Committee Feed
https://feeds.library.caltech.edu/people/Hall-J-F/committee.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 15:22:37 +0000Element-by-Element Solution Procedures for Nonlinear Transient Heat Conduction Analysis
https://resolver.caltech.edu/CaltechETD:etd-01252007-132425
Authors: {'items': [{'id': 'Winget-James-Michael', 'name': {'family': 'Winget', 'given': 'James Michael'}, 'show_email': 'NO'}]}
Year: 1984
DOI: 10.7907/G7VB-EV65
<p>Despite continuing advancements in computer technology, there are many problems of engineering interest that exceed the combined capabilities of today's numerical algorithms and computational hardware. The resources required by traditional finite element algorithms tend to grow geometrically as the "problem size" is increased. Thus, for the forseeable future, there will be problems of interest which cannot be adequately modeled using currently available algorithms. For this reason, we have undertaken the development of algorithms whose resource needs grow only linearly with problem size. In addition, these new algorithms will fully exploit the "parallel-processing" capability available in the new generation of multi-processor computers.</p>
<p>The approach taken in the element-by-element solution procedures is to approximate the global implicit operator by a product of lower order operators. This type of "product" approximation originated with ADI techniques and was further refined into the "method of fractional steps." The current effort involves the use of a more natural operator split for finite element analysis based on "element operators." This choice of operator splitting based on element operators has several advantages. First, it fits easily within the architecture of current FE programs. Second, it allows the development of "parallel" algorithms. Finally, the computational expense varies only linearly with the number of elements.</p>
<p>The particular problems considered arise from nonlinear transient heat conduction. The nonlinearity enters through both material temperature dependence and radiation boundary conditions. The latter condition typically introduces a "stiff" component in the resultant matrix ODE's which precludes the use of explicit solution techniques. Implicit solution techniques can be prohibitively expensive. Instead, the matrix equations are solved by combining a modified Newton-Raphson iteration scheme with an element-by-element preconditioned conjugate gradient subiteration procedure. The resultant procedure has proven to be both accurate and reliable in the solution of medium-size problems in this class.</p>
https://thesis.library.caltech.edu/id/eprint/337Experimental Studies of Dynamic Response of Foundations
https://resolver.caltech.edu/CaltechETD:etd-08152006-091708
Authors: {'items': [{'email': 'ben@hushmand-associates.com', 'id': 'Hushmand-Behnam', 'name': {'family': 'Hushmand', 'given': 'Behnam'}, 'show_email': 'YES'}]}
Year: 1984
DOI: 10.7907/6TJK-6088
<p>An investigation was made into the behavior of rigid foundations and structures resting on the surface or embedded in a cohesionless soil and subjected to transient active or passive excitations and forced vibrations using the centrifuge modeling technique. The investigation was aimed at studying both low and high amplitude vibrations of foundations under machine type loadings, earthquake or wave induced vibrations, and other sources of dynamic loads. Rigid "prototype" foundations of mass and size comparable to foundations of a low rise building were simulated in the centrifuge at a centrifugal acceleration of 50g. Rigid model structures (aluminum towers) attached to foundations of different shapes, sizes, masses, and moments of inertia were tested. The effect of soil depth, boundary conditions, and depth of foundation embedment were investigated. Mainly rocking and horizontal modes of vibration were studied. The impulse rocking-horizontal excitation of the models was provided by actively perturbing the model structures using explosive energy or by passively exciting them by shaking the whole soil bucket using a hydraulic shaking system. The forced vibration was produced by a miniature air-driven counterrotating eccentric mass shaker mounted on the model structures. During the tests detailed measurements of the static and dynamic contact pressure distributions, displacement components of the model, and acceleration amplitudes at different elevations of the model structure were obtained. The acceleration ratios were used to determine the modes of vibration of the foundation systems. Natural frequencies and damping coefficients of the modes were calculated by fitting the amplitude-frequency response of a single degree of freedom mass-spring-dashpot oscillator to the experimental response curves derived from the test data. Experimental results provided information regarding the influence of different geometrical, inertial, and loading conditions on the vibrational characteristics of the soil-structure system. In particular the effect of foundation embedment was to increase the model resonant frequencies and to cause an appreciable change in contact pressure distribution underneath the footing. However, the resonant frequencies predicted by the lumped parameter analysis for a simple two-degree-of-freedom (rocking and translation) model were about 20 to 55 per cent higher than those measured experimentally. These results were consistent with the comparisons made in similar theoretical and experimental studies such as those performed by Morris in the Cambridge centrifuge and those performed on full-scale footings by Stokoe and Richart. Damping ratios of the rocking-sliding vibration did not change considerably when footing size or depth of embedment changed. The existence of rigid boundaries around the soil mass in the bucket, and an inefficient contact between soil and the foundation side walls and lower surface could account for these observations. Uplift and nonlinear large amplitude vibrations were consistently observed during the steady-state vibration tests. Uplift led to a softer vibrating system which behaved non-linearly. As a result the frequency of vibration decreased with the amount of lift-off. In transient vibration uplift reduced the intensity of higher frequency vibration. Soil around the foundation edge yielded and plastic deformations and subsequent softening of the contact soil increased the material damping while it decreased the resonant frequency of the system. It was concluded that elastic half-space theory does not satisfy the needs for analysis of foundation behavior under high amplitude vibrations and more sophisticated methods of analysis are required.</p>https://thesis.library.caltech.edu/id/eprint/3131Analysis of the Observed Earthquake Response of a Multiple Span Bridge
https://resolver.caltech.edu/CaltechTHESIS:11262018-124513833
Authors: {'items': [{'id': 'Wilson-John-Charles', 'name': {'family': 'Wilson', 'given': 'John Charles'}, 'show_email': 'NO'}]}
Year: 1984
DOI: 10.7907/5321-6p57
<p>Accelerograms obtained during the 1979 Coyote Lake, California earthquake are used to examine the response of a multiple-span, steel girder bridge to strong earthquake loading. The structure studied, the San Juan Bautista 156/101 Separation Bridge, is typical of many highway bridges in seismic regions of the United States. Although the bridge was not damaged, the strong-motion records are of significant engineering interest as they are the first to be recorded on such a structure.</p>
<p>An engineering seismology study suggests that long-period ground displacements at the bridge site were caused by Rayleigh waves. A three-second period, pseudostatic response of the superstructure is attributed to small amounts of differential support motion induced by the surface waves.</p>
<p>A time-domain technique of system identification is used to determine linear models which can closely replicate the observed bridge response. Using time-invariant models, two structural modes at 3.50 and 6.33 Hz, are identified in the horizontal direction. Each mode, having approximately ten-percent damping, involves coupled longitudinal and transverse motions of the superstructure. Time-variations of frequency and damping in the horizontal response are also identified using a moving-window analysis.</p>
<p>A three-dimensional finite element model which includes soil-structure interaction predicts several important features of the dynamic response of the bridge. The first two computed horizontal frequencies are found to be in excellent agreement with the observed responses provided the model's expansion joints are locked, preventing relative translational motions from occurring across the joints. Locking is confirmed by the observed deformations of the structure in the fundamental mode. Fundamental vertical frequencies of the individual spans, predicted by the finite element model, are in very good agreement with ambient vibration test data. Results of the strong-motion data analysis and the finite element modeling are used to recommend a plan for expansion of the strong-motion instrumentation array on the bridge.</p>https://thesis.library.caltech.edu/id/eprint/11283Cumulative Damage of Structures Subjected to Response Spectrum Consistent Random Processes
https://resolver.caltech.edu/CaltechTHESIS:04052019-121036771
Authors: {'items': [{'id': 'Jeong-Garrett-Duane', 'name': {'family': 'Jeong', 'given': 'Garrett Duane'}, 'show_email': 'NO'}]}
Year: 1985
DOI: 10.7907/97b3-2427
<p>A theoretical analysis of the effect of duration on the damage of structures subjected to earthquakes is presented. Earthquake excitation is modeled as a nonstationary random process. Estimates of the first-passage probability of a simple oscillator are employed to choose modulated Gaussian random processes consistent with a prescribed response spectrum. The response spectrum is assumed to be specified independent of the duration. Expressions for the mean damage of a structure are derived using an approach similar to the Miner-Palmgren rule for failure caused by cyclic loads. The expected damage expressions are then evaluated for a structure subjected to modulated Gaussian random processes of varying duration.</p>
<p>Two types of structures are examined: a steel structure and a reinforced concrete structure. Results are presented for systems with constant linear stiffness and a particular form of softening behavior. The nonlinearity of the softening system is accounted for by statistical linearization. The level of expected damage is found to be a strong function of both the duration of the excitation and the ductility of the response.</p>https://thesis.library.caltech.edu/id/eprint/11441Range Dependent Signals and Maximum Entropy Methods for Underwater Acoustic Tomography
https://resolver.caltech.edu/CaltechETD:etd-04092008-080843
Authors: {'items': [{'id': 'Kendall-Elizabeth-Ann', 'name': {'family': 'Kendall', 'given': 'Elizabeth Ann'}, 'show_email': 'NO'}]}
Year: 1985
DOI: 10.7907/hrqs-cv92
<p>A new method for simulating underwater acoustic signals in range dependent environments is presented, and the approach utilizes Maslov asymptotic theory as developed by C. H. Chapman for synthetic seismograms. The simulated range dependent signals are then used in active underwater acoustic tomography exercises, where changes in observed acoustic transmissions are inverted to obtain information about ocean sound velocity structure. The inversions are performed with both the generalized inverse and the maximum entropy inverse, and a new numerical method for finding the maximum entropy inverse with noisy data is presented. The numerical technique follows the ε statistic approach proposed by Bryan and Skilling.</p>
https://thesis.library.caltech.edu/id/eprint/1319A Model for the Rigid Body Motions of Skew Bridges
https://resolver.caltech.edu/CaltechTHESIS:02132019-102055741
Authors: {'items': [{'id': 'Maragakis-Emmanuel', 'name': {'family': 'Maragakis', 'given': 'Emmanuel'}, 'show_email': 'NO'}]}
Year: 1985
DOI: 10.7907/YWES-NJ36
<p>This thesis investigates the rigid body motions of skew bridges, concentrating on the in-plane translational and rotational displacements of the bridge deck induced by impact between the deck and the abutments. Experience in the San Fernando Earthquake of February 9, 1971 demonstrates that this feature is particularly important for skew bridges.</p>
<p>A simple model, in which the bridge deck is represented by a rigid rod restricted by column and abutment springs is examined first. This model illustrates the mechanism by which in-plane rotational vibrations is triggered after the closure of the gap between the bridge deck and the abutment. It also shows that the force-deflection relations of the columns and the abutments are particularly important features for the response of the bridge. Methods for the exact and approximate estimation of the elastic stiffness of elastically founded, tapered bridge columns with octagonal cross section are presented next. The methods are applied to a bridge used later as an example. In addition, the yielding of the columns is examined and the force-deflection relations for bending about two orthogonal axes are estimated.</p>
<p>The abutments are treated as rigid bodies and the soil embankments as Winkler Foundations with elastic spring constants increasing with depth. For the examination of the yielding of soil the Rankine theory is used. Based on these assumptions an approximate force deflection relation for the abutments is constructed.</p>
<p>The response of a more complicated bridge model applied to a bridge near Riverside, California is examined at the end of the thesis and examples of the results are given. This model, in which the bridge deck is still represented as a rigid rod, has three in-plane degrees of freedom: two orthogonal displacements and a rotation, and is capable of capturing many of the more important features of the nonlinear, yielding response of skew bridges during strong earthquake shaking.</p>https://thesis.library.caltech.edu/id/eprint/11391System Identification of Hysteretic Structures
https://resolver.caltech.edu/CaltechTHESIS:12192018-105342164
Authors: {'items': [{'id': 'Cifuentes-Arturo-O', 'name': {'family': 'Cifuentes', 'given': 'Arturo O.'}, 'show_email': 'NO'}]}
Year: 1985
DOI: 10.7907/p43n-j428
<p>This thesis is concerned with the earthquake response of hysteretic structures subjected to strong ground acceleration. Several earthquake records corresponding to different instrumented buildings are analyzed. Based on these observations, a new model for the dynamic behavior of reinforced concrete buildings is proposed. In addition, a suitable system identification algorithm to be used with this new model is introduced. This system identification algorithm is based upon matching the restoring force behavior of the structure rather than the time history of the response. As a consequence, the new algorithm exhibits significant advantages from a computational point of view. Same numerical examples using actual earthquake data are discussed.</p>https://thesis.library.caltech.edu/id/eprint/11322A Simple Strain-Space Plasticity Model for Clays
https://resolver.caltech.edu/CaltechTHESIS:01022019-123640478
Authors: {'items': [{'id': 'Chelvakumar-Kasivisvanathan', 'name': {'family': 'Chelvakumar', 'given': 'Kasivisvanathan'}, 'show_email': 'NO'}]}
Year: 1985
DOI: 10.7907/kcgf-c188
<p>This thesis develops and demonstrates a simple strain-space constitutive model for wet clays. It has been seen that a strain-space formulation of the constitutive behavior of engineering materials facilitates the solution of boundary value problems involving these materials. Soil, because of its multi-phase granular constitution poses challenging problems in constitutive modeling. Although several stress-space plasticity models exist for soils, they are not used commonly in engineering practice due to their complexity. It is attempted herein to develop and test a simple model which could result in simplified solutions for some soil problems.</p>
<p>The model is based on the experimentally observed physical behavior of soil. Certain approaches alien to conventional plasticity. Are employed so that the material behavior is closely predicted without sacrificing the simplicity of the model.</p>
<p>The model is initially developed for triaxial load systems. Its predictions are then tested against other model predictions and experimental data. The model is then generalized. The generalization renders the model capable of handling general stress-strain states and finite deformations.</p>
<p>Finally, the generalized model is used to solve an idealization of a practical problem. The problem of a pile driven into a soil medium is idealized as an expanding cavity in a homogeneous infinite medium. The solution predicted by the strain-space model is compared with other model predictions and test results.</p>https://thesis.library.caltech.edu/id/eprint/11330Tsunamis: The Response of Harbours with Sloping Boundaries to Long Wave Excitation
https://resolver.caltech.edu/CaltechETD:etd-03142008-080030
Authors: {'items': [{'email': 'jeffreyz@broadpark.no', 'id': 'Zelt-Jeffrey-Alan', 'name': {'family': 'Zelt', 'given': 'Jeffrey Alan'}, 'show_email': 'NO'}]}
Year: 1986
DOI: 10.7907/QBE6-5409
<p>The influence of sloping boundaries on the long wave response of bays and harbours is studied in this work. Laboratory experiments are performed to help validate the theoretical analysis which is applicable to nonbreaking waves.</p>
<p>A set of long wave equations in the Lagrangian description is derived which includes terms to account for nonlinear, dispersive, and dissipative processes for wave propagation in two horizontal coordinates. A finite element model is developed based on these equations which is capable of treating arbitrary geometry and the runup of nonbreaking waves on a beach.</p>
<p>An analytical harbour response model, capable of treating narrow rectangular harbours with variable bathymetry and sidewall geometry, is developed and applied to several simple geometries. The model shows that for a given harbour length and entrance width, the resonant frequencies and the response of a harbour are very dependent on the harbour sidewall geometry and bathymetry.</p>
<p>Some of the nonlinear effects of the runup of nonbreaking periodic waves on a plane beach are discussed. In particular, the time average of the water surface time history at a fixed spatial location is negative and the wave crests are smaller than the troughs. Nonlinear effects do not alter the runup maxima or minima and the maximum fluid acceleration occurs at the point of maximum rundown of the
wave.</p>
<p>Laboratory experiments were performed to determine the long wave reponse of a narrow rectangular harbour whose still water depth decreases linearly between the harbour entrance and the shoreline. Good agreement with the finite element model was obtained, including the prediction of the depression of the mean water level within the harbour.</p>
<p>A three-dimensional application of the finite element model treats the runup of solitary waves on a coastline with variable bottom topography and a curved shoreline. The results indicate that the model can predict the trapping of wave energy along a sloping coastal margin, a process of fundamental importance for predicting potential tsunami damage.</p>https://thesis.library.caltech.edu/id/eprint/955Analysis of Unanchored Liquid Storage Tanks under Seismic Loads
https://resolver.caltech.edu/CaltechTHESIS:10232019-110854163
Authors: {'items': [{'id': 'Peek-Ralf', 'name': {'family': 'Peek', 'given': 'Ralf'}, 'show_email': 'NO'}]}
Year: 1986
DOI: 10.7907/w43k-hj54
<p>Because of cost, cylindrical, ground supported liquid storage tanks are often not fixed to their foundation, even in seismic areas. For such an unanchored tank made of steel, the weight of the cylindrical shell is mostly insufficient to prevent local uplift due to seismic overturning moments. Although, for properly designed connecting pipes, uplift itself is not a problem, it results in larger vertical compressive stresses in the tank wall at the base, opposite to where the uplift occurs. These compressive stresses have often caused buckling, even in earthquakes which did not cause much damage to other structures.</p>
<p>Various investigators have studied the behavior of unanchored tanks experimentally, but, due to the complexity of the problem, so far very little theoretical work has been done. Two methods of analysis for static lateral loads are presented: An approximate one in which the restraining action of the base plate is modeled by nonlinear Winkler springs, and a more comprehensive one in which the two dimensional nonlinear contact problem is solved by the finite difference energy method. The theoretical results are compared with existing experimental results and with the approach from current U.S. design standards. The theoretical peak compressive stresses are in good agreement with the experimental results, but in some cases exceed those calculated by the code method by more than 100%.</p>
<p>Finally, a new design concept, by which the tank wall is preuplifted all around its circumference by inserting a ring filler is described. It will be shown theoretically and experimentally that this preuplift method substantially improves the lateral load capacity.</p>https://thesis.library.caltech.edu/id/eprint/11848Two-Phase Soil Study: A. Finite Strain Consolidation. B. Centrifuge Scaling Considerations
https://resolver.caltech.edu/CaltechETD:etd-03082008-084249
Authors: {'items': [{'id': 'Tan-Thiam-Soon', 'name': {'family': 'Tan', 'given': 'Thiam-Soon'}, 'show_email': 'NO'}]}
Year: 1986
DOI: 10.7907/agt3-mf84
<p>Two different aspects of the behavior of soil as a two-phase medium are studied, namely, the consolidation of soil and scaling relations for soils in centrifuge testing.</p>
<p>PART A</p>
<p>First a consistent approach is presented that unifies all current theories of consolidation of soil. For one-dimensional finite strain consolidation, a Lagrangian finite element scheme is then given and tested against three different experiments and found to give consistent results. For a quick solution to a particular problem, the regular perturbation method applied to the formulation in which the dependent variable is the natural strain is shown to give the most consistent results. For the Eulerian formulation, the material derivative contains a convective term. This convective effect is then analytically studied and found not to be negligible for a final natural strain greater than 10%. A method is then introduced that can account for both the moving boundary and the convective effect. This method is tested in a finite difference scheme and found to give identical results with the Lagrangian finite element scheme for the one-dimensional case. Finally the method is used for the axisymmetric problem of consolidation by vertical drain. The solution to this case suggests that arching and subsequent load redistribution should be considered.</p>
<p>PART B</p>
<p>Conceptually, when a centrifuge is used to test models, the centrifuge is assumed to produce an equivalent ng gravitational field (as on another planet) and the behavior of the model in the ng field is then assumed to be similar to that of the prototype. For most static problems, the centrifuge does model the prototype well but for some dynamic problems, these assumptions can break down. To investigate this, the similarity requirements are examined for the case of a single particle moving in a fluid. It is found that for the post-liquefaction process and for seepage flow, unless the Reynolds number is much less than one in both model and prototype, the centrifuge is not a good simulation of the prototype situation. But, perhaps contrary to expectations, the breakdown is due to the fact that the behavior in the ng planet is not similar to the prototype ig planet, whereas the centrifuge does simulate the ng planet well. Further, it is shown that the concept of "modeling of models" can lead to misleading results. Lastly, for cratering experiments, it is concluded that the centrifuge will only model the crater shape just after an explosion and not the final crater shape.</p>https://thesis.library.caltech.edu/id/eprint/902Dynamic Fracture Initiation and Propagation in Metals: Experimental Results and Techniques
https://resolver.caltech.edu/CaltechETD:etd-03052008-085910
Authors: {'items': [{'id': 'Zehnder-Alan-T', 'name': {'family': 'Zehnder', 'given': 'Alan T.'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/f3zy-j846
<p>Dynamic fracture initiation and propagation in ductile and brittle materials was studied experimentally using the optical method of caustics in conjunction with high speed photography. The drop weight impact test, previously used only for studies of fracture initiation, was adapted to study both dynamic fracture initiation and dynamic fracture propagation.</p>
<p>The results show that for a relatively brittle, quenched and tempered, high strength 4340 steel the dynamic fracture propagation toughness depends on crack tip velocity through a relation that is a material property. In addition, the effect of stress waves on the dynamic response of different specimen geometries is discussed and the micromechanisms of failure for this heat treatment of 4340 steel are investigated.</p>
