Browsing by Author "Veletsos, Anestis S."
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Item Dynamic impedances of soil layers and piles(1988) Dotson, Kirk Wayne; Veletsos, Anestis S.The objectives of the studies reported in this dissertation are: (1) to evaluate the dynamic impedance of horizontally inhomogeneous, thin soil layers and of piles embedded in such layers; and (2) to provide improved insight into the response of vertically excited piles bearing on a stratum of arbitrary stiffness. The dissertation consists of two major parts. The first part, reported in Chapters 2 through 5, deals with the analysis of the impedances of inhomogeneous soil layers. Vertical, torsional, and horizontal modes of vibration are investigated. Primary emphasis is placed on assessing the effect of the radial variation of the soil shear modulus. The numerical solutions for layer impedances are obtained for several different radial variations of the shear modulus, and are compared with each other and that applicable to a homogeneous soil. The importance of the inertial effects of the soil zone near the pile and the consequences of discontinuous radial variations of soil modulus are clarified. The layer impedances are then used to evaluate the manner and extent to which the dynamic stiffness and damping capacity of piles may be influenced by the weakening of the soil near the pile-soil interface. The second part of this dissertation, comprised of Chapters 6 and 7, deals with the dynamic impedance of a vertically excited pile which is driven through a uniform layer and bears on a stratum of finite depth and arbitrary stiffness. The solution takes due account of the variation of the soil reactions with depth by expressing them in terms of the characteristic functions of the two-layer medium. Comprehensive parametric studies are made to assess the effects of the various parameters involved and to evaluate the applicability of the solution based on the idealization of the soil medium as a series of thin horizontal layers. Comparisons are also made with previously reported solutions for statically and dynamically loaded piles. The results provide improved insight into the mechanism of soil-pile interaction.Item Dynamic response of certain elastoplastic systems of the shear beam type(1967) Sung, Leonard Chiuan-Lin; Veletsos, Anestis S.; Vann, W. P.A quantitative study is made by means of a digital computer of the dynamic behavior of certain elastoplastic systems of the shear beam type. The systems considered have from two to five degrees of freedom and particular distributions of mass, stiffness, damping, and yield level characteristics, but wide ranges of the fundamental frequency and the magnitude of the yield level are examined. The systems are subjected to two prescribed motions of the base, a simple pulse and an earthquake record. The response of each elastoplastic system is compared to that of an associated elastic system. Particular attention is given to the following aspects of the response: the location of the spring in which the maximum deformation occurs, the magnitude of the maximum deformation, and the effect of the number of degrees of freedom on the maximum deformation trends.Item Dynamic response of force-excited systems(1999) Ghosh, Rupak; Veletsos, Anestis S.The elastic and inelastic responses of the force-excited single-degree-of-freedom (SDOF) systems are investigated from the gross characteristics of forcing functions such as the frequency content, the integrals and the amplitudes. The studies are carried out initially with simple version of force-excitation and then, using progressively complicated version of forcing functions having more than one frequency. The forces are chosen in such a way that they resemble the simple version of wave-induced forces, thus, paying a special attention to compliant systems. The responses due to actual wave-induced forces are also studied considering the surface record from the Gulf of Mexico during Camille hurricane (August, 1969). The responses are displayed in tripartite response spectra and explained from the characteristics of the forces. The effect of the frequency content in the forces and their integrals are addressed while interpreting the results. The studies are also made based on the parameters such as damping of the systems and the periodicity of forcing functions. An attempt is made to approximate the elastic responses from the characteristics of the integrals of forcing functions in low and low-medium frequency region. The inelastic responses are studied based on the gross characteristics of forcing function and they are compared with the elastic responses. Low-frequency systems of high ductility and low ductility are given a special attention due to current interest in compliant systems.Item Dynamic response of hazardous liquid-waste storage tanks used in nuclear facilities(1996) Padmanaban, Shivakumar; Veletsos, Anestis S.This dissertation presents a comprehensive study of the dynamic response of storage tanks containing high-level radioactive wastes in nuclear facilities. Of the many issues that are peculiar to these systems, four important ones have been studied. Accordingly, the study is presented