The 9/11 Forum

Dear 911forum readers:

Over the past few days I have been engaged in a "hot and heavy" debate with some of the authors of the recent paper by Harrit el al. The debate started out on-topic but soon shifted to a discussion of the collapse of WTC 1 as viewed by myself, David Benson, Bazant, etc, vs. the views expressed in well-known papers/videos by Tony Szamboti, Frank Legge and David Chandler.

Many of the issues have been kicked around a lot on some of the threads on this Forum. However, I thought it would be useful to present this latest round of discussions:

The set of e-mails begin with Frank Legge's most recent e-mail to me. (Yes, there are two Franks involved in these e-mails so I have tried to make it clear which one is me):

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From Frank Legge to Frank Greening:

You say: "I would say that Chandler's slight of hand is the implied notion that Newton's 3rd Law is universally applicable"

I previously said that it would be a bold position to refute Newton's third law. Now you have done it. Your courage amazes me. How about writing a paper on that? No, I would advise you against that. I suggest you just try to figure out Chandler's logic and save yourself embarrassment.

You say: "Chandler argues that there was (obviously!) no resistance offered by the lower section of WTC 1, only some mysterious force that made the upper block move downwards with a uniform acceleration of 64 % of g."

No he doesn't, and the upward force is not mysterious. The only thing mysterious is why the upward force should be so small, about 10% of the design strength of the columns. If you don't rush too quickly to print I am sure you will be able to work out what could have made the columns so weak. I trust you will enjoy your epiphany.

Frank (Legge)

Frank Greening wrote:

Frank (Legge) and Gregg,

To save me the trouble of answering your questions twice, I will respond to you both with this:
As we all know, Newton's 3rd Law states that to every action there is an equal and opposite reaction. From this universal Law Mr. Chandler concludes that there must have been no net force acting between the lower (stationary) section of WTC 1 and the upper descending block because the lower section must (always) provide an equal and opposite reaction force to any downward accelerating force. However, because there was indeed an observed uniform downward acceleration of the upper block, Chandler argues that there was (obviously!) no resistance offered by the lower section of WTC 1, only some mysterious force that made the upper block move downwards with a uniform acceleration of 64 % of g.

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.

Think of it this way: suppose I place a concrete block on a tall cardboard box, and let's say that the box can just support the concrete block. Now I pour some water over the cardboard box making it all wet and soggy. Pretty soon my little construct will collapse as the cardboard box looses strength and is no longer able to support the weight of the concrete block.

Have I violated any of Newton's Laws? I don't think so!

Another worthwhile analogy would be a tower made out of wooden dominoes weakly glued together at each end. Now suppose I douse some of the dominoes near the top of the tower with gasoline and set them on fire. After a while the gasoline-soaked dominoes would burn through and start to crumble. At some point the block of dominoes above the fire-affected zone would drop down on to the section below this zone. I feel confident in predicting that the motion of the upper block could initiate a total collapse of the entire structure.

Again, I have to ask, have I violated any of Newton’s Laws?

Now Gregg, you claim that Chandler is correct because, in your words, “The lower surface of the upper block and the upper surface of the lower block are not moving up or down relative to each other, hence the forces between them are equal.” This is totally irrelevant! The fact is that the roof of the tower is descending with a finite acceleration! What do you think causes this? I would say that the columns and/or the column connections in the damaged/fire-affected zone have lost enough of their strength so that the upper block can no longer be fully supported by the weakened columns. Thus the upper block of floors moves downward relative to the base of the tower. Is this such an outlandish proposition?

Are you familiar with the verinage technique of building demolition? In this technique hydraulic rams, placed on an upper floor of a building, are used to push columns over causing the entire block of floors above the critical floor to drop onto the structure below. When done correctly this causes the entire building to collapse. Now you guys would tell me that this is impossible, so may I suggest you check out this video and follow the collapse sequence frame by frame.

http://www.linternaute.com/video/57824/ ... -batiment/

So, to recap: Newton’s Laws apply to the external forces acting between interacting bodies in closed systems. Newton’s 3rd Law does not apply to the internal forces causing an open-structured body to collapse in on itself. Thus, during the collapse of a building, elastic strain energy and gravitational potential energy are being released and converted to kinetic energy and/or heat. That is what needs to be discussed and evaluated, not Chandler’s video of disinformation!

As for the “missing jolt” of MacQueen and Szamboti, I have already discussed this at length on the911forum.freeforums. Sure, Bazant and Zhou’s model may lead one to expect a “jolt” at the first impact of the upper block on the lower structure, but as I keep saying, the collapse was not that simple! First of all, as in the verinage of the building in France, two floors were probably involved in collapse initiation. The build up of KE over a drop of 2 x 3.7 meters would have been tremendous. Secondly, there was some tipping motion of the upper block of WTC 1, which means that the first impact was not sudden, but spread out over perhaps ½ second. This has the effect of smoothing out any velocity changes so that the net effect was an apparent constant acceleration of about ¾ g over the first few seconds of collapse.

