WTC: Rate of Fall (rate of crush)

Jazzy

Closed Account
Sorry but massive things are alot slower than lightning
Massive things fall at the same speed as light things - were it not for air resistance. That tower dropped as swiftly as a ping-pong ball if you like. Remember Galileo?

The transferred loadings would have moved around the structure at the speed of sound in steel, which is about 5 miles per second. That is similar to the speed of lightning.

It doesn't matter if the intact structure stops the upper one or not - it most definitely has to impede its progress somewhat. Can we agree on that much?
Of course we agree on that. WTC2 fell at about 0.7 G, or 22.5 feet per second per second, WTC1 started lighter and had further to go, and recorded 0.64 G, or 20.6 feet per second per second.
 

Jazzy

Closed Account
No, that is not clear at all. If we know it didn't fall by gravity alone then something had to remove the structure in the floors below the impact.
Not true at all. Do you know what buckling means?

It happens to columns with perfectly adequate compressive stress capability if they lose their lateral connections. They fail for reasons of stability, not stress. They fail silently. No jolt occurs.

http://en.wikipedia.org/wiki/Buckling

Earlier in this thread Mick linked a very informative video. I recommend it to you.

Once the top "assembly" dropped a floor it would be doing about 20 feet per second. How energetic and easy to stop is 120,000 tons of steel at twenty feet per second?
 

George B

Extinct but not forgotten Staff Member
No you have it wrong. . . .these three buildings were the only structures in the world that were engineered to collapse the way they did. . . you cannot find any other structures of their size and construction that ever have histirically or ever will collapse this way again (it was the perfect storm, unprecedented in 4,000 years of human history). . . They can prove it because it was captured on video and tons of other evidence supplied by the authorities who decided how everything would be researched, who would do the research, what data to release when the collapse was simulated, and only reported to the public years and years after the fact to make sure they had it all harmonized and correct of course . . . why should you question such a thing??? If you do you are obviously a nut case and do not understand basic physics . . . Sorry to be so blunt but you know who you are. . . .


See if you don't know how something happened (and are not allowed to consider unusual, alternative explanations) . . . you start with the end result and back engineer it so the assumed process explains the result. . . .as in developing a theory and simulation that fits your preconceived ideas. . . and of course withholding the information you used to construct your simulation . . . You then tell everyone this information and data was withheld for security reasons. . . .
I am sooo sorry I am in error or could be in error of my statement regarding the unprecedented collapse in 4,000 years (I should have said 5,999 years) of human history. . . Seems in Genesis Chapter 11, verses 1-9 (note the coincidence) it seems God may well have Verniage demolished a Tower named Babel. . . .

 

Mick West

Administrator
Staff member
Not true at all. Do you know what buckling means?

It happens to columns with perfectly adequate compressive stress capability if they lose their lateral connections. They fail for reasons of stability, not stress. They fail silently. No jolt occurs.

http://en.wikipedia.org/wiki/Buckling

Earlier in this thread Mick linked a very informative video. I recommend it to you.
Worth posting again, methinks"

 

lee h oswald

Banned
Banned
I am sooo sorry I am in error or could be in error of my statement regarding the unprecedented collapse in 4,000 years (I should have said 5,999 years) of human history. . . Seems in Genesis Chapter 11, verses 1-9 (note the coincidence) it seems God may well have Verniage demolished a Tower named Babel. . . .


Lol!!!!

The old official account up there makes no mention of Verinage, G. And I'm not sure it would work on such a large brick structure. Plus, old G Himself (the other one) must have had a bit of help - there must be a heavenly whistle (or trumpet?) blower somewhere - so many would have been involved - someone would have given the game away by now.....
 

George B

Extinct but not forgotten Staff Member
Lol!!!!

