Showing papers in "Journal of Engineering Mechanics-asce in 1978"

Closure to “Instability, Ductility, and Size Effect in Strain-Softening Concrete”

Abstract: Analysis of structural instability due to strain-softening (i.e., declining branch of the stress-strain diagram) is presented. In a continuum, strain-softening is impossible; it can exist only in a heterogeneous material. Failure occurs by unstable localization of strain or beam curvature, in which the stored strain energy of the structure is transferred into a small strain-softening region whose size is several times the aggregate size, or the spacing of reinforcement, or the depth of the beam. The existence of a lower limit on the size of this region permits ductility, along with its dependence on the size and stored energy, to be predicted by a stability analysis. Calculations of limit loads and moment redistributions in strain-softening beams and frames must include instability checks of possible curvature localization. The same applies to finite element analyses of reinforced concrete structures with account of tensile cracking, and predictions of limit loads of these structures which are questionable because they depend on the size of the finite elements.

Dynamic Soil Reactions for Plane Strain Case

Abstract: Resistance of soil to harmonic motion of an infinitely long cylinder is investigated theoretically in terms of linear viscoelasticity. Closed-form solution is obtained from which the complex stiffness can be evaluated. The numerical results agree with those obtained by means of the correspondence principle.

Pore Pressure and Drying of Concrete at High Temperature

Abstract: A mathematical model for water transfer in concrete above 100°C is developed. Drying tests of heated concrete are reported and material parameters of the model are identified from these tests as well as other test data available in the literature. It is found that water transfer is governed principally by the gradient of pore pressure, which represents the pressure in vapor if concrete is not saturated. Permeability is found to increase about 200 times as temperature passes 100°C, which could be explained by a loss of necks on migration passages. The pore volume available to free water increases as dehydration due to heating progresses and as the pore pressure is increased. The temperature effect on pressure-water content (sorption) relations is determined. Thermodynamic properties of water are used to calculate pore pressures. A finite element program for coupled water and heat transfer is developed and validated by fitting test data.

Plasticity theory for soil stress-strain behavior

Abstract: A general analytical model that describes both drained and undrained, anisotropic, elastoplastic, path-dependent stress-strain-strength properties of inviscid saturated soils is presented. For any loading (or unloading) history, the instantaneous configuration of the field of yield surfaces is determined by calculating the translation and contraction (or expansion) of each yield surface during successive changes in load. The material behavior can thus be determined for complex, and, in particular, for cyclic loading paths. The inverse stress-strain relations always exist and are uniquely defined if and only if the yield surfaces do not overlap. In order to avoid such overlappings, a new isotropic/kinematic hardening rule is introduced which couples the simultaneous translation of consecutive surfaces. The isotropic/kinematic hardening of the outer surfaces is thus made compatible with any isotropic/kinematic hardening rule assumed for the inner surfaces.

Impedance Functions of Piles in Layered Media

Abstract: An approximate analytical solution is established that makes it possible to calculate impedance functions (stiffness and damping) of a single pile embedded in layered media. The pile can be of stepwise variable cross section and feature any tip condition. The impedance functions are complex and frequency dependent but can be obtained by means of a computer program that is simple and very inexpensive to run. A comparison of the theory with experiments indicates the potential of the theory and demonstrates the need to consider the variation of soil properties with depth and lack of the fixity of the tip for short piles.

Finite Element Analysis of Contact Problems

Abstract: A finite element procedure for modeling the interaction of contacting bodies is developed and illustrated. The model is capable of accounting for both slippage and separation of the mating surfaces. In addition, the bond springs, which in certain situations are used in the nonslip model, can be used to capture local deformation phenomena such as edge effects and local deformation caused by asperities, etc. The capabilities of the model for representing reinforced concrete bond, reinforced soil bond, soil structure interaction, and testing machine-sample interaction are considered. Numerical examples are given for modeling of concrete-reinforcement bond, pile-soil interaction, and footing-soil interaction; the results for the first two examples are compared to available experimental results.

Generalized Stress-Strain Relations for Concrete

Abstract: Triaxial experimental data on the behavior of concrete have indicated that while only volumetric changes occur under pure hydrostatic stress, both distortional and volumetric changes occur under pure deviatoric stress. By analyzing these data and assuming that such experimental data in general can only be relied upon to represent the material behavior up to the level at which continuous cracks begin to form within the structure of the material, it has been possible to express mathematically the deformational characteristics of concrete in terms of the bulk and shear moduli and a correction function that allows for volume changes unde deviatoric stress. These expressions have been formulated to be consistent with use in computer-based methods of structural analysis and they completely define the state of strain corresponding to any state of stress.

