So many here claim that the towers didn't

[wildbilln864]

fall at near free fall speed. So I want to ask what would be the fall time in seconds if they indeed were to fall at freefall? Thanks!
:hi:

[LARED]

1. Try for yourself

http://hyperphysics.phy-astr.gsu.edu/hbase/traj.html

[wildbilln864]

2. I don't need to...

Admit you have no answer other than the collapse times.

[LARED]

3. Huh?

What are you babbling about?

There is a theoretical free fall time (9.2 sec), and then there is the actual fall time of the towers. (~ 15+ sec)

[wildbilln864]

6. Ok....

don't get your panties in a wad!
15 seconds sounds exagerated!
:yoiks:

[LARED]

16. I'll tell you what

go find a video of the towers collapse and time it yourself. I recommended Loose Change because you nearly get the full view of the towers as they fell. As I recall 15+ sec is a very conservative time.

But go see for yourself and let me know how you make out.

[jberryhill]

4. Why worry about time

...when still pictures show you a built-in free-fall clock right NEXT to the towers:



Notice how the debris in free fall approaches the ground faster than the collapse front of the tower.

Are you suggesting that the gravity around the tower is enhanced, so that free falling objects fall faster than free fall?

And, yes, I freehanded the red lines, but you can easily see that the debris is approaching the ground faster than the tower is collapsing.

[wildbilln864]

5. Good point.

Thanks.

[jgraz]

7. Excellent pics, thanks for posting them n/t

[mogster]

8. What's astounding is

What's pushing the fall, what's driving the momentum? What's on top, pushing so hard downwards that debris are jetting out of the building like that?

[vincent_vega_lives]

20. Its simple

Mass * acceleration due to gravity = a shit load of force.

[Show_Me _The_Truth]

28. OMG

I just spilled my coffee that was so funny.

Thanks for a laugh based on humor in this forum. I usually just laugh at all the "physics" and "theories" based on "evidence."

[greyl]

32. Was it loaded with 4 shots of Kahlua or Sambuca? nt

[greyl]

37. was mine? I see your point now. ;) nt

[jberryhill]

27. Hundreds of tons of stuff

Namely, the upper portion of the tower - about 20 stories worth.

We can't really see through the dust, but those hundreds of tons of stuff had to go somewhere. The floors were basically thin slabs of concrete which readily shatter and create a lot of concrete dust. Ditto for gypsum wallboard. But what wasn't pulverized on impact remained pretty darn heavy.

[nebula]

9. Nonsense

"Notice how the debris in free fall approaches the ground faster than the collapse front of the tower."

The debris below the collapse front is the debris from the upper floors.

So obviously, debris from the uppers floor would fall faster than the collapse front because it got a head start and began falling BEFORE the collapse front. Duh.

[rman]

10. Ok but how come the debris did get a head start on the collapse front

in the first place?
It means that at least initially the collapse was slower than free fall speed (or more accurately: slower than free fall acceleration).

Not that this lends any credence to the OCT.
The fact that the collapse front moves at a more or less constant speed is easily explained by having a fixed delay between the demo charges on each floor.

[greyl]

12. "Not that this lends any credence to the OCT."

Holy crap man, you're catching on!

[nebula]

13. It's pretty simple.

"how come the debris did get a head start on the collapse front"


As the video shows, the floors are destroyed one at at time, from the top down.

Therefore, the debris from the top floors will reach the ground faster than the debris from the lower floors. Because debris from the upper floors began its fall before the collapse line and before the debris of the lower floors.

[vincent_vega_lives]

19. That simply makes no sense

The debris originates from the collapse point. The debris beats the collapse point as it progresses down the building. The debris is moving at free fall. Therefor the collapse is NOT at free fall.

[freedom fighter jh]

30. Nebula's right, Vince.

As has been pointed out, "free fall" refers to acceleration, not velocity.

What if you dropped a ball from the top of a high tower? By the time the ball got halfway down the tower, it would be moving pretty fast, right? And if you dropped a ball from a window halfway down the tower, at that point that ball would not be moving at all, because gravity would not have had a chance to accelerate it.

