Engineering Mechanics

This paper reports on the pull-in behavior of nonlinear microelectromechanical coupled systems. The generalized differential quadrature method has been used as a high-order approximation to discretize the governing nonlinear integro-differential equation, yielding more accurate results with a considerably smaller number of grid points. Various electrostatically actuated microstructures such as cantilever beam-type and fixed-fixed beam-type microelectromechanical systems (MEMS) switches are studied. The proposed models capture the following effects: (1) the intrinsic residual stress from fabrication processes; (2) the fringing effects of the electrical field; and (3) the nonlinear stiffening or axial stress due to beam stretching. The effects of important parameters on the mechanical performance have been studied in detail. These results are expected to be useful in the optimum design of MEMS switches or other actuators. Further, the results obtained are summarized and compared with other existing empirical and analytical models.

A particle model for brittle aggregate composite materials such as concretes, rocks, or ceramics is presented. The model is also applicable to the behavior of unidirectionally reinforced fiber composites in the transverse plane. A method of random computer generation of the particle system meeting the prescribed particle size distribution is developed. The particles are assumed to be elastic and have only axial interactions, as in a truss. The interparticle contact layers of the matrix are described by a softening stress-strain relation corresponding to a prescribed microscopic interparticle fracture energy. Both two-and three-dimensional versions of the model are easy to program, but the latter poses, at present, forbidding demands for computer time. The model is shown to simulate realistically the spread of cracking and its localization. Furthermore, the model exhibits a size effect on: (1) The nominal strength, agreeing with the previously proposed size effect law; and (2) the slope of the post-peak load-deflection diagrams of specimens of different sizes. For direct tensile specimens, the model predicts development of asymmetric response after the peak load.

This paper proposes a probabilistic formulation to assess the effectiveness of the fiber reinforced polymer (FRP) retrofit schemes in enhancing the structural performance of reinforced concrete (RC) bridge columns. Two probabilistic models are proposed to predict the deformation capacities of retrofitted columns. One deformation model corresponds to the flexural failure and the other considers the bond failure in the lap-splice region. A Markov Chain Monte Carlo (MCMC) simulation method is used to estimate unknown model parameters in the context of a Bayesian updating approach. The probabilistic capacity models are used to estimate the fragility curves of three example columns. In this paper, fragility is defined as the conditional probability of failure for given deformation demand. The results compare the column fragilities before and after the application of the retrofit measure. The results from the example columns indicate that the use of FRP composites considerably reduced the fragility for the bond failure mode and is also beneficial but with a moderate impact when considering the flexural failure.

The modeling of deteriorating hysteretic behavior is becoming increasingly important, especially in the context of seismic analysis and design. This paper presents the development of a versatile smooth hysteretic model based on internal variables, with stiffness and strength deterioration and with pinching characteristics. The theoretical background, development, and implementation of the model are discussed. Examples are shown to illustrate the features of the model. Many inelastic constitutive models in popular use have been developed independently of each other based on different behavioral, physical, or mathematical motivations. This paper attempts to unify the concepts underlying such models. Such a holistic understanding is essential to realize limitations in application of inelastic models and to extend 1D models to 3D models featuring interaction between various stress resultants.

Two independent experimental investigations of the behavior of turbulent boundary layers with increasing Reynolds number were recently completed. The experiments were performed in two facilities, the Minimum Turbulence Level (MTL) wind tunnel at Royal Institute of Technology (KTH) and the National Diagnostic Facility (NDF) wind tunnel at Illinois Institute of Technology (IIT). Both experiments utilized oil-film interferometry to obtain an independent measure of the wall-shear stress. A collaborative study by the principals of the two experiments, aimed at understanding the characteristics of the overlap region between the inner and outer parts of the boundary layer, has just been completed. The results are summarized here, utilizing the profiles of the mean velocity, for Reynolds numbers based on the momentum thickness ranging from 2500 to 27 000. Contrary to the conclusions of some earlier publications, careful analysis of the data reveals no significant Reynolds number dependence for the parameters describing the overlap region using the classical logarithmic relation. However, the data analysis demonstrates that the viscous influence extends within the buffer region to y+≈200, compared to the previously assumed limit of y+≈50. Therefore, the lowest Reθ value where a significant logarithmic overlap region exists is about 6000. This probably explains why a Reynolds number dependence had been found from the data analysis of many previous experiments. The parameters of the logarithmic overlap region are found to be constant and are estimated to be κ=0.38, B=4.1 and B1=3.6 (δ=δ95).

