Moderated Continuation - Why a one-way Crush down is not possible

[Klimax]

?? The structure below, part A, is exactly the same as the structure above, part C, with the exception that A carried C before (and not the other way around). That is clear from all my papers.

So A is slightly stronger than C. Or C is weaker than A. You agree?

Don't suggest that you can one-way crush down a stronger structure, part A, with a weaker one, part C.

I am bit confused by your usage of labels,so say:A - top part,which lost support and C - part to be impacted by A.

First: A is moving.
Second: C is composed of smaller parts which should be treated separately in simulation and not as one entity.(Floor beneath first(hit) floor is not supporting the first floor)

So I say that if you treat C as solid thing,you have already failed.
And since A will be always more then single part of C(usually floor) in case of towers,than C will be crushed. One part at a time...

 

[Heiwa]

Are you going to modify your axiom to exclude unfinished structures? It seems that if the axiom were correct, all that would be necessary would be for the lower part to be sturdier than the upper part for collapse to be arrested. And the fact that the unfinished structure is standing is proof of that.

Also, I can pretty much guarantee that there are some unfinished structures that are sturdier than some finished structures...especially very old finished structures that are on the verge of collapse. Does your axiom exclude those as well?

My axiom is, as you know:

A smaller part of an isotropic or composite 3-D structure, when dropped on and impacting a greater part of same structure by gravity, cannot one-way crush down the greater part of the structure.

So we talk about finished, existing structures. Unfinished structures were never included at all!
Note that the parts have same structure! No part is sturdier than the other.

Old structures, subject to wear and tear, etc, for many years do not qualify! Reason being that wear and tear change the structure in unpredictable ways and you have lost control of the situation/structure. But to be frank, I have never seen or heard of an old structure suddenly being crushed down by an old part being dropped on it. Have you?

 

[A W Smith]

My axiom is, as you know:

A smaller part of an isotropic or composite 3-D structure, when dropped on and impacting a greater part of same structure by gravity, cannot one-way crush down the greater part of the structure.

wrong

So we talk about finished, existing structures. Unfinished structures were never included at all!
Note that the parts have same structure! No part is sturdier than the other.

The french hydraulic demolitions took place with buildings that had identical floors top to bottom.

Old structures, subject to wear and tear, etc, for many years do not qualify! Reason being that wear and tear change the structure in unpredictable ways and you have lost control of the situation/structure. But to be frank, I have never seen or heard of an old structure suddenly being crushed down by an old part being dropped on it. Have you?

You just moved your goalposts in the span on a single post which disqualifies your own "axiom" which you quoted above.


http://en.wikipedia.org/wiki/Moving_the_goalpost

Moving the goalpost, also known as raising the bar, is an informal logically fallacious argument in which evidence presented in response to a specific claim is dismissed and some other (often greater) evidence is demanded. In other words, after an attempt has been made to score a goal, the goalposts are moved to exclude the attempt. This attempts to leave the impression that an argument had a fair hearing while actually reaching a preordained conclusion. Moving the goalpost can also take the form of reverse feature creep, in which features are eliminated from a product, and the goal of the project is redefined in such a way as to exclude the eliminated features.

 

[aggle-rithm]

My axiom is, as you know:

A smaller part of an isotropic or composite 3-D structure, when dropped on and impacting a greater part of same structure by gravity, cannot one-way crush down the greater part of the structure.

So we talk about finished, existing structures. Unfinished structures were never included at all!

Can you point me to the part of the axiom that says unfinished structures are not included? Are you simply redefining "structure" to mean only a very specific type of structure?

Old structures, subject to wear and tear, etc, for many years do not qualify! Reason being that wear and tear change the structure in unpredictable ways and you have lost control of the situation/structure.

Can you point me to the part of the axiom that says old structures are not included? Are you simply redefining "structure" to mean only a very specific type of structure?

