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

[Dave Rogers]

I'm glad that you agree that 'Smith's Law' points up what should be bleedin' obvious to any person with a working brain.

Unfortunately, it's also bleedin' useless. Look up the word "tautology".

Dave

 

[NobbyNobbs]

Newt,


.
No problem. Talking about it here is why I brought it up.

My point is that I'm not talking (yet) about the PROCESS by which it got crushed, or the energy that this process will take.

For now, I am talking about the fact that, if you take a structure such as floors of an office building, the debris formed when it has been crushed & compacted has a higher density than the material before it was crushed & compacted.

Or imagine dropping a bookcase full of books (3 shelves, 2' apart) from a height of 32'. (Easy math.) Assume that shell (i.e., top, bottom & sides) of the bookcase happens to weigh the same as each shelf with its books. When it hits the ground, it's doing 32 ft/sec. Imagine three cases:

1. When the book case hits the floor, imagine that the support brackets on each shelf instantly give way.

2. The bookcase is packaged as delivered by IKEA with all the books stacked & wrapped with packing tape on top. A nice tight bundle.

3.The strength of all the components of the bookcase are sufficient that it survives the drop perfectly intact. The shelves & frame flex like crazy, but nothing breaks. Amazing bookcase.

Case 1. The initial impact delivers the momentum of the shell of the bookshelf ONLY. The shelves all break free, and continue towards the floor. The second impact (bottom shelf) happens about (3 ft / 32 ft/sec =) 0.1 second later, the next 0.05 seconds after that, & the last 0.05 seconds after that. (I've assumed the books are 1' high.) In this case, then you have 4 separate impacts, each delivering 1/4th the total momentum of the entire bookcase.

Case 2. This is the equivalent of a "100% crushed & compacted" bookcase. There is no flex, no give, & no opportunity for anything to break, except in the collision itself. This collision delivers the entire momentum to the ground in one instant.

Case 3. This is intermediate between the first two cases. All of the flexing stretches out the impact in time.

In each case, the total momentum that the bookshelf delivers to the ground is identical. (The mass of the bookshelf x it's terminal velocity.) There is a theorem that equates the "impulse" (force integrated over time) to the change in momentum of the bookcase. And Newtons "equal & opposite forces" law says that the force the ground exerts on the bookcase is equal to the force the bookcase delivers to the ground.

On a force (y axis) vs. time (x axis) graph, the impulse is the area under the curve. So, keeping the areas under the curves the same for all three cases, stretching out the duration of the impact in time automatically lowers the peak and average forces.

Parts breaking & subcomponents colliding separately (like the shelves when the supports break), stretches out the collision time a lot. Structures flexing stretches out the time duration to a lesser amount. But both reduce both the max force & average force delivered to the ground.

"Tah dah...!"

Oh, excuse me, academics in the house. "QED".

Later, we'll get into the subtler aspects, like strength of the structural connections prior to crushing, and the strength of the interdigitation after crushing.

Tom

tfk--

Am I correct in saying that this is the same reason cars have seat belts, air bags, and crush zones? To spread out the transfer of momentum over time, thus reducing the peak force?

 

[bill smith]

tfk responded "Your statement is a lie. I presented to the readers at Topix NIST's CLEAR presentation of their analyses in NCSTAR1-2, which show the plane losing about 70% of its velocity while penetrating the outer wall."

If that's wrong, copy+paste his responses from topix that show "broad agreement" with your view about lack of aircraft deceleration at WTC. If you're right, you have it in your power to show tfk to be the liar, in which case you will receive my most sincere apologies.

Or post here with your topix usernames, and I'll have a look myself.

Well I used the secret name 'bill smith' and he used the name 'Tomk65' or something like that. Be warned though. That single thread has well over 100,000 posts.

Incidentally I am not concernd with proving anyone a liar. I assume that others are capable of making that distinction for themselves over time. Drawing attention to it is unneccessary. I would show those posts but I mostly do not not keep records of posts unless there is some detail i especially want to remember .

