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re: domes lighter once built than their component parts?
14 may 1998
the butterfly   wrote:

>does anyone remember the reference for how a dome weighs less once it's
>build than its component parts, due to the lifting force of the air mass
>inside it?

i don't recall reading that anywhere, but it makes sense. if the dome
is airtight at the top and warmer and/or more humid than the outdoors,
the air inside will have a bouyant force, like a hot air balloon. 

the 1993 ashrae handbook of fundamentals says 70 f moist air weighs
0.073 lb/ft^3, and 30 f moist air weighs 0.081, and a 450' hemisphere
contains 23.9 million cubic feet of air, so a 70 f dome might weigh
23.9x10^6(0.081-0.073) = 180k pounds or 90 tons less on an average
30 f january day in philadelphia. 

how big does a "lightweight" dome have to be to float?


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