Validation of a novel virtual reality platform for investigating pedestrian-pedestrian interaction in the context of structural vibration serviceability

Pedestrian-pedestrian interaction (PPI) is one of the fundamental mechanisms purported to influence the amplitudes of structural response under the action of a walking crowd. This is because a pedestrian is likely to alter their gait due to the presence of other pedestrians, which in turns alters the magnitude of structural loading. However, little empirical data are currently available to assess the effect of PPI in the context of vibration serviceability. This is mainly due to logistical challenges in assembling and instrumenting a crowd of walking pedestrians, and the associated cost. To this end, a novel virtual reality platform is developed for experimental investigation of pedestrian-pedestrian interaction. In comparison to real-world crowd testing, the platform enables experimental protocols to be implemented repeatedly in a highly controlled environment while collecting a rich set of data on pedestrian behaviour. The platform incorporates state-of-the-art technology for motion capture, artificial intelligence and three-dimensional computer modelling, and comprises of three core modules: (i) the environment, (ii) the crowd and (iii) the user interface enabling real walking behaviour. To assess the validity of the platform for investigating PPI, tests were conducted to quantify gait synchronisation between a pair of walking pedestrians. The pair of pedestrians consisted of either two real humans or a real human and an avatar generated within a fully immersive VR environment. The test subject was either not explicitly asked to or specifically asked to synchronise their gait while walking side-by-side or front-to-back. It was found that walking with an avatar yields qualitatively the same results as walking with a real person, whether that is with or without the instruction to synchronise gait. However, the results differ quantitatively in terms of the synchronisation strength and the directionality

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

Optimal Design for the Blunt Trailing-Edge Profile of Wind Turbine Airfoils under Glaze Ice Conditions

Glaze ice is more likely to occur on the rotating blade, and greatly decreases the energy utilization efficiency of wind turbines. Moreover, due to its complex and irregular shape, a high-quality grid and more grid cells are needed in aerodynamic calculation. To improve this situation, this study develops a novel multiobjective optimization method for the blunt trailing edge of airfoils under glaze ice conditions. The parametric representation of the asymmetric trailing-edge profile is given by the B-spline function. The aerodynamic coefficients of the airfoils without and with glaze ice are calculated using the computational fluid dynamics (CFD) method and back propagation (BP) neural network, respectively. The update mode of the potential well center of nonoptimal particles is modified by the social learning and the optimal particle position is identified using the Lévy flight and greedy algorithm for quantum particle swarm optimization (QPSO) algorithm. The optimizer based on the improved QPSO algorithm integrated with CFD method and BP network seeks the trailing-edge control parameters maximizing the lift coefficient and lift-drag ratio. The lift and drag coefficients, lift-drag ratios, and pressure contours of the original and optimized airfoils are investigated before and after icing. Significant improvements of the aerodynamic performance are achieved in this process, confirming that the presented method constitutes a valuable tool for the airfoil design of wind turbines operating in icing conditions

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

Dynamic performance verification of the Rędziński Bridge using portable camera-based vibration monitoring systems

The assessment of dynamic performance of large-scale bridges typically relies on the deployment of wired instrumentation systems requiring direct contact with the tested structures. This can obstruct their operation and create unnecessary risks to the involved personnel and equipment. These problems can be readily avoided by using non-contact instrumentation systems. However, the cost of off-the-shelf commercial products often prevents their wide adoption in engineering practice. To this end, the dynamic performance of the biggest one-pylon cable-stayed bridge in Poland is investigated based on data from a consumer-grade digital camera and open access image-processing algorithms. The quality of these data is benchmarked against data obtained from conventional wired accelerometers and a high-end commercial optical motion capture system. Operational modal analysis is conducted to extract modal damping, which has a potential to serve as an indicator of structural health. The dynamic properties of the bridge are evaluated against the results obtained during a proof loading exercise undertaken prior to the bridge opening. It is shown that a vibration monitoring system based on consumer-grade digital camera can indeed provide an economically viable alternative to monitoring the complex time-evolving dynamic behaviour patterns of large-scale bridges