Collapse of the World Trade Center - Some Engineering Aspects

The University of Sydney - Department of Civil Engineering ^ | The University of Sydney - Department of Civil Engineering

[imberedux]

Once the steel frame on one floor had melted, it collapsed downwards, inflicting massive forces on the already-weakened floor below. From then on, the collapse became inevitable, as each new falling floor added to the downward forces. Further down the building, even steel at normal temperatures gave way under the enormous weight - an estimated 100,000 tonnes from the upper floors alone. 

"It was as if the top of the building was acting like a huge pile-driver, crashing down on to the floors underneath," said Chris Wise. 

Early in the unfolding horror, some office workers were told to stay where they were - dreadful advice, said Professor Knapton. 

The towers withstood impact but not inferno. People's only hope was to run and keep running - reaching open ground. The building could have fallen over sideways, he points out, potentially bringing even greater devastation. 

Another 47-storey building belonging to Salomon Brothers caved in later, weakened by the earlier collapses, and more nearby buildings may still fall, say engineers. 

But the eventual collapse of the twin towers was so predictable that the order should have been given to withdraw emergency services within an hour, said Professor Knapton. He watched in horror, knowing the building would fall within two hours. The hundreds of dead firemen and police officers should simply not have been there, he said. 

"I think they should not have gone in at all," he said. "If they did decide to take the risk, they should have been pulled out after an hour." 

But in the panic and horror, the order was never given for rescue workers to abandon the building. "Mistakes were made," said Professor Knapton. 

"It was like a horror film and I think people's rationale had gone" Professor John Knapton 

"It sounds harsh - this had never happened in the world, so you can hardly criticise them. But I would have given the order to get out. You would have thought someone with technical expertise would have been advising them." But he acknowledged that the sheer scale of the tragedy probably overwhelmed the operation commanders. "I think everyone was not thinking. It was like a horror film and I think people's rationale had gone," he said. 

The building's design was standard in the 1960s, when construction began on what was then the world's tallest building. At the heart of the structure was a vertical steel and concrete core, housing lift shafts and stairwells.   Steel beams radiate outwards and connect with steel uprights, forming the building's outer wall. All the steel was covered in concrete to guarantee firefighters a minimum period of one or two hours in which they could operate - although aviation fuel would have driven the fire to higher-than-normal temperatures. The floors were also concrete. The building had to be tough enough to withstand not just the impact of a plane - and the previous bomb attack in 1993 - but also of the enormous structural pressures created by strong winds. 

Newer skyscrapers are constructed using cheaper methods. But this building was magnificent, say experts, in the face of utterly unpredictable disaster. 

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Trade Center architect discusses buildings Taken from CNN http://www.cnn.com/2001/US/09/12/wtc.architect.cnna/index.html

(CNN) -- When they were completed in the early 1970s, the Twin Towers of New York's World Trade Center were the tallest buildings in the world. That designation didn't last long -- Chicago's Sears Tower took the title in 1974, a year after Two World Trade Center was finished -- but the buildings' standing as a New York City landmark, anchors amid the office-tower canyons of Manhattan's financial district, remained unchallenged. 

Tuesday, the buildings -- daytime home of more than 50,000 workers -- were destroyed when two hijacked passenger jets were flown into the structures. 

CNN's Leon Harris spoke with Aaron Swirsky, part of the architectural team led by World Trade Center chief architect Minoru Yamasaki, on the way the building was designed. 

LEON HARRIS, CNN ANCHOR: So many of us had thought for so long that the Twin Towers were invincible. We had heard for so many times over the years that the buildings have been built to withstand an impact from the crash of a plane. 

Let's talk right now on the telephone with Aaron Swirsky. He's in Jerusalem. He was one of the original architects of the complex, as I understand it. 

Is that the case, Mr. Swirsky? 

AARON SWIRSKY, ARCHITECT: I was working with Minoru Yamasaki, who is the architect of the building. But I was one of the workers with him. We were a team of 14 architects, and I was one of the members of the team. 

HARRIS: As a member of the team, and having such insight to how this building was constructed, could you believe that a plane could bring these buildings down? 