in four parts. The first part deals with the dynamic response of flexible tanks that are placed inside concrete vaults and are attached to the vaults both at the top and the bottom. The tanks are presumed to be fixed at the base and supported by either a roller or a hinge at the top. The response quantities examined include the natural frequencies of vibration, the hydrodynamic pressures and the induced tank forces. The general trends are established by comparing the response quantities for the top-constrained systems with those for base-excited cantilever systems. The second part deals with the response of rigid and flexible tanks containing stratified liquids. The liquid is considered to be arranged in layers with varying layer thicknesses and mass densities, or to be inhomogeneous with a continuously varying mass density. In addition to the free vibrational sloshing characteristics of the liquid, the responses examined include the vertical displacements at the free surface, and the impulsive and convective components of the hydrodynamic wall pressures and associated tank forces. A simplified analytical procedure that estimates the response quantities for the layered systems from corresponding solutions for homogeneous systems is also presented. The third part presents an exploratory study for assessing the effects of tank-base flexibility on the response of vertically excited liquid storage tanks. A suitable model of the tank-liquid-foundation-soil system is considered, and the natural frequencies, the associated damping ratios and the induced pressures are studied for a wide range of base-flexibility values. The final part estimates the effects induced by the impact of the sloshing liquid on the tank roof. Consideration is given not only to the effects that are induced on the roof but also to the effects that are transferred to the side-wall. Both rectangular and cylindrical systems are considered and the effect of changing the slope of the roof on the induced impact effects is also studied.Item Dynamic response of liquid storage tanks subjected to coherent and incoherent ground motions(1992) Padmanaban, Shivakumar; Veletsos, Anestis S.A comprehensive study of the response of liquid storage tanks subjected to ground induced lateral excitations is presented. Wall flexibility affects the response coefficients of tall tanks significantly. In general, response coefficients for tanks are relatively insensitive to super-structure parameters. A simplified procedure for evaluating inertial interaction effects is considered. While foundation translation dominates the response of broad tanks, foundation rocking is important for tall tanks. Exact values and approximate expressions reveal that the effects of interaction are a function of the relative flexibilities of the medium and super-structure. Effects of ground motion incoherence are evaluated by formal stochastic and approximate semi-deterministic approaches. Kinematic interaction reduces the response of tanks. The reductions are generally small, particularly for tall tanks. Inertial interaction effects are generally substantially larger than corresponding kinematic interaction effects. Inertial interaction normally reduces the response of broad tanks, but can increase the response of tall tanks.Item Dynamic response of simple pile-structure systems(1990) Khurana, Sandeep; Veletsos, Anestis S.This dissertation is composed of two parts. In the first part, a comprehensive study is made of the harmonic response of a rigid mass or a mass-sprint system supported on a pile driven in a homogeneous elastic medium. The objectives are to assess the effects of the various factors that affect the response of the system. The second part deals with the harmonic response of rigid and elastically supported simple systems on pile groups. The main aim is to incorporate pile-to-pile interaction and investigate its effect on response of such systems. The system is shown to respond essentially as a viscously damped single-degree-freedom oscillator. Plots are presented for the amplification factor and the resonant frequency of the system. With the information presented herein significant aspects of the dynamic response of vertically and laterally excited simple structure-pile systems may be evaluated readily.Item Dynamic response of simple systems to periodic forces(1983) Dotson, Kirk Wayne; Veletsos, Anestis S.; Lutes, Loren D.; Austin, Walter J.A study of the response of viscously damped single-degree-of-freedom systems to non-harmonic periodic excitations is presented. The objectives have been (1) to assess the effects of the various factors that affect the response of such systems; and (2) to present information and concepts with which the salient features of the response may be identified readily. The following aspects of the response are examined: (a) the steady-state response, which is the response obtained after the free vibrational component is damped and the resulting motion repeats itself; (b) the absolute maximum response, which is generally obtained prior to the attainment of the steady-state response; (c) the rate of "build-up" of the response; and (d) the effects of possible cessation of the excitation. The