Seismic analysis of lock–soil–fluid systems by hybrid BEM–FEM

A study of soil–structure–fluid interaction (SSFI) of a lock system subjected to harmonic seismic excitation is presented. The water contained lock is embedded in layered soils supported by a half-space bedrock. The ground excitation is prescribed at the soil–bedrock interface. The response is numerically obtained through a hybrid boundary element (BEM) finite element method (FEM) formulation. The semi-infinite soil and the fluid are modeled by the BEM and the lock is modeled by the FEM. The equilibrium equation for the lock system is obtained by enforcing compatibility and equilibrium conditions at the fluid–structure, soil–structure and soil–layer interfaces under conditions of plane strain. To the authors’ knowledge this is the first study of a lock system that considers the effects of dynamic soil–fluid–structure interaction through a BEM–FEM methodology. A numerical example and parametric studies are presented to examine the effects of the presence of water, lock stiffness, and lock embedment on the response.     

Numerical study of characteristics of underground blast induced surface ground motion and their effect on above-ground structures. Part II. Effects on structural responses

Studies of structural responses and damage to high-frequency blast motion are very limited. Current practice uses some empirical allowable ground vibration limits in assessing structural performance. These empirical limits overlook the physical parameters that govern structural response and damage, such as the ground motion characteristics and inherent structural properties. This paper studies the response of RC frame structures to numerically simulated underground blast-induced ground motions. The structural response and damage characteristics of frame structures to ground motions of different frequencies are investigated first. The effects of blast ground motion spatial variations and soil–structure interaction on structural responses are also studied. A suitable discrete model that gives accurate response prediction is determined. A damage index defined based on the accumulated plastic hinge rotation is used to predict structural damage level. Numerical results indicated that both the low structural vibration modes (global modes) and the first elemental vibration mode (local) might govern the dynamic structural responses depending on the ground motion frequency and structural response parameters under consideration. Both ground motion spatial variations and soil–structure interaction effects are prominent. Neglecting them might yield inaccurate structural response prediction. The overall structural response and damage are highly ground motion frequency dependent. Numerical results of structural damage are also compared with some test results obtained in a previous study and with code specifications. Discussions on the adequacy of the code allowable ground vibration limits on RC frame structures are also made.     

Seismic soil–structure interaction of massive flexible strip-foundations embedded in layered soils by hybrid BEM–FEM

A study on the seismic response of massive flexible strip-foundations embedded in layered soils and subjected to seismic excitation is presented. Emphasis is placed on the investigation of the system response with the aid of a boundary element–finite element formulation proper for the treatment of such soil–structure interaction problems. In the formulation, the boundary element method (BEM) is employed to overcome the difficulties that arise from modeling the infinite soil domain, and the finite element method (FEM) is applied to model the embedded massive flexible strip-foundation. The numerical solution for the soil–foundation system is obtained by coupling the FEM with the BEM through compatibility and equilibrium conditions at the soil–foundation and soil layer interfaces. A parametric study is conducted to investigate the effects of foundation stiffness and embedment on the seismic response.     

The influence of dynamic soil–structure interaction on traffic induced vibrations in buildings

This paper deals with the dynamic response of buildings due to traffic induced wave fields. The response of a two-storey single family dwelling due to the passage of a two-axle truck on a traffic plateau is computed with a model that fully accounts for the dynamic interaction between the soil and the structure. The results of three cases where the structure is founded on a slab foundation, a strip foundation and a box foundation are calculated and a comprehensive analysis of the dynamic structural response is performed. A methodology is also proposed to calculate the structural response, neglecting the effects of dynamic soil–structure interaction. A comparison with the results of calculations where dynamic soil–structure interaction is accounted for shows that a good approximation is obtained in the case of a rigid structure resting on a soft soil.     

Dynamic soil–structure interaction effects on the seismic response of asymmetric buildings

An approach is formulated for the linear analysis of three-dimensional dynamic soil–structure interaction of asymmetric buildings in the time domain, in order to evaluate the seismic response behaviour of torsionally coupled buildings. The asymmetric building is idealized as a single-storey three-dimensional system resting on different soil conditions. The soil beneath the superstructure is modeled as linear elastic solid elements. The contact surface between foundation mat and solid elements of soil is discretised by linear plane interface elements with zero thickness. An interface element is further developed to function between the rigid foundation and soil. As an example, the response of soil–structure interaction of torsionally coupled system under two simultaneous lateral components of El Centro 1940 earthquake records has been evaluated and the effects of base flexibility on the response behaviour of the system are verified.     

