[Closed] 9/11 documentary

CharlieMungus - Member

yes, i meant in the collapse

referenced above, but copied again

Nearly every large building has a redundant design that allows for loss of one primary structural member, such as a column. However, when multiple members fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down.

The perimeter tube design of the WTC was highly redundant. It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. Many structural engineers believe that the weak points—the limiting factors on design allowables—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure (see Figure 5). With a 700 Pa floor design allowable, each floor should have been able to support approximately 1,300 t beyond its own weight. The total weight of each tower was about 500,000 t.

As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell. The floor below (with its 1,300 t design capacity) could not support the roughly 45,000 t of ten floors (or more) above crashing down on these angle clips. This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of 200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h.1 It has been suggested that it was fortunate that the WTC did not tip over onto other buildings surrounding the area. There are several points that should be made. First, the building is not solid; it is 95 percent air and, hence, can implode onto itself. Second, there is no lateral load, even the impact of a speeding aircraft, which is sufficient to move the center of gravity one hundred feet to the side such that it is not within the base footprint of the structure. Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity. To summarize all of these points, a 500,000 t structure has too much inertia to fall in any direction other than nearly straight down.

[quote=sbob ]There were huge lateral forces when the plane hit the building; shook the whole tower. Took the resultant fire to take it down though.

Still not huge enough to shift the CoG outside the footprint. In fact I suspect even if you had a succession of airliners crashing into the tower during the 10s it took to collapse that still wouldn't have been enough to prevent it collapsing straight down.

Before anybody asks, no I haven't done the maths and I CBA.

It couldn't really have collapsed any other way, that does NOT mean that the collapse was predictable or likely. Just the manner of collapse.

Yes, i did mean the manner of collapse. That if someone said a plane would explode in there and the subsequent fire would cause a collapse, you would reasonably say, Yeah, it will pretty much fall on itself

Unless you're aware of some significant lateraa force that we've all missed?

Only ones i can imagine would be the unpredictable nature of one thing falling on another. tends not to be just vertical forces at play

Only ones i can imagine would be the unpredictable nature of one thing falling on another. tends not to be just vertical forces at play

No, it really is just the vertical bit you need to worry about in this situation.

The force is gravity, and the mass is mahooosive, any lateral reaction forces from bumping into stuff on the way down are many orders of magnitude smaller, consequently you cna pretty much disregard them, hence...

"Yeah, it will pretty much fall on itself"

Only ones i can imagine would be the unpredictable nature of one thing falling on another. tends not to be just vertical forces at play

Yeah but the mass under the influence of gravity swept everything in one direction, there was considerable deflection mass - lots of stuff being ejected in the videos, but the general trend is going to be in the direction of the insanely dominant accelerating force of gravity.

ahhh cross post with amedias

jivehoneyjive - at times with your various forum posts [i][b]very[/b] occasionally[/i] you touch on some almost credible (albeit tenuous) points. But with all the nonsense spouted on topics like this you're discrediting anything you say I'm afraid. I wish you well, the world needs cynics but you need to pick an appropriate target, this is on a par with faked moon landings and lizard Royal families.

This is one of those situations where the numbers are BIG, and humans are generally speaking, terrible at envisioning BIG. We think we can, but we can't, hence why we need maths to prove stuff to us once it gets outside of the realms of what our brains can deal with.

Logic and experience tells us that if you bump into stuff you get deflection*, but in everyday situations we're not encountering stuff BIG enough, or realising the BIGness of the difference between things.

Plug the numbers in and it becomes a lot clearer!

*yes some of the rubble gets spat out sideways, but the bulk of the mass goes in the same direction.

The force is gravity, and the mass is mahooosive, any lateral reaction forces from bumping into stuff on the way down are many orders of magnitude smaller, consequently you cna pretty much disregard them, hence...

makes the straight down seem very unusual

It's not though.

This really is a case of your perception vs reality. There may be lots of stuff in the way for it to bump into in a chaotic fashion, but the horizontal forces are still orders of magnitude smaller than the one going down, so down it goes, and remember its not a rigid structure, even if some part of it gets a punt sideways, it's not transferred to the mass as a whole.

You really need to get your head around the fact that there are no [i]external[/i] lateral* forces during the collapse. Once you get your head round the fact that there is no force acting laterally on the mass then you can see why there's no reason to expect lateral movement of the mass as a whole.

*The only lateral forces are horizontal components of reaction forces due to the down wards motion, and these are tiny by comparison.

The chaotic nature at the small scale (say the size of a brick or book) is going to largely cancel out. Within the core of the building there'd be so much material and so little time between a floor collapsing and the material of that floor impacting the next that lateral forces would be minimal. At the edges there'd be less constraint so you'd get material ejected but it would be relatively constant on all four sides so the resulting lateral forces would still be too small to have any significant effect.

As noted earlier, the largest lateral force the buildings had to cope with were the plane strikes: 280 tonnes at 400-500 knots vs 450,000 static tonnes.

