When I was in the military, we would stop marching in unison when marching over a bridge, even small foot bridges.
We were told that if we all marched in synch over the bridge, it would fail or fall apart.
Can bridges be destroyed by marching feet?

Already tested with a suspension type bridge. They couldn't get it to break.

The command, "Route Step March" for bridge crossing is true in the US Army as you just described.

Apparently there have been a few small or weakened bridges that have collapsed in the 1800's.

Do not know if the collapses were because of marching or overload combined with other factors but if you think about it, evryone's left foot hitting at the same time over a long weakened bridge with a few Companies does put a little more stress on the bridge.

Can it happen with modern bridges? Doubt it.

It could however, happen in Third World Countries in the poorer areas and similar situations.

Why risk it?

I think they tested it twice. I dont remember the results.

One test were with small robots and the other test was Adam on 2 x 4 bridge.

I have an alternative way to test this myth. If the MB's is interested.. Id be happy to share. I got a final to take.

I'd always heard that route step was only needed on the quick-setup pontoon bridges (or whatever they're called), so as not to shake them apart where the sections are connected.

I know they tested this myth but I was not impressed with their approach. There are a number of variables that come into play (as usual). The length of the bridge between spans, the rate of the footsteps and the overall mass of the troops. Stand on the end of a diving board and start swinging your arms in unison. You may begin to see the board move up and down in step with your arms. Now increase the speed at which you swing your arms. You will notice that some speeds are more effective than others. Some will do almost nothing. Now place a 5 lb weight in each hand and do the same thing.

I don’t know that a bridge can be brought down with this method, but I think under certain conditions a bridge could begin to move a bit more than normal. I suppose that it could even move beyond what was intended.

1 word: wrong concept

adan and jamie could resolved that one without build this little rangers and two bridges !

its simple: if a bridge dont falls when cars cross over, why should a infantry could destroy it?

let me explain better, lets talk about geometry:

what is a circle?

a circle is a group of small straight lines or a polyghon with manny small sides

what is a perfect circle?

a perfect circle is a group of infinite straight lines or a polighon with infinite sides

in real life, we use perfect circles? no

so, we can say the tire of a veichle is a polyghon with manny small sides

now, to broke a bridge u must have a intense and very high frequence of hits during all the action

the frequence that makes a veichle tire, considering the milion of small sides and
considering that the sides are so small that
is almost impossible the hits could be unison,
because u allways have a surface of this wheel
hitting the floor, is very very higher than the frequence made by a infantry

difficult to understand ?

when a thing like this

http://wwwb.click21.mypage.com.br/hosp_cliente_sc/M/a/c/MachoYbrain.myblog.com.br/P1010005.jpg

or like this

http://www.rica.com.br/Rolo.gif

cross the street of your house, u know what happend allright? everything starts to shake

its basicly a heavy wheel without tires

the frequency is so hight that all de soil around starts to shake, the wheight by itself was unable to do this effect. is a combination of much wheight and hight frequency so

now think about how many cars, trucks and everything who have wheels cross a very used bridge everyday. if it dont broke a bridge, a infantry wont do anyway

so, i have a explanation about..

[When I was in the military, we would stop marching in unison when marching over a bridge, even small foot bridges.]

the explanation is: for any army, cross a bridge is a crucial and deadly task, because, when a army is in the middle of cross, one bomb and it will be lost in its majority thousands of soldiers can die. so, to cross a bridge, the army must walk more silently but fast

maybe they tell the "fall bridge story" to distract them about the real danger, i dont know...

I'm going on faded memories here, but the Brooklyn Bridge was the first long suspension bridge. To prove the concept, an Army unit was told to march lock step across it. I believe that to really prove or disprove the myth, a suspension bridge should not be used.

On the show, they referred to Galloping Gertie, the original Tacoma Narrows bridge. They mistakenly said resonance was the cause of her collapse. Again, if my memory serves, the cause of that collapse was simple aerodynamics - during windstorms, the bridge tried to fly.

