Fire-proofing on twin towers 'was not tested'

American federal investigators have been unable to find evidence that tests were ever made on fire-proofing material in the World Trade Centre, a potential factor in the buildings' collapse on 11 September 2001.

American federal investigators have been unable to find evidence that tests were ever made on fire-proofing material in the World Trade Centre, a potential factor in the buildings' collapse on 11 September 2001.

According to an interim report by the National Institute of Standards and Technology (NIST), released on Wednesday, documents show that builders in 1969 directed contractors to coat the floor supports with half an inch of fluffy, spray-on fireproofing. Though the fireproofing was later upgraded, apparently no tests were conducted to see how it handled intense flames.

In 1999, the building's owner, the Port Authority of New York and New Jersey, issued guidelines to upgrade fire-proofing by tripling the thickness. In 2000, officials were still redoing the fire-proofing, noting in one assessment that some areas could withstand only one hour in a fire. By 11 September 2001, fire-proofing had been upgraded on floors in the areas of the towers where the hijacked planes struck. The north tower fell after an hour and 45 minutes and the south tower about an hour after being struck. Nearly 2,800 people were killed.

In the 1960s, the architects chose to build according to fire resistance standards that required floors to last for two hours and columns to hold for three, NIST found.