Railway Track & Structures - January 2009 - (Page 17)

TTCI R&D New developments in fatigue life estimation of railroad bridges under unit trains Understanding the toll heavier loads and fatigue take on steel bridge quarter spans. nderstanding fatigue life of steel railroad bridge components is of particular importance. Some steel girders may have reduced section areas at or near quarter-span locations. The designers may not have anticipated today’s heavier axle loads or considered fatigue. Transportation Technology Center, Inc., tested a steel railroad bridge to validate a recent development in fatigue theory that is particularly applicable to unit trains on steel railroad bridges. The test results show the need to evaluate fatigue ratings at quarter-span locations, in addition to the traditional mid-span U location for many typical railroad spans. While the mid-span location is usually the area that governs steel girder design for bending and is most commonly checked for rating, this study shows that the quarter-span area should also be checked when fatigue is considered. Fatigue life depends on the size and number of load cycles. For a certain carto-span length ratio, the number of cycles may be higher at quarter-span locations than at midspan locations. In that case, the bridge components at or close to a quarter-span location may accumulate higher fatigue over the service life of the bridge than those components at or close to a midspan location. The effect of the car-to-span length ratio on fatigue life is a recent theory and has been discussed in detail in references 1 and 2. Theoretical fatigue life due to unit Figure 1, left, the BNSF Fountain River Bridge used for validation testing. Figure 2 shows a unit grain train crossing 115-foot test span. trains was studied for spans up to 160 feet. This study provides a range of spans in which the fatigue life may be lower for typical unit coal trains. For certain bridge spans, unit coal and grain trains appear to reduce fatigue life at quarter-span locations to less than half that at midspan locations. Test bridge To validate the theory, a 115-foot span bridge over the Fountain River in Pueblo, Colo., was instrumented and by Duane Otter, principal investigator, Muhammad Akhtar, senior engineer, Brian Doe, engineer, TTCI Railway Track & Structures January 2009 17 www.rtands.com http://www.aar.com http://www.rtands.com

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Railway Track & Structures - January 2009 Contents On Track Industry Today Supplier News AREMA News NRC News TTCI R&D Railroads Continue to Budget Major Capital Projects Battling Brush and other Vegetation Steel Mills Increasing Metallurgical Quality of Rail Steels Products and Literature People Calendar Advertisers Index Sales Representatives Website Directory Professional Directory Classified Advertising Chicago Perspective

Railway Track & Structures - January 2009

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