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.