In the past two decades, meshfree methods have emerged into a new class of computational methods with considerable success. In addition, a significant amount of progress has been made in addressing the major shortcomings that were present in these methods at the early stages of their development. For instance, essential boundary conditions are almost trivial to enforce by employing the techniques now available, and the need for high order quadrature has been circumvented with the development of advanced techniques, essentially eliminating the previously existing bottleneck of computational expense in meshfree methods. Given the proper treatment, nodal integration can be made accurate and free of spatial instability, making it possible to eliminate the need for a mesh entirely. Meshfree collocation methods have also undergone significant development, which also offer a truly meshfree solution. This paper gives an overview of many classes of meshfree methods and their applications, and several advances are described in detail.

Service life of concrete structures is limited by the susceptibility of the reinforcement to corrosion. Oxidation of iron leads to the formulation of various products (such as ferrous and ferric oxides), some of which occupy much greater volume than the original iron that gets consumed by the corrosion process. As corrosion progresses, these products accumulate, thereby generating expansive pressures on the surrounding concrete. The pressure builds up to levels that cause internal cracking around the bar and eventually leads to through cracking of the cover and spalling. Loss of cover marks the end of service life for corrosion-affected concrete structures, because at that stage the reinforcement loses its ability to develop its forces through bond and is no longer protected against further degradation from corrosion. In this paper, a simple analytical model is formulated to demonstrate the mechanical consequences of corrosion-product buildup around the bar. Service life is estimated as the time required for through cracking of the cover, which is identified in the model by a sudden drop of the internal pressure exerted by the corroding bar eventually relaxing to zero. Cracking time is found to be a function of cover, material properties of the surrounding concrete, and rust product, and is controlled by the rate of rust accumulation. In formulating the associated boundary-value problem, the governing equation expressed in terms of radial displacements is discretized using finite differences, whereas cracked concrete is treated as an orthotropic material. Calculated cracking times are correlated against published experimental data. The parametric sensitivity of the model is established with reference to published experimental evidence, and the role of the important design variables in the evolution of this mechanical problem is identified and discussed.

A time domain approach for predicting the coupled flutter and buffeting response of long span bridges is presented. The frequency dependent unsteady aerodynamic forces are represented by the convolution integrals involving the aerodynamic impulse function and structural motions or wind fluctuations. The aerodynamic impulse functions are derived from experimentally measured flutter derivatives, aerodynamic admittance functions, and spanwise coherence of aerodynamic forces using rational function approximations. A significant feature of the approach presented here is that the frequency dependent characteristics of unsteady aerodynamic forces and the nonlinearities of both aerodynamic and structural origins can be modeled in the response analysis. The flutter and buffeting response of a long span suspension bridge is analyzed using the proposed time domain approach. The results show good agreement with those from the frequency domain analysis. The example used to demonstrate the proposed scheme focuses on the treatment of frequency dependent self-excited and buffeting force effects. Application to nonlinear effects will be addressed in a future publication.

The concrete material model developed in the preceding Part I of this study is formulated numerically. The new model is then verified by comparisons with experimental data for compressive and tensile uniaxial tests, biaxial tests, and triaxial tests, as well as notched tests of mode I fracture and size effect.

This paper develops plane strain boundary equations for axisymmetric shear and dilation waves based on an approximation of the form of the outward travelling waves. These boundary equations are shown to be equivalent to mechanical systems with frequency independent components. Examples are presented illustrating the accuracy of the truncating boundaries. ---------------------------------Abstract Finite element analysis of dynamic foundation problems requires the use of transmitting boundaries to model the radiation of waves from the finite element mesh into the far field. Problems involving inelastic behaviour of the soil in the near field are most readily solved in the time domain. The standard viscous boundary is widely used in such situations. However, in axisymmetric situations this boundary is inappropriate. This paper develops plane strain boundary equations for axisymmetric shear and dilation waves based on an approximation of the form of the outward travelling waves. These boundary equations are shown to be equivalent to mechanical systems with frequency independent components. The complex stiffnesses of the new boundaries are compared with the equivalent viscous and plane strain boundary stiffnesses, and the new boundaries are found to agree closely with the plane strain boundaries. The response of an extended axisymmetric finite element mesh subjected to a transient force of the type generated by pile hammers is computed and compared to the response of the same mesh truncated with various transmitting boundaries.

