Was the stability of the WTC1&2 core columns dependant on the diaphragmic action of the building's floors?

Question

I am currently in a discussion with a so called '9/11 truther' who claims there is no adequate explanation for the collapse of the central core columns of the WTC towers. (Sorry to beat a dead horse)

Based on my viewing of a documentary, I argued that the composite floors added a 'stiffening' effect (i presume engineers call this 'diaphragmic action'?), and that because the floors collapsed first, this compromised the structural integrity of the columns and caused the main columns to collapse quickly after the floors. This documentary is can be seen here and the explanation is at 45:10. https://www.youtube.com/watch?v=DgwlFY-4Txc

I'm however not 100% whether this explanation is satisfying enough. It seems logical enough for the perimeter columns, but is it also true of the core? Is the idea of of your building core being stabilised by diaphragmic action of the floors something that is common or should a framed tube building core be designed completely independant of any stabilising effect this action might provide?

My discussion partner claims that it is absurd for a building core to be dependant on the floors like this. It might be a tall order, but I wonder if someone experienced in this subject can provide some clarity.

Just for context, I am not an engineer, but a draughtsman in the construction industry.

Answer 1

Yes, it was. The structure was designed on the basis of the pipe-in-pipe principle. Concrete core housing elevators and stairs get their lateral strength and stability from the perimeter columns which act together like a pipe with a huge moment of inertia. For a hollow square tubing with the wall thickness, T, and base of $B\cdot B$, the moment of inertia which is a measure of lateral stiffness is

$$I\propto T\frac {B\cdot B^3}{12}$$

So the outer pipe is more effective in resisting lateral force by a power of four. $$\frac{I_{outer}}{I_{core}}=(\frac{B_{outer}}{B_{inner}})^4$$ The slabs played a dual task of supporting the floor load and shear transfer beams to connect the outer pipe to the inner pipe. The softening of the slab trusses and yielding due to the heat was one of the major components of the collapse.