Showing papers on "Soil structure interaction published in 2012"

Centrifuge Modeling of Bridge Systems Designed for Rocking Foundations

Abstract: In good soil conditions, spread footings for bridges are less expensive than deep foundations. Furthermore, rocking shallow foundations have some performance advantages over conventional fixed-base foundations; they can absorb some of the ductility demand that would typically be absorbed by the columns, and they have better recentering characteristics than conventional reinforced-concrete (RC) columns. Foundations designed for elastic behavior do not have these benefits of nonlinear soil-structure interaction. One potential disad- vantage of rocking systems is that they can produce significant settlement in poor soil conditions. Centrifuge model tests were performed to account for the interaction between soil, footing, column, deck and abutments systems. Bridge systems with rocking foundations on good soil conditions are shown to perform well and settlements are small. An improved method for quantification of settlements is presented. The model tests are described in some detail. One of the important factors limiting the use of rocking foundations is the perception that they might tip over; experiments show that tipping instability is unlikely if the foundations are properly sized. In one experiment, a column for a system with large fixed-base foundation collapsed while the systems with smaller rocking foundations did not collapse. DOI: 10.1061/(ASCE)GT .1943-5606.0000605. © 2012 American Society of Civil Engineers. CE Database subject headings: Earthquakes; Shallow foundations; Bridges; Centrifuge models. Author keywords: Earthquake; Shallow foundation; Rocking; Bridge; Centrifuge modeling.

A modified scaled boundary finite element method for three‐dimensional dynamic soil‐structure interaction in layered soil

Abstract: SUMMARY This paper is devoted to the analysis of elastodynamic problems in 3D-layered systems which are unbounded in the horizontal direction. For this purpose, a finite element model of the near field is coupled to a scaled boundary finite element model (SBFEM) of the far field. The SBFEM is originally based on describing the geometry of a half-space or full-space domain by scaling the geometry of the near field / far field interface using a radial coordinate. A modified form of the SBFEM for waves in a 2D layer is also available. None of these existing formulations can be used to describe a 3D-layered medium. In this paper, a modified SBFEM for the analysis of 3D-layered continua is derived. Based on the use of a scaling line instead of a scaling centre, a suitable scaled boundary transformation is proposed. The derivation of the corresponding scaled boundary finite element (SBFE) equations in displacement and stiffness is presented in detail. The latter is a nonlinear differential equation with respect to the radial coordinate, which has to be solved numerically for each excitation frequency considered in the analysis. Various numerical examples demonstrate the accuracy of the new method and its correct implementation. These include rigid circular and square foundations embedded in or resting on the surface of layered homogeneous or inhomogeneous 3D soil deposits over rigid bedrock. Hysteretic damping is assumed in some cases. The dynamic stiffness coefficients calculated using the proposed method are compared with analytical solutions or existing highly accurate numerical results. Copyright © 2011 John Wiley & Sons, Ltd.

Model Container Design for Soil-Structure Interaction Studies

Abstract: Physical modelling of scaled models is an established method for understanding failure mechanisms and verifying design hypothesis in earthquake geotechnical engineering practice. One of the requirements of physical modelling for these classes of problems is the replication of semi-infinite extent of the ground in a finite dimension model soil container. This chapter is aimed at summarizing the requirements for a model container for carrying out seismic soil-structure interactions (SSI) at 1-g (shaking table) and N-g (geotechnical centrifuge at N times earth’s gravity). A literature review has identified six types of soil container which are summarised and critically reviewed herein. The specialised modelling techniques entailed by the application of these containers are also discussed.

Dynamic Winkler modulus for axially loaded piles

Abstract: The problem of axial dynamic pile–soil interaction and its analytical representation using the concept of a dynamic Winkler support are revisited. It is shown that depth- and frequency-dependent Winkler springs and dashpots, obtained by dividing the complex-valued side friction and the corresponding displacements along the pile, may faithfully describe the interaction effect, contrary to the common perception that the Winkler concept is always approximate. An axisymmetric wave solution, based on linear elastodynamic theory, is then derived for the harmonic steady-state response of finite and infinitely long piles in a homogeneous viscoelastic soil stratum, with the former type of pile resting on rigid rock. The pile is modelled as a continuum, without the restrictions associated with strength-of-materials approximations. Closed-form solutions are obtained for: (a) the displacement field in the soil and the pile; (b) the stiffness and damping (‘impedance') coefficients at the pile head; (c) the actual, dep...

