Journal of Engineering Mechanics

A new methodology is presented for the generation of spatially correlated nonstationary ground motion time histories that are compatible with a prescribed response spectrum using the spectral representation method. The method introduces two important improvements over the current state of the art in that it preserves the coherence among the ground motion histories and enables the incorporation of both time and frequency modulation by upgrading the evolutionary power spectral density function with random pulse-like perturbations. An example is provided for the simulation of design spectrum–compatible, uniformly modulated nonstationary acceleration time histories at three locations on the ground, and the results are compared directly with an existing state-of-the-art methodology.

A new three-dimensional (3D) elastoviscoplastic (EVP) constitutive model for both normally consolidated and overconsolidated clays is presented. In developing the new model, first the time effects on clays were connected with the change of overconsolidation degree according to the concepts of aging time and the instant normal compression line. This made it convenient to combine a logarithmic creep function with the reloading function of the unified hardening (UH) model (a model for overconsolidated clays), and, thus, an isotropic EVP relationship was built. Second, a time variable derived from the isotropic EVP relationship was embedded into the current yield function of the UH model, and, thereby, a time-dependent current yield function is proposed. Based on the time-dependent current yield function and the flow rule, an EVP model for triaxial compression stress states was built. The EVP model for triaxial compression stress states then was extended to a 3D EVP model for general stress states through the transformed stress method. The proposed 3D EVP model can describe time-dependent behaviors, such as creep, relaxation, and loading rate effect, for both normally consolidated and overconsolidated clays, with two additional parameters compared with the modified Cam-clay model. Finally, numerous experimental results in the literature (such as the results of triaxial undrained creep tests, triaxial undrained compression and extension tests at the constant strain rate, and triaxial undrained tests at the stage-changed strain rate) were used to validate the new model.

Slender columns subjected to compressive stress are common in many civil structures. The rapid in situ measurement of this stress may prevent structural buckling. In this study, the authors applied an artificial neural network (ANN) to process numerical data that describe the coupling mechanism between highly nonlinear solitary waves (HNSWs) propagating along a granular system and a beam in contact with the granular medium. The aim is to evaluate the ability of HNSWs to measure stress in thermally loaded structures and to estimate the neutral temperature, i.e., the temperature at which the stress is null. Nonlinear solitary waves are compact nondispersive waves that can form and travel in nonlinear systems such as one-dimensional chains of particles, where they are conventionally generated by the mechanical impact of a striker. The authors numerically investigated a straight chain of spherical particles in contact with a prismatic beam subjected to thermal stress. The effect of the neutral temperature on certain features of the waves was examined. These features then were fed into an ANN with the aim of estimating the neutral temperature. In the future, the findings presented in this paper may be used to develop a novel sensing system for the nondestructive prediction of neutral temperature and thermal buckling.

One of the important goals of structural health monitoring is damage detection. Although many methods have been proposed to detect the existence of structural damage, relatively few studies are found on higher-level damage diagnosis such as identification of the location and extent of damage. In this paper, multiple substructural damage identification models based on regression between internal responses and boundary responses of individual beam elements in either plane or three-dimensional space are derived. Three damage indexes are defined from regression model characteristics, and two change-point analysis methods are adopted to capture changes in damage index sequences which are extracted from structural monitoring data sets from healthy and unknown states. Possible damage locations are identified as where the most significant changes in the damage indexes occur, and a voting scheme is used to synthesize the results from different algorithms. This damage detection approach is straightforward and efficient, with the regression coefficients directly related to the structural stiffness properties. The numerical and experimental application results show that the method successfully identifies and locates structural change in most of the cases.

