Evaluation of the safety ratio in the Eurocode for deck acceleration limit on ballasted railway bridges

Abstract

Ballasted railway bridges are subject to dynamic excitation from passing trains, which can cause excessive vibrations in the ballast bed depending on the train speed, resulting in track instability. The Eurocode EN 1990 uses vertical deck acceleration as an indicator of safety, limited it to 3.5 m/s2 for ballasted bridges. Since experimental studies show that ballast instability occurs at about 7.0 m/s2, the normative limit seems to be arbitrarily based on a safety ratio of 2.0. The present paper examines the suitability of a lower safety ratio. The proposed methodology compares the physical acceleration limit with the design acceleration calculated at a critical speed corresponding to a failure probability of 10-4. An algorithm for the efficient assessment of critical speeds based on subset simulation is introduced, together with a parametric study for its optimization. A sensitivity analysis of the random variables of ballasted bridges allows the definition of two design scenarios in accordance with the Eurocode EN 1991-2. Results from the application of the methodology to four case study bridges show that design accelerations greater than the limit defined in the Eurocode can be found in ballasted bridges within the target probability of failure, suggesting that the safety ratio may be set lower than 2.0.