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Twisting the Doughnut

CHINA

THERE are many buildings under construction in China at the moment that have the potential to take the breath away. But when it comes to the truly spectacular it is hard to beat the new headquarters of CCTV, the Chinese national broadcaster.

Architect Rem Koolhaas has a reputation for outlandish structures but his proposal for CCTV, developed with Arup, pushed the envelope of structural engineering so much that the client had its doubts whether it could be built. The so-called 'twisted doughnut' design looked to laymen's eyes like it should be out of balance and collapse under its own weight.

'In 2002 we were working with [Koolhaas practice] OMA on the competition for CCTV.

By August it was getting clear that our bid was going to be the winner. I was working on another job at the time but I was told to hotfoot it over to China because although the client liked the design they were concerned about whether it could actually be built, and particularly how it would stand up to the forces in an ear thquake, ' says A rup CCTV project director Rory McGowan.

After cajoling the client into a position where it was happy the building was structurally possible the team, which also included architects from local design institute ECADI, decamped to London to work up construction documents and drawings for the building.

By September 2004 these documents were complete and contractor China State started work on the groundworks. The first stage in the project was to excavate out a three storey basement under the site. Although the surface area where the base meets the ground is only around 60 m by 40 m, the foundation raft covers a square with sides of 100 m and is 7.5 m thick. This provides a toe for the building, resisting the huge turning forces acting on it.

The raft then sits on a series of 1.2 m thick piles at 3 m centres that sink 50 m into the Beijing soil.

With the groundworks complete the steel frame is now beginning to take shape. But given the massive cantilevered top section of the looping structure and the angled nature of the towers, this has been extremely challenging for the whole team.

'We can draw parallels with bridge construction, ' says Mr McGowan 'I don't believe anything like this has been done with a building.' As the two towers of the 234 m high building r ise it would be normal to expect them to be based on a concrete core, as is typical of high rises across the world. But the 6 degree angle of the towers and the overhanging section that joins them meant that this system would be impossible.

'The overturning forces would be so dominant that the cores could never have enough strength and stiffness, ' says Mr McGowan. 'Instead all of the st reng th is in the facade, which is a triangulated steel mesh.' The building consists of a 'tube' with bracing on all sides of the facade running in cont inuous planes th roughout the building to provide st iffness. Steel sect ions of up to 30 tonnes will be lifted into place to form this facade, tak ing up to 48 hou rs to weld into place the largest of these sections.

When the two towers are near to their final height the team will begin work on the cantilevering sections that will join them together. The stiffness of the towers will mean that no temporary works will be needed to hold this section up during construction. Even so the towers will bend slightly in towards the centre as the overhanging load increases, but this effect has been built into the calculations for the structure.

'It's a bit like the Channel Tunnel, the two teams are working towards each other in the hope that they meet at the middle. The difference is that on the channel tunnel the tolerance was in the hundreds of millimetres.

We are much, much tighter than that. Even temperature becomes important. We would have to do the last sect ion of the overhang in the early morning when it is cool or else the building would have moved out of place as it heated up, ' says Mr McGowan. No wonder he describes CCTV as the most analysed building Arup has ever worked on.