An efficient comparative beam-shell framework for assessing the vibration characteristics of corroded monopiles in soil–structure interaction

This study investigates the global (GSV) and local (LSV) structural vibration behavior of hollow steel monopiles embedded in soil. The monopile is modeled as a beam in the global formulation and as a cylindrical shell in the local formulation, while the surrounding soil is represented using a two-parameter Winkler-shear foundation. The governing equations are derived using Hamilton’s principle and solved numerically through the generalized differential quadrature technique (GDQT). A comprehensive parametric study is conducted to evaluate the effects of pile length, wall thickness, axial compressive load, boundary conditions (BCs), site classification, soil stiffness distribution patterns, corrosion-induced thickness reduction, and foundation stiffness on the inherent vibration frequencies (IVFs) of the monopile system. The results indicate that IVFs increase with mode number and soil shear wave velocity, while they decrease with increasing pile length, compressive axial load, and reduced boundary restraint. End-bearing configurations exhibit significantly higher frequencies than floating configurations due to increased tip restraint and effective lateral stiffness. The influence of wall thickness is found to be mode-dependent, with the first mode showing frequency reduction associated with modal mass-stiffness redistribution effects. The study further demonstrates that non-uniform soil stiffness distributions with low near-surface stiffness reduce higher-mode frequencies even when the depth-averaged shear wave velocity remains constant. Corrosion effects are shown to depend strongly on damage location relative to modal curvature, leading in some cases to frequency increases due to localized mass reduction. Overall, the results demonstrate distinct vibration characteristics predicted by the beam and shell formulations and highlight the importance of selecting an appropriate formulation for evaluating the vibration behavior of hollow monopiles.