Journal of Engineering Mechanics

A methodology to construct probabilistic capacity models of structural components is developed. Bayesian updating is used to assess the unknown model parameters based on observational data. The approach properly accounts for both aleatory and epistemic uncertainties. The methodology is used to construct univariate and bivariate probabilistic models for deformation and shear capacities of circular reinforced concrete columns subjected to cyclic loads based on a large body of existing experimental observations. The probabilistic capacity models are used to estimate the fragility of structural components. Point and interval estimates of the fragility are formulated that implicitly or explicitly reflect the influence of epistemic uncertainties. As an example, the fragilities of a typical bridge column in terms of maximum deformation and shear demands are estimated.

A computational fluid dynamics (CFD)-based computational tool, named STREAM, is used to analyze fluid flow in a centrifugal blower. The unsteady interaction of the flow in the rotating impeller (rotor) and the stationary volute (stator) is modeled via quasi-steady rotor–stator models. Two such models are developed: one based on a local exchange of information and the other based on a circumferential averaging procedure at the rotor–stator interface. Due care is exercised to ensure that inadequate grid resolution and numerical dissipation do not smear out the small pressure rise typical of the blower considered here. Computed results based on the proposed models with multiblock structured grids are presented; global quantities such as static pressure rise, horsepower, and static efficiency, available from test data for different mass flow rates, are used to evaluate the trends predicted by the CFD results. Overall, the predictions by the proposed models are satisfactory.

This paper describes a class of high-order Taylor-Galerkin methods for solving problems of linear acoustics characterized by the classical (scalar) wave equation. The methods provide high-order temporal accuracy and unconditional stability on arbitrary nonuniform finite element meshes of varying element sizes and shape functions.

Free vibrations of a taut cable with an attached linear viscous damper are investigated in detail using an analytical formulation of the complex eigenvalue problem. This problem is of considerable practical interest in the context of stay-cable vibration suppression in bridges. An expression for the eigenvalues is derived that is independent of the damper coefficient, giving the range of attainable modal damping ratios and corresponding oscillation frequencies in every mode for a given damper location without approximation. This formulation reveals the importance of damper-induced frequency shifts in characterizing the response of the system. New regimes of behavior are observed when these frequency shifts are large, as is the case in higher modes and for damper locations further from the end of the cable. For a damper located sufficiently near the antinode in a given mode, a regime of solutions is identified for which the damping approaches critical as the damper coefficient approaches a critical value. A regime diagram is developed to indicate the type of behavior in each mode for any given damper location.

Free vibrations of a taut cable with a nonlinear power-law damper attached near the end are considered. An approximate analytical solution for the amplitude-dependent effective damping ratios in each mode is developed by assuming the same form of solution as for the linear damper and minimizing the mean-square error in the force equilibrium at the damper. An asymptotic approximate solution for small frequency shifts reveals a nondimensional grouping of parameters allowing the development of an amplitude-dependent “universal estimation curve” for the power-law damper. The shape of the universal curve is slightly different for each value of the damper exponent, but for a given exponent the curve is nearly invariant over the same range of parameters as the universal curve for the linear damper. This formulation yields insights into the dependence of nonlinear damper performance on mode number and amplitude of oscillation, suggesting potential advantages that may be offered by a nonlinear damper over a traditional linear damper.

A method which combines two nonintrusive imaging techniques, particle tracking velocimetry and laser induced fluorescence, was used to make simultaneous measurements of velocity and concentration in a neutrally buoyant turbulent round jet. The measurements were made at two different Reynolds numbers (R), 360 and 4,210, at a Schmidt number of 1,930. The mean velocity 〈u〉, mean concentration 〈c〉, Reynolds stress $-〈u^{\prime }{v}^{\prime }〉\text{,}$ and turbulent scalar flux $〈v^{\prime }{c}^{\prime }〉$ were obtained and the eddy viscosity, eddy diffusivity, and turbulent Prandtl number $\left({\mathrm{Pr}}_t\right)$ calculated from these measurements. Both the low and high Reynolds number results show self-similar characteristics that are dependent on R with ${\mathrm{Pr}}_t$ a function of radial position. For the R=4,210 case, it was found that $0.7<{\mathrm{Pr}}_t<0.9$ for $0<y/x<0.15$ (y is the radial coordinate and x the streamwise coordinate) with ${\mathrm{Pr}}_t$ decreasing monotonically for $y/x>0.12\text{.}$ For the R=360 case, it was found that ${\mathrm{Pr}}_t\approx 0.4$ for $0.06<y/x<0.15$ but significantly smaller at the center (approximately half of the peak value) and outer regions of the jet profile. Comparatively, for the R=4,210 case, ${\mathrm{Pr}}_t$ at $y/x=0$ is only slightly smaller than the peak value (0.7 versus 0.9). In both R cases, the maximum of ${\mathrm{Pr}}_t$ occurred at approximately $y/x=0.1\text{.}$

