Since the World Trade Center disaster in 2001, various theories have been proposed as to what caused the observed rapid collapse progression ending in the total collapse of both towers. According to the theory now widely accepted, the columns in the aircraft impact zone lost their load-bearing capacity due to the effects of fire. As a result, the upper part of the building fell over the height of at least one story. The resulting impact forces greatly exceeded the buckling load of the columns near the impact zone, which led to the release of a new portion of potential energy. It was also shown that the energy absorbed by the columns during buckling was significantly less than the potential energy released during this process. This led to the conclusion that the total collapse was inevitable once initial failure occurred.However, there are some examples of buildings, in which the columns of an entire story failed and no collapse progression occurred. Such cases were observed in the 1995 earthquake in Kobe, Japan. There were also some failed controlled demolition attempts, in which a progressive collapse to be triggered by the destruction of vertical load bearing elements in lower levels did not occur. No significant deformations of vertical elements were observed in these cases. This raises the question if there are energy absorption mechanisms other than column buckling that may play a role in the structural response to initial failure. Numerical simulations of simple systems are presented showing that there are such mechanisms indeed and that column buckling is not the only possible way to absorb the energy at impact. An analytical approach based on a simplified model is also described. 284 Structures Congress 2013
It is generally taken as a given that there is no reasonable design concept that could have prevented the collapse of the Twin Towers, once it was initiated, from progressing all the way down to the ground. This view is rooted in the idea that the force generated during the inevitable impact between what may be called the intact upper section (IUS) and the intact lower section (ILS)-meaning the building sections above and below the initially lost columns, respectively-will exceed by at least one order of magnitude the capacity of the latter. On closer inspection, this turns out to be only partially correct-it is correct with regard to the topmost floor plate of the ILS but not with regard to the columns below this floor plate. This paper shows that if the ILS in the Twin Towers had been topped by a stronger-than-ordinary floor plate allowing the columns below to respond properly, rather than be bypassed, these columns-and with them the ILS-would likely have survived. The paper subsequently proposes a building design concept consisting in the insertion of strengthened floor plates in intervals of 10-20 stories.
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