Prishati Raychowdhury

Application and Validation of Practical Tools for Nonlinear Soil-Foundation Interaction Analysis

Practical guidelines for characterization of soil-structure interaction (SSI) effects for shallow foundations are typically based on representing foundation-soil interaction in terms of viscoelastic impedance functions that describe stiffness and damping characteristics. Relatively advanced tools can describe nonlinear soil-foundation behavior, including temporary gap formation, foundation settlement and sliding, and hysteretic energy dissipation. We review two tools that describe such effects for shallow foundations and that are implemented in the computational platform OpenSees: a beam-on-nonlinear-Winkler foundation (BNWF) model and a contact interface model (CIM). We review input parameters and recommend parameter selection protocols. Model performance with the recommended protocols is evaluated through model-to-model comparisons for a hypothetical shear wall building resting on clay and model-data comparisons for several centrifuge test specimens on sand. The models describe generally consistent moment-rotation behavior, although shear-sliding and settlement behaviors deviate depending on the degree of foundation uplift. Pronounced uplift couples the moment and shear responses, often resulting in significant shear sliding and settlements. Such effects can be mitigated through the lateral connection of foundation elements with tie beams.

Sensitivity of Shallow Foundation Response to Model Input Parameters

From a predictive point of view, it is desirable to characterize the effect of varying model input parameters on the seismic response of soil-foundation systems. In this paper, this issue is studied for shallow foundation systems in dry dense sand with varying vertical factors of safety, embedment depths, demand levels, and moment to shear ratios. Response parameters considered are the moment, shear, sliding, settlement, and rotation demands of the foundation. First-order sensitivity analyses indicate that among the soil input parameters, the friction angle has the most significant effect on capturing the foundation force and displacement demands. Furthermore, the uncertainty in friction angle contributes 80% of the variance of the settlement demand and 40% of the variance of the moment demand. It is also found that the uncertainty in Poissons ratio has a marginal effect in predicting the studied foundation response. Although the findings of this study are limited to the parameter space considered herein and care should be taken for broader applicability, it does shed light on which parameters uncertainty should be minimized.

Nonlinear Material Models for Winkler-based Shallow Foundation Response Evaluation

When a foundation is subjected to earthquake ground motions, its slid- ing, settling and rocking movements can provide a mechanism for dissipating energy at the soil-structure interface and thus reduce the force demand to the structure. One potential approach to capturing this behavior involves modeling the foundation sub- grade as a Beam-on-Nonlinear-Winkler-Foundation (BNWF) with a system of discrete, mechanistic, uncoupled springs having nonlinear inelastic behavioral response. This type of model is useful in engineering practice, due to its simplicity and ease of im- plementation in a general purpose finite element platform. However, the nonlinear backbone curves for spring interface elements have only been evaluated against lateral pile load test data. In this study, nonlinear inelastic springs for use in capturing shal- low footing response under seismic loading are calibrated against centrifuge and field tests. The effects of the updated backbone curves is demonstrated on a model footing subjected to reversed cyclic rotation.

Effect of nonlinear soil-structure interaction on seismic response of low-rise SMRF buildings

The nonlinear behavior of a soil-foundation system may alter the seismic response of a structure by providing additional fl exibility to the system and dissipating hysteretic energy at the soil-foundation interface. However, the current design practice is still reluctant to consider the nonlinearity of the soil-foundation system, primarily due to lack of reliable modeling techniques. This study is motivated towards evaluating the effect of nonlinear soil-structure interaction (SSI) on the seismic responses of low-rise steel moment resisting frame (SMRF) structures. In order to achieve this, a Winklerbased approach is adopted, where the soil beneath the foundation is assumed to be a system of closely-spaced, independent, nonlinear spring elements. Static pushover analysis and nonlinear dynamic analyses are performed on a 3-story SMRF building and the performance of the structure is evaluated through a variety of force and displacement demand parameters. It is observed that incorporation of nonlinear SSI leads to an increase in story displacement demand and a significant reduction in base moment, base shear and inter-story drift demands, indicating the importance of its consideration towards achieving an economic, yet safe seismic design.

Shallow Foundation Response Variability due to Parameter Uncertainty

Uncertainty in soil parameters may play a crucial role in response variation of foundations and the supporting structures and, consequently, may control several design decisions. It is, therefore, extremely important to identify and characterize the relevant parameters. Furthermore, it is also important to identify the sources and extent of uncertainty of soil and model input parameters, along with the effect of their uncertainty on the shallow foundation response. This chapter intends to investigate the effect of soil and model parameter uncertainty on the response of shallow foundation-structure systems resting on dry dense sand. In this study, the soil-foundation interface is modeled using Winkler-based concept, where the soil-foundation interface is assumed to be an assembly of discrete, nonlinear elements composed of springs, dashpots, and gap elements. The sensitivity of both soil and model input parameters on various force and displacement demands of the foundation-structure system is investigated using first-order second-moment analysis and Latin hypercube technique. It has been observed that the force and displacement demands of the foundation-structure system are highly sensitive to the soil and model parameters.