The old official account up there makes no mention of Verinage, G. And I'm not sure it would work on such a large brick structure. Plus, old G Himself (the other one) must have had a bit of help - there must be a heavenly whistle (or trumpet?) blower somewhere - so many would have been involved - someone would have given the game away by now.....
You think?? . . . no thermite then . . . maybe a scalar weapon . . . Hmmm. . . guess the people involved are intimidated by . . . you know . . . the big guy . . .
 

Oxymoron

Banned
Banned
But what exactly is it that makes the top of the building "bounce off" the bottom part? Have you actually done some calculations to quantify this "highly engineered structural steel" property?
Maybe this may help:
http://s1.zetaboards.com/LooseChangeForums/topic/3873416/1/
 

TWCobra

Senior Member.
In fairness the 707 model used to complete the analysis was probably the -320 which actually had a very similar weight and fuel load to the 767-200's that hit the WTC. The fuel loads on those flights also were not full loads. More like 75% full at takeoff less whatever was burnt before impact.

The 767-200 has a slightly larger wingspan and a much larger cabin diameter.
 

George B

Extinct but not forgotten Staff Member
In fairness the 707 model used to complete the analysis was probably the -320 which actually had a very similar weight and fuel load to the 767-200's that hit the WTC. The fuel loads on those flights also were not full loads. More like 75% full at takeoff less whatever was burnt before impact.

The 767-200 has a slightly larger wingspan and a much larger cabin diameter.
Here are the actual figures. . . .707-320 and 767-200 next

 

Jazzy

Closed Account
It doesn't make any sense to suggest that the WTC architects would use a fully fuel-laden Boeing 707 in their structural calculations. Nor would they envisage a collision velocity of 600 miles per hour.

They would have assumed a 707 low on fuel, traveling at a maximum allowable speed of 250 knots, lost in fog, in search of the local airport. The plane that hit the Empire State Building was in such a state. It is a plausible one...

There had been no hijackings of aircraft before or during the time the towers were designed. Only the Japanese had had any previous practice immolating themselves using aircraft.

The architect Leslie Pearson, in fact, claims they made no calculations at all involving the fuel such aircraft carry. None at all.
 

tjohnson_nb

New Member
Bazant's theory starts with the assumption that there was a complete and sudden failure of the structure resulting in a freefall impact of the upper floors on the lower floors. What basis is there for this assumption? This is a huge assumption and without it the rest of the theory falls apart.
 

Mick West

Administrator
Staff member
Bazant's theory starts with the assumption that there was a complete and sudden failure of the structure resulting in a freefall impact of the upper floors on the lower floors. What basis is there for this assumption? This is a huge assumption and without it the rest of the theory falls apart.
It's a simplification for the sake of the model. Of course the reality was a bit more complex, especially with WTC2.

But if you look at the videos of the collapses, you can see there's a pretty rapid onset. It only makes a difference to calculations in the first half a second or so.
 

Jazzy

Closed Account
Bazant's theory starts with the assumption that there was a complete and sudden failure of the structure resulting in a freefall impact of the upper floors on the lower floors. What basis is there for this assumption? This is a huge assumption and without it the rest of the theory falls apart.
The structure failed completely, suddenly, and silently, due to buckling.

This isn't an assumption, because you can see it quite clearly here:

[video=youtube_share;TJJPYTVjxug]http://youtu.be/TJJPYTVjxug[/video]
 

tjohnson_nb

New Member
I'm sorry, but I just can't believe all columns failed simultaneously and completely so as to allow a virtual free fall of the upper structure onto the lower structure. Are you suggesting that ALL columns on the perimeter and in the core bent inwards enough to fracture simultaneously? How were the joist attached to these columns? Would the shear strength of this attachment be stronger than that of the columns?
 

Mick West

Administrator
Staff member
I'm sorry, but I just can't believe all columns failed simultaneously and completely so as to allow a virtual free fall of the upper structure onto the lower structure. Are you suggesting that ALL columns on the perimeter and in the core bent inwards enough to fracture simultaneously? How were the joist attached to these columns? Would the shear strength of this attachment be stronger than that of the columns?
They all failed within a reasonably small period of time. Enough for the model to be valid.

