CONTROVERSY STILL surrounds the exact collapse mechanism of the Twin Towers, despite three years of detailed investigation by the National Institute of Standards & Technology (NIST) team.
Some engineers believe the collapse was influenced by factors other than the fires caused by burning aviation fuel which weakened vital structural steel elements. And they have accused NIST of repeatedly changing its explanation of the collapse mechanism.
'In this latest version, the 'hat trusses' on top of the towers play a crucial role in the redistribution of stresses after the impact, ' one leading US structural engineer told NCEI in New York. 'In earlier versions they are hardly mentioned.' (NCEI December 2004).
The trusses were originally designed to carry loads from TV towers on the roofs - but only WTC 1 ever had such a tower.
NIST now believes the trusses helped the upper storeys of the towers to stand up after the aircraft impacts.
Consultant rup as equently criticised NIST's analysis of the collapse mechanism.
Last month Arup went on the attack again. In a statement, Arup associate Dr Barbara Lane said: 'We consider thermal expansion in fire to be a leading factor in a collapse.
'This does not seem to have been addressed in NIST's recommendations, but we hope it will become part of the ongoing discussions going forward.'