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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 30 Nov 2023 19:29:14 +0000Fatigue Strength of Metal Sandwich Type Construction
https://resolver.caltech.edu/CaltechETD:etd-01282009-095113
Authors: Lamb, William Emerson
Year: 1949
DOI: 10.7907/YEGJ-XT59
An investigation was conducted to determine the fatigue strength of a new type of all metal sandwich panel structure, designed and furnished by Western Engineering Associates of Los Angeles. This structure consisted basically of a new type of embossed core attached by spot welding to one or more smooth sheets of the same metal. Specimens tested were all of the single core, single skin type. Metal used was 24ST aluminum alloy of 0.032 inch thickness. This construction affords high structural rigidity for its weight. Two patterns of the embossed core were tested. One of the specimens had its core embossed with a triangular pattern, the other with a square pattern.
The specimens were tested by utilizing them as simply supported beams, loaded in the center by a sinusoidally varying load of constant maximum magnitude. This load was applied by means of loading bars one on the top of the specimen, the other on the bottom. These bars had flat, one inch wide surfaces, contacting the specimen. To prevent the sharp edges of the bars from causing local failures of the specimens, a layer of one-eighth inch thick koroseal was used between the bars and the specimen. Specimens tested had a length between end supports of 16.7 inches. Their width was 9 inches.
The loading obtained was a combination of bending and shear. The shear stress was of such a low magnitude, however, that it could be neglected.
Failure of the specimens was deemed as occurring at the time the first crack appeared. A method was devised for crack detection that consisted of laying down a conducting strip over a thin insulating layer in a network fashion, covering all saddle points of the core in the central area, since previous testing had disclosed the fact that failure occurred at these points first. Any crack in one of these saddles caused a break in the conducting strip which changed the bias on the controlling electronic tube to a cut off value. This tube was part of an Eccles-Jordan Trigger circuit which with associated tubes allowed current flow through a thyratron relay circuit opening the starting circuit of the testing machine, causing it to stop.
The data shows that for the specimens oriented in a normal fashion, that is, with one of the sides of the square or triangle of the core parallel to the loading bar, which was the situation for most of the tests, the square pattern is vastly superior to the triangular one, as regards fatigue strength. For specimens oriented in this way the square pattern withstood a bending moment of 13.45 inch pounds per inch of width; whereas the triangular pattern withstood only 9.28. Two tests conducted with the square core having the sides of the squares at an angle of forty-five degrees to the loading bars gave results about midway between. However, the effective EI for this configuration was considerably reduced, and consequently actual failure stress was probably about equal to that for the ease where the sides were parallel to the loading bar.
The triangular pattern was actually much worse than the curve shows. At the higher loads cracks occurred with very few cycles of loading. Automatic cut-off feature was not in use for these tests. In service it is highly possible that overloads of short duration might cause small cracks which would become focal points of fatigue failure, thus reducing the fatigue strength well below the design point. This weakness of the triangular pattern arises from the fact that smooth fillets or saddles, joining depressed and elevated portions of the core, are harder to obtain in this pattern than in the square one. Wrinkles and tool marks were present in almost every one of the triangular specimens tested.https://thesis.library.caltech.edu/id/eprint/393A Critical Study of Spin-Up Drag Loads on Aircraft Landing Gears
https://resolver.caltech.edu/CaltechETD:etd-01232009-153114
Authors: Brown, John R.
Year: 1949
DOI: 10.7907/PDPG-ET80
This report attempts to analyze in detail the spin-up drag loads imposed upon an aircraft main landing gear. Other factors in the landing gear problem are ignored except insofar as they affect this one type of loading. As an instrument for study, one model of aircraft was chosen for which extensive flight test and drop test data were available.
The main parameters which enter into the spin-up drag load are the landing weight of the aircraft, the rate of descent at contact, the ground speed, the time interval from initial contact to attainment of maximum vertical load, and the coefficient of friction between the tire and the runway surface. Minor parameters which may affect the drag load are the tire pressure, moment of inertia of the rolling stock, oleo pressure, and quantity of oil in the hydraulic shock absorber.
