Crowd-induced lateral bridge vibration

• Main Author: Carroll, Seàn P.
• Published: University of Nottingham 2013
• Subjects: 624.252; TG Bridge engineering
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.594602

Vibration induced by walking pedestrians has motivated research in the civil engineering community for many years. An area within this broad field that has received particular attention is the dynamic interaction that can occur between pedestrians and laterally flexible bridge structures. Perhaps the most notable example occurring on the opening day of London's Millennium Bridge. The enduring interest in this research problem is fuelled by two of its key features; (i) the sensitivity and adaptability of human balance to lateral motion and (ii) the spatial and temporal variation in flow characteristics exhibited by a pedestrian crowd. Both of these features are addressed herein. In this project an experimental campaign was executed with the aim of identifying the interaction mechanism by which pedestrians produce force harmonics, that resonate with the oscillating structure on which they walk. These so-called self-excited forces have been experimentally identified by others but the underlying reason for their existence has remained an open question. In an effort to address this, human balance behaviour while walking on a laterally oscillating treadmill was recorded using 3-dimensional motion capture equipment. Subsequent analysis revealed that human response to sinusoidal base motion is dominated by periodic alteration of foot placement position. This produces amplitude modulation of the lateral component of the ground reaction force and is ultimately responsible for the self-excited force harmonics. It was further revealed that human centre of mass motion while walking on an oscillating structure is predominantly passive. The passive inverted pendulum model is thus an excellent model of pedestrian frontal plane balance. The second facet of this work is concerned with developing a crowd-structure interaction model that builds upon the current state of the art. The model presented utilises the understanding of human-structure interaction identified above and employs an agent-based modelling approach. Thus, the resulting 'virtual crowd' is capable of simulating key crowd features, such as inter-subject variability and emergent velocity-density flow behaviour. Using this model, it is shown that the experimentally identified human-structure interaction mechanism can lead to large amplitude lateral deck oscillations, consistent with field observations reported in the literature. The model successfully predicts the multi-mode instability of Bristol's Clifton Suspension Bridge in the absence of step frequency tuning among the crowd. This provides supporting evidence for the model's validity. The work described above has resulted in a clearer understanding of the feedback between pedestrian balance behaviour and bridge response. Furthermore, the modelling techniques developed have potential for application in the wider study of crowd-induced vibration of dynamically susceptible structures.