GregoryUrich
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WTC7 Revisited
- Thread starter GregoryUrich
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SDC, I didn't provide an opinion. I pointed out the sticking points many people would naturally have with the fall of WTC 7.
I provided a photo library and asked if anyone has the originals of the following 2 images. That's all.
One of these 2 images is a fake. Someone has tried to lie to us. This is a fact and not an opinion.
Given the original images we can learn which one is a fake.
Why is this information useless?
I provided a photo library and asked if anyone has the originals of the following 2 images. That's all.
One of these 2 images is a fake. Someone has tried to lie to us. This is a fact and not an opinion.
Given the original images we can learn which one is a fake.
Why is this information useless?
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GregoryUrich
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That can not happen in the fantasy world of 9/11 truth.
Examples, using facts, can only ruin the fantasy of making it up yourself.
Nothing ever fails in the world of 9/11 truth unless it was done by some NWO bush/clinton cabal of keebler elves.
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GregoryUrich
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Good point MT. I didn't see what you were getting at before. I dare say that "it looks like" the photos have different damage.
GregoryUrich
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Dr. Rosenpxnis, I presume. WTC1 dissipated nearly 60% of the PE on resistance. WTC7 dissipated 15%. Sure they were different buildings, but as I believe I have the 15% is only the energy required to expel the air. Do you honestly believe the structure itself should provide no resistance.
GregoryUrich
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Mancman
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The east penthouse collapse must have caused internal damage.
See here: http://www.youtube.com/watch?v=Y5_e5yROoqY
Beneath the penthouse we see a shockwave travel to ground level, the building bulges and flexes, then global collapse ensues.
GregoryUrich
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Thanks for that Mancman. I think you prove my point. I agree you can see the building flex and rebound. It would look much different if the mass if the penthouse went falling through the building and floors fell because the windows would get blown out.
Mancman
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So what do you think causes the flex which travels downwards directly below the penthouse?
DGM
Skeptic not Atheist
One thing you also notice in that video is the lack of significant air movement. Could the building have been so badly fractured inside as to let the majority of the air stay put?
Both images are genuine. I debated this in great detail with Christopher7 last year here (post #3673) and on the following pages including post #3751 in that thread.
You will find the photo on the left hand side in NIST interim report on WTC 7. And the photo on right hand side you will find here, it is the last one before the night shots at the bottom of the page:
http://www.amanzafar.com/WTC/index.shtm
For some real good examples on light refraction effects look at the photos on page 66 and 63 (PDF page 78 and 76) in the report on One Meridian Plaza fire.
That's it from me on this issue.
I assume you meant the the south face of WTC 7 and not the north face here, since the north face was undamaged after the collapse of WTC 1. Just a friendly correction from me.
jaydeehess
Penultimate Amazing
The worst problem is that we cannot see the lowest floors of the north or south side.
(BTW Sabrina, its the south side that faced the towers and had many broken windows.)
The east penthouse falls due to failure of one or more columns directly under it. The debris from this crushes through the building and impacts the heavy trusses at the 5 and 7th floors. This reduces the strength of the core system, those heavy trusses are there for a reason. The northern core columns support the heavy beams over the pre-existing Con-ed building and cantilever the north facade.
Fail those core columns and the entire north half of the building will be falling on top of the Con-ed building which would be akin to a person stepping on an egg carton. The south half of the building had already suufered some significant damage to the perimeter system and had little to arrest a fall if the core was damaged at the 5th to 7th level. That would mean that the entire structure would essentially fall 7 floors with very little to arrest that fall. After falling that distance the impact with the ground would essentially shatter the structure.
I believe that there was one witness that described the lower north wall as pushing out just as global collapse started. That would be consistent with those beams over the Con-ed building losing their core support.
Was this a design flaw? Perhaps, in that the initiating event would seem to have been the result of the office fires going un-fought and causing a local column failure. However would the designers ever have considered a fire that would go on unfought for so long? Would the designers have ever considered what the effect of large parts of WTC 1 falling on it? Probably not.