The results of this study indicate that the maximum gear drag load is primarily a function of the time required to reach maximum vertical load, and that further study of this parameter, using drop test data for several types of aircraft, might well lead to some valuable empirical information essential to landing gear design. The value of the coefficient of friction was seen to vary widely in test landings but a maximum value of 0.55 appears to be satisfactory for limit design calculations.https://thesis.library.caltech.edu/id/eprint/315Correlation of Fatigue Data to Determine Stress Concentration Factors in 76s-T Aluminum Alloys
https://resolver.caltech.edu/CaltechETD:etd-02092009-081322
Authors: Kane, John Clarence
Year: 1950
DOI: 10.7907/FHCF-VX77
<p>The purpose of this study was to determine the stress concentration factor in a shoulder under a cyclic tensile load. The test specimens were models of a propeller blade root section similar to the blade design now employed in the fans of the Cooperative Wind Tunnel. All specimens were given a 5 microinch finish and all were made of 76S-T aluminum alloy taken from an actual propeller blade forging. Two series of tests were conducted. One series consisted of a cyclic tensile load varying from zero to maximum over a nominal stress range from 21,000 psi to 28,000 psi. The other series consisted of a cyclic load, wherein the amplitude of the cyclic stress was varied from 20 to 50 percent of the maximum developed stress. These two conditions would correspond respectively to the "start-stop" cycle of wind tunnel operation and to constant speed tunnel operation.</p>
<p>It was found that the stress concentration factor determined from the fatigue test results closely approximated the results obtained from the three dimensional static tests of the actual blade.</p>
<p>This work was carried out by the authors at the Guggenheim Aeronautical Laboratory, California Institute of Technology, under the supervision of Dr. E. E. Sechler.</p>
https://thesis.library.caltech.edu/id/eprint/579Internal Friction in Metals
https://resolver.caltech.edu/CaltechETD:etd-03272007-080746
Authors: Alford, Jack Leland
Year: 1950
DOI: 10.7907/J7H1-BJ17
A summary of the current knowledge on the subject of internal friction is presented. An experimental investigation of the variation of specific damping capacity with torsional stress for three metals is described. It is found that the specific damping capacity can be represented. Calculations are made of the effect on specific damping capacity of type of stress and of non-uniform distribution of stress; the calculations are compared with the available experimental data. An expression is proposed to describe the manner in which energy dissipation varies with stress amplitudehttps://thesis.library.caltech.edu/id/eprint/1166A Study of Single and Polycrystalline Ingot Iron Under Repeated Stress
https://resolver.caltech.edu/CaltechETD:etd-03272007-130713
Authors: McClintock, Frank Ambrose
Year: 1950
DOI: 10.7907/A49D-6315
Specimens of single and polycrystalline ingot iron were tested in a rotary bending fatigue machine and examined microscopically in order to check previous observations of slip and to study the nature of crack formation and growth. As has been reported, slip takes place on surfaces of high shear stress parallel to a <111> direction in the crystal lattice. In contrast to recent work, the slip was found to occur on wavy surfaces rather than pairs of crystallographic planes.
Fatigue cracks were found to follow surfaces of high shear stress containing a <111> direction. The conclusion that fatigue failures are primarily shear failures is strengthened by a statistical explanation of the mechanism by which a microscopic shear failure can lead to the apparent tension failure commonly observed in rotary bending tests on polycrystalline iron. Microscopic observation of the polycrystalline specimens revealed that cracks originate at the grain boundaries before 30,000 cycles in a specimen which would very probably run for at least 106,000 cycles.https://thesis.library.caltech.edu/id/eprint/1174Transient Response of a Multi-Frequency Reed Gage
https://resolver.caltech.edu/CaltechETD:etd-03302009-082744
Authors: Shapiro, Henry
Year: 1952
DOI: 10.7907/1V1D-ET24
The purpose of this thesis is to extend the usefulness of a multifrequency reed gage, a simple mechanical instrument, which records essential features of transient motions. This instrument, which can be considered a dynamic model of a multiple degree of freedom structure, consists of e set of one degree of freedom cantilever beams having various frequencies. The information from the instrument consists of the maximum positive and negative displacement, of each beam. It is desired to obtain as many features as possible about the exciting motion from this information alone.
The instrument response is obtained analytically for various exciting pulses. Both symmetrical pulses and non-symmetrical pulses with rise time to decay time ratios from zero to infinity are considered. The impulsive response is considered separately.
From a study of these responses a correlation is made between the response and the excitation. Table #5, page 32, summarizes the information about these studies and indicates the methods for obtaining, maximum information from the reed gage.
https://thesis.library.caltech.edu/id/eprint/1212Fatigue Stress Concentration Studies on Aluminum Alloys
https://resolver.caltech.edu/CaltechETD:etd-03232009-161109
Authors: Dervishyan, Aram Ohannes
Year: 1952
DOI: 10.7907/4BCE-M652
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Fatigue tests in reversed bending were conducted in 75S-T6 and 17S-T6 aluminum alloys to determine whether Neuber's theory on fatigue stress concentration factors in notches was applicable to these materials. The results of the tests indicate confirmation of the theory within engineering accuracy providing the value of […] (a material constant) is determined experimentally.
