Showing papers on "Soil structure interaction published in 2007"

Nonlinear Soil-Abutment-Bridge Structure Interaction for Seismic Performance-Based Design

Abstract: Current seismic design of bridges is based on a displacement performance philosophy using nonlinear static pushover analysis. This type of bridge design necessitates that the geotechnical engineer predict the resistance of the abutment backfill soils, which is inherently nonlinear with respect to the displacement between soil backfill and the bridge structure. This paper employs limit-equilibrium methods using mobilized logarithmic-spiral failure surfaces coupled with a modified hyperbolic soil stress-strain behavior (LSH model) to estimate abutment nonlinear force-displacement capacity as a function of wall displacement and soil backfill properties. The calculated force-displacement capacity is validated against the results from eight field experiments conducted on various typical structure backfills. Using LSH and experimental data, a simple hyperbolic force-displacement (HFD) equation is developed that can provide the same results using only the backfill soil stiffness and ultimate soil capacity. HFD is compatible with current CALTRANS practice in regard to the seismic design of bridge abutments. The LSH and HFD models are powerful and effective tools for practicing engineers to produce realistic bridge response for performance-based bridge design.

Influence of dynamic soil–structure interaction on the nonlinear response and seismic reliability of multistorey systems

Abstract: A set of reinforced concrete structures with gravitational loads and mechanical properties (strength and stiffness) representative of systems designed for earthquake resistance in accordance with current criteria and methods is selected to study the influence of dynamic soil–structure interaction on seismic response, ductility demands and reliability levels. The buildings are considered located at soft soil sites in the Valley of Mexico and subjected to ground motion time histories simulated in accordance with characteristic parameters of the maximum probable earthquake likely to occur during the system's expected life. For the near-resonance condition the effects of soil–structure interaction on the ductility demands depend mainly on radiation damping. According to the geometry of the structures studied this damping is strongly correlated with the aspect ratio, obtained by dividing the building height by its width. In this way, for structures with aspect ratio greater than 1.4 the storey and global ductility demands increase with respect to those obtained with the same structures but on rigid base, while for structures with aspect ratio less than 1.4 the ductility demands decrease with respect to those for the structures on rigid base. For the cases when the fundamental period of the structure has values very different from the dominant ground period, soil–structure interaction leads in all cases to a reduction of the ductility demands, independently of the aspect ratio. The reliability index β is obtained as a function of the base shear ratio and of the seismic intensity acting on the nonlinear systems subjected to the simulated motions. The resulting reliability functions are very similar for systems on rigid or on flexible foundation, provided that in the latter case the base rotation and the lateral displacement are removed from the total response of the system. Copyright © 2006 John Wiley & Sons, Ltd.

Influence of the soil–structure interaction on the fundamental period of buildings

Abstract: This paper includes an investigation of the influence of the soil-structure interaction (SSI) on the fundamental period of buildings. The behaviour of both the soil and the structure is assumed to be elastic. The soil-foundation system is modelled using translational and rotational discrete springs. Analysis is first conducted for one-storey buildings. It shows that the influence of the SSI on the fundamental frequency of building depends on the soil-structure relative rigidity K ss . Analysis is then extended for multi-storey buildings. It allows the generalization of the soil-structure relative rigidity K s to such complex structures. Charts are proposed for taking into account the influence of the SSI in the calculation of the fundamental frequency of a wide range of buildings.

