Professor S. Kitipornchai

Are tall buildings in Hong Kong safe from terrorist plane crashes?

by Professor S. Kitipornchai

No-one will ever forget September 11, 2001 – a day when the most powerful country in the world was caught completely off guard. Groups of well co-ordinated terrorists managed with relative ease to hijack a number of commercial planes and slam them into the 110-storey twin towers of the World Trade Centre and a section of the Pentagon.

The two Boeing 767 planes that crashed into the twin towers had just taken off, and both carried full tanks of fuel (91,000 litres). The twin towers were completely destroyed with the loss of over 6000 lives. The north tower, which was hit first, at about the 90th floor, stood for 1 hour and 45 minutes, whilst the south tower, which was hit at about the 60th floor, collapsed after only 45 minutes. Both towers collapsed spectacularly in front of millions of television viewers around the world. This raises some fundamental questions: why and how did the towers collapse, and are tall buildings in Hong Kong safe from similar attacks?

Engineers have anticipated planes crashing into tall buildings

The twin towers were the tallest buildings (416 m tall) in the world when they were completed in the 1970s. The buildings, 63.4m x 63.4m in plan, were framed in structural steel with closely spaced exterior steel columns, each of which were 476 mm wide and 560 mm apart, forming an exterior hollow tube wall. This wall acted as a structural frame, and provided the necessary lateral resistance. The central steel core of the building was designed to carry vertical or gravity loads. Horizontal steel trusses that spanned 18.3 m from exterior tube wall to the core supported the concrete floor, which also acted as a rigid diaphragm at each level.

Engineers have long anticipated the scenario of planes crashing into tall buildings. In 1945, a USAF bomber crashed into the 79th floor of the 102-storey Empire State Building in Manhattan. The crash occurred during a misty night, and the damage was restricted to the impact area. There have also been a number of near misses.

One of the criteria used in the design of the World Trade Centre was that if a Boeing 707 should hit either of the towers, then it would go right through without damaging the other storeys. This would be like punching a hole through the wall of a hollow tube without making the tube collapse. Calculations can easily show that such an impact will have little or no effect on the overall structural integrity of the building because the mass of the plane is very small compared to the mass resistance of the building. In a similar manner, the explosion of a terrorist bomb in the north tower basement during 1993 created a large hole, but did no real damage to the overall structural integrity of the building.

However, the Boeing 767s that hit the towers were much larger than expected, and had much greater fuel capacities. The impact of the fuel explosion and the ensuing fire was not considered in the building design.

Buildings collapsed because of explosions and fire

Video evidence suggests that the initial impact would have caused only local structural damage. The towers would have survived the impacts had it not been for the ensuing explosions and fire. The 250 or so fire fighters and police officers were ordered into the towers in the belief that they would not collapse – sadly none of them survived. This was a tragic error of judgment. An order for immediate evacuation should have been made at the very beginning.

With so much fuel, the temperature inside the buildings would have exceeded 1000°C. It was only a matter of time before the steel columns and the supporting floor trusses softened and lost all strength, precipitating the inevitable collapse of at least one complete storey at the level of impact. Once this happened, the huge mass of all the floors above would have simply crushed the intact floors below, resulting in the collapse of one storey after another like a row of dominoes. The south tower collapsed first because the plane entered the building at the corner, cutting through the exterior structural frame to either side, whereas the plane that hit the north tower entered more favourably near the middle of the building. It was fortunate that both towers collapsed almost vertically, otherwise there would have been much more devastation.

Are tall buildings in Hong Kong safe from terrorist plane crashes?

The attack on the World Trade Centre towers is thought to have been in the planning for some years, and seems to have been the result of perceived American foreign policy bias in the Middle East. There is no reason to suggest that Hong Kong will be targeted in a similar manner. However, we need not be complacent. Tall buildings in Hong Kong or anywhere else in the world would have met the same fate as did the towers in New York. There was nothing structurally wrong with the design of the twin towers. On the contrary, the buildings performed extremely well in the circumstances. Most other buildings hit by a Boeing 767 with a capacity fuel load would have collapsed almost immediately. The high degree of redundancy inherent in the exterior structural frame delayed the collapse of the buildings by about an hour, thus allowing thousands of occupants the precious time to escape.

It would be hard to imagine how any tall building could be designed to withstand attacks of this nature. Such buildings would be massive and the cost prohibitive. Only nuclear power plants are designed to withstand explosions and fire. Even if we can design and construct new buildings to resist such attacks, how do we make safe a multitude of existing buildings all over the world?

The answer may well lie in the design of aircraft. How do we keep planes out of the hands of terrorists? The challenge is to design them in a manner that would prohibit unauthorised entry into the cockpit even if terrorists could get on board. Only in that way could we arrest the tragedy of passenger planes being used as deadly bombs.

AIIB Newsletter, October edition.

Professor S. Kitipornchai is a Chair Professor in the Department of Building and Construction at the City University of Hong Kong

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