Material size effect was investigated and found to exist. This was an unexpected result since other sources (see References 3 and 4) indicated that no size effect existed for aluminum alloys.
Ignoring the correction due to size effect a value of the Neuber's constant […] of approximately 0.05" gave reasonable checks with the experimental data for both 75S-T6 and 17S-T6 aluminum alloys. This may indicate that this value of […] is the correct material constant for aluminum alloys but additional data on other alloys is needed to confirm this conclusion.https://thesis.library.caltech.edu/id/eprint/1082Transient Response of Uniform Beams
https://resolver.caltech.edu/CaltechETD:etd-04212003-111122
Authors: Anderson, Roger Alan
Year: 1953
DOI: 10.7907/ZVM1-7T63
Several special topics relating to the transient flexural vibrations of a uniform beam predicted by the usual elementary or Bernoulli-Euler equation are discussed. The effect on the beam response of the concentration of an applied transient force in space and in time is studied. In the case of an applied step force, it is shown that the dynamic team response can be larger than twice the response to an equal force statically applied. It is demonstrated that the beam response in the higher modes is independent of the boundary conditions.
A new, general series solution of the mode superposition type is given for the flexural vibrations of a uniform beam according to the more refined Timoshenko equations including the secondary effects of shear deflections and rotatory inertia. As a special case, the solution is presented for a pin-ended beam. These solutions are characterized by two series, each of the form of the series solution of the Bernoulli-Euler equation.
For the special case of a concentrated transient force applied at the midpoint of a pin-ended beam, the bending moment and shear force solutions for the Timoshenko and Bernoulli-Euler equations are compared.
https://thesis.library.caltech.edu/id/eprint/1432The Existence and Stability of Periodic Motions in Forced Non-Linear Oscillations
https://resolver.caltech.edu/CaltechETD:etd-04072003-115022
Authors: Caughey, Thomas Kirk
Year: 1954
DOI: 10.7907/6MNJ-8N71
A general first order theory is presented for treating forced oscillations in multiple degree of freedom quasi-linear systems. It is shown that under suitable conditions ultraharmonic or subharmonic motion may exist in addition to the harmonic motion which a linearized theory would predict. A general study of the stability of such motions reveals that a sufficient condition for the instability, and consequent jump phenomena, of forced oscillations, is that the amplitude frequency response curves possess a vertical tangent. By considering some fairly general two degree of freedom systems it has been shown that a necessary and sufficient condition for stable forced oscillations in non-linear passive systems is that the amplitude lie outside the region enclosed by the loci of vertical tangency. For systems containing an energy source there is, in addition, a restriction on the magnitude of the non-linear damping force.
The general theory has also been applied to ultraharmonic and subharmonic motion in a one degree of freedom system having a cubic non-linearity in the restoring force. It has been shown here also, that a necessary and sufficient condition for stability is that the amplitude of forced oscillation lie outside the region enclosed by the loci of vertical tangency.
A study of the dependence of the motion on the initial conditions reveals that, while ultraharmonic and harmonic motions are relatively insensitive to the initial conditions, the existence of subharmonic motion can be achieved only for a rather restrictive set of initial conditions.https://thesis.library.caltech.edu/id/eprint/1283Transient Response of Non-linear Spring-Mass Systems
https://resolver.caltech.edu/CaltechETD:etd-12032003-092544
Authors: Ergin, Erdem I.
Year: 1954
DOI: 10.7907/0528-K973
The purpose of this thesis is:
1) To investigate the applicability and to compare the accuracy of existing perturbation methods of non-linear mechanics for the solution of transient response problems, and
2) To describe a new analytical approximate method for the solution of certain types of non-linear problems involving pulse excitation. This new method combines the advantages of engineering accuracy with ease of applicability.
In the course of this study it is found that the solution of homogeneous non-linear equations can be obtained readily and with sufficient accuracy by the perturbation methods of Kryloff and Bogoliuboff or Lindstedt, even for large nonlinearities. Greater accuracy can be attained by the use of the newly developed bi-linear approximation. The advantage of the bi-linear method becomes more pronounced when the step function or the single pulse response of the system is investigated. It is shown that the bi-linear method is the only convenient analytical approximate method available for the solution of general pulse excitation problems involving non-linear spring-mass systems.https://thesis.library.caltech.edu/id/eprint/4730An Investigation of Ground Accelerations Produced by Machines
https://resolver.caltech.edu/CaltechETD:etd-12032003-094611
Authors: Flygare, Richard Wilson
Year: 1955
DOI: 10.7907/H63X-5M60
Machines with reciprocating or rotating parts initiate stress waves which propagate through the ground and produce measurable ground motions in the vicinity of the machines. In many respects these ground motions may be objectionable. Consequently it is desirable to have a body of precise information on the ground motion produced by various common machines.