SWIRSKY: No, as a matter of fact, one of the rationales of the structure of the building was that it would be built as a pipe. And that proved itself to work during the explosion of 1993, when a hole was brought into the building, and it survived. But somehow, nobody could foresee anything like (Tuesday's incident). 

Also, at that time, the planes were not like these types of planes that we have now. I think the biggest plane was a 100-passenger plane, something like that, and the fuel capacity of those planes was not like they are today. 

The criterion was that if a plane hits, it would go right through it. And nobody could foresee something like that. The tower was protected in such a way that the damage would be limited to one story, but it wouldn't travel to the other stories. 

HARRIS: The planes that crashed yesterday were much bigger than that. They were 757s. 

SWIRSKY: And also the fuel capacity is much more tremendous. 

HARRIS: Exactly. That's what I want to ask you about. Which was it that made the biggest difference? Was it the impact felt from the larger plane, or was it the heat generated by the burning and that much fuel. 

SWIRSKY: I imagine, when I saw the pictures of the implosion of the building, it looks like the fuel must have leaked right to the core of the building, and from there it was the massive explosion that caused the building to collapse. So it was something completely unforeseen, so far as the design criteria was (concerned). 

HARRIS: Let me ask one final question, if I may. Considering what you know about the building -- you say it was constructed like a pipe, these two buildings -- and the manner in which we saw them collapse, does that give you any hope at all that the way it collapsed, there will be more packets inside, at the bottom, where survivors could be found? 

SWIRSKY: Well, I sure hope so. We pray that there will be survivors and that this won't happen again. It's a terrible, terrible, incredible tragedy. 

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[imberedux]

Fix font?

[imberedux]

FYI

[Real Cynic No More]

I am an engineer. You have done us a great service by posting this engineering analysis.

[Kevin Curry]

Once one storey collapsed all floors above would have begun to fall. The huge mass of falling structure would gain momentum, crushing the structurally intact floors below, resulting in catastrophic failure of the entire structure.

It appears that you called it exactly right, _Jim. It's too bad that on the thread where you explained this you were pooped on by a bunch of anarchist, conspiracy-sniffing loons.

[Roscoe]

Thanks for the heads up.

[Not now, Not ever!]

Outstanding compilation, Thank you.

[imberedux]

Thank you. The rumors, disinformation and lies were begining to bug me.

[bjs1779]

Also, at that time, the planes were not like these types of planes that we have now. I think the biggest plane was a 100-passenger plane, something like that, and the fuel capacity of those planes was not like they are today.

What is this guy talking about? The 747 was flying in 1969.

[imberedux]

I hope the loons realize how they are helping our enemies. And I hope they stop. They won't but I can hope.

[LarryLied]

Click to enlarge:

SPOT Releases Satellite Image of World Trade Center
 Fires SPOT - Reston, VA--SPOT released a satellite image
 of Manhattan, acquired on September 11 at 11:55 AM EST,
 3 hours after two planes crashed into the World Trade
 Center (see URLs below). The colors result from the use of
 infrared bands to identify the actual fire hot spots (see red
 spots near the base of the smoke plume). The .tif and .jpg
 files were created with SPOT 20 meter resolution imagery.
 The SPOT satellites orbit at an altitude of 822 km.
 http://www.spot.com/home/news/NYC-091101.jpg
 http://www.spot.com/home/news/NYC-091101.tif This
 copyrighted image may be used free of charge, with the
 appropriate copyright attribution, as follows: CNES/SPOT
 Image 2001 For further information: Clark Nelson
 (703.715.3131) or nelson@spot.com

[bjs1779]

I hope the loons realize how they are helping our enemies. And I hope they stop. They won't but I can hope.

Very true. It is getting to the point that no Jew was ever killed by the Nazis either, according to them.

[imberedux]

Thanks!

[_Jim]

It's too bad that on the thread where you explained this you were pooped on by a bunch of anarchist, conspiracy-sniffing loons.

I allow two types of living beings to 'poop' on me on account of *who* they are -

- one group is the 1) anarchy-bound, conspiracy-sniffing loons and the other is 2) a member of the Cockatiel (bird) family.

Both groups possess about the same intelligence level, one group, however, *is* working at maximum intellectual capacity ...

