Characterisation of crowd lateral dynamic forcing from full-scale measurements on the Clifton Suspension Bridge

Lateral loading of bridges by a crowd of walking pedestrians is of serious concern as it can lead to a sudden growth in the amplitude of structural oscillations, i.e. lateral dynamic instability. A vibration amplitude threshold, marking a qualitative change in pedestrian behaviour, is then usually proposed beyond which the likelihood of structural instability is said to increase. To verify this presumption, measurements were taken during a crowd loading event on Clifton Suspension Bridge in Bristol, UK. Two lateral modes of the bridge were studied, previously found susceptible to pedestrian-induced excitation. A novel procedure is proposed based on time-frequency analysis enabling, for the first time, the average equivalent added mass per pedestrian to be identified from measurements on a full-scale structure. Previous measurements on Clifton Suspension Bridge during crowd loading leading to the onset of large-amplitude vibrations revealed an increase in the natural frequency of one from the two considered modes. The proposed time-frequency analysis procedure has successfully identified the additional mass, due to the pedestrians, that is effectively negative. Cycle-by-cycle energy analysis per mode confirms the presence of additional damping of the pedestrians at low vibration amplitudes, that is also effectively negative. Although some of the results are uncertain quantitatively, there is no evidence of the amplitude threshold at which the human-structure interaction phenomenon occurs.