I can accept the difference in magnitude, but the chaotic nature of bumping into stuff, especially the building itself, lots and lots of times makes the straight down seem very unusual

Assuming every level was blown to get it to drop - would this look any different? All the concrete is gonna break up and be sent in all directions but will get normalised into the downward direction.

forces are still orders of magnitude smaller

there is always a gravitational force on the mass, the deflection forces get cancelled/minimised/deflected again or the part gets destroyed/broken again, when they hit something else, the gravity force is still there.
It's lie leaves blowing int he wind, there is turbulence, collisions and stuff getting ejected sideways, but the majority of the mass is going in the direction of the prevailing wind.

Assuming every level was blown to get it to drop - would this look any different? All the concrete is gonna break up and be sent in all directions but will get normalised into the downward direction.

Not sure how it works, but by blowing it, isn't bumping reduced because the thing it might bump into has gone?

In isolation, many of the theories regarding the towers collapse as a result of plane impact and fire do seem perfectly reasonable, all we're ever likely to have on that front is informed conjecture from either camp

That said, when looking at cases which bear some similarities, it's not unreasonable to question not only the mode of collapse, but the factors which led to it.

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the deflection forces get cancelled/minimised when they hit something else,

except at the edges, where they will tend to be in the same direction.
and only if collapse is uniform

Not sure how it works, but by blowing it, isn't bumping reduced because the thing it might bump into has gone?

So you have concrete and steel layers, an external skin, and inner cores columns. The layers (the floors) are tagged onto the inner core columns. If you cut all the columns or de-tag them from the columns its gonna go down right. But there is still most of the existing mass.

Do they not collapse from the ground up?

jivehoneyjive - Member

In isolation, many of the theories regarding the towers collapse as a result of plane impact and fire do seem perfectly reasonable, all we're ever likely to have on that front is informed conjecture from either camp

Not really.
I saw the plane impact the building, I saw the fire, I saw the collapse.
They didn't collapse before the plane impact and fire, did they?

except at the edges

The Windsor building in Madrid is a different construction to that of the WTC. The Windsor building is/was a traditional stanchion and beam construction so each floor has multiple columns supporting it. The WTC were an inner core and outer skin joined by very lightweight lattice beams.

If you don't understand the difference then you won't understand why one collapsed and the other didn't. Of course I don't remember a passenger jet aircraft striking the Windsor building, do you?

lots of stuff got ejected sideways, have you seen the videos.

Yes but it wasn't all from one side so any force resulting from material being ejected from the north side would be largely counteracted by that being ejected from the south. Similarly for east and west.

CharlieMungus - Member
Do they not collapse from the ground up?

Well yeah but the demolition proponents say they were blown in order just in advance of each floor dropping. I am giving them latitude.

Had we not established that mechanically at least, the aircraft impact was not relevant?

lots of stuff got ejected sideways, have you seen the videos.

There's videos of the collapse????

forces are still orders of magnitude smaller

Except they are not.

The horizontal motion from 'bumping into stuff' is a small horizontal reaction component of the downwards momentum after collision, there is still the resulting downwards component of momentum + the force from gravity downwards, and that is [i]still [/i]acting, the horizontal force [i]stops [/i]after collision.

Take a single piece of rubble as an example, even if 100% of the downwards momentum were converted to 100% lateral, there is nothing pushing that bit of rubble sideways after it hit whatever it hit, but gravity is still pulling it downwards, overall path is down...

I'll say this bit again:

There are no [i]external [/i]lateral forces. There is an external vertical force, gravity.

CharlieMungus - Member

Not sure how it works, but by blowing it, isn't bumping reduced because the thing it might bump into has gone?

If you blow it into dust, sure. But that's not what controlled demolitions do. In either situation the bulk of the building and the bulk of the strength and resistance to the falling material is still going to be there.

Perhaps you could do a massive demolition which did have a bigger impact on the strength and the rate of collapse- but people would have noticed when the building blew up like a melon with a grenade in it, rather than collapsing like a collapsing building. Or, you could strip out the contents and leave just a skeleton- like in a real demolition- but again that'd be kind of noticable.

I'll say this bit again:

There are no external lateral forces. There is an external vertical force, gravity.

No need to say that again I haven't proposed any external lateral forces

Take a single piece of rubble as an example, even if 100% of the downwards momentum were converted to 100% lateral, there is nothing pushing that bit of rubble sideways after it hit whatever it hit,

Had we not established that mechanically at least, the aircraft impact was not relevant?

nothing pulling it back either.

No need to say that again I haven't proposed any external lateral forces

Yet you think there should be (bulk) lateral movement. How can there be lateral movement with no lateral force?

nothing pulling it back either

There's nothing pulling it no.... but there is something pushing back against it....air resistance, which is why it decelerates horizontally, probably not *that* much there is no force acting on it any more pushing it further out where as vertically gravity is still acting on it vertically.

