Bazant's crush-down/crush-up model

[Major_Tom]

In the paper BL there is no doubt that Bazant believes that the concept of crush down, then crush up applies to WTC1 and WTC2.

Bazant & Le
Closure to "Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions"
http://www.civil.northwestern.edu/pe... Replies.pdf

In the section titled "Reply to R Gourley", points #4 and 5 clearly show that Dr Bazant believes the concept of crush down, then crush up can be applied quite literally to the cases of WTC1 and 2.

Point 4 entitled "Why cannot crush up proceed simultaneously with crush down?"

Point 5 entiled "Why cannot crush up begin later?"


There is no doubt that Dr Bazant is applying his idea of crush down, then crush up literally to the case of WTC1 and 2. It is also clear from the quote from BLGB in my last post.

 

[Major_Tom]

SO who is correct, the posters who believe that crush down, then crush up cannot be literally applied to WTC1 or 2 , or Dr Bazant?


Ironically, you are right and your expert is wrong in this case.

 

[Oystein]

SO who is correct, the posters who believe that crush down, then crush up cannot be literally applied to WTC1 or 2 , or Dr Bazant?
...

Would I go out on a limb if I answered "both, to some extent"?

The Bazant model is, of course, a model: It assumes some ideal condictions, like a homogeneous, horizontal crush front. It neglects some mass loss due to lateral ejections. It ignores tilt. It ignores the fact that the core is stronger than the rest of the structure and actually managed to penetrate the rubble front. Etc.

As a result, one would expect the crush-up-crush-down scenario, only dirtier due to many deviations of the real world from model.

 

[Cuddles]

Remain on topic and avoid making personal attacks.

Replying to this modbox in thread will be off topic  Posted By: Cuddles

 

[Oystein]

By the way: Interesting approach to changing the point of view, and excellent arguments from R.Mackey! Thanks all!
I did read (well, skim) the Bazant papers, including the annotations, but didn't really get it until now why the top part should experience less stress and destruction.

 

[leftysergeant]

I think Bazant went badly astray when he referred to "crush-up." This does not invalidate his assertion that there was potential energy enough acvailable to cause total collapse. All I see occuring is crush-down. The top block is constantly crushing its own lower regions ab initio. As I understand such things, it does, however, remain a "geometric solid." It is just not a homogenous solid. Whether discombobulated rubble and steel elements, or a nicely-constructed box, it still has mass and volumn. That mass and volumn continue to increase as collapse continues.

And it still hammers the structure below it.

My problem with the rocket anology is that the rocket must be constantly accelerating, as was the top block in each of the towers.

 

[Oystein]

I think Bazant went badly astray when he referred to "crush-up." This does not invalidate his assertion that there was potential energy enough acvailable to cause total collapse. All I see occuring is crush-down. The top block is constantly crushing its own lower regions ab initio. As I understand such things, it does, however, remain a "geometric solid." It is just not a homogenous solid. Whether discombobulated rubble and steel elements, or a nicely-constructed box, it still has mass and volumn. That mass and volumn continue to increase as collapse continues.

And it still hammers the structure below it.

I guess the point of the theory, besides being purely interesting (because counter-intuitive) on its own merits, is that it shows that not much kinetic energy was "wasted" on structural deformatin in the top block, and thus more was available to crush the lower portion. Even though that is not necessary in order to explain why total collapse occurred at all (there was enough energy around to make that inevitable), but it might add a little velocity to the event.

My problem with the rocket anology is that the rocket must be constantly accelerating, as was the top block in each of the towers.

Uhm, that's what rockets do as long as they fire...

 

[Furcifer]

I did read (well, skim) the Bazant papers, including the annotations, but didn't really get it until now why the top part should experience less stress and destruction.

At some point in the collapse the core pierced the upper section and it was "crushed up". Bazant's just shown mathematically it doesn't matter. It's amusing how this frustrates the hell out of the truthers.

 

[leftysergeant]

I did read (well, skim) the Bazant papers, including the annotations, but didn't really get it until now why the top part should experience less stress and destruction.

The top part had only to resist the downward acceleration of its own mass (which mass was decreasing as it disintegrated) while the bottom had to resist the mass of the top part, plus the increasing amount of its own mass that was being accelerated downward.

 

[femr2]

At some point in the collapse the core pierced the upper section and it was "crushed up". Bazant's just shown mathematically it doesn't matter. It's amusing how this frustrates the hell out of the truthers.

Bazants model assumes crush-up does not occur until after full-cycle crush-down. It is clear that no such behaviour actually ocurred.

Even the terms crush-up and crush-down can only be loosely applied to the towers...

In the real WTC situation, we don't have a true "crush down / crush up" anyway. What actually happens is the core and perimeter structure of the lower block funnels falling material onto the floors. The truss floors preferentially fail downward, whereas the beam-framed floors in the core preferentially fail upward.

