It's just what falls out the end of the data really. Applying the scaling metrics used by NIST results in acceleration way over freefall. Refining those metrics and including modification for perspective brings it down a bit, but still above-G. Still looking for additional factors to reduce it further. Doubt any factors will bring it to or below G though.
The physics toolkit
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It's just what falls out the end of the data really. Applying the scaling metrics used by NIST results in acceleration way over freefall. Refining those metrics and including modification for perspective brings it down a bit, but still above-G. Still looking for additional factors to reduce it further. Doubt any factors will bring it to or below G though.
Evidence for a collapse led by the core with the corners being pulled down after a delay is shown in the linked gif:
http://img687.imageshack.us/img687/7150/bowingnorthface2.gif
An obvious global deformation of the perimeter that looks nothing like the NIST simulations shown here:
http://img534.imageshack.us/img534/3061/nistani5.gif.
The corner delay shows how an additional downward force could accelerate the corner faster than gravity.
Possibly.
http://img687.imageshack.us/img687/7150/bowingnorthface2.gif
An obvious global deformation of the perimeter that looks nothing like the NIST simulations shown here:
http://img534.imageshack.us/img534/3061/nistani5.gif.
The corner delay shows how an additional downward force could accelerate the corner faster than gravity.
Possibly.
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W.D.Clinger
Philosopher
a homely analogy for the lurkers
Consider a parachutist who's about to jump from an airplane. Imagine that he holds his arm out at waist level to hold on to the door frame before he jumps. Imagine that, as he jumps, he continues to hold onto the door frame until he has fallen so far that his hand is above his head; only then does he let go of the door frame.
If you run the numbers, and look only at the motion of the parachutist's hand, you'll find that the parachutist's hand accelerates faster than free-fall at the beginning of its descent. On the other hand, the parachutist's center of gravity never accelerates faster than free-fall.
That appears to be what's going on with the northwest corner of WTC7. The WTC7 roof as a whole corresponds to the parachutist as a whole, and the northwest corner to his hand.
The following analogy might assist some lurkers who have been wondering what this is all about.
Consider a parachutist who's about to jump from an airplane. Imagine that he holds his arm out at waist level to hold on to the door frame before he jumps. Imagine that, as he jumps, he continues to hold onto the door frame until he has fallen so far that his hand is above his head; only then does he let go of the door frame.
If you run the numbers, and look only at the motion of the parachutist's hand, you'll find that the parachutist's hand accelerates faster than free-fall at the beginning of its descent. On the other hand, the parachutist's center of gravity never accelerates faster than free-fall.
That appears to be what's going on with the northwest corner of WTC7. The WTC7 roof as a whole corresponds to the parachutist as a whole, and the northwest corner to his hand.
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Have added some column headings...
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Not perfect, but better. Still have to apply pixel to real world units translation. It's just the raw pixel data at the mo.
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Seymour Butz
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Can that even be possible with steel?
I could understand that in the parachutist, the somewhat flexible tendons and ligaments can stretch and come under tension, and then snap back. Or muscles can come under tension when the arm become pulled on, and then accelerate the hand towards the body when the hand rips loose. This would be the aforementioned mechanical storage device.
Can there be any circumstance where steel could do that?
The most likely candidate is observational error, agreed?
Absolutely, steel can do that.
Given the right dimension, it can do it exceedingly well. It's one reason that they make springs out of steel.
It's a matter of the "compliance" of the structure as a whole.
Buildings, as built, have a small amount of compliance to reduce wind & earthquake stress peaks. Once the building is 75% destroyed (the core has collapsed), then there is guaranteed to be a boatload more compliance in the structure than was in the original design.
So this is another mechanism that seems extremely likely: stored energy in deformation of the structure.
For a simple version: Imagine a thin steel ruler, about 3 feet long. It is retained on each end by a pin, and it has some load on it that causes it to deform a bit in the middle.
Now you press downwards slightly in the middle. The ruler flexes more, slips off of the pins. During the deformation time, the ends stay still. (The ends might even rise if the pins are in-board from the ends.)
