9/11 Physics from Non-Experts

You are redirecting. Which means you don't want to admit you are wrong.

I asked you if there was any separation in the wtc case?
 
GregoryUrich said:
You are redirecting. Which means you don't want to admit you are wrong.

I asked you if there was any separation in the wtc case?
Click to expand...
You're squirming. You've already tripped over yourself with this answer:
"The relative velocity and the COR assuming like objects."
And now you refuse to answer a VERY simple question.
You may want to correct that answer first, then answer my question.
 
3bodyproblem said:
You're squirming. You've already tripped over yourself with this answer:
"The relative velocity and the COR assuming like objects."
And now you refuse to answer a VERY simple question.
You may want to correct that answer first, then answer my question.
Click to expand...

For anyone curious, the relative velocity is the COR, it is the ratio of the separation velocity and the velocity of approach. I told Greg this WHEN HE SAID HE WOULD EXPLAIN IT TO ME :) (then he asked me what it was)
As to my other question, the mass of the upper section would be the determining factor of the velocity of separation. He had better than 50% chance at just guessing this.
And yes Greg, there was separation in the WTC, 3.7m You should probably know this by now, I bet Bazant does :)
 
GregoryUrich said:
The equation for P is:

P = 1 + sqrt (1 + 2mghC)

which is the solution of the quadratic equation:

P^2 - 2P - 2mghC = 0

which is simply a restatement of:

mg[h + (P/C)] = P^2/2C

Why not drop the distracting blather and focus on the fact that Newton was wrong about the displacement.
Click to expand...

You need to not quote mine and understand what the variables mean. The term "C" is a spring stiffness of the WHOLE LOWER PORTION. Which means the stiffness of the first floor to the floor just above impact acting in series. h+P/C is the displacement, h is the distance it fell (one floor), and P/C is the displacement of the ENTIRE structure. Hence, he uses the whole lower mass. Do you want to retract your statement about me being wrong? This isn't that complicated, do you really need me to walk you through the energy balance and show you how he figured out the force?

As far as #3, we're considered with ENERGY, not force. Assuming a complete pancake collapse, the upper part of the structure can only absorb so much plastic strain energy. It will unload after impact, however if the lower floor failed, then the upper block is certainly at least in a plastic region of the stress-strain curve and hence not able to absorb that much energy.

MY collision rule is: don't let big objects collide inside of my building.
 
Newtons Bit said:
You need to not quote mine and understand what the variables mean. The term "C" is a spring stiffness of the WHOLE LOWER PORTION. Which means the stiffness of the first floor to the floor just above impact acting in series. h+P/C is the displacement, h is the distance it fell (one floor), and P/C is the displacement of the ENTIRE structure. Hence, he uses the whole lower mass. Do you want to retract your statement about me being wrong? This isn't that complicated, do you really need me to walk you through the energy balance and show you how he figured out the force?

As far as #3, we're considered with ENERGY, not force. Assuming a complete pancake collapse, the upper part of the structure can only absorb so much plastic strain energy. It will unload after impact, however if the lower floor failed, then the upper block is certainly at least in a plastic region of the stress-strain curve and hence not able to absorb that much energy.

MY collision rule is: don't let big objects collide inside of my building.
Click to expand...

Sorry I think we are having terminology problems. I think Bazant would call P/C (which I have also called displacement) deflection. I agree with you up to that point. Sorry for stating you were wrong about that.

Are you assuming that C some way incorporates the mass of the lower section? I don't think you are, I am just checking to make sure we understand each other. Bazant defines C as "stiffness" which is not commonly used for inertia:

What is its stiffness C? It can vary greatly with the distribution of
the impact forces among the framed tube columns, between these
columns and those in the core, and between the columns and the
trusses supporting concrete floor slabs.
Click to expand...

So my understanding is that Bazant uses only the stiffness in the energy equation. Sure, P/C is the deflection of the entire structure but Bazant doesn't take into account that in order to have that deflection a certain portion of the mass of the lower section must be displaced.

As you say, Bazant balances the maximum loss of PE of the top section against the strain energy.

Bazant: 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.
Click to expand...

Do you suggest that strain energy somehow incorporate the mass of the object under strain?


Regarding the pancakes:

The floor by floor pancake collapse is so far from reality that I can't believe you are bringing it up at this point. A pancake collapse with momentum transfer only (zero energy loss due to elastic work, plastic work, and ejection of debris) takes 15.5 seconds with infinitely thin floors. Are you saying NIST was wrong about the collapse time?

By the way I noticed after rereading my issues that I misstated my first issue.

