Debunk This
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Mancman
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I was specifically referring to videos. The only videos we have of the WTC1 collapse from the south are taken from a distance, and the failure of the wall cannot be clearly seen.
gumboot
lorcutus.tolere
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Perhaps an engineer can help here...
The claim is CDs fall faster than freefall because the "implosion" creates a vacuum and sucks the building down.
Now... for now let's ignore the stupidity of this theory, and only look at the theoretical physics...
Is this correct?
It is my understanding that "freefall" can only be obtained by an object falling in a vacuum. Surely if a vacuum magically appeared in the base of a building, the building would not be "sucked" down, but would freefall through the vacuum at freefall speed?
-Gumboot
The claim is CDs fall faster than freefall because the "implosion" creates a vacuum and sucks the building down.
Now... for now let's ignore the stupidity of this theory, and only look at the theoretical physics...
Is this correct?
It is my understanding that "freefall" can only be obtained by an object falling in a vacuum. Surely if a vacuum magically appeared in the base of a building, the building would not be "sucked" down, but would freefall through the vacuum at freefall speed?
-Gumboot
No, so long as the building was standing in ambient air.
Say we build a house of cards. Then we run a vacuum cleaner right to the bottom of the house of cards, and turn it on.
The cards will fall "faster than freefall," whether we take that to mean falling under the influence of gravity only, or whether we take drag into account. That's because there is low pressure ahead of the cards and high pressure behind them. This differential accelerates them, in addition to gravity.
If we do this on the moon, then the vacuum cleaner has no effect, and they fall "as fast as freefall."
Say we build a house of cards. Then we run a vacuum cleaner right to the bottom of the house of cards, and turn it on.
The cards will fall "faster than freefall," whether we take that to mean falling under the influence of gravity only, or whether we take drag into account. That's because there is low pressure ahead of the cards and high pressure behind them. This differential accelerates them, in addition to gravity.
If we do this on the moon, then the vacuum cleaner has no effect, and they fall "as fast as freefall."
qarnos
Cold-hearted skeptic
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You're right about the gravity of the situation (pun intended
), but the argument is that the upper floors are pressurised whilest the lower floors are a vacuum. The pressure differential would, IMHO, give a slight increase over the gravity-only speed. How much is debateable, but I think it would be negligable.Well we have a video out there of the "Landmark" Tower Controlled Demolition. We know how tall it was, and we can time the demolition.
Since the claim made is that most CDs are ment to occur "Faster than Free Fall" we can use this example as a test case.
TAM
Since the claim made is that most CDs are ment to occur "Faster than Free Fall" we can use this example as a test case.
TAM
Anti-sophist
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I am highly skeptical of the claim that you can use explosives to make a building fall faster than freefall. My skepticism grows once you add in the requirement that they have to be relatively quiet.
ktesibios
Worthless Aging Hippie
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Well, if d= 1/2*A*t2
(2*d)/A= t2
t= sqrt(2*d/A)
Then if d= 116m and A= 9.8 m/s2, t= 4.865 s.
So 7-8 seconds would be longer than it should take an object to free fall 116 meters.
gumboot
lorcutus.tolere
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Wikipedia has a very useful entry for this sort of stuff:
Equations for a Falling Body
I use it all the time...
-Gumboot
Equations for a Falling Body
I use it all the time...
-Gumboot
Loss Leader
I would save the receptionist., Moderator
I just got finished reading the dictionary. Turns out the zebra did it.
Credit: Steven Wright
defaultdotxbe
Drunken Shikigami
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you should read the phone book, not much of a plot but a great cast of characters
Arkan_Wolfshade
Philosopher
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Yeah, the ending's a real zinger, isn't it?
Dave Rogers
Bandaged ice that stampedes inexpensively through
Trumpman critique, part 3
Here are my comments on the third part of Trumpman's paper, "Cloud Analysis".
Early in this section Trumpman states that "Explosions are occurring before any observable signs of collapse". This is in direct contradiction to the sequence of events given at the beginning of the previous section, in which he states that the tower antenna moves before any explosions are seen. Having viewed videos of the WTC1 collapse I have not seen the explosions prior to the collapse that Trumpman asserts are visible; however, as the building was on fire and emitting large volumes of smoke at the time, this could have been confused with explosions. The emissions of material from below the collapse zone, believed by many to be window breakage due to overpressure and the resulting ejection of debris, are referred to as explosions.
