The Collapse of World Trade Center: The Complete Physics

[Dan Burt]

Stumbled across this video this morning. Was uploaded 4 days ago. It helped me understand better why the towers collapsed in the way they did.



Source: https://www.youtube.com/watch?v=1NkBfLBov5Q

 

[Dan Burt]

Basically shows what Mick has shown over the years in his garage. But in a nice 15 minute video.

 

[Mendel]

The animation at 10:14 seems counterfactual to me, showing the crushing at the base of the tower and not near the top where it buckled.

 

[Matt33]

The animation at 10:14 seems counterfactual to me, showing the crushing at the base of the tower and not near the top where it buckled.

That is being criticized in the comments under the video and the creator admits that the animation is wrong.

Yah, sorry for the animation mistake. I was a little careless while guiding the animator during this scene. However, we showed the collapse right in the simplified collapse (inside white rectangle). We will correct this mistake in the dubbed videos.

Source: https://www.youtube.com/watch?v=1NkBfLBov5Q&lc=UgxTaD6-sPnoM-EptOp4AaABAg.9vOw879fYz49vWJr2L-4yK&ab_channel=Lesics

 

[econ41]

The animation at 10:14 seems counterfactual to me, showing the crushing at the base of the tower and not near the top where it buckled.

You are correct. The collapse actually progressed top down inside the "tube" of the lower tower There was none of the gross crushing at the base as the lower tower dropped bodily as shown by the video.

However, overall the explanation is as good as I have seen - let's say 95% technically accurate and a comprehensive coverage.

The gross feature mentioned above the only serious error.

The vulnerability of the joist <> column connectors was a key feature driving the collapse. AFAIK first identified and shown in this crude 2007 graphic. Physically modelled by Mick in more recent years as seen in other threads.



... we could have used this new video back then - 6 years of debate had been led astray over that simple issue.

Just a couple of minor misleading points. The video on, I think, 6 occasions refers to jet fuel causing the heating - most of the energy came from office fit out and content material. Jet fuel primarily acting as accelerant.

The explanation does not distinguish the stages of collapse and therefore is not clear as to the fundamental difference of the initiation mechanism from the progression which is explained.

Also there are in inferences that all or most of the steel was heated to softening. That was not so.

But minor points - overall a video which we could have used with advantage if we had it in 2007 -08 -09 onwards.

 

[Oystein]

If nit-picks are allowed:

1. The section on the "kangaroo cranes", while well-done and educational, is a distraction, as it does not illuminate any aspect of the collapses.
2. While he points out and stresses the importance of spray-on fireproofing, which helped the towers to survive as long as they did, he could have mentioned that some of that fireproofing was compromised, if not removed, by the plane crashes - the towers could have survived longer had the fireproofing stayed on better.
3. At 10:30 minutes, e claims that the plane-crash szenario was "tested before the towers were constructed? In a plane crash test, this tower design stood well enough..." <- Not true! The engineers merely did a couple of back-of the envelop calculations, no test, as the illustration with a tower model and plane model suggests.
4. Immediately following this, video claims that the "size of the plane" assumed for the alleged "crash test" (actually: calculation), a Boeing 707, "...was half the size of the one that actually hit" <- Not true! The two calculations that the WTC engineers did consider a) only the wingspan of the plane and b) only its lateral force on the tower, which is proportional to its momentum. As for a), the wingspans of a 707 were between 130 and 145 (depending on variant), and 156 ft for both the 767s that actually crashed. That's at most 20% than what the 1960s educated guesses assumed. As for b), the empty weight of the crasing 767s was near 265,000 lbs, while the empty weight plus full fuel capacity of the smallest 707 variant would have been about 190,000 lbs - not quite 30% less; but this difference is mitigated by the engineers assuming a speed of 600 mph rather than "just" 450 mph, making the momentum of both the actual 767s and the assumed 707 roughly equal.
5. The illustration at 13:15 min of the 767 crash (next to the B25 that struck the ESB) suggests that the wingspan was almost the entire width of the tower - it was "only" about 3/4th thereof

And to repeat what has been pointed out before:

• The animated illustrations are merely that - illustrations, not accurate physics simulations of the actual structures; particularly the "choice" to have the tower fail at the bottom is lamentable.
• Stressing heat from jet fuel while ignoring that a lot more heat was released inside by burning office contents is lamentable

Also:

• The video goes to some length to claim that the North Tower stood longer because there was less mass above the fire zone / crash area. Is this true? I have always believed that the more significant differenc was that the North Tower was hit in the center of a face, such that load distribution paths were available on both sides of the gash; whereas the South Tower had a corner clipped. The difference in total load would mostly be mitigated by the thicker steel plates lower in the structure.

 

[econ41]

@Oystein - I agree with your comments which are more comprehensive than mine.