A “smart” base isolation strategy is proposed and shown to effectively protect structures against extreme earthquakes without sacrificing performance during the more frequent, moderate seismic events. The proposed smart base isolation system is composed of conventional low-damping elastomeric bearings and “smart” controllable (semiactive) dampers, such as magnetorheological fluid dampers. To demonstrate the advantages of this approach, the smart isolation system is compared to lead-rubber bearing isolation systems. The effectiveness of the isolation approaches are judged based on computed responses to several historical earthquakes scaled to various magnitudes. The limited performance of passive systems is revealed and the potential advantages of smart dampers are demonstrated. Two- and six-degree-of-freedom models of a base-isolated building are used as a test bed in this study. Smart isolation is shown to achieve notable decreases in base drifts over comparable passive systems with no accompanying increase in base shears or in accelerations imparted to the superstructure. In contrast to passive lead-rubber bearing systems, the adaptable nature of the smart damper isolation system provides good protection to both the structure and its contents over a wide range of ground motions and magnitudes.

Developing membrane structures for space-based applications is an effort to reduce launch mass and stowed volume. Accurate modeling of the shape after deployment is very important to the high-precision inflatable structures for space applications. This paper presents a numerical study of the inflation of an initially plane membrane with circular boundary. The simulations were conducted using the nonlinear FEM code ABAQUS. Two types of boundary conditions are imposed. The conditions are either prescribed boundary movements during the inflation or prescribed spring constants for the springs that are attached to the rim of the membrane. Zero tensile stress is considered as the condition on the verge of forming wrinkles. When a compression zone appears, wrinkles are formed. Numerical results show that there is a limited amount of inward movement at the edge before the formation of wrinkles. The numerical results are also compared with an analytical solution.

Hot mix asphalt (HMA) is a composite material that consists of mineral aggregates, asphalt binders, and air voids. A finite element model of the HMA microstructure is developed to study the influence of localized strain distribution on the HMA mechanical response. Image analysis techniques are used to capture the HMA microstructure. Due to limitations on the image resolution, the microstructure is divided into two phases: aggregates larger than 0.3 mm and mastic (binder and aggregates smaller than 0.3 mm). A viscoelastic constitutive relationship is used to represent the mastic phase of the HMA microstructure. The mastic viscoelastic properties are obtained from the results of testing asphalt binders in a dynamic shear rheometer and microstructure analysis of idealized mastic. A step-wise finite element procedure is employed in order to account for the influence of the localized high strains on the mastic viscoelastic properties, and HMA mechanical response. The mastic and binder elements of the microstructure are shown to exhibit high strain values within the nonlinear viscoelastic range. The HMA viscoelastic properties are calculated at different strain levels, and the results are compared with experimental data obtained from the frequency sweep shear test.

This problem studies the effect of gravity and initial stress on the propagation of torsional surface waves in dry sandy medium. The mathematical analysis of the problem has been dealt with the Whittaker function. Assuming the expansion of the Whittaker function up to linear term, it is concluded that the gravity field will always allow torsional waves to propagate. The expansion of the Whittaker function up to quadratic terms shows that two such wave fronts may exist in the medium. Finally, it is concluded that the sandy medium without support of a gravity field cannot allow the propagation of torsional surface waves, where as the presence of a gravity field always supports the propagation of torsional surface waves regardless of whether the medium is elastic or dry sandy.