The message of the speech was a simple one: "The greatest obstacle in the development of multipurpose stadiums is that there is no economical system for a retractable roof." With a system to shut out the weather, stadiums can host many more revenue generating activities. For a start, a roof would prevent sports events being disrupted by heavy rain and concerts could be held in the arena whatever the season. And with the roof closed, there would be less noise to disturb people living near the venue. As Jahanpour says: "A completely different business plan can be put together for a stadium with a roof." Jahanpour was convinced there must be an engineering solution. As leader of Building Design Partnership's elite Special Structures Group, this was the kind of challenge he relished. In the days following the presentation Jahanpour mulled over all kinds of possible solutions at his desk and at home in the evenings while practising his hobby of Persian drumming in his attic.
This is not the first time Jahanpour has had to find a structural solution to a difficult roof design problem. The elegant Polo-mint-shaped shell-structure roof on Wimbledon's Number 1 court was his design, as was the delicate glass and steel canopy covering the central courtyard of the National Maritime Museum at Greenwich. He was also the engineer that proposed the alternative construction method that was the basis for BDP's bid to build a rival to the London Eye in Prague.
The solution, when it came to him three weeks later, was simple – why not suspend the retractable roof elements from cables and pull them into position to cover the opening over the pitch? Jahanpour thinks his innovative solution, which has the rather less-than-innovative name of Skylid, will revolutionise the design of sports stadiums around the world.
One advantage of the system is that it reduces the distance each section of the roof has to span, which in turn reduces the weight of the roof. "It will be 80% lighter than conventional systems, which means it'll be cheaper to build and will needs less support. It will open and close in 10 minutes and it can be added to an existing building," he says proudly.
Like his earlier achievements, the design of the retractable roof is based on Jahanpour's knowledge of structures and materials. "The key to the concept is to use conventional structural materials at their optimum efficiency," he explains. Existing movable roofs weigh about 250 kg/m2. But through clever use of materials, Skylid is much lighter: "It weighs about 50 kg/m2," explains Jahanpour.
Although the concept is simple, explaining the its operating principle to his colleagues at BDP and, more importantly, to potential clients, proved far more difficult. So rather than producing pages of diagrams and sketches, Jahanpour returned to his attic where he built a model of a stadium and its retractable roof from scraps of wood and pieces of metal and string. The sawn-off base from an old Persian drum formed the circular stadium, triangular wooden offcuts formed the movable roof sections and kitchen string was used in place of steel for the supporting cables. With the installation of two electric motors, Jahanpour was able to demonstrate the workings of his design.
The concept is based on a domed circular roof with an aperture in the middle – like a hubcap with a hole cut in its centre. This centre section of the hubcap is then divided into four evenly sized quadrants. These quadrants are designed to slide in and out on rails like four Dairylea cheeses. When closed, the quadrants form a circle to fill the open aperture; in the open position the cheeses rest on the fixed perimeter roof.
The perimeter roof is a shell structure – similar in design to the roof of the Number 1 Court – so it is light and strong. This will support the rear section of the cheeses. Rails mounted on the roof allow the quadrants to slide in and out while a cat's cradle of steel cable will serve to pull the roof sections into position and to support the suspended quadrants as they are tugged into position. In the closed position the quadrants lock together to form a separate shell structure.
Jahanpour claims his design can be adapted to accommodate any configuration of playing field. The movable quadrants can even be split into smaller sections that can be telescoped out to reduce the area of permanent roof necessary for parking the sections on when the roof is open. "This would deal with the problem of shadows on the pitch," he explains.
All that remains now is for Jahanpour to test his solution on a real installation. And because this solution promises to be much cheaper than any other design currently on the market, he is hopeful that he will not have to wait too long.
How the cables open and close the roofThe cable system is the most innovative part of Jahanpour’s solution. The two rear corners of each quadrant rest on slide rails attached to the fixed roof, while the leading corner of each quadrant is hung from the cable system. This loops and criss-crosses the roof-opening between four supporting masts spaced evenly around the stadium’s perimeter. As well as supporting the quadrants, two separate loops of cable also pull the movable roof sections in and out. Each loop works independently and connects a pair of facing quadrants. The loop of cable follows a convoluted route through a series of pulleys housed within the support towers and around a winch at the base of one of each pair towers. By turning the drum on the winch, the cable loop can be pulled one way to close the roof and another to open it. To hold the movable roof sections firmly in position and to stop them moving sideways, the cable is looped back around the same route a second time. The two loops of cable at each supporting tower are separated so that the cable forms a triangle with the tip of the quadrant at its peak to stabilise each quadrant. The domed shape of the roof creates the problem of uplift: in a strong wind the quadrants would be sucked off the roof. To prevent this, a second, smaller loop of cable runs beneath the roof to tie the quadrants to the fixed section. This second loop acts in tension like the main cables but pulls in the opposite direction to the supporting cables to hold the panels tightly down.
See the Skylid in action at www.bdp.co.uk/skylid