Flexible Foundation Research Articles

Two-dimensional translation, rocking, and waves in a building during soil-structure interaction excited by a plane earthquake P-wave pulse

A two-dimensional model of a building supported by a rectangular, flexible foundation embedded in the soil is analyzed for excitation by an incident plane P-wave. The building, the foundation, and the soil have different material properties. The building is assumed to be anisotropic and linear, while the soil and the foundation are assumed to be isotropic and can experience nonlinear deformations. In general, the work spent for the development of nonlinear strains in the soil can consume a significant part of the input wave energy and thus less energy is available for the excitation of the building. However, in the case of excitation by a plane P-wave pulse, we show that the nonlinear response in the soil and the foundation does not significantly change the nature of excitation in the base of the building. It is noted that the response of a building can be approximated by base translation and rocking for excitation by long, strong motion waves.

Development and validation of p‐y modeling approach for seismic response predictions of highway bridges

SummaryThis study aims to realistically simulate the seismic responses of typical highway bridges in California with considerations of soil–structure interaction effects. The p‐y modeling approaches are developed and validated for embankments and pile foundations of bridges. The p‐y approach models the lateral and vertical foundation flexibility with distributed p‐y springs and associated t‐z and q‐z springs. Building upon the existing p‐y models for pile foundations, the study develops the nonlinear p‐y springs for embankments based on nonlinear 2D and 3D continuum finite element analysis under passive loading condition along both longitudinal and transverse directions. Closed‐form expressions are developed for two key parameters, the ultimate resistant force pult and the displacement y50, where 0.5pult is reached, of embankment p‐y models as functions of abutment geometry (wall width and height, embankment fill height, etc.) and soil material properties (wall‐soil friction angle, soil friction angle, and cohesion). In order to account for the kinematic and site responses, depth‐varying ground motions are derived and applied at the free‐end of p‐y springs, which reflects the amplified embankment crest motion. The modeling approach is applied to simulate the seismic responses of the Painter Street Bridge and validated through comparisons with the recorded responses during the 1992 Petrolia earthquake. It is demonstrated that the flexibility and motion amplification at end abutments are the most crucial modeling aspects. The developed p‐y models and the modeling approach can effectively predict the seismic responses of highway bridges. Copyright © 2016 John Wiley & Sons, Ltd.

Soil–structure interaction for a SDOF oscillator supported by a flexible foundation embedded in a half-space: Closed-form solution for incident plane SH-waves

A closed-form analytical solution is presented for the dynamic response of a SDOF oscillator, supported by a flexible foundation embedded in an elastic half-space, and excited by plane SH waves. The solution is obtained by the wave function expansion method. The solution is verified for the special case of a rigid foundation by comparison with published results. The model is used to investigate the effect of the foundation flexibility on the system response. The results show that the effect is significant for both foundation response and structural relative response. For a system with more flexible foundation, the radiation damping is smaller, the foundation response is larger, especially for obliquely incident waves, while the structural relative response is smaller, and the system frequency shifts towards lower frequencies. This simple model may be helpful to obtain insight into the effects of soil–structure interaction for a slim structure on an extended flexible foundation.

A hybrid approach for modelling dynamic behaviours of a rotor-foundation system

A new hybrid approach is presented to study the dynamic behaviour of a rotor- foundation system, in which a shaft coupled with various discontinuities are connected to a flexible foundation via discrete spring subunits. By modelling the rotor with the modified transfer matrix method and describing the flexible foundation through the appropriate modal model, the proposed technique facilitates a computationally efficient modelling approach where a mixture of theoretical, numerical or experimental models can be incorporated into one overall numerical model. Particularly, the present model enables one to conveniently consider both the free and forced vibrations as well as effects of various combinations of discontinuities encountered in the rotor. Some results are compared with available results in previous publications and those from the finite element method to validate the model. Parametric studies are also performed to demonstrate the accuracy and versatility of the developed method for substructure coupling analysis.

Biesterfeld to sell Heroc aseptic/antimicrobial masterbatches worldwide

Designing support structures for offshore wind turbines is a complex task as these are highly dynamic systems subjected to long-term cyclic loads with variable amplitude. Long-term cyclic loading may cause stiffening or softening of the soil around the pile foundation of an offshore wind turbine, which leads to variations in the foundation stiffness and accumulated permanent rotation of the pile. Although variations in the foundation stiffness can negatively impact the fatigue life, the long-term variability of the soil conditions is normally not considered in the fatigue damage assessment. The main objective of this study is the investigation of the impact of changes in soil parameters on the fatigue lifetime for an offshore wind turbine founded in loose sand. For this purpose, a generic monopile based offshore wind turbine with flexible foundation model was used. The soil-pile interaction was modeled with a distributed spring model using nonlinear API p-y curves. Integrated analyses in the time domain were performed and fatigue damage was assessed in terms of a damage equivalent bending moment at mudline. The fatigue lifetime varies between -9 percent and +4 percent when considering changes in soil conditions, depending on the assumption of soil softening or stiffening, respectively. These results indicate that changes in soil parameters should be taken into account in the fatigue damage calculations of offshore wind turbines for more precise fatigue lifetime estimation. Moreover, it is emphasized that more accurate modeling of soil-pile interaction is required in the design and optimization of offshore wind turbines.