<p>Extension of the optical method of caustics to applications in elastic-plastic fracture was studied with the goal of learning how to measure dynamic fracture initiation toughness in tough, ductile materials. Static experiments were performed on different specimen geometries of a ductile 4340 steel and 1018 cold rolled steel, and were compared to small scale yielding, plane stress, finite element results. Issues studied that are related to the applicability of caustics are the extent of the dominance of the plane stress HRR field, the effect of plasticity on the accuracy of caustics from the elastic region outside the plastic zone, and the extent of the crack tip region of three dimensionality.</p>
<p>The above approach to caustics in ductile materials was based on the assumption of validity of the HRR field. A novel approach to the use of caustics with ductile materials was taken that eliminates the concerns over the region of dominance of the HRR field, etc. In this approach a calibration experiment was performed relating the caustic diameter to the J integral for a particular specimen geometry under conditions of large scale yielding. This approach was successfully applied to optically measure for the first time the J integral under dynamic loading. Measurement of the J integral by means of strain gages was developed and applied to obtain J simultaneously with the caustics measurement.</p>
<p>At the same time (and on the same specimens) additional measurements were made including, load, load-point displacement, strains near the crack tip and out of plane displacements (measured with interferometry). These results are compared with excellent agreement to a three dimensional finite element simulation of the specimen.</p>https://thesis.library.caltech.edu/id/eprint/879The Response of Stick-Slip Systems to Random Seismic Excitation
https://resolver.caltech.edu/CaltechTHESIS:04122019-165004448
Authors: {'items': [{'id': 'Moser-Michael-Anthony', 'name': {'family': 'Moser', 'given': 'Michael Anthony'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/ntjg-hg45
<p>This thesis examines the response of stick-slip, or frictional, systems to harmonic and random excitation. Two frictional models are considered: constant slip force, or Coulomb, friction, and displacement dependent slip force, used to model a caster, or pivoting wheel. The response to harmonic excitation of systems exhibiting both frictional models is determined using the method of slowly varying parameters. Changes in the response amplitude of both systems caused by the addition of a linear centering mechanism are also examined.</p>
<p>The response of the system with displacement dependent slip force is examined under Gaussian mean zero white noise excitation using the generalized equivalent linearization method. It is shown that a lower bound is obtained from the Coulomb friction system's response.</p>
<p>For filtered random excitation, linearization methods are shown to predict erroneous displacement trends for the Coulomb system when the input has no spectral content at zero frequency. When the excitation is modeled as a Poisson pulse process, an approximate method exhibiting the proper displacement trends can be constructed. The method is shown to be accurate over a broad range of input parameters if overlaps in the input pulses are considered. A set of excitation parameters consistent with seismic events is then used to estimate final rms displacements as a function of coefficient of friction.</p>https://thesis.library.caltech.edu/id/eprint/11457Structure Measurements in a Synthetic Turbulent Boundary Layer
https://resolver.caltech.edu/CaltechETD:etd-03192008-085206
Authors: {'items': [{'email': 'jaywant@mecheng.iisc.ernet.in', 'id': 'Arakeri-Jaywant-H', 'name': {'family': 'Arakeri', 'given': 'Jaywant H.'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/FABW-YZ77
<p>Extensive hot-wire measurements have been made to determine the structure of the large eddy in a synthejc turbulent boundary layer on a flat-plate model. The experiments were carried out in a wind tunnel at a nominal free-stream velocity of 12 m/s. The synthetic turbulent boundary layer had a hexagonal pattern of eddies and a ratio of streamwise scale to spanwise scale of 3.2:1. The measured celerity of the large eddy was 84.2 percent of the free-stream velocity. There was some loss of coherence, but very little distortion, as the eddies moved downstream. Several mean properties of the synthetic boundary layer were found to agree quite well with the mean properties of a natural turbulent boundary layer at the same Reynolds number.</p>
<p>The large eddy is composed of a pair of primary counter-rotating vortices about five δ long in the streamwise direction and about one δ apart in the spanwise direction, where δ is the mean boundary-layer thickness. The sense of the primary pair is such as to pump fluid away from the wall in the region between the vortices. A secondary pair of counter-rotating streamwise vortices, having a sense opposite to that of the primary pair, is observed outside of and slightly downstream from the primary vortices. Both pairs of vortices extend across the full thickness of the boundary layer and are inclined at a shallow angle to the surface of the flat plate. The data show that the mean vorticity vectors are not tangential to the large-eddy vortices. In fact, the streamwise and normal vorticity components that signal the presence of the eddy are of the same order of magnitude. Definite signatures are obtained in terms of the mean skin-friction coefficient and the mean wake parameter averaged at constant phase. Velocities induced by the vortices are partly responsible for entrainment of irrotational fluid, for transport of momentum, for generation of Reynolds stresses, and for maintenance of streamwise and normal vorticity in the outer flow. A stretching mechanism is important in matching spanwise vorticity close to the wall to variations in turbulent shearing stress. Regions where the stretching term is large coincide with regions of large wall shearing stress and large turbulence production.</p>https://thesis.library.caltech.edu/id/eprint/1012Modeling and Identification in Structural Dynamics
https://resolver.caltech.edu/CaltechTHESIS:02272019-105053083
Authors: {'items': [{'id': 'Jayakumar-Paramsothy', 'name': {'family': 'Jayakumar', 'given': 'Paramsothy'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/1wwx-ca82
<p>Analytical modeling of structures subjected to ground motions is an important aspect of fully dynamic earthquake-resistant design. In general, linear models are only sufficient to represent structural responses resulting from earthquake motions of small amplitudes. However, the response of structures during strong ground motions is highly nonlinear and hysteretic.</p>
<p>System identification 1s an effective tool for developing analytical models from experimental data. Testing of full-scale prototype structures remains the most realistic and reliable source of inelastic seismic response data. Pseudo-dynamic testing is a recently developed quasi-static procedure for subjecting full-scale structures to simulated earthquake response. The present study deals with structural modeling and the determination of optimal linear and nonlinear models by applying system identification techniques to elastic and inelastic pseudo-dynamic data from a full-scale, six-story steel structure.</p>
<p>It is shown that the feedback of experimental errors during the pseudo-dynamic tests significantly affected the higher modes and led to an effective negative damping for the third mode. The contributions of these errors are accounted for and the small-amplitude modal properties of the test structure are determined. These properties are in agreement with the values obtained from a shaking table test of a 0.3 scale model.</p>
<p>The nonlinear hysteretic behavior of the structure during strong ground motions is represented by a general class of Masing models. A simple model belonging to this class is chosen. with parameters which can be estimated theoretically, thereby making this type of model potentially useful during the design stages. The above model is identified from the experimental data and then its prediction capability and application in seismic design and analysis are examined.</p>https://thesis.library.caltech.edu/id/eprint/11411Some Observations on the Random Response of Hysteretic Systems
https://resolver.caltech.edu/CaltechTHESIS:04152019-163242410
Authors: {'items': [{'id': 'Paparizos-Leonidas-G', 'name': {'family': 'Paparizos', 'given': 'Leonidas G.'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/w5wv-5f87
<p>In this thesis, the nature of hysteretic response behavior of structures subjected to strong seismic excitation, is examined. The earthquake ground motion is modeled as a stochastic process and the dependence of the response on system and excitation parameters, is examined. Consideration is given to the drift of structural systems and its dependence on the low frequency content of the earthquake spectrum. It is shown that commonly used stochastic excitation models, are not able to accurately represent the low frequency content of the excitation. For this reason, a stochastic model obtained by filtering a modulated white noise signal through a second order linear filter is used in this thesis.</p>
<p>A new approach is followed in the analysis of the elasto-plastic system. The problem is formulated in terms of the drift, defined as the sum of yield increments associated with inelastic response. The solution scheme is based on the properties of discrete Markov process models of the yield increment process, while the yield increment statistics are expressed in terms of the probability density of the velocity and elastic component of the displacent response. Using this approach, an approximate exponential and Rayleigh distribution for the yield increment and yield duration, respectively, are established. It is suggested that, for duration of stationary seismic excitation of practical significance, the drift can be considered as Brownian motion. Based on this observation, the approximate Gaussian distribution and the linearly divergent mean square value of the process, as well as an expression for the probability distribution of the peak drift response, are obtained. The validation of these properties is done by means of a Monte Carlo simulation study of the random response of an elastoplastic system.</p>
<p>Based on this analysis, the first order probability density and first passage probabilities for the drift are calculated from the probability density of the velocity and elastic component of the response, approximately obtained by generalized equivalent linearization. It is shown that the drift response statistics are strongly dependent on the normalized characteristic frequency and strength of the excitation, while a weaker dependence on the bandwidth of excitation is noted.</p>https://thesis.library.caltech.edu/id/eprint/11467On the Start Up of Supersonic Underexpanded Jets
https://resolver.caltech.edu/CaltechETD:etd-03042008-081340
Authors: {'items': [{'email': 'nehemias@alumni.caltech.edu', 'id': 'Lacerda-Nehemias-Lima', 'name': {'family': 'Lacerda', 'given': 'Nehemias Lima'}, 'show_email': 'YES'}]}
Year: 1987
DOI: 10.7907/W3CX-2Z48
<p>An impulsively started jet can be formed by a gas confined in a high pressure reservoir that escapes suddenly through an exit orifice, into a controlled atmosphere. Supersonic gas jets of this type are unsteady and differ from the steady jet that develops later by the presence of a bow shock, a jet head and a nonstationary Mach disk. The effects of the pressure ratio between the high pressure gas inside the reservoir and the lower pressure atmospheric gas, as well as the gas combination used, are studied experimentally. The gases used for the jet and the atmosphere were selected from helium, nitrogen and sulfur hexafluoride.</p>
<p>The data acquisition consisted of: high resolution flash photography to obtain detail from the pictures; high-speed movie pictures to obtain the time development of selected features; and fast-response pressure transducers located at the reservoir end plate, the tank end plate and the jet exit. The initial development of the jet is highly time dependent. During this phase, the shape that the jet assumes varies with pressure ratio and with the choice of gas. In particular an extremely light gas exhausting into a heavy atmosphere, exhibits an uncommon shape. It develops as a bubble wrapped by the bow shock, that increases its volume with flow time and pressure ratio. As the pressure ratio increases, it becomes more tightly wrapped by the bow shock. At later times the jet assumes conventional linear growth.</p>
<p>After the jet starts, a Mach disk is observed close to the jet exit which moves downstream as the exit pressure builds up. The monotonic increase in exit pressure is caused by the slow breaking of the diaphragm. The position of the Mach disk is furthest from the jet exit when the exit pressure is a maximum. After that it oscillates around the location predicted by the steady theory of Ashkenas and Sherman (1966) at a frequency close to one of the resonant frequencies of the reservoir. The features observed for the inner structure of the jet were verified to agree with those obtained for impulsive flow generated by a muzzle blast.</p>
<p>The frontal part of the jet forms the jet head, whose shape changes with the flow conditions. The initial evolution of the jet head is linear but after propagating a distance of around ten exit diameters, it reaches asymptotic behavior with an evolution that is approximately proportional to square root of time. The head creates a bow shock ahead of it that propagates downstream and increases the pressure of the atmospheric gas. This bow shock was found to be less attenuated than in spherically symmetric explosions. The asymptotic behavior of the bow shock was reached after about eight exit diameters.</p>https://thesis.library.caltech.edu/id/eprint/865Mode I, Plane Stress Crack Initiation and Growth in Elastic-Plastic Solids: A Finite Element Analysis
https://resolver.caltech.edu/CaltechETD:etd-11062003-094350
Authors: {'items': [{'id': 'Ramarathnam-Narasimhan', 'name': {'family': 'Ramarathnam', 'given': 'Narasimhan'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/RG0C-WF30
<p>A detailed finite element analysis of crack initiation and stable crack extension is performed under Mode I plane stress, small-scale yielding conditions. A small strain, J<sub>2</sub> incremental plasticity theory is employed and both elastic-perfectly plastic materials and power law hardening materials are considered.</p>
<p>Some issues pertaining to the stationary plane stress crack problem, such as the range of dominance of the asymptotic stress and deformation fields and the amount of non-proportional loading near the crack tip are addressed. Special attention is devoted to the perfectly plastic idealization, by performing a separate singular finite element analysis, to clarify some details about the asymptotic fields near the stationary crack tip. The full-field numerical solution is used to simulate synthetic (optical) caustic patterns at different distances from the crack tip, which are compared with experimental observations and with asymptotic analytical results.</p>
<p>A nodal release procedure is used to simulate quasi-static crack extension. It is found that the asymptotic angular extent of the active plastic zone, surrounding the propagating crack tip, is from θ = 0 to about θ = 45° for the perfectly plastic case. The near-tip angular stress distribution within the active plastic zone is in good agreement with the variation in a centered fan, as predicted by a preliminary asymptotic analysis by Rice, for the perfectly plastic case. It is also observed that the σ<sub>rr</sub> stress component has a strong radial variation within the active plastic zone. The angular extent of active yielding around the moving tip increases with hardening, while its maximum radial extent ahead of the tip decreases. Clear evidence of an elastic unloading region following the active plastic zone is found, but no secondary (plastic) reloading along the crack flank has been numerically observed for any level of hardening.</p>
<p>The crack tip opening profile during growth is obtained for various levels of hardening. A ductile crack growth criterion is employed to investigate the nature of the J resistance curves under plane stress. Finally, the influence of hardening on the potential for stable crack growth is examined.</p>https://thesis.library.caltech.edu/id/eprint/4416Failure of Slopes
https://resolver.caltech.edu/CaltechETD:etd-03012008-132659
Authors: {'items': [{'id': 'Burridge-Paul-Brian', 'name': {'family': 'Burridge', 'given': 'Paul Brian'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/2C4G-6R71
<p>The dynamic mechanism of slope failure is studied both experimentally and analytically to establish the spatial and temporal process of failure initiation and propagation during collapse of a natural or man-made slope.</p>
<p>Model slopes, constructed of a brittle cemented sand material, are tested to collapse in a geotechnical centrifuge and the dynamics of failure recorded by motion picture film and mechanical detectors within the slope specimen. Shear failure is observed to initiate at the toe and propagate rapidly to the crest in the presence of crest tension cracking.</p>
<p>A finite difference approach is taken to numerically solve the plane strain slope stability problem under gravity, based on unstable material behavior. Using a Lagrangian differencing scheme in space and explicit integration in time with dynamic relaxation, the numerical method finds the equilibrium state of the slope as the large-time limit of a dynamic problem with artificial parameters. The solution predicts localized shear failure zones which initiate at the slope toe and propagate to the slope crest in the manner and geometry observed in the centrifuge tests. In so doing, the finite difference algorithm also demonstrates an apparent ability to predict shear failure mechanisms in solid continua in general.</p>
https://thesis.library.caltech.edu/id/eprint/825Steady-State and Transient Methods for Modeling Chemical Reactions on Supported Catalysts
https://resolver.caltech.edu/CaltechETD:etd-06142006-131434
Authors: {'items': [{'id': 'Prairie-Michael-Roland', 'name': {'family': 'Prairie', 'given': 'Michael Roland'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/x3zp-aa54
<p>A systematic experimental strategy based on fluid-phase measurements is proposed for modeling dynamic behavior of heterogeneous catalytic reactions. The strategy utilizes steady-state rate, step-response, cycled-feedstream, and feedback-induced bifurcation techniques. Ethylene hydrogenation on Pt/Al<sub>2</sub>O<sub>3</sub> was studied using this strategy. In addition, transmission infrared spectroscopy is applied to investigate support effects which accompany ethylene hydrogenation on Pt/Al<sub>2</sub>O<sub>3</sub>, and to the detailed study of CO adsorption, desorption and oxidation on Rh/Al<sub>2</sub>O<sub>3</sub>. The proposed experimental strategy combined with surface infrared spectroscopy provides a very powerful means for identification and validation of dynamic kinetic models.</p>
<p>Observed bifurcation behavior can be accurately attributed to a model for the catalytic reaction only if each dynamic element in the closed-loop experimental hardware is properly accounted for. Accordingly, time delay and feedback gain were the manipulated parameters in a feedback-induced bifurcation scheme aimed at validating a dynamic model for an experimental gas-phase reactor flow system without reaction. The apparatus consists of an isothermal, stirred, fixed-bed reactor, mass flow controllers, an infrared gas analysis system, and a computerized data acquisition and control system. Experimental bifurcations to sustained oscillations show that the stability of the reactor system is strongly influenced by delay. The relationships of time delay to Hopf bifurcation gains and frequencies provide a very sensitive basis for model comparisons.</p>
<p>Steady-state, step-response, feedback-induced Hopf bifurcation and forced concentration cycling experiments were applied to study ethylene hydrogenation over 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> at 80°C. Step-response experiments indicate a time scale of 5000 s which is associated with chemisorbed hydrogen. Conversely, feedback-induced Hopf bifurcation data indicate this time scale to be on the order of 1 s in magnitude. In the overall strategy of dynamic modeling, the two techniques are complementary since each inherently focuses on an opposite region in the spectrum of time scales for the reactor system. Cycling the feedstream composition resulted in improvement of the time-average reaction rate for the ethylene hydrogenation reaction compared to steady-state reactor operation.</p>
<p>Steady-state, step-response and Hopf bifurcation data are also presented for 0.5% Pt/Al<sub>2</sub>O<sub>3</sub> at 30°C and compared with results for the 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> catalyst. A single value of 2.5 s for the surface time constant associated with chemisorbed hydrogen is sufficient for modeling behavior on 0.5% Pt/Al<sub>2</sub>O<sub>3</sub>, whereas the lower-loaded 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> catalyst requires two very different values. In addition, the 0.5% catalyst was used to demonstrate the general result that small discrepancies between the actual and chosen reference steady state give rise to imperfect, cusp-like bifurcations. Steady-state bifurcation data are also shown to be useful for discriminating among rival kinetic models.</p>
<p>Ethylene hydrogenation on spillover-activated alumina is proposed as an explanation for the very slow transient behavior observed for 0.05% Pt/Al<sub>2</sub>O<sub>3</sub>. Transmission infrared spectroscopy was used to study hydrogen spillover dynamics on 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> at 80°C via hydroxyl/deuteroxyl exchange. Ethylene in the gas-phase markedly slows the rate of spillover. The presence of ethylene likely reduces the concentration of platinum-adsorbed hydrogen adatoms, the precursors of hydrogen spilled onto alumina, due to catalytic hydrogenation on the platinum. Surface transport of hydrogen atoms on spillover-activated alumina is proposed as an explanation for the very slow transient behavior observed for ethylene hydrogenation on 0.05% Pt/Al<sub>2</sub>O<sub>3</sub>. Infrared spectra exhibit characteristics of both hydroxyl and deuteroxyl groups for reactor feed containing only D<sub>2</sub> and C<sub>2</sub>H<sub>4</sub>. This observation confirms the existence of a dissociative ethylene adsorption process.</p>
<p>A section of the thesis unrelated to ethylene hydrogenation investigates modeling applications of transmission infrared spectoscopy (TIR) by applying it to study adsorbed CO on Rh/Al<sub>2</sub>O<sub>3</sub> during CO chemisorption, steady-state, step-response, and forced-cycling oxidation experiments at 900 torr. At 300°C, the catalyst initially supported primarily a dicarbonyl CO species, but after use exhibited spectra characteristic of a surface mostly covered by linearly bound CO. A model that describes transient, diffusion-influenced CO adsorption and desorption for the supported catalyst is presented. It suggests that the CO desorption energy depends linearly on coverage, and that the magnitude of this dependence is a function of temperature. Observed rate dependence on bulk CO concentration for O<sub>2</sub> effluent levels of 0.5% and 0.25% is interpreted considering the effects of internal and external mass transport at 300°C. Step-response and forced-cycling oxidation experiments across stoichiometric conditions exhibit oxygen and CO storage effects characteristic of CO oxidation catalysts. Data indicating autonomous oscillation of CO coverage and CO<sub>2</sub> production are also presented.</p>https://thesis.library.caltech.edu/id/eprint/2590Asymptotic Analysis of Thin Plates Under Normal Load and Horizontal Edge Thrust
https://resolver.caltech.edu/CaltechTHESIS:03212013-094948659
Authors: {'items': [{'email': 'taraathan@gmail.com', 'id': 'Brewster-Mary-Elizabeth', 'name': {'family': 'Brewster', 'given': 'Mary Elizabeth'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/DDP9-KW92
<p>We consider the radially symmetric nonlinear von Kármán plate equations for circular or annular plates in the limit of small thickness. The loads on the plate consist of a radially symmetric pressure load and a uniform edge load. The dependence of the steady states on the edge load and thickness is studied using asymptotics as well as numerical calculations. The von Kármán plate equations are a singular perturbation of the Fӧppl membrane equation in the asymptotic limit of small thickness. We study the role of compressive membrane solutions in the small thickness asymptotic behavior of the plate solutions. </p>