(BTW Sabrina, its the south side that faced the towers and had many broken windows.)
The east penthouse falls due to failure of one or more columns directly under it. The debris from this crushes through the building and impacts the heavy trusses at the 5 and 7th floors. This reduces the strength of the core system, those heavy trusses are there for a reason. The northern core columns support the heavy beams over the pre-existing Con-ed building and cantilever the north facade.
Fail those core columns and the entire north half of the building will be falling on top of the Con-ed building which would be akin to a person stepping on an egg carton. The south half of the building had already suufered some significant damage to the perimeter system and had little to arrest a fall if the core was damaged at the 5th to 7th level. That would mean that the entire structure would essentially fall 7 floors with very little to arrest that fall. After falling that distance the impact with the ground would essentially shatter the structure.
I believe that there was one witness that described the lower north wall as pushing out just as global collapse started. That would be consistent with those beams over the Con-ed building losing their core support.
Was this a design flaw? Perhaps, in that the initiating event would seem to have been the result of the office fires going un-fought and causing a local column failure. However would the designers ever have considered a fire that would go on unfought for so long? Would the designers have ever considered what the effect of large parts of WTC 1 falling on it? Probably not.
Reflection on Gregory's Calculation
I want to discuss two different topics, namely the air pressurization, and the effective strength of the remaining structure. Let me also say that some of the slings and arrows aimed at Gregory appear unearned. There is an opportunity to learn from this, guys.
---
1. Air Pressurization
The question of energy spent expelling the air is not simple. Consider the following: In an ideal, inviscid fluid, there is actually no resistance offered by the air in this situation! How can this be?
Think of the question like this: We have air that is initially moving with the building, passes through a constriction, and ends up in free space. What velocity do we use for the air? Do we use it at the constriction? No, we don't. That instantaneous velocity of air is much higher than the bulk velocity, but it's a temporary cost. Air gets pushed through the constriction by the much slower air behind it, and it expands into a space left by the external air, which is moving too. And there's no cost to start the bulk air moving, because it falls with the structure -- we've already accounted for its potential energy. So, had our structure been filled with superfluid helium, the fluid resistance would be zero. There would be no effective backpressure inside the structure.
Of course, we are not dealing with superfluid helium. What happens with a real fluid is known as the Venturi Effect. The fluid only gets free acceleration through the constriction until it runs out of ambient pressure to accelerate it. This pressure energy gets returned immediately outside the restriction, but since the pressure is finite, there's a finite amount of acceleration it can handle. Once we exceed this -- and we will -- we enter a blocked flow condition.
When the flow is blocked at the restriction, it will cause a backpressure inside the structure, and this backpressure will indeed oppose the collapse. The rising backpressure means additional pressure to oppose the Venturi Effect, so flow increases until pressure and flow rate balance. We can then estimate the energy loss during collapse -- because (most of) this energy returns once the air has left the structure -- either from the Bernoulli energy density at the restriction, or the adiabatic gas energy contained in the pressurized region.
Solving this rigorously depends strongly on the size and location of the restrictions. And there's yet another issue, namely that the structure can only handle a small amount of backpressure before ruptures start appearing all over. How much? Only a couple of PSI -- anything more will surely break windows upstairs, and may crack or even burst damaged walls and floors. So we can put a very rough upper limit on the opposing force, and from that the total energy expended against the gas, by assuming say 10 kPa backpressure times the floor area, integrated over the height of the structure, which works out to about 7.7 GJ.
This number is actually in decent agreement with Gregory's figure -- my calculation could easily be off by a factor of two or three in either direction. Working out the correct answer will be difficult.
I speculate that the actual number is closer to mine or even less, simply because the condition of the building was very poor. Fires had broken windows on over ten floors. The structure had ventilation to begin with. There were large elevator shafts running the height of the building, and we know the roof experienced substantial collapses prior to the rest of the structure. Thus, there were many escape paths for overpressure, and perhaps it could only handle 0.5 PSI or thereabouts? Who knows.
