When two Boeing 767s crashed into the Twin Towers, Tower One took 103 minutes to fall while Tower Two only lasted for 53 minutes. New York, home of the skyscraper, was now the site of the world's most horrific high rise tragedy. A few lucky people managed to escape before these two great edifices of modern American life collapsed, killing thousands in their wake.
The World Trade Center was in fact a complex of seven buildings. Six of them have now partially or entirely collapsed in the wake of the attacks. Construction of the complex began in 1966 and was completed in 1973, at a total cost of US$1.5 billion. The Twin Towers took the title of the tallest building in the world from the Empire State Building, also in New York. They held on to it for just one year, before the Sears Tower in Chicago took the title. The Petronas Towers in Kuala Lumpur, Malaysia, are now the tallest, each standing at 452m.
The famous Twin Towers of the WTC were among the first high rises to use a tube structure, rather than the frame structure used in earlier skyscrapers. Indeed, they were the first very tall buildings designed without any masonry at all. Tube structure buildings are made of a rigid hollow tube of closely packed steel columns, with floor trusses that extend from the perimeter of the building to its core. The tube structure of modern skyscrapers allows them to withstand higher winds. It also eliminates the need for interior columns, allowing the use of more floor space.
While most skyscrapers built since the 1970s also have the tube structure, the Twin Towers were unique in other ways. Light floor trusses had been used in high rise buildings before, but not to span as much as 18m (60 feet), as they did in the towers. The core and elevator system of the building were also unusual. Because it was feared the pressure created by the buildings' high speed elevators might cause conventional elevator shafts to buckle, engineers used a plaster board system fixed to a steel core to house the elevators. This made the shafts more flexible, though also more flammable.
Cause of the collapse
There is no simple answer to the question of why the Twin Towers collapsed. Engineers, academics and demolition experts have not found agreement on the subject.
Charles Clifton, structural engineer at the New Zealand Heavy Engineering Research Association, believes 'the impact damage, not the severity of the fire was the principal cause of the ultimate collapse'. This view is shared by Gregory Fenves, professor of civil engineering at the University of California.
However, Eduardo Kausel, professor of civil and environmental engineering at Massachusetts Institute of Technology (MIT), believes that fire was primarily responsible. So does Robert McNamara, president of the US structural engineering firm, McNamara and Salvia.
But Oral Buyukozturk, another professor of civil and environmental engineering at MIT and Mark Loizeaux, president of US demolition company Controlled Demolition Incorporated, suspect a combination of factors caused the towers to fall.
When the planes hit the towers, they inflicted major damage on the structure of the buildings. As many as 40 vertical columns at the perimeter of each building were knocked out. It's likely that the impact damaged columns at the core of the building too – preventing people from escaping down stairwells, which were in the core. However, the towers were – initially at least – able to withstand this damage. Indeed, the force with which the planes hit the buildings was 95% of the wind load which they were designed to withstand.
What is not known for sure, is how much damage the planes inflicted on the buildings internally. Writing in New Steel Construction, Charles Clifton argues that 'having penetrated the perimeter frames, the planes would have done much more than just stripping the fire protection off the columns... The effect would have been to completely shatter and eliminate large areas of floor slabs and many of the internal supporting columns... leaving the rest vulnerable to fire attack.'
It's important to note that the terrorists had either done their homework or were very lucky: they struck the towers at precisely the right height. Had the planes hit the buildings any higher up, it's likely that the weight of the floors above the crash site might not have been sufficient to bring the building down. And lower down on the building, the vertical columns are thicker, and fewer would have been destroyed by the impact.
The terrorists certainly made sure they hijacked planes which had plenty of fuel on board. A Boeing 767 at the start of a long haul flight would be carrying around 24,000 gallons of fuel.
although much of the fireproofing was removed on impact, it isn't clear whether it would have been able to withstand an aviation fuel fire anyway. The fireproofing had been designed to protect the building from the type of fire expected in an office building: one fueled by paper, desks, and other office furniture. But this fire was different. The temperature of hydrocarbon fires rises much more rapidly and reaches much higher levels than most building fires. It may have reached 1200 to 1500ºC. Water sprinklers are relatively ineffective in combating a hydrocarbon fire, which is usually fought with chemical foam.
The structure of the WTC towers is crucial when considering the impact of the fire. The towers, being lightweight and devoid of concrete, were difficult to protect from fire. The weak links were the steel floor trusses – they spanned considerable distances relative to their thin construction, meaning they would have heated up quickly.
The fact that the offices were open plan increased the fire hazard. The floors of the towers spanned 40,000 square feet, yet fire chiefs argue that it's impossible to fight a fire in an open plan floor space of half that square footage.
The remaining undamaged columns were capable of bearing considerable loads, but to some extent depended on support from the floors to do so. Once the floors had succumbed to the heat of the fire, the integrity of the building was threatened. Core columns were not only bearing extra loads, but were also subject to intensely high temperatures. Once they began to buckle, the crash site floor collapsed onto the floor underneath. The effect was similar to dropping one multi-storey building onto another: each floor collapsed onto the one below, and so on. Once this domino effect had begun, it took seconds for the towers to be reduced to rubble.
Opinions are divided on the issue of whether other buildings could have survived an attack such as those of 11 September. Charles Clifton says that 'the very light and open structure probably made the buildings more vulnerable to collapse from the aircraft impact than would have been the case for a heavier structural system'. However, some architects argue that an old style frame building would have collapsed immediately, and that the tube structure saved thousands of lives.
However, fire chiefs and structural engineers agree that the fireproofing which existed was insufficient. KAFKO, a mineral-based fireproofing applied to the steel columns of the building, was difficult to apply to the floor trusses. Leslie Robertson, the engineer largely responsible for the structure of the Twin Towers, has admitted that although a plane crash was considered when designing the building, aviation fuel explosion and fire were not. This, believes Eduardo Kausel, is 'a key design omission.'
Looking at the WTC Twin Towers collapse, it's difficult to draw firm conclusions. Had the building been made of a more solid frame structure, it may collapsed more quickly. But a concrete structure would have been more resistant to fire than the steel tube structure of the towers.
Dr Graham Owens, director of the Steel Construction Institute, believes that 'society has the right to expect engineers and the construction industry to respond effectively to their requirements for buildings to resist terrorist attack.' However, he points out that attempting to terrorist-proof a building will be expensive. Though he adds that it could be possible to build 'vertical bomb shelters' – buildings with an extremely tough concrete core. This would be a safe haven where the occupants of a building could take refuge from a disaster.
'Refuge floors', fireproofed empty spaces every 10 or 15 storeys, are being installed in the Shanghai World Financial Center, currently under construction. But it has been pointed out that if these existed in the Twin Towers the loss of life may have been greater. Occupants may have taken shelter from the fire on these floors, only to be crushed once the building collapsed. The Shanghai Center will also have dedicated fire lifts, allowing the emergency services to access the building without hampering evacuation of the occupants. Stairways are likely to be made wider in the future. Though this will be costly, since it will reduce the amount of commercial space available in a building.
The sad truth is that it might be impossible to construct completely safe structures, buildings impenetrable to any disaster man or nature can throw at us. As Charles Thornton, designer of the Petronas Towers, says: 'For every building that we can conceive, someone can come up with a scenario to knock it down.'
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