Model for lateral excitation of footbridges by synchronous walking

It was observed on a pedestrian cable-stayed bridge that the girder vibrated laterally with a frequency of about 0.9 Hz, the natural frequency of the first lateral mode, when a large number of people crossed the bridge. This girder response depended on the number of pedestrians but some pedestrians occasionally felt uncomfortable and unsafe. The girder was excited by the lateral dynamic force which was produced by the zigzag movement of pedestrians. Once the bridge started to vibrate, some of the pedestrians synchronized with the girder vibration, which further increased the girder response. However, when the vibration reached a certain level, pedestrians modified their walking and the girder response stayed at that level. A dynamic model was proposed to evaluate the pedestrians' synchronized dynamic forces. Overall behavior of the girder response calculated by this dynamic model agreed with the measured values on the bridge. It was found by parametric studies that the smaller bridge damping and the smaller bridge mass produced the larger girder response. The higher pedestrian density also increased the girder response unless they were too crowded to walk normally.