CWR IN WINTER
CWR IN WINTER:
RAIL REPAIRS AND RNT
T
his article presents an engineering analysis of the thermal behavior of track. The results provide a rigorous basis for managing rail neutral temperature (RNT) after repairs to winter rail breaks. An illustrative example is provided. Rail steel, like most metal materials, expands when heated and contracts when cooled. When rail is subjected to temperature variations, it moves relative to the ground surface. But it does not move freely because it is connected to ties that are embedded in ballast. As shown in Figure 1, this restraint from the ballast causes axial forces to develop in the rail. Drawing C, shaded green, illustrates the track segment at its natural, unstressed length: 8 Railway Track & Structures // November 2023
that is, at its rail neutral temperature or RNT. Drawing B shows the segment at its freely expanded, unrestrained length associated with some temperature above its RNT. Drawing D shows the segment at its freely contracted, unrestrained length associated with some temperature below its RNT. When considering drawings B and D, one should imagine that the track segment is not installed in ballast, rather the bottoms of the ties rest upon a frictionless surface. This allows the steel rail to assume the length it would naturally have at the given temperatures above or below its RNT. Stated another way, drawings B and D show the rail “where it wants to be” at those temperatures— longer and stress-free when heated above its
RNT or shorter and stress-free when cooled below its RNT. Now consider that the ties are confined within deformable ballast, and, by various means, the rail is attached to the ties. Consequently, the rail cannot be “where it wants to be.” Drawing A, shaded orange in Figure 1, shows the final deformed position of the rail at a given temperature above its RNT. The rail is held shorter than “where it wants to be” by some amount that depends upon the movement between the ties and ballast and the movement between the rail and the ties. It is being restrained and experiences axial compressive force as a result. Drawing E, shaded blue in Figure 1, shows rtands.com
Photo Credit: Shutterstock.com/Stock video footage
By Gary T. Fry, Ph.D., P.E.