Brazil: Evonik – precipitated silica

Designing support structures for offshore wind turbines is a complex task as these are highly dynamic systems subjected to long-term cyclic loads with variable amplitude. Long-term cyclic loading may cause stiffening or softening of the soil around the pile foundation of an offshore wind turbine, which leads to variations in the foundation stiffness and accumulated permanent rotation of the pile. Although variations in the foundation stiffness can negatively impact the fatigue life, the long-term variability of the soil conditions is normally not considered in the fatigue damage assessment. The main objective of this study is the investigation of the impact of changes in soil parameters on the fatigue lifetime for an offshore wind turbine founded in loose sand. For this purpose, a generic monopile based offshore wind turbine with flexible foundation model was used. The soil-pile interaction was modeled with a distributed spring model using nonlinear API p-y curves. Integrated analyses in the time domain were performed and fatigue damage was assessed in terms of a damage equivalent bending moment at mudline. The fatigue lifetime varies between -9 percent and +4 percent when considering changes in soil conditions, depending on the assumption of soil softening or stiffening, respectively. These results indicate that changes in soil parameters should be taken into account in the fatigue damage calculations of offshore wind turbines for more precise fatigue lifetime estimation. Moreover, it is emphasized that more accurate modeling of soil-pile interaction is required in the design and optimization of offshore wind turbines.

Influence of Soil Parameters on the Fatigue Lifetime of Offshore Wind Turbines with Monopile Support Structure

Designing support structures for offshore wind turbines is a complex task as these are highly dynamic systems subjected to long-term cyclic loads with variable amplitude. Long-term cyclic loading may cause stiffening or softening of the soil around the pile foundation of an offshore wind turbine, which leads to variations in the foundation stiffness and accumulated permanent rotation of the pile. Although variations in the foundation stiffness can negatively impact the fatigue life, the long-term variability of the soil conditions is normally not considered in the fatigue damage assessment. The main objective of this study is the investigation of the impact of changes in soil parameters on the fatigue lifetime for an offshore wind turbine founded in loose sand. For this purpose, a generic monopile based offshore wind turbine with flexible foundation model was used. The soil-pile interaction was modeled with a distributed spring model using nonlinear API p-y curves. Integrated analyses in the time domain were performed and fatigue damage was assessed in terms of a damage equivalent bending moment at mudline. The fatigue lifetime varies between -9 percent and +4 percent when considering changes in soil conditions, depending on the assumption of soil softening or stiffening, respectively. These results indicate that changes in soil parameters should be taken into account in the fatigue damage calculations of offshore wind turbines for more precise fatigue lifetime estimation. Moreover, it is emphasized that more accurate modeling of soil-pile interaction is required in the design and optimization of offshore wind turbines.

Effect of numerical soil-foundation-structure modeling on the seismic response of a tall bridge pier via pushover analysis

This paper examines the role of the numerical modeling of soil-foundation-structure (SFS) interaction on the seismic response of a tall, partially embedded, flared bridge pier. For this purpose, static, pushover, nonlinear, finite-element, stand-alone analyses are performed on nine different models of one of the two piers of the Mogollon Rim Viaduct, a long-span, reinforced-concrete bridge supported on pile foundations. Structural modeling considerations, such as selection of concrete constitutive models, material properties, and bond-slip and P-Δ effects, on the nonlinear response of this pier are investigated. p-y, t-z and Q-z nonlinear curves are applied to model the soil-pile interaction, and equivalent nonlinear springs are developed to reproduce the soil-pile cap interaction. In addition, the effects of the partial pier embedment and the slope of the ground surface on the lateral resistance of the pier and the total capacity of the SFS system are examined. The results illustrate how structural and geotechnical modeling approaches for the SFS interaction can affect the nonlinear response of tall bridges, and may lead to differences in the numerical prediction of local or global failure. For the case analyzed herein, the partial pier embedment and foundation flexibility can dramatically modify the structural response, and influence the bond-slip effect at the pier-pile cap connection.

