The National Academies

Rail Safety IDEA Project 45 [Completed (IDEA)]

Development of a Fatigue Load for Railway Bridges

  Project Data
Funds: $100,000
Staff Responsibility: Velvet Fitzpatrick
Research Agency: Purdue University
Principal Investigator: Robert J. Connor
Completion Date: 10/4/2022
Fiscal Year: 2020

Fatigue of steel railway bridges has been an issue since steel was first used for railway bridges. While the mechanism driving fatigue cracking has been understood since the 1970s, there is a lack of knowledge on the cyclic behavior in terms of railway service loading. The purpose of this research was to fill the knowledge gap on cyclic behavior of railway loadings. Trains consist of a series of loads, and different train types require different analysis techniques and solutions to obtain cycle counts from a typical train for each type.  Also, an important interaction exists between the length of each railcar and the length of the span where each position on the span will have a unique reaction (e.g., quarter point versus mid-span). Results at one location differ from other locations. The point of maximum fatigue effects moves from the generally assumed mid-span location because of the axle load interactions. The products of this research were a computer software for fatigue analysis of railroad bridges and a proposed fatigue design load for railroad bridges. The software, named CyclRR, performs bending moment beam analysis for simply supported structures under a series of moving loads reflective of actual railroad equipment and develops a bending moment time-history and an associated bending stress time-history to document the effects of the train passage. It then performs rainflow cycle-counting analysis on the stress cycles above a user-defined threshold. The Root-Mean-Cube (RMC) stress range and RMC moment range are calculated along with the associated number of cycles. The software can run a simple one-time train for a typical unit-train of all similar cars or for Monte Carlo simulations with development of mixed train models. The mixed train models are crucial for railway bridge fatigue analysis. The fatigue design load is the second product of this IDEA research. Railroad bridge design and rating recommendations are published by the American Railway Engineering and Maintenance-of-Way Association (AREMA). Current fatigue analysis uses the Cooper E80 design load with an adjusted number of equivalent stress cycles to estimate the effects of actual traffic. The Cooper Load provides adequate overall design quantities for moments and forces but does not act like actual railway loads, a key difference for a fatigue load. The proposed fatigue design load resembles a railcar and provides moment magnitudes and cyclic behavior such that it can serve as a reference for developing a fatigue design load and quantifiable rating system for fatigue. In conjunction with the fatigue load development, sample calculation examples are provided in the report along with the tables necessary to perform those calculations. Application of the fatigue life estimation method was also included in the report to demonstrate the ability to perform different types of calculations from the results of CyclRR. The addition of cycling data for different types of trains should be very useful to anyone involved in engineering and maintenance of railway bridges for large railroads, short lines, or public agencies in charge of commuter and other passenger operations.

The final report is available.

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