Some aspects of the dynamic soil-structure interaction of a portal frame bridge

Abstract: For certain bridge types, the influence of soil-structure interaction (SSI) may have an important contribution to the stiffness and damping of the structural system. From a design point of view, this influence may be both conservative and non-conservative and therefore, an increased knowledge within this field could lead to better design assumptions. In terms of maintenance, assessment and upgrading of existing structures, an increased knowledge of the phenomena and parameters which govern the soil-structure interaction, may lead to more realistic models and thereby, to more precise information for the decision makers and railway system owners and administrators.SSI appears to be most important for short and relatively stiff structures such as portal frame bridges. Dynamic analyzes of this bridge type have shown a large sensitivity in the choice of boundary conditions, where applying elastic constraints on the vertical degree of freedom at the support, compared to fixing this degree of freedom, may increase the maximum vertical bridge deck acceleration by as much as a factor of three.In this thesis, numerical analysis procedures for the computation of dynamic stiffness functions describing the frequency dependency of the foundation-soil interface have been explored under the assumption that the analysis can be performed using linear theories alone. The numerical solution of the equations of motion of structural systems, including such frequency dependent parameters, is performed using an integration scheme based on the discrete Fourier transform. Furthermore, preliminary experimental work on a newly built portal frame bridge is described. This portal frame bridge is subject to a case study in which the the computational techniques mentioned above are applied on a two dimensional model of the bridge. Theoretically, the damping of the SSI is shown to give a large contribution to those modes of vibration which excite the foundations much. These structural modal damping ratios may be much larger than those prescribed by the design codes. Those modes of vibration which do not excite the foundations much are similar to those obtained using clamped or constant elastic boundary conditions and in these cases, the contribution to the modal damping ratio of the structure is only a fraction of that prescribed by the design codes. A very rough analysis of measurements taken from the bridge indicate a similar behavior, but the amplitudes of vibration in many of the estimated modes are quite small (in the order of the quantization error of the measurement system) and therefore, the errors in the damping ratio estimates may be substantial. The work with this thesis have raised many questions, the answers to which are believed to substantially improve our understanding of resonance phenomena and also our possibilities to update numerical models of existing railway bridges using dynamic measurements.From the simplified analysis of a portal frame bridge performed within this project, it has been concluded that when the elastic modulus of the soil is increased, the total structural damping ratio when dynamic SSI is included decreases. Furthermore, with respect to vertical bridge deck accelerations, clamped boundary conditions are certainly not conservative as compared with static and dynamic SSI.