Nonetheless, Gregory's number is plausible.
2. "Resistance Factor"
Regarding the remaining strength of the building, what you've done here is basically confirmed the result of Dr. Heikki Kurttila. Like you, he estimated that the surviving structure resisted the collapse with an average force equal to only 16% of the pre-collapse weight.
I explored this in Appendix B of my whitepaper. Basically, the mathematical result is correct, but the interpretation is not. Quoting from my whitepaper:
In other words, the structure is not very flexible. The total energy dissipated -- the total work done by the structure -- is the product of the resisting force times the distance over which it resists. Because it isn't flexible, the distances are actually very small. Therefore, the collapse is really a series of brief jerks and snapthroughs, and if you average this out over the entire duration of the collapse, you will indeed get a low number compared to the ultimate static strength of the building prior to collapse.
3. Energy Comparison
Does this make sense? Could it be true that the energy to displace the air is comparable to or even greater than that to smash the building?
Why, yes, actually. It's counterintuitive, but it is possible. Two points to consider:
First, above we spoke of pressurizing the building to 1 PSI or so during the collapse. 1 PSI is 144 pounds per square foot, and that is actually comparable to the strength of the building! (About half of the ultimate load on a given floor, roughly). Also, unlike the structure, this really is a continuous opposing force, not one that goes to zero every time a new set of columns gives way. Integrated over the whole collapse, this is indeed competitive.
Second, we're not quite comparing the same things here. The air displacement is all the energy the air is ever going to get. We're considering the full kinetic energy of the air. In contrast, the energy expended on the structure during collapse is only part of it. There will still be a considerable amount of energy left over, in the form of kinetic energy after local failure, that continues to grind the pieces together at the ground.
Thus, it is indeed possible for the air to absorb more energy than the structure itself during collapse. This result will probably surprise a lot of people! But this is why it's important to trust in your calculations, and not shoot the messenger. There's a lesson in there for the JREF stalwarts.
4. Comparison to WTC 1 and 2
Also from Appendix B of my whitepaper, I estimate that the Towers absorbed closer to 50% of their gravitational energy as they collapsed, compared to only 16% in WTC 7. Why?
The answer is in the style of collapse. WTC 1 and 2 both experienced a top-down progressive collapse, whereas WTC 7 was more bottom-up. In WTC 1 and 2, the structure is really no more able to oppose the collapse through sheer strength, but instead, you have the contact plane way up in the air, and that means momentum transfer. In addition to breaking columns, the descending mass also has to overcome the inertia of the lower block. As it does so, it expends more energy, which means more total destruction of things like concrete floors.
WTC 7, on the other hand, has the contact interface at or near ground level. Momentum transfer is to the Earth, and so rather than slow down the descending "upper block," it merely destroys everything at the ground level all at once. None of the Earth's inertia gets transmitted up. As a result, the "momentum sink" of WTC 1 and 2 just doesn't happen.
Similarly, there is less energy expenditure high in the structure. This is why we don't see concrete comminution or drywall devastation in the same degree.
Regarding destruction of materials, it is also important to note that the gravitational potential energy per kilogram, or per square meter, will be much lower in WTC 7 than in the Towers. Thus, we predict a less thorough destruction of material, and this is indeed exactly what we see.
5. Closing Comments
I want to stress again that this kind of calculation is fraught with uncertainties, and therefore this is really not a good way to estimate the building properties. However, after some consideration, I don't see anything in GregoryUrich's calculation that appears implausible. I speculate that his results might be a tad high, but he's got the right order of magnitude.
Consideration of this problem, even using greatly simplified models and rough numbers, actually exposes some features of mechanics that are not obvious at first glance. This is a useful exercise. This also highlights the peril in arguing from common sense. We just don't have experience with these kinds of events in our daily lives.