I have a slightly different perspective on your final comment:

Also:

• The video goes to some length to claim that the North Tower stood longer because there was less mass above the fire zone / crash area. Is this true? I have always believed that the more significant differenc was that the North Tower was hit in the center of a face, such that load distribution paths were available on both sides of the gash; whereas the South Tower had a corner clipped. The difference in total load would mostly be mitigated by the thicker steel plates lower in the structure.

... I'm unsure either way - I cannot guess whether greater load or off-centre impact would dominate the cause of the faster WTC2 collapse initiation.

 

[Mendel]

... I'm unsure either way - I cannot guess whether greater load or off-centre impact would dominate the cause of the faster WTC2 collapse initiation.

Thankfully we don't need to decide.

Generally, I feel that people who'd be able to raise @Oystein 's nit picks are not the target audience for the video. The more interesting question is, does either of you feel that something important has been left out?

I think the idea that "multiple large fires raging" was an important phase that caused the subsequent collapse should've been featured more.

 

[Scaramanga]

The vulnerability of the joist <> column connectors was a key feature driving the collapse. AFAIK first identified and shown in this crude 2007 graphic. Physically modelled by Mick in more recent years as seen in other threads.

It's a key point the conspiracists forget. The towers had massive vertical load strength, but far less strong horizontal connector strength, as the horizontal connectors were not holding the entire weight above but largely holding the vertical beams in place. The loss of the horizontal connectors allows the vertical beams to bend inwards or outwards....and the towers collapse. Essentially those horizontal struts are the Achilles heel of the entire building. Lose a floor's worth of those and all that vertical load strength is negated as the slightest force can then skew or dislodge the beams.

 

[Mendel]

It's a key point the conspiracists forget. The towers had massive vertical load strength, but far less strong horizontal connector strength, as the horizontal connectors were not holding the entire weight above but largely holding the vertical beams in place. The loss of the horizontal connectors allows the vertical beams to bend inwards or outwards....and the towers collapse. Essentially those horizontal struts are the Achilles heel of the entire building. Lose a floor's worth of those and all that vertical load strength is negated as the slightest force can then skew or dislodge the beams.

I don't think I support that conclusion.

NCSTAR 1-6:



A few pages later, the report states that WTC2 lost "1/3 of the connections to the east exterior wall on floor 83".

But the main mechanism inducing instability is the column buckling induced by the sagging floors pulling columns inward through the intact connections. The collapse didn't start because a floor detached; the collapse started because, like the coke can in the video, the exterior wall buckled and collapsed under the load, which set the entire upper section of the building at a tilt and started it moving downward as the adjacent walls rip apart, too.

It is unclear to me how that collapse initiation would have differed if the floor connections had all remained intact.

 

[econ41]

Thankfully we don't need to decide.

True.

Generally, I feel that people who'd be able to raise @Oystein 's nit picks are not the target audience for the video.

It's 'too little - too late' for a 2023 layperson audience but may still be of interest to the few 'newbies' who still arrive on the scene. It may have been useful back in those earlier days.

The more interesting question is, does either of you feel that something important has been left out?

It is out of balance for a serious, new, layperson audience. My own preferences should be well known to longer-term members of Metabunk. I would frame the explanation explicitly in the stages. The two main ones - 'initiation' and 'progression' for overview explanations. The full four stages are essential for anyone who wants detailed understanding OR in discussions with serious truthers who need rebuttal of the nitpicking false anomalous claims they may have been spoon-fed by truther sources.

I think the idea that "multiple large fires raging" was an important phase that caused the subsequent collapse should've been featured more.

It has to be the main premise. My own iconoclastic doubts about whether aircraft initial impact damage was necessary can be set aside. (I also doubt the sprinkler failure was an essential factor. Few would agree with me on either point. ) But the intensity and scope of fires was the key feature. And those fires could not have occurred without aircraft impact. So my doubts are moot.
I've only made brief comments so far - a full critique would serve little purpose - I, or 'we', can address any detailed queries if newer members raise them.

I don't think there is anything new and of significant interest the video that was not known 2013 or earlier. Yes, there are a few blanks and several concerns about the 'balance' of the importance of several factors.

 

[econ41]

It's a key point the conspiracists forget.

Not just the conspiracists. In the early years of CT debate - whilst the two sides polarisation was developing and even thru to about 2009 both sides were not clear on the two related points. Vix: (1) the crucial factor in 'speed' of progressive collapse was that the falling material missed the columns - fell on the floor spaces. HENCE (2) the related criticality of the joist to column connectors.

However the same failure also occurred in the core areas where the beam-to-column connections were more robust. The available energy was simply overwhelming. The 'debunker' side debate from 2001 thru 2009-10>> was dominated by assumptions that the columns were crushed or buckled. Led astray by misreading of Bazant & Zhou's 2001-2 paper, and confirmed when Bazant and Verdure in 2007 proposed 'crush down/crush up' which an astonishing number of people still accept.

