falling debris severely crippeled its upper structure and started fires, 5 hours later the lower structure weakened by fire and stressed by additional load not being taken by the upper floors caused chain failure in the main structure.
Source: had an entire section on it in one of my college's classes written and taught by a civil and materials engineer who investigated it.
WTYP did a looooong episode on 9/11, but the part where the talk about building 7 is pretty short: https://www.youtube.com/watch?v=f7Qop_64qqk&t=2h44m22s
Debris from WTC did though. And apparently sprinklers weren't working because the water supply had been fucked up because of the twin tower incident.
The building had a weird design because it was built on top of a electrical substation from like the 1950s. So the building had to straddle this thing with its foundations, which resulted in the building having several failure critical columns. As far as I remember the fire didn't actually melt the column, but made one of the critical steel columns expand enough that it shifted and dropped the beam it was supporting which lead to progressive failure of the structure.
Our building practices in this country go hand-in-hand with the insurance industry.
It was built by NYC contractors in the 70s. I don't know why anyone thinks it's collapse was surprising.
