1200 Architects And Engineers

reply to post by Gorman91


Gorman why don't you stop making up your own physics and educate yourself? There's plenty of free material on the web.

Simply put, big thing falls, makes bigger force than if big thing was laying down stationary. If you reject that, I don't care about the rest of your post.

I reject that.

Now that Gorman has stopped reading I'll explain why. For some reason Gorman doesn't really understand the concept of force, especially how it relates to velocity (he maintains it is proportional to velocity rather than acceleration). When the "big thing falls" it only applies a greater force (than when it was stationary) to the thing trying to hold it up IF the thing trying to hold it up decelerates the big falling thing, as F=ma. For the big falling thing to uniformly and constantly accelerate the thing holding it up MUST apply less upwards force than if it were only supporting the big things stationary weight.

Reason why big thing fell is because fires made the materials incapable of generating resistant force. Material property changes in different conditions.

It appears Gorman also can't read, as I've told him numerous times I don't have a problem with collapse initiating, only the extreme behavior of collapse. I'm only concerned with forces in the undamaged structure.

Failure to understand that is also disturbing.

So what forces? We're done with that topic. How about you prove that those forces at work remain constant in different temperatures.

So we're done with the topic of forces because Gorman has realized he is unable to adequately describe the basic forces he thinks are involved, and was caught out attempting to pass off gravitational potential energy as a force.

It is understandable Gorman wants to shy away from the topic of forces because a little bit of physical reasoning shows how the undamaged lower structure had to apply less force to the top section than when it was stationary.

reply to post by Azp420


Well that's nice and all but I already explained to you why the collapse happened the way it did. The hiearchy detached.

Now maybe you reject the physics of a big thing falling making more force than the same big thing stationary. But the forces of nature disagree. because every time you look at the moon, the proof is right there in the craters.

You can claim I don't understand, but you have yet to say how. First you have to explain how something in motion hitting does not generate more force than the same thing stationary resting on something. Being 5 am and me not honestly giving a damn to look up the exact science does not make me wrong. It makes me lazy. The fact remains, however, is that you just rejected the notion of force applied due to speed.

FYI, I'm typing. Typing is the same thing. My fingers are in motion, causing more force than if they simply were resting on the key.

I really fail to see how hard it is to understand. Big thing moves and hits thing. More force than if big thing was resting on the thing.

In the WTC, the big thing was the part that fell. The little thing was the inner structure. The floors are irrelevant to strength at that point. The hierarchy detached, the inner structure fell. The outer structure stood but fell a few moments after because nothing was supporting it. A tesseract.

Your taking a situation seriously that I honestly cannot find serious enough to care about. Yes my math is sometimes wrong. Yes the exact term I use is many times wrong. But the basic idea has not changed nor has been disproven. And the videos I posted prove that, which you conveniently ignore.

Honestly I stopped taking this issue seriously years ago. So sorry if I'm wrong, but in order to care about being 100% right I'd actually have to care about dedicating my life to the topic. I do not. So the basic idea remains true.l if the terms I use are not perfect, ok then. I don't care.

reply to post by Gorman91

Well that's nice and all but I already explained to you why the collapse happened the way it did. The hiearchy detached.

Your explanation violates basic physical principles.

Now maybe you reject the physics of a big thing falling making more force than the same big thing stationary. But the forces of nature disagree. because every time you look at the moon, the proof is right there in the craters.

Because the moon decelerated (how many times till you get this?) the masses that hit it, hence equal and opposite forces and all that.

You can claim I don't understand, but you have yet to say how.

Carefully read my posts rather than quickly skimming over. It's all in there.

First you have to explain how something in motion hitting does not generate more force than the same thing stationary resting on something.

Object A in motion falling on and impacting Object B will generate more force than Object A resting on Object B in most cases because in most cases Object B will decelerate Object A. There is a very rare case when Object B won't decelerate Object A and that is when Object B is controlled demo'd out of the way just as Object A is about to impact it.

FYI, I'm typing. Typing is the same thing. My fingers are in motion, causing more force than if they simply were resting on the key.

Yup, your fingers are accelerating the keys. Isn't it amazing how things can be explained with F=ma?

I really fail to see how hard it is to understand.

