Invloed van dynamische grond-structuurinteractie op de respons van periodische bruggen met meerdere overspanningen

Dynamic soil-structure interaction (SSI) affects the modal characteristics and response of civil engineering structures such as buildings, bridges, tracks and tunnels. The flexibility of a structure's foundation lowers its natural frequencies and increases its modal damping ratios, resulting in a possible positive or negative effect on its response. For railway bridges, analyzing the influence of dynamic SSI is important for three main reasons: (1) to ensure a safe and cost-effective design, (2) to improve the estimation of modal characteristics as commonly used in structural health monitoring applications and (3) to predict ground-borne vibration in urban environments. The effect of dynamic SSI on a structure's response is usually assessed with numerical models. From a computational point of view, 3D element-based models are only feasible for short bridges with a few spans. Longer bridges, however, often consist of repeated identical spans, enabling the use of periodic structure theory to reduce the computational effort by modeling only a reference cell. This effectively reduces computation time and memory usage. This thesis develops numerical models based on periodic structure theory for long, continuous multi-span bridges supported by piled foundations, incorporating dynamic SSI and foundation-soil-foundation interaction (FSFI), i.e. the interaction between neighboring foundations through the soil. A sub-structure approach is employed, where the dynamic stiffness of a piled foundation is precomputed using finite element-boundary element (FE-BE) models or finite element-perfectly matched layer (FE-PML) models. First, a model based on the Floquet transform is developed. The structure's response is computed in the wavenumber-frequency domain using an FE-BE or FE-PML model of the reference cell. The inverse Floquet transform is then evaluated to determine the response in any cell. As integral transforms are used, the model implicitly assumes that the structure is infinitely long. It is demonstrated that dynamic SSI mainly affects the torsional and vertical bending modes with in-phase motion in neighboring spans. The effect of FSFI on the bridge response is generally negligible, especially when compared to the overall effect of dynamic SSI which is much more important. To model periodic bridges of finite length, the wave finite element method (WFEM) is used. This model allows for any boundary conditions at the bridge ends and can also be employed to compute the response of quasi-periodic bridges. Additionally, the method directly provides the dispersion curves of the structure from the free wave characteristics of the reference cell. It is demonstrated that the natural frequencies decrease significantly due to dynamic SSI, especially for the first torsional mode with in-phase motion in neighboring spans. The modal damping ratios increase, particularly for the vertical bending modes. It is also shown that, in case of fixed footings, the response is higher for short bridges. By accounting for dynamic SSI, however, short and long bridges show a similar response, indicating that infinitely long bridge models could be used to predict the response of short bridges in this case.