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Reply #104: WTC test [View All]

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lcwilson Donating Member (1 posts) Send PM | Profile | Ignore Wed Jul-05-06 01:31 PM
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104. WTC test
I consulted a standard handbook - Perry's Chemical Engineers'
Handbook, Sixth Edition, page 23-65.  It has a nice graph of
the stress (load on the beam due to the weight of the
building above it) versus temperature giving the rate of
creep (beam starting to deform and fail).  Two important
points that are broad generalizations: below about 800 F, the
rate of creep is so low that you don't need to worry about it.
 At about 1200 F, carbon steel can only support a stress of
600 psi with an acceptable rate of creep, versus the typical
design spec of 30,000 psi at room temperature.  This is a
reduction in the strength of the beam of 50 times, well past
any safety factor.  

The reason the rabbit hutch test failed is because the WTC
had walls and kept the heat in.  Redo the test with a
thermocouple to see how hot the fire can get when you have
good insulation around it and a good chimney to draw the air
in.  My son's forge regularly gets to the point that the
steel starts to melt (2800 F), using only charcoal and a
blower.  When the steel gets hot enough, it will fail.  When
your test reproduces the conditions in the WTC, it will fall
down as well.

Perhaps we can get the opinion of a real mechanical engineer
or fire safety engineer.  I am a chemical engineer and know
enough to stay away from serious designs and get the right
guy to do the design for me if I need something like this. 
However, I have worked in labs for 20+ years and done some
experiments at these temperatures, etc. so this critique of
the rabbit hutch experiment is based on sound science.
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