A lucky choice of design for the World Trade Center towers reduced the death toll caused by their destruction, say engineers.
Each tower was struck by a passenger aeroplane, hijacked by suicidal terrorists, but remained upright for nearly an hour. Eventually raging fires melted the supporting steel struts, but the time delay allowed hundreds of people to escape.
"Most buildings would have come down immediately," says John Hooper, principal engineer in the company that provided engineering advice when the World Trade Center was designed.
Hooper added that engineers at Skilling, Ward, Magnusson, Berkshire, based in Seattle, had been devastated by Tuesday's events. "We're just trying to get through this," he told New Scientist.
Skyscrapers like the World Trade Center are not built to withstand direct hits by large aeroplanes, he says.
Furthermore, the fire suppression system in the towers did not include the foam sprinklers that could to deal with the jet fuel fires. Both the crashed aeroplanes were fully fueled for trans-continental flights, making them ideal "flying bombs".
However, the design of the 415 metre WTC towers did help prevent a greater catastrophe. The outside of the towers consisted of closely spaced steel columns forming a giant "steel tube". The columns were less than a metre apart in the upper stories and could withstand the entire load placed on the building by high winds.
This meant that the building's internal columns bore only the load of gravity. Whether it was the external frame or the internal supports that kept the building standing for an hour is not known, but the "double support" saved many lives.
Many skyscrapers have vertical columns up to six metres apart and rely on combined diagonal struts to bear loads. Destroying these structures would probably collapse a building immediately.
The collapse of the WTC towers looked like a classic controlled demolition, said Mike Taylor of the National Association of Demolition Contractors in Doylestown, Pennsylvania.
"If there's any good thing about this it's that the towers tended not to weaken to one side," said Taylor. "They could have tipped onto other buildings or into the river across the West Side highway."
The collapse of the WTC towers mirrored the strategy used by demolition experts. In controlled demolitions, explosives are placed not just on the lowest three floors but also on several consecutive floors about a third of the way up the building.
The explosions at the higher floors enable the collapse to gain downward momentum as gravity pulls the full weight of unsupported higher floors down into lower floors in a snowballing effect.
On Tuesday, the impacts of aeroplanes on the higher floors replaced the explosives. The collapse of the higher floors caused the floors below to be crushed. "It cascaded down like an implosion," says Taylor.
The lack of collapse in higher stories was one reason why the 454 kilogram bomb detonated in the underground garage of the World Trade Center in 1993 failed to destroy the building.