The effect of load sequence on GRMS measurements

The Gage Restraint Measurement System (GRMS) is an instrumented railroad car which measures the effectiveness of the ties and fasteners in maintaining gage under load. This involves applying a gage-widening load and measuring the resultant change in gage. In some measurements on weak track a directional dependence of the GRMS loaded gage measurement has been observed. Measurements taken from a forward traveling GRMS showed a consistent difference from data taken from the GRMS that backs up over the same section of track. A numerical model was developed to analyze the problem of a rail subjected to a combination of vertical and lateral loads similar to those produced by the GRMS test car. This model represents a rail as a beam supported by a continuous elastic foundation in the vertical direction and by friction elements at tie spacing intervals in the lateral direction. These supports are intended to represent the restraint behavior of the ties. Railhead displacements relative to the ties, caused by the applied vertical and lateral loads, are predicted by the model. Model predictions show that the presence of Coulomb friction results in a loaded gage measurement which depends on the proximity of an adjacent vertical load and whether or not this load trails or leads the applied lateral load during vehicle motion. The difference in the loaded gage measurement is 0.021 inches (14.1%) for the loading configurations presented in this paper.