Showing papers in "Journal of Engineering Mechanics-asce in 1982"

Identification of Linear Structural Dynamic Systems

Abstract: This paper studies methods of parameter estimation for linear multi-degree-of-freedom structural dynamic systems, based on observed records of the external forces and the structural responses. The auto-regressive and moving-average (ARMA) model is used for this purpose. It is found that the ARMA model is a convenient model representing linear multi-degree-of-freedom structural dynamic systems and that the model is highly compatible with the instrumental variable method and the maximum likelihood method of identification. In order to check the accuracy of the estimation methods, analytical simulation studies are performed on the basis of simulated data dealing with the aerodynamic coefficient matrices that appear in the equations of motion of a two-dimensional model of a suspension bridge. Then, these methods are applied to the same equations to identify the coefficient matrices using the field measurement data yielding good estimates of the system parameters even under large output noise conditions.

Three-Parameter Failure Criterion for Concrete

Abstract: A general, three-dimensional failure criterion for plain concrete and mortar is proposed. This criterion is formulated in terms of the first and the third stress invariants of the stress tensor, and it involves only three independent material parameters. Although these parameters interact with one another, each parameter corresponds to each of three failure characteristics of concrete behavior. These material parameters may be determined from simple tests such as uniaxial compression and triaxial compression or biaxial tests. For the purpose of including reasonable values of tensile strengths in the failure criterion, it is advisable to include the uniaxial tensile strength in the parameter determination. A simple expression for evaluation of the uniaxial tensile strength on the basis of the uniaxial compressive strength is given. Twenty-two sets of good quality data for concrete and mortar have been included in this study, and comparisons between the proposed failure criterion and the experimental data are made in biaxial, triaxial, octahedral, and τ\N - σ\N planes. The ability of this criterion to capture the characteristics of failure in concrete and mortar appears to be quite good with accuracies generally within the natural scatter of the test data.

Flexural-Torsional Stability of Curved Beams

Abstract: General fourth order coupled differential equations are presented to determine the elastic buckling loads of in-plane as well as out-of-plane buckling modes of spatial thin-walled curved beams. Included are the warping contribution to the buckling loads, and the effects of antisymmetry of cross-section (found to be significant). The governing differential equations are based on the variational principle. Critical load formulas are given for a few special combinations of loadings and boundary condition. Comparative studies were made and possible sources of discrepancies were traced.

Constitutive Relationships for Fluid-Solid Mixtures

Abstract: Constitutive equations are derived for the rapid shear flow of a mixture formed from granular solids in a fluid. The stress state within the mixture is considered to be created by the momentum exchange between colliding solids. The constitutive equations describe the shear and normal stresses as a function of the velocity gradient, friction and restitution coefficients of the solids, the fluid drag coefficient for the particle shape and the density of the solid and fluid constituents. The theoretical results agree well with the results obtained from numerous laboratory experiments.

Suspension Bridge Response to Multiple-Support Excitations

Abstract: The earthquake-induced vertical response of suspension bridges when subjected to, correlated or uncorrelated, multiple-support excitations is analyzed by means of random vibration theory. Appropriate ground motions describing the basic inputs to such structures are defined through finite Fourier transforms of existing recorded ground displacements at locations within distances similar to the spans of such structures. Spectral correlation analysis of the multiple-support excitations are estimated via these Fourier transforms. It is found that studies of earthquake-induced vertical motion of suspension bridges need to take into account the influences of ground excitations in the longitudinal directions of such bridges. An example is presented in which the response of an existing bridge (in a seismically active area) to earthquake ground motions with characteristics significantly different at the supports is examined. It is observed that the participation of higher modes in the total response is essential to reliably assess the seismic behavior of such structures and that the response values associated with correlated multiple-support excitations are significantly different from those obtained through the uncorrelated case.

Dynamic Characteristics of Liquid Storage Tanks

Abstract: A reliable and computationally effective method for calculating the dynamic characteristics of ground-supported cylindrical tanks is developed. The method offers a rigorous treatment of the interaction problem and provides a starting point for the consideration of the complicating factors which influence the dynamic behavior of tanks. The liquid region is treated analytically and only the shell is modeled by finite elements. In this approach, the number of unknowns is substantially less than in those analyses where both tank wall and liguid are subdivided into finite elements. The reliability of the analysis is illustrated by computing modes and natural frequencies of full-scale tanks and comparing them with the results of vibration tests. The effectiveness of the method has led to a development of an improved seismic design procedure for tanks.

