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How to damp a wobbly bridge

Updated Tuesday 27th September 2011

Inspired by the Millennium Bridge in London, Adam Hart-Davis investigates how to damp a wobbly bridge

In this clip from the OU/BBC programme Science Shack, Adam Hart-Davis and his team investigate ways to 'damp' a bridge.

The Millennium Bridge in London was first opened in 2000, but initially suffered from being too wobbly. Using a scaled-down version, the team look at why bridges wobble, and methods that bridge builders use to reduce it.

You can also read a diary of the full three-day experiment.



Adam Hart-Davis

Now, our simulated bridge is quite hard to understand because there’s so much scaffolding, so I built a toy version here lined up exactly with it, so the river is flowing that way and the bridge is going across that way.  We’re interested in a section of decking which on the real bridge is four metres wide.  It’s going to be exactly the same on our simulated bridge.  The critical thing is the suspension, it’s a suspension bridge, we’ve got it hung on bits of string and that allows it to wobble like that.  Now, our simulated bridge we couldn’t hang it on string, we didn’t think the string would take the weight and so it’s actually suspended on these box sections of steel, this black stuff here, and we hope that will allow this to wobble in exactly the same way as the big bridge behind me.

It’s really intriguing because none of these people is trying to drive it, they’re all having some difficulty walking and the only way you can walk comfortably is by getting in step, but then of course everyone is driving the bridge.  You can’t help it.  You’re actually forced by the movement of the bridge to get into step and therefore to drive it to move further!

Let me show you how you might damp a wobbly bridge.  Here we have two sections of a wobbly bridge, alright, and they wobble together like that.  They are suspended in exactly the same way so they wobble almost exactly the same and now I’m going to damp one of them.  Here is a little bath of oil, just cooking oil, I put that on there and then in here I dip a spoon so it dips into the oil, a bent spoon.  Now, I’ll pull them back the same amount and let go and you’ll see that this has stopped wobbling in about a swing and a half whereas the other one goes on and on.  So, very quickly what happens is that all the energy is dissipated into moving the oil, and the neat thing is you can tune the damping to match the oscillations of the bridge by changing either the viscosity of the fluid or the size of the spoon.

I want to measure scientifically how efficient these dampers are, so we’ve got two women up here to drive the bridge and we’ve got Linda down here with a chart recorder and she’s going to record the movements of the bridge.  Okay, are you all ready, poised?

First, without the dampers.

One, two, three, go!  And you can see the movements building up as they walk backwards and forwards or sway anyway and we’ve got a lovely sine wave and we’re now reaching roughly maximum amplitude.  So, walkers, one, two, three, stop!  And that was where they stopped and you can see the trace dying away, one, two, three, four, the fifth away was just about dead so it’s taken five complete sine waves to die away virtually to nothing.  Now, the question is; what will happen when we put the dampers in?

Now, we’ve got our five dampers in.  How much difference will that make to the rate of decay of swing?  Okay, swingers get swinging.  Right, chart woman.  Come on!  Come on!  Put some welly into it!  They are having some difficulty in getting it going so obviously the dampers are having some effect.  I don’t think you’ve had enough breakfast, you two.

So, the dampers are flattened to the sideways movement of the bridge.

I think we need some more weight on the bridge.  Could we have some more volunteers to swing please?  Yeah, push from below.  That’s better.  We’re getting some movement now.  Good.  Okay, swingers, one, two, three, stop! 

Hahaha! That’s absolutely staggering!  Right, let’s have the original one.  Thank you very much.  Now, this was what it looked like without the dampers, alright, just wind on a bit winder woman.  Go on, go on.  Stop there, lovely.  Now, look at the colossal difference.  That’s where I stopped it.  Without the dampers on we did five swings.  With the dampers on, well it’s about one.  It’s just gone splat, the whole thing has completely stopped swinging so clearly the dampers can damp down the movement very rapidly indeed, and indeed I suspect with these two young things on there we don’t need as many as five, but perhaps with a big multitude on the bridge five would be safe.  Terrific!  Triumph for damping!



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