Abstract:
Over the last few years, the trend in footbridge design has been towards greater spans and lightness. Once followed, such trend gives increased flexibility in dynamic behaviour. As a consequence, stiffness and mass sometimes decrease and lead to smaller natural frequencies. In practice, such footbridge has particularly been found to be more sensitive to dynamically imposed pedestrian loads. The reason behind the sensitivity in movement is known to be related with coincidence of fundamental natural frequency of superstructure with the dominant frequencies of the pedestrian load. In such cases footbridge has the potential to suffer excessive vibrations.
The vertical and horizontal forces that pedestrians impart to a footbridge are considered in the current work for using a modeling procedure in finite element technique to obtain design of some prototype footbridges by considering biomechanics of pedestrian movement and human-structure interaction induced synchronization effects. The work started with a literature review of dynamic loads induced by pedestrians. Design criteria and load models proposed by several widely used standards were introduced and a comparison was made. Dynamic analysis of two footbridges having different structural system has been performed using several modeling techniques to make comparisons. Available solutions to vibration problems and improvements in design procedures were exemplified.
The work further investigates the optimization of a structural system and its effect, the effect on different stiffening mechanisms, vibration modes and the fundamental natural frequencies using finite element models. Different patterns of pedestrian loading have been imposed and dynamic response of as-built structure is compared with analytical predictions. The synchronization effect due to pedestrian movement has been also investigated for the prototype cases. Human perception of vertical and horizontal vibration and their interaction with bridge movement has been studied with respect to vibration serviceability. To this end, the complex issues of human reactions to vibration and next walkup modes are discussed.
Available solutions to vibration problems and improvements of design procedures are studied. It is shown that the requirements in the codes for design of this class of structure widely varies because of the poor understanding of the complex human-structure interaction phenomena and associated bio-mechanical problems. The study and results indicate the necessity of further field measurements to analyze the human-structure dynamic interaction in footbridges to further rationalize the available design codes. In spite of this, the study indicates a better rationality in BS 5400 and ISO 10137 than other codes for taking care of lateral and vertical vibration modes. In order to resist such vibrations, structural system needs to be optimized in the way either by adjusting mass and stiffness in the dominant mode of vibration or by increasing the damping properties. Based on all such observations and keeping the variability of material, support and boundary conditions, the study have further shown the possibility of having a better performance in a hanger supported structure than in a longer span simply supported one.
