Revealed: Why London's Millennium Bridge wobbled

[davedlg 0]

http://news.yahoo.com/s/nm/20051102/sc_nm/science_bridge_dc

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LONDON (Reuters) - A natural phenomenon rather than a design fault caused London's Millennium Bridge to wobble and sway, forcing its closure just two days after opening in 2000.

The elegant pedestrian walkway was conceived as a blade of light linking the south bank of the River Thames to the City of London.

But as large crowds walked across the steel structure on opening day in June 2000, the 320-meter long bridge swayed from side to side because of a phenomenon known as collective synchronisation.

"The phenomenon was that people who were walking at random, at their own favorite speed, not organized in any way spontaneously synchronized," said Steven Strogatz, of Cornell University in Ithaca, New York.

"That's the phenomenon. Why did they all start moving in step? They did it unconsciously. That is what nobody had thought about and engineers did not anticipate."

The applied mathematician and expert on the phenomenon said collective synchronisation is now something engineers will have to consider when designing bridges.

He and colleagues at Cornell and other universities in the United States, Britain and Germany have devised a theory based on what happened to the Millennium Bridge to estimate how much damping or stabilization is needed in footbridges.

Their findings are published in the science journal Nature.

"We think our theory will provide some guidance to help engineers avoid the problem," Strogatz said in an interview.

Certain coincidences must occur for collective synchronisation to occur. In the case of London's wobbly bridge, it was large crowds walking across a flexible footbridge that vibrated at a frequency of one cycle per second, which just happened to be the same frequency as humans walking.

"The people were resonating with the bridge," said Strogatz.

As the bridge started to move, people would get in step with the sway to steady themselves. They widened their stance to make it more comfortable to walk and inadvertently made the wobbling worse.

"A lot of people were blaming it on the beautiful innovative structure, the design of the Millennium Bridge itself, which was a radical design," said Strogatz.

"But that is not true."

Collective synchronisation occurs in nature when crickets start chirping in unison. In some parts of the world, fireflies blink on and off in perfect synchrony like a Christmas tree. The monthly cycles of women living together have also been known to synchronize.

"It is always very striking and almost spooky because it is like order coming out of chaos," said Strogatz.

After 5 million pounds worth of modifications to steady the structure and 20 months of closure, the Millennium Bridge successfully reopened in February 2002.

[AlexCrowley 0]

I've always heard of a similar story regarding a bridge near a military training ground. On exercises the squads have to march out of step so the bridge doesnt shake itself apart.

Resonant frequencies are fun.

TV's got them images, TV's got them all, nothing's shocking.

[Mike111 0]

i wouldn't be surprised if it still wobbled even after the 5 million investment

[riddler 0]

It makes sense to build a footbridge that naturally dampens resonant frequencies in the range of human footsteps.

I wonder about everyone walking in step; what if people felt the sympathetic frequency of everyone else underfoot, then started walking in time subconsciously? Kinda like when people tap their toes to music without thinking about it.

Trapped on the surface of a sphere. XKCD

[davedlg 0]

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I wonder about everyone walking in step; what if people felt the sympathetic frequency of everyone else underfoot, then started walking in time subconsciously? Kinda like when people tap their toes to music without thinking about it.

I think you're right...

http://news.bbc.co.uk/go/pr/fr/-/1/hi/england/london/4400010.stm

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Few people 'caused' bridge wobble
London's Millennium Bridge would have wobbled with as few as 160 people walking in time, new research shows.
All that was needed was a smallish crowd to stride in step, to trigger the now infamous wobble, the research by Professor Steve Strogatz shows.

Writing in the journal Nature, Prof Strogatz said he hoped his analysis would help the design of safeguards.

After opening in June 2000, the £18.2m bridge was closed for 20 months while giant shock absorbers were fitted.

About 80,000 people crossed the suspension bridge spanning the Thames from the Tate Modern to St Paul's Cathedral on its opening day, with as many as 2,000 on it at any one time.

