The dynamic interaction between a civil infrastructure and the soil beneath is crucial for seismic risk assessment. Due to the increased computational capacity, more frequently, this problem is starting to be addressed by 3D modelling based on the finite element method (FEM). However, because of the interaction between the stresses and strains in the orthogonal directions of the soil volume, i.e. the Poisson effect, it is not trivial to achieve specific spectral ordinates at the surface of the FEM model, after propagation from the bedrock. This study introduces a novel method aimed at obtaining surface-level ground motions with specific spectral intensities by using 3D FEM models. This method integrates spectral matching, filters, deconvolution using 1D models, and frequency modulation techniques, to address misalignments between the outcomes of 1D and 3D models, particularly focusing on high-frequency spectral amplification in soil response. It has been tested by analysing two seismic scenarios, which have been characterized from a probabilistic perspective. The proposed approach ensures the development of ground motion records accurately producing specific spectral intensities at the surface, enhancing seismic risk assessments and structural analysis. The study emphasizes the importance of accurate seismic hazard characterization, providing valuable insights for earthquake engineering practices.

Electric transformers are major components of electrical systems, and damage to them caused by earthquakes can result in significant financial loss. The current study modeled a three-dimensional (3D) isolated electrical transformer under horizontal and vertical records from different earthquakes. Instead of using fixed coefficients, an improved wavelet method has been used to create the greatest compatibility between the response spectra and the target spectrum. This method has primarily been used for dynamic analysis of isolated structures with spring-damper devices because it has shown greater accuracy in predicting the response of such structures. The effect of the nonlinear soil-structure interaction on the probability of transformer failure also has been investigated. Soil and structure interaction modeling was carried out using a beam on a nonlinear Winkler foundation. The effect of the nonlinear soil-structure interaction during dynamic analysis of transformers revealed that the greatest increase in the probability of transformer failure was in the fixed-base condition when the structure was located on soft soil. This intensified the response of the structure and increased the probability of transformer failure by up to 27% for far-field and up to 95% for near-field ground motions. A comparison of the results indicates that the use of 3D isolation systems in transformers in areas with soft clay that are subject to near-field ground motions can strongly reduce the probability of failure and improve the seismic performance of the transformer.