This mathematical 1-D interaction described in the latter Bazant papers has come to be taken quite literally as the process that actually happened to each tower. Block mechanics is now an integral part of the technical historic record of the collapses. As a result there is no accurate portrayal of the true collapse progression mode within any academic or government literature. A literal interpretation of the blocks described by Bazant serves as the current description of how the buildings collapsed within recorded technical history.
The Core Literature Reinforcing Block Mechanics
Once this more detailed information is considered, it is enlightening to take advantage of the gift of hindsight to spot simple mistakes in each of these papers:
All 4 Bazant papers BZ, BV, BL and BLGB, linked and reviewed here
Frank Greening, Energy Transfer in the WTC Collapse, linked and reviewedhere
Keith Seffen, Progressive Collapse of the World Trade Centre: a Simple Analysis, linked and reviewed here
Gordon Ross: Momentum Transfer Analysis of the Collapse of the Upper Storeys of WTC 1 linked here
It is easy to see that the tools now available allow for a far better understanding of specific properties of the collapses. These authors had no clear concept of anything like a ROOSD mechanism when each of these papers were written.
From the current vantage point it can be seen that each of these authors were for the most part grappling in the dark and were only able to see the grossest features of the collapse processes.
They would embrace what some of us now recognize as cartoons seemingly without realizing how badly their conceptions were contradicted within the visual record.
They seemed to have no specific conception of mechanism at all. The actual process of propagation was treated only in generalities.
They had no visual mappings in the sense that the author uses the expression today. They had no access to mappings of the rubble. No access to detailed mappings of collapse fronts. No mappings of how massive the core remnants actually were.
Lacking mappings and any specific concept of a propagation mechanism, it is absolutely impossible to understand overpressurization patterns within the visual record. If so, the authors couldn't possibly deal with the understanding or explanation of various isolated overpressurization events in a remotely realistic way.
Building movements were described only in the most general terms, as "blockish". Overpressurizations were understood as blockish, as a super-piston.
COUNTER-ARGUMENTS TO THE COLLAPSE PROPAGATION MODEL ALSO RELY ON BLOCK MECHANICS
Block mechanics, which is a provably incorrect way to describe the collapses of the Twin Towers, has resulted in a number of claims and counter-claims also based on block mechanics. For example, the underlying description of collapse progression by AE911T is also based on block mechanics.
The irony of such counter claims is that by phrasing collapse mechanics in terms of blocks just as Bazant had, counter-arguments reinforce the false historic description of the collapse modes of the Twin Towers in terms of crushing blocks. Therefore, while they appear to counter the Bazant formulation of collapse mechanics at a superficial glance, they actually reinforce it and support it by embracing the same block approach. Common arguments both for and against CD rely on the mistaken need to destroy the whole lower and upper portions with demolition devices. Some argue that the upper block cannot crush the lower portion and will somehow come to rest before striking earth while others argue it can crush the block below before crushing itself when striking the earth. But both approaches express collapse progression in terms of interacting blocks.
Block mechanics occupies a central part of the AE911T evidence list, which will be reviewed in section 2.8, and is a key component within their main arguments for demolition. It is an integral part of each polarized position within the false dichotomy by which the history of the collapses is currently represented.
2.71 VARIOUS DESCRIPTIONS OF BLOCK MECHANICS PRESENTED AND FACT-CHECKED
Literal interpretations of the papers that utilize and reinforce "block mechanics" has created a whole range of grossly incorrect descriptions of the WTC twin towers collapse mechanics. These incorrect descriptions have caused much confusion, in their respective authors as well as readers who are susceptible to believing in them literally. A wide variety of examples are given.
RYAN MACKEY BLOCK MECHANICS
Ryan Mackey presents himself as an engineer who works for NASA and speaks and writes publicly about the mechanics of the collapses of the WTC towers. In the direct quote which follows serves as an excellent example of taking 1-D block mechanics quite literally. Ryan Mackey takes block 1-D block mechanics so literally that he believes he can visualize the process in intricate detail.
(Comments by me in red)
R Mackey describing crush down, then crush up in his own words1 (Comments by the author in red):
Think of it in terms of impulse -- the total change of momentum at a particular impact. Impulse is equal and opposite, by conservation of momentum. Impulse is equal to F delta-T (force times the time over which the force is applied), or M delta-V (the raw change of momentum in its familiar definition P = m V).
