The 9/11 Forum

I hope you can watch this 3 minute video by David Chandler, I was hoping to see if anyone could see if it was correct, as far as I can see on the video, chandler is right in saying the top section starts to crush itself before the lower half (visibly starts to collapse) but is this not what we would expect?

I think what Chandler is suggesting is that the top block destroys itself before the lower part starts to fall, therefore ''something'' else must be destroting the lower block other than the top block, I'm not so sure any thoughts on this?

http://m.youtube.com/watch?gl=US&hl=en- ... jSd9wB55zk


Thanks

Chandler is either dishonest or delusional. There were no blocks... it's pure nonsense to describe what we saw like that. He's probably both. He's no scientist... that's for sure.

Hi sander,

Do you mean how he describes the top section as a ''block'' as in a solid object?

Yes... first of all the top section was not solid nor was the bottom. The towers were, like most high rise structure or even most buildings mostly air. The structure, in a steel frame is a rather small percentage of the mass of the building. The floors themselves would only occupy 2.6% of the total building volume. The steel was less than 1% of the total volume of the towers.

So there was not much there and there certainly were anything like solid blocks.

If you think of a blimp... a light metal frame with a skin stretched over it filled with helium gas... a massive volume but not solid by any means.

The towers look like blocks because the were rectangular solids... defined by the 4 planes of the facde. They were no blocks... but columns... a thin slabs suspended between the facade and the outer columns of the core.

When they core failed the various parts of the structure above disintegrated... dropped onto the floor below which failed set of a progressive destruction of the floors...

There were no blocks and anyone who sees blocks needs to have the eyes checked and their head examined... they are not in touch with reality.

My point entirely Sander, What I have got myself confused about and I do not know about you, is when Dr greening said that the 3rd law did not apply to this situation, (or something similar). I think he was saying because it is not a block it didn't apply, what's your position on this?

thanks

I have no idea what Dr G was referring to. One simply can't pretend that the towers were blocks... if you, me or Dr G looks at the interactions... on the micro level... we will see that all of the laws of thermodynamics apply. How could they not?

Check this on DR G:

newton-s-3rd-law-and-the-collapse-of-wtc-1-t153.html

I would agree it must apply, but he was also saying how the sections of buildings were made of up components etc and not just two solid blocks.

Illuminist14 wrote:I hope you can watch this 3 minute video by David Chandler, I was hoping to see if anyone could see if it was correct, as far as I can see on the video, chandler is right in saying the top section starts to crush itself before the lower half (visibly starts to collapse) but is this not what we would expect?

This particular topic has come up a number of times, most notably in the thread you linked to above.


There was a fair bit of meandering around the central points in that thread, so they may be obscured. I'll try to summarize it.

1) the average resistive force provided by the lower section is indeed less than the weight of the upper section
2) this is expected

There are many more details, but that's the core of it.

Essentially, everything Chandler says is correct up until 2:36 in the video. At this point, he mentions he has "been using the term 'block' loosely" (as if there's any other way to use it) and has the opportunity to provide the explanation for the phenomena he's been discussing, but instead veers off in a bad direction. Let's examine the statements he makes and then go on to the real reasons for the phenomena observed.

Chandler video transcript wrote:What we actually see here is the falling section of the building turning to dust before our eyes.

It would be generous to call that hyperbole. The upper section disintegrates or dissociates, yes, but by no means turns to dust. There is also a lot of dust. Is that a surprise? Not really. But keep the following in mind...

A lot of what looks like dust in videos is not dust
Consider the scale of the structure and the distance involved in most videos. Anything smaller than about 10cm is indistinguishable visibly from dust in images. The means of distinguishing fine particulate from chunks is the former display diffusion mechanics in air and the latter display projectile mechanics. The former spreads and settles, the latter falls. There's a lot more visible mass falling than diffusing and, implicitly but directly inferred, way more mass falling than what's visible.

Smoke rises, chunks fall, dust spreads. Does anything else spread or settle slowly? Anything with high surface area to mass. What were both towers packed with? Paper. Paper is seen settling everywhere post-collapse. From a mile away, paper looks like dust. It gets entrained in air flow, disperses, and settles.

Clearly there's only a small fraction of the upper section converted to dust over the period in question, so the notion of a large portion the entire upper section turning to dust - let alone all of it - is false.

Concrete is but a small portion of dust-producing materials
The big source is gypsum wallboard. Easily fragmented and crushed with a high proportion going to fine particulate. Plenty of other fragile materials. All of the easily crushed materials would begin producing large volumes of dust immediately in any collapse, and a relatively small amount of dust can expand into a large volume. Ceramic and other brittle materials crush more readily than concrete and will produce some proportion of dust.

Naturally, there was a huge amount of concrete and that will produce dust as it fractures, too. In demolitions where contents and non-structural members have been removed, there are still huge volumes of dust, generally of the order of the building volume or significantly greater. This is true even in verinage where no explosive is used. The classic case is the Balzac-Vitry but there are many others. Lots of dust, and very early on.