I see no reason now or before to suspect explosives were involved. This should have been obvious -- Chief Nigro can tell you that WTC 7 was coming down, explosives or no explosives, so what's the point? Just to make it fall faster? Nonsense. Neither did Gregory suggest this, to his credit. This is just what happens when tall buildings exceed their limits.
I want to discuss two different topics, namely the air pressurization, and the effective strength of the remaining structure. Let me also say that some of the slings and arrows aimed at Gregory appear unearned. There is an opportunity to learn from this, guys.
---
1. Air Pressurization
The question of energy spent expelling the air is not simple. Consider the following: In an ideal, inviscid fluid, there is actually no resistance offered by the air in this situation! How can this be?
Think of the question like this: We have air that is initially moving with the building, passes through a constriction, and ends up in free space. What velocity do we use for the air? Do we use it at the constriction? No, we don't. That instantaneous velocity of air is much higher than the bulk velocity, but it's a temporary cost. Air gets pushed through the constriction by the much slower air behind it, and it expands into a space left by the external air, which is moving too. And there's no cost to start the bulk air moving, because it falls with the structure -- we've already accounted for its potential energy. So, had our structure been filled with superfluid helium, the fluid resistance would be zero. There would be no effective backpressure inside the structure.
Of course, we are not dealing with superfluid helium. What happens with a real fluid is known as the Venturi Effect. The fluid only gets free acceleration through the constriction until it runs out of ambient pressure to accelerate it. This pressure energy gets returned immediately outside the restriction, but since the pressure is finite, there's a finite amount of acceleration it can handle. Once we exceed this -- and we will -- we enter a blocked flow condition.
When the flow is blocked at the restriction, it will cause a backpressure inside the structure, and this backpressure will indeed oppose the collapse. The rising backpressure means additional pressure to oppose the Venturi Effect, so flow increases until pressure and flow rate balance. We can then estimate the energy loss during collapse -- because (most of) this energy returns once the air has left the structure -- either from the Bernoulli energy density at the restriction, or the adiabatic gas energy contained in the pressurized region.
Solving this rigorously depends strongly on the size and location of the restrictions. And there's yet another issue, namely that the structure can only handle a small amount of backpressure before ruptures start appearing all over. How much? Only a couple of PSI -- anything more will surely break windows upstairs, and may crack or even burst damaged walls and floors. So we can put a very rough upper limit on the opposing force, and from that the total energy expended against the gas, by assuming say 10 kPa backpressure times the floor area, integrated over the height of the structure, which works out to about 7.7 GJ.
This number is actually in decent agreement with Gregory's figure -- my calculation could easily be off by a factor of two or three in either direction. Working out the correct answer will be difficult.
I speculate that the actual number is closer to mine or even less, simply because the condition of the building was very poor. Fires had broken windows on over ten floors. The structure had ventilation to begin with. There were large elevator shafts running the height of the building, and we know the roof experienced substantial collapses prior to the rest of the structure. Thus, there were many escape paths for overpressure, and perhaps it could only handle 0.5 PSI or thereabouts? Who knows.
Nonetheless, Gregory's number is plausible.
2. "Resistance Factor"
Regarding the remaining strength of the building, what you've done here is basically confirmed the result of Dr. Heikki Kurttila. Like you, he estimated that the surviving structure resisted the collapse with an average force equal to only 16% of the pre-collapse weight.
I explored this in Appendix B of my whitepaper. Basically, the mathematical result is correct, but the interpretation is not. Quoting from my whitepaper:
R. Mackey said:
In other words, the structure is not very flexible. The total energy dissipated -- the total work done by the structure -- is the product of the resisting force times the distance over which it resists. Because it isn't flexible, the distances are actually very small. Therefore, the collapse is really a series of brief jerks and snapthroughs, and if you average this out over the entire duration of the collapse, you will indeed get a low number compared to the ultimate static strength of the building prior to collapse.