The towers had massive vertical load strength, but far less strong horizontal connector strength, as the horizontal connectors were not holding the entire weight above but largely holding the vertical beams in place. The loss of the horizontal connectors allows the vertical beams to bend inwards or outwards....and the towers collapse. Essentially those horizontal struts are the Achilles heel of the entire building. Lose a floor's worth of those and all that vertical load strength is negated as the slightest force can then skew or dislodge the beams.

There is a far more complex issue implicit in the 'Achilles Heel' situation. It was only an 'Achilles Heel' under a situation of gross trauma - well in excess of reasonable design parameters. The overriding aspect of resistance to disproportionate progressive collapses is still under debate at the leading edge of professional practice. (I think there are several threads elsewhere on Metabunk discussing these issues of design philosophy.)

 

[Oystein]

Generally, I feel that people who'd be able to raise @Oystein 's nit picks are not the target audience for the video.

May be true.
I was picturing a situation where there is bunk present in need of debunking, or an audience already exposed to bunk. Such audiences may well be influenced by "bunkers" (truthers), and in time I guess some Truthers will publish a nit-pick to the video and make it available to that new audience.
In such a situation, we, debunkers, do well to already know the nit-picks - and that they are minor, do not invalidate the major points of the video.

The more interesting question is, does either of you feel that something important has been left out?

As @econ41 pointed out, the fact that collapse progression featured falling debris missing the (strong) columns and instead loading the (vertically much weaker) trusses and their seats, this could have been fleshed out more.

 

[Henkka]

I've never heard anyone suggest the towers should have tilted over at the base. That's pretty absurd. The question is why the top floors did not stop or even decelerate a little when impacting the undamaged steel below. And even that question only becomes relevant if we accept that there is nothing dubious about the initiation of the collapse, that it is sufficiently explained by sagging trusses or whatever. This question is only addressed by a few sentences in the video:

The sudden impact force of this huge weight over the floor right below it trashed the lower floors. The perimeter tubes also almost immediately buckled since the impact force was huge and compressive in nature. This caused an unfortunate chain reaction and one by one, the floors kept crashing down.

Meanwhile in reality, we know that there was no huge "impact force" from the fact that measurements of the roofline show no deceleration. If you want to say the floors below were smashed to bits by an impact, you must show what that impact slowed down to get the required force. Just like in a car crash, the cars slow down on impact when the deformation occurs. You're not going to see a car smoothly go through another car.

Source for the claim that the roofline did not decelerate:
Source: https://www.youtube.com/watch?v=ZjSd9wB55zk


You're going to see some things in this video that are uncommon in mainstream or debunking videos:

1) Real footage of the event
2) Measurements based on the real footage
3) Actual physics principles applied to the measurements

The videos like the one OP posted are always the same. There are several in this genre on Youtube. They're always these hand-animated digital cartoons, with zero actual physics applied to the problem.

 

[Oystein]

I've never heard anyone suggest the towers should have tilted over at the base. That's pretty absurd.

But that happens to be exactly the scenario the structural engieers who designed the WTC structures had in mind when they considered whether or not the towers would withstand a high-speed crash of a 707. That's the scenario 4. b) in my post #6 above, i.e. estimating the the total lateral force.
You see, they stopped at estimating the lateral force the plane's crash (its deceleration by structural resistance) would exert on the structure as a whole - and they compared that to the wind load (also a lateral force on the entire tower) of the maximum designed-for hurricane-force wind. Because that max. wind load is something they had already calculated, and had already deterined to not push the tower over. From those calculations (and probably tests also - if I remember correctly, they did some wind tunnel tests with tower models, although I am not 100% what they tested for) they already knew where the critical load path was going, what part of the structure would be the first to be overloaded: the leeward side of the tower, only a little bit above ground level.

In fact, I'd bet that these engineers, as they were world-class structural engineers in their time and eminently experienced with the peculiarities of highrise designs, already KNEW before doing work on the WTC that wind loads will stress towers the most near teir base levels.

And so, I give no weight watsoever to your personal and uneducated disbelief.

The question is why the top floors did not stop or even decelerate a little when impacting the undamaged steel below. ... This question is only addressed by a few sentences in the video:
...

Yeah, you have a point there: This could have been explained better, or at all.

 

[Mendel]

Just like in a car crash, the cars slow down on impact when the deformation occurs. You're not going to see a car smoothly go through another car.

Have you ever seen a train hit a car on a railroad crossing? The train is "going smoothly" nonetheless.

 

[Mendel]

Meanwhile in reality, we know that there was no huge "impact force" from the fact that measurements of the roofline show no deceleration.

The greater the "impact force", the less deceleration.
High school physics.

 

[FatPhil]

The greater the "impact force", the less deceleration.
High school physics.

