  A model of a gravity-driven collapse of a tall building has been proposed by
Ba\v{z}ant et al. We apply this model to the collapse of the North Tower of the
World Trade Center to determine the energy dissipation per storey during the
collapse. Our findings are (at most) 250\,MJ during the first 4.6 sec. In the
time interval between 4.6 and 7.7 sec after collapse initiation we find an
additional energy dissipation per storey of 2500\,MJ. Because the steel columns
increase in strength towards the ground this value corresponds to a value of
2000\,MJ for the storeys in the aircraft impact zone. After 7.7 sec the value
reduces again. These results have two possible interpretations: (1) If due to
the building design (column strength, shape etc.) the energy dissipation per
storey cannot reach the high values which we observed, then the collapse cannot
be described by the gravity-driven collapse model. (2) If the collapse is
described correctly by the gravity-driven collapse model, then then we fond
direct evidence that the collapse mechanism did not follow the same pattern
during the whole of the collapse. The possible amount of energy dissipation was
reduced by an order of magnitude during two long time time intervals. In both
cases there is no a priori reason to justify the sometimes expressed belief
that the collapse was inevitable even after the falling top section had gained
a significant amount of momentum. In fact, if the amount of energy dissipation
had stayed only little longer on the high level, then a gravity-driven collapse
would have arrested. Note that (1) implies that if in principle the
gravity-driven collapse model describes gravity-driven collapses of tall
buildings, then the collapse was not gravity-driven.