Vibration of Flexible Plate on Viscoelastic Medium

Abstract: The dynamic response of a massless flexible circular plate with a rigid core supported on a layered viscoelastic half-space and subjected to harmonic vertical and rocking excitation is studied. The mixed boundary-value problem for the case of relaxed contact conditions between the plate and the half-space is reduced to Fredholm integral equations of the second kind which are solved numerically. The effects of flexibility of the plate on the force-displacement relationship, on the motion of different points on the plate, and on the distribution of contact stresses beneath the plate are studied numerically. In general, it has been found that all of these aspects of the response of the plate are highly dependent on the flexibility of the plate relative to that of the supporting half-space.

Suspension Bridge Vibration: Continuum Formulation

Abstract: The methodology of the free vibration analysis of suspension bridges with horizontal decks is refined and extended, utilizing a continuum approach to include both the coupled vertical-torsional vibration and the effect of cross-sectional distortion. Variational principles are used to obtain the coupled equations of motion in their most general, and nonlinear form. The general equations are linearized, eliminating the coupling effect, and solved for a specific bridge; the resulting natural frequencies and modes of vertical and torsional vibrations are compared with those calculated using the finite element approach and with those estimated from low-amplitude full-scale ambient vibration tests which were conducted on the bridge. In addition, the general, nonlinear equations of motion governing the lateral vibration of suspension bridges are presented and the linearized forms are obtained; a numerical example and a comparison among the analytical, numerical and full-scale test results are also presented.

Complications in Free Vibration Analysis of Tanks

Abstract: A simple and computationally effective method for computing the dynamic characteristics of ground-supported, cylindrical liquid storage tanks has been developed. A generalization of this method is presented herein to include some complicating factors which affect these characteristics. It has been shown that the initial hoop stress due to the hydrostatic pressure and the in-plane rigidity of the roof system may affect considerably the cosnΘ-type modes of the tank wall. In addition, the flexibility of the foundation soil can reduce measurably the fundamental natural frequency of the cosΘ-type modes due to the rocking motion. It is also concluded that the coupling between liquid sloshing modes and shell vibrational modes is weak; and consequently, the convective pressure can be evaluated with reasonable accuracy by considering the tank wall to be rigid, and the impulsive pressure can be determined by analyzing the liquid-shell system and neglecting the sloshing motion. The validity of the method of analysis has been confirmed by both scale model testing and field measurements of the vibrational characteristics of full-scale tanks.

Control of Tall Buildings Under Earthquake Excitation

Abstract: A method of analysis for tall buildings implemented by active control systems and excited by earthquake ground motions is proposed. The effectiveness of the active mass damper and the active tendon system is investigated. Demonstrations show that both control systems can be used to substantially reduce the response of tall buildings under strong earthquake excitations, if the control parameters are designed appropriately. The control law suggested herein is referred to as the semi-active control in the sense that on-line computations to regulate active control forces are not needed. Thus, the control systems investigated are more reliable, although the control law is not optimal. Numerical examples for two tall buildings are worked out to demonstrate the feasibility of active control systems.

Bridge Impact due to Wheel and Track Irregularities

Abstract: The impact factors in bridge members due to flat wheels and track irregularities were investigated. The maximum wheel flat height was taken as 0.2 in. (5 mm), maximum pier settlement as –0.25 in. (–6.3 mm) and maximum camber error as –0.5 in. (–12.5 mm). The track roughness spectra for class 6 track was used in the analysis, and only selected members of the 175 ft. pinned connections truss bridge were studied. The train of three 70 ton freight cars with spring constants of 11.2 kip/in. (1.96 kn/mm) per wheel were used. The train was moving at various speeds and with initial displacements of 0.25 in. (6.3 mm) and roll of 0.02 rad. Damping of the bridge and vehicle were neglected. The impact factors for the train moving at 50 mph (22.35 m./sec.) seem to agree fairly well with the current American Railway Engineering Association specification. Other conclusions are summarized.