<p>We give evidence for the existence of a singular compressive solution for the circular membrane and show by a singular perturbation expansion that the nonsingular compressive solutions approach this singular solution as the radial stress at the center of the plate vanishes. In this limit, an infinite number of folds occur with respect to the edge load. Similar behavior is observed for the annular membrane with zero edge load at the inner radius in the limit as the circumferential stress vanishes. </p>
<p>We develop multiscale expansions, which are asymptotic to members of this family for plates with edges that are elastically supported against rotation. At some thicknesses this approximation breaks down and a boundary layer appears at the center of the plate. In the limit of small normal load, the points of breakdown approach the bifurcation points corresponding to buckling of the nondeflected state. A uniform asymptotic expansion for small thickness combining the boundary layer with a multiscale approximation of the outer solution is developed for this case. These approximations complement the well known boundary layer expansions based on tensile membrane solutions in describing the bending and stretching of thin plates. The approximation becomes inconsistent as the clamped state is approached by increasing the resistance against rotation at the edge. We prove that such an expansion for the clamped circular plate cannot exist unless the pressure load is self-equilibrating.</p>https://thesis.library.caltech.edu/id/eprint/7539Generalized Modal Identification of Linear and Nonlinear Dynamic Systems
https://resolver.caltech.edu/CaltechTHESIS:03132013-163747278
Authors: {'items': [{'id': 'Peng-Chia-Yen', 'name': {'family': 'Peng', 'given': 'Chia-Yen'}, 'show_email': 'NO'}]}
Year: 1988
DOI: 10.7907/e70y-gz66
<p>This dissertation is concerned with the problem of determining the dynamic characteristics of complicated engineering systems and structures from the measurements made during dynamic tests or natural excitations. Particular attention is given to the identification and modeling of the behavior of structural dynamic systems in the nonlinear hysteretic response regime. Once a model for the system has been identified, it is intended to use this model to assess the condition of the system and to predict the response to future excitations.</p>
<p>A new identification methodology based upon a generalization of the method of modal identification for multi-degree-of-freedom dynaimcal systems subjected to base motion is developed. The situation considered herein is that in which only the base input and the response of a small number of degrees-of-freedom of the system are measured. In this method, called the generalized modal identification method, the response is separated into "modes" which are analogous to those of a linear system. Both parametric and nonparametric models can be employed to extract the unknown nature, hysteretic or nonhysteretic, of the generalized restoring force for each mode.</p>
<p>In this study, a simple four-term nonparametric model is used first to provide a nonhysteretic estimate of the nonlinear stiffness and energy dissipation behavior. To extract the hysteretic nature of nonlinear systems, a two-parameter distributed element model is then employed. This model exploits the results of the nonparametric identification as an initial estimate for the model parameters. This approach greatly improves the convergence of the subsequent optimization process.</p>
<p>The capability of the new method is verified using simulated response data from a three-degree-of-freedom system. The new method is also applied to the analysis of response data obtained from the U.S.-Japan cooperative pseudo-dynamic test of a full-scale six-story steel-frame structure.</p>
<p>The new system identification method described has been found to be both accurate and computationally efficient. It is believed that it will provide a useful tool for the analysis of structural response data.</p>https://thesis.library.caltech.edu/id/eprint/7516On the Collapse of Long Thick-Walled Circular Tubes under Biaxial Loading
https://resolver.caltech.edu/CaltechETD:etd-11062007-080516
Authors: {'items': [{'email': 'madhavan1@fuchinobe.oilfield.slb.com', 'id': 'Madhavan-Raghu', 'name': {'family': 'Madhavan', 'given': 'Raghu'}, 'show_email': 'NO'}]}
Year: 1988
DOI: 10.7907/VPAC-QV51
<p>The collapse phenomenon of long, thick-walled tubes subjected to axial tension and external pressure is investigated. A combined experimental and analytic approach is adopted. The diameter to thickness ratio (DA) of the tubes studied is in the range 10-40.</p>
<p>A series of collapse tests are conducted using thick-walled, small diameter tubes of two different materials. Careful measurements of geometrical and material parameters are carried out before each collapse test. Tension-Pressure collapse envelopes are obtained for tubes of different D/t and loading paths. Collapse tests involving initially ovalized tubes are also carried out. The results show that collapse strength is strongly influenced by initial ovality.</p>
<p>A two-dimensional model is used for predicting the collapse strength. The limit point behavior of a long tube with initial geometric imperfections has been modeled. The tube is assumed to be under generalized plane strain conditions and the possible variations of material and geometric parameters along the length are not considered. Hill's anisotropic plasticity theory involving a quadratic yield function is used to model the anisotropies in yield shown by drawn tubes. A power law creep model is employed to assess the effect of primary creep on collapse strength.</p>
<p>The interaction between collapse pressure and tension is found to depend on both material and geometric parameters. The yield behavior of the tube material strongly affects the collapse phenomenon. Initial ovality of the tube is shown to be a very important geometric parameter that influences collapse strength. The effect of primary creep on collapse is shown to be not very significant, for the type of materials used (304 stainless steel and 6061-O aluminum).</p>
https://thesis.library.caltech.edu/id/eprint/4421Compression Failure of Fibrous Laminated Composites in the Presence of Stress Gradients: Experiment and Analysis
https://resolver.caltech.edu/CaltechETD:etd-11062003-092741
Authors: {'items': [{'email': 'dcw@umich.edu', 'id': 'Waas-Anthony-Marius', 'name': {'family': 'Waas', 'given': 'Anthony Marius'}, 'orcid': '0000-0002-5258-2749', 'show_email': 'YES'}]}
Year: 1988
DOI: 10.7907/PP86-PE50
<p>A series of experiments were performed to determine the mechanism of failure in compressively loaded laminated plates in the presence of stress gradients generated by a circular cutout. Real time holographic interferometry and in-situ photomicrography of the hole surface, were used to observe the progression of failure.</p>
<p>The test specimens are multi-layered composite flat plates, which are loaded in compression. The plates are made of two material systems, T300/BP907 and IM7/8551-7. Two different lay-ups of T300/BP907 and four different lay-ups of IM7/8551-7 are investigated.</p>
<p>The load on the specimen is slowly increased and a series of interferograms are produced during the load cycle. These interferograms are video-recorded. The results obtained from the interferograms and photo-micrographs are substantiated by sectioning studies and ultrasonic C-scanning of some specimens which are unloaded prior to catastrophic failure, but beyond failure initiation. This is made possible by the servo-controlled loading mechanism that regulates the load application and offers the flexibility of unloading a specimen at any given instance in the loadtime history.</p>
<p>An underlying objective of the present investigation is the identification of the physics of the failure initiation process. This required testing specimens with different stacking sequences, for a fixed hole diameter, so that consistent trends in the failure process could be identified.</p>
<p>It is revealed that the failure is initiated as a localized instability in the 0° plies at the hole surface, approximately at right angles to the loading direction. This instability emanating at the hole edge and propagating into the interior of the specimen within the 0° plies is found to be fiber microbuckling. The microbuckling is found to occur at a local strain level of ≃ 8600 µstrain at the hole edge for the IM material system. This initial failure renders a narrow zone of fibers within the 0° plies to loose structural integrity. Subsequent to the 0°-ply failure, extensive delamination cracking is observed with increasing load. The through thickness location of these delaminations is found to depend on the position of the 0° plies.</p>
<p>The delaminated portions spread to the undamaged areas of the laminate by a combination of delamination buckling and growth, the buckling further enhancing the growth. When the delaminated area reaches a critical size, about 75-100% of the hole radius in extent, an accelerated growth rate of the delaminated portions is observed. The culmination of this last event is the complete loss of flexural stiffness of each of the delaminated portions leading to catastrophic failure of the plate. The levels of applied load and the rate at which these events occur depend on the plate stacking sequence.</p>
<p>A simple mechanical model is presented for the microbuckling problem. This model addresses the buckling instability of a semi-infinte layered half-plane alternatingly stacked with fibers and matrix, loaded parallel to the surface of the half-plane. The fibers are modelled using Bernoulli-Navier beam theory, and the matrix is assumed to be a linearly elastic foundation. The predicted buckling strains are found to overestimate the experimental result. However, the dependence of the buckling strain on parameters such as the fiber volume fraction, ratio of Youngs moduli of the constituents and Poisson's ratio of the matrix are obtained from the analysis. It is seen that a high fiber volume fraction, increased matrix stiffness, and perfect bonding between fiber and matrix are desirable properties for increasing the compressive strength.</p>https://thesis.library.caltech.edu/id/eprint/4415The Application of the Multigrid Method to the Finite Element Solution of Solid Mechanics Problems
https://resolver.caltech.edu/CaltechETD:etd-08102006-090020
Authors: {'items': [{'email': 'dennis.parsons@live.com', 'id': 'Parsons-Ian-Dennis', 'name': {'family': 'Parsons', 'given': 'Ian Dennis'}, 'show_email': 'YES'}]}
Year: 1988
DOI: 10.7907/MVMM-ED69
<p>A multigrid algorithm is described that can be used to obtain the finite element solution of linear elastic solid mechanics problems. The method is applied to some simple two and three dimensional problems to evaluate its strengths and weaknesses. The usefulness of the method is demonstrated by solving some large three dimensional problems of practical interest.</p>
<p>When conditions of near incompressibility are encountered, the multigrid method performs poorly due to a combination of a reduction in the smoothing effect of the Gauss-Seidel relaxation method and coarse mesh locking. These problems can be partially cured by using the Jacobi preconditioned conjugate gradient method to smooth the error, and assembling the coarse mesh stiffness matrices using a reduced integration scheme.</p>
<p>It is also found that the bending behavior of the linear brick and quadrilateral elements used in this thesis slow the convergence of the multigrid method. This effect also causes nonuniform meshes to yield computation times that are not proportional to the problem size; however, the linear dependence can be recovered by increasing the refinement of the finite element meshes. It is demonstrated that reduced integration techniques become less effective in relieving the stiffness of the coarse mesh for nonuniform meshes as the problem size is increased. The solution of a well-conditioned three dimensional test problem shows that the multigrid algorithm requires far less computational effort than a direct method, and that its performance is comparable to that of the Jacobi preconditioned conjugate gradient method.</p>
<p>The usefulness of the multigrid method is demonstrated by applying it to the finite element solution of two solid mechanics problems of engineering interest: the elastostatic state near a three dimensional edge crack, and the relationship between the average offset and the stress drop for two and three dimensional faults in a half-space. The features of the solution to these problems are extensively discussed. It is found that the multigrid method is faster than the Jacobi preconditioned conjugate gradient method when applied to these practical problems.</p>
<p>The investigations described in this thesis reveal some interesting features of the performance of the multigrid method when it is applied to the finite element solution of solid mechanics problems. In particular, the storage requirements of the method are linearly proportional to the problem size. The constant of proportionality depends only on the dimension of the problem. The solution times of the multigrid method are found to be linearly proportional to the problem size if uniform meshes are used. However, this is not true for most of the problems that are solved with nonuniform meshes. The constant of proportionality in the relationship between the problem size and the solution time depends on the particular problem under consideration.</p>
https://thesis.library.caltech.edu/id/eprint/3088Experimental and Finite Element Studies of a Large Arch Dam
https://resolver.caltech.edu/CaltechTHESIS:03142013-082003398
Authors: {'items': [{'id': 'Durón-Ziyad-Hassan', 'name': {'family': 'Durón', 'given': 'Ziyad Hassan'}, 'show_email': 'NO'}]}
Year: 1988
DOI: 10.7907/e0rt-g677
<p>Forced vibration field tests and finite element studies have been conducted on Morrow Point (arch) Dam in order to investigate dynamic dam-water interaction and water compressibility. Design of the data acquisition system incorporates several special features to retrieve both amplitude and phase of the response in a low signal to noise environment. These features contributed to the success of the experimental program which, for the first time, produced field evidence of water compressibility; this effect seems to play a significant role only in the symmetric response of Morrow Point Dam in the frequency range examined. In the accompanying analysis, frequency response curves for measured accelerations and water pressures as well as their resonating shapes are compared to predictions from the current state-of-the-art finite element model for which water compressibility is both included and neglected. Calibration of the numerical model employs the antisymmetric response data since they are only slightly affected by water compressibility, and, after calibration, good agreement to the data is obtained whether or not water compressibility is included. In the effort to reproduce the symmetric response data, on which water compressibility has a significant influence, the calibrated model shows better correlation when water compressibility is included, but the agreement is still inadequate. Similar results occur using data obtained previously by others at a low water level. A successful isolation of the fundamental water resonance from the experimental data shows significantly different features from those of the numerical water model, indicating possible inaccuracy in the assumed geometry and/or boundary conditions for the reservoir. However, the investigation does suggest possible directions in which the numerical model can be improved.</p>https://thesis.library.caltech.edu/id/eprint/7517Nonlinear Seismic Analysis of Arch Dams
https://resolver.caltech.edu/CaltechETD:etd-06112008-155515
Authors: {'items': [{'email': 'mike.dowling@arup.com', 'id': 'Dowling-Michael-John', 'name': {'family': 'Dowling', 'given': 'Michael John'}, 'show_email': 'NO'}]}
Year: 1988
DOI: 10.7907/DJ4P-9393
<p>A nonlinear finite element procedure for arch dams is described in which the gradual opening and closing of vertical contraction joints and predetermined horizontal cracking planes are considered. A special joint element approximately represents the deformations due to plane sections not remaining plane at each open joint and allows a single shell element discretization in the thickness direction to be used for the dam. Compressive and sliding nonlinearities are not included. Finite element treatments are also used for the water, assumed incompressible, and for the foundation rock, assumed massless, with all degrees of freedom (dof) off the dam condensed out. For efficiency in the computations, the condensed water and foundation matrices are localized in a way which maintains good accuracy. The response of Pacoima Dam to the 1971 San Fernando ground motion recorded on a ridge over one abutment and scaled by two-thirds is computed first for water at the intermediate level that existed during the 1971 earthquake and then for full reservoir. In the first analysis, the dam exhibits pronounced opening and separation of the contraction joints, allowing violation of the no-slip assumption. The presence of a full reservoir greatly increases the dam response, enough to bring some of the assumptions of the analysis into question. Reducing the ground motion scale to 0.44 with full reservoir drops the response back to a reasonable level, but the contraction joint separations remain.</p>
https://thesis.library.caltech.edu/id/eprint/2556Considerations for the Design of Gas-Lubricated Slider Bearings
https://resolver.caltech.edu/CaltechETD:etd-11132007-092151
Authors: {'items': [{'email': 'drpwsmith@aol.com', 'id': 'Smith-Paul-Wesley-Jr', 'name': {'family': 'Smith', 'given': 'Paul Wesley, Jr.'}, 'show_email': 'YES'}]}
Year: 1988
DOI: 10.7907/FYJ3-X454
<p>An approach is developed that simplifies calculation of the dynamic characteristics of a self-acting, gas-lubricated slider bearing system. This technique avoids a lengthy simultaneous solution of the equations of motion of the slider and the time-dependent Reynolds' equation, while providing additional design information that is otherwise unobtainable.</p>
<p>The equilibrium pressure distribution in the gas film is obtained using the Bunov-Galerkin formulation of the finite element method. By considering small perturbations of the slider bearing system about equilibrium, two coupled, second-order partial differential equations are derived, which define the in-phase and out-of-phase perturbation pressures in the gas film. These perturbation pressures are integrated to obtain the frequency dependent, non-symmetrical stiffness and damping matrices for the slider bearing.</p>
<p>Using the stiffness and damping properties of the gas bearing and slider support, the equations of motion for the entire slider bearing system are derived. The frequency dependence of the stiffness and damping matrices renders the eigenvalue problem nonlinear, and the eigensolutions are obtained iteratively using Brent's method.</p>
<p>Because of the non-symmetrical stiffness and damping matrices, a similarity transformation based on the left and right modal matrices is used to decouple the equations of motion. This decoupling is approximate because of the frequency dependence of the stiffness and damping matrices, but the resulting damped natural frequencies are shown to be in excellent agreement with published experimental data. Fractions of critical damping obtained for several slider geometries also successfully predict observed instabilities.</p>
<p>The mode shapes of slider oscillation, unobtainable with other methods, permit calculation of the center of rotation for the coupled, pitch-heave modes; this information can be used to determine the optimum location for the magnetic transducer. Closed-form solutions are obtained for the response to disk surface displacement, and for the response to a random force applied to the slider body. These forced-response solutions are useful in identifying the critical parameters of slider design.</p>
https://thesis.library.caltech.edu/id/eprint/4539Nonlinear Earthquake Response of Concrete Gravity Dam Systems
https://resolver.caltech.edu/CaltechETD:etd-06072007-132404
Authors: {'items': [{'id': 'El-Aidi-Bahaa', 'name': {'family': 'El-Aidi', 'given': 'Bahaa'}, 'show_email': 'NO'}]}
Year: 1989
DOI: 10.7907/n2f8-rv33
<p>The earthquake response of concrete gravity dam systems is investigated with emphasis on the nonlinear behavior associated with tensile concrete cracking and water cavitation. A single dam-monolith is considered and is assumed to respond independently as a two-dimensional system under plane stress conditions. The two-dimensional assumption is also extended to model the compressible water body impounded upstream of the dam. Standard displacement-based finite element techniques are used to spatially discretize the field equations and produce a single symmetric matrix equation for the dam-water system. Energy dissipation in the reservoir, through radiation in the infinite upstream direction and absorption at the bottom, is approximately accounted for, and a set of numerical examples is presented to demonstrate the accuracy of the present formulation in modeling the linear earthquake response of infinite reservoirs. An approximate procedure to account for dam-foundation interaction is incorporated based on the response of a rigid plate attached to a three-dimensional viscoelastic half-space.</p>
<p>Water cavitation is modeled by a smeared approach which uses a bilinear pressure-strain relation. It is shown that the water response becomes dominated by spurious high frequency oscillations upon closure of cavitated regions, and improved results can be obtained by using some stiffness-proportional damping in the water reservoir. As demonstrated in an example analysis of Pine Flat Dam (linear dam), cavitation occurs in the upper part of the reservoir along the dam face, unlike other investigations which show cavitated regions at considerable distances from the dam, and both the tensile pressure cutoffs and compressive impacts have a minor effect on the dam response.</p>
<p>Tensile cracks are incorporated using the smeared crack approach, and sliding along closed cracks is allowed. Coupling effects inherent in the finite element formulation are explained, and their influence on open and closed cracks is investigated. Propagation of cracks is monitored in an interactive environment which uses an equivalent strength criterion and allows for user input; remeshing is avoided. The algorithm adopted here produces narrow cracks, unlike many other investigations which show large zones of cracking. An extensive numerical study of Pine Flat Dam demonstrates some interesting features of the nonlinear response of the system, identifies potential failure mechanisms, and reveals a number of difficulties that the analysis encounters. Although no instability of the dam occurs, the numerical difficulties will have to be overcome before definite conclusions regarding stability can be made. It is shown that cracking reduces the hydrodynamic pressures in the reservoir and, hence, reduces water cavitation.</p>https://thesis.library.caltech.edu/id/eprint/2510Interaction of Water Waves with a Density-Stratified Fluid in a Rectangular Trench
https://resolver.caltech.edu/CaltechETD:etd-02152007-151955
Authors: {'items': [{'id': 'Ting-Francis-Chi-Kin', 'name': {'family': 'Ting', 'given': 'Francis Chi Kin'}, 'show_email': 'NO'}]}
Year: 1989
DOI: 10.7907/8bgx-4e53
<p>The interaction of normally incident time-periodic water waves with a density-stratified fluid in a rectangular trench is studied experimentally and theoretically; the fluid outside the trench is homogeneous.</p>
<p>This investigation has focused on the excitation of internal waves in the trench by surface waves, and the effects of the internal oscillations on the waves on the free surface. The study shows that, when the frequency of the incoming surface waves corresponds to the natural frequency of oscillation of the internal waves in the trench, the amplitude of the internal waves becomes large compared to the amplitude of the surface waves. The effects of the internal waves on the surface waves were very small in the experiments.</p>
<p>A two-layer model and a three-layer model are developed and applied to a particular constant-depth channel and trench configuration used in the experiments. The two-layer model is also applied to a rectangular trench in an infinite region. These models treat steady-state wave motions of infinitesimal amplitude for all ranges of fluid depth relative to the wavelength of the surface waves, and include a vigorous treatment of the effects of energy dissipation in the laminar boundary layers adjacent to the solid surfaces and at the density interface. In the two-layer model the stratified fluid in the trench is represented by two homogeneous fluids of different densities; in the three-layer model these two fluids are separated in between by a transition region of linear density variation.</p>
