NIST FAQ: WTC 7 north face had a 2.25-second free-fall stage

Observation

NIST measured a 2.25-second free-fall stage of WTC 7's north face and described negligible support from the structure below.

Interpretation

The measurement is an official observation. Whether free fall implies explosives or NIST's buckling sequence is the H005/H006 split. Do not treat the FAQ as a demolition admission; do not treat it as closing the question.

Source text

OBSERVATION: NIST's own FAQ, after its draft had been criticised for a collapse time 40% longer than free fall, analyses 5.4 seconds of north-face descent in three stages. Stage 2 is 2.25 seconds of gravitational acceleration — 'essentially in free fall' — 'indicating negligible support from the structure below.'

NIST says this is consistent with exterior-column buckling in its model.

Alternative investigators treat free-fall through a steel-framed high-rise as physically indicating simultaneous removal of support, i.e. explosives (H006).

Hypothesis linkage

• H005indeterminateseeded

  NIST free-fall stage is compatible with its buckling model and with demolition arguments.

• H006supportsseeded

  2.25 s free fall with 'negligible support from the structure below.'

• H018supportsseeded

  Free-fall stage as the physical counterpart of a pull.