factors investigated include the characteristics of the structure and the excitation. Special attention is paid to the behavior of low-frequency systems. For a number of excitations, closed-form expressions are also presented for the steady-state response of undamped systems.Item Dynamic response of soil-wall systems(1997) Giarlelis, Christos M.; Veletsos, Anestis S.A two-part study of the dynamic response to horizontal base shaking of vertical, rigid and flexible walls retaining a uniform soil stratum is presented. The first part deals with available pseudostatic, limit-state methods of analysis; specifically, the Mononobe-Okabe method and its extension by Richards and Elms. Following a detailed review of these methods and of their underlying assumptions, comprehensive numerical data are presented that elucidate the effects and relative importance of the various parameters involved. Next, the design provisions for such systems recommended in different building codes are reviewed, and their interrelationship is discussed. Both unyielding and yielding walls are examined. In the second part of the study, the different experimental programs on retaining walls that have been carried out over the years are reviewed, and the results obtained for selected tests on walls with a non-deflecting base are compared with appropriate theoretical predictions. The agreement between the experimental results and analytical solutions that do provide for the flexibility of the wall is found to be reasonable.Item Dynamic soil pressures on deeply embedded structures(1994) Younan, Adel Helmy; Veletsos, Anestis S.The studies reported herein are motivated by the need to gain improved understanding of the response to earthquakes of deeply embedded and underground cylindrical tanks storing nuclear wastes, and develop rational but simple methods of analysis and design for such systems. The first part of the study, reported in Chapters 2 and 3, is concerned with the dynamic modeling and response of soil-wall systems. Simple approximate expressions for the dynamic pressures and associated forces induced by ground shaking on a rigid, straight, vertical wall retaining a semi-infinite, uniform viscoelastic layer are developed, and comprehensive numerical data are presented which elucidate the effects of the numerous parameters involved. These solutions are then compared with those obtained by use of a popular approximate model, and the accuracy of the model is assessed. It is shown that the model may lead to substantial inaccuracies. The sources of the errors are identified, and a modification is presented that defines correctly the action of the system. The modified model is then used to evaluate the response of more involved soil-wall systems. The second part, comprised of Chapters 4 and 5, deals with the response to horizontal shaking of an upright, circular, rigid vault that is embedded in a uniform viscoelastic stratum of constant thickness and infinite extent in the horizontal plane. Both the vault and the stratum are presumed to be supported on a non-deformable base. Simple approximate expressions for the dynamic pressures and associated forces induced in the vault are developed, and comprehensive numerical solutions are presented. Furthermore, the accuracy of a popular model in which the medium is represented by a series of independent thin layers is investigated. The errors arising from such an idealization are identified, and a modification is proposed that describes correctly the behavior of the stratum. The last part, reported in Chapter 6, deals with the response to a lateral base shaking of a rigid cylinder containing a viscoelastic solid. Simple approximate expressions for the resulting dynamic wall pressures and tank forces are developed, and comprehensive numerical solutions are presented which elucidate the effects of the numerous parameters involved.Item Dynamic soil pressures on embedded walls(1995) Parikh, Vimal H.; Veletsos, Anestis S.A comprehensive study is made of the response to horizontal base shaking of a uniform viscoelastic soil stratum retained by two vertical rigid walls. The system is assumed to experience a space-invariant horizontal motion along its base. Both harmonic and earth-quake ground motions are considered. The study is motivated by the need for a simple method of analysis and numerical data which may be used reliably and cost effectively in the design of such structures. A simple, approximate method of analysis is employed and its accuracy assessed by comparing its predictions with those obtained by a more nearly exact method. The study is presented in two parts. In the first part, the walls are assumed to be fixed at the base, whereas in second part, they are considered to be elastically constrained against rotation. The response quantities examined include the dynamic wall pressures and the associated forces and moments. Comprehensive numerical data are presented which elucidate the effects and relative importance of the numerous parameters involved. The results presented provide a convenient framework for the analysis of more complex soil-wall systems as well.Item Dynamic soil pressures on rigidly and elastically supported embedded