In the scenario above, shortly after initiation there is no real semblance of upper block remaining. There are several zoned debris avalanches causing essentially internal pancaking, with a tangled mess of core lagging behind and sections of perimeter either being ejected or peeling even further behind. In addition, whilst floor destruction progressed to ground, core destruction arrested.

Could you expand on what you mean by *it doesn't matter* ?

 

[femr2]

it shows that not much kinetic energy was "wasted" on structural deformatin in the top block, and thus more was available to crush the lower portion. Even though that is not necessary in order to explain why total collapse occurred at all (there was enough energy around to make that inevitable), but it might add a little velocity to the event.

It sounds like you are applying the Bazant model behaviour to the real-world events.

As we have agreed the Bazant model behaviour does not and was not intended to match real world behaviour.

Could you clarify your intent ?

 

[Oystein]

It sounds like you are applying the Bazant model behaviour to the real-world events.

As we have agreed the Bazant model behaviour does not and was not intended to match real world behaviour.

Could you clarify your intent ?

I did not participate in any such agreement. See my post 23.

 

[Furcifer]

Bazants model assumes crush-up does not occur until after full-cycle crush-down. It is clear that no such behaviour actually ocurred.

Actually that's not technically correct. With the mathematical proof it's no longer an "assumption" per se, but a validated simplification.

It's clear that it adequately describes the actual behaviour.

 

[femr2]

I did not participate in any such agreement. See my post 23.

Okay, then...

it shows that not much kinetic energy was "wasted" on structural deformatin in the top block

Do you agree that in the real world there was no real semblance of *top block* shortly after initiation ?

and thus more was available to crush the lower portion.

Do you agree that core destruction and floor *pancaking* propogation was separated by considerable distance ?

there was enough energy around

Agreed, and imo the primary valid result of the model. Other behavioural properties of the model simply don't match the real world events.

but it might add a little velocity to the event.

Floor destruction propogation (ROOSD) attained a fairly constant velocity after about 4s. Core destruction arrested.

Are you sure you're not inadvertently applying model behaviour to the real world inappropriately ?

 

[femr2]

Actually that's not technically correct. With the mathematical proof it's no longer an "assumption" per se, but a validated simplification.

No. Floor destruction propogation did not involve an *upper block* at all, and was basically driven entirely by a *rubble layer*. Core destruction arrested. Perimeter essentially peeled.

It's clear that it adequately describes the actual behaviour.

As the limiting case, it shows there was enough energy available and given it's simplfying assumptions that propogation to ground would ensue in those conditions, but it does not describe the actual behaviour at all.

 

[Furcifer]

No. Floor destruction propogation did not involve an *upper block* at all, and was basically driven entirely by a *rubble layer*. Core destruction arrested. Perimeter essentially peeled.


As the limiting case, it shows there was enough energy available and given it's simplfying assumptions that propogation to ground would ensue in those conditions, but it does not describe the actual behaviour at all.

I'm not sure how you figure breaking it up makes any difference? Bazant has already shown it doesn't matter. You have a variety of ways to get from A to B. Either way you end up at B.

 

[femr2]

I'm not sure how you figure breaking it up makes any difference?

I'm highlighting to you that stating *it adequately describes the actual behaviour* is not correct. The real world behaviour and the behaviour of the Bazant model are very different, obviously.

Bazant has already shown it doesn't matter.

What doesn't matter ? Please cite where Bazant uses a model without the *top block*, and also includes early *crush up*. You can infer from the energetics results that there was enough energy available, sure. *Showing it doesn't matter* in this context is your inference, not something Bazant has shown. The energetics inferences are totally different to stating that the model behaviour matches real world behaviour. That's just not true.

You have a variety of ways to get from A to B.

Sure, and my point is that it is erronious to apply crush down/up behaviour of Bazants model to the real world events literally. Again, as I'm sure you'll listen to Ryans' words more than mine...

In the real WTC situation, we don't have a true "crush down / crush up" anyway. What actually happens is the core and perimeter structure of the lower block funnels falling material onto the floors. The truss floors preferentially fail downward, whereas the beam-framed floors in the core preferentially fail upward.

This description does not match the Bazant model behaviour well at all. The vast majority of floor destruction does not involve a top block at all, and simply a *rubble layer*.

Either way you end up at B.

Bazant does not mention tipping cores. IF the large core remnant of WTC 1 that remained after core destruction arrest had not dropped, would you still describe the outcome as *B* ? That core portion was not destroyed by *crush-down*, and so is not accounted for by the model.

 

[Furcifer]

I'm highlighting to you that stating *it adequately describes the actual behaviour* is not correct.
Sure, and my point is that it is erronious to apply crush down/up behaviour of Bazants model to the real world events literally.