<p>Fresh water and salt water were used to model density stratification in the experiments. The effects of surface wave amplitude and density distribution on the internal motion in the trench were investigated for small density differences compared to the density of water. A new technique using a scanning laser beam and detector system was developed to measure internal wave amplitudes. Satisfactory agreement with the theoretical predictions was obtained. The effects of nonlinearity and viscous dissipation on the internal motions were more pronounced when the depth of the heavier fluid was small compared to the wavelength of the internal waves in the trench.</p>
<p>For a trench in an infinite region, the two-layer model also predicts that large surface wave reflections occur when the trench is "at internal resonance," and a significant portion of the incident wave energy can be dissipated within the trench.</p>
<p>The investigation has provided insight with regard to both the dynamics of wave-trench interaction and the design of navigation channels in density-stratified fluids for reducing the potential of wave-induced internal resonance.</p>https://thesis.library.caltech.edu/id/eprint/644Experimental Investigation of the Nonlinear Seismic Response of Concrete Gravity Dams
https://resolver.caltech.edu/CaltechETD:etd-05232007-074318
Authors: {'items': [{'id': 'Donlon-William-Patrick-Jr', 'name': {'family': 'Donlon', 'given': 'William Patrick, Jr.'}, 'show_email': 'NO'}]}
Year: 1989
DOI: 10.7907/4mqw-6z67
<p>The nonlinear seismic response of concrete gravity dams is investigated experimentally through the use of small-scale models. Of primary interest is crack formation, crack opening and closing, and sliding along crack planes. Also of concern is the stability of the structure after cracking. Three small-scale models (length scale - 115) of a single monolith of Pine Flat Dam are tested to determine the extent of such behavior and its effect on structural stability. The models are constructed of one polymer-based and two plaster-based materials developed for these experiments. The plaster-based materials fulfill the strength, stiffness, and density requirements established by the laws of similitude, while the polymer-based material fulfills only the stiffness and density requirements and is used only in the lower part of the dam where cracking is not expected. The excitation is a modified version of the N00E component of the 1940 Imperial Valley earthquake, applied to each model's base in the stream direction through a vibration table with high-frequency capability. Tests are performed with and without water in the reservoir. The response of each earthquake test is presented in the form of acceleration and displacement time histories, Fourier spectra, and frames taken from high-speed films of the model's response. The results of the experiments indicate that the neck region of a concrete gravity dam is most susceptible to cracking, although crack profiles can differ as a result of variations in excitation, material properties, and construction techniques. These results also indicate alternate design techniques which could improve the seismic stability of a cracked gravity dam.</p>https://thesis.library.caltech.edu/id/eprint/1982Effect of Nonuniform Seismic Input on Arch Dams
https://resolver.caltech.edu/CaltechTHESIS:06042013-091930444
Authors: {'items': [{'id': 'Nowak-Paul-Scott', 'name': {'family': 'Nowak', 'given': 'Paul Scott'}, 'show_email': 'NO'}]}
Year: 1989
DOI: 10.7907/zbd0-ty45
<p>Standard earthquake analyses of civil engineering structures use uniform ground motions even though considerable variations in both amplitude and phase can occur along the foundation interface for long-span bridges and large dams. The objective of this thesis is to quantify the effect that these nonuniformities have on the structural response.</p>
<p>The nonuniform, free-field motions of the foundation interface are assumed to be caused by incident plane body waves. The medium in which these waves travel is a linear, elastic half-space containing a canyon of uniform cross section in which the structure is placed. The solutions for the free-field motions that are due to incident SH, P and SV waves are calculated using the boundary element method.</p>
<p>An analysis of Pacoima (arch) dam located near Los Angeles, California, is performed for both uniform and nonuniform excitations. The important effect of nonuniformities in the free-field motions, sometimes leading to a decrease in the dam response and sometimes to an increase, is quantified.</p>https://thesis.library.caltech.edu/id/eprint/7830Dynamic Crack Propagation in Elastic-Plastic Solids
https://resolver.caltech.edu/CaltechETD:etd-11062003-112730
Authors: {'items': [{'email': 'deng@engr.sc.edu', 'id': 'Deng-Xiaomin', 'name': {'family': 'Deng', 'given': 'Xiaomin'}, 'show_email': 'NO'}]}
Year: 1990
DOI: 10.7907/WHJV-C644
<p>The present finite element study addresses several issues of interest pertaining to the phenomenon of dynamic crack propagation in elastic-plastic solids. Three classes of materials, namely elastic-perfectly plastic materials, linear hardening materials and power-law hardening materials, are considered. The materials are assumed to obey the von Mises yield criterion and the associated flow rule.</p>
<p>Under conditions of Mode I, plane stress, steady state and small scale yielding, we investigated the structures of the near-tip stress and deformation fields. A preliminary asymptotic analysis for crack-tip stress and velocity fields in elastic-perfectly plastic solids was provided to reveal and explain some special features of the crack tip fields observable only in the case of rapid crack propagation. We studied the theoretical basis of a fracture criterion based on the dynamic stress intensity factor for crack growth in materials which fail in a locally ductile manner. We explored the behavior of crack tip fields under non-<i>K</i>-dominance conditions and its effects on the dynamic fracture toughness vs. crack propagation speed relationship.</p>
<p>An Eulerian finite element scheme is employed. Finite element meshes with extremely small elements near the crack tip are carefully designed. The ratio of the crack tip plastic zone size to that of the element nearest to the crack tip is of the order of 1.6 x 10⁴. In order to overcome numerical difficulties associated with crack-tip strain singularities and the use of small near-tip elements, an efficient stress integration algorithm is devised. The existing stress state determination procedure is modified to prevent the occurrence of negative plastic flow and to avoid mistakenly treating elastic unloading as plastic flow. The above measures are proven to be essential for the convergence of the numerical solution.</p>https://thesis.library.caltech.edu/id/eprint/4417Soil Stress Field Around Driven Piles
https://resolver.caltech.edu/CaltechETD:etd-02212007-130942
Authors: {'items': [{'id': 'Allard-Marie-Agnès', 'name': {'family': 'Allard', 'given': 'Marie-Agnès'}, 'show_email': 'NO'}]}
Year: 1990
DOI: 10.7907/GGE1-GC92
<p>The description, equipment, and results of a series of pile-driving experiments conducted in a centrifuge using a model pile driven in dry sand are presented.</p>
<p>The work was conceived on the basis of the modelling of a soil-structure system under an artificially generated gravitational field, and motivated by the need for experimental data for a better understanding of the complex phenomena involved in the pile-soil interaction during driving. The behavior of the pile itself has been the focus of more attention in the past, but few full-scale or model experimental results have been obtained to the present concerning the soil stress field during pile driving. These are necessary for comparison with analytical and theoretical work. The work presented here appears to be the first attempt to obtain dynamic response of the soil during driving. The objective was to obtain a good understanding of the physical phenomena occurring in the soil and pile during driving.</p>
<p>In order to achieve these objectives both dynamic (transient) and static responses of the soil and pile were measured by means of transducers: accelerometers and strain gages for the pile, pressure transducers for the soil. In particular, the relations between static and dynamic data were explored, which resulted in the modelling of the soil-transducer interaction with a non-linear, history-dependent, model.</p>
<p>Results were obtained regarding pile dynamics, soil dynamics, and soil stress field (radial and vertical distribution, stress contours). Both linear and soil-cell model assumptions were used, which enabled a comparison between the two, leading to an estimate that each constitutes a bound of the real stress field, with the linear giving the higher, and the non-linear the lower bound, and the true stress being closer to the lower bound.</p>
<p>The soil response during driving is obtained, filling the gap in the study of the pile-soil system, where only the pile response was known. Recommendations for further work and better experimental procedures are given.</p>https://thesis.library.caltech.edu/id/eprint/696Time-Temperature Response of Multi-Phase Viscoelastic Solids Through Numerical Analysis
https://resolver.caltech.edu/CaltechETD:etd-10292003-112909
Authors: {'items': [{'email': 'cbrinson@northwestern.edu', 'id': 'Brinson-Lynda-Catherine', 'name': {'family': 'Brinson', 'given': 'Lynda Catherine'}, 'show_email': 'YES'}]}
Year: 1990
DOI: 10.7907/SYAS-2A50
<p>A numerical model has been constructed for the study of the properties of multi-phase viscoelastic composites. The model utilizes the dynamic correspondence principle of viscoelasticity in a finite element program to solve boundary value problems simulating uniaxial tension or simple shear and obtains the global complex Young's or shear moduli of the composite.</p>
<p>Each phase of the composite is considered to be thermorheologically simple. The resulting modulus properties of the composite however, are thermorheologically complex and this investigation examines the nature of time-temperature behavior of multi-phase composite materials. The specific composite considered throughout this study contains viscoelastic inclusions embedded in a different viscoelastic matrix material. The deviation of the composite moduli from thermorheologically simple behavior of the matrix material is shown to occur at frequencies and temperatures where the glass-to-rubber transition of the included phases are reached.</p>
<p>Properties of polystyrene and polybutadiene are used to investigate the thermorheological complexity (non-shiftability) of a Styrene-Butadiene-Styrene (SBS) block copolymer. To achieve congruence of the results with experimental data, it is necessary to consider a transition phase of properties "intermediate" to those of styrene and butadiene. Using accurate physical information on the individual phase properties and on the interphase region, it is possible to utilize the numerical model to predict long term properties of multi-phase composites from short term laboratory data. Lacking detailed information on the properties of a particular phase (e.g., the interphase), but knowing the time dependent properties for the composite material at a broad range of temperatures, it is also possible to use the numerical tool to solve an inverse problem and determine the unknown properties of the phase in question.</p>https://thesis.library.caltech.edu/id/eprint/4297Transfer of solutes into and out of streambeds
https://resolver.caltech.edu/CaltechETD:etd-07092007-074127
Authors: {'items': [{'email': 's.elliott@niwa.co.nz', 'id': 'Elliott-A-H', 'name': {'family': 'Elliott', 'given': 'Alexander H.'}, 'show_email': 'NO'}]}
Year: 1991
DOI: 10.7907/JBTC-ZX91
Laboratory experiments were conducted to determine the mechanisms and rates of bed/stream exchange of non-reactive solutes for beds of medium and fine sand. Experiments were conducted under steady flow conditions with and without sediment transport in a recirculating flume. Flat beds and beds covered with ripples or triangular bedforms were studied. The net mass exchange was determined by measuring concentration changes in the main flow. The penetration of the solute (fluorescent dye) into the bed was also observed visually.
Two key exchange mechanisms, 'pumping' and 'turnover', were identified. Pumping is the movement of pore water into and out of the bed due to flows induced by pressure variations over bedforms (ripples and dunes). Turnover occurs as moving bedforms trap and release interstitial fluid.
Predictive models based on the details of the exchange processes were developed. A residence time distribution approach was used. The models do not require calibration. Appropriate scaling variables were identified.
With stationary bedforms the exchange is strongly influenced by pumping. The predictions of net mass exchange based on models of pumping with periodic bedforms show good agreement with the measured exchange in the initial stages of the experiments (hours to days). The models under-predict the exchange later in the experiments. The deviation is associated with the large-scale and somewhat random features in the penetration of the dye cloud. Such features are expected to influence the net exchange for large time in natural streams.
When the bedforms move slowly in relation to the characteristic pore water velocity, turnover can be neglected and pumping dominates. A model based on a random distribution of bedform sizes provides a good prediction of the mass exchange with slowly-moving bedforms.
With rapidly-moving bedforms, turnover dominates the exchange at the start of the experiments, when the solute penetration is limited to the maximum bedform scour depth. The scour depth can be predicted well. Later the depth of penetration is greater than the scour depth and the model predictions.https://thesis.library.caltech.edu/id/eprint/2836Nonlinear seismic behaviour of steel planar moment-resisting frames
https://resolver.caltech.edu/CaltechTHESIS:10062011-075420910
Authors: {'items': [{'id': 'Challa-Venkata-Ramana-Murty', 'name': {'family': 'Challa', 'given': 'Venkata Ramana Murty'}, 'show_email': 'NO'}]}
Year: 1992
DOI: 10.7907/jk1f-2d70
<p>The nonlinear response of steel planar moment-resisting frames during strong earthquakes poses a strong need for accurately modelling inelastic behaviour and large displacements. This thesis attempts to provide realistic and efficient analytical tools to aid this study.</p>
<p>Two large-displacement small-strain beam-column models are employed to include material and geometric nonlinearities. The first model assumes lumped plasticity, and discretises an element into segments. Axial force-Bending Moment strength interaction and flexural bowing are considered. Ten characteristic segment states are identified. An efficient numerical scheme is suggested to solve the nonlinear governing equations. This model only approximately represents the strength and stiffness of beam-columns.</p>
<p>A comprehensive finite element beam-column model is developed to more accurately model the strength and stiffness. A beam-column is discretised into segments, and further, each segment into one-dimensional fibres. A uniaxial cyclic constitutive law valid under arbitrary transient loading is proposed for structural steel. This physically motivated law incorporates the initial yield plateau, and provides explicit expressions for stress in terms of strain throughout the hysteretic path. This law is used to control the hysteretic loading of fibres.</p>
<p>A simple semi-empirical model is employed to analytically describe the highly nonlinear hysteretic behaviour of flexible joint panel zones in steel planar frames. Some modelling assumptions that may be made in frame analyses are evaluated. Numerical study of a building frame with flexible joints indicates that its collapse is sensitive to the joint panel zone design in addition to the ground motion.</p>
https://thesis.library.caltech.edu/id/eprint/6707Tsunamis : forces on a vertical wall caused by long waves, bores, and surges on a dry bed
https://resolver.caltech.edu/CaltechETD:etd-03242005-150131
Authors: {'items': [{'email': 'Ramsden@pbworld.com', 'id': 'Ramsden-J-D', 'name': {'family': 'Ramsden', 'given': 'Jerald Day'}, 'show_email': 'YES'}]}
Year: 1993
DOI: 10.7907/HVQR-ZH55
The major objective of this study has been to investigate experimentally the forces and overturning moments produced by tsunamis on vertical walls. The experimental results are compared with several analytical and numerical models. Several types of waves were used in a horizontal tank including solitary waves, undular bores, turbulent bores, and surges on a dry bed. Bores produced from breaking solitary waves in a tilting wave tank were also investigated. Various measurements were made, including the incident wave celerity, the wave profile, the runup, force, overturning moment, and pressure time histories. The impact process of the bores in the tilting wave tank were recorded with high-speed movies.
The wave profiles in the horizontal tank were defined using a laser induced-fluorescence system (LIF) which allows the free surface on a two-dimensional plane in the center of the wave tank to be recorded. This method was developed to measure accurately the surface elevation profile of turbulent high-speed flows which is difficult to measure reliably either with conventional flow visualization techniques or intrusive devices such as wave gages. The LIF method was also used to determine the runup on the wall.
Strong vertical accelerations were shown to occur during the reflection of bores and steep solitary waves at a vertical wall. These reduced the force on the wall relative to a hydrostatic force computed from the maximum runup height on the wall. The accelerations also cause the maximum force to occur before and after the maximum runup for steep solitary waves and bores, respectively. For these cases, the maximum measured force and overturning moment were always less than computed from the maximum measured runup on the wall using hydrostatic considerations. The maximum force due to surges on a dry bed was also less than the hydrostatic force calculated from the maximum runup height on the wall. For all the dry bed cases studied, the maximum runup height on the wall was between 1.46 and 1.62 times the velocity head computed from the celerity of the incident surge. For the entire range of wave conditions of this study, the maximum relative runup occurred for a bore with a relative wave height of 1.23, and produced a runup equal to 3.8 times the velocity head computed from the wave celerity.
The maximum measured water surface slopes along the front of long waves, bores, and dry bed surges were computed from the measured wave profiles. At the transition from undular bores to turbulent bores, there was a discontinuity in the maximum water surface slope where the slope increased by a factor of 2.5 to three for turbulent bores. This discontinuity corresponded with a rapid increase in the measured runup, force, and moment on the wall.
The properly normalized force on a vertical wall due to the impingement of a bore on a mildly sloping beach is shown to be equivalent to the force produced by a bore of constant volume on a horizontal bed. This implies the results from the horizontal wave tank experiments can be used to estimate the loads expected from bores propagating on mild beaches with slopes ranging up to 0.02m/m.
Two numerical models were compared with the experimental results. A boundary integral element model, which solves the potential flow problem subject to the full nonlinear free surface boundary conditions, predicted the loads imposed on the wall due to steep solitary waves quite well. A finite difference model of the Navier-Stokes equations was also used to simulate the reflection of solitary waves and mild turbulent bores at a vertical wall. This finite difference model predicted the solitary wave loads quite well; however, it over-predicted the steepness of the incident bore profiles and produced a force-time history with a high amplitude and short-duration peak, which was not observed in the measurements. Except for this sharp peak, the agreement of the finite difference model with the experimental results was quite reasonable.https://thesis.library.caltech.edu/id/eprint/1095Parsimonious modeling of inelastic systems
https://resolver.caltech.edu/CaltechTHESIS:11192012-102847690
Authors: {'items': [{'email': 'dchiang@mail.ncku.edu.tw', 'id': 'Chiang-D-Y', 'name': {'family': 'Chiang', 'given': 'Dar-Yun'}, 'show_email': 'YES'}]}
Year: 1993
DOI: 10.7907/75ZY-2K10
<p>Analytical modeling of one-dimensional hysteresis and general multi-axial cyclic plasticity is studied, with particular emphasis on the parsimony of model parameters and the physical consistency of model behavior. General criteria for good models are proposed to provide guidelines to the modeling studies conducted in this research.</p>
<p>Various one-dimensional hysteretic models are examined in detail, including both deteriorating and non-deteriorating models. A general formulation for modeling of degrading systems is presented based on the formulation of the Distributed-Element Model (DEM) and the introduction of a damage index function. A new class of deteriorating Masing models, whose behavior can be completely described by a few simple mathematical rules and the extended Masing rules, is also developed to substitute for a special class of deteriorating DEMs, so that their applicabilty to system identification studies is improved.</p>
<p>The one-dimensional DEMs are extended to the multi-dimensional case for constitutive modeling of cyclic plasticity, while preserving the concept of modeling plasticity by an assemblage of simple ideal elasto-plastic elements. In the generalization, a new invariant-yield-surface theory is proposed, in which no kinematic hardening rule is needed to account for the subsequent yielding and strain hardening behavior. A general theory is also developed to elucidate some important properties of material behavior based on the proposed multi-dimensional DEMs. The establishment of the theory provides instructive insight into the elastic-plastic response mechanisms of real materials under complicated loading conditions. Based on the insight, the Masing rules for one-dimensional hysteresis are extended to the multi-dimensional case by introducing a composition of plane-geometry transformations to a response formula developed for initial loading. This transformation method serves as an efficient way of implementing the classical multi-yield-surface theory with the Mroz kinematic hardening rule. Validity of the new formulations are confirmed by comparison with experimental results from the literature.</p>https://thesis.library.caltech.edu/id/eprint/7274A nonlinear thermoviscoelastic stress and fracture analysis of an adhesive bond
https://resolver.caltech.edu/CaltechETD:etd-10182005-153006
Authors: {'items': [{'email': 'cong.n.duong@boeing.com', 'id': 'Duong-C-N', 'name': {'family': 'Duong', 'given': 'Cong N.'}, 'show_email': 'NO'}]}
Year: 1994
DOI: 10.7907/3HF2-K703
The evolution of residual stresses resulting from cooling an adhesive bond configuration on its lateral surfaces at a constant rate through the glass transition of the polymer are considered. A nonlinear, viscoelastic (free-volume) model serves for the thermoviscoelastic characterization of the polymer. The simultaneous solution to the heat diffusion and the transient thermoviscoelatic problems are addressed. Both an infinite (one-dimensional) and a finite (two-dimensional) domain are studied. A "critical" cooling time exists, in the present case on the order of a few seconds, which separates the control of the solidification process according to whether the relaxation or thermal diffusion time scale governs. The short time "quenching process," i.e., when the time scale is governed by thermal diffusion, leads to essentially constant residual stresses. Slower cooling increasingly invokes the time and rate sensitive properties of the polymer and leads to monotically decreasing residual stresses with longer cooling times. To reduce residual stresses by a factor of two from their maximal values requires cooling times on the order of one or two days. These results are not drastically altered by changes in the thicknesses of the bond components. Apart from singular behavior of the stress components in the two-dimensionally finite domain "quenching" has the effect of producing significantly different stress distributions (including stress "spikes") than slow or thermoelastic analyses would suggest. This observation is attributed to the interaction of the bending response of the metal components early in the cooling history under the high thermal gradients, which deformations are then partially frozen in during the subsequent cooling of the polymer. Implications of these results for systems possessing geometric and material differences subjected to various thermal cooling ranges are also discussed. The results demonstrate the importance of knowing the bulk relaxation or creep spectrum for the polymer.
