Numerical and experimental study of crowd-structure interaction.

For slender and lightweight structures, vibration serviceability is a matter of growing concern, often constituting the critical design requirement. In recent years, special interest emerged for structures subjected to human-induced loading, in particular for footbridges where the design freedom is restricted by fewer functional and structural demands. Currently, designers are forced to rely on - what are assumed to be conservative - equivalent load models, upscaled from single-person force measurements. The concerns of vibration comfort and safety are strengthened by unexplored human-structure interaction (HSI) phenomena. With designs governed by the dynamic performance under high crowd densities, a strong demand exists for the verification and refinement of the currently available load models.
This work therefore addresses the experimental identification and analytical modelling of crowd-induced loading with a specific focus on the vertical component and corresponding HSI phenomena. Given the scope of the vibration serviceability problems of footbridges, focus is on the low-frequency dynamic behaviour (<10 Hz) of the footbridge and the pedestrians.  An extensive experimental study is performed to identify the relevant dynamic properties of the footbridge and the human body. In addition, a methodology is developed for the ambulatory characterisation of the walking behaviour using an innovative 3D inertial motion tracking technique. By comparing the measured and predicted structural response, it is demonstrated that the in situ identified pacing rate and synchronisation among the pedestrians constitute an essential input for the simulation and verification of the human-induced loads.
A comprehensive parametric study is performed to investigate the mechanical interaction between the crowd and the supporting structure. It is found that HSI-effects are primarily determined by the natural frequency of the footbridge and the crowd to structure mass ratio. The most significant effect of HSI is in the effective damping ratio of the coupled system which is much higher than the inherent structural damping. A comprehensive full-scale experimental study is performed on two real footbridges to verify the numerical findings. It is shown that the coupled human-structure model is able to reproduce the experimentally identified modal characteristics of the coupled system.
Finally, a numerical model for pedestrian excitation including HSI is proposed. The impact of HSI on the structural response is evaluated for various pedestrian densities and footbridge parameters. It is shown that by taking into account HSI, the structural response is reduced. Furthermore, it is demonstrated that the unrealistic high acceleration levels as predicted by simplified force models are not reached as the result of HSI. It is concluded that the mechanical interaction with the crowd is relevant for the vertical low-frequency dynamic behaviour of footbridges. Moreover, the associated reduction in structural response is sufficiently large for consideration in design. Design procedures which disregard HSI may therefore lead to over-conservative designs.