> troops are ordered not to walk in step, because of the stress it places on the bridge
Not really, not because of the "stress." The walking rhythm may match the natural oscillation of the bridge thus resulting in a resonance effect. At least that's what they taught us in high school.
I've always been told this, and it's always seemed plausible, but it was declared busted by Mythbusters.
Having said that, as quoted elsewhere, the Millennium Bridge across the Thames in London did exhibit resonant swaying as pedestrians crossed it, and extra dampers needed to be added.
Added in edit ...
Auto-hoist, petard-wise. It seems some people have assumed that I meant that this never happens, and cannot ever happen, simply because it was the subject of a Mythbusters episode in which they declared it busted, even though I then added that something similar has in fact happened in real life.
I thought the implication was clear, but obviously not. So let me say more explicitly ...
It is clear that people walking on a bridge can cause resonance, it has been seen in real life.
Further, that was a non-extreme case, so it is totally plausible that more vigorous marching, more definite synchronisation, and closer matching to the structure's resonant frequency is not only possibly going to cause problems, but likely to cause problems.
It is, however, unlikely that marching will match the resonant frequency. Structures these days don't resonate at 2 Hz. Football stadiums are designed to resonate at above 12 Hz. Further, structures often have explicit energy dissipation system built in to them. Taipei 101, for example, has a tuned mass damper to change the resonant frequencies and help absorb, then dissipate energy (yes, I'm speaking loosely here):
So yes, I knew all that, and yes, a single Mythbusters episode is not proof of anything, but it does provide a collection of data from which to start forming your own hypotheses, designing your own experiments, and understanding that things aren't necessarily as simple as you thought.
That episode seemed particular weak to me. The "myth" is that this can happen under particular circumstances - but they seemed to test the hypothesis that marching in lock step will determinedly bring down any bridge.
Another famous example would be the collapse of the Tacoma Narrows Bridge in 1940, widely believed to have been caused by forced resonance. However, as per Wikipedia, the real cause of the collpase was aeroelastic flutter.
Oh, goodie, case closed then. Mythbusters have this tendency to "bust" stories that revolve around rare and unique circumstances by running just few experiments. Obviously that's a flawed approach.
And then there was the time they tested shock absorbers on a car's bumper exploding in a fire and sending the bumper flying away. They couldn't replicate it.
They spoke to a woman with scars on her legs from a bumper that hit her after shock absorbers on a burning car exploded and sent the bumper flying away.
So they called it confirmed, since it could happen, they just couldn't get everything right in the lab.
I know it's entertainment first and science second, but a complete reversal of policy just because there is someone you're not willing to look at and call a liar? Weak.
Not really, not because of the "stress." The walking rhythm may match the natural oscillation of the bridge thus resulting in a resonance effect. At least that's what they taught us in high school.