Human-structure dynamic interaction - Wobbly bridges and bouncy grandstands Seminar

On its opening day, the London Millennium Bridge experienced unexpected large oscillations due to crowd loading. This form of pedestrian-structure interaction has since been identified on several other bridges of various structural forms. The mechanism has commonly been attributed to ‘pedestrian synchronous lateral excitation’. However, some site measurements show a lack of evidence of pedestrian synchronisation, including on the Clifton Suspension Bridge, results from which will be presented. In contrast to other research on the dynamic excitation, the approach to be presented considers the basic mechanism of human balance, adopting a simple model from the biomechanics field. This gives the surprising result that pedestrians walking randomly, keeping balance as normal, can effectively act as negative dampers to the bridge motion. This is in agreement with the empirical model developed by Arup from measurements on the London Millennium Bridge, leading to divergent amplitude vibrations above a critical number of pedestrians. Results from a state-of-the-art instrumented oscillating treadmill rig will also be presented, which compare favourably with the proposed model. The presentation will also include recent analysis of a different form of human-structure dynamic interaction – jumping on a vertically oscillating structure such as a grandstand. Analysis of a simple jumping model exhibits chaos, similar to features observed in laboratory tests in which it was not possible to jump regularly at resonance with the structure.