[gleeaikin]

Real cause of the sudden collapse of the Twin Towers.

I saw an interesting analysis of the collapse the other evening on PBS. Apparently as the metal floor joists began to melt from the high heat they pulled the outer wall inwards, as much as 5 feet at which point they fractured out of the wall and the whole collapse sequence began.

[Robert A Cook PE]

The floor joists did NOT melt, but merely had to "slightly soften" (an orange glow indicating 800-110 degrees) in metal means it has NO strength, and bends under very little pressure) so that the walls fell in on themselves.

If I heat the center of an 1/2 sq bar to a yellow heat with my torch, I can bend and twist the bar with my thumb and one finger's pressure at the other end.