Acceleration of the falling top blocks.

leftysergeant

Penultimate Amazing
leftysergeant
Joined
Jul 13, 2007
Messages
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I have been thinking a great deal over the last couple of days about the rate of acceleration when the top blocks of the twin towers began collapsing.

Everybody seems to assume that the core columns all broke more or less at the same time, to allow the top block to fall more or less at free-fall acceleration. Has that actually been measured?

Only in the context of free-fall does Tony's "missing jolt" make the least bit of sense.

But the seems to me to be an alternate mode of failure that would not produce any jolt.

Think for a moment of the huge "horseshoe" column, smoothly bent with not much cracking. This can, of course, only occur in very hot steel. Is it possible that the columns just sagged enough to let the top blocks come to rest catawampus on the perimeter columns and start to break up, thus wedging the perimeter columns open?
 
leftysergeant said:
I have been thinking a great deal over the last couple of days about the rate of acceleration when the top blocks of the twin towers began collapsing.

Everybody seems to assume that the core columns all broke more or less at the same time, to allow the top block to fall more or less at free-fall acceleration. Has that actually been measured?...
Click to expand...
It can often help understanding to approach a problem from a different direction. Try this one:

Once the top block had started to fall all the support columns had already failed. some of them the top bit already falling past the bottom bit. Others still plastic bending before breaking. But all had failed. And none of them landed top bit on bottom bit and stayed there. (Several points in that paragraph which need careful thinking to understand why they must be true... :rolleyes: )

Now how had they got to that stage - still working backwards. Your comment "more or less at the same time" is not specific enough. The failure is best explained as a "cascade failure" to which a a zip fastener pulling apart may be a suitable analogy. i.e. columns failing in a sequence where as any one column fails the load it was carrying transfers to other columns and either causes those columns to collapse or brings them closer to collapse load. So, just as each catch of a zip is easy to pull apart bit by bit in rapid succession working from one end BUT the whole zip fastener is not easy to separate in one go. Similarly a small load transfer can cause one extra column to transfer fail but never enough to cause all remaining columns to fail at once. (And, yes, there could well be situations occurring in the progressive cascade failure where a small group of columns fail "more or less at the same time". That exception does not disprove the principle.)

So the whole process was very fast once the critical stage was reached. But it was still sequential failures not "all at once". Hence my comment about "more or less at the same time" not being specific enough.

At that point my explanation parts company with many others but....
leftysergeant said:
...Only in the context of free-fall does Tony's "missing jolt" make the least bit of sense....
Click to expand...
The base problem with the "Missing Jolt" explanation is that it makes some assumptions then uses circular logic to prove the assumptions. :(
leftysergeant said:
...But the seems to me to be an alternate mode of failure that would not produce any jolt....
Click to expand...
Yes. And the concept of "one storey free fall" is highly sus whether proposed by a truther or a Bazant follower.
leftysergeant said:
Think for a moment of the huge "horseshoe" column, smoothly bent with not much cracking. This can, of course, only occur in very hot steel. Is it possible that the columns just sagged enough to let the top blocks come to rest catawampus on the perimeter columns and start to break up, thus wedging the perimeter columns open?
Click to expand...
I'm not sure what you are saying. BUT recall my first comments "Once the top block had started to fall all the support columns had already failed.... And none of them landed top bit on bottom bit and stayed there."
 
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I was thinking that the crucial columns may have sagged quickly and greatly, rather than snapped.It may have just set the load down on the floors below, rather than dropping them with a thud. Is there a record of more than one "horseshoe column?"
 
leftysergeant said:
I was thinking that the crucial columns may have sagged quickly and greatly, rather than snapped.It may have just set the load down on the floors below, rather than dropping them with a thud...
Click to expand...
I think that you are heading in the right direction - we are using different words but what you are starting to spell out is much the same as I have suggested. BTW it puts us at odds with both sides of the usual arguments - whether Bazantish OR truther. The background issue is that you are joining a small group which includes me who try to explain the collapses by describing the mechanisms of collapse. Contrast with the vast majority of debunkers who are Bazant followers and who work primarily from abstract modelling and energy considerations. And give little credence to the actual details of mechanisms. The "set down" rather than "with a thud" is not as gentle as it may seem. Remember that the cascade failure went very quickly despite being sequential failures of columns. So the "set the load down" was fast enough to be a really quite sharp "thud" - just not the thud which would come from a free fall drop
leftysergeant said:
Is there a record of more than one "horseshoe column?"
Click to expand...
Sorry - I don't know.
 
