Analysis of dynamic wheel load signals in the frequency domain

Dynamic load tests were performed on three medium-span bridges with fundamental natural frequencies f = 4.4Hz, 3.0 Hz and 1.6 Hz, respectively. The test vehicle was a 45 ton five-axle tractor-semitrailer. The vehicle's suspension system could be changed between steel leaf and air, its body bounce frequency accordingly changing between f = 3 Hz (steel) and f = 1.5 Hz (air). All ten wheels of the vehicle, as well as the bridges, were instrumented to continuously measure the dynamic wheel loads and the vertical bridge response signals during the vehicle's crossing of the bridge. Subsequently, vehicle-bridge interaction was investigated through analysing the vehicle body bounce frequency for the phases 'before' and 'during' the crossing of the bridges. Dynamic interaction, or frequency coupling, between vehicle and bridge, resulting in a shift of the vehicle's body bounce frequency (and of the bridge response frequency), was observed for the cases where the fundamental frequencies of the vehicle and the bridge were close to each other. This frequency shift was clearly identified for the steel leaf suspended vehicle on the 3 Hz bridge.