Measurements of the ground acceleration produced by drop hammers, punch presses, railway trains, heavy automotive trucks, a cutting shear, a hydraulic press, and a pneumatic hammer were made. The vertical and two horizontal components of acceleration were measured at various distances from the source. The maximum ground acceleration component observed was 64.4 feet per second per second near a drop hammer. Ground accelerations as great as 1.6 feet per second per second were measured at a distance of one hundred feet from the source.
The influence of ground acceleration on equipment, on machines, and on human beings is largely dependent on the maximum value of the ground acceleration but also may be markedly influenced by the character of the ground motion. The detailed character of the ground acceleration varies widely with the different sources of the motion, with distance from the source, and with the geology of the transmitting medium. To cover this aspect of the problem sixty-two acceleration time records of the ground motion are reproduced in the thesis.https://thesis.library.caltech.edu/id/eprint/4733Transient Motion Measurements for Structural Response Analysis
https://resolver.caltech.edu/CaltechETD:etd-06252004-104122
Authors: Rubin, Sheldon
Year: 1956
DOI: 10.7907/ND1B-3V27
The theory of the design of instruments suitable for the measurement of the acceleration-time history of a transient motion is considered. It is assumed that the primary purpose of the acceleration-time record is the calculation of structural response. As a criterion of instrument behavior, it is proposed that the correct reproduction of the response spectrum of the transient motion be adopted. The application of this response spectrum criterion to some selected acceleration pulses indicates that the optimum instrument parameters for a transient measurement are essentially the same as for the measurement of periodic motions.
A general theory for the multiple degree of freedom seismic accelerometer characteristics is established. The particular case of a two degree of freedom accelerometer is studied and it is shown that the sensitivity to acceleration can be increased to about five times that of a single degree of freedom instrument having the same useful frequency range.
An instrument which can be used to obtain structural response information directly, without the necessity of an acceleration-time recording is studied. This instrument, the reed gage, gives directly information concerning the transient response of a single degree of freedom system or of individual modes of a complex structure. The total response of a complex structure can be determined from reed gage information by the superposition of the maximum responses in the individual modes. An analysis is made of the errors in total response inherent in this superposition process. It is found that in many practical problems the distribution of mode frequencies and the form of the excitation is such that the total error due to superposition is not a significant factor in the problem.https://thesis.library.caltech.edu/id/eprint/2724The Dynamic Response of Bilinear Hysteretic Systems
https://resolver.caltech.edu/CaltechETD:etd-10052004-162528
Authors: Iwan, Wilfred Dean
Year: 1961
DOI: 10.7907/Y32J-KT73
A study is made of the dynamic response of one and two degree of freedom systems having a bilinear hysteretic restoring force. In the case of the one degree of freedom system exact steady state solutions are obtained for both square wave and trigonometric excitation. It is thereby shown that the system exhibits a soft type resonance and that there exists a critical level of excitation above which the system displays unbounded resonance. An approximate steady state theory for the one degree of freedom system is investigated and on the basis of this theory it is found that the system is stable and possesses a single locus of vertical tangency. The results of the exact and approximate steady state theories are supplemented by electric analog studies of both the harmonic and ultraharmonic response.
The response of the one degree of freedom system to transient excitation of finite duration is also examined and it is noted that certain rather general conclusions may be made about the final state of the system without reference to the specific time history of the excitation.
A first order approximate theory for the steady state response of the two degree of freedom system is formulated and it is shown that there are two critical levels of excitation for unbounded resonance. The existence of loci of vertical tangency is demonstrated and the stability problem is treated in limiting cases. Direct numerical integration of the equations of motion is carried out for a number of specific cases as a check of the approximate theory.https://thesis.library.caltech.edu/id/eprint/3924Mode Superposition Methods Applied to Linear Mechanical Systems Under Earthquake Type Excitation
https://resolver.caltech.edu/CaltechETD:etd-04132006-153621
Authors: Merchant, Howard Carl
Year: 1961
DOI: 10.7907/KMKT-B363
The determination of the maximum dynamic responses of a multidegree of freedom mechanical system under earthquake type excitation using mode superposition methods is the general problem considered. The experimental work was carried out using a special purpose electronic differential analyzer involving a three degree of freedom system, or a three mode approximation to a larger system.