Retrofitting of steel pile-abutment connections of integral bridges using CFRP

Integral bridges are typically designed with flexible foundations that include one row of piles. The construction of integral bridges solves difficulties due to the maintenance of expansion joints and bearings during serviceability. It causes integral bridges to become more economic comparing with conventional bridges. Research has been focused not only to enhance the seismic performance of newly designed bridges, but also to develop retrofit strategies for existing ones. The local performance of the pile to abutment connection will have a major effect on the performance of the structure and the embedment length of pile inside the abutment has a key role to provide shear and flexural resistance of pile-abutment connections. In this paper, a simple method was developed to estimate the initial value of embedment length of the pile for retrofitting of specimens. Four specimens of pile-abutment connections were constructed with different embedment lengths of pile inside the abutment to evaluate their performances. The results of the experimentation in conjunction with numerical and analytical studies showed that retrofitting pile-abutment connections with CFRP wraps increased the strength of the connection up to 86%. Also, designed connections with the proposed method had sufficient resistance against lateral load.

Two-dimensional translation, rocking, and waves in a building during soil-structure interaction excited by a plane earthquake SV-wave pulse

A two-dimensional (2-D) model of a building supported by a rectangular, flexible foundation embedded in the soil is analyzed for excitation by an incident plane SV-wave. The incidence is below the critical angle. The building is assumed to be anisotropic and linear while the soil and the foundation are assumed to be isotropic and can experience nonlinear deformations. In general the work spent for the development of nonlinear strains in the soil can consume a significant part of the input wave energy and thus less energy is available for the excitation of the building. We show that the energy distribution in the building depends on the nature of the incident wave and differs substantially between the cases of incident P- and SV-waves. However, for both excitation by a plane SV-wave pulse and excitation by a P-wave, we show that the nonlinear response in the soil and the foundation does not significantly change the nature of excitation of the base of the building. It is noted that the building response can be approximated by translation and rocking of the base only for excitation by long, strong motion waves.

Time-domain uncoupled analyses for seismic assessment of land-based wind turbines

Seismic analysis plays an important role in the design of land-based and offshore wind turbines in areas at seismic hazard. For seismic assessment, International Standards and Guidelines allow combining two separate analyses, one under wind and another under earthquake only, as alternative to computationally expensive, fully-coupled time-domain simulations. In these uncoupled analyses, the separate earthquake response is generally computed using the standard acceleration response spectrum, upon including an additional damping referred to as aerodynamic damping. By a response-spectrum approach, however, important sources of nonlinearity, such as those related to foundation flexibility, cannot be properly accounted for.Focusing on land-based wind turbines, this paper investigates a time-domain implementation of uncoupled analyses, which may involve a nonlinear foundation model. The case study is a 5MW baseline wind turbine, resting on a pile foundation modeled by nonlinear springs. For different earthquake records and wind velocities, comparisons with fully-coupled simulations show that the combination of uncoupled analyses implemented in the time domain yields accurate results, provided that an appropriate level of aerodynamic damping is included in the model. Notably, it is seen that such aerodynamic damping level agrees with the one generally recommended for response-spectrum based uncoupled analyses.

Numerical Analysis of Deformations of the Shallow Square Foundations with Variable Relative Stiffness

The paper deals with numerical analysis of deformations and relative displacements (relative settlement, relative deflection and flexibility) of the shallow square foundations depending on the variable relative stiffness. For solution of the problem finite element method was used with theoretical assumptions of the linearly elastic half-space. Foundation-subsoil contact was modelled by one-directional and bi-directional bond effect. Results of the solution are presented by graphical forms.

Human-Robot Collaboration Dynamic Impact Testing and Calibration Instrument for Disposable Robot Safety Artifacts.

The Dynamic Impact Testing and Calibration Instrument (DITCI) is a simple instrument with a significant data collection and analysis capability that is used for the testing and calibration of biosimulant human tissue artifacts. These artifacts may be used to measure the severity of injuries caused in the case of a robot impact with a human. In this paper we describe the DITCI adjustable impact and flexible foundation mechanism, which allows the selection of a variety of impact force levels and foundation stiffness. The instrument can accommodate arrays of a variety of sensors and impact tools, simulating both real manufacturing tools and the testing requirements of standards setting organizations. A computer data acquisition system may collect a variety of impact motion, force, and torque data, which are used to develop a variety of mathematical model representations of the artifacts. Finally, we describe the fabrication and testing of human abdomen soft tissue artifacts, used to display the magnitude of impact tissue deformation. Impact tests were performed at various maximum impact force and average pressure levels.

