Why a one-way Crush down is not possible

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Heiwa said:
C. The only things that fluctuate are the internal, dynamic forces in parts C and A after impact. Mass cannot fluctuate! So just study these forces and how they displace the elements or break elements/connections locally.
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These absolutist statements such as "Mass cannot fluctuate" will be the death of you some day.

I'll try to make it simple for you: You have two buckets half-filled with sand. Bucket A and Bucket B.

You pour twenty grams of sand from A to B. The mass within A and the mass within B have now FLUCTUATED. Like it or not. One has been reduced by twenty grams, the other has increased by twenty grams.

You may reply, "But the original mass didn't fluctuate! It just moved from one place to another!"

I never claimed that it did otherwise. When I talk about the moving portion of the building, I am not talking about a portion that has always and will always be moving. I am talking about the parts of the building that are in motion at a particular moment in time.

Follow me so far?

When I talk about the stationary portion of the building, I am talking about that portion of the building that is still intact and is not moving. I am not talking about the section that was intact at the moment the upper section became mobile.

Get it? It's a process.

When I say that the mass of each fluctuates, I am saying that as time progresses, some of the mass that was formerly in the stationary category moves into the mobile category, and vice versa. Clearly, in the case of the WTC towers, most of this switching of categories occured in one direction: the mobile mass tended to grow larger with time, while the stationary mass grew smaller, until equilibrium was reached.

There! Was that so hard? Now you can't pretend that you don't understand what I'm talking about.


After a while the forces become static! Arrest has occurred.
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I agree. At some point the moving mass encounters debris that is so compacted that it cannot continue to fall. By the time the WTC towers reached that point, they had completely collapsed.
 
I would say yes, based on my admitted limited engineering understanding. But, my dad is a SE, and he has agreed.
 
Everyone,

Sorry about my troll-feeding. I'm sure this has all been explained to Heiwa many times, ad nauseum.

I have no illusions that Heiwa will somehow "get it". It's just that I find this discussion interesting.
 
aggle-rithm said:
Can you give us some examples?
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What about fruit and cardboard?

How better to model a structure like the WTC than with a bunch of lemons or some pizza boxes?

Fruit and cardboard....engineering analysis at its finest......
 
aggle-rithm said:
These absolutist statements such as "Mass cannot fluctuate" will be the death of you some day.

I'll try to make it simple for you: You have two buckets half-filled with sand. Bucket A and Bucket B.

You pour twenty grams of sand from A to B. The mass within A and the mass within B have now FLUCTUATED. Like it or not. One has been reduced by twenty grams, the other has increased by twenty grams.

You may reply, "But the original mass didn't fluctuate! It just moved from one place to another!"

I never claimed that it did otherwise. When I talk about the moving portion of the building, I am not talking about a portion that has always and will always be moving. I am talking about the parts of the building that are in motion at a particular moment in time.

Follow me so far?

When I talk about the stationary portion of the building, I am talking about that portion of the building that is still intact and is not moving. I am not talking about the section that was intact at the moment the upper section became mobile.

Get it? It's a process.

When I say that the mass of each fluctuates, I am saying that as time progresses, some of the mass that was formerly in the stationary category moves into the mobile category, and vice versa. Clearly, in the case of the WTC towers, most of this switching of categories occured in one direction: the mobile mass tended to grow larger with time, while the stationary mass grew smaller, until equilibrium was reached.

There! Was that so hard? Now you can't pretend that you don't understand what I'm talking about.




I agree. At some point the moving mass encounters debris that is so compacted that it cannot continue to fall. By the time the WTC towers reached that point, they had completely collapsed.
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What if part A had been 200 floors ? Where would collapse arrest have taken place then ?
 
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phunk said:
In a design like the WTC, probably at the ground no matter how tall you made it.
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That means that 5 % of a building would crush te other 95% down level with the ground
. See it as a stack of 20 identical items with the top one crushing the other 19. It has never happened and it never will.
 
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Phunk,

I was thinking the same thing. Somewhere near the earth would be my guess. But, it could have been at the first floor, im not certain.
 