When the ruler does slip off of the pins, the ends whip back to a neutral position from their flexed position. During this time, the acceleration of the ends will be greater than g, even if the center of gravity of the ruler itself is falling at an acceleration equal to g.
A ruler has little dampening, so you'd expect to see several oscillations about an average acceleration equal to g. With a period characteristic of the natural frequency of the ruler for that oscillation mode. But if the oscillation frequency were low, or there were lots of other dynamics going on, you'd likely lose that in the noise.
There is a big lesson here:
There are LOTS of possible, real world data collection & interpretation complications that would allow the external wall to APPEAR to be descending faster than "g" for short periods of time.
There are also LOTS of possible, real world structural complications that would allow a portion of the external wall to REALLY be descending faster than "g" for short periods of time.
[ETA: And by extension, it is also possible for the entire wall to accelerate at levels >g. If that wall is just one portion of a larger structure falling in just the right way.]
The physics says that it is an isolated, free falling body that falls at "g".
In physics, it is often the custom to "Imagine a cow to be a frictionless, perfect sphere..."
In this particular case, is it a fool's errand to imagine that the north outer wall of WTC7 is isolated from the structure behind it.
Tom
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Seymour,
If you look at the gif that Major Tom linked:
http://img687.imageshack.us/img687/7150/bowingnorthface2.gif
... you can see how much flex there was to the north wall.
Tom
If you look at the gif that Major Tom linked:
http://img687.imageshack.us/img687/7150/bowingnorthface2.gif
... you can see how much flex there was to the north wall.
Tom
Are you telling me that NIST's simulation is unable to correspond, feature by feature, with a process of chaotic nature such as the collapse seen in the the video?
By the way, in your first GIF it looks to me that the NE corner actually twists towards the camera. Pretty clearly, the distance between the black rectangle's left edge and the NE edge of the building grows significantly, which, assuming it's rigid enough as to not be an
That's an added confusion factor, as in that case, since the corner goes in the camera's direction, it "deceives" the camera about its actual height, making it seem higher than it actually is. Maybe that could explain in part why that corner seems to fall more slowly than the NW corner?
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Seymour Butz
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Yup, pretty much what the NIST engineers said:
East [ETA:
Can see the sky thru the windows in the upper northeast corner. Clearly the roofs gone in that area.
Then the core collapses, bowing the north wall.
Interesting that the southern end of the west wall also seems to move with the core, but not the north end.
Then the north wall begins to fall ... starting from a flexed state.
Well, Tom, I know you'll be shocked, shocked I say, that I disagree with you.
The NIST sim shows the whole tower. The video shows only the top 1/3rd of it (about 16 out of 47 stories). If you cover up the bottom 2/3rd of the NIST sim, and compare only the top 1/3rd, the sim doesn't look too bad.
Roof gone. Collapse of the West Penthouse. Inward bowing of the central section of the north wall. The northeast & northwest corners maintaining their elevation. Looks like there might even be some action happening on the west wall.
Of course, if one expects that sim to predict the exact sequence down to individual elements, one is certain to be disappointed. And possibly get all atwitter.
Fortunately, the NIST engineers were pros, no doubt with a healthy dose of skepticism about the ability of that model to predict fine detail. Probably used it for the gross effects, and took that "individual columns" stuff with a giant grain of salt.
I'll let you elaborate, rather than run the risk of misinterpreting what you're saying here.
Tom
PS. A question about that video clip.
Is the dark black vertical line on the north west vertical corner in the original video? Or has someone added that?
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W.D.Clinger
Philosopher
In addition to the flexing mechanism that tfk explained so well, there's the simpler (but evidently less intuitive) mechanism I first proposed.
Applied to the parachutist, imagine that his grip on the door frame is extremely relaxed, that he's just barely touching it, but remains in contact with it until his arm is fully extended above his head. At that moment, the momentum acquired by the parachutist's body creates a very large accelerating force that is applied to his hand via tension of the bones and ligaments that attach the hand to the body. As you say, some stretching and snapping back will occur within a human body, which smears the large, above-free-fall acceleration of the hand over time.