Concrete was pulverized, but to a lesser extent early on
There was a final distribution of particle sizes in the rubble pile, but one should not infer the degree of pulverization was the same at all points and times during the collapse. This is not just a bad idea, it must necessarily be wrong according to thermal physics. There is more energy available to do work in pulverization in the latter stages of collapse, therefore there will be more pulverization towards the end, period.

The greater kinetic energy of the rubble in the later stages, plus (and especially) the huge whomp of inelastic collision at the end when debris is basically brought to rest, must necessarily dissipate a huge portion of energy which originally came from the potential energy of the erect towers. That would be true explosives or otherwise (I'd be happy to elaborate on that if you wish but it's out of scope here).

Chandler's statement is at best hyperbole and at worst nonsense. You'd expect a large amount of dust, and there is. But he says the top 'dustifies' before our eyes. Not to my eyes it doesn't. I'd like to think what he really means to say is the top disintegrates but, if that's what he meant, he should have said it.

Chandler video transcript wrote:But what is happening to the upper section of the building, behind the dust clouds, doesn't really affect this analysis.

Let's cut to the chase: What is happening behind the 'dust' clouds has a HUGE bearing on the real mechanics. Any analysis which fails to capture this aspect will be unable to explain the dynamics of the actual collapse.

The validity of this statement depends on the context (his analysis), but is false in general and specifically false in this context. His analysis IS a simplified blocks analysis. It's not even an analytical work, it's forensic. He has prepared a block model and applied empirically observed acceleration to it, which is fine, but it IS a block model of the crudest variety theoretically possible and he can't just wave that away with a statement about how he's been referring 'loosely' to a block.

It's either a block, or it isn't. The real tower was not a pair of blocks, so we're only talking about a model, you know? He chose a block model and with that comes certain responsibilities. Failure to properly frame a model can make a model inaccurate or useless.

His model is the crudest possible because it's 1D and there are two blocks. There are only two components; you can't even model a collapse with one component, a single block. There is only one dimension, nothing to pare away there! His model doesn't even account for accretion of mass to the upper block as the collapse progresses. There's nothing to his model except the upper block pushing on the lower and vice versa, one degree of freedom and two (effectively point) masses.

This would be okay so long as all of this is kept in mind. Someone well-grounded in physics would immediately understand that not much could be expected of such a model. By coincidence, it may deliver good results, either in terms of predictive power or backfitting, but only if it's handled properly, and such is not the case here.

Chandler does not get to use the term block loosely, it is the crux of his model.

Here we get to the final statements of the video, and root of Chandler's error.

Chandler video transcript wrote:Given the fact that it is accelerating downward, the top section of the building - whatever its condition - cannot possibly be destroying the lower section of the building. The destruction of the building must be caused by something else.

The conclusion does not follow from the premise.

A valid conclusion from the premise would be: Given the fact that it is accelerating downward, the top section of the building - whatever its condition - is experiencing an average force of roughly one-third of its weight over the measurement interval. That's simply F = ma based on the measurements.

This is pigeonholing an event which is extremely inhomogenous in both time and the three spatial dimensions into two simple blocks, but that's all the model is capable of accommodating. For all of its shortcomings, the model gives some pretty good results (others, myself, have used it to good success) when handled properly.

It really comes down to this: are the following two statements equivalent?

1) The upper block experienced an average force of roughly one-third of its weight
2) The upper block could not have been destroying the lower section

At the very least, Chandler did not bridge the gap to show how the two are synonymous. In fact, he cannot, for they are not. It is his unspoken and mistaken assumptions which lead to his error, and these are things he apparently takes to be self evident.

Next, we'll see what those axiomatic 'givens' are and why they are wrong.

The biggest unspoken assumption of Chandler's is that the resistive force provided by the lower section is too low; so low as to require assistance of some kind to cause what was observed. This is really the only way to logically move from the premise to the conclusion. It is an argument based on force, yet he provides no mention let alone explanation on why the observed force should be considered anomalous.

Why?

I can only guess, but it's a pretty reasonable guess. He believes the intact portion below should afford the same resistance it did when both blocks were static. This is wrong in so many ways and on so many levels, it's hard to know where to begin.

Demand-to-capacity (DCR) represents the ratio of STATIC load imposed to STATIC load capacity. For argument let's say the towers had a typical DCR of 0.5, which very roughly represents a factor of safety of 2 (not actually but not important). If the total load above a certain point were doubled, the support would barely hold it, but there is plenty of margin there for the regular static load. In perfect alignment.

But what happens if the load is in motion or support members are misaligned?

If the supports have an elastic response to small compression, as steel columns do, simply bringing a load into contact with a support and releasing it will cause twice the peak deflection as the same load sitting there statically. If the support doesn't fail, the system will undergo a rapidly damped oscillation to the static deflection position at equilibrium. If a column has a DCR of 0.5, merely bringing the design load into contact and releasing it will bring a column to the end of its elastic response and the onset of plastic yield and technical failure.

