Journal of Engineering Mechanics

In this paper, the transient response of a damped oscillator subjected to cycloidal pulses is investigated. The response is computed analytically by considering viscous and friction damping, and response spectra for relative and absolute quantities are presented for the linear viscous and sliding oscillator. The study complements the list of numerous shock spectra mostly published for the undamped linear oscillator. Subsequently, a numerical procedure based on a state-space formulation is developed to compute the response of damped oscillators when subjected to ground motions recorded near the source of strong earthquakes. It is found that although in several occasions such motions resemble to cycloidal pulses, the response of structures with low to moderate periods is substantially affected by the high-frequency fluctuations that often override the long duration pulse.

A general discrete thin rod model to study the preslip response of a strand of helical wires having wires-to-core friction contacts under constant curvature free bending has been developed for the case of Coulomb stick friction that considers together (a) all the three wire forces and three wire couples; (b) more appropriate equations for the wire shear forces; (c) the wire normal force as one of the contributors to strand bending moment; (d) wire external equilibrium; and (e) wire rolling with or without zero net shear on the strand cross section. The individual contributions of wire forces and couples to the strand bending moment for 10 models including EPRI model are compared. Several earlier models are shown to be special cases of the present general model.

Experimental verification of structural control strategies is essential for eventual full-scale implementations. However, few researchers have facilities readily available to them that are capable of even small-scale structural control experiments. This paper demonstrates that appropriately constructed bench-scale models can be used to study important aspects of full-scale structural control implementations, including control-structure interaction, actuator and sensor dynamics, actuator saturation effects, limited availability of sensors, output feedback design, digital control implementation, and control spillover. The active control experiment considered herein is shown to be an effective tool for familiarizing students, researchers, and practitioners with control system design and the associated challenges. Moreover, this experiment provides an excellent platform for conducting proof-of-concept tests of control strategies, providing impetus for the most promising approaches to be verified at larger-scale facilities.

Recently, several authors have treated the problems of bending of beams of bimodulus materials. The present paper, applies Levinson beam theory, which includes shear deformation and warping of the cross section, to bending analysis of thick rectangular beams with bimodulus materials. Many numerical results are obtained by use of the transfer matrix approach and compared with the methods of Bernoulli-Euler beam theory, Timoshenko beam theory, and Levinson beam theory with various boundaries. Also, the neutral-surface location and displacements for beams of bimodulus materials are calculated.

Rigid body dynamics is used to determine the deformation of a fixed-end, rigid-plastic beam subjected to uniformly distributed impulsive loading. The proposed solution methodology allows calculations of deformations at plastic hinges and can be used to establish rigid-plastic fracture criteria for rigid-plastic beams. Unlike previous solutions to this problem, rotary inertia and the shear deformations at the support are considered. The solution for beam deformations is described in three phases: shear, bending, and membrane. Each phase ends when the corresponding component of the strain rate vector vanishes. The initial shear phase is completed when the transverse shear velocity at the support vanishes. The beam then undergoes only rigid body rotation and axial stretching at plastic hinges in the bending phase. The bending phase ends when the angular velocity vanishes. In the membrane phase, the beam acts like a string until the transverse velocity vanishes. It has been found that beams subjected to low impulse velocity attain permanent deformation in the bending phase, while beams subjected to high impulse velocity reach permanent deformation in the membrane phase. The predictions of the beam deflections using the proposed methodology are within 15% of the experimental results.

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.

The nonlocal probabilistic theory developed in Part I is applied in numerical studies of plain concrete beams and is compared to the existing test data on the modulus of rupture. For normal size test beams, the deterministic theory is found to dominate and give adequate predictions for the mean. But the present probabilistic theory can further provide the standard deviation and the entire probability distribution (calculated via Latin hypercube sampling). For very large beam sizes, the statistical size effect dominates and the mean prediction approaches asymptotically the classical Weibull size effect. This is contrary to structures failing only after the formation of a large crack, for which the classical Weibull size effect is asymptotically approached for very small structure sizes. Comparison to the existing test data on the modulus of rupture demonstrates good agreement with both the measured means and the scatter breadth.