'In sync'

Prof Strogatz described the wobble as a chicken-and-egg problem.

"What came first, the bridge movements or the synchronised strides?" he said.

The bridge had two factors working against it, he explained: its flexible structure and its natural frequency was close to that of human walking.

"If the people are initially disorganised and random, if a few of them get into sync by accident, the bridge would become unstable," he said.

When a certain number of pedestrians was reached, the wobbling became marked enough to force everyone into stride.

Prof Strogatz, a mechanics expert from Cornell University in New York, US, said the analysis should help engineers solve the problem before they build a bridge.

Story from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/1/hi/england/london/4400010.stm

[leroydb 0]

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The applied mathematician and expert on the phenomenon said collective synchronisation is now something engineers will have to consider when designing bridges.

this is wrong in that I saw an old black and white vide where collective synchronisation happend to a bridge in the 1940(-50's?)

Leroy


..I knew I was an unwanted baby when I saw my bath toys were a toaster and a radio...

[ReBirth 0]

Yeah...they did an experiment on Mythbusters regaring this based on a story from that time frame.

[Stumpy 284]

I think i know the film you mean - wasn't it a suspension bridge that was made to hit its resonant frequency by the wind and it literally ripped itself apart? amazing bit of film

Never try to eat more than you can lift

[Remster 30]

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this is wrong

No its not.

Resonnance freq is a well know physical attribute that all bridges have. Armies know about this and break step when crossing bridges.

What is new is the sympatetic occurance of it when people, unknowingly start getting in sync.

Edit: Good lord I cant spell!

Remster

[leroydb 0]

let me rephrase... it is wrong as it is not a sudden find... "Resonnance freq" has been know about...

Leroy


..I knew I was an unwanted baby when I saw my bath toys were a toaster and a radio...

[davedlg 0]

Yup - The old Tacoma Narrows bridge was vibrated to resonant frequency by wind and tore itself apart. Think about what happens when you blow on a blade of grass or a harmonica to make noise. This is the same thing highly multiplied.

[Kris 0]

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let me rephrase... it is wrong as it is not a sudden find... "Resonnance freq" has been know about...

Yes, it has, but the article is talking about the collective aspect of it, which had never been observed before now.

They didn't know that people would automatically, and collectively, sync themselves up to each others gait like this before.

Sky, Muff Bro, Rodriguez Bro, and
Bastion of Purity and Innocence!™

[Remster 30]

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sync themselves up

Resistance is futile

Remster

[ntrprnr 0]

Then explain how 36,000 runners crossing over the Verazanno bridge in NYC during the marathon all at once don't even make the bridge wobble a little bit?

_______________
"Why'd you track away at 7,000 feet?"
"Even in freefall, I have commitment issues."

[Broke 0]

Different resonance

Divot your source for all things Hillbilly.
Anvil Brother 84
SCR 14192

[kelpdiver 2]

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Then explain how 36,000 runners crossing over the Verazanno bridge in NYC during the marathon all at once don't even make the bridge wobble a little bit?

marathon runners have pretty diverse gaits. Also, some diliberately try to switch to a shorter one for such a long race. It's also a much larger bridge.

Instead of spending 5M, couldn't they just have put lots of post barriers on the bridge to break people's walking stride?

[ReBirth 0]

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let me rephrase... it is wrong as it is not a sudden find... "Resonnance freq" has been know about...

Yes, it has, but the article is talking about the collective aspect of it, which had never been observed before now.

They didn't know that people would automatically, and collectively, sync themselves up to each others gait like this before.

Ok...well that was covered in the beginning of "The Dead Poets Society"

[NickDG 23]

Galloping Gertie . . .

http://www.ketchum.org/bridgecollapse.html

NickD BASE 194

[Ashtanga 0]

It's because of this chick.

http://www.dropzone.com/cgi-bin/forum/gforum.cgi?do=post_attachment;postatt_id=54546;