Abdurrahman ONAY

Investigations of soil-structure interaction in framed railway bridges

The development of the existing railway network for high-speed trains provides new challenges in the dynamic design of new and especially existing bridge structures. High-speed train crossings can cause resonance phenomena in bridges, leading to large deformations, accelerations and stresses in the bridge structure. As a consequence, the maintenance intervals must be reduced, and in the worst case, the train may derail or the structure may fail. Damping of the structure has a great influence on the response at critical resonance state. In comparison to measured damping values, the damping values used in structural analysis are chosen very conservatively based on design guidelines and codes. Especially in the case of portal frame bridges with short span widths, economic dimensioning is often not possible. Due to their integration into the subsoil, they show a strong soil-structure interaction during train passage, which leads to a high geometric damping due to wave propagation in soil. This master thesis investigates the dynamic soil-structure interaction of portal frame bridges and the geometric damping effect of the soil.

For the present numerical studies, simple plane finite element models are created, consisting of the frame structure and the soil. These models are used to perform parametric studies by varying the geometry of the frame structure and the soil properties. The portal frame bridge model is excited by an impulse in the center of the span and the frame-wall. The subsequent the decay behavior of the free vibration is used to identify the geometric damping. The soil-structure interaction of railway bridges is evaluated based on natural frequencies, mode shapes and global damping coefficients. In particular, the first horizontal and the first vertical bending mode of the frame, are considered.

It is shown that the natural frequency of the horizontal bending mode experiences a significant stiffening due to a lateral bedding of the frame-walls. In contrast, the natural frequency of the vertical bending mode is not noticeably influenced by the surrounding soil. It is shown that the damping coefficient of the first vertical bending mode is significantly affected by soil-structure interaction if the spans are very short. For spans larger than 20 m this effect is negligibly small. The damping value depends on the soil stiffness and is higher for soft soil. With increasing frame height and integration into the soil damping becomes larger.



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