Help me understand the collapse a bit more

[Undesired Walrus]

Hey.

Even though I am goddam awful at maths, I wish to understand how the buildings fell in the way they did, as building construction and structual collapse fascinate me.

I am amused when Avery backs himself into a corner on Hardfire and says his 'common sense' argument:
Avery: "I cannot see how 14 floors can destroy 90"
Mark: "Well that's why you're not a structual engineer".

Is it hard to comprehend because we imagine them as solid structures?

If Dylan is reading, and I am sure he often does, explain it to him, and to me.
One difference is, I will be listening.

 

[Brainache]

Stop me if I get too technical here, but: BLOODY GREAT PLANES CRASHED INTO THEM.

Sorry if I sound a bit mad. It's not directed at you, just at the weirdos who think that the towers shouldn't have collapsed after BLOODY GREAT PLANES CRASHED INTO THEM.

Oh yeah and there was also fire. Lots of fire.

 

[Undesired Walrus]

Let me make clear, I understand the floor trusses, and everything that caused collapse, just not all that clear on the mechanics behind the entire collapse.

Why?

Because I aint a structual engineer.

(If only more truthers said that)

 

[rwguinn]

Let me make clear, I understand the floor trusses, and everything that caused collapse, just not all that clear on the mechanics behind the entire collapse.

Why?

Because I aint a structual engineer.

(If only more truthers said that)

Walrus:
I'm going to propose that you do a little experiment. go down to the local lumber/hardware/Wal-Mart, and pick yourself up a couple of wooden dowels, about 1/8th inch (3.1mm) diameter.
Take 1 of them, and clamp it vertically. Add weight to the top till it breaks. Now, take the other one, and hold the center in a jet of steam from the teapot, or other source, and slowly start bending it (You don't want to hear any crackling sounds at all) until it is about an inch or so crooked. Let it cool down.
Repeat the experiment with the weights. See how much less it will hold up?
That is essentiall what happened to the towers. The floors sagged, pulling the vertical columns inward. At that point, they could withstand a much lesser force than the straight, vertical, columns could. This happened on one side of the building (The Tilt of the top portion at the start of collapse is visible in many of the videos) This overloaded the columns on the other side, causing them to fail, and the total force at the next floor due to impact from the upper portion was many times greater than what the structure could take. Things progressed from there.

 

[Undesired Walrus]

But if I add weight from the top (14 floors), presumably would I not need to create a larger weight (90 floors) on the bottom, underneath the top? I think this is what many cannot comprehend. The idea a smaller weight can create a larger weight underneath to fail. The buildings are not solid structures, but doesnt that also mean the weight above is not solid?

In many videos you see what appears to me to be floors collapsing in the middle of the structure, many floors below the collapse wave. Did the floors essentially, 'beat' the upper, more dramatic and visible masses to the ground?

 

[Anti-sophist]

Well, the reason that 14 floors can destroy 90 is the difference between static and dynamic forces. This difference can be demonstrated very easily in your home gym. Take a 15 pound dumbell and rest it on your foot. Now take a 15 pound dumbell and drop it from 10 feet, onto your foot. Notice a difference?

 

[Undesired Walrus]

Well, the reason that 14 floors can destroy 90 is the difference between static and dynamic forces. This difference can be demonstrated very easily in your home gym. Take a 15 pound dumbell and rest it on your foot. Now take a 15 pound dumbell and drop it from 10 feet, onto your foot. Notice a difference?

So, is my foot essentially 1 floor?

And does that foot then essentially add onto the weight of the mass going down? Like 14 floors becomes 15, then 16 then so and so forth? If the 14 floors had been designed 10 feet apart, and then the bottom 90 floors were filled on every floor with concrete, would it have stopped the top portion?

 

[uk_dave]

Maybe the way to look at it is not as 14 floors destroying 90, but rather...

14 floors destroying 1 then...

15 floors destroying 1 then...

16 floors destroying 1.....

etc etc

Because, apart from the debris ejected outside of the collapse zone (overflowing it, if you will) the structure, being made up of individual elements such as columns and floor trusses, all mechanically fixed to each other, once the building stops performing as designed, i.e floor loads transferred to exterior and interior columns and from there down to the foundations, the individual elements are subject to stresses far in excess of their design, for example a vertical column subject to lateral forces from the accumulation of falling debris pushing outwards.

 

[Anti-sophist]

So, is my foot essentially 1 floor?

Well, your foot is simply demonstrating that the weight an object can hold up statically is nothing compared with the forces it experiences during a collision.

And does that foot then essentially add onto the weight of the mass going down? Like 14 floors becomes 15, then 16 then so and so forth?

Yes. Essentially, after "one" cycle, you've gained energy from falling and you've gained mass from all the stuff you've broken. You've also lost some energy actually breaking things, and you've lost some mass "out the sides". I believe if you actually estimate these 4 factors you'll find that the system is gaining energy as it goes, not losing it, and therefore a full collapse is inevitable. The energy is snowballing.

