A study of suspension bridge vibrations induced by heavy vehicles

The Lysefjord suspension bridge (Norway) is instrumented with accelerometers, anemometers, weather station and a GPS monitoring system, making it a full-scale test bed for research on dynamic bridge behavior and wind characteristics in a complex terrain. In this study we investigate bridge vibrations induced by heavy vehicles crossing the bridge. As the bridge is located on the outskirts of the urban environment, the traffic across the bridge is generally intermittent, allowing for investigating loading effects of individual heavy vehicles during days with low wind velocity. The findings suggest that heavy vehicles cause a considerable dynamic load effects, especially when entering and exiting the bridge, especially at higher modes of vibration with natural frequencies above 1 Hz. Spectral analysis of the traffic induced acceleration time series agree with results from previous studies showing that the acceleration bridge response to vehicles consists of a combination of both low and higher frequency modes. Truck induced excitation has been used to estimate the modal parameters of the bridge, such as critical damping ratios for the vertical modes of vibration assuming a viscous-equivalent damping and a logarithmic decay of the bridge motion as the trucks exit the bridge. The identified damping ratios as well as natural frequencies are found to be comparable to values obtained from general ambient vibration data. However, there are significant variations in the results depending on measurement location and the driving pattern of the truck along the bridge.