(BEIJING, June 14) -- On a clear day you can see … Beijing!

And it was clear to reporters this week that construction of the National Stadium's outer membrane structure had begun – clearly as construction contractors and scaffolding everywhere in sight.

The membrane structure will encase the steel structure 20-30 cm from the outer to inner rims and also cover the spectator stands.

Extra sound-absorbing layer

The National Stadium will differ from the National Aquatics Center (or Water Club) because it will have a double layer membrane structure. The outer layer will be constructed of 11 sq m of rain-resistant ETFE film and an inner layer of sound-absorbing PTFE film will be constructed 12 m under the ETFE layer. The PTFE layer is - in essence - a sound insulation screen with added acoustic technology, which will keep noise under control;

Final construction of this membrane skin is slated for completion during the latter half of 2007.

Hidden water tanks

Some 1,000 sheets of ETFE line the top of the National Stadium, creating over a thousand mini rooftops. With rain, these mini rooftops face a drainage dilemma.

Li Xinggang and other designers of the National Stadium have come up with a solution: install rain gutters that will allow rain water to drain into 120 water tanks hidden throughout the stadium's steel structure. The water collected in the tanks is then carried away with the siphon elimination system already in place.

"Cold fireworks" for the Opening and Closing Ceremonies

Fireworks during the Opening and Closing ceremonies of the Games could pose a problem for the membrane structure because the outer ETFE film can only withstand Celsius temperatures in the 100s. Ordinary pyrotechnics reach about 400 C when lit, and should they come into contact with the membrane structure, the outer film would melt.

Cold fireworks, however, only reach temperatures of about 270 C when lit and would cool to about 100 C when they come in contact with the membrane structure and are planned for use during the Opening and Closing Ceremonies next August 2008.