Please try to avoid associating this question with baseless controlled demolition theories
I suppose the former 7 World Trade Center and it's fate does not need an introduction.
The progressive collapse started with the failure of 2 or 3 columns in the east end of the core at the lower levels. This caused the floors supported by these columns to cave in due to the bean connections being unable to redistribute loads. This is the official version, and is quite intuitive to the layman. It can also be deduced from the observed motion of the eastern penthouse.
What is confusing is what happened next. After 5 seconds of nothing happening, the whole building suddenly collapsed.
We can tell that all the remaining core and exterior columns failed near simultaneously by observing the motion of the western penthouse. This penthouse, just like the other one, was supported by the core and had no connection to the exterior framed tube other than the floor beams. Yet it moved almost in sync with the facade.
The official version holds, and common sense suggests, that these two events are causally connected. But how?
What caused all of these columns to buckle at once?
The fire was not hot enough to weaken any them at any point in time, according to the official version.
The floors bracing them remained mostly in place, as can be seen in the NIST simulations.
- They were not all hit by falling debris because nothing was falling around them.
Then why did the the columns shown in the above screenshot buckle?