Sincerely, Frank G.

P.S. Are we ever going to get back to discussing thermite, red chips and microspheres!

----- Original Message -----

From: Frank Legge
To: Frank Greening
Cc: harrit ; Gregg Roberts ; James Gourley ; Steven Jones ; Kevin Ryan ; Niels Harrit ; Jeffrey Farrer ; Danny Farnsworth ; Brlbu

Sent: Saturday, April 18, 2009 11:56 PM
Subject: Re: important criticism by Dr. Greening

To: Frank Greening,

In the case of the Missing Jolt paper, even if there was an error in the calculation of energy consumption of 50% would not the reduction in velocity still have made a very visible deviation in the velocity graph?

Even if we make the absurd assumption that there is no energy lost whatsoever due to compression of the steel, elastically and plastically, and in pulverization of the concrete, there is still the undeniable effect of conservation of momentum to be considered as the upper block encounters the inertia of each floor. You showed this yourself in an early paper. That reduction alone would show a visible deviation in the velocity graph, as illustrated in the graph below.

No such deviation is observed, hence there was no impact, hence something else is destroying the structure.

In the case of Chandler's video you are mistaken in describing his second force diagram as showing the top block "resting". Of course you didn't mean this - it was just an unfortunate choice of words - you are aware that this diagram indicates the forces exerted by the upper and lower portions of the building on one another as the top falls and accelerates through the bottom. If you say they are not exerting equal and opposite forces on one another you are denying the truth of Newton's third law - a very bold position to take.

I am sure you are not denying Newton's third law so must be confused about what Chandler's earlier diagram is are implying. I suggest you take his second diagram, undeniably valid, and work back from there in order to understand what Chandler was getting at.

Again the smoothness of his velocity graph indicates that no impact can be detected, hence something else was destroying the structure.

My assessment of this discourse is that this is your last chance to admit error and redeem yourself. If you do not, and in view of the fact that I regard you as highly intelligent, I will be forced to consider whether your arguments are merely sleight of hand for the purpose of deception.

You will of course not be concerned about what I might think of you, and if you continue your present efforts you can be sure that some very influential people will think highly of you. You might be very pleased with that but a lot of people would not. The choice is yours.

Kind regards
Frank Legge

Frank Greening wrote:

Frank (Legge):

First of all I have discovered a serious computational error on page 26 of MacQueen & Szamboti’s paper that changes his estimate of the velocity decrement considerably. I pointed this out to the authors in a post on the911forum.freeforums and Mr. Szamboti immediately disappeared from the site and started posting on the less technically focussed JREF site.

However, I also have problems with the way the energy consumption is, more or
less, "doubled-up" in the M&S paper -without much justification I might add - to supposedly allow for a
non-rigid upper block.

I say, go ahead and do this, but may I remind Mr. Szamboti to halve his spring constant because

Ktot = 1/[1/k1 + 1/k2].

If, as a first approximation, we take k1 = k2,

Ktot = k1/2

Now since the buckling energy of a "two-spring" system is given by:

Eb = Energy to compress k1 + Energy to compress k2

We wind up with Eb = Constant x (k1/2 + k1/2) = Constant x k1,

which is the same as the buckling energy in a "one-spring" model!

In the end, it's the single interaction between an upper and lower block
that matters. Hence there is no need to get hung-up on the upper block as a
neglected energy sink. It's much better, (and more realistic!) to work with
ONE impact and ONE energy sink .....

However, until M&S correct their computational error I really have nothing more to say about that paper.

I am quite amazed at the volume of comments generated by Chandler’s WTC 1 video on web forums such as 911Blogger, JREF, and LooseChange, to name but a few I have checked out. I think this level of interest stems from the outrageous claims being made by Chandler. Unfortunately, for most viewers of the video, I suspect that the contorted arguments presented by Chandler, which may sound convincing to some, serve only to confuse, bother and bewilder most of us! So I would like to have a go at critiquing Chandler’s presentation to see if I can offer a simple explanation as to what is wrong with his analysis.

First of all, Chandler measures a uniform downward acceleration of 6.31 m/s^2 for the upper block of WTC 1 and correctly states that this is equal to 64 % of g. From this observation Chandler concludes that during the collapse of WTC 1 there was an upward, resistive force of 36 % of W, where W is the weight of the falling section of the building.

At this point in his presentation Chandler gives a diagram of the upper block showing a force W acting downwards together with a force equal to 0.36W acting upwards. This diagram therefore correctly indicates that there was a net downward force of 0.64W acting on the upper block.