vaults(1993) Attia, Magda Youssef; Veletsos, Anestis S.A comprehensive study is made of an approximate model for the computation of the dynamic pressures and associated forces induced by ground shaking on rigid embedded vaults (2, 10). For rigidly supported vaults, it is shown that, due to the lack of shearing resistance of the medium surrounding the vault, the model is unable to predict the forces induced under static conditions of loading, and that it generally leads to much higher results than those obtained by available exact solutions (20). The soil surrounding the vault is hence modelled by a new set of impedances (20). It is shown then that the effect of base flexibility is to reduce the forces induced on massless vaults. Evaluation is also made of the effect of the vault mass. It is finally shown that the difference in the responses computed by the original and modified models decreases in importance with increasing flexibility.Item Effect of out-of-roundness on the dynamic response of liquid storage tanks(1978) Turner, James Ward; Veletsos, Anestis S.The objective of this study is to examine the possible effects that an initial out-of-roundness could produce in the radial displacement response of liquid storage tanks subjected to earthquake ground motions. Theory predicts that such structures vibrate in modes proportional to cos when excited laterally. Model tests conducted recently indicated that the radial response was dominated by functions proportional to cos 3 and cos 4. It was these unpredicted results that inspired the study given here. Two possible response mechanisms are considered. An out-of-roundness could produce higher order cos pressures which in turn produce higher order deflections. This analysis was shown to be deficient in explaining the magnitudes of the experimental results. The other mechanism considered is that filling an initially out-of-round tank removes a portion of its magnitude and that dynamic pressures can cause conditions such that the removed portion is partially recovered. This analysis shows that a high order of initial out-of-roundness such as cos 8 or greater is significantly removed when the tank is filled with water. This is the result of hoop forces which tend to stretch the tank wall, thus removing a portion of the irregularity. Under dynamic excitation, the hydrodynamic pressures are dominated by a function proportional to cos 8. In the region where the hydrostatic pressure is relieved, there is an accompanying reduction in the hoop force which allows the recovery of a portion of that which was removed initially. The experimenters were then most likely measuring this partial recovery for their response function. Due to the small number of radial sensors used in the test, this high order complex response was then erroneously interpreted to be cos 3 and cos 4 dominant. The results of this analysis are of a magnitude comparable to the experimental results indicating that the proper response mechanism was chosen.Item First passage probability for two-mode systems(1979) Chen, Yutang; Lutes, Loren D.; Veletsos, Anestis S.; Holt, Edward C.This study deals with the first-passage time and maximum response statistics of linear systems with one and two degrees of freedom (SDOF and 2-DOF). Emphasis is placed on empirical studies of the first-passage time of 2-DOF systems. The empirical results were obtained from a digital computer simulation, with the Gaussian white noise excitation obtained from a random number generator subroutine. Maximum response distribution is compared with the Gumbel type 1 distribution as well as Poisson crossings, Vanraarcke's and Mark's analytical approximate results. A simple empirical approximation equation is determined for SDOF systems from the study of the probability of first-passage time distribution. This equation is shown to be in good agreement in general, with the empirical data and with numerical results obtained by Roberts. The Gumbel type 1 distribution is compared with the empirical data. It appears to fit quite well for moderate failure levels and diverge for very large failure levels for both SDOF and 2-DOF systems. The results of Poisson crossings, Vanmarcke's and Mark's analytical theories do not give good approximations to the maximum response distribution of 2-DOF systems, particularly for small damping values. A considerable amount of empirical data is obtained for first-passage probability of 2-DOF systems. Emphasis is placed on the limiting decay rate (a) of survival probability, since this has been shown to be a useful parameter to characterize first-passage probability in SDOF systems. In particular, the dependence of a on the barrier level (b) is studied, and similarities with and differences from SDOF results are considered. No general formula is obtained for predicting a from knowledge of b, and system parameters.Item Free vibration of arches considering axial load effects(1976) Aydin, Ali; Veletsos, Anestis S.An analysis of the in-plane, free vibration of uniform, circular arches is made considering the effects of both the axial load and the rotatory inertia in the arch. Both hinged- and fixed-ended arches are considered. From a study of numerical solutions, it is