Two big rectangles and 2 large Newtonian force vectors. This is the block formulation of WTC collapse mechanics.
When we look at the "upper block," it's delta-V is smaller than the delta-V experienced by the newly broken part of the lower block. As you say, the upper block decelerates by an average 1/3 g, while the lower block accelerates by an average 2/3 g. This is because the participating part of the lower block masses less than the participating part of the upper block -- it really is the compacted mass and upper block versus a small number of floors at a time, not the entire lower block.
The pper block is surviving because he read it in a Bazant paper and believed it. Now he can visualize the whole imaginary process in his mind and teach others, too.
The reason only part of the lower block participates at any given time is because the lower block is still a mostly intact sparse structure of braced columns. When it's hit, the columns lose bracing, get loaded eccentrically, shear their welds and bolts, and in some cases are totally overwhelmed and fracture entirely. These pieces break at a stress much too low to actually support the descending mass. This also has nothing to do with the strength of the perfectly intact building -- the descending rubble heap isn't contacting the lower structure at its strongest points, and it's introducing brand new failure modes, so the effective opposing strength of the lower structure is far lower than its ideal carrying capacity. Furthermore, where the lower structure does resist at or near its ideal strength, it can only do so for a very brief delta-T -- until reaching its failure strain, which takes only about ten milliseconds at the speeds of collapse -- and this is not enough to amount to all that much total impulse.
The upper chunk, in contrast, is cushioned by a thick layer of rubble. This is compacted about as far as it can, thus it doesn't have those complex failure modes and it doesn't suffer much more "damage" even at much higher stresses.
No complex failure modes at all? Hmmm...
So the rubble pile remains, and the lower structure gives way. This is for the same reason you don't sink into the ground, even though you can push your finger easily through a cupful of soil.
The "upper block," what remains of it, rides on top of this cushion of debris. It is supported pretty well. It also only decelerates at that lower rate, thanks to the much greater inertia of the upper block + debris. So the only real force it suffers is the inertial force, i.e. its own self-weight times its deceleration, again about 1/3 g. It can be expected to survive this deceleration.
"It is supported pretty well". By...what? The description is based on the assumption of homogeneity popularized in Dr Bazant's descriptions of collapse progression.
It's only when the rubble pile has nowhere else to go and the upper block has to suddenly stop, dissipating all of its momentum in mere milliseconds, that it totally fails.
"Upper block" only fails when contacting earth. Homogenous crush front assumption throughout the collapse progression until the "upper block" strikes earth.
Again, this is slightly idealized, but you get the point. Unless you're a Truther.
Within the passage Ryan Mackey is describing the motion of a building as it appears in the Bazant papers on collapse progression with the assumption of homogeneity.
All the popular memes are present in his description:
Survival of an intact 'upper block'
Lower columns being crushed by a 'zone B' while upper columns ride upon 'zone B'
A homogenizing layer of debris which separates 'upper block' and 'lower block'
Newtonian force vectors being applied to 'upper' and 'lower' blocks
He makes the same homogenizing assumption as Bazant and Chandler.
From the website:
"It is quite simple! A Structure cannot be one-way crushed-down from above by a small Piece of itself driven by Gravity"
"The WTC Towers could not have been one-way crushed down and destroyed by their upper parts dropping down on 9/11/2001"
"A skyscraper can only get destroyed from bottom up! A skyscraper can never collapse from top down!"
Anders Borkman's idea of entanglement:
He comes pretty close to ROOSD in these images if he doesn't allow the flooring to hang there from one end. Notice the similarity between his hanging floors and the gentleman below:
The image requires that the flooring connectors can hold at extreme bending angles. If such an assumption was removed, entanglement between upper and lower portions as drawn ceases to exist.
Collapse progression modes remain unidentified within all official and academic literature. The claims based on the assumption of homogeneity had led to a number of counter claims which are based on the same faulty assumption of homogeneity.
The Bazant papers published in 2007-2008, though incorrect in their assertion that block formulations can be applied to the WTC towers, have had a major influence in describing the collapse progression mode of the twin towers. The crush down, then crush up mechanics proposed is now taken quite literally as the actual collapse progression mode for both towers.
