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Meeting their match

Milton Keynes stadium - Designers of Milton Keynes' new football stadium are accommodating changes by making it beautifully simple, Andrew Mylius discovers.

Pete Winkleman takes apparent delight in confessing himself to be 'a nightmare client'.

Now chairman of the UK's Milton Keynes Dons football club, better known until recently as south London's Wimbledon FC, he gained some fame and fortune as a heavy rock music promoter, and still has a manic rock'n'roll intensity.

Winkleman is the driving force behind construction of Milton Keynes' £43M ($76M) new football stadium. Plans for a stadium in Milton Keynes had been kicked around since the town's inception in 1973, but were never put into play.

Winkleman decided to turn the need for a decent ground into a personal quest in 1998, and has since then agitated relentlessly for funding and planning support, chasing potential financial backers and local councillors.

When he selected and briefed architect HOK and structural engineer SKM Anthony Hunts, Winkleman already had a very clear idea that he wanted the stadium to 'look like a spaceship that had crashed into the ground'. But there was a huge amount of detail to pin down.

Part of Winkleman's challenge was figuring out exactly what he could afford to build, as potential backers blew hot and cold. 'We looked at how other stadiums in the country had been built: nearly all had been done with enabling developments.

Otherwise, you can't justify something with such high capital costs and such low revenue.' As first retail giant Asda and then Ikea agreed to build stores alongside the stadium, Winkleman's ambitions grew, leading to design changes.

Changes have also been made as Winkleman has looked for ways to maximise the use of the stadium. It will now stage rugby as well as football, and will host concerts and conferences.

As the project has advanced, he has shoehorned in a hotel, a children's party room, a swimming pool, bars, cafés and shops.

'We have a lump sum design and build contract that Kevin [Underwood, project manager for contractor Buckingham] lets me mess with far more than he needs to, ' says Winkleman.

'The contract is scheduled to complete Christmas next year, but I've put builders and everybody else under no pressure to finish by then.

There's no absolute deadline, ' he adds. He will be happy if the stadium is ready for football by June 2007.

SKM Anthony Hunts structures manager Nick Ling says: 'He knows that if he's going to make constant variations, he has to provide slack. On the one hand he's the nightmare client, and on the other he's the best you could have.' The future home of the MK Dons is a 30,000 seat venue that differs from other stadiums of the same size in having two tiers of terraces all the way round. 'I wanted something like [Barcelona's] Noucamp stadium, or the Moocamp, as we might call it here', Winkleman says, making reference to Milton Keynes' famous concrete cows.

First design was for a stadium with a large concourse running the full circumference of the lower tier, 8m above ground level. Overflying the lower and upper tiers was a space frame roof, clad top and bottom with translucent polycarbonate.

It was beautiful, but would have blown Winkleman's budget to smithereens, says Underwood.

'To save money we proposed an alternative design, which was to excavate a 4m deep pit and build up a 4m high bund around it with the cut material.' By sinking the pitch level below that of the surrounding site, Buckingham was able to form the lower terraces on cut and made ground - the bottom 4m is a simple ground-bearing slab. The slab for the upper 4m of the bowl is supported on radial ground beams cast into the bund. Bitumen emulsion was sprayed on to the ground to stabilise it before concrete was poured.

Buckingham's rethink of the lower bowl eliminated a huge amount of structure, and with it the need for extensive piling.

The only piling being carried out on site is for two concentric rings of columns on a 7.2m grid, from which the roof is cantilevered and which support the upper terraces with a third, inner ring of shorter columns (see section). Two to three CFA piles of 450mm and 600mm diameter provide necessary load bearing capacity.

The upper tier of terraces is partially supported off the roof structure. Initially, the space frame roof design was simply propped on slender steel columns. However, steel for the space frame was priced at $7/t, leading to a rapid re-evaluation of structural options.

A more conventional lightweight cantilever roof could be built, using square, U-C and circular hollow section steel costing $3.5/t. SKM and HOK worked hard to maintain the original roof design's hovering 'anti-gravity' quality as they reworked it.

Sitting atop spindly legs, the roof cantilevers forward 35m. Principal members are dart shaped trusses, 3m deep at the centre of rotation and diminishing to a fine point at their outer extremities. To simplify fabrication as far as possible, the upper chords are 200mm 2 U-C section steel joined to a square hollow section lower chord by 140mm diameter circular hollow section bracing (CHS).

'Use of flat sided top and bottom chords enabled the fabricator Rowecord Engineering to do straight cuts for the CHS joints, ' says Underwood.

Simplifying things further, the tips of the trusses are fabricated from plate steel. No tower crane is necessary; Buckingham is erecting the roof with a single mobile crane and a pair of cherry pickers.

To make construction manageable using such lightweight equipment the trusses have been broken down into three elements, each weighing no more than 5t. Each cantilever is built out by bolting one element to the last.

Underwood has been working equally hard on the way the lower terraces are constructed.

Conventionally, terrace steps are cast insitu in narrow strips using 'staircase' formwork. Casting one full flight of steps after another, the concreting contractor works along the stand.

But at Milton Keynes, Buckingham has devised what it believes is a new - and seemingly obvious - technique. It is casting a single step at a time over large lengths of each stand.

Starting at the bottom of the bowl, concrete subcontractor EP Hayes erects a single, long, low wall of shuttering, gets it plumb with the aid of adjustable feet, and pours a step. Once this has gone off and been finished, the next step can be cast behind it.

'It would have cost $440,000 to hire staircase shuttering, and a tower crane would have been needed to place the concrete.

Then you get problems with hydrostatic load when you poker vibrate concrete in an 8m high staircase. Our system can be moved by two blokes, and you're casting half a metre at a time, ' says Underwood.

As Winkleman's ongoing interventions in the stadium's design add complexity to the project, simplification is being pursued by engineer and contractor, almost in inverse proportion. Buckingham has set out to eliminate any danger of different trades stepping on one another's toes by splitting the project into two.

Groundworks for the more complex west and south sides of the stadium will start next year, while concrete and steelwork on the north and east part is completed. With groundworks out of the way, steel and concrete works will move to the opposite side of the pitch, allowing fit out contractors unfettered access to the north and east stands.

With working at heights regulations in mind, Buckingham is fixing as much of the lighting and PA equipment as possible to the trusses on the ground. Meanwhile, in the space between the roof support columns behind the upper tier of terraces, new facilities may one day be added. To allow new structure to be effortlessly adapted, it is being 'future proofed'. Holes are pre-drilled for future connections, and movement joints can be modified to account for possible changes in the structure's dynamics.

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