Railway Track & Structures - January 2009 - (Page 22)

TTCI R&D Figure 5, left, shows girder stresses under a tank train with 43- to 45-foot cars. Figure 6 shows the relative fatigue life for a 115-foot girder span under three unit train types. expected, the shorter tank cars produced the shortest fatigue life at the midspan location. The longer grain cars produced the longest fatigue life at the midspan location. For ease of comparison, all fatigue life calculations were normalized to those for the common 53-foot coal car at the midspan location. The comparisons between measured and theoretical fatigue life for the midspan location are reasonably good, considering some of the assumptions made in the analysis. The measured fatigue life estimates were made using the measured stress histories for the three train types. The theoretical calculations assumed uniform trains of 286,000-pound cars for all three train types and uniform car lengths of 44 feet for tank cars, 53 feet for coal cars and 59 feet for grain cars. As noted above, the actual trains had some variations in car lengths and weights, particularly for the tank and grain trains. These differences likely contribute to the variations between the theoretical and measured values, as Figure 6 shows. Note that in Figures 3 and 4, for the coal and grain trains, the peak stresses at the quarter-span locations are nearly as high as those at the midspan location, due to the reduced cross section of the cover-plated girder at the quarter points. And there is more cyclic activity, adding to the amount of fatigue experienced at the quarter-span locations. One might expect the fatigue life at the quarter points to be less than that at the mid22 Railway Track & Structures January 2009 span location for the coal and grain trains. Figure 6 also shows the computed fatigue life at the quarter-span locations for all three train types on this 115-foot girder span. The fatigue life at the quarter-span locations for the unit coal and grain trains is less than half that at the midspan location for those trains. In fact, the fatigue life for the coal and grain trains at the quarter-span locations is about the same as that shown for the midspan location for the tank train. when fatigue is considered. The car-tospan length ratio is a key parameter that controls the potential for fatigue to govern near the quarter-span locations rather than the midspan location. For light bridge designs and certain car-to-span length ratios, the fatigue life at the quarter-span locations might be less than that at the midspan location by a significant amount. Built-up girders with cover plate cut-offs are more likely to be affected than girders with a constant cross section. Assumptions and caveats The above analysis is only for the 115-foot span tested under the three train types shown. Results may vary with spans and cars of different lengths. For this particular bridge, the design is quite heavy and the peak stresses are relatively low. Since the stresses are so low compared to the constant amplitude fatigue limit, most of the stress cycles at the quarter point would not normally be counted. But for a bridge of lighter design, most or all of the stress cycles might be appropriately counted. So, for purposes of illustration, the above analysis included all stress cycles greater than 0.5 ksi in the fatigue stress computations. Depending on the type of detail, peak stress levels, and other factors, stress cycles of magnitude less than half of the constant amplitude fatigue limit might be ignored. Acknowledgements We gratefully acknowledge the technical and logistical help of BNSF Railway in testing the steel bridge, especially to Steve Millsap, Ed Ferguson, Charlie Mincic, and Ben Marquez, as well as to Dr. Stephen Dick for his technical assistance. The information reported here would not have been possible without their help. References 1. Dick, Stephen M. 2002. “Bending Moment Approximation for Use in Fatigue Life Evaluation of Steel Railway Girder Bridges.” Ph.D Dissertation, Department of Civil, Environmental and Architectural Engineering, University of Kansas, Lawrence, Kan. 2. Dick, Stephen M., and Steven L. McCabe. 2002. “Fatigue Analysis of Steel Railway Girder Bridges.” Proceedings, American Railway Engineering & Maintenance-of-Way Association 2002 Annual Conference. 3. American Railway Engineering and Maintenance-of-Way Association. 2008. Manual for Railway Engineering, “Chapter 15 – Steel Structures,” Lanham, Md. www.rtands.com Bridge design, rating implications While the midspan location is usually the area that governs girder design for bending and is most commonly checked for rating, this study shows that the quarter-span area should also be analyzed http://www.aar.com http://www.rtands.com

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Railway Track & Structures - January 2009

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