I would say that this diagram should have ended Chandler’s presentation but instead he now pulls a sleight of hand by showing a new diagram of the upper block resting on the lower section of WTC 1 with a force of 0.36W acting upwards (ok!), AND a force of 0.36W acting downwards, (not ok!). I say this downward force is not ok because Chandler has already shown that the downward force was W, not 0.36W. The net force of 0.64W explains why the upper section was accelerating relative to the lower, immovable section at 64 % of g.

But let’s be very clear on this one point: if Chandler’s new diagram correctly represented the forces acting inside WTC 1 during its collapse, the upper section would not have been capable of motion relative to the lower section since there would have been no net force acting between the sections. This, of course, is the situation in a stable, undamaged building. However, WTC 1 was not stable but quite severely damaged! Furthermore, only if the entire building was accelerating at 36 % of g, (somewhere in outer space perhaps!), would the forces between the upper and lower sections be 0.36W.

I think Chandler’s confusion, (or deliberate obfuscation!) arises from the fact that he is only considering forces acting within a tower where no parts are moving relative to one another. Obviously, if you make a horizontal cut at any height along such a static structure, the downward force acting on the lower section must, by Newton’s 3rd Law, be balanced by the upward, reactive force acting on the upper section. However, everything changes if you consider a tower where some parts are moving relative to other parts – something that must happen in a collapsing building! The very fact that the upper section of WTC 1, once it started to descend, appears to weigh less than Mg, is simply because it is accelerating downwards. (Try standing on a weigh scale in an elevator!) In fact, if the upper section happened to be accelerating downwards at precisely g, we would say it was “weightless” and there would truly be no reactive force. However, because the upper block was accelerating at less than g, there must have been a reactive force even if Chandler tries very hard to hide it!

Now, as for your paper, Frank (Legge), on the collapse acceleration of WTC 1, I believe everyone is getting too hung-up on trying to “disprove” Bazant’s first 9/11 paper, (the one written with co-author Zhou). I get the feeling that you guys think that if you are able to discredit Bazant’s work you have somehow proved that the Twin Towers were brought down by CD. This is a serious misconception on all your parts! That first paper by Bazant, (and my “Energy Transfer” article), were first stabs at developing approximate models of the collapse of WTC 1&2 that provided computational methodologies and allowed some characteristics of the collapse dynamics to be quantified. But a lot more work has been done since then, and a lot more work is still needed!

For example, at the present time I am working on a computer program that predicts the rate of column buckling after first motion. The program allows one to determine the velocity of the upper block up to the moment of its impact with the floor below. The impact velocity looks to be about 6 - 7 m/s. I have also been comparing the calculated results with some excellent (high precision) measurements of the drop vs. collapse time for WTC 1 taken from the well-known Sauret video by a guy who calls himself "OneWhiteEye" on the911forum. I believe these measurements are accurate to better than 0.5 meters for the first 3.5 seconds of collapse motion and are far superior to the measurements reported by Mr. Szamboti or by you Frank! More significantly, OneWhiteEye’s measurements show that the initial motion of the upper block of WTC 1 was exponential with the drop distance ~ 0.015e^2t for the first 2.5 seconds. This is hardly consistent with an “explosive” demolition.

In conclusion let me add that we are supposed to be discussing THERMITE! Unfortunately the mysterious Prof. Harrit has taken us well off topic with his comments about my knowledge of Newton’s Laws. That’s his prerogative I guess, but I find it quite telling that after venturing out from his ivory tower to throw a few stones at me, the learned Professor simply disappears, leaving his surrogates to respond to my rebuttal of his bluster….

Is this how academics normally behave at the University of Copenhagen? If so, I must say:

“Something is rotten in the state of Denmark”!

Hamlet Act 1, Scene 4

----- Original Message -----

From: Frank Legge
To: Frank Greening
Cc: harrit ; Gregg Roberts ; James Gourley ; Steven Jones ; Kevin Ryan ; Niels Harrit ; Jeffrey Farrer ; Danny Farnsworth ; Brlbu
Sent: Saturday, April 18, 2009 11:04 AM
Subject: Re: important criticism by Dr. Greening

Frank (Dr Greening)

Following on from Prof Harrit's remark I quote the Abstract to your paper with Bazant et al, revised 31 March 2008.

"... it remains to be checked whether the recent allegations of controlled demolition have any scientific merit. The present analysis proves they do not."

Firstly consider the general appearance of the videos.
http://www.911research.com/wtc/evidence ... lapse.mpeg

Do the videos not show that the top of WTC 1 disintegrates first, allowing the roof to fall with a steady acceleration for about half its height before the lower unheated, undamaged section of the building starts to give way? Does this not mean that the impact which the falling block delivers must be much less than used in your calculations? If so, does this not mean that any conclusion you may have arrived at regarding the ability of the falling block to destroy the lower section is unfounded? If the top section is not a solid block but is disintegrating at several levels, as the videos show, how can the material of the top deliver any significant impact? Yet we see the bottom section starting to disintegrate and fall out of the way of the falling roof. Why would it do that if there is little or no impact?

