Comment by lutusp | Hacker News Reader

Comment by lutusp | Hacker News Reader

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> There is a place in the UK where this happens with spooky regularity with people walking.

Sometimes in the military, when crossing a bridge, troops are ordered not to walk in step, because of the stress it places on the bridge.

http://en.wikipedia.org/wiki/Angers_Bridge

A quote: "As usual in crossing that bridge, the soldiers had been ordered to break step and to space themselves farther apart than normal."

More dramatic:

http://en.wikipedia.org/wiki/Broughton_Suspension_Bridge

A quote: "On 12 April 1831, the 60th Rifle Corps carried out an exercise on Kersal Moor under the command of Lieutenant P. S. Fitzgerald, the son of John Fitzgerald. As a detachment of 74 men returned to barracks in Salford by way of the bridge[6] the soldiers, who were marching four abreast, felt it begin to vibrate in time with their footsteps. Finding the vibration a pleasant sensation some of them started to whistle a marching tune, and they began to "humour it by the manner in which they stepped", causing the bridge to vibrate even more.[6] The head of the column had almost reached the Pendleton side when they heard "a sound resembling an irregular discharge of firearms".[6] Immediately, one of the iron columns supporting the suspension chains on the Broughton side of the river fell towards the bridge, carrying with it a large stone from the pier to which it had been bolted. The corner of the bridge, no longer supported, then fell 16 or 18 feet into the river, throwing about 40 of the soldiers into the water or against the chains. As the water was only about two feet (60 cm) deep at that point none of the men were killed, but 20 were injured, including six who suffered severe injuries including broken arms and legs, severe bruising, and contusions to the head."

> 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.

> The walking rhythm may match the natural oscillation of the bridge thus resulting in a resonance effect.

Yes, and that stresses the bridge. If it didn't, there would be no reason to avoid it.

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.

http://en.wikipedia.org/wiki/Millennium_Bridge_%28London%29#...

http://www.youtube.com/watch?v=eiaM_LZUsqM

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):

http://en.wikipedia.org/wiki/Taipei_101#Structural_design

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.

> but it was declared busted by Mythbusters

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.

> declared busted by Mythbusters

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.

The pneumatic "marching" rig they used was anything but synchronized or controlled. I would put little faith in that particular Mythbust.

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.

Video of bridge collapse : http://www.youtube.com/watch?v=xox9BVSu7Ok Wikipedia: http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge#cite_note...

london's "millennium bridge" mentioned briefly in the first article.. http://www.youtube.com/watch?v=eiaM_LZUsqM

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