Seismic hazard and site-specific ground motion for typical ports of Gujarat

Abstract: Economic importance of major ports is well known, and if ports are located in seismically active regions, then site-specific seismic hazard studies are essential to mitigate the seismic risk of the ports. Seismic design of port sites and related structures can be accomplished in three steps that include assessment of regional seismicity, geotechnical hazards, and soil structure interaction analysis. In the present study, site-specific probabilistic seismic hazard analysis is performed to identify the seismic hazard associated with four typical port sites of Gujarat state (bounded by 20°–25.5°N and 68°–75°E) of India viz. Kandla, Mundra, Hazira, and Dahej ports. The primary aim of the study is to develop consistent seismic ground motion for the structures within the four port sites for different three levels of ground shaking, i.e., operating level earthquake (72 years return period), contingency level earthquake (CLE) (475 year return period), and maximum considered earthquake (2,475 year return period). The geotechnical characterization for each port site is carried out using available geotechnical data. Shear wave velocities of the soil profile are estimated from SPT blow counts using various empirical formulae. Seismicity of the Gujarat region is modeled through delineating the 40 fault sources based on the seismotectonic setting. The Gujarat state is divided into three regions, i.e., Kachchh, Saurashtra, and Mainland Gujarat, and regional recurrence relations are assigned in the form of Gutenberg-Richter parameters in order to calculate seismic hazard associated with each port site. The horizontal component of ground acceleration for three levels of ground shaking is estimated by using different ground motion attenuation relations (GMAR) including one country-specific GMAR for Peninsular India. Uncertainty in seismic hazard computations is handled by using logic tree approach to develop uniform hazard spectra for 5% damping which are consistent with the specified three levels of ground shaking. Using recorded acceleration time history of Bhuj 2001 earthquake as the input time motion, synthetic time histories are generated to match the developed designed response spectra to study site-specific responses of port sites during different levels of ground shaking. It is observed that the Mundra and Kandla port sites are most vulnerable sites for seismic hazard as estimated CLE ground motion is in order of 0.79 and 0.48 g for Mundra and Kandla port sites, respectively. Hazira and Dahej port sites have comparatively less hazard with estimated CLE ground motion of 0.17 and 0.11 g, respectively. The ground amplification factor is observed at all sites which ranges from 1.3 to 2.0 for the frequency range of 1.0–2.7 Hz. The obtained spectral accelerations for the three levels of ground motions and obtained transfer functions for each port sites are compared with provisions made in Indian seismic code IS:1893-Part 1 (2002). The outcome of present study is recommended for further performance-based design to evaluate the seismic response of the port structures with respect to various performance levels.

Seismic Performance of Pile-Supported Wharf Structures considering Soil-Structure Interaction in Liquefied Soil

Abstract: Many existing pile-supported marginal wharves within ports along the West Coast of the United States were designed in the late 1960s and early 1970s using the seismic design criteria available then...

Earthquake induced pounding between adjacent buildings considering soil-structure interaction

Abstract: Many closely located adjacent buildings have suffered from pounding during past earthquakes because they vibrated out of phase. Furthermore, buildings are usually constructed on soil; hence, there are interactions between the buildings and the underlying soil that should also be considered. This paper examines both the interaction between adjacent buildings due to pounding and the interaction between the buildings through the soil as they affect the buildings’ seismic responses. The developed model consists of adjacent shear buildings resting on a discrete soil model and a linear viscoelastic contact force model that connects the buildings during pounding. The seismic responses of adjacent buildings due to ground accelerations are obtained for two conditions: fixed-based (FB) and structure-soil-structure interaction (SSSI). The results indicate that pounding worsens the buildings’ condition because their seismic responses are amplified after pounding. Moreover, the underlying soil negatively impacts the buildings’ seismic responses during pounding because the ratio of their seismic response under SSSI conditions with pounding to those without pounding is greater than that of the FB condition.

On the behaviour of flexible retaining walls under seismic actions

Abstract: This paper describes an experimental investigation of the behaviour of embedded retaining walls under seismic actions. Nine centrifuge tests were carried out on reduced-scale models of pairs of retaining walls in dry sand, either cantilevered or with one level of props near the top. The experimental data indicate that, for maximum accelerations that are smaller than the critical limit equilibrium value, the retaining walls experience significant permanent displacements under increasing structural loads, whereas for larger accelerations the walls rotate under constant internal forces. The critical acceleration at which the walls start to rotate increases with increasing maximum acceleration. No significant displacements are measured if the current earthquake is less severe than earthquakes previously experienced by the wall. The increase of critical acceleration is explained in terms of redistribution of earth pressures and progressive mobilisation of the passive strength in front of the wall. The experimental data for cantilevered retaining walls indicate that the permanent displacements of the wall can be reasonably predicted adopting a Newmark-type calculation with a critical acceleration that is a fraction of the limit equilibrium value.