DAVID CHANDLER BLOCK MECHANICS
The David Chandler Third Law argument as an extreme example of block mechanics used to describe the WTC collapse progression.
1) the average resistive force provided by the lower section is indeed less than the weight of the upper section
2) this is expected
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.
ON THE APPEARANCE OF DUST
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, much more mass falling than what is visible.
Smoke rises, chunks fall, dust spreads. Does anything else spread or settle slowly? Anything with high surface area to mass. Both towers were 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 is 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 large source is gypsum wallboard, easily fragmented and crushed with a high proportion going to fine particulate. There are plenty of other fragile materials, too. 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-style demolitions where no explosives are used. The classic case is the Balzac-Vitry demolition 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.
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 with explosives or otherwise.
Chandler's statement is at best hyperbole and at worst nonsense. One would expect a large amount of dust, and this is observable. But, he says the top 'dustifies' before our eyes. Perhaps what he really means to say is the top disintegrates but, if that's what he meant, he should have stated it clearly.
Vague and Generalized Mechanisms
From the Chandler video transcript:
"But what is happening to the upper section of the building, behind the dust clouds, doesn't really affect this analysis."
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 is not even an analytical work, it is 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 cannot just wave that away with a statement about how he's been referring 'loosely' to a block.
It is either a block, or it isn't. The real tower was not a pair of blocks, so he is only talking about a model. 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 is one dimensional and there are two blocks. There are only two components; one cannot even model a collapse with one component, a single block. There is only one dimension, nothing to pare away there! His model does not even account for accretion of mass to the upper block as the collapse progresses as the Bazant block model does. There is 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 is handled properly and with caution, and such is not the case here.
Chandler does not get to use the term block loosely; it is the crux of his model.
Acceleration does not mean there is no capacity for destruction at the "contact surface"
Please consider the final statements of the video, and root of Chandler's error.
Chandler video transcript:
"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 is simply F = ma based on the measurements.
This is pigeon-holing an event which is extremely inhomogenous in both time and the three spatial dimensions into two simple blocks, but that is all the model is capable of accommodating. For all of its shortcomings, the model gives some pretty good results (others 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 synonymous. It is his unspoken and mistaken assumptions which lead to his error, and these are things he apparently takes to be self evident.
Is The Resistive Force in the Lower Portion Too Low?
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? He appears to believe 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 is 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.
If the load is dropped 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 is a large spike in resistive force at initial compression then virtually no support through the rest of the fall.
That is 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 is 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!
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 cannot. The properly developed nominal case block 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.
Reduction of Support Capacity
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. Columns above will experience no retarding force from the column below. For example, if half the columns missed each other at any point, there is 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 dependent. 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 is 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.
In summary, Chandler fails to account for the distinction between:
1) static capacity versus dynamic load-displacement response
2) Peak force versus average force
3) as-built capacity versus non-nominal (collapse) capacity
These things completely explain the difference between his expectations and the real world. Because of his misunderstanding, he believes he has found an incontrovertible smoking gun in the physics of collapse.
The reality is, his errors are so bad one really must question whether he should be teaching high school physics. But such confusion is more widespread than the argument of David Chandler.
Dialog Within The Context of Block Mechanics: Physics Gone Wild!
The posts are riddled with misconception. Smart people don't get this. Physicists, engineers, and PhD chemists don't get this. In the matter of physics-based objections, this is a big deal. It forms the basis of most of the variants of CD claims or 'proofs' by way of alleged violations of physics, like The collapses were too fast... they shouldn't have accelerated... there should have been a jolt... the resistive force was less than the capacity... the 3rd law requires equal and opposite destruction...
A lot of these points are academic. It is important to understand these ideas to have a good foundation but many things argued simply don't matter at all. It is the blocks mechanics thing again - the notion that, somehow, the sections really were blocks, the collapses really were 1-D, and so on.
"I would say that Chandler's slight of hand is the implied notion that Newton's 3rd Law is universally applicable, even to a collapsing building. The fact is that when a building is collapsing by multiple floor failures the reaction force obviously fails to balance the downward force because the yield strength of the failing columns is being exceeded."
