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

TTCI R&D Automation of rail neutral temperature readjustment methodology for improved CWR performance TTCI has developed software to enhance rail neutral temperature maintenance practices. s part of the Association of American Railroads’ Rail Stress Management Strategic Research Initiative, Transportation Technology Center, Inc., has been investigating the behavior of rail neutral temperature (RNT) in continuous welded rail (CWR) 1 and developing guidelines for RNT maintenance. A computer program has been written based on the results of this research to provide track maintenance forces with basic information that is needed to readjust the RNT to a specified condition that is usually not known or readily available in the field. A Figure 1 shows RNT readjustment methodology concepts related to a cold weather rail break. Importance of RNT maintenance The RNT is the rail temperature at which there is zero thermal force in the rail. Rail temperatures above the RNT produce compression as rail expansion is constrained by the rail anchors/fasteners and ballast, while temperatures below it produce tension as rail contraction is constrained. Controlling the RNT allows thermal forces to be managed so the potential for track and rail damage during periods of very high or low temperatures is minimized. Railroads have established designated rail laying temperatures (DRLT) that control the RNT of CWR as it is installed. Over time, the as-installed RNT can change substantially due to longitudinal rail creep, lateral curve movement, vertical track settlement and track maintenance. www.rtands.com Of particular concern is rail that is added during cold weather rail repairs. Tensile forces that are released when the rail breaks or is cut at temperatures below the RNT produce a gap in the rail and cause reduction of the RNT in the vicinity of the break/cut. Rather than trying to retension the rail and close the gap immediately, it is often necessary to install a plug rail that eliminates the gap by adding rail. It is important to remove the added rail and readjust the RNT before the onset of warm weather to avoid high compressive forces and the possibility of buckled track. Removing excess rail restores the RNT to the pre-break condition, but does not necessarily readjust it back to the DRLT or other target value. To readjust the RNT to a given value requires knowledge of the pre-break RNT, the length of rail over which the RNT was affected by the release of tension, the required distance to deanchor and total gap width to recover. In some cases, track maintenance personnel will know the amount of rail that was added but may not know or have access to the additional information needed to readjust to a target RNT. The critical readjustment information can be estimated from relationships that exist between the rail thermal force, temperature and longitudinal resistance. Technology in the form of an RNT readjustment methodology has been developed that defines and quantifies these relationships. 2,3,4 RNT readjustment methodology Figure 1 shows basic concepts of the RNT readjustment methodology: • Rail tension is determined by the rail area and the difference between the RNT and rail temperature ( T). • The width of the broken/cut rail gap is a function of the rail tension and the longitudinal resistance provided by rail anchors/fasteners and the ballast section. The gap width is an indicator of the preby David Read, TTCI principal investigator and Andrew Kish, Ph.D., Kandrew, Inc. Railway Track & Structures May 2008 19 http://www.aar.com http://www.rtands.com

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Railway Track & Structures - May 2008

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