| A method was developed to study the dynamic response of a railway bridge caused by the vehicle-track-bridge interactions in both vertical and lateral directions. Analytical models for the bridge and nonlinear freight car were developed. Responses in selected bridge members, due to random inputs from track irregularities, were evaluated in terms of forces and impact. Fatigue life of a critical hanger in the bridge was determined.; The conclusions were: (1) The impact factors are quite small for the case of vehicles running on a smooth track with no irregularities. (2) In most bridge members, the impact factors are slightly greater in the case when both vertical and lateral inputs from track irregularities are considered, as compared to the case when only vertical input is considered. (3) In general, the "web members" in the bridge truss receive more impact than the chord members. (4) The dynamic forces and impacts increase significantly as the track class on the bridge approaches is lowered from 6 to 4. The impact factors in hanger members are within the American Railway Engineering Association (AREA) recommended value only when 2% bridge damping is included. In certain members with small static force, the impact factors are much higher than those specified by the AREA. However, the dynamic forces and stresses in these members remain quite low. (5) With a few exceptions, the impact factors decrease with the inclusion of bridge damping, and increase with the increase in train speed. (6) The impact factor is influenced by the vehicle weight and type, and other associated parameters, such as axle spacing, suspension system stiffness, truck center distance, etc. (7) With the assumed lateral stiffness, the dynamic forces in the lower lateral bracing members are quite small, compared to their allowable values computed from the old AREA Specifications. (8) With Class 6 track simulated on the bridge and its approaches, four freight trains, consisting of 40 loaded and 29 empty, 100-ton cars, were assumed to cross the bridge per day at a speed of 50 mph. The fatigue life of the critical hanger was found to be 67 years. |
