This study uses a two -stage finite element method to analyze offshore wind turbine (OWT) support structures under soil liquefaction during earthquakes. First, a three-dimensional cuboid u -p analysis with the soil cap yield criterion provides time -history soil accelerations and pore water pressure ratios. The second stage models the OWT support structure using traditional finite element methods, incorporating seismic displacement fields into p -y, t -z, and q -z nonlinear springs, with excess pore water pressure ratios reducing their stiffness. Numerical simulations were performed on 10, 15, and 20 MW OWT structures, yielding key insights. Ground accelerations near the soil surface cause frequent soil liquefaction, effectively mitigating seismic forces on the OWT foundation. When pile length is sufficient, the analysis with the u -p model requires less steel than traditional methods without considering soil liquefaction. Moreover, the reduction in sandy soil stiffness extends into deeper layers, making settlement a critical concern during soil liquefaction. Analysis accounting for soil liquefaction consistently reports greater settlements than traditional approaches. Consequently, the critical factor for deep pile OWT support structures with regard to soil liquefaction is foundation settlement rather than the design of the steel structure section.

This work presents a simplified method for the nonlinear analysis of the load-displacement response of piles in multi-layered soils. As a starting step, a new interface model based on the disturbed state concept (DSC) is put forth to simulate the interface shear stress-displacement relationship by considering the nonlinear hardening-softening behaviour. In the new model, input parameters can be conveniently calibrated using conventional interface shear tests or on-site tests. The good agreement between predictions and experimental data from interface direct shear tests validated the performance of the proposed DSC model. The DSC model performed better in terms of predictions when compared to the hyperbolic one. Next, the soil-structure interface model and bearing capacity theory are coupled to provide a theoretical framework for the analysis of pile load-transfer in saturated and unsaturated multi-layered soils, where the DSC model is employed to represent base resistance as well as skin friction. This work also discusses the profile of steady-state in-situ matric suction, soil-water characteristic curve, and pore-water pressure of unsaturated soils. The proposed method has the advantage of being used in practice as it is simple to obtain input parameters from laboratory tests, as well as Standard Penetration or Cone Penetration Tests. The proposed framework is finally applied to the analysis of five welldocumented case studies. The proposed approach and the static load test results from the field measurements are found to be in satisfactory agreement, indicating that the proposed method performs well. The proposed method is suggested to be utilised for preliminary analysis, planning a suitable programme of loading tests, as well as optimizing the pile design by back analysis of the load test results.

The safety and reliability of wind turbines subjected to multiple loads has recently attracted great attention. To investigate how soil-structure interaction (SSI) affects the seismic performance of operating wind turbines, a wind tunnel-shaking table test platform has been built which can realize applying wind load and seismic load simultaneously. A 1:100-scaled wind turbine model with two different kinds of foundations (soil-structure interactive foundation and rigid foundation) has been tested under four seismic excitations (El-Centro and Taft, input in two directions) when it keeps operating in wind excitations. Nacelle displacement with the soil-structure interactive foundation was significantly larger than that of the rigid foundation, reaching 3-5 times at a peak ground acceleration (PGA) of 0.8 g. The maximum nacelle displacement of the scaled model with soil-structure interactive foundation always occurs in the direction of incoming wind, unlike the rigid one occurring in the direction of seismic input direction. The rigid foundation model presents a strong whipping effect with an acceleration amplification factor of the tower top (APF = 3). In contrast, the model with soil-structure interactive foundation shows mild whipping effect due to smaller foundation stiffness (APF of the tower top = 1.5). The study demonstrates SSI could weaken dynamic responses, reducing bending moments but inducing excessive nacelle displacement, risking structural damage. This study underscores the importance of SSI in evaluating the safety and reliability of wind turbines subjected to the wind and seismic loads and provides experimental results for future designs.

The seismic response analysis of subway underground structures only considers the impact of a single mainshock, ignoring the aftershocks, which may cause more serious secondary damage within a short time after strong earthquakes. To investigate the seismic performance of underground structures under sequential earthquakes and improve the understanding of aftershocks, we analyzed the dynamic response of subway station structures under mainshock-aftershock sequences. A typical two-story and three-span subway station was selected as the prototype, and a finite element model of a soil-underground structure interaction was established. Based on the basic concept of endurance time and design response spectrum of relevant seismic standards in China, an endurance time acceleration function was generated, and nine mainshock-aftershock sequences were constructed. Thereafter, the seismic responses of subway stations under the action of mainshock-aftershock sequences, such as lateral deformation characteristics and damage failure law, were discussed and analyzed. The results show that the endurance time method can be used as a new and efficient method to study the seismic performance of underground structures subjected to mainshock-aftershock sequences. The effects of aftershocks on the damage and lateral displacement of underground structures were preliminarily revealed. The structural deformation response of each layer varies greatly due to the different damage states of the mainshock. The maximum story drift theta of the structure increases with the increase of loading seismic intensity. From the perspective of seismic damage and relative deformation, the structural dynamic response, considering the effect of sequential earthquakes, is greater than that of the mainshock alone, which reflects the disadvantage of aftershocks in seismic design. The results provide a reference for seismic analysis and damage assessment of structures under sequential earthquakes.