Debris from near the top of the tower would be moving much faster than debris halfway down the tower that had only just started moving. Debris falling into the collapse front could have accelerated the lower floors some, but -- even if the structure put up no resistance at all -- could not have accelerated the lower floors to the speed at which the upper floors would have fallen unimpeded.

And of course the buildings would have put up a great deal of resistance, which would have slowed their fall substantially.

[vincent_vega_lives]

34. Wow

Not the top of the tower...the point of collapse...where the debris originates from. Egads.

http://re3.mm-a4.yimg.com/image/3053877084

[petgoat]

38. According to Dr. Judy Wood, the freefall time of a billiard

ball from the top of the tower was 9.2 seconds.

The freefall time of a pancake collapse where each floor's collapse was
initiated by the arrival of the floor above it is 88 to 97 seconds.

Now obviously at some point the accumulated weight of all the falling
debris would so far outstrip the ability of the standing floors to
resist that the structure would not slow down the collapse any more.
(This doesn't explain why the cores came down, though.)

Dr. Wood cites this photo: note the dust emerging from the windows
below the zone of falling debris.



Note also what appears to be squibs below the dust.

[Bolo Boffin]

39. Petgoat: you've almost got it!

at some point the accumulated weight of all the falling
debris would so far outstrip the ability of the standing floors to
resist that the structure would not slow down the collapse any more.

You see, petgoat, that point was reached after the falling debris had fallen the distance of a SINGLE STORY. Once it got going, it was unstoppable by anything the lower floors could resist with.

And the core couldn't stand without the bracing provided by the perimeter columns. They had zero lateral force resistance, or as close to zero as possible. No perimeter columns, no core.

No building.

You've almost got it!

[petgoat]

40. The core had zero lateral force resistance?

Baloney.

The floors internal to the core would have provided lateral resistance
even if they weren't designed to provide it.

This looks pretty well braced laterally to me:

[Bolo Boffin]

41. Shows what you know.

The core was designed strictly for bearing load.

The perimeter was designed to handle some load and all lateral forces.

That's why they could only build the core up about that far from the rest of the structure. Any further and the core would be tilting too far out of spec for construction purposes. Get much too far up and they'd watch the core tumble down.

Do your homework. Your gut isn't educated enough to make those kinds of assessments.

[petgoat]

42. The core was designed strictly for bearing load.

I didn't say it wasn't.

I said the presence of the core flooring would have braced
the core laterally whether it was designed to do so or not.
Also note that in such a tall building any springiness
whatsoever in the flooring would have been unacceptable,
so the floors would need to be heavily overbuilt.

I also said that in my picture the presence of lateral bracing
elements is quite conspicuous.

[Bolo Boffin]

43. And as I said

the core built up any further than you see in pictures would have thrown off the building.

The core floor may have given some lateral support to the core itself - but nothing that could have supported it standing very high.

Therefore, as the upper falling mass tore the perimeter and outer floor away, the core had nothing left to support it and would have had to collapse eventually - even without the upper section still falling into it.

You do almost have it. I'm surprised to see you so close to the tipping point.

It appears that you are surprised as well.

[petgoat]

49. "the core built up any further than you see in pictures would have thrown off the building."

That's absurd. The core was 80 feet by 140 feet. So you're saying that an 80 foot by 140
foot conventionally steel framed structure can not be built more than ten stories high
because it will fall down? And that this one in particular could not be built that high
even though it was built to bear the weight of 90 stories above it?

Where did you learn such nonsense? Or do you assume structural constraints limited the
allowable height of the core above the floors, even though limiting the height would make
sense simply from the standpoint of keeping the cranes close to the floor elements and
perimeter columns they were raising?

I'd maintain that the core could have been self-supporting for forty or fifty floors
at least, and that under natural collapse, the danger that, stripped of the external
floors, the core would topple into One Liberty, WFC, or the Verizon Building was very
great. I'd thus suggest that demolishing the core was necessary even if pancaking
floors could have brought down the non-core elements.

[dailykoff]

51. Yes, absurd is an understatement. (n/t)

[Bolo Boffin]

56. Not without proper bracing.

You say you see enough bracing there. I don't see any diagonal bracing in the core whatsoever.

You have no idea how the WTC towers worked to support themselves.

[dailykoff]

63. Um, what do you think all these diagonal struts are?

C-4 coated rebar?

