3 The Millennium Bridge UKOctober 20, 2009
The Millennium Bridge a suspension footbridge across the Thames River in the heart of London costing $Aus 32 million opened on 10th June 2000. It closed half an hour later as pedestrians were being knocked off their feet by the swaying bridge.
The Maths Error: Designing 3D Bridge in 2D.
The three big mistakes, often catastrophic, in engineering are maths, materials or human error (ie. Hitting the pylon of a bridge with a ship. Tasman Bridge Collapse Hobart.. 1975). The Millennium Bridge was a maths problem. The bridge was designed in 2D. The engineers allowed for up and down movement but not sideways movement. Any kid running across suspension bridge in a playground knows that as you run it wobbles sideways! The bridge’s movements were caused by a ‘positive feedback’ phenomenon, known as Synchronous Lateral Excitation or wobbles. It cost $Aus 9 million to dampen the bridge wobble.
A similar maths problem with suspension bridge design involves harmonics. If the wind keeps adding an extra nudge each time the bridge sways and the bridge has not been designed to dampen this effect it can break apart. (See Galloping Gertie. 1940 Bridge Tacoma Narrows Bridge Collapse. Watch for guy who saves dog from car. )