Variation in measured force density levels of light rail vehicles.

Abstract

The procedure recommended by the Federal Transit Administration for detailed predictions of ground-borne vibration is based on measurements at an existing rail system and measurements at the target site. A force density level (FDL) is derived from train vibration and line source transfer mobility (LSTM) measurements at the existing system. This FDL is then combined with the LSTM measured at the target site to develop predictions. This empirical procedure usually provides more reliable predictions than computer modeling procedures. FDL is assumed to characterize the trains and track support system and LSTM is assumed to characterize the effects of local geology. Implicit assumptions when applying this procedure are that FDL is independent of the local geology and that the vehicle and track support system will be the same at the target site as they are at the vehicle test site. This paper considers the differences in FDLs measured at light rail systems in Los Angeles, Minneapolis, Phoenix, and Portland. Issues discussed are how to adjust for train speed, why the speed adjustments are system specific, causes for FDL variations when the vehicles and track support systems are nominally the same, and procedures for obtaining consistent FDL measurements.