Moderated Steel structures cannot globally collapse due to gravity alone

[Christopher7]

Why do you think air pressure can hurl a 4 ton framework section up to 600 feet?

Why do you imagine I think that?

Because that's what the BLGB theory proposes.
pg 7
Resisting Forces Due to Ejecting Air and Solids
The air mass within the confines of one story, which is . . . , gets accelerated from 0 to velocity v_a as it exits the tower perimeter. The kinetic energy acquired by the escaping air of one story just outside the tower perimeter is . . . . where . . . = initial volume of air within the story.
The energy dissipated by viscosity of flowing air and by boundary friction is estimated to be negligible. Therefore, virtually all of the kinetic energy of escaping air must be supplied by gravity, . . .
<snip>
The average over-pressure of air within the tower is . . . .
The pressure peaks near the end of squeezing of a story are doubtless much higher, as already mentioned, and thus must contribute to the break up of many floor slabs (theoretically, the pressure in a thin layer of viscous gas between two colliding parallel flat slabs approaches infinity at the end).

The mass that is shed from the tower, characterized by k-out, exits at various velocities ranging from nearly 0 to almost either the air ejection velocity, for fine dust, or to roughly z˙, for large steel pieces.

 

[Heiwa]

(bolding mine)

Heiwa, if all load carrying elements have suffered a local structural failure over a two floor section of the towers, what would then restrain the upper section from being accelerated by gravity downwards? Surely columns can't both fail and also carry load?

OK, an intact column is always subject to gravity load/force from above but luckily there is always an opposite reaction load/force to maintain equilibrium inside the column. The result is compressive stress in and elastic compression of the column. You agree?

So a part of the column disappears - no load/force can be transmitted through it. Where does the load/force go?

It is transferred somewhere else!

Normally through other structural members of the upper part still connected to the lower structure as outlined above. The load remains in place.

If not, the load (or rather mass above) drops and the force is 100% used to produce the drop. There is no reaction force acting on the loose mass. If free falls.

But it will not last long. As soon as this loose mass contacts something below a reaction force develops and this reaction force affects the loose mass, e.g. deforms it, produce failures of it, etc.

You are 100% right - a removed column cannot carry any load.

 

[A W Smith]

Yes, the force of gravity that was pulling straight down resulted in the top section falling down and to the side.

Once in motion down and to the side, the top section will continue falling down and to the side unless there is another force applied.

The top section suddenly stopped falling to the side.

Something arrested the fall to the side.

Therefore, a force other than gravity was applied to stop the sideways motion.

Are you with me so far?

The mass was accelerating downward faster than the tilt was accelerating sideways. The only reason it tilted in the first place was the hinging effect of the columns, once that hinge was lost there was no connection to the lower body to tilt upon. You would need an external tilting force that would have to exceed the inertia of its gravitational downward acceleration.

 

[e^n]

The mass that is shed from the tower, characterized by k-out, exits at various velocities ranging from nearly 0 to almost either the air ejection velocity, for fine dust, or to roughly z˙, for large steel pieces.

I don't understand what you believe you are criticising, this passage shows that gas ejection and steel ejection are separate? They are both treated as energy sinks as they use up energy from the collapse. Where do the authors specify the mechanism of ejection?

OK, an intact column is always subject to gravity load/force from above but luckily there is always an opposite reaction load/force to maintain equilibrium inside the column. The result is compressive stress in and elastic compression of the column. You agree?

I do.

So a part of the column disappears - no load/force can be transmitted through it. Where does the load/force go?

It is transferred somewhere else!

Normally through other structural members of the upper part still connected to the lower structure as outlined above. The load remains in place.

Sure, this is accurate for situations where alternate load paths are available and have sufficient capacity.

If not, the load (or rather mass above) drops and the force is 100% used to produce the drop. There is no reaction force acting on the loose mass. If free falls.

I don't agree with this however. I know what you are attempting to say here but it has been phrased clumsily. A mass accelerating at free-fall under gravity has no resistive force being applied to it.

But it will not last long. As soon as this loose mass contacts something below a reaction force develops and this reaction force affects the loose mass, e.g. deforms it, produce failures of it, etc.

Indeed, in these examples, the floor below.

You are 100% right - a removed column cannot carry any load.

