Abstract
Concrete is the dominant sleeper material choice for demanding locations on heavy axle load (HAL) freight railroads with steep grades, sharp curves, and high annual gross tonnage. Center flexural cracking is one of the most common factors limiting the service life of concrete sleepers in North America, and rail seat cracking has also been documented as a performance concern. As such, development and implementation of a structural design method that enables optimization of sleeper design for varied applications and loading environments will reduce initial capital cost and recurring maintenance expense. Field instrumentation has been developed to reliably capture revenue service field flexural demands, facilitating a probabilistic design method for the flexural capacity of concrete sleepers with bending data as the primary input. This paper presents a design process based on structural reliability analysis concepts whereby target values for reliability indices (β) for new designs are obtained and compared with existing designs for further design optimization. New (proposed) designs are quite different from current ones. For HAL freight, a reduction in rail seat bending capacity of approximately 40% is justified, reducing the size of the rail seat cross section by approximately the same magnitude. In most cases the proposed designs have fewer prestressing wires and a higher centroid of prestressing steel. In all cases the flexural capacities at the sleeper center and rail seat are better balanced from a structural reliability standpoint.
Highlights
Documentation of the need for concrete sleeper structural design optimization in the United States can be found as early as 1970 (RMSA, 1970), coinciding with their initial installation
Wolf et al (2015) built on the earlier findings of McHenry (2013) in an effort to revise AREMA to better account for non-uniform sleeper support conditions when designing for rail seat positive bending, which resulted in updated recommendations in 2017 version of the American Railway Engineering and Maintenance-of-Way Association [AREMA], 2017
Equations 5–12 are used as limit state functions for a first order reliability method (FORM) analysis to generate reliability indices (Zhao and Ono, 1999)
Summary
Documentation of the need for concrete sleeper structural design optimization in the United States can be found as early as 1970 (RMSA, 1970), coinciding with their initial installation. By deploying sleepers instrumented with load cells at the rail seats and the sleeper bottom in various field service conditions, Sadeghi (2008) concluded that dynamic load factors and rail seat loads are accurately estimated by AREMA, but sleeper support conditions are best estimated using UIC (2004) methods Realizing this deficit, Wolf et al (2015) built on the earlier findings of McHenry (2013) in an effort to revise AREMA to better account for non-uniform sleeper support conditions when designing for rail seat positive bending, which resulted in updated recommendations in 2017 version of the American Railway Engineering and Maintenance-of-Way Association [AREMA], 2017. There are no known design practices that require incorporation of empirical field bending moment data, largely due to the scarcity of these data and the challenges associated with interpolation and extrapolation of field results to the variety of applications in which concrete sleepers are used
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