Drop the load from any height at all, a DCR 0.5 column will fail decisively. Decrease the DCR (increase factor of safety) and there will be some increased drop height which will fail the column. Once a column is compressed axially past a certain point, its capacity diminishes drastically. Even though the load is the same, capacity is reduced greatly, so the DCR becomes greater than 1. At DCR greater than one, the load will accelerate downward continuously.

Axial compression is the column failure mode which would dissipate the greatest energy in collapse and yet the columns can't even hold the static load beyond a certain (rather small) degree of compression. So, even if the collapse were a perfect 1D two block collision, the total energy required to fail all the columns axially over one story is less than the potential energy lost in descending through that story. There's a big spike in resistive force at initial compression then virtually no support through the rest of the fall.

That's in a perfect, axial 1D blocks world.

A decent analysis, using engineering mechanics and properties of materials, shows that steel columns in axial compression would provide an average resistive force of 15-30% of the static load associated with a perfectly aligned pair of blocks. Already, a properly developed block model can get within error bands of Chandlers measurements, predictively.

But that's an approximation which doesn't include inelastic accretion. With that included, Chandler's 0.65g is right in the fat band of expectation from a crude model only one step up from his. Without it, but with sound engineering and physics foundation, his block model would predict a faster collapse than what was observed!

So what is Chandler's problem?

It has to be that he expects the intact lower section to provide resistance of at least the magnitude of the static load - ALL THE TIME. It can't. The properly developed nominal case blocks model with accretion predicts fall times close to what is observed precisely because it derives the expected AVERAGE resistive from empirically determined relations and finds force to be much less than the equivalent static load of the descending block.

Chandler assumes, erroneously, that the average force must be greater than it is observed to be. A simple 1D mechanical approximation says otherwise and makes a good prediction of collapse rates. Such an analysis is also very optimistic for survival because it unrealistically assumes perfect alignment.

How much is capacity reduced when column ends miss each other entirely? The reduction is total, the capacity goes from X times the static load to zero. DCR is undefined/infinite. Column above will experience no retarding force from the column below. For example, if half the columns missed each other at any point, there's half the capacity right there. Eventually, things collide, but will there ever be another load path for any given column as good as the one it was built with?

Move any column an inch in either horizontal dimension and there will be a significant reduction in capacity. Tilt a column a few degrees off plumb and there will be a significant reduction in capacity. Do these things in combination and the effects are multiplicative (e.g, 2 reductions by half is quarter capacity). Relatively small translational and rotational misalignments can mean two fully intact columns will provide a small fraction of as-built capacity.

In Chandler's video, one can directly see evidence of total perimeter misalignment at initiation over large stretches of two sides. Not only does this show that expected capacity during collapse progression will be far less than a perfectly aligned block model, but it also introduces another factor: global eccentricity.

If an otherwise intact tower were to have half of its columns on one story removed, would it stand? According to one of the principals involved in the construction, yes - IF every other column was removed. That is an even distribution of capacity to match the imposed load.

If, on the other hand, all the columns on one half of the building were cut, the total number of intact columns would be the same but the upper section would have no support on one side. All of the load would be on the few columns in the center until they gave enough to put load onto the others. That means a tilt, however slight, and an upper block in rotational motion. This is a dynamic system with positive feedback because - while the load is a constant - the capacity is very much geometry dependendent. As the geometry changes from nominal design, the capacity will not stay constant but reduce further. The progression will continue and accelerate unless capacity is somehow restored to a level greater than that required to hold the static load.

In other words, the top will tilt until there is very little capacity and then it will drop. Whether anything can stop it later is another matter, but the half-story is not going to hold it up.

The building with one half sliced away is extreme, for sure, but it's all in how capacity is distributed. It has the same overall capacity as the every-other-column-cut scenario, but it has far less effective total capacity. The blocks model, with uniform DCR of 0.5, would predict both cases would barely stand exactly on the threshold of collapse, where it is only true for one case.

Factors which come under the heading of suboptimal available capacity distribution (translation, rotation, eccentricity) represent a large reduction in capacity, multiplied on top of the best case 1D axial capacity which, on average, is just a fraction of the total intact static capacity.

If one accounts for the peak/trough nature of the support capacity in an ideal 1D alignment and the retarding effect of inelastic accretion, using exemplar values for material properties, the observed average acceleration over a story displacement would range between 85% of g initially to about 45% at terminal asymptotic state.

If one then applies the further reduction in capacity due to suboptimal loading, the expected average force from the lower section is based on the average degree of misalignment, with some dithering on what happens to accretion. Assume the average reduction is to 25% of base capacity, quite generous considering a few inches of misalignment will net that much reduction. The naive 1D model with no velocity dependent forces would then predict >90% g initial acceleration converging on a stable mean of just over 60% of g.

Pretty close to what is observed for the early phase.