Determination of internal forces in human hand

Abstract: Three-dimensional force analysis of finger tendons and joints in isometric hand functions was formulated as a statically indeterminate problem. This problem was solved adjunctly by the permutation-combination technique and the method of linear programming. Optimal solutions were compared with those produced by the permutation-combination technique and justified based on clinical and physiologic assessments. The results obtained are within the expected range and the muscle force distribution coincides with the data established in previous EMG studies and muscle volume analyses. The validity of the technique used and its potential applications to force analysis of the entire human hand in various functional configurations were considered.

Ambient Vibration Tests of Suspension Bridge

Abstract: Extensive experimental investigations were conducted on the Vincent-Thomas Suspension Bridge at Los Angeles Harbor to determine natural frequencies and mode shapes of vertical, torsional, and lateral vibrations of the structure. These ambient vibration tests involved the simultaneous measurements of both vertical and lateral vibrational motions caused by traffic. Measurements were made at selected points and different cross sections of the stiffening structure. Comparison with previously computed mode shapes and frequencies showed good agreement with the experimental results, thus, confirming both the accuracy of the experimental determination and the reliability of the method of computation.

Amplitude Reduction of Surface Waves by Trenches

Abstract: The effect of trenches and barriers in reducing the disturbances created by a time-harmonic line load acting on the surface of a linearly elastic, layered slab is investigated. In this approach, boundary conditions are applied which take into account the stresses due to disturbances propagating in the adjacent, regular, semi-infinite regions. The dimensions and the location of the barrier are systematically varied for several frequencies of excitation, and the results are found to be in agreement with experimental observation. It is shown that the trench effectiveness is primarily a function of the ratio of its depth to the wavelength of the propagating disturbances, h/λ. For a value of h/λ greater than 0.6 the disturbances are significantly reduced. The results indicate that the location of the trench and the shape of its cross section, however, are not important parameters. The influence of backfilled trenches is also investigated and these are found to be less effective in reducing the vibration amplitudes than open trenches.

Spectrum-Consistent Time-History Generation

Abstract: A method is presented for generating an acceleration time history to match a specified absolute acceleration response spectrum. The relationship between the change in time history required to effect a specified change in its spectrum is established and forms the basis for an iterative procedure for generating a spectrum consistent time history. It is shown that a high degree of accuracy can be achieved in a few applications of the iterative procedure.

Structural Control by Pole Assignment Method

Abstract: Most of the application of active control theory to the control of civil engineering structures were based on a trial and error method, optimal control method, and modal control method. In this paper, the pole assignment method and its application to the control of a long span bridge under a deterministic moving load are shown. Although the pole assignment method is similar in concept to the modal control method, the former provides a more general and systematic design procedure available for a system of any order and any number of measurements.

Effects of Turbulence on Bridge Flutter Derivatives

Abstract: Bridge flutter derivatives have heretofore been obtained from models almost exclusively tested under oncoming laminar flow conditions This paper demonstrates how a section model teest may be set up and the results interpreted to obtain bridge deck flutter derivatives under conditions of oncoming turbulent flow New theory is necessary in the turbulent flow case, and this is developed in the paper The technique described amounts to a system identification method in which aerodynamic results are inferred from a theoretical model plus measured results of turbulent wind components, together with the motion they cause in the model Flutter derivative results for an arbitrary model shape, obtained under a particularly pronounced form of turbulence that does not necessarily duplicate the natural turbulent wind over a prototype, are compared with analogous results obtained under laminar flow

Buckling of Imperfect Rigid-Jointed Frames

Abstract: The buckling of multistory, multibay frames is investigated. A method of analysis is presented for estimating directly critical conditions, for both bifurcational and limit point type of instability. This method is based on a nonlinear formulation, and on the simultaneous solution of the equilibrium and buckling equations, and it is demonstrated through two simple frame problems.

Active Tendon Control of Structures

Abstract: A method is developed for the analysis of a continuous structure implemented by an active tendon control under the action of stochastic wind loads. The transfer matrix technique is employed to obtain the frequency response function, in a closed form, of the actively controlled structure. This permits an examination of the behavior of all the vibrational modes in the frequency domain with respect to the variation of control parameters. It is shown that both the displacement and the acceleration of the structure can be reduced significantly in the entire frequency domain only if the feedback control parameters are designed appropriately. It is further shown that although some vibrational modes can be suppressed completely by the active control devices, other vibrational modes may be magnified if the feedback control parameters are not designed carefully.