Now consider the scientific analyses which have been presented of the motion of the roof. I do not know how many such studies there are and will use just three, starting with my own.

1. http://journalof911studies.com/articles ... ion_20.pdf
This is early work and a bit rough. I have since recalculated the acceleration several times and see an acceleration of the roof of about 7m/s/s. The essential point to note is that the acceleration is almost exactly uniform right from the start until the roof goes out of sight in the dust. The rate is significantly less than free fall, but as we assert that the fall is caused by explosives there is no need for it to have any particular rate, the rate being determined by the timing of the detonation sequence. I suggest that the rate was chosen so that the debris would fall a little faster than the explosion front, thus obscuring the line of explosives. This was successful most of the time but the first three can be clearly seen as they occurred before substantial debris had been produced.

2. Next we look at the work of MacQueen and Zamboti, the Missing Jolt paper.
http://journalof911studies.com/volume/2 ... gJolt7.pdf

They converted their data to show the roof velocity. Uniform acceleration will of course produce a straight line velocity graph. It is possible to calculate when the falling top block should hit the lower structure.

You make calculations to determine the initial potential energy and hence kinetic energy at the moment of arrival of the falling top block onto the lower portion of the building. You base your case on the adequacy of the kinetic energy to do various things, like pulverize the concrete and destroy the steel structure. Do you agree that if the falling block is to do these things on impact it must give up kinetic energy? Do you agree that if a falling body gives up kinetic energy on impact its acceleration will sharply decline? If the acceleration sharply declines there will of course be a deviation in the velocity graph. The authors found no such deviation. Does this not mean that there was no impact? If there was no impact does this not mean that the lower portion gave way due to some other cause?

3. David Chander has provided a summary of his views about WTC 1 in this video.

http://www.youtube.com/watch?v=xG2y50Wyys4

David reminds us that the net force on a body can be deduced from its acceleration. David shows that the supporting force the bottom is exerting on the falling top is about one third that of gravitational force and that, due to Newton's third law, that is also the force which the falling block will be exerting on the lower section. Given that the tower was designed to be over three times as strong as needed to stand up against gravity, this shows that the lower section was exerting only about 10% of its design strength. Clearly this is impossible unless something is turning it into mush.

The first of these three papers was written in 2006 and your paper was revised in 2008 so it should have taken this evidence into account. You would have been wise to withdraw your name from the paper. However perhaps you missed it. But now we have three studies showing that there was no discernible impact of the top block on the lower portion of the building, all drawn to your attention. Do you acknowledge that you have made a mistake? Will you be posting corrections to your previous statements?

Frank Legge

Frank Greening wrote:

Dear Prof. Harrit,

I do not believe that any of my work "violates" any of Newton's Laws.
Besides, I mostly use an energy analysis.

In any case my "Bible" is "Elements of Newtonian Mechanics" by J.M. Knudsen
and P.G. Hjorth. I very excellent book.

These guys are at the University of Copenhagen, ..... perhaps you know them?

Frank

----- Original Message -----

From: harrit
Cc: "Frank Greening", etc, etc .....

Sent: Friday, April 17, 2009 6:01 PM
Subject: Re: important criticism by Dr. Greening

Frank

As for Frank Greenings mistakes I could suggest violation of Newtons
second and third laws when being a co-author with Bazant on the
pile-driver theory.
That is some kind of mistake, I would say.

Niels Harrit

Re: 3rd law. See Bazant & Le, ASCE J. Engg. Mechanics, Oct 2008.

A member at STJ911 used a similar line of argument (based on not understanding basic physics)with me regarding the conservation of momentum. I suggested that this member check with Tony Szamboti, Greg Jenkins or Frank Legge (all members) whom I assumed could help explain. To my amusement I found out that the member WAS Frank Legge.

A fascinating back-and-forth, thanks for sharing this. And thanks for the good review of the displacement data.

Even more interesting is this:

For example, at the present time I am working on a computer program that predicts the rate of column buckling after first motion. The program allows one to determine the velocity of the upper block up to the moment of its impact with the floor below.

Any details or results from this would always be most welcome.

PS what is the distance of initial buckling descent before impact?

Just received this from David Chandler.

Hmm ............... I guess he missed the gist of my argument ....

Anyway here it is, (unedited!); hope it's not too confusing because Mr. Chandler mixes his comments in with my comments from my e-mail to Harrit and Jones. I have placed some initials to clarify who said what:

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DC:

I thought Greening was a chemist and chemists are supposed to know physics. The comments in this letter are beyond belief. I'm ready for a difference of opinion, but I'm not prepared to have a rational discussion with someone who denies Newton's Laws in a non-relativistic, non-quantum mechanical realm.