Dynamic Behavior of a Geotextile-Reinforced Pond Ash Embankment

ABSTRACT The present study investigates the applicability of woven geotextile in improving the seismic performance of a pond ash embankment located in Renusagar, in seismic zone III of India. Reinforcing materials used are woven geotextiles, placed at 1 and 3 layers of pond ash embankment. To obtain the material models, strain-controlled cyclic triaxial tests had been carried out. A general expression was developed for shear modulus and damping ratio of pond ashes reinforced with and without woven geotextiles considering the data taken from seismic zones of III and IV. The proposed equations have been compared with the reported results of other pond ashes located in India. Finite element model of the pond ash embankment reinforced with geotextiles has been developed. A fluid solid porous material model has been employed to incorporate pore pressure build up during cyclic loading. A parametric study has also been performed to study the effect of reinforcements on the seismic behavior of Renusagar pond ash embankment. Results showed that using the three layers of geotextile, the lateral displacement reduces up to 17%, and the excess pore pressure reduces as much as 30–45%. From the study, it can be concluded that use of woven geotextile in pond ash dykes can be beneficial for its application as reinforcing material in highway embankments, building fills, and earth dams.

Seismic active earth pressure on bilinear retaining walls using a modified pseudo-dynamic method

BackgroundProper understanding of seismic behavior of retaining structures is crucial during a strong earthquake event. In particular, response of retaining walls with bilinear backface, where a sudden change in the inclination along its depth make the problem more complex. This study focuses on estimating the seismic earth pressure coefficients of a retaining wall with bilinear backface using a modified pseudo-dynamic method.MethodsIn this method, the backfill soil is modeled as a visco-elastic Kelvin–Voigt material. A frequency-dependant amplification function is derived for the waves traveling along the backfill using well-established one-dimensional ground response analysis theory. A rigorous parametric study has been carried out to understand the effect of various parameters such as amplitude of base acceleration, direction of vertical acceleration, soil shear resistance angle, soil-wall friction angle, wall inclination, frequency ratio, and damping ratio on the seismic active earth pressure.ResultsIt has been observed that the damping ratio of the backfill soil plays an important role, particularly when the frequency of wave is close to the natural frequency of the backfill. Further, the seismic active thrust is found to increase in both upper and lower segments of the wall when the frequency of the primary wave is greater than that of the shear wave. Comparison of results with the previous studies indicates that the conventional pseudo-dynamic methods significantly underestimate the seismic coefficients and seismic pressures, particularly for the high-intensity motions.ConclusionsThe results of the study show that the natural frequency and damping of the backfill soil have significant effect on the seismic active earth pressure coefficients. Comparison with conventional pseudo-static and pseudo-dynamic methods indicates that the previous methods largely underestimate seismic coefficients and seismic pressures (as much as 48%). This under-estimation is more prominent for higher-intensity motions and less-damped soil, where the soil amplification effects pose most importance. This modified pseudo-dynamic approach can further be used for design of bilinear retaining structures.

Effect of Soil-Structure Interaction on Low to Medium-Rise Steel Frames Through Shake Table Experiments

The present study investigates the effect of soil-structure interaction on the dynamic response of low to medium-rise steel-moment-resisting-frame (SMRF) structures supported by isolated shallow footings resting on loose dry Ganga sand bed. A series of shake table experiments is conducted on three-dimensional steel models of a 3-storey and a 6-storey building for this purpose. The model frames are placed in a laminar container filled with dry Ganga sand of 36% relative density and subjected to a number of base excitations. Further, the same structures are tested under the fixed base conditions (i.e., clamped with the base plates) in order to isolate the effect of soil-structure interaction. Various engineering demand parameters such as storey displacement, base moment and foundation rocking are emphasized. It is observed that SSI causes significant increase in the storey displacement and reduction in the base moment of the building. The roof displacement is observed to increase up to twice for the 3-storey structure and up to 37% for the 6-storey structure for structure being placed on the Ganga sand bed compared to its fixed-base position. On the contrary, a reduction in base moment up to 65% and 90% is observed due to SSI effect for the 3-storey and 6-storey structure, respectively. The energy dissipations due to rocking movement are estimated up to about 36% and 42.6% for the 3-storey and 6-storey structure, respectively.

Effect of Nonlinear SSI on Seismic Response of Low-Rise SMRF Buildings

Nonlinear behavior of soil-foundation system may alter the seismic response of a structure by providing additional flexibility to the system and dissipating hysteretic energy at the soil-foundation interface. However, the current design practice is still reluctant to consider the nonlinearity of the soil-foundation system, primarily due to lack of reliable modeling techniques. This study is motivated toward evaluating the effect of nonlinear soil-structure interaction (SSI) on the seismic responses of low-rise steel moment-resisting frame (SMRF) structures. In order to achieve this, a Winkler-based approach is adopted, where the soil beneath the foundation is assumed to be a system of closely spaced, independent, nonlinear spring elements. Static pushover analysis and nonlinear dynamic analyses are performed on a 3-story SMRF building, and the performance of the structure is evaluated through a variety of force and displacement demand parameters. It is observed that incorporation of nonlinear SSI leads to increase in story displacement demand and reduction in base moment, base shear, and inter-story drift demands significantly, indicating the importance of its consideration toward achieving an economic yet safe seismic design procedure.