Showing papers on "Soil structure interaction published in 2011"

Vibrations of wind-turbines considering soil-structure interaction

Abstract: Wind turbine structures are long slender columns with a rotor and blade assembly placed on the top. These slender structures vibrate due to dynamic environmental forces and its own dynamics. Analysis of the dynamic behavior of wind turbines is fundamental to the stability, performance, operation and safety of these systems. In this paper a simplied approach is outlined for free vibration analysis of these long, slender structures taking the soil-structure interaction into account. The analytical method is based on an Euler-Bernoulli beam-column with elastic end supports. The elastic end-supports are considered to model the flexible nature of the interaction of these systems with soil. A closed-form approximate expression has been derived for the first natural frequency of the system. This new expression is a function of geometric and elastic properties of wind turbine tower and properties of the foundation including soil. The proposed simple expression has been independently validated using an exact numerical method, laboratory based experimental measurement and field measurement of a real wind turbine structure. The results obtained in the paper shows that the proposed expression can be used for a quick assessment of the fundamental frequency of a wind turbine taking the soil-structure interaction into account.

Seismic Behavior of Soil-Pile-Structure Interaction in Liquefiable Soils: Parametric Study

Abstract: Nonlinearity of the soil medium plays a very important role on the seismic behavior of soil-pile-structure interaction. The problem of soil-pile-structure interaction is further complicated when the piles are embedded in liquefiable soil medium. A finite-element code was developed in MATLAB to model three-dimensional soil-pile-structure systems. Frequency dependent Kelvin elements (spring and dashpots) were used to model the radiation boundary conditions. A work-hardening plastic cap model was used for constitutive modeling of the soil medium. The pore pressure generation for liquefaction was incorporated by a two-parameter volume change model reported in the literature. In this paper, a 2×2 pile group in liquefiable soil is considered and a parametric study is conducted to investigate its seismic behavior. The effects of loading intensity and stiffness of the soil on the seismic behaviour of the soil-pile system are investigated, considering nonlinearity and liquefaction of the soil medium for a wide ran...

Efficient Longitudinal Seismic Fragility Assessment of a Multispan Continuous Steel Bridge on Liquefiable Soils

Abstract: The increased failure potential of aging U.S. highway bridges and their susceptibility to damage during extreme events necessitates the development of efficient reliability assessment tools to prioritize maintenance and rehabilitation interventions. Reliability communication tools become even more important when considering complex phenomena such as soil liquefaction under seismic hazards. Currently, two approaches are widely used for bridge reliability estimation under soil failure conditions via fragility curves: liquefaction multipliers and full-scale two- or three-dimensional bridge-soil-foundation models. This paper offers a computationally economical yet adequate approach that links nonlinear finite-element models of a three-dimensional bridge system with a two-dimensional soil domain and a one-dimensional set of p-y springs into a coupled bridge-soil-foundation CBSF system. A multispan continuous steel girder bridge typical of the central and eastern United States along with heterogeneous liquefiable soil profiles is used within a statistical sampling scheme to illustrate the effects of soil failure and uncertainty propagation on the fragility of CBSF system components. In general, the fragility of rocker bearings, piles, embankment soil, and the probability of unseating increases with liquefaction, while that of commonly monitored components, such as columns, depends on the type of soil overlying the liquefiable sands. This component response depen- dence on soil failure supports the use of reliability assessment frameworks that are efficient for regional applications by relying on simplified but accepted geotechnical methods to capture complex soil liquefaction effects.

Seismic analysis of deep tunnels in near fault conditions: a case study in Southern Italy

Abstract: The importance of underground structures in transportation and utility networks makes their vulnerability to earthquakes a sensitive issue. Underground facilities are usu- ally less vulnerable to earthquakes compared to above-ground structures, but the associated risk may be relevant, since even a low level of damage may affect the serviceability of a wide network. Seismic analysis of tunnels close to seismogenic faults is a complex prob- lem, which is often neglected at the design stage for the lack of specific codes or guidelines for the design of underground structures in seismic conditions and also because, as men- tioned above, underground structures are considered less vulnerable to earthquake loading. This paper investigates the seismic response of deep tunnels focusing on the required steps for a proper design under both static and dynamic loading. The study aims at contributing to improve the methods currently used for the seismic analysis of underground structures. At this purpose, the seismic response of a deep tunnel in Southern Italy has been investi- gated along the transversal direction. The infrastructure is part of the railway switch line connecting Caserta to Foggia in the Southern Apennines which is one of the most active seismic regions in Italy. The seismic response in the transversal direction has been analysed by using the pseudo-static approach as well as through advanced numerical modeling using the spectral element method coupled with a kinematic approach for finite fault simulations. The pseudo-static approach has been implemented using a closed-form analytical solution.