[Bolo Boffin]

67. Um, the scaffolding holding up the cranes?

Nice try.

[dailykoff]

68. No, and before you mention elevator guide rails,

it's not that either. :)

[Bolo Boffin]

69. Um, yes. eom

[petgoat]

70. Bolo are you blind? I see 7 cells on the long side of the core.

3 of those cells at floor level have big honking diags.
3 of those cells at floor level plus 2 have big honking diags.

And that's not counting the crane towers or the truss connecting the crane
towers--which may be a moment frame.

[Bolo Boffin]

71. That is all temporary bracing.

http://www.youtube.com/watch?v=qJ11i6fi7KQ

Notice the lack of diagonal braces in this computerized rendition of the WTC towers, based on as many architectural drawings as they could find.

Face it - the core was not designed to take any lateral loads. That was the perimeter columns' job. Go back and read the NIST report - I recommend section 5.4.2 of the main report. Nothing about core - the lateral load resistance was all about the perimeter.

[petgoat]

73. Since when do 14" X 36" steel columns only five stories tall,

built of steel plate 4" thick, connected by floor girders
massively over-engineered for the purpose of holding up
the floors require temporary diagonal bracing?

[Bolo Boffin]

75. When they're being built up above their lateral support system

and are being asked to participate in holding up four cranes.

[petgoat]

80. I see no connection between the core columns and the crane towers.

Of six angle braces I see, only one is connected to a corner
column. Three are in interior cells nowhere near the crane
towers.

[dailykoff]

50. That's nonsense. The cores took the wind loads

which were staggering. Conventional wisdom is that the perimeter columns bore the wind loads, and they did -- by transferring them to the cores, which transferred them to the ground.

Claiming that the cores had "zero lateral force resistance" is a good example of what happens when flacks take over for the silent engineers, and they have been very silent.

:rofl:

[Bolo Boffin]

55. The "silent" engineers spoke in the NIST report.

I suggest you read it.

[dailykoff]

61. That's amusing. The handful of professional shills

who put their names on that interminable whitewash at least know enough about structures not to make completely foolish claims like that one.

Edit to add: and if in Appendix 5 of version 4.9 they do make such a claim, that reinforces the point that the entire report is a profile in shameless shilling.

[rman]

22. not if the delay between each subsequent collapsing floor

is in accordance with freefall acceleration - which is a matter of timing the demo charges:
the collapse of second floor to would then be timed to coincide with debris from the first collapsing floor arriving at the floor below it.

Unlike your argument implies ("floors are destroyed one at at time, from the top down") it's not just about sequence, it is about the timing of the sequence.

[jberryhill]

25. But

...if your point is that the tower came down according to some definition of "free fall", then how can you say:

"Therefore, the debris from the top floors will reach the ground faster than the debris from the lower floors. Because debris from the upper floors began its fall before the collapse line and before the debris of the lower floors."

If it comes down at "free fall" then the debris from the upper floors will hit the ground at the same time as the collapse front.

(random aside: I always thought Debris would be a great name for a girl)

[vincent_vega_lives]

35. Debris Spears? (n/t)

[greyl]

11. Since you agree, why are you calling it nonsense?

One thing you forgot to mention is how obvious it is that the upper floor debris is also accelerating at a higher rate* than the collapse front. The span between red lines is way larger in the second photo than in the first. It looks exactly like the collapse began where the fires & damage caused by the plane impacts occurred, not on any of the lower floors.

*colloquially true, not technically

[nebula]

14. As you can see from the 2nd photo

Debris from the right side is falling faster than debris from the left side.


So the seemingly larger gap between the collapse front and the debris in the 2nd photo only applies to debris on the right side. The gap on the left side is approximately the same height in both photos.

[greyl]

15. Yeah, the laws of physics were different on the right side!

Way to go.

[nebula]

17. I don't see what those two pictures prove

What is the point of the photos?

To prove that the buildings didn't collapse at free fall speed?

So what? The OP never said they did. He said they fell at near free-fall, and that they certainly did.

[LARED]

18. On a technical note

exactly how is near free-fall defined?

[vincent_vega_lives]

21. somewhere between

the speed of paint drying and light.

[LARED]

29. I never thought of that

Of course it could be I have a cognitive deficit preventing me from seeing this possibility.