Frank Greening wrote:
Frank and Gregg,

To save me the trouble of answering your questions twice, I will respond to you both with this:

As we all know, Newton's 3rd Law states that to every action there is an equal and opposite reaction.

DC:

This is a commonly misunderstood concept by students and also some textbook authors. I prefer to put it in these terms:

"All forces are actually interactions. Whenever object A exerts a force on object B, object B exerts and equal but opposite force on object A."

It fails to hold in relativistic electrodynamics, but interestingly conservation of momentum, which is derived from Newton's 3rd law does continue to hold, so ultimately it is a more fundamental statement of the principle. Nothing approaching relativistic conditions is at work here, however, so I will stick with the applicability of Newton's 3rd Law.

Dr. G:

From this universal Law Mr. Chandler concludes that there must have been no net force acting between the lower (stationary) section of WTC 1 and the upper descending block because the lower section must (always) provide an equal and opposite reaction force to any downward accelerating force.

DC:

You clearly did not pay attention to the argument. I never made any such claim. Consider for a moment a free body diagram of the upper block. I said that since the NET force is in the same direction as the acceleration, it must be downward. There are two forces acting: gravity downward and the normal force from the lower section of the building acting upward. Since the net force is downward, the upward force must be less than the downward force (i.e. the weight) acting on the upper block. Newton's law kicks in when we turn our attention to the lower section of the building. The downward force exerted on the lower section of the building must equal the force it exerts upward on the upper block: substantially less than the weight of the upper block. Your comment constitutes a flat-out misunderstanding of what I said.

Dr. G:

However, because there was indeed an observed uniform downward acceleration of the upper block, Chandler argues that there was (obviously!) no resistance offered by the lower section of WTC 1

DC:

Bullshit.

Dr. G:

, only some mysterious force that made the upper block move downwards with a uniform acceleration of 64 % of g.

DC:

Bullshit. What I said was perfectly clear. Go back and watch the video a few more times.

Dr. G:

I would say that Chandler's slight of hand...

DC:

Bullshit.

Dr. G:

is the implied notion that Newton's 3rd Law is universally applicable, even to a collapsing building.

DC:

Newton's 3rd Law IS (!!!) universally applicable except for the relativistic exception I mentioned above. You are revealing that you are not competent in basic physics. This reminds me of the famous paradox of a horse pulling a cart. Since the horse and cart exert equal and opposite forces on each other they shouldn't be able to move. The paradox arises from a misunderstanding of Newton's 3rd law. This may be the same misunderstanding that is trapping you here (or maybe not...It's hard to trace all the implications of an irrational argument.) If you need me to explain the paradox for you, just ask.

Dr. G:

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.

DC:

Bullshit. When a running football player is blocked, the one who slows his progress is also accelerated in the interaction. When a falling section of the building stresses the columns beneath it, the columns push back. Since the columns were designed to support more than the weight of the upper section, the push-back force should be greater than the downward force of gravity so the net force should be upward. This should decelerate the upper block. It is the deceleration of the falling block that is observationally missing in this case. (Go reread Tony Szamboti's and Graeme MacQueen's Missing Jolt article, which is very parallel to mine.) Newton's Laws apply in every detail.

Dr. G:

Think of it this way: suppose I place a concrete block on a tall cardboard box, and let's say that the box can just support the concrete block. Now I pour some water over the cardboard box making it all wet and soggy. Pretty soon my little construct will collapse as the cardboard box looses strength and is no longer able to support the weight of the concrete block.

DC:

Except in our case, the lower section of the building is undamaged below the plane impact point prior to the collapse.

Dr. G:

Have I violated any of Newton's Laws? I don't think so!

Another worthwhile analogy would be a tower made out of wooden dominoes weakly glued together at each end.

DC:

We are arguing the pile driver theory here, acting to destroy the columns, not the pancake collapse. NIST already rejected pancakes.

Dr. G:

Now suppose I douse some of the dominoes near the top of the tower with gasoline and set them on fire. After a while the gasoline-soaked dominoes would burn through and start to crumble. At some point the block of dominoes above the fire-affected zone would drop down on to the section below this zone. I feel confident in predicting that the motion of the upper block could initiate a total collapse of the entire structure.

DC:

The problem here is the lower section of the building was designed to support a larger load than the weight of the upper block. Therefore, when the upper section comes crashing down, the upward force should be greater than its weight, so the net force should be at least momentarily upward, and it should decelerate. Furthermore, you are back to describing pancake-like floors without any substantial vertical support.

Dr. G:

Now Gregg, you claim that Chandler is correct because, in your words, “The lower surface of the upper block and the upper surface of the lower block are not moving up or down relative to each other, hence the forces between them are equal.”

DC:

I would correct Gregg on this point, if this is actually what he said. The relative forces are equal because of Newton's 3rd law regardless of any other condition whatsoever. It doesn't matter what's moving or not moving. If two objects are interacting, the forces between them are equal and opposite at every instant.