Bridge–embankment interaction under transverse ground excitation

Abstract: Seismic performance and dynamic response of bridge–embankments during strong or moderate ground excitations are investigated through finite element (FE) modelling and detailed dynamic analysis. Previous research studies have established that bridge–embankments exhibit increasingly flexible performance under high-shear deformation levels and that soil displacements at bridge abutment supports may be significant particularly in the transverse direction. The 2D equation of motion is solved for the embankment, in order to evaluate the dynamic characteristics and to describe explicitly the seismic performance and dynamic response under transverse excitations accounting for soil nonlinearities, soil–structure interaction and imposed boundary conditions (BCs). Using the proposed model, equivalent elastic analysis was performed so as to evaluate the dynamic response of approach embankments while accounting for soil–structure interaction. The analytical procedures were applied in the case of a well-documented bridge with monolithic supports (Painter Street Overcrossing, PSO) which had been instrumented and embankment participation was identified from its response records after the 1971 San Fernando earthquake. The dynamic characteristics and dynamic response of the PSO embankments were evaluated for alternative BCs accounting for soil–structure interaction. Explicit expressions for the evaluation of the critical embankment length Lc are provided in order to quantify soil contribution to the overall bridge system under strong intensity ground excitations. The dynamic response of the entire bridge system (deck–abutments–embankments) was also evaluated through simplified models that considered soil–structure interaction. Results obtained from this analysis are correlated with those of detailed 3D FE models and field data with good agreement. Copyright © 2007 John Wiley & Sons, Ltd.

Impedance of flexible suction caissons

Abstract: The dynamic response of offshore wind turbines is affected by the properties of the foundation and the subsoil. The aim of this paper is to evaluate the dynamic soil-structure interaction of suction caissons for offshore wind turbines. The investigations include evaluation of the vertical and coupled sliding-rocking vibrations, influence of the foundation geometry and examination on the properties of the surrounding soil. The soil is simplified as a homogenous linear viscoelastic material and the dynamic stiffness of the suction caisson is expressed in terms of dimensionless frequency-dependent coefficients corresponding to different degrees of freedom. The dynamic stiffness coefficients for the skirted foundation are evaluated using a three-dimensional coupled boundary element/finite element model. Comparisons with known analytical and numerical solutions indicate that the static and dynamic behaviours of the foundation are predicted accurately using the applied model. The analysis has been carried out for different combinations of the skirt length, Poisson's ratio of the subsoil and the ratio of the soil stiffness to the skirt stiffness.

Earthquake fault rupture—shallow foundation interaction in undrained soils: a simplified analytical approach

Abstract: The interaction between normal or reverse fault ruptures and shallow foundations resting on a homogeneous undrained soil layer is investigated. After performing a thorough set of non-linear finite element simulations, three simple kinematic mechanisms are proposed to predict the conditions for the fault rupture to be diverted by the foundation. The results obtained by both numerical and analytical approaches are in good agreement, and support the adequacy of the proposed mechanisms. A relationship is established for assessing the minimum foundation bearing load needed for diversion of the fault rupture trace, either normal or reverse. This relationship is proved to be independent of the fault type and dip angle, so that it can be applied easily for engineering applications, even in the absence of specific tectonic information.

Modified Topographic Amplification Factors for a Single-Faced Slope due to Kinematic Soil-Structure Interaction

Abstract: In this paper, we evaluate the additive effects of topography, soil nonlinearity, and soil-structure interaction SSI along the crest of an idealized 40 m high cliff-type topographic feature with slope inclination 300, where excessive damage was observed during the Athens 1999 earthquake. The objective of this paper is to investigate the relative contribution of topographic amplification, and kinematic SSI as a function of the incident motion frequency content and geotechnical site conditions for a surface and an embedded structure located at the cliff crest. For this purpose, we perform elastic parametric and nonlinear site-specific two-dimensional finite element simulations using three profiles and six input motions. We illustrate the role of SSI in altering the response at the location of peak topographic amplification potential behind the crest, the effects of incident motion incoherency on the transient structural response, and the beneficial contribution of structural embedment. We finally suggest that empirical models for base-slab averaging of shallow foundations, developed as a function of site conditions, structural dimensions and center line location, could be combined with topographic amplification factors to predict realistic design spectra for structures located on irregular topographic features.

