David Chandler jumps the shark

[Oystein]

okay, so ignoring your "duh stuff" bs, how would this achieve a constant acceleration? You said it would get back up to 19mph. that would presume a fall of about 13.5 ft at freefall from rest every time the upper block impacts the floor directly below it. There's "duh" for ya.
There's no way that the tower can fall in anything like 14s given your "gets it right back to 19mph" presumption.

ETA - I think I see where you went wrong now - you got constant speed confused with constant acceleration.
DUH

95% of each story is air, and 98% of the horizontal area is air, except for the 4 inches of light-weight concrete

What's going to stop most of the falling rubble and steel from accelerating at practically g during the 13 ft drop between slabs?

g is constant acceleration. It doesn't take much energy to hammer the floor joists loose. Most of the kinetic energy goes into inelastic deformation of the floor slabs and core beams. If you do the math (conservation of energy & conservation of momentum) you'll find that, with the collapse starting with 15 top floors, acceleration will average about 2/3 of g.

 

[GlennB]

okay, so ignoring your "duh stuff" bs, how would this achieve a constant acceleration?

Because it's a chaotic collapse, not a synchronised one.

But either way, your concept of "It should arrest within 6 floors" is nonsense, for the simple reason I gave.

 

[Grizzly Bear]

okay, so ignoring your "duh stuff" bs, how would this achieve a constant acceleration? You said it would get back up to 19mph. that would presume a fall of about 13.5 ft at freefall from rest every time the upper block impacts the floor directly below it. There's "duh" for ya.
There's no way that the tower can fall in anything like 14s given your "gets it right back to 19mph" presumption

If the calcs done upthread are being used as the metric, for this part of the discussion then the loss is only 1 or meters per second maximum at the instant the impact took place, and then the fall goes for Nother floor level allowing for a net gain in velocity and KE so the dynamics at the moment of subsequent impacts is actually greater unless you want to argue that theres a reason for the opposite to happen.

It was probably slightly more "loss" at each impact if you account for the real collapse dynamics but clearly as observed the simplified conservation of momentum model makes a sufficient enough point; you appear along side notconvinced to grossly overestimate the localized capacity of the system to disperse the amount of force and all involved

 

[gerrycan]

95% of each story is air, and 98% of the horizontal area is air, except for the 4 inches of light-weight concrete

What's going to stop most of the falling rubble and steel from accelerating at practically g during the 13 ft drop between slabs?

g is constant acceleration. It doesn't take much energy to hammer the floor joists loose. Most of the kinetic energy goes into inelastic deformation of the floor slabs and core beams. If you do the math (conservation of energy & conservation of momentum) you'll find that, with the collapse starting with 15 top floors, acceleration will average about 2/3 of g.

Yeah 2/3g means just under 14s as I said for the collapse.
And "floor joists"?
Not sure what you mean by this though "Most of the kinetic energy goes into inelastic deformation of the floor slabs and core beams."

 

[gerrycan]

Because it's a chaotic collapse, not a synchronised one.

But either way, your concept of "It should arrest within 6 floors" is nonsense, for the simple reason I gave.

NO glenn, You said that the arrested block would "the next one-floor drop gets it right back to 19mph" It's simple for sure, but bereft of reason.

And you have the audacity to say "this is duh stuff". ??

 

[DGM]

No Dave. The top block gets to 19mph in falling one floor height. Maybe you can explain the energy balance. It should arrest within 6 floors.
As for looking stupid, aren't you the guy who backed down from debating me live? Do it. I'll ram your own arrogance right through you and we'll see who is stupid.

Does it come to a full stop at each floor? If not, why would it not be accelerating?

 

[Dave Rogers]

No Dave. The top block gets to 19mph in falling one floor height. Maybe you can explain the energy balance. It should arrest within 6 floors.

So you too don't believe in gravity. Falling objects convert potential energy into kinetic energy, and the amount of kinetic energy absorbed by the collapsing of a floor is less than that gained by falling the distance to the next one; as a result, collapse doesn't arrest at all.