But to be frank, I have never seen or heard of an old structure suddenly being crushed down by an old part being dropped on it. Have you?

Reality is not defined by what either you are I have heard of.

 

[MIKILLINI]

My axiom is, as you know:

A smaller part of an isotropic or composite 3-D structure, when dropped on and impacting a greater part of same structure by gravity, cannot one-way crush down the greater part of the structure.

So we talk about finished, existing structures. Unfinished structures were never included at all!
Note that the parts have same structure! No part is sturdier than the other.

Old structures, subject to wear and tear, etc, for many years do not qualify! Reason being that wear and tear change the structure in unpredictable ways and you have lost control of the situation/structure. But to be frank, I have never seen or heard of an old structure suddenly being crushed down by an old part being dropped on it. Have you?

Considering what you have said above, the WTC's were 30 year old structures subject to losing control of their situation/structure. Even more, they were struck by airliners and subsequent fire after the collisions.
You have debunked yourself with that statement.

 

[aggle-rithm]

Considering what you have said above, the WTC's were 30 year old structures subject to losing control of their situation/structure. Even more, they were struck by airliners and subsequent fire after the collisions.
You have debunked yourself with that statement.

I think the axiom is about to get very convoluted. He's going to say that "30 years old is not 'many years' old", so now the axiom will have to define the term "many years". Inevitably, it will also have to exclude poorly designed or poorly constructed buildings. It will be an odd sort of axiom once all the exclusions are defined.

 

[paulheinze]

Two thoughts about one-way crush down

I see two problem with the one-way crush-down theory:
1. Symmetry
Bazant-Zhou treats two equal structures differently. Whereas the upper Block is assumed as rigid, the lower block is regarded as non-rigid, despite of having quite the same structure. I want to point out, that the rigidity is not proven in his papers but merely an assumption derived by the force of the collision between the columns.

2. Stability
BZ creates a 1-dimensional model regarding the perfect column-on-column collapse as "the most optimistic assumption". But he does not provide any calculation on the stability of the path of the upper block. Supposed the upper columns do not hit the lower columns perfectly, it might be probable that the deflection increases over time. The perfectly centered vertical path of tbe upper block would be an unstable solution of the moving equation. An increasing deflection would lead to a partial collapse of the structure not to a total collapse. At least a 2-dimensional model would be necessary to clarify whether his assumption is justified.

 

[triforcharity]

So, tell us, oh new one, how SHOULD the TTs have reacted?? What SHOULD they have done.


You say Bazant's calculations are wrong, but provide none of your own. Can you explain, with some math, why its wrong??

 

[TruthersLie]

crush down, crush up.

the french verinage videos.

do you mean those real world examples are impossible?

 

[Dave Rogers]

I see two problem with the one-way crush-down theory:
1. Symmetry
Bazant-Zhou treats two equal structures differently. Whereas the upper Block is assumed as rigid, the lower block is regarded as non-rigid, despite of having quite the same structure. I want to point out, that the rigidity is not proven in his papers but merely an assumption derived by the force of the collision between the columns.

The main points to be made here are that the two structures are not exactly equal, and that Bazant is considering not two bodies but three. It's absolutely crucial to Bazant's analysis that there is a third body present, which is the accumulated mass of debris between the upper and lower blocks, and that any debris created by collapse of the two relatively intact parts of the building is added to this mass. Once that's understood, then the source of the asymmetry becomes clear. The mass of debris plays the main part in crushing down the lower block, and since its dynamic loading is greater than the resistance of the lower structure, it accelerates downwards. Since it is accelerating downwards, the reaction force it exerts on the upper block is less than the weight of the upper block. The tendency is therefore for crush-down to increase and crush-up to decrease, reaching a stable condition where only crush-down occurs. This condition persists until the debris reaches a structure capable of decelerating it - namely, the ground - at which point crush-up predominates.

