Derailment loads in nearly all U.S. transit design criteria documents are similar but not based on any rational foundation. The loads have been modified numerous times by different agencies between 1985 and 2005 with no documented or defined basis. Currently, vertical impact is defined as 100% acting on any truck in the train consist, while horizontal derailment load is taken as 40% of a single car load acting on one truck. Lateral excursion in both cases is taken as a maximum of 3 ft, which assumes that a vehicle will not travel farther after derailment. Neither of these loads has been experimentally verified and appears to be based on an arbitrary definition that has become the norm in the U.S. industry. It is not known if this load is reasonably accurate, over-conservative, or even unconservative. [Lobo and MacNeill (“Dynamic Amplification of Transit Loads due to Derailment Impact,” Joint Rail Conference Proceedings, ASME, April 2022) show that derailment loads may be significantly higher than 100% impact with reported values ranging from 500% to 700% for derailment impact factor.]
Research is needed to formulate a rational load that is within acceptable limits of accuracy to ensure the efficient and safe design of bridges. The transit vehicle industry has the available tools to simulate the entire vehicle derailment and use these models to check the safety of the vehicles. It will be relatively simple to leverage their expertise and knowledge and extend their models to determine the loads on the bridge in both vertical and lateral directions.
This research will determine a load that is accurate and representative of actual transit vehicle loads. It will be defined in a manner that permits the efficient and safe design of structures. This research will also develop load factors and load combinations for derailment load conditions in line with the safety and reliability levels that underpin Load Resistance Factor Design (LRFD) design for bridges.
AASHTO has developed a notional load for transit vehicles in their The AASHTO LRFD Bridge Design Specifications (2019). An approach to tie the derailment load to the same notional load may be developed along with the train-specific loads. This will reduce extraneous analysis and maintain a consistent approach to these loads as the AASHTO load becomes the national norm for U.S. transit design.