The results indicate that a suitably weighted average of the sum of the absolute values and the square root of the sum of the squares of the individual mode contributions gives a practical design criterion for the base shear forces. For critical designs this weighted average reduces to the absolute sum of the modes, which will be close to the true value for a significantly high percentage of the cases. The base moment may be more accurately approximated than the base shear by use of the first mode alone.https://thesis.library.caltech.edu/id/eprint/1374Post-elastic dynamic response of mild steel structures
https://resolver.caltech.edu/CaltechETD:etd-04152003-093000
Authors: Hanson, Robert Duane
Year: 1965
DOI: 10.7907/76XX-4687
The current philosophy of earthquake resistant design is that a structure should be able to withstand an extremely strong motion earthquake without collapsing, even though a certain amount of damage is incurred. To make such a design requires a knowledge of the dynamic behavior of structures under large amplitude vibrations. The objective of the work reported here was to investigate the dynamic frequency response characteristics of a mild steel structure vibrating in the plastic range. Comparisons were made of the static, the dynamic, and the theoretical responses of the yielding structure.
The behavior of single-story structures having structural steel columns was investigated experimentally by means of horizontally applied forces generated by a shaking machine. The experimentally determined dynamic response showed the decrease in resonant frequency for increases in deflection amplitude which is characteristic of a "softening spring" type of nonlinearity. Ultraharmonic response was also observed. Under steady-state oscillations the fraction of equivalent viscous damping varied from 0.0016 at small, elastic deflections to 0.089 at large, plastic deflections. Structural deterioration was observed in both the static and dynamic experiments with a recovery of strength occurring between tests. A completely stable hysteretic loop was not attained at large deflection amplitudes, and the hysteresis loop did not become completely stable until the deflection amplitude was reduced to almost the initial yield value. However, the change in the hysteretic loop per cycle for the large deflections was small enough to permit assuming that a steady-state dynamic condition existed over a limited number of cycles.
Discrepancies between the theoretical and the experimental virgin force-deflection curves were found and these are shown to be the result of simplifying approximations introduced in the structural analysis. It was found that the resonant vibrational amplitude of the structure can be predicted within 20%, and the resonant natural frequency within 2-1/2%, on the basis of the static virgin force-deflection curve. For large, plastic deflections at an excitation frequency of 3 cps, it was found that the differences between the dynamic and the static hysteresis loops were less than the changes in the static loops resulting from the deterioration caused by repeated cycles of loading.https://thesis.library.caltech.edu/id/eprint/1390Studies of response to earthquake ground motion
https://resolver.caltech.edu/CaltechTHESIS:09152015-111110090
Authors: Brady, Arthur Gerald
Year: 1966
DOI: 10.7907/48HF-MX48
<p>A study is made of the accuracy of electronic digital computer calculations of ground displacement and response spectra from strong-motion earthquake accelerograms. This involves an investigation of methods of the preparatory reduction of accelerograms into a form useful for the digital computation and of the accuracy of subsequent digital calculations. Various checks are made for both the ground displacement and response spectra results, and it is concluded that the main errors are those involved in digitizing the original record. Differences resulting from various investigators digitizing the same experimental record may become as large as 100% of the maximum computed ground displacements. The spread of the results of ground displacement calculations is greater than that of the response spectra calculations. Standardized methods of adjustment and calculation are recommended, to minimize such errors. </p>
<p>Studies are made of the spread of response spectral values about their mean. The distribution is investigated experimentally by Monte Carlo techniques using an electric analog system with white noise excitation, and histograms are presented indicating the dependence of the distribution on the damping and period of the structure. Approximate distributions are obtained analytically by confirming and extending existing results with accurate digital computer calculations. A comparison of the experimental and analytical approaches indicates good agreement for low damping values where the approximations are valid. A family of distribution curves to be used in conjunction with existing average spectra is presented. The combination of analog and digital computations used with Monte Carlo techniques is a promising approach to the statistical problems of earthquake engineering.</p>
<p>Methods of analysis of very small earthquake ground motion records obtained simultaneously at different sites are discussed. The advantages of Fourier spectrum analysis for certain types of studies and methods of calculation of Fourier spectra are presented. The digitizing and analysis of several earthquake records is described and checks are made of the dependence of results on digitizing procedure, earthquake duration and integration step length. Possible dangers of a direct ratio comparison of Fourier spectra curves are pointed out and the necessity for some type of smoothing procedure before comparison is established. A standard method of analysis for the study of comparative ground motion at different sites is recommended. </p>
https://thesis.library.caltech.edu/id/eprint/9154Transmission matrices and lumped parameter models for continuous systems
