International Journal of Soil Dynamics and Earthquake Engineering最新文献

The characteristics of the near-source ground motion in two-dimensional anti-plane and plane-strain kinematic fault models are examined in some detail. In particular, the relative importance of the near-source starting phases, passage of the rupture front phase and stopping phases is determined. At short distances to the fault, at low or intermediate frequencies, and, away from the ends of the fault, the rupture passage phase dominates the synthetic motion. It is shown that this phase can be calculated by use of a simple steady-state dislocation model. A method of solution which decomposes the near-source ground motion into starting, stopping and rupture passage phases is also presented. Finally, a simple steady-state anti-plane shear fault model in a two-material medium is used to determine the amplification effects associated with shock waves refracted into softer materials.

An efficient finite element procedure to analyze wave propagation phenomena in fluid-saturated porous media is presented. The saturated porous medium is modelled as a two-phase system consisting of a solid and a fluid phase. Time integration of the resulting semi-discrete finite element equations is performed by using an implicit-explicit algorithm. In order to remove the time step size restriction associated with the presence of the stiff fluid in the mixture, the fluid contribution to the equations of motion is always treated implicitly. The procedure allows an optimal selection of the time step size independently of the fluid. Depending upon the particular intended applications (e.g., seismic, blast loading,...) the fluid may be assumed incompressible or compressible. Numerical results which demonstrate the accuracy and versatility of the proposed procedure are presented.

The stationary response of the Duffing oscillator excited by white noise is considered. Based on the associated Fokker-Planck equation, the joint moments, of all orders, of the displacement and velocity are obtained in closed-form, in terms of parabolic cylinder functions. An asymptotic expansion, valid for large values of a dimensionless parameter, is also presented. It is shown that all moments of the Duffing oscillator are bounded by the corresponding moments of the linear oscillator.

For a significant class of soil-structure interaction problems, soil media have nonuniform properties. This includes both layered sites and soil continua with depth-dependent stiffness characteristics. An approximate method to characterize such sites is proposed as an aid to preliminary analysis.

Although numerical solution techniques exist for nonuniform media analysis, there are also appoximate methods based on uniform media. To aid in applying the latter method to a nonuniform medium, its properties are approximated by those of an equivalent uniform medium. In vertical motion, the nonuniform medium is modeled by a series of springs as influenced by stress level with depth. The spring system is then replaced by a single spring and hence, a single uniform half-space. The results of this type of analysis are extrapolated to degrees of freedom other than vertical motion. The method is applied to both stiffness and damping, giving a different set of equivalent uniform properties for each degree of freedom in either stiffness or damping.

An example problem indicates that the proposed method yields reliable results for impedance values. The approximation is acceptable within the limits of geotechnical accuracy. The only unconservatism occurs for values of translational damping because the method does not consider layering effects on radiation damping. A blanket damping reduction factor is recommended to account for those effects.

An analysis is presented here to give the soil impedance on a vertically vibrating floating pile. The soil is assumed to be a homogeneous elastic halfspace. The analysis uses the slenderness aspect of the pile to assume that the pile action is that due to a line distribution of vertical loads. Also, it is assumed that the wave length of the waves generated in the soil is much larger than the pile diameter. The analysis leads to an expression for the shear force acting along the length of pile. This expression is similar to the one given by Baranov. Whereas the Baranov expression assumes that only horizontally propagating shear waves are generated by the vibrating pile, the present analysis incorporates axisymmetric waves generated by the pile of finite length. When the pile length is made infinite the present result leads to the Baranov expression. Through a numerical consideration it is shown that the latter expression is valid near the soil surface only and that there is considerable departure in the interior of the soil.

Results of linear and nonlinear static analyses of gravity-induced stresses in several typical dam cross-sections, in conjunction with published experimental correlations of shear modulus versus confining pressure for a large variety of soils, reveal that the average shear modulus across the width of earth/rockfill dams may be expressed as a power m of depth, with m ranging from 0.35 to 0.90 and depending on material and geometric parameters. A general inhomogeneous shear beam model is developed to account for any possible such variation of modulus with depth. Perhaps somewhat surprisingly, closed-form analytical expressions are derived for natural frequencies, modal displacements, participation factors, and steady-state response functions for all values of the inhomogeneity factor m. Parametric results are presented in tabular and graphical form and conclusions are drawn of practical significance. Finally, a comprehensive comparative study is undertaken to investigate the validity of the inhomogeneous shear beam (SB) models. For five different dam cross-sections, each excited by four recorded accelerograms, it is shown that plane-strain finite-element analyses yield fundamental periods and peak displacements within the dam which are in very good accord with the predictions of a ‘consistent’ inhomogeneous SB model. A companion paper¹ extends the present work and focuses on seismic shear strains and seismic coefficients within dams and embankments.

Estimation of peak stresses in buried pipelines due to ground shaking is studied in this paper. Particular attention has been focused on the analysis of the stiffnesses of springs replacing the surrounding ground. It is shown here that these stiffnesses are functions of the wavelengths and frequencies of the seismic disturbance. They also depend nontrivially on the rigidity and the Poisson's ratio of the surrounding ground. NUmerical results for the stiffnesses are presented graphically showing these dependencies. Numerical results are also presented for the maximum axial and hoop stresses induced in a pipeline due to moving and fixed axial and transverse loads. It is found that these depend primarily on the ratio (M) of the rigidities of the ground and the pipe. The stress amplitudes increase as M decreases. For axial load the axial stress is found to be l arger than the hoop stress. But for the transverse load, it is found that when M is small the hoop stress is larger than the axial stress. This behaviour reverses for large M.

The annual probability of exceeding levels of the root mean square acceleration (RMS_a) of strong ground shaking is evaluated for downtown San Francisco. Attenuation relationships for RMS_a are developed for use in the seismic hazard analysis. A Bayesian seismic hazard analysis that incorporates the variability in seismicity parameters and the attenuation uncertainty is used.

For a strip wall erected on a rigid strip foundation and supported on the surface of the ground, the dynamic soil-structure interaction under the action of horizontal ground motion is investigated. The ground motion is idealized as vertically propagating, horizontal steady-state motion. Because the horizontal ground motion brings about sliding vibration of the foundation as well as rocking vibration, the coupled rocking and sliding vibration of the soil-structure system is considered in this paper. For the contact between the ground and the foundation, the following assumptions are made: (a) the contact is assumed to be welded, that is to say, the motion of the foundation is consistent with the ground; (b) the horizontal translation at each point on the bottom surface of the foundation is equal to a constant; (c) the distribution of the normal displacements under the foundation remains linear in the rocking vibration. For comparison, the case of uncoupled vibration is considered also. The use of Fourier transform method yields dual integral equations (for the case without coupled effect) or simultaneous dual integral equations (for the case with coupled effect). Both of them are solved by means of infinite series of orthogonal functions, and Jacobi polynomials. The numerical results show that there are significant differences between the displacement of the foundation, the relative displacements of the top of the wall with respect to its base, and the distributions of contact stresses beneath the foundation, for the cases with and without the coupling effect.