The spray-on fireproofing might have been the stuff that could be 'sneezed' off. Or, with the way things often work in the modern world, perhaps the stuff wasn't properly applied. Or in this case, there simply wasn't a reliable way to protect the steel from the effects of a catastrophic airliner impact. In any event, according to one of the original architects interveiwed for a PBS Frontline documentary on the WTC, the buildings were simply not designed to survive the type of disaster that occured on 9-11.

Happy Motoring, Mark

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DrDKW