https://resolver.caltech.edu/CaltechTHESIS:10192015-155737101
Authors: Rocke, Richard Dale
Year: 1966
DOI: 10.7907/4K3H-1E57
<p>The use of transmission matrices and lumped parameter models for describing continuous systems is the subject of this study. Non-uniform continuous systems which play important roles in practical vibration problems, e.g., torsional oscillations in bars, transverse bending vibrations of beams, etc., are of primary importance.</p>
<p>A new approach for deriving closed form transmission matrices is applied to several classes of non-uniform continuous segments of one dimensional and beam systems. A power series expansion method is presented for determining approximate transmission matrices of any order for segments of non-uniform systems whose solutions cannot be found in closed form. This direct series method is shown to give results comparable to those of the improved lumped parameter models for one dimensional systems.</p>
<p>Four types of lumped parameter models are evaluated on the basis of the uniform continuous one dimensional system by comparing the behavior of the frequency root errors. The lumped parameter models which are based upon a close fit to the low frequency approximation of the exact transmission matrix, at the segment level, are shown to be superior. On this basis an improved lumped parameter model is recommended for approximating non-uniform segments. This new model is compared to a uniform segment approximation and error curves are presented for systems whose areas very quadratically and linearly. The effect of varying segment lengths is investigated for one dimensional systems and results indicate very little improvement in comparison to the use of equal length segments. For purposes of completeness, a brief summary of various lumped parameter models and other techniques which have previously been used to approximate the uniform Bernoulli-Euler beam is a given. </p>
https://thesis.library.caltech.edu/id/eprint/9230Vibration tests of a multistory building
https://resolver.caltech.edu/CaltechETD:etd-09302002-101427
Authors: Kuroiwa, Julio Horiuch
Year: 1967
DOI: 10.7907/NW3D-FE77
<p>Vibration tests were performed on a 9 story reinforced concrete building with basement, in order to investigate its dynamical characteristics, by exciting the building with 2 vibration generators installed on its 9th floor.</p>
<p>The natural periods of vibration, the value of the damping, and the mode shapes, in the N-S and E-W directions and in torsion, were determined by measurement. Before the main part of the testing was carried out, some preliminary tests were made to check the correctness of some assumptions which would simplify the main test procedure.</p>
<p>It was possible to investigate in detail only the first mode of each type of motion, because of the relatively high rigidity of the building and a limitation on the maximum frequency at which the shakers could be driven.</p>
<p>The periods measured were quite short for a 9 story building, 0.505 sec in the N-S direction, 0.662 sec in the E-W direction, and 0.346 sec in torsion, and their values increased by about 3 per cent when the tests were performed at the highest force levels.</p>
<p>The damping, which consistently increased as the exciting force increased, varied between 0.70 and 2.00 per cent of the critical viscous damping. The periods and damping values were also determined at very low force levels by exciting the building with a rhythmical movement of the operator's body. The periods measured in this way were slightly smaller than those found using the shakers, and the damping varied between 0.6 and 0.9 per cent of the critical viscous damping.</p>
<p>The mode shape did not seem to be well defined for the lower force levels, but after the force level reached a certain minimum value, the normalized mode shape remained unchanged, both with further increases in the forces, and with changes in the frequency of excitation. However, in both the N-S direction and in torsion, the horizontal displacements of the first and basement floors consistently increased, on the order of 3 per cent with respect to the displacements of the upper floors as the exciting force increased.</p>
<p>Some aspects of the dynamical behavior of buildings, which have not been studied by other investigators in previous tests, were examined. It is a common practice in the seismic analysis of structures to assume that the floor systems act as rigid diaphragms when the building is acted upon by horizontal forces, and also to assume that the structure is fixed at the ground level. It was found that the first assumption was correct, but instead of the second it is more accurate to assume that the building is fixed at the foundation, and not at ground level.</p>
<p>The vibration of the ground in the vicinity of the building was also measured, together with the vibrations of the basement and first floor. It was also possible to measure the acceleration at the top of one of the units of the air conditioning equipment located on the roof. The acceleration at the top of this unit was about 8.5 times that of the roof.</p>https://thesis.library.caltech.edu/id/eprint/3825The response of nonlinear multi-story structures subjected to earthquake excitation
https://resolver.caltech.edu/CaltechETD:etd-09182002-085426
Authors: Giberson, Melbourne Fernald
Year: 1967
DOI: 10.7907/W8D4-3W95
The dynamic responses of a 20-story nonlinear structural frame representative of a modern high rise building are analyzed with the aid of a digital computer. Related analytical studies of continuous systems are carried out. Quantitative information is provided on the importance of a wide range of modes to the various responses of a multistory structure during an earthquake. The effect of yielding on the response is observed. The magnitude of the structural responses are compared with common measurements of earthquake strength.