Fluctuating Wind Loads on Buildings

Abstract: A method for evaluating the fluctuating wind forces and their distribution on buildings has been developed and applied to the understanding and quantification of crosswind loads on a square cross-section building. Wind loads are determined by integrating simultaneously monitored pressure fluctuations on a building model surface in a boundary layer wind tunnel. An example is presented to illustrate that a suitable combination of a limited number of pressure transducers can be used to monitor pressure fluctuations for the estimation of multi-level wind loads on building models. This methodology also provides information on the local spatio-temporal variation of pressure fluctuations which can provide useful input to the cladding design. Power spectral density and chordwise variation of co-spectra between locations at one level on the model surface are presented. The cross power spectral density matrices of the crosswind forcing function are developed for urban and suburban flow conditions. An increase in the incident turbulence has dampening effects on the crosswind forcing function, that is, a slight spectral peak. Methods of random vibration analysis can be used to compute the probabilistic response of a building modeled as a single or a multi-degree-of-freedom system.

Scattering of SH Waves by Subsurface Topography

Abstract: Horizontally polarized shear waves in an elastic alluvial valley of arbitrary shape perfectly bonded to a linearly elastic, homogeneous and isotropic half space are considered. The valley is subjected to a steady state horizontal displacement field. Total displacement field is evaluated along the interface between the half space and alluvial valey. Comparison with known exact solutions for simple geometry provided the following conclusions: 1) the method provides excellent results for wide range of frequencies, 2) for a fixed number of sources, the results are more accurate at lower frequencies, and 3) as the number of sources increases, the accuracy increases.

Mechanics of Three-Dimensional Soil-Structure Interaction

Abstract: A finite element procedure for the general problem of three-dimensional soil-structure interaction involving nonlinearities caused by material behavior, geometrical changes, and interface behavior is presented. The formulation is based on the updated Lagrangian or approximate Eulerian approach with appropriate provision for constitutive laws. Consideration is given to the mathematical and experimental aspects related to the important topic of development and adoption of a constitutive law for the geologic medium. development and use of an interface element to describe the three-dimensional interface behavior are described. The influence of cracking and tensile stress conditions in the soil and interfaces is allowed for by using the stress transfer approach. The procedure is verified by comparing predictions with observation of a structure (tool) pushed in the soil in a special soil-bin test facility.

Boundary Element Analysis of Fluid Domain

Abstract: The boundary element method of analysis is applied to the calculation of hydrodynamic pressures in a reservoir impounded by a gravity dam and subjected to a harmonic ground motion., It is shown that the differential equation governing the small amplitude vibrations of the reservoir reduces to the familiar Helmholtz equation. This equation is converted to an integral equation which involves integrations only on the boundary of the reservoir. The integrals are evaluated numerically by using a series of straight line elements to represent the boundary. The resulting simultaneous equations are solved by standard techniques to obtain the pressures and pressure derivations on the boundary. Analytical results are presented to show that the proposed method gives results that are in good agreement with the known classical solutions. The method is then applied to the solution of vibration problems of reservoirs with irregular boundaries for which classical solutions do not exist. The method is also successful in dealing with cases of infinite reservoirs and in modelling the energy loss in the waves moving to the infinity.

Large Deflections of Beams with Combined Loads

Abstract: Closed-form solutions are presented for the problem of large deflection of a cantilever beam with a couple and a concentrated load acting simultaneously at the free end Dimensionless charts are provided which relate the variables maximum vertical deflections, maximum horizontal deflections, couple and force

Cylindrical Shells of Bimodulus Composite Materials

Abstract: Certain fiber-reinforced materials, especially those with slightly curved fibers in very soft matrices, exhibit considerably smaller stiffnesses when loaded in compression than when loaded in tension. Examples are tire cord-rubber, wire-reinforced solid propellants, and certain soft biological tissues. For purposes of analysis and design, such materials can be modeled as a bimodulus material, i.e., one having one set of stiffnesses when the fiber direction strain is tensile and another set when this strain is compressive. Using the fiber-governed bimodulus-material model introduced several years ago and verified for cord-rubber composites, the previous work is extended on the deflection of single-layer and cross-ply laminated rectangular plates to circular cylindrical shells of the same construction. A closed-form solution is presented for the case of a simply supported cylindrical panel under sinusoidally distributed loading. Numerical results are presented to show the effect of shell curvature on the neutral-surface position and deflection.

