The excessive vibration of footbridges caused by walking pedestrians has generated great public concern in recent years. However, it has not been explored how crowd size quantitatively influences structural dynamic properties, including human dynamic properties. This paper provides an alternative method for understanding how crowds excite the excessive sway motion of a large-span structure when walking on it. In this study, pedestrians are modeled by a walking dynamic bipedal system. The crowd-structure system is established based on the bipedal model, for which the dynamic properties from pedestrians are considered. The vertical dynamic property equations of structure are calculated under crowd-induced vibration, based on the assumption of a uniformly distributed crowd. Through the proposed framework, the changes of frequency and damping of structure induced by walking pedestrians are studied. The increase of the crowd size can reduce structural frequency, but increase its damping. The impact tendency of crowd size on structural dynamic properties is consistent with measured results. This research provides insight as to how crowd size quantitatively influences the change of structural dynamic properties.
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