Responses of Buildings with Different Structural Types to Excavation-Induced Ground Settlements

Abstract: This paper compares the responses of buildings with different structural types on shallow foundations subjected to excavation-induced ground settlements and provides a better understanding of the complex soil-structure interaction in building response. Investigated structures include brick-bearing structures, open-frame structures, and brick-infilled frame structures. These structures are often encountered near a construction area, and the different structures may show very different behaviors to excavation-induced ground settlements. In this research, numerical studies were carried out to evaluate the responses of single brick-bearing walls and frame structures (both open and brick infilled) subjected to an identical progressive ground settlement and to provide key features of building responses in different soil conditions, structure conditions, and structural types. Each structure, which is four stories high, was modeled numerically with two different soil conditions, and the response was compared among other types of structures and between elastic and crackable conditions for the brick-bearing and brick-infilled frame structures. Comparison of building responses was investigated by using distortions and crack damages induced to the structures by excavation-induced ground settlements. The structures were modeled by using the two-dimensional (2D) universal distinct element code (UDEC 3.1) in which each brick unit was modeled as a separate unit, with the contacts between brick units having stiffness and strength characteristics of mortar. The numerical studies indicated that the structural response to excavation-induced ground settlements is highly dependent on structural type, cracking in a structure, and soil condition; therefore, their effects should be considered to better assess building response to excavation-induced ground settlements.

Large-Scale Earthquake Simulation: Computational Seismology and Complex Engineering Systems

Abstract: Methods commonly used to generate artificial ground histories can't deal with the complex interactions that occur during earthquakes, including the seismic source and wave's path, site conditions, and the presence of the built environment. To address this problem, researchers in Carnegie Mellon University's Quake Group have used high-performance computing to simulate earthquakes at regional scales including complex engineering systems.

Optimized frequency-based foundation design for wind turbine towers utilizing soil–structure interaction

Abstract: This study illustrates design optimization for multiple wind towers located at different villages in Alaska. The towers are supported by two different types of foundations: large mat or deep piles foundations. Initially, a reinforced concrete (RC) mat foundation was proposed. Where soil conditions required it, a pile foundation solution was devised utilizing a 30 in thick RC mat containing an embedded steel grillage of W18 beams and supported by 20–24 in grouted or un-grouted piles. For faster installation and lower construction cost, all-steel foundations were proposed for these remote Alaska sites. The new all-steel design was found to reduce the natural frequencies of the structural system due to softening the foundation. Thus, the tower–foundation system could potentially become near-resonant with the operational frequencies of the wind turbine. Consequently, the likelihood of structural damage or even the collapse is increased. A detailed 3D finite-element model of the tower–foundation–pile system with RC foundation was created using SAP2000. Soil springs were included in the model based on soil properties obtained from the geotechnical investigation. The natural frequency from the model was verified against the tower manufacturer analytical and experimental values. When piles were used, numerous iterations were carried out to eliminate the need for the RC and optimize the design. An optimized design was achieved with enough separation between the natural and operational frequencies. The design successfully avoids damage to the structural system, while eliminating the need for any RC in most cases.

Applicability of Conventional p-y Relations to the Analysis of Piles in Laterally Spreading Soil

Abstract: This paper presents a kinematic analysis of a single pile embedded in a laterally spreading layered soil profile and discusses the relevancy of conventional analysis models to this load case. The research encompasses the creation of three-dimensional (3D) finite-element (FE) models using the OpenSees FE analysis platform. These models consider a single pile embedded in a layered soil continuum. Three reinforced concrete pile designs are considered. The piles are modeled using beam-column elements and fiber-section models. The soil continuum is modeled using brick elements and a Drucker-Prager constitutive model. The soil-pile interface is modeled using beam-solid contact elements. The FE models are used to evaluate the response of the soil-pile system to lateral spreading and two alternative lateral load cases. Through the computation of force density-displacement (p-y) curves representative of the soil response, the FE analysis (FEA) results are used to evaluate the adequacy of conventional p-y curve rel...

