Serviceability assessment of footbridges when subjected to vibrations induced by running pedestrians

Sammanfattning: Vibration serviceability in the design of footbridges is gathering enormous prominence as comfort restrictions get enhanced. Comfort verifications are often becoming critical when considering human induced dynamic loading on lightweight structures, which are increasing in slenderness and flexibility. The aim of this work was to build up understanding about the running load effects on the response of footbridges and proving that it could imply a critical load case that would require verification. Additionally, the accuracy of potential models to estimate the structural response was evaluated. Finally, aiming for a practical application, this work provides a step forward towards the possibility of adopting a simplified design methodology to be included in the future guidelines and an insight into the potential effects of a marathon event. While the walking load case is a well-studied phenomenon, not much attention has been paid to the running induced excitation. Guidelines motivate that there is no need for verification and exceptionally, some get to suggest a time domain load model definition. The interaction phenomena as well as the effects of groups of runners in the dynamic response of the structure remain still unknown. Limiting the work to the vertical component of the response and force and based on a large set of additional assumptions, experimental and numerical analyses were performed. Three footbridges were tested and subjected to tests involving different motion forms; jumping, walking and running. On the other hand, the time domain load models available in the literature were applied accounting for the spatial displacement of each of the pedestrians along the footbridge. In the most advanced of the models, aiming to account for interaction effects, the subjects were modelled as independent mechanical systems. The results derived from the experimental study helped characterizing the running load effect on the footbridge's response and proved that there may be structures in which running could comprise a critical load case. Furthermore, the numerical analyses allowed to verify the accuracy of the suggested models and the improvement that the human structure interaction effects involve. The analyses resulted in complementary sets of conclusions that built up understanding about the running load effects on footbridges; such as the sensitivity of the estimated response to the structure's modal properties and the influence of the parameters that characterize the running motion. Finally, the suggested simplified design methodology was able to estimate, with a very reasonable error for the current case study, the calculated response by the most accurate of the models. To sum up, this work serves as a motivation to include the running load case in the guidelines and establishes a starting point for further research and simplified design methodologies based on the strategy and models suggested in this work.