Resonance occurs when the natural frequency of an offshore wind turbine matches its rotational or blade passing frequency, potentially causing severe structural damage. Existing research on the resonance frequency characteristics of offshore wind turbines has mainly focused on elastic analysis, neglecting the nonlinear dynamic interaction between the foundation and soil. Based on the dynamic Winkler foundation model, the hyperbolic soil resistance around the pile-lateral displacement (p-y) backbone curve was used to consider the stiffness nonlinear of the pile-soil system. The shear strain-dependence of hysteretic damping was considered for soil energy dissipation. A simplified nonlinear frequency domain analysis method for calculating the resonance frequency of monopile-supported offshore wind turbines was proposed. The validity of the method was confirmed through comparisons with model test results and field measurements from the Lely (A2) offshore wind turbine. A parametric study was conducted to investigate the influence of sand density, pile length and pile diameter on the nonlinear resonance frequency of offshore wind turbines. The results show that the resonance frequency of the offshore wind turbine system decreases with increasing loading amplitude. Additionally, the influence of soil nonlinearity on the resonance frequency for systems is more obvious when the sand is looser, the pile length is shorter, or the pile diameter is smaller.

Noida, located within India's National Capital Region and near the tectonically active Himalayan region, is highly susceptible to seismic activity. Past moderate to high-intensity seismic events emphasize the need for detailed subsurface characterization to enhance seismic hazard assessments. This study investigates seismic site effects in Noida using microtremor measurements and the Nakamura technique to develop spatial distribution maps for seismic amplification, fundamental frequency, and seismic vulnerability index. A total of 129 microtremor data points were collected, with 54 meeting the SESAME criteria for reliable Horizontal-to-Vertical Spectral Ratio (HVSR) analysis. The analysis reveals that the predominant frequency at most sites falls within the range of 0.63-1.10 Hz, indicating the widespread presence of thick, soft sediments in the area. To avoid structural damages caused by the resonance of soil and structure and a table is prepared to showcase the approximate building frequency of various storey in order to avoid soil-building resonance phenomenon. The maximum amplification (A(0)) observed ranges from 4.53 to 5.17 at a few sites, whereas the majority of the study area experiences low to moderate amplification. The calculated seismic vulnerability index (K-g) for the 54 studied locations ranges from 2.27 to 23.60, with higher values found in regions with soft alluvial deposits, identifying them as fragile zones likely to suffer infrastructure damage during an earthquake. Lower K-g values correspond to areas with stiffer substrates. This study provides a preliminary assessment for urban planning and highlights the need for further research into the socio-economic and structural seismic vulnerabilities of the Noida region.

The fundamental resonance frequency is a key parameter for seismic risk assessment. Avoiding proximity between soil and building resonance frequencies is crucial to prevent dual resonances that exacerbate earthquake-induced damages. Additionally, establishing the initial fundamental resonance frequency of a building serves as a baseline for post-event damage assessment, offering a quantifiable metric alongside qualitative evaluations. This study focuses on collating ambient vibration measurements conducted in Nice's buildings since the 2000s. These measurements encompass diverse structural types, such as masonry-isolated or in block and reinforced concrete buildings. The streamlined configurations of the measurements chosen is a setup of three sensors at the top of building to be able to determine the fundamental resonance frequency and to check any possible torsion that could increase the vulnerability of the structure. We conducted a comparable analysis on all data, involving frequency selection from average Fourier spectra to identify the first mode. Our findings reveal a linear correlation between masonry buildings and height, expressed by the formula T = 0.014 H. For reinforced concrete buildings, the correlation is T = 0.015 H. These correlations align well with previous studies.