Let's try some probabilities

1)Probability that it would collapse 'neatly' given that it was a controlled explosion:

2)Probability that it was a controlled explosion, given that it collapsed neatly:

some estimates will do just to get started

Yet you think there should be (bulk) lateral movement. How can there be lateral movement with no lateral force?

Off centre mass would be a possibility. Think Jenga, no external lateral force, still results in bulk lateral movement

There's nothing pulling it no.... but there is something pushing back against it....air resistance, which is why it decelerates horizontally, there is no force acting on it any more. Where as vertically gravity is still acting on it vertically.

This isn't towers dropping in a vacuum!

You've just gone down the rabbit hole CM...

Off centre mass would be a possibility. Think Jenga, no external lateral force, still results in bulk lateral movement

Nooooo! the Towers are NOT like Jenga.

re-read the earlier posts re: pivoting and rigid/load transferred structures. Jenga blocks are solid, the Towers were mostly hollow. The upper stack of a Jenga tower can pivot due to being a semi-rigid structure with good load transference. The Towers could not do that.

air resistance acting in any significant way on a brick ejected from the building?

you'd be surprised, but you're missing the point. In the absense of an acting force the brick (or whatever) will continue horizontally at the same speed, it won't get any faster, it would infact slow down due to air resistance, but the amount is not the point, the point is that gravity is STILL acting on it vertically so it will accelerate vertically.

Off centre mass would be a possibility. Think Jenga, no external lateral force, still results in bulk lateral movement

This deserves response because I know you are not trolling, you are robustly questioning.

In jenga, the energy during the fall, in the down direction, is much less than the energy required to smash/crush/destroy the jenga blocks.So the falling blocks deflect off relatively very strong block below, cos they have nowhere else to go.
In wtc, the energy required to crush any level below, is utterly insignificant to the energy during the fall.

In isolation, many of the theories regarding the towers collapse as a result of plane impact and fire do seem perfectly reasonable, all we're ever likely to have on that front is informed conjecture from either camp

That said, when looking at cases which bear some similarities, it's not unreasonable to question not only the mode of collapse, but the factors which led to

I'm fairly sure that only one camp is using informed conjecture.

Out of the buildings shown above what fundamental design similarities do they have, and importantly what differences?

what do the fires have in common? E.G. were any of them caused by the flash ignition of 90,000 litres of fuel or did they start in one small location and spread from there over time? I'm fairly sure you know the answer already

you'd be surprised, but you're missing the point. In the absense of an acting force the brick (or whatever) will continue horizontally at the same speed, it won't get any faster, it would infact slow down due to air resistance, but the amount is not the point, the point is that gravity is STILL acting on it vertically so it will accelerate vertically.

…with magic explosives that don’t make any sound. Aye. Right.…with magic explosives that don’t make any sound. Aye. Right.

...oh you were there?

This deserves response because I know you are not trolling, you are robustly questioning.

In jenga, the energy during the fall, in the down direction, is much less than the energy required to smash/crush/destroy the jenga blocks.
In wtc, the energy required to crush any level below, is utterly insignificant to the energy during the fall.

indeed, imagine jenga where the blocks are 95% air and they are physically joined together. Now structurally weaken them rather than trying to topple it.

As mentioned early as a very good analogy, if you want to simulate the aircraft impact on your jenga then flick a sugar lump at it.

...oh you were there?

Nope but there were one or two other people there and I'm led to believe that there might have been some news coverage too. Spookily enough no one has ever reported hearing explosives which, had they been used, would certainly have been heard.

I have now lost track of who are the truthers and who are the sane ones!

What's all this sugar cube business?

[quote=CharlieMungus ]Except they are not. They are of the same magnitude, they may be smaller. Unless you reckon the angle of the 'bump' is very small. It would just be a factor of Cosine of the bump angle, wouldn't it?

If the whole of one side of a structure between floors stays intact whilst the other side collapses, then given the numbers here the sin of the angle is 4/70 - still an order of magnitude less horizontal forces even resulting from a collapse in such a fashion. The reality is that nowhere near that much offset happened during the collapse. I'm not sure where you think you're getting a bigger bump angle than that affecting the whole structure, unless you're completely discounting the floor by floor collapse model.

1)Probability that it would collapse 'neatly' [s]given that[/s] if it was a controlled explosion:

high, being that that is the aim of a controlled demolition and a building of that construction type cant really collapse any other way, it can't topple so what other way for it to collapse is there?

2)Probability that it was a controlled explosion, given that it collapsed neatly:

Impossible to answer as this is a non-sequitur, since the only manner in which a building of that construction type could collapse is 'neatly' (in this context) whether controlled or not you cannot infer a probability of the former from the latter.

indeed, imagine jenga where the blocks are 95% air and they are physically joined together. Now structurally weaken them rather than trying to topple it.

oh, and set fire to them too, just because it'd be fun 😉