Dynamic Behaviour of Footbridges Subjected to Pedestrian-Induced Vibrations Studies

Abstract: Over the last years, the trend in footbridge design has been towards greater spans and increased flexibility and lightness. As a consequence, stiffness and mass have decreased which has lead to smaller natural frequencies and more sensitivity to dynamic loads. Many footbridges have natural frequencies that coincide with the dominant frequencies of the pedestrian-induced load and therefore they have a potential to suffer excessive vibrations under dynamic loads induced by pedestrians. The main focus of this thesis was on the vertical and horizontal forces that pedestrians impart to a footbridge and how these loads can be modelled to be used in the dynamic design of footbridges. The work was divided into four subtasks. A literature study of dynamic loads induced by pedestrians was performed. Design criteria and load models proposed by four widely used standards were introduced and a comparison was made. Dynamic analysis of the London Millennium Bridge was performed using both an MDOF-model and an SDOF-model. Finally, available solutions to vibration problems and improvements of design procedures were studied. The standards studied in this thesis all propose similar serviceability criteria for vertical vibrations. However, only two of them propose criteria for horizontal vibrations. Some of these standards introduce load models for pedestrian loads applicable for simplified structures. Load modelling for more complex structures, on the other hand, are most often left to the designer. Dynamic analysis of the London Millennium Bridge according to British and International standards indicated good serviceability. An attempt to model the horizontal load imposed by a group or a crowd of pedestrians resulted in accelerations that exceeded serviceability criteria. The most effective way to solve vibration problems is to increase damping by installing a damping system. Several formulas have been set forth in order to calculate the amount of damping required to solve vibration problems. However, more data from existing lively footbridges is needed to verify these formulas.