Dr. G:

This is totally irrelevant! The fact is that the roof of the tower is descending with a finite acceleration! What do you think causes this?

DC:

The real answer here is that the supposed pile driver is disintegrating before our eyes, and so is the rest of the building. Observationally, there is no pile driver.

Dr. G:

I would say that the columns and/or the column connections in the damaged/fire-affected zone have lost enough of their strength so that the upper block can no longer be fully supported by the weakened columns. Thus the upper block of floors moves downward relative to the base of the tower. Is this such an outlandish proposition?

DC:

Yes, if you think it can avoid Newton's laws in the process. Why don't you draw your own free body diagrams and do a real analysis step by step instead of relying on analogies to guide your thinking?

Dr. G:

Are you familiar with the verinage technique of building demolition? In this technique hydraulic rams, placed on an upper floor of a building, are used to push columns over causing the entire block of floors above the critical floor to drop onto the structure below. When done correctly this causes the entire building to collapse. Now you guys would tell me that this is impossible, so may I suggest you check out this video and follow the collapse sequence frame by frame.

http://www.linternaute.com/video/57824/ ... -batiment/

DC:

It just so happens that I did exactly this video analysis on, I believe, exactly the same building you refer to here. Guess what happens? When the top section crashes into the bottom section it decelerates. By the way both sections disintegrate simultaneously. No need for "crush up" to wait for the completion of "crush down"!

Dr. G:

So, to recap: Newton’s Laws apply to the external forces acting between interacting bodies in closed systems. Newton’s 3rd Law does not apply to the internal forces causing an open-structured body to collapse in on itself.

DC:

Bullshit!!! Total absolute bullshit!!!!!!!! I can't believe I'm reading this from someone who claims to be a scientist. You get an F in my class.

Dr. G:

Thus, during the collapse of a building, elastic strain energy and gravitational potential energy are being released and converted to kinetic energy and/or heat. That is what needs to be discussed and evaluated, not Chandler’s video of disinformation!

DC:

I'm supposed to reply to that politely? %&#%#&^^*^%&^

--David Chandler

My reply:

..........................................................................................
David:

You quoted me:
I would say that the columns and/or the column connections in the damaged/fire-affected zone have lost enough of their strength so that the upper block can no longer be fully supported by the weakened columns. Thus the upper block of floors moves downward relative to the base of the tower. Is this such an outlandish proposition?

To which you responded:

Yes, if you think it can avoid Newton's laws in the process. Why don't you draw your own free body diagrams and do a real analysis step by step instead of relying on analogies to guide your thinking?

Well, here's my reply:

David, you cannot deny that an upper block of floors moved downward relative to the base of the tower.

And yet you consider this to be an outlandish proposition!

It looks to me like you are the one having trouble explaining what was actually observed that day!

I think you need to get out of your classroom more often and take a look at real world problems.

Newton's 3rd Law applies to bouncing billiard balls not the interiors of collapsing buildings ........

DAVID: During the collapse of a building, elastic strain energy and gravitational potential energy are being released and converted to kinetic energy and/or heat.

Please explain to me how these processes are covered by Newton's Laws!

But these phenomena are what need to be discussed and evaluated, not your classroom physics vainly trying to apply rigid-body Newtonian mechanics to processes such as crushing, buckling, fracture and comminution.

DC wrote:All forces are actually interactions. Whenever object A exerts a force on object B, object B exerts and equal but opposite force on object A.

He is correct about this. The law should be stated as equal and opposite reaction forces. It applies to bodies in motion and collision. Two bodies in collision will exert equal and opposite impulses on each other at the contact interface. That really says little about the resulting motion, which depends on the net force on each object, obviously.

However, I believe he's inappropriately applying d'Alembert's principle and overlooking losses to internal degrees of freedom in the upper and lower block. It's hard to blame him since part of the controversy is over the rigidity of the upper block. I understand B&L treats this, but not having worked through it myself, I can't comment.

More as I gather my thoughts.

Slightly OT for a moment but renders the issue nearly moot for me. Two things, nothing new:

- Refusal to recognize that tilting occurred and how it can affect the failure dynamics, both in spreading energy dissipation out over time and reducing the amount of energy needed to induce failure.

- Insisting on axial impacts and optimal loading of the structure below. Capacity is reduced by misalignment. It has to be!

Ignoring these factors in an energetic model is one thing, ignoring them when examining displacements is quite another. Until these issues are addressed, discussion of jolts are purely academic.

Newton's third law applies to the point of contact between two bodies. It is only d'Alembert's principle that says distance between points in a rigid body remains constant. Real bodies are not rigid. Ignoring deformation of the upper block is questionable, and I have indeed questioned the validity of treating the upper block as rigid body in the work of Bazant et al. I'm assuming that B&L actually did deal with this issue of upper block rigidity correctly and that two internal degrees of freedom (at a minimum) - top and bottom of crush front - are not significantly different from one. Haven't worked it out myself, so no comment.

