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Collapse of the World Trade Center Towers

 G Charles Clifton, HERA Structural Engineer
17th September 2001, revised 19th September, minor revision on impact force made 8th October, minor revisions made 11 December (see elaboration)

Introduction

Construction of the World Trade Center Towers began on August 5 1966 and they were officially opened on April 4 1973. Fig 1. shows the two towers prior to the attack. As will have been forever seared on the memory of all readers, they were destroyed in a terrorist attack on 11 September 2001. The method of destruction was simple and devastating, namely suicide attack by aircraft. The resulting images of the towers burning and collapsing were ones no-one ever expected to see.

The first airplane hit the North Tower at 8.46am local time and that tower collapsed at 10.28 am or 1 hours after the impact.

The second tower was hit at 9.03 am but collapsed more quickly, at 9.59 am.




















Fig 1 World Trade Center Towers Before Attack  (Associated Press photo).

My background has given me some insight into what may have happened to these towers under the much more severe event of a direct hit from a near fully loaded large modern airplane. It is important to note that the explanation given is only my considered opinion, based on the information available six to eight days after the event.

Subsequent material received from then until 11th December has led to minor changes in terms of • Correcting the times of impact and collapse and a brief note on the orientation of the map presented in Fig 2

  • Elaboration on the structural load distribution in the North Tower after the impact ( given that the initial explanation is overly simplistic)
  • Correcting some typographical errors

Before presenting those details, some details of the building are given, followed by brief details of the impact. The effect of the impacts can only be assessed in light of these details, in particular the devastatingly high local impact force on the buildings from the planes. This is followed by my assessments of the effects of this impact on each of the two towers, which showed some significant differences.

There has already been considerable speculation on the severity of the fire and its role in the collapses. On the basis of what I have seen and heard reported to date, it is my opinion that the effect of the fire was of much less importance than the effect of the initial impact, especially on the first tower to be hit ( the North Tower). The reasons behind this opinion follow details of the effects of the impacts on each tower and the article ends with a personal footnote on the tragedy and a reference.

 continued...

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