Darby & Zona,
indazona,
Excellent job with your response. Outstanding.
I agree. And I'm trusting him to design the buildings at my Colorado property, so he'd better know his stuff! /ttiforum/images/graemlins/smile.gif
I'll leave this one up to RMT because he has the data...
What was the approximate kinetic energy transferred from the aircraft to the buildings on impact?
Yes, I am remiss in providing this analysis, as I had promised it earlier in this thread. Essentially, one of the most instructive analyses we could perform on this issue relates to comparing how much kinetic energy was considered for an airplane impact for the WTC building design to the amount of kinetic energy that the buildings were actually exposed to on 9-11. The difference is quite astounding, and what is even more astounding is the fact that the buildings continued to stand after both of these impacts! :eek: But before I present the numbers of this comparison it is important that we review some quotes. Let's begin with the quotes made by Leslie Robertson after 9-11. He was one of the people involved in the WTC structural design. I have highlighted the important points of what he said:
One of my jobs was to look at all of the possible events that might take place in a highrise building. And of course, in New York there had been two instances of aircraft impacts, the most famous being on the Empire State Building. Now, we were looking at an aircraft that was not unlike the Mitchell that ran into the Empire State Building. We were looking at an aircraft that was lost in the fog, trying to land. It was a low-flying, slow-flying 707, which was the largest aircraft of its time. And so we made calculations not anywhere near the level of sophistication that we could today. But inside of our ability, we made calculations of what happens when an aircraft goes in and it takes out a huge section of the outside wall of the building. And we concluded that it would stand. It would suffer but it would stand. And the outside wall would have a big hole in it, and the building would be in place. What we didn't look at is, what happens to all that fuel? And perhaps we could be faulted for that, for not doing so. But that, for whatever reason, we didn't look at that question of what would happen to the fuel.
–Leslie Robertson, interviewed in New York: A Documentary Film, by Ric Burns
Two things here are important. First is his admission that they did not look at the fuel burning. All they looked at was the initial impact. Since the buildings did not fall immediately, but only after the fires had burned for an hour or more, then it is instructive to note that the initial analysis did NOT consider anything beyond the airplane impact.
The second thing he says is very important... that they considered a scenario of an airplane that was "low-flying, slow-flying." This is very important because as Indazona has pointed out, kinetic energy scales with the SQUARE of the velocity! Furthermore, there is a very good reason why they would NOT bother analyzing an aircraft flying into the buildings at 500 mph or more... that reason has to due with air traffic LAWS that restrict how fast an airplane is permitted to fly when below an altitude of 10,000 feet.
It is against the law for an airplane to fly in US airspace at a speed greater than 250 knots (about 288 mph) while below 10,000 feet. This is an FAA operating restriction, and it is also an operating restriction that all other countries on earth adhere to. Why is this important? Because if the design scenario that they were considering for the WTC design was related to "
an aircraft that was lost in the fog, trying to land" (no one has EVER said they were considering a terrorist action in the design!), then it stands to reason that since the WTC towers are well below 10,000 feet that the fastest an airplane would ever be traveling when it hit the towers would be 250 knots (288 mph, or about 422 feet per second). Now, before we do the comparative energy analysis, let's look at some other things that Robertson had to say:
From: Robertson, Leslie E. (2002). Reflections on the World Trade Center. The Bridge Volume 32, Number 1. National Academy of Engineering. Retrieved on 2006-07-28.
According to Robertson, the modeled aircraft weighed 263,000 lb (119 metric tons) with a flight speed of 180 mph (290 km/h), as in approach and landing, which would have been much slower than the actual impacts of 9/11.