A good mechanics textbook will go over all these concepts slowly and in detail. Remember, F=ma.

n the WTC, the big thing was the part that fell. The little thing was the inner structure. The floors are irrelevant to strength at that point. The hierarchy detached, the inner structure fell. The outer structure stood but fell a few moments after because nothing was supporting it. A tesseract.

I'll say it again: I have no problem with collapse initiating.

Your taking a situation seriously that I honestly cannot find serious enough to care about.

I don't take life seriously, but I care to be accurate in my beliefs as to did and did not play a hand in 9/11.

Yes my math is sometimes wrong. Yes the exact term I use is many times wrong. But the basic idea has not changed nor has been disproven. And the videos I posted prove that, which you conveniently ignore.

Force will never be directly proportional to velocity.

I've not seen any relevant videos.

Honestly I stopped taking this issue seriously years ago. So sorry if I'm wrong, but in order to care about being 100% right I'd actually have to care about dedicating my life to the topic.

Nah man its easy as, it doesn't take a life's work. Just an open mind and a solid understanding of Newtonian physics.

reply to post by Azp420


HOW do they? You can't merely say they do. Define your terms.

The masses accelerated. This is called gravity. The moon has no atmosphere to decelerate. Items either stayed at constant speed because moon gravity is borderline irrelevant or sped up. Upon impact, their motion was transfered into heat energy and the items came to a hault. That would be your deceleration. But that does not change the fact that energy was transfered.

When an item in motion hits a stationary item that will not move in response, one of two things will happen. The item moving will damage the pinned item, or the pinned item will make the moving item bound back. The example you are using is not related to items that cannot move to respond.

The deceleration you speak of is the transfer of force. And if this transfer of force is straight down onto a pinned item, the two will break and collapse down together. Yes they will decelerate, bit once the pinning is broken, they both free fall due to the impact breaking the pin. Structural failure.

Please learn what you speak.

reply to post by Gorman91

HOW do they? You can't merely say they do. Define your terms.

Sorry, I'm not sure what this is referring to.

The masses accelerated. This is called gravity. The moon has no atmosphere to decelerate. Items either stayed at constant speed because moon gravity is borderline irrelevant or sped up.

I mean at the moment of impact. The fast moving mass is very quickly decelerated which results in a large force as per the F=ma equation.

Upon impact, their motion was transfered into heat energy and the items came to a hault. That would be your deceleration. But that does not change the fact that energy was transfered.

Bingo, that's the deceleration. Yes, of course the kinetic energy was transferred into other forms. Conservation of energy is simply a different method to the force acceleration method.

When an item in motion hits a stationary item that will not move in response

There's not much in this world that won't move in response at some scale.

The example you are using is not related to items that cannot move to respond.

Yes it is, F=ma applies here too, just as it did in the moon example.

The deceleration you speak of is the transfer of force.

Now you're getting it! The next step is to try to find that deceleration as the top section impacted the undamaged lower section...

And if this transfer of force is straight down onto a pinned item, the two will break and collapse down together.

Be careful making assumptions. Buildings transfer forces all day without breaking.

Please learn what you speak.

What?

reply to post by Azp420


The transfer of energy in motion and impact when the massive falling piece hit the top of the inner structure created massive bending force. It could not resist that. The impact itself would make the structure of the floors useless. That's the only thing holding the two pieces together. The hierarchy detached, the inner structure could not hold it alone, and so fell.

That is the only reason why the outer structure at the bottom was intact. If there were explosives used, then the outer structure would have been blasted to pieces, not staying mostly intact at the foundation.

reply to post by Gorman91


IMO-The moon was a bad example- way less gravity.I am trying to see your logic believe it or not. WTC1 and 2 had an unbelievably weak structure outside of the core. Pancaking to the basement would be possible if the core was left intact. Like dropping doughnuts over a catchup bottle. However the stronger core denigrated as it fell, Therefore no catchup bottle. ( Was wtc7 hit- in the middle by force? On the left side be (debris)? How does it collapse into it's basement. The fire was not exponential, as the firemen were called out of the building. Hours later, it fell middle first on to it's own footprint.

reply to post by Tribble


A column is pretty damn strong. You could probably put a lot on there. But as the old saying goes, the stronger they are the stronger they fall.

Once the structure reaches a certain level of stress, it all comes down. Indeed it can be incredibly strong. But being incredibly strong means you are incredibly weak once your main strength is over-played. Once the iner structure was tasked with doing the job of the entire structure, it reached its max and fell to pieces.

