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The World Trade Center’s Twin Towers were built to withstand aircraft crashing into them, suggested a panel on the top floor visitor center; but on 11th September 2001, less than two hours after they were hit, both towers tumbled to the ground after being targeted in a terrorist attack.
The panel was correct up to a point; engineers expected the towers to survive. The design of the revolutionary towers was advanced for its time. They featured a central core, a tube-framed structure with floor trusses extending out to the perimeter. It was designed to withstand tremendous pressures, under predictable events such as high wind and earthquakes, but the terrorist attack was not a predictable event, and its structure couldn’t prevent its eventual collapse.
The World Trade Centers
The Twin Towers were built in 1966-7 and officially declared open in April 1973; they soared 110 stories over New York and stood just 60m apart. At 8.45 am on the 11th September, the 92nd floor of the North Tower was struck by a hijacked Boeing 767; the South Tower’s 83rd floor was hit by a Boeing 757 at 9.03 am.
The strikes were accompanied by a fireball; the South Tower collapsed at 10.05 am and the North Tower at 10.28 am.
The mechanism of collapse and sequence of events was originally unclear as the buildings were enshrouded by dust and smoke, but it is believed that the floors – like many in high-rise buildings – succumbed to progressive vertical collapse caused by one floor dropping down onto another as a result of the fire.
The attack made structural and fire engineers consider how to limit the consequences of such events on heavily-populated, high-rise buildings.
There are two main structural forms used in tall buildings, a cantilever core and a frame-tubed structure, although hybrids also exist. For any given building footprint, the idea is to minimize the area taken up by the core, and maximize the usable floor space.
Buildings with a cantilever core have a large robust core that it structurally wind-resistant; it is typically a heavily reinforced concrete tube. It features external columns widely spaced around the perimeter, a small external column section size, large column spacing, and a small floor plan.
The NatWest office tower in London is a perfect example of a building with a cantilever core; it stands at 173 m tall, and its design suggests it would withstand an aircraft impact.
The World Trade Center was a framed-tube structure; the towers had a small, non-structured core and a non-robust core enclosure. Its structure was a grillage of steel beams and columns which formed the external walls; it had a large external column section size, deep external beams, and small column spacing.
In such structures, like One Canada House (Canary Wharf, London) the service core can be constructed on simple beam and post principles and can be clad in lightweight construction. This can be vulnerable to damage from horizontal missiles, such as the engines from planes.
The Boeing 757 and 767 passenger jets are both similar; they are low wing aircraft with engines under each wing, which acts like missiles.
The force arising from impact is calculated using Newton’s Second Law of motion – force equals mass times deceleration. One jet engine can hit with an incredible amount of force and can continue to move at speed once impact is made.
Any passenger aircraft big enough to cause serious impact damage to tall buildings will be carrying vast amounts of fuel, meaning the impact will cause a fireball. Leakage of fuel can make the situation worse by igniting furniture, carpets and so on.
Every building is different, and every missile and its parts are different depending on the aircraft design, cargo, and fuel. It is therefore difficult to guide on how to make a building more resistant to aircraft damage, although total collapse is less likely with a cantilever core than a frame-tubed design.
There are, however, some steps that can be taken:
- Limit the height of buildings, or ensure no single building towers over its neighbors too much to prevent targeting of a single one.
- Risk assessments should consider extreme events and allow for impact at any level and any orientation. Designs should be able to cope with large horizontal impacts and take into account of how external walls and floors might behave as a missile enters and exits.
- Design the service core to act as a laterally robust structural cantilever and avoid delicate frame-tubed construction.
Sources and Further Reading