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January 2009 £5.50 19 US$20 · www.ribajournal.com

The Cosy comeback

journal

116/1 January 2008

Richard MacCormac, Christophe Egret, Sarah Wigglesworth, Rogers Stirk Harbour, the Manser family. Plus: Palladio, Arts and Crafts, and Wimbledon's wondrous roof

Cliff Richard devotees will be devastated, but for everyone else the new moveable roof that will close off Centre Court from the rain is an engineering triumph. By Jan-Carlos Kucharek

Photographs by Thomas McNulty

Cliff fans, look away now O

ne might be forgiven for thinking that little has changed at the All England Lawn Tennis and Croquet Club since architect Captain Stanley Peach first constructed Wimbledon's Centre Court in 1922; in so doing creating the iconic setting for arguably the world's most important tennis match. Peach was an avid bird shooter, and an apocryphal story tells that the Centre Court's superlative sightlines and ball-viewing quality were derived from the maximum distance that a shooter would be able to identify a grouse hen or cock. The story might be a myth, but an undisputed fact is that Peach's 13,509 capacity stadium was built in nine months flat for the 1922 finals, all for £140,000. Since its construction, it has served Wimbledon well, being extended five times in its lifetime; but taking shape now is the most radical change to occur to the complex since it opened. In summer 2009, Wimbledon officials will be able to close off Centre Court to the elements in 10 minutes. The disappointment felt by the nation when `rain stopped play' will be a thing of the past, and one of the UK's most traditional organisations

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will be owner of one of the world's most technologically advanced roof structures. Mike Bridges, director for main contractor Galliford Try, who has worked on all the development projects at the Wimbledon site since 1981, says the club itself was happy with the open Centre Court arrangement until the demands of the broadcast media and the expectations of millions of viewers precipitated a change in thinking. While it would have been easier to demolish the Centre Court and start again, a historical connection with Peach's original design ruled this out from the start. The engineering challenge, co-ordinated by Edge Structures consultant engineer John Westmuckett (retained by Capita Symonds to complete the project), was to devise a strategy that would allow the original design to be structurally modified so it could take not only the 1,700 tonne load of the new fixed roof (the original weighed only 350 tonnes), but the additional load of the moving one. Bridges also notes that the development history of the Wimbledon site has been dictated by one condition ­ namely, the opening day of the championships. He says there is no

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roofing

Above: The trusses of the Centre Court new roof, in its semi-retracted state. Left: Visualisation of the Centre Court with the roof open, closing and fully closed. RIBA Journal | January 2009 51

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roofing

Creating the roof has also yielded extra tiers in the Centre Court itself, taking capacity from 13,500 to 15,000. The trusses are on sliding bearings to take account of movement in the box girders as the roof starts to deploy (top). Above: the methodology of the `batwings' opening on the sides took a lot of design development to ensure that the opening sequence is smooth and unobstructed.

such thing as a delayed practical completion date, and contracts are negotiated on a prime cost basis to give the contractor the flexibility to do what is necessary to accommodate the works and complete. Bridges says that has even meant construction cranes being taken down before the tournament and reinstalled within 24 hours of the men's singles trophy being raised. `So the team was faced with three challenges,' he explains, `The programme, the club's requirement that the existing levels of UV exposure and ventilation should be no worse than with the existing roof, and the plan-

ners' restrictions on the additional height of the Centre Court itself.' These demands were doubly onerous for engineer Westmuckett, as the roof not only needed to generate minimal shadows during the two weeks of the championships, but in its `parked' position, there were to be no adverse effects on the growing regime of the grass. `You might get away with substandard grass for rugby and football, ` he says, `but that is simply not an option with tennis. We were as concerned with the time the championship's weren't being played, as the UV exposure throughout the year is key to ensuring perfect growing conditions for the court's grass surface.' This meant co-ordinating the roof design with architect HOK Sport to conduct daylight studies. The new roof is supported on four huge trusses spanning the space between four `supercolumns' that have been driven through the existing structure, chunky enough to take the roof's 1,100 tonne structural load, as well as an additional 400 tonnes of mechanical plant built into the static part of the roof. Westmuckett explains that `loads on the two northern supercolumns are greater than at the south, where the loads are shared by two existing columns to reduce the depth of the new structure, in deference to the only "original" south facade of the court'. It is these huge trusses that will take the load of the 10 Gore `Tenara' PTFE fabric-covered folding steel trusses, each spanning 77m over the Centre Court, rising 6m from the roof eaves line, and which will run on bogeys along a track on the east and west sides. `These moving trusses are connected to each other via specially designed 16.5 tonne "end arms" ­ basically a huge hinge with a bearing in the middle driven by elecro-mechanical actuators manufactured by Moog, which

