R-3 Repository :: Browsing by Author "Lutes, Loren D."

Browsing by Author "Lutes, Loren D."

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A characterization of surface roughness
(1982) Abdul Rahim, Abdallah Adel; Merwin, John E.; Lutes, Loren D.; Bourland, Hardy M.
A study is made on the surface finish of five ground specimens in an attempt to improve on their surface roughness characterization. Surface finish is characterized by a single statistic referred to in practice as the Center Line Average height value (CLA). This CLA height value is a measure of the roughness contribution to surface texture. A fourth order, low pass Butterworth filter is used to separate the contribution of roughness, waviness and error of form that are present in the surface texture. Applying this filtering technique eliminates the need of varying meter cutoff lengths to maximize and minimize those contributions. A statistical analysis provided a choice of ranges of cutoff periods where the CLA height values appear to be more representative of the roughness characteristics of the surface texture. Additional studies using other surfaces are necessary to improve on the single number characterization of surface roughness.
A comparison of fatigue life improvements obtained by weld reinforcement and toe grinding
(1981) Mullen, Christopher Louis; Merwin, John E.; Lutes, Loren D.; Bourland, Hardy M.
A study is made of two treatments which help to improve the fatigue life of a welded structure. Experimental data are examined to assess the relative magnitude of the improvements under a bending type of loading. The results of this investigation are discussed in the light of stress distribution information obtained from a finite element analysis of two welds having different weld shapes. Reinforcement is found to achieve reductions in peak stress at the weld toe not only by reducing the angle at which the weld bead meets the plate surface but also by increasing the leg length which reduces the moment developed at the weld toe location. These reductions in stress are accompanied by reductions in strain which, in the presence of repeatedly applied tensile forces, make the weld toe more resistant to the formation and propagation of cracks. Toe grinding is shown to modify the shape and surface finish of the weld toe. These modifications vary significantly with the tool used in the grinding. At low load levels toe grinding with a tool that not only provides a beneficial curvature to the toe but also removes potential crack initiation sites may lead to improvements which are greater than those obtained by reinforcement.
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.
Effect of a slab on the behavior of exterior beam to column connections
(1985) Zerbe, Hikmat Edward; Durrani, Ahmad J.; Lutes, Loren D.; Austin, Walter J.
In reinforced concrete frame buildings, slabs are usually cast monolithically with the floor beams. The currently used recommendations for the design of beam to column connections for seismic loading, however, completely ignore the presence of the slab. Even though, previous studies have shown slabs to have a significant effect on the performance of connections, it has been believed that the test data are not adequate to justify revisions of the design recommendations. In this experimental investigation, the interaction of a slab with exterior beam to column connections is studied. The experimental program consisted of tests under simulated earthquake type loading on seven large-scale exterior beam to column connections. Except for the slab width, which was varied in each specimen, the basic design and configuration of subassemblages remained the same and conformed to the current design criteria. Based on the test results, it is concluded that it may be nonconservative to ignore the slab in the design of connections. It is suggested that a certain width of the slab effectively acts as a part of the floor beam. It is also concluded that at larger levels of interstory drift, transverse beams may not be effective in confining the joint. Ignoring either of these effects can lead to nonconservative connection designs.
Equivalent linearization for random vibrations of a building with yielding girders
(1988) Wang, Chein-Lee; Lutes, Loren D.
The stochastic response of a yielding 2DF system subjected to a stationary Gaussian white noise excitation is investigated. The particular model considered represents a structure with yielding girders, so is more realistic than the common rigid floor model. The responses considered are the stationary root mean square (RMS) distortions of columns and rotations of girders. Simulation results are presented, as well as an equivalent linearization to approximate the response levels. A numerical integration scheme is presented to obtain simulation data on the stationary RMS distortions and rotations. The stationary RMS response is obtained by averaging a long sample time history (assuming ergodocity) of the response after the response has reached stationarity. The equivalent linearization approach is based on a substitute structure concept. Values of linear parameters are determined from simulation data for the SDF system and are functions of ductility. The Lyapunov equation is then written to calculate the stationary RMS response of the linearized system. (Abstract shortened with permission of author.)
Equivalent linearization of a randomly excited yielding oscillator
(1973) Takemiya, Hirokazu; Lutes, Loren D.
