Railway Track & Structures - January 2009 - (Page 19)

girders was affected, particularly girders with variable cross sections. The effects are most significant for ~100-foot girders. This span length is approximately twice the length. Figure 1 shows the bridge tested. Span length is under 113 feet from center-tocenter of bearings. The bridge has several deck plate girders with a ballasted deck. The girders are of riveted fabrication with flanges made of several cover plates. The girder cross section varies, changing at cover plate cut off locations. Traffic over the bridge includes numerous unit coal trains, occasional unit grain trains, and unit tank trains, as well as manifest freight trains. Strain gauges were attached to both girders of the eastern-most span. Gauges were located near the midspan location and also near the east quarter points of the girders. Data were collected under the passage of unit coal, grain and tank trains, as Figure 2 shows. The different car lengths in these trains provided the opportunity to make additional comparisons to the theory regarding the ratio of car-to-span length. predicted by the theory.1 The cars in both of these trains are very close to half the span length. Based on the span length from center-to-center of bearings, the carto-span length ratio is 0.47 for the coal cars and about 0.52 for the grain cars. The car-to-span length ratio for the tank cars is about 0.39. As expected, the shorter cars in the tank train produce the highest strains at the midspan location. They also generate the largest per-car cyclical stress variations at the midspan location. It is reasonable to expect that they will accumulate the greatest amount of fatigue at the midspan location. Figure 6 compares fatigue life calculations from both theoretical and measured stress ranges for the three car types at the midspan of the 115-foot girder. As Test results Figures 3, 4 and 5 show strain data, converted to girder stresses, from the passage of a coal train, a grain train and a tank train, respectively. The coal train had uniform car lengths of 53 feet. The coal cars also appear to have been uniformly loaded. All cars in the coal train were stenciled for 286,000-pounds gross rail load (GRL). The grain train had car lengths of about 58 feet to 61 feet. Some cars in the grain train were stenciled for 286,000pounds GRL, while others were stenciled for 263,000-pounds GRL. Some variation in girder stresses is evident in the data, likely corresponding to differences in car loadings. The tank train had car lengths of about 43 feet to 45 feet. As with the grain train, some cars in the tank train were stenciled for 286,000-pounds GRL, while others were stenciled for 263,000-pounds GRL. Again, some variation in girder stresses is evident in the data, likely corresponding to differences in car loadings. The data collected under the coal and grain trains illustrate the phenomenon www.rtands.com Railway Track & Structures January 2009 19 http://www.rtands.com

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Railway Track & Structures - January 2009

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