Even if the first floor collapse was not in free-fall, then the second one was. The initiation was more complex than the model, but that's models for you.

How do you think it would have been modified? Would it change the outcome?
 

tjohnson_nb

New Member
They all failed within a reasonably small period of time. Enough for the model to be valid.
You can't simply gloss over it by saying this - it is CRUCIAL to the the theory! The gravity collapse theory relies on there being enough kinetic energy from a free fall to crush the structure beneath.
I am not going to propose any other theories, I only wish to discredit this one.
 

Mick West

Administrator
Staff member
You can't simply gloss over it by saying this - it is CRUCIAL to the the theory! The gravity collapse theory relies on there being enough kinetic energy from a free fall to crush the structure beneath.
I am not going to propose any other theories, I only wish to discredit this one.
But it only required a drop of a few inches for there to be enough energy to break the floor below, and then it's in full free-fall for the next floor. So even if the first floor collapse was somewhat uneven then the building is still going to collapse.
 

tjohnson_nb

New Member
But it only required a drop of a few inches for there to be enough energy to break the floor below, and then it's in full free-fall for the next floor. So even if the first floor collapse was somewhat uneven then the building is still going to collapse.
My point is that it is a huge assumption to assert ANY amount of freefall - not even a 1/1000 of an inch.
 

Mick West

Administrator
Staff member
My point is that it is a huge assumption to assert ANY amount of freefall - not even a 1/1000 of an inch.
What do you think would happen when some columns buckle? If a column buckles it has basically lost all of its supporting strength. It's not going to slowly fold in two. It's going to collapse.


Even if the first floor collapse were NOT pure freefall (and it probably was not), then consider the energy the upper mass has when it moves through the distance from one floor to the next. That energy has to go somewhere. It's the same amount of energy regardless of how long it takes to get there.
 

Jazzy

Closed Account
I'm sorry, but I just can't believe all columns failed simultaneously and completely so as to allow a virtual free fall of the upper structure onto the lower structure.
Had all the columns failed simultaneously then the tower would have fallen straight down.

The logical place for the columns to have begun to fail was at the weakest column.

When that column buckled, its loading was immediately transferred to its neighbors. The tower mast of WTC 1 showed a circular movement as buckling failure moved clockwise round that tower level.

Are you suggesting that ALL columns on the perimeter and in the core bent inwards enough to fracture simultaneously?
No.

How were the joists attached to these columns?
They were bolted.

Would the shear strength of this attachment be stronger than that of the columns?
Stronger than the bend force required to initiate buckling in the column, yes, obviously.

You have to remember that this mode was multiple. There were many floor connections pulling in on many columns simultaneously. There were lateral tension members spreading loads laterally also. Even so, there would have been a single point where the maximum load occurred, albeit briefly.

You can't simply gloss over it by saying this - it is CRUCIAL to the the theory!
It is not a "glossing over". Ignoring how material buckles is crucial to your assumption.

The gravity collapse theory relies on there being enough kinetic energy from a free fall to crush the structure beneath.
All engineers agree there is. The top of the tower was doing 20 feet per second when it hit the structure beneath - off the structure line.

I am not going to propose any other theories, I only wish to discredit this one.
When you are greatly ill-equipped to do so. No understanding of buckling and an argument from ignorance/disbelief do you no good.

My point is that it is a huge assumption to assert ANY amount of freefall - not even a 1/1000 of an inch.
And my point is that you believe buckling to be a "huge assumption" even when it has been demonstrated to you, both model and reality.

Buckling has been understood by engineers for two hundred and sixty years.
 

tjohnson_nb

New Member
What do you think would happen when some columns buckle? If a column buckles it has basically lost all of its supporting strength. It's not going to slowly fold in two. It's going to collapse.


