A series of large-scale shaking table tests were conducted to investigate the dynamic response and damage characteristics of the variable- single pile foundation in liquefiable soil-rock interaction strata under seismic loading. The test results show that the seismic responses of the excess pore pressure ratio under seismic excitations are divided into four stages, among which the difference in the sustained liquefaction stage is the most significant. Pile acceleration amplification is governed by dual coupling effects of soil-pile interaction and structural stiffness. The pile body bending moment distribution features dual-peak characteristics, the largest peak arises at the soil layers interface, while the other peak occurs at the variable-section. Increased seismic excitation accelerates the liquefaction of the saturated sand layer, yet simultaneously slows down the dissipation of the excess pore pressure. As the seismic excitation increases, the acceleration response and displacement response of the pile top are most significant, though maximum bending moment positions remain stable. The stress overrun damage occurs gradually in the variable- zone under strong earthquakes. Based on the analysis results and the Fourier spectrum modal characteristics of the pile top, the damage mechanism of the pile body is revealed and verified. This study will provide an essential reference for further understanding the seismic response and damage of the variable- single pile foundation in liquefiable soil-rock interaction strata.

Increasing numbers of complex structures are being constructed with the acceleration of urbanization. The complex dynamic characteristics pose challenges to the seismic design of large chassis. This paper investigates the seismic response and damage evolution of complex structures using linear and nonlinear dynamic explicit analysis under obliquely incident SV waves. A twodimensional finite element model considering soil-structure interaction (SSI) is developed using fiber beam elements. Elastic and elastoplastic damage constitutive models are employed. A comprehensive numerical analysis is conducted to investigate the influence of key parameters, including incidence angles, ground motion characteristics, and site types, on the seismic response and damage evolution of complex structures. The results of this study indicate that, in the elastic stage, the seismic response of the frame-shear wall structure is reduced in the case of oblique incidence compared to vertical incidence. Specifically, the inter-story drift ratio is reduced by 60% at an incidence angle of 30 degrees. In comparison to vertical incidence, the inter-story drift ratio and horizontal acceleration of the underground structure are reduced under oblique incidence. Conversely, in the elastic stage, the beam-end vertical displacement ratio and vertical acceleration exhibit increases of 57% and 36%, respectively. In the elastoplastic stage, as the incidence angle increases, the damage to the beams of the underground structure becomes more significant, while the damage to the frame-shear wall structure relatively decreases. Low-frequency ground motion and soft soil amplify the structural response compared to high-frequency and hard soil.

The wind resistance of transmission towers is not only affected by wind load, but also by service environment. This study uses the world's second Ultra High Voltage Direct Current transmission project - Xinjiang & PLUSMN;800 kV Tianzhong Line UHV DC transmission project - to develop a fragility analysis method for transmission towers in saline soil under wind loads to investigate the change of wind loads fragility of transmission towers in long-term service in the saline soil environment. It develops a tower-line-foundation (3 T-2L-F) system model considering soil-structure interaction. In addition, this research addresses the durability damage of the transmission tower using field investigation data and material degradation models and analyzes the influence of various durability damage components on the natural vibration mode of the basic of 3 T-2L-F model. Finally, it builds the structure-wind samples utilizing a Latin hypercube sampling method and explores the time histories analysis, the pushover analyses, and the time-varying fragility analyses considering the uncertainty of materials and wind loads. The findings indicate that the 3 T-2L-F model accurately simulates the actual situation of the transmission tower. The fragility of a transmission tower subjected to wind loads is proportional to the degree of material damage and the strength of wind loads.

Seismic fragility analysis is a crucial tool for assessing the seismic performance of buildings. In areas with dense clusters of tall buildings, the significant site-city interaction (SCI) effect alters wave propagation mechanisms, influencing the seismic fragility of structures. However, a significant increase in computational workload results from the need for detailed modeling of sites and building clusters for the SCI analysis. To address this challenge, this work first investigates the minimum number of earthquake waves required to characterize SCI-induced response changes. The Central Business District of Shanghai is analyzed. A table for the recommended minimum number for a given accuracy requirement and prediction reliability is provided. Moreover, a seismic fragility analysis method considering the SCI effect is proposed for low-rise buildings. The case study indicates that, buildings with similar height will exhibit various fragility changes after considering SCI. For the complete damage state, the mean intensity value of the fragility curve can be 14.4 % smaller than that without SCI. In addition, this approach provides significant computational workload reduction. For the case study, the computational workload of the proposed method is roughly 1/50 of that using traditional IDA method.

The unfrozen water content is crucial to soil's physical and mechanical properties. Soils on the Qinghai-Tibet Plateau are frequently subjected to freeze-thaw (F-T) cycles. The quantitative relationship between F-T effects and the unfrozen water content of soils requires further investigation. This study employs a nuclear magnetic resonance (NMR) scanner with a temperature-control module to measure the unfrozen water content of silty clay during multiple F-T cycles. The soil freezing characteristic curves (SFCC) of silty clay are derived from the T2 (transverse relaxation time) distribution curves based on NMR measurements. Two distinct T2 cutoff values are used to classify three types of water in soils: bound water, capillary water, and bulk water. The impact of F-T cycles on the evolution of unfrozen water content as temperatures decrease has been analyzed. The testing results indicate that the SFCC of silty clay can be segmented into three stages: super-cooling, fast-declining, and stable. As the number of F-T cycles increases, capillary water content decreases while bulk water content increases during the super-cooling stage. The damage coefficient, derived from pore volume measurements, increases sharply during the first four F-T cycles before stabilizing gradually. Additionally, there is a negative linear correlation between the damage coefficient and the initial capillary water content, and a positive linear correlation with the initial bulk water content. This study offers valuable insights for the quantitative analysis of unfrozen water content in seasonally frozen regions and serves as an essential guide for geotechnical construction projects in cold areas.