In the second part of the thesis the effect of the residual stresses on fracture behavior of an adhesive bond are addressed within the context of linear fracture mechanics for dissimilar materials. The crack faces are found to be in contact at the fractured end during the (residually stress) unloading process. A significantly error results if this contact zone is not taken into account. The combined effect of the mechanical loads and the residual stresses on the energy release rate is also studied. The total energy release rate from the combined effect is not necessarily higher or equal to the sum of the individual contribution from external loads and from residual stresses separately.
https://thesis.library.caltech.edu/id/eprint/4162Investigation of Base-Isolated Structures During Recent Earthquakes and Computer Simulations Utilizing Near-Source Long-Period Ground Motions
https://resolver.caltech.edu/CaltechETD:etd-10152007-141910
Authors: {'items': [{'id': 'Halling-Marvin-Wilford', 'name': {'family': 'Halling', 'given': 'Marvin Wilford'}, 'show_email': 'NO'}]}
Year: 1995
DOI: 10.7907/HWVH-DM02
<p>Base isolation is a recently applied technology for building structures in the United States. To date, the three base-isolated buildings considered in this study have been subjected to earthquakes of varying magnitudes and epicentral distances. The records obtained from these instrumented buildings demonstrate low levels of excitation and small structural responses. In all cases, the maximum relative displacement of the roof to the foundation is less than 3 cm. However, an increasing quantity of near-source strong-motion records produces large spectral displacements of up to approximately 50-55 cm in the 2 to 2.5 sec period range for 15% damping. This suggests that long-period structures such as base-isolated structures would be vulnerable to these near-source ground motions.</p>
<p>The current study contains two major parts. Part One consists of the identification and analysis of three existing base-isolated buildings in Southern California. The identification and analysis utilize the recorded motions of these structures from past earthquakes. System identification is useful for understanding the extent to which the structures enter the nonlinear realm and how much their properties change.</p>
<p>Models are constructed assuming completely elastic three-dimensional superstructures, with idealized bi-linear hysteretic elements for the isolating bearings. The properties used in the bearing models were taken from tests of the actual bearings before installation. The models were then verified by comparing their responses computed using the various recorded foundation ground motions, with the recorded responses of the actual structures. The models were adjusted to minimize the error of several response quantities.</p>
<p>Part Two contains computer simulations for the three structural models developed in Part One subjected to large-amplitude near-source ground motions. These structural models were subjected to two classes of ground motions. The first is a sampling of near-source recorded motion from past moderate-to-large earthquakes. The second is a group of synthetic near-source motions generated for a hypothetical M 7.0 earthquake. In some cases, the lateral response of the models exceeds the isolation gap, indicating that the displacement barrier would be impacted.</p>
<p>In order to further study base-isolated buildings when the isolation bearings undergo large displacements, a typical base-isolated building (TBIB) model is used and the computer program 2D-BUMP is developed. This program includes the effects of a fully nonlinear superstructure, nonlinear springs acting as displacement barriers which engage at specified distances, and a tri-linear model for the elastomeric bearings. Using this model, several conclusions are drawn regarding the probable areal extent of damaging near-source ground motions from the M 7.0 event, as well as the behavior of base-isolated structures due to these near-source long-period ground motions.</p>https://thesis.library.caltech.edu/id/eprint/4102Mode-Like Properties and Identification of Nonlinear Vibrating Systems
https://resolver.caltech.edu/CaltechTHESIS:12012011-113838877
Authors: {'items': [{'email': 'lhuang1306@gmail.com', 'id': 'Huang-Liping', 'name': {'family': 'Huang', 'given': 'Liping'}, 'show_email': 'NO'}]}
Year: 1995
DOI: 10.7907/707b-jf67
<p>A study is made of mode-like properties and identification of nonlinear systems and their applications in structural seismic analysis.</p>
<p>In the thesis, mode-like behavior of nonlinear systems is examined. The modal frequencies and mode shapes of nonlinear systems are found to be dependent on the
response. Based on approximation, amplitude-dependent mode shape is defined and approximate methods for calculation of modal frequencies and mode shapes (instantaneous and amplitude-dependent) are presented. Based on amplitude-dependent modal relationship, amplitude-dependent models of modal equations which are valid in large range of response
and suitable for unique identification are proposed and the corresponding modal identification procedures are developed. The applicability of the new models and
identification algorithms is tested through the analysis of an ideal 3DOF nonlinear system.</p>
<p>As applications, the seismic responses of a 47-story building and a 4-story building are investigated using the presented methods. The modal parameters and modal equations
of the structures are identified.</p>https://thesis.library.caltech.edu/id/eprint/6747The evolution of damage in ceramic matrix composites
https://resolver.caltech.edu/CaltechETD:etd-01072008-112449
Authors: {'items': [{'id': 'Walter-M-E', 'name': {'family': 'Walter', 'given': 'Mark E.'}, 'show_email': 'NO'}]}
Year: 1996
DOI: 10.7907/w4b4-dx66
In an effort to better understand the evolution of damage in brittle matrix composites, the mechanical behavior of a ceramic matrix composite, unidirectional SiC/CAS (SiC fibers reinforcing a calcium aluminosilicate matrix), was studied. The presented results are based on uniaxial tension experiments for specimens with the fibers aligned in the loading direction. Post-test optical and scanning electron microscopy was also used to identify the various micromechanisms of damage; axial and transverse strain gauges on all four gage section surfaces and in situ acoustic emission and ultrasonic wave speed measurements were used to monitor the evolution of damage. The experimental results demonstrate the existence of "zones of deformation" which are associated with the onset of different damage mechanisms. The energy dissipated in each of these zones was calculated. It is shown that the observed stress-strain behavior can be qualitatively explained in terms of the material properties of the matrix and the fiber, the material processing, and the postulated zones of deformation.
The experimental results for SiC/CAS were compared with an existing shear-lag model, and the shortcomings of the model are discussed. By approximating matrix cracks as penny shaped cracks, a micromechanical model was used to estimate the change in the axial modulus of the composite. These results also present another way to interpret the acoustic emission data.
The evolution of damage in the SiC/CAS experiments was found to be strain rate dependent even within the quasi-static strain rate regime. For higher rate experiments, the transition from elastic to matrix cracked occurred at a stress level that was nearly twice that of the same transition in the lower rate experiments. This phenomenon and the mechanisms which cause it was further investigated with a model material system (a brittle epoxy resin sandwiched between aluminum strips). In situ quantification of the stress during damage initiation and propagation was realized by the optical method of Coherent Gradient Sensing. Based on these results, the reasons for strain rate dependence of the composite are postulated.
Detailed understanding of aspects of the evolution of in brittle matrix composites was achieved with finite element simulations. This modeling was based on an axisymmetric unit cell composed of a fiber and its surrounding matrix. The unit cell was discretized into linearly elastic elements for the fiber and the matrix and cohesive elements which allow cracking in the matrix, fiber-matrix interface, and fiber. The cohesive elements failed according to critical stress and critical energy release rate criteria (in shear and/or in tension). After failing, the cohesive elements could slide with Coulomb friction. The tension and shear aspects of failure were uncoupled. The cohesive elements were used to simulate a Dugdale penny shaped crack in a homogeneous cylinder; results compared well to the analytical solution. In order to solve the composite axisymmetric unit cell problem, inertia and viscous damping were added to the formulation. The resulting dynamic problem was solved implicitly using the Newmark Method. Results were compared to the experiment by assuming that only a given number of unit cells were active at any point during the simulation. The effects of changing material properties (e.g., interface strength and toughness and matrix toughness) and loading rate are discussed. Several aspects of the experimentally observed material response of SiC/CAS composite were reproduced by the numerical simulations.
https://thesis.library.caltech.edu/id/eprint/58Response Control of Structural Systems Using Semi-Actively Controlled Interactions
https://resolver.caltech.edu/CaltechTHESIS:01032013-113224489
Authors: {'items': [{'id': 'Hayen-Jeffrey-Clyde', 'name': {'family': 'Hayen', 'given': 'Jeffrey Clyde'}, 'show_email': 'NO'}]}
Year: 1996
DOI: 10.7907/2c70-e383
<p>The objective of the research described herein is to demonstrate conditions under which controlled interactions between two structures or structural components can be made effective in reducing the response of structures that are subjected to seismic excitation. It is shown that the effectiveness depends upon such factors as the control
strategy implementation, the interaction element mechanical properties, and the parameters which characterize the dynamic behavior of the structural systems.</p>
<p>A study is conducted to examine the performance of a structural response control approach referred to as Active Interface Damping (AID). This control approach utilizes
controlled interactions between two distinct structural systems or different components of a single structural system to reduce the resonance buildup that develops
during an external excitation. Control devices or elements may be employed to physically produce the interactions between the systems. The proposed control approach
differs from some other control approaches in that the sensors, processors, and switching components all operate actively, whereas the interaction elements function passively. The major advantage of this semi-active control technology is that relatively large control forces can be generated with minimal power requirements, which is of prime importance for the control of relatively massive systems, such as structures.</p>
<p>In the most simple form, the strategy of the control approach is to remove energy associated with vibration from only one system (the primary system). This process is
accomplished through the transfer of energy to another system (the auxiliary system) by means of interaction elements, the dissipation of energy directly in the interaction elements, or a combination of both these methods. In a more complex form, the control strategy may be to minimize some composite response measure of the combined primary-auxiliary system. Only the most simple form of the control strategy is considered in the present study.</p>
<p>Several physical interpretations of the control approach are possible: one is that the systems represent two adjacent multi-story buildings; another is that the primary system represents a single multi-story building, while the auxiliary system could represent either an externally- situated resilient frame or a relatively small, unrestrained mass - or even be completely absent (in this latter scenario, the interaction elements are internally-mounted control devices). The interactions consist of reaction forces that are developed within and transmitted through the elements which are located between the two systems (or different points of a single system). The mechanical properties of these elements can be altered in real time by control signals, so the reaction forces applied to the systems may be changed, and the response control objective is achieved by actively changing the
interactions at the interface of the two systems (or different points of a single system).</p>
<p>Initially, a preliminary study of the proposed control approach is conducted within the specialized setting of linear single-degree-of-freedom (SDOF) primary and auxiliary
systems. Numerical simulations are performed for a series of control cases using horizontal ground accelerations from an ensemble of earthquake time histories as excitation input. Subsequently, a follow-on study of the proposed control approach is conducted for linear multiple-degree-of-freedom (MDOF) primary and auxiliary systems intended to represent actual structural systems. Based upon the investigation and insight obtained from the preliminary study, a limited number of control cases are considered
which include those deemed most effective and implementable. Numerical simulations are again performed using the same excitation input as for the SDOF systems. The
control approach is targeted at reducing the response contribution from the fundamental or dominant mode of vibration associated with the primary system. Uniformly-discretized models of a 6-story primary structural system capable of only lateral deformations are considered in most cases. A few cases involving models of a 3-story primary
structural system are also examined.</p>
https://thesis.library.caltech.edu/id/eprint/7365On the Dynamic Response of Nonlinear Uncertain Systems
https://resolver.caltech.edu/CaltechTHESIS:10282019-124732191
Authors: {'items': [{'id': 'Huang-Ching-Tung', 'name': {'family': 'Huang', 'given': 'Ching-Tung'}, 'show_email': 'NO'}]}
Year: 1996
DOI: 10.7907/2xpp-3s50
<p>This thesis presents an approach for performing second moment analyses of nonlinear dynamic systems with parameter uncertainty. The uncertain parameters are modeled as time-independent random variables. The set of orthogonal polynomials associated with the probability density function is used as the solution basis. When a deterministic excitation source is considered, the response variables are expanded in terms of a finite sum of these polynomials with time-dependent coefficients. The weighted residual method is employed to derive a set of deterministic nonlinear differential equations that can be solved numerically for evaluations of response statistics.</p>
<p>This solution approach is further extended to nonlinear continuous systems involving inhomogeneous random media. A discrete representation is obtained via a spatial discretization procedure for the continuous response variables as well as the random continuum. Thus, the continuous random system can then be treated as in the case of the discrete random systems. The solution approach is applied to a study of a nonlinear random shear-beam model subjected to a near-field earthquake ground motion.</p>
<p>The response uncertainty for nonlinear uncertain systems subjected to external stochastic excitation is also investigated. A general solution procedure based on equivalent linearization is presented. In this solution methodology, the instantaneous equivalent stiffness and damping matrices are approximated as quadratic random functions. The resulting Liapunov system with explicit random coefficients can then be solved using the newly developed solution approach. Applications to single-degree-of-freedom uncertain systems are given and the accuracy of the results is validated.</p>https://thesis.library.caltech.edu/id/eprint/11867Earthquake Response of Steel Braces and Braced Steel Frames
https://resolver.caltech.edu/CaltechThesis:03142014-103017013
Authors: {'items': [{'id': 'Gan-Wenshui', 'name': {'family': 'Gan', 'given': 'Wenshui'}, 'show_email': 'NO'}]}
Year: 1997
DOI: 10.7907/6zt3-2k51
<p>This thesis consists of three parts. Chapter 2 deals with the dynamic buckling behavior of steel braces under cyclic axial end displacement. Braces under such a loading condition belong to a class of "acceleration magnifying" structural components, in which a small motion at the loading points can cause large internal acceleration and inertia. This member-level inertia is frequently ignored in current studies of braces and braced structures. This chapter shows that, under certain conditions, the inclusion of the member-level inertia can lead to brace behavior fundamentally different from that predicted by the quasi-static method. This result is to have significance in the correct use of the quasi-static, pseudo-dynamic and static condensation methods in the simulation of braces or braced structures under dynamic loading. The strain magnitude and distribution in the braces are also studied in this chapter.</p>
<p>Chapter 3 examines the effect of column uplift on the earthquake response of braced steel frames and explores the feasibility of flexible column-base anchoring. It is found that fully anchored braced-bay columns can induce extremely large internal forces in the braced-bay members and their connections, thus increasing the risk of failures observed in recent earthquakes. Flexible braced-bay column anchoring can significantly reduce the braced bay member force, but at the same time also introduces large story drift and column uplift. The pounding of an uplifting column with its support can result in very high compressive axial force.</p>
<p>Chapter 4 conducts a comparative study on the effectiveness of a proposed non-buckling bracing system and several conventional bracing systems. The non-buckling bracing system eliminates buckling and thus can be composed of small individual braces distributed widely in a structure to reduce bracing force concentration and increase redundancy. The elimination of buckling results in a significantly more effective bracing system compared with the conventional bracing systems. Among the conventional bracing systems, bracing configurations and end conditions for the bracing members affect the effectiveness.</p>
<p>The studies in Chapter 3 and Chapter 4 also indicate that code-designed conventionally braced steel frames can experience unacceptably severe response under the strong ground motions recorded during the recent Northridge and Kobe earthquakes.</p>https://thesis.library.caltech.edu/id/eprint/8133Active interaction control for civil structures
https://resolver.caltech.edu/CaltechThesis:03142014-105804007
Authors: {'items': [{'id': 'Wang-L-J', 'name': {'family': 'Wang', 'given': 'Luo-Jia'}, 'show_email': 'NO'}]}
Year: 1997
DOI: 10.7907/zsgm-ve62
<p>This thesis presents a civil engineering approach to active control for civil structures. The proposed control technique, termed Active Interaction Control (AIC), utilizes dynamic interactions between different structures, or components of the same structure, to reduce the resonance response of the controlled or primary structure under earthquake excitations. The primary control objective of AIC is to minimize the maximum story drift of the primary structure. This is accomplished by timing the controlled interactions so as to withdraw the maximum possible vibrational energy from the primary structure to an auxiliary structure, where the energy is stored and eventually dissipated as the external excitation decreases. One of the important advantages of AIC over most conventional active control approaches is the very low external power required.</p>
<p>In this thesis, the AIC concept is introduced and a new AIC algorithm, termed Optimal Connection Strategy (OCS) algorithm, is proposed. The efficiency of the OCS algorithm is demonstrated and compared with two previously existing AIC algorithms, the Active Interface Damping (AID) and Active Variable Stiffness (AVS) algorithms, through idealized examples and numerical simulations of Single- and Multi-Degree-of Freedom systems under earthquake excitations. It is found that the OCS algorithm is capable of significantly reducing the story drift response of the primary structure. The effects of the mass, damping, and stiffness of the auxiliary structure on the system performance are investigated in parametric studies. Practical issues such as the sampling interval and time delay are also examined. A simple but effective predictive time delay compensation scheme is developed.</p>https://thesis.library.caltech.edu/id/eprint/8135Probabilistic Robust Control: Theory and Applications
https://resolver.caltech.edu/CaltechTHESIS:03042014-093439011
Authors: {'items': [{'id': 'May-Bennett-Scott', 'name': {'family': 'May', 'given': 'Bennett Scott'}, 'show_email': 'NO'}]}
Year: 1998
DOI: 10.7907/19jn-c337
<p>In this work, the development of a probabilistic approach to robust control is motivated by structural control applications in civil engineering. Often in civil structural applications, a system's performance is specified in terms of its reliability. In addition, the model and input uncertainty for the system may be described most appropriately using probabilistic or "soft" bounds on the model and input sets. The probabilistic robust control methodology contrasts with existing H∞/μ robust control methodologies that do not use probability information for the model and input uncertainty sets, yielding only the guaranteed (i.e., "worst-case") system performance, and no information about the system's probable performance which would be of interest to civil engineers.</p>
<p>The design objective for the probabilistic robust controller is to maximize the reliability of the uncertain structure/controller system for a probabilistically-described uncertain excitation. The robust performance is computed for a set of possible models by weighting the conditional performance probability for a particular model by the probability of that model, then integrating over the set of possible models. This integration is accomplished efficiently using an asymptotic approximation. The probable performance can be optimized numerically over the class of allowable controllers to find the optimal controller. Also, if structural response data becomes available from a controlled structure, its probable performance can easily be updated using Bayes's Theorem to update the probability distribution over the set of possible models. An updated optimal controller can then be produced, if desired, by following the original procedure. Thus, the probabilistic framework integrates system identification and robust control in a natural manner.</p>
<p>The probabilistic robust control methodology is applied to two systems in this thesis. The first is a high-fidelity computer model of a benchmark structural control laboratory experiment. For this application, uncertainty in the input model only is considered. The probabilistic control design minimizes the failure probability of the benchmark system while remaining robust with respect to the input model uncertainty. The performance of an optimal low-order controller compares favorably with higher-order controllers for the same benchmark system which are based on other approaches. The second application is to the Caltech Flexible Structure, which is a light-weight aluminum truss structure actuated by three voice coil actuators. A controller is designed to minimize the failure probability for a nominal model of this system. Furthermore, the method for updating the model-based performance calculation given new response data from the system is illustrated.</p>https://thesis.library.caltech.edu/id/eprint/8102Finite-Element Simulations of Earthquakes
https://resolver.caltech.edu/CaltechThesis:04292016-090209111
Authors: {'items': [{'email': 'baagaard@usgs.gov', 'id': 'Aagard-Brad-Thomas', 'name': {'family': 'Aagaard', 'given': 'Brad Thomas'}, 'show_email': 'NO'}]}
Year: 2000
DOI: 10.7907/T65C-9C94
<p>This thesis discusses simulations of earthquake ground motions using prescribed ruptures and dynamic failure. Introducing sliding degrees of freedom led to an innovative technique for numerical modeling of earthquake sources. This technique allows efficient implementation of both prescribed ruptures and dynamic failure on an arbitrarily oriented fault surface. Off the fault surface the solution of the three-dimensional, dynamic elasticity equation uses well known finite-element techniques. We employ parallel processing to efficiently compute the ground motions in domains containing millions of degrees of freedom.</p>
<p>Using prescribed ruptures we study the sensitivity of long-period near-source ground motions to five earthquake source parameters for hypothetical events on a strike-slip fault (M<sub>w</sub> 7.0 to 7.1) and a thrust fault (M<sub>w</sub> 6.6 to 7.0). The directivity of the ruptures creates large displacement and
velocity pulses in the ground motions in the forward direction. We found a good match between the severity of the shaking and the shape of the near-source factor from the 1997 Uniform Building Code for strike-slip faults and thrust faults with surface rupture. However, for blind thrust faults the peak displacement and velocities occur up-dip from the region with the peak near-source factor. We assert that a simple modification to the formulation of the near-source factor improves the match between the severity of the ground motion and the shape of the near-source factor.</p>
