Research On Pedestrian-Induced Vibration Of Footbridge

With increasing use of light and high-strength materials and aesthetic demand of structural appearance, newly constructed modern footbridges tend to have larger spans, less mass and lower damping. In recent years several footbridges have suffered from excessive pedestrian-induced lateral vibration which severely influences the comfort of pedestrians on the footbridges or even the structural safety. Vibration serviceability of footbridges has been concerned on in bridge engineering. Literature survey shows that the characteristic of lateral pedestrian forces have not been well understood up to now. Analysis on human-induced vibration of footbridges was based on many hypotheses and simplification, but the results do not accord with actual situation very well. In this thesis, the characteristic of lateral pedestrian forces, the mechanism of human-induced vibration of footbridges and the method to assessing vibration serviceability were studied. The main contents of the research include:1. The experiments for measuring pedestrian forces were carried out by using a shaking table test and a SDOF structural model free vibration test. The pedestrian forces induced by a person were measured when the shaking table or the SDOF model harmonically oscillated with various frequencies and amplitudes. And the pedestrian walking manner under the lateral vibration was observed. It shows that the influence of the lateral vibration is more effective to the lateral pedestrian forces than to vertical forces. The lateral forces of synchronized pedestrians gradually increase with the amplitude of lateral vibration until the pedestrians cannot walk normally. An approximate linear relationship between the 1st DLF of the lateral force and vibration amplitude was founded. When the pedestrians synchronize with the lateral vibration, it shows that they try to keep a steady relative movement with the vibration which can be quantified by the phase lag between lateral forces and the swaying displacement. The probability of synchronization of pedestrians is also dependent on the frequency and amplitude of vibration in lateral direction. According to the experimental results, a model of self-excited synchronized lateral forces varying with vibration was proposed which can be seen as additional stiffness and damping.2. Analytic theoretics on pedestrian-induced vibration of footbridges was compared and discussed in detail, including forced resonance vibration, self-excited vibration, parametric vibration and random vibration. If pedestrian forces are expressed by Fourier series, then pedestrian induced vibration can be simplified as forced resonance vibration. While the conventional theory of forced vibration cannot describe human-structural interaction and mechanism of pedestrian synchronization. According to the model of synchronized lateral forces based on the experimental study and the expression of random walking force based on random vibration theory, a self-excited vibration method for simulating pedestrian-induced vibration of footbridges was established which reflected the whole responses of random walking and synchronized walking. Along with the vibration amplitude increases, the lat- eral pedestrian forces gradually transform to synchronization from random walking. It shows that the response of lateral vibration induced by pedestrian-footbridge interaction could be well estimated by using this method. Parametric vibration was introduced recently to explain pedestrian induced vibration of some footbridges. The analysis to London Millennium footbridge shows that its lateral vibration and vertical vibration are coupled with each other, so it is potential to induce parametric lat- eral vibration by vertical pedestrian forces for some formal footbridges. Pedestrian induced vibration of footbridges can also be analyzed in frequency domain with random vibration theory.3. The evaluating methods and comfort criteria of pedestrian subjected to bridge vibration were summarized and compared with the methods and criteria for floors and tall buildings. The crowd loads and comfort criteria for footbridges were discussed. Based on the studies on vibration serviceability and deficiency of current methods, an improved method to evaluating pedestrian-induced vibration serviceability of footbridges was established.4. As an example, the vibration serviceability of a footbridge was evaluated. The mode shapes and natural frequencies were analyzed. It shows that there are several natural frequencies close to frequencies of pedestrian forces. The lateral modes close to 1Hz are potentially excited by synchro- nized human walking. The time histories of response were simulated with using forced resonance, self-excited vibration or random excited vibration methods. Finally vibration serviceability of the footbridge was assessed.