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In recent times a number of footbridges have suffered from lateral vibration induced by pedestrians. Field measurements were carried out on a Japanese pedestrian suspension bridge, the M-bridge, to clarify the dynamic properties of this lateral vibration. The bridge vibrated in either the third asymmetric mode with a natural frequency of 0.88Hz or the fourth symmetric mode with a natural frequency of 1.02Hz, depending on the distribution of the pedestrians on the bridge. The maximum lateral displacement reached about 45mm. The measurements showed that
when a pedestrian walked on the vibrating girder, the person synchronised to the girder frequency with a phase shift between 120° and 160° ahead of the girder. According to the ambient vibrations, the synchronisation is unlikely to occur at the girder natural frequency under 0.6Hz.
By analysing the vibration data of the three pedestrian bridges which suffered from lateral vibration, it is ascertained that the smaller bridge mass and damping produces the largest girder response. When the girder response reached about 45mm, equivalent to the velocity of
250mm/sec or the acceleration of 1350mm/sec2, pedestrians felt unsafe and could not walk normally. This condition is thought to be the serviceability limit for the lateral girder vibration.
Shun-ichi Nakamura, PhD, DIC, PE, MIABSE, MASCE, FJSCE
Professor, Department of Civil Engineering, Tokai University, Japan
Visiting Professor, School of Engineering, University of Surrey, UK