Study of the Group Effects on the Vibration Serviceability of Slender Footbridges

Long span, slender footbridges can be susceptible to excessive vibrations due to pedestrian movements. The vibrations when pedestrians cross a footbridge at a speed near its resonance frequency can become excessive and cause concern about the structural integrity. This paper presents the results of the experimental studies on two steel footbridges with different structural systems and architectural configurations, susceptible to vibrations due to human movements. The resonance frequencies of each structure was measured. Several vibration tests were conducted during which various number of pedestrians crossed the footbridges at different speeds (random, resonance frequency of the structure, and most probable speed). Various vibration evaluation parameters such as peak acceleration, maximum transient vibration value (MTVV) and vibration dose value (VDV) were computed for each measured record. The frequency-weighting functions from different standards were used for the computation of each evaluation parameter. To assess the group effects for each case, an enhancement factor was defined as the ratio of the particular evaluation parameter when a group of pedestrians crossed the footbridge divided by the same from only one pedestrian. This study showed the relative consistency between the results of the tests on the two structures. However, it became evident that the variations of the enhancement factors are not consistent with those reported in the existing literature.

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