You haven't answered my question directly, but you seem to be tacitly agreeing with me.

If the upper block falls from a large enough height to overwhelm the columns' load carrying capacity, then those columns will fail in both the upper and lower blocks. The upper block, being free of any support, will continue to descend along with the debris consisting of broken columns and various other elements.

The question is how much load must be applied to fail the columns in the first place, BLBG seems to address this admirably.

I don't see where we disagree Heiwa, but we clearly do.

The mass was accelerating downward faster than the tilt was accelerating sideways. The only reason it tilted in the first place was the hinging effect of the columns, once that hinge was lost there was no connection to the lower body to tilt upon. You would need an external tilting force that would have to exceed the inertia of its gravitational downward acceleration.

I'm not so sure I agree with this in principle. An item rotating around its centre of mass with no other forces will continue to rotate. However it is a mistake to think of the upper block of either tower as being a rotating mass with no external forces being applied. It would seem reasonable that the upward resistive force from the lower block was not distributed evenly due to the tilt, and the 'leading edge' of the tower would experience more resistance. This would work against the direction of rotation and lead to the upper block slowing its rotation which appears to be what happened.

Layman's view as always.

 

[A W Smith]

I'm not so sure I agree with this in principle. An item rotating around its centre of mass with no other forces will continue to rotate. However it is a mistake to think of the upper block of either tower as being a rotating mass with no external forces being applied. It would seem reasonable that the upward resistive force from the lower block was not distributed evenly due to the tilt, and the 'leading edge' of the tower would experience more resistance. This would work against the direction of rotation and lead to the upper block slowing its rotation which appears to be what happened.

Layman's view as always.

I agree, Much like a gyroscope would continue to spin despite acceleration. My acceleration assumption was in error,

 

[Dave Rogers]

Because that's what the BLGB theory proposes.

"The mass that is shed from the tower, characterized by k-out, exits at various velocities ranging from nearly 0 to almost either the air ejection velocity, for fine dust, or to roughly z˙, for large steel pieces."

Two different fractions of mass, two different mechanisms, and no suggestion that large steel pieces are ejected by air movement. Chris, you have repeatedly tried to justify statements by adding quotes that either do not justify the statement or that directly contradict it. This is beyond lying, and gives the appearance of irrationality. You are hurting your case by continuing to argue it this way.

Please continue.

Dave

 

[Christopher7]

The mass was accelerating downward faster than the tilt was accelerating sideways. The only reason it tilted in the first place was the hinging effect of the columns, once that hinge was lost there was no connection to the lower body to tilt upon. You would need an external tilting force that would have to exceed the inertia of its gravitational downward acceleration.

You are not considering that the weight crushing one side of the tower is about 3 times as much as the other side as this illustration shows.

This much greater weight/force will crush that side faster and the top section will continue to tilt more in that direction.

 

[Dave Rogers]

You are not considering that the weight crushing one side of the tower is about 3 times as much as the other side as this illustration shows.

http://img13.imageshack.us/img13/1237/tiltgraphic2fx4.jpg

This much greater weight/force will crush that side faster and the top section will continue to tilt more in that direction.

The ratio of weights on the two sides depends on the location of the hinge, the angle of tilt at the time the hinge collapses (which must be very small), the angular velocity and the time after collapse initiation. Just guessing "about 3" on the basis of a drawing that has no physical justification isn't particularly convincing. If the hinge collapses very quickly, as would be expected, the top section doesn't hinge sideways as you've drawn it, but instead rotates about its centre of mass while crushing the structure below. In that case, the weight on the two sides is roughly equal. However, the resistance of the two sides is not quite the same, because the strength of the structure is greater for lower storeys, and therefore the leading edge experiences a slightly greater resistance than the trailing edge. One would expect, then, that the falling mass would continue to rotate, but at a slightly decreasing angular velocity. That's more or less what was in fact observed up to the time it was obscured by dust.

Dave

 

[Christopher7]

Two different fractions of mass, two different mechanisms, and no suggestion that large steel pieces are ejected by air movement.

What two mechanisms? The section "Resisting Forces Due to Ejecting Air and Solids" only speaks of air pressure and it includes "large steel pieces".

"The mass that is shed from the tower, characterized by kout, exits at various velocities ranging from nearly 0 to almost either the air ejection velocity, for fine dust, or to roughly z˙, for large steel pieces."

