Dynamic properties of two pedestrian woodn bridges incluiding seasonal effects

Abstract

The dynamic properties of two pedestrian wooden bridges situated in the Stockholm area have been determined experimentally. The main purpose was to investigate if these properties are season dependent. For that, tests were performed during the winter and the spring. 17 accelerometers were used to measure both vertical and horizontal vibrations. An impact hammer was used to excite the bridges. For the Vega bridge, a large variation for the lowest natural frequencies was observed, both for the first vertical and for the first transversal mode. For the Hägernäs bridge, a large variation was observed for the first transversal mode but not for the first vertical mode. With exception for the first transversal mode for the Hägernäs bridge, no significant differences regarding the damping between winter and spring were observed. Finite element models of the two bridges have also been implemented and a good agreement with experimental results have been obtained. The numerical models have shown that the connections between the different components of the bridges have a significant influence on the dynamic properties. The models have also shown that the variation of the stiffness for the asphalt layer may explain the differencies for the natural frequencies between spring and winter. However, due to the incertainties in the modelling of the asphalt, this conclusion must be taken with caution.