The impact-echo method has been developed over the past 20 years and is now widely used in the nondestructive evaluation of concrete. However, some practical issues remain unresolved, such as the physical basis for the empirical correction factor ͑␤͒ used to obtain thickness mode frequency. A new approach based on guided wave theory is proposed in this paper: that the impact-echo resonance in plates corresponds to the zero-group-velocity frequency of the S 1 Lamb mode. A numerical model is developed, verified by experiment, and then shown to adequately simulate the dynamic response of a concrete plate. Using this model the thickness resonance mode is identified and found to accurately match that particular Lamb mode in terms of shape and frequency. New values for ␤ based on the Lamb mode model are computed and dependence on material Poisson's ratio is demonstrated.

This paper investigates the seismic response control of a nonlinear benchmark building with a new re-centering variable friction device (RVFD). The RVFD consists of three parts: (i) a friction generation unit, (ii) a piezoelectric actuator, and (iii) shape memory alloy wires. The friction unit and piezoelectric actuator compose the first subcomponent of the hybrid device that is a variable friction damper (VFD). The clamping force of the VFD can be adjusted according to the current level of ground motion by adjusting the voltage level of piezoelectric actuators. The second subcomponent of this hybrid device consists of shape memory alloy (SMA) wires that exhibit a unique hysteretic behaviour and full recovery following post-yielding deformations. In general, installed SMA devices have the ability to re-center structures upon end of the motion and VFDs can increase the energy dissipation capacity of structures. The full realization of these devices into a singular, hybrid form which complements the performance of each device is investigated. A neuro-fuzzy model is used to capture rate-and temperature-dependent nonlinear behaviour of the SMA components of the hybrid device. A fuzzy logic controller is developed to adjust voltage level of VFDs for favourable performance in a RVFD hybrid application. Numerical simulations of seismically excited nonlinear benchmark building are conducted to evaluate the performance of the hybrid device. Results show that the RVFD modulated with a fuzzy logic control strategy can effectively reduce interstory drifts without increasing acceleration response of the benchmark building for most cases.

A Bayesian methodology to construct probabilistic seismic demand models for the components of a structural system is developed. Existing deterministic models and observational data are used. The demand models are combined with previously developed capacity models for reinforced concrete (RC) bridge columns to estimate the seismic fragilities of bridge components and systems. The approach properly accounts for all relevant uncertainties, including model error. Application to two bridge examples typical of modern California practice is presented.

The micromechanics design theory has realized random short fiber-reinforced cement composites showing pseudostrain hardening (PSH) behavior with over 5% of strain capacity under tension. Nevertheless, this existing theory currently is limited to specific constituent properties, which does not account for chemical bond and fiber rupture. This article presents a new design theory that eliminates this restriction, achieving fiber rupture type PSH-random short fiber-reinforced cement composites with high-performance hydrophilic fibers like polyvinyl alcohol fibers. Uniaxial tensile tests are conducted employing polyvinyl alcohol fiber composites, the results of which support the validity of the proposed theory. Furthermore, parametric study employing the proposed theory quantitatively evaluates the effects of composite's micromechanics parameters, such as bond strength and fiber strength, on composite performance. This parametric study reveals that continuously increasing the degree of fiber rupture (fiber rupture intensity) enhances the strength performance of composites but not energy performance. However, an optimum rupture intensity exists for maximizing energy performance, which is critical for PSH behavior. The consistency between theoretical predictions and experimental results consequently demonstrates that the proposed theory can be utilized practically as a powerful and comprehensive tool for PSH composite design.

This is a practical paper which consists of investigating fracture behavior in asphalt concrete using an intrinsic cohesive zone model ͑CZM͒. The separation and traction response along the cohesive zone ahead of a crack tip is governed by an exponential cohesive law specifically tailored to describe cracking in asphalt pavement materials by means of softening associated with the cohesive law. Finite-element implementation of the CZM is accomplished by means of a user subroutine using the user element capability of the ABAQUS software, which is verified by simulation of the double cantilever beam test and by comparison to closed-form solutions. The cohesive parameters of finite material strength and cohesive fracture energy are calibrated in conjunction with the single-edge notched beam ͓SE͑B͔͒ test. The CZM is then extended to simulate mixed-mode crack propagation in the SE͑B͒ test. Cohesive elements are inserted over an area to allow cracks to propagate in any direction. It is shown that the simulated crack trajectory compares favorably with that of experimental results.