The previous section was:
Velocity of Air Ejected from the Tower
and the following section is:
Energy Dissipated by Comminution (or Fragmentation and Pulverization)

They do not offer another mechanism for the "large steel pieces".

 

[tfk]

Heiwa,

Thanks, tk, for long post. Where do I provide a DEFINITION to prove a conclusion?

When you assert that a single point failure will never lead to a progressive collapse because ... single point failures don't lead to progressive collapses.

Circular. Illogic.

I noticed that you did not respond to the examples that I provided of cases where fires DID progress to total or near-total collapse.

Perhaps you'd care to discuss the ways in which the cable failure on the Tacoma-Narrows bridge lead to total, progressive, catastrophic collapse of that steel structure?

Or more to the point, how a single point failure on the Windor Towers led to a total, progressive, catastrophic collapse of all NON-INSULATED steel structures. While leaving all insulated steel structures still standing.

In all my articles about structural damage analysis I simply point out that you have to establish the path of all structural failures, step by step, to understand what happened. One structural failure causes a second structural failure and so on.

The attitude that you are suggesting - "Either you know everything or you know nothing" - may be appropriate for a lawyer or a political hack. But it has no place in the world of competent engineering. Engineering has ALWAYS been the art & science of deriving USEFUL knowledge in the absence of TOTAL knowledge. I am surprised that an alleged engineer with your experience would suggest what you did.

The choice of when to decide that you've got a handle on events is an engineering judgment. IMO, the level of detail provided by NIST's analysis was appropriate.

Re WTC1 one failure produced by gravity cannot produce the million of failures we see on all videos. The first failures would be arrested at once, which I clearly describe. No crush-down is possible. And that applies to all steel structures! You cannot destroy a steel structure by dropping a piece of it from above on itself!

This is simple & utter nonsense. It violates several fundamental principles of mechanics, specifically the definition of the term "toughness", which specifies that parts fail (i.e., rupture) when they are subject to input strain energies that exceed limits that are defined by a part's materials & size.

Perhaps a review of Charpy or Izod impact testing methods might help clarify this for you, since both of these are standardized "impact to failure" tests.

It is trivial to prove that your assertion is nonsense, either intuitively or rigorously.

Rigorously, one would use the equivalence of work & energy. In order to halt the fall of the upper segment, the lower segment would have to bring the upper one to a halt. In doing so, it would have to absorb the kinetic energy of the upper segment. This kinetic energy is equal to mass x acceleration x distance of fall.

The maximum amount of energy that a structure can absorb is fixed by its materials & assembly configuration. The amount of energy that the falling body can deliver can be increased by allowing it to fall larger distances. You will always be able to find some maximum distance that the upper body can fall, which the lower structure will just barely be able to arrest its fall. Any higher, and the lower structure will fail.

Your assertion that "You cannot destroy a steel structure by dropping a piece of it from above on itself" completely ignores the distance the upper body has fallen. As it ignores the amount of energy that the lower structure must absorb.

It is therefore trivially incorrect.

Intuitively, one would fully expect the lower structure to arrest the upper structure if it was (somehow magically) "dropped" one millimeter. One would be nothing short of astonished if the lower structure could successfully catch upper block after it was dropped 100 feet.

The little piece you drop will be destroyed prior to major failures of the structure below. Happens every time. Or the small piece will just bounce!

The "little piece" that you are referring to weighs something on the order of 25 thousand tons.

I don't care what sort of building onto which you are dropping something of that weight. From above a sufficient distance (that one author calculated as a couple feet), that structure WILL collapse.

Your silly assertion notwithstanding.

NIST thinks otherwise but ignores, e.g. friction between failed parts. So the NIST report becomes science fiction (no friction!). Include friction in the analysis and you will find why structural failures are arrested.

Really. So, it appears that you believe that friction somehow reduces the amount of energy that a falling body accumulates. And therefore the amount of energy that the lower structure will have to absorb.

This is, of course, rubbish. There is no friction term in the expression for the kinetic energy of a falling body.

Bazant is worse. His model is 1-D, a line, that shortens itself.

