The Snare Of Stride Synchronicity

Whoa:

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.

[emphasis and links mine]
In my experience, we’d call this “collective synchronization” an emergent behavior. What is strange to me is that, even in a multi-million £ project, even after such well-known disasters as the Tacoma Narrows Bridge, well-trained, well-paid engineers still fail to take resonance frequencies into account. Why did no one say “Hey, this thing has a resonance frequency of 1 Hz. And it’s going to host foot traffic. And the human stride averages out to about 1 Hz. So, given that we could see some bizarre resonance effects, let’s work on dampening said effects”?
Thankfully, no one was hurt and, as the article states, the bridge was repaired in 2002. Still, consider me gobsmacked at the level of engineering cluelessness that would be required to let a screwup of this magnitude go by.
[/Insert witty comment on the dangers of people walking in lockstep here]
(Via the indispensible Jeff Goldstein.)