Debris from the North Tower started fires on the lower floors. The water supply for the sprinklers there came from city mains broken by the tower collapses. NIST found that heat from the uncontrolled fires made floor beams on the east side expand, pushing a girder on Floor 13 off its seat at Column 79. Floors fell onto those below, Column 79 buckled, and the failure spread across the building. [1][2]
NIST called WTC 7 “the first known instance of a tall building brought down primarily by uncontrolled fires,” and said its fires were similar to those in other tall buildings where sprinklers did not work. It added that the debris damage did not start the collapse; the fires did. [2]
NIST considered and rejected a blast. It calculated that the smallest charge able to cut the critical column would have produced 130 to 140 decibels half a mile away; no such sound is on any recording, and no witness reported one. It also rejected diesel fuel stored in the building as a cause. [2]
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