Equivalent linearization of bilinear hysteretic systems subjected to a white noise-excitation is attempted by using 2nd and 3rd order linear systems. The bilinear hysteretic systems considered have the slope ratio between the initial and the reduced stiffness of a = 1/2 (moderately nonlinear case) and a = 1/21 (nearly elasto-plastic case). The technique is to match both energy dissipation per unit of time and average frequency between the original system and its equivalent linear system in stationary motion. In the 2nd order linearization these criteria are essentially the same as the requirements from the Krylov-Bogoluibov method. However, special attention is given to the estimation of the hysteretic energy dissipation per unit of time, resulting in improved predictions of stationary levels of root-mean-square displacement and velocity response. Satisfying the above matching criteria does not require explicit specification of the parameters in the equivalent linear system. In this investigation several linearization matching the above criteria are considered. These include: the usual 2nd order linear system, a model with two uncorrelated 2nd order modes whose undamped natural frequencies correspond to the initial stiffness and the reduced stiffness of the bilinear hysteretic system, a 3rd order linear system which has the same stiffness arrangement as the bilinear hysteretic system model but replaces the Coulomb friction slider in the original system by a viscous damper, and a model with two uncorrelated 3rd order modes which have the same root-mean-square displacement. A severe test of the equivalence to the original system is executed by comparing the response power spectral densities. After getting the specified equivalent linear systems, their transient root-mean-square responses are compared with the experimental results. Tor this response analyses, the Rice method is applied for the 2nd order linear systems and the Markov process approach is taken for the 3rd order linear systems. As a result, a correlation between the stationary response power spectral density matching and the transient root-meansquare response matching is found. As a whole, the two-mode 3rd order linear system proves to be the best linearization among those considered herein.
Fatigue analysis for non-normal stochastic stress
(1982) Hu, Sau-Lon James; Lutes, Loren D.; Merwin, John E.; Pharr, George M.
The basic goal of this study is to determine the relationship between the fatigue damage predicted for Gaussian and nongaussian processes when both are analyzed by the rainflow counting method. Due to a lack of references on nongaussian process simulation, some initial effort is devoted to finding a simple way of generating nongaussian processes. In this study, in addition to mean and variance (the usual two parameters of a normal distribution), kurtosis is chosen as a third parameter to indicate the degree of non-normality. A theoretical prediction of the fatigue damage due to a nongaussian process is also obtained for the special situation of a narrow-band process with the exponent in the S-N curve limited to integer values. The effect of non-normality on the empirical rainflow results (for any bandwidth process) is shown to be approximately the same as the effect predicted theoretically for the corresponding narrow-band process. A practical example associated with the fatigue life design of an offshore platform is given. It incorporates non-normality considerations into the design procedure. It is concluded that the effect of non-normality should not be neglected. It is also noted that the influence of nonnormality is affected not only by the kurtosis of the stress process, but also by the slope of the S-N curve (which is a material property).
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.
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.
Random process simulation for stochastic fatigue analysis
(1988) Larsen, Curtis Eliot; Lutes, Loren D.
A simulation technique is described which directly synthesizes the extrema of a random process and is more efficient than the Gaussian simulation method. Such a technique is particularly useful in stochastic fatigue analysis because the required stress range moment, E(R$\sp{\rm m}$), is a function only of the extrema of the random stress process. The family of autoregressive moving average (ARMA) models is reviewed and an autoregressive model is presented for modeling the extrema of any random process which has a unimodal power spectral density (psd). The proposed autoregressive technique is found to produce rainflow stress range moments which compare favorably with those computed by the Gaussian technique and to average 11.7 times faster than the Gaussian technique. The autoregressive technique is also adapted for processes having bimodal psd's. The adaptation involves using two autoregressive processes to simulate the extrema due to each mode and the superposition of these two extrema sequences. The proposed autoregressive superposition technique is found to be 9 to 13 times faster than the Gaussian technique and to produce comparable values for E(R$\sp{\rm m}$) for bimodal psd's having the frequency of one mode at least 2.5 times that of the other mode. A key parameter in the autoregressive model is the correlation coefficient $\rho\sb1$ between adjacent extrema. A linear regression of $\rho\sb1$ on Vanmarcke's bandwidth parameter is presented as a practical description of $\rho\sb1$'s dependence on bandwidth for both unimodal and bimodal psd's. The effect of psd shape on the expected fatigue damage rate is also investigated. For bimodal psd's, the contribution of the two frequency components to the damage rate is determined for frequency ratios from 1.5 to 15. The relative contribution of the two modes is measured by a parameter b which is the ratio of the mean squared value of the high frequency component to that of the other component. It is found that both components must be considered for b values from 0.01 to 10. The effect of high frequency truncation of the psd on the expected damage rate is also studied for two unimodal psd's.
Random vibration of simple models for reinforced concrete structures
(1988) Senthilnathan, Appasamy; Lutes, Loren D.
