Vibration Performance of Two FRP Footbridge Structures in the United Kingdom

Footbridges are increasingly made of fibre-reinforced polymer (FRP) composites due to advantages they bring, such as high-strength and light-weight nature of the material, fast installation and low maintenance costs. It is often argued, however, that the low mass of these structures might lead to high sensitivity to vibration, especially when the low mass is coupled with a low natural frequency and/or low damping ratio for one or more modes of vibration. This paper investigates dynamic behavior of two FRP footbridges – each having a deck that is made of pultruded units that were interlocked and bonded together. The two structures have different spans (51.3m vs 16.9m) and structural systems (suspension vs simple beam) resulting in distinctly different natural frequencies. Six vibration modes were identified for the suspension bridge in the frequency range up to 5Hz, whilst there was one mode only for the simple beam bridge. Comparison of the vibration behavior under human-induced excitation, however, revealed that they exhibit similar responsiveness to walking excitation due to the suspension footbridge having higher mass per unit length and the beneficial role of nodal points in the vibration modes. A walker reported being more sensitive to vibrations of the suspension bridge, which is, given the vibration frequencies in question, opposite to the (well known) effect on a standing person. This paper demonstrates that even extremely light (e.g. the simplebeam) bridge can exhibit satisfactory vibration performance.

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