This paper presents an experimental and analytical study on a steel slit damper designed as an energy dissipative device for earthquake protection of structures considering soil-structure interaction. The steel slit damper is made of a steel plate with a number of slits cut out of it. The slit damper has an advantage as a seismic energy dissipation device in that the stiffness and the yield force of the damper can be easily controlled by changing the number and size of the vertical strips. Cyclic loading tests of the slit damper are carried out to verify its energy dissipation capability, and an analytical model is developed validated based on the test results. The seismic performance of a case study building is then assessed using nonlinear dynamic analysis with and without soil-structure interaction. The soil-structure system turns out to show larger seismic responses and thus seismic retrofit is required to satisfy a predefined performance limit state. The developed slit dampers are employed as a seismic energy dissipation device for retrofitting the case study structure taking into account the soil-structure interaction. The seismic performance evaluation of the model structure shows that the device works stably and dissipates significant amount of seismic energy during earthquake excitations, and is effective in lowering the seismic response of structures standing on soft soil.

The absence of a defined allowable pile ductility in integral abutment bridges (IABs) creates a critical gap in determining the maximum safe bridge length. This paper introduces a design aid procedure to assist bridge engineers in establishing the length limits of jointless bridges. Numerical and analytical approaches were used in formulating the design aid procedure. A total of 66 finite difference models were established to obtain pile equivalent cantilever length considering various design parameters (soil stiffness, pile size, pile orientation, axial compressive load, and lateral displacement magnitude). The analytical approach incorporates a strain compatibility and equilibrium model to generate moment -curvature diagrams and load -deflection curves for standard HP sections commonly used in IABs construction. The validity of the developed design aid procedure was examined and tested with available experimental and numerical results. Lateral buckling displacement capacity of HP sections ranged from 50 to 100 mm (2 - 4 in.). Based on these displacement capacities, length limits for IABs were established and compared with existing studies. The maximum length limits for steel integral bridges fall within the range of 162 - 320 m (530 - 1050 ft), while concrete integral bridges have limits ranging from 210 to 390 m (680 - 1285 ft). These limits depend on factors such as pile size, soil stiffness, and climate conditions.

The issue of SSI involves how the ground or soil reacts to a building built on top of it. Both the character of the structure and the nature of the soil have an impact on the stresses that exist between them, which in turn affects how the structure and soil beneath it move. The issue is crucial, particularly in earthquake regions. The interaction between soil and structure is an extremely intriguing factor in increasing or reducing structural damage or movement. Structures sitting on deformable soil as opposed to strong soil will experience an increase in static settlement and a decrease in seismic harm. The engineer must take into account that the soil liquefaction problem occurs for soft ground in seismic areas. A reinforced concrete wall -frame dual framework's dynamic reaction to SSI has not been sufficiently studied and is infrequently taken into consideration in engineering practice. The structures' seismic performance when SSI effects are taken into account is still unknown, and there are still some misconceptions about the SSI idea, especially regarding RC wall -frame dual systems. The simulation study of the soil beneath the foundations significantly impacts the framework's frequency response and dynamic properties. Therefore, the overall significance of SSI in the structural aspect and sustainability aspects will be reviewed in this research.

In recent years, strong earthquakes have caused a lot of damage around the world. In order to prevent such damage, proper evaluation of the seismic performance of buildings is absolutely necessary. However, the current analysis procedure in seismic design assumes fixed boundary conditions for the foundation and neglects the influence of the substructure on the superstructure. Previous studies have shown that the type of foundation affects structural responses during earthquakes. However, most of these studies have focused on single-degree-of-freedom (SDOF) structures and have not considered variations in response according to different substructure types. This study aims to investigate the effects of different substructures on ground motion and corresponding responses of the superstructure. Centrifugal simulations were conducted on a multi-degree-of-freedom (MDOF) superstructure, including a Half-embedded with Pile foundation, a fixed deep basement, and a Shallow foundation. The experimental results indicate that in the case of a half-substructure with a pile foundation, there was no significant difference between free field motion and foundation motion due to the pile foundation. However, in the case of a fixed deep basement, the embedment effect was most pronounced, especially in the short period range of 0.1 s to 0.5 s in the response spectrum. This resulted in a notable reduction in the spectrum. The analysis of the response spectra of foundation motion and free field motion revealed that the reduction effect was absent in the half-embedded with a pile foundation, but it was prominent in the fixed deep basement. Notably, the ratio of response spectrum increased in the fundamental period of the substructure. In the case of a shallow foundation, it was observed that foundation motion experienced larger amplification compared to free field motion. Shallow foundations have a relatively low stiffness of the substructure and are influenced by the inertial forces of the superstructure. Additionally, this tendency is believed to be more prominent due to the imperfectly fixed boundary conditions of shallow foundations to the ground. However, apart from the increase in foundation motion, the response of the superstructure was not proportional to it. These results contribute to a better understanding of the changes in seismic load and the response of multi-degree-of-freedom superstructures according to the type of substructure. The seismic design of the superstructure is safer and more reasonable when considering the effects of the type of substructure.

Considering the effect of dynamic interaction between liquefiable soil and underground structure, simulating the nonlinear seismic response of underground structures through simple, convenient, and reliable 3D numerical model is still a great challenge at present. A three-dimensional numerical simulation method for dynamic response of underground structure in liquefiable site is proposed based on the centrifuge shaking table test designed for a subway station buried in liquefiable interlayer site carried out in the previous period. The numerical model takes into account the characteristics of sand prone to large deformation after liquefaction and the nonlinear characteristics of the contact between saturated soil and structure. The typical numerical calculation results are compared with the experimental results to verify the correctness of the numerical model, and the extended analysis of the structural damage is carried out, which visually shows the damage distribution of structure under different functional states after earthquake. The analysis of structural damage shows that the junction position of middle column and longitudinal beam is more prone to tensile damage under the horizontal earthquake action; while after considering the horizontal-vertical seismic effect, the middle column will produce additional compressive damage due to the vertical inertia force, and the end of middle column should be paid attention to in seismic design.