Tools have been downed on the first of three phases to install a new moving roof over Court 1 at Wimbledon Tennis Club as the tournament gets underway. Designer Thornton Tomasetti has spoken to New Civil Engineer about the complex engineering challenges behind the feat.
Work began to design the new roof in 2013 and it takes a similar form to that of its counterpart on Centre Court, with a new moving concertinaed cover over the field of play. However the court 1 roof will be longer than its predecessor with an extra bay of fabric to cover the opening.
Although the arena had an existing roof, this has had to be removed to make way for the new covering which has had to adhere to some strict constraints. A planning restriction placed a limit on the overall height of the new roof and the opening had to be big enough to let enough natural light to get to the grass court below. Sight lines from commentators and spectators and a need to be visually undistracting for the players also dictated where the structure could be placed.
Two new, 80m long, 9m deep prismatic main trusses supported at each corner of the arena will allow the new roof to slide open and closed when rain threatens to stop play.
The existing dodecagonal continuous seating bowl is split into four separate stands behind, each with two reinforced concrete cores at either end. Because the original arena included a provision for a future moving roof, each of the cores has 15,000kN of additional capacity.
Each of the main trusses span north to south and at their rear are supported by the two existing cores in both the east and west stands and at the front by four new points of support. In the south-east, south-west and north-east corner three new ‘super’ columns between the stands support the front of the trusses, however in the north-west corner, due to restrictions in the basement a column could not be installed. Here the truss is supported by a complex, cranked transfer truss spanning between the cores in the adjacent stands.
The covering is then supported by secondary 6.6m deep, 1.5m wide trusses which span 74m between the main trusses.
A further planar truss in the north of the arena, spanning east to west, supports a new plant deck.
Conditioning the space
“That’s been one of the big challenges on this job,” said Thornton Tomasetti associate director Michael Roberts.
“Basically as soon as you close the roof it becomes an indoor space, so it needs to be conditioned to stop condensation forming on the grass and on the underside of the roof.
“There is a fantastic amount of mechanical and electrical equipment which has to be accommodated in the roof structure to make sure it’s functions correctly.”
Weekly co-ordination meetings are held to avoid clashes between the tangled web of services which are necessary to condition the space and the complex structure.
The roof itself will take around two and a half minutes to close, but to allow the space to be conditioned, play will be stopped for at least 15 minutes.
Movements and deflections
To design the roof around 1500 static load case combinations were analysed with an additional 150 combinations for the moving roof.
“In terms of how we had to look at it there are so many positions in which the moving roof can be in and then couple that with the varying wind condition, snow conditions, thermal conditions it really does become quite complex,” he said.
One of the other major challenges said Roberts, has been to control the movements and deflections of the structure to allow the roof to function smoothly.
“We’ve got a moving roof which is powered by metal mechanised components machined to fractions of a millimetre by the specialist SCX,” he said. “But then that moving roof is sat on long spans trusses that themselves, as the roof deploys, deflect by about 120 mm. They are then supported by an existing building which has its own movements.
“So the biggest challenge was getting to grips with how things were moving and interacting.”
Despite these relatively large deflections, Roberts said the smooth opening of the roof relied more on the relative gradients of the structure.
“It’s more relative than global,” he said. “What has been important, is looking at the gradient of the rails and making sure as the roof takes to its different positions along the rails, we are not getting any gradients that are too steep for mechanization.”
The fabric for the roof is a special coated woven fabric called Tenara.
“There are a number of things which are quite important about the fabric,” said Roberts. “It needs to have the right amount of light transmittance to have the same feeling of natural daylight coming through.
“But then at the same time, the underside has to have a certain amount of reflectivity because it’s backlit from the underside as well.”
To aid construction of the roof and save space around its congested perimeter, contractor Sir Robert McAlpine, came up with a design to mount the tower cranes onto four of the cores. To do this however, it first had to post-tension the cores to make sure they will not overturn during the cranes’ operation.
The cranes have been taken down during the tournament but will be reinstated in the next phases of work.
Tensioned bars run inside the steel framing within the lift shaft and are anchored into the side walls in the lift pit. A pre-compression is then put into the bars to stop the cores going into tension.
“We’ve had quite a close relationship with a temporary works designer,” said Roberts. “It’s a balance to make sure we have enough compression in the core to counteract the overturning of the tower crane, but not too much so as to overstress the core.”
The project is currently at the end of its first phase of three, nine month long packages of work – split to work around the two week tournament – with the new roof planned to be showcased in the 2019 tournament.
Roberts said: “If you look at the Wimbledon website they say they had a global news audience of a billion so this is very much a structure which has to work first time every time there’s no room for error.”