The kinematics and kinetics of pedestrians on a laterally swaying footbridge

Progress in understanding human-structure interaction (HSI) on footbridges has been hampered by the shortage of quality data collected in realistic environments. This paper reports a novel experiment conducted on a naturally-swaying 7 m footbridge of frequency 0.67 Hz and amplitudes up to 125 mm. Subjects crossed the bridge while wearing infrared motion-tracking markers and pressure-sensing insoles. The bridge was fitted with bespoke force plates, allowing investigators to simultaneously record kinematic and kinetic reactions to the structure's motion, which was assumed simple harmonic. The bridge was naturally excited by the test subjects, who were allowed to walk at a comfortable self-chosen pace. The data show that the subjects adopted a fixed-in-space Centre of Mass (CoM) strategy but their Centres of Pressure (CoP) were highly correlated to the position of the bridge deck within its lateral oscillation cycle (henceforth 'bridge phase'), allowing for the prediction of wide and crossed steps. Ground forces generally correlated to CoM-CoP separation, which reflected the phase of the bridge at the previous heel-strike. Instantaneous step width was correlated to the bridge phase and is proportional to the offset in the Medial-Lateral (M-L) ground force between consecutive steps. The Inverted Pendulum Model (IPM) was evaluated using the data, exhibiting a limited fit to the recorded ground forces. Finally, the pedestrian-induced work on the bridge and system stability boundaries are also calculated, revealing mechanisms for bridge instability to occur.