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September 10, 2008 3:12 am

Flaw in Twin Towers revealed

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Scientists have discovered the fundamental reason why the Twin Towers collapsed on September 11 2001. The steel used in the buildings softened fatally at 500ºC – a temperature far below its melting point – as a result of a previously unsuspected magnetic change in the metal.

The finding, announced at the BA Festival of Science in Liverpool on Tuesday, should lead to a new generation of safer steels capable of retaining strength at much higher temperatures.

Sergei Dudarev and colleagues at the UK Atomic Energy Authority’s Culham laboratory made the discovery while researching special steels for use in future nuclear fusion reactors.

Testing structural steel from the New York World Trade Centre, the UKAEA researchers found that the metal underwent an “alpha-gamma phase transition” – an internal realignment of its atoms and consequently its magnetic properties – that greatly sapped its strength at temperatures above 500ºC. Strangely, the transition was completed when the steel reached 911ºC. (The metal melts at about 1500ºC.)

The steel would have weakened and softened sufficiently in the 9/11 fire for the two buildings to collapse, although it was not hot enough actually to have melted the steel, said Dr Dudarev.

“Understanding how materials behave means we can find the right ‘medicine’ to make steel stronger at high temperatures,” he said.

Dr Dudarev was confident that affordable changes in the composition of steel alloys would enable the construction industry to adopt more heat-resistant materials quite quickly. “It might take a few years but not decades,” he said.

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