Is it true than if a bunch of people walk over a bridge at the exact same pace the bridge will collapse ?
I heared that when soldiers cross a bridge they purposely don’t walk at the same pace so the bridge won’t collapse …
Thanks for answering this one,
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What oddities are being circulated now?
Welcome to the SDMB, diabless55. Yes, it’s called resonance (see this How Stuff Works article).
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I question whether a large bridge would have its resonant frequency driven to the point of destriction by people marching in step. The frequencies that cause structural damage a quite low. Several seconds to complete one cycle in the case of the Tacoma Narrows bridge in Ice Wolf’s cite. However, it is possible in theory and could possibly happen to smaller bridges if the frequency of the steps is a close match to one of the bridge resonances.
Notice all the hedges?
And, by the way, asuspension bridge, like the Golden Gate Bridge might be more sensitive than atruss bridge. The cite is a model railway bridge, but this is what a real truss bridge looks like.
A suspension bridge is essentially a roadway suspended from cables and is a relatively flexible structure with a low resonant frequency. A truss bridge is a relatively inflexible, stiff structure with a higher resonant frequency.
A couple of susceptible bridges in London… the Albert Bridge and the Millenium Bridge.
In basic training we were ordered to rout step[walk anyway you want] when crossing even small to medium footbridges[like the ones you see built over highways so pedestrians don’t have to dodge cars] for this very reason. Hey, anything to stop marching for a bit. Isn’t there a tale/urban legend about not having a cat walk directly down the center of a suspension bridge? Cats, being “perfect walkers”[their hind paws being placed on the same spot that their front paws have just vacated] would set up a momentum, which would grow in width and speed, and the bridge would ultimately collapse. Anybody want to have their Mr. Whiskers check it out for us?
The problem with the Milennium Bridge was not only that the resonant frequency was fairly close to that of the average human footfall, but also that the side-to-side resonance of the bridge caused people to react by placing their feet further apart and walking in step with the movement - a feedback effect that rapidly increased the severity of the resonance.
Watching a video of the phenomenon for the first time, it is easy to imagine that the people crossing the bridge, waddling from side to side in near-perfect unison, were doing so deliberately, but they weren’t - they were just trying to stay upright.
A great article from the Village Voice about the very issue.
GQ.
Have you ever felt a major football stadium sway, because of a large number of people stomping their feet in unison?
I have! SCARY!
Trinopus
Although there were serious engineering mistakes made, the collapse of the Kansas City Hyatt Regency walkway (July, 1981) was partially attributed to the number of dancers stamping in time to the music.
To be politically incorrect, I wonder if enough people in Kansas City can keep with the beat to make this happen.
Alright, alright, before everyone gets their panties in too much of a knot over resonance, it is important to note that all of these bridges acting like springs/oscillators/whatever still behave in a non-conservative manner in regards to movement. Although there may be a certain frequency that happens to be the perfect frequency to get the most motion out of a bridge, that motion will still absorb energy, so, unlike what quiltguy154 suggested:
No, a cat walking in perfect harmony with the natural frequency of a bridge won’t cause it to collapse, the input forces still have to be sufficiently large to do damage.
Strong bridges are still stronger than weak ones.
The whole thing is one more nugget of pseudo-knowledge which is very convenient for know-little school teachers to impress young students. In spite of how often this is repeated it is just plain folklore and UL. Any bridge which is at any risk of damage from any number of people walking in step is a bridge which is at more severe risks from other natural factors. No sound bridge in history has collapsed due to this or even come close to collapsing. Other forces exert much more stresses on a bridge than marching in step ever could. There is just no way in the world that any sound bridge could even be made to oscillate to the point where the wind can make it oscillate. No way.
Not to mention that one doesn’t see armies marching in step on roads, (them having become aware of the invention of the internal combustion engine and the jet fighter) and much less on bridges.