I came across this pice while debating this subject with another group:

From:

http://lava.ds.arch.tue.nl/lava/people/rob/slany/

TUBULAR SYSTEMS

@@@ A recent development in structural design is the concept of tubular behaviour introduced by Fazlur Khan of S.O.M. At present, four of the world's five tallest buildings are tubular systems. They are the Hancock Building, the Sears Building, and the Standard Oil Building in Chicago, and the World Trade Center in New York.

Tubular systems are so efficient that in most cases the amount of structural material used per square foot of floor space is comparable to that used in conventionally framed buildings half the size.

Tubular design assumes that the facade structure responds to lateral loads as a closed hollow box beam cantilevering out of the ground. Since the exterior walls resist all or most of the wind load, costly interior diagonal bracing or shear walls are eliminated.

The tube's walls consist of closely spaced columns around the perimeter of the building tied together by deep spandrel beams. This facade structure looks like a perforated wall. The stiffness of the facade wall may be further increased by adding diagonal braces, causing trusslike action. The rigidity of the tube is so high that it responds to lateral loading similar to a cantilever beam.

As we see later, the exterior tube alone can resist all lateral loads entirely, or it can be further stiffened by adding interior bracing of some kind. The earliest application of the tubular concept, was first used in the 43-story Dewitt Chestnut Apartment Building in Chicago (S.O.M., 1961).

In this Vierendeel tube system the exterior walls of the building, consisting of a closely spaced rectangular grid of beams and columns rigidly connected together, resist lateral loads through cantilever tube action without using interior bracing.

The interior columns are assumed to carry gravity loads only and do not contribute to the exterior tube's stiffness. The stiff floors act as diaphragms with respect to distributing the lateral forces to the perimeter walls.

Other examples of hollow framed tube buildings are the 83-story Standard Oil Building in Chicago and the 110-story World Trade Center in New York.

Although these buildings have interior cores, they act as hollow tubes because the cores are not designed to resist lateral loads.

[Real Cynic No More]

"The rumors, disinformation and lies were begining to bug me."

They bugged me too. On the night of 9/11 and on 9/12 I posted some responses to some other goofy speculations, in which I discussed the possible failure mechanism, with the principal culprit being the fire, but not to the level of technical detail as you have collected here. I repeat, you have done us all a great service. Thank you very much.

[KC Burke]

Great compilation and research to find complete reports this soon that are this extensive. Others should see this.

It's an interesting figure I plucked from a quick read that the building had an extimated mass of 1,000,000 tons. If contents were 70 tons per floor and we have 220 floors between the two towers, that brings the debris to 1,154,000 tons. If building 7 and the hotel and others are thrown in that is approximately 1,500,000 tons.
As of today they have said they have moved 20,000 tons of debris which means that they are only 1.33 Percent through the pile.

Damn!

[livius]

Thanks for that very informative post. My brother, who still lives in NY and is an architecture junkie, told me that basically, in layman's terms, it was a matter of the outer supporting piers simply melting and dropping the concrete slabs towards the middle, which could not support them. He also said, BTW, that the architect who designed the project gave a talk in Europe, perhaps in Brussels, a few months ago, where he boasted that the Towers could withstand even a direct hit by a 707. Unfortunately, he was thinking of an accidental strike by a nearly empty (of fuel) plane going about 200 mph towards a landing. The reality - a fuel laden plane going about 600 mph with every intention of crashing - could never have occurred to any architect.

[ICU812]

The building could have fallen over sideways, he points out, potentially bringing even greater devastation.

This occured to me too. The fact that it fell straight down(almost like a planned demolition) saved untold thousands of lives.

[imberedux]

Where do you put a million plus tons of rubble?

[yarddog]

I am not knowlegeable on architecture but skyscrapers just plain look dangerous to me. I remember when our class went to New York and the Worlds Fair, we also went to the top of the Empire State Building. Just looking at those buildings so tall for their base scared me, although I did make the trip to the top.

I know they are supposed to withstand tremendous stresses and some have withstood the test of time, but it still seems to me that A Pyramid shape would be much more sturdy. I particularly like the Ziggurat style where there are tiers.

Of course it would require a little more real estate but I am certain that a pyramid shape would withstand much more stress. I even like the BankAmerica pyramid although it does not have a large enough base.