re Harmonic resonance on bridges: We were taught in CCF (uk = OTC in US I think, = toy soldiers at school, right?) to break step on bridges for that very reason.
And the recent problems with the "Millenium Bridge" in London bear witness to the phenomenon. Whereas the designers had thought to control vertical harmonics in this footbridge they discovered as soon as they opened it that a slight breeze caused a small sway (allowed for) and that this caused all the pedestrians to walk in a wider gait and in step, setting up a positive feedback, and making it very uncomfortable. It has been closed for about a year now, but I believe they have designed a solution and have started to fit it.
Rod