Fiber-reinforced cementitious matrix (FRCM) composites represent a newly developed promising technique for strengthening RC structures. The FRCM composites are comprised of high-strength fibers applied to the concrete substrate through an inorganic cementitious matrix. In this work, single-lap direct-shear tests were carried out on FRCM strips, comprised of one layer of fiber net embedded within two layers of matrix, bonded to a concrete block. The weakness of FRCM-concrete joints was observed to be the debonding at the matrix-fiber interface. The experimental results indicated that the role of each matrix layer is different. The stress-transfer mechanism between the fiber filaments and the matrix layers on either side of the fiber net was studied by means of a fracture mechanics approach, and three models of the interfacial cohesive material law were proposed for each matrix-fiber interface.

Crossties and cable networks are used on cable-stayed bridges to mitigate wind-induced stay vibration. A nonlinear free-vibration analysis of a cable network with random oscillation amplitudes is presented in this study. Specifically, a nonlinear restoring-force spring model is introduced at the crosstie to simulate in-plane network free vibration at large amplitudes. The current constitutive model of the crosstie is also combined with taut-cable theory to simulate the dynamics in the stays and is solved by the equivalent linearization method. Stochastic functions are introduced in the model because the measure of the amplitudes produced by wind- and rain-wind–induced vibrations in the network can be affected by various uncertainties. The stochastic approximation (SA) algorithm is applied to find the roots of the characteristic polynomials associated with a stochastic vibration amplitude parameter. Brute force Monte Carlo methods are also used to analyze the SA convergence properties.

This paper presents a study on the input parameters used in the discrete-element simulations with gravity. Input parameters of the discrete-element method (DEM) include particle mass (density), damping, gravity constant, strain rate, and contact relationship. This paper focuses on particle mass and damping. The samples consisted of two kinds of ellipsoidal particles. The unit weight of the particles is constant. Samples are prepared by depositing particles under gravity. The final bulk densities of the samples are different for different damping and particle mass. The effect of the reduction of particle density is similar to that of the reduction of damping in sample preparation. For the samples, static equilibrium can be achieved with a damping ratio greater than 0.24%. When the damping ratio is less than 0.24%, some particles are oscillating. The oscillation cannot be reduced with further relaxation. Undrained simulations are carried out on these samples. Shear strength increases with the increase of damping as expected. Different undrained stress paths are found for simulations of loose samples with different damping. For simulations of loose samples with the same damping ratio, particle density affects the results slightly. However, the effect of damping and particle mass is negligible for dense samples. The ultimate shear strength of the granular material is not affected by either damping or particle mass.

The problem of estimation of the time-variant reliability of actively controlled structural dynamical systems under stochastic excitations is considered. Monte Carlo simulations, reinforced with Girsanov transformation–based sampling variance reduction, are used to tackle the problem. In this approach, the external excitations are biased by an additional artificial control force. The conflicting objectives of the two control forces—one designed to reduce structural responses and the other to promote limit-state violations (but to reduce sampling variance)—are noted. The control for variance reduction is fashioned after design-point oscillations based on a first-order reliability method. It is shown that for structures that are amenable to laboratory testing, the reliability can be estimated experimentally with reduced testing times by devising a procedure based on the ideas of the Girsanov transformation. Illustrative examples include studies on a building frame with a magnetorheologic damper–based isolation system subject to nonstationary random earthquake excitations.

A simple computational strategy for finite-element implementation of a finite-strain viscohyperelasticity model for rubber-like materials was developed. The constitutive model has had a strong physical significance because of the explicit consideration of the nonlinear dependence of viscosity through internal variables (e.g., past maximum overstress and current deformation). To simulate the stress-strain response for particular one-dimensional boundary value problems, a scheme for solving the first-order differential equation representing the viscosity-induced strain-rate effect of rubber was proposed. The scheme was successful in reproducing experimental results obtained from high-damping rubber specimens. In addition, the wider applicability of the proposed strategy in simulation was tested by verifying the numerical results with independent experiments on full-scale high-damping rubber bearings with different geometries and loading rates. The effect of shape factor on bearing responses was examined through numerical examples obtained from different finite-element models subjected to the same load and loading rate. Finally, the proposed computational strategy was applied to locate the regions of stress concentrations in steel plate laminated rubber expansion joints used widely to transfer reactions at central-hinge locations on balanced cantilever highway bridges.

