An Old Problem Resurfaces

Innovations sometimes don’t work well. The Millennium Bridge in London, which was a new twist on suspension bridge design, moved noticeably when it was built and needed a retrofit to reduce the sway.

An article on the intersection of structural dynamics, statistics, and the biomechanics of the human gait – Walking Crowds on a Shaky Surface by Varun Joshi and Manoj Srinivasan – makes a good case that this was an odd coincidence that happened at this bridge because of its unique design.

The early history of modern suspension bridges (those with decks that are not supposed to move the way rope bridges do) is full of collapses from resonance created by soldiers march in formation. (As seen above, resonance was also a concern on temporary bridges.) The Millennium Bridge problem was odd because it occurred from random people crossing.

Vibration is a tough problem structurally. If you make a structure stronger, you typically decrease the amplitude of vibrations while increasing the frequency. Higher frequencies may make the vibration more perceptible and can even make resonance worse depending on the natural frequency of the structure at hand. The best solution, developed in the nineteenth century, is to add strength and weight, and let the weight damp out the vibrations. A more elegant solution, as was used in the Millennium retrofit, is to add mechanical damping.