The Truss Failure, or Zipper and Domino, Theory
MIT materials science professor Thomas Eagar has eagerly championed
the truss failure theory with helpful metaphors.
He was interviewed by NOVA in
The Collapse: An Engineer's Perspective
Professor Eagar explains his zipper theory:
Once you started to get angle clips to fail in one area, it put extra load
on other angle clips, and then it unzipped around the building
on that floor in a matter of seconds.
Eager suggests the towers were designed to survive only a trashcan fire:
If it had only occurred in one little corner, such as a trashcan
caught on fire, you might have had to repair that corner,
but the whole building wouldn't have come crashing down.
The problem was, it was such a widely distributed fire,
and then you got this domino effect.