[greyl]

33. If our favorite psycho analyst shows up again, maybe they can clear it all up.nt

[jberryhill]

26. Nope

Look at the right side in the upper photo.

Now look just to the right of WTC 7.

Now look just to the right of WTC 7 in the lower photo. See stuff in the air, down at about the level of the lower red line, that wasn't in the upper photo?

[jberryhill]

24. Follow your own logic

"debris from the uppers floor would fall faster than the collapse front because it got a head start"

A "head start" on what? If the point is that the collapse front is moving at free fall, then how did any debris get a "head start" on the collapse front?

At all points, we see a mushroom of debris ejected along the collapse front. If the collapse front were moving as fast as the falling debris, then the "cap" of that mushroom would not be convex upward, it would be flat, since stuff outside of the tower wouldn't be moving any faster downward than the collapse front.

What you have here is a twenty-story building dropped onto an eighty-story building. Why anyone believes that would not be sufficient to cause collapse is really beyond me.

[Beam Me Up]

84. No.

What is happening is EXPLOSIVE FORCE IS EXCLERATING THE DEBRIS. The debris field isn't merely "falling" it is being EJECTED. That is the point.

[jberryhill]

23. I'm pretty convinced the only physics people took here

...was introductory.

In a vacuum, things will fall according to (1/2)gt^2, and that's where the analysis always quits in these "free fall" discussions.

In air, over any appreciable distance, that is not going to be true.

Things falling in air will reach their own "terminal velocity" over a fairly short distance. Presumably, there are people here who think a parachute is an anti-gravity device.

The comments about the large piece on the right side, and how it seems to fall more slowly, is pretty exemplary of something with a wide cross-section having more wind resistance.

Given a "bunch of stuff" falling, the one with the highest terminal velocity wins.

http://hypertextbook.com/facts/JianHuang.shtml

[greyl]

31. Do you think terminal velocity was reached by the initial debris in the 1st pic?

Or between 1st and 2nd?

[jberryhill]

36. depends on which

piece of debris you are talking about:

http://www.geocities.com/operation_rising_star/mis.tve.htm

As noted there, a human body reaches terminal velocity within 5 seconds of falling.

[AZCat]

85. But even that's a generalization.

Terminal velocity is highly dependent on apparent cross section, and a human body has quite a wide range of cross sectional areas. IIRC the maximum recorded speed of a falling human is about twice the speed of a human falling spread-eagled (>300 mph versus ~160 mph). All this, of course, depends on the density of a particular human.

[jberryhill]

87. Good points all..

which further leads to the question of what is meant by "almost free fall speed" relative to the towers.

Skydivers modulate their speed doing various acrobatics all the way down

[freedom fighter jh]

44. Free Fall Times

If you dropped a ball off the top of a tower 1360 feet tall (isn't that about how tall they were?) and if the ball encountered no resistance, even from air, it would hit the ground in t = square root of (2d/g) seconds, where d is distance, here 1360 feet, and g is the acceleration of gravity, 32 ft/second squared. That comes to 9.2 seconds.

One of the towers did indeed start to collapse from the top down. So free fall time for it would have been 9.2 seconds.

The other tower collapsed, I believe, from the 85th floor down, or something close to that. Assuming all the floors are about equal in height, that tower collapsed from about 1051 feet. Substitute 1051 for d and you get 8.1 seconds.

Of course, the real collapse was more complex than the free fall of a ball; the towers could not collapse straight down unimpeded, because the lower floors were in the way.

Underlying the issue of the speed of the collapse is the question of energy. If the towers had collapsed at free fall speed, then all of their gravitational energy would have gone into accelerating them downward. That raises the question of the source of the rest of the energy necessary to the fall. The concrete was pulverized, the columns were shattered and ejected with such force that some of them lodged themselves into buildings across the street, and there must have been drag (air resistance) to overcome. What was the source of the energy that did that?

It's not entirely clear exactly how much time it took the towers to collapse. The highest estimate I've seen is 16 seconds, about twice free fall time. If they fell in twice free fall time, then their average falling velocity was half free fall velocity. Since energy is equal to 1/2 m v squared, that would mean that only 1/4 of the towers' gravitational energy went into accelertaing the towers; the rest went elsewhere in the collapse.