Greening makes a couple of mistakes, both stemming from the same thing. First, he chooses to say the 3rd law is not universally applicable where it is applicable in all classical mechanics, under which progressive collapse falls. Second, by using the term "reaction force" instead of resistive force, he miscategorizes the argument he really wants to make. Newton's 3rd is a reflexive relation, how can the reaction force not be of the same magnitude as its complement? His objection to Chandler's claims is valid, but he mis-states it and starts the entire discussion off on a bad footing.
Steven Jones wrote:
"No. This is a blatant and fundamental error. I have caught many a student on the equivalent of this nonsense, as I taught Newtonian Mechanics for over 21 years. Newton's 3rd law is always applicable, even in the case you mention, Frank. The key is that the "equal and opposite forces" must act on DIFFERENT bodies. Suggest you consult a basic physics or mechanics text if you don't understand that."
Jones correctly points out that the law is applicable to collapsing buildings, so score one for the overunity guy. On the face of it, it is not clear what Jones is talking about when he mentions the "key" being the force acting on different bodies, and I'm still not exactly sure. One can guess, based on subsequent discussion, that he is referring to the horse and cart example. Again, technically correct but in actuality not only failing to address Greening's argument but also working against Jones and Chandler!
Because of Greening's poorly chosen language (which, all due respect, does reflect a misunderstanding of the situation to some degree), Jones is essentially tilting at windmills. Jones is correct and partially addresses what Greening actually said, but the conversation is already off course from the start. The real issues will be lost amongst the uproar that Greening thinks the 3rd law doesn't apply to collapses (and it was).
Without going on to recap the whole torrid exchange, captured at 911Blogger, it seems that neither Greening nor Chandler/Jones could ever manage to get a shot off in each other's direction. Chandler, full of huff and arrogance, is ignorant of the application of physics to progressive collapse, cascading failure and so on. This is odd for someone schooled in physics. Greening, for his part, is an excellent chemist but not a physicist and is unable to accurately articulate what is wrong with Chandler's position. This is odd from someone published on the subject of progressive collapse.
The general reaction to the debate, on the other hand, is armchair physics gone wild!
Jones, Tony Szamboti and Gordon Ross all share two common characteristics - they support Chandler and yet they make statements which come within a hair of refuting Chandler. Most interesting. It is like the strange disconnect between this subject and The Missing Jolt. Do these guys talk to each other? Do they read each other's articles?
Szamboti understands peak-and-trough nature of the load displacement. He understands the difference between peak resistance and average resistance and how the latter is less than even static capacity. Szamboti's argument is that the jolt from this peak resistance is missing. The argument is flawed on a number of counts but all the same Szamboti has the tools to refute Chandler's misunderstanding, yet does not.
Jones knows that Newton's 3rd law can be misapplied by failing to properly identify the bodies on which particular forces act. This is part of Chandler's error and Szamboti's flaw in the missing jolt - that the upper section really acted like a rigid block in 1D instead of a deforming complex body in 3D.
Ross knows that a monolithic rigid block on top is unrealistic, but doesn't make the connection to the arguments of his associates.
If these four individuals could pool their collective understanding, they would mutually refute each other and be on the right track.
HOW TO MOVE BEYOND BLOCK MECHANICS TO A REALISTIC SENSE OF GLOBAL MOVEMENT
A realistic collapse mechanics is not difficult if one loosens the constraint of homogeneity and examines the visual record more carefully. Once non-symmetric progression and non-homogenous crush fronts are allowed as a possibility, ROOSD becomes a pretty natural idea that seems to match all observables.
I think the best way to try to understand the twin towers collapse progression processes or explain them to somebody else is through the following series of links.
Once this more detailed information is considered, it is enlightening to take advantage of the gift of hindsight to spot simple mistakes in each of the papers which mutually reinforce the simplified model of block mechanics:
It is an essential addition to the complete visual record because it allows ordinary people the ability to understand the global mass flow of both towers better than the "experts".. It is easy to see that the tools now available allow for a far better understanding of specific properties of the collapses. These authors had no clear concept of anything like a ROOSD mechanism when each of these papers were written.
They would embrace what some of us now recognize as cartoons seemingly without realizing how badly their conceptions were contradicted within the visual record.
They seemed to have no specific conception of mechanism at all. The actual process of propagation was treated only in generalities.
They had no visual mappings in the sense that the author uses the expression today. They had no access to mappings of the rubble. No access to detailed mappings of collapse fronts. No mappings of how massive the core remnants actually were.
