From the official website of the Beijing Olympic Committee

External steel structure of Bird's Nest completed in one yearUpdated：2008-06-28
(BEIJING, June 28) -- Chairman of the Board for both the Beijing State-Owned Assets Management Co., Ltd. and the National Stadium Co., Ltd., Li Aiqing, announced the completion of the National Stadium's interwoven steel roof which broke ground in September 2005.
According to the official statement, engineering and installation of the crisscrossed steel roof is the most important and technically challenging portion of the construction project. A framework of 24 supporting piers forms the outer layer of the nest. The scale of the steel skeleton is most notable, weighing a total of 42,000 tons and spanning east to west 298 meters and north to south 333 meters. The completed building is 69 meters at its highest point and 40 meters at the lowest. The engineering and construction of this highly complex structure drew upon the highest level of domestic and international expertise.
In his statement, Li Aiqing explained that all work on the facility's support structure, roofing, and primary installations had reached completion. He further promised that finishing touches to the stadium were well underway and would be finalized as planned before the 2008 Olympics.
Completion of the steel skeleton represents a major milestone in the stadium project. The last stage of construction was reached this morning with the final dismantling of temporary support towers. The 78 steel support columns, weighing over 1.4 tons, were erected to stabilize the steel exoskeleton during construction. The process of disconnecting and dismantling the temporary supports occurred in several stages and involved lifting and setting the structure with a computer controlled hydraulic system. The successfully disconnection of the steel structure from its temporary supports was a proud moment for the engineering and construction teams; seeing the bird's nest stand on its own "feet" was a culmination of years hard work and planning.