Increasing strength of new structural materials and longer spans of new footbridges, accompanied with aesthetic requirements for greater slenderness, are resulting in more lively footbridge structures. In the past few years this issue attracted great public attention. The excessive lateral sway motion caused by crowd walking across the infamous Millennium Bridge in London is the prime example of the vibration serviceability problem of footbridges. In principle, consideration of footbridge vibration serviceability requires a characterisation of the vibration source, path and receiver. This paper is the most comprehensive review published to date of about 200 references which deal with these three key issues.The literature survey identified humans as the most important source of vibration for footbridges. However, modelling of the crowd-induced dynamic force is not clearly defined yet, despite some serious attempts to tackle this issue in the last few years.The vibration path is the mass, damping and stiffness of the footbridge. Of these, damping is the most uncertain but extremely important parameter as the resonant behaviour tends to govern vibration serviceability of footbridges.A typical receiver of footbridge vibrations is a pedestrian who is quite often the source of vibrations as well. Many scales for rating the human perception of vibrations have been found in the published literature. However, few are applicable to footbridges because a receiver is not stationary but is actually moving across the vibrating structure.During footbridge vibration, especially under crowd load, it seems that some form of humanstructure interaction occurs. The problem of influence of walking people on footbridge vibration properties, such as the natural frequency and damping is not well understood, let alone quantified.Finally, there is not a single national or international design guidance which covers all aspects of the problem comprehensively and some form of their combination with other published information is prudent when designing major footbridge structures. The overdue update of the current codes to reflect the recent research achievements is a great challenge for the next 5-10 years. Abbreviations:ASD-auto spectral density; DLF-dynamic load factor; DOF-degree of freedom; FE-finite element;FRF-frequency response function; MDOF-multiple-degree-of-freedom;MTMD-multiple tuned mass damper; RMS-root-mean-square;SDOF-single-degree-of-freedom; TLD-tuned liquid damper; TMD-tuned mass damperThis paper has been published under the following reference:Živanović, S., Vibration serviceability of footbridges under human-induced excitation: a literature review.

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A limitation of existing walking models used for vibration serviceability assessment of structures carrying pedestrians is that they are typically based on direct measurements of single footfalls replicated at precise intervals. This assumption of 'perfect periodicity' allows walking forces to be modelled as a Fourier series of sine waves having frequencies of the walking pace and its integer multiples. The true imperfection and randomness in walking is currently not taken into account even in more advanced dynamic loading codes of practice and leads to an unknown degree of conservatism.Having this in mind, this paper examines real continuous walking forces obtained from an instrumented treadmill and the effect of their random imperfection through time and frequency domain simulations of structural response.The main conclusions are that there are significant differences between responses due to the imperfect real walking forces and the 'equivalent' perfectly periodic simulation.These differences are most significant for higher harmonics where the simulated vibration response overestimates the real-life behaviour, sometimes significantly. This is mainly due to random imperfections in real walking. Given that a more realistic representation of imperfect walking is an auto-spectral density function, the random character naturally leads to a stochastic approach to treatment of pedestrian loading applied in the frequency domain. The approach can be used for single pedestrians but the 3 benefits are greater for crowd loading where correlation between pedestrians as well as statistics of their pacing rates can be applied directly.

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