Probabilistic Seismic Assessment of Structure, Foundation and Soil Interacting Systems

Abstract: This report presents research on the probabilistic seismic performance evaluation of a structural-geotechnical interacting system. The system comprises a bridge, its foundation, and the supporting soil. The investigation includes a study on probabilistic performance evaluation methodologies, development of a multiplatform and hybrid simulation framework, verifications of numerical models of structural and geotechnical systems in comparison with measured data, and the derivation of fragility curves of a bridge in Central and Eastern United States. Seismic performance evaluation procedures are studied using a benchmark threestory, reinforced concrete (RC) building structure. Three probabilistic performance evaluation methods are applied: the Monte Carlo simulation, response surface, and SACFEMA methods. The analysis of benchmark structure shows that the effect of random variability in structural materials is small compared to the effect of input ground motion. When Peak Ground Acceleration (PGA) is used as an intensity measure, the derived vulnerability curves highly depend on ground motion sets. Three different simulation methods results in similar vulnerability curves. The computational cost are the most expensive when the Monte Carlo simulation is adopted. Methodologies for soil-structure-interaction analysis are introduced, including the newly developed multiplatform, multiresolution hybrid simulation framework. These methodologies and numerical models of soil-structure-interaction systems are verified through comparison with field measurements and experimental results. The soil-structure interacting system is verified through analyses of a heavily instrumented bridge which recorded several sets of ground motions. The verification study of soil-structure interacting system shows that detailed and meticulously developed analytical models are capable of replicating measurements of the response of complex bridge systems subjected to strong ground motion. Seismic vulnerability curves of a reference bridge in the Central and Eastern United States (CEUS) are derived employing the aforementioned methods with and without soilstructure interaction. A typical highway over-crossing bridge representing one of the most common bridge types in the CEUS is selected. Four different approaches of SoilStructure Interaction (SSI) are tried: (a) Abutments and foundations are assumed to be fixed, (b) Conventional lumped spring approaches are adopted to model abutments and foundations, (c) Lumped springs for abutments and foundations are estimated from Finite Element (FE) analysis of geotechnical system, and (d) Multiplatform simulation is conducted. All four of the methods shows that abutment bearings in transverse direction are most vulnerable components. Failure probability of the bridge system is highly dependent on the failure probability of abutment bearings. Considering that simplified methods for SSI analysis include larger assumptions than fully coupled methods and that the multiplatform simulation is verified with measured responses from instrumented bridge, the use of multiplatform simulation is suggested if computational power and resources for FE modeling are affordable.

Centrifuge modeling of seismic response of layered soft clay

Abstract: Centrifuge modeling is a valuable tool used to study the response of geotechnical structures to infrequent or extreme events such as earthquakes. A series of centrifuge model tests was conducted at 80g using an electro-hydraulic earthquake simulator mounted on the C-CORE geotechnical centrifuge to study the dynamic response of soft soils and seismic soil–structure interaction (SSI). The acceleration records at different locations within the soil bed and at its surface along with the settlement records at the surface were used to analyze the soft soil seismic response. In addition, the records of acceleration at the surface of a foundation model partially embedded in the soil were used to investigate the seismic SSI. Centrifuge data was used to evaluate the variation of shear modulus and damping ratio with shear strain amplitude and confining pressure, and to assess their effects on site response. Site response analysis using the measured shear wave velocity, estimated modulus reduction and damping ratio as input parameters produced good agreement with the measured site response. A spectral analysis of the results showed that the stiffness of the soil deposits had a significant effect on the characteristics of the input motions and the overall behavior of the structure. The peak surface acceleration measured in the centrifuge was significantly amplified, especially for low amplitude base acceleration. The amplification of the earthquake shaking as well as the frequency of the response spectra decreased with increasing earthquake intensity. The results clearly demonstrate that the layering system has to be considered, and not just the average shear wave velocity, when evaluating the local site effects.

Damage Analysis of Structures on Elastic Foundation

Abstract: A performance-based design methodology is aimed at controlling the structural damage based on precise estimations of the seismic response of the whole building–foundation system. This work presents a simplified procedure for practical damage analysis of structures considering the soil–structure interaction effects, with potential application to performance-based design of new buildings as well as to performance-based evaluation of existing buildings. A damage model based on maximum displacement and dissipated energy under monotonic loading is proposed, with the effects of cyclic load reversals being estimated by using a modified Park–Ang index. To simplify the consideration of the soil–structure interaction effects, an equivalent fixed-base oscillator with the same yield strength and energy dissipation capacity as the actual flexible-base structure is applied. Selected numerical results are presented in terms of dimensionless parameters for their general application, using a set of appropriate earthquake ...