If the 14 floors had been designed 10 feet apart, and then the bottom 90 floors were filled on every floor with concrete, would it have stopped the top portion?

My suspicion is yes, but I haven't done any math to prove it.

 

[Undesired Walrus]

Maybe the way to look at it is not as 14 floors destroying 90, but rather...

14 floors destroying 1 then...

15 floors destroying 1 then...

16 floors destroying 1.....

etc etc

Because, apart from the debris ejected outside of the collapse zone (overflowing it, if you will) the structure, being made up of individual elements such as columns and floor trusses, all mechanically fixed to each other, once the building stops performing as designed, i.e floor loads transferred to exterior and interior columns and from there down to the foundations, the individual elements are subject to stresses far in excess of their design, for example a vertical column subject to lateral forces from the accumulation of falling debris pushing outwards.

So, the only thing truthers have going is the time of the collapse?

They all surely understand that 14 floors on 1 floor collapse it instantly right?

What's all this conservation of energy stuff, momentum etc?

 

[uk_dave]

'truthers' probably imagine it like a hollywood film where the actual failure would be slow enough to represent as part of the film. So they would imagine a pause as the structure is overloaded, dramatic sounds of snapping etc and then failure of the structure.

But with 15 storeys overloading 1 which was designed to support a fraction of the load it is now exposed to, the single storey will fail almost instantaneously.

And as the 15 storeys become 20 or 30 or 40 the speed of failure of each lower floor can only become faster.

 

[Undesired Walrus]

'truthers' probably imagine it like a hollywood film where the actual failure would be slow enough to represent as part of the film. So they would imagine a pause as the structure is overloaded, dramatic sounds of snapping etc and then failure of the structure.

But with 15 storeys overloading 1 which was designed to support a fraction of the load it is now exposed to, the single storey will fail almost instantaneously.

And as the 15 storeys become 20 or 30 or 40 the speed of failure of each lower floor can only become faster.

I fail to see how truthers cannot understand this.

 

[uk_dave]

Because they don't understand how the wtc towers were built. I seriously suspect that some of them imagine the columns to be continuous pieces of steel from the ground to the top (How many times have we heard the 'truthers' comment on the 'convenient' lengths of the steel within the pile. They see short lengths of steel as proof that the steel was cut by their mythical thermite devices and that the evil doers planned it so the clean up operation would have easily handled lengths to dispose of. Their tiny brains can't concieve that perhaps the steel arrived at the construction site on the back of a lorry, in easily handled lengths!), or that they really do think the towers would behave like a tree, They also have little or no concept of the scale of the buildings.

I know we try to avoid careless analogies, as these are often picked up and spun against us, but the scale of the building and the relative size of the individual members is something the 'truthers' cannot grasp.

Take an example:

The footprint of a tower was about 63m x 63m
The concrete floor decks were 0.1m thick

Now if you scale this down to something more easily visualised you could try to imagine it as a room of say 6.0m x 6.0m (or 20ft x 20ft). Still a very large space. But the slab would be only 10mm thick.

Makes ya think.

 

[Apollo20]

I think the biggest problem with explaining the collapse of WTC 1 & 2 is that many people cannot get beyond how they THINK it would look. And their IMAGINATION tells them that if the upper section collapsed onto the lower section it would take a finite amount of time to destroy the floor below. And they IMAGINE that this finite time must be similar to the time it would take to bend a metal bar in the way a circus "strong man" bends a metal bar. That's why you hear people claim that is should take a second, for example, to destroy each floor or 100 seconds for the entire building to come down.
Now calculations show that a floor can be destroyed in more like 10 to 20 milliseconds. That's the fact that needs to be conveyed to people who claim that the towers fell "too fast".

 

[Undesired Walrus]

Ok, crude but..

 

[Kryptos]

Hey.
Is it hard to comprehend because we imagine them as solid structures?

WTC was a much much lighter, lower density structure than older buildings such as the Empire State Building. As I think everyone knows, the WTC and other newer buildings used spray-on fireproofing and gypsum (drywall) as "fireproofing" to protect the stairwells and other structural elements. The structures were not nearly as solid as older buildings, such as the Empire State Building is perhaps overbuilt and is a very heavy/dense structure, using concrete and masonry for fireproofing. The WTC buildings and many others built since then are lightweight structures, built that way for economical reasons.

The new 7 World Trade Center still uses spray-on fireproofing, but it's a different material -- portland cement based material, which is denser than what was used in the original WTC buildings. It's also much thicker, with a concrete core which obviously provides better protection for the stairwell sand other structural elements.

http://www.skyscrapersafety.org/html/article_11092001.html - Article published in The New Yorker (November 2001)

http://www.na.graceconstruction.com...red_articles/pdf/Fire-resistive_materials.pdf - about fireproofing in the new 7 WTC

 

[Kryptos]

The idea a smaller weight can create a larger weight underneath to fail. The buildings are not solid structures, but doesnt that also mean the weight above is not solid?

