Case Studies

Do the Locomotion With Me – Virtual Reality Interface for Locomotion on Flexible Structures

Author: Dr Mateusz Bocian

Virtual Reality is an enabling technology rapidly growing in its sophistication and reach. Further research studies using virtual reality as found in VSimulators will enable better characterisation of pedestrian behaviour on flexible structures such as footbridges, allowing the whole complexity of bi-directional human-structure and human-human interaction to be captured. In this case study, Dr Mateusz Bocian at the University of Leicester explores research developments in this field, from the early use of virtual reality and a treadmill through to the development of the VSimulators research facility.

Walking is a seemingly simple everyday activity serving to move the body in space. This is achieved by pushing feet against the ground, generating ground reaction forces. The nature of these forces is well understood – we are pushing up against the ground to support the body against the action of gravity, we are pushing back to propel the body forward, and we are pushing sideways to enable bipedal gait and ensure balance is maintained. Nevertheless, modelling pedestrian ground reaction forces remains a challenge in the structural engineering community.

The forces exerted by walking pedestrians on the ground are the primary concern in the design of many modern structures. The canonical examples are slender lightweight footbridges incorporating long spans along one dominant dimension. The structural form renders these structures susceptible to response due to the presence of walking pedestrians. This can be easily verified if you stand close to the midspan between supports of any footbridge built in the last 20 years – you are likely to feel vibrations coming through your feet. Under certain circumstances, the vibrations’ amplitude can grow rapidly beyond the point marking the comfort zone of bridge users, causing them to stop in their stride. In extreme cases, this can lead to panic, stampedes and loss of life.

There have been numerous cases of bridge instability reported in recent years all over the world. The prima facie explanation of this phenomenon was simple synchronisation of pedestrian footsteps to the bridge motion, causing direct resonance. However, there is now a growing body of evidence challenging this notion. It is evident that pedestrians do not act as purely external loads to the structure but can modify their behaviour, hence loading, in response to the bridge motion. This can, in turn, modify structural behaviour, and so a feedback loop emerges. Another complexity arises, as walking in the presence of other people can significantly alter pedestrians' behaviour.

A new approach to investigating human behaviour on structures is therefore required, allowing the whole complexity of bi-directional human-structure and human-human interaction to be captured. This should enable a continuous measurement of ground reaction forces over many gait cycles, while preserving the ecological complexity of real world settings. The latter condition must be met because humans are highly adaptive systems, capable of sensing and reacting to environmental changes to optimise their performance.

A viable solution to this challenge could be achieved by integrating a platform capable of delivering a wide range of motion representative of real structural behaviour with a self-paced instrumented treadmill. To ensure sensory experience representative of real world environment, virtual reality (VR) can be provided.

The first attempt at realising this ambition was made during a doctoral project of Dr Mateusz Bocian, now a lecturer at the University of Leicester, co-supervised by Dr John Macdonald and Dr Jeremy Burn, both from the University of Bristol. The results from an experimental campaign with pedestrians walking on a laterally-oscillating treadmill, representing conditions during the instability period of the London Millennium Footbridge, revealed the influence of visual information on the magnitude of pedestrian loading. A new “phase pulling” mechanism in pedestrian stepping behaviour was also discovered which, in the absence of convincing evidence of widespread synchronisation, could be responsible for generating large magnitude destabilising forces to the structure.

VR is an enabling technology rapidly growing in its sophistication and reach. Further studies utilising VR are expected to enable better characterisation of pedestrian behaviour on flexible structures. The ultimate goal with the VSimulators facilities is the introduction of new provisions for modelling structures against human-induced loading and inclusion of these into codified design guidelines and best practice recommendations.

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