Daniel Claff

Doctor of Philosophy, Keble College, University of Oxford, Michaelmas Term 2015

Pedestrian-Induced Lateral Excitation of Footbridges

This thesis investigates human-structure interactions between pedestrians and oscillating footbridges via experimental kinematic and kinetic tests. The first aspect was to improve and validate a simple frontal plane gait model, the Inverted Pendulum Model (IPM), based on kinematic and kinetic gait data for stable ground walking. Next, test subjects were recorded while crossing a laterally swaying footbridge in order to examine kinematic and kinetic walking patterns and assess the model's accuracy at predicting unstable gait.

Participants were recorded walking over force plates in a gait laboratory so their normal ground forces could be compared to each other and the IPM. High inter-subject variability and low intra-subject variability were observed. The IPM did not reproduce transient components of the ground forces. An analysis of the IPM's inherent assumptions revealed that some were inappropriate. A Modified Inverted Pendulum Model (MIPM) is proposed, eliminating some of the IPM's assumptions. For all samples examined, the correlation between the real ground forces and the MIPM was higher than that of the IPM.

Custom-designed force plates were installed into a novel laboratory footbridge rig. The footbridge was excited naturally by the participants' walking and the participants responded naturally to the swaying of the bridge. The participants' step widths could be predicted by the phase of the structure at the previous heel strike. At high structural amplitudes, CoP and ground force patterns were dominated by the motion of the structure. Centreof Mass (CoM) motion was found to be `Fixed-in-space' with patterns dissimilar to those anticipated by the IPM. The MIPM was typically better than the IPM at predicting ground forces on the moving base.

Finally, a spherical model was compared to the two-dimensional MIPM. The model exhibited few discrepancies to the spherical kinematic data, but the predicted medial-lateral ground forces were significantly different to theforce plate data.

Thesis (8.6Mb, pdf)