First off, I realize this may seem kinda morbid. I hope I’m not upsetting anyone by posting this, but I know others have had the same question.
The planes that hit the World Trade Center hit pretty high on the building. Would that be enough to take it out, or do you think explosives were needed?
There were ten or twenty stories of building above the point where the planes hit. Their support got burned out from under them, and they collapes pretty much intact onto the 80th-90th floors, which got crushed and caused the buildings to collapse.
The videos I’ve seen of buildings being imploded get pretty dramatic when the explosives go off–there’s no doubt that explosives were used in those cases. Here with the WTC, it was just the sheer impact of a 10- or 20-story building landing on another one that caused the total collapse.
The thought that kept running through my mind–“Thank god they didn’t topple over.” They would have crushed buildings for five or six blocks, including all the firefighters, EMT types and evacuees waiting in the streets.
It doesn’t take much in the way of explosives to take down a duildsing in a PLANNED explosion… It’s all a matter of placement, taking out key structures, and then gravity does the rest. I believe that is BASICALLY what happened here. The planes weakened the structure, gravity did the rest. I’d imagine (and this is just a guess on my part) that the first tower collapsing may have weakened the second in some way, as well.
Another factor was that the palnes were cross country flights just taking off from the eastern USA - so totally loaded w/ fuel
What about the reports of secondary exploseions in the buildings before they collapsed?
The two towers shared a common foundation. Once the first tower fell, it’s easy to imagine the damage done to the base of the other. Otherwise the north tower looked like it might have withstood it’s attack.
I am not a structural engineer and what I am about to relate was gathered from the TV news on this event so take it FWIW.
Someone mentioned on the news that these buildings (specifically the World Trade Center but I assume most other skyscrapers as well) are designed to implode rather than topple over. Given the pictures just prior to the towers’ collapse you could see the building leaning quite noticeably yet they (mostly) fell straight down when they collapsed.
Also notice that both towers withstood the initial impact. They also mentioned on the news that the fires that were burning weakened the supports further (hot steel gets soft) till eventually they gave way.
The WTC was designed so that the outer walls were load-bearing. The plane crashing into them weakened the structure and, when coupled with damage from the fires, it wasn’t able to hold up its own weight. Thus, collapse.
It’s as if the building was held up by cables. The walls were the cables. Once weakened, it fell.
OTOH, if the plane had struck the Empire State Building, it probably would have survived (it did the last time that happened). The load on the ESB is carried by the inner framework and, from a structural point of view, the outer walls are just there to keep the wind out.
Out of curiosity I did a few calculations. The site http://www.british-airways.com/flights/factfile/airfleet/docs/767.shtml lists the statistics of a 767 (both planes which hit the towers were 767’s). The max takeoff weight is 400,000 lbs. And max fuel is listed at 73,000 kg.
Landing speed is listed as just under 200 mph, so a back of the envelope calculation is that the kinetic energy of the plane alone is on the order of a half ton of TNT (note that at 320 mph that goes to a full ton of TNT, and at cruising speed of 540 mph that’s 3 tons of TNT–looks like the plane was maneuverthough, so I think it was going much slower than cruising). I don’t know the energy content of airplane fuel so I can’t estimate that energy, but the KE was delivered directly to supporting structures in the second tower hit. The second tower was also hit lower down, and collapsed before the other one did.
Anyone have figures for energy content of airplane fuel?
(Was going to start a thread on this myself if it hadn’t been started already.)
I think that you have this backwards.
The weight of the aircraft would have been a contributor as well. You can’t just drop 400,000 lbs on a floor of a building and expect it to hang in there. I believe both aircraft stayed inside the buildings.
ABC has a good article on the design and structure of the towers, and why the plane crashes were enough to destroy them.
Use the same figure as for kerosene - that’s more or less what jet fuel is.
Also, at least the second plane was reported to be at cruise when it hit.
Wow. Well, I ran some numbers, and here’s what I got:
http://www.chevron.com/prodserv/fuels/bulletin/aviationfuel/pdfs/chapter4.pdf
Shows that the energy content for various fuels runs 40-45 MJ/kg. So I took 42 MJ/kg and plugged it in. The result says equiv of 700 tons of TNT. That doesn’t seem right, since it was only 80 tons of fuel–is fuel 10x more energetic than TNT? I don’t know.
Oops, a non-pdf link for the same table of info is:
http://www.chevron.com/prodserv/fuels/bulletin/aviationfuel/4_at_fuel_comp.shtm
and at second look, suggests 44 MJ/kg is a better number than 42 for kerosene.
I started a thread on this here —
How much fuel does a large commercial plane carry, and how much explosive power?
The impact of those 2 towers collapsing quite probably severely damaged some of the surrounding structures as well, as is evidenced by the collapse of the 43-story WTC building #7 just a little while ago.
As for the energy content of Jet fuel, you could probably compare it to deisel fuel or kerosene, as it is pretty close to either of those in form.
No. I was talking about the Empire State Building, which is different from the World Trade Center.
This is very likely what happened. One of the design considerations made when the World Trade Center towers were built was that they should be able to survive an impact by an aircraft (since they are located near flight paths). They withstood the initial impacts from the aircraft as they had been designed, but buckled when the supports finally collasped from the extreme heat.
Here are two good links that point to discussions of the WTC collapses’ architectural and engineering issues. The second one is better. (Thanks to Phil Agre and the RRE digest.)
http://home.actlab.utexas.edu/pipermail/discuss/2001-September/000226.html
http://chicagotribune.com/news/nationworld/chi-010911kamin-towers.story