Fatigue Capacity for Concrete Trough Bridges at the Ore Line

Sammanfattning

Ballast is a critical construction material in railway bridges, as it distributes heavy loads from the rails tothe supporting structure. According to the Eurocode, axle loads should be distributed through the ballastwith a load distribution ratio of 4:1, while Swedish standards recommend a ratio of 2:1. This discrepancybetween the Eurocode and Swedish standards forms the basis of the first research question in this thesis,which aims to determine the impact of this difference on the capacity of railway bridges.With increasing traffic volumes on Malmbanan (the Ore Line from Luleå on the Baltic to Gällivare andKiruna, extending to Narvik on the North Sea), there is a need to raise the allowable axle load on thetrack. Consequently, the capacity of railway bridges must be analyzed and enhanced if necessary. Thepurpose of this study is to analyze the effects of increased axle loads on railway bridges and their fatiguecapacity.Trafikverket considers the issue of increasing railway bridge capacity to be significant and is funding anongoing project at LTU. This project involves conducting fatigue tests on a reinforced concrete troughbridge to gain a better understanding of the fatigue capacity of railway bridges and how ballast transferaxle loads.Hand calculations of the load effects from dead weight and axle loads are performed to evaluate thecapacity of the bridge being tested at LTU. The axle loads analyzed in this study are 25, 30, 32.5, 35, and40 tons. The load effects are examined using Eurocodes, Model Codes issued by the International Fed-eration of Concrete Structures (fib), and Swedish standards to verify the bridge's capacity concerning bothfatigue and ultimate limit states.The calculations for the ultimate limit state reveal no significant difference between the load distributionratios of 4:1 and 2:1. Furthermore, Ultimate Limit State (ULS) calculations indicate that the axle load onMalmbanan can be increased to 35 tons without risking bending or shear failure in railway bridges. Acritical aspect is the assessment of the shear capacity of the slab connection to the longitudinal beam.The fatigue analysis of the longitudinal bending reinforcement suggests that the axle load should notexceed 30 tons according to the proposed revised Eurocode for concrete structures (pR EN 1992-1-1,2020). This conclusion is valid under the condition that the number of trains per day or wagons per trainremains unchanged (currently 28 trains per day, each with 68 wagons). However, the fib Model Code2020 allows a higher load, 40 ton.The shear fatigue of the connection of the slab to the longitudinal beams is less studied and the codes aremore conservative and only allows 1 million load cycles for an axle load of 30 tons. However, no shearcracks have appeared after 10 million load cycles, so the shear capacity is probably higher. This willhopefully be shown in the further testing of the bridge.