2. Stability
BZ creates a 1-dimensional model regarding the perfect column-on-column collapse as "the most optimistic assumption". But he does not provide any calculation on the stability of the path of the upper block. Supposed the upper columns do not hit the lower columns perfectly, it might be probable that the deflection increases over time. The perfectly centered vertical path of tbe upper block would be an unstable solution of the moving equation. An increasing deflection would lead to a partial collapse of the structure not to a total collapse. At least a 2-dimensional model would be necessary to clarify whether his assumption is justified.

That reasoning doesn't follow. It should be fairly clear that column-on-column impact is the most optimistic assumption, in that it allows the columns (the strongest component) of the lower structure to resist collapse, rather than the weaker floor trusses. It should also be fairly clear that, without the lateral restraint from the floor trusses, the perimeter columns are incapable of standing over a significant fraction of the height of the tower. Therefore, any lateral deflection of the upper block will decrease the ability of the lower structure to resist collapse, unless there is sufficient lateral deflection that the upper structure completely misses a section of the lower core, allowing some part of the floor system and perimeter columns to remain standing. It's by no means certain that any such remnant would be stable anyway, but in any case this requires that the upper block be moved laterally by over 100 feet in less than ten seconds. Where is the lateral force to come from to accelerate a mass of over 30,000 tons to a speed of 10 feet per second? There wasn't any structural element in the buildings capable of exerting the required lateral force.

There is, also, a rather powerful counter-argument to doubts over the one-way crush-down theory, and that is that it is clearly no longer a theory. The videos posted over the last few days of buildings demolished by the French Verinage technique show very clearly that one-way crush-down is in fact an observation, and a highly reproducible one.

Dave

 

[paulheinze]

More thoughts

It might be that I do not understand the crush-down crush-up theory completely. You might help me out. I would expect a simultaneous crush-down crush-up. I have to admit that (at the moment) I cannot rule out the resulting debris crushing the building.

Other thoughts:
1. Body 1 is the small part "falling down" and Body 2 is the remaining structure connected to earth. The calculations of Bazant done from the reference frame of Body 2. In this case Body 1 is accelerated by g. Changing to the (accelerated) reference frame of Body 1 Body 2 is the one accelerating with g. Why not first crush-up then crush-down? Why not regarding in the accelerated reference frame Body 2 as rigid?

2. In my opinion it is not consistent to assume a column-column collision and a rigid body and later depending on a compressed rubble pile.

3. It is correct that the columns are the strongest part of the building. It is possible that the assumption of BZ columns colliding perfectly centered. But in the case of a non-centric collision we would see a different collapse in reality. If the assumption of BZ is unlikey because of instability of the assumed movement of the top it would not be a sufficient explanation of the collapses observed.

 

[TruthersLie]

It might be that I do not understand the crush-down crush-up theory completely. You might help me out. I would expect a simultaneous crush-down crush-up. I have to admit that (at the moment) I cannot rule out the resulting debris crushing the building.

Other thoughts:
1. Body 1 is the small part "falling down" and Body 2 is the remaining structure connected to earth. The calculations of Bazant done from the reference frame of Body 2. In this case Body 1 is accelerated by g. Changing to the (accelerated) reference frame of Body 1 Body 2 is the one accelerating with g. Why not first crush-up then crush-down? Why not regarding in the accelerated reference frame Body 2 as rigid?

2. In my opinion it is not consistent to assume a column-column collision and a rigid body and later depending on a compressed rubble pile.

3. It is correct that the columns are the strongest part of the building. It is possible that the assumption of BZ columns colliding perfectly centered. But in the case of a non-centric collision we would see a different collapse in reality. If the assumption of BZ is unlikey because of instability of the assumed movement of the top it would not be a sufficient explanation of the collapses observed.

The Bazant Zhou model is extremely simplified, and it is the BEST case scenerio in the collapse. It is NOT designed to show what REALLY happened. It is designed to show that NO MATTER WHAT once the collapse started, it was going to be complete. That is what their model shows.