Beams, Frames and Shells on Elastic Foundations

Abstract: Explicit stiffness matrix, present matrix and nodal forces are used as a basis for computer analysis of beams, rings, simple frames, and branched shells of revolution on a three-parameter foundation model. This technique demands neither auxiliary handling nor does it result in a decrease in accuracy when applied to the beam or shell with a general variation of the elastic modulus, thickness, or foundation factors along the beam or shell meridian.

Dynamic Response of Plate on Elastic Half-Space

Abstract: Analytical results are presented for the dynamic response of a plate bearing on an elastic half-space and subjected to harmonic forces. The present work represents a departure from existing analyses in that herein both the flexibility and three-dimensionality of the plate are taken into account. Displacements and contact stresses are presented for square plates having a practical range of flexural stiffness. The harmonic analysis is conducted within the framework of a global stiffness solution, in which the plate and subgrade impedance matrices are formulated independently in accordance with a prescribed discretization pattern. Then compatibility of displacements and equilibrium of forces are enforced at the plate-subgrade interface. Solutions are presented for massless square plates subjected to harmonic point, uniform pressure, and moment loadings. The effect of the plate's mass on its response is studied by starting with a perfectly flexible massive plate and subsequently increasing the stiffness toward the limit of a completely rigid plate.

Simplified Bending Theory for Wire Rope

Abstract: A theory has been developed to predict the static response of wire rope subjected to tension, torsion and bending This type of loading occurs frequently in ropes wrapped around sheaves Since the rope is generally restrained against rotation, a twisting moment is developed in the rope in addition to the tension and bending Equilibrium requires that a uniformly distributed couple per unit length be applied to the rope Expressions are presented for the significant stresses in the rope The results can be extended so as to include more complex cross sections other than the single lay rope considered herein

Scattering of Elastic Waves by an Alluvial Valley

Abstract: Plane strain model for scattering of steady state plane waves by an elastic alluvial valley of arbitrary shape is presented. The valley is presumed to be perfectly bonded to a linearly homogeneous and isotropic half space. Total displacement and stress fields are evaluated along the interface between the alluvial valey and elastic half space using the source method. The results are compared with known exact solutions for simple geometries to determine the accuracy of the proposed method. It was found that the proposed method provides very good results. As an illustrative example diffraction of a plane P-wave by an elastic semi-elliptical alluvial valley is presented in detail.

Stresses on Rapidly Sheared Fluid-Solid Mixtures

Abstract: Constitutive equations were derived describing the stress in a rapidly sheared mixture of granular cohesionless solids and a Newtonian fluid. These equations are considered to describe the flow of mixtures such as coal and oil in a pipeline or sediment suspended in a river. The stress is found to depend upon the following properties of the solid: diameter and density of the granular material, coefficient of kinetic friction, coefficient of restitution, and volumetric concentration. The density and viscosity of the interstitial fluid was also found to affect the stress within the granular mixture. The theoretically predicted shear stresses were found to be in good agreement with measured stresses determined from experimental studies where rapid shear flows were produced in mixtures formed from polystyrene spheres in air, polystyrene spheres in water, glass spheres in air and polystyrene spheres in air.

Dynamic Stability Boundaries for Shallow Arches

Abstract: The interactive effects of multiple dynamic loads on the snap-through instability of shallow elastic arches are investigated. Impulse loads and step loads of infinite duration are considered. The loads are applied either as concentrated loads at the quarter points of the arch, or as concentrated loads at the third points together with a uniformly distributed load. The arch is sinusoidal and has pinned ends. Critical load combinations, based on the Budiansky-Roth criterion, are computed numerically and plotted as interaction curves (i.e., stability boundaries). Curves of constant maximum response are also presented. The results for step loads are compared to the static critical loads, while the critical impulse loads are compared to some lower bounds. In addition, a single concentrated load at an arbitrary position of the arch is treated, and the locus of critical loads is determined for various arch rises. Both for step and impulse loads, the positions corresponding to minimum critical load are usually near the third points of the arch.

System Reliability Bounding by Conditioning

Abstract: Series systems that may fail in one of several unimodes and which have normally distributed unimode safety margins may be analysed with respect to reliability in terms of bounds reported in literature. For the majority of the correlation coefficients between the safety margins smaller than about 0.5 these bounds are very narrow Provided the total reliability is high and even if there is not just some very few dominating unimodes. It is shown how these bounds may be applied in a method of conditional sounding to give close bounds also in a case where most or all of the correlation coefficients are high. In a specific example the method is compared to another known method based on product form correlation coefficients.