I don't think either of your statements are correct as stands. However, that depends on what "impact force" is supposed to mean. It sounds like it might be some quantity derived from force, and all that I can imagine is the summing of force over time - namely the impulse, or net change in momentum. But that definitely doesn't gel with it being higher implying less acceleration. Someone running and skidding across a frozen pond would undergo the same impulse as they slowed down from running speed to stationary no matter if they did it slowly as a slide, or if they did it instantaniously upon hitting a "no skating" sign. Same change of momentum, different accelerations.

 

[econ41]

I don't think either of your statements are correct as stands.

They are both down the same false trail as has confused debate throughout the history of these discussions.

There is an implied assumption that the "Top Block" DROPPED then IMPACTED. This leads to the assumption that a noticeable JOLT would result.

The error has a long history of both sides of the polarised debate being wrong.

In brief, it started from a paper by Bazant & Zhou released 9/13 2001 > Yes. TWO days after the event. It was a limit case assessment that assumed the Top Block dropped to impact. Neither of those assumptions, 'droppped' or 'impact', was correct but they legitimately defined a LIMIT case, Not what actually happened.

But for the next 8 years, confusion reigned because too many - both truther and debunker side - assumed it was the actual mechanism. Tony Szamboti - leading truther side engineer - wrote a paper about "the Missing Jolt" which dominated the debate for years. He made the same two errors.

Bazant himself fell for the same mistake when in 2007 in association with Verdure he lent his name to a further paper that introduced the "Crush Down/Crush Up" hypothesis which is wrong. It has four fatal false assumptions, three of them independently fatal. But, given Bazant's status as the omniscient head of engineering debunking, his wrong explanations are still accepted as gospel by far too many debunkers.

So that is the history.
And the David Chandler graph makes the same errors. Both Chandler and Szamboti, AFAIK, persist to this day repeating the same false claims based on those two false premises.

Let me state it bluntly:
The Top Block did NOT drop to impact the lower tower. There was no massive, single 'jolt' deceleration to look for.

The actual mechanism was a sequenced process of failures somewhat analogous to toppling a row of dominoes. One failure leads to the next. Then the next. Then the next. EXCEPT each domino topple is a single binary event. Triggered and it topples. The column failures were at least three grades more complex overlays of analogue events.

Leave it there - we can explore it in more detail if anyone needs to understand it more fully. There is no point in even attempting an explanation before relevant members accept the starting pair of premises - viz The Top Block did not "Drop" to "Impact". Therefore the large-scale gross and obvious "Jolt" was not possible. The claim and its debate are down a false trail.

I've many times explained that the process of the initiation stage was driven by a cascading sequence of column failures.

Explaining what actually happened goes IMO too far off the topic of this thread which is a critique of a new video.

Meanwhile @FatPhil your comments head in the right direction:

However, that depends on what "impact force" is supposed to mean. It sounds like it might be some quantity derived from force, and all that I can imagine is the summing of force over time - namely the impulse, or net change in momentum. But that definitely doesn't gel with it being higher implying less acceleration. Someone running and skidding across a frozen pond would undergo the same impulse as they slowed down from running speed to stationary no matter if they did it slowly as a slide, or if they did it instantaniously upon hitting a "no skating" sign. Same change of momentum, different accelerations.

Reality was a distributed in time sequence of small impulses. NOT one ruddy big and obvious jolt.

 

[Mendel]

I don't think either of your statements are correct as stands. However, that depends on what "impact force" is supposed to mean. It sounds like it might be some quantity derived from force, and all that I can imagine is the summing of force over time - namely the impulse, or net change in momentum. But that definitely doesn't gel with it being higher implying less acceleration. Someone running and skidding across a frozen pond would undergo the same impulse as they slowed down from running speed to stationary no matter if they did it slowly as a slide, or if they did it instantaniously upon hitting a "no skating" sign. Same change of momentum, different accelerations.

F=m×a

a=gravity
m=a lot

A large mass hitting a stationary small mass undergoes a change of momentum, but its change in velocity is smaller if the ratio if masses is higher. Henkka imagines a car hitting another car (~ equal mass), resulting in a large change of velocity, but I imagine a locomotive hitting a car (big difference), resulting in very little change of velocity. In your analogy, it matters if a skater hits that sign, or a truck does.

And that's even without considering that the perceived motion of the "roofline" depends on perspective (the tilt direction matters!), and that the motion of the building shell may not represent the motion of its interior (the penthouse dropped first on WTC7).

 

[econ41]

F=m×a

a=gravity
m=a lot

A large mass hitting a stationary small mass undergoes a change of momentum, but its change in velocity is smaller if the ratio if masses is higher.

That much is correct if we treat the event as the impact of two discrete masses allowing for the differences in mass/weight.

That is NOT the situation which happened at WTC Twin Towers on 9/11. It is a false analogy.