Some energy quantities are difficult or impossible to estimate, at least given the limited information that I have seen. How much energy went into shooting the pieces of columns into the air and across the street? That would depend on the masses of the columns pieces and the speed at which they were ejected. I can't even guess. Drag? Who knows? And I don't know enough about how the columns shattered to estimate the energy in that.

Jim Hoffman has estimated the energy needed to pulverize the concrete and expand the dust clounds. See http://911research.wtc7.net/papers/dustvolume/volumev3.html . Hoffman finds (see the table near the bottom of the page) that this energy was at least 20 times the gravitational energy available. If you can't follow all the heavy thermodynamics (I can't), then consider just the energy needed to pulverize the concrete. In the second paragraph in the introduction, Hoffman conservatively estimates this energy at 135,000 KWH, while each tower's gravitational energy is 110,000 KWH. So the energy to pulverize the concrete alone was greater than the tower's gravitational energy. That would mean that the collapse itself did have enough energy to crush the concrete floors. And if all the energy of collapse did to into crushing the concrete floors, then none was left over to shatter the columns, shoot them out at high speeds, and bring the towers down at half the speed of free fall.

Since the towers did come down at at least half the speed of free fall, there had to be another source of energy. Airplane crashes cannot explain the collapse of the twin towers.

[Bolo Boffin]

45. Ok then...

Let's grant your premise (or Hoffman's premise that you reproduce here). The energy needed to pulverize the concrete and expand the dust clouds was 20 times greater than the gravitational energy available in the towers. (I don't believe this in the slightest, but I'm granting the premise for the sake of the argument.)

The gravitational energy, then, is 5% of the energy required. The other 95% of the energy comes from somewhere - it must be explosives.

Calculate this:

How much explosive material is needed to provide the remaining energy? Calculations for TNT, C4, and thermite would be most comprehensive here. Since we are making up a deficit that is 19 times the energy of the falling towers, prepare for your answers to be staggeringly large.

Next, explain how that large quantity of explosive (no matter what your final choice of explosive) was smuggled past William Rodriguez and his fellow workers in the WTC towers.

Next, explain just how all of that large quantity of explosives were hidden in the towers - was there enough space in the towers to actually hold the amount of explosives that Hoffman requires to pulverize the concrete and expand the dust cloud?

[freedom fighter jh]

46. I don't know

Please note that "20 times" was not an essential part of my argument.

And I don't know anything about explosives. But apparently you do, or else you wouldn't characterize the answer as "staggeringly large."

So could you clue me in? How did you come to that? And how do you know you're using the most compact explosives?

[Bolo Boffin]

47. It is where your argument went.

If the towers had collapsed at free fall speed, then all of their gravitational energy would have gone into accelerating them downward.

What would happen to it after it was done accelerating the mass of the towers downward?

19 times the energy of the falling towers contained in explosives is going to be staggeringly large. I believe that's just common sense. Wouldn't you agree?

Feel free to include the most compact explosive you can live with in your calculations.

[freedom fighter jh]

48. The calcs are up to you.

In your first question, do you mean what would happen to the energy? I don't know. I suppose it would dissipate inside the earth.

Yes, 19 times the gravitational energy in the falling towers would be staggeringly large. You would need a hge amount of energy to explain those dust clouds that rolled down the street.

Oh, and it's on you to come up with explosive data, since you're the one making the assertion about it.

[petgoat]

52. I don't understand the argument that a zillion tons of explosives

would be required to produce the dust clouds, and therefore
no explosives produced the dust clouds, but a natural collapse
did.

If the natural collapse could produce the dust clouds, then
an explosive collapse could produce the dust clouds by the same
mechanism without massive quantities of explosives.

[Bolo Boffin]

54. The argument is based on the absurd premises of Hoffman.

As I took pains to point out, I don't believe those premises in the slightest. But you neatly avoided my disclaimers there, and no wonder you're confused.

I think the fall of the towers produced more than enough (well beyond more than enough) energy to account for what I see. It is the idiotic assertion to the contrary that I am showing to be absurd.

[freedom fighter jh]

58. And how you figure the fall of the towers produced enough . . .

. . . energy is?

