Pedestrian-induced Vibration And Its Control For Footbridges

With the increasingly application of new high-strength materials and theimprovement of structural design and construction technologies, as well as tendancyof using slender structures due to its elegant appearance, the footbridge naturalfrequencies become lower. When footbridges have natural (vertical or lateral)frequencies that coincide with the dominant frequencies of the human-induced loadand therefore they have a potential to suffer excessive (vertical or lateral) vibrationsunder dynamic loads induced by pedestrians, which could affect user comfort or evendamage the structures. The problem of vertical vibration of footbridges induced bywalking pedestrians is addressed in this study. By taking a lively footbridge inMingyang, China, the accuracy of three methods for analyzing pedestrian-inducedvertical vibrations is compared, and the practical problems in using tuned massdampers (TMD) for vibration control is discussed. The main contents and conclusionsof this thesis are listed as follows:(1) A detailed introduction into the human-induced force and time-domain forcemodel is presented, and then the pedestrian load models and vibration serviceabilityapplications in four foreign footbridge design codes/guidelines are also introducedand compared the differences with each other.(2) The vertical vibration of footbridge caused by passing of a single pedestrianis formulated based on the forced vibration theory. Three methods for analyzingfootbridge vertical vibrations due to pedestrian streams are introduced, which includeMonte Carlo method, random vibration method and the response spectrum method.The vertical vibrations of a lively footbridge are analyzed for the case of a singlepedestrian and the case of pedestrian streams. The results indicate that all the threemethods give similar results when the mean pacing rate is close to the footbridgefundamental frequency and that the result of the response spectrum method is notaccurate when the pacing rate differs significantly from the fundamental frequency ofthe bridge.(3) The footbridge response is also analyzed under two crowd densities and twomean pace frequencies. When the crowd density is0.2people/m~2, the verticalfootbridge responses conform to all codes requirements for comfort level under bothstep frequencies. While the crowd density is1.0people/m~2, the comfort level of the footbridge is acceptable for the mean step frequency of1.82Hz, but it is unacceptablefor the frequency of2.0Hz according to the ISO10137and Eurocode5.(4) The principles of tuned mass dampers as well as optimal parameter design areintroduced and tuned mass dampers for control the footbridge are designed. Theeffectiveness of the designed TMD is confirmed by numerical prediction of equivalentdamping ratio and vertical vibration of footbridge caused by pedestrian streams afterinstalling TMDs. The effect of change of TMD-to-structure frequency ratio, whichmay be caused by change of footbridge frequency, TMD frequency, or both, oncontrol performance is analyzed.