Note, however, that you don't need the "snapping-back" phase to get an acceleration greater than free-fall.
If the arm had infinite yield strength and did not stretch at all, the hand would be accelerated instantaneously to match (very nearly) the velocity at which the rest of the parachutist's body was already falling. That implies infinite instantaneous acceleration, which isn't physically possible; everything stretches a little bit, which smears the otherwise infinite instantaneous acceleration over time.
Suppose, however, that there is no snapping back, for whatever reason; you might imagine that the arm (or steel) had exceeded its yield strength and was just barely able to hold itself together, without returning to its previous dimensions. You would still see the greater-than-free-fall acceleration of the hand during the stretching phase, as its velocity rose rapidly from zero to match the velocity of the already-falling body.
In my opinion, the snapping-back phases of the mechanisms proposed by both tfk and femr2 are entirely plausible. I'm just pointing out that a small portion of the structure can accelerate faster than free fall even without release of stored mechanical energy.
Several of us thought so at first, which is one of the reasons femr2 has taken so much flack, but his (and NIST's) data have held up well enough under attack to convince some of us that the explanation is physical. As tfk said, there are many possible mechanisms once we abandon our overly simplistic spherical cowWP approximations.
Something that should not be overlooked is the similar over-G results of tracing the alternate view below, which does not suffer from perspective as much as the Camera #3 viewpoint...
A potential source of error is the NIST supplied building measurement data, which is used as part of the translation from pixel data to real-world units (feet-n-inches).
The NIST values used are...
NIST said:
...and...NIST said:
NIST said:
NISTs measurement from the top of the windows of floor 29 is used as the base metric, with multiples of 12 ft 9 in subtracted or added as required, depending upon the view and the horizontal feature.
If anyone has additional building measurements data in real world units, it will be very handy.
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Whilst I'm there...
Bearing in mind the pre-*release* flexing, this strikes me as a very poor method to determine t0.
As flexing logically reduces in magnitude the lower in the building you go, I may consider tracing some features as far below the roofline as possible, to determine a t0 with minimal flexing to mess-up the vertical motion data.
NIST said:
Bearing in mind the pre-*release* flexing, this strikes me as a very poor method to determine t0.
As flexing logically reduces in magnitude the lower in the building you go, I may consider tracing some features as far below the roofline as possible, to determine a t0 with minimal flexing to mess-up the vertical motion data.
To you perhaps, but does it mean they could not get close enough, for their needs? If so can you please, show evidence of that?
Bound to be some disagreement, but...
Tracing the following region...
...results in the following...
I've placed a suggested t=0 at the last point of inflexion.
The vertical position returns to *zero*, suggesting the prior motion is not vertical descent, but the side-effect of *flexing*.
Tracing the following region...
...results in the following...
I've placed a suggested t=0 at the last point of inflexion.
The vertical position returns to *zero*, suggesting the prior motion is not vertical descent, but the side-effect of *flexing*.
My previous post should suffice.
A couple-o notes:
a) As the trace data shows the low magnitude flexing behaviour, it should be obvious that pixels on the (poorly defined) roofline will vary in HSL throughout the entire trace period.
b) I'll replicate the process using different trace points, and should end up with a very similar value.
c) I result in a t=0 time before that suggested by NIST.
d) Different horizontal positions will each have a slightly differing t=0. I suggest the NW corner t=0 is ~1.5s later.
Point (a) invalidates the NIST process.
I've yet to extract static point trace data, but it should not affect the end result.
Maybe I'm just being dense, but how exactly does this negate the conclusions NIST has arrived at?
Here is their method described...
NIST said:
a) There is no pixel near the center of the roofline adjacent to sky until after at least the East penthouse has descended...
b) Are they using a point on the edge of the west penthouse roof ? If so, it descends with a different t=0 to the roofline. Are they using a portion of the roofline after the East penthouse has descended ? If so, the *white* they mention is a halo effect from their contrast modifications.
c) Their method does not quantify movement, simply brightness change, so there is no way to differentiate between pre-release flexing and vertical drop, or indeed any other cause of pixel data change.
It kinda works, but is a poor method imo.