concluded that the axial load significantly decreases the natural frequencies of the predominantly inextensional modes, but may increase the frequencies of the extensional modes, especially of the higher extensional modes. The conditions of occurrence of each case are explained, and for several cases, simple approximate formulas are presented with the aid of which the natural frequencies of such systems may be estimated readily. In the course of this investigation, a study has also been made of the buckling load of the arch considering the extensibility of the arch axis. The buckling loads of fixed- or hinged-ended arches with 9° angle of opening are presented as a function of the slenderness ratio. The work reported herein is complementary to earlier studies carried out at Rice University on this subject. In the other studies, the effects of the axial load were not considered.Item Higher order equivalent linearization in random vibration(1979) Hseih, Jeder; Lutes, Loren D.; Merwin, John E.; Veletsos, Anestis S.This study considers third-order linear systems and a two mode linear system as candidates for equivalence to the bilinear hysteretic system subjected to a Gaussian white noise excitation. In each linear system some parameters are chosen to match stationary mean squared levels of both displacement and velocity response to those of the bilinear hysteretic system (using empirical values for the nonlinear system). The particular types of equivalence being sought here involve the power spectral density (psd) of stationary response and transient buildup of mean squared response. The values predicted by the linear models are compared with empirical data for the nonlinear system. The bilinear hysteretic systems considered have the slope ratio between post-yielding and pre-yielding stiffnesses of a - 1/2 (moderately nonlinear system) and a = 1/21 (nearly elastoplastic system). Two ways of choosing the parameters of the third-order linear system are considered. One method involves choosing two damping factors for equivalence and the other involves choosing one damping factor and one spring stiffness for equivalence. The two mode linear system is a model with two uncorrelated modes whose undamped natural frequencies correspond to the initial stiffness and the reduced stiffness of the bilinear hysteretic system, and with two damping factors chosen for equivalence. Based on the comparisons of both psd and transient response, it is concluded that the third-order system is the better equivalent linearization overall (arbitary a), although the two mode linear system is somewhat better in the particular case of a nearly elasto-plastic system.Item Seismic response of uplifting liquid storage tanks(1992) Malhotra, Praveen Kumar; Veletsos, Anestis S.A systematic study is carried out on several aspects of the dynamic response of liquid containing, cylindrical steel tanks that may experience partial uplifting of their base during intense ground shaking. The tanks are presumed to rest on a rigid base and to be subjected to unidirectional horizontal ground motion. The objective is to formulate a method of analysis with which the dynamic response of such systems may be evaluated reliably and cost-effectively. The study consists of two parts: the first deals with the static uplifting resistance of the flexible base plate, and the second deals with the dynamic response of the uplifting system. In Chapters 2 and 3, an insight into the behavior of the uplifting plate is gained with the help of a prismatic beam solution. In Chapter 2, the solution is implemented 'exactly' whereas, in Chapter 3 it is implemented approximately by use of the Ritz energy procedure with a set of judiciously selected deflection shape functions. In Chapter 4, the 'exact' solution of the axisymmetrically uplifted plate is obtained. A detailed comparison is then made between the plate and the beam solutions in order to ascertain the suitability of the beam model for the plate analysis. The insight gained from this comparison is then utilized in the formulation of an approximate method of analysis for the solution of an asymmetrically uplifted plate. In obtaining the beam and plate solutions, due consideration is given to the effects of membrane forces associated with large deflections, plastic yielding in the base plate, and the restraining action of the tank wall at the plate boundary. In Chapter 5, an approximate method of analysis is formulated for efficiently computing the seismic response of uplifting tanks. Both unanchored tanks and partially anchored tanks for which the number of anchor bolts at the base is insufficient to ensure full fixity are considered. The responses examined include the hydrodynamic pressures, the base uplift at critical locations, the plastic rotation at the plate-shell junction, and the maximum compressive stress in the tank wall. The parameters that are varied include the intensity of ground shaking, the geometric and material properties of the tank, and the degree of base fixity.Item Simple equivalent linearization of yielding MDF structures subjected to random excitation(1984) Hsieh, Chang Chun; Lutes, Loren D.; Holt, Edward C.; Veletsos, Anestis S.A simple equivalent