Showing papers on "Soil structure interaction published in 1995"

Effects of Soil-Structure Interaction on Inelastic Seismic Response of Bridge Piers

Abstract: The investigation reported in the paper is an attempt to assess the relevance of soil-structure interaction (SSI) effects on the dynamic response of bridge piers responding in the inelastic range and, in particular, on the maximum required ductilities in the critical regions of the superstructure. The problem is dealt with reference to a simple structural configuration: a vertical cantilever carrying a mass at the top, representing realistic cases of bridge piers of common geometry with spread footing foundations. A relatively large number of parameters has been considered in the analysis, with a total of 240 cases examined, and with ductility demands on the piers ranging between 1.5 and 7. The results indicate that although in most cases SSI produces an increase of the maximum displacements, this effect is not very significant, and the inelastic demand in terms of curvature essentially remains unaffected by soil-structure interaction, showing a tendency to decrease. Finally, the stability of these conclusions against rather extreme variations of the parameters is confirmed by the results of additional bounding analyses, characterized by unusually soft soil conditions and by a seismic excitation multiplied by a factor of two.

Efficient modal analysis for structures with soil‐structure interaction

Abstract: An efficient methodology is presented which uses modal analysis implemented in the frequency domain to obtain the structural response of a system with soil-structure interaction. The interaction effects are represented using a free-field ground motion modification factor, derived for each mode of vibration and used in the determination of structural response. Applying this algorithm, the advantages of the modal superposition method are fully exploited, and the interaction problem can be solved easily and effectively within the framework of the conventional frequency domain analysis for a fixed-base structure. In addition, this method produces accurate approximation with less computational effort due to consideration of only the first few vibration modes of the structure.

Observation and numerical analysis of soil-structure interaction of a reinforced concrete tower

Abstract: A vast amount of earthquake response records of an observation tower are used together with microtremor data to investigate various aspects of the dynamic behaviour of the soil-structure system. It is found that separation of the soil from the structure occurs under large dynamic loads, leading to changes in the predominant frequency of the system. As a result of the decreasing of the soil support at the side walls of the foundation, the stress caused by the structural weight on the bottom soil increases during earthquakes. With regard to its practical applicability, a linear sway-rocking model is applied for numerical modelling of the soil-structure system. Alterations in the soil support as a result of soil non-linearity and separation of the structure from the soil are investigated by comparing recorded and simulated structural response. The influence of each of these factors on the softening of the soil support is distinctly assessed. An empirical relationship between the peak ground velocity and the soil constants for earthquake excitations of different magnitude is presented.

Soil-structure interaction: FEM computations

Abstract: Publisher Summary The chapter reviews aspects of the soil–structure interface behavior at the element level and the numerical integration of the corresponding interface constitutive models. The design of structures subjected to soil–structure interaction and to contact with friction should be tackled using soil–structure interface constitutive equations. These laws differ from the laws for soils because of three main features: the size of the relative displacements and of relative rotations between grains, the high level of dilatancy and contraction under shearing, and the presence of an intense degradation effect resulting from localization in the pattern of a shear band. The elastoplastic interface constitutive equations are easy to use but do not modelize all these effects. The incrementally non-linear interface constitutive equations are versatile for modeling all these interdependent phenomena. In addition, applications to piles under tension loading are presented to illustrate the results of these procedures.

Discrete model for foundation-soil-foundation interaction

Abstract: A new discrete model for the study of dynamic interaction phenomena between adjacent, rigid foundations on a homogeneous, linear elastic half-space is presented. Each dynamic degree of freedom of the foundations consists of a mass connected to a rigid support through frequency independent springs and dashpots. The interaction between the foundations is achieved by imposing spring and damping couplings developed in this work. The time lagging effects of coupled dynamic input due to wave propagation is also considered through a proposed modified vector approach.

Nonlinear seismic response of single piles

Abstract: A macroscopic model that consists of distributed hysteretic springs and frequency dependent dashpots is utilized to model the soil-pile interaction and a practical method based on one dimensional finite element formulation is developed to compute the nonlinear seismic response of single piles.