The previously developed microprestress-solidification theory for concrete creep and shrinkage is generalized for the effect of temperature ͑not exceeding 100°C͒. The solidification model separates the viscoelasticity of the solid constituent, the cement gel, from the chemical aging of material caused by solidification of cement and characterized by the growth of volume fraction of hydration products. This permits considering the viscoelastic constituent as non-aging. The temperature dependence of the rates of creep and of volume growth is characterized by two transformed time variables based on the activation energies of hydration and creep. The concept of microprestress achieves a grand unification of theory in which the long-term aging and all transient hygrothermal effects simply become different consequences of one and the same physical phenomenon. The microprestress, which is independent of the applied load, is initially produced by incompatible volume changes in the microstructure during hydration, and later builds up when changes of moisture content and temperature create a thermodynamic imbalance between the chemical potentials of vapor and adsorbed water in the nanopores of cement gel. As recently shown, this simultaneously captures two basic effects: First, the creep decreases with increasing age at loading after the growth of the volume fraction of hydrated cement has ceased; and, second, the drying creep, i.e., the transient creep increases due to drying ͑Pickett effect͒ which overpowers the effect of steady-state moisture content ͑i.e., less moisture-less creep͒. Now it is demonstrated that the microprestress buildup and relaxation also captures a third effect: The transitional thermal creep, i.e., the transient creep increase due to temperature change. For computations, an efficient ͑exponential-type͒ integration algorithm is developed. Finite element simulations, in which the apparent creep due to microcracking is taken into account separately, are used to identify the constitutive parameters and a satisfactory agreement with typical test data is achieved.

This study presents micromechanical finite-element ͑FE͒ and discrete-element ͑DE͒ models for the prediction of viscoelastic creep stiffness of asphalt mixture. Asphalt mixture is composed of graded aggregates bound with mastic ͑asphalt mixed with fines and fine aggregates͒ and air voids. The two-dimensional ͑2D͒ microstructure of asphalt mixture was obtained by optically scanning the smoothly sawn surface of superpave gyratory compacted asphalt mixture specimens. For the FE method, the micromechanical model of asphalt mixture uses an equivalent lattice network structure whereby interparticle load transfer is simulated through an effective asphalt mastic zone. The ABAQUS FE model integrates a user material subroutine that combines continuum elements with viscoelastic properties for the effective asphalt mastic and rigid body elements for each aggregate. An incremental FE algorithm was employed in an ABAQUS user material model for the asphalt mastic to predict global viscoelastic behavior of asphalt mixture. In regard to the DE model, the outlines of aggregates were converted into polygons based on a 2D scanned mixture microstructure. The polygons were then mapped onto a sheet of uniformly sized disks, and the intrinsic and interface properties of the aggregates and mastic were assigned for the simulation. An experimental program was developed to measure the properties of sand mastic for simulation inputs. The laboratory measurements of the mixture creep stiffness were compared with FE and DE model predictions over a reduced time. The results indicated both methods were applicable for mixture creep stiffness prediction.

Aerodynamic forces on bridges are commonly separated into static, self-excited, and buffeting force components. By delving into the relationships among force descriptors for static, self-excited, and buffeting components, novel perspectives are developed to unveil the subtle underlying complexities in modeling aerodynamic forces. Formulations for airfoil sections and those based on quasisteady theory are both considered. The time domain modeling of unsteady aerodynamic forces including their frequency-dependent characteristics and spanwise correlation is presented, which are often neglected in current time domain analyses due to modeling difficulty. A nonlinear aerodynamic force model is proposed to take into account the nonlinear dependence of the aerodynamic forces on the effective angle of incidence. The nonlinear aerodynamics may become increasingly critical when the aerodynamic characteristics of innovative bridge deck designs, with attractive aerodynamic performance, exhibit significant sensitivity with respect to the effective angle of incidence and with the increases in the bridge span. Clearly, in these cases one may be pushing the envelope of the current linear aerodynamics which has successfully served thus far. The synergistic review of the writers' recent work in bridge aerodynamics presented here, in light of the current state-of-the-art in this field, may serve as a building block for developing new analysis tools and frameworks for the accurate prediction of the response of long span bridges under strong wind excitation.