I'm still stuck near the beginning of your post:

leftysergeant said:
Everybody seems to assume that the core columns all broke more or less at the same time, to allow the top block to fall more or less at free-fall acceleration.
Click to expand...

Really? I think, going all the way back to Bazant and Zhou, they conclude that the columns can't have dissipated very much energy, but they don't assume as fact that the core columns all fractured at some moment. Often the models make assumptions that aren't intended to be factual. (And sometimes people aren't very clear -- perhaps even in their own minds -- which assumptions are intended to be factual.)

Has that actually been measured?
Click to expand...

If you're referring to the acceleration right at the beginning of the collapses, the measurement problems are formidable. I think it's accepted by everyone who has really looked that the tops fell at average acceleration substantially less than g. In the video under discussion on tfk's thread, Chandler says that the north tower fell at 2/3 g. Why Chandler thinks that is evidence for CD, instead of evidence against CD, I cannot tell.

I don't think it's facially unreasonable to argue that the 'almost constant' acceleration observed at the rooflines argues against certain naive collapse models.
 
leftysergeant said:
I have been thinking a great deal over the last couple of days about the rate of acceleration when the top blocks of the twin towers began collapsing.

Everybody seems to assume that the core columns all broke more or less at the same time, to allow the top block to fall more or less at free-fall acceleration. Has that actually been measured?

Only in the context of free-fall does Tony's "missing jolt" make the least bit of sense.

But the seems to me to be an alternate mode of failure that would not produce any jolt.

Think for a moment of the huge "horseshoe" column, smoothly bent with not much cracking. This can, of course, only occur in very hot steel. Is it possible that the columns just sagged enough to let the top blocks come to rest catawampus on the perimeter columns and start to break up, thus wedging the perimeter columns open?
Click to expand...

I do not understand why people have such problems understanding the collapse of the towers. There is no "missing jolt" because the impact of the upper floor block was not directly on the columns below, but rather into the floor plates and adjacent spaces. The floor plates were only designed for a 100+ psf , but they provided the lateral bracing for the columns. The floor plates failed due to extreme overloading, the columns failed because of no lateral support. The horseshoe bends are not unexpected, with everything going on, it would not be surprising to have some columns remained fixed at on end while an extreme load was places on the other end.
 
Acceleration is a very good topic. Acceleration takes time.
If, say, the upper twenty stories fell one story, it would take a
certain amount of time for them to hit the story below, the
better part of a second, actually.

Then, once the top twenty-one floors started falling, it would
take that same fraction of a second for them to hit the lower
floor.

Then, once the top twenty-two floors started falling, it would
once again take the same fraction of a second to hit the next
lower floor.

The distance from floor to floor was about twelve feet, and it
takes about 0.75 seconds to accelerate through that distance
even without including the resistance that the bottom floors
and wall sections of the building would have provided.

Add up all these fractions of a second, and you get something
very different from what was seen on 9/11. We saw the buildings
go away in about a dozen seconds. There wasn't enough time
for a floor-by-floor gravity accelerated collapse. The buildings
fell too fast for this to be a proper explanation.
 
WTC Dust said:
Acceleration is a very good topic. Acceleration takes time.
If, say, the upper twenty stories fell one story, it would take a
certain amount of time for them to hit the story below, the
better part of a second, actually.

Then, once the top twenty-one floors started falling, it would
take that same fraction of a second for them to hit the lower
floor.

Then, once the top twenty-two floors started falling, it would
once again take the same fraction of a second to hit the next
lower floor.

The distance from floor to floor was about twelve feet, and it
takes about 0.75 seconds to accelerate through that distance
even without including the resistance that the bottom floors
and wall sections of the building would have provided.

