The design of the World Trade Center saved thousands of lives by standing for well over an hour after the planes crashed into its twin towers, say structural engineers.
It was the fire that killed the buildings - nothing on Earth could survive those temperatures with that amount of fuel burning
Structural engineer Chris Wise
The steel and concrete structures performed amazingly well, said John Knapton, professor in structural engineering at Newcastle University, UK.
"I believe tens of thousands of lives have been saved by the structural integrity of the buildings," he told BBC News Online.
"They had a lot of their structure taken out, yet they remained intact for more than an hour, allowing thousands to escape."
Temperatures at 800C
But as fires raged in the towers, driven by aviation fuel, the steel cores in each building would have eventually reached 800C - hot enough to start buckling and collapsing.
The protective concrete cladding on the cores would have been no permanent defence in these extraordinary circumstances - keeping the intense heat at bay for only a limited timespan.
Nothing is designed or will be designed to withstand that fire
World Trade Center construction manager
"The columns would have melted, the floors would have melted and eventually they would have collapsed one on top of each other."
The buildings' construction manager, Hyman Brown, agreed that nothing could have saved them from the inferno.
"The buildings would have stood had a plane or a force caused by a plane smashed into it," he said.
I would have given the order to get out - you would have thought someone with technical expertise would have been advising them
Professor John Knapton, Newcastle University
Once the steel frame on one floor had melted, it collapsed downwards, inflicting massive forces on the already-weakened floor below.
Science of collapse
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.
Other buildings - including the 47-storey Salomon Brothers building - 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
"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.