At the ends of each girder and column of the structural frame are yield hinges which have bilinear bending moment-rotation hysteretic characteristics. Two beam models having such characteristics are studied; one of these models can treat curvilinear hysteretic behavior. Three definitions of ductility factor are discussed, one of which is applicable to both bilinear and curvilinear hysteresis loops. In the computer program, the frame is subjected to the time history of an earthquake accelerogram, the equations of motion are stepwise integrated, and the various structural responses - displacement, bending moments, incurred yielding, etc. - are determined.
The agreement between the response parameters resulting from excitation by seven different earthquake ground motions indicates that these response characteristics are determined more by the properties of the structure than by the earthquake. These results throw some light on extreme value statistics of the response of yielding structures subjected to earthquakes. The characteristic patterns observed in the computed responses of the nonlinear structure can be related to analytical studies of linear elastic, shear-type, uniform and tapered continuous cantilever beamshttps://thesis.library.caltech.edu/id/eprint/3604Strong Motion Accelerograph Evaluation
https://resolver.caltech.edu/CaltechTHESIS:06082017-105754537
Authors: Peters, Rex Bredesen
Year: 1969
DOI: 10.7907/TEJJ-XX37
<p>A brief study is made of the effect of common instrument
errors on the accuracy of data obtained from strong motion earthquake
accelerographs. Error sources considered include zero drift,
tilts, nonlinearities, cross-axis sensitivity, lack of initial conditions,
noise, and time base errors. It is concluded that most data of current
engineering interest are not critically affected by the level of errors
found in existing accelerographs. Techniques are suggested for
reducing or eliminating many of these errors by instrument design
changes.</p>
<p>An experimental study is made of a new strong motion
accelerograph during its engineering development. This new accelerograph
is designed to record an FM analog of ground acceleration
on magnetic tape, providing a record which may be rapidly and
automatically converted to digital form. The accuracy limits of the
accelerograph are explored and the design reasons for these limits
investigated. The more significant findings may be briefly
summarized:</p>
<p>(1) Static accuracy. The sensitivity and linearity of the instrument
are found to depend critically on a series of interdependent adjustments.
Reasonable care will bring errors in both of these
quantities to within ±2% of 1/2 g full scale. Higher accuracies
are possible, but require much more time and care, primarily
due to the limiting effect of mechanical drift in the accelerometers.</p>
<p>(2) Zero point drift. Uncertainties in the accelerometer zero
point arise from both mechanical and electronic drifts. Long
term drifts may be related to temperature or relative humidity, or
may be entirely random. Short term drifts of up to 2% of full scale
may occur during the course of a typical record. The total
variation may be as much as ± 30% of full scale for a 100°F range
of temperatures. These variations require adjustment of the data
before processing, but are not sufficient to interfere with operation
of the accelerograph.</p>
<p>(3) Noise. Random noise in the system as tested amounted to
1.4% of full scale, RMS, and was mostly due to the tape recording
system. By comparison with optical accelerographs, this noise
figure is marginal, but acceptable, and can be improved by changes
to the compensation system.</p>
<p>(4) Timing. The advantages of an effectively continuous time base
over discrete time marks were discovered and means devised to
obtain such a base from the test accelerograph. This method of
timing is a qualitative improvement over the best system which is
practical on optical recorders.</p>
<p>The overall performance of the test accelerograph is
adequate to yield acceptably accurate acceleration vs. time records
and Fourier spectra within the range of frequencies which are of
current engineering interest. It is able to produce useful displacement
records only for periods shorter than several seconds. The
reasons for this latter limitation are sufficiently fundamental that
markedly superior instruments are not expected to be available
within the next ten years.</p>https://thesis.library.caltech.edu/id/eprint/10316Investigation of strong earthquake ground motion
https://resolver.caltech.edu/CaltechTHESIS:02192016-120838514
Authors: Trifunac, Mihailo Dimitrije
Year: 1969
DOI: 10.7907/FE4G-RF61
<p>The pattern of energy release during the Imperial Valley, California, earthquake of 1940 is studied by analysing the El Centro strong motion seismograph record and records from the Tinemaha seismograph station, 546 km from the epicenter. The earthquake was a multiple event sequence with at least 4 events recorded at El Centro in the first 25 seconds, followed by 9 events recorded in the next 5 minutes. Clear P, S and surface waves were observed on the strong motion record. Although the main part of the earthquake energy was released during the first 15 seconds, some of the later events were as large as M = 5.8 and thus are important for earthquake engineering studies. The moment calculated using Fourier analysis of surface waves agrees with the moment estimated from field measurements of fault offset after the earthquake. The earthquake engineering significance of the complex pattern of energy release is discussed. It is concluded that a cumulative increase in amplitudes of building vibration resulting from the present sequence of shocks would be significant only for structures with relatively long natural period of vibration. However, progressive weakening effects may also lead to greater damage for multiple event earthquakes. </p>