Add up all these fractions of a second, and you get something
very different from what was seen on 9/11. We saw the buildings
go away in about a dozen seconds. There wasn't enough time
for a floor-by-floor gravity accelerated collapse. The buildings
fell too fast for this to be a proper explanation.
Click to expand...

Wrong, wrong and wrong........but that is what is expected from troofers :rolleyes:
 
I'd like to know which part of what I'm saying you think is wrong.
And, yes, I was a 9/11 Truther before it was cool to be a 9/11 Truther,
because I smelled the burning building, and knew something was wrong
with the explanation.

It's made for an interesting, if insult-filled, ten years.

But enough about me. Talk about the acceleration.

Animal said:
Wrong, wrong and wrong........but that is what is expected from troofers :rolleyes:
Click to expand...
 
WTC Dust said:
Acceleration is a very good topic. Acceleration takes time.
If, say, the upper twenty stories fell one story, it would take a
certain amount of time for them to hit the story below, the
better part of a second, actually.

Then, once the top twenty-one floors started falling, it would
take that same fraction of a second for them to hit the lower
floor.

Then, once the top twenty-two floors started falling, it would
once again take the same fraction of a second to hit the next
lower floor.

The distance from floor to floor was about twelve feet, and it
takes about 0.75 seconds to accelerate through that distance
even without including the resistance that the bottom floors
and wall sections of the building would have provided.

Add up all these fractions of a second, and you get something
very different from what was seen on 9/11. We saw the buildings
go away in about a dozen seconds. There wasn't enough time
for a floor-by-floor gravity accelerated collapse. The buildings
fell too fast for this to be a proper explanation.
Click to expand...

You are wrong, that was not physics, it was nonsnese.

Here is the physics for floor to floor impact model - reality steps in.
0.869
0.397
0.314
0.272
0.245
0.226
0.212
0.201
0.192
0.184
0.177
0.171
0.166
0.162
0.157
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0.150
0.147
0.144
0.141
0.138
0.136
0.134
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0.099
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0.097
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0.096
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0.094
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0.092
0.091
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0.088
0.088
0.087
0.086
0.086
0.085
0.085
0.084
0.084
0.083
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0.082
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0.081
0.081
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0.080
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0.079
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0.078
0.077
0.077
0.076
0.076
0.076
0.075
0.075
0.075
0.074
0.074
0.073
0.073
0.073
0.072
12.081 total time - slower than free fall, a lot slower if you think of this as a race.
WTC Dust said:
I'd like to know which part of what I'm saying you think is wrong.
...
Click to expand...
All of it.
 
Last edited:
WTC Dust said:
Acceleration is a very good topic. Acceleration takes time.
If, say, the upper twenty stories fell one story, it would take a
certain amount of time for them to hit the story below, the
better part of a second, actually.

Then, once the top twenty-one floors started falling, it would take that same fraction of a second for them to hit the lower floor.
Then, once the top twenty-two floors started falling, it would once again take the same fraction of a second to hit the next lower floor. The distance from floor to floor was about twelve feet, and it
takes about 0.75 seconds to accelerate through that distance
even without including the resistance that the bottom floors
and wall sections of the building would have provided.

Add up all these fractions of a second, and you get something
very different from what was seen on 9/11. We saw the buildings
go away in about a dozen seconds. There wasn't enough time
for a floor-by-floor gravity accelerated collapse. The buildings
fell too fast for this to be a proper explanation.
Click to expand...

Highlighted part is wrong.

From my article on tower collapse times:

If the block falls from rest, the time to drop a distance h is
time2fall_0.jpg


But, if the block is already falling at some speed v0, then the time to drop a distance h is
time2fall_v0.jpg


As v0 increases, the drop time decreases. Try it on a spreadsheet.

This is why Beachnut's times decrease exactly as they do just above.

Analogy: how long does it take to drive 12 feet?

It takes a lot longer if your car is not moving at first, as opposed to it already going quite fast (say, 60 mph).

You're welcome.
 
