The whole business of the bridge collapse has been discussed for many
years, notably in AJP February 1991. That article points out that the
bridge and wind constitute a dynamically coupled system with many
degrees of freedom. The resulting motion is much more complicated than
that of a simple oscillator with an external driving force that is
unaffected by the motion. Whether or not the analysis is completely
correct, it should certainly makes one reluctant to present the
collapse as a case of driven harmonic motion. I believe this was
Leigh's point, and one that is well taken.