STEELWORK FOR the leaning and cantilevered China Central TV headquarters in Beijing is now 12 storeys above ground and rising fast.
The structure, designed by structural engineer Arup with Dutch architect OMA, consists of two 223m tall towers leaning 10infinity from the vertical.
They will be linked at their tops by a 15 storey 'bridge' which, rather than connecting in a straight line, will have a 90infinity elbow.
'The lean of the towers combined with the cantilever of the top story connection will exert an overturning force equivalent to 10 times wind loading, ' said Arup director Rory McGowan.
Arup is structural engineer on the project.
To cope with this and with potentially high seismic loading, the building has a structural system of mega columns at 10m centres, and diagonally braced diaphragm floors at every other level.
McGowan said that rather than changing the dimensions of the bracing according to loading and stress variations around the building, bracing will be added or subtracted as required.
'It'll be possible to see which parts of the building are working hardest. In areas that are highly stressed there'll be double or even quadruple the density of bracing and in areas that are very lightly stressed there will be half or quarter the density.
The approach has enabled Arup to 'fine tune' the structure and win some efficiencies in the tonnage of steel used, McGowan added.
He said that the cost of the building compared favourably with the cost of providing the same 560,000m 2 of office, studio, production and broadcast facilities for a conventional high rise building.
But the structure is still consuming 100,000t of steel.
The most heavily loaded columns are 1.2m wide on the facade and 2m deep. They have double fl anges 100mm thick, and four webs, also 100mm thick.
'They're big enough to support a 200 storey tower, ' McGowan said.
Contractor China State Construction is erecting steelwork in two storey sections.
The towers have been designed to ensure that construction of the leaning towers compensates for bending of the under self weight.