Journal of Engineering Mechanics

This paper describes a model for the spatial variation of the elastic modulus of parallel strand lumber (PSL) that is based on bending experiments and also describes a validated stochastic computational model that incorporates orthotropic elasticity and uncertainty in strand geometry and material properties. The PSL exhibits significant variability both within members and between members, but this variability is less than that of equivalent sawn-wood members, and decreases with increasing member size. The correlation length of the elastic modulus is found to be several meters and is independent of the cross-sectional size. The variance of PSL elastic modulus is found to scale inversely with the number of strands in the cross section. The validated computational model is flexible enough to allow preliminary exploration of the properties of new mixes of species and strand sizes in PSL material design.

The presence of air voids, moisture, and chlorides inside tendons or ducts was cited as a reason for the early age strand corrosion and failure in the Mid-bay, Sunshine Skyway, and Niles Channel posttensioned (PT) bridges in Florida, United States. Although rare, these incidents call for frequent inspection and structural reliability assessment of PT bridges exposed to moisture and chlorides. This paper develops and presents probabilistic strand capacity models that are needed to assess the structural reliability of such PT bridges and recommends a time frequency of inspection. A total of 384 strand test specimens were exposed to various void, moisture, and chloride concentration conditions for 12 and 21 months; the remaining tension capacities were then determined. Using this experimental data and a Bayesian approach, six probabilistic capacity models were developed based on the void type. The mean absolute percentage errors of these models are less than 4%, indicating that reasonably accurate prediction of the strand capacity is possible, when void, aggressive moisture, and chloride conditions are present.

Clay material can be considered as a collection of clusters, which interact with each other mainly through mechanical forces. From this point of view, clay is modeled by analogy to granular material in this paper. An elastoplastic stress-strain relationship for clay is derived by using the granular mechanics approach developed in previous studies for sand. However, unlike sand, clay deformation is generated not only by the mobilizing but also by compressing clusters. Thus, in addition to the Mohr-Coulomb’s plastic shear sliding and a dilatancy type flow rule, a plastic normal deformation has been modeled for two clusters in compression. The overall stress-strain relationship can then be obtained from the mobilization and compressing of clusters through a static hypothesis of the macro-micro relations. The predictions are compared with the experimental results for clay under both drained and undrained triaxial loading conditions. Three different types of clay, including remolded and natural clay, have been selected to evaluate the model’s performance. The comparisons verify that this model is capable of accurately reproducing the overall behavior of clay, which accounts for the influence of key parameters such as void ratio and mean stress. A section of this paper is devoted to show the model’s capability of considering the influence of inherent anisotropy on the stress-strain response under undrained triaxial loading conditions.

This paper addresses an advanced and unified thermomechanical constitutive model for soils. Based on experimental evidence showing the nonlinear and irreversible thermomechanical responses of saturated soils, the constitutive equations of the developed model, Advanced Constitutive Model for Environmental Geomechanics-Thermal effect (ACMEG-T), are presented. In the context of elastoplasticity and critical state theory, the model uses the multimechanism plasticity and bounding surface theory. Nonlinear thermoelasticity is joined with two coupled thermoplastic dissipative processes. The yield functions, the dissipative potentials and the plastic multipliers are introduced. Attention is particularly focused on the coupling between both plastic mechanisms, an isotropic and a deviatoric one, which are in agreement with the consistency condition for multiple dissipation. As far as isotropic mechanism is concerned, a unique thermomechanical yield surface reproduces the thermoplasticity observed at low and intermediate overconsolidation ratios, as well as the plasticity under mechanical loading in an framework unifying mechanical and thermal hardenings. Finally, the efficiency of ACMEG-T is proven by validation tests on drained and undrained thermomechanical paths.