Let's consider Henkka's example and show how it differs from WTC.

Henkka imagines a car hitting another car (~ equal mass), resulting in a large change of velocity, but I imagine a locomotive hitting a car (big difference), resulting in very little change of velocity. In your analogy,

Whether it is car<>car or locomotive<>car - let's use locomotive. The first impact is between the "cow catcher" guard on the front of the loco. It hits the side panel of the car. Then over fractions of a second it pushes, distorts and penetrates the car as the car starts to accelerate sideways. In a fraction of a second the minor impacts and distortions are overwhelmed and the car goes bodily sideways.

What happened at WTC corresponds only to those first few milliseconds. Though it took minutes>>tens of seconds >> fractions of seconds in an exponentially faster process at WTC on 9/11. And the interactions were only between those structural elements that were in contact and were failing. The interaction between the two portions of the structure never reached the stage of dynamic impact between the full weights of either. The Top Block started to descend and the two parts immediately started a process of mutual dismantling.

The loco<> car impact analogy is only valid before the full impact of the loco hits. Whilst the cowcatcher is penetrating the body panels.

Here is one single frame static image from a motion gif of that 'transition stage' process.


Yes it is relatively late in the process - I don't have the full sequence of diagrams to illustrate the full process.
Note that the wall of perimeter columns is moving downwards - left side - to impact the floor joists of the lower tower. The impact from those perimeter columns has available most of the weight of the top block. BUT the force needed to shear the joists off the columns is far less than that full weight - possible 1/30th. So there was no impact of the full weight of Top Block landing on anything strong enough to give the "equal and opposite" reaction needed cause the jolt that everyone seems to be looking for. The jolt was never there in 2001, 2006, 2007 or later when people were looking for "The Jolt That Never Was" because it never could have occurred. And nothing has changed now in 2023 despite Henkka recycling a long rebutted false explanation by David Chandler. It is a wrong now as it was when D Chandler first published it.

(And, yes, I'm taking a few shortcuts to simplify the explanation. I can provide extended rigorous proof if necessary.)

it matters if a skater hits that sign, or a truck does.

Sure - but we are outside the range of validity of the analogy.

And that's even without considering that the perceived motion of the "roofline" depends on perspective (the tilt direction matters!), and that the motion of the building shell may not represent the motion of its interior (the penthouse dropped first on WTC7).

But we don't need to consider the roof line motion in order to understand the actual mechanism OR the errors resulting from assumed false premises in the argument presented by Chandler and many others and recycled by Henkka.

 

[FatPhil]

F=m×a

a=gravity
m=a lot

A large mass hitting a stationary small mass undergoes a change of momentum, but its change in velocity is smaller if the ratio if masses is higher. Henkka imagines a car hitting another car (~ equal mass), resulting in a large change of velocity, but I imagine a locomotive hitting a car (big difference), resulting in very little change of velocity. In your analogy, it matters if a skater hits that sign, or a truck does.

And that's even without considering that the perceived motion of the "roofline" depends on perspective (the tilt direction matters!), and that the motion of the building shell may not represent the motion of its interior (the penthouse dropped first on WTC7).

I'm not seeing your analogy. Isn't the mass of the remaining building and the earth it's rigidly embedded in somewhat higher than the mass of the falling upper block? These are not elastic collisions.

 

[econ41]

These are not elastic collisions.

Exactly. Hence my attempt to explain what actually happened. Getting lost in a macro overview treating the "impact" as between homogeneous blocks is the central issue that has confused much debate over many years.

 

[Henkka]

Have you ever seen a train hit a car on a railroad crossing? The train is "going smoothly" nonetheless.

Yeah if you have a collision between two objects of vastly differing mass, the one with higher mass will easily destroy the one with lower mass. The problem is that when you apply this analogy to the North Tower, the intact tower below is the "train", and the falling floors are the "car".

 

[Mendel]

I'm not seeing your analogy. Isn't the mass of the remaining building and the earth it's rigidly embedded in somewhat higher than the mass of the falling upper block?

no, the lack of rigidity of the floor seats has been discussed above

 

[Mendel]

These are not elastic collisions.

Are you claiming that these collisions lose so much energy to heat and deformation that the speed of the falling top section should be noticeably affected?

 

[FatPhil]

Are you claiming that these collisions lose so much energy to heat and deformation that the speed of the falling top section should be noticeably affected?

Can you show me where you think I'm claiming that precise thing? If you cannot, then that might be because I'm not making such a claim.

I am claiming that your train analogy is not close enough to be illuminating of the situation. Anything perpendicular to gravity, thus not turning potential energy into additional kinetic energy and momentum in the same direction as the original impulse, will be a poor analogy. It works as far as "the big thing breaks the small thing made of similar materials", but little further.

