Experimental Investigation of the Vibration Susceptibility of Footbridges for Subcritcal Vibration Modes

A lot of slender and/or long span footbridges display vertical vibration modes with natural frequencies below the frequency range that is specified to be critical by various guidelines. The presented study aims at assessing the susceptibility of vibration modes in a sub—critical frequency range within the scope of serviceability aspects. The assessment is based on vibration tests on 2 slender long-span footbridges which revealed modes and frequencies below normal footfall rates. During the vibration tests accelerations with a significant participation of the sub-critical modes were observed in particular for randomly and slowly walking occupants on the bridge deck though the overall acceleration levels were within acceptable values. In addition vibration tests with an intended resonance-like excitation of the relevant modes were performed. The measured dynamic response was then qualitatively compared with the calculated dynamic responses for which methods of current guidelines and experimentally determined Dynamic Load factors for sub-critical footfall rates and for subharmonics have been applied. Based on this comparison the relevance of sub-critical vibration modes for normal loading and vandalism was assessed.

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