Investigation of the Effects of Tie Pad Stiffness on the Impact Loading of Concrete Ties in the Northeast Corridor


This investigation was conducted in response to the discovery in June 1980, that rail seat bending cracks had developed in many concrete ties which has been in service on the Amtrak Northeast Corridor track for periods of a few months. The study was initiated by the Federal Railroad Administration to investigate the cause of the cracks and to identify a solution which would arrest further crack development while minimizing the retrofitting of components. The study was conducted in three phases:
a. Existing load and tie strain data were examined over a large frequency bandwidth (0-1200 Hz) to determine whether impact loads could be identified at level above that known to cause tie cracking. Such high-frequency impacts were found on a small percentage of high-speed passenger train wheels. These impacts were caused by wheel tread irregularities.
b. Laboratory impact tests verified that major reductions in tie pad stiffness could significantly reduce the tie bending moments resulting from impact loads.
c. Field measurements of impact loads and tie bending moments were conducted in a test zone where more flexible tie pads were substituted. The tests verified that flexible tie pads which are adaptable to the current fastener system could reduce the occurrence of bending strains which can crack ties. However, it was shown that elimination of tie cracking will require both a more flexible pad and a program to eliminate the wheel conditions causing the highest impact loads on Northeast Corridor passenger trains.