<p>For simulations with dynamic failure on a strike-slip fault or a thrust fault, we examine what constraints must be imposed on the coefficient of friction to produce realistic ruptures under the application of reasonable shear and normal stress distributions with depth. We found that variation of the coefficient of friction with the shear modulus and the depth produces realistic rupture behavior in both homogeneous and layered half-spaces. Furthermore, we observed a dependence of the rupture speed on the direction of propagation and fluctuations in the rupture speed and slip rate as the rupture encountered changes in the stress field. Including such behavior in prescribed ruptures would yield more realistic ground motions.</p> https://thesis.library.caltech.edu/id/eprint/9690Structural Design under Seismic Risk Using Multiple Performance Objectives
https://resolver.caltech.edu/CaltechThesis:05052016-115652196
Authors: {'items': [{'email': 'ayhan@alumni.caltech.edu', 'id': 'Irfanoglu-Ayhan', 'name': {'family': 'Irfanoglu', 'given': 'Ayhan'}, 'orcid': '0000-0001-8334-6717', 'show_email': 'NO'}]}
Year: 2000
DOI: 10.7907/W5WE-TD86
<p>Structural design is a decision-making process in which a wide spectrum of requirements, expectations, and concerns needs to be properly addressed. Engineering design criteria are considered together with societal and client preferences, and most of these design objectives are affected by the uncertainties surrounding a design. Therefore, realistic design frameworks must be able to handle multiple performance objectives and incorporate uncertainties from numerous sources into the process.</p>
<p>In this study, a multi-criteria based design framework for structural design under seismic risk is explored. The emphasis is on reliability-based performance objectives and their interaction with economic objectives. The framework has analysis, evaluation, and revision stages. In the probabilistic response analysis, seismic loading uncertainties as well as modeling uncertainties are incorporated. For evaluation, two approaches are suggested: one based on preference aggregation and the other based on socio-economics. Both implementations of the general framework are illustrated
with simple but informative design examples to explore the basic features of the framework.</p>
<p>The first approach uses concepts similar to those found in multi-criteria decision theory, and directly combines reliability-based objectives with others. This approach is implemented in a single-stage design procedure. In the socio-economics based approach, a two-stage design procedure is recommended in which societal preferences are treated through reliability-based engineering performance measures, but emphasis is also given to economic objectives because these are especially important to the structural designer's client. A rational net asset value formulation including losses from uncertain future earthquakes is used to assess the economic performance of a design. A recently developed assembly-based vulnerability analysis is incorporated into the loss estimation.</p>
<p>The presented performance-based design framework allows investigation of various design issues and their impact on a structural design. It is a flexible one that readily allows incorporation of new methods and concepts in seismic hazard specification, structural analysis, and loss estimation.</p>
https://thesis.library.caltech.edu/id/eprint/9702On the Solution of First Excursion Problems by Simulation with Applications to Probabilistic Seismic Performance Assessment
https://resolver.caltech.edu/CaltechTHESIS:03062014-085528325
Authors: {'items': [{'email': 'ivanau@ntu.edu.sg', 'id': 'Au-Siu-Kui', 'name': {'family': 'Au', 'given': 'Siu-Kui'}, 'orcid': '0000-0002-0228-1796', 'show_email': 'NO'}]}
Year: 2001
DOI: 10.7907/C0JQ-G051
<p>In a probabilistic assessment of the performance of structures subjected to uncertain environmental
loads such as earthquakes, an important problem is to determine the probability that the structural response exceeds some specified limits within a given duration of interest. This problem is known as the first excursion problem, and it has been a challenging problem in the theory of stochastic dynamics and reliability analysis. In spite of the enormous amount of attention the problem
has received, there is no procedure available for its general solution, especially for engineering
problems of interest where the complexity of the system is large and the failure probability is small.</p>
<p>The application of simulation methods to solving the first excursion problem is investigated in this dissertation, with the objective of assessing the probabilistic performance of structures subjected to uncertain earthquake excitations modeled by stochastic processes. From a simulation perspective, the major difficulty in the first excursion problem comes from the large number of uncertain parameters
often encountered in the stochastic description of the excitation. Existing simulation tools are examined, with special regard to their applicability in problems with a large number of uncertain parameters. Two efficient simulation methods are developed to solve the first excursion problem. The first method is developed specifically for linear dynamical systems, and it is found to be extremely efficient compared to existing techniques. The second method is more robust to the type of
problem, and it is applicable to general dynamical systems. It is efficient for estimating small failure probabilities because the computational effort grows at a much slower rate with decreasing failure probability than standard Monte Carlo simulation. The simulation methods are applied to assess the probabilistic performance of structures subjected to uncertain earthquake excitation. Failure
analysis is also carried out using the samples generated during simulation, which provide insight into the probable scenarios that will occur given that a structure fails.</p>
https://thesis.library.caltech.edu/id/eprint/8106Model Selection, Identification and Robust Control for Dynamical Systems
https://resolver.caltech.edu/CaltechTHESIS:02082012-113016272
Authors: {'items': [{'email': 'kvyuen@umac.mo', 'id': 'Yuen-Ka-Veng', 'name': {'family': 'Yuen', 'given': 'Ka-Veng'}, 'orcid': '0000-0002-1755-6668', 'show_email': 'YES'}]}
Year: 2002
DOI: 10.7907/YEV9-8X44
<p>To fully exploit new technologies for response mitigation and structural health monitoring, improved system identification and controller design methodologies are desirable that explicitly treat all the inherent uncertainties. In this thesis, a probabilistic framework is presented for model selection, identification and robust control of smart structural systems under dynamical loads, such as those induced by wind or earthquakes. First, a probabilistic based approach is introduced for selecting the most plausible class of models for a dynamical system using its response measurements. The proposed approach allows for quantitatively comparing the plausibility of different classes of models among a specified set of classes.</p>
<p>Then, two probabilistic identification techniques are presented. The first one is for modal identification using nonstationary response measurements and the second one is for updating nonlinear models using incomplete noisy measurements only. These methods allow for updating of the uncertainties associated with the values of the parameters controlling the dynamic behavior of the structure by using noisy response measurements only. The probabilistic framework is very well-suited for solving this nonunique problem and the updated probabilistic description of the system can be used to design a robust controller of the system. It can also be used for structural health monitoring.</p>
<p>Finally, a reliability-based stochastic robust control approach is used to design the controller for an active control system. Feedback of the incomplete response at earlier time steps is used, without any state estimation. The optimal controller is chosen by minimizing the robust failure probability over a set of possible models for the system. Here, failure means excessive levels of one or more response quantities representative of the performance of the structure and the control devices. When calculating the robust failure probability, the plausibility of each model as a representation of the system's dynamic behavior is quantified by a probability distribution over the set of possible models; this distribution is initially based on engineering judgement, but it can be updated using the aforementioned system identification approaches if dynamic data become available from the structure. Examples are presented to illustrate the proposed controller design procedure, which includes the procedure of model selection, identification and robust control for smart structures.</p> https://thesis.library.caltech.edu/id/eprint/6817A Statistical Approach to Equivalent Linearization with Application to Performance-Based Engineering
https://resolver.caltech.edu/CaltechETD:etd-06012003-123539
Authors: {'items': [{'id': 'Guyader-Andrew-Charles', 'name': {'family': 'Guyader', 'given': 'Andrew Charles'}, 'show_email': 'NO'}]}
Year: 2003
DOI: 10.7907/Z9HZ-9M41
<p>A new methodology for calculating optimal effective linear parameters for use in predicting the earthquake response of structures is developed. The methodology is applied to several single-degree-of-freedom inelastic structural models subjected to a suite of earthquake acceleration time histories. Separately, far-field and near-field earthquakes are analyzed. Error distributions over a two-dimensional parameter space of period and damping are analyzed through a statistical approach with optimization criterion most applicable to structural design. Four hysteretic models are analyzed: bilinear, stiffness degrading, strength degrading and pinching. Initial structural periods are analyzed in groups for several second slope ratios (alpha) at different levels of ductility. It was discovered that as ductility increases, the accuracy of the effective parameters decrease but the consequences of bad parameter selection become less severe.</p>
<p>The new effective parameters are intended for use in displacement-based structural analysis procedures as used in Performance-Based Engineering. Of the several procedures available, Nonlinear Static Procedures, such as the Capacity Spectrum Method, are widely used by structural engineers because the nonlinear characteristics of both structural components and the global structure are utilized without running a nonlinear time history analysis. Effective linear parameters are used in the Capacity Spectrum Method to calculate the expected displacement demand, or Performance Point, for a structure. Because several sources of error exist within the Capacity Spectrum Method, an analysis that isolates the error from the effective linear parameters is performed. The new effective linear parameters show considerable improvement over the existing effective linear equations. The existing linear parameters are extremely unconservative at the lower ductilities and conservative at the higher ductilities. The new parameters lead to a significant improvement in both cases.</p>
<p>A modification to the Capacity Spectrum Method is introduced to account for the new effective linear period. Currently, the Capacity Spectrum Method uses the secant period as the effective linear period. The modification preserves the basic Performance Point calculation. Finally, a new, entirely graphical solution procedure using a Locus of Performance Points provides crucial insight into the effects of strengthening, stiffening and increasing building ductility not available in the current procedure.</p>https://thesis.library.caltech.edu/id/eprint/2346Dynamic Characteristics of Woodframe Buildings
https://resolver.caltech.edu/CaltechETD:etd-06092003-150851
Authors: {'items': [{'id': 'Camelo-Vanessa-Sabrina', 'name': {'family': 'Camelo', 'given': 'Vanessa Sabrina'}, 'show_email': 'NO'}]}
Year: 2003
DOI: 10.7907/GPHK-KA52
A database of dynamic characteristics of woodframe buildings was developed through analysis of recorded earthquake response and by forced vibration and shake-table testing. Modal identification was performed on eight sets of strong-motion records obtained from five buildings, and forced vibration tests were performed on five other buildings. The periods identified were sensitive to the amplitude of shaking, due to the reduction in lateral stiffness at stronger shaking levels. The equivalent viscous damping ratios were usually more than 10% of critical during earthquake shaking. A regression analysis was performed on the earthquake and forced vibration test data to obtain a simple, but reasonably accurate, period formula for woodframe buildings at low drift levels (less than 0.1%). Data obtained from the UC San Diego and UC Berkeley full-scale shake-table tests illustrate the shift in periods due to increasing shaking amplitude. Forced vibration tests of the UC Berkeley 3-story building before and after the shake-table tests showed how the periods and modeshapes shift due to damage. A simple analytical model of masses and springs was used to model the UC Berkeley test structure. The effects of diaphragm stiffness and mass distribution assumptions were evaluated and found to have a significant effect on the model torsional response. This model was used to find the equivalent wall stiffnesses giving frequency-response curves that best-fit the experimental data. These spring values were used to quantify the stiffness loss resulting from severe shaking of the structure, and the observed damage corresponded to stiffness losses of over 75%. The correlation between stiffness loss and damage to woodframe buildings has potential structural health monitoring implications.https://thesis.library.caltech.edu/id/eprint/2524Energy Radiation from a Multi-Story Building
https://resolver.caltech.edu/CaltechETD:etd-06032004-143147
Authors: {'items': [{'id': 'Favela-Javier', 'name': {'family': 'Favela', 'given': 'Javier'}, 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/SNYE-QG70
<p>Damping limits the resonance of vibrating systems and thus higher anelastic damping is generally favored for engineered structures subjected to earthquake motions, because it means that a structure can dissipate a larger percentage of its energy per oscillation cycle. However, there are elastic processes that can mimic the effects of anelastic damping. In particular, buildings lose kinetic energy when their motion generates elastic waves in the Earth, which is referred to as radiation damping. Unlike anelastic damping, strong radiation damping may not always be desirable, as reciprocity can be used to show that buildings may be strongly excited by elastic waves of similar characteristics to those generated by the building's forced vibrations. As a result, it is important to quantify the radiation damping of structures to be able to improve their design.</p>
<p>Several experiments using Caltech's nine story Millikan Library as a controlled source were performed to investigate the radiation damping of the structure. The building was forced to resonate at its North-South and East-West fundamental modes, and seismometers were deployed around the structure in order to measure the waves generated by the library's excitation. From this "local" data set, we determine the elastic properties of the soils surrounding the structure and estimate what percentage of the total damping of the structure is due to energy radiation. Using Fourier transforms, we were also able to detect these waves at distances up to 400 km from the source using the broadband stations of the Southern California Seismic Network. This "regional" data set is used in an attempt to identify arrival times and to constrain the type of waves being observed at regional distances.</p>https://thesis.library.caltech.edu/id/eprint/2417Structural Damage Evaluation: Theory and Applications to Earthquake Engineering
https://resolver.caltech.edu/CaltechETD:etd-05172004-101516
Authors: {'items': [{'id': 'Shaikhutdinov-Rustem-Vil', 'name': {'family': 'Shaikhutdinov', 'given': 'Rustem Vil'}, 'orcid': 'Shaikhutdinov', 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/5SA6-4414
The further development of performance-based earthquake engineering (PBEE) is on the current agenda of the earthquake engineering community. A part of assessing the seismic performance of civil engineering structures involves estimation of seismic damage. The conventional approach to damage estimation is based on fragility functions that relate some chosen parameters of structural response to incurred damage. Therefore, damage prediction is based exclusively on the knowledge of the chosen structural response parameters, meaning that damage analysis is uncoupled from the structural analysis. The structural response parameters selected for use in damage analysis are usually referred to as engineering demand parameters (EDP). In the present study, it is shown that for structural damage estimation, the uncoupled damage analysis has deficiencies that lead to less accurate damage prediction. These shortcomings originate from two sources: first, dependence of practically all EDPs on structural damage and second, inexact damage description. To overcome these deficiencies, another approach to structural damage estimation is proposed. The proposed approach, besides using an EDP, uses all information available from structural analysis that is relevant to the damage to be assessed, implying that damage analysis is coupled with structural analysis. It is shown that utilization of this additional information provides more accurate damage prediction. The difference between the two approaches is studied by comparison of results of damage estimation performed for a 2-D structural model of a reinforced-concrete frame. The results show that difference between uncoupled and coupled damage analysis estimates could be significant and that it depends on specific characteristics of the chosen structural model and the damage model in a complex way, preventing the possibility of estimating this error in a general form that is applicable to all practically possible cases. Damage estimation is performed for various damage models that include both single and multiple damage states. Since the final goal of seismic performance evaluation is estimation of decision variables such as repair cost, downtime, etc., the two approaches to damage estimation are also compared in terms of repair cost that is calculated for the reinforced-concrete frame. A case where structural damage prediction is based on observation of EDP alone, without a structural model available, is also studied. It is shown that incorporating site-specific information can significantly change the damage estimates and, therefore, may be worth doing.https://thesis.library.caltech.edu/id/eprint/1844Structural Control Using Regenerative Force Actuation Networks
https://resolver.caltech.edu/CaltechETD:etd-06012004-063432
Authors: {'items': [{'email': 'jscruggs@umich.edu', 'id': 'Scruggs-Jeffrey-Thomas', 'name': {'family': 'Scruggs', 'given': 'Jeffrey Thomas'}, 'orcid': '0000-0002-1560-6211', 'show_email': 'YES'}]}
Year: 2004
DOI: 10.7907/W3M9-ZW72
<p>A Regenerative Force Actuation (RFA) Network consists of multiple electromechanical forcing devices distributed throughout a structural system and actuated in such a way as to reduce the response of the structure when subject to an excitation. The associated electronics of the devices are connected together such that they are capable of sharing electrical power with each other. This makes it possible for some devices to extract mechanical energy from the structure, while others re-inject a portion of that energy back into the structure at other locations. The forcing capability of an RFA network is constrained only by the requirement that in the aggregate the total network must always dissipate energy.</p>
<p>The electromechanical currents generated by RFA networks must be controlled to create the desired structural forces. This control is facilitated by the alternation of a multitude of power-electronic transistor switches in the electrical network. In this study, a sliding-mode switching controller is proposed for realizing zero-error force command tracking. It is shown that parameter uncertainty is a critical issue for force commands which require the network to operate near its optimum transmissive efficiency.</p>
<p>RFA networks can be used to create velocity-proportional damping forces in structures. However, unlike traditional structural damping, RFA networks have the ability to create non-local and asymmetric damping forces. It is shown that this more generalized damping capability can lead to significant improvements in the forced response of a structure, as compared with traditional linear damping.</p>
<p>RFA networks may also be used for feedback control. In this context, the forcing capability of the RFA network is constrained by its physical limitations. In this study, a systematic method of nonlinear control design called "Damping-Reference" control is proposed, which guarantees a certain level of quadratic performance for the structural response. Variants of the control law synthesis are proposed for quadratic regulation, stochastic control, and H[infinity] control contexts.</p>
<p>These ideas are illustrated in the context of earthquake engineering through a simulation example, involving a three-story structure with a two-actuator RFA network installed. In this example, it is shown that the "power sharing" nature of the RFA network has a significant influence on the response.</p>https://thesis.library.caltech.edu/id/eprint/2347Modern Digital Seismology: Instrumentation, and Small Amplitude Studies in the Engineering World
https://resolver.caltech.edu/CaltechETD:etd-05202004-225044
Authors: {'items': [{'email': 'jclinton@sed.ethz.ch', 'id': 'Clinton-John-Francis', 'name': {'family': 'Clinton', 'given': 'John Francis'}, 'orcid': '0000-0001-8626-2703', 'show_email': 'YES'}]}
Year: 2004
DOI: 10.7907/DVSS-2290
<p>The recording of ground motions is a fundamental part of both seismology and earthquake engineering. The current state-of-the-art 24-bit continuously recording seismic station is described, with particular attention to the frequency range and dynamic range of the seismic sensors typically installed. An alternative method of recording the strong-motions would be to deploy a velocity sensor rather than an accelerometer. This instrument has the required ability to measure the strongest earth motions, with enhanced long period sensitivity.</p>
<p>An existing strong motion velocity sensor from Japan was tested for potential use in US seismic networks. It was found to be incapable of recording strong motions typically observed in the near source of even moderate earthquakes. The instrument was widely deployed near the M8.3 Sept 2003 Tokachi-Oki earthquake. The dataset corroborated our laboratory observations of low velocity saturations. The dataset also served to show all inertial sensors are equally sensitive to tilting, which is widespread in large earthquakes. High rate GPS data is also recorded during the event. Co-locating high-rate GPS with strong motion sensors is suggested to be currently the optimal method by which the complete and unambiguous deformation field at a station can be recorded.</p>
<p>A new application of the modern seismic station is to locate them inside structures. A test station on the 9th floor of Millikan Library is analysed. The continuous data-stream facilitates analysis of the building response to ambient weather, forced vibration tests, and small earthquakes that have occurred during its lifetime. The structure's natural frequencies are shown to be sensitive not only to earthquake excitation, but rainfall, temperature and wind. This has important implications on structural health monitoring, which assumes the natural frequencies of a structure do not vary significantly unless there is structural damage.</p>
<p>Moderate to small earthquakes are now regularly recorded by dense, high dynamic range networks. This enhanced recording of the earthquake and its aftershock sequences makes possible the development of a Green's Function deconvolution approach for determining rupture parameters.</p>https://thesis.library.caltech.edu/id/eprint/1894Three-Dimensional Nonlinear Analysis of Tall Irregular Steel Buildings Subject to Strong Ground Motion
https://resolver.caltech.edu/CaltechETD:etd-02252004-181515
Authors: {'items': [{'email': 'swami.krishnan@manhattan.edu', 'id': 'Krishnan-Swaminathan', 'name': {'family': 'Krishnan', 'given': 'Swaminathan'}, 'orcid': '0000-0002-2594-1523', 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/A00K-RQ42
<p>Strong ground motion from a nearby fault has frequency content in the same range as the natural frequencies of tall buildings. This may have serious repercussions and is the topic of this dissertation.Buildings are designed per building code standards. But, are the code provisions adequate? Strong motion from large earthquakes has been recorded only in recent times in the near-source region. Have the current codes used this information to update tall structure design guidelines? Considerable damage has been observed in tall buildings from the Northridge, Kobe, Turkey, and Taiwan earthquakes. How will tall buildings designed per the latest code regulations perform if they were to be shaken by any of these earthquakes? This thesis attempts to answer these questions.</p>
<p>Tall buildings by their nature are computationally intensive to analyze. They consist of thousands of degrees of freedom and when subjected to strong ground motion from a nearby source, exhibit inelastic response. Modeling this inelastic response requires an iterative approach that is computationally expensive. Furthermore, a large class of buildings, classified as irregular, exhibits complex behavior that can be studied only when the structures are modeled in their entirety. To this end, a three-dimensional analysis program, FRAME3D, has been developed incorporating two special beam-column elements -- the plastic hinge element and the elastofiber element that can model beams and columns in buildings accurately and efficiently, a beam-column joint element that can model inelastic joint deformation, and 4-noded elastic plane-stress elements to model floor slabs acting as diaphragms forcing the lateral force resisting frames in a building to act as one unit. The program is capable of performing time-history analyses of buildings in their entirety.</p>
