Effects of pedestrian excitation on two short-span FRP footbridges in Delft

Reported in this paper is an evaluation of the vibration behaviour of two footbridges in The Netherlands having main spans of about 15 m. Short-span footbridges over canals and rivers are embedded in the landscape of Delft and elsewhere. Increasingly, these bridges are made of Fibre-Reinforced Polymer (FRP) composites, utilising the high-strength and light-weight nature of the material, and taking advantage of fast installation and low maintenance costs. Due to low mass, these FRP bridges might be sensitive to dynamic excitation by human actions. The two footbridges investigated in this paper have the main bearing structure which consists of either two or four longitudinal beams made of vacuum infused FRPs with foam cores connected by an FRP deck. Modal testing revealed that fundamental vertical natural frequency of the two structures at 4.8 and 6.1 Hz is in the range typical of similar structures made of concrete and steel, whilst the corresponding damping ratio for the wider, slightly cambered, bridge was exceptionally high at 7.9%. The vibration response to dynamic force by people walking was typically up to 1 m/s2. While both of these light-weight structures performed satisfactorily under the regular pedestrian loading, the higher frequency – higher damping structure represents an example of successful control of pedestrian-induced vibrations by means of longitudinal restraints at the end supports and slightly curved shape of the main structure.