| New Civil Engineer 22/09/2005 |
A ROW over the causes of the World Trade Center twin tower collapses on 11 September 2001 broke out between British and American fire engineers last week.
British engineers strongly disputed official American claims that the towers became more vulnerable to collapse after the hijacked aircraft scraped vital fire protection from their steel frames.
The twin towers collapsed when each caught fire after terrorists flew hijacked Boeing passenger jets into them.
The disagreement provoked a strong exchange of views at a major conference held at Gaithersburg near Washington DC to discuss the official findings of America's National Institute for Standards & Technology (NIST) investigation into the 9/11 collapses.
"We don't believe that NIST has satisfactorily demonstrated that the loss of fire proofing was the deciding factor in the collapse, " said Arup associate director Dr Barbara Lane.
We have carried out computer simulations which show that the towers would have collapsed after a major fire on three floors at once, even with fireproofing in place and without any damage from plane impact." Lane said the difference of opinion was significant because clients had begun to demand that designs had NIST-compliant fire protection (NCE 30 June).
NIST is now recommending that all structural elements of tall buildings have the same degree of fire protection.
Firms like Arup have developed international reputations for producing designs which avoid the need for such extensive fi re protection.
Lane commended NIST's modelling exercise and agreed with most of its recommendations.
"But they have not taken proper account of the thermal expansion of the structural elements, especially the floors.
"As a result, there is too much emphasis on passive fire protection in NIST's recommendations and not enough on the benefits of good design." Dave Parker, Gaithersburg (See analysis p23) . ST's report into the baffling collapse of the 47 storey WTC7 tower on 9/11 has been delayed again, NIST confirmed last week. A draft for public comment is now not expected until May next year. In June NIST promised that the report would be out by the end of 2005 at the latest. It blames the latest delay on the need to concentrate resources on finalising the report into the twin towers collapse.