The compression and bending modulus of elastomeric bearings, commonly applied as vibration isolators, are important design considerations. Analytical solutions have demonstrated that the sensitivity to the compressibility of the elastomer can be significant and begins at relatively low shape factors. These analytical solutions, which include the effects of compressibility on the compression and bending modulus, are often complex and not suitable for design purposes. Alternatively, an ad hoc approximation has been recommended that expresses the compression and bending modulus, including the compressibility of the elastomer, by assuming incompressibility and correcting with the bulk modulus. It is demonstrated that the ad hoc approximation provides an unconservatively large value of the compression and bending modulus for infinite strip, square, circular, and annular pad geometries. A correction factor to the ad hoc approximation is determined by expanding and simplifying the analytical solutions. The proposed approximations typically reduce the magnitude of the error while also providing a conservatively lower estimate of the compression and bending modulus.

A three-dimensional (3D) coupled potential + viscous flow model based on a domain decomposition method is developed and applied to study shoaling and breaking of a solitary wave on a nonuniform bathymetry. The flow domain is decomposed into two subdomains separated by an interface: a wave generation and propagation subdomain modeled by a potential-flow (PF) solver and a shoaling and surf zone subdomain modeled by a Navier-Stokes equation (NSE) solver. The PF solver, based on a boundary-element method, is capable of modeling the motion of a piston wavemaker and the propagation of the resulting wave downstream. After the wave passes through the interface, it continues to propagate in the viscous-flow (NSE) subdomain, which is solved by a FEM. A free-surface capturing feature of the NSE solver allows simulation of wave breaking and the postbreaking behavior. Numerical results show good agreement with those of a large-scale wave-basin experiment of a solitary wave run-up on a 3D wedge. The simulation shows that breaking criteria and categorization for plane slopes do not apply to this particular bathymetry. The velocity and vorticity patterns during wave breaking are also examined to demonstrate the capabilities of the viscous-flow solver.

The aim of this paper is to investigate the effect of soil grading on the stress-strain and the critical-state behavior of granular materials from idealized spheres to natural soils as well as from discrete to continuous modeling. The three-dimensional discrete-element method has been applied to study the mechanical behavior of idealized granular materials. The results confirm that the critical-state line (CSL) shifts downward as grading broadens with an increase of the coefficient of uniformity $C_u$. An exponential relationship between the critical-state parameters and the coefficient of uniformity $C_u$ can then be established. Furthermore, experimental investigations on an artificial material (glass beads) and a natural material (Hostun sand) were carried out. The experimental results confirm the grading dependency of the critical-state behavior, and a similar relationship between CSL and $C_u$, for both glass beads and natural sand, extending previous literature results for high $C_u$ values up to 20. Moreover, the experimental results show that the CSL tends to stabilize when the coefficient of uniformity $C_u$ becomes higher than 10. Based on all test results, a simple elastoplastic model accounting for the influence of the grain size distribution (GSD) on the mechanical behavior of granular materials has been developed within the framework of critical-state soil mechanics. Comparisons between experimental results and simulations show that the model can accurately reproduce the mechanical behavior of granular materials with different GSDs.

This paper presents closed-form solutions for the nongeometric spectral characteristics of nonstationary stochastic processes representing the response of linear elastic structural models subjected to fully nonstationary excitation processes. These spectral characteristics provide a complete description of the nonstationary stochastic processes representing the dynamic response of linear structural models when the input excitation is a Gaussian process. In particular, the nongeometric spectral characteristics can be used to evaluate the time-variant central frequency and bandwidth parameters of nonstationary stochastic processes. The closed-form solutions derived in this paper are used to evaluate the time-variant statistics and the first-passage probability of the response of single-degree-of-freedom and multidegree-of-freedom linear elastic structural models subjected to a well-known, fully nonstationary earthquake ground motion model. The new analytical results presented in this paper can be used in a number of structural dynamic applications.