Using waste glass as an aggregate in concrete can cause severe damage because of the alkalisilica reaction (ASR) between the alkali in the cement paste and the silica in the glass. Recent accelerated 2week tests, conducted according to ASTM C 1260, revealed that the damage to concrete caused by expansion of the ASR gel, which is manifested by strength reduction, depends in these tests strongly on the size of the glass particles. As the particle size decreases, the tensile strength first also decreases, which is expected because of the surface-to-volume ratio of the particles, and thus their chemical reactivity increases. However, there exists a certain worst (pessimum) size below which any further decrease of particle size improves the strength, and the damage becomes virtually nonexistent if the particles are small enough. The volume dilatation due to ASR is maximum for the pessimum particle size and decreases with a further decrease of size. These experimental findings seem contrary to intuition. This paper proposes a micromechanical fracture theory that explains the reversal of particle size effect in the accelerated 2-week test by two opposing mechanisms: (1) The extent of chemical reaction as a function of surface area, which causes the strength to decrease with a decreasing particle size; and (2) the size effect of the cracks produced by expansion of the ASR gel, which causes the opposite. The pessimum size, which is about 1.5 mm, corresponds to the case where the effects of both mechanisms are balanced. For smaller sizes the second mechanism prevails, and for sizes <0.15 mm no adverse effects are detectable. Extrapolation of the accelerated test (ASTM C 1260) to real structures and full lifetimes will require coupling the present model with the modeling of the reaction kinetics and diffusion processes involved.

Long cables, such as are used in cable-stayed bridges, are prone to vibration due to their low inherent damping characteristics. Several studies have investigated the use of an optimal viscous damper attached transversely to dampen such vibration. This paper investigates the potential for improved damping using a semiactive device. The equations of motion of the cable/damper system are reviewed and an improved modeling formulation is introduced to dramatically reduce computational burden. The response of a cable with linear viscous, active, and semiactive dampers is studied. A semiactive damper is found to decrease response by 51% compared to the optimal linear viscous damper, thus demonstrating the efficacy of a semiactive damper for absorbing cable vibratory energy.

An exact three-dimensional analysis for steady-state dynamic response of an arbitrarily thick, isotropic, and functionally graded plate strip due to the action of a transverse distributed moving line load which is propagating parallel to the infinite simply supported edges of the plate at constant speed is presented based on the linear elasticity theory. The inhomogeneous plate is approximated by a laminate model, for which the solution is expected to gradually approach the exact one as the number of layers increases. The problem solution is derived by using Fourier transformation with respect to a moving reference frame in conjunction with the classical transfer matrix approach entailing the continuity of displacement and stress components at the interfaces of neighboring layers. The analytical results are illustrated with numerical examples in which a metal-ceramic (ZrO 2 -Al) FGM plate strip of unit width is subjected to a half-sine normal line load of constant amplitude traveling along the strip at uniform speeds. Four types of FGM plate strips are configured, and the effects of load velocity, material compositional gradient, and plate thickness on the basic dynamic field quantities are evaluated and discussed. Also, the response curves for the FGM plates are compared with those of equivalent bilaminate plates containing comparable total volume fractions of constituent materials. Limiting cases are considered and good agreements with the solutions available in the literature are obtained.

The nonlocal generalization of Weibull theory previously developed for structures that are either notched or fail only after the formation of a large crack is extended to predict the probability of failure of unnotched structures that reach the maximum load before a large crack forms, as is typical of the test of modulus of rupture (flexural strength). The probability of material failure at a material point is assumed to be a power function (characterized by the Weibull modulus and scaling parameter) of the average stress in the neighborhood of that point, the size of which is the material characteristic length. This indirectly imposes a spatial correlation. The model describes the deterministic size effect, which is caused by stress redistribution due to strain softening in the boundary layer of cracking with the associated energy release. As a basic check of soundness, it is proposed that for quasibrittle structures much larger than the fracture process zone or the characteristic length of material, the probabilistic model of failure must asymptotically reduce to Weibull theory with the weakest link model. The present theory satisfies this condition, but the classical stochastic finite-element models do not, which renders the use of these models for calculating loads of very small failure probabilities dubious. Numerical applications and comparisons to test results are left for Part II.