Even if the first floor collapse were NOT pure freefall (and it probably was not), then consider the energy the upper mass has when it moves through the distance from one floor to the next. That energy has to go somewhere. It's the same amount of energy regardless of how long it takes to get there.
It's anybody's guess how the failure would manifest itself as columns began buckling - I assume the upper structure would begin tilting to the side where the failure begins. IMO, it would probably tilt until it reached an equilibrium or else completely fell off the lower structure. Apparently the antenna weighed over 700 tons so if the upper structure started tilting I wouldn't be surprised if it just fell right off.
 

Mick West

Administrator
Staff member
It's anybody's guess how the failure would manifest itself as columns began buckling - I assume the upper structure would begin tilting to the side where the failure begins. IMO, it would probably tilt until it reached an equilibrium or else completely fell off the lower structure. Apparently the antenna weighed over 700 tons so if the upper structure started tilting I wouldn't be surprised if it just fell right off.
It can't keep tilting without crushing the floors beneath it. That will arrest the tilt. Buy that time it's moving down fast enough to just go straight down.
 

tjohnson_nb

New Member
Btw, I have been checking some of Bazant's references in the Mar 2007 paper from ASME and one reference is to "floors failed by progressive collapse; the 2000 Commonwealth
Ave. tower in Boston in 1971, triggered by punching of insufficiently hardened slab". IMO, this seriously puts his credibilty into question. I can find almost no similarity between this event and the collapse of WTC and to suggest there is any makes me very suspicious about his analysis in general. You can read about it here
 

tjohnson_nb

New Member
It can't keep tilting without crushing the floors beneath it. That will arrest the tilt. Buy that time it's moving down fast enough to just go straight down.
I agree that its possible the top could fall off sideways and accelerate to the ground afetr snapping off the central core, but no way could it fall straight down through the path of highest resistance.
 

Mick West

Administrator
Staff member
I agree that its possible the top could fall off sideways and accelerate to the ground afetr snapping off the central core, but no way could it fall straight down through the path of highest resistance.
But if you factor in gravity, it's actually the path of least resistance. There is no force acting sideways on the building, so why would it move sideways? There is the massive weight of the mass of the building acting straight down, so that's naturally the way it would move.

Why woud you expect it to move sideways? What are the forces?
 

tjohnson_nb

New Member
But if you factor in gravity, it's actually the path of least resistance. There is no force acting sideways on the building, so why would it move sideways? There is the massive weight of the mass of the building acting straight down, so that's naturally the way it would move.

Why woud you expect it to move sideways? What are the forces?
Yes, there is massive weight acting straight down and there is equally massive force holding it up. But if the centre of gravity of the top gets shifted enough to one side then its possible for it fall unimpeded over the side. I'm assuming some perimeter columns would fail first so the sideways motion is introduced as it pivots on the core structure.
 

Mick West

Administrator
Staff member
Yes, there is massive weight acting straight down and there is equally massive force holding it up. But if the centre of gravity of the top gets shifted enough to one side then its possible for it fall unimpeded over the side. I'm assuming some perimeter columns would fail first so the sideways motion is introduced as it pivots on the core structure.
Again, this is confusing static and dynamic force.

When the lower building is holding up the upper part (the "block"), it's just supporting the static force. Of course it's not going.

But when the upper part of the building starts moving downwards, then it's a dynamic force, and to stop it without the lower floor collapsing it has to exert an upwards force MUCH greater than before, because the acceleration needed is many many times that of one gravity.

Try this illustrative experiment. Take something like a can of soda, hold it in your hand. Easy right? The downwards force is very small, and very easy to counteract, just the acceleration due to gravity.

Now take the same can, throw it up in the air ten feet, then put your arm back in the same position, and try to catch the can without your hand moving downwards more than 1/4 of an inch.

Notice the difference in the force, static vs. dynamic.
 

tjohnson_nb

New Member
Again, this is confusing static and dynamic force.

When the lower building is holding up the upper part (the "block"), it's just supporting the static force. Of course it's not going.

