Merged Applicability of Bazant's model to the real world

[Major_Tom]

When Bazant says "The gravity-driven progressive collapse of a tower consists of two
phases—the crush-down, followed by crush-up"


Does he really mean "The gravity-driven progressive collapse of a theoretical scenario based on successive upward or downward axial impacts which represent the case most optimal for survival and does not represent the actual behavior of the towers consists of two phases—the crush-down, followed by crush-up"

Is that what he really means? No it is not.

 

[Grizzly Bear]

Is Bazant applying his crush down, then crush up model to the WTC towers in this quote and in the figures, or is he only describing and and illustrating the theoretical scenario best able to survive as an extreme case (which we do not expect to match the actual towers)?

I see no mention of this only as a theoretical case most optimal for survival. Are we to assume so?

He never refers to the diagrams as only an extreme, theoretical case based on assumptions most optimal for survival.

Just to point out...

http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/405.pdf

This paper presents a simplified approximate analysis of the overall collapse of the towers of World Trade Center in New York on September 11, 2001. The analysis shows that if prolonged heating caused the majority of columns of a single floor to lose their load carrying capacity, the whole tower was doomed.

For our purpose, we may assume that all the impact forces go into the columns and are distributed among them equally. Unlikely though such a distribution may be, it is nevertheless the most optimistic hypothesis to make because the resistance of the building to the impact is, for such a distribution, the highest. If the building is found to fail under a uniform distribution of the impact forces, it would fail under any other distribution. According to this hypothesis, one may estimate that C'71 GN/m ~due to unavailability of precise data, an approximate design of column cross sections had to be carried out for this purpose.

More prominent posters such as Tony Szamboti give me no indication of having acknowledged this, and frankly I'm not sure how the author of that paper could have been any more clear. This is in the first page of the paper... It's rather difficult to overlook if you carefully read it.

 

[Major_Tom]

Grizzly Bear, your quote if from BZ in 2002. Different argument, different paper. You people cannot be that stupid.

 

[DGM]

Grizzly Bear, your quote if from BZ in 2002. Different argument, different paper. You people cannot be that stupid.

Sure we can, That's why there's all the fuss in the engineering world.

 

[Major_Tom]

Dave Rogers provides another example of applying the concept of crush down, then crush up to blocks and buildings here:

"If you were to actually read Bazant's papers for comprehension, rather than deciding what strawman you want to attribute to them, you would see that the analysis predicts that crush-up and crush-down will initially both occur, but that the rate of crush-up will quickly decay to zero while the rate of crush-down increases. This is an idealised case, and the presence of damage to the structure above or below the collapse initiation zone will modify the precise amount of crush-up initially observed, but at no point is Bazant claiming that there is no crush-up until crush-down is complete; he's determining that a very small amount of initial crush-up occurs, but that it self-terminates until the upper block contacts the ground. As a result, a small proportion of the rubble comes from the upper block, and the majority from the lower."

From this post http://www.internationalskeptics.com/forums/showpost.php?p=5941193&postcount=92

We once again have an upper block being destroyed by crush up only after it hits the earth.

The upper block once again preserved by the strength of it's columns and the magic zone B. Bazant proved it in BL, no?

with no awareness of the irony, Dave concluded:

"Funnily enough, no, the rest of us can't see that, because the dust ejected by the collapse obscures vision of the upper block part way into the collapse. Since we can't tell what happens to things we can't see, we can either make up something that fits our prejudices or model the physics of the collapse mathematically and draw provisional conclusions. The first approach seems to be working very nicely for you, to the extent that you've convinced yourself you can see something that's hidden in a dust cloud. For the rest of us, we'd rather start from what we can see then decide on a conclusion, than start from a conclusion then decide what we can see."

That part about the upper block was made up.

 

[DGM]

Major Tom:
In all seriousness, Why do you think the engineering (as applied to buildings) world is not paying any attention to you?

I don't think I can make this anymore plain.

 

[pgimeno]

Grizzly Bear, your quote if from BZ in 2002. Different argument, different paper. You people cannot be that stupid.

Oh! So are you arguing that Bazant became dumber as he published more papers?

A good idea is to read the OP more carefully. Your examples are bogus.

