In its August 2008 draft NIST said the north face took 5.4 seconds to fall 18 stories, 40 percent longer than free fall would take. Asked in the public comment period to explain the difference in detail, NIST tracked a point on the roofline frame by frame. [1]
Its final report divides the descent into three stages: a slow start of about 1.75 seconds as the exterior columns buckled; a second stage of about 2.25 seconds, from 1.75 to 4.0 seconds, when the face fell at gravitational acceleration, about eight stories; and a third stage of slowing. NIST says the free fall was consistent with its model because the interior structure had already collapsed and the buckled exterior columns provided almost no support. [1][2]
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