The Wobbly Bridge
The sun sets between a cloud bank and the horizon making for an unusually brilliant backdrop. Most commuters paid me (and my blackberry camera) no mind but several did give me a wary eye. And now they are on my blog. We are looking northwards at St Paul's crossing the Millennium Bridge.
The MB cost about £18 million to build and mostly paid for by the millennium commission and the London Bridge Trust. It opened on 10 June 2000. Unexpected lateral vibration (resonant structural response) caused the bridge to be closed on 12 June for modifications. Attempts were made to limit the number of people crossing the bridge: this led to long queues, but damped neither public enthusiasm for what was something of a white-knuckle ride, nor the vibrations themselves. The closure of the bridge three days after opening attracted public criticism, as another high-profile British millennium project suffered an embarrassing setback. Example A: the Millennium Dome. Modifications eliminated the wobble which has not recurred since the bridge reopened in February 2002.
The bridge's movements were caused by a 'positive feedback' phenomenon, known as Synchronous Lateral Excitation. The natural sway motion of people walking caused small sideways oscillations in the bridge, which in turn caused people on the bridge to sway in step, increasing the amplitude of the bridge oscillations and continually reinforcing the effect. Opening day saw 90,000 people, with up to 2,000 on the bridge at any one time - because the lateral motion caused the pedestrians loading the bridge to directly participate with the bridge, the vibrational modes had not been anticipated by the designers. The lateral vibration problems of the Millennium Bridge are unusual, but not entirely unique - the greater the number of people, the greater the amplitude of the vibrations. Exhibit B, C and D: the Birmingham NEC Link bridge, the Groves Suspension Bridge in Chester; and the Auckland Harbour Road Bridge -- all collapsed, sometimes spectacularly.
The MB fixed by the retrofitting of 37 fluid-viscous dampers (energy dissipating) to control horizontal movement and 52 tuned mass dampers (inertial) to control vertical movement. This took six months and cost £5m. The MB re-opened on 22 February 2002 and has not been subject to significant vibration since. For Londoners, it is forevermore the "wobbly bridge."
(source: Wiki and various)
