Constantine C. Spyrakos

Dynamic response of flexible strip-foundations by boundary and finite elements

A time domain Boundary Element-Finite method is employed to determine the dynamic response of flexible surface two-dimensional foundations under conditions of plane strain placed on an elastic soil medium and subjected either to transient external forces or to obliquely incident seismic waves. The elastic, isotropic, and homogeneous soil medium is treated by the time domain Direct Boundary Element Method, while the flexible foundation is treated by the Finite Element Method. The two methods are appropriately combined through equilibrium and compatibility considerations at the soil-foundation interface. Parametric studies examining the effect of the relative stiffness between the foundation and the soil and the spatial distribution of the dynamic disturbances on the foundation response are presented.

SEISMICALLY ISOLATED BRIDGE PIERS ON SHALLOW SOIL STRATUM WITH SOIL STRUCTURE INTERACTION

Abstract The objective of this study is to assess the effects of soil–structure interaction on the response of seismically isolated bridge piers founded on a shallow soil stratum overlying a rigid bedrock and to develop a method that considers soil–structure interaction and can be easily applied to the preliminary design of bridges. The relative importance of several parameters of the bridge-isolators-soil system is examined. Cases in which soil–structure interaction needs to be incorporated in seismically isolated bridge design are identified and ways to take advantage of soil–structure interaction in order to enhance the safety level and reduce design costs are recommended.

Time domain boundary element method approaches in elastodynamics: a comparative study

Abstract Boundary Element Method formulations of elastodynamic problems under plane strain/plane stress conditions are presented. The formulations are performed in the time domain allowing consideration of loads with transient time variation. The advantages, efficiency and accuracy of the methods are depicted through comparative studies of a representative soil-structure interaction problem.

Seismic behavior of bridge piers including soil-structure interaction

Abstract As a rule, widely used practices do not consider soil-structure interaction (SSI) in the seismic design of bridges. This study attempts to assess the significance of SSI for the design of bridge piers placed on either a homogeneous deep soil stratum or a shallow soil stratum overlying a rigid bedrock. The objective is pursued through a simple, yet capable to capture the effects of the most crucial physical parameters, representation of a bridge pier-soil system. Cases in which SSI needs to be considered in the design are identified, and recommendations that can lead to more economical and safer bridge designs are provided.

ASSESSMENT OF SSI ON THE LONGITUDINAL SEISMIC RESPONSE OF SHORT SPAN BRIDGES

Abstract Current practice usually neglects the effects of soil-structure interaction (SSI) in the seismic analysis and design of bridges. This work attempts to assess the significance of SSI on the seismic responses of short span bridges. The focus is placed on pier behaviour, since piers together with the abutments are the most critical elements in securing the integrity of bridge superstructures during earthquakes. The study is based on a simple representation of a soil-bridge pier system, yet one able to capture the effects of the most significant physical parameters. It has been found that SSI greatly affects the dynamic behaviour of bridge piers leading to more flexible systems, increased damping and larger total displacements. Besides a thorough investigation of the relative significance of various physical parameters on the system response, an easy-to-use approach that can be incorporated for a preliminary design of bridges concurrent with the AASHTO specifications is presented. The study concludes that safer and more economical bridge designs can be obtained by properly accounting for SSI.

Seismic behavior of a post-tensioned integral bridge including soil–structure interaction (SSI)

Current practice usually pays little attention to the effect of soil– structure interaction (SSI) on seismic analysis and design of bridges. The objective of this research study is to assess the significance of SSI on the modal with geometric stiffness and seismic response of a bridge with integral abutments that has been constructed using a new bridge system technology. Emphasis is placed on integral abutment behavior, since abutments together with piers are the most critical elements in securing the integrity of bridge superstructures during earthquakes. Comparison is made between analytical results and field measurements in order to establish the accuracy of the superstructure –abutment model. Sensitivity studies are conducted to investigate the effects of foundation stiffness on the overall dynamic and seismic response of the new bridge system. q 2002 Published by Elsevier Science Ltd.

Closed form series solutions of boundary value problems with variable properties

Abstract A Fourier cum polynomial series solution scheme is presented herein which is applicable to a broad class of initial/boundary value problems in engineering mechanics. Special emphasis is given to those problems that are represented by linear partial or ordinary differential equations with variable coefficients and not amenable to exact analytic solutions. In the proposed approach, the coefficients of the polynomials are simultaneously solved as functions of the undetermined Fourier coefficients by satisfying the initial/boundary conditions. A Fourier series expansion of the variable coefficients and application of orthogonality conditions leads to the evaluation of the undetermined Fourier coefficients through the solution of simultaneous equations or summation equations. The summation equations are solved in closed form by a new and highly efficient algorithm developed by the authors. Representative engineering mechanics problems are examplified to elucidate the features of the method and demonstrate its advantages over other techniques.

Seismic analysis of towers including foundation uplift

Abstract A seismic analysis procedure including soil-structure interaction and partial foundation uplift for tower structures is developed. The nonlinear equations of motion are derived with the aid of Lagranges equation. Parametric studies are performed to evaluate the effects of factors such as: soil stiffness, ratio of tower height to foundation width, and partial separation of the foundation from the soil. The studies indicate that the effects of soil stiffness on a short tower are greater than on a slender one. The height-width ratio affects the seismic response of the tower significantly, especially for a tower at a rock-like location. The allowance of foundation uplift may substantially reduce the seismic response of moment and foundation rotation in the case of hard soil and a slender tower, or may greatly increase the seismic response of shear in the case of hard soil and a short tower. The study concludes that uplift is not always beneficial and its effects could be significant for structures under strong seismic motions.

Dynamic analysis of flexible strip-foundations in the frequency domain☆

Abstract This work deals with the development and presentation of a frequency domain hybrid numerical method for determining the dynamic response of surface strip-foundations under conditions of plane strain placed on an elastic soil medium and subjected to either externally applied forces or seismic disturbances. The elastic, isotropic, and homogeneous soil medium is treated with the boundary element method, while the flexible and massive foundation is treated with the finite element method. The two methods are appropriately combined through equilibrium and compatibility conditions at the soil-foundation interface. Several numerical examples are worked out to demonstrate and attest the efficiency and accuracy of the method.

Uplifting-sliding response of flexible structures to seismic loads

Abstract This study presents the development of a BEM-FEM methodology to analyze flexible structures on an elastic halfspace allowed to simultaneously uplift and slide under seismic excitations. The methodology combines the Boundary Element Method (BEM) applied to model the semi-infinite soil medium and the Finite Element Method (FEM) to model the foundation and the super-structure. The two methods are combined with appropriate force equilibrium and displacement compatibility requirements through the utilization of FEM interface elements at the foundation-soil interface. All four modes of interface deformations, i.e., stick, debonding, rebonding and sliding, are accounted for to accurately simulate uplift and sliding of the foundation-structure system from the underlying soil medium. The sliding mode of deformation is associated with Coulomb friction at the soil-structure interface. The methodology is employed to investigate the response of a nuclear containment structure subjected to the El Centra earthquake of 1940. The results revealed that the base shear reduces significantly if the structure is allowed to slide. Further, parametric studies for various values of the friction coefficient are conducted in order to investigate the structure response under varying friction conditions.