German structural engineer Knut Göppert of the firm Schlaich Bergermann und Partner has given a detailed explanation about the design and engineering of the new Green Point stadium roof.

According to Goppert, whose firm is currently working on four of South Africa’s 2010 stadiums, the design was a unique challenge. Not only would the top of the stadium have to be unobtrusive due to its impact on Cape Town's iconic skyline, it would also have to be strong enough to handle the city's notorious winds.

The brief for the roof was for it to be elegant, understated, and to respect the site, he told a crowd at the Green Point Visitor's Centre on Thursday, 13 November.

The site in Cape Town is really special” he said, comparing it to most stadium sites that are built on flat land located outside a city, not in the middle of them.

He also pointed out that as the stadium is highly visible from the surrounding mountains, the roof needs to be “good to look at, and easy to maintain and clean.”

With this in mind, the architects and roof engineers decided on a 36 000 square metre hanging roof, supported by an outer compression ring and inner tension ring.

Göppert used a bicycle wheel to demonstrate how the roof will work, with the outer compression ring resting on the pillars, and an inner tension ring.

Height, lighting and noise reduction also had to be taken into account, given the fact that the stadium is situated in a residential area.

One of the major factors to be considered during the design of the roof was Cape Town’s notorious wind, particularly the South Easter.

Göppert explained that the turbulence created by these winds is higher than most strong winds, and was therefore a significant factor in the roof’s design. The designers visited the National Wind Tunnel facility in Pretoria to conduct tests on the location of the stadium on a scale model of Cape Town. They discovered that most of the wind, and resulting turbulence, comes not from the sea, but from Table Mountain and Signal Hill, and amended the design accordingly.

One of the main design elements to prevent wind damage is the use of heavy glass for the roof, while another is the use of a truss system on top of the compression ring, which will reduce turbulence. It also helps create the shape of the roof, and distributes the uneven loads evenly onto the cables.

The design of the roof also had to take into account heavy rainfalls, and ensure that the water drains to the two lowest points effectively.

Construction work on the R430-million roof started in September, with the erection of the first compression ring. It will take a year to complete.