The dynamic response of stiffness degrading models subjected to random excitations is investigated. Monte Carlo simulation is performed to determine the effect on the response of various system parameters and to provide results for testing the applicability of approximate analytical techniques. The principal focus is on the single degree of freedom Clough model for reinforced concrete structures with white excitations lasting eight nominal periods of the structure. A heuristic equivalent linearization method is proposed to estimate the response of degrading structures through approximate analysis. The equivalent linear stiffness and damping at any time are computed from certain weighted averages of the secant stiffness and the hysteretic energy dissipation associated with limiting hysteresis loops corresponding to the maximum amplitude at that time. The drift component of the displacement is not treated explicitly, but is approximately accounted for by assigning appropriate weighting functions in this scheme. The equivalent linear coefficients are expressed as functions of some measure of the maximum response statistics and are modified at specified increments of time. The problem of predicting such maximum response statistics is also investigated for the simpler process representing the nonstationary response of linear structures. An approximate method is developed and compared with two existing approximate methods. The new method is found to yield reasonably accurate estimates of the mean and the variance of the response maximum with significant improvements in computational efficiency as compared to simulation. The applicability of a closure approach for finding the response of degrading models is also investigated. A modified Gaussian closure procedure is shown to yield good approximations for two simpler problems which embody some aspects of the Clough model: the maximum value problem for linear systems, and the random response of bilinear hysteretic structures. For the Clough model also, this technique has been applied successfully.
Shear strength of prestressed concrete t-beams with welded wire fabric as shear reinforcement
(1985) Robertson, Ian Nicol; Durrani, Ahmad J.; Austin, Walter J.; Lutes, Loren D.
In thin webbed prestressed concrete T-beams, the placement of conventional stirrups is difficult and time consuming. Welded wire fabric appears to provide a relatively economical alternative. In this study, the shear strength of prestressed T-beams reinforced with welded wire fabric as shear reinforcement is investigated. Thirteen large-scale bonded and unbonded prestressed T-beams were tested. The test variables included the type, amount and anchorage conditions of the shear reinforcement. These beams were tested under static third-point loading over a span of eleven feet. In all beams, shear failures occured along shallow diagonal tension cracks at loads in excess of the code predicted values. Anchorage of the welded wire fabric by means of two horizontal wires at top and bottom of the vertical web wires as recommended in the code performed well. However, the quality of the welded wire fabric was found to be critical for satisfactory performance.
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.
Stochastic dynamics of yielding two-story frames
(1980) Brinkman, Carl Rhys; Lutes, Loren D.; Holt, Edward C.; Merwin, John E.
An investigation is performed to determine the response of yielding two-story frames subjected to a white noise ground acceleration. Column stiffnesses of the structural model exhibit a bilinear hysteretic behavior. Empirical data for general yielding systems is obtained by a numerical integration procedure which measures mean square displacement and velocity values at each floor level of the structural model. Results are compatible with known data for linear and nonlinear single degree of freedom systems. Attempts at analytical solutions are limited to two-story frames in which only the first floor yields. The first theoretical solution attempts to predict the behavior of a nonlinear system by modifying stiffness and damping characteristics and studying the resulting system as a linear model. It is found that interstory mean square response is underpredicted with this procedure. The second analytic approach defines an equivalent linear system by matching its response parameters with those obtained from numerical results and calculating frequencies and mode shapes of that equivalent system. Results from this interpretive approach indicate that for systems for which the second slope of the bilinear stiffness relation is a small percentage of the initial slope, a linear system which exhibits the same mean square response levels as the original yielding system does not exist.
Stochastic fatigue of structural steel at a hole
(1988) Sun, Yuanhui; Lutes, Loren D.
Nine steel specimens, each containing a small hole have been tested under three sets of variable amplitude loadings to determine the effect of amplitude variability on fatigue life. Twenty-three constant amplitude tests have also been run at basically four different combinations of mean stress and stress range to determine the S-N curve and to check the variable amplitude test results against a commonly used analytical technique--the Rayleigh approximation. The analytical prediction is based on an S-N curve corrected for the effect of mean stress. The specimens consist of hot rolled A36 Grade 81A steel bar stock of dimension 0.75 inch by 1.50 inch. The loading gives a uniform bending moment about the axis of minimum moment of inertia and the small central hole is orthogonal to this axis. The stochastic loadings used in this study are so-called pseudo-narrowband loadings. The loadings vary from broadband to extremely narrowband. (Abstract shortened with permission of author.)
Stochastic stress history simulation for fatigue analysis
(1983) Zimmerman, James J.; Lutes, Loren D.; Merwin, John E.; Bourland, Hardy M.; Pharr, George M.