Analytical model for the prediction of building deflections induced by ground movements

Abstract: The objective of this study is to develop an analytical model that can predict the building-relevant deflections induced by tunnelling or mining subsidence. The model takes into account soil–structure interactions due to differences in stiffness between the ground and the building. The ground is modelled by the Winkler model with an initial ground curvature equivalent to the free-field ground movements. The building is modelled by a horizontal beam with uniform loading. The static and cinematic equilibrium of both the ground and the building are then calculated to assess the final building and ground shape, and the building deflection is derived. The resulting analytical model is used to investigate the influence of the ground and the building's mechanical properties, the building load and the initial value of the free-field ground curvature (hogging or sagging). The model appears to be more comprehensive than those reported elsewhere that address the problem with numerical models. In particular, the analytical model makes it possible to distinguish two different final situations—with continuous or discontinuous contact between the ground and the building. The model is compared with numerical results and used to analyse a case study. Copyright © 2010 John Wiley & Sons, Ltd.

Simulation of the response of base-isolated buildings under earthquake excitations considering soil flexibility

Abstract: The accurate analysis of the seismic response of isolated structures requires incorporation of the flexibility of supporting soil. However, it is often customary to idealize the soil as rigid during the analysis of such structures. In this paper, seismic response time history analyses of base-isolated buildings modelled as linear single degree-of-freedom (SDOF) and multi degree-of-freedom (MDOF) systems with linear and nonlinear base models considering and ignoring the flexibility of supporting soil are conducted. The flexibility of supporting soil is modelled through a lumped parameter model consisting of swaying and rocking spring-dashpots. In the analysis, a large number of parametric studies for different earthquake excitations with three different peak ground acceleration (PGA) levels, different natural periods of the building models, and different shear wave velocities in the soil are considered. For the isolation system, laminated rubber bearings (LRBs) as well as high damping rubber bearings (HDRBs) are used. Responses of the isolated buildings with and without SSI are compared under different ground motions leading to the following conclusions: (1) soil flexibility may considerably influence the stiff superstructure response and may only slightly influence the response of the flexible structures; (2) the use of HDRBs for the isolation system induces higher structural peak responses with SSI compared to the system with LRBs; (3) although the peak response is affected by the incorporation of soil flexibility, it appears insensitive to the variation of shear wave velocity in the soil; (4) the response amplifications of the SDOF system become closer to unit with the increase in the natural period of the building, indicating an inverse relationship between SSI effects and natural periods for all the considered ground motions, base isolations and shear wave velocities; (5) the incorporation of SSI increases the number of significant cycles of large amplitude accelerations for all the stories, especially for earthquakes with low and moderate PGA levels; and (6) buildings with a linear LRB base-isolation system exhibit larger differences in displacement and acceleration amplifications, especially at the level of the lower stories.

Nonlinear seismic behaviour of wall-frame dual systems accounting for soil–structure interaction

Abstract: SUMMARY The aim of this paper is to study the effects of soil–structure interaction on the seismic response of coupled wall-frame structures on pile foundations designed according to modern seismic provisions. The analysis methodology based on the substructure method is recalled focusing on the modelling of pile group foundations. The nonlinear inertial interaction analysis is performed in the time domain by using a finite element model of the superstructure. Suitable lumped parameter models are implemented to reproduce the frequency-dependent compliance of the soil-foundation systems. The effects of soil–structure interaction are evaluated by considering a realistic case study consisting of a 6-storey 4-bay wall-frame structure founded on piles. Different two-layered soil deposits are investigated by varying the layer thicknesses and properties. Artificial earthquakes are employed to simulate the earthquake input. Comparisons of the results obtained considering compliant base and fixed base models are presented by addressing the effects of soil–structure interaction on displacements, base shears, and ductility demand. The evolution of dissipative mechanisms and the relevant redistribution of shear between the wall and the frame are investigated by considering earthquakes with increasing intensity. Effects on the foundations are also shown by pointing out the importance of both kinematic and inertial interaction. Finally, the response of the structure to some real near-fault records is studied. Copyright © 2012 John Wiley & Sons, Ltd.

