CHAPTER 7
Summary and Recommendations for Research

This chapter summarizes the major findings from the research for this project. The findings were based on the range of rail vehicle classes, bridge structures, impact locations, and derailment scenarios presented throughout this report. A vertical drop from a 115RE rail to a direct fixation plinth (drop height of 6.25 in.) was considered. These variables were selected to capture a “typical” realistic range of what is found in practice. However, modifying these variables may significantly lead to different findings. The following recommendations are intended to account for some uncertainty in these variables.

7.1 Bridge Derailment Simulation Findings

Train car derailment simulations were performed to study the effect of derailment on bridges. These simulations progressed from simplified bridge and derailment modeling to more detailed bridge modeling. The first round of simulations modeled the bridge as a rigid surface. This step was performed to understand the effects of the rigid body kinematics of the derailment in isolation on a bridge structure. The second round of simulations modeled the bridge as a simple SDOF system by using rigid body kinematics from the first round of simulations as the initial conditions. This step was performed to understand the primary effects of the principal variables (i.e., derailment scenario, class of rail car, weight of passenger loads, and bridge frequency). The final round of simulations included detailed modeling of bridges with detailed train models in motion across the bridge before derailment. The final round of simulations was intended to have the greatest level of accuracy. In addition, these results were intended to inform the research teamʼs recommended methodology for derailment loading in bridge design and analysis. The following are specific findings from the bridge derailment simulations:

• The peak vertical rigid body velocities varied less than 40% for the different derailment scenarios, rail car classes, and passenger loads considered in the research teamʼs analysis. Peak rotational pitch rigid body velocities varied by as much as 90%. These results indicated that rotational pitch velocity is more sensitive to the variables than vertical velocity.
• The peak displacement of the simplified SDOF bridge system did not vary significantly for the different derailment scenarios for a given rail car class and passenger weight. This result indicated that the type of derailment (e.g., single rail break, double rail break, rail climb) is not a primary consideration in determining the derailment loading in bridge design and analysis.
• Increasing the stiffness of a bridge structure (and thus increasing its natural frequency) increases the DAF on the bridge structure. This finding indicated that the dynamic properties of a bridge structure are a primary consideration in determining derailment loading in bridge design and analysis.
• The dynamic properties of a bridge structure vary based on the impact location. In the teamʼs simulations, impacts occurring closer to bridge piers or abutments had higher dynamic

• frequency responses and higher contact force impulses. This result indicated that increasing the stiffness of the underlying structure effectively increased the COR for the derailment impact.
• The dynamic motion of the bridge before the derailment impact did not significantly affect the peak bridge displacement due to the derailment impact. This result indicated that the derailment impact may be considered in isolation from the dynamic motion of the bridge due to normal train operation prior to the derailment.
• Peak contact forces and impulses in the horizontal direction due to derailment impact were much smaller than those in the vertical direction. Contact forces in the transverse direction were consistent with friction due to the static wheel load. Frictional forces in the longitudinal direction were of smaller magnitude due to wheel rotation. Governing horizontal impact forces do not occur simultaneously with governing vertical impact forces.
• Compared to the industry practice of using 100% for the DAF, the teamʼs detailed modeling indicates equivalent DAFs can range between 91% and 359% over the range of considered parameters.

7.2 Suggested Revisions to Current Codes

Two different methodologies for vertical derailment impact load that could be incorporated into the AASHTO LRFD Guide were developed [1]:

1. The static vertical load method presented in Section 6.1 calculates a DAF to be applied to a single truck of a design train. The DAF is not intended to be applied to a notional live load, such as LRT-16, which was specified in the AASHTO LRFD Guide. This method is intended to directly replace the 100% vertical derailment impact load currently specified in the AASHTO LRFD Guide. This method is a simplified approach that accounts for the primary variables affecting the magnitude of derailment impact loads on bridges.
2. The dynamic vertical impact method presented in Section 6.2 is intended to offer bridge designers and analysts an option that can be incorporated into a dynamic bridge model. The AASHTO LRFD Guide does not currently include any analysis options similar to this method.

The research team suggests incorporating the static vertical load method presented in Section 6.1 into Section 3.2.4 (Derailment Load: DE) of the AASHTO LRFD Guide. The team expects that most bridge designers and analysts will prefer to continue using a methodology reasonably similar to currently used methodologies. Vertical derailment impact load should be used with a load factor of 1.0. In addition, vertical derailment impact load does not need to be applied simultaneously with any horizontal derailment impact loads. These suggestions are applicable to both direct fixation and ballasted track. Revising the AASHTO LRFD Guide to incorporate the dynamic vertical load method presented in Section 6.2 is not recommended. However, agencies may incorporate this methodology to provide bridge designers and analysts with an option to perform a more accurate analysis with less inherent conservatism.

Likewise, revising the AASHTO LRFD Guide to incorporate the horizontal impact method presented in Section 6.6 is not recommended. The simulation results show that vertical derailment impact forces are more significant than horizontal impact forces. The longitudinal braking loads prescribed by the AASHTO LRFD Guide govern the longitudinal impact loads calculated from Equation 6.10. The research does not justify modifying the barrier wall loads prescribed by the AASHTO LRFD Guide, and these loads govern the transverse friction force loads calculated using Equation 6.10.

7.3 Suggested Future Research

This research primarily focused on the effects of derailment impact on bridge structures immediately after the derailment impact occurs. The off-track post-derailment behavior of trains is difficult to predict. The research team has not performed simulations of more rare

and potentially catastrophic derailment scenarios that could result in rail cars tipping over. The suggested revisions to the current codes are expected to provide an indirect measure of safety against the more catastrophic derailment scenarios by ensuring bridges meet the design criteria for the more common and realistic derailment impact scenarios. However, additional research into the effects of the off-track post-derailment behavior of trains can help make bridges safer. The following topics are proposed as potential areas for future research:

• The effects of rail car tip-over on bridge safety;
• Transverse loading due to rail car collisions with bridge barrier walls;
• Loading due to the interaction of derailed cars with rail ties, fasteners, plinths, and other track structures; and
• The effects of guardrails on derailment prevention, derailment impact, and post-derailment behavior.