20 September 2001
As the world still struggles to comprehend the terrorist attacks on New York and Washington, Catalyst investigates why the World Trade Centre towers collapsed the way they did.
Narration: When the World Trade Centre was built in 1973, its twin towers were the tallest buildings in the world. Part of their revolutionary new design made them resistant to strong winds and allowed for huge open office spaces. Their strength depended on a steel shell. But no skyscraper in the world is designed to withstand this.
Dr Andy Davids: Steel looses strength and stiffness as its temperature increases past about 1500 degrees. The ferocity of that fire would have been well and truly enough to push it past it's elastic limit if you like and it would have started to soften, and yield
Narration: Dr Andy Davids is one of a handful of people who designs the world's tallest buildings. He's a structural engineer and a friend of Les Robertson who engineered the world trade centre.
Dr Andy Davids: You can see clearly on the footage that that top 25 stories moves almost as a rigid object a single block - it rotates over quite a long way - actually several meters - and drops as a single object. The mass of debris just keeps feeding on itself as it drives down the building and the building just basically unzips.
Narration: So, was the lightness of the steel frame a fatal flaw?
Dr Andy Davids: The failure was due to the consuming nature of the fire and how that weakened the steel structure. I think it's not possible to design a structure that's failure proof.
Narration: Yet, 56 years ago, a near neighbour of the world Trade Centre, withstood another aerial attack. On 18 July, 1945 a B25 bomber flew into the Empire State building in heavy fog.
Dr Andy Davids: It was a much smaller aircraft, a World War two bomber and it weighed about 10 tonne as compared to the 767 as I understand hit the World Trade Centre, which weighed probably to the order of 100 to 150 tonnes.
Karina Kelly: So 10 times bigger - but the Empire state building is still standing today - why is that?
Dr Andy Davids: Yes the structural system of the Empire State is similar in one way to the World Trade Centre in that it is a steel frame, however on the Empire State those steel beams and columns had all been in filled with heavy masonry panels so the building was a very stiff building; a very dense building; had a lot of mass in which to absorb the inertia of the aircraft striking it and limit the damage.
Karina Kelly: Does that mean the Empire State building was a better made building because it could withstand this plane crashing into the side of it?
Dr Andy Davids: Man-made objects such as building are evaluated on many criteria and the striking of a fully loaded aircraft was not one of those criteria.
Karina Kelly: But how would more modern buildings cope with a plane slamming into them?
Dr Andy Davids: The buildings that we design today are probably not much better in that regard. Buildings by their very nature require support along their perimeter and also in the centre. I think the main difference is that the more modern high rise buildings that we design and build today tend to have a large solid reinforced concrete core in the centre, which is the main stability element which prevents the building from moving in response to wind and earthquakes and impacts such as from aircraft.
Karina Kelly: So if you have a core of reinforced concrete then a collapse like that wouldn't happen
Dr Andy Davids: I wouldn't say it wouldn't happen because I think the actual collapse was due to the fire in fact, rather than the impact of the aircraft. So I think that if we are to learn any lessons from this tragedy it would be that the control of massive fire in these types of buildings really needs to be reconsidered.