Structure, soil–structure response and effects of damage based on observations of horizontal-to-vertical spectral ratios of microtremors

The microtremor horizontal-to-vertical-spectral-ratio (HVSR) technique is widely used in the urban environment to assess the fundamental frequency response of the ground. Extensive literature exists about case histories using HVSR for microzonation in several cities, but no systematic studies have been devoted to check the presence of soil–structure interaction effects, and even less attention to study building behaviour after earthquake damage. To evaluate the above-mentioned effects, a series of experiments are reported in this article.We first made a series of microtremor measurements on buildings and civil structures to evaluate the reliability of fundamental frequency determinations. Then, we considered several case studies to evaluate the effect of soil–structure interaction in estimates of site response in the presence of tall buildings. Finally, an experiment on the frequency change due to damage was performed. It was possible to confirm that HVSR is able to detect building fundamental modes and once known the building frequency, it is also possible to detect the presence of soil–structure interaction. Thus, once the presence of the building natural frequency is identified, it is possible to infer the site response from free field measurements. We also found that the HVSR technique is equally useful for detecting structural damage by determining the frequency shift of the buildings.     

Some analytical results on lateral dynamic stiffness for footings supported on hysteretic soil medium

The nonlinearity of the soil affects soil–structure interaction to a considerable extent. For a reliable and safe analysis of soil interaction effects on the dynamic response of structures, a more realistic and relatively straightforward method incorporating the nonlinear hysteretic nature of the underlying soil–foundation system needs to be developed. The present paper models the soil–foundation system as a single degree of freedom spring–dashpot system with nonlinear hysteresis in form of elasto-perfectly plastic behavior. Analytical results for the lateral dynamic stiffness on footing have been presented. An example study has been carried out in case of circular footings. It is shown how the analytical results can be used to get a preliminary idea of the lateral dynamic stiffness of footings on a soil medium prior to a detailed computational geo-mechanics analysis provided the static nonlinear load–deformation characteristic of the soil medium is known and can be modeled by a hysteretic elasto-plastic behavior. The corresponding results are presented in a graphical form. The results have been computed showing parametric variations with the change in the amplitude and dimensionless frequency of the non-dimensional excitation force. Analytical results are also presented for the asymptotic cases at low and very high values of dimensionless frequency parameter.     

Numerical simulation of dynamic soil–structure interaction in shaking table testing

This paper provides an insight into the numerical simulation of soil–structure interaction (SSI) phenomena studied in a shaking table facility. The shaking table test is purposely designed to confirm the ability of the numerical substructure technique to simulate the SSI phenomenon. A model foundation–structure system with strong SSI potential is embedded in a dry bed of sand deposited within a purpose designed shaking-table soil container. The experimental system is subjected to a strong ground motion. The numerical simulation of the complete soil–foundation–structure system is conducted in the linear viscoelastic domain using the substructure approach. The matching of the experimental and numerical responses in both frequency and in time domain is satisfying. Many important aspects of SSI that are apparent in the experiment are captured by the numerical simulation. Furthermore, the numerical modelling is shown to be adequate for practical engineering design purposes.     

Numerical simulation of liquefaction effects on seismic SSI

The present paper deals with the influence of soil non-linearity, introduced by soil liquefaction, on the soil–foundation–structure interaction phenomena. The objective is to reveal the beneficial or unfavourable effects of the non-linear SSI on both structural drift and settlement of a given structure. Factors such as the signal modification due to liquefaction, and ratios of fundamental frequencies of soil, structure and signal may play an important role on the damage of the structure. The importance of each of these factors is evaluated through a significant parametric study. A 2D coupled finite element modelling is carried out using an elastoplastic multi-mechanism model to represent the soil behaviour. This paper presents the research work we did in the framework of the European Community project NEMISREF (New methods of mitigation of seismic risk on existing foundations, GRDI-40457), to study possible retrofitting measures using GEFDYN computational tools.     

International standard (ISO) on seismic actions for designing geotechnical works–An overview

The seismic performance of geotechnical works is significantly affected by ground displacement. In particular, soil–structure interaction and effects of liquefaction play major roles and pose difficult problems for engineers. An International Standard, ISO23469, is being developed for addressing these issues in a systematic manner within a consistent framework. The objective of this paper is to give an overview of this International Standard.In this International Standard, the seismic actions are determined through two stages. The first stage determines basic seismic action variables, including the earthquake ground motion at the site, the potential for earthquake-associated phenomena such as liquefaction and induced lateral ground displacement. These basic variables are used, in the second stage, for specifying the seismic actions for designing geotechnical works. In the second stage, the soil–structure interaction plays a major role. Types of analyses are classified based on a combination of static/dynamic analyses and the procedure for soil–structure interaction classified as follows:

– simplified: soil–structure interaction of a global system is modeled as an action on a substructure;

– detailed: soil–structure interaction of a global system is modeled as a coupled system.