A case study on the soil–pile–structure interaction of a long span arched structure

Different concepts for modelling of soil-foundation in complete dynamic interaction analysis for a 110-m height 70-m span arched structure on 180 piles were investigated in this paper. The modelling approaches consisted of a sophisticated procedure to account for soil compliance and foundation flexibility by defining frequency-dependent springs and dashpots; namely, flexible-impedance base model. The results of this model were compared with those of the conventional modelling procedures; namely, fixed base model and flexible base model by defining frequency-independent springs. In the flexible-impedance base model, the substructure approach was employed through finite element modelling. To account for the kinematic interaction, the numerical model of the soil, foundation and piles were developed using a verified finite element model in ABAQUS. The free field time history and design spectrum were modified to obtain the foundation input motion. The impedance of pile groups with different length was obtained by the finite element model to assess the inertial interaction. The comparison of the results of the employed models showed that rocking and torsional responses were greatly affected by soil–structure interaction, indicating redistribution of seismic demands. It was also proven that the internal demands of the conventional model considering frequency-independent Winkler springs might be higher than those of the model including pile–soil–structure interaction effects.

Seismic performance of a bridge with tall piers

This paper presents the study of seismic performance evaluation of a proposed railway bridge with tall piers. The study is mainly focused on the central span of bridges having 100-m high piers. The primary aim of the study is to examine the applicability of the response reduction factors, R, given in seismic design codes, for bridges with tall piers. The bridge is designed with increasing values of response reduction factors and its performance is evaluated considering different detailing practices. Moment curvature behaviour and shear resistance of hollow reinforced concrete piers is also discussed. The effect of foundation flexibility on seismic performance of the bridge is explored by performing a sensitivity study with varying modulus of subgrade reaction of the founding soil. It is observed that the response reduction factors prescribed in the Indian code also yield satisfactory performance for tall bridges. In contrast to buildings, currently, displacement is not considered as a design criterion for bridges; however, in most of the cases considered in the present study, the bridges behaved elastically or nearly elastically, with excessive displacement at the top. Foundation–soil flexibility has a significant effect on the displacement of tall bridges, and results in a change of seismic performance level of the considered bridge, in some cases.

Closed form solution of Eigen frequency of monopile supported offshore wind turbines in deeper waters incorporating stiffness of substructure and SSI

Offshore wind turbines (OWTs) are dynamically loaded structures and therefore the estimation of the natural frequency is an important design calculation to avoid resonance and resonance related effects (such as fatigue). Monopiles are currently the most used foundation type and are also being considered in deeper waters (>30m) where a stiff transition piece will join the monopile and the tapered tall tower. While rather computationally expensive, high fidelity finite element analysis can be carried to find the Eigen solutions of the whole system considering soil–structure interaction; a quick hand calculation method is often convenient during the design optimisation stage or conceptual design stage. This paper proposes a simplified methodology to obtain the first natural frequency of the whole system using only limited data on the WTG (Wind Turbine Generator), tower dimensions, monopile dimensions and the ground. The most uncertain component is the ground and is characterised by two parameters: type of ground profile (i.e. soil stiffness variation with depth) and the soil stiffness at one monopile depth below mudline. In this framework, the fixed base natural frequency of the wind turbine is first calculated and is then multiplied by two non-dimensional factors to account for the foundation flexibility (i.e. the effect of soil–structure interaction). The theoretical background behind the model is the Euler–Bernoulli and Timoshenko beam theories where the foundation is idealised by three coupled springs (lateral, rocking and cross-coupling). 10 wind turbines founded in different ground conditions from 10 different wind farms in Europe (e.g. Walney, Gunfleet sand, Burbo Bank, Belwind, Barrow, Kentish flat, Blyth, Lely, Thanet Sand, Irene Vorrink) have been analysed and the results compared with the measured natural frequencies. The results show good accuracy (errors below 3.5%). A step by step sample calculation is also shown for practical use of the proposed methodology.

Seismic performance of a 10-story RC box-type wall building structure

The purpose of this study is to evaluate the seismic performance of high-rise reinforced concrete (RC) box-type wall structures commonly used for most residential buildings in Korea. For this purpose, an analytical model was calibrated with the results of the earthquake simulation tests on a 1:5 scale 10-story distorted model. This calibrated model was then transformed to a true model. The performance of the true model in terms of the stiffness, strength, and damage distribution through inelastic energy dissipation was observed with reference to the earthquake simulation test results. The model showed high overstrength factors ranging from 3 to 4. The existence of slab in this box-type wall system changed the main resistance mode in the wall from bending moment to tension/compression coupled moment through membrane actions, and increased the overall resistance capacity by about 25~35%, in comparison with the common design practice of neglecting the slab´s existence. The flexibility of foundation, which is also commonly neglected in the engineering design, contributes to 30~50% of the roof drift in the stiff direction containing many walls. The possibility of concrete spalling and reinforcement buckling and fracture under the maximum considered earthquake (MCE) in Korea appears to be very low when compared with the case of the 2010 Concepcion, Chile earthquake.

