This paper examines the issues relating to the use of 6”x 8” timber crossties in low-density heavy axle load operations, such as found on short lines, regional railways or railroad branch lines. This application is of particular importance in light of the requirement for low-density lines to operate 286,000 lb. heavy axle load cars. This paper focused on the engineering and economic issues associated with the use of the smaller size (and footprint) crossties.
Specifically, this paper summarizes the results of an analytical study of 6”x 8” timber crossties track performance as related to (1) ballast and subgrade bearing stresses under the tie and (2) tie bending stresses.
In the area of ballast and subgrade bearing stresses under the tie, the paper shows the results of an analysis of tie-ballast bearing stress as a function of tie width and axle load for different track support (track modulus) configuration. It also presents an analysis of ballast-subgrade bearing stress, based on 9” and 12” ballast (and sub ballast) sections.
Using the differences in stresses between 6×8 and 7×9 cross-ties, the effect on track surface (geometry) degradation is determined, together with the corresponding change in track surfacing cycle, as a function of traffic density (MGT) per year. This change in track surfacing cycle is then used, together with the cost differential between the 6×8 and 7×9 ties, to calculate the relative economics of 6×8 cross-ties as a function of traffic density and track support condition (defined in terms of track modulus).
In the areas of tie bending stress, the tie-bending stresses are presented as a function of tie width and axle load for different track support (modulus) configuration and those conditions where the bending strength stress levels exceed allowable levels (for different wood types) are identified.