Optimal Grillage Layout for Given Natural Frequency

Abstract: The problem of minimizing the volume of structural material of an elastic grillage subject to a prescribed value or the fundamental frequency of free transverse vibrations, is considered. The domain and the boundary conditions are given, and the grillage is equipped with a specified, distributed non-structural mass, which is large in comparison with the structural mass. It is the objective to determine the optimal layout of the grillage, and to optimize the beams, which are of given depth and varying width. It is shown that the optimal grillage layout is the same as that found earlier for three different types of design problems, namely plastic design for given collapse load, elastic design for given permissible stress, and elastic design for given compliance. An an example, a simply supported, square grillage with uniformly distributed mass loading is optimized, and it is shown that the amount of structural material of the optimal grillage is significantly smaller than that to be used for a uniform plate with the same fundamental natural frequency of transverse vibrations.

Strain-Rate Effect in Rapid Triaxial Loading of Concrete

Abstract: Presented is a generalization of time-independent nonlinear triaxial constitutive relations for concrete to model short-time viscoelastic effects. The effect of strain-rate magnitude upon the initial elastic modulus, the peak stress, and the sharpness of the stress peak is taken into account. To obtain a model that also exhibits short-time rapid creep and stress relaxation, the incrementally linear constitutive law fully characterized by tangential moduli is replaced by a nonlinear first-order differential equation in stress, strain, stress rate and strain rate. This requires introduction of the (apparent) instantaneous modulus, the relaxation time and retardation time. The irreversibility at unloading, caused by the short-time viscoelastic mffects, is also analyzed. The model allows more realistic dynamic finite element analysis of concrete structures.

Finite Layer Analysis of Nonhomogeneous Soils

Abstract: A finite layer approach capable of analysing both layered and continuously varying cross- anisotropic elastic soil profiles is presented. The method of analysis is suited for soil deposits in which there is no variation in properties in the horizontal plane. For this type of soil, the proposed technique provides an accurate and computationally more economical alternative to the use of the finite element method. The benefits of this approach are particularly evident for three dimensional problems and for problems involving deep soil layers. The application of the theory is illustrated by analysing the behaviour of uniformly loaded strip and circular footings resting on a soil with homogeneous crust underlain by a main deposit whose stiffness increases linearly with depth. The results of this analysis indicate the importance of a weathered crust upon the settlement of surface footings and provide a convenient means of estimating the effect of this crust and the underlying non-homogeneity upon settlement for a limited range of soil profiles (A). (TRRL)

Sandwich Beam-Columns with Interlayer Slips

Abstract: Elasticity solution for sandwich beam-columns bonded with non-rigid adhesives is presented. The solution satisfies the equilibrium equations of the face and core elements, the compatibility equations of stresses and strains at the interfaces, and the relevant boundary conditions. To investigate the effects of adhesive stiffnesses on the responses of sandwich beam-columns, parametric studies are conducted. The numerical results show that the adhesive stiffness, up to a certain level, has a strong effect on the beam deflection and the column buckling load. Beyond this level, the usual assumption of perfect bonding used in classical theories is quite acceptable. An answer to what constitutes perfect bonding is found in terms of the ratio of the core stiffness to the adhesive stiffness. To facilitate the appication of the present solution, simple expressions are presented for the maximum normal force in facings and interlayer shear flux of sandwich beams subjected to various loading types.

Noise in Earthquake Accelerograms

Abstract: The amplitudes of processing and digitization noise in strong motion earthquake accelerograms are reviewed for hand and automatic digitization. By finding the period bands for which the signal-to-noise ratio in digitized accelerograms is greater than one, dependence of cut-off periods for accelerogram data processing is presented for amplitude scaling in terms of: (1) Earthquake magnitude and epicentral distance; and (2) modified mercalli intensity at the recording site.

Active Control of Large Structures

Abstract: In this paper, it is shown that the pole assignment method through which one can control high degrees of freedom structures can be used to determine an optimal control law. The finding of this paper is that among all possible gain matrices which can be found by the pole assignment method there is only one gain matrix which is optimal. This result is proven when the poles of an optimal control system are assigned as the desired poles and the optimal gain matrix is found among the all possible gain matrices. This finding may solve the difficulties met for controlling large structures in an optimal way. An application to optimal control of two-story building frame is given.