<p>Six 19-story irregular steel moment frame buildings (with buildings 2A and 3A being variants of buildings 2 and 3, respectively) have been designed per the latest code (Uniform Building Code, 1997). Two of these buildings have reentrant corners and the other two have torsional irregularity. Their strength and ductility are assessed by performing pushover analyses on them. To assess their performance under strong shaking, FRAME3D models of these buildings are subjected to near-source strong motion records from the Iran earthquake (Mw = 7.3, Tabas Station) of 1978, the Northridge earthquake (Mw = 6.7, Sylmar Station) of 1994 and the Kobe earthquake (Mw = 6.9, Takatori Station) of 1995. None of the buildings collapsed under these strong events in the computer analyses. However, when compared against the acceptable limits for various performance levels in FEMA 356 document, the damage in terms of plastic deformation at the ends of beams and columns and at joints would render the buildings inadequate in terms f life safety in quite a few cases and would even indicate possible collapse in a couple of cases. Thus, in these terms, the code falls short of achieving its life safety objective, and the near-source factors introduced in the code in 1997 in recognition of the special features of near-source ground motion seem to be inadequate.</p>
<p>The ductility demand, in terms of plastic rotation at the ends of beams and columns and in joints, on these buildings during this class of earthquakes is up to 6% of a radian, which is far greater than a typical limiting plastic rotation of 3% associated with fracture and consequent failure of large wide-flanged steel sections during experiments. Thus, if strength degradation due to fractures, local buckling, etc., were to be included in the analysis, then the results would likely to be worse, as far as the ability of these buildings to withstand these earthquakes without collapse is concerned.</p>
<p>Due to damage localization, the peak drifts observed in the structure far exceeded the inelastic drift limit in the code of 0.02 (in some cases up to 3 times). This points to serious non-structural damage to facades, interior dry wall, etc. Furthermore, large roof permanent offsets after the events indicate significant post-earthquake repair requiring considerable disruption and building closure.</p>
<p>Column yielding was minimal thus validating the strong-column weak-beam criterion in the code. Redundancy factors used to assess the redundancy in the system need to take into account the case of torsionally sensitive structures where frames in both principal directions are simultaneously activated. Stress concentration was not observed at the reentrant corners in L-shaped buildings.</p>
<p>Finally, the data catalogued in this work could be useful for future code development and tall structure design guidelines.</p>https://thesis.library.caltech.edu/id/eprint/755Nonlinear Analysis of Pacoima Dam with Spatially Nonuniform Ground Motion
https://resolver.caltech.edu/CaltechETD:etd-10292004-155829
Authors: {'items': [{'id': 'Alves-Steven-Wayne', 'name': {'family': 'Alves', 'given': 'Steven Wayne'}, 'show_email': 'NO'}]}
Year: 2005
DOI: 10.7907/QVFR-MV06
<p>Spatially uniform ground motion is an assumption that has often been made for structural analysis of arch dams. However, it has been recognized for many years that the ground motion in a canyon during an earthquake is amplified at the top of the canyon relative to the base. Pacoima Dam has been strongly shaken by the 1971 San Fernando earthquake and the 1994 Northridge earthquake. The acceleration records from both of these events demonstrate the spatial nonuniformity of the ground motion, but the amount and quality of the data made it difficult to study in detail. An opportunity to do so arose on January 13, 2001, when a relatively small magnitude 4.3 earthquake was recorded by an upgraded accelerometer array at Pacoima Dam.</p>
<p>Frequency-dependent topographic amplification is apparent at locations along both abutments at 80% height of the dam relative to the base. Also, the ground motion is delayed at the abutment locations compared to the base. The delays are consistent with seismic waves traveling upward along the canyon, and the waves appear to be dispersive since the delays are frequency-dependent. Both of these effects are quantified in this thesis by several approaches that involve varying degrees of approximation. A method for generating nonuniform ground motion from a single 3-component ground motion specified for one location in the canyon, e.g., at the base, is developed using transfer functions that quantify the amplification and phase delay. The method is demonstrated for the 2001 earthquake and the Northridge earthquake with several variations in the transfer functions.</p>
<p>The 2001 earthquake records were also used for system identification. These results do not agree with results from a forced vibration experiment, which indicate a stiffer system. The earthquake must induce nonlinear vibrations, even though the excitation is quite small. This observation has implications for applications of structural health monitoring.</p>
<p>The generated nonuniform ground motions are supplied as input to a finite element model. The results indicate that the method for generating nonuniform input produces ground motion that yields reasonable modeled responses, but there is some evidence that the time delays may be larger for stronger ground motion. Comparisons of the responses from ground motions generated with various implementations of amplification and time delays were made. For modeling purposes, accuracy of the amplification appears to be more important than the delays, which can be dealt with using a simpler approximation. The nonuniform input produces a response that is substantially different than the response produced by uniform input. The major difference is that while the pseudostatic response is a rigid body motion for uniform input, it causes deformation of the dam, mostly close to the abutments, for nonuniform input. In order to refine the proposed method for generating nonuniform ground motion, more data is required from Pacoima Dam and other structures with instrumentation coverage along the abutments.</p>https://thesis.library.caltech.edu/id/eprint/4304An Ounce of Prevention: Probabilistic Loss Estimation for Performance-Based Earthquake Engineering
https://resolver.caltech.edu/CaltechETD:etd-05282007-233606
Authors: {'items': [{'email': 'jmitrani@gmail.com', 'id': 'Mitrani-Reiser-Judith', 'name': {'family': 'Mitrani-Reiser', 'given': 'Judith'}, 'orcid': '0000-0002-3985-6848', 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/JXPV-1Q19
<p>Performance-based earthquake engineering (PBEE) is a methodology that incorporates desired performance levels into the design process. Performance in PBEE can be expressed in economic terms, or as elapsed downtime, or in terms of life and building safety objectives. These performance objectives are relevant to various types of stakeholders. They should be addressed in building loss estimation procedures because after an earthquake, the repair cost will not be the only "loss" suffered by building stakeholders. In a sizeable earthquake, there will likely also be some losses due to business interruption during the repair effort, building closure taken as a post-earthquake safety precaution, and human casualties caused by building failures during the seismic event.</p>
<p>An analytical approach for PBEE is developed and implemented to evaluate the performance of a new reinforced-concrete moment-frame office building. The PBEE approach used is consistent with the Pacific Earthquake Engineering Research (PEER) center?s modular framework, which is divided into four core analytical stages: hazard analysis, structural analysis, damage analysis, and loss analysis. Future losses of the building are uncertain because they depend on uncertain quantities, such as the shaking intensity of the earthquake, the mechanical properties of the facility, and the uncertain damageability and unit repair costs of the facility. An analytical approach is developed to propagate these uncertainties. This work presents the mathematical foundation for the damage and loss analyses, and a description of its implementation into software. The results from running this software on multiple design variants of the building are presented, including seismic vulnerabilities as a function of shaking intensity and corresponding expected annual losses.</p>
<p>The methodology developed and implemented in this work estimates the direct economic losses due to repair costs as well as two types of indirect economic losses, those produced by building downtime and by human fatalities. A procedure for a virtual inspection is used to assess the safety of buildings, based on current damage assessment guidelines. Additionally, a model is established to estimate human fatalities caused by the partial and global collapse of buildings, using probabilities of fatality based on relevant empirical data and the results of the virtual inspection process. A simplified methodology is presented for estimating building downtime after seismic events, including mobilization delays before construction begins and the elapsed time needed to repair damaged building components. The losses due to downtime and human fatalities are then added to the building repair cost in order to estimate the total building loss, which is then used to perform a benefit-cost analysis of the benchmark building. The work presented, is to our knowledge, the most faithful attempt to estimate the main decision variables (termed the 3 Ds?dollars, deaths, and downtime), proposed by PEER and the ATC-58 Project for performance assessment of structures.</p>
https://thesis.library.caltech.edu/id/eprint/2207Time-Frequency Analysis of Systems with Changing Dynamic Properties
https://resolver.caltech.edu/CaltechETD:etd-11292006-214839
Authors: {'items': [{'email': 'samuel.c.bradford@jpl.nasa.gov', 'id': 'Bradford-Samuel-Case-V', 'name': {'family': 'Bradford', 'given': 'Samuel Case, V'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/HMK7-FJ81
<p>The Wigner-Ville Distribution, and related refinements, represent a class of advanced time-frequency analysis tools that are distinguished from Fourier and wavelet methods by an increase in resolution in the time frequency plane. Time-frequency analysis provides a set of exploratory tools for analyzing changing frequency content in a signal, which can then be correlated with damage patterns in a structure.</p>
<p>For systems of interest to engineers, investigating the changing properties of a system is typically performed by analyzing vibration data from the system, rather than direct inspection of each component. Nonlinear elastic behavior in the force-displacement relationship can decrease the apparent natural frequencies of the system - these changes typically occur over fractions of a second in moderate to strong excitation and the system gradually recovers to pre-event levels. Structures can also suffer permanent damage (e.g., plastic deformation or fracture), permanently decreasing the observed natural frequencies as the system loses stiffness. Advanced time-frequency representations provide a set of exploratory tools for analyzing changing frequency content in a signal, which can then be correlated with damage patterns in a structure. Modern building instrumentation allows for an unprecedented investigation into the changing dynamic properties of structures: a framework for using time-frequency analysis methods for instantaneous system identification is discussed.</p>https://thesis.library.caltech.edu/id/eprint/4689Application of Stochastic Simulation Methods to System Identification
https://resolver.caltech.edu/CaltechETD:etd-05222007-152843
Authors: {'items': [{'id': 'Muto-Matthew-Mokihana', 'name': {'family': 'Muto', 'given': 'Matthew Mokihana'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/9JKZ-Z422
<p>Reliable predictive models for the response of structures are a necessity for many branches of earthquake engineering. However, the process of choosing an appropriate class of models to describe a system, known as model-class selection, and identifying the specific predictive model based on available data, known as system identification, is difficult. Variability in material properties, complex constitutive behavior, uncertainties in the excitations caused by earthquakes, and limited constraining information make system identification an ill-conditioned problem. In addition, model-class selection is not trivial, as it involves balancing predictive power with simplicity.</p>
<p>These problems of system identification and model-class selection may be addressed using a Bayesian probabilistic framework that provides a method for combining prior knowledge of a system with measured data and for choosing between competing model classes.</p>
<p>Similar approaches have been used in the field of system identification, but these methods (referred to as asymptotic-approximation-based methods) often focus on the model defined by the set of most plausible parameter values and have difficulty dealing with ill-conditioned problems, where there may be many models with high plausibility. It is demonstrated here that ill-conditioned problems in system identification and model-class selection can be effectively addressed using stochastic simulation methods.</p>
<p>This work focuses on the application of stochastic simulation to updating and comparing model classes in problems of: (1) development of empirical ground motion attenuation relations, (2) structural model updating using modal data for the purposes of structural health monitoring, and (3) identification of hysteretic structural models, including degrading models, from seismic structural response.</p>
<p>In cases where asymptotic approximation-based methods are appropriate, the results obtained using stochastic simulation show good agreement with results from asymptotics. For cases involving ill-conditioned problems based on simulated data, stochastic simulation methods are successfully applied to obtain results in situations where the use of asymptotics is infeasible. Finally, preliminary studies using stochastic simulation to identify a deteriorating hysteretic model with sparse real data from a structure damaged in an earthquake show that the high-plausibility models demonstrate behavior consistent with the observed damage, indicating that there is promise in applying these methods to ill-conditioned problems in the real world.</p>https://thesis.library.caltech.edu/id/eprint/1959A Plasticity Model to Predict the Effects of Confinement on Concrete
https://resolver.caltech.edu/CaltechETD:etd-05212008-221749
Authors: {'items': [{'email': 'julie@wolfkatz.com', 'id': 'Wolf-Julie-Anne', 'name': {'family': 'Wolf', 'given': 'Julie Anne'}, 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/XMA5-MK90
A plasticity model to predict the behavior of confined concrete is developed. The model is designed to implicitly account for the increase in strength and ductility due to confining a concrete member. The concrete model is implemented into a finite element (FE) model. By implicitly including the change in the strength and ductility in the material model, the confining material can be explicitly included in the FE model. Any confining material can be considered, and the effects on the concrete of failure in the confinement material can be modeled. Test data from a wide variety of different concretes utilizing different confinement methods are used to estimate the model parameters. This allows the FE model to capture the generalized behavior of concrete under multiaxial loading. The FE model is used to predict the results of tests on reinforced concrete members confined by steel hoops and fiber reinforced polymer (FRP) jackets. Loading includes pure axial load and axial load-moment combinations. Variability in the test data makes the model predictions difficult to compare but, overall, the FE model is able to capture the effects of confinement on concrete. Finally, the FE model is used to compare the performance of steel hoop to FRP confined sections, and of square to circular cross sections. As expected, circular sections are better able to engage the confining material, leading to higher strengths. However, higher strains are seen in the confining material for the circular sections. This leads to failure at lower axial strain levels in the case of the FRP confined sections. Significant differences are seen in the behavior of FRP confined members and steel hoop confined members. Failure in the FRP members is always determined by rupture in the composite jacket. As a result, the FRP members continue to take load up to failure. In contrast, the steel hoop confined sections exhibit extensive strain softening before failure. This comparison illustrates the usefulness of the concrete model as a tool for designers. Overall, the concrete model provides a flexible and powerful method to predict the performance of confined concrete.https://thesis.library.caltech.edu/id/eprint/1929Bayesian Learning for Earthquake Engineering Applications and Structural Health Monitoring
https://resolver.caltech.edu/CaltechETD:etd-12052007-141434
Authors: {'items': [{'email': 'ockoogi@gmail.com', 'id': 'Oh-Chang-Kook', 'name': {'family': 'Oh', 'given': 'Chang Kook'}, 'show_email': 'YES'}]}
Year: 2008
DOI: 10.7907/RFD5-7Y72
<p>Parallel to significant advances in sensor hardware, there have been recent developments of sophisticated methods for quantitative assessment of measured data that explicitly deal with all of the involved uncertainties, including inevitable measurement errors. The existence of these uncertainties often causes numerical instabilities in inverse problems that make them ill-conditioned.</p>
<p>The Bayesian methodology is known to provide an efficient way to alleviate this ill-conditioning by incorporating the prior term for regularization of the inverse problem, and to provide probabilistic results which are meaningful for decision making.</p>
<p>In this work, the Bayesian methodology is applied to inverse problems in earthquake engineering and especially to structural health monitoring. The proposed methodology of Bayesian earning using automatic relevance determination (ARD) prior, including its kernel version called the Relevance Vector Machine, is presented and applied to earthquake early warning, earthquake ground motion attenuation estimation, and structural health monitoring, using either a Bayesian classification or regression approach.</p>
<p>The classification and regression are both performed in three phases: (1) Phase I (feature extraction phase): Determine which features from the data to use in a training dataset; (2) Phase II (training phase): Identify the unknown parameters defining a model by using a training dataset; and (3) Phase III (prediction phase): Predict the results based on the features from new data.</p>
<p>This work focuses on the advantages of making probabilistic predictions obtained by Bayesian methods to deal with all uncertainties and the good characteristics of the proposed method in terms of computationally efficient training, and, especially, prediction that make it suitable for real-time operation. It is shown that sparseness (using only smaller number of basis function terms) is produced in the regression equations and classification separating boundary by using the ARD prior along with Bayesian model class selection to select the most probable (plausible) model class based on the data. This model class selection procedure automatically produces optimal regularization of the problem at hand, making it well-conditioned.</p>
<p>Several applications of the proposed Bayesian learning methodology are presented. First, automatic near-source and far-source classification of incoming ground motion signals is treated and the Bayesian learning method is used to determine which ground motion features are optimal for this classification. Second, a probabilistic earthquake attenuation model for peak ground acceleration is identified using selected optimal features, especially taking a non-linearly involved parameter into consideration. It is shown that the Bayesian learning method an be utilized to estimate not only linear coefficients but also a non-linearly involved parameter to provide an estimate for an unknown parameter in the kernel basis functions for elevance Vector Machine. Third, the proposed method is extended to a general case of regression problems with vector outputs and applied to structural health monitoring applications. It is concluded that the proposed vector output RVM shows promise for estimating damage locations and their severities from change of modal properties such as natural frequencies and mode shapes.</p>https://thesis.library.caltech.edu/id/eprint/4802Steel Moment-Resisting Frame Responses in Simulated Strong Ground Motions: or How I Learned to Stop Worrying and Love the Big One
https://resolver.caltech.edu/CaltechETD:etd-05202008-201030
Authors: {'items': [{'email': 'annao@caltech.edu', 'id': 'Olsen-Anna-H', 'name': {'family': 'Olsen', 'given': 'Anna H.'}, 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/Y42R-V333
This thesis studies the response of steel moment-resisting frame buildings in simulated strong ground motions. I collect 37 simulations of crustal earthquakes in California. These ground motions are applied to nonlinear finite element models of four types of steel moment frame buildings: six- or twenty-stories with either a stiffer, higher-strength design or a more flexible, lower-strength design. I also consider the presence of fracture-prone welds in each design. Since these buildings experience large deformations in strong ground motions, the building states considered in this thesis are collapse, total structural loss (must be demolished), and if repairable, the peak inter-story drift. This thesis maps these building responses on the simulation domains which cover many sites in the San Francisco and Los Angeles regions. The building responses can also be understood as functions of ground motion intensity measures, such as pseudo-spectral acceleration (PSA), peak ground displacement (PGD), and peak ground velocity (PGV). This thesis develops building response prediction equations to describe probabilistically the state of a steel moment frame given a ground motion. The presence of fracture-prone welds increases the probability of collapse by a factor of 2--8. The probability of collapse of the more flexible design is 1--4 times that of the stiffer design. The six-story buildings are slightly less likely to collapse than the twenty-story buildings assuming sound welds, but the twenty-story buildings are 2--4 times more likely to collapse than the six-story buildings if both have fracture-prone welds. A vector intensity measure of PGD and PGV predicts collapse better than PSA. Models based on the vector of PGD and PGV predict total structural loss equally well as models using PSA. PSA alone best predicts the peak inter-story drift, assuming that the building is repairable. As "rules of thumb," the twenty-story steel moment frames with sound welds collapse in ground motions with long-period PGD greater than 1 m and long-period PGV greater than 2 m/s, and they are a total structural loss for long-period PGD greater than 0.6 m and long-period PGV greater than 1 m/s.https://thesis.library.caltech.edu/id/eprint/1900Nonlinear Responses of High-Rise Buildings in Giant Subduction Earthquakes
https://resolver.caltech.edu/CaltechETD:etd-04072009-042113
Authors: {'items': [{'email': 'jingy2008@gmail.com', 'id': 'Yang-Jing', 'name': {'family': 'Yang', 'given': 'Jing'}, 'show_email': 'NO'}]}
Year: 2009
DOI: 10.7907/JDRP-HR74
With the exception of the 2003 Tokachi-Oki earthquake, strong ground recordings from large subduction earthquakes (Mw > 8.0) are meager. Furthermore there are no strong motion recordings of giant earthquakes. However, there is a growing set of high-quality broadband teleseismic recordings of large and giant earthquakes. In this thesis, we use recordings from the 2003 Tokachi-Oki (Mw 8.3) earthquake as empirical Green’s functions to simulate the rock and soil ground motions from the 2004 Sumatra-Andaman earthquake and a scenario Mw 9.2 Cascadia subduction earthquake in the frequency band of interest to flexible and tall buildings (0.075 to 1 Hz). The effect of amplification by the Seattle basin is considered by using a basin response transfer function, which is derived from deconvolving the teleseismic waves recorded at rock sites from basin sites at the SHIP02 experiment. These strong ground motion time histories are used to simulate of the fully nonlinear response of 20-story and 6-story steel moment-frame buildings designed according to both the U.S. 1994 Uniform Building Code and the 1987 Japanese building code. We consider several realizations of the hypothetical subduction earthquake. The basin amplification and the down-dip limit of rupture are of particular importance to the simulated ground motions in Seattle. At rock sites, if slip is limited to offshore regions, the building model responses are mostly in the linear range. However, if rupture is extended beyond the Olympic Mountains, large deformations occur in the high-rise buildings models, especially those with brittle welds. At basin sites, our simulations indicate the collapse of all building models for a source model with rupture beyond the Olympic Mountains, whereas buildings with perfect welds avoid collapse for simulations based on a source model with rupture limited to offshore. The synthetic ground motions all have very long durations (more than 5 minutes at soil sites), and our building simulations should be considered as a low estimate since we the degradation model used in our simulation did not consider local flange buckling.
