Graceful and very tall, Guangzhou’s new TV and Sightseeing Tower reached the dizzy heights of 612m earlier this year. Jessica Rowson reports on the structural form of this willowy wonder.
Television towers tend, by their nature, to be tall. Sticking out above the skyline, they can come to embody a city − think of the Telecom Tower in London, or the Fernsehturm in Berlin. The new TV and Sightseeing Tower in Guangzhou is no exception. With an elegant and unusual twisted lattice structure, it will act as a symbol for China’s third largest city.
A port on the Pearl River 120km north west of Hong Kong, Ghangzhou is the capital of Guangdong province. It has a population of 10M and cutting edge telecommunications are vital to the city’s developing prosperity. But more than that, having a TV tower that ranks among the most striking in the world is a matter of municipal pride.
From a distance, Ghangzhou’s TV tower has a discernibly feminine form, growing from a broad elliptical base at ground level to a tapering waist and then widening slightly up to a second, smaller elipse at the main roof, 450m above. The 162m-long mast takes the final height of the tower to 612m. The shape was developed by imagining that the two ovals were connected by straight lines, forming a cylinder which narrowed, after which the ovals were twisted 35º relative to each other.
“Hence the twisted lady look,” says Arup director Man Kang. Arup’s London office collaborated on the Ghangzhou project with Information Based Architecture (IBA) of Amsterdam, for which the tower is a first major project. The concept was developed by Arup and IBA in Amsterdam, after which an increasing amount of work was done by Arup’s Chinese offices in association with the local design institute.
The tower’s geometry was refined using parametric associative software, which can generate geometrical and structural models based on a set of variable parameters and link the geometrical data to the analytical and drafting software. The result is a form that is both simple and complex.
The main load carrying elements of the tower’s structural lattice are its steel columns which taper as they climb and twist up the building. Infilled with concrete, these are 1.8m in diameter at the bottom reducing to 800mm at the top. Horizontal rings of steel attached inside the columns keep these in the right position and help balance the forces created by the fact that they effectively slope. Diagonal members then give the structure added rigidity.
“By creating a triangulated framework, the sloping columns are stabilised while their elegance is maintained,” explains Kang.
“By creating a triangulated framework, the sloping columns are stabilised while their elegance is maintained. For buckling, we needed to look at joint stiffness.”
Man Kang, Arup
A matter which needed particular care was how the joints between the columns, rings and diagonals were treated as the connections affect the way in which the structure behaves. “For buckling, we needed to look at joint stiffness and for that we needed to look at adjoining members,” says Kang.
The waistline tightening caused by the rotation between the two ellipses makes the central portion of the tower very dense − the lattice structure being much more porous and spacious at the bottom and top of the tower.
There is an open-air staircase with views over the city in the waist area, where it does not make financial sense to build floors. “We’re not looking at a building here. It’s primarily a tower that has some floors,” says Kang.
The structure’s columns have to work harder in this area due to this lack of floors. Columns are normally restrained from moving laterally by floorplates. Where there are floors, these are constructed of steel beams with a composite deck supported at the tower perimeter by the external lattice and internally by a central concrete core.
“When you break the structure down, it’s simply a concrete core plus tubes, where the tubes are the columns, rings and diagonals,” says Kang. “Using steel externally with a concrete core can be a bit of a challenge as they have different properties: you have to take into account long term deformation, shrinkage and temperature effects. Differential shortening comes into the equation. It all adds to the complexity of the analysis.”
Unusual and unpredictable
The tower is different from many other tall structures in that it is not a sealed building. In some areas, in particular the waist, the wind can blow straight through the structural lattice. This made it more difficult to determine exactly what wind loads would be imposed on the building.
“This is the tallest building in Guangzhou,” says Kang. “We had to go back to the beginning. Because the external skin is open lattice and a concrete core, we had to look at what areas we should be considering.”
Arup decided to go one step further to test the robustness of the design. As well as testing a typical scale model of the tower with its surrounding buildings, it also built a larger scale model to further investigate the behaviour of the structure.
“We had to go back to the beginning. Because the external skin is open lattice and a concrete core, we had to look at what areas we should be considering.”
Man Kang, Arup
As the structure is perforated it interacts with the wind in a different manner to a normal building − wind can go through it as well as around it. Work started on site in 2005 and structural work was finished in May this year. The fitting out continues and the tower is due to open in 2010.
The design team worked hard to ensure the complexity of the structure would not be reflected by the complexity of the build. “We looked at construction as part of the design,” says Kang.
“We looked at how to decompose the shell into parts by size and weight. We divided the structure into three elements − rings, verticals and diagonals.” The columns and diagonals were supplied to site in 8m to 12m lengths and the ring pieces in 3m to 6m lengths; the components weighing 25t to 30t.
How high is high?
With talk of 1km-high towers coming out of Dubai, perspective on what makes a structure a really tall one has become distorted. To put the 612m height of the Guangzhou tower into perspective, the Burj Dubai was topped out at 818m earlier this year.
The CN Tower in Ontario, Canada, which was the world’s tallest completed freestanding structure before the Burj, reaches 554m. The Shanghai World Financial Center is said to have the world’s highest roof at 488m.