Crowd loading of bridges can become a source of undesirable vibrations. A canonical example of this is the widely publicised wobbly behaviour of the London Millennium Footbridge on its opening day. Much effort has been made in recent years to understand the cause of this problem in the case of walking pedestrians. However, little attention has been given to the effects of running crowds.
With the proclaimed “renaissance in running”, running crowds are becoming a common sight in urban environments. An increasing number of marathons are organised from year to year in major cities around the globe. These events are often celebrated by large crowds of spectators, accompanied by music (scroll to 2:10 to see a brass band crossing Bosporus Bridge during Istanbul marathon in 2014) across major bridges. While it is well known that spectators can react to the music, often synchronising their actions to its beat, how the music affects the behaviour of runners is uncertain. Uncertainty also exists in interactions between runners that might prompt synchronisations of their footsteps. However, synchronised groups of runners could input large amplitude resonant force to the bridge, causing a build-up of undesirable vibration, similar to those evidenced on the London Millennium Footbridge.
In order to shed light on this problem, a series of tests have been conducted at Exeter Arena. Wireless headphones were used to provide auditory signals, prompting some runners in a group to synchronise their steps, while some other runners were unaware of this stimulus, hence allowed to act freely. Data on the motion of each runner was collected using wireless sensors. This data was used to analyse synchronisation between runners, as can be seen on the plots below.
The top image: patches with alternating colours indicate two runners stepping at different frequencies (taking different number of steps to cover the same distance in a given time). The bottom image: red patches indicate periods at which two runners were stepping at the same frequency.
The presented preliminary results indicate that synchronisation between runners can indeed occur spontaneously or prompted by the beat of music, hence the problem of excitation of structures under running crowds requires further investigation.