David Chandler jumps the shark

[gerrycan]

Well, no: energy is the driver and velocity is the result. Estimating the velocity just unnecessarily complicates the problem, in fact, because then you have to try to figure out how much force there is in the impact. I admit I thought you would need to do that, too, but reading Bazant's 2002 energy argument was like "doh!" -- if you can't dissipate the energy due to gravity, then the collapse proceeds, and the velocity is an irrelevant detail.

Nah, that's not what is being said at all. In the kinematic equations, velocity is the value that is squared and so is the main driver of energy in that instance.
If you want to say that energy is the driver, then velocity is the force multiplier.

 

[Dave Rogers]

Maybe you could just throw the figures in there and I can look at them?

Perhaps, since you're the one making the claim, you could support it yourself. Given all the questions you're asking, it seems you don't know very much about the mass distribution, the acceleration profile against time, the collision dynamics, or even the basic definition of kinetic energy, yet you've managed to come up with the surprisingly precise value of six floors to arrest collapse. Some of us might think you're just making things up without any foundation in actual data; or, to use a technical term, "lying".

On reflection you may be correct.

You reluctantly concede that I may be correct about my own position, and you accuse me of arrogance? Priceless.

Dave

 

[gerrycan]

I ask questions that are too inconvenient? I understand your feelings then.

No, it's nothing that subtle. I just don't like you.

 

[WilliamSeger]

Nah, that's not what is being said at all. In the kinematic equations, velocity is the value that is squared and so is the main driver of energy in that instance.
If you want to say that energy is the driver, then velocity is the force multiplier.

Yes, I want to say that energy is the driver, but then, David Chandler wasn't my high school physics teacher. Do you understand Bazant's energy argument?

 

[gerrycan]

You reluctantly concede that I may be correct about my own position, and you accuse me of arrogance? Priceless.

Dave

Nothing reluctant about it. Look back at what I said.....
I reflected that the guy who was thinking about debating was stupid, but not quite that stupid. You distinguished yourself.

 

[ozeco41]

I think you fail to understand something extremely basic:

Every part of the tower's top that is moving down has ALREADY lost practically all structural contact with the lower, still standing part.
In other words, by the time you see the top part falling, ALL columns have ALREADY failed and are no longer supporting the top.
All these failed columns are passing each other - the lower ends of the top part have ALREADY moved below the top ends of the bottom part.
All the descending mass, so far as it isn't falling outside of the bottom part's perimeter, keeps dropping mostly on floor slabs. So the collapse dynamics is totally dominated by the response of the floor slabs to this dynamic loading - not by the structural strength of the standing columns!

Hey. I like that.

Now we only need to get:
1) all debunkers agreeing on that lot AND
2) gerrycan to state clearly what he is claiming THEN
3) maybe the discussion could move on to discuss what really happened
--- not all this circling around in abstract application of physics to an undefined event
--- which was NOT what happened at WTC on 9/11.

 

[gerrycan]

Yes, I want to say that energy is the driver,

Keep trying, you'll manage it eventually

but then, David Chandler wasn't my high school physics teacher.

More's the pity.

Do you understand Bazant's energy argument?

Cut and pasteExplain it to me. Start at the bit where a whole floor disappears and allows the top block to get to 19mph before impacting the lower floors.

 

[WilliamSeger]

Cut and pasteExplain it to me. Start at the bit where a whole floor disappears and allows the top block to get to 19mph before impacting the lower floors.

I already did: if you can't dissipate the energy due to gravity then the collapse proceeds. But you really should read it yourself, since it answers your specific question.

 

[Oystein]

No, it's nothing that subtle. I just don't like you.

You don't have to like me to answer this:

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?

This is asking you to reflect on the mechanics of structural resistance in the vertical collapse progression of the towers.
Trying to answer the question will help you learn something.

 

[gerrycan]

I already did: if you can't dissipate the energy due to gravity then the collapse proceeds. But you really should read it yourself, since it answers your specific question.

In what way does it account for the absorption of the impact by floors lower than the uppermost floor of the lower block?

 

[GlennB]

The velocity is the driver really though. So, given that I previously presumed the top block falling at freefall for 13.5ft before impacting the floors below, what would a more realistic figure be for a real world calculation?
Obviously, in a fire/damage scenario, there would be no possibility of a whole floor disappearing and allowing a freefall of 13.5 ft for the top block.
Does 10mph sound fair?

No. The first fall was subject to the same restraints as subsequent falls. I think you're picturing that first drop as unique in the set and leading to a higher velocity impact than the others.

In fact (as has been pointed out many times) that initial impact was the easiest for the building to resist. Thereafter residual velocity and added mass gave the floors below even less of a chance.

 

[WilliamSeger]

In what way does it account for the absorption of the impact by floors lower than the uppermost floor of the lower block?

From the section entitled Elastic Dynamic Analysis:

For a short time after the vertical impact of the upper part, but after the elastic wave generated by the vertical impact has propagated to the ground, the lower part of the structure can be approximately considered to act as an elastic spring.

Anything else I can look up for you now that I have it open?

 

[Tony Szamboti]

I already did: if you can't dissipate the energy due to gravity then the collapse proceeds. But you really should read it yourself, since it answers your specific question.

I think GerryCan is asking how Bazant can claim free fall through the initiating story, because that is what would be necessary to reach 19 mph after a one story drop.