The authors couldn't possible deal with the understanding or explanation of various isolated overpressurization events in a remotely realistic way.
Building movements were described only in the most general terms, as "blockish". Overpressurizations were understood as blockish, as a super-piston.
It is impossible to discuss the question of demolition of either of the Twin Towers without a knowledge of the general mass flow (the collapse propagation mechanism). As the public debate has proven many times, if a person does not have a good grasp of the movements within the photographic record, it is impossible for them to have a knowledgeable discussion on the subject of demolition.
A block approach to collapse mechanics demonstrates poor observational skills as numerous and rather sizable clues within the visual record need to be ignored to support it.
Blocks cannot be applied as representing building sections in a literal way
It is a simple thing. Why is it so hard to explain?
In a typical first year physics course (physics 101), these are some examples of simple problems involving blocks:
Inclined plane
Colliding blocks
The actual WTC 1 and 2 structures, on the other hand, look like this:
of which this is a small section:
What happens when two of these more complex structures are rammed against each other? Consider 2 collision scenarios: No gravity and gravity
1) Collision with no gravity (two pieces with this structure rammed together in empty space)
How will core columns ram into the opposing structure?
How will rows of perimeter columns ram the opposing structure?
How will flooring systems ram against the opposing structure?
If 2 equal structures rammed into each other in the absence of a gravitational field, one can expect roughly equal and opposite damage depending on how the contact surfaces collide.
2) Now gravity is added
In the case of the WTC towers the outside walls are observable. What can be learned of how the perimeter walls ram against each other by looking at the collapses? It can be known that for each of the 8 perimeter walls, the upper portion fell either outside or inside the lower portion.
(In fact, this can be verified know for WTC1: Upper north wall fell outward, upper west wall fell outward, upper south wall fell outward, north side of upper east wall fell outward, the rest of the east wall in unknown.
For WTC2: Upper east wall fell inward, upper west wall fell outward, east side of north and south walls fell inward, upper west sides of north and south walls fell outward.)
The ramming together of the flooring or core columns are, obviously, not visible from outside the structures. The way in which the floor units ram against each other is not too difficult to guess. The way in which the core columns would ram against the opposing structure has been the object of debate. Some say the core columns must inevitably ram against the opposing structure in a way that that column butts against column in a massive "whack". Our experience with rows of perimeter columns shows us that many columns can fall either outside or inside the the opposing structure. Also, it is possible for opposite sides of the perimeter to both fall outside the opposite structure. For example, in the case of WTC1 both the north and south walls of the upper structure fell outside the opposite structure. If many perimeter columns are allowed to bypass one another when rammed together, there is no reason to believe similar interactions cannot happen with the core columns as well.
How can one apply the simple physics of blocks shown in the physics 101 block images to the complex process of building sections ramming against each other?
If a person tries to apply basic physics with blocks, simple collisions and Newton's 3 laws to the WTC towers, they will naturally try to apply the equation F=ma to the complex interactions between the two structurally complex building sections.
They may reason thus:
"the upper portion looks like a large cube and so does the lower portion. Therefore I'll draw the upper and lower sections as 2 blocks and I'll attach force vectors to each block just like I would with inclined planes and simple collisions between blocks."
Then they may draw the complex interactions between upper and lower portions of the WTC towers as an interaction between 2 simple blocks like this:
Instead of realistically asking about the interactions between the complex surfaces of building sections consisting of perimeter, core and flooring, they will simply ask how one "block" manages to be supported by the other. Descriptions of the complex interactions between perimeter and perimeter, or between flooring and flooring, or the complex behavior of core columns acting as spears are never addressed. Instead, the building is considered as one simple block resting on top of another simple block exactly as the physics 101 block systems shown earlier.
With such simplfied concepts, they may reason thus:
Chandler:
"It is difficult to imagine how an upper block exerting a force of only 36% of its static weight could crush the larger, stronger undamaged lower section of the building to the ground, when the building, at any level, was designed to support several times the weight above it."
or like this:
Chandler:
"Assuming a safety factor of between 3 and 5, the observed acceleration implies that close to 90% of the strength of the lower section of the building must have been eliminated by forces other than the supposed "pile driver", suggesting that some sort of controlled demolition was at work."
