Friday 14 September, 2001
Super-skyscraper safety
The dramatic way in which the World Trade Centre's towers collapsed in New York, just an hour or so after they were hit by the hijacked aeroplanes, has raised the question of whether any building could be made safe against such attacks.
In recent years it has become standard practice to protect large buildings against earthquakes; but is there any technology that can safeguard against terrorism? BBC Science reports.
World Trade Centre collapse
The World Trade Centre’s towers were as much symbols of man's engineering mastery as of international trade and capital markets. Nearly half a kilometre high they were, at the time of their construction, the tallest buildings on Earth.
But on 11th September 2001 disaster struck. At 0845 Eastern Daylight Time American Airlines Flight 11 crashed into the north tower of the World Trade Centre in New York.
The United Airlines Flight 175 crashing into the south tower of the Trade Centre, causing a devastating explosion, quickly followed this at 0903 EDT.
Holding up for just over an hour, at 0950 EDT the south tower of the World Trade Centre tower collapsed and by 1029 EDT the north tower was also in ruins. The World Trade Centre was no more.
High impact
Structural engineers have said that the design of the Centre did in fact save thousands of lives as it managed to stay standing for some time after the initial impact, but could it have ever been built to withstand being hit by an aeroplane?
Henry Guthard was an engineer who worked with the architect of the World Trade Centre. Having watched the building fall he described to The New York Times his amazement that it stood for as long as it did. He commented:
‘To hit the building; to disappear inside the building; to have pieces come out the other side; it was amazing the building stood. To defend against 5,000 gallons of ignited fuel in a building of 1,350 feet is not possible.’
However, according to Scott Steedman, a structural engineer in London, buildings including the World Trade Centre are regularly designed to take enormous impacts. He explains:
‘Buildings are routinely designed for impact from aircraft, that’s quite normal. In the nuclear industry it is absolutely routine and these sorts of tower buildings, these buildings in particular, were designed to take the impact of large airliners.’
| ‘The impact of an aeroplane is relatively small in comparison to the winds blowing in a hurricane that a tall building can experience.’ | |
Inferno
So what went wrong? Well in the case of the World Trade Centre, what the designers had not foreseen was the fire that quickly followed the attack.
Despite the concrete cladding on the building’s steel frame temperatures probably reached around 1000 degrees Celcius.
According to Chris Wise of the London-based company Expedition Engineering, it was this that brought the towers of the World Trade Centre crashing quickly to Earth. He explains:
| ‘The columns would have melted, the floors would have melted and eventually they would have collapsed one on top of the other.’ | |
‘Gradually the weight of the top of the World Trade Centre, which was probably about 100,000 tones, collapsed down on to the structure underneath it.’
But the World Trade Centre was an unusual building, with a frame built of steel tubes - which has raised in some peoples' minds the question of whether whoever planned the attack had some knowledge of civil engineering, and selected their target deliberately.
Perhaps other kinds of structure might have survived the inferno longer?
Ahmed Elghazouli of London's Imperial College researches the science of protecting buildings. He believes that few structures could endure such an attack:
‘Nothing is impossible in terms of structural design, but it would probably mean much more in terms of cost. It would mean more use of fire protection and it may also mean a substantial change in terms of the design philosophy overall.’
| ‘If such a situation occurred in any building, it is likely that there would be a total collapse.’ | |
Modern super-skyscrapers
So where does that leave the more modern super-skyscrapers that have been built since the World Trade Centre's opening in 1973?
Are buildings like the Petronas Towers in Kuala Lumpur, and the Sears Tower in Chicago similarly vulnerable to such an attack?
Structural engineer, Scott Steedman, thinks not:
‘Those buildings are more recent structures and advantages have definitely been made in those times. But I do think that this event will require a reassessment of a number of structures.’
Indeed re-examination and re-assessment seems likely for the tallest buildings of today, but what about those of tomorrow?
In a world where skyscrapers reach ever higher it is likely that what has happened in New York will change building design forever. As Henry Guthard recently told The New York Times:
‘The World Trade Centre was designed in a period when that kind of terrorist activity was not anticipated by civilised designers. Surely, they’ll be a rethinking about what target a building exhibits.’
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| Shanghai World Financial Centre |
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When completed the Shanghai World Financial Centre will stand at 1,509ft tall.
At 95 storeys high, it is due to pip the current tallest building (Petronas Towers in Kuala Lumpur, Malaysia) by seven floors and around 26 feet.
The huge cylindrical void, or ‘moon gate’ is designed to prevent the tower buckling in the wind.
The floor space will be converted into offices, shops, a gallery, a hotel and an observation deck at the very top. |
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