Accounting for soil nonlinearity in three-dimensional seismic structure-soilstructure-interaction analyses of adjacent tall buildings structures

The interactive effects of adjacent buildings on their seismic performance are not frequently considered in seismic design. The adjacent buildings, however, are interrelated through the soil during seismic ground motions. The seismic energy is redistributed in the neighboring buildings through multiple structure-soil-structure interactions (SSSI). For example, in an area congested with many nearby tall and/or heavy buildings, accounting for the proximity effects of the adjacent buildings is very important. To solve the problem of SSSI successfully, researchers indicate two main research areas where need the most attention: 1) accounting for soil nonlinearity in an efficient way, and 2) spatial analysis of full 3D soil-structure models. In the present study, three-dimensional finite element models of tall buildings on different flexible foundation soils are used to evaluate the extent of cross interaction of adjacent buildings. Soil nonlinearity under cyclic loading is accounted for by Equivalent Linear Method (ELM) as to conduct large parametric studies in the field of seismic soil-structure interaction, the application of ELM is preferred over other alternatives (such as application of complicated constitutive soil models) due to the efficiency and reliability of its results. 15 and 30 story steel structures with pile foundations on two sandy and clayey sites are designed according to modern codes and then subjected to several actual earthquake records scaled to represent the seismicity of the building sites. Results show the cross interaction of adjacent buildings on flexible soils, depending on their proximity, increases dynamic displacements of buildings and reduces their base shears.

Uji Beban dan Analisis Lendutan Model Pelat Fleksibel yang Didukung Tiang-Tiang pada Tanah Pasir

Sand commonly has a good bearing capacity. Problems may occur when sand has low density and thick deposit. Flexible plate foundation may be used in this soil but plate deflection may be still high. To reduce deflection and improving soil density, piles were used to support the plate. Installing piles made foundation system stiffer. The objectives of this study are to studies about behavior of plates and plate with a pile of sand on the ground, influence of plate thickness , pile length and spacing of the pole deflection, and the influence of the bond between the plate and the deflection of the pile .In this study is used test box of 1.2 m depth, 1.2 m width, 1.2 m length, and 1.0 m depth filled out with sand. Plate models were made from plexiglas of rectangular and square geometry. Piles were of steel pipesof 2.5cm diameter. Some parameters were as follows : plate thickness, plate geometry, pile length (L), pile spacing (s), bounding between plate and pile (fix or free), ’piled’ and freestanding foundation, and base pile enlargement. The behavior of the plateswere observed under loading (point load). The result shows that plate deflections were affected by the method of pile installation, plate thickness and pile length. For a ticker plate, contact surface between plate and soil was wider. For the 40cmx10cm plates with fix end pile, deflectionswere founf to reduced up to 70% compared with free end pile. The ’piled foundation’ on 40cmx10cm plates, L=20cm, s=4d, deflections were reduced 83,63% compared with free standing foundation.

Analisis Lendutan Model Pelat Fleksibel dengan Tiang Perbesaran Ujung dan Pelat Tidak Rapat Tanah Pada Tanah Pasir

Problems in sandy soil may occur when sand has low density, uniform gradation and thick deposit. Flexible plate foundation may used in this condition but plate deflection still high. To reduce deflection and to improve soil density, piles were used to support the plate. Installing piles made foundation system stiffer. The objectives of this study are to studies about behavior of plates and plate with pile on sandy soil. Plate deflection was observed with variation of plate thickness, bottom pile enlargement, and soil-plate-pile interaction (free standing pile and piled foundation). 1,2 x 1,2 x 1,2 m box container filled with sandy soil was used as soil media. Square and rectangular plexiglass plate were used to modelled plate. Steel pipe with 2,5 cm in diameter were used as pile model. The behavior of the plates were observed under loading (point load). The results shows that plate deflections were affected on plate thickness, bottom pile enlargement and soil-plate-pile interaction. For a ticker plate, contact surface between plate and soil was wider. For the 40 cm x 10 cm plates with base pile enlargement, deflections were found to reduced up to 21,26%. The ‘piled foundation’ on 40 cm x 10 cm plates, (installing with 20 cm pile length, and 10 cm spacing between pile), deflections were reduced 83,63% compared with free standing foundation.