The London 2012 Olympic stadium seems to have sprung up on the East London skyline. Piling started a little over a year and a half ago but already design and build consortium Team Stadium is putting the finishing touches to the cable net roof.
Everything is going to plan and the Olympic Games deadline looks within easy reach for the project team of contractor Sir Robert McAlpine, designer Buro Happold and Populous.
The idea for a cable net roof came relatively late in the design process, so keeping the roof structure separate from the main stadium bought the design team extra time to progress the idea as well as lending itself to demountability post 2012.
“In 2007 we had a different roof, a much more muscular one,” says Populous associate principal Mark Craine.
“In 2007, we had a much more muscular roof. At the end of the year Buro Happold suggested a cable net roof. We liked it− it embraced the lightweight structure.”
Mark Craine, associate principal at Populous
“At the end of 2007, Buro Happold suggested a cable net roof. We liked it− it embraced the lightweight structure. We detached the roof from the bowl so by the end of 2007 we could start issuing information about the bowl to the precast manufacturers, leaving time to design the roof. Separating the two was a significant part of the design, it allowed us to keep to programme.”
The separation of the two worked well structurally− the steelwork of the main stadium needs movement joints to release any pressure generated when the structure moved thermally. However, the cable net roof cannot be broken up by movement joints.
.“It works well structurally,” says Buro Happold senior structural engineer Rastislav Bartek.
“The bowl steelwork has movement joints for when it expands and contracts. The roof [truss] is a compression structure so it can’t have movement joints in it. The two can expand and contract independently.”
The first of the 15m high, 85t steel sections of the supporting structure was lifted into place at the end of January last year.
“The jacking of the cable net had to be carefully controlled. Initially, the whole net could be pulled off the scaffold.”
John Calland, Watson Steel Structures
The giant white truss that runs around the outside of the stadium supports the roof and is structurally independent from the rest of the bowl steelwork− the lower tier of precast concrete seating rests on precast concrete rakers and the upper tier rests on steel rakers.
The last of the 28 steel sections was in place in July and the compression truss, which sits on the top of the external shell, was closed at the end of September.
After that, work could start on the cable net roof. The roof comprises the external steel truss compression ring and the inner tension ring, which is made from circular cables. The tension ring is suspended from the compression ring by 28 sets of main suspension cables − an 80mm diameter cable runs from the tension ring to the top of the compression truss and a 70mm diameter cable runs from the tension ring to the bottom of the compression truss.
Strand jacks were used to erect the roof. First, the tension ring was laid out on a temporary support scaffold erected around the stadium bowl. Some 28 working platforms were then erected on the terraces− one for each jack.
“When the system is full tensioned, there’s 1,300t in the tension ring, which is a huge force.”
Rastislav Bartek, senior structural engineer at Buro Happold
The suspension cables were then unreeled on the ground and lifted into position with tall tower cranes− draping down from their connection at the top of the external compression truss to the strand jacks sitting on working platforms on the terraces halfway between the compression truss and tension ring.
The strand jacks connect back to the tension ring by 19mm diameter strands. To raise the roof, the strands are fed through the jacks, making the lengths of cables shorter, forcing the tension ring to rise in the air.
“The first stage was trying to get the tension ring off the scaffold without adding horizontal load,” says fabricator Watson Steel Structures liftmaster John Calland. “The jacking of the cable net had to be carefully controlled. Initially [if the loads were not controlled], the whole net could have been pulled off the scaffold. It wasn’t designed for a huge lateral load.”
The loads were monitored at each stage and compared with results from staged structural analyses by both Watson Steel and Buro Happold. When the numbers tied up, Calland gave the nod to progress to the next stage of lifting.
Taking it in stages
The north and south side lifted off first, with the east and west coming after, due to the shape not being circular.
The sequenced jacking continued until the tension ring reached the level of the bottom chord of the compression truss. At that point, the lower 70mm diameter cables were secured and the jacking of the suspension cables continued until the tension ring reached full height.
“When the system is fully tensioned, there’s 1,300t in the tension ring, which is a huge force,” says Bartek. “There is 200t in each cable that runs between the tension ring and compression ring.”
The jacking operation was completed before Christmas and Team Stadium is now removing the jacking equipment.
At this point there is just one activity left for the tower cranes to do. This month they will start raising access gantries into place. The tension ring has already been raised with a radial access gantry in place. The tower cranes will lift eight more gantries in place which will span from the bottom of the compression truss to the tension ring and radial access gantry.
Then the tower cranes will come down as the rest of the work takes place at a height beyond their reach. Fourteen huge lighting towers weighing 35t each are to be installed on top of the cable net roof.
Stadium of light
“There are few stadia like this in the world, but none have had these lighting towers,” says Bartek. “It adds much more complexity− these towers are being supported by something elastic and soft. It’s structurally challenging to have such a big weight at the tip of the cantilever. These towers have 18m long legs, making them around 30m tall altogether.”
These triangular lighting towers are being constructed on the field of play− constructing them at ground level is safer and easier. Crawler cranes will be raising these up to 60m above the field of play in February.
“The flood lighting has to be a certain height so not to glare into the TV cameras,” says Craine. “We took this approach to comply with lighting standards. We have to maintain camera angles.”
These 28m tall rigs will sit on the inner tension ring. In their final position they will be secured by suspension cables running back to the compression truss and a circular suspension cable linking the top of all the towers. However, the rigs will also need to be stabilised in the temporary condition, by a method that Team Stadium is still finalising.
Lastly, but not least, the fabric will be installed on the roof. Fabric contractor Seele is due on site in April. Polyvinylchloride (PVC) coated polyester will be used for the roof fabric.
“The fabric has to be the last activity, otherwise it would be damaged,” says Bartek. “The structural fabric has to be tensioned, like a large curtain stretched at all four points. We’re using PVC coated polyester as we only need low design life as the stadium is a temporary structure. If the design life was 50 years, we would be using Teflon.”
The temporary design brief has led the design in other ways. Where there might have been welds, there are bolts to allow for disassembly. Temporary steel fixings for the strand jacking operations have been left in place to allow the process to be reversed.