Dynamic Properties of Footbridges: Influence of Asphalt Pavement and Support Conditions

Natural frequencies and damping ratios are important parameters in the design of footbridges against vibrations. Although not normally considered in calculations, non-structural elements such as the asphalt pavement and the supports can have a significant influence on the dynamic properties. In this article we present results from vibration tests conducted on four footbridges over the last few years covering a wide range of asphalt temperatures. The experiments show that both the natural frequencies and the damping ratios can change significantly with temperature. In one particular case, a variation in natural frequency of 25% has been observed between high and low asphalt temperatures. An additional damping of around 1% due to the asphalt pavement has been detected. To more clearly understand the experimental results, the bridges were also analysed numerically using a visco-elastic material for the asphalt. The analysis of the experimental results also revealed that other non-structural elements can have a large influence on dynamic properties of bridges. In two cases, the supports did not perform as expected and lead to surprising results.