Gerhard Fiedler wrote:
> James Newtons Massmind wrote:
>
>   
>> Others have said that the weight of the top floor would accelerate the
>> lower floors and account for the rapid fall. That is an interesting
>> point, which may very well be correct. I don't know. I would love to
>> hear from someone who does. 
>>     
>
> If it falls from the top, the weight of the top ceiling crashes on the
> floor below. There it "shares" its energy and momentum, and both continue
> to fall, however slower as if there were no floor below. And so on. To fall
> nearly like free fall, I'm pretty sure the whole steel frame has to crash
> at nearly the same time (between all floors). 
>
>   
>> However, from all the videos, it is very clear that it started falling
>> from the bottom, not the top. If fires heated the steel, why wouldn't
>> the top have fallen first?
>>     
>
> Maybe because the load is much higher at the bottom, and a much lower
> temperature is needed to soften it to a point where it gives in? But even
> then, in order to fall in a time that's close to free fall, the whole steel
> frame has to give in at nearly the same time. Which may be possible.
>
> Gerhard
>
>   
How much of a slow down would you find acceptable? That many thousands
of tonnes of stuff falling down is going to make most of the stuff under
it get outta the way pretty dern quick.
Everybody seems to also be going on and on about how each floor should
have absorbed the impact of all the stuff above it.
My understanding of that building is that the floors were not much more
than bits of paper strung across a fairly tough outer skin. Their only
real contribution to the structure of the building was to stop the
outside bits from wobbling. When the melted bit in the middle gave way
as i recall it toppled at least a little. The misalignment of load would
mean that the top bit would fall down largley missing the "structural"
components below them, knocking out their weak supports meaning those
bits too fall down. Yes the fall should be slower than gravity in a
vacuum but not by much.

My Impression of the structure is something like a tube of skewers held
together with sheets of tissue paper. The paper keeps the skewers
pointing up and incidentally supports things like photocopiers and
people. But when the stack topples slightly the whole thing collapses
pretty much at the same time.

In your average brick type building the floors actually play a
significant role in the structure of the building and the loss of one
structural member wont cause a "house of cards" collapse. I view the
towers collapse more like the failure of a composite part, no
deformation, no damage until a certain point then "bang" and its all
over in one big cascade failure.

Also looking at stuff falling next to the building and saying "ooh look
it goes at the same rate as the building" doesn't count for much. Air
has a density of ~1kg/m^3 concrete is about 6000 steel is about 8000.
(your average human is about 1000 btw for those of you working out
terminal velocities). So the air is basically going to play a small role
in slowing the main collapse. Its more likley to get dragged along for
the ride sucking stuff along with it (making debris seem to fall real
quick like) as I recall when they collapsed there wasn't much dust left
where they were standing as they fell. It all shot up again afterwards.

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