 

[bill smith]

tfk responded "Your statement is a lie. I presented to the readers at Topix NIST's CLEAR presentation of their analyses in NCSTAR1-2, which show the plane losing about 70% of its velocity while penetrating the outer wall."

If that's wrong, copy+paste his responses from topix that show "broad agreement" with your view about lack of aircraft deceleration at WTC. If you're right, you have it in your power to show tfk to be the liar, in which case you will receive my most sincere apologies.

Or post here with your topix usernames, and I'll have a look myself.

Check this out Glenn. I put it on the general discussion thread because it will drive things off topic here.

http://www.internationalskeptics.com/forums/showthread.php?t=144954&page=16

 

[tfk]

tfk--

Am I correct in saying that this is the same reason cars have seat belts, air bags, and crush zones? To spread out the transfer of momentum over time, thus reducing the peak force?

.
Hey Nobbs,

For seat belts & crush zones, you are not only correct.
You're DAMN correct.

At 60 mph, you're travelling 88 ft/sec = 1060 in/sec. Which is equivalent to a little more than one millisecond per inch of travel.

I'd guess that your forehead could withstand about 1/2 inch of deflection before fracturing. (When you see bodies deform in slow motion, it's actually amazing, gruesomely amazing, how far bones and limbs will bend before they snap. (Anyone watching a major league pitcher's arm in slow motion has seen this. Another example: Joe Theisman.)

So, a half inch of travel will take about 1 millisecond. (You're slowing down from 1000"/sec to 0"/sec, so your average velocity is about half the max.)
(This is unreasonably simplified, because the object that you hit is going to deform as well. For a simple approximation, assume equal deformation in your head & the dash board. Double the time to 2 milliseconds.)

And while the difference in timing between 2 millisecond and say, 50 milliseconds (with crush zones) may barely perceptible to a human, it reduces the force by a factor of 25. Way cool.

By the way, a crush zone does nothing for you if you're not wearing your seat belt. You get thrown with all the force against the dashboard & window.

I also believe (but I'm not sure) that several of the earlier seat belt systems attempted to incorporate "snubbers" into the design. These are devices that allow the belt to slow down quickly but gradually, instead of just "locking up" like most modern ones. It seems that the mechanisms for those were a bit too complicated, and most today are just an inertia activated locking mechanism. (A weight on a pivot attached to a locking cam.) The car's crush zone now gradually slows down the ANCHOR POINT of the seat belt, which gradually slows down the passenger.

The air bag is a bit different. They found that, with seat belts, even tho you stopped the body & chest, the head pivoted forward (inertia again), hyperextending & seriously damaged the neck muscles & nerves. The air bag's main function is to prevent this injury. Plus they work even if you're not wearing your seat belt. I think that this was it's main selling point: "works even if you're stupid enough not to put on your seat belt."

Of course, a second benefit is that it ALSO does exactly what you say. When wearing just a seat belt, a lot of people said that being in an accident was like being hit in the chest with a baseball bat. Adding surface area (like a bag does) spreads out the contact tremendously, and also - just as you point out - the bag deformation hugely increasing the deceleration time. All the while, keeping the head & chest nearly aligned. All of the above greatly increased the speed of an accident that a person could survive. And frequently, amazingly, walk away from.

One final note: there is a limit to the speeds at which these systems will work. Even tho you can slow down the external surface of the body, the internal organs are still obeying Newton's laws. They have their own inertia. And with too many G's of acceleration, they bounce & slosh around inside you. This causes lots of brain injury (for pro boxers too). And when people jump off of bridges over water, it's frequently the heart ripping free from its supporting tissue, severing the aorta, that causes death.

This is one of the reasons that you're not likely to see seat belts in planes any time soon. (Just one reason. There are lots of others...)

Gruesome physics. But stuff those guys gotta learn in order to help save lives.

Tom

 

[Newtons Bit]

We always argue Bazant here but I sometimes wonder whether we really should be doing that.