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allow for the concertina action of the roof,' says Westmuckett. `To allow for UV exposure, the roof will normally be "parked" with all 10 trusses tucked up within 300mm of each other on the north side. But during the finals, five trusses will be moved to the south side in preparation for deployment.' The bogeys, on which sit the sliding bearings for the trusses, are all individually driven ­ as are the actuatorcontrolled hinged arms that open out across the top of the roof to `lock' it in position once deployed. All driving mechanisms and actuators are electronically calibrated with each other, ensuring that the mechanism rolls the roof out in a measured, self-correcting manner. The actuators, normally used in electronic aeronautics and defence applications, have micromillimetre precision.

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Testing the mechanism

Westmuckett explains that the smooth running of the mechanism was a crucial aspect of the design, as the All England Club would only accept the proposal if it was convinced it would work. So it decided to build a full-scale prototype with fabricator SCX in Brinsworth, near Sheffield, a year before any structure was installed above the Centre Court. `This allowed us to test every aspect of the design, from the castings and fabrication of the main structure, to the primary motive elements, to the fixing of the fabric. It was basically a huge R&D project, where we tweaked and tested everything before installation. The construction occurring on site is actually nothing more than a mere assembly operation, albeit on a large scale,' adds Bridges. The necessity for the roof to provide a virtually sealed environment meant a lot of research by the design team into the origami-like folding `batwings' that open out within the end arms to seal the roof as it deploys. There is also the consideration of this side interface with the drainage gutter that runs along both sides ensuring all rainwater is distributed to the edges to be stored below the Centre Court and pumped out to the Wimbledon balancing lake across the road. The edge condition is important because of the air conditioning that kicks in the moment the roof begins to close, designed to hold the internal environment at around 24ºC, with 50% relative humidity. Maintaining this will be a fresh air input of 120,000 litres/sec, pumped in at high level by the 14 air-handling units installed within the main trusses of the fixed roof, and fed by nine remote air-cooled chiller units. These will distribute air evenly to the court via nozzles in two main ways ­ across the underside of the roof fabric to prevent the forming of condensation, and down over the seating at initially high velocity, to dry out the court surface. Once playing conditions are stabilised, within 30 minutes of deployment, rates are lowered to reduce noise, but always maintain a positively pressurised internal environ54 RIBA Journal | January 2009

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Installation of the new roof has been a logistical nightmare, with cranes even being removed for the championships and then reinstalled. The final two trusses, being welded at ground level, will soon be ready to be lifted into position. Commissioning will start soon afterwards.

ment, to take account of the open vomitories within the raked seating. It is obvious from the quality of the build, the level of the prototyping, consultant coordination and incredibly complex site logistics, that the new Centre Court came at a price, even though costs are not mentioned. But like wearing anything but tennis whites at the championships, that conversation would seem a little undignified, given what has been achieved, and what the new roof means for Wimbledon. `The programme forced us to design and

construct this project in parallel, on one set of foundations, with no extension of time. That's similar to the "groundbreaking" contract on T5 ­ but we've been doing that since 1984. Everybody who worked on this project knew the conditions, and has happily worked within them for years,' notes Bridges. A labour of love for all the team then, in realising a project that will secure the viability of the All England Club for the foreseeable future; and contractually, a rare example, but maybe a fitting one for the game of tennis, of the efficacy of a `gentleman's agreement'.

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