I read with interest Adrian Greeman's article on the overhaul of the US fire codes in response to the findings of the federal investigation into the 9/11 attacks on the WTC (NCE 24 June).
Irrespective of the variation in the sprayed concrete protection, the steel columns of the WTC towers failed because there was no protective skin that could withstand temperatures up to 800infinityC. Future buildings made of refractory concretes and aided by structural composites can withstand such onslaughts better.
Further, the aircraft collision into the two WTC towers was quite different and, hence, their failures differed. The aircraft hit the WTC1 between 90-96 floors and the impact was almost centred. However, WTC2 was hit between the 75th and 84th floors and the offcentred impact affected the corner portion of the tower heavily. This impact could have produced a torque, which might have also influenced the tilting of the upper part of WTC2.
Hence, WTC2 collapsed more quickly than WTC1. More than the lack of fire protection, the structural behaviour of the two towers in response to the aircraft collision played a predominant part.
K Sivakumar (G), Division of Civil Engineering, University of Dundee, Dundee DD1 4HN