Grass ceiling

A roof over Wimbledon’s Centre Court ensures rain doesn’t stop play — but designing the 1,000-tonne retractable cover wasn’t simple. Berenice Baker reports

The new roof on Wimbledon’s Centre Court has had tennis fans for once praying that rain clouds would threaten so they could see the one-of-a-kind covering slide in place.

Although this year’s tournament was one of the driest on record, the white steel and translucent fabric opened for the first time during a game on 29 June in a match between Amelie Mauresmo and Dinara Safina, then memorably had Andy Murray battling Stanislas Wawrinka by floodlight late into the night.

The roof, the total cost of which is a closely guarded secret, was a triumph of collaboration between a project team of mainly UK companies under lead contractor Galliford Try. Its main structure consists of 10 trusses, each weighing 100 tonnes, but its near-silent operation belies the 1,000-tonne mass supported over the heads of tennis fans. Its design is claimed to be unique, with the Toshiba stadium in Tokyo the nearest previous equivalent.

Centre Court’s grass is reputedly the most expensive turf in the world and protecting it was as high a priority for the contractors as ensuring play goes on. Iain Chisholm of Moog Controls, which supplied the equipment that moves the roof into place, said: ‘The biggest problem was how to get such a big structure in such a small space, while ensuring enough UV gets to the grass. We did discuss using hydraulics too, but there was the risk it could leak oil and kill the grass.’

To ensure that the grass has full exposure to the sun, Centre Court is designed so the stand dips lower at the south end. To allow the sun to continue to work its magic, the roof is designed in two halves. For much of the time it will remain in parked mode, with both halves nestled together to the north of the court. When matches are playing, one half is moved south and locked in place ready to be deployed at a moment’s notice in what is dubbed championship mode.

When the rain starts, groundsmen still rush to cover the court with tarpaulins and play is suspended. The truss end deploys, then each truss section moves in alternately over the 77m span, a process that takes around eight minutes at a rate of 214mm/s. When parked, it does so with an accuracy of 0.1mm over the entire span. Air conditioning and relative humidity controls avoid moisture gathering on court and keep the temperature around a comfortable 21°C.

The solution Moog delivered features 148 axes of electrical control; 78 end-arm electric actuators that each produce 35 tons, to move the roof one inch per second; and 40 restraint arm actuators that each deliver 14 tonnes of force and lock the trusses into position.

The trusses form an inverted triangle that is inherently unstable. Motors on the bogies at the base take the weight of the truss. At the end arms, four actuators controlled via a M3000 servo controller on each side pull the centre of each fold up, like a book lying open face-down.

Before work began at Wimbledon, a test rig consisting of a single truss was set up at Moog’s facility in Sheffield. Chisholm said that, given the nature of the project, the design started by working through the safety logic. ‘We worked through the “what-ifs?” — what if the drive fails, or the brakes are damaged?’ He said. ‘We can drive through some error modes, but the roof can’t be stopped dead as that would send shockwaves down through the entire building.’

The roof is controlled via a control-panel housing a supervisory system developed by Fairfield Controls that features a programmable logic control (PLC) interface with M3000 servo controller. The roof position is monitored via a Rockwell SCADA (Supervisory Control and Data Acquisition) graphic interface and CCTV, which also gives an enviable view of the tennis action. The roof position is calculated via absolute encoders rather than positional sensors, and the system monitors errors to feed in a stop to the system if a critical situation is detected, for example if a person gets onto the roof.

Five big green buttons allow one-touch access to the main modes: championship, parked, sunshade deploy, deploy and retract to championship. It also has lights and air-conditioning controls. The first of its kind to deploy with 15,000 people inside, the roof can withstand wind speed up to 25m/s; once deployed, restraint arms provide a line of force holding the structure together.

The maintenance cycle is designed to be work-through. ‘We want the actuators there as long as possible, as they were put there with giant cranes,’ explained Chisholm. ‘They have had the equivalent of 12 years’ testing done over five years. Although they will only be deployed around 52 times a year, inspection will be carried out four times a quarter to see if anything needs replacing.’

To ensure that things keep moving, the roof has a pre-test mode in which each actuator is moved by just a millimetre and the bogeys move 3mm, which makes sure the locks are off. It is moved every week for maintenance.

The roof fabric is Tenara by Gore, made in the US and fabricated in Germany and Poland by three designers. It was selected as being the only fabric suitable for the constant articulation and it allows 40 per cent of available daylight through, meaning in daylight the additional flood lights are only needed to support HDTV coverage.

The bearings were supplied by FAG, part of the Schaeffler Group (UK). Derek Peasley, Schaeffler’s product support engineer, said: ‘The Wimbledon roof is a concertina-type design and there are a number of folding sections. In the same way as a door has to open and close, then the concertina action of the roof also needs bearings at those folding points.’

Contracted through previous customer StreetCraneXpress, which put the structure in place, Schaeffler supplied 300 separate rolling element bearings; spherical roller bearings in the hinges of the separate bays of the folding sections and the spherical roller thrust bearings to support the high tension imposed by the wire, which is stretched across the width of the bay to pre-tension the roof and reduce the amount of deflection across the span.

The radial loads on the spherical bearings were in the order of 300kN of 30 tonnes; the axial loads on the spherical radial thrust bearings were about 600kN. These were all put into Schaeffler’s load programs and the effect of those on the bearings was calculated.

Engineering manager Dr Steve Lacey said: ‘Our top priority was the safety-critical nature of it — being over everybody’s head we could ill afford to have anything go wrong. And if those bearings didn’t rotate or decided not to move during a Centre Court match on a Saturday or Sunday final when it starts raining, it would be embarrassing.’

Although opinion remains divided on this recent upgrade to a 1923 iconographic structure, the roof’s ability to keep tennis championships going in the face of inclement weather has won its detractors over — game, set and match.