Large Deflection of Tapered Annular Plates by Dynamic Relaxation

Abstract: It is asserted that the use of fictitious densities based on Gerschgorin bounds in the Dynamic Relaxation (DR) method of analysis leads to improved convergence characteristics. The validity of this assertion is demonstrated by comparing large deflection, variable thickness, circular plate results produced by Murthy and Sherbourne's DR program, which is based on arbitrarily chosen fictitious densities, and the present program. The latter is used to produce a number of solutions to variable thickness annular plates subject to a uniform lateral load. Results are presented for two hole sizes and three thickness taper ratios.

Cyclic Metal Plasticity: Experiments and Theory

Abstract: Stress-strain curves are presented for random inelastic cyclic loadings of 2-in. (51-mm) diameter, thin-walled ASTM A106 seamless steel tubes subjected to uniaxial and torsional experiments. The mechanical properties of the selected steel resemble those of the A36 steel widely used in building construction. The experiments simulate the extreme conditions which may occur in some elements of structural steel frames during severe earthquakes. The uniaxial cyclic tests correspond to the conditions which may develop in the flanges of beams; whereas the torsional tests simulate those in the panel zones of the beam-column joints. Employing the effective stress and the effective plastic-strain invariants and a hereditary concept a procedure is described for using the experimental results from uniaxial tests for obtaining consititutive relations for generalized loadings, including complete load reversals. Comparisons of the calculated results with experiments are given showing good agreement.

Response Spectra of Earthquake Ground Motion

Abstract: In this paper a method is presented for direct scaling of Pseudo Relative Velocity Spectra (PSV) in terms of: (1)Earthquake magnitude M and epicentral distance R; or (2)Modified Mercalli Intensity (MMI) at a site. These models also depend directly on the geologic site conditions and are presented for horizontal and for vertical ground motions. This scaling is realized by means of “coefficient” functions which are determined through regression analysis of computed PSV spectra from recorded accelerograms. One of the principal advantages of the proposed method is that the ambiguities associated with the scaling of the fixed shape spectra by means of peak amplitudes of ground motion are now completely eliminated. The 91 direct regressions of horizontal and vertical spectra, at 91 periods, smoothed over all periods, lead to more complete and reliable sampling of the frequency-dependent characteristics of strong ground motion than the correlations of peak amplitudes alone.

Stress and Strain in the Lung

Abstract: The stress-strain relationship of the lung tissue is described. There are two sources of stresses in the lung: that due to elasticity of the material, and that due to surface tension on the interfaces between the moist interalveolar wall and the air. A theoretical formula is derived for the stress-strain relationship on the basis of an idealized alveolar structure. Evidences supporting the validity of the formula are quoted. This formula is then used as the basis for analyzing the stress and strain distribution in the lung. An exact solution is obtained for a vertical lung supporting its own weight by hydrostatic pressure distributed on the pleura (the outer surface of the lung).

Compliance Monitoring of Crack Growth in Concrete

Abstract: The method of compliance measurement is sometimes used to determine the length of cracks in fatigued metal specimens. This paper considers an application of this method to plain, concrete beams subjected to sinusoidal loading in three point bending. Three beams, prenotched by cutting with a concrete saw, were used to determine a compliance calibration curve. This curve was then used to determine the extent of cracking in eight beams that were fatigue loaded. Of the beams precracked by fatigue loading, six were subsequently statically loaded to failure. A method for estimating the effective fracture toughness based on a test procedure for metal specimens was used to obtain experimental data as input to a finite element model. Results, including fracture surface appearance, indicate that the compliance method is a convenient and suitable testing technique for monitoring crack growth in concrete beams due to fatigue.

Active Control of Building Structures

Abstract: The purpose of this study is to find a feasible feedback comfort control function for building structures which are subjected to wind force excitations. A vibrational model of multistory building structures subjected to wind force excitation and comfort control was formulated, and solutions were obtained with the use of the Monte Carlo method. An effective and feasible comfort control law was found and applied to a 40-story steel building structure. Results of the analysis showed that: (1)The comfort control law performed satisfactorily according to the comfort and safety criteria; and (2)it was practically feasible to use such an active control law in designing future building structures.

Bilinear Model for Coated Square Fabrics

Abstract: An idealized composite model of a square biaxially loaded coated fabric is proposed. The model consists of straight linearly elastic yarns embedded in a thin layer of a linearly elastic isotropic coating material with the system loaded in the plane of the membrane. Governed by the assumptions of small strains, equations describing the material behavior are derived for a general state of two-dimensional loading. The resulting system of nonlinear simultaneous equations are then solved numerically for a range of coating and yarn properties. For relatively low stresses, the response is governed by the mechanism of crimp interchange and coating deformation, and at higher stresses the response is governed by yarn and coating properties. The general nonlinear result is further approximated by a set of bilinear equations and a procedure for evaluating definitive model parameters from data obtained from uniaxial tests is presented.