There is room for opinion on whether, how and why a model can be applied to the real world, but doing so is in not necessarily a mistake. That is what the OP is about. You are claiming that the model can't be applied to the real world for extracting certain aspects out of it, without providing any valid reason. Your reasoning is that it is invalid because it doesn't match certain aspects of the real collapse more closely. It's like claiming that Newtonian physics are invalid and can't be applied to the real world, because they don't match with relativistic effects.

And that makes no sense.

 

[Grizzly Bear]

Grizzly Bear, your quote if from BZ in 2002. Different argument, different paper. You people cannot be that stupid.

Please review his subsequent papers as they either reference his original 2001 paper or state succicntly the purpose of the model:

http://www.civil.northwestern.edu/p...TC Collapse - What Did & Did Not Cause It.pdf

Abstract: Previous analysis of progressive collapse showed that gravity alone suffices to explain the overall collapse of the World Trade Center (WTC) towers.

To explain the collapse, it was proposed (on September 13, 2001; Baˇzant 2001; Baˇzant and Zhou 2002) that viscoplastic buckling of heated and overloaded columns caused the top part of tower to fall through the height of at least one story, and then shown that the kinetic energy of the impact on the lower part must have exceeded the energy absorption capacity of the lower part by an order of magnitude.

http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/466.pdf

A simplified one-dimensional analytical solution of the collapse front propagation will be presented. It will be shown how this solution can be used to determine the energy absorption capability of individual stories if the motion history is precisely recorded.

In broad terms, this scenario was proposed by Bažant 2001, and Bažant and Zhou 2002a,b on the basis of simplified analysis relying solely on energy considerations.

There is no indication that Bazant is using anything other than a limiting case model, no assumptions about this are necessary. He's makes it clear - usually within the first page or two - what he was setting up. It would be relevant for you to be reminded that you asked for clarification on whether or not Bazant's crush up/down model was in fact, a limiting case - the answer to which is yes it was. And yes he made it clear it was. "Different argument, different paper"? Sorry, no... reread what you asked. I gave you the answer.


Such a model may not represent what happened in reality, but it is applicable providing the scope of it is explained properly.

 

[pgimeno]

Grizzly Bear, please read the OP. It deals precisely with where and how Bazant et al are applying the model to the real world collapse in order to extract some conclusions out of it, and not as a limiting case.

 

[Grizzly Bear]

Grizzly Bear, please read the OP. It deals precisely with where and how Bazant et al are applying the model to the real world collapse in order to extract some conclusions out of it, and not as a limiting case.

Editing

 

[leftysergeant]

All the videos I have seen of verinage show the upper block disintegrating more slowly than the lower. There were, for example, more floors above the break in Balzac-Vitry, but the roof is seen to be in tact until nearly the last seconds.

The upper floors of the twin towers acted somewhat differently because they were more fragile and had that huge spindle shoved up the middle. The joints in the upper core columns were probably badly compromised already because they were more badly skewed and had most of the stress of the hat trusses twisting them. Somewherte here we have videos of some buildings in Mecca being pulled. The taller, thinner buildings begin to fold as they tip over, just as did the south tower.

The crush-down, crush-up model generally apllies. The rate is just a little different in varying types of construction. Crush-up appears to occur when resistance in the lower block reaches a certain threshhold. That occurred very early in the north tower because the lower portion was not as badly compromised, thus more resistant, than the top. The building still had no chance of survival because by the time crush-up was cpmpleted, enough debris had accumulated that it might as well have been an undamaged building falling on the lower floors. Those hat trusses were freaking MASSIVE compared to the rest of the steel structures that were falling.

 

[ozeco41]

... Those hat trusses were freaking MASSIVE compared to the rest of the steel structures that were falling.

Even without the maths, had the top block hung up on the core I would be pretty confident that the outer columns and floors of the top block would have stayed in place hanging off the hat truss.

 

[bill smith]

Wishful thinking there Bill on your three points:
1) "We" are all agreed that Bazant was right - there was sufficient energy to complete the global collapse of the twin towers.
2) "We" are all agreed that NIST was right - no demolition of the towers AND once collapse started "global collapse was inevitable" AND
3) recent discussions here show that the only place explosives are on the table is where honest professionals explain the "no demolition" case to genuine sceptics.

And "we" don't need to make progress but we have. There are few genuine sceptics still enquiring about truther rubbish.

...and few, if any, genuine truthers still posting...

