Help me understand the collapse a bit more

I'll give it a go:

As things started falling onto other things, it shook the sh[rule8] out of the other stuff that was still standing, which only served to make them weaker, so that when the falling stuff hit them, they broke easier.
 
Anti-sophist said:
Well, the reason that 14 floors can destroy 90 is the difference between static and dynamic forces. This difference can be demonstrated very easily in your home gym. Take a 15 pound dumbell and rest it on your foot. Now take a 15 pound dumbell and drop it from 10 feet, onto your foot. Notice a difference?
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Yeah, but wouldn't that mean your foot would actually be a 150 pound hunk of iron, and the 15 pounds above would be like a VW Beetle crashing into 40 buses, it wouldn't destroy all the buses, would it?
 
The Kooger said:
Yeah, but wouldn't that mean your foot would actually be a 150 pound hunk of iron, and the 15 pounds above would be like a VW Beetle crashing into 40 buses, it wouldn't destroy all the buses, would it?
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Then how about this analogy? Rest the 15 pounds on your head. No problem. Now drop the 15 pounds from a few feet above onto your head.
 
3bodyproblem said:
Well I proposed there were 2 collapse waves. The first wave strips the floors from the exterior and interior columns, approximately 3-4 stories before the second wave, which is the bottom of the falling upper section. This first collapse wave can best be visualized by the "pancaking" or "bagelling" of the floors. Due to the construction of the WTC floor and truss system this is somewhat of a misnomer, as the floors would not have remained intact during the collision. Instead it would have been more of a churned up mess of cement and steel. The second wave would have been responsible for destruction of the core, and peeling away the now detached exterior columns. I personally think this theory is supported by the video evidence and can account for the freestanding core. I proposed this over at PhysOrg but was immediately shot down due to the following: the inertia of the floors would have slowed that collapse wave down and allowed the upper section to catch up. I contended that this would manifest as an oscillation, in the sense that if the second wave caught up to the floor collapse wave, the pressure the upper section exerted would accelerate the floor collapse wave ahead of it. The period of oscillation would of course be very short due to the speed of the collapse. Again I didn't get much of a response from that so I kinda dropped it. Not much of a point developing a model if you're not going to get any support :) It's still the way I visualize the collapse until I get any new information on the matter.
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This is pretty much how I see the collapse occurring. I believe it's also supported by the debris pile.

-Gumboot
 
BeAChooser said:
Then how about this analogy? Rest the 15 pounds on your head. No problem. Now drop the 15 pounds from a few feet above onto your head.
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And you shatter, into a pulp, leaving just your boots?
Yup, it's a good analogy to use to spot the weaknesses in certain theories.
 
scooby said:
And you shatter, into a pulp, leaving just your boots?
Yup, it's a good analogy to use to spot the weaknesses in certain theories.
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No, but you would collapse to the floor. The reason a person wouldn't break apart is because, percentage wise, they tend to be bit less hollow than buildings.
 
Redtail said:
No, but you would collapse to the floor. The reason a person wouldn't break apart is because, percentage wise, they tend to be bit less hollow than buildings.
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Though some people have hollow heads, don't they scooby?
 
Undesired Walrus said:
What's all this conservation of energy stuff, momentum etc?
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I'm not sure anyone's given the simple explanation you're looking for on these two points, so I'll give it a try.

First: Conservation of energy. When the top block falls on the floor below, it has a certain amount of energy due to the speed at which it's falling (the kinetic energy of the block). In order to break the supports of the floor below, a certain amount of energy needs to be used up (the fracture energy of the supports). If the kinetic energy is more than the fracture energy, then the top block can break the supports and then just keep on going. If it's less, then all the kinetic energy will be used up breaking just some of the supports, and the collapse will stop.

Gordon Ross argues that if you assume that the top block falls perfectly straight and square on to the floor below, so that the columns all line up, and if you ignore two-thirds of the kinetic energy for a bogus reason, then the remaining one-third of the kinetic energy isn't quite enough to break the supports, as long as you assume that the supports are springier than they could possibly be. There are a couple of flaws in his reasoning, as you can see.

Second: Momentum. This is an interesting one, and something it took me a while to figure out. Because the collapse of the twin towers started from the top, it was at first only the top bit that was accelerated downwards by gravity. When the top part hit the first floor below it, it had to speed up that floor so that everything was falling at the same speed. As a result, the top block was slowed down a bit. After the floor supports collapsed, then the whole lot carried on speeding up until it hit the next floor, where the same thing happened. Dr. Frank Greening (Apollo20 on this forum) analysed how much effect this had on the collapse time - an excellent piece of work IMHO, BTW, Dr. Greening - and found that it was what made the main difference between the time the towers actually took to collapse, and the time it would have taken for the top block to free-fall if the rest of the tower hadn't been there. Several people in the truth movement have repeated the calculations and got the same result, which they then have to find some very clever ways to ignore.

