A television quiz

"Take four Canary Wharf towers. Tilt two of them at an angle so they lean over, crank another at 90º and stick it on the top of the first two, then crank the last and stick it on the bottom." Rory McGowan, structural engineer and director of multidisciplinary consultant Arup is trying to convey the sheer scale of the task he has been grappling with for the past two years. It is the headquarters for Central Chinese State Television and it looks like Ascending and Descending, MC Escher's perpetually rising/falling staircase. Its designer, Rem Koolhaas, is not the type of man to come up with something straightforward.

This landmark building will be the flagship of a new complex for CCTV. The broadcaster needed the facilities to realise its ambition of rivalling other international networks, such as the BBC and CNN. The 553,000 m² complex is to be built in Beijing's new central business district and features the main 420,000 m² headquarters, which is envisaged as a complete television production community. It will house studios, production facilities and canteens, indeed everything required to make and sell television programmes. Next door is the Television Cultural Centre, also by Koolhaas' Office of Metropolitan Architecture. This is a public building containing a hotel, a theatre and conference rooms. Two other buildings on the site contain security and other services.

The 231 metre high, 54-storey building has the structure of a continuous loop: two offset vertical sections, leaning by 6°, are joined at the top by a 15-storey horizontal link cranked in the middle by 90º. The vertical sections are also joined at the base with a link cranked 90° in the opposite direction from the top. The towers lean over at 6° in two axes. According to McGowan, this is equivalent to it leaning over by 10º in one axis. To add a little enhanced flavour to the recipe, the building is located in an earthquake zone and has been designed to withstand a "level three" earthquake – expected once every 2475 years.

Even though McGowan is no Koolhaas virgin, this still took his breath away. "I've been working with Office for Metropolitan Architecture on other leaning buildings for 13 years, and even though we are the falling-over-building specialist this was in another league." McGowan's main problem was that leaning buildings need stiff structures, but earthquake-resistant ones tend to be flexible. The other challenge with the unusual form was that there was no internationally agreed standards for building something like it in a seismic zone. This meant that the team had to convince the Chinese that their final solution would stand up – the approval authorities wanted to see every last detail before granting permission.

Flexible tall buildings perform better in earthquakes than stiff ones. All buildings have a natural frequency of their own and will vibrate – move from side to side – at a frequency determined by how flexible they are when excited by a force such as an earthquake. "The stiffer the building, the lower its frequency, and the higher the earthquake loading it attracts," says McGowan. "So what we needed to do was increase the building's natural frequency as much as possible to decrease the amount of seismic load." McGowan's challenge was therefore to find a degree of stiffness that would allow the structure to stand up under normal circumstances yet flex sufficiently to accommodate an earthquake.

The team considered several solutions to the problem. The first idea was to use sloping cores, complete with sloping elevators, inside the leaning towers. This proved too flexible, so the team added beams to connect the core to the outer skin, but this still was not stiff enough to resist the forces generated by the building's lean.

The next idea was to use the outer skin of the building – a hollow, continuous square tube – as the structural frame. This was drawn up on a computer as a mesh – a diagonal arrangement of steel without vertical columns. This was then analysed to see where the stresses were concentrated. "No building has ever been analysed like this one," laughs McGowan. The pattern density of the mesh was then adjusted to take account of the varying stresses – it is more concentrated where the stresses are highest, and looser where the stresses are low. This solution clearly went down well with Koolhaas, as the structural solution is visible in the building's final form. Because a structure based on the stresses that an earthquake places on buildings appears random, it still manages to intrigue the eye.

Further analysis showed that a pure mesh structure would still make the building too stiff to perform well in an earthquake. To overcome this, the steelwork used to make up the diagonals has been reduced in size, making the structure lighter and more flexible. Vertical columns have been introduced to help take the loads that would have been carried by the original, mesh-only structure. One added benefit of introducing columns is that they also help increase the natural frequency of the building.

The building was now fine-tuned to optimise the size of the main structural elements. For example, the columns on the inside of the leaning towers support the equivalent of a 200-storey building, whereas the outer columns support the equivalent of 20 storeys.

Thicker steel plate is being used to beef up the more heavily loaded columns, but where the plate's thickness reaches 120 mm, the columns will be made deeper to cope. The external frame supports the cantilevered top link. Internal load-bearing columns are supported by huge, three-storey-deep transfer trusses at the base of the link. These transfer the loads to the external frame.

The leaning towers will sit on 100 m² raft foundations at a depth of 7.5 m. These will be supported by piles more than 50 m long.

Although tall buildings often have damping devices at their top to stop the structure vibrating excessively in an earthquake, the final value for the stiffness of this building was sufficient to avoid this becoming a necessity. In strong winds, CCTV will have a horizontal movement of just 37 mm, and in a level one earthquake (with a probability of one every 50 years), it will move 135 mm. This degree of movement is too small for damping devices to work anyway.

McGowan says that before China opened up economically, architects and engineers had to be incredibly careful with how they used the nation's resources. "The story goes that if you overdesigned you got locked up and if you underdesigned you also got locked up," he says.

Although the situation is different today, Arup still had a long struggle to persuade the Chinese authorities that the design would meet its performance requirements. As mentioned above, every last aspect of the building was checked to ensure that it would behave as predicted in an earthquake. The team also had to prove to the Chinese authorities that the building wouldn't fall down if an earthquake occurred during construction. Indeed, the design had to take account of construction-induced stresses, something made harder for McGowan's team by the fact that they didn't know who the contractor was going to be, or exactly how they would tackle the building. However, Arup has overdesigned the building so that stress that builds up during construction can amount to up to 10% of the total stress that the structure can withstand. The contractor will have to find a method that fits within this design parameter.

Another factor that the team has had to contend with is the sun. During construction, it will heat each tower in turn, causing them to lean over by an extra metre. This means that joining the two towers together is a critical operation. "Joining the two is a major task. It will have to be done just before dawn so there are no differential stresses," explains McGowan.

Work will start this year and is due to finish in time for the Beijing Olympics in 2008, when the ambitions of Chinese Central Television to become one of the world's premier broadcasters will be tested to the full.