Trumpman asserts that the clouds of dust emitted early in the collapse are concrete dust, based solely on their colour, but does not discuss how their appearance differs from plaster or drywall dust. He states that the presence of explosives is demonstrated by the emission of concrete dust before floor 98 impacts floor 97, but if the dust expelled at this time is from plaster and drywall this conclusion is invalid. In any case it is not clear to me from viewing videos that this happens at all, and Trumpman includes no still pictures to illustrate his assertions.
Trumpman analyses the volumes of the dust clouds after expulsion from the towers, working from the assumption that mixing of air between the emitted dust clouds and the air initially outside the towers is limited to 1/3 of the final volume. His justification for this is that "It has been asserted that one takes about 1/3 off to account for expansion by air turbulence", but there is no source given for this assertion nor any discussion of how it was arrived at.
Trumpman next calculates the fall velocities at each floor impact, and attempts to estimate the energy absorption at each impact by determining the difference between the expected kinetic energy had the upper section fallen in free fall and the actual kinetic energy from his velocity estimates. There are three crucial errors in his analysis here. Firstly, due to his incorrect value for the weight of the tower, all his kinetic energies are only 40% of the correct values. Secondly, he calculates the impact velocity at each floor as being inversely proportional to the fall time, whereas for the case of constant acceleration it should be proportional to the square root of the fall time. Most seriously, however, his energy terms are incorrect. For a proper analysis it would be necessary to determine the velocity just after impact rather than just before impact, and to subtract the post-impact kinetic energy from the pre-impact kinetic energy. This is not equivalent to Trumpman's calculation; in particular it leads to an incorrect estimate of zero energy loss at the first impact, as the first floor collapse would be expected to proceed at free fall. Interestingly, though, this method gives a reasonable estimate, for each stage of the collapse, of the energy lost in the previous stage; obviously for the first stage this is zero.
Trumpman next calculates the energy required to collapse a single floor. I do not have the expertise to comment on his methodology, but qualitatively his result that the energy is a very small fraction of the kinetic energy available appears reasonable by comparison with the work of other authors. Indeed, Trumpman makes the point that the column collapse energy was relatively small: "I observed in my research that people mistakenly believe columns would have resisted the collapse significantly. It is true, they can support large loads. But they cannot handle that same weight dropped from a height." He then calculates, based on his incorrect figures for energy loss, the amount of energy available for pulverisation of concrete, and determines that this would be able to pulverise none of the concrete from the 97th floor, 28% of that from the 96th, 55% from the 95th and 20% from the 94th. He makes no estimate of the effect of random errors in time measurement on these numbers. Given that his energies are only 40% of what they should be and his assumptions are incorrect, it is possible to re-calculate these numbers to give the result that sufficient energy was available to pulverise 70% of the 97th, 137% of the 96th and 50% of the 95th floor, an average of 85% of the concrete in these three floors.
Trumpman now goes on to state that, since there was insufficient energy lost in each collapse stage to pulverise the concrete in that stage totally, there could therefore have been no energy left over for expansion of the dust cloud. This is based on the completely unjustified and unrealistic assumption that all the concrete in every floor was pulverised to fine dust. The existence of large slabs of compacted concrete from multiple floors in the WTC rubble shows that this assumption is incorrect. He then calculates that the dust cloud expands by 2.5 times as the building collapses, comparing this with Hoffman's figure of 3.4 times, and states that there is no possible source for this expansion. His analysis in this section becomes extremely confusing and disjointed, punctuated with an irrelevant rant about the cost of the NIST enquiry, so it is hard to determine what his point is. If, however, he is arguing, as does Hoffman, that the dust cloud expansion was due to thermal expansion of the air, there is a serious logical flaw in this argument. He estimates the average temperature in a floor of WTC1 as 148C, or 421 kelvin. For a 2.5 times expansion this would have to be heated to 1053 kelvin, or 779C, a temperature sufficiently high to cause serious (probably fatal) burns to anyone exposed to this dust cloud as it fell to ground level. This same issue has caused Hoffman to revise his theory of dust cloud expansion to include the postulate of 2,000 tons of water whose origin is not known. Indeed, Hoffman's latest version of his dust cloud analysis has been under development since January 2004, suggesting that this discrepancy is proving extremely difficult to resolve.