That being the case, I'm not defending any particular theory, only examining the issue of Newton's third and jolts.

Consider a model consisting of discrete masses connected vertically in series by overdamped springs (like spring and shock absorber in automobile suspension), where the bottom is fixed and the springs can break. A spring is removed and the upper block descends through height = story_height - crushed_height, at which time a fully inelastic collision between end masses occur. If one takes the collision as instantaneous, no spring displacement occurs during collision and the final momenta and KE of the contacting masses are the same and half that, respectively, of the bottommost mass of the upper block just prior to collision. Call this time immediately post-collision t0.

At t0, the rest of the upper block is still moving down with the initial speed of the fall while the contacting masses have half that speed, below them is still stationary. From here, I'm sure it's no problem to set up a free-body diagram and solve for the displacement over time of the topmost mass. Eh?

Here's a much simplified arrangement with only two masses in one block, fixed at both ends, no damping, no breakage, no gravity:

http://farside.ph.utexas.edu/teaching/336k/lectures/node114.html

Looks like the 110 story model WITH all the above might be a bit of a drag to set up and solve.

Without working out the details, I think the above 1D model has a better shot of describing the motion of the 'roofline' than does a Bazantine model, which was not crafted with roofline displacement specifically in mind. An impulse at one end of a deformable body does not translate to the same acceleration over the entire structure.

OneWhiteEye wrote:Without working out the details, I think the above 1D model has a better shot of describing the motion of the 'roofline' than does a Bazantine model, which was not crafted with roofline displacement specifically in mind. An impulse at one end of a deformable body does not translate to the same acceleration over the entire structure.

Good! You start to learn the basics. Upper part C consists of many elements, joints and masses where the elements deform at impact with part A, of similar nature but more elements. And it can be studied in 1-D.

Only rigid part in this system is the ground, but you can be sure that it also absorbs energy at impact C/A. But let's assume that ground does not displace at impact C/A, so we can use it as base.

So let's connect 97 springs on top of each other with the bottom one at ground and the top one at level 97. Between each spring is a mass m and one of top (total 97 m). The spring parameters are adjusted to permit same static stress/compression (bottom spring carries 111 m, spring at level 97 15 m. This is lower part A. Its top mass m is the floor to be impacted.

Upper part C level 98 and up consists of 13 springs with mass m between them and one on top - the roof line and one on bottom - the hammer floor. This is upper part C. Also these springs are adjusted to carry 13 to 1 m.

Note that roof line m is only connected to its spring below that only carries 1 m.

Note that spring #98 between levels 97/98 is missing - the one that could carry 14m!

Now we accelerate complete part C, so its hammer floor (#98) impacts, with velocity v = 8.52 m/s, the top floor (#97) of part A.

Before impact part A is decompressed to carry 14 m less than design, e.g. A spring #1 carries 97 m in lieu of 111 and A spring #97 1 m in lieu of 14 m. Before impact all part C springs are decompressed to zero compression/stress. They are under free fall!

Each m in part C has potential energy E=mv²/2 at impact, when C floor #98 contacts A floor #97.

So, at impact C/A only E is applied at contact A/C and not 14 E as assumed by Benson (and Bazant) in their simplified model (no springs in part C).

The energy E of the roof m of part C is just applied to the spring below. Therefore top roof m will not decelerate from speed 8.52 m/s so fast as any other m below. &c. But you can be sure that it will slow down = jolt!

And that's one of the reasons, why part C can never one-way crush down part A in this model.

The first spring to break in this model due to big E I assume is C spring #98.

You just have to check the stresses in all springs after C/A impact.

Dr. G wrote:Dear 911forum readers:


Another worthwhile analogy would be a tower made out of wooden dominoes weakly glued together at each end. Now suppose I douse some of the dominoes near the top of the tower with gasoline and set them on fire. After a while the gasoline-soaked dominoes would burn through and start to crumble. At some point the block of dominoes above the fire-affected zone would drop down on to the section below this zone. I feel confident in predicting that the motion of the upper block could initiate a total collapse of the entire structure.

Again, I have to ask, have I violated any of Newton’s Laws?

LOL!

No, at every contact between a fire damaged domino (sub part of part C) and the structure below (part A) Newton's third law apply. But a small loose domino cannot damage an intact structure of glued together dominoes. The same applies to an assembly of glued together dominoes (part C) that you drop on a similar but bigger assembly (part A) of glued together dominoes.

If all dominoes in parts C and A have same mass and the strength of all joints between dominoes are exactly the same (two joints - one at each end), what happens at contact C/A?

Normally C bounces on A. It's a big jolt!