The vertical stiffness and deformation theories of rubber isolators in compression and compression-shear states are systemically researched in the paper, a series of basic concepts, such as origin compression stiffness, origin compression longitudinal elastic modulus, offset vertical stiffness, etc. are suggested with corresponding theoretical formula and experimental estimation method. Based on the basic concepts and newly suggested calculating theories, the deformation calculating theory related to pure compression state and compression-shear state of isolating bearing is established. The vertical stiffness, offset vertical stiffness and deformation tests are performed with nature rubber bearings and lead plug rubber bearings total 16 original specimens to verify the new concepts and computation model of rubber isolators. All test results show that the theories established in the paper are suitable for analyzing the vertical stiffness and deformation of rubber isolators.

This paper describes a theoretical analysis of a type of thermal expansion bridge bearing which could be used as a lightweight low-cost elastomeric seismic isolator for application to housing, schools, and other public buildings in earthquake-prone areas of the developing world. The analysis covers the effect of the frictional resistance of the supports on the vertical stiffness of this type of isolator. The most important aspect of these bearings is that they do not have end plates, which reduces their weight, but also means that they are not bonded to the upper and lower support surfaces and are held in place only by friction. This at first sight might seem to be a deficiency of this design, but it has the advantage that it eliminates the presence of tensile stresses in the bearings. It is these tensile stresses and the bonding requirements that arise from them that lead to the high costs of conventional isolation bearings. A theoretical analysis of the response of these bearings to vertical load shows that slip between the unbonded surfaces and rigid supports above and below can have a significant influence on the vertical stiffness and the internal pressure distribution.

A model has been developed that can predict the load-deformation response of a reinforced concrete (RC) member subjected to torsion combined with bending and shear to spalling or ultimate capacity. The model can also be used to create interaction surfaces to predict the failure of a member subjected to different ratios of applied torsion, bending, and shear. The model idealizes the sides of an reinforced concrete member as shear “wall panels.” The applied loads are distributed to the wall panels as uniform normal stresses and uniform shear stresses. The shear stress due to an applied torsional moment and shear force are summed over the thickness of the shear flow zone. Stress-strain relationships are adopted for tension stiffening and softened concrete in compression. The crack alignment rotates to remain normal to the principal tensile stress and the contribution of concrete in shear is neglected. The model has been validated by comparing the predicted and experimental behavior of members loaded under torsion combined with different ratios of bending and shear. The torque-twist behavior, reinforcement stress, and concrete surface strain predicted by the model were in agreement with experimental results.

A model was recently proposed for predicting the load-deformation response of a reinforced concrete member under torsion combined with bending and shear to spalling or ultimate. This paper shows the application of the model to create interaction surfaces to predict the failure of a member subjected to different ratios of applied torsion, bending, and shear. The model was validated by comparing the predicted and experimental behavior of 28 members from three experimental studies available in the literature. The members were loaded under torsion combined with different ratios of bending, and shear. The torque-twist behavior, reinforcement stress, and concrete surface strain predicted by the model were in close agreement with the experiments. This paper also describes how the model can be applied to create interaction surfaces. The interaction curves predicted by the model were validated by comparing the predicted and experimental capacities of 17 specimens available in the literature.

A numerical model that uses a nonlinear cracked hinge was developed to characterize the flexural behavior of a beam element composed of hybrid fiber-reinforced concrete (HyFRC). Deflection hardening performance criteria were established based on an average tensile strain capacity of 0.002 and a deterministic sensitivity analysis of the model. A HyFRC mix design is proposed that exceeds these performance criteria. The HyFRC flexural behavior and its flexural stiffness are compared against conventional reinforced concrete. Implications of the material flexural performance on performance-based structural design methods are discussed.

Turbulence-induced changes in aerodynamic force characteristics on an oscillating rectangular prism were investigated in this study. It encompassed examination of aeroelastic quantities, i.e., flutter derivatives, and the buffeting components of the integral forces. A forced-vibration system was employed to extract the aerodynamic characteristics of an oscillating prism using a model instrumented with multiple pressure transducers for synchronous scanning of the pressure field. Chordwise distributions of self-excited pressure amplitudes were measured and associated phases were derived to examine the anatomy of turbulence effects on the aerodynamics of the prism rather than simply discern the influence of turbulence on the integrated forces. The resulting changes in flutter derivatives were traced back to a turbulence-induced upstream shifting of the regions of maximum pressure amplitudes. This upstream shifting was consistent with earlier research showing that turbulence increases the radius of curvature of separated shear layers and moves reattachment upstream. In this study, turbulence was found to have a stabilizing effect on the aerodynamics of the prism. The broad band character of the buffeting forces was found to be quite similar to that of stationary prisms with body motion slightly increasing energy content.