To my knowledge, no steel-frame skyscraper has ever collapsed like a house of cards except in controlled demolition and on 9/11. Would you know of a case other than on 9/11 when a steel-frame skyscraper collapsed like a house of cards and was not demolished intentionally?

[Bolo Boffin]

59. Do you know of a case...

...where a large passenger jet with fuel for a transcontinental flight flew into a steel-frame skyscraper and the skyscraper remained standing?

I like these figures:

So, here's how I did my calculations: Assuming the top portion is 2/3 of the way up (as westprog did), I get a value of (rounding to two significant figures):

( (500,000,000 kg)/3 )* (9.8 m/s2) * (411 * 0.677 m) =
67,000,000 * 9.8 * 274 = 450,000,000,000 J =
~98,000 kilograms of TNT =
108 tons of TNT

http://www.convert-me.com/en/convert/energy

This is about 74 times what you got, Westprog (6.1 * 74 = ~450), which makes sense, because I am taking all 74 floors from the ground to the impact zone into account (i.e., the lower 2/3rds of the building), whereas you are just calculating for the height of one floor. (Tons of TNT convert also: your value of 1.5 * 74 = 111, about what I get.)

----

Now, to go back to R.Mackey's post, if you take my value of 98,000 kg of TNT in that top portion of the South Tower (this exercise relates much better to the ST where the impact zone was close to a third of the way down; not exactly, but a reasonable estimate for now).

In comparison to my value of 450 x 10<9> J, Mackey calculates 1010 x 10<9> J, which is in the same ballpark. Of course, he's calculating for the whole tower, whereas I'm calculating for the top portion only.

The sources of error in my value are (1) the tower got lighter with height (as Mackey pointed out), whereas I used the simplifying assumption that the tower had equal weight density through its entire height, and (2) I didn't integrate over the entire height of the top portion, but rather took its lowest point as my "h" value. (1) tends to make my value too large, whereas (2) tends to make my value too small, so it's hard to say which way my error will tend overall.

I'm having difficulties with your value for kilograms of TNT, R.Mackey. The energy conversion factor I found (http://www.convert-me.com/en/convert/energy) was 4612 Joules per gram of TNT, unlike your value of 4184. Where did you get your value? As a result, my calculated value of kg of TNT (98,000) for just the top portion of the towers exceeds your value (72,000) for the entire tower.

Then, as you say, we must apply a factor such as 30% to get that energy applied to the instantaneous breakup of the tower as it fell (although, as you point out in Note 3, the tower breaks up upon impact with the ground and itself and the kinetic energy dissipates as that energy of breakup and as heat, so less energy is lost than 70%, no doubt).

Anyway, I come up with a value of 0.30 * 98,000 = ~29,000 kg of TNT, for the value of the potential energy in the top of the South Tower available for "collapse alone," using that 30% analogy.

---

I'd like to go back to my original energy value of 450 x 10<9> J, or 98,000 kg of TNT, as I'm not at all convinced that all that energy isn't used in some way in the collapse (let's hash it out if you like). Anyway, all that energy is available for something, so it's significant, imho.

On another forum, to convey the huge magnitude of this energy, I converted it first to foot-pounds (an old British unit, I believe) and then to a unit I made up (ha), called "ton-miles," because it seems like an intuitive way to look at it:

450 x 10<9> Joules (http://www.convert-me.com/en/convert/energy)
= 331,900,000,000 foot-pounds ( / 2000 pounds/ton)
= 166,000,000 foot-(short) tons (http://www.convert-me.com/en/convert/length)
= ~31,000 mile-tons

To me, this conveys just as "graphically" as kilograms of TNT how much energy is involved in just the top portion of the South tower: Enough energy to push a ton of material for 31,000 miles, i.e., all the way around the equator of the earth, plus some.

To me, this value conveys well the INCREDIBLE amount of potential energy that became available for "work," as the tops of the towers came free from their bearings when their support structures failed.

[freedom fighter jh]

64. Thanks for the calcs

But this doesn't answer the question. If you don't like Hoffman's calcs, can you say what's wrong with them? Can you support the position that gravitational energy was enough to bring the towers down -- and support it with more than just "otherwise you would need more explosives that could be managed"?