And if C does not bounce on A. What happens? Well, equal amounts of dominoes/joints in both parts will be affected/broken. And as A>C, C will be broken in pieces before A. And broken pieces of C will not damage A further.

Google on Björkman's Axiom for structures and you'll understand why!

Heiwa wrote:Good! You start to learn the basics.

Actually, I did know the basics well before posting the above. Didn't really think about it much until after 9/11 but, if I had, I would have said the same thing at any time in the last couple of decades, or more. Now was just the time to say it.

Maybe you think I didn't know these basics before because of my solid mechanics sims, but you must have forgotten or not noticed when I said that the invisible joints were just very stiff springs. I need to post an animation with a close-up view of a tower base when it gets hits from above with an upper block. The floors all displace downward, just as expected. But the structures are brittle, like glass, and do not have a plastic deformation phase as of yet. Still, an elastic joint that fails at a high enough peak force can absorb the same energy in failure as a lossy elastic/plastic joint with lower yield and ultimate strength. Correct?

Therefore it will not decelerate so fast as any other spring. &c.

I'm with you all the way through this point, no argument at all.

And that's one of the reasons, why part C can never one-way crush down part A in this model.

And this is where we part company. I don't see how this necessarily follows from everything preceding it. Not denying what you say as it pertains to this particular slab-spring model, I just don't know, and it would take solving this monstrosity analytically or at least numerically to convince me. Maybe some people would have no problem dealing with this system, but it looks like a bear to me. That's why I resort to the solid mechanics simulations!

The first spring to break in this model due to big E I assume is C spring #98.

Most reasonable to assume, but still only an assumption until the actual dynamics are worked out. You may be right, for this model and its parameters. Qualitative arguments might push me into accepting it, but they would have to be very good.

You just have to check the stresses in all springs after C/A impact.

This is exactly what I attempted with the simulations, though I admit they're little more than curiousities unless the behavior is better characterized and limitations more thoroughly explored. I find them more compelling, personally, than waving my hands in front of my face, though far less credible than a good analytical approach.

Therefore top roof m will not decelerate from speed 8.52 m/s so fast as any other m below. &c.

This, I believe, is exactly Dr. G's point, for which he's getting a goodly amount of lip. You are clear on the fact that the 'masses' interspersed with the 'springs' provide both material inertia and internal flexure, thus satisfying Newton's 3rd while yet assuring that the roofline will display the least effect of the resistance impulse delivered below.

But you can be sure that it will slow down = jolt!

Yes, but a lot less than the contact interface. Which is Dr. G's point, and Mr. Chandler is not getting it. I think the suggestion that the 3rd law doesn't apply to deformable bodies did not sit well initially, for understandable reasons, and he hasn't been able to get past that. Since you understand the manner in which it does apply to the internal degrees of freedom in a deformable body, perhaps you could explain it to him.

I'd suggest starting with putting a lossy, ratcheted spring between the horse and cart.

Is this about his recent youtube video we discussed here before ? I though that case was already closed.

Dr.G wrote:Think of it this way: suppose I place a concrete block on a tall cardboard box, and let's say that the box can just support the concrete block. Now I pour some water over the cardboard box making it all wet and soggy. Pretty soon my little construct will collapse as the cardboard box looses strength and is no longer able to support the weight of the concrete block.

Have I violated any of Newton's Laws? I don't think so!

Sorry but I'm going to have to chime in with my layman/amateur/non scientific expertise here, even at the risk of sounding like a complete moron. (Well I'm Stundie after all! )

Your analogy is totally inapplicable to the argument of the collapse of the WTC (Especially WTC1) in that the weaker/lighter object (Cardboard Box) is supporting the stronger/heavier object (Concrete block) above it. You are not violating any of Newtons laws, you are violating the rules of an equal comparator to make your argument and prove your point.

Lets say that the collapse was initiated at floor 93 and is completely wiped out allowing the drop distance of a single floor. Then we have 17 floors above it vs 92 floors below it, all of them undamaged. The upper/lower portions are made of the same materials, so concrete vs cardboard is a poor analogy to start with but lets roll with it.

So if we are rolling with this analogy, surely the concrete block should be at the bottom representing the 93 floors and the cardboard at the top representing the 17 floors. This would be more accurate as a comparator.

Swap it around and do this experiment again, pour water over the entire model and tell me if the concrete block collapses under the weight of the cardboard box? Then ask me if I have violated any of Newtons laws? lol

If I were to go to the JREF forum and use the above as a comparative model to argue in favour of a collapse arrest, I would be laughed at, mocked and called all kinds of questionable names.

I would suggest that you change the concrete box above to another cardboard box or change the bottom to a concrete block and run your thought experiment again.

Would we see a crush down from the upper portions? I highly doubt it...otherwise wouldn't that violate newtons law? In that 2 objects made from the same materials but different masses would mean the larger object always wins.

Or in this case...the lower portion??

Cheers

Stundie