The real fact of the matter is that more than 85% of the core and perimeter columns that connected part A and part C were still intact after the crash and right up to collapse initiation. But somehow we still we had a collapse at virtual freefall speed.

So would you like to share your engineering thoughts on what might have happened to the 250-odd columns that held to the last second before failing 100%. Did they kneel or concertina ? We do not see an overlap of the two blocks so kneeling seems to be out of the question. Over to you Newton.

Columns deflect under compression and lateral loads. At a certain point the column splices fail (they're brittle, so it's sudden).

It's not complicated.

 

[bill smith]

Columns deflect under compression and lateral loads. At a certain point the column splices fail (they're brittle, so it's sudden).

It's not complicated.

Is that so ? lol

 

[bill smith]

Columns deflect under compression and lateral loads. At a certain point the column splices fail (they're brittle, so it's sudden).

It's not complicated.

So did they deflect by kneeling or by conertina-ing ? I hope you understand the terninology.

Remember here was no visible overlap of the top and bottom blocks of WTC1 to indicate kneeling.

 

[Newtons Bit]

So did they deflect by kneeling or by conertina-ing ? I hope you understand the terninology.

Remember here was no visible overlap of the top and bottom blocks of WTC1 to indicate kneeling.

I think I'll pull a Heiwa: Please read my paper, all information contained within!

It wasn't "concertina". And I'm not sure what you're referring to by "kneeling".

 

[bill smith]

I think I'll pull a Heiwa: Please read my paper, all information contained within!

It wasn't "concertina". And I'm not sure what you're referring to by "kneeling".

Deflection by bending to the floor somewhat resembling the bended knee, or 'kneeling'.

 

[Newtons Bit]

Deflection by bending to the floor somewhat resembling the bended knee, or 'kneeling'.

They wouldn't actually make it that far before the splices would break.

The splices, being welded together and having a total strength less than that of the total column section, will fracture before the column goes into inelastic buckling.

 

[bill smith]

They wouldn't actually make it that far before the splices would break.

The splices, being welded together and having a total strength less than that of the total column section, will fracture before the column goes into inelastic buckling.

I don't think they knelt anyway because for some in the middle to bend in one direction all the perimeter columns would have to bend too collectively carying Part C visibly out of line with part A which did not happen according to the video evidence.

So if no concertina and no bend what hapened to the core columns between parts A and C ?

 

[Newtons Bit]

I don't think they knelt anyway because for some in the middle to bend in one direction all the perimeter columns would have to bend too collectively carying Part C visibly out of line with part A which did not happen according to the video evidence.

So if no concertina and no bend what hapened to the core columns between parts A and C ?

When a column nears buckling stresses it begins to bend. This internal bending moment is equal to the maximum plastic bending strength of the column. The splices cannot resist this force and they sever before the column bends much.

I've already said this.

 

[bill smith]

When a column nears buckling stresses it begins to bend. This internal bending moment is equal to the maximum plastic bending strength of the column. The splices cannot resist this force and they sever before the column bends much.

I've already said this.

What if they were hot from your raging fires ? Would the metal not become ductile enough to bend without fracturing ?

 

[Dave Rogers]

What if they were hot from your raging fires ? Would the metal not become ductile ductile enough to bend without fracturing ?

If so, then this would necessarily absorb less energy than the fracture of a weld. The cold splice fracture energy is therefore an upper limit to the energy absorption.

Dave

 

[Newtons Bit]

What if they were hot from your raging fires ? Would the metal not become ductile ductile enough to bend without fracturing ?

Not the welds.

 

[tfk]

I can see that Bill's up to his usual...

Ol' T is an inveterate archiver. You have seen him dredge up obscure posts from the distant past from the topix forum and repost them here in his many failed attempts to discredit me and perhaps others. You may have noticed that I invited him to repost the arguments we had about the plane entering the buillding with no apparent loss of speed. You may have further noticed that he did not take the opportunity to do so. Draw ur own conclusions. I'm sure others will.

Simple. To the point, and then you're back to "Ignore", Bill...

