The Bump at the End of the Railway Bridge
Nicks, Jennifer Elizabeth
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The bump at the end of the railway bridge is a result of differential movement between the bridge deck and the approach embankment. The movement can have the form of a bump or a dip. Either defect in the track geometry can cause significant problems in track performance. The current state of practice was evaluated by conducting a literature review and an industry survey. According to the survey, approximately half of all railway bridges are affected by the bump/dip. The total annual cost for repairing these bridge transition problems is estimated at $26 million. This does not take into account the considerable cost resulting from speed reductions that railroads must place on trains at these locations. In addition to the increased maintenance costs, the bump/dip leads to higher impact loads, uncomfortable rides and possible safety hazards. The track response due to the bump at the end of the bridge was evaluated by creating a 4-D finite element model of the train, track structure and track substructure. The motion of the train model across a bridge/approach transition, with and without a bump/dip, was then simulated using LS-DYNA. It was found that a track modulus differential alone (no bump/dip) at a bridge/approach location leads to impact forces as well as increased ballast and subgrade pressures on the approach. This instigates the formation of a bump or dip in the track. The track response is increased when a bump/dip is present in the track profile. A parametric study looking at the influence of train direction, train speed, bump/dip size, approach embankment soil modulus, approach tie material, bridge tie material, bridge deck type, ballast thickness and approach tie length on the magnitude of impact forces, track deflection, ballast and subgrade pressures was also performed with the model. Finally, a design solution to minimize the bump at the end of the bridge is proposed. The solution involves installing varying length steel bars into a soft subgrade approach embankment. The solution addresses both the settlement and track modulus differential between the bridge and the embankment. A full-scale field test of this prototype solution is underway.