1. Bazant models the plastic energy for compression of the lower part of the structure as a spring. The upper part should also be modeled the same way (i.e. two springs exerting pressure on each other). At maximum compression there is enormous force applied to the debris between the springs which likely accounts for the ejection of so much debris. This is also an energy sink.
Click to expand...

The issue should read:

1. Bazant models the elastic (strain) energy for compression of the lower part of the structure as a spring. The upper part should also be modelled the same way (i.e. two springs exerting pressure on each other). At maximum compression there is enormous force applied to the debris between the springs which likely accounts for the ejection of so much debris. This is also an energy sink.

Anyway, it can be clearly seen in the videos that the impacting floors of the upper section fail. This means that the energy from unloading of the top floors cannot effectively be transmitted to the intact structure of the lower section. Where does this energy go? Further destruction of debris from failed floors and ejection of debris is my hypothesis.
 
3bodyproblem said:
For anyone curious, the relative velocity is the COR, it is the ratio of the separation velocity and the velocity of approach. I told Greg this WHEN HE SAID HE WOULD EXPLAIN IT TO ME :) (then he asked me what it was)
As to my other question, the mass of the upper section would be the determining factor of the velocity of separation. He had better than 50% chance at just guessing this.
And yes Greg, there was separation in the WTC, 3.7m You should probably know this by now, I bet Bazant does :)
Click to expand...

I believe you are misinterpreting me and confusing the terminology.

I stated that:

(The separation velocity) is dependent on the relative velocity and the COR. I am referring to the approach velocity of the colliding objects relative to each other.

I didn't say I would explain what COR is and then ask you what it was. This is just plain misrepresentation on your part.

Regarding separation, it occurs after the impact as the objects move away from each other at separation velocity. This phenomena was not observed in the wtc collapse.
 
twinstead said:
Wrong. I don't mean the experts here. I mean the experts involved in the NIST report.

Here's a couple of them:[/LIST]These folks disagree with you. Why is that?
Click to expand...

Twinstead:

that is a great list, which I would like to add to our list of scientists supporting the Official account of 9/11. DO you have a source please? Is it simply taken from a section of the NIST credits? Thanks.

TAM:)
 
T.A.M. said:
Twinstead:

that is a great list, which I would like to add to our list of scientists supporting the Official account of 9/11. DO you have a source please? Is it simply taken from a section of the NIST credits? Thanks.

TAM:)
Click to expand...

The list is courtesy of Gravy, and I think it is indeed from the credits.
 
GregoryUrich said:
I believe you are misinterpreting me and confusing the terminology.
Click to expand...

I think you are misrepresenting yourself and confused as well.

Did I misinterpret something?

The COR is Newton's collision rule. The COR defines the type of collision. Bazant has elastically calculated the forces in the columns caused by the vertical impact of the upper part, this means there is separation, and the COR is unity by definition.
If you bothered to go back and learn something you would see that the impulse of deformation on the upper section is negligible in comparison to the impulse of deformation on the lower. This is why Bazant is justified in assuming the upper section is a rigid body. I think perhaps Newtons Bit has explained this to you as an engineer would, me in physics. If you hit a spring with a hammer, the spring will break before the hammer deforms. There is no need to calculate the amount of deformation in the hammer.
 
GregoryUrich said:
Sorry I think we are having terminology problems. I think Bazant would call P/C (which I have also called displacement) deflection. I agree with you up to that point. Sorry for stating you were wrong about that.

Are you assuming that C some way incorporates the mass of the lower section? I don't think you are, I am just checking to make sure we understand each other. Bazant defines C as "stiffness" which is not commonly used for inertia:

So my understanding is that Bazant uses only the stiffness in the energy equation. Sure, P/C is the deflection of the entire structure but Bazant doesn't take into account that in order to have that deflection a certain portion of the mass of the lower section must be displaced.

As you say, Bazant balances the maximum loss of PE of the top section against the strain energy.
Click to expand...
Apology accepted.

To move on, he does look at the lower portion of the tower as deflecting, that's P/C, and then equates the loss in potential energy of the tower deflecting to the strain energy of the lower spring 0.5 * P^2 / C. This resolves:

"Bazant doesn't take into account the momentum transfer due to the mass of the spring (the intact lower structure) being accellerated."
Click to expand...

The entire purpose of energy methods is that you don't need to look at F=ma. I don't know how else I can explain this.

If you really want to look at the mass of the lower sections displacing as you want, then they lose potential energy and the strain energy goes up creating an even larger force acting upon the top levels, however the actual individual displacement of each floor is very small. It also makes the math involved painful.
 
Newtons Bit said:
It also makes the math involved painful.
Click to expand...

