The National Stadium — Bird’s Nest

The Beijing National Stadium,also known as the National Stadium,the “Bird’s Nest” is a stadium finished form the Olympic Games in Beijing.The “Bird’s Nest”,the main venue for Beijing’s Olympic Games,is perhaps the most iconic structure of the 2008 Olympics.It is located east of the Beijing Natinal Aquatics Centre.

The stadium will host the main track and field competitions form the 2008 Summer Olympics,as well as the opening and closing ceremonies.The stadium can seat as many as 91,000 spectators during the Olympics.The capacity will then be reduced to 80,000 after the Games.It has replaced the original intended venue of the Guangdong Olympic Stadium.the stadium is 330 meters long by 220 meters wide,and is 69.2 meters tall.The stadium uses 258,000 square meters of space and has a usable area of 204,000 square meters.It was built with 36 km of unwrapped steel,with a combined weight of 45,000 tons.The stadium will cost up to 3.5 billion Yuan.

The National Stadium is dubbed the “Bird’s Nest” because ofits 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.The constructors have made 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 weights 42,000 tons,with the roof and the hanging parts around it accounting for 11,2000 tons.To bear such a heavy load,78 supporting structures were temporarily installed and distributed in different points under stress.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 innercircle.In other words,35 mini-steps are needed to complete the whole process.

To unload a supporting structure,or to load the insitu 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.