1. You said that you "had measured the deceleration of the plane".
http://www.internationalskeptics.com/forums/showthread.php?postid=4940035

You've never measured that. You've never attempted to measure anything. You wouldn't know how to measure it if you wanted to.

You lied.

2. You also said that I "performed acceleration measurements on the plane". And implied that I agreed with you that it didn't slow down. I have never done, nor said that I've done, this measurement.

You lied again.

2. You KNOW you lied. It wasn't a memory slip. You KNOW that you haven't a clue how to measure deceleration on the plane. You KNOW that you don't have the software necessary to do those measurements. You KNOW that you wouldn't know how to use it, even if you did. These aren't mistakes. These aren't "misremembering", bill.

These are lies.

3. You KNOW that we've never agreed on any significant aspect of the events of 9/11. Your opinions are driven by hatred, paranoia and ignorance. Mine by facts & engineering. We discussed the topic of the plane deceleration at great length, and you KNOW that we NEVER agreed. Most certainly, I never suggested with a single word that I agreed with your nonsense.

More lies.

4. You lied, just as you casually, effortlessly lie on a daily basis. Nobody needs to go running off, doing research, to find that you lie daily.

Yesterday on this very thread:

Dave Rogers has been exquisitely clear what he thinks of "Smith's Law".
In direct contradiction to what he has told you, you say:

I'm glad that you agree that 'Smith's Law' points up what should be bleedin' obvious to any person with a working brain.

That's another lie, bill.

5. Yesterday on this very thread:

... we still we had a collapse at virtual freefall speed.

That's not a mistake, bill.

That's a lie.

6. Yesterday on this very thread:

... the topic of this thead is why a one-way crush down of WTC1 is impossible. We are already 20 pages into this second thread on this exact subject and still nobody has has come close to showing how it could be done in the official way.

That's not a mistake, bill. You KNOW that you possess none of the knowledge required to make that judgment. You KNOW that you don't even know what the words (like "rigid body") mean. Much less, understand the concepts that would determine the answer to that question. So, that's not a "mis-statement", bill.

That's another lie.

7. Today on this very thread:

... You have seen him dredge up obscure posts from the distant past from the topix forum and repost them here in his many failed attempts to discredit me ...

I don't "attempt to discredit you", bill. That's not a lie. Just a glaring misperception. The facts discredit you. Science discredits you. Engineering discredits you. But mostly, YOUR OWN WORDS discredit you. Your arrogance discredits you. Your insincerity discredits you. Most of all, your casual lying discredits you.

But there IS another lie in that sentence, bill. That would be the word "failed" in: "... failed attempts to discredit me..."


8. Yesterday on this very thread:

Whatever I do Beachnut I don't lie.

The irony drips & pools.
Like molten metal...

ONE day. ONE thread. Quite the tally.
__

When you lie about yourself, or about events, that's your own business. I could not possibly care less. It's your reputation that is DOA.

You lied about me. Then told me that the burden was MINE to prove you wrong.

In the list of adjectives that describe the true nature of your habitual lying, I guess we can simply add "lazy", "slandering", and "apathetic" to "carefree" & "pathological".


BTW, Bill, I am PERFECTLY HAPPY to have the folk (what was that again... Oh yeah...) "Draw ur own conclusions". I am counting on it.

 

[bill smith]

Not the welds.

Do you mean that the welds were more brittle than the columns they joined ?

 

[Newtons Bit]

FYI, welds are extremely brittle. In areas of high seismic activity, we generally want our lateral force resisting members to go into post-yielding behavior (which is typically what you laymen call "failure") as this bleeds extremely large amounts of energy. To do this, the welds connecting the members are generally designed for 50% more than the absolute maximum force that the yielding members can ever deliver. This allows for some pretty spectacular energy absorption from the earthquake. The buildings are generally a little bit... tilted afterwards, but everyone lives.

 

[Newtons Bit]

Do you mean that the welds were more brittle than the columns they joined ?

Orders of magnitude more brittle.