CRAZY BOUNCING BRIDGE in flooded river.

Do NOT try this at home! Unless you have a suspension bridge in your back yard. Been meaning to upload footage of this crazy bridge for ages. Got the chance when I took the camera down to the waterfalls in the Crocodile River to film some of the effects of the tropical cyclone Dando that passed through the area and led to extensive flooding in the Kruger National Park downstream of here.
Probably not a good idea, but hey, stupid can be fun sometimes. The river was in full spate, with the waterfall just downstream, so if this thing had indulged in a spectacular failure a la the Tacoma Narrows bridge, I would have been up the proverbial creek without a paddle.
Suspension bridges are particularly prone to this kind of sympathetic resonance effect. It just seems like bad design to have the resonant frequency very close to that of a normal human stride. I think Mythbusters did an episode on this, featuring the myth of a platoon (or whatever the latin name is, will need to check my Asterix books to make sure) of Roman soldiers walking across a bridge and the bridge collapsing as a result of the bridge being "tuned" to the frequency of the marching feet.
I don't think this one was likely to collapse, but just in case I hung onto the suspension cables while I was doing the insane jumpy thing. Figuring that the deck would collapse rather than the main cable snapping.
Of course the REAL danger here might not have been a bridge failure or a trip down a flooded waterfall. It's not called the Crocodile River for nothing....
The tend to congregate in the pools below the rapids after heavy rains, at the confluence of the Croc River and Nels River. Sometimes see them there on the walk in to the climbing crags in the Nels River gorge. Was supposed to go climbing but the base of the entire cliff was under water, which would have meant an uncomfortable hanging belay, amongst other inconveniences. So that plan was aborted. This provided an alternative adrenaline rush.
I am always reluctant to capitalise on tragedy for views. A little unethical in my view. I know all the major news channels thrive on it, and many famous youtubers have jumped on the bandwagon. So I waited a few days before posting this. The tropical storm Dando caused a few dozen deaths in Southern Africa, mostly in Mocambique. But this is just me running on a badly-designed bridge, so i figure it's far enough removed not to count. Sad part is, there's another storm that might hit the already waterlogged region. Depending on steering winds, which are always unpredictable. As of today, Funso is already a category 4 Cyclone (also known as a hurricane or typhoon) and is gathering strength over the warm waters of the Mocambique channel. If it veers westwards a bit we're in for interesting times, because the ground is already waterlogged and the rivers and dams are all full.
Because I start running from the ends, the initial mechanical forces are near the midpoint of half the span, so that leads to the second harmonic oscillation you can see here. This is simple mechanical excitation in one plane, with a simple vibration pattern.The much-studied failure of the "Galloping Gertie" bridge on November 7, 1940 in Washington State was rather more complex, with a torsional component adding to the complexity. And that one had more to do with strong wind and aeroelastic flutter (like pinching a blade of grass in each hand and stretching it in front of your mouth, then blowing) than with a guy acting like a demented monkey and hanging from the suspension cables.
The deck of the bridge is essentially acting like a guitar string here, although the physics is a bit more complicated because the deck and the main cable and the staying cables all need to be considered as a dynamic system. The complexity of the problem is illustrated by the presence of a travelling wave (lengthwise) in the handrails, seen at 0:34.
In my opinion this bridge represents a bit of an engineering fail. It is a bad design,because the natural resonant frequency corresponds too closely to that of a person walking. Also, the deck structure is probably too light and too shallow. I doubt that it would make it if the river rose those extra couple of feet. Or if there were an extra couple of (Roman soldier) feet, marching in time to mine.
The cables supporting the deck of the bridge are elastic, as are the beams of the deck. The cables act basically like a series of springs or bungee cords, subject to Hooke's law. F=-kx For steel, the law would apply throughout its range of elascticity. (For rubber, it gets a bit more complicated.) The potential problem would occur if one could get the steel cables to extend beyond their elastic limit, which would generally lead to catastrophic failure.
Sometimes the problem can be solved by "tying the bridge down" with cables, like an inverse suspension system.

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[yellowmetalcyborg]

Those bridges are designed to cope with resonant energy transfer brought about by children and under-grown adults :P

So you can bounce up and down for as long as you want and the bridge won't fail.

Isn't technology wonderful?

[ROBwithaB]

@yellowmetalcyborg

Yes, IF they are designed according to the codes, they shouldn't fail.

But sometimes, unexpected things happen.

The Millenium Bridge in London did something odd when it was opened to pedestrian traffic.Turned out that it wasn't that unusual, just that earlier reports of it had been buried in engineering and scientific journals.

My main concern with this bridge is that the resonant frequency is to0 close to that of a normal human gait.

[AndrettiUA]

Did you shoot a video how to start fire with a bridge?

[ROBwithaB]

@AndrettiUA

I know this is supposed to be a joke, but I HAVE given a lot of thought to the concept of using a bouncing motion like this to generate fire.

The problem with most methods of friction fire is that they rely heavily on upper body strength. So, traditionally, only men made fire.

And women were obliged to maintain embers back at the cave until the men came back from the hunt.

I'm looking at a simple device that uses the strong leg muscles and a bouncy action.

For ze babes.

[bebbodux]

you should break the pace when walking on bridges

[ROBwithaB]

@bebbodux

I concur. That's very sensible advice.

[AndrettiUA]

I definitely see now Rob with a Bridge

[5wanderingroses]

@RobwithaB a spinning wheel uses a bouncing motion in a way.

[SPuDlyDUdLEEpudles]

why does this remind me of indiana jones?

[divinefart]

hey robwithaB, have a look at the capilano suspension bridge, in BC canada, looks similar, you should visit :) cheers and nice beard ..

[kingcollie]

@ROBwithaB Thanks for the comment!

[HikerJohn316]

Rob, as I recall the US Army field manuel on drill and ceremonies FM 22-5 ? notes that when marching over a bridge a unit should be marching in "route step" which means that you dont have to be in step - to prevent the situation you described.

Beautiful area and river!

Sincerely missing Roberta,

HikerJohn316

[yellowmetalcyborg]

@ROBwithaB You're right, but the bridge dissipates most of the energy transferred to it by one person. If you get a crowd to all walk in phase, then that's another story.

[watcbd]

@ROBwithaB Oh yes, monkey face as well. Also, I screamed "Oonga Boonga, I have made fire" ....needless to say that was the end of that relationship.