DOCTOR Bazant has developed a working one dimensional model for progressive collapse. This is a tremendously USEFUL model that any real engineer would appreciate when analyzing or predicting collapse. In precisely the same way that a 1D MODEL of a beam is a tremendously useful (albeit simplified) MODEL for a beam segment in FEA.

I have debunked it at http://heiwaco.tripod.com/nist3.htm . Pls read it and point out any errors in my conclusions.

I scanned your work. "Debunked" is about the last word that occurred to me. "Bunk" is the first.

Sorry, I have seen here exactly how disrespectfully you interact with other competent engineers who have devoted time & effort to pointing out your errors. I'll not be wasting my time further than I already have.

A&E911truth.org liked that article very much and made me their personality for February 2009. Great honour.

Let's see: "Birds of a feather". "Ship of Fools". "Damned by Faint Praise".

Feel free to choose your homily.

Re Dresden ... Very few steel structures there! Most of it was stone and bricks.

Perhaps instead of dismissing this quite so glibly, you might want to wander over to the History Museum in Dresden and look at some of the photos & exhibits they have of the steel buildings that WERE there. You might learn something.

Perhaps you'd care to explain why the building industry spends billions of dollars per year on the insulation of steel structures if, as you appear to claim, this is needless.

Or perhaps you'd care to explain this column, from the base of WTC5.
http://911research.wtc7.net/mirrors/guardian2/wtc/fig-4-17.jpg
Buckling produced ONLY by heat from fire.

tk

 

[tfk]

Read my article about it.

I am glad to confirm that dropping a part of something on something will not crush something. There will just be local failures at contact. Or if something has homogeneous density, etc, it will be a bounce.

Heiwa,

A small rock landed here.

Apparently it did not read your thesis on the fact that it should NOT be able to damage the somewhat larger rocks beneath it. Or "just bounce".

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

tk

 

[NobbyNobbs]

Why do you think air pressure can hurl a 4 ton framework section up to 600 feet?

You might be surprised what air pressure can do.

 

[Heiwa]

Heiwa,

...

The maximum amount of energy that a structure can absorb is fixed by its materials & assembly configuration. The amount of energy that the falling body can deliver can be increased by allowing it to fall larger distances. You will always be able to find some maximum distance that the upper body can fall, which the lower structure will just barely be able to arrest its fall. Any higher, and the lower structure will fail.

Your assertion that "You cannot destroy a steel structure by dropping a piece of it from above on itself" completely ignores the distance the upper body has fallen. As it ignores the amount of energy that the lower structure must absorb.

It is therefore trivially incorrect.

Intuitively, one would fully expect the lower structure to arrest the upper structure if it was (somehow magically) "dropped" one millimeter. One would be nothing short of astonished if the lower structure could successfully catch upper block after it was dropped 100 feet.

The "little piece" that you are referring to weighs something on the order of 25 thousand tons.

I don't care what sort of building onto which you are dropping something of that weight. From above a sufficient distance (that one author calculated as a couple feet), that structure WILL collapse.

Your silly assertion notwithstanding.



tk

Sorry, you are 100% wrong! We have a steel structure of columns/beams which we call parts A + B. B < A height wise. Now we drop B on A. A is fixed on ground. A is the lower part. B is the upper part. Nothing collapses! And B cannot even crush A!

Why is that? Because when B contacs A and applies force F on A, A applies force -F on B. A also apply a force on the ground. Guess what force it can be?

Case 1 - If F only compresses A elastically after a perfect contact/impact, it is likely that -F also only compresses B elastically and the result is that B bounces on A. You agree? Structure is same in A and B.

Case 2 - If F causes local damages to A at the contact areas, -F will also produce equal local damages to B in the contact areas. You agree?

Now you may argue that A also applies a force to the ground and that the ground will apply an equal but opposite force to A and that A should also be damaged at ground level when, hit by B at the other end ... but I will leave it you to find out why it does not happen. Case 1 may help. Ground bounces also.

Reason why in Case 2 local failures occur in interface A/B at contact is that stronger sub-parts damage weaker sub-parts of the two structure parts. The structure crumbles at interface A/B. This does not happen at interface ground/A, as there parts are better aligned. And as A>B, part B will crumble completely before A does the same. Sorry, B cannot crush A.

And nothing collapses!

 

[Heiwa]

Heiwa,

A small rock landed here.

