Re: Physics of the 9/11 building collapses?


hetware wrote:
ABarlow wrote:

Ever tried blacksmithing, out of curiousity? It is worth the experience
if you haven't.

There are many problems with the notion that heat was responsible for the
total collapse of the buildings. For one, the cores were in an area that
did not contain a lot of office furniture. Most of the jet fuel burned off
outside the building and (according to official models) virtually all the
jet fuel was consumed in the first few minutes. The temperature needed to
compromise the integrity of the structure seems to be at the high end of
the possible range for the kinds of fires possible in those buildings.
That heat would have to have been communicated to primary support members.
A heat induced structural compromise may be in the realm of possibilities,
but handwaving arguments are not sufficient to persuade me at this point.

If you are going to argue the the WTC should have been able to
withstand the flames and heat from 5,000 gallons of flaming jet fuel, I
think that physics can very easily prove you wrong.

I don't think that is the point he's trying to make. I think his
problem is largely an energy concern: when one floor hits the next,
kinetic energy will be lost in demolishing the lower floor, which will
slow the rate of collapse of the structure. If the rate of collapse is
very close to, for instance, the same rate of fall as a piece of debris
leaving the building, that would imply that very little energy is being
lost in the collapse process.

Exactly. The numbers I have right now for the period of collapse of 1 WTC
and 2 WTC is around 15 seconds (though many have said closer to 9 seconds,
and a few have put it as high as 20 seconds.) These folks seem like some of
the more sober investigators, so I'm drawing from their data:
http://911research.wtc7.net/ [Note that NIST and FEMA have, AFAIK, not
even mumbled about the issue of what made the portions of the building that
wasn't fire compromised by fire collapse, other than some sweeping
"progressive collapse" mumbo-jumbo.]

There would have been air resistance acting to retard the descent.
Additionally, the complete destruction of the steel skeleton of the
building would have required a lot of work = force x distance. The
concrete was converted into a powder similar to confectionary sugar.

I think the effect of air resistance would be fairly small since air
resistance typically is a function of velocity. The main energy loss
would be the demolition of the structure itself, particularly since, we
must note, a fair amount of the support structure *that was not heated
by the jet fuel* was nonetheless destroyed in the collapse. If it were
just the floors collapsing (which seems more logical, actually) then
most of the support structure should still have been intact.

One thing you have to remember about the energy loss thing is that
conservation of momentum predicts that the change in velocity will get
smaller as the kinetic energy of the falling parts increase. Think of
it this way: if you crash a VW Beetle into a parked car at 30 mph, your
speed will be reduced quite significantly by the impact; if you crash
an 18 wheeler going 100 mph into the same car, you will barely notice
any loss in speed at all.

There's a huge amount of energy driving this pyroclastic flow:
http://home.comcast.net/~jeffrey.king2/spire_dust-3.jpg
http://www.geo.mtu.edu/volcanoes/hazards/primer/images/volc-images/sthelenserup.jpg

Could all of that have come from gravitational potential energy? There was
enough energy to not only pulverize the concrete, but also to drive a huge
cloud of the resulting material outword with considerable force.

Possibly, it is difficult to say. It's not an easy thing to compute.

Steel frame building are notorious for collapsing when there is a fire.
Ask a fireman. I-beams aint so tough at 800 or 1,000 Farenheit.

Since you asked, there have only been 6 incidents of a large reinforced
steel building collapsing due to fire from 1970-2003. Four of them
occured on September 11 (WTC 1, 2, 5, 7).
http://www.haifire.com/presentations/Historical_Collapse_Survey.pdf

WTC 5? The two examples I see in PDF you linked which are not part of the
WTC crime scene were not steel frame structures.

Look at the table near the end of the document. I think the other two
were one in Montreal in '86, and another in New York in '70. Both were
partial collapses.

A.

There is no way that those buildings could tolerate that much burniong
jet fuel. I knew that they were coming down as soon as I saw it live on
Squakbox, and it's an insult to turn it into some damn UFO story.

Which begs the question: what about WTC 5 and 7?

As well as begging the original questions. I've built computer simulations
of steel frame structures and those do not collapse in a way remotely
similar to what happened with the WTC. But computer animation is not
proof. Especially when there is insufficient data to construct a truly
representative model. Nonetheless, the models I built had nowhere near the
amount of structural redundancy of 1, 2 WTC had.

--
http://www.vho.org/GB/c/DC/gcgvcole.html
http://www.vho.org/GB/Books/dth/
http://www.germarrudolf.com/
http://www.ice.gov/graphics/news/newsreleases/articles/051115chicago.htm

.

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