The columns in that initiating story would have to be buckling according to Bazant's scenario and buckling columns over one or even two stories have a significant minimum resistance.

We refer to this Bazant claim as the vanishing story in the paper "Some Misunderstanding Related to WTC Collapse Analysis" and show it is a fatal flaw in his analysis.

 

[Oystein]

...
In fact (as has been pointed out many times) that initial impact was the easiest for the building to resist...

Yes.
The descend through the first story height commenced when the structure's capacity had dropped below 1.0x load. From that point on, those columns that had not yet been severed needed to buckle further until they were severed. This conversion from just under 1.0x load to zero capacity dissipated some energy and slowed the overall acceleration during the first few feet of descent.

After this was accomplished, and before top part had descended a full story's height, no more column needed to be buckled.
The first floor "impact"^[*] had the advantage of being with a KE-diminished top part - KE decreased due to column buckling, a circumstance that played no significant role in any further floor "impacts".



^[*] Scare quotes around "impact" to indicate this wasn't, of course, a full, clean and even square on impact uniformly across the whole footprint. Which is by the way why calculating forces and accelerations during that "impact" is an academic excercise, only useful to illustrate average orders of magnitude. The main factor is energy (and momentum), as William Seger correctly points out - more relevant, and more easily computed, as energy and momentum are extensive properties that can be added up from all the bits and pieces of the more chaotic real collapse.

 

[Oystein]

I think GerryCan is asking how Bazant can claim free fall through the initiating story, because that is what would be necessary to reach 19 mph after a one story drop.

The columns in that initiating story would have to be buckling according to Bazant's scenario and buckling columns over one or even two stories have a significant minimum resistance.

We refer to this Bazant claim as the vanishing story in the paper "Some Misunderstanding Related to WTC Collapse Analysis" and show it is a fatal flaw in his analysis.

Yes, and to pretend that the starting assumptions of B&Z are descriptive of the real event is the birth defect of MJ that renders it 100% disabled.

The relevant points in the real world are that
- In fact the top block had some velocity, and thus KE and momentum, by the time it had descended the height of one floor
- Most of the mass of the now completely detached upper part never hit a column end of the standing lower part. Instead, most of the mass that impacted anything impacted the floors
- It took comparatively VERY little energy to fail the floors (preferentially: truss seat failures)
- Once you realize that most of the energy is dissipated by impacting floor slabs, not columns, it becomes almost trivial to compute, via CoE/CoM, how much energy actually got dissipated that way

The result will be: An average ~1/3 of kinetic energy dissipated by shattering floors.
Conversely: Falling mass will accelerate at an average of 2/3g

 

[ozeco41]

Yes, and to pretend that the starting assumptions of B&Z are descriptive of the real event is the birth defect of MJ that renders it 100% disabled.

Beat me to it Oystein.

How Tony Szamboti can repeat that asinine nonsense beggars belief.

I admire your restraint I'm tempted to break my own rules and pour ridicule on it.

Tony cannot be serious. But the alternative is also unpleasant.

 

[GlennB]

I think GerryCan is asking how Bazant can claim free fall through the initiating story, because that is what would be necessary to reach 19 mph after a one story drop.

The columns in that initiating story would have to be buckling according to Bazant's scenario and buckling columns over one or even two stories have a significant minimum resistance.

And that's where gerrycan is going wrong. He's assuming a resistance-free 'first drop'.

 

[gerrycan]

And that's where gerrycan is going wrong. He's assuming a resistance-free 'first drop'.

No Glenn. That is what I am arguing against. It is the exact polar opposite to what I am arguing. Scroll up.
ETA nice try though

 

[WilliamSeger]

I think GerryCan is asking how Bazant can claim free fall through the initiating story, because that is what would be necessary to reach 19 mph after a one story drop.

Hmmm, that's not what I recall, but since I have it open, let me check... Nope, I was right; Bazant makes no such claim:

The downward displacement from the initial equilibrium position to the point of maximum deflection of the lower part (considered to behave elastically) is h1+(P/C) where P = maximum force applied by the upper part on the lower part and h = height of critical floor columns (= height of the initial fall of the upper part) ~=3.7 m. The energy dissipation, particularly that due to the inelastic deformation of columns during the initial drop of the upper part, may be neglected, i.e., the upper part may be assumed to move through distance h almost in a free fall (indeed, the energy dissipated in the columns during the fall is at most equal to 2pi X the yield moment of columns, X the number of columns, which is found to be only about 12% of the gravitational potential energy release if the columns were cold, and much less than that at 800°C). So the loss of the gravitational potential energy of the upper part may be approximately equated to the strain energy of the lower part at maximum elastic deflection.

Now, if the "collapse/no collapse" decision is close, obviously you would want to tighten up any approximations, but it turns out that it isn't anywhere near close, so it isn't "necessary to reach" anywhere near "19 mph." If you take the real scenario rather than Bazant's limit case, it's even worse, because that would absorb less energy.

 

[GlennB]

No Glenn. That is what I am arguing against. It is the exact polar opposite to what I am arguing. Scroll up.
ETA nice try though

Yes, my mistake. I skimmed your intervening blather.

Can we take it, then, that you no longer have 'energy balance' worries? That if the first floor collapses in similar fashion to subsequent floors the collapse is bound to progress, what with residual velocity at each one but with gathering mass?