The 47-storey third tower, known as Tower Seven, collapsed seven hours after the twin towers.
Investigators are expected to say ordinary fires on several different floors caused the collapse.
.. That would make it the first and only steel skyscraper in the world to collapse because of fire.
.. Building Seven is the smoking gun of 9/11. A sixth grader can look at this building falling at virtually freefall speed, symmetrically and smoothly, and see that it is not a natural process.
“Buildings that fall in natural processes fall to the path of least resistance”, says Gage, “they don’t go straight down through themselves.”
.. There are a number of facts that have encouraged conspiracy theories about Tower Seven.
- Although its collapse potentially made architectural history, all of the thousands of tonnes of steel from the skyscraper were taken away to be melted down.
- The third tower was occupied by the Secret Service, the CIA, the Department of Defense and the Office of Emergency Management, which would co-ordinate any response to a disaster or a terrorist attack.
- The destruction of the third tower was never mentioned in the 9/11 Commission Report. The first official inquiry into Tower Seven by the Federal Emergency Management Agency was unable to be definitive about what caused its collapse.
- In May 2002 FEMA concluded that the building collapsed because intense fires had burned for hours, fed by thousands of gallons of diesel stored in the building. But it said this had “only a low probability of occurrence” and more work was needed.
.. With no steel from Tower 7 to study, investigators have instead made four extremely complex computer models worked out to the finest detail.
.. It says Tower Seven had an unusual design, built over an electricity substation and a subway; there were many fires that burnt for hours; and crucially, fire fighters could not fight the fires in Tower 7, because they didn’t have enough water and focused on saving lives.
.. They think fires burnt long enough to weaken and break many of the connections that held the steel structure together.
Most susceptible were the thinner floor beams which required less fireproofing, and the connections between the beams and the columns. As they heated up the connections failed and the beams sagged and failed, investigators say.
.. The collapse of the first of the Twin Towers does not seem to have caused any serious damage to Tower Seven, but the second collapse of the 1,368ft (417m) North Tower threw debris at Tower Seven, just 350ft (106m) away.