The quest for clean, renewable energy resources has given a global rise in offshore wind turbine (OWT) construction. As OWTs are more exposed to harsh environmental conditions, the dynamic behavior of OWTs with jacket support structures under critical loading scenarios is crucial yet least understood, which becomes more convoluted with the consideration of soil-structure interaction (SSI) effects. In addition, the seismic characteristics of such systems heavily depend on the excitation characteristics like frequency content, a feature that is still ambiguous. This research aims to examine the influence of seismic frequency contents on the dynamic characteristics and damage modes of jacket-supported OWT systems including SSI effects. The numerical model is established and validated based on a previous study, which ensures the accuracy of the numerical modeling framework. Upon validation, extensive numerical analyses are performed under earthquakes with varying frequency contents. Results reveal the relationship among the ground motion frequency, SSI, and the dynamic and damage behavior of jacket-supported OWTs, offering important insights for the improved seismic design and analysis of jacket-supported OWTs.

In seasonally frozen areas, freezing of topsoil is detrimental to the seismic response of bridge substructures and may cause bridge damage. In order to counter this problem, this paper proposes the use of a temperature-insensitive composite material (PolyBRuS) to replace the soil around the with the aim of preventing seismic damage brought about by the seasonally frozen soil, which is named as the replacement method. Firstly, a three-dimensional finite element model was built based on the model tests, and the results of the model tests were used for verification and calibration. Secondly, based on the finite element model, a time-history analysis of the seismic response of the bridge substructure was carried out to explore the nonlinear seismic response of the bridge foundation in different seasons and with or without replacement conditions. The result of numerical simulations showed that frozen soil significantly reduced the extent of the plastic zone of the soil under seismic loading and affected the seismic response of the bridge substructure, including an increase of foundation acceleration (19% increase), a decrease of foundation displacement (32% decrease), and an increase of foundation bending moment (10% increase). Notably, it can be found that the replacement method can reduce the seismic acceleration, increased column deformation (21% increase), and reduced column bending moment of the winter bridge foundations (9% decrease), consequently reducing the risk of seismic damage to the bridge substructure. Meanwhile, the compressive stress and compressive strain characteristics of the PolyBRuS material on the column side under seismic action are similar to those of unfrozen soil in summer. Above all, the adverse effects of surface freezing on bridge substructures can be effectively mitigated by the replacement method, and the bridge foundations will have similar seismic responses in winter and summer. This achievement has practical application prospects and is expected to provide a new seismic strategy for bridge engineering in seasonally frozen soil areas.

Here, a seismic-response analysis model was proposed for evaluating the nonlinear seismic response of a pile-supported bridge pier under frozen and thawed soil conditions. The effect of a seasonally frozen soil layer on the seismic vulnerability of a pile-supported bridge pier was evaluated based on reliability theory. Although the frozen soil layer inhibited the seismic response of the ground surface to a certain extent, it exacerbated the acceleration response at the bridge pier top owing to the low radiation damping effect of the frozen soil layer. Furthermore, the frozen soil layer reduced the lateral displacement of the bridge pier top relative to the ground surface by approximately 80%, thereby preventing damage caused by earthquakes, such as falling girders. Compared to the thawed state of the ground surface, the bending moment of the bridge pier in frozen ground increases. However, the bending moment of the pile foundation in frozen ground decreases, thereby lessening the seismic vulnerability of the bridge pile foundation. The results of this can provide a reference for the seismic response analysis and seismic risk assessment of pile-supported bridges in seasonally frozen regions.