Posted on 09/14/2001 12:27:43 AM PDT by t-shirt
How the World Trade Center fell
Thursday, 13 September, 2001, 12:59 GMT 13:59 UK
By BBC News
Sheila Barter
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
But the towers' ultimate collapse was inevitable, as the steel cores inside them reached temperatures of 800C - raising questions as to why hundreds of rescue workers were sent into the doomed buildings to their deaths.
The steel and concrete structure 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 core 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
"It was the fire that killed the buildings. There's nothing on earth that could survive those temperatures with that amount of fuel burning," said structural engineer Chris Wise.
"The columns would have melted, the floors would have melted and eventually they would have collapsed one on top of each other."
The building's construction manager, Hyman Brown, agreed that nothing could have saved it from the inferno.
"This building 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
"But steel melts, and 24,000 gallons (91,000 litres) of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire."
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.
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.
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
"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.
Steel-core design
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.
During my last read I considered that some poor grieving soul might come by the forum and read my thoughts. I didn't have the heart to post them.
In time there will be questions asked. Not all the answers are going to be ones everyone will like.
Because we are human, disasters like this only happen in the movies or so we thought, nobody but nobody ever thought we would see this type of calamity.
The selfless acts of those firemen and police proved that we are good and we are right.
:(
The credit should be given to the builders.
Terrible!
The huge pile-driver would have to be the heavy computer-controlled concrete block at the very top of each tower used to control the sway of the building and to keep people from feeling nauseous. I haven't heard anyone discuss it.
I think that it was architectural arrogance to make buildings so tall and to hold so many people , "just because we can do it, and we want to build the tallest buildings in the world" Some of the survivors said that it took them an entire half hour to run all the way down. It was confusing because the stairwell that some people were in ended on the 72nd floor. They had thousands of people cramming into tiny stairwells, while a normal sized building would have had many fewer. In the end the loss of life was so enormous because our enemy knew that there were tens of thousands of americans highly concentrated in a tiny space. What an inviting target for a mad suicide bomber.
Additionally, the buildings proved to be the demise of the buildings around them. One 40 story building collapsed Tuesday. There are now a number of other buildings on the verge of collapse, and they weren't the target. Their damage was caused by neighboring buildings that towered hundreds of feet above before they came crashing down on top of them.
I am sure that there would have been much greater loss of life at the Pentagon if it had the same people capacity, but were a skyscraper. It was great that it was all spread out, with many exits.
I will never forget the images of the poor people above the impact sites at the World Trade Center, hanging out windows, doomed, or jumping to their deaths. That didn't need to happen.
I wonder if there would be any way of constructing a building in such a way that between every ten floors or so there was a space which could crush either elastically or plastically in the event of structural failure above it? Otherwise any skyscraper is going to have the risk of a sufficiently bad fire anyway pancaking the entire building.
Partin also says the buildings would crash at some point after the steel reached enough heat. He says once several floors crashed at the top, the lower floors would crash in a chain reaction.
(To me it seems strange that the lower floors would crash if they had not even been exposed to tremendous heat yet.)
But I am not the expert.
I am surprised they don't used asbestos to line the structure.
Or maybe I'm not surprised, steel and concrete are relatively cheap compared to asbestos.
It might greatly increase the cost of the building.
I don't believe it is the arrogance of man to build these things. It is the ignorance of man that destroys them.
Should say "... concern, do you say the same thing..."
the entire 110 floor structure weighed in excess of one billion pounds, but the load can only be carried if it is relatively static
once a chunk of the building is in downward motion, the force exerted becomes greater than the tensile strength of the room temperature steel in the lower levels, and it snaps and shears
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