A new study says that thicker insulation might have prevented the collapse of the WTC towers, or at least allowed them to remain standing for much longer.

It is widely accepted that the collapses were caused by the failure of the buildings' steel structure as it was weakened by the heat of the fires. But Jim Quintiere of the University of Maryland, College Park, thinks the thickness of the surviving fire insulation, rather than the destruction of insulation during the impacts, explains why the towers collapsed when they did.

The south tower was the first to fall even though it was hit after the north tower. The insulation on its burning floors was only half as thick. According to Quintiere's calculations, if the insulation had matched that in the north tower, the south tower would have stayed standing longer.

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No one doubts that the planes killed many people on impact and started the fires that led to the buildings' collapse, says Quintiere. But if both towers had had insulation over 50 millimetres thick, he says, they might not have collapsed at all. His analysis calls into question the safety of other buildings constructed to the same standards as the twin towers. However, the Port Authority of New York, the owner of the twin towers, rejects his theory

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If the NIST tests back Quintiere's theory, attention will turn to why the insulation was thinner in the south tower than the north tower. The New York City building code stipulates that the insulation on steel structures should be at least 38 millimetres thick. However, the Port Authority's special legal status means it does not have to comply with the code.

When the twin towers were built in the early 1970s, fire insulation just 19 millimetres thick was sprayed onto the trusses. But in 1996, Lombardi recommended the thickness be doubled. "I made the decision, since there was a question from a general contractor as to how much thickness is needed to provide a two-hour fire rating of the floor joists and floor assembly that would be in conformance with New York City building code," he says.

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Despite the recommendations by Lombardi, thicker insulation had been applied to fewer than a third of the trusses in the twin towers by 11 September. This, Lombardi says, was because it could only be done as floors became empty.

This is another example of how monstrous it (still) is to exempt the Port Authority from city regulations. The WTC complex was exempt from city smoking laws, and all PA territory, including the airports, is considered "private property" and exempt from free speech rights such as demonstrations or leafletting.