Dynamic Characterization and Vibration Serviceability Assessment of a Highly Trafficked, Lively Pedestrian Bridge

Pedestrian bridges represent a class of structures that are not typically heavily loaded, but may experience significant vibrations during operation due to pedestrian traffic and wind loads. To investigate the effect of these vibrations, pedestrian bridges can be characterized based on their dynamic properties such as natural frequencies, mode shapes and damping ratios. Existing design codes address the vibration levels either by ensuring the critical frequency ranges associated with typical pedestrian bridges are outside the fundamental frequencies of the structure or by restricting the maximum accelerations below the limits for pedestrian comfort. This research study discusses vibration serviceability assessment of a highly trafficked, lively pedestrian bridge based on the experimental tests and numerical analysis. The selected bridge is an approximately 190-ft long three-span steel structure with a continuous reinforced concrete slab supported on two longitudinal steel girders.
First, a finite element model of the pedestrian bridge was developed to setup key instrumentation parameters for the field tests. Experimental tests consisting of impact hammer tests, ambient vibration tests and pedestrian interaction tests were then conducted to obtain the modal characteristics of the structure. It was observed that the fundamental frequency of the bridge in the vertical direction obtained through field tests was within the critical range described by available design guidelines. This required further analysis to assess the performance of the bridge relative to the maximum acceleration threshold. In addition to the peak dynamic response obtained from the pedestrian interaction tests, peak acceleration values based on current design guidelines were computed analytically and compared to the comfort limits. Results from the study suggest that the footbridge possesses satisfactory serviceability performance under low and dense traffic conditions, but the comfort level was classified as minimum under very dense traffic loads.