I'd say, to properly test this myth, they should build a non-suspension bridge, and try the army soldiers on it. Just my $.02...

Oh, also, Marcus' description of a circle is completely wrong for this discussion. A circle being an infinite number of straight lines is a mathematical concept that was used to approximate area, volume, etc. It's not how a compass draws a circle, that's certain.

Yes, Marcus the conclusion is likely the same but the reasoning was way out in outer space. I think the "test" the MB did was a complete waste of time but the myth is pretty far out there in the first place and I do not think anyone really thinks its possible. Like many of the weaker myths it depends on to many factors. A large army on a weak bridge sure. How large and how weak are testable but not very interesting.

I would like to point out the fact that they broke the bridge but not the way they had expected it to.

I saw the episode of when they tested this Myth but it was "BUSTED"

they should retest the myth the the tesla idea on the new episode. the tested it on a real bridge and felt the vibrations all along the bridge and it wasent a suspension bridge.

I would like to see these myths tested on an actual suspension bridge. Adam built a beautiful bridge with cables on it, but the road deck was supported by a truss.

A suspension bridge hangs the weight of the road deck from catenary cables anchored in enormous concrete blocks. The road deck on such a bridge is not very rigid. If three vertical cables in a row were to be severed, that section of bridge would fall into the water.

The Mythbusters catenary bridge cables were not anchored at all. The video shows the ends of those cables loose on the ground. And the deck was strong enough that the scale model bridge spanned the gap with no cables at all.

I'd love to see this one tested on an actual suspension bridge.

The ends of the bridge were not anchored. Much of the kinetic energy that was transfered to the bridge from the boots was simply thrown off at the unconnected ends of the bridge.

The roadbed was not properly anchored to the uprights. In the MB video the road bed pivots. In the Tacoma Bridge disaster you can see that there is very little relative energy transfer beyond the uprights. The energy is contained between the uprights allowing the kinetic energy to accumulate.

Also the lack of anchoring the suspension cables allowed transfer of some of the energy to the uprights and bridge ends.

There was to great a span between the uprights. The increased span allowed some of the resonance to dissipate.

The roadbed was not wide enough to sustain the resonance. The wider roadbed would allow the twist at the center, between the uprights, that would be a key ingredient of the deflection/resonance ratio.

The roadbed was not sufficiently attached to the trussing, causing dampening.

The roadbed was to thick and to stiff for the scale. This would dampen the resonance.

The roadbed may have been to heavy. MB used 1" of what looked like drywall. On that scale it would translate to what, 2' or 3' of asphalt?

The second resonance device was poorly concieved. It did not introduce a twisting resonance like people marching instep, but a hammer effect as though they were all standing in the center of the span and jumping up and down.

Part of the energy that the resonance is derived from is not simply the pounding, but the side to side motion of the weight. The way the mythbusters had it set up there would have been an ungodly amount of weight over a height of 10 to 15 stories. The things would have toppled off before they were able to get any resonance going.

I think the material that was used for trussing would be to thick. I am not sure that at the scale MB built their bridge at it would probably have to use not much more than 24 to 28 guage steel. Some of the struts in the trusses looked to be 1/4" or more thick. This would offer to much resistance.

That said, I don't think this could happen on modern bridges. Resonance is one of the factors that are calculated into the design of the bridge. Besides, I can't think of a reason why you would want to march 5000+ troops over a bridge anyway. To destroy it you bomb it. To get the troops to the other side, trucks are faster.

God Bless
Frank

I saw the show and would like to relate a bridge collapse from the 1840s.

My son just finished a pubic presentation project (4H) about the bridges in Keeseville NY. One bridge was a chain (not cable) suspension bridge, crossing the Ausable River. In 1848, (I think, we will check the facts for exact dates) a militia was marching across the bridge when it collapsed, sending 50 into the water, killing 13. The bridge was rebuilt, using chains. The bridge was replaced in the 1880s with a cable suspension bridge that still stands today. Chains were a product of local iron mines and blooming mills. Chains are also only as strong as the weakest link.