Later in the paper, it should be pointed out, Trumpman suggests that the additional air volume was created by the gases released in the explosion of 1.4 tonnes of HMX high explosive. This is a more reasonable suggestion than that of thermal expansion of the dust cloud. Even so, it rests on the fundamental assumption that mixing with external air is limited to 1/3 of the final volume of the cloud, and this is not satisfactorily established in this paper.
Dave
Here are my comments on the third part of Trumpman's paper, "Cloud Analysis".
Early in this section Trumpman states that "Explosions are occurring before any observable signs of collapse". This is in direct contradiction to the sequence of events given at the beginning of the previous section, in which he states that the tower antenna moves before any explosions are seen. Having viewed videos of the WTC1 collapse I have not seen the explosions prior to the collapse that Trumpman asserts are visible; however, as the building was on fire and emitting large volumes of smoke at the time, this could have been confused with explosions. The emissions of material from below the collapse zone, believed by many to be window breakage due to overpressure and the resulting ejection of debris, are referred to as explosions.
Trumpman asserts that the clouds of dust emitted early in the collapse are concrete dust, based solely on their colour, but does not discuss how their appearance differs from plaster or drywall dust. He states that the presence of explosives is demonstrated by the emission of concrete dust before floor 98 impacts floor 97, but if the dust expelled at this time is from plaster and drywall this conclusion is invalid. In any case it is not clear to me from viewing videos that this happens at all, and Trumpman includes no still pictures to illustrate his assertions.
Trumpman analyses the volumes of the dust clouds after expulsion from the towers, working from the assumption that mixing of air between the emitted dust clouds and the air initially outside the towers is limited to 1/3 of the final volume. His justification for this is that "It has been asserted that one takes about 1/3 off to account for expansion by air turbulence", but there is no source given for this assertion nor any discussion of how it was arrived at.
Trumpman next calculates the fall velocities at each floor impact, and attempts to estimate the energy absorption at each impact by determining the difference between the expected kinetic energy had the upper section fallen in free fall and the actual kinetic energy from his velocity estimates. There are three crucial errors in his analysis here. Firstly, due to his incorrect value for the weight of the tower, all his kinetic energies are only 40% of the correct values. Secondly, he calculates the impact velocity at each floor as being inversely proportional to the fall time, whereas for the case of constant acceleration it should be proportional to the square root of the fall time. Most seriously, however, his energy terms are incorrect. For a proper analysis it would be necessary to determine the velocity just after impact rather than just before impact, and to subtract the post-impact kinetic energy from the pre-impact kinetic energy. This is not equivalent to Trumpman's calculation; in particular it leads to an incorrect estimate of zero energy loss at the first impact, as the first floor collapse would be expected to proceed at free fall. Interestingly, though, this method gives a reasonable estimate, for each stage of the collapse, of the energy lost in the previous stage; obviously for the first stage this is zero.
Trumpman next calculates the energy required to collapse a single floor. I do not have the expertise to comment on his methodology, but qualitatively his result that the energy is a very small fraction of the kinetic energy available appears reasonable by comparison with the work of other authors. Indeed, Trumpman makes the point that the column collapse energy was relatively small: "I observed in my research that people mistakenly believe columns would have resisted the collapse significantly. It is true, they can support large loads. But they cannot handle that same weight dropped from a height." He then calculates, based on his incorrect figures for energy loss, the amount of energy available for pulverisation of concrete, and determines that this would be able to pulverise none of the concrete from the 97th floor, 28% of that from the 96th, 55% from the 95th and 20% from the 94th. He makes no estimate of the effect of random errors in time measurement on these numbers. Given that his energies are only 40% of what they should be and his assumptions are incorrect, it is possible to re-calculate these numbers to give the result that sufficient energy was available to pulverise 70% of the 97th, 137% of the 96th and 50% of the 95th floor, an average of 85% of the concrete in these three floors.