It's pretty clear it's not meant to describe the events literally, but it's adequate.

It seems to me you're just playing a game of semantics?

 

[femr2]

It's pretty clear it's not meant to describe the events literally

Of course. That is why I asked you to clarify *It's clear that it adequately describes the actual behaviour.*, and pointed out the actual behaviour was very different.

but it's adequate.

For it's intended purpose, limiting case energy availablility, it's okay. Not a whole lot different in that sense to other simplified models kicking around. Applying it to actual behaviour is, again, wrong.

Minimal axial column impacts.
Minimal column buckling.
No true crush-down or crush-up in the Bazantian sense.
No *upper block* shortly after initiation.
Separate and different floor, core and perimeter modes of destruction.
Arrest of core destruction for a significant proportion of the core.
etc.

It seems to me you're just playing a game of semantics?

Not at all. I'm simply making sure that the model is not used incorrectly. As there is enough energy in the limiting case for complete destruction, does that mean the model can be used to state there is enough energy for alternate modes of destruction...sure, but does it mean that the behaviour matches alternate modes of destruction (such as the actual real world event) ? No.

Perhaps you could highlight a number of real world behaviours that do match the behaviour of the Bazant model ?

 

[pgimeno]

Think about your model. The rocket engines are accelerating the "ground." That means the only parts that accelerate with the rocket engine are those for which there is a stable load path going back to the ground. Everything else is not accelerating, and being hit by this structure.

But that (the bolded part) is not accurate because the top is accelerating as is the rubble, otherwise in the gravity version the top and the rubble would have fallen in freefall, or what am I missing?

In the freefall case it's obvious that the top wouldn't be crushed until reaching the ground.

The rubble does not have a stable load path to ground. They are free objects. They are not reliably carried by the lower structure's columns, but are instead impacting all over the place -- columns, floors, eccentrically, and so on. Thus, the rubble counts as part of the detached mass, which includes the upper block.

My problem keeps being that both the rubble and the top are also pushed by the bottom, experimenting an acceleration (if the top fell at 2/3 g in the gravity version, the acceleration of the top and rubble in the rocket version would be 1/3 g).

Yes, there is. Think of it like this -- F = m a, right? The forces on the upper block, once there is a significant rubble layer, are inertial forces. If you measure the aggregate deceleration of the upper block, you know the stress in that block.

As the upper block + rubble layer increases, m increases. F, on the other hand, is decreasing in your reframe of the problem -- the rockets have to throttle down. As a result, a decreases. That means the stress in the surviving upper block decreases as the collapse continues.

Hum... That makes sense. But that doesn't prevent the possibility of a few more floors destroyed, does it?

In my view, ignoring the ejection of mass from the system and the non-homogeneous nature of the rubble layer, the total thrust of the rockets will be a simple function of the intact bottom mass which must accelerate at g, and the rest of the mass which must accelerate at 1/3 g. That's my way of understanding that the thrust must decrease in order to maintain the acceleration: as the mass of the bottom decreases and the mass of the rest increases, the required force will be less because the increasing mass requires less acceleration. That result is surprising and counterintuitive for me, but that's life

The peak stress in the upper block is the failure stress of the floor at the collision interface. It simply cannot transmit any higher stress than that. So we expect to see some damage to the upper block at the initial contact, followed by very little damage to the upper block afterward, until it again hits the "immovable" rocket platform at which point it will fail floor-by-floor in the crush up phase.

I see why there's less damage but I don't see why there's very little damage to the upper block afterward.

I still would understand it if it were the parameters from the WTC and maybe other buildings that make the crush-up not to happen but not in general, but you said that's not the case. Wonder what I'm missing.

I've read the explanation to Gourley from Bazant. I didn't understand most of it, but I see that he uses conservation of momentum and energy and that he introduces factors such as mass shedding ratio and mass compaction ratio, which makes me wonder if the rubble layer acts like a cushion as it is compacted, that dampens the dynamic load on the top, making it able to support itself when it's an "almost static" load with the additional advantage of reduced acceleration.

It also seems relevant to me that he employs real world data to feed the equations instead of solving them in general, which would settle the universality of the phenomenon.

In the real WTC situation, we don't have a true "crush down / crush up" anyway. What actually happens is the core and perimeter structure of the lower block funnels falling material onto the floors. The truss floors preferentially fail downward, whereas the beam-framed floors in the core preferentially fail upward. I wrote a cartoon describing this in the supporting presentation for my debate with Tony Szamboti, which you can read here: http://www.911myths.com/index.php/Ryanmackey (pages 20-25 of the presentation, downloadable as PPT or PDF).

Thank you very much. Not fully understood but let's leave that for another occasion