But when the upper part of the building starts moving downwards, then it's a dynamic force, and to stop it without the lower floor collapsing it has to exert an upwards force MUCH greater than before, because the acceleration needed is many many times that of one gravity.

Try this illustrative experiment. Take something like a can of soda, hold it in your hand. Easy right? The downwards force is very small, and very easy to counteract, just the acceleration due to gravity.

Now take the same can, throw it up in the air ten feet, then put your arm back in the same position, and try to catch the can without your hand moving downwards more than 1/4 of an inch.

Notice the difference in the force, static vs. dynamic.
Thanks Mick, I'm aware of all that. :cool: As I said at the outset I don't think its reasonable to assume any dynamic loading on the lower structure. We have to always make assumptions in science but the rule of thumb is to make the least number of them and simplest as possible. IMO, assuming a negligible resistance to the upper structure falling and thus accelerating onto the lower one is a very large, and I would venture, unpredecented assumption.
 

Mick West

Administrator
Staff member
Thanks Mick, I'm aware of all that. :cool: As I said at the outset I don't think its reasonable to assume any dynamic loading on the lower structure. We have to always make assumptions in science but the rule of thumb is to make the least number of them and simplest as possible. IMO, assuming a negligible resistance to the upper structure falling and thus accelerating onto the lower one is a very large, and I would venture, unpredecented assumption.
It's not negligible, it's huge, it's just smaller than the local dynamic forces.
 

Jazzy

Closed Account
Yes, there is massive weight acting straight down and there is equally massive force holding it up. But if the centre of gravity of the top gets shifted enough to one side then its possible for it fall unimpeded over the side. I'm assuming some perimeter columns would fail first so the sideways motion is introduced as it pivots on the core structure.
That really isn't possible because it is trying to fall sideways against the core, which is a strongly cross-braced structure which still threads the collapsing floors.

sim1.gif
 

tjohnson_nb

New Member
It's not negligible, it's huge, it's just smaller than the local dynamic forces.
Here is what Bazant wrote; (emphasis mine)
As a result, the upper part of the tower fell, with little resistance, through at least one floor height, impacting the lower part of the tower.
Why not tilt to the side where the outer columns are weakest?
 

tjohnson_nb

New Member
That really isn't possible because it is trying to fall sideways against the core, which is a strongly cross-braced structure which still threads the collapsing floors.
Why couldn't the entire core in the upper structure deflect from vertical as the perimeter columns buckle?
 

Mick West

Administrator
Staff member
Here is what Bazant wrote; (emphasis mine)
Why not tilt to the side where the outer columns are weakest?
It did. But once it has tilted, that means that ALL the columns on that floor are no longer vertical, and so they cannot support the weight that's directly above them. They have buckled, so the upper block falls.

Sure this is initially asymmetric, but since the side that falls faster encounters more resistance it evens out and stops rotating.

And it's really even a bit more complex than that. The block can't really hinge very rigidly, because the floors are not strong enough. The horizontal members are vastly weaker than the vertical members.
 

Jazzy

Closed Account
Why couldn't the entire core in the upper structure deflect from vertical as the perimeter columns buckle?
It would have deflected from the vertical. But steel is much more elastic than plastic. There would have been many core columns that merely experienced bending. There would have been some that snapped, and some that buckled.

Even so, as they began to fall they were being restrained back to the vertical by the as-yet unbroken core column assembly beneath. That assembly did get broken in the end, leaving a few free-standing core columns. These were also unstable, and finally buckled.

In the simulation image above, which admittedly is no more than roughly accurate, most of the floors have already fallen (due to being well off the vertical), leaving a top which is merely a shell. This was true also of WTC 7, where the internal structure had collapsed before the exterior was seen to do so.
 

Mick West

Administrator
Staff member
That assembly did get broken in the end, leaving a few free-standing core columns. These were also unstable, and finally buckled.
They were quite large too, half the height of WTC1:



 
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