Buried Pipelines Subject to Subgouge Deformations

Abstract: Engineers often model pipe/soil interaction events based on the concept of subgrade reactions originally proposed by Winkler. Engineering models often utilize beam and nonlinear/plastic spring elements to represent pipelines and the surrounding soil medium, respectively. The spring formulations, defining soil resistance to deformations in three-dimensional space, are usually assumed to be independent and the responses are discrete between adjacent soil zones. However, this idealization does not truly replicate a soil medium behavior. This study presents coupled numerical analyses of pipeline for the specific problem of subgouge deformations due to ice gouge events. Three dimensional continuum analyses of coupled pipe/soil/ice keel interaction using an explicit arbitrary Lagrangian finite- element approach were performed. The study compares the continuum finite-element results with Winkler-type analysis for the specific analyzed problem. A Lagrangian adaptive meshing technique was employed to model very la...

Flexural response of piles under liquefied soil conditions

Abstract: The paper pertains to the development of a generalized procedure to analyze and predict the flexural behavior of axially and laterally loaded pile foundations under liquefied soil conditions. Pseudo-static analysis has been carried out taking into consideration the combined effect of axial load and lateral load. Based on the available literature effect of degradation on the modulus of subgrade reaction due to soil liquefaction has been incorporated in the analysis. The developed program was calibrated and validated by comparing the predicted behavior of the pile with theoretical and experimental results reported in literature. The predicted behavior has been found to be in excellent to very good agreement with the theoretical and observed values in the field, respectively. The present study highlights the importance of considering the axial load from the superstructure along with the inertia forces from the superstructure and the kinematic forces from the liquefied soil in the design of pile foundations in liquefiable areas. The significance of densification of the soil in the liquefiable areas and presence of an adequate top non-liquefied soil cover causing appreciable reduction in deflection and bending moment experienced by the piles has been highlighted.

Elastoplastic medium for foundation settlements and monotonic soil–structure interaction under combined loadings

Abstract: Foundation settlements and soil–structure interaction are important problems to structural and geotechnical engineers. This study introduces a novel elastoplastic three-degree-of-freedom medium which models foundations settlements under combined loadings. A soil–structure interaction problem can then be solved by replacing the soil mass with this three-degree-of-freedom elastoplastic medium, thus reducing significantly the size of the problem. The model was developed by extending the classical plasticity concepts to the force-deformation level. Its ability to predict foundation deformations was evaluated using finite element solutions of a typical shallow foundation problem and was found reasonably accurate while producing significant time savings. Copyright © 2006 John Wiley & Sons, Ltd.

Centrifuge modelling of the effects of soil loading on flexible sewer liners

Abstract: This paper presents an experimental study of the effects of soil loading on large-diameter, close-fitting, non-bonded, flexible sewer liners as typified by ‘cured-in-place pipes’ (CIPP). CIPP is the process whereby a polymeric pipe lining is cast directly against the wall of a deteriorating sewer pipe. The transfer of soil loading to CIPP liners is strongly influenced by the interaction between the existing host pipe and the surrounding soil. In adjusting to deterioration events of the host pipe and possible void formation in the soil, the existing pipe-soil structure deforms and interacts with the liner inside it. By monitoring deformations of the liner and host structure the magnitude of load transfer can be inferred. The paper describes 1/30th scale physical models tested in dry sand under 30g of centrifugal acceleration. The models included: (a) a host pipe with two simulated modes of deterioration; (b) an instrumented close-fitting, non-bonded, flexible liner; (c) voids in the soil; and (d) moving, h...

Coupled Macroelement Model of Soil-Structure Interaction in Deep Foundations

Abstract: The principal objective of this study is the development and calibration of a macroelement model for soil-pile interaction under simultaneously applied lateral and vertical loads. Herein, we focus on cast-in-drilled-hole single piles that are partially or fully embedded in soil, which are frequently used as support structures in highway construction. The model is calibrated and verified using primarily three-dimensional finite-element simulations and, whenever possible, with experimental data obtained from open literature. These data indicate that lateral loads significantly affect the vertical response of single piles, whereas the converse coupling is negligible. The proposed macroelement model is capable of mimicking this phenomenon. As such, it is a computationally efficient alternative to finite-element analyses, and is feasible to be utilized in practical applications.

