Pedestrian lateral foot placement and lateral dynamic instability of bridges

The most often purported mechanism causing the lateral dynamic instability of the London Millennium Footbridge is the synchronisation of footsteps to the lateral structural motion. However, evidence from full-scale measurements and treadmill tests has challenged this notion. Instead, an active control of foot placement is advocated to be the source of destabilising forces to the structure, occurring even without synchronisation. This is to say that, while walking on a laterally oscillating surface, pedestrians maintain their balance primarily by controlling the position of their feet, rather than adjusting the timing. Similar behaviour was previously observed in experimental tests measuring the response of pedestrians to an impulsive perturbation of gait. The analysis of the collected data suggested a simple linear foot placement control law, whereby the position of the foot at the instant of foot placement immediately following the perturbation depends on the instantaneous lateral velocity of the centre of mass and a constant offset. However, it is has been uncertain whether the same foot placement control law applies while walking on laterally oscillating structures. To test this proposition, an experimental campaign was conducted on a laterally oscillating treadmill with a test subject monitored with an optical motion capture system. The motion of the body centre of mass and the position of the feet were identified and analysed. It was found that a simple linear foot placement control law applies. Further tests were conducted to test the influence of the visual information on pedestrian stepping behaviour using virtual reality delivered via a head mounted display. It was found that the identified foot placement control law is very robust for different walking surface conditions and visual environments

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 pedestrians’ 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