Lateral Pedestrian Excitation of Footbridges

The phenomenon of synchronized lateral pedestrian excitation of footbridges has been highlighted in recent years by the widely publicized problems encountered with the London Millennium Footbridge. On opening day, during which a large number of pedestrians crossed the bridge, it exhibited alarming and unstable amplitudes of lateral vibration, which resulted in an 18 months closure and a 7 million dollar retrofit. Subsequent investigations revealed that the dynamic characteristics of the bridge did not differ significantly from those of other footbridges of similar span, and that the phenomenon of synchronized lateral excitation had recently been exhibited by several other bridges. An analysis of lateral pedestrian excitation of bridges, based on the governing differential equations of motion, is presented. Analytical models of lateral pedestrian loading are developed which can be used to assess both the subcritical response and the critical number of pedestrians likely to induce synchronized excitation. Theoretical predictions of synchronization are compared, and show close correlation, with the results of full-scale tests on the Millennium Footbridge.