As for looking stupid, aren't you the guy who backed down from debating me live? Do it. I'll ram your own arrogance right through you and we'll see who is stupid.

"Backed down" would imply that I suggested an interest in doing so and then withdrew. If you believe that I did, then you're as deluded about that as about 9/11.

Dave

 

[GlennB]

NO glenn, You said that the arrested block would "the next one-floor drop gets it right back to 19mph" It's simple for sure, but bereft of reason.

I was looking at it from your pov :

The initial drop breaks the floor below, leading to collapse progression to some extent at least. The next one-storey fall has exactly the same effect as the first, but with added mass and extra velocity (the first impact didn't halt collapse, as you happily concede).

There is no reason for the collapse to halt for reasons of what you call 'energy balance'. Each storey-worth of collapse liberates 1 storey-height of PE to be turned into KE. In this theoretical floor-hits-next-floor-squarely view of things, the only impact that could be arrested is the first. After that things only get worse.

 

[gerrycan]

So you too don't believe in gravity.

Eh ??

Falling objects convert potential energy into kinetic energy, and the amount of kinetic energy absorbed by the collapsing of a floor is less than that gained by falling the distance to the next one; as a result, collapse doesn't arrest at all.

Not at all. There's an energy deficit problem in your assertion. Maybe you could just throw the figures in there and I can look at them?

"Backed down" would imply that I suggested an interest in doing so and then withdrew. If you believe that I did, then you're as deluded about that as about 9/11.

On reflection you may be correct. The character I was remembering was wrong, but his assertions were not quite as dumb.

 

[Oystein]

gerrycan, you avoided my question - please answer it:

95% of each story is air, and 98% of the horizontal area is air, except for the 4 inches of light-weight concrete
What's going to stop most of the falling rubble and steel from accelerating at practically g during the 13 ft drop between slabs?

Yeah 2/3g means just under 14s as I said for the collapse.

Yes. Cool.

And "floor joists"?

I see a question mark, but I can't discern what your question is. Full sentence, perhaps?

Not sure what you mean by this though "Most of the kinetic energy goes into inelastic deformation of the floor slabs and core beams."

Exactly that.
Do you not understand what "inelastic deformation of the floor slabs and core beams" means?
Floor slabs: Mostly fracture (think "pulverisation")
Core beams: All sorts of failure modes of bolts and welds, and, well, permanent deformation.
Or perhaps I should have been more pedantic, saying "most of the kinetic energy dissipated during vertical collapse progression (~1/3 of potential energy) is turned into concrete fracture and steel deformation (and most the remaining 2/3 are dissipated at the end of the collapse, on the ground, as deformation, heat and seismic waves)".
Better?

And don't forget:
What's going to stop most of the falling rubble and steel from accelerating at practically g during the 13 ft drop between slabs?

 

[DGM]

Not at all. There's an energy deficit problem in your assertion. Maybe you could just throw the figures in there and I can look at them?

Maybe you can "throw some figures". It looks to me like you keep dipping into the first impact to help the next (the reason I asked about a "full stop"). After the first impact there's a complete reset with new mass and added velocity (what's left over from the first).

 

[gerrycan]

Can we agree on these first to make this easier -----
The buildings structural mass per floor increases from top to bottom. ie half the mass in the first 35 or so floors. (i'll try to get this more precise if you dispute it - it's my estimate)
The floors trusses were not load bearing.
The corner floor truss arrangements were transverse, so much stronger there than at the middle of each face.
The upper block impacts will not just have an effect on the topmost impacted floor below.

 

[Oystein]

Can we agree on these first to make this easier -----
The buildings structural mass per floor increases from top to bottom. ie half the mass in the first 35 or so floors. (i'll try to get this more precise if you dispute it - it's my estimate)

Probably a bit too extreme.
Gregory Urich wrote a paper years ago, tallying all building elements and adding up their mass. He determined the total mass of the north tower to be 288,100 tons, and he also gave a total for potential energy of 480,600 MJ. Since PE = m*g*h, it is straightforward to compute the height of the center of mass from that as h = PE/m/g, and that works out to 170 m (rounded to full meters), or 41% of the height of WTC1, or ~45 floors.
The columns are heavier in the lower floors, but the floors are more or less uniform from bottom to top.

