Railway Track & Structures - August 2008 - (Page 19)

Figure 5, above, shows track strength distribution and is from a presentation to the Federal Railroad Administration Rail Safety Advisory Committee by Dr. Andy Kish, July 2007, reprinted with permission of author. Figure 6 shows thermal rail forces on the 300-foot span. through the track fastening system. This is of particular concern where longitudinal resistance is stiff and the expansion bearing of a long bridge abuts the bridge approach, where track is often weaker than typical open track.7 Figure 5 shows a distribution of track strengths that can be expected on the North American system. Track strength is expressed in terms of degrees above rail neutral temperature. A differential ( T) of 60°F corresponds to single rail compression of approximately 150,000 pounds for 136-pound rail. A T of 110°F corresponds to rail compression of about 275,000 pounds. Because bridge approaches are potential trouble spots, rail compression that exceeds 150,000 pounds can be considered at risk for rail buckling. Figure 6 shows predicted rail forces for a 300-foot single-span steel bridge with the rail and span at 45°F above neutral temperature. The black dotted line shows the underlying rail thermal force of 112,000 pounds, with the additional compression caused by bridge expansion indicated by the arrows. Note that both anchoring conditions exceed the 150,000pound maximum – exceeding the minimum expected track strength. Figure 7 shows the maximum rail force at the approach results for a number of similar single-span bridges of various www.rtands.com lengths. The red line indicates the minimum expected track strength against buckling. Based on these predictions, fully anchored riveted top spans may be at risk for approach buckling on spans of 100 feet or more. Fully anchored smooth top spans may be at risk of buckling on spans of 200 feet or more. A number of lower restraint anchoring conditions have been proposed to reduce buckling risk. Rail force predictions for two special conditions are presented in Figure 7. Both cases propose that the only ties that are attached to the superstructure be box anchored for the first 100 feet from the fixed bearing end of the span. Anchoring Case 1 applies to a riveted top structure and Anchoring Case 2 applies to a smooth top structure. Clearly, Anchoring Cases 1 and 2 should effectively control rail compression for all of the spans considered. However, as Figure 8 indicates, reducing longitudinal resistance to reduce forces into the rail may increase the potential cold weather broken rail gap to unacceptable levels. In cases such as these, rail expansion joints may be required to ensure acceptable performance. the analytical model to investigate cwr interaction on numerous combinations of bridges and anchoring conditions and will recommend overall guidelines for use of rail anchors and expansion joints. This work is being conducted as part of the Association of American Railroads’ SRI to reduce the stress state of railroad bridges. References: 1. Akhtar, M., D. Otter, and B. Doe. May 2005. “Preliminary Impact Load Assessment of Ballast Deck Prestressed Concrete Bridges.” Technology Digest TD-05-013, Association of American Railroads, Transportation Technology Center, Inc., Pueblo, Colo. 2. Daniels, L. January 2000. “Longitudinal Force on Long Bridges and Aerial Structures.” Report to Transportation Research Board, Committee A2M01, Track Design, Washington, D.C. 3. Fryba, L. 1996. Dynamics of Railway Bridges. Chapter 8, pp 139-141, Thomas Telford Services Ltd., London. 4. Esveld, C. 1989. Modern Railway Track. Chapter 5, MRT-Productions, Duisburg, West Germany. 5. Joy, R.B. et al. July 2001. “FAST Railway Track & Structures August 2008 19 Going forward TTCI researchers are currently using http://www.rtands.com

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Railway Track & Structures - August 2008

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