Moment of truth for centre court
As Wimbledon once again prepares to host the world’s most prestigious tennis tournament, an army of contractors waits to see if their addition to the hallowed centre court will be a winner. By Kevin Walsh
Client:All-England Lawn Tennis Club
Main Contractor:Galliford Try
Architect: Populous (formerly HOK)
Steelwork contractors:Watson Steel Structures
Start Date: June 2007 (Enabling works June 2005)
End Date: May 2009
Each year the modern greats of men’s and women’s tennis descend on London to compete for the coveted Wimbledon trophy, enduring injury, occasionally questionable umpiring and even Cliff Richard entertaining spectators during heavy downpours.
But as of this year, they need not worry about the unpredictable British weather thanks to the installation of Centre Court’s much-anticipated retractable roof.
After seven years of work the All-England Lawn Tennis Club (AELTC) has followed the example set by other tournaments around the world so that this year’s Wimbledon competitors can now play through bad weather and, if necessary, continue well into the evening thanks to powerful lighting attached to the roof.
The multi-million pound retractable roof, covering an area of 5,200 sq m once deployed, consists of nine moving steel trusses supporting a folding concertina-style translucent fabric.
Once heavy rain starts the 100-tonne trusses can be moved using bogies attached to each end at speeds of up to 13 m per minute in a carefully controlled closing operation. The process of closing the 3,000-tonne roof can take less than ten minutes, though this belies the complexity of the control systems needed.
Capita Symonds director of structural engineering, Jean-Philippe Cartz explains: “There’s a number of elements that work simultaneously make the roof move. You have the bogies, the end-arms and you have the actuators, and all that’s working together on a structure which is almost 80 m long that has to close to within 5 mm.
“All these elements have to work in tandem, so you’re pulling and pushing at the same time in different positions on the truss. You can imagine all the software behind that, self-correcting software, sensors measuring all the timings. The number of accidents and problems that could go wrong!”
To design moving elements that would enable the roof to move quickly while under carefully monitored controls, Capita Symonds enlisted the help of mechanical handling specialists Street Crane Express (SCX), and it has been impressed by the results.
“They are very specialised and the stuff they’re doing for the software that controls this is fantastic,” says Mr Cartz.
SCX designed bespoke control software to regulate the roof’s movement, and is capable of reacting to any potential faults during the opening and closing processes.
SCX Project manager Dan Salthouse says: “There was a lot of initial computer modeling, calculations and analysis of each and every individual structure and component, based on the structural analyses that had been done to make sure that it was strong enough. Watson Steel then built three of the trusses and we fitted all our equipment to them and subjected it to all the movement criteria that it has to do on top of the roof.”
With the help of subcontractors Moog Incorporated and Fairview Controls, synchronisation of the moving elements of the roof was achieved with surprisingly few problems for a project of this size, in no small part due to the tight controls on the closed loop control system.
“It did go very well. We were trying to control 108 actuators, 40 motors and about 80 locks, synchronising and coordinating all of those elements. Mostly we had to relax the tolerances that we were working to rather than increase them because we found we had too precise a control system for the structure. When you’re moving something that’s 77 metres long, weighing 100 tonnes and is reasonably flexible, if you try and tie it down too tightly you just start to set vibrations off.”
With all the plant and equipment attached, the finished roof structure weighs 3,000 tonnes, so to accommodate the additional load, two load sharing columns were added to support the original foundations.
These columns are designed in the same manner as a spring, so instead of the weight sitting on them and loading completely, the design allows a certain degree of flexibility with how much of the total load it supports at any given time. This works as a cut-off mechanism to ensure that they do not take on too much of the total load.
“It is designed to the normal codes,” says Mr Cartz, “but you’re talking about 33 different load combinations, it’s a really complex list of scenarios. There’s something like 50 different combinations that we had to look at to see what kind of movement you get under different conditions.”
When the roof is not needed, the 100-tonne trusses travel along bogies attached to their ends to the north and south sides of the roof, allowing maximum sunlight onto the carefully maintained lawns. But even when closed the special Tenara fabric, provided by Gore, allows 40 per cent of natural light through while keeping out the rain, to keep the lawn in good shape.
The importance of this cannot be overemphasised, as the AELTC’s Chris Conroy explains: “The biggest worry in tennis which you don’t have so much with football or rugby is players slipping on the grass. With football and rugby at least they’ve got studs on. So we have got moisture sensors and humidity sensors in the grass and around the bowl that record the relative humidity and grass water content, they tell us when it’s okay to play. Then the referee comes out, puts his hand on the grass and makes the final decision.”
With this year’s tournament only a month away, the AELTC has all but completed the transformation of the grounds for championship mode, and the one of the most important checks left to be carried out is the testing of the new ventilation system for Centre Court when the roof is closed.
To ensure the best quality playing surface, the grass at centre court has to be kept within certain parameters as it is sensitive to changes in temperature. A few degrees too high without adequate ventilation and the grass will start to sweat, creating an inconveniently slippery playing surface.
To this end nine chillers were installed in the car park along with 14 air handling units built into the enclosed fixed roof space, that will deliver 143,000 litres of conditioned air per second to regulate the temperature under the closed roof.
Furthermore, the air delivery system has been designed and tested by Capita Symonds to ensure that it is sound insulated, so while it is in operation any noise it generates should be at a lower level than the ambient noise from a capacity crowd.
As Construction News went to press, the ventilation system and new roof was due to undergo final testing at a one-off event on 17 May. Former stars of international tennis Andre Agassi, Tim Henman, Stephanie Graf and Kim Clijsters were scheduled to be the first to play under its shadow.
Although expected to perform, it is unknown how well the ventilation system will cope with regulating humidity amongst a capacity crowd of 15,000.
“With a big task like this, once you’ve closed the roof there will be 15,000 people seated in here, and the big test on 17 May is in going from outdoor configuration to indoors configuration quickly,” comments Mr Cartz. “The celebration would be very short-lived if you start getting condensation, so ventilation is key. The challenge is to see how long it takes to get the right playing conditions after it switches on.”
The entire closing operation, including humidity regulation inside the enclosed centre court is estimated to take up to a maximum of 40 minutes, ten minutes to close the roof, and a further 30 minutes before the internal temperature is right to commence play. Once rain has stopped it takes just ten minutes to open the roof, with an additional ten minutes beforehand to let the high-power lighting cool down, thereby avoiding heat transfer to the roof fabric.
Preliminary smoke tests are being carried out this week to monitor the airflow within the bowl in advance of the event.
Principal contractor Galliford Try has had a long-standing relationship with Wimbledon and have had a presence there since 1973. The culmination of years of work, Galliford Try project manager Mike Bridges is pleased with the end result: “From our point of view this is just part of the long-term development plan, and we’ve got more work to do next year. The job’s gone fantastically, it has been a great team build and team effort from a design point of view. We’ve been working on this for six years, bringing it on from a concept to completion, and it’s been the most fun I’ve had on a construction site for years.”