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Resonance in Bridges

01/24/2008 6:36 AM

We had learnt that soldiers break steps when marching over bridges, to prevent collapse due to resonance. Why does it not happen with vehicular traffic

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#1

Re: resonance in bridges

01/24/2008 6:46 AM

A number of cars with identical speed wobbles should do the trick.

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#2

Re: resonance in bridges

01/24/2008 7:29 AM

Vehicles don't stamp their feet. e.g They do not apply impulses of force, it is a smooth application of force.
Like Hendrik says, if someone was daft enough to put speed bumps on a bridge at regular intervals, and the traffic speed was right I daresay you could get a nice resonance.

The Millennium foot bridge across the Thames is still known as the 'Wobbly Bridge' due to it's teething troubles caused by resonance even tho' the people weren't walking in step!

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#5
In reply to #2

Re: resonance in bridges

01/24/2008 11:40 PM

You are only imagining situations in advanced contries where no pot holes and bumps are there.

But in developing countries or underdeveloped world, pot holes and bumps or rammbles is common. Still, bridge will not collapse because of vehicular trafic, as all vehicles will not be moving at same speed, and can not generate any sychronous vibrations. Each vehicle will generate different viration pattern and effectively, good dampening takes place.

So no body should worry

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#12
In reply to #5

Re: resonance in bridges

01/27/2008 4:01 AM

27.1.2008

Aint not too sure about this one. There is this bridge, a rather longish one, could be upwards of a mile,on the river Narmada at Bharuch, Gujrat India. I once got caught in a trafic jam bang on the bridge. Believe me, the amplitude of vibration was more than 3 inches. I put in a silent thanks giving prayer when done with xing the river.

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#9
In reply to #2

Re: resonance in bridges

01/25/2008 7:07 PM

Like Hendrik says, if someone was daft enough to put speed bumps on a bridge at regular intervals, and the traffic speed was right I daresay you could get a nice resonance.

Interesting to mention it. While driving on many highway bridges in Houston, I noticed that the gaps between the plates that make the upper structure (the one on which the car spins the wheels) produce a rithmic sound. More, can feel a kind of oscillation on the vertical (that, on a longer bridge like HW59 in downtown area). On land part, here are, still, gaps that produce the rithmic sound but not oscillation. Now, I am sure that this problem has been taken care of, but a man thinks in the future....

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#11
In reply to #2

Re: resonance in bridges

01/26/2008 4:16 PM

Leave the speed bumps behind, I have the way to tes this theory! We will get a few lowrider car clubs together, after closing the bridge of course, and have them start bumping their hydraulics in unison (this, along with the hi hop rhythms coming from the cars, again they would have to be playing a synched up rhythm). Cheers

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#3

Re: Resonance in Bridges

01/24/2008 10:40 AM

I believe what you have 'learnt' is purely a myth.

Impossible for a normal modern day bridge (say built in last 100Years) to be destroyed by marching.

In order to get enough people on the bridge, to even attempt to stress it, the bridge would have to be very long. A long bridge means a lower resonance, meaning the marching has to be very slow. A bridge with a shorter length, will have a better chance of having a resonance around the typical march pace, but then not enough soldiers fit on the bridge to even threat to cause damage.

Also since the marchers are spread out, over the width of the bridge, their steps cancel any possible side to side resonance.

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#4

Re: Resonance in Bridges

01/24/2008 11:00 AM

I agree that modern traffic bridges have been so designed as to eliminate the low frequency resonance of marching (somewhere near 2 Hz unless they're doing the airborne shuffle), but footbridges have often not been designed this way. And, until recently, it apparently didn't occur to designers to consider side shear in gait (thus, the Millenium Bridge difficulty).

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#6

Re: Resonance in Bridges

01/25/2008 2:33 AM

I am not a mechanical engineer but I think that the traffic moving mass can modify the resonant peak of the bridge.

This fact added to a river flow whose energy is concentrated at very low frequencies can turn out to be rather destructive.

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#7

Re: Resonance in Bridges

01/25/2008 3:12 AM

The reference is for elemental or primative suspension bridges, or pontoon bridges. Any rythemic impulse - marching in step, would or could set up a harmonic effect and cause the bridge to collapse.

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#8

Re: Resonance in Bridges

01/25/2008 3:30 AM

Pointless to even think it! Not unless cars can bounce. It would take an great amount of force say the wind to do such damage.

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#10

Re: Resonance in Bridges

01/26/2008 2:10 AM

In case of soldiers, each of them have the same resonance at the same timing, inwhich a great magnification occurs to become a high resonance. Where in case of vehicles, each of them have its own resonance which differs from the others (due to weight, RPM, ... etc.) and without equating the timing of the individuals resonance, so there is no magnification to the resultant resonance.

The bridge resonance depends on its design, dimensions, shape, suspension and weight.

The damage of bridge occurs when the resultant external resonance equal to the natural resonance of the bridge.

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#13

Re: Resonance in Bridges

02/09/2009 5:28 PM

what steps do engineers take to prevent resonance from occuring in bridges and buildings???

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Users who posted comments:

Abdel Halim Galala (1); Anonymous Poster (1); Del the cat (1); Ferris (1); gowdy87 (1); gsuhas (1); Hendrik (1); indel (1); isloorkar (1); Mevel123 (1); OLD F**T (1); qfteer (1); TVP45 (1)

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