The world was shocked by the terrorist atrocities in New York and Washington. As engineers we need to reflect long and hard on the engineering issues arising from the collapse of the World Trade Center twin towers and part of the Pentagon building. We should not react in a knee jerk uninformed manner. However it is our duty to keep the public informed about the possible risks to large buildings.
I would offer the following comments at this stage. The attacks were unprecedented and from a structural viewpoint the structures initially succeeded after the aircraft impacts because neither tower collapsed immediately: in fact they stood for about an hour after initial impact. The consensus is that the collapses resulted from the fireball that engulfed the buildings. The planes had only been in the air for a short time and would have been carrying large quantities of aviation fuel, which ignited on impact. The temperatures generated would have overwhelmed the fire protection in the buildings. As the steel, some already damaged, heated up it would have softened and lost strength. Eventually it became too weak to sustain the weight of the building above. At this stage a large mass of material was moving under gravity, and its kinetic energy could not be absorbed by the remaining structure and the whole building collapsed vertically.
Eventually engineers around the world will need to rethink the construction of skyscrapers. Should we continue to construct these dramatic towers or would it be better to stick to lower, less vulnerable buildings? This is for the future, at the moment we can only extend our sympathy to the families affected and hope that this terrible threat can be eliminated.
I have to report a distressing incident. We discovered through a member of the ICE that a paper published in the Structures and Buildings Journal had been published earlier in an almost identical form in the Structural Engineer. Both Journals took this matter up with the authors and have received apologies from them. We have relied on authors being honest about the originality of their papers but this incident has shown that we can no longer do so. In future authors of papers accepted for publication will be required to sign a declaration of originality. It is regrettable that unprofessional behaviour of this nature should have occurred and we intend to prevent its recurrence.
