What would it have taken to prevent the total collapse of the Twin Towers?

Have we learnt all the lessons the collapse of the World Trade Center Twin Tower on 11 September 2001 has to offer? Uwe Starossek, author of Progressive Collapse of Structures, Second edition, on tall building design and progressive collapse.

There are two main questions that need to be asked from an engineering standpoint regarding the Twin Towers’ collapse:

• First, how did the internally intact building section above the aircraft impact zone—we can call this section ‘intact upper section’ (IUS)—begin its descent?
• And the second, once the IUS was set in motion, how did it manage to destroy the entire intact lower section (ILS) of the building?

If the Twin Towers are viewed as fortresses under siege, which should hardly strain the imagination, the first of the above questions addresses the outermost defence line of a fortress. Another—a posterior, but no less important defence line, is addressed by the second question.

Had this posterior defence line been adequate in the Twin Towers, the consequences of penetrating the previous defence line would have been far less severe, since the ILS, which made up the larger part of both towers, would have been saved.

It is, thus, rather curious that in the attempts to draw lessons from the Twin Towers’ collapse, the first of the above questions has attracted far more attention. Of course, a fortress must have an adequate outermost defence line, but relying on this defence line alone is not wise, especially when dealing with unpredictable enemies.

Let’s apply the terminology of my book, the Second edition of Progressive Collapse of Structures. Using the outermost defence line means dealing with the property of collapse resistance only. The favourable property of robustness, on the other hand, is only addressed when posterior defence lines are involved—and this has not been sufficiently done since 2001.

So, what would it have taken to prevent the Twin Towers’ total collapse? To properly address this question, it must first be recognised that the amount of resistance the ILS could offer during the inevitable impact with the IUS descending from above, was determined by the load-bearing capacity of the weaker of the following two components:

• the topmost floor plate of the ILS
• the columns supporting this floor plate.

Obviously, the topmost floor plate of the ILS was the far weaker component. This floor plate, designed to carry its own dead and live loads only, was utterly unable to take the mere weight of the ILS, not to mention dynamic forces resulting from the impact. Since this was the case for every other floor plate on the way down, the total collapse of the ILS was inevitable. The columns of the ILS, far stronger than the dictating floor plates, were simply bypassed.

This insight gives the first important hint at what could have saved the ILS in the Twin Towers:

• a stronger-than-ordinary floor plate at the top of the ILS—a floor plate allowing the columns below to be properly activated, rather than bypassed.

Now, assuming such a floor plate was present in the Twin Towers, would the capacity reserves of the columns in the ILS have been sufficient to cope with the force resulting from the impacting IUS?

The short answer is probably. The extensive considerations behind this answer are presented in Nikolay Lalkovski’s, Hamburg University of Technology, Germany, doctoral dissertation, which was elaborated under my guidance.

Looking for more information on progressive and/or disproportionate collapse? My book Progressive Collapse of Structures, Second edition, available in print and as an eBook, covers the underlying structural concepts and offers a general approach for designing structures against progressive collapse.