3. Energy Comparison
Does this make sense? Could it be true that the energy to displace the air is comparable to or even greater than that to smash the building?
Why, yes, actually. It's counterintuitive, but it is possible. Two points to consider:
First, above we spoke of pressurizing the building to 1 PSI or so during the collapse. 1 PSI is 144 pounds per square foot, and that is actually comparable to the strength of the building! (About half of the ultimate load on a given floor, roughly). Also, unlike the structure, this really is a continuous opposing force, not one that goes to zero every time a new set of columns gives way. Integrated over the whole collapse, this is indeed competitive.
Second, we're not quite comparing the same things here. The air displacement is all the energy the air is ever going to get. We're considering the full kinetic energy of the air. In contrast, the energy expended on the structure during collapse is only part of it. There will still be a considerable amount of energy left over, in the form of kinetic energy after local failure, that continues to grind the pieces together at the ground.
Thus, it is indeed possible for the air to absorb more energy than the structure itself during collapse. This result will probably surprise a lot of people! But this is why it's important to trust in your calculations, and not shoot the messenger. There's a lesson in there for the JREF stalwarts.
4. Comparison to WTC 1 and 2
Also from Appendix B of my whitepaper, I estimate that the Towers absorbed closer to 50% of their gravitational energy as they collapsed, compared to only 16% in WTC 7. Why?
The answer is in the style of collapse. WTC 1 and 2 both experienced a top-down progressive collapse, whereas WTC 7 was more bottom-up. In WTC 1 and 2, the structure is really no more able to oppose the collapse through sheer strength, but instead, you have the contact plane way up in the air, and that means momentum transfer. In addition to breaking columns, the descending mass also has to overcome the inertia of the lower block. As it does so, it expends more energy, which means more total destruction of things like concrete floors.
WTC 7, on the other hand, has the contact interface at or near ground level. Momentum transfer is to the Earth, and so rather than slow down the descending "upper block," it merely destroys everything at the ground level all at once. None of the Earth's inertia gets transmitted up. As a result, the "momentum sink" of WTC 1 and 2 just doesn't happen.
Similarly, there is less energy expenditure high in the structure. This is why we don't see concrete comminution or drywall devastation in the same degree.
Regarding destruction of materials, it is also important to note that the gravitational potential energy per kilogram, or per square meter, will be much lower in WTC 7 than in the Towers. Thus, we predict a less thorough destruction of material, and this is indeed exactly what we see.
5. Closing Comments
I want to stress again that this kind of calculation is fraught with uncertainties, and therefore this is really not a good way to estimate the building properties. However, after some consideration, I don't see anything in GregoryUrich's calculation that appears implausible. I speculate that his results might be a tad high, but he's got the right order of magnitude.
Consideration of this problem, even using greatly simplified models and rough numbers, actually exposes some features of mechanics that are not obvious at first glance. This is a useful exercise. This also highlights the peril in arguing from common sense. We just don't have experience with these kinds of events in our daily lives.
I see no reason now or before to suspect explosives were involved. This should have been obvious -- Chief Nigro can tell you that WTC 7 was coming down, explosives or no explosives, so what's the point? Just to make it fall faster? Nonsense. Neither did Gregory suggest this, to his credit. This is just what happens when tall buildings exceed their limits.
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Bazant & Zhou does treat momentum transfer and plastic energy. Their load capacity is a bit flaky, weight distribution and total is too simple, spring constant debatable, and no heating etc., but correcting these factors only eats into the gross overload they predict, and does not lead to an expectation of arrest.
BLBG treats all of these issues. Mass is still arguably high, but it will divide out, as we've discussed several times.
GregoryUrich
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Thanks for your thoughtful response. I'm not sure I'll get a chance to think through your comments until this weekend. I just wanted to mention that as I was dosing off last night I realised that the shape of the building makes a significant difference. I think my result for air resistance will be revised downward by 40-50%.