 

[FatPhil]

no, the lack of rigidity of the floor seats has been discussed above

If you're going to be pedantic about the meaning of the word "rigid", then *nothing* in the universe is "rigid". I clearly meant that it was able to supply *some* resistive force, even if it's not the same resistive force it was spec'ed for. The floors have no degree of freedom in the up-down direction - they are "rotation fixed and translation fixed", if you want more precision.

 

[econ41]

Can you show me where you think I'm claiming that precise thing? If you cannot, then that might be because I'm not making such a claim.

I am claiming that your train analogy is not close enough to be illuminating of the situation.

Correct. It is not a good analogy. Only the first micro or milliseconds of the initial impact train>>car are at all analogous to the start of WTC collapses. And by "start" I refer specifically to the first portions of downwards motion of the Top Block - where David Chandler's explanation is wrong.

Anything perpendicular to gravity, thus not turning potential energy into additional kinetic energy and momentum in the same direction as the original impulse, will be a poor analogy.

It's a damn sight easier to understand what actually happened.p

It works as far as "the big thing breaks the small thing made of similar materials", but little further.

Possibly true BUT the relevant impact for WTC is the multiple partial impacts between the multiplicity of structural elements. It was NOT an impact of one homogeneous body on another. Even the reference to elastic or non elastic is taking a much too "global" overview.

 

[Oystein]

Remember where the discussion comes from:

Henkka's imagining that the forces of resistance that crush the crush zone ought to translate into noticable deceleration (or perhaps at least jolts) at the roofline, that should be measurable from video analysis.

Members are trying to intruduce and refine, others to throw out, an analogy "crash car vs locomotive".

I first suggest that you define rigidly what in the WTC tower represents "car", what represents "locomotive"
I secondly suggest that you think for a moment how the effects on the locomotive are different from effects at the end of the train, as there are a number of connections and elastic/plastic elements between the crash front and the end of the train. Obviously, "end of train" would correspond to "roofline".

Now, as for my first suggestion, I think the key is to keep at the forefront of mind at all times the fact that, for the most part, columns did not impact columns.
Instead
Columns impacted floors (train <> car)
Floors impacted columns (car <> train)
Floors impacted floors (car <> car)
(and core beams interacted with core beams etc; but that is secondary, as the roofline is the upper end of the wall "train", i.e. the perimeter columns)

A column<>floor impact would see an extreme load exerted by the very strong columns on a very small area (1.4 square feet, 1/8th of s square meter in the case of a perimeter box column) which is not nearly suited to resist such a force - the heavy column would surely interact with only a small floor area / mass that is at the same time failing, punching a hole, then meet little further resistance. This would register as only a small deceleration at the roof line, considering there is also a lot of elastic response between impact point and top of the column.
When floor hits floor, these amount to a "head-on car vs car" analogy, where they exchange momentum fairly: The previously stationary floor assumes half the momentum of the falling one, which in turn loses half its momentum, give or take. That would correspond to a HUGE momentary acceleration (+/-) of the floor slabs. BUT, and this is the important point: The deceleration of a falling floor translates only very imperfectly to a decelration of the columns it is still attached to, as such a deceleration of the (very heavy!) columns would require a very large force - but forces exerted by floors on columns are limited by the maximum force the floor seats can exert.

And that's the thing to understand: Massive forces, accelerations, change of momentum, jolts that surely DO happen within the crush zone just don't translate to much of an acceleration at the roofline - because the main interaction between crush zone and roofline is "massive trains (columns) hitting flimsy cars (floors) at their wheels (truss seats)"

------------------

And also, I should point out that David Chandler, Toni Szamboti et al, in their video analyses, smooth their data (by doing running averages or even just simply drawing a straight line through velocity values) before concluding that there is no change in acceleration corresponding to anything hitting anything.
BUT that is an artificial, inevitable result of the smoothing!
If you look at the raw data, if you just take two adjacent values for velocity which you derived by in turn dividing delta-h by delta-t of adjacent frames, you will find VERY LARGE changes of acceleration. And these would be to a significant degree be due to measurement error, so some sort of smoothing seems in order - but the smoothing removes the very signal you are looking for.
TL;DR: The video analysis methods that Chandler and friends didn't aren't able to find the signal they are looking for. A fool's errant.

 

[DavidB66]

I am not a 9/11 expert, and I have only given the video one casual watch. My impression was that it was quite good at explaining the rapid collapse of the 'floors', but not so much in explaining what happened to the central core. An equally casual viewer (i.e. most of the likely audience) may wonder why the core (or a large part of it) didn't stay upright while the rest of the building collapsed around it, or why, if it was bound to fall, it didn't fall in one piece, or at least in bigger chunks than seems to have been visibly the case. And why did it (apparently) fall at the same rate as the floors, despite their very different structure? (I'm aware that a substantial part of one core did stay intact and upright, so the question is: why not more of it? )

Of course these comments are not intended as a contribution to the technical analysis, just as an indication how the video may appear to an average uncommitted viewer.