Equivalent linear substructure approximation of soil–foundation–structure interaction: model presentation and validation

An equivalent linear substructure approximation of the soil–foundation–structure interaction is proposed in this paper. Based on the inherent linearity of the approach, the solution of the structural and the soil domain is obtained simultaneously, incorporating the effects of the primary and secondary soil nonlinearities. The proposed approximation is established theoretically and then validated against centrifuge benchmark soil–foundation–structure interaction tests. The equivalent linear substructure approximation is proved to simulate efficiently the effects of the nonlinear soil behavior on the soil–foundation–structure system under a strong earthquake ground motion.     

Investigation of seismic behavior of fluid–rectangular tank–soil/foundation systems in frequency domain

Main purpose of this study is to evaluate the dynamic behavior of fluid–rectangular tank–soil/foundation system with a simple and fast seismic analysis procedure. In this procedure, interaction effects are presented by Housner's two mass approximations for fluid and the cone model for soil/foundation system. This approach can determine; displacement at the height of the impulsive mass, the sloshing displacement and base forces for the soil/foundation system conditions including embedment and incompressible soil cases. Models and equations for proposed method were briefly explained for different tank–soil/foundation system combinations. By means of changing soil/foundation conditions, some comparisons are made on base forces and sloshing responses for the cases of embedment and no embedment. The results showed that the displacements and base shear forces generally decreased, with decreasing soil stiffness. However, embedment, wall flexibility, and soil–structure interaction (SSI) did not considerably affect the sloshing displacement.     

Earthquake damage detection in the Imperial County Services Building III: Analysis of wave travel times via impulse response functions

The majority of structural health monitoring methods are based on detecting changes in the modal properties, which are global characteristics of the structure, and are not sensitive to local damage. Wave travel times between selected sections of a structure, on the other hand, are local characteristics, and are potentially more sensitive to local damage. In this paper, a structural health monitoring method based on changes in wave travel times is explored using strong motion data from the Imperial Valley Earthquake of 1979 recorded in the former Imperial County Services (ICS) Building, severely damaged by this earthquake. Wave travel times are measured from impulse response functions computed from the recorded horizontal seismic response in three time windows—before, during, and after the largest amplitude response, as determined from previous studies of this building, based on analysis of novelties in the recorded response. The results suggest initial spatial distribution of stiffness consistent with the design characteristics, and reduction of stiffness following the major damage consistent with the spatial distribution of the observed damage. The travel times were also used to estimate the fundamental fixed-base frequency of the structure f₁ (assuming the building deformed as a shear beam), and its changes during this earthquake. These estimates are consistent with previous estimates of the soil–structure system frequency, f_sys, during the earthquakes (f₁<f_sys as expected from soil–structure interaction studies), and with other estimates of frequency (f₁ from ETABS models, and f_sys from ambient vibration tests, and “instantaneous” f₁ from high-frequency pulse propagation).     

Effects of rainfall on soil–structure system frequency: Examples based on poroelasticity and a comparison with full-scale measurements

In this paper, a simple two-dimensional soil–structure interaction model, based on Biot's theory of wave propagation in fluid saturated porous media, is used to explain the observed increase of the apparent frequencies of Millikan library in Pasadena, California, during heavy rainfall and recovery within days after the rain. These variations have been measured for small amplitude response (to microtremors and wind excitation), for which Biot's linear theory is valid. The postulated hypothesis is that the observed increases in frequency are due to the water saturation of the soil. The theoretical model used to explore this hypothesis consists of a shear wall supported by a circular foundation embedded in a poroelastic half-space. This rigid foundation model may be appropriate only for the NS response of Millikan library. This paper presents results for the foundation stiffness, and for the system response for model parameters similar to those for Millikan library (located on alluvium with shear wave velocity of about 300 m/s). The foundation impedance matrix, foundation input motion and system response are compared for dry and fully saturated half-space, with permeable and impermeable foundation. The results show that for embedded foundations, the effects of saturation on the horizontal foundation stiffness are as significant as for the vertical stiffness, contrary to what has been known for surface foundations investigated by other authors. Further, the results suggest a 1–2% increase in system frequency of the first two modes of vibration, depending on the drainage condition along the foundation–soil interface. Such increases agree qualitatively with the observations.     

Nonlinear response of surface soil and NTT building due to soil–structure interaction during the 1995 Hyogo-ken Nanbu (Kobe) earthquake

The 1995 Hyogo-ken Nanbu (Kobe) earthquake brought about enormous damage to structures in the Hanshin and Awaji areas. In this paper the importance of investigating the relationship between ground motion and structural damage is pointed out.

Strong seismic motion was observed at the NTT (Nippon Telegraph and Telephone) Building during this earthquake. The structural damage to this building was relatively slight. In order to evaluate the relationship between ground motion and structural damage, it is necessary to assess the effects of the soil–structure interaction. In this study, the seismic response of the building and of the surface soil were evaluated by means of a nonlinear soil–structure interaction analysis using FEM.