How did you get these numbers? From someone else, or did you
derive them yourself?

beachnut said:
You are wrong, that was not physics, it was nonsnese.

Here is the physics for floor to floor impact model - reality steps in.
0.869
0.397
0.314
0.272
0.245
0.226
0.212
0.201
0.192
0.184
0.177
0.171
0.166
0.162
0.157
0.154
0.150
0.147
0.144
0.141
0.138
0.136
0.134
0.131
0.129
0.127
0.125
0.124
0.122
0.120
0.119
0.117
0.116
0.114
0.113
0.112
0.110
0.109
0.108
0.107
0.106
0.105
0.104
0.103
0.102
0.101
0.100
0.099
0.098
0.097
0.097
0.096
0.095
0.094
0.094
0.093
0.092
0.091
0.091
0.090
0.089
0.089
0.088
0.088
0.087
0.086
0.086
0.085
0.085
0.084
0.084
0.083
0.083
0.082
0.082
0.081
0.081
0.080
0.080
0.079
0.079
0.078
0.078
0.078
0.077
0.077
0.076
0.076
0.076
0.075
0.075
0.075
0.074
0.074
0.073
0.073
0.073
0.072
12.081 total time - slower than free fall, a lot slower if you think of this as a race.

All of it.
Click to expand...
 
Thank you for responding! :-)

You're model doesn't account for the fact that the bottom
floors can't move until the top floors reach them to deliver the impact.

The gravity-only collapse model is quite clear about this.
If floors impacting on other floors caused subsequent floors to
fall, they can't fall before the impact, and it takes a certain,
non-zero amount of time for the top floors to fall the distance
of one floor.

Add this all up and you get a much slower collapse time than
observed in real life.

DaveThomasNMSR said:
Highlighted part is wrong.

From my article on tower collapse times:

If the block falls from rest, the time to drop a distance h is
[qimg]http://www.nmsr.org/time2fall_0.jpg[/qimg]

But, if the block is already falling at some speed v0, then the time to drop a distance h is
[qimg]http://www.nmsr.org/time2fall_v0.jpg[/qimg]

As v0 increases, the drop time decreases. Try it on a spreadsheet.

This is why Beachnut's times decrease exactly as they do just above.

Analogy: how long does it take to drive 12 feet?

It takes a lot longer if your car is not moving at first, as opposed to it already going quite fast (say, 60 mph).

You're welcome.
Click to expand...
 
WTC Dust said:
Thank you for responding! :-)

You're model doesn't account for the fact that the bottom
floors can't move until the top floors reach them to deliver the impact.

The gravity-only collapse model is quite clear about this.
If floors impacting on other floors caused subsequent floors to
fall, they can't fall before the impact, and it takes a certain,
non-zero amount of time for the top floors to fall the distance
of one floor.

Add this all up and you get a much slower collapse time than
observed in real life.
Click to expand...

Well, in real life...

Oh, never mind. It's not even worth the effort.
 
WTC Dust said:
Thank you for responding! :-)

You're [sic] model doesn't account for the fact that the bottom
floors can't move until the top floors reach them to deliver the impact.

The gravity-only collapse model is quite clear about this.
If floors impacting on other floors caused subsequent floors to
fall, they can't fall before the impact, and it takes a certain,
non-zero amount of time for the top floors to fall the distance
of one floor.

Add this all up and you get a much slower collapse time than
observed in real life.
Click to expand...

Wrong, wrong, wrong. The upper sections gather mass, and fall faster and faster. The bottom sections are at rest until they are impacted and incorporated into the falling upper mass, taking on its speed.

In your weird world, an avalanche would not gather speed as it flows downhill, it would just poke along at constant velocity.

Wrong.

You're welcome.
 
If you look at the videos you see the side walls peeling away like a banana. And this would cause several floors at a time to become unsupported. In WTC 1 a good portion of the core remained intact with the side walls fallen around it, with 12 people alive inside as well. You can see the core is the last parts to fall.

Each floor does not need to wait to start a downward fall. An avalanche is a good analogy. An avalanche of steel concrete and building contents.
 
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