[Bolo Boffin]

66. Hoffman's calcs

The main thing that should be tipping you off is the whole "19 times the energy of the falling building" nonsense. Something's got to be wrong with that. Even controlled demolitions use strategic charges to undermine the structure and let the gravitational energy destroy the building. That's how they work -- OF COURSE the gravitational energy is enough to destroy the building! More than enough, well beyond what is needed.

The question is: what causes the collapse to start? What conditions are needed to start a global collapse?

And the answer to that is in the NIST report.

[freedom fighter jh]

72. strategic charges to undermine the structure

That's just it: The structure won't fall unless something undermines it. Why? Because it takes energy to destroy the structure. In a controlled demolition that something is strategic charges. In the case of the towers, it's been argued that that something was the impact of the upper floors on the lower ones.

The trouble was the huge amount of energy that was released in the collapse. All the concrete was pulverized. The columns were shattered and shot out in all directions. And the towers collapsed in seconds. Gravitational energy is not enough to account for these things.

[Bolo Boffin]

74. in key places.

The explosives in a controlled demolition destroy only the ability of the structure to remain standing. The structure then destroys itself under the energy induced by gravity.

In the case of the towers, damage from both the airplane strike and the subsequent fires destroyed the ability of the towers to remain standing. After collapse was initiated, gravity did the rest. Even with the conservative figures I quoted, the equivalent of ~29,000 kg of TNT were available as potential energy in the upper portion of the South Tower. Do you think we could pulverize concrete with that much TNT?

[Bolo Boffin]

53. Me do the calculations for your assumptions??

Get real.

Where else is all of that energy going to come from, then? Space beams from Alpha Centauri? Please produce your best guess as to where the rest of that energy came from, since it so obviously came from somewhere.

[freedom fighter jh]

57. You're the one who said "explosives"

I told you I don't know anything about explosives. And I should not have to know anything about explosives to point out that the OCT does not make sense. If you think that explosvies are the only alternative and that that alternative is not feasible, then you show that.

[Bolo Boffin]

60. OK, then: your favorite theory has an energy deficit.

Explain it. If you don't like explosives, then where did your extra energy come from?

Space beams from Alpha Centauri?

[freedom fighter jh]

62. I didn't say I don't like explosives.

I said I don't know where the extra energy came from. It *could* have been explosives. But really I don't know. What I do know is that there is, as you say, an energy deficit. It was something other than airplanes that collapsed those buildings.

[Bolo Boffin]

65. There is, as I say, an energy deficit IN YOUR ASSUMPTIONS.

There is no actual one.

Come on, guy, you're looking for an energy source that can deliver 19 times the energy of the buildings falling down. Your assumptions demand it! Let's do a little speculating. What could it be?

[vincent_vega_lives]

82. Why do you assume that?

Energy is energy. A multi ton aircraft at velocity carries A LOT of energy, as does a major fuel initiated office fire. Over time the two dumped an enormous amount of energy into the structures.

Explosives use instantaneous and focused energy to compromise structural integrity in ECD, but the result can be much the same.

[eyl]

76. You realize that those figures

require between an explosion (or explosions) of 2-10 kilotons?

(anyway, who the hell measures gravitational energy in KWH?)

[Bolo Boffin]

77. KWH - is that Kilowatts per Hour?

That surprised me too. Maybe it means something else in Hoffman's brain.

[eyl]

78. That's the only

unit of energy I know of to which the acronym fits - but I've never encountered a scientist or engineer using it (other than in the context of power grids and such)

[AZCat]

86. I've used it before in building energy analyses.

Since electric rates are usually $/KWh, I have to convert whatever unit of energy I'm using at some point anyway in order to figure out yearly electrical costs.

[freedom fighter jh]

79. KWH is kilowatt hours

And that measures energy.

Power is energy expended per unit time. Power can be measured in watts. A kilowatt his one thousand watts. A kilowatt hour is the number of kilowatts expended in an hour. Units of energy per unit time, times time, gives you energy. It's kind of a convoluted way of thinking of energy, but it works, and I used it because I was quoting someone who used it.

[eyl]

81. I'm aware of that

It's your source's use of the term that I find puzzling - as I noted above, I've never encountered any scientific or engineering work using KWH for energy.

[vincent_vega_lives]

83. Yes usually it is using Joules or MJ (n/t)