Abstract Summary
Slender footbridges are predominantly sensitive to human-induced vibrations. While in the past two decades the main focus was on pedestrian-induced vibrations, topical research questions investigate the dynamic impact of running actions. In comparison to the walking load, these running actions involve higher load amplitudes, a different dominant frequency content and discontinuous contact between the feet and the footbridge. The load model that is currently applied for running actions in the Vibration Serviceability Assessment (VSA) of footbridges is only applicable for a single runner and does not account for Human-Structure Interaction (HSI) phenomena. These phenomena involve (1) active HSI where the body locomotion is influenced by the motion of the supporting structure and (2) passive HSI where the human body acts as a mechanical system resulting in a coupled human-structure system with modified modal parameters. For walking, HSI has a normative impact on the resulting structural response and therefore on the VSA of footbridges. The question now is whether HSI also occurs for running actions and if this is relevant to be accounted for in the VSA of footbridges. To facilitate the identification, characterization and eventually the development of load models including HSI for running, experimental data is required. The present work presents the results of an extensive measurement campaign that has been performed involving 8 participants to investigate active HSI for running actions. A treadmill is placed on a vibrating footbridge that is excited by a shaker at a desired frequency and structural acceleration level. For reference purposes, the measurements are repeated on a rigid laboratory surface. The applied measurement setup is suitable for both indoor and infield measurements and allows the simultaneous registration of the contact forces, the body motion and the structural response. The participants are equipped with in-shoe pressure sensors and an accelerometer fixed to the lower back. In addition, a treadmill is instrumented with an optical movement analysis system to capture multiple relevant running motion metrics, such as the flight and contact time, on a step-by-step basis. Analysis of the running metrics indicates the existence of active HSI and the vital role it may play in the VSA of footbridges. Depending on the amplitude of the footbridge vibrations and ratio between the pacing rate and the frequency of the footbridge vibrations, the normal running locomotion of the participants is modified. The modification in running motion furthermore has a clear impact on the resulting structural response.
