When working with vibration and sound, resonance is a quite vital thing to always keep in consideration. Even when not into our field of business, most people have tried out acoustic resonance – when blowing into a bottle, making a flute. Or, felt the power of mechanical resonance and perhaps seen dramatic consequences of it like short videos of when the Tacoma Narrows bridge collapsed in 1940.
The understanding of resonance is essential for vibration control and preventing fatigue and consequences thereof. And to avoid it, the knowledge of the eigenfrequencies to the mechanism or structure is the key factor for a solution. It is when the force applied coincides with one of the eigenfrequencies, that resonance occurs. To design things so that it gets eigenfrequencies that it is very unlikely to be exposed to, may seem like a clever thing to do.
I guess that the engineers constructing the Millennium Bridge in London, a footbridge that opened in June 2000, took that among other things into consideration. What they did not take into consideration, was that the human way of walking would cause “resonance”. The interest of the bridge was huge at the grand opening day, and the bridge was crossed by 90.000 people, with up to 2.000 on the bridge at any one time. The natural “swaying” movement of the human walk did excite the bridge construction – making it moving sideways. This movement caused in turn the people walking on the bridge, to sway even more to compensate in order to keep their balance – and they synchronized as well – amplifying the bridge movement!
By adapting ideas originally developed to describe the collective synchronization of biological oscillators such as neurons and fireflies, there are now approaches to help engineers to estimate the damping needed to stabilize exceptionally crowded footbridges against synchronized walking.