You can demonstrate this on your own. Find a sturdy stick on a dry hot day. push along its height. Feel its strength as it bends. But merely push a bit more, and it cracks and all that strength is lost. And it happens so fast too. Because it's so strong to begin with.

reply to post by Gorman91


You are on a logical path. There was extreme heat, and the burnable insulation installed between floors (so much so that it raised eyebrows from the port authority. I am now asking for your thoughts on tower 7. I said enough about it in my last post.

Originally posted by Azp420
Now that Gorman has stopped reading I'll explain why. For some reason Gorman doesn't really understand the concept of force, especially how it relates to velocity (he maintains it is proportional to velocity rather than acceleration). When the "big thing falls" it only applies a greater force (than when it was stationary) to the thing trying to hold it up IF the thing trying to hold it up decelerates the big falling thing, as F=ma. For the big falling thing to uniformly and constantly accelerate the thing holding it up MUST apply less upwards force than if it were only supporting the big things stationary weight.

I just lost a huge reply to this post so I am going to summarise it very briefly.

1. The WTC was not homogeneous. The force is not applied constantly, but periodically as floors impact one another
2. You are ignoring conservation of momentum. While force scales to the acceleration, momentum scales to velocity.

If you take these two factors into account, you end up with instantaneous forces well in excess of column capacity. By averaging out the acceleration and ignoring the construction your case is superficially plausible, but proper analysis has been done by Bazant et al and shows your case to be incorrect.

If you want to discuss this in more detail, please lay out exactly the initial conditions and impacts you expect to occur and what values you have used for them.

reply to post by Tribble


Best I can get from it is that It had a bunch of stuff happen to it ranging from internal heating explosions of some kind, a chunk of WTC tower falling into it, a destabilization of exterior structure (search google images to see the giant chunks missing), and probably some thermite reactions as the fires grew. i'm not going to deny it's strange, but I am going to say that, like the WTC towers, the lower outer structure remained mostly intact from the collapse. Not exploded to pieces as would come from foundation demolitions. I won't deny the chances that some mischievous people might have done something to make the fires worst to make some parts explode in order to get some insurance money, but to say demos I think not.

reply to post by Azp420


You have to realize, when arguing physics with Gorman, this is someone who was talking not even a month ago about throwing a bowling ball to the top of the towers at 9.82 mph, as if that would somehow have relevance to anything.

9.82 mph.... ?

And before that he posted videos of the towers re-building themselves out of oil barrels and 4 planes flying out of the top of them, and called those "simulations" of the collapse.


Gorman, you have a great imagination, but your technical knowledge does not exist.

reply to post by exponent

1. The WTC was not homogeneous. The force is not applied constantly, but periodically as floors impact one another

I recognize this. For the pancake theory I would expect decelerations to occur as each floor was impacted (with accelerations between floors). The main deceleration I would have expected to see would be when the top section first impacts undamaged lower structure. Planes and fires damage the initiation zone to the point where it gives way, and is so damaged it provides such little resistance to the falling top section that it is able to accelerate at ~2/3rds free fall. That's fair enough. But when the top section first impacts the undamaged lower section, there is no way I would expect anything near this kind of acceleration to be uniformly and constantly maintained. There was very little difference in the resistance provided by the heavily damaged and on fire initiation zone and the resistance provided by the undamaged structure. I would have expected the net upwards reactional force provided by the lower structure to be greater than ~1/3rd of the net upwards reactional force provided before collapse had begun. I would have expected the top section to begin to crush the undamaged lower structure with a net force greater than ~1/3rd of the weight of the top section.

There is no way I would have expected the leading edge of the wave of ejections to accelerate down the building overall at the same rate that the top section accelerated through the initiation zone.

2. You are ignoring conservation of momentum. While force scales to the acceleration, momentum scales to velocity.

I'm not ignoring this, I'm well aware of it. All I am describing is how the forces are related to the accelerations. Momentum or velocity is not needed for this, as momentum is not a force.

If you take these two factors into account, you end up with instantaneous forces well in excess of column capacity.

For such a symmetrical collapse I would expect the columns (particularly the inner) to be more or less equally loaded. As the net combined upwards force of all structural elements during collapse was only ~1/3rd of what it was before collapse, and the capacities are much greater than what they were taking before collapse, I find it doubtful that the falling top section was the only thing responsible for symmetrically overloading structural elements level by level as it progressed down the structure.