It was found that, the nonlinearity of surface soil near the building had a great effect on the soil–structure interaction, especially the rocking of the building.     

Ambient vibration tests of a seven-story reinforced concrete building in Van Nuys, California, damaged by the 1994 Northridge earthquake

Results of two detailed ambient vibration surveys of a 7-story reinforced concrete building in Van Nuys, California, are presented. Both surveys were conducted after the building was severely damaged by the 17 January 1994, Northridge earthquake (M_L=5.3, epicenter 1.5 km west from the building site) and its early aftershocks. The first survey was conducted on 4 and 5 February 1994, and the second one on 19 and 20 April 1994, about one month after the 20 March aftershock (M_L=5.3, epicenter 1.2 km north–west from the building site). The apparent frequencies and two- and three-dimensional mode shapes for longitudinal, transverse and vertical vibrations were calculated. The attempts to detect the highly localized damage by simple spectral analyses of the ambient noise data were not successful. It is suggested that very high spatial resolution of recording points is required to identify localized column and beam damage, due to the complex building behavior, with many interacting structural components. The loss of the axial capacity of the damaged columns could be seen in the vertical response of the columns, but similar moderate or weak damage typically would not be noticed in ambient vibration surveys. Previous analysis of the recorded response of this building to 12 earthquakes suggests that, during large response of the foundation and piles, the soil is pushed sideways and gaps form between the foundation and the soil. These gaps appear to be closing during “dynamic compaction” when the building site is shaken by many small aftershocks. The apparent frequencies of the soil–foundation–structure system appear to be influenced significantly by variations in the effective soil–foundation stiffness. These variations can be monitored by a sequence of specialized ambient vibration tests.     

Response of L-shaped rigid foundations embedded in a uniform half-space to traveling seismic waves

A simplified indirect boundary element method is applied to compute the impedance functions for L-shaped rigid foundations embedded in a homogeneous viscoelastic half-space. In this method, the waves generated by the 3D vibrating foundation are constructed from radiating sources located on the actual boundary of the foundation. The impedance functions together with the free-field displacements and tractions generated along the soil–foundation interface are used to calculate the foundation input motion for incident P, S and Rayleigh waves. This is accomplished by application of Iguchi's averaging method which, in turn, is verified by comparison with results obtained rigorously using the relation between the solutions of the basic radiation (impedance functions) and scattering (input motions) problems. Numerical results are presented for both surface-supported and embedded foundations. It is shown how the seismic response of L-shaped foundations with symmetrical wings differs from that of enveloping square foundations. The effects of inclination and azimuth of the earthquake excitation are examined as well. These results should be of use in analyses of soil–structure interaction to account for the traveling wave effects usually overlooked in practice.     

Seismic response analysis of pipes by a probabilistic approach

Seismic response of buried pipes in longitudinal direction is studied. The effect of the variation of geotechnical properties of the surrounding soil on the stiffness, mass and damping of the soil is considered. The soil–structure interaction depends on pipe stiffness, joint stiffness, the variation of the soil stiffness and the soil mass and damping. Variations of the properties of the surrounding soil along the pipe are described by the random field theory. A numerical model is developed in order to simulate the effects of the variation of the soil on displacements, bending moments in the pipe and also to carry out a statistical analysis. The influence of different parameters regarding design and safety level of the pipe is conducted.     

Plain strain soil–structure interaction model for a building supported by a circular foundation embedded in a poroelastic half-space

A simple theoretical model for soil–structure interaction in water saturated poroelastic soils is presented, developed to explore if the apparent building–foundation–soil system frequency changes due to water saturation. The model consists of a shear wall supported by a rigid circular foundation embedded in a homogenous, isotropic poroelastic half-space, fully saturated by a compressible and inviscid fluid, and excited by in-plane wave motion. The motion in the soil is governed by Biot's theory of wave propagation in fluid saturated porous media. Helmholtz decomposition and wave function expansion of the two P-wave and the S-wave potentials is used to represent the motion in the soil. The boundary conditions along the contact surface between the soil and the foundation are perfect bond (i.e. welded contact) for the skeleton, and either drained or undrained hydraulic condition for the fluid (i.e. pervious or impervious foundation). For the purpose of this exploratory analysis, the zero stress condition at the free surface is relaxed in the derivation of the foundation stiffness matrix, which enables a closed form solution. The implications of this assumption are discussed, based on published comparisons for the elastic case. Also, a closed form representation is derived for the foundation driving forces for incident plane (fast) P-wave or SV wave. Numerical results and comparison with the full-scale measurements are presented in the companion paper, published in this issue.