Some Misunderstandings Related to WTC Collapse Analysis

[Tony Szamboti]

Now not only does there have to be a dynamic load to start the column yielding, but there has to be enough kinetic energy available to completely collapse the column, because as the column deforms it absorbs energy, and in the case of large steel columns lots of it. The fall of the upper section of the North Tower should not have progressed, because the column energy absorption capacity was significantly greater than the kinetic energy that was available. In other words, there wasn't enough energy available to break through the columns after a one or even two story drop.

Is it possible the inner core/outer frame structure caused a lateral force that was exacerbated by the floor platforms falling into the void caused by this lateral shift and ensuing removal of the floor platform supports that caused an unexpected massive kinetic force that drove the collapse of the inner core therefore defeating the expected resistance models you cite? In other words the resistance wasn't there because the outer frame and inner core both shifted away from the floor supports causing that mass to plunge unobstructed therefore causing the force it would have taken to overcome the column energy absorption you cite? When those inner core columns were compromised by that lateral force they would no longer possess the vertical resistance you cite. This failure would be almost instantaneous and not possess the expected deceleration you cite.

And what about the South Tower?

I don't believe what you are contemplating is possible for several reasons.

1. The only mechanisms acting on a building which can provide lateral forces are wind loads and seismic loads and a building would be designed to handle these without shifting between floors. So I don't know where the lateral force required would come from and how it could accomplish what you are saying. It would have to be applied to only the upper section and would have to be enormous to shift the upper section due to inertia and with its columns still connected to the lower section.

2. The columns would not buckle after losing support for one story. They all could go at least three stories without lateral support before buckling. It is hard to see how this contemplated shift could ever be sufficient to dislodge three stories worth of floors from their vertical supports. The shift would have to be massive and even then it would only potentially separate the floors from their vertical supports in one direction, since it would only affect the floors that are normal to the shift not those parallel to it. What would really need to happen, for what you are contemplating, is for the core to shrink in girth and the exterior to expand. That is even less possible than the shift.

3. For the lower section columns not to provide resistance to the fall of the upper section their lateral support needs to be completely removed before the upper section hits it.

4. The core floors would not be susceptible and the core was self supporting, so it would not fall without providing resistance.

I would say what you are contemplating would be on par with a magic trick. It just has no basis for occurring in reality.

The paper is about the North Tower. If there is a serious problem with it then it is not hard to imagine a problem elsewhere.