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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