London's Millennium Bridge was closed not for structural reasons, but due to a “small perceived
risk to pedestrians of losing balance and coming to harm in crowded situations” due to movements of “unexpected amplitudes”. Videos show up to 50
mm. of flex with a frequency of .77 cycles/second at the ends and 70 mm. flex at .95 cycles/sec for the center span, which caused pedestrians to stop and hold onto the
balustrades to regain their balance. (data and quote from subsequent investigation)
Mechanical tests validated the computer simulations done before construction-- unfortunately British bridge building standards only specify vertical excitation and designers did not take into account the structure’s natural frequency, which proved to be similar to the horizontal effect of crowds walking in step. Research and implementation of an expensive plan to add viscous and tuned horizontal dampeners has kept the Millennium Bridge closed to the public for over a year. The tabloids have shown no mercy in exposing this politically embarrassing situation, considering the success of the nearby Tate Modern gallery, the bridge's French connection, and the failure of the Dome.
At long last the bridge is open to the public and is a popular, if notorious, landmark.