This paper is concerned with the in-plane elastic stability of arches with a symmetric cross section and subjected to a central concentrated load. The classical methods of predicting elastic buckling loads consider bifurcation from a prebuckling equilibrium path to an orthogonal buckling path. The prebuckling equilibrium path of an arch involves both axial and transverse deformations and so the arch is subjected to both axial compression and bending in the prebuckling stage. In addition, the prebuckling behavior of an arch may become nonlinear. The bending and nonlinearity are not considered in prebuckling analysis of classical methods. A virtual work formulation is used to establish both the nonlinear equilibrium conditions and the buckling equilibrium equations for shallow arches. Analytical solutions for antisymmetric bifurcation buckling and symmetric snap-through buckling loads of shallow arches subjected to this loading regime are obtained. Approximations for the symmetric buckling load of shallow arches and nonshallow fixed arches and for the antisymmetric buckling load of nonshallow pin-ended arches, and criteria that delineate shallow and nonshallow arches are proposed. Comparisons with finite element results demonstrate that the solutions and approximations are accurate. It is found that the existence of antisymmetric bifurcation buckling loads is not a sufficient condition for antisymmetric bifurcation buckling to take place.

Multicorrelated stationary random processes/fields can be decomposed into a set of subprocesses by diagonalizing their covariance or cross power spectral density ͑XPSD͒ matrices through the eigenvector/modal decomposition. This proper orthogonal decomposition ͑POD͒ technique offers physically meaningful insight into the process as each eigenmode may be characterized on the basis of its spatial distribution. It also facilitates characterization and compression of a large number of multicorrelated random processes by ignoring some of the higher eigenmodes associated with smaller eigenvalues. In this paper, the theoretical background of the POD technique based on the decomposition of the covariance and XPSD matrices is presented. A physically meaningful linkage between the wind loads and the attendant background and resonant response of structures in the POD framework is established. This helps in better understanding how structures respond to the spatiotemporally varying dynamic loads. Utilizing the POD-based modal representation, schemes for simulation and state-space modeling of random fields are presented. Finally, the accuracy and effectiveness of the reducedorder modeling in representing local and global wind loads and their effects on a wind-excited building are investigated.

This paper presents a two-stage structural health monitoring methodology and applies it to the Phase I benchmark study sponsored by the IASC-ASCE Task Group on Structural Health Monitoring. In the first stage, modal parameters are identified using measured structural response from the undamaged system and then from the ͑possibly͒ damaged system. In the second stage, these data are used to update a parametrized structural model of the system using Bayesian system identification. The approach allows one to obtain not only estimates of the stiffness parameters but also the probability that damage in any substructure exceeds any specified threshold expressed in terms of a fractional stiffness loss. It successfully identifies the location and severity of damage in all cases of the benchmark problem.

A theory that accurately describes tensile strength of wet sand is presented. A closed form expression for tensile strength unifies tensile strength characteristics in all three water retention regimes: pendular, funicular, and capillary. Tensile strength characteristically increases as soil water content increases in the pendular regime, reaches a peak in the funicular regime, and reduces with a continuing water content increase in the capillary regime. Three parameters are employed in the theory: internal friction angle ͑at low normal stress͒ t , the inverse value of the air-entry pressure ␣, and the pore size spectrum parameter n. The magnitude of peak tensile strength is dominantly controlled by the ␣ parameter. The saturation at which peak tensile strength occurs only depends on the pore size spectrum parameter n. The closed form expression accords well with experimental water retention and tensile strength data for different sands.

A new popular method for retrofitting reinforced concrete beams and slabs is to bond fiberreinforced plastic (FRP) plates to the soffit. An important failure mode for such strengthened members is the debonding of the FRP plate from the member due to high interfacial stresses near the plate ends. Accurate predictions of the interfacial stresses are a prerequisite for designing against debonding failures. In this paper, a theoretical interfacial stress analysis is presented for simply supported beams and slabs bonded with a thin FRP composite or steel plate and subjected to a uniformly distributed load in combination with a uniform bending moment. The analysis leads to an exact closed-form solution, in which a plane stress model is used for beams and a plane strain model is used for slabs. The salient features of the new analysis include the consideration of nonuniform stress distributions in and the satisfaction of the stress boundary conditions at the ends of the adhesive layer. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution over existing ones and to illustrate the main characteristics of interfacial stress distributions in beams and slabs.