But once the upper section is set in motion, through what physical mechanism is force transferred between upper and lower sections? What is actually pushing against what?
Before motion begins, when the buildings are intact and standing, the columns of the upper section are pushing against the columns of the lower section. But once the building is in motion, columns can no longer push against columns. So what is now pushing against what? Would anyone reasonably expect the safety factor of between 3 and 5 to be maintained once the building is in motion? If columns can no longer push against columns, no "safety factor" can be maintained.
Factor of safety is a property of the supporting columns. The term can have no meaning once a building is in a dynamic (moving) state, in which columns can no longer directly push against columns.
1-D block models do not reflect the complex interface between the portions of the actual towers undergoing collisions. The real towers are a complex mesh of perimeter, flooring and core. The upper and lower plates of each perimeter wall collide in a complex way which cannot be accurately reflected in a simple 1-D block model. Floor on floor collisions are also highly complex, with specific weak nodes in their connection strength to core and perimeter columns. This complex behavior also cannot be expressed in a simple 1-D model with an upper block and a lower block.
The single common misunderstanding with 1-D approaches is the reckless abandonment with which various 1-D scenarios are taken to represent real building qualities of the WTC towers.
1-D models and complex 3-D actual behavior are interchanged with impunity.
The people that jump back and forth between their 1-D model and actual complex behavior do not seem to see the need to qualify their statements accordingly, and leave it to the reader to do the actual work of verifying the claim.
If I were the author of one of these block papers and I began to apply qualities of a simplified 1-D model to real WTC building behavior, I would feel the need to justify the use of 1-D qualities to the very complex collapse being considered. I wouldn't ask the reader to just assume I am right in doing so with no qualifying argument.
The author cannot take it as a simple given that 1-D simplified models reflect real world behavior and leave it to the poor reader to untangle that mess for them. If the author of any paper cannot clearly explain why they feel justified in ascribing 1-D attributes onto the complex nature of the WTC towers, then they shouldn't expect the reader to figure that out for them.
A person cannot hop between simplified 1-D and the complex mass flows possible in 3-D as if they are skipping rope. Yet a free application of simple 1-D behavior to the complexity of the actual mass flows forms the basis of most of the variants of CD claims or 'proofs' by way of alleged violations of physics, like:
The collapses were too fast
they shouldn't have accelerated
there should have been a jolt
the resistive force was less than the capacity
the 3rd law requires equal and opposite destruction
Crush down happens before crush up
Assumption of homogeneity
These are all products of 1-D models but they are indiscriminately applied to the real WTC towers by various authors without any valid explanation as to why they can be applied.
In pretty much every case, the author of the claim assumed the 1-D quality could be applied to the actual towers and considered the assumption to be so self-evident that the reader is offered no explanation for the literal application.
1-D block descriptions cannot explain 2 stage processes. In the case of ROOSD scenario described in part 2.1 of this book, global mass flow is explained as the flooring being stripped away from the core columns and the inside of the perimeter. In what way can 1-D block set-up represent a ROOSD process? It cannot. A block set-up ignores the possibility that column strength of the lower portion was simply bypassed through a stripping process.
Or consider how an complex collision between upper and lower portions can dislodge enough debris within the OOS regions to allow for an inependent local ROOSD process to begin. How can this possibility be addressed with a simple 1-D, 2 block model?
1-D block descriptions of the WTC towers collapsing do not take into account a realistic collision interface between the two building sections. The buildings clearly do not collide while maintaining a homogenous interface.
page 10:
"Explicitly invoking Newton's Third Law puts this result in another light. Since the forces in the interaction are equal and opposite, the falling block exerts a force of only 36% of its weight on the lower section of the building. In other words, as long as the falling block is accelerating downward we have the counter-intuitive result that the force it exerts on the lower section of the building is significantly less than its static weight. It is difficult to imagine how an upper block exerting a force of only 36% of its static weight could crush the larger, stronger undamaged lower section of the building to the ground, when the building, at any level, was designed to support several times the weight above it. Assuming a safety factor of between 3 and 5, the observed acceleration implies that close to 90% of the strength of the lower section of the building must have been eliminated by forces other than the supposed "pile driver", suggesting that some sort of controlled demolition was at work."
The "strength" of the building was bypassed and left standing or peeling.