Spatial variations in soil properties and their influence on structural reliability

Abstract: The longitudinal variation of soil properties induces stresses and/or displacements that cannot be predicted when assuming soil homogeneity. The reliability analysis may require a comprehensive modelling on soil–structure interaction, accounting for the spatial distribution of soil properties. Modelling must account for: (a) the soil spatial variability; and (b) the soil–structure interaction, which is case specific. It is shown that the magnitude of induced effects depends on three factors: (a) magnitude of soil variability; (b) a soil–structure stiffness ratio; and (c) a soil–structure length ratio, which combines the soil fluctuation scale and a structural characteristic length. A reliability analysis is performed in two cases: a buried set of pipes, and a piled raft. Since a ‘worst value’ of the soil–structure length ratio can be identified, some very practical engineering rules can be derived regarding the definition of characteristic values of the soil properties, which can directly be used in simpl...

Displacement-Based Seismic Design of Drilled Shaft Bents with Soil-Structure Interaction

Abstract: The Direct Displacement-Based Design method is implemented for performance based seismic engineering of drilled shaft bents with consideration of soil-structure interaction effects. This was accomplished by defining an equivalent model that allows the prediction of yield displacement, displacement ductility and equivalent viscous damping for the in-plane and out-of-plane response of bents embedded in soft clay and sand. The utilization of the model is simple and requires the input of geometry, basic soil properties, target performance in terms of top displacement, ductility or strain limits and seismic demand in the form of displacement response spectra. Examples are presented to demonstrate the application of the procedure.

Simple prediction of the undrained displacement of a circular surface foundation on non-linear soil

Abstract: Textbooks and university courses teach elasticity and plasticity as separate methods for analysing the stiffness and strength of a shallow foundation. The behaviour of real soil is neither linear elastic nor perfectly plastic. In this paper, two simple techniques for incorporating non-linearity in routine design have been validated using finite element (FE) analysis. These two techniques—Atkinson's method and the mobilisable strength design (MSD) approach—assume that the responses of an individual soil element and the boundary value problem being considered are self-similar. Using this assumption, the soil element response can be scaled to predict the response of the boundary value problem. Atkinson's method is based on elasticity whereas MSD uses plasticity. Non-linearity has been captured in the FE analysis using a power law soil model. This approach uses minimal parameters, but is shown to capture accurately the undrained stress–strain response of typical clays under monotonic loading. Comparison with ...

Cursory seismic drift assessment for buildings in moderate seismicity regions

Abstract: This paper outlines a methodology to assess the seismic drift of reinforced concrete buildings with limited structural and geotechnical information. Based on the latest and the most advanced research on predicting potential near-field and far field earthquakes affecting Hong Kong, the engineering response spectra for both rock and soil sites are derived. A new step-by-step procedure for displacement-based seismic hazard assessment of building structures is proposed to determine the maximum inter-storey drift demand for reinforced concrete buildings. The primary information required for this assessment is only the depth of the soft soil above bedrock and the height of the building. This procedure is further extended to assess the maximum chord rotation angle demand for the coupling beam of coupled shear wall or frame wall structures, which may be very critical when subjected to earthquake forces. An example is provided to illustrate calibration of the assessment procedure by using actual engineering structural models.

Soil-structure interaction effects on an instrumented building

Abstract: The paper highlights the use of fem and bi-directional lumped-mass-storey-stiffness numerical models for the study of the soil–structure interaction (ssi) effects on an instrumented building. Data on the structural response have been obtained through the project for seismic instrumentation of a 16-storey r/c cast-in-place dwelling building (Chisinau, Republic of Moldova) during a series of earthquakes (Gutenberg–Richter MGR = 5.0−6.7). The effect of soil–structure interaction is clearly observed comparing the responses recorded on foundation and free-field. ssi becomes more pronounced for higher level of ground shaking amplifying the natural period of the structure and slightly suppressing high frequences on the foundation in comparison with the free-field motion.