Interaction of caisson foundations with a seismically rupturing normal fault: centrifuge testing versus numerical simulation

Abstract: Dramatic failures have occurred in recent earthquakes as a result of the interplay of surface structures with outcropping fault ruptures, highlighting the need to account for fault-induced loading in seismic design. Current research into the mechanisms of fault rupture–foundation–structure interaction has revealed a potentially favourable role of caissons in comparison with other foundation types. This paper explores the mechanisms of normal fault rupture interaction with rigid caisson foundations, with an integrated approach using both experiments and analysis. A series of centrifuge model tests were first conducted to study the response of a square (in plan) caisson foundation of dimensions 5 m × 5 m × 10 m, founded on a 15 m thick layer of dry dense sand. A non-linear three-dimensional numerical simulation of the problem was then developed, and adequately validated against centrifuge test results. Depending on its position relative to the fault, the caisson interacts with the fault rupture, sometimes m...

Non-linear finite element analysis for prediction of seismic response of buildings considering soil-structure interaction

Abstract: . The objective of this paper focuses primarily on the numerical approach based on two-dimensional (2-D) finite element method for analysis of the seismic response of infinite soil-structure interaction (SSI) system. This study is performed by a series of different scenarios that involved comprehensive parametric analyses including the effects of realistic material properties of the underlying soil on the structural response quantities. Viscous artificial boundaries, simulating the process of wave transmission along the truncated interface of the semi-infinite space, are adopted in the non-linear finite element formulation in the time domain along with Newmark's integration. The slenderness ratio of the superstructure and the local soil conditions as well as the characteristics of input excitations are important parameters for the numerical simulation in this research. The mechanical behavior of the underlying soil medium considered in this prediction model is simulated by an undrained elasto-plastic Mohr-Coulomb model under plane-strain conditions. To emphasize the important findings of this type of problems to civil engineers, systematic calculations with different controlling parameters are accomplished to evaluate directly the structural response of the vibrating soil-structure system. When the underlying soil becomes stiffer, the frequency content of the seismic motion has a major role in altering the seismic response. The sudden increase of the dynamic response is more pronounced for resonance case, when the frequency content of the seismic ground motion is close to that of the SSI system. The SSI effects under different seismic inputs are different for all considered soil conditions and structural types.

Behavior of Circular and Square Reinforced Concrete Bridge Columns under Combined Loading Including Torsion

Abstract: Reinforced concrete (RC) bridge columns can be subjected to flexural, axial, shearing, and torsional loading during earthquake excitations, resulting in complex failure modes. However, in spite of its explicit occurrence during earthquake excitations and significant effect on final failure modes, the torsional loading effect on the seismic performance of RC columns has not been studied in depth. In particular, understanding interacting behavior of each loading condition should be preceded to satisfy the increased demand for seismic design. In this context, this study aims at improving the knowledge of seismic performance of RC bridge columns under combined loading, including torsion through the experimental study carried out by Missouri University of Science and Technology. The experimental study focused on investigating the effect of different cross-sectional shapes (circle and square), hysteretic torsional and flexural response, damage distribution, and ductility characteristics with respect to various torsion-to-bending moment (T/M) ratios. Finally, interaction diagrams were established based on experimental results.

Predicting the Earth Pressure on Integral Bridge Abutments

Abstract: The soil adjacent to integral bridge abutments experiences daily and annual temperature-induced cyclic loading due to expansion and contraction of the bridge deck. This causes a particular soil response and complicated soil-structure interaction problem, with considerable uncertainties in design. This paper describes a method of calculating the effects of thermal cycling by using the results of laboratory cyclic stress path testing within a numerical model. Samples of stiff clay and sand were tested in the triaxial apparatus under stress paths typical for behind an integral abutment. Distinct behavior was observed for the two soils, with stiff clay showing relatively little buildup of lateral stress with cycles, whereas for sand stresses continued to increase, exceeding at-rest and approaching full passive pressures. To explore the implications of these findings on the soil-abutment interaction and to estimate the lateral stresses acting on the abutment as a whole, a numerical (finite difference) model was developed with a soil model reproducing the sand behavior at element level. The numerical model gave good agreement with published centrifuge and field data, and indicated that the stress profile specified in some current standards is conservative. Influence of abutment stiffness and wall friction is also quantified.

