What does it take to build the world’s largest freestanding dome?

The world’s largest free-standing dome wasn’t built to be record-breaking, says Mike King, Arup’s lead structural engineer on the project. Only after finishing the design and engineering stages of the 310 metre dome atop Singapore’s National Stadium, did the engineers check the record books to see they had created the largest in the world.

The National Stadium is part of a world-class sports precinct called the Singapore Sports Hub, which opened in 2014 on 35 hectares of land inside Singapore’s CBD. The project was developed by a consortium that includes Arup (Sports Venues), DP Architects (Non-Sport Venues) and AECOM (landscape design). Its brief was ambitious in size and purpose. The stadium needed to seat 55,000 people at full capacity and accommodate a range of sports including soccer, cricket, rugby and athletics. It also needed to represent the Singapore of today - a highly developed country and leader in the Asia Pacific region. Moreover, the stadium had to contend with a site of reclaimed swampland and Singapore’s equatorial and monsoonal climate.

The dome

The huge dome was born out of the need to have a retractable, efficient roof that was supported without any external supports extending beyond the seating bowl. The resulting design is supported by an external ring beam of reinforced concrete. “The purpose of the ring beam is to restrain the dome structure from spreading, and is a key element in terms of achieving a very lightweight steel roof structure,” says King. “It also means that no external support structures beyond the stadium perimeter are required.”

Allowing for public space

At Singapore Sports Hub the area around the perimeter of the stadium is important public space. The Sports Promenade is open to the public all year round. Having activity in the precinct during non-event days was a key concern of the client, says King. “The trouble with a lot of stadia around the world is that they are very lively and active when there's an event and just before and after an event but for the rest of the time they can be something of a ghost town.” On any given day, there will be people jogging around the Sports Promenade, taking part in aerobics and Zumba classes or walking to the nearby Kallang Wave Mall.

Climate control

To mitigate the humid air inside the stadium, the roof can retract to allow cool air into the seating but it can also close against Singapore’s monsoonal rains. At 2000 tons, King describes the roof as one of the lightest of its kind, but evidently still heavy. As the retractable roof rolls backwards and forwards across 50-odd-meters, it creates “quite a massive varied load on what we call the fixed roof” says King. “So we needed to have a very efficient structure to support that moveable roof.”

Unstable land

The site’s geology also meant that the dome had to be self-contained. Singapore reclaimed the land in the 1920s, leaving it with about 15-18 metres of marine clay, which is “structurally equivalent to a grey tooth paste,” says King. “It's terrible stuff.” Because of this, lateral supports for the dome would never be entirely stable. “The whole of the stadium and all the other buildings there essentially supported on long piles down to a decent stratum,” describes King.

Any load-bearing supports on the surface level would move as the marine clay settles about 300 - 500mm in the next 30 years. Instead, the weight of the dome is contained in a kilometre-long, structurally stiff, concrete ring beam around the perimeter of the stadium. “That had around 40 mega newtons of post tension in it to contain the weight of the roof or to contain the spread of the roof” says King.

The dome is quite an engineering feat, but other aspects of the National Stadium are also remarkable. The climate control system, for instance, pumps cool air from underneath spectator’s seats and uses bowl cooling rather than air conditioning. Over half the seats move to accommodate different sporting fields - 30,000 seats can move in or out by approximately 12m to change the space from soccer to athletics. (Watch a video here.)

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