This is the Millenium Bridge, otherwise known as the wibbley, wobbley bridge. It is called such because when it was first opened and thousands of people started walking across it, it began to sway from side to side. So they had to shut it and make the alterations so that it was safe. It was opened a year later, costing millions in repairs. I just can't believe that they didn't check such a basic architectural property as the resonant frequency. They made the soldiers march out of step in the olden days to make sure that the marching would not be at the common resonant frequency of the bridge. If you don't know anything about resonant frequency, it is is the frequency of vibration of a structure at which continued vibration will case the oscillations of the structure to continue to build and build, and not cancel each other out. It's how you smash windows with high pitched singing. The note that is struck is at the resonant frequency of the widows and so the vibrations of the window just keep getting larger til it smashes. Thus ends the crash course in physics!