Originally posted by Azp420
But when the top section first impacts the undamaged lower section, there is no way I would expect anything near this kind of acceleration to be uniformly and constantly maintained. There was very little difference in the resistance provided by the heavily damaged and on fire initiation zone and the resistance provided by the undamaged structure. I would have expected the net upwards reactional force provided by the lower structure to be greater than ~1/3rd of the net upwards reactional force provided before collapse had begun. I would have expected the top section to begin to crush the undamaged lower structure with a net force greater than ~1/3rd of the weight of the top section.

There is no way I would have expected the leading edge of the wave of ejections to accelerate down the building overall at the same rate that the top section accelerated through the initiation zone.

The problem is that you are still making the same mistake as you were making before, in fact I will even quote it explicitly:

momentum is not a force

Let me come up with a hypothetical scenario, to describe why you are wrong. I will use some example values that have no relation to reality, if you want real values I suggest you check Bazant's series of papers where he deals with this as a structural engineer.

Ok, so we'll assume that there were 3 damaged floors, totalling 11m of vertical space. We'll assume the drop was completely symmetric and uniform, and that the descent rate is 2/3g for the damaged section. We will ignore debris and deal only with symmetric column on column impacts.

After descending 11m, the upper block will be at sqrt(2*6.5*11) or approximately 12m/s. If its mass is 100,000kg (yeah right!) then its momentum p is 1,200,000kgm/s.

Now, the columns this block is about to impact can only contract a certain amount before they fracture or buckle. Once fractured or buckle these columns will hold a negligible load, but we will not consider that.

If these columns can shorten 5% before fracture, and are 11m long (columns in the WTC were each 3 storeys long) then this momentum must be dissipated within 0.55m. As you know, Force = mass * acceleration. Ignoring for now the mass of the columns as they are compressed we can work out the deceleration required to come to a halt within 0.55m, that is:

0 = 12² + 1.1*a
rearranged to
1.1a = 12²
1.1a = 144
a = 131m/s

Now we know that the force required is 131 * 100,000 or 13,100,000N. Compare this to the original required load of 9.81*100,000 or 981,000N we find that with this three storey drop, the columns would be overwhelmed by a factor of over 13.

Now, obviously the columns cannot survive this (in a hypothetical scenario of course) and if they cannot survive it, they will not halt the block completely. The question now becomes: is the distance between this storey and the next enough for the block to accelerate to a high enough energy to destroy the columns on the floor below?

Again I will ask you to refer to the various Bazant papers which analyse this topic much more in depth.

Hopefully my sums are all correct here, but it is 5:30am here and I have just finished a major reconfiguration, so I am about to head to bed. Let me know if you spot any mistakes.

reply to post by exponent


Well lets look at some evidence...



The top did not pancake the lower floors.



The top did not pancake the lower floors.

If you can not figure out why from those two pics then you have no experience with mechanics and should not be supporting the OS that you fail to understand.

Originally posted by ANOK
reply to post by exponent

 

Well lets look at some evidence...

The top did not pancake the lower floors.



The top did not pancake the lower floors.

Except those pictures show it doing just that, you just posted proof of it pancaking the lower floors!

Firstly, I'm no engineer, but using my own logic, I am suprised, despite have read and watched tons of info on the 9/11 incident, that no one seems to have mentioned the following, in relation to the collapsing of the towers....

I believe they were brought down by explosives, and this is the point I want to make in support of that:

If they fell due to the 'planes' impact and fires, then considering standard physics, surely as the affected floor gave way due to the buckling of the beams through intense heat, the floors above would collapse downwards. However, below the fires the beams were unaffected, so as with car crashes and the deflection and absorption of impact energy, the floors above would meet ever increasing resistance to their collapse, and this resistance, floor by floor, would slow up the collapse. The material decending, still having downward energy, would then be forced outwards, leaving even less weight and therefore pressure, on the floors below, and so the collapse would stop, say, part way down, with all debris above falling outside of the remaining building structure.

So, seeing the way they collapsed so uniformly, surely the only possible explanation is... detonation!

can any engineers reading this please confirm, or disprove my theory?

reply to post by exponent

The problem is that you are still making the same mistake as you were making before, in fact I will even quote it explicitly:

momentum is not a force

I don't see how this is a mistake. Momentum is not a force and is therefore not considered in my free-body diagram.