I have a variation to suggest: In the movies, the hero(s) are often being chased through mountains/jungle by murderous bad guys, and come to a rotting, decaying suspension bridge over a horrible chasm. Our hero runs across the bridge, which starts to break behind him, piece by piece, but he runs fast enough to keep ahead of it as it breaks, making it safely to the other side and foiling the bad guys.

Question: would a rotten suspension bridge really break like that, allowing the guy to keep running, or would it simply experience sudden catastrophic failure and go down all at once?

Vibration is a very serious consideration on the analysis of any structure. A relatively low oscillating force can cause very large amplitudes of resonance. Even though a marching group of soldiers seem like they would not affect the bridge like a much larger one time force would, it is the steady oscillating force provided by marching that enables this to happen. The amplitude is maximized when the frequency of the oscillating force is equal to the natural frequency of the structure. More specifically:

Amplitude= Force/[sqrt{(k-m*omega^2)+(b^2*omega^2)}]

omega is the frequency of the oscillating force, m is the mass of the structure, k is a constant related to the geometry and material properties of the structure, and b is a constant that is proportional to the ability of the structure to dampen oscillations.

The natural frequency = sqrt(k/m). When omega equals the natural frequency the equation becomes:

Amplitude= Force/[b*omega]

This explains why some structures seem uneffected by oscillating forces, because they have a very high dampening constant. I do not remember how the suspension bridge was setup on MB, however, someone earlier mentioned the ends of the cables were not anchored. This lack of anchoring could result in very high dampening qualities of the bridge they built, resulting in their bridge being very difficult to attain high amplitudes on.

In any engineering structural analysis there are many factors. First of all, being able to hold the load. In the case of a bridge, it has to hold the weight of the cars. In addition to testing for failure loads, a structural analysis also estimates the natural frequency. Depending on the application it may be desired for the natural frequency to be low or high. All this said to make the point that from an engineering prospective, vibration is a very big consideration.

I was amazed at the fact that they show'd the millenium bridge in london (wobble brigde) and didn't link it to what they were testing. What happened there is basicly the same thing as they were trying to test. Resonance is basicly no different than you pushing a swing. You don't push it every second, because than you wouldn't go any higher. You push it at different intervals to go higher and higher. When people walked over the millenium bridge all they did was push it like a swingset. Resonance does the same thing. It's not gonna happen every time you march over a bridge. There's just that chance that you'll hit its natural frequency and then it's bye bye bridge.

The Tacoma Narrows bridge collapsed because the wind was pushing it like a swingset. It just accidently pushed it at it's natural frequency.

There are numerous reasons why i think the test on mythbusters failed (I do like the show by the way). First off all, the natural frequency off a bridge is the determant by all its parts togheter (more precisly by it's mass and stiffness) The bridge they build on the show wasn't connected very well. You could see the different parts off the deck just move to one another. So you might as well have tested it on one off those beams. Also i didn't like the robots in the first test. To hit the natural frequency of anything you have to apply a force as a sinusfunction (i don't know if this is the right word in english). That's very hard to do with mechanical stomping feet.

The thing i did like was the method they tested it the second time. Turning weights in circles is probably the closest thing you'll get to the correct force.

I'd really like to see them try it again on a real bridge. In theory it should even word on little footbridges. So If they could find one they could destroy, i'd love to see that happen.

[quote]The Tacoma Narrows bridge collapsed because the wind was pushing it like a swingset. It just accidently pushed it at it's natural frequency.[/quote]

Wind doesn't have a frequency; it is a steady force. In this case, it was also an utterly huge force, which drove the bridge to oscillate in it's second fundamental frequency, but that had nothing to do with resonance; it is an example of an overdriven system shedding energy in the simplest way possible.

The Tacoma Narrows bridge collapse was due to high winds, poor design and even poorer construction. Not resonance.

http://www.vibrationdata.com/Tacoma.htm