So Robertson even tells us that the speed they considered was quite a bit slower than the 250 knot (288 mph) speed limit by the FAA. Clearly, this speed they used is definitely along the lines of an approach and landing speed for an aircraft (and yes, I think I ought to know!). So now let us run some numbers for comparison. We do know that the mass (weight) of a 707 was fairly close to the weight of the 767's that hit the WTC, so there is really no point in using various weights, especially since it is VELOCITY which has such a large impact (pardon the pun) on the kinetic energy that the buildings would have to absorb. So let's use the same weight (263,000 pounds, or about 8174 slugs of mass) for all three of the following calculations:
1) First let us calculate the ideal kinetic energy of an airplane impacting the WTC at the speeds that Robertson said they considered in the design (180 mph, which is about 264 feet/sec):
Kinetic Energy = (1/2)*Mass*Velocity^2
Kinetic Energy = (1/2)*(8174 slugs)*(264 feet/sec)^2
Kinetic Energy = 284,847,552 pound-feet = 386.2 MegaJoules
2) Now let us calculate the ideal kinetic energy of an airplane impacting the WTC at the maximum speed allowed by Air Traffic Control when below 10,000 feet altitude (250 knots, which is about 422 feet/sec):
Kinetic Energy = (1/2)*(8174 slugs)*(422 feet/sec)^2
Kinetic Energy = 727,829,308 pound-feet = 986.8 MegaJoules (wow, quite a bit higher, huh?)
3) Finally, let us calculate the ideal kinetic energy of the fastest airplane that hit either of the two WTC towers on 9-11, which was approximately 520 mph (about 763 feet/sec):
Kinetic Energy = (1/2)*(8174 slugs)*(763 feet/sec)^2
Kinetic Energy = 2,379,324,703 pound-feet = 3226 MegaJoules!!!
So they hit the buildings with OVER 800% MORE ENERGY than Robertson claims they considered during the design! That is an awful lot of energy, and what is more amazing is that the towers ACTUALLY DID withstand that initial impact without toppling over!
Now, there have been some claims that the design of the WTC towers considered the 707's flying at their top cruise speed, which was about 600 mph. It is one thing to SAY that some analysis was done that "proved" the buildings would not fall at 600 mph, but it is another thing to actually produce that analysis so it can be reviewed for its technical accuracy. Let's see what NIST said about such a claim:
http://fire.nist.gov/bfrlpubs/fire05/PDF/f05119.pdf
A three-page document from the Port Authority of New York and New Jersey (PANYNJ or Port Authority) Indicates that the impact of a Boeing 707 flying at 600 mph was analyzed during the design stage of the WTC towers in February/March 1964 (Letter with an attachment dated November 13, 2003 from John R. Dragonette(Retired Project Administrator, Physical Facilities Division, World Trade Department) to Saroj Bhol (Design and Engineering Department, PANYNJ).
No documents on the aircraft impact analysis are available to review the criteria and method used in the impact analysis of a Boeing 707 aircraft on the WTC tower and to verify the assertion in the three-page document that "...such collision would result in only local damage which could not cause collapse or substantial damage to the building and would not endanger the lives and safety of occupants not in the immediate area of impact." Without the original calculations of the aircraft impact analysis, any comment on the document would be a speculation.
Again, I have emboldened the parts that are most crucial. No one can produce a document that has this analysis. So to simply trust this 3 page document from PANYNJ that talks about the analysis, but not be able to see and validate the analysis itself, it truly WOULD be speculative, just as NIST has said. Moreover, we also do not know if this 600 mph analysis considered anything more than the impact only (such as the weakening of steel columns by unextinguished fires). And furthermore, we already have Robertson himself stating that they DID NOT consider the impact of fires after an aircraft impact.
But even if we do accept the 600 mph analysis, we have already seen (from 9-11) that the towers DID withstand the initial impact of the airplanes. The towers DID NOT come down right away, and the fact that they came down only after a period of time where fires were raging on the damaged floors does lend technical support to the NIST conclusions that the towers fell as a result of the initial damage, the transferring of total loads to the remaining columns, and the weakening of the steel due to the heat of the fires.
Now, the thing I have not addressed here (as yet) is what Darby related to in his last post: How all this "ideal kinetic energy" was dissipated at and after the impact. Obviously, the large part of the energy dissipation was as mechanical forces which were responsible for severing the columns and wreaking havoc inside the building as the remains of the airplane cut through it. But in such collisions like this, there is also quite a good deal of energy dissipated as HEAT due to friction. The heat of metal-on-metal friction at speeds in the range of 520 miles per hour is very significant! Significant enough that a large number of sparks would have been generated to ignite jet fuel, but also to create enough heat that would actually begin to melt the aluminum from the aircraft!!!
I think that's enough for now...
RMT