The basic goal of this study is to find an alternate, more efficient method of simulating stochastic stress histories for fatigue analysis. Stress histories are generated from power spectral densities made up of either one or two rectangular blocks. The currently popular simulation technique produces a normal signal by summing sine waves with random phase angles. The fatigue damage predicted from stress histories simulated by this method is used as the basis of comparison for three other techniques. Two techniques which simulate correlated, Rayleigh distributed peaks and valleys are investigated. Another technique produces correlated peaks which have S. O. Rice's peak distribution. The rainflow method of cycle counting is used to determine the stress ranges from all the stress histories and Miner's rule is used to predict fatigue damage. It is concluded that fatigue damage from processes having single block power spectral densities can be efficiently and accurately predicted from a sequence of correlated peaks which have the peak distribution. This technique is three to four times faster than the currently popular technique. Simulation techniques which generate Rayleigh distributed peaks are found to be overly conservative in their prediction of fatigue damage. None of the three techniques investigated can be used for simulation of processes with two block power spectral densities. It is also concluded that a stress process cannot be sufficiently characterized solely by the spectral width parameter for fatigue damage predictions.
Stress ranges produced by stochastic loads
(1981) Corazao, Miguel; Lutes, Loren D.; Merwin, John E.; Bourland, Hardy M.; Zettlemoyer, Nicholas
Fatigue damage is an important cumulative effect of random loads, such as those caused by earthquakes, wind and waves. The widely used Palmgren-Miner rule provides a simple procedure to quantify the damage produced by complicated time histories if a satisfactory cycle counting method such as the rain-flow algorithm is available. A cost effective alternative is to assume a probability distribution of stress ranges and to use it to solve the cycle counting problem. The shapes of six commonly used distributions were compared with that of rain-flow stress ranges, and it was found that acceptable results can be expected only in stress histories of narrow to medium band widths. It was also found, however, that the damage computed on the basis of the Rayleigh distribution can be adjusted to provide results that accurately agree with those achieved using the rain-flow method.
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.
Techniques to obtain seismic time histories of coupled systems
(1988) Singhal, Ajay; Lutes, Loren D.
Accuracy and efficiency are investigated for several different methods to obtain time histories of response of coupled linear or nonlinear systems subjected to seismic excitations. Solutions for the composite system of equations are considered, as well as iterative solutions of coupled equations for primary and secondary subsystems. Two new formulations involving stiffness coupling of the subsystems are introduced. For linear systems, the Newmark method for the composite system seems generally to give slightly better results than the primary-secondary methods. For nonlinear systems, the direct stiffness coupled subsystem approach is found to be more efficient than the other methods, especially if the nonlinearity is in one of the smaller subsystems. It is also shown that only Rayleigh damping can simultaneously lead to classical normal modes for the composite and for each subsystem without imposing any restrictions on the stiffness or mass matrices.
The development of transportation in Belize, Central America
(1977) Cooper, Daniel H.; Lutes, Loren D.
The development of Belize with emphasis on the role of transportation is considered. The past and present situations are summarized, including a study of the various economic, social and cultural aspects. The historical growth of transportation is noted and related to the economic development of the country, and the present situation and immediate needs are analyzed. Finally, a plan for future development is presented. In this plan growth concepts are discussed and recommendations are made for efficient interaction between transportation and national development. The historical development of Belize was built around the lumber industry and mercantile activities with Belize City evolving as the center of population, transportation and government. The decline in marketable timber and the absence of natural mineral resources has forced Belize to turn towards agricultural production as a source of export commodities as well as a means for reducing national dependence on imported food-stuffs. The most important potential agricultural areas are located inland and were, until recently, almost totally inaccessible. The immediate transportation needs center around improved highway networks and port facilities, since air travel is developing in an adequate fashion and rail transport does not appear to be justified. There is a clear need for an improved highway network that will allow convenient access to remote parts of the country and provide a means of marketing agricultural produce. In addition an improved highway system that is designed to connect to the existing Central American network would free Belize from its now isolated position. The existing port facilities at Belize City and Commerce Bight are inadequate at present. The principle port at Belize City is required to handle all import and export items by means of lighters, since no deep water port facilities exist. The current governmental proposals for construction of new deep water port facilities at Belize City and Commerce Bight are discussed and possible alternatives examined. Finally, considerations are made concerning the future development of transportation in Belize. Future needs are based on both the continued economic growth of the country and the need for a more complete regional transportation network. The identification of areas of unique economic potential and socio-economic regions is made, along with suggestions for establishing urban growth centers and their corresponding transportation links.

Browsing by Author "Lutes, Loren D."

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