Small-Scale Modeling of Reinforced Concrete Structural Elements for Use in a Geotechnical Centrifuge

Abstract: This paper discusses the modeling of reinforced concrete structural elements for use in geotechnical centrifuge modeling of soil-structure interaction problems. Centrifuges are employed in geotechnical modeling so that the nonlinear constitutive behavior of soil in small-scale models can be correctly modeled at prototype scale. Such models typically necessitate large scale factors of between 1∶20 and 1∶100, which is significantly larger than most conventional small-scale structural modeling. A new model concrete has been developed consisting of plaster, water, and fine sand as a geometrically scaled coarse aggregate that can produce a range of model concretes with cube compressive strengths between 25–80 MPa. Reinforcement is modeled using roughened steel wire (beams) or wire mesh (slabs). To illustrate the validity of the modeling technique, a series of three- and four-point bending tests were conducted on model beams designed to represent a 0.5×0.5 m square section prototype beam at 1∶40 scale, and mod...

Seismic soil–structure interaction in multi-span bridges: Application to a railway bridge

Abstract: The effects of soil-structure interaction on the seismic response of multi-span bridges are investigated by means of a modelling strategy based on the domain decomposition technique. First, the analysis methodology is presented: kinematic interaction analysis is performed in the frequency domain by means of a procedure accounting for radiation damping, soil–pile and pile-to-pile interaction; the seismic response of the superstructure is evaluated in the time domain by means of user-friendly finite element programs introducing suitable lumped parameter models take into account the frequency-dependent impedances of the soil–foundation system. Second, a real multi-span railway bridge longitudinally restrained at one abutment is analyzed. The input motion is represented by two sets of real accelerograms: one consistent with the Italian seismic code and the other constituted by five records characterized by different frequency contents. The seismic response of the compliant-base model is compared with that obtained from a fixed-base model. Pile stress resultants due to kinematic and inertial interactions are also evaluated. The application demonstrates the importance of performing a comprehensive analysis of the soil–foundation–structure system in the design process, in order to capture the effects of soil-structure interaction in each structural element that may be beneficial or detrimental. Copyright © 2011 John Wiley & Sons, Ltd.

A simplified 3D model for soil-structure interaction with radiation damping and free field input

Abstract: This paper addresses the time history finite element analysis of rock-structure interaction. Modeled is not only the lateral energy dissipation, but also the interaction between the far field and the numerical model itself. This is accomplished by a preliminary analysis of the far field as a shear beam (for lateral excitation), and then velocities and displacements are transferred to the model as nodal forces through damping and stiffness matrices respectively. Details of the finite element implementation are given, along with an extensive series of simulations comparing this method, with the one of Lysmer for both 2D and 3D models. The model is derived from the principle of virtual work, and its implementation does not require any modification of existing finite element codes, only clever pre and postprocessing of results are needed.

Investigation of Load-Transfer Mechanisms in Geotechnical Earth Structures with Thin Fill Platforms Reinforced by Rigid Inclusions

Abstract: The reinforcement of soft soils by rigid inclusions is a practical and economical technique for wide-span buildings and the foundations of embankments. This method consists of placing a granular layer at the top of the network of piles to reduce vertical load on the supporting soil and vertical settlement of the upper structure. The study focuses on the modeling of load-transfer mechanisms occurring in the reinforced structure located over the network of piles with a coupling between the finite-element method (geosynthetic sheets) and discrete element method (granular layer; concrete slab in some cases). The importance of granular layer thickness to increase load-transfer intensity and to reduce vertical settlement was observed. However, without a basal geosynthetic sheet, the compressibility of soft soil has a great influence on the mechanisms. A method predicting the intensity of load transfers was proposed, based on Carlsson’s solution. The main parameters concerned are the geometry of the work and the peak and residual friction angles of the granular layer.

The effects of Soil–Structure Interaction on a reinforced concrete viaduct

Abstract: This paper presents a numerical strategy to model a three-pier viaduct made of prestressed concrete. The viaduct was tested pseudodynamically in ELSA laboratory (JRC Ispra, Italy). During the experimental campaign, only the three piers were tested, whereas the behaviour of the deck was simulated using the finite element method. The first part of the paper presents a numerical model of the viaduct based on the Timoshenko multifibre beam elements and non-linear constitutive laws. Comparisons with the experimental results show the good performance of the approach. In the second part, a parametric study is carried out showing the influence of Soil-Structure Interaction (SSI). Various types of soils are considered using a recently developed macro-element representing a rigid shallow foundation. The macro-element is suitable for dynamic (seismic) loadings and it takes into account the plasticity of the soil, the uplift of the foundation, P-theta effects and the radiative damping. Finally, the numerical results are compared with the ones coming from a classical engineering approach using linear elastic springs at the base of the piers. This comparison shows that SSI is a complex phenomenon inducing displacements and internal forces in the structure that are difficult to predict with the linear approach. Based on the results obtained in this paper, it seems now possible to use this approach to investigate numerically the behaviour of a wider variety of configurations.

