9. Pedestrian Induced Lateral Vibrations with Emphasis on Modal Energy Transfer

Already on the opening day of the Lardal Bridge in 2001 were large lateral vibrations observed. These excessive vibrations were seen as relatively dense and continuous flow of people was crossing the bridge. This type of observation has later been made for several other bridges, old as well as new. These observations have initiated an extensive investigation program quantifying structural properties such as natural frequencies, mode shapes and damping and their influence on the pedestrian induced vibration. Thus, the pedestrian lateral load phenomenon has equally been thoroughly investigated. As part of the pedestrian load investigation at the Lardal Bridge were a large number of time series with different sized groups of people recoded. This paper aims to re-analyze these data with the emphasis on the energy transfer between modes. It is clear from these response recordings that the lateral displacement is often not initially initiated, rather vertical or torsion motions. This is also true for several crossings of larger groups of pedestrians. This raises some interesting questions of how and when the pedestrian induced energy transfers between modes. From short time Fourier transforms (STFT) it can be seen that energy transfers after certain levels of response are reached. Interestingly, the horizontal mode, a reversed pendulum motion, includes a small vertical component with twice the frequency of the horizontal mode. The investigation explores the system sensitivities to group size, initially triggered response modes and the pacing frequencies of the pedestrian motion.