The initial stresses existing in the natural ground are anisotropic in the sense that the vertical stress is typically larger than the lateral stresses. The construction activities, such as embankments and excavation, induce anisotropy in the stress system. The stressdeformation behavior and excess pore water pressure response of soils are affected by the inherent and induced stress anisotropy. This paper presents an improved soil model based on the anisotropic critical state theory and bounding surface plasticity. The anisotropic critical state theory of Dafalias was extended into three-dimensional stress space. In addition to the isotropic hardening rule, rotational and distortional hardening rules were incorporated into the bounding surface formulation with an associated flow rule. The projection center that is used to map the actual stress point to the imaginary stress point was specified along the K 0 line instead of the hydrostatic line or at the origin of the stress space. A simplified form of plastic modulus was used and the proposed model requires a total of 12 material parameters, the same number as that of the single-ellipse time-independent version of the Kaliakin-Dafalias model. The model was validated against the undrained isotropic and anisotropic triaxial test results under compression and extension shearing modes for Kaolin Clay, San Francisco Bay Mud, and Boston Blue Clay. The effects of stress anisotropy and overconsolidation were well captured by the model. The time effect was not included in the formulations presented in this paper.

In order to detect intermittent first-and higher-order correlation between a pair of signals in both time and frequency, a wavelet-based coherence and bicoherence technique was developed. Due to the limited averaging in a time-frequency coherence estimate, spurious correlated pockets were detected due to statistical variance. The introduction of multiresolution, localized integration windows was shown to minimize this effect. A coarse ridge extraction scheme utilizing hard thresholding was then applied to extract meaningful coherence. This thresholding scheme was further enhanced through the use of ''smart'' thresholding maps, which represent the likely statistical noise between uncorrelated simulated signals bearing the same power spectral density and probability-density function as the measured signals. It was demonstrated that the resulting filtered wavelet coherence and bicoherence maps were capable of capturing low levels of first-and higher-order correlation over short time spans despite the presence of ubiquitous leakage and variance errors. Immediate applications of these correlation detection analysis schemes can be found in the areas of bluff body aerodynamics, wavestructure interactions, and seismic response of structures where intermittent correlation between linear and nonlinear processes is of interest.

An improved form of a recently derived energetic-statistical formula for size effect on the strength of quasibrittle structures failing at crack initiation is presented and exploited to perform stochastic structural analysis without the burden of stochastic nonlinear finite-element simulations. The characteristic length for the statistical term in this formula is deduced by considering the limiting case of the energetic part of size effect for a vanishing thickness of the boundary layer of cracking. A simple elastic analysis of stress field provides the large-size asymptotic deterministic strength, and also allows evaluating the Weibull probability integral which yields the mean strength according to the purely statistical Weibull theory. A deterministic plastic limit analysis of an elastic body with a throughcrack imagined to be filled by a perfectly plastic "glue" is used to obtain the small-size asymptote of size effect. Deterministic nonlinear fracture simulations of several scaled structures with commercial code ATENA ͑based on the crack band model͒ suffice to calibrate the deterministic part of size effect. On this basis, one can calibrate the energetic-statistical size effect formula, giving the mean strength for any size of geometrically scaled structures. Stochastic two-dimensional nonlinear simulations of the failure of Malpasset Dam demonstrate good agreement with the calibrated formula and the need to consider in dam design both the deterministic and statistical aspects of size effect. The mean tolerable displacement of the abutment of this arch dam is shown to have been approximately one half of the value indicated by the classical deterministic local analysis based on material strength.

In comparison with atmospheric boundary -layer winds, which are generally regarded as stationary, windstorms such as hurricanes and thunderstorms/downbursts have strong non-stationary features characterized by rapid changes in wind speed and direction. The averaging interval associated with turbulent wind characteristics in boundary-layer winds is typically varied between 10-min. and 1-hr. A fixed averaging interval has been effective in characterizing the fluctuating component of boundary-layer winds, but question remains whether the user-defined interval is appropriate for non-stationary winds. For better understanding of the characteristics of non-stationary winds, fixed averaging interval (FAI) and variable averaging interval (VAI) approaches are assessed in this study. Non-stationary and transient datasets measured during the passage of the hurricane Lili and a rear-flank downdraft (RFD) obtained from the 2002 thunderstorm outflow field experiment, respectively, are utilized to examine FAI and VAI schemes and the associated turbulent wind characteristics. This study highlights the characteristics of non-stationary winds based on the traditional FAI scheme for boundary-layer winds, an alternative FAI scheme for non-stationary winds, and several VAI schemes. In addition, the definitions for gust factor and turbulence intensity in the non-stationary wind field are revisited.