I'll leave the rest to the real structural engineers.

 

[triforcharity]

Excuse me, paulheinze, would you care to address my questions?? Thanks.

Edited by Gaspode: 

Corrected username.

 

[Newtons Bit]

3. It is correct that the columns are the strongest part of the building. It is possible that the assumption of BZ columns colliding perfectly centered. But in the case of a non-centric collision we would see a different collapse in reality. If the assumption of BZ is unlikey because of instability of the assumed movement of the top it would not be a sufficient explanation of the collapses observed.

Non-concentric collisions will place additional bending moments on the columns. The columns will thus fail sooner.

A collapse model where the columns don't collide at all, and instead only impact the floors below, is the least conservative of all collapse models. It is also most certainly what actually happened.

 

[Dave Rogers]

It might be that I do not understand the crush-down crush-up theory completely. You might help me out. I would expect a simultaneous crush-down crush-up. I have to admit that (at the moment) I cannot rule out the resulting debris crushing the building.

That was my first impression too, that crush up and crush down should be simultaneous. If you only consider two bodies, that's the intuitive impression you're bound to get. It's only when you realise that the crushed material between the two not only can be, but has to be, treated as a third body that the rather more counter-intuitive result becomes clearer.

Other thoughts:
1. Body 1 is the small part "falling down" and Body 2 is the remaining structure connected to earth. The calculations of Bazant done from the reference frame of Body 2. In this case Body 1 is accelerated by g. Changing to the (accelerated) reference frame of Body 1 Body 2 is the one accelerating with g. Why not first crush-up then crush-down? Why not regarding in the accelerated reference frame Body 2 as rigid?

Firstly, the two frames of reference aren't equivalent, because the reference frame of body 2 isn't inertial. It shouldn't be surprising, therefore, that the results aren't symmetrical between the two bodies. Secondly, there's the rather major asymmetry that body 2 is fixed at its outer end to a mass large enough that it may be treated as immovable, whereas body 1 isn't; body 1 can therefore move freely under the action of the forces acting upon it, whereas body 2 can only crush.

2. In my opinion it is not consistent to assume a column-column collision and a rigid body and later depending on a compressed rubble pile.

That depends on what the assumption of a rigid body actually means. In this context, it means that elastic and plastic deformation of body 1 is negligible, not that body 1 is indestructible. Body 1 is still allowed to crush; the absence of crush-up is a result, not an assumption.

3. It is correct that the columns are the strongest part of the building. It is possible that the assumption of BZ columns colliding perfectly centered. But in the case of a non-centric collision we would see a different collapse in reality. If the assumption of BZ is unlikey because of instability of the assumed movement of the top it would not be a sufficient explanation of the collapses observed.

Nor was it ever intended to be a sufficient explanation. It is, rather, a limiting case. There is an infinite range of possible collapse modes, with varying energy balances. The BZ calculation is based on the one collapse mode that is most optimistic to survival; however unlikely that mode may be, if it is found to result in collapse, then any other collapse mode must also result in collapse.

In reality, it's perfectly clear that the impacts were not column-on-column; it's likely that there were several processes going on simultaneously, including funnelling of rubble into the inside of the perimeter column tube, floor collapse, and buckling failure of unsupported column sections trailing the interior collapse. There's also the role of the hat truss to consider, in presenting a stronger top end of the falling block and quite possibly doing considerable damage to the core as it collided with parts of it. However, what we can say with reasonable certainty is that the real collapse mode was energetically more favourable than the theoretical limiting case analysed by BZ, and that therefore global collapse was the expected outcome. Detailed analysis of the collapse to show that it differs from BZ in fine details is irrelevant to this conclusion.