Effect of Initial Base Motion on Response Spectra

Abstract: The problems involved in converting earthquake ground acceleration records from analog to digital form for use in structural response calculations are widely recognized. Several different balancing or base line corrections are widely recognized. Several different balancing or base line correction procedures have been proposed over the years. Recent studies have resulted in a standard correction procedure for accelerograms, which was used in producing the EERL.

Vibration of Towers Due to Galloping of Iced Cables

Abstract: Wind tunnel experiments were conducted with stationary and vibrating models whose cross sections were similar to those of some ice formations observed. Aerodynamic data established for iced cables were introduced into the analysis of the whole guyed tower. The quasi-study theory of galloping instability was extended to this end. It was found that the energy input generated by galloping of one cable can be sufficient to cause large amplitudes of the whole tower even at low wind speeds.

Tunnel Closure for Nonlinear Mohr-Coulomb Functions

Abstract: A closed-form analysis for elastic-plastic response of a cylindrical tunnel in rock under axisymmetric loading at infinity shows the effect of yielding according to a nonlinear Mohr-Coulomb-type function in which the angle of internal friction decreases with increasing mean normal stress. Explicit expressions for stresses and deformation are obtained by approximating the nonlinear function by a series of straight line segments. Comparisons of tunnel closures calculated with nonlinear functions with those from single straight line approximations show that tunnel closure is sensitive to plastic behavior nearest the tunnel. This means that the low-stress part of the yield function, where nonlinearity is usually greatest, should be modeled as accurately as possible.

Cyclic Shear Yielding of Wide-Flange Beams

Abstract: The results of experiments with wide-flange steel beams subjected to cyclic shear yielding are described. The tests indicate that beams that yield in shear show stable behavior at large cyclic deflections, and they dissipate large amounts of energy. The significance of cross-sectional warping is shown, and the beneficial effect of the formation of cyclic diagonal tension fields is described. An analytical model is developed for predicting the inelastic behavior of shear yielding elements which shows good correlation with the test results. An application for accurately predicting the global behavior of eccentrically braced steel frames subjected to cyclic loading characteristic of the problem in aseismic design illustrates the usefullness of the developed analytical model.

Seismic Assessment of High-Rise Buildings

Abstract: A method is presented by which the seismic resistance of a structure can be assessed and which leads to results with a rather high level of confidence. It is based on the notion of the critical excitation of a structure. This is an excitation that drives one of the structural design variables to a larger response peak than any other, in some given set of credible excitations. Evidence is presented which shows that it is possible to reduce the idea to a method with considerable promise for application in practice, but that substantial modifications are necessary in it. The evidence consists of the results of analyses of four high-rise buildings, three in existence and one in the planning stage. The results show that the method leads to designs that are somewhat conservative, but that appear to be consistent with established good engineering practice. Its application therefore seems most appealing to structures that are sufficiently important to demand high confidence in their survival or integrity during an earthquake.

Post-Buckling Analysis of Cylindrical Shells

Abstract: A method is presented for the calculation of the critical load and post-buckling displacements of elastic structures, and is demonstrated on circular cylindrical shells. The critical load is found by successive extrapolations of the load with the aid of the tangent stiffness matrix determinant. The buckling mode shape is calculated at the instability point and the prebuckling displacements are perturbed a small amount proportional to this mode shape in order to determine an equilibrium state in the post-buckling range. A modified Newton-Raphson iteration technique is utilized to trace the post-buckling load-displacement curve.

Statistical Stability Effects in Concrete Failure

Abstract: Using a uniaxial localization model, it is shown that parallel elastic restraint increases ductility, while an increase in support flexibility or length of specimen reduces ductility. Statistical macroscopic nonhomogeneity of the specimen is modeled by a system of uniaxial parallel elements of random properties following the normal distribution. The stability analysis and Monte Carlo simulations explain that in such a system an increase of length or support flexibility reduces not only ductility but also the strength of the system. The effect on strength depends on the number of elements (width of specimen), which represents a new non-classical statistical size effect, and on the standard deviation of peak stress values within the parallel system. Existence of an inflection point and the prolonged tail on the descending branch is explained by the nonhomogeneity of the specimen, and the shape of the descending branch, along with the location of the inflection point, is obtained as a function of machine stiffness, parallel elastic restraint, and specimen length and width.

Porous Medium Deformation in Multiphase Flow

Abstract: The deformation of a porous medium occupied by more than one fluid phase can be described using classical point equations integrated over the porous medium. The resulting system of governing equations are expressed in terms of macroscopic variables that have precise mathematical and physical definitions. It is found that the momentum balance for the entire porous medium contains terms involving surface tension that have heretofore been neglected.