If these columns can shorten 5% before fracture, and are 11m long (columns in the WTC were each 3 storeys long) then this momentum must be dissipated within 0.55m.

I'm not saying the falling top section should have come to an abrupt halt within half a meter. I'm saying as it applied enough force to yield each of the structural members level by level, at the very least it should have accelerated at an overall significantly lower rate than when it was accelerating through the almost resistance-less initiation section.

Now, obviously the columns cannot survive this (in a hypothetical scenario of course) and if they cannot survive it, they will not halt the block completely.

Of-course, but they should have applied to the block an equal and opposite force equal to the amount it takes to fail the column, decelerating the block in the process.

The question now becomes: is the distance between this storey and the next enough for the block to accelerate to a high enough energy to destroy the columns on the floor below?

Agreed, there should have been accelerations between floors and decelerations as floors impacted, especially when undamaged structure was impacted for the first time.

Again I will ask you to refer to the various Bazant papers which analyse this topic much more in depth.

I've read some of Bazant's work. There is no explanation as to how the top section is able to accelerate through undamaged structure at the same rate that it accelerates through the highly damaged initiation zone.

reply to post by dereks

You're joking. It's leaning over at an absurd angle, not stacking up floor by floor.

reply to post by ckitch

can any engineers reading this please confirm, or disprove my theory?

That's pretty much along the lines of my thinking. The undamaged structure should have been able to provide significantly more resistance to collapse than the heavily damaged initiation zone.

WTC7 also achieved massive accelerations (including a period of sustained free fall), meaning its structure also provided minimal resistance to collapse (in another perfectly symmetrical fashion despite asymmetrical damage).

Originally posted by Azp420

The problem is that you are still making the same mistake as you were making before, in fact I will even quote it explicitly:

momentum is not a force

I don't see how this is a mistake. Momentum is not a force and is therefore not considered in my free-body diagram.

No, it's not a mistake.

Force is measured in Newtons.

Momentum is Newton-seconds.

In effect momentum is a function of force over time and so there is no way to express it instantaneously in a free-body diagram. The only thing you can express instantaneously is the instantaneous force which is measured in Newtons.

Originally posted by Azp420
I don't see how this is a mistake. Momentum is not a force and is therefore not considered in my free-body diagram.

Yeah this is my mistake, I should have mentioned it more explicitly. Momentum is not a force, but the momentum of an object does lead to certain forces having to be overcome. In my excuse it was some horrible time in the morning.

I'm not saying the falling top section should have come to an abrupt halt within half a meter. I'm saying as it applied enough force to yield each of the structural members level by level, at the very least it should have accelerated at an overall significantly lower rate than when it was accelerating through the almost resistance-less initiation section.

This could perhaps have happened if the assumptions I used in the above scenario held in reality. That is, if the upper block descended symmetrically, causing column on column impacts.

However, in reality we both know that it did not and tilted significantly. In this situation, the mass per floor to be accelerated is still the same, and so while the impact floors might cause a minor lowering of required force, the momentum transfer is still identical.

Of-course, but they should have applied to the block an equal and opposite force equal to the amount it takes to fail the column, decelerating the block in the process.
...
Agreed, there should have been accelerations between floors and decelerations as floors impacted, especially when undamaged structure was impacted for the first time.

Well again this is very hypothetical, we know that this did not occur in real life, and if the most conservative estimates (allowing for direct column-column impact for example) still causes building failure, then we have to assume that a less optimistic analysis also results in collapse.

I've read some of Bazant's work. There is no explanation as to how the top section is able to accelerate through undamaged structure at the same rate that it accelerates through the highly damaged initiation zone.

That's certainly true, but the analysis carried out of energy sinks in the tower should be enough to convince you that no matter what the columns did not have enough capacity to resist the collapse.

WTC7 also achieved massive accelerations (including a period of sustained free fall), meaning its structure also provided minimal resistance to collapse (in another perfectly symmetrical fashion despite asymmetrical damage).

This is also not true, the total distance over which WTC7 achieved free fall was 8 floors, and the collapse mechanism is not symmetrical, nor was the damage it sustained.

Even though we disagree Azp. At least you are actually thinking about this from a technical perspective. I appreciate that and will try and point you to whatever sources I think are relevant.