Is that all that Bazant brought then ? His theory that there was enough energy in a short drop of the top one-tenth of the building to crush the other nine-tenths of the structure down flat on the ground ? If so what a horrible waste of time and space Bazant has been.

Your number two point can be ignored as completely absurd.

Recent discussions here are serving to put Bazant where he should be put. Firmly on the rearmost back burner if not altogether out with the trash.

 

[leftysergeant]

Is that all that Bazant brought then ? His theory that there was enough energy in a short drop of the top one-tenth of the building to crush the other nine-tenths of the structure down flat on the ground ? If so what a horrible waste of time and space Bazant has been.

It strikes down little Dickie's argument that you can't break a big box by dropping a small one on it.

 

[Major_Tom]

I've been trying to figure out why people associate a crush up, crush down model with the argument in BZ (2002). The crush up crush down model was published in 2007 in the paper by Bazant and Verdure (2007).

When I use the phrase "Bazant model", I think most of the posters have no idea what I am referring to, so to clear up confusion let's think of there being 2 different Bazant models.


In BZ, there is a "model" used for the analysis of inelastic energy dissipation of a single story. THIS IS NOT A CRUSH UP, CRUSH DOWN MODEL. It only analyzes a single collision after a 12 ft fall to see if the resulting kinetic energy can be totally absorbed by the underlying columns. Not much of a surprise that it cannot.


In BV (2007) there is a crush up, crush down progressive collapse model presented in the form of separate crush down and crush up equations of motion. THIS IS A CRUSH DOWN, CRUSH UP MODEL.



There is no crush up, crush down progressive collapse model in BZ. Only a single perfect axial collision between columns is analyzed.

I began posting on this subject in May and it is now September, yet many people still cannot tell the difference between BZ single collision analysis and the crush up, crush down model given in BV (2007).





Hint: THE CRUSH UP, CRUSH DOWN MODEL IS FIRST EXPLAINED IN BV (2007), IT IS REFINED IN BL (2008) AND APPLIED TO THE WTC TOWERS IN BL (2008) AND BLGB (2008).

IT DID NOT EXIST IN 2002 SO PLEASE DO NOT REFER TO BZ QUOTES WHEN DISCUSSING IT.

 

[Major_Tom]

Posters like Ozeco and Leftysergeant are referring to the energetics argument in BZ. That is fine and I have no argument with them. The OOS collapse propagation model suggests a path of least resistance down the OOS regions, so I am obviously not arguing for collapse arrest.


Leftysergeant, Bazant's crush down then crush up model presented in BV (2007) predicts that crush down must run to completion before significant crush up can occur.

Not one verinage demo that I have observed does this. We see different crush up, crush down ratios but no examples of complete crush down before crush up. It is not the question of energetics but in the order of crush down and crush up that matters whan discussing BV, BL and BLGB.

 

[Major_Tom]

BZ is not useful for matching or predicting real observable features of the WTC collapses, but the arguments in BV, BL and BLGB can be used to match or predict specific features of the collapsing towers, and Bazant seems to do just that in BL and BLGB.

Let's keep the BZ "model" separate from the BV crush up, crush down progressive collapse model published in 2007 or you won't understand what Bazant's progressive collapse model is.

 

[bill smith]

So the 2007 model does not act as a continuation or oart 2 of the 2002 model ?

You mean that you cannot describe the two together as ' the Bazant model '? Why not ?

 

[Major_Tom]

Because the BZ (2002) paper discusses the energetics of the collapse of a single story with conditions most favorable for survival.

In BV (2007), Bazant tries to derive crush down and crush up equations which can be used to measure the roofline and crush front motion of real buildings if 4 simplifying assumptions are met.

He applies a modified form of these same equations to the towers in BLGB and tries to match the actual roofline descent of WTC1 with the model in BLGB.

 

[bill smith]

Because the BZ (2002) paper discusses the energetics of the collapse of a single story with conditions most favorable for survival.

In BV (2007), Bazant tries to derive crush down and crush up equations which can be used to measure the roofline and crush front motion of real buildings if 4 simplifying assumptions are met.

He applies a modified form of these same equations to the towers in BLGB and tries to match the actual roofline descent of WTC1 with the model in BLGB.

So the two, BZ and BV cannot be be connected and seen collectively as 'the Bazant Model' ? (Even loosely ?) That seems kind of silly of Bazant doesn't it ? A lost opportunity to make a complete contiguous collapse sequence.