Dynamic Amplification Factor for Light Rail Vehicle Transiting Box-Girder Bridge

Dynamic response of a prestressed concrete, box-girder bridge to passage of a light rail vehicle at design speed is expressed as a Dynamic Amplification Factor (DAF) - the ratio of effect of a moving vehicle (i.e. dynamic displacement of the bridge) to effect of the static, dead load of the vehicle. If the lowest natural frequency of the bridge indicates it may interact dynamically with the suspension of the moving vehicle - if lowest bridge frequency is about 3 Hz or less - the Owner's design criteria require a refined analysis. The present paper describes a detailed, dynamic three-dimensional finite element analysis combining models of the bridge, including curvature, and of a moving vehicle including multi degree-of-freedom suspension system. A main advance of the analysis is the procedure by which we move the model vehicle over the model bridge using the contact feature of ANSYS. The structure is a slender, prestressed reinforced concrete box girder bridge that is part of a metropolitan light-rail system. The bridge is a dual track light rail bridge with three spans that will span a total of 584 feet. To evaluate the design of the bridge, a rolling stock analysis was carried out to determine the DAF of a Light-Rail Vehicle (LRV) traveling over the bridge at 55 mph. The DAF analysis is based on a three-dimensional ANSYS model that combines the bridge and the LRV, including mass distribution of wheel sets, trucks, and vehicle body, and the primary and secondary suspension. Based on preliminary analysis of the natural frequencies of the bridge (flexural frequency 2.2 Hz) and the LRV, Owner's criteria require a detailed DAF analysis. We find that DAF is 1.06, less than the threshold value of 1.33 that would require stiffening of the bridge.