It feels so stupid to have to argue the reason why the top item in a stack of 20 identical items will never crush the other 19 down flat with the ground using gravity alone. Just the fact that nobody can design any structure to demonstrate that it can happen or the fact that it has never happened in the entire history of worldwide construction either before or after 9/11 should be enough to end any debate.

So when somebody (or apparently almost everybody on the jref ) says it can happen without providing example or precedent you can guess what that tells me. That fact is just as self-evident as the fact that the top item in a stack of 20 identical items will never crush the other 19 down to the ground by gravity alone..
 
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bill smith said:
That means that 5 % of a building would crush te other 95% down level with the ground
. See it as a stack of 20 identical items with the top one crushing the other 19. It has never happened and it never will.
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Bill, it's not like standing on top of a pop can, bending over, tapping the sides of the can, then your weight crushes the can to a small package.

I think crush is the wrong term.

As I and others have said before, the truss connections on the inside of the perimeter columns and core columns are what get the brunt of the weight of the top section collapsing.

The "L" shaped truss connection cannot handle the sudden impact of the load from above so they either bend downward or shear off.

Heiwa's description of the columns being "between the floors" is wrong on his site. The perimeter columns were around the outside of the floor and the core columns were on the inside of it, not underneath each floor.
 
Gamolon said:
Heiwa's description of the columns being "between the floors" is wrong on his site. The perimeter columns were around the outside of the floor and the core columns were on the inside of it, not underneath each floor.
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Is it? I clearly say that the intact floors are just hanging on the columns; like pictures on a wall. Or a pin for a bird to sit on in a cage.

And if a picture drops from the wall it doesn't one-way crush down the wall. Same for the pin! The bird has no pin to sit on. And the cage does not collapse.
 
Heiwa said:
Googling on fluctuating mass I found following.

http://ias-spes.org/SPESIF2009/Presentations/Woodward_1.pdf

Maybe someone should build in this contraption in a structure so that the top part C one-way crushes the bottom part A. You never know!

I will have a try with my pizza boxes.
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Don't forget the lemon test!!!

Maybe you could also try other types of fruit.....

Fruit tests seem to be quite popular when trying to analyze and model the performance of complex structures....
 
Heiwa said:
Not really! It seems paranormal and is thus welcome at JREF.

Suggest you design a structure with fluctuating mass and enter it in The Heiwa Challenge thread.
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Well, I guess I have underestimated your ability to pretend not to understand.
 
bill smith said:
That means that 5 % of a building would crush te other 95% down level with the ground
. See it as a stack of 20 identical items with the top one crushing the other 19. It has never happened and it never will.
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Nope. I'm sure this has been explained to you, but I will try again.

The top item crushes the item below it.

There are now two items in motion.

Then the top two items crush the item below them.

There are now three items in motion.

Then the top three items crush the item below them.

There are now four items in motion.

Then the top four items crush the item below them.

...should I go on? Or can you figure it out from here?
 
Heiwa said:
Is it? I clearly say that the intact floors are just hanging on the columns; like pictures on a wall. Or a pin for a bird to sit on in a cage.

And if a picture drops from the wall it doesn't one-way crush down the wall. Same for the pin! The bird has no pin to sit on. And the cage does not collapse.
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Please answer the question I asked before. If I were to pound a perimeter column into the ground and then welded an "L" shaped connection to said column and then hit it with a sledgehammer, where is the most stress going to occur?
 
bill smith said:
What if part A had been 200 floors ? Where would collapse arrest have taken place then ?
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Do you know why there are no 200 story buildings?

It's because the taller a building is, the greater the tendency to fall down.

Engineering a tall building is all about overcoming this tendency. They haven't yet figured out how to do this for a 200-story building.

With this in mind, it's easy to see that the taller a building is, the MORE likely it will collapse completely if the top part of it starts to move downwards.
 
Gamolon said:
Please answer the question I asked before. If I were to pound a perimeter column into the ground and then welded an "L" shaped connection to said column and then hit it with a sledgehammer, where is the most stress going to occur?
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Heiwa's babbling again. It goes in cycles.

Soon he will disappear for a few days, then return and make the same unsupported arguments all over again.
 
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