Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Beijing brain teaser

Structures - The new HQ for China's national broadcaster is an eye catching megastructure which looks like it ought to fall over. Andrew Mylius reports.

The teetering angles and outrageous cantilevers of China Central TV's (CCTV) new headquarters in Beijing seem anything but rational.

But with Dutch architect OMA on board it should come as no surprise. OMA has a history of skewing structural elements out of conventional horizontal and vertical planes with the intent of provoking and unsettling its buildings' users and neighbours.

When it entered the broadcaster's design competition in 2002 it had been working on ideas for tilted buildings for some time, says collaborator and structural engineer for the project, Arup director Rory McGowan.

McGowan admits that the structural gymnastics have a purely aesthetic justication. But there is an organisational logic to the building, he insists.

'The building's total oor area is 560,000m 2, which is vast.

It will provide space for every conceivable requirement of a national broadcaster ? studios, ofces for management, sales, production and post-production, broadcasting, technical back up, mobile broadcasting, vehicle eets ? things which, with the BBC, are scattered across London in 20 different locations.' At 55 storeys and 223m tall, the CCTV building is the same height as One Canada Square at Canary Wharf in London's Docklands. Describing it, McGowan says: 'Imagine taking four Canary Wharfs and planting two at 10¦ off the vertical. These are connected by two more Canary Wharfs, turned on their sides and bent through 90¦, one at the bottom and one at the top.' The overturning forces resulting from the cant of the towers and the jutting 15 storey cantilever structure connecting the tops of the towers have posed an extreme structural engineering challenge, says McGowan.

They have also vastly increased the building's cost compared to structures of a similar height ? though the client was convinced that, were the same floor space provided in a single very tall tower, it would end up paying the same.

During early development of the design, Arup looked at a number of structural systems to counteract the CCTV building's wildly eccentric loading.

'Because overturning forces are equivalent to 10 times wind loading, the building needed very robust support, ' McGowan states.

Mega diamond bracing using composite steel and concrete members looked attractive.

Mega braces, triangulating between massive corner columns, would have been visually dominant and imposed a rigid geometric order on the building's facade.

OMA wanted a structural system that was more sympathetic to the building's drunken attitude.

'The mega bracing established the idea of a tube structure, with all of the support provided around the building's perimeter and we started looking for other ways the tube could manifest itself, ' says McGowan.

He says that a breakthrough was made when the tube was subjected to computer modelling.

'We placed a triangular mesh over the structure and subjected it to wind, overturning and dead load, and measured the flow of forces around it. We found that some areas were very highly stressed and others not loaded much at all.

'We agreed to express the concentration of forces by translating the mesh from the computer model to the built structure ? we triangulated the facade over two floors between regularly spaced columns and beams at the floor edges, ' McGowan explains.

The building's reinforced concrete cores are selfsupporting only.

'Following the force analysis, in areas that were highly stressed we doubled or even quadrupled the density of the mesh, and in areas that are lightly stressed we subtracted members from the mesh, reducing its density by 50% or 75%. The conventional way to have dealt with varying forces would have been to increase or reduce member sizes.' On the CCTV building bracing is a constant box section of 300mm by 400mm.

Adding structure where it is needed but stripping it out where it would be redundant helped reduce total steel tonnage, and improved ductility. This was vital to meeting seismic design requirements, McGowan says.

On China's seismic scale of 1 to 9, the CCTV building has been designed to withstand an event measuring 8.5.

McGowan describes the building's composite steel/ concrete columns as animals.

'In one corner you have an elephant and in the opposite corner a mouse. In between there are all sorts of other beasts.' The most highly loaded columns are vast, with four anges and double webs, all 100mm thick.

'They're the sort of thing you'd expect to find in a 200 storey tower.' Those taking least load are more in scale with buildings of 20 storeys, with twin anges and double webs of 30mm to 40mm thick. Columns measure 1.2m wide on the facade, and can be up to 2m deep. There are 28 different column congurations.

Contractor China State Construction is now 14 storeys above ground, McGowan says, and the structure is rising in two storey leaps. Steelwork is fabricated in Shanghai and transported to Beijing by road.

On site it is lifted into position by a pair of 80t tower cranes.

Connections are welded, with the largest columns requiring up to 78 hours of continuous welding.

In total the CCTV building will consume 10,000t of steel.

'Because the building's leaning there is more and more bending added, ' McGowan notes. 'To combat that, the contractor is having to predict bending and build the towers in a reverse banana shape.' As the steelwork's self weight bend, the towers straight, stress will inevitably be locked into the structure. Arup calculates locked in stress will account for up to 10% of the building's static loading.

The cantilever was initially intended to be strand jacked into place, as deck sections of cable stayed and suspension bridges are.

However, China State Construction has opted to build it piece by piece insitu. 'The two bottom floors of the cantilever form a 3,000t transfer deck which will pick up the columns supporting all of the upper floors, ' says McGowan.

CCTV is scheduled to be completed next year, ready for the broadcaster to set up shop in time for the 2008 Olympic Games.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.