Experimental Characterisation of Dynamic Properties of an All-FRP Truss Bridge

Fibre Reinforced Polymers (FRPs) have increasingly been utilised for construction of pedestrian bridges due to high strength- and stiffness-to-weight ratios, low maintenance costs and quick installation. Their relatively low mass and stiffness make these bridges potentially susceptible to vibration serviceability problems, which increasingly govern the design. Currently, the wider application of FRPs in civil engineering is hindered by the lack of experimental insight in dynamic performance of as-built structures. This paper presents an experimental investigation on a 25 m long glass-FRP truss footbridge in Italy. Ambient vibration tests were conducted to identify the dynamic properties. The peak-picking method and stochastic subspace identification approach were employed for modal parameter identification. The two methods produced very consistent results. Eight vibration modes were identified in the frequency range up to 10 Hz. Two lateral flexural vibration modes having natural frequencies of 5.8 and 9.6 Hz were identified, as well as two vertical flexural modes (at 7.5 and 8.1 Hz) and four torsional modes (at 2.1, 2.7, 4.8 and 9.3 Hz). Damping ratios for all modes up to 10 Hz except the eighth mode were above 1.2%.