, although work will continue right up to the end of January 2006, just three months before the start of the Commonwealth Games.

While most construction jobs have just one deadline to contend with, the MCG rebuild has had to skirt around major cricket and football matches every six months.

'We have to maintain at least 70,000 seats during the reconstruction and 80,000 for key events, ' says Grocon design manager David Waldren.

For this reason, redevelopment is phased over five stages which involves demolishing and rebuilding half the stands, improving the drainage beneath the cricket field, laying a new athletics track beneath the grass pitch and building the new conference, dining and sporting facilities. The new stand will also have 3,700 more seats, to bring the capacity of the MCG to 100,000.

'We are dealing with every form of construction at each stage of this project - commissioning kitchen facilities, laying foundations, tensioning roof cables - all while allowing the public to pass through, ' says Grocon project manager Steve Richardson.

Not surprisingly, design and construction has been heavily influenced by the tight time windows.

For the roof structure, too much time spent constructing and demounting scaffolding would eat into the programme and restrict public access. The solution was to go for a tension structure supporting a steel frame roof canopy over the seating area.

'The roof had to be light enough to span the depth of the canopy without deflecting too much, but heavy enough to withstand wind loads and overturning, ' says Arup senior associate Frank Gargano.

The roof canopy is made up of steel box girders up to 42m long, connected to the back of the concrete seating by a pinned joint. The other end is held in place by a cable connected to the top of a tubular steel mast which is itself anchored back with cables to the concrete structure behind the new seating.

Steel masts are located every 16m along the length of the stands, varying in height from 13m to 27m. They also taper from 800mm in the centre to 400mm at the ends. Backstay and forestay cables are 65mm to 80mm in diameter and are made from galvanised spiral bound steel strands.

The upper tier of seating is also supported by tubular steel elements which sit on the concrete structure beneath. The tubular steel theme is carried through the stadium and atria.

Steel fabricator is Australian company Alfasi.

'The mast and back stay cables are put up first using temporary supports. The rafter is then craned into position and connected to the back of the stand at one end and to the top of the mast via a forestay cable at the other, ' explains Gargano.

The cables, he points out, come from Doncaster, UK and are the same as those which feature on the new Wembley Stadium roof.

But the whole structure only works if the cables stay in tension. To maintain this, the box girder rafters and circumferential box girders are filled with concrete to ballast them against lifting in gusts of wind.

A pecial superflow' concrete mix had to be developed by Grocon to ensure it could be placed in the hollow steel rafters using hoppers without being vibrated. Prefilling the rafters with concrete would make them too heavy to be lifted by tower cranes.

Before e oncrete is placed, the end of the rafter is tied down using temporary cables to allow adjustment of the structure's geometry and prevent uplift.

A network of 22mm diameter cables ties the rafters together.

This cable net helps stabilise the mast and redistributes wind loads evenly across the whole structure. Arup's in-house structural analysis software was used to model the effect of wind on the structure.

The cable net also restrains the top of the mast, removing the need for a separate bracing system, and ensures the design brief for a 'light, elegant, transparent structure' is maintained.

Metal sheeting is used to clad the back half of the canopy while the front is covered in 17mm thick heat strengthened laminated glass.

Each 16m bay can be built independently, following on behind the last 'like sausages in a sausage machine. When one bay is completed the next begins', says Grocon design manager David Waldren.

Most importantly, sections can be completed in bite sized stages to fi t in with the MCG's operational programme.

The result is worth all the trouble, says Connell Mott MacDonald senior associate Mark Sheldon: 'What you've got here is world class sight lines - you can see action on the boundary, even from the back seat.' With such tight deadlines and logistical problems of working around an operating stadium, specifying prefabricated columns, floor units and walls was another way of facilitating quick, neat construction.

'It took a real team approach to work out what could be precast and what couldn't. Sometimes there wasn't enough craneage or room to place elements, ' says Richardson. Since all the cranes had to be on the outside of the stadium, accessing all parts of the arena interior was impossible.

Lessons were also learned from the construction of the western scoreboard at the start of the project in 2002. The fivestorey structures took longer to construct than programmed, so the eastern scoreboard, currently being built, is incorporating top down basement construction to hasten the superstructure.

Over the next few months construction activities will dodge a weekly series of Australian Football League games, but will have to halt to allow reinstatement of full stadium capacity for the grand final at the end of September.

Work will then begin to remove the entire playing surface so the new athletics track can be laid. The pitch will be re-turfed and the centre wicket installed in time for the 2006 Boxing Day test.

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