14. Time-Frequency Analysis of Crowd Lateral Dynamic Forcing from Full-Scale Measurements on the Clifton Suspension Bridge

Pedestrian-induced vibrations have caused significant issues on various footbridges, over the past few decades, as it can lead to an abrupt growth in the amplitude of structural oscillations, i.e. lateral dynamic instability. Measurements were taken during two separate crowd loading events on the Clifton Suspension Bridge, UK, to characterise the human-structure interactions observed. Two lateral modes of the bridge were studied, previously found susceptible to pedestrian-induced excitation. A comparative study is performed to identify the different interactions observed for the differing bridge deck amplitude responses through a novel procedure based on time-frequency analysis. This enables the identification of the fluctuations on the instantaneous modal amplitude and resonant frequency during pedestrian loading. Previous measurements of Clifton Suspension Bridge during crowd loading leading to the onset of large-amplitude vibrations revealed a significant increase in the natural frequency of the two modes considered. The instantaneous frequency, of both modes, appeared to roughly mirror the displacement amplitude response during significant loading, >250 pedestrians. Recent measurements of Clifton Suspension Bridge during crowd loading, illustrated tentative evidence for the presence of human-structure interactions during low-amplitude responses. For high amplitude responses on the bridge the modal frequencies were observed to increase with an increase in amplitude displaying a complex non-linear hardening effect for the two modes of vibration.