Structural Element Approaches for Soil-Structure Interaction

Abstract: The emphasis within this study regards structural element models for soil-structure interaction (SSI). The methods are compared and calibrated against an elastic continuum modelled with solid elements, which in the study is used as the “correct” solution. Main interest is the influence on results of simplifications in the method often used today, with springs representing the subgrade (Winkler model). In the study this model is modified to better capture the soil’s behaviour. In a Winkler model the springs act independently of each other, while the soil in reality is a continuous medium that also transfers shear stresses. To achieve a better behaviour in a structural element model, different kinds of interaction elements are included, which couple the springs. The biggest shortcoming, identified in this thesis, for a Winkler model with uniform foundation stiffness is that the soil around the superstructure is not taken into account. This can result in major underestimation of the foundation’s stiffness towards the superstructure’s edges which normally, at the edges, leads to conservative sectional forces in the ground slab and unconservative ground pressure. It can also result in a convex settlement profile, where a concave would be more realistic. Surrounding soil can be taken into account by increasing the foundation’s stiffness towards the superstructure’s edges, alternatively the different models described in the thesis can be implemented. The different models are applied in two simple cases, one representing a footing and the second a slab. The best correlation to the elastic continuum was achieved by applying an interaction element between the springs in a Winkler model that only transfers shear deformations. This shear layer model is also evaluated in 3D for a case study of “Malmö Konsert, Kongress och Hotell”. A simple method, for practical use in 2D, to determine this shear layer model’s parameters is developed by the authors. Analyses indicate that the shear layer’s stiffness can be determined independently of the superstructure’s geometry. Therefore only the soil’s properties and depth is needed to determine the shear layer’s stiffness. A relation for homogenous and elastic soil is presented to determine the stiffness. The relation is based on 2D analysis and is not verified for 3D. Further study and verification is needed to make the method complete for practical use.

Discrete element method simulations of the seismic response of shallow foundations including soil‐foundation‐structure interaction

Abstract: SUMMARY A novel three-dimensional particle-based technique utilizing the discrete element method is proposed to analyze the seismic response of soil-foundation-structure systems. The proposed approach is employed to investigate the response of a single-degree-of-freedom structure on a square spread footing founded on a dry granular deposit. The soil is idealized as a collection of spherical particles using discrete element method. The spread footing is modeled as a rigid block composed of clumped particles, and its motion is described by the resultant forces and moments acting upon it. The structure is modeled as a column made of particles that are either clumped to idealize a rigid structure or bonded to simulate a flexible structure of prescribed stiffness. Analysis is done in a fully coupled scheme in time domain while taking into account the effects of soil nonlinear behavior, the possible separation between foundation base and soil caused by rocking, the possible sliding of the footing, and the dynamic soil-foundation interaction as well as the dynamic characteristics of the superstructure. High fidelity computational simulations comprising about half a million particles were conducted to examine the ability of the proposed technique to model the response of soil-foundation-structure systems. The computational approach is able to capture essential dynamic response patterns. The cyclic moment–rotation relationships at the base center point of the footing showed degradation of rotational stiffness by increasing the level of strain. Permanent deformations under the foundation continued to accumulate with the increase in number of loading cycles. Copyright © 2011 John Wiley & Sons, Ltd.

Physical and Numerical Modeling of Seismic Soil-Structure Interaction in Layered Soils

Abstract: The structural response of buildings subjected to seismic loads is affected by local site conditions and the interaction between the structure and the supporting soil media. Seismic centrifuge model tests were conducted on two layered clay soil profiles at 80 g field to investigate soil-structure interaction and dynamic response of foundation. Several earthquake-like shaking events were applied to the models using an electro-hydraulic shaking table to simulate linear and nonlinear soil behavior. Results showed that the foundation input motion was significantly amplified in both models, especially for weak earthquake motions. Seismic soil-structure interaction was found to have an important effect on structure response by increasing the amplification of foundation input motion. A 3D finite difference numerical model was also developed to simulate the response of centrifuge model tests and study the parameters that affect the characteristics of earthquake at the base of the structure. The results indicated that the stiffness and stratification of the soil profiles had a significant effect on modifying the foundation input motion.

On the performance of lumped parameter models with gyro‐mass elements for the impedance function of a pile‐group supporting a single‐degree‐of‐freedom system

Abstract: SUMMARY Lumped parameter models with a so called “gyro-mass” element (GLPMs) have been proposed recently in response to a strong demand for efficiently and accurately representing frequency-dependent impedance functions of soil–foundation systems. Although GLPMs are considered to be powerful tools for practical applications in earthquake engineering, some problems remain. For instance, although GLPMs show fairly close agreement with the target impedance functions, the accuracy of the transfer functions and the time-histories of dynamic responses in structural systems comprising GLPMs have never been verified. Furthermore, no assessment has been performed on how much difference appears in the accuracy of dynamic responses obtained from GLPMs and those from conventional Kelvin–Voigt models comprising a spring and a dashpot arranged in parallel with various frequency-independent constants. Therefore, in this paper, these problems are examined using an example of 2×4 pile groups embedded in a layered soil medium, supporting a single-degree-of-freedom system subjected to ground motions. The results suggest that GLPMs are a new option for highly accurate computations in evaluating the dynamic response of structural systems comprising typical pile groups, rather than conventional Kelvin–Voigt models. Copyright © 2011 John Wiley & Sons, Ltd.