[Full disclosure: I just cheated and watched the video again. I still don't think it explained what happened to the core.]

 

[Henkka]

It was NOT an impact of one homogeneous body on another.

Neither are cars or trains, though. I think you guys are getting too caught up in the details. Like nothing in reality is this simple:

Even if you have a cardboard box on a ramp, it's not really like that diagram. The bottom of the box is wobbly and uneven, and the ramp will have tiny fluctuations in friction all over its surface. But we can simplify and approximate reality with diagrams like this and do calculations that still give us 99,9% the correct answer. So I'm not claiming that this (If the image doesn't load, it's David Chandler's diagram from the video I linked above):

Is literally, exactly what happened. Obviously in the real collision there were thousands of parts impacting, obviously the structure of the building is not homogeneous, and so on. But I'm not understanding why a diagram like this is not an appropriate, simplified model of what happened.

 

[Henkka]

[Full disclosure: I just cheated and watched the video again. I still don't think it explained what happened to the core.]

Watch more mainstream Youtube videos purporting to explain the collapse and you'll find this is a common theme.

 

[econ41]

It was NOT an impact of one homogeneous body on another.

Neither are cars or trains, though. I think you guys are getting too caught up in the details. Like nothing in reality is this simple:

I'm not one of the members who are trying to force fit a poor analogy - as I have already explained.
It is not simple but I have already outlined sufficient of the necessary bits of a valid explanation of what actually happened.

 

[Landru]

I'm not one of the members who are trying to force fit a poor analogy - as I have already explained.
It is not simple but I have already outlined sufficient of the necessary bits of a valid explanation of what actually happened.

I agree. This reiterates why analogies are often a poor way to put forward an idea.

 

[noy]

Stumbled across this video this morning. Was uploaded 4 days ago. It helped me understand better why the towers collapsed in the way they did.



Source: https://www.youtube.com/watch?v=1NkBfLBov5Q

Not too bad of an explanation...
But it fails to show or explain what happened with the core beans. How such a massive solid mesh structure was reduced to pieces and fell like the rest of the buildings.
Also... What about WTC 7?

 

[econ41]

I agree. This reiterates why analogies are often a poor way to put forward an idea.

Thank you.
In this case the analogy starts from a false premise - viz that the "significant big jolt" was possible. It wasn't. THEN the analogy makes two fatal errors:
(1) It assumes a moving locomotive >> impacts with car. The Top Block of WTC started from ZERO speed and NEVER achieved impact speed in the valid time frame. i.e. in the time period when column on column impact was plausible. (Yes 'plausible'. It was NEVER possible for reasons we don't need at this time.)
(2) By the time the Top Block was moving downwards the opportunity for a significant impact was already past. The dropping Top Block columns had already missed the lower tower columns.

The analogy does NOT represent the actual event.

And the OP Topic video is silent on the related issues.

 

[Henkka]

(1) It assumes a moving locomotive >> impacts with car. The Top Block of WTC started from ZERO speed and NEVER achieved impact speed in the valid time frame.

Yes I think in 9/11 discussions, we often approach this issue by assuming the top block somehow got to fall pretty much unimpeded for a moment, and then impacted the lower block. Then we debate about what this impact should result in, truthers saying it should slow down and stop the collapse, and debunkers saying the impact was enough to cause a chain reaction that destroyed the entire tower.

But obviously there was no way for an unimpeded fall to occur in the first place. The core columns in the impact zone didn't go anywhere. They didn't suddenly cleanly snap in half, or vaporise or whatever. Some of them were damaged by the impact (Steel vs. aluminium though...), and then softened by the heat of the fire. But gradual softening should not cause a sudden, smooth, symmetrical collapse. Maybe you would expect the top block to slowly slump down or something like that instead, if the fire was really so hot.

The dropping Top Block columns had already missed the lower tower columns.

Yeah I remember you had this theory about the columns "missing". But I remember in a previous thread asking you how you get the core columns detached and moving in the first place so they can "miss", and you didn't have a satisfactory answer imo.

 

[econ41]

Yes I think in 9/11 discussions, we often approach this issue by assuming the top block somehow got to fall pretty much unimpeded for a moment, and then impacted the lower block.

That was the assumption made by Bazant & Zhou in their 2001/2 limit case hypothesis. It is a valid worst case BUT NOT what happened. Then most of the debate assumed it was what really did happen. And the debate derailed for 7 or 8 years. And it is one of the false assumptions which underpins several of the AE911 'experts' specifically T Szamboti, D Chandler (in the video you posted) and IIRC J Cole.

The Top Block did NOT either 'drop' or 'impact'.

Then we debate about what this impact should result in, truthers saying it should slow down and stop the collapse, and debunkers saying the impact was enough to cause a chain reaction that destroyed the entire tower.