linear multi-degree-of-freedom (MDF) model is proposed to approximate the stationary rms response levels of yielding MDF structures subjected to Guassian random base excitation. The linear model is characterized by an equivalent stiffness for each yielding element, and a damping matrix which is proportional to that in the unyielded system. The yielding systems under study herein have nearly elasto-plastic bilinear hysteretic elements. No exact analytical solutions are known for such systems. A semi-empirical approach is used to determine the equivalent parameters of the simple model used here. The equivalent stiffness values used here are not based on response velocity values, but rather on the distribution of rms distortion over the height of the structure. The new equivalent damping is taken as r times the original damping, and the factor r is evaluated using the assumption that the damping matrix affects the magnitudes of rms story distortions,- but not their distribution over the structure. With the equivalent stiffness and damping, the Lyapunov covariance matrix equation of the simple linear model is then solved to compute the stationary rms story distortions. Overall, the analytical results thus obtained by this model appear to agree quite well with simulation values. An apparently satisfactory analytical relationship is found between story distortion and equivalent stiffness. Further study will be required to find a corresponding relationship for the damping factor r. It is shown, however, that the equivalent damping is quite small for a variety of MDF systems.Item Soil-structure interaction effects of simple structures supported on rectangular foundations(1992) Wu, Wen-Hwa; Veletsos, Anestis S.This thesis deals with the effects of soil-structure interaction, both kinematic and inertial, on the responses of seismically excited rectangular foundations and simple structures supported on such foundations. The ground motion considered is defined stochastically by a local power spectral density function and a spatial incoherence function. The structures examined are considered to have one lateral and one torsional degree of freedom in their fixed-base condition. The response quantities investigated include the ensemble means of the peak values of the lateral and torsional components of the foundation input motion and of the associated structural deformations. These responses are evaluated and compared with those obtained for no soil-structure interaction and for kinematic interaction only. The information and concepts presented elucidate the nature and relative importance of the two effects and make it possible to assess readily the influences of the more important parameters involved.Item Studies of dynamic response of a model of a compliant offshore platform(1991) Horng, Jyhkuen; Veletsos, Anestis S.Using simple structural and hydrodynamic models, a parametric study is made of the effect of the various parameters that influence the dynamic response of a compliant offshore structure in 2,000 ft of water. The parameters examined include the effects of different sea states, the stretching scheme used to approximate the near surface fluid kinematics, the horizontal variation of fluid kinematics; the effects of fluid-structure interaction and current; and the number of modes considered in the computation of the response. The dynamic responses of the structure, including maximum displacements, shears, and bending moments, are presented both in tabular and graphic forms. The sensitivity of the responses to the various parameters involved is examined and the practical implication of the results is discussed.Item Studies of dynamic response of structures to wave loadings(1983) Bordinhao, Ricardo de Carvalho; Veletsos, Anestis S.; Lutes, Loren D.; Austin, Walter J.Studies of the dynamic response of structures to wave-induced forces are presented. The objective has been to assess the effects and relative importance of some of the factors that affect the response and to evaluate the sensitivity of the response to some of these factors. The structures are modeled as viscously damped single-degree-of-freedom systems, and the wave forces are defined by Morison's equation using a Pierson-Moskowitz wave spectrum and linear wave theory to generate the fluid kinematics. Only responses within the linearly elastic range of deformation are investigated. A simpler excitation, composed of a sequence of a few pulses only, also is considered, and its effects are compared with those obtained for the more realistic representation of the wave loading. The responses of the systems are evaluated over a broad range of conditions, and the practical significance of the results are discussed. The factors investigated include the natural frequency and damping of the structure, the relative magnitudes of the inertia and drag components of the wave forces, the initial conditions of the forcing function and the structure, the phase angles involved in the definition of the fluid kinematics, the degree of periodicity of the wave excitation, and the degree of coupling or interaction between the oscillating fluid and the structure. The results are displayed in the form of response spectra.