The National Stadium in Beijing, the main stadium for the 2008 Olympic Games, is dubbed the "bird's nest" because of its innovative grid formation. The twig-like structural elements and the bowl-shaped roof are the masterpiece of the project, yet they pose great challenges to technicians and workers who need to make the building stand on its own feet.
Workers pull a rope at the site of the National Stadium, dubbed the "Bird's Nest," in Beijing where construction work passed a crucial stage when supports underpinning the steel structure were removed yesterday. [AFP]
Three years and eight months have passed since earthworks started in December 2003, and now the constructors are making decisive efforts to let the "bird's nest" support its own weight without relying on any of the supporting structures.
The large steel skeleton of the project weighs 42,000 tons, with the roof and the hanging parts around it accounting for 11,200 tons. To bear such a heavy load, 78 supporting structures were temporarily installed and distributed in different points under stress, i.e. 24 supporting structures along the outer circle, 24 in the middle circle and 30 in the inner circle. The current task is to discharge all these supporting objects from a weight of 11,200 tons.
Through accurate calculation and careful argumentation, experts came to understand that the unloading process of the supporting frameworks should be divided into seven steps and each step should abide by the sequence of outer circle, middle circle, inner circle, middle circle and inner circle. In other words, 35 mini-steps are needed to complete the whole process.
To unload a supporting structure, or to load the in-situ steel skeleton, a lifting jack is used. Jacking pads with a height of 100mm to 200mm are placed on the top of the supporting structures. When the lifting jack rises, it replaces the structure to bear the load of the steel skeleton, and a jacking pad is removed. Then the jack descends slowly to give the remaining load again to the column, thus repeating a total of 35 times until the steel skeleton is able to bear its own weight.
Accuracy is required during the process. The maximum descent of the outer circle is limited to 68-286mm, the middle circle 161-178mm and inner circle 208-286mm. If the descent of the steel structure, which relies on its own supporting capability, exceeds the extent, or cracks appear somewhere in the skeleton, then problems might exist in the design, manufacturing or construction stages in the past three years. In a word, the unloading process is a proof of "the bird's nest" quality.