Trumpman now goes on to state that, since there was insufficient energy lost in each collapse stage to pulverise the concrete in that stage totally, there could therefore have been no energy left over for expansion of the dust cloud. This is based on the completely unjustified and unrealistic assumption that all the concrete in every floor was pulverised to fine dust. The existence of large slabs of compacted concrete from multiple floors in the WTC rubble shows that this assumption is incorrect. He then calculates that the dust cloud expands by 2.5 times as the building collapses, comparing this with Hoffman's figure of 3.4 times, and states that there is no possible source for this expansion. His analysis in this section becomes extremely confusing and disjointed, punctuated with an irrelevant rant about the cost of the NIST enquiry, so it is hard to determine what his point is. If, however, he is arguing, as does Hoffman, that the dust cloud expansion was due to thermal expansion of the air, there is a serious logical flaw in this argument. He estimates the average temperature in a floor of WTC1 as 148C, or 421 kelvin. For a 2.5 times expansion this would have to be heated to 1053 kelvin, or 779C, a temperature sufficiently high to cause serious (probably fatal) burns to anyone exposed to this dust cloud as it fell to ground level. This same issue has caused Hoffman to revise his theory of dust cloud expansion to include the postulate of 2,000 tons of water whose origin is not known. Indeed, Hoffman's latest version of his dust cloud analysis has been under development since January 2004, suggesting that this discrepancy is proving extremely difficult to resolve.
Later in the paper, it should be pointed out, Trumpman suggests that the additional air volume was created by the gases released in the explosion of 1.4 tonnes of HMX high explosive. This is a more reasonable suggestion than that of thermal expansion of the dust cloud. Even so, it rests on the fundamental assumption that mixing with external air is limited to 1/3 of the final volume of the cloud, and this is not satisfactorily established in this paper.
Dave
Newtons Bit
Penultimate Amazing
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Real safety factors of modern buildings is roughly 1.65. It's hard to put an exact number on it based on how safety is determined. For instance, a typical member is designed to take 1.2*DeadLoad + 1.6*LiveLoad. with a 0.9 factor on the column strength. (that's just one load combination).
Sorry if someone else answered this, I didn't read the rest of the post.
Already answered. And we'd better make clear that you're talking about a very simple example here, aren't you?
imstellar28
Scholar
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I was correct about the safety factors, from the NIST report figures I gave earlier in the thread, as was I in my implementation of them:
The safety factor for yielding and buckling is:
• 1.67 and 1.92 for core columns in the original design and SOP cases, and for all columns in refined NIST estimate case.
• 1.26 and 1.44 for perimeter columns in the original design and SOP case (discounting the 1/3 increase in allowable stress under wind loads).
• After reaching the yield strength, structural steel components continue to have significant reserve capacity, thus allowing for load redistribution to other components that are still in the elastic range.
• On September 11, the towers were subjected to in-service live loads, which are considered to be approximately 25 percent of the design live loads.
• On September 11, the wind loads were minimal, thus allowing significantly more reserve capacity for the exterior walls (demand on exterior columns was about 1/5 their capacity).
As I stated, after the planes impacted, the loads shifted effectively and the structure would have continued to stand as the steel was still well within its load-bearing limits due to the combination of reduced loads on 9/11 and high safety factor.
However, the potential problem with my oversimplified model, was not a misunderstanding or miscalculation of forces acting on the steel, it was that my model ignored the potential changes in geometry due to thermal expansion.
For you to refute this model, you have to show that the thermal expansion associated with a fire at X degrees was sufficient to cause deflection to the point where the steels yield strength decreased below the value of live and dead loads.
The safety factor for yielding and buckling is:
• 1.67 and 1.92 for core columns in the original design and SOP cases, and for all columns in refined NIST estimate case.
• 1.26 and 1.44 for perimeter columns in the original design and SOP case (discounting the 1/3 increase in allowable stress under wind loads).
• After reaching the yield strength, structural steel components continue to have significant reserve capacity, thus allowing for load redistribution to other components that are still in the elastic range.
• On September 11, the towers were subjected to in-service live loads, which are considered to be approximately 25 percent of the design live loads.
• On September 11, the wind loads were minimal, thus allowing significantly more reserve capacity for the exterior walls (demand on exterior columns was about 1/5 their capacity).
As I stated, after the planes impacted, the loads shifted effectively and the structure would have continued to stand as the steel was still well within its load-bearing limits due to the combination of reduced loads on 9/11 and high safety factor.
However, the potential problem with my oversimplified model, was not a misunderstanding or miscalculation of forces acting on the steel, it was that my model ignored the potential changes in geometry due to thermal expansion.
For you to refute this model, you have to show that the thermal expansion associated with a fire at X degrees was sufficient to cause deflection to the point where the steels yield strength decreased below the value of live and dead loads.
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