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Science, Maths & Technology

Millennium Bridge Diary

Updated Friday 2nd June 2006

Science Shack takes on the Millennium Bridge wobble, aided by a Vauxhall Astra and some olive oil. Their diary will take you behind the scenes, warts and all.

Jem and his drawings Copyrighted image Icon Copyright: The Open University

26th Nov

9am
The rustic simplicity of the Science Shack shed stands in stark contrast to the awe-inspiring bulk of the Tate Modern Gallery. Producer Marty looks forward to ‘getting the show on the road’.

10am
Adam Hart-Davis lends the Science Shack team a hand, as they kick-start the power generator to provide electricity to light the shed. Does it really take four men to press one switch?

Lynda and Ruth are sticking plastic drinking straws together, building a metaphorical ‘Wobbly Bridge’ which will later demonstrate the fragility of Man’s ‘designs’ on the millennium.

10.30am
Jem gives one of his rare (short) interviews. He proudly displays his most recent oeuvre (‘drawings for the building of a scaffolding platform to do the Millenium Bridge Experiment on 2001’). He explains these are only photocopies because the originals were taken from him, free of charge, by the curator of the Tate Modern, who needed to clear them with the site manager. Jem tells us that he was impressed by the understated way the curator did this ‘art deal’ with him and looks forward to an exhibition of his works at the Tate Modern.

1.30pm
Lynda and Ruth’s Straw Bridge gets its TV debut as Adam explains the theory behind this week’s experiments. Their handiwork is a scale model (or rather an artistic interpretation - well we are at the Tate!) of the “Science Shack Millennium Bridge Simulator” or “Scaffold Bridge”. Adam and the team will be looking at the sideways movement of Millennium Bridge and how people’s footsteps affect the “wobble”.

2pm
Our scaffolder friends have got the Science Shack Millennium Bridge Simulator Wobbling. The movement increases as more weight is put from one foot to another in a rhythm. It’s a bit like those playground rockers you used to play on that you could push back and forward to gain momentum.

3pm
The theory the team are testing is that the more weight you put on at each sideways “swing” the more wobbly the bridge becomes. The question is: do people naturally, subconsciously, start to walk ‘in sympathy’ together (in step) when they are massed together? Could the effect of such rhythmic walking set up a swing in the bridge which is exaggerated by this rhythm? Adam ropes in some volunteers to test the theory. The results show that human beings do, indeed, tend to walk in rhythm together.

4pm
Adam explains ‘resonance’. In this example he shows that each of these poles has its own resonance (based on their length). If you very gently wobble the base (mounted on a roller system) the tallest pole starts to swing like crazy. A little faster and just the middle pole moves. Faster still and the short one moves. When the movement starts to be in sympathy with the natural resonance of each length of pole the swinging begins. So, if people walking in step ‘en masse’ across the Millennium Bridge happen to hit the natural resonance of the bridge wobble occurs.

4.30pm
Adam is not only a master of verbal communication skills. He is also able to ‘speak’ through gestures. Using the rubber balls from the experiment before, he makes a sign, meaning ‘I have not been given enough lunch and I am looking forward to my tea’.

5pm
Adam eyes the baked beans longingly, but manages to concentrate on explaining the pendulum principal. Which is that whatever the weight of the pendulum load its rate of swing is constant. It is only by changing the length of the string that you can change the swing rate. This he tests by setting two very differently weighted bean cans swinging together with the same length of string.

6pm
You heard it here first – a half plastic bottle with olive oil in it is the solution to the Millennium Bridge wobble. This is one kind of damper that can break down the effect of the resonance set up by lots of people walking across the Millennium Bridge. By adjusting the viscosity in the oil or by changing the size of the spoon or paddle you can counteract the resonance to any degree you want.

7.30pm
Filming in the winter is always a race because the days are so short. But the Science shack team have overcome this by having their own mobile studio – the Shack – parked outside the Tate Modern.

 
The team on the simluator Copyrighted image Icon Copyright: Production team

27th November

8am
As the team set up for the days filming, Adam appears out of the Science Shack. Has he been working through the night on solutions to the bridge wobble or was he taking a sneaky coffee break whilst the rest of the team were hard at work?

9am
Nine year old Alex Gocke talks to Adam about his experience of being on the Millennium Bridge when it wobbled. Alex crossed on the second day the bridge was open with his Dad. He describes how the bridge wobbled and you had to walk in a funny way to get across. He also remembers having to queue for the bridge to get across, not because it was too crowded but because they were limiting the amount of people on the bridge because of the wobble.

Alex is the first to have a go on our Millennium Bridge Simulator, he shows us what happens if you try to walk normally across the bridge and wobbles rather quickly across to the other side. Luckily Alex developed a slightly more controlled way of getting across the real Millennium Bridge or he might have ended up in the Thames!

As Alex demonstrates, to get across a wobbling bridge you have to waddle rather than walk, putting your weight from one side to another. This kind of walk accentuates the rhythmic stepping the team looked at yesterday. If other people on the bridge were walking like this, could the stepping have added to the wobble?