* The plane impact took out numerous perimeter columns and core columns, shifting the load (weight of the upper floors) on to the fewer, remaining columns.

* At the same time, fireproofing was knocked off columns and floor trusses, making them more susceptible to heat and fire. We know that steel quickly loses strength as its heated.

* Also, we know there were existing gaps in fireproofing:

The Port Authority also had problems keeping up with maintenance and repair of fireproofing in the World Trade Center buildings. Thus, it is likely there were many gaps in the fireproofing, with some steel elements exposed. Frederick W. Mowrer, an associate professor in the fire protection engineering department at the University of Maryland, looked through an archive of photographs and other evidence. The photographs were taken during inspections relating to asbestos lawsuits. He found that large areas of fireproofing were missing from the core columns. Roger G. Morse, an architect who took the photos, said "the problems were far more widespread than that, probably because the fireproofing had been applied improperly to rusty steel."

Alan Reiss, a Port Authority official since 1984 who rose to director of the department in charge of day-to-day operations at the trade towers, said engineers had long been aware of difficulties in keeping some of the fireproofing, in the core of the building around the elevator shafts, on the large structural columns there. Reiss said the problems were caused by the swaying of the buildings in the wind and the impact of elevator cables against the beams. "It was an ongoing maintenance headache," he said. Although measures were repeatedly taken to prevent the problem, he said, "every March and April when you had these windstorms and the building rocked back and forth, you would still knock some of the fireproofing down." (Glanz, James and Michael Moss. "Faulty Fireproofing Is Reviewed as Factor in Trade Center Collapse", The New York Times, December 13, 2001.)

Larger load applied to individual columns which had reduced strength means the columns are severely strained and will deform. Eventually these forces reach a breaking point, where the stress is too great and there will be structural failure.

 

[lapman]

Here's another analogy that shows the difference between how the towers were built compared to how most other buildings are built. Most buildings are constructed where the columns are spread throughout the floor area to support the floor above. So, in effect, each floor supports the floors above it. Not so in the towers. Each floor is it's own entity supported only by the core and the outer wall.

So, take a piece of paper. Hold it in the center and have a friend drop a 5 pound weight onto the paper from 4 feet. You should be able to catch the weight with no problem since the paper is supported from the center as the floors on most buildings are. Now, take that same paper, hold it on each end with just your thumb and forefinger and have your friend drop the weight. One of two things will happen. Either the paper will tear or it will be pulled out of your hands. Watch what your hands do when that happens.

 

[Minadin]

Also, to really simplify the steel strength that Rwguinn was talking about, because he's getting into some pretty complex vector analysis of loads (albeit with a delightfully simple example that's easy to reproduce) you can think about it this way:

In order to be economically and spatially efficient with the design of the structure, architects and engineers typically will load a steel member in a building to about 2/3 of its total capacity, as a rule of thumb. (There are other limiting factors, and the structural engineering experts will tell you that they have a myriad of different safety factors for different types of loading conditions, as well as things like moment, shear, deflection, etc. But, that's why we have architects to translate that stuff to the lay people anyway.)

The fires in the WTC were hot enough to reduce the strength of the steel by about half. So now you have steel members in there which are potentially loaded to 66%, but only have the strength to hold 50%. Not a good thing. Couple that with the impact damage - the forces that those members used to be carrying are now de facto redistributed to surviving members, some of which are potentially damaged and / or on fire. So in reality, they're not just subject to their original design load, but are now experiencing extra loads, which are coming to them in less than optimal ways.

In addition to this, you have the expansion of various building materials due to the heat of the fires, and different materials expand at different rates. So you have added forces at work there as well.

And that's the basic condition of the impact area when thngs started to fail. The floors sagged and pulled in on the weakened columns. Like Rwguinn was saying, once those aren't vertical anymore, they aren't able to support the weight as well as they should. With the weakened state (besides just the bending) and the extra loads, it is completely understandable that they failed.

 

[rwguinn]

But if I add weight from the top (14 floors), presumably would I not need to create a larger weight (90 floors) on the bottom, underneath the top? I think this is what many cannot comprehend. The idea a smaller weight can create a larger weight underneath to fail. The buildings are not solid structures, but doesnt that also mean the weight above is not solid?

In many videos you see what appears to me to be floors collapsing in the middle of the structure, many floors below the collapse wave. Did the floors essentially, 'beat' the upper, more dramatic and visible masses to the ground?

What the laymen cannot see to understand, and truthers in particulat, cannot seem to grasp, is that the weight below the collapse zone does not matter, except in that steel does have mass, and therefore weight.
What does matter is strength. the supports holding up, say, floor 78, were designed to hold up floors 78 through floor 113, plus the roof and its parasitic mass. when all that stuff started moving down and impacted floor 78, what those supports saw was 10 or more times what floors 78 through 113 plus. Locally (individual columns) they saw much more than that.