  Ba\v{z}ant et al. have proposed a model of a gravity-driven collapse of a
tall building which collapses in a progressive-floor collapse after the failure
of a single storey. The model allows the re-computation of the structural
resistance of the building once the downward movement of the building has been
quantified. We give a physically more sound version of the collapse model, and
determine the downward movement of the North Tower of the World Trade Center.
Thereby we reproduce a value for the upward resisting force during the collapse
that is similar to what has been achieved by Ba\v{z}ant et al. for the first
three seconds of the collapse. However, our method of measurement also includes
data up to 9 seconds after collapse initiation. These data show a much bigger
upward resistance force between 4 and 7 seconds after collapse initiation