Structural vibration isolation by rows of piles

Abstract: Three-dimensional passive structural vibration isolation by rows of piles is studied numerically by the frequency domain boundary element method. The source of soil vibration is assumed to be a vertical force harmonically varying with time. The piles and the soil material behaviour are assumed to be linear elastic or viscoelastic. Coupling between the soil and piles is accomplished through equilibrium and compatibility at their interfaces. Both continuous and discontinuous quadratic quadrilateral elements are employed and advanced direct numerical integration schemes are used for the treatment of the various singular integrals. The full-space dynamic fundamental solution is used and this requires a discretization of not only the substructure interfaces but also a finite portion of the free soil surface around the vibration isolation system. Symmetry and antisymmetry considerations reduce the complexity of the problem considerably. The above methodology is tested for accuracy by solving a problem of active vibration isolation by trenches for which there exist numerical solutions and then applied to the problem of structural vibration isolation by rows of piles and compared with an existing approximate analytical solution.

Structural damping for soil-structure interaction studies

Abstract: A soil-structure interaction formulation is used here which is based on consideration of the dynamics of the structure with a free, rather than a fixed, base. This approach is shown to give a quite simple procedure for coupling the dynamic characteristics of the structure to those of the foundation and soil in order to obtain a matrix formulation for the complete system. In fixed-base studies it is common to presume that each natural mode of the structure has a given fraction of critical damping, and since the interaction formulation uses a free-base model, it seems natural for this situation to assign the equal modal damping values to free-base modes. It is shown, though, that this gives a structural model which is significantly different than the one having equal modal damping in the fixed-base modes. In particular, it is found that the damping matrix resulting in equal modal damping values for free-based modes will give a very significantly smaller damping value for the fundamental distortional mode of the feued-base structure. Ignoring this fact could lead one to attribute dynamic effects to interaction which are actually due to the choice of damping.

Dynamic soil–structure interaction of coupled shear walls by boundary element method

Abstract: For a class of civil engineering structures, that can be accurately represented by ‘coupled shear walls’ (CSWs), a discrete model for the analysis of the dynamic interaction with the underlying soil is proposed. The CSWs, with one or more rows of openings, rest on a rigid foundation embedded in the elastic or viscoelastic half-space. A hierarchical finite element model based on an equivalent continuum approach is adopted for the structure. A frequency-domain boundary element method is used to represent the half-space. Finally, the set of equations governing the response of the coupled soil-structure system to harmonic lateral loads acting on the structure is also given. The frequency deviation effect with respect to the fixed-base structure and the effects of radiation and material damping in the soil are presented for different characteristics of the structure and different soil properties.

Embankment train track on soil stratum and wave impeding block (wib) measured for vibrations reduction

Abstract: Aiming at the assessment of train induced vibrations in the neighborhood, a preliminary theoretical investigation is conducted by assuming a Ricker wave form loading with probable period on an embankment track. The track is assumed to be constructed on a soil stratum. The transient responses are computed by applying a hybrid method of finite elements (FE) and boundary elements (BE). The time domain Green function for a stratum is implemented into the latter formulation. The response reduction measure called WIB (wave impeding block), which is based on the cut-off frequency of wave field, is investigated when installed in the embankment track. Parameter studies are performed for the rigidity ratio between embankment and supporting soil. The vibration assessment of a nearby structure is made in terms of the velocity response from the human perception.

Soil structure interaction analysis of buried tank subjected to vertical excitations

Abstract: Underground High Level Waste Storage Tanks are subjected to strigent seismic requirements At some DOE sites, many existing waste storage tanks are of the double-shell tank design. In this configuration, the concrete outer structure acts as the vault and provides secondary confinement for the primary steel waste storage tank. To ensure the safety of the design and a good understanding of the seismic response of the concrete confinement structure, seismic analysis, including the effects of Soil-Structure Interaction (SSI), is generally performed with special purpose SSI computer analysis programs. Generally, the seismic SSI response due to vertical excitation is considered to be secondary to those of the horizontal excitation. In this paper, a detailed evaluation of the SSI response due to vertical excitation is presented and is shown to merit equal consideration relative to the horizontal excitation. The geometry and relative dimensions (i.e. flexibility) of the structure can have significant influence on the vertical seismic SSI response in local region(s) of the concrete structure.