Dynamic Design of Bridges on High-Speed Railway Lines

Abstract: In recent years, high-speed railway has been on the upswing in the public debate, as Swedish authorities have affirmed the plans of a high-speed corridor connecting Stockholm, Gothenburg and Malm¨o. The track speed limit is not fully determined, but will be in the range of 250-320 km/h. At these higher speed levels, it is harder to ensure traffic safety and passenger comfort, and dynamic analyses of the included bridges are required. Substantial research has already been published on the subject of high-speed railway dynamics; yet, there are still areas that need further investigation to understand the behaviour of bridges that undergo this type of loading. In this thesis, parametric studies were performed on a three-spanned slab bridge and on a portal frame bridge. The main objective was to investigate how the response of the bridges changed when the train speed, the span length and the cross-section height werevaried. The analyses were executed in BRIGADE/Plus and the response was primarily evaluated in terms of vertical accelerations of the bridge deck. However, verifications of vertical deflection, torsion, rotation at bearings, and section forces, were also covered in the study. Previous research has shown that the representation of the boundary between the foundation and the surrounding soil is a critical part of the modelling. In the major part of this study, the conventional method using fixed boundary condition was adopted in all DOFs except for those representing the rotation around the in-plane axes, where linear elastic rotational springs were inserted. A slightly more realistic approach with an increased Young’s modulus of the soil and an additional spring in the vertical DOF, was also evaluated. In addition, a minor literature study of more comprehensive works on the topic of Soil-Structure Interaction (SSI), was conducted. The results suggest that none of the bridges in the study meet the dynamic design requirements for a track speed interval of 250-350 km/h. Furthermore, an increase in crosssectional height seems to be an efficient way of reducing the maximum accelerations, whereas an enlargement of the span length may drastically increase the response. A 10 % increase in span length can increase the acceleration by a factor 2 for the slab bridge, and by a factor 3 for the portal frame bridge. It has also been shown that using fixed boundary conditions at the soil-structure interface can yield non-conservative results. For the frame bridges of different span lengths, the accelerations were significantly larger for the case with vertical springs; however, no such trend was able to be identified for the slab bridges. In contrast to the existing body of research, the acceleration was not the decisive parameter in all aspects of this study. The section forces, and particularly the choice of dynamic enhancement factor, were critical for the shorter bridges in the track speed interval 125-250 km/h.