Performance based assessment of dynamic soil-structure interaction effects on seismic response of building frames

Abstract: Soil-Structure Interaction (SSI) has progressed rapidly in the second half of 20th century stimulated mainly by requirements of the nuclear power and offshore industries to improve the seismic safety. In this study, a fifteen storey moment resisting building frame is selected in conjunction with three different soil deposits with shear wave velocity less than 600m/s. The design sections are defined after applying dynamic nonlinear time history analysis based on inelastic design procedure using elastic-perfectly plastic behaviour of structural elements. These frames are modelled and analysed employing Finite Difference approach using FLAC 2D software under two different boundary conditions, namely fixed-base (no soil-structure interaction), and considering soil-structure interaction. Fully nonlinear dynamic analyses under the influence of different earthquake records are conducted and the results of inelastic behaviour of the structural model are compared. Variations of the shear modulus ratio with the shear strain are included in the nonlinear dynamic analysis. The results indicate that the inter-storey drifts of the structural model resting on soil types De and Ee (according to the Australian standard) substantially increase when soil-structure interaction is considered for the above mentioned soil types. Performance levels of the structures change from life safe to near collapse when dynamic soil-structure interaction is incorporated. Therefore, the conventional inelastic design procedure excluding SSI is no longer adequate to guarantee the structural safety for the building frames resting on soft soil deposits. Design engineers need to address the effects of dynamic SSI precisely in their design especially for construction projects on soft soils.

In-structure shock assessment of underground structures with consideration of rigid body motion

Abstract: The present study assesses the in-structure shock of an underground structure induced by a nearby subsurface detonation. Both the rigid body motion of the entirely buried structure and the local response of the structural element are considered in characterizing the in-structure shock. Soil-structure interaction, behaving as an interfacial damping, is taken into account, and the effect of different surrounding soils is investigated. A response spectrum is plotted for assessing in-structure shock induced by a typical subsurface detonation, and subsequently the safety of the internal facilities and equipment mounted on the buried structure is evaluated. For safety purposes, the protective structures are better constructed in a site with small acoustic impedance and a large attenuation factor. Results show that the proposed in-structure shock assessment method is effective and can be used as a supplement to TM-5-855-1 and TM-5-1300.