Apparently it did not read your thesis on the fact that it should NOT be able to damage the somewhat larger rocks beneath it. Or "just bounce".

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

tk

Thanks, your example proves exactly what I always say. In this case a small rock, part B, has dropped on a big rock (the earth), part A. The result is local damages to both parts, an indent in part A, while small part B was pulverized. Actually the indent in part A is a bit of A that also got pulverized.

Reason for this is when B contacted A with force F, A applied force -F on B and pulverized B. Force F could just pulverize a bit of A - what is seen as a crater.

See, it is not difficult.

 

[Heiwa]

Heiwa,

Perhaps instead of dismissing this quite so glibly, you might want to wander over to the History Museum in Dresden and look at some of the photos & exhibits they have of the steel buildings that WERE there. You might learn something.

Perhaps you'd care to explain why the building industry spends billions of dollars per year on the insulation of steel structures if, as you appear to claim, this is needless.

tk

By chance I have a house at Freiberg, 37 kms from Dresden, so I am often at Dresden and have visited all the museums there. Plenty to learn. Big fire February 1945! What a waste! Dropping napalm bombs on civilians!

Reason why we insulate steel structures is to delay the transmission of heat into the structure at fires. Pls, do not suggest that I appear to claim that it is needless. Because I have never done it. Why do you suggest things that I have never claimed? An appearance? Hallucinations?

 

[Bell]

nvm

 

[lapman]

By chance I have a house at Freiberg, 37 kms from Dresden, so I am often at Dresden and have visited all the museums there. Plenty to learn. Big fire February 1945! What a waste! Dropping napalm bombs on civilians!

It wasn't napalm. It was many tons of thermite.

 

[GlennB]

Thanks, your example proves exactly what I always say. In this case a small rock, part B, has dropped on a big rock (the earth), part A. The result is local damages to both parts, an indent in part A, while small part B was pulverized. Actually the indent in part A is a bit of A that also got pulverized.

Reason for this is when B contacted A with force F, A applied force -F on B and pulverized B. Force F could just pulverize a bit of A - what is seen as a crater.

See, it is not difficult.

So - if a meteorite (smaller than a house) landed at 10,000 kph on a house then action=reaction (F = -F) and all is well? The meteorite's impact is halted? It stops on the roof?

You have a long history of turning serious questions into jokes. This is a serious question and it might test your apparent beliefs. Can you cope with this?

Would the meteorite :

a)stop when it hits the roof?

b)utterly demolish the house and create a large crater where the house used to be?

A serious answer please. No jokes.

 

[Mince]

Steel structures cannot globally collapse due to gravity alone.

Are you referring to your pizza box steel structure?

 

[NobbyNobbs]

Sorry, you are 100% wrong! We have a steel structure of columns/beams which we call parts A + B. B < A height wise. Now we drop B on A. A is fixed on ground. A is the lower part. B is the upper part. Nothing collapses! And B cannot even crush A!

Why is that? Because when B contacs A and applies force F on A, A applies force -F on B. A also apply a force on the ground. Guess what force it can be?

Case 1 - If F only compresses A elastically after a perfect contact/impact, it is likely that -F also only compresses B elastically and the result is that B bounces on A. You agree? Structure is same in A and B.

Case 2 - If F causes local damages to A at the contact areas, -F will also produce equal local damages to B in the contact areas. You agree?

Now you may argue that A also applies a force to the ground and that the ground will apply an equal but opposite force to A and that A should also be damaged at ground level when, hit by B at the other end ... but I will leave it you to find out why it does not happen. Case 1 may help. Ground bounces also.

Reason why in Case 2 local failures occur in interface A/B at contact is that stronger sub-parts damage weaker sub-parts of the two structure parts. The structure crumbles at interface A/B. This does not happen at interface ground/A, as there parts are better aligned. And as A>B, part B will crumble completely before A does the same. Sorry, B cannot crush A.

And nothing collapses!

Are you honestly suggesting that one object can be dropped on another and... regardless of what the objects are made of, how they're built, or the height from which it is dropped...the lower object will never, ever collapse? Is that what you are honestly suggesting?

If that is the case, could you please explain why this person doesn't bounce?



Also, if this is what you are suggesting, could you please mention at what university you received your engineering degree and what physics courses you were required to take while you were there? Thanks.