Oscillatory flows over a flat plate are studied by using Large Eddy Simulation (LES) and Reynolds-Average Navier-Stokes (RANS) methods. A dynamic subgrid scale (SGS) model is employed in LES, while the Saffman's turbulence model in RANS. The mean velocity profile, the turbulence intensity, and the wall shear stress are computed and compared with earlier experimental and numerical works. The results indicate that the flow behaviors are quite different during the accelerating and decelerating phases of the oscillating cycle. The transition from laminar to turbulent is also investigated as a function of the Reynolds number, R, which represents the square of the ratio of the oscillation amplitude at free stream to the thickness of the Stokes layer at the plate. The present results both from LES and RANS show that the transition occurs in the range of 5 × 10⁴ < R < 5 × 10⁵. The evolution of the flow structure in the Stokes layer during the transition from laminar to turbulent is clearly demonstrated from the numerical results. The friction coefficient of the amplitude of oscillatory surface shear stress varies as R^−0.5 with a phase angle of 45° in laminar regime and transition to R^−0.23 with a phase angle of about 10° in turbulence regime. These variations in the surface shear stress with the Reynolds number are in excellent agreement with the earlier experimental results of Kamphuis and the numerical results of Blondeaux. The excellent agreement between the LES and RANS demonstrated that Saffman's turbulence model, as originally intended by Saffman, is applicable for unsteady flows.

The arrival time of the first P-wave is, among other parameters, of prime importance for pinpointing the location of an acoustic emission (AE) event source. However, it seems that determining the first P-wave arrival times from digitized signals has been overlooked in previous works. In this paper, a new method is proposed based on digital signal processing techniques. AE analyses are then performed for both normal-strength concrete and high-strength concrete in uniaxial tension to study the damage initiation and progression by interpreting the activities and locations of AE events. The results indicate that using the proposed method to determine the P-wave arrival time of AE signals is quite successful. It is found that microcracking starts early before peak stress. Localization is clearly observed for both normal-strength concrete and high-strength concrete. The examination of AE activities and source location maps reveal that the internal microcracking behavior is consistent with the macroscopic deformation behavior of the materials.

An optimal control algorithm using neural networks is proposed. The controller neural network is trained by a training rule developed to minimize cost function. Both the linear structure and the nonlinear structure can be controlled by the proposed neurocontroller. A bilinear hysteretic model is used to simulate nonlinear structural behavior. Three main advantages of the neurocontroller can be summarized as follows. First, it can control a structure with unknown dynamics. Second, it can easily be applied to nonlinear structural control. Third, external disturbances can be considered in the optimal control. Examples show that structural vibration can be controlled successfully.

The Green's functions of a point dislocation, a concentrated moment, a normal point force, a couple moment, and a couple bending force applied to an infinite plate with an arbitrarily shaped hole under a displacement boundary condition are derived in this paper. The closed-form stress functions are obtained by using the technique of the rational mapping function and the complex stress function approach. In the derivation, the analytical continuation and Cauchy integral are used for the different actions. Without loss of generality, some calculated results are shown for a square hole under a fixed boundary. The solutions show that the stress functions have different orders of singularity for the different actions. In order to illustrate the stress level due to bending moment and shear force clearly and efficiently, the effective stress of thin plate bending is shown.

This note comments on a strain rate pattern found in tensile tests. This strain rate pattern readily suggests that the Cowper-Symonds equation relating the static and dynamic flow stresses with the strain rate is not valid for a broad range of strains unless its coefficients change. A modification of the Cowper-Symonds equation is proposed, and good prediction of the dynamic flow stress at any strain and strain rate level was obtained.

For a simplified model of a structure-pile-soil system, semiexplicit forms of the responses to random earthquake inputs are derived together with sensitivity expressions of the random responses with respect to a certain model parameter. Originally this model has a non-tridiagonal stiffness and damping matrices with single earthquake input. It is shown for the first time that independent treatment of the free-field ground and multiple imposition of the free-field ground response into the structure-pile system allow for a transformation of the governing equations so that the final form has tridiagonal stiffness and damping matrices. Closed-form expressions of the inverse of the tridiagonal matrix lead to semiexplicit expressions of the random responses and their sensitivities with respect to a certain model parameter.