Seismic Performance Assessment in Dense Urban Environments

Abstract: Author(s): Mason, Henry Benjamin | Advisor(s): Bray, Jonathan D | Abstract: In seismically active, densely populated areas, buildings within a city block interact with one another during an earthquake. This phenomenon, whereby two adjacent buildings interact with each other through the surrounding soil during an earthquake, is often called structure-soil-structure interaction (SSSI). SSSI effects are less understood than soil-foundation-structure interaction (SFSI) effects. There are a lack of high-quality case histories that clearly show SSSI, which is a key reason that SSSI is less understood than SFSI. SSSI effects can potentially be detrimental and lead to more damage within the soil-foundation-structure system. Accordingly, it is important to understand when SSSI effects are important, and include them in engineering analysis and design when necessary.This dissertation describes three centrifuge tests designed to simulate SSSI and SFSI case histories. All centrifuge test described within this dissertation were performed at the University of California at Davis Center for Geotechnical Modeling (UCD-CGM). The first test, Centrifuge Test-1, examined two inelastic moment-resisting frame structures atop a bed of dry, dense sand. One frame structure represented a prototypical three-story moment-resisting frame structure founded on spread footings. The other frame structure represented a prototypical nine-story moment-resisting frame structure founded on a three-story basement. The two structures were located a significant distance apart, and thus, SSSI effects were masked. Accordingly, the purpose of Test-1 was to examine SFSI effects of inelastic frame structures and to serve as a baseline test (i.e., a control test). The second test, Centrifuge Test-2, examined the same two structures atop a bed of dry, dense sand. In Test-2, however, the two structures were located adjacent to each other. Therefore, the purpose of Test-2 was to examine SSSI effects. By comparing results from Test-1 with results from Test-2, insights into SSSI effects were made.The third test, Centrifuge Test-3, examined three structures atop a bed of dry, dense sand. Two of the structures were identical, and represented prototypical three-store moment-resisting frame structures founded on spread footings. These structures were nearly identical to the three-story structures used during Test-1 and Test-2. The third structure was a rigid rocking wall founded on a large mat foundation, which was identified as the transmitter structure. One frame structure, which was identified as the receiver structure, was located adjacent to the transmitter structure. The other frame structure, which was identified as the control structure, was located a significant distance away from the transmitter-receiver pair of structures. The design goal of the transmitter-receiver pair was to maximize interaction between the two structures. By comparing the seismic response of the control structure with the seismic response of the receiver structure, insights into SSSI were made.The earthquake motions employed during the three centrifuge tests described within this dissertation are critically important. A preliminary centrifuge test (Test-0) was performed after an earthquake motion selection process. The purpose of Test-0 was to calibrate a suite of earthquake motions that could be used at the UCD-CGM. This dissertation describes an earthquake motion selection and calibration process that future researchers can use to create test-specific earthquake motions for their research projects.Kinematic SFSI and SSSI effects were examined during Test-1 and Test-2. Specifically, the earthquake motions recorded in the free-field at the surface, which is the earthquake motion most often used by practicing engineers for dynamic analyses, was compared to the earthquake motion recorded under the basement, in the soil. Because of kinematic interaction effects, which include base slab averaging and embedment effects, the earthquake motion recorded under the basement has smaller amplitude and smaller high-frequency content than the earthquake motion recorded in the free-field at the surface. This is an established observation, and Test-1 and Test-2 data corroborate with current kinematic interaction estimation procedures. When comparing the results from Test-2 with Test-1, however, it was seen that basement-level earthquake motion differed less from the free-field surface motion during Test-2. This result indicates that kinematic interaction effects may be masked in urban environments.The seismic responses of the shallowly embedded frame structure footings were also examined during Test-1, Test-2, and Test-3. More specifically, the vertical displacement (settlement and uplift), horizontal displacement (sliding), and rocking were examined. By comparing results from Test-2 with results from Test-1, it was seen that the deeply embedded basement "restrains" the adjacent footings. In other words, the adjacent footings displace and rotate less than the footings that are not adjacent to the basement (i.e., the free footings). This asymmetrical footing response leads to additional demands on the superstructure, which may be unacceptable. In addition, the seismically-induced column moments measured above the restrained footings are larger than those measured above the free footings. Therefore, SSSI effects were seen to be potentially detrimental (i.e., lead to more superstructure damage) during Test-2.During Test-3, the same footing restraining effect observed in Test-2 was found to be not as large. However, there is evidence that the transmitter structure affected the seismic response of the adjacent receiver structure. More specifically, as the transmitter structure rocked and settled during the higher-intensity earthquake motions, the adjacent footings of the receiver structure did uplift, and this caused asymmetry in the superstructure. A general observation from Test-3 is that the seismic footing response of frame structures founded on shallowly-embedded footings is erratic. Future work in this area will examine possible explanations for the observed erratic response.

Evaluation of the influence of interface elements for structure - isolated footing - soil interaction analysis

Abstract: In this study, two extreme cases of compatibility of the horizontal displacements between the foundation and soil are considered, for which the pressure and settlements of the isolated footings and member end actions in structural elements are obtained using the three dimensional models and numerical experiments. The first case considered is complete slip between foundation and soil, termed as the uncoupled analysis. In the second case of analysis, termed as the coupled analysis, complete welding is assumed of joints between the foundation and soil elements. The model and the corresponding computer program developed simulate these two extreme states of compatibility giving insight into the variation of horizontal displacements and horizontal stresses and their intricacies, for evaluation of the influence of using the interface elements in soil-structure interaction analysis of three dimensional multiscale structures supported by isolated footings.

Seismic soil-structure interaction of buildings on hill slopes

Abstract: In hilly regions, engineered construction is constrained by local topography resulting in the adoption of either a step-back or step-back-set-back configuration as a structural form for buildings. The adopted form invariably results in a structure which is irregular by virtue of varying column heights leading to torsion and increased shear during seismic ground motion. To capture the real behavior of buildings on hill slope a 3-D analysis of the building is required. In the present study, static pushover analysis and Response spectrum analysis (RSA) have been conducted on five building i.e. three step back buildings and two step back-set back buildings with varying support conditions. These buildings have been analyzed for different soil conditions (hard, medium and soft soils) idealized by equivalent springs. The response parameters, i.e. total base shear (V), displacement from pushover analysis (δ performance point), displacement from RSA (δ elastic) and response correction factor (R’) have been studied with respect to fixed base analysis to compare the effect of soil springs. In general it is found that response reduction factor decreases with increasing time period, but is expected to be constant beyond a certain value of time period.