Influence of plane strain soil-structure interaction on the eigenfrequency of railroad frame bridges

The effects of soil-structure interaction (SSI) on dynamic characteristics is of particular interest in the evaluation of resonance risk in railroad frame bridges. To identify the fundamental SSI mechanisms subject to the first vertical natural frequency, a plane interaction model of half-space and frame bridge is designed using the finite element method in Abaqus. Via parameter studies with variation of geometry and soil stiffnesses, the span widths and superstructure thickness is identified as governing parameters. The system reacts more robustly than expected to a variation of the soil stiffnesses. Contrary to expectations, the parameter study also shows that the combination of massive abutment and soft foundation soil causes the first natural frequency to increase due to counter-phase abutment vibrations and the system changes from a one-mass oscillator to a two-mass oscillator state. Recommendations for modeling with indirect replacement models and a hand formula are given for practical construction applications.