10am
Ian Fletcher from Epsom also talked about his experience of the bridge. He was one of the first people across in the afternoon and felt rather unnerved by the experience. “I felt like we would all end up in the river!” A few days before, Ian had joked to his friends that he wanted to cross it when it opened as he didn’t reckon it would stay up long!

Ian has a go on our simulator and talks to Alan about whether we’ve got our wobble right. As none of the team were there on the days the Millennium Bridge wobbled, Ian’s input is very interesting. Although we’ve got the basic movement right, he remembers the movement being more like a roll on a ship and there being an element of vertical movement too. The Science Shack team are left to puzzle this out and conclude that some of this extra movement may be caused by the amount of people on the bridge compared to ours. A thousand extra people on our bridge may create the same effect, but there wouldn’t be much space for walking!

11am
The Millennium Bridge Simulator swings from side to side as the Science Shack team attempts to walk up and down it in a straight line. It is impossible to avoid walking both in step with each other and in step with the sideways movement – exaggerating the effect.

12pm
Allan McRobie works at Cambridge University. He is an engineer who specialises, among other things, in ‘bridge dynamics’. He designed our simulator and by putting exercise treadmills on our scaffolding bridge gets it to simulate the movement of a very long bridge. Allan has tremendous enthusiasm for bridges in general and the Millennium Bridge in particular. He loves the fact that ‘the biology took over from the nuts and bolts’ when it was first used. He explains that when it was first tested the bridge began to work like a biological system as opposed to an engineered one. ‘There was more flesh and blood on the bridge than concrete and steel’, he says.

‘The Millennium Bridge is a slender, daring bridge – so its dynamics come to the fore. We like to describe most engineered systems with simple laws so we can do the maths – but many things are ‘non-linear’ especially biological systems. Non-linear means you get a disproportionate change resulting from an input. Rio Ferdinand the Leeds United player’, he adds, ‘cost eighteen million quid – so did this bridge’. We can safely say which one Allan thinks is truly worth the money!

2pm
Joan Lasenby, a lecturer at Trinity College and expert at capturing motion digitally, helps Science Shack understand the subtle ways people walk and the forces involved in varying kinds of gait. She attaches fairy lights to volunteer Pete and tracks them using three cameras hooked up in a very clever way to her computer. Hollywood animators use exactly the same technique, but their machinery can cost millions. Joan and her colleagues have developed a more basic (and affordable) system that is helping medics understand a range of diseases related to movement. Because she has fewer cameras than Hollywood she has to ‘help’ the computer by ‘pre-installing’ into the computer’s memory information about the way a skeleton controls and restricts movement. This helps the computer calculate what is happening faster (and ultimately cheaper!)

4pm
Pete, a volunteer who came out to help us yesterday and was immediately exploited to the maximum by Jem, models the Science Shack high tech solution for fixing Allan’s Millennium Bridge Simulator – a half dozen Astra car suspension units. Will they stand up to the harsh test we have waiting for them?

5pm
The local pearly King and Queen arrive to cut the ribbon and declare our – newly fixed - scaffolding bridge open. It is hard to stop them breaking into song at a moment’s notice.

As the Pearly Kings and Queens sing ‘Roll Out The Barrel’ our dancing girls start strutting their stuff on the scaffolding bridge and do their damnedest to make it swing. But Science Shack technology defeats them and the bridge is declared officially viable.

 

Seismograph Copyrighted image Icon Copyright: Production team 28th Nov
Today Adam and the Science Shack team are on the banks of the River Thames in London, outside the Tate Modern Gallery.

11am
First thing this morning, the team are back on the Science Shack Millennium Bridge Simulator to carry out more tests. We know from yesterday's opening that the Astra suspension acted as dampeners and held the bridge still, but this morning the team want to quantify the success. A seismograph type gadget is attached to the bridge and the team get it wobbling without dampeners attached. Our gadget produces a very satisfying “tremor – like” pattern, when the team stop moving it takes 5 swings for the bridge to stop swaying.

Marty and Jem fix the Astra shock absorbers back on ready to test the bridge swing again, if effective, the swing on the bridge will be less on the seismograph pattern. The Astra suspension dampeners are in fact so effective that Sophia and Ruth can’t get the bridge swinging to start the test. They call in an extra swinger – Jem to get the bridge moving. The pattern shows that the bridge is only swinging very slightly, and that’s with extra help to get it moving. When the team stop moving the bridge sways only once – “A triumph for dampers!”

1pm
The Science Shack sits empty as Adam and the team head onto the real Millennium Bridge to find out how they are fixing the wobble problem, could our experiments and findings give them a hand?

Adam talks to Roger, the Millennium Bridge project Director. It seems they have already thought of our idea of dampers and they hope it will be as successful at fixing their wobble as it was for ours. He also explains that the lateral movement caused by people’s feet and the increase in sideways movement as people walk in time has been experienced by other bridges in the past. However, the makers of the Millennium Bridge claim they’re the first to investigate it and try to fix it. They reckon it’s now the most damped bridge in the world!

Arup’s dampers come in two varieties. Vertical – to stop the up and down movement and Lateral dampers – to stop the sideways wobble. Engineer Ron Hall takes up the decking and shows us how they work.

1.30pm
Filming ends.

 

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