Determination of scoured bridge natural frequencies with soil–structure interaction

• We developed a FEM to find bridge natural frequencies with soil–fluid–structure interactions.
• Problems with large degrees of freedom can be solved efficiently.
• A test was used to validate the calculated natural frequencies in good agreement.
• The water effect may be neglected in the bridge natural frequency analysis.
• If scour depth is over the pile cap, natural frequency changes with the scour depth are more obvious.

This study developed a finite element method with the effect of soil–fluid–structure interaction to calculate bridge natural frequencies. The finite element model includes bridge girders, piers, foundations, soil, and water. The effective mass above the soil surface was then used to find the first natural frequency in each direction. A field experiment was performed to validate that the natural frequencies calculated using the proposed finite element method had acceptable accuracy. The calculated natural frequencies with the fluid–structure interaction effect are always smaller than those without this effect. However, the frequency change due to the fluid effect is not obvious, so using the soil–structure interaction model is accurate enough in the bridge natural frequency analysis. The trend of the frequency decreases with the increase of the scour depth, but the curve is not smooth because of non-uniform foundation sections and layered soils. However, when the scour depth is such that pile cap is exposed, the changes in natural frequency with the scour depth are more obvious, and this is useful for measurement of the depth using bridge natural frequencies.