When you first learn Newtonian Mechanics the world is a wonderful place of massless ropes, perfect pulleys, geometrically centered masses, ideal collisions and frictionless surfaces. By third year this sense of wonder is completely gone and the tedium of reality sets in. Ropes suddenly have mass, pulleys get hot, density is no longer uniform, collisions get oblique and the ground gets dirty. This is when the math gets painful. You must quickly learn to evaluate what is significant and what isn't in every equation.

If it isn't significant you drop it. Good luck Greg
 
GregoryUrich said:
I believe I have reached agreement on a number of issues with the experts here. I assume it is those experts you mean.

1. I don't think anyone who has been following this discussion will still suggest that the actual weight of one wtc was 500,000 tons. They may not agree with my value but we know it is substancially less than 500,000 tons.

2. I believe I have convinced most people that my linear scaling of steel mass is reasonable, although slightly in favor of a higher PE.

3. I think most people agree that the live loads are closer to an empty building than the full design capacity. We don't know how much closer but I will look into it. I did get some good references.

4. I think anyone who has looked at the actual blueprints and done some calculations will agree that the amount of empty space (not even a floor) in the core is around 30% average for the entire building. This I will also be calculated more carefully in the next version of my paper.

There are a number of valid issues raised by the experts here which I have acknowledged. These I will also take into account in the next version of my paper.
Click to expand...

I disagree with all your points.
 
Originally Posted by GregoryUrich View Post
I believe I have reached agreement on a number of issues with the experts here. I assume it is those experts you mean.

1. I don't think anyone who has been following this discussion will still suggest that the actual weight of one wtc was 500,000 tons. They may not agree with my value but we know it is substancially less than 500,000 tons.

2. I believe I have convinced most people that my linear scaling of steel mass is reasonable, although slightly in favor of a higher PE.

3. I think most people agree that the live loads are closer to an empty building than the full design capacity. We don't know how much closer but I will look into it. I did get some good references.

4. I think anyone who has looked at the actual blueprints and done some calculations will agree that the amount of empty space (not even a floor) in the core is around 30% average for the entire building. This I will also be calculated more carefully in the next version of my paper.

There are a number of valid issues raised by the experts here which I have acknowledged. These I will also take into account in the next version of my paper.

Not yet :D
 
Last edited:
Apollo20 said:
Augustine:

"Collapse was inevitable...."

Would that be with or without x-amount of steel at greater than 400 deg C?

And on the topic of live loads, three classic papers of relevance to the WTC live loads question would be:

Jong-Cherng Peir et al., "Spatial and Temporal Variability of Live Loads"

J. of the Structural Division... May 1973,

R. K. McGuire et al., "Live Load Effects in Office Buildings"

J. of the Structural Division... July 1974,

and,

B. Ellingwood, "Analysis of Live Loads in Office Buildings",

J. of the Structural Division... Aug 1977.

The numbers quoted in these papers are quite revealing...
Click to expand...

Thanks Apollo,

I checked out R. K. McGuire et al., "Live Load Effects in Office Buildings". In the chapter, "Results of live load survey - Variance of sustained load", the mode is given as 0.45 kPa and 90% of the samples are less than 1.0 kPa. The average appears to be around 0.55 kPa = 11.5 psf. This would indicate that my use of 25 psf live load for the area outside of the core is more likely an overestimate than an underestimate.

Is anyone aware of more recent developments that would contradict this?
 
Gregory Urich:

Glad I could help. (I do recommend reading up on the split Hopkinson bar)

However, don't let the JREF "illegitimi" grind you down or railroad you....., the JREF "hit-team" are mostly "paper tigers"!

PHYSORG forum, while not perfect, is a better place to post your ideas.

This site is all about stifling debate.
 
Apollo20 said:
Gregory Urich:

Glad I could help. (I do recommend reading up on the split Hopkinson bar)

However, don't let the JREF "illegitimi" grind you down or railroad you....., the JREF "hit-team" are mostly "paper tigers"!

PHYSORG forum, while not perfect, is a better place to post your ideas.

This site is all about stifling debate.
Click to expand...

Dr. Greening, I've seen you post essentially the same ad hominem argument on several threads. I'm sure you know what's coming next:

Please provide evidence for your claim, bolded above.
 
Apollo20 said:
However, don't let the JREF "illegitimi" grind you down or railroad you....., the JREF "hit-team" are mostly "paper tigers"!

PHYSORG forum, while not perfect, is a better place to post your ideas.

This site is all about stifling debate.
Click to expand...

I don't know if Greg is ready to bring his ideas to PHYSORG, but if he does please let me know I am curious as to how that all turns out. It certainly couldn't hurt.
 
Mackey: I and many others too numerous to mention are the evidence!

More condescension from you too!
 
You must log in or register to reply here.

Back
Top Bottom