Yes - those are the two common claims. Both are wrong but for different reasons.

But obviously there was no way for an unimpeded fall to occur in the first place.

That is true - your next comment is not - >>

The core columns in the impact zone didn't go anywhere.

The mechanism of initiation was a cascading sequence failure of ALL columns. Perimeter AND core. No 'special rules' to prevent the core columns from failing. So of ALL the columns whether perimeter or core a few were pre-cut by aircraft impact. the rest failing in a combination of heat weakening effects and load redistribution. (NOTE: That is TRUE whether or not there was any CD - all that CD changes is the ratio of pre-cut columns to columns that failed in cascade load redistribution..)

This next bit is confused:

They didn't suddenly cleanly snap in half, or vaporise or whatever. Some of them were damaged by the impact (Steel vs. aluminium though...), and then softened by the heat of the fire. But gradual softening should not cause a sudden, smooth, symmetrical collapse. Maybe you would expect the top block to slowly slump down or something like that instead, if the fire was really so hot.

It is easier if you track through the logic in legitimate taxonomic sequence of factors. So START with the known fact - the columns FAILED in a cascadfing sequence - whether or not there was CD involved. Then understand what drives such a cascading sequence. I have for many years used a sequence of diagrams to illustrate the PRINCIPLE. Start with this one and remember we are explaining PRINCIPLES - not WTC specific details.

So a single row ofcolumns and we cut the first two with explosives - just to make it easier for learning purposes. This scenario.

We can use that graphic for discussion if necessary. Assume uniform column loading before cuttinmg./ Where does the load from 'A' and 'B' go? How much extra goes onto 'C'? Does the load on 'D' increase?

Now let's trigger the cascade by heat weakening - it is a couple of grades more complicated in the details tho not much harder to comprehend as a broad principle:

Yeah I remember you had this theory about the columns "missing".

Take care that you don't confuse and conflate the collapsed stages. The main effect of columns 'missing' was to allow the 2/3rds 'G' runaway of the progression stage. BUT the column ends 'missing' or 'bypassing' was set up in the initiation stage. And this current debate is about 'initiation' - moving into 'transition'.
There were two causes of 'missing' - call them, 'micro' >> small scale - if a single column gets shorter by overload it will do so either by gross buckling over a couple of storeys length OR by localised 'folding' if there is sufficient heat >> we can explain those in more detail if necessary. Then 'macro' scale missing - look at the graphic a few posts back showing the first dropping of the Top Block. The yellow arrows show how far the Top Block parts of the columns were away from alignment with their lower parts.

Whether by local 'micro' buckling or bigger scale 'macro' shifting of the base of the top block - EITHER proves column misalignment and THEREFORE - no possibility of the large scale noticeable impact jolt.

But I remember in a previous thread asking you how you get the core columns detached and moving in the first place so they can "miss", and you didn't have a satisfactory answer imo.

So I didn't [persuade you on a previous occasion. Remember my persistent advice: "Understand the collapse mechanism". This thread is focussed on false analogies because most discusion is not recognising the actual collapse mechanism.

 

[Henkka]

So I didn't [persuade you on a previous occasion. Remember my persistent advice: "Understand the collapse mechanism". This thread is focussed on false analogies because most discusion is not recognising the actual collapse mechanism.

Well, I found the exchange:

But you need to get the columns disconnected and moving in the first place, so the missing can take place, no?

You: Yes, And, surprisingly, that is the key issue that few people have looked at or considered seriously.

Agreed... Like if you look at the NIST FAQ on the towers, the word 'core' is found only 8 times. And as far as I can tell, this is the only sentence in the FAQ that directly discusses what happened to the core: "Thus, yielding and buckling of the steel members (floor trusses, beams, and both core and exterior columns) with missing fireproofing were expected under the fire intensity and duration determined by NIST for the WTC towers."

The focus is always on the floor trusses and the perimeter columns. But when the antenna starts moving down, the core under it must have already failed. Now I don't want to get too conspiratorial, but it almost feels as if they don't want you to think about it too hard.

You: Which is part of the reasoning why I decided to never rely on NIST's arguments.
From here: https://www.metabunk.org/threads/wo...-fire-alone-without-plane-impact.12533/page-8

So it's not really surprising I wasn't persuaded, since you didn't say much. What I found surprising there was your casual statement that "few people" have seriously considered such a "key issue". "Key issue" feels like an understatement, it's like the most important thing! But you can barely find any answers in videos like in the OP, or even the NIST FAQ. It always feels like this: "Well you see, the intense fires made the floor trusses sag, causing them to pull on the exterior columns. This eventually made them snap, and at the same time, uhh, something happened to the core columns, don't think about it too hard, ok?"

As for the rest of your post with the diagrams, I don't know, I found it hard to follow/understand honestly.