ETA: Details in my other blog: http://oystein-issues.blogspot.de/2010/09/potential-energy-of-mass-of-each-twin.html /ETA

The floors trusses were not load bearing.

Exactly - and that's why they failed so easily, without dissipating much energy at all!

The corner floor truss arrangements were transverse, so much stronger there than at the middle of each face.

What failed was the floor truss seats. I don't think the seats were stronger in the corners than in the middle.

The upper block impacts will not just have an effect on the topmost impacted floor below.

Let's say the upper block is 15 floors.
The first floor it impacts is the 15+1st floor from top. That impacted floor has all its floor truss seats fail in a snap.
The descending top block (which has the 15th and 15+1st floors partially rubblized by now) now consists of 16 floors that fall in unison. These 16 floors start their fall not from zero but from some residual velocity.
The second impact happens at a velocity greater than the first impact, and with greater mass. It will bring the falling mass to 17 floors, which start out at a higher velocity still.
The third impact happens at a velocity greater than the second impact, and with greater mass. It will bring the falling mass to 18 floors, which start out at a higher velocity still.
Etc.
Etc.
Etc.
Etc.

 

[DGM]

Can we agree on these first to make this easier -----
The buildings structural mass per floor increases from top to bottom. ie half the mass in the first 35 or so floors. (i'll try to get this more precise if you dispute it - it's my estimate)

Sure but, it doesn't mater as it is not in motion.

The floors trusses were not load bearing.

Close enough considering your first point is moot.

The corner floor truss arrangements were transverse, so much stronger there than at the middle of each face.

As above.......

The upper block impacts will not just have an effect on the topmost impacted floor below.

No idea what this means. You're not suggesting that the rest of the building will aid in stopping the effected floors (on the local level).

 

[JSanderO]

Can we agree on these first to make this easier -----
The buildings structural mass per floor increases from top to bottom. ie half the mass in the first 35 or so floors. (i'll try to get this more precise if you dispute it - it's my estimate)
The floors trusses were not load bearing.
The corner floor truss arrangements were transverse, so much stronger there than at the middle of each face.
The upper block impacts will not just have an effect on the topmost impacted floor below.

Your mass distribution is wrong.

 

[gerrycan]

Your mass distribution is wrong.

Ok, I did kind of put a disclaimer in there - what floor do you reckon represents the 50% mass mark?

 

[jaydeehess]

We still await consise explanations for "KE... minus p", and upward mass vector.

Looks like this isn't going to happen.
Qu'elle suprise.

 

[Tony Szamboti]

Ok, I did kind of put a disclaimer in there - what floor do you reckon represents the 50% mass mark?

Except for the mechanical floors and hat truss area, the floors were all about the same. It was the column mass which changed with height. You can find a story by story mass breakdown at this link http://www.journalof911studies.com/volume/200703/GUrich/MassAndPeWtc.pdf

With the pdf up go to chapter 5.2 and pick the hyperlink for the actual mass analysis spreadsheet which would be the xls file.

I think the midpoint in mass would be around the 50th story of the 110 story building as the steel in the lower stories is heavier.

 

[DGM]

Except for the mechanical floors and hat truss area, the floors were all about the same. It was the column mass which changed with height. You can find a story by story mass breakdown at this link http://www.journalof911studies.com/volume/200703/GUrich/MassAndPeWtc.pdf

With the pdf up go to chapter 5.2 and pick the hyperlink for the actual mass analysis spreadsheet which would be the xls file.

I think the midpoint in mass would be around the 50th story of the 110 story building as the steel in the lower stories is heavier.

Good job Tony.

Now explain what this has to do with the mass that actually was in motion.

You guys remind me of the dogs in the move "up". Except you expect us to follow the squirrels.

 

[gerrycan]

With the pdf up go to chapter 5.2 and pick the hyperlink for the actual mass analysis spreadsheet which would be the xls file.

Hi Tony, and thanks. I make it to be between fl 43 and 44 according to that.