The stability characteristics of a jet with a low-frequency time-dependent periodic swirl are determined. An analytical dispersion relation for the perturbations is obtained and solved numerically. Growth rates and celerities determined from this formulation are compared with predictions based on applying steady state analysis to instantaneous velocity profiles, i.e., the quasisteady approach. The results show that periodic unsteadiness in the swirl causes variations in the growth rates and celerities of the unstable modes. Furthermore, it is shown that the quasisteady approach is not applicable and fails to predict the magnitudes of these variations.

Representation theorems for isotropic functions are used to construct a closure model for the fluctuating pressure-strain-rate correlation tensor. In contrast to alternative proposals in the literature, the present model does not include a dependence on the vorticity in inertial frames or on the intrinsic spin tensor in noninertial frames because it is shown that this dependence violates objectivity which has been previously suggested to be an essential property of a turbulence closure. The proposed model is calibrated using data from homogeneous shear flows in inertial frames and is tested against data from a variety of turbulent shear flow, in both fixed and rotating frames, with generally very encouraging results. In particular, the results conclusively demonstrate that the absence of dependence on vorticity and intrinsic spin in the model for the pressure-strain-rate correlations does not adversely impact its ability to accurately predict the effects of system rotation on the turbulence.

The space-time variation in contaminant concentration in unsteady flow in a homogeneous finite aquifer subjected to point source contamination is analytically derived under two conditions: (1) the flow velocity in the aquifer is of sinusoidal form; and (2) the flow velocity is an exponentially decreasing function. The analytical solution is illustrated using an example. Analytical solutions are perhaps most useful for benchmarking numerical codes and solutions.

Experimental tests exhibit a strong interaction between material damage and transport properties of concrete. There are at least two asymptotic cases where some theoretical modeling exists: in the case of diffuse cracking, the material permeability should be controlled by damage, e.g., by the decrease of average stiffness due to microcracking. In the case of localized microcracking, and after a macrocrack has formed, permeability should be controlled by a power function of the crack opening (Poiseuille flow). For quasi-brittle materials with evolving microstructure due to mechanical loads, a transition regime on the evolution of permeability between these two asymptotic cases is expected. In this contribution, we define a relationship between permeability and damage that is consistent with the two above configurations. One of the key issues is to relate the crack opening to the state variables in the continuum approach, so that the two asymptotic cases are expressed in the same variable system and can be matched. A simplified approach is used for this purpose. The permeability law is then derived using a mixing formula that weights each asymptotic regime with damage. To illustrate the influence of the matching law on structural response, finite-element simulations of a Brazilian splitting test and a comparison with existing test data are presented.

A new membrane boundary that applies realistic fluid confining pressure has been developed for modeling triaxial tests on intact rock by using the discrete element method. To realistically simulate the confining pressure, the new approach applies updated boundary force rather than a “wall” boundary. The applied forces only act on the boundary particles, which are identified and updated periodically. Comparisons between rigid-wall boundary and membrane boundary show that rigid-wall boundary can significantly alter the material response especially the material strength. The effect of confining pressures on the simulated results of triaxial tests is also investigated.

The use of glass ballotini as a granular material provides the opportunity to simultaneously study internal stress fields and internal fields of deformation as a sample is submitted to boundary perturbations. Digital image correlation makes use of the visible fabric of the material to deduce a field of displacements from one digital photographic image to the next. If the glass granules are immersed in a fluid having the same refractive index, then observation with polarized light exploits the photoelastic properties of the glass to reveal information about the stresses. Again, comparison of digital photographs enables changes in stress conditions from one image to the next to be discovered. Tests performed in a simple loading device which forces rotation of principal axes in parts of the granular mass are presented to demonstrate the unique potential of this dual experimental configuration.