Dave

 

[aggle-rithm]

I see two problem with the one-way crush-down theory:
1. Symmetry
Bazant-Zhou treats two equal structures differently. Whereas the upper Block is assumed as rigid, the lower block is regarded as non-rigid, despite of having quite the same structure. I want to point out, that the rigidity is not proven in his papers but merely an assumption derived by the force of the collision between the columns.

Look at it this way:

Imagine you're trying to model a head-on collision between a race car and a race horse, specifically to determine how much damage will be done to the horse. You have a choice between calculating all the elastic and plastic deformations of the car to determine EXACTLY how much of the impact the horse is spared, or you can assume the car is rigid. It should be obvious to laymen that the difference would be miniscule.

The difference between a rigid and non-rigid block in a building collapse is not as self-evident to laymen, but it is to engineers.

 

[rwguinn]

The Bazant Zhou model is extremely simplified, and it is the BEST case scenerio in the collapse. It is NOT designed to show what REALLY happened. It is designed to show that NO MATTER WHAT once the collapse started, it was going to be complete. That is what their model shows.

I'll leave the rest to the real structural engineers.

Exactly.
Every assumption in that model is biased toward collapse arrest. It is not supposed to be realistic(e) with respect to the actual collapse--it is a n "Envelope" case, defining the best possible outcome for the conditions.
Think of a falling rock problem--a rock falling from the cliff can, when it hits your car, either hit the glass windshield, or the metal roof.
Bazant set it up so it always did the equivalent of hitting the metal--and it still killed the driver...

 

[Dave Rogers]

Firstly, the two frames of reference aren't equivalent, because the reference frame of body 2 isn't inertial.

Too late to edit, but this should read "the reference frame of body 1 isn't inertial."

Dave

 

[TruthersLie]

That was my first impression too, that crush up and crush down should be simultaneous. If you only consider two bodies, that's the intuitive impression you're bound to get. It's only when you realise that the crushed material between the two not only can be, but has to be, treated as a third body that the rather more counter-intuitive result becomes clearer.



Firstly, the two frames of reference aren't equivalent, because the reference frame of body 2 isn't inertial. It shouldn't be surprising, therefore, that the results aren't symmetrical between the two bodies. Secondly, there's the rather major asymmetry that body 2 is fixed at its outer end to a mass large enough that it may be treated as immovable, whereas body 1 isn't; body 1 can therefore move freely under the action of the forces acting upon it, whereas body 2 can only crush.



That depends on what the assumption of a rigid body actually means. In this context, it means that elastic and plastic deformation of body 1 is negligible, not that body 1 is indestructible. Body 1 is still allowed to crush; the absence of crush-up is a result, not an assumption.



Nor was it ever intended to be a sufficient explanation. It is, rather, a limiting case. There is an infinite range of possible collapse modes, with varying energy balances. The BZ calculation is based on the one collapse mode that is most optimistic to survival; however unlikely that mode may be, if it is found to result in collapse, then any other collapse mode must also result in collapse.

In reality, it's perfectly clear that the impacts were not column-on-column; it's likely that there were several processes going on simultaneously, including funnelling of rubble into the inside of the perimeter column tube, floor collapse, and buckling failure of unsupported column sections trailing the interior collapse. There's also the role of the hat truss to consider, in presenting a stronger top end of the falling block and quite possibly doing considerable damage to the core as it collided with parts of it. However, what we can say with reasonable certainty is that the real collapse mode was energetically more favourable than the theoretical limiting case analysed by BZ, and that therefore global collapse was the expected outcome. Detailed analysis of the collapse to show that it differs from BZ in fine details is irrelevant to this conclusion.

Dave

What he said. (and the other engineers. I would have tried to explain it that way, but I didn't want to make Newton's Bit cranky. )

 

[Newtons Bit]

What he said. (and the other engineers. I would have tried to explain it that way, but I didn't want to make Newton's Bit cranky. )

I'm always cranky

My crankiness only gets applied to others when they claim that buildings could or couldn't collapse because of a number that they just pulled out of their... err, posterior.