<p>The model with surface Love waves propagating through a single layer as a surface wave guide is studied. It is expected that the derived properties for this simple model illustrate well several phenomena associated with strong earthquake ground motion. First, it is shown that a surface layer, or several layers, will cause the main part of the high frequency energy, radiated from the nearby earthquake, to be confined to the layer as a wave guide. The existence of the surface layer will thus increase the rate of the energy transfer into the man-made structures on or near the surface of the layer. Secondly, the surface amplitude of the guided SH waves will decrease if the energy of the wave is essentially confined to the layer and if the wave propagates towards an increasing layer thickness. It is also shown that the constructive interference of SH waves will cause the zeroes and the peaks in the Fourier amplitude spectrum of the surface ground motion to be continuously displaced towards the longer periods as the distance from the source of the energy release increases.</p>
https://thesis.library.caltech.edu/id/eprint/9565Investigation of Earthquake and Microtremor Ground Motions
https://resolver.caltech.edu/CaltechThesis:08262021-203716987
Authors: Udwadia, Firdaus Erach
Year: 1972
DOI: 10.7907/4cv1-wy73
<p>The nature of strong earthquake ground shaking has been investigated based on a study of 15 accelerograms recorded at El Centro in southern California. It is concluded that the characteristics of the source mechanism and the transmission path play a dominant role in determining the details of strong ground shaking at the site. No local site periodicities could be clearly identified, which suggests that source and transmission path effects overshadow the influence of local site conditions.</p>
<p>The method of using microtremor measurements to determine local site characteristics has been tested by direct comparison with strong motion measurements. Microtremor ground motions were recorded at five sites in the El Centro area and measurements were repeated at three of these sites after a period of 24 hours. These low amplitude ground motions have been found to be widely different from the motions caused by strong earthquake ground shaking. Their nonstationary nature over a period of a day or so makes the interpretation of such data from a single microtremor measurement very unreliable. It has been concluded that these microtremor ground motions are forced oscillations of the ground caused by nearby sources of excitation. The microtremor acceleration spectra do not indicate prominent peaks that could be correlated with local site conditions. At this site the use of microtremor measurements to define local subsoil conditions would evidently not be feasible.</p>https://thesis.library.caltech.edu/id/eprint/14343Near-Field Ground Motion from the Landers Earthquake
https://resolver.caltech.edu/CaltechTHESIS:12012011-110028500
Authors: Chen, Xiaodong
Year: 1995
DOI: 10.7907/v3ye-3c10
<p>In order to investigate near-field ground motions, an important Lucerne Valley record from the Landers earthquake is studied. The Lucerne Valley record was recorded on the Kinemetrics SMA-2/EMA instrument located 2 km from the fault. Since the characteristics of the SMA-2/EMA instrument were not completely understood and the conventional data processing procedures have difficulty in recovering long-period information from near-field earthquake accelerograms, an instrument test on the SMA-2/EMA is conducted and a new data processing procedure is developed to perform the instrument and baseline corrections.</p>
<p>For an electro-magnetic transducer, an additional parameter of corner frequency, other than natural frequency, electronic damping ratio and sensitivity, should be considered during instrument correction of the SMA-2/EMA recorded accelerograms. For this purpose, a special instrument correction filter was derived in support of
instrument correction and a laboratory test of the SMA-2/EMA accelerograph was conducted for obtaining the characteristic parameters of the instrument. The possible
error sources in data recording and playback procedure were also examined and an appropriate baseline correction scheme was formulated for effectively removing the nonphysical trend involved in the earthquake data.</p>
<p>The new data processing procedure was verified by a set of SMA-2/EMA simulated long-period accelerograms and then applied to the Lucerne Valley record. The results of new data processing revealed the important features of near-field ground motion, which were a displacement offset parallel to the fault and a large pulse-like motion perpendicular to the fault. The response spectra and Fourier spectra were also calculated and compared to those of the conventionally processed record. With these investigations, a number of important conclusions are obtained and several suggestions for future studies are given.</p>https://thesis.library.caltech.edu/id/eprint/6745