The interesting bit, and the bit I didn't see at first, is that when the main part of WTC7 fell, the collapse started near the bottom. As a result, most of the building was accelerated all together, and there weren't any non-collapsing bits of floor that needed to be speeded up. That meant that the time for WTC7 to collapse was a lot closer - about half a second rather than a few seconds - to the free-fall time. That's something the truth movement like to fixate on, because CDs fall in close to freefall times for the same reason - the collapse starts at the bottom. But in fact it proves nothing, because you'd expect the same for any collapse that started low down, for any reason.

Hope this helps.

Dave
 
Dave Rogers:

Thanks for the kind remarks!

Since we are all looking for useful models/analogies, it helps to remember that the upper section of WTC 1 weighed about 50,000 tons - that's about the same weight as the great ship the TITANIC.

Now think of the TITANIC dropping 12 feet onto the top of a building.....
 
The Kooger said:
Yeah, but wouldn't that mean your foot would actually be a 150 pound hunk of iron, and the 15 pounds above would be like a VW Beetle crashing into 40 buses, it wouldn't destroy all the buses, would it?
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Let's add gravity to that analogy, since gravity was known to be present at Ground Zero.

Park the 40 buses end to end on a very steep slope, with 11 feet of space between each bus and the next.

Adjust their parking brakes so that the brakes can resist 60% more force than is needed to hold each bus in place. (So, if for example you increased the weight of a bus by 50%, the brakes would still hold, but if you doubled it, the brakes would slip.) Also, design the parking brakes so that if any bus moves a foot or more, its parking brake releases completely.

Leave their transmissions/gearboxes in neutral, and their wheels aimed straight down the slope.

Now crash that VW beetle into the first one at the top of the slope.

Don't forget to videotape it, I want to watch.

Respectfully,
Myriad
 
Who dropped the titanic on the buildings ? Was it Godzilla ?

Again the main question is why energy is dissipated at the crushing front..
 
Myriad said:
Park the 40 buses end to end on a very steep slope, with 11 feet of space between each bus and the next.

Adjust their parking brakes so that the brakes can resist 60% more force than is needed to hold each bus in place. (So, if for example you increased the weight of a bus by 50%, the brakes would still hold, but if you doubled it, the brakes would slip.) Also, design the parking brakes so that if any bus moves a foot or more, its parking brake releases completely.

Leave their transmissions/gearboxes in neutral, and their wheels aimed straight down the slope.

Now crash that VW beetle into the first one at the top of the slope
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There's a problem with your analogy. If the portion below the impact zone of the towers is represented by 40 buses, then it is not a VW that crashes into the top one but a bunch of other buses. Each tower had 110 stories. In the case of WTC 1, which was hit between the 93rd and 99th floors, the top block would be equivalent to at least 4 buses! In the case of WTC 2, which was hit between the 77th and 85th floors, the top block would be equivalent to at least 9 buses. Now do we really have to do your experiment or can common sense prevail?
 
Were the floors that pulled inwards (On the videotapes) intact floors, undamaged by the plane? Was it just the floor trusses that are visible that caused the collapse? As they seem to be below the impact point on the south tower.

Was the reason the floors did not collapse from above because there were missing floors to do this from above because of the destruction?



Also
 
Is there a simple model that can be built to show the start of collapse? I thought I saw one on the internet somewhere.
 
BeAChooser said:
There's a problem with your analogy. If the portion below the impact zone of the towers is represented by 40 buses, then it is not a VW that crashes into the top one but a bunch of other buses. Each tower had 110 stories. In the case of WTC 1, which was hit between the 93rd and 99th floors, the top block would be equivalent to at least 4 buses! In the case of WTC 2, which was hit between the 77th and 85th floors, the top block would be equivalent to at least 9 buses. Now do we really have to do your experiment or can common sense prevail?
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Well, my point was that a runaway chain reaction ("collapsing" all the buses down the hill) would probably occur even if you started with only the aforementioned Beetle (though the first reaction in the chain might be a little iffy, it would depend on the Beetle's speed). But you're correct, starting with a few rolling buses would be a better analogy.

Respectfully,
Myriad
 
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Yah, as Dave pointed out the Gordon Ross theory relies on a perfectly symetrical collision. Since we're all throwing anologies around, the best one I can think of is dropping a full glass of water on the ground. Much like Ross' theory, if you drop the glass perfectly there is a "sweet spot" where it hits the ground and doesn't spill a drop. As long as the bottom of the glass remains horizontal to the ground at impact, in theory, it won't tip or spill. This was the image I got in my head when I first read his paper, and still the reason I laugh out loud when I think about it.
 
Undesired Walrus said:
Were the floors that pulled inwards (On the videotapes) intact floors, undamaged by the plane? Was it just the floor trusses that are visible that caused the collapse? As they seem to be below the impact point on the south tower.

Was the reason the floors did not collapse from above because there were missing floors to do this from above because of the destruction?
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There's a good summary of the simulated sagging in NCSTAR 1-6, it is a combination of trusses sagging and exerting pull in forces and trusses becoming disconnected with the outer walls decreasing lateral stability.
 
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