https://thesis.library.caltech.edu/id/eprint/1298Hysteretic Characteristics of Wood-Frame Structures Under Seismic Motions
https://resolver.caltech.edu/CaltechETD:etd-05172009-153304
Authors: {'items': [{'email': 'dsutoyo@gmail.com', 'id': 'Sutoyo-Daniel', 'name': {'family': 'Sutoyo', 'given': 'Daniel'}, 'show_email': 'NO'}]}
Year: 2009
DOI: 10.7907/R92X-6R80
<p>In order to develop seismic codes that can effectively mitigate damage to wood-frame construction under seismic activity, the dynamic characteristics of wood-frame buildings must be well understood. Funding of full-scale structure experimental tests can be costly and may not be a true replica of real life scenarios. Therefore, data interpretation projects focusing on dynamic behavior of low-rise wooden shearwall buildings under large seismic motions have become increasingly important. Procedures include determining the modal parameters and extracting hysteretic characteristics from the available records. The results help extend the understanding of wood-frame structures and update building codes. Furthermore, the amount of information extracted can help evaluate the effectiveness of the current instrumentation program.</p>
<p>This work focuses on the seismic records from wood-frame structures during the 2004 Parkfield Earthquake. Studies involve verifying the amplitude dependence of modal parameters and retrieving pinching hysteresis curves that are common in wood-frame structures. Modal parameters are identified with a robust routine called MODE-ID. Equivalent viscous damping estimates in wood-frame buildings can range from 5% - 10% in largely linear behavior and 10% - 20% in significant nonlinear behavior. The discrepancies of damping estimates reported in the past are a result of inappropriate comparisons without understanding 1) the degree of nonlinear response and 2) the system identification methods used for the studies. By studying the hysteretic curves, insights can be obtained to reveal and to resolve the damping estimate discrepancies. Since displacement time histories of structures are not typically measured, the hysteretic curves are extracted from acceleration time histories. The proposed process accounts for inherent double integration errors and phase delay through filtering. It is still being debated that if the double integration can provide meaningful structural relative displacement time histories. In a laboratory setting with unilateral ground motion, the extraction process provides accurate hysteretic curves. However, this dissertation demonstrates that if the building experiences bi-directional ground motions, the nonlinear behavior of the diaphragm tampers with this process.</p>
<p>The results from modal identification and hysteresis curves serve as a basis for creating numerical models. Direct and gradient search methods were used for model updating. Bayesian updating and model selection provided the best results for dealing with hysteretic structural models. This probabilistic framework demonstrates potential benefits in a seamless integration with a seismic database. The selected hysteretic model showed great resemblance to the measured responses and had evidence of pinching hysteresis. Insights on the structure’s deformations and dissipation of energy can be inferred from the model.</p>
https://thesis.library.caltech.edu/id/eprint/1851Virtual Earthquake Engineering Laboratory with Physics-Based Degrading Materials on Parallel Computers
https://resolver.caltech.edu/CaltechTHESIS:01302012-081909374
Authors: {'items': [{'email': 'ihcho@caltech.edu', 'id': 'Cho-In-Ho', 'name': {'family': 'Cho', 'given': 'In Ho'}, 'show_email': 'YES'}]}
Year: 2012
DOI: 10.7907/49W9-PY43
<p>For the last few decades, we have obtained tremendous insight into underlying microscopic mechanisms of degrading quasi-brittle materials from persistent and near-saintly efforts in laboratories, and at the same time we have seen unprecedented evolution in computational technology such as massively parallel computers. Thus, time is ripe to embark on a novel approach to settle unanswered questions, especially for the earthquake engineering community, by harmoniously combining the microphysics mechanisms with advanced parallel computing technology.</p>
<p>To begin with, it should be stressed that we placed a great deal of emphasis on preserving clear meaning and physical counterparts of all the microscopic material models proposed herein, since it is directly tied to the belief that by doing so, the more physical mechanisms we incorporate, the better prediction we can obtain.</p>
<p>We departed from reviewing representative microscopic analysis methodologies, selecting out “fixed-type” multidirectional smeared crack model as the base framework for nonlinear quasi-brittle materials, since it is widely believed to best retain the physical nature of actual cracks. Microscopic stress functions are proposed by integrating well-received existing models to update normal stresses on the crack surfaces (three orthogonal surfaces are allowed to initiate herein) under cyclic loading.</p>
<p>Unlike the normal stress update, special attention had to be paid to the shear stress update on the crack surfaces, due primarily to the well-known pathological nature of the fixed-type smeared crack model–spurious large stress transfer over the open crack under nonproportional loading. In hopes of exploiting physical mechanism to resolve this deleterious nature of the fixed crack model, a tribology-inspired three-dimensional (3d) interlocking mechanism has been proposed. Following the main trend of tribology (i.e., the science and engineering of interacting surfaces), we introduced the base fabric of solid particle-soft matrix to explain realistic interlocking over rough crack surfaces, and the adopted Gaussian distribution feeds random particle sizes to the entire domain. Validation against a well-documented rough crack experiment reveals promising accuracy of the proposed 3d interlocking model.</p>
<p>A consumed energy-based damage model has been proposed for the weak correlation between the normal and shear stresses on the crack surfaces, and also for describing the nature of irrecoverable damage. Since the evaluation of the consumed energy is directly linked to the microscopic deformation, which can be efficiently tracked on the crack surfaces, the proposed damage model is believed to provide a more physical interpretation than existing damage mechanics, which fundamentally stem from mathematical derivation with few physical counterparts.</p>
<p>Another novel point of the present work lies in the topological transition-based “smart” steel bar model, notably with evolving compressive buckling length. We presented a systematic framework of information flow between the key ingredients of composite materials (i.e., steel bar and its surrounding concrete elements). The smart steel model suggested can incorporate smooth transition during reversal loading, tensile rupture, early buckling after reversal from excessive tensile loading, and even compressive buckling. Especially, the buckling length is made to evolve according to the damage states of the surrounding elements of each bar, while all other dominant models leave the length unchanged.</p>
<p>What lies behind all the aforementioned novel attempts is, of course, the problem-optimized parallel platform. In fact, the parallel computing in our field has been restricted to monotonic shock or blast loading with explicit algorithm which is characteristically feasible to be parallelized. In the present study, efficient parallelization strategies for the highly demanding implicit nonlinear finite element analysis (FEA) program for real-scale reinforced concrete (RC) structures under cyclic loading are proposed. Quantitative comparison of state-of-the-art parallel strategies, in terms of factorization, had been carried out, leading to the problem-optimized solver, which is successfully embracing the penalty method and banded nature. Particularly, the penalty method employed imparts considerable smoothness to the global response, which yields a practical superiority of the parallel triangular system solver over other advanced solvers such as parallel preconditioned conjugate gradient method. Other salient issues on parallelization are also addressed.</p>
<p>The parallel platform established offers unprecedented access to simulations of real-scale structures, giving new understanding about the physics-based mechanisms adopted and probabilistic randomness at the entire system level. Particularly, the platform enables bold simulations of real-scale RC structures exposed to cyclic loading–H-shaped wall system and 4-story T-shaped wall system. The simulations show the desired capability of accurate prediction of global force-displacement responses, postpeak softening behavior, and compressive buckling of longitudinal steel bars. It is fascinating to see that intrinsic randomness of the 3d interlocking model appears to cause “localized” damage of the real-scale structures, which is consistent with reported observations in different fields such as granular media.</p>
<p>Equipped with accuracy, stability and scalability as demonstrated so far, the parallel platform is believed to serve as a fertile ground for the introducing of further physical mechanisms into various research fields as well as the earthquake engineering community. In the near future, it can be further expanded to run in concert with reliable FEA programs such as FRAME3d or OPENSEES. Following the central notion of “multiscale” analysis technique, actual infrastructures exposed to extreme natural hazard can be successfully tackled by this next generation analysis tool–the harmonious union of the parallel platform and a general FEA program. At the same time, any type of experiments can be easily conducted by this “virtual laboratory.”</p>
https://thesis.library.caltech.edu/id/eprint/6894A New Ground Motion Intensity Measure, Peak Filtered Acceleration (PFA), to Estimate Collapse Vulnerability of Buildings in Earthquakes
https://resolver.caltech.edu/CaltechTHESIS:10212013-164416621
Authors: {'items': [{'email': 'songsy04@gmail.com', 'id': 'Song-Shiyan', 'name': {'family': 'Song', 'given': 'Shiyan'}, 'show_email': 'YES'}]}
Year: 2014
DOI: 10.7907/J5YB-AR86
<p>In this thesis, we develop an efficient collapse prediction model, the PFA (Peak Filtered Acceleration) model, for buildings subjected to different types of ground motions.</p>
<p>For the structural system, the PFA model covers modern steel and reinforced concrete moment-resisting frame buildings (potentially reinforced concrete shear wall buildings). For ground motions, the PFA model covers ramp-pulse-like ground motions, long-period ground motions, and short-period ground motions.</p>
<p>To predict whether a building will collapse in response to a given ground motion, we first extract long-period components from the ground motion using a Butterworth low-pass filter with suggested order and cutoff frequency. The order depends on the type of ground motion, and the cutoff frequency depends on the building’s natural frequency and ductility. We then compare the filtered acceleration time history with the capacity of the building. The capacity of the building is a constant for 2-dimentional buildings and a limit domain for 3-dimentional buildings. If the filtered acceleration exceeds the building’s capacity, the building is predicted to collapse. Otherwise, it is expected to survive the ground motion.</p>
<p>The parameters used in PFA model, which include fundamental period, global ductility and lateral capacity, can be obtained either from numerical analysis or interpolation based on the reference building system proposed in this thesis.</p>
<p>The PFA collapse prediction model greatly reduces computational complexity while archiving good accuracy. It is verified by FEM simulations of 13 frame building models and 150 ground motion records.</p>
<p>Based on the developed collapse prediction model, we propose to use PFA (Peak Filtered Acceleration) as a new ground motion intensity measure for collapse prediction. We compare PFA with traditional intensity measures PGA, PGV, PGD, and Sa in collapse prediction and find that PFA has the best performance among all the intensity measures.</p>
<p>We also provide a close form in term of a vector intensity measure (PGV, PGD) of the PFA collapse prediction model for practical collapse risk assessment.</p>https://thesis.library.caltech.edu/id/eprint/8002New Applications that Come from Extending Seismic Networks into Buildings
https://resolver.caltech.edu/CaltechTHESIS:03182014-225151551
Authors: {'items': [{'email': 'minghei.cheng@gmail.com', 'id': 'Cheng-Ming-Hei', 'name': {'family': 'Cheng', 'given': 'Ming Hei'}, 'show_email': 'NO'}]}
Year: 2014
DOI: 10.7907/STB2-XR07
This thesis describes engineering applications that come from extending seismic networks into building structures. The proposed applications will benefit the data from the newly developed crowd-sourced seismic networks which are composed of low-cost accelerometers. An overview of the Community Seismic Network and the earthquake detection method are addressed. In the structural array components of crowd-sourced seismic networks, there may be instances in which a single seismometer is the only data source that is available from a building. A simple prismatic Timoshenko beam model with soil-structure interaction (SSI) is developed to approximate mode shapes of buildings using natural frequency ratios. A closed form solution with complete vibration modes is derived. In addition, a new method to rapidly estimate total displacement response of a building based on limited observational data, in some cases from a single seismometer, is presented. The total response of a building is modeled by the combination of the initial vibrating motion due to an upward traveling wave, and the subsequent motion as the low-frequency resonant mode response. Furthermore, the expected shaking intensities in tall buildings will be significantly different from that on the ground during earthquakes. Examples are included to estimate the characteristics of shaking that can be expected in mid-rise to high-rise buildings. Development of engineering applications (e.g., human comfort prediction and automated elevator control) for earthquake early warning system using probabilistic framework and statistical learning technique is addressed. https://thesis.library.caltech.edu/id/eprint/8145A Retrofitting Framework for Pre-Northridge Steel Moment-Frame Buildings
https://resolver.caltech.edu/CaltechTHESIS:05302014-102237282
Authors: {'items': [{'email': 'arnarbjorn@gmail.com', 'id': 'Bjornsson-Arnar-Bjorn', 'name': {'family': 'Bjornsson', 'given': 'Arnar Bjorn'}, 'show_email': 'NO'}]}
Year: 2014
DOI: 10.7907/3Z6T-2H83
<p>In the 1994 M<sub>w</sub> 6.7 Northridge and 1995 M<sub>w</sub> 6.9 Kobe earthquakes,
steel moment-frame buildings were exposed to an unexpected flaw. The commonly utilized
welded unreinforced flange, bolted web connections
were observed to experience brittle fractures in a number of buildings, even at
low levels of seismic demand. A majority of these buildings have not been retrofitted
and may be susceptible to structural collapse in a major earthquake.</p>
<p>This dissertation presents a case study of retrofitting a 20-story pre-Northridge steel
moment-frame building. Twelve retrofit schemes are developed that present some
range in degree of intervention. Three retrofitting techniques are considered: upgrading
the brittle beam-to-column moment resisting connections, and implementing either conventional
or buckling-restrained brace elements within the existing moment-frame bays.
The retrofit schemes include some that are designed to the
basic safety objective of ASCE-41 Seismic Rehabilitation of Existing Buildings.</p>
<p>Detailed finite element models of the base line building and the retrofit schemes
are constructed. The models include considerations of brittle
beam-to-column moment resisting connection fractures, column splice fractures,
column baseplate fractures, accidental contributions from ``simple'' non-moment resisting
beam-to-column connections to the lateral force-resisting system, and composite
actions of beams with the overlying floor system.
In addition, foundation interaction is included through nonlinear translational springs
underneath basement columns.</p>
<p>To investigate the effectiveness of the retrofit schemes, the building models are
analyzed under ground motions from three large magnitude simulated earthquakes that
cause intense shaking in the greater Los Angeles metropolitan area, and under
recorded ground motions from actual earthquakes.
It is found that retrofit schemes that convert the existing moment-frames into braced-frames
by implementing either conventional or buckling-restrained braces are effective in limiting
structural damage and mitigating structural collapse. In the three simulated earthquakes,
a 20% chance of simulated collapse is realized at PGV of around 0.6 m/s for the base line model,
but at PGV of around 1.8 m/s for some of the retrofit schemes. However, conventional braces are observed
to deteriorate rapidly. Hence, if a braced-frame that employs conventional braces survives
a large earthquake, it is questionable how much service the braces provide in potential
aftershocks.</p>https://thesis.library.caltech.edu/id/eprint/8440Toppling Analysis of Precariously Balanced Rocks under Earthquake Excitation
https://resolver.caltech.edu/CaltechTHESIS:05252015-121408938
Authors: {'items': [{'email': 'swetha.veeraraghavan@gmail.com', 'id': 'Veeraraghavan-Swetha', 'name': {'family': 'Veeraraghavan', 'given': 'Swetha'}, 'show_email': 'NO'}]}
Year: 2015
DOI: 10.7907/Z98W3B9Z
Toppling analysis of a precariously balanced rock (PBR) can provide insights into the nature of ground motion that has not occurred at that location in the past and, by extension, realistic constraints on peak ground motions for use in engineering design. Earlier approaches have targeted simplistic 2-D models of the rock or modeled the rock-pedestal contact using spring-damper assemblies that require re-calibration for each rock. These analyses also assume that the rock does not slide on the pedestal. Here, a method to model PBRs in three dimensions is presented. The 3-D model is created from a point cloud of the rock, the pedestal, and their interface, obtained using Terrestrial Laser Scanning (TLS). The dynamic response of the model under earthquake excitation is simulated using a rigid body dynamics algorithm. The veracity of this approach is demonstrated by comparisons against data from shake table experiments. Fragility maps for toppling probability of the Echo Cliff PBR and the Pacifico PBR as a function of various ground motion parameters, rock-pedestal interface friction coefficient, and excitation direction are presented. The seismic hazard at these PBR locations is estimated using these maps. Additionally, these maps are used to assess whether the synthetic ground motions at these locations resulting from scenario earthquakes on the San Andreas Fault are realistic (toppling would indicate that the ground motions are unrealistically high).https://thesis.library.caltech.edu/id/eprint/8885Quantifying Earthquake Collapse Risk of Tall Steel Braced Frame Buildings Using Rupture-to-Rafters Simulations
https://resolver.caltech.edu/CaltechTHESIS:05252015-122512887
Authors: {'items': [{'email': 'ramses.mourhatch@gmail.com', 'id': 'Mourhatch-Ramses', 'name': {'family': 'Mourhatch', 'given': 'Ramses'}, 'orcid': '0000-0001-8037-726X', 'show_email': 'YES'}]}
Year: 2015
DOI: 10.7907/Z9DV1GTG
<p>This thesis examines collapse risk of tall steel braced frame buildings using rupture-to-rafters simulations due to suite of San Andreas earthquakes. Two key advancements in this work are the development of (i) a rational methodology for assigning scenario earthquake probabilities and (ii) an artificial correction-free approach to broadband ground motion simulation. The work can be divided into the following sections: earthquake source modeling, earthquake probability calculations, ground motion simulations, building response, and performance analysis. </p>
<p>As a first step the kinematic source inversions of past earthquakes in the magnitude range of 6-8 are used to simulate 60 scenario earthquakes on the San Andreas fault. For each scenario earthquake a 30-year occurrence probability is calculated and we present a rational method to redistribute the forecast earthquake probabilities from UCERF to the simulated scenario earthquake. We illustrate the inner workings of the method through an example involving earthquakes on the San Andreas fault in southern California.</p>
<p>Next, three-component broadband ground motion histories are computed at 636 sites in the greater Los Angeles metropolitan area by superposing short-period (0.2~s-2.0~s) empirical Green's function synthetics on top of long-period ($>$ 2.0~s) spectral element synthetics. We superimpose these seismograms on low-frequency seismograms, computed from kinematic source models using the spectral element method, to produce broadband seismograms. </p>
<p>Using the ground motions at 636 sites for the 60 scenario earthquakes, 3-D nonlinear analysis of several variants of an 18-story steel braced frame building, designed for three soil types using the 1994 and 1997 Uniform Building Code provisions and subjected to these ground motions, are conducted. Model performance is classified into one of five performance levels: Immediate Occupancy, Life Safety, Collapse Prevention, Red-Tagged, and Model Collapse. The results are combined with the 30-year probability of occurrence of the San Andreas scenario earthquakes using the PEER performance based earthquake engineering framework to determine the probability of exceedance of these limit states over the next 30 years.</p>https://thesis.library.caltech.edu/id/eprint/8886Quantifying Earthquake Collapse Risk of Tall Steel Moment Frame Buildings Using Rupture-to-Rafters Simulations
https://resolver.caltech.edu/CaltechTHESIS:06042015-132126668
Authors: {'items': [{'email': 'hemanth.iitm@gmail.com', 'id': 'Siriki-Hemanth', 'name': {'family': 'Siriki', 'given': 'Hemanth'}, 'show_email': 'NO'}]}
Year: 2015
DOI: 10.7907/Z9TX3CB4
<p>There is a sparse number of credible source models available from large-magnitude past earthquakes. A stochastic source model generation algorithm thus becomes necessary for robust risk quantification using scenario earthquakes. We present an algorithm that combines the physics of fault ruptures as imaged in laboratory earthquakes with stress estimates on the fault constrained by field observations to generate stochastic source models for large-magnitude (Mw 6.0-8.0) strike-slip earthquakes. The algorithm is validated through a statistical comparison of synthetic ground motion histories from a stochastically generated source model for a magnitude 7.90 earthquake and a kinematic finite-source inversion of an equivalent magnitude past earthquake on a geometrically similar fault. The synthetic dataset comprises of three-component ground motion waveforms, computed at 636 sites in southern California, for ten hypothetical rupture scenarios (five hypocenters, each with two rupture directions) on the southern San Andreas fault. A similar validation exercise is conducted for a magnitude 6.0 earthquake, the lower magnitude limit for the algorithm. Additionally, ground motions from the Mw7.9
earthquake simulations are compared against predictions by the Campbell-Bozorgnia NGA relation as well as the ShakeOut scenario earthquake. The algorithm is then applied to generate fifty source models for a hypothetical magnitude 7.9 earthquake originating at Parkfield, with rupture propagating from north to south (towards Wrightwood), similar to the 1857 Fort Tejon earthquake. Using the spectral element method, three-component ground motion waveforms are computed in the Los Angeles basin for each scenario earthquake and the sensitivity of ground shaking intensity to seismic source parameters (such as the percentage of asperity area relative to the fault area, rupture speed, and risetime) is studied.</p>
<p>Under plausible San Andreas fault earthquakes in the next 30 years, modeled using the stochastic source algorithm, the performance of two 18-story steel moment frame buildings (UBC 1982 and 1997 designs) in southern California is quantified. The approach integrates rupture-to-rafters simulations into the PEER performance based earthquake engineering (PBEE) framework. Using stochastic sources and computational seismic wave propagation, three-component ground motion histories at 636 sites in southern California are generated for sixty scenario earthquakes on the San Andreas fault. The
ruptures, with moment magnitudes in the range of 6.0-8.0, are assumed to occur at five locations on the southern section of the fault. Two unilateral rupture propagation directions are considered. The 30-year probabilities of all plausible ruptures in this magnitude range and in that section of the fault,
as forecast by the United States Geological Survey, are distributed among these 60 earthquakes based on proximity and moment release. The response of the two 18-story buildings hypothetically located at each of the 636 sites under 3-component shaking from all 60 events is computed using 3-D nonlinear time-history analysis. Using these results, the probability of the structural response exceeding Immediate Occupancy (IO), Life-Safety (LS), and Collapse Prevention (CP) performance levels under San Andreas fault earthquakes over the next thirty years is evaluated. </p>
<p>Furthermore, the conditional and marginal probability distributions of peak ground velocity
(PGV) and displacement (PGD) in Los Angeles and surrounding basins due to earthquakes occurring primarily on the mid-section of southern San Andreas fault are determined using Bayesian model class identification. Simulated ground motions at sites within 55-75km from the source from a suite of 60 earthquakes (Mw 6.0 − 8.0) primarily rupturing mid-section of San Andreas fault are considered for PGV and PGD data.</p>https://thesis.library.caltech.edu/id/eprint/8985SteelConverter and Caltech VirtualShaker: Rapid Nonlinear Cloud-Based Structural Model Conversion and Analysis
https://resolver.caltech.edu/CaltechTHESIS:10052015-133333291
Authors: {'items': [{'email': 'chrisjanover@gmail.com', 'id': 'Janover-Christopher-George', 'name': {'family': 'Janover', 'given': 'Christopher George'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9SF2T3V
<p>STEEL, the Caltech created nonlinear large displacement analysis software, is currently used by a large number of researchers at Caltech. However, due to its complexity, lack of visualization tools (such as pre- and post-processing capabilities) rapid creation and analysis of models using this software was difficult. SteelConverter was created as a means to facilitate model creation through the use of the industry standard finite element solver ETABS. This software allows users to create models in ETABS and intelligently convert model information such as geometry, loading, releases, fixity, etc., into a format that STEEL understands. Models that would take several days to create and verify now take several hours or less. The productivity of the researcher as well as the level of confidence in the model being analyzed is greatly increased.</p>
<p>It has always been a major goal of Caltech to spread the knowledge created here to other universities. However, due to the complexity of STEEL it was difficult for researchers or engineers from other universities to conduct analyses. While SteelConverter did help researchers at Caltech improve their research, sending SteelConverter and its documentation to other universities was less than ideal. Issues of version control, individual computer requirements, and the difficulty of releasing updates made a more centralized solution preferred. This is where the idea for Caltech VirtualShaker was born. Through the creation of a centralized website where users could log in, submit, analyze, and process models in the cloud, all of the major concerns associated with the utilization of SteelConverter were eliminated. Caltech VirtualShaker allows users to create profiles where defaults associated with their most commonly run models are saved, and allows them to submit multiple jobs to an online virtual server to be analyzed and post-processed. The creation of this website not only allowed for more rapid distribution of this tool, but also created a means for engineers and researchers with no access to powerful computer clusters to run computationally intensive analyses without the excessive cost of building and maintaining a computer cluster. </p>
<p>In order to increase confidence in the use of STEEL as an analysis system, as well as verify the conversion tools, a series of comparisons were done between STEEL and ETABS. Six models of increasing complexity, ranging from a cantilever column to a twenty-story moment frame, were analyzed to determine the ability of STEEL to accurately calculate basic model properties such as elastic stiffness and damping through a free vibration analysis as well as more complex structural properties such as overall structural capacity through a pushover analysis. These analyses showed a very strong agreement between the two softwares on every aspect of each analysis. However, these analyses also showed the ability of the STEEL analysis algorithm to converge at significantly larger drifts than ETABS when using the more computationally expensive and structurally realistic fiber hinges. Following the ETABS analysis, it was decided to repeat the comparisons in a software more capable of conducting highly nonlinear analysis, called Perform. These analyses again showed a very strong agreement between the two softwares in every aspect of each analysis through instability. However, due to some limitations in Perform, free vibration analyses for the three story one bay chevron brace frame, two bay chevron brace frame, and twenty story moment frame could not be conducted. With the current trend towards ultimate capacity analysis, the ability to use fiber based models allows engineers to gain a better understanding of a building’s behavior under these extreme load scenarios. </p>
<p>Following this, a final study was done on Hall’s U20 structure [1] where the structure was analyzed in all three softwares and their results compared. The pushover curves from each software were compared and the differences caused by variations in software implementation explained. From this, conclusions can be drawn on the effectiveness of each analysis tool when attempting to analyze structures through the point of geometric instability. The analyses show that while ETABS was capable of accurately determining the elastic stiffness of the model, following the onset of inelastic behavior the analysis tool failed to converge. However, for the small number of time steps the ETABS analysis was converging, its results exactly matched those of STEEL, leading to the conclusion that ETABS is not an appropriate analysis package for analyzing a structure through the point of collapse when using fiber elements throughout the model. The analyses also showed that while Perform was capable of calculating the response of the structure accurately, restrictions in the material model resulted in a pushover curve that did not match that of STEEL exactly, particularly post collapse. However, such problems could be alleviated by choosing a more simplistic material model. </p>
https://thesis.library.caltech.edu/id/eprint/9198A Hybrid-Parallel Framework for the Nonlinear Seismic Analysis of Very Tall Buildings
https://resolver.caltech.edu/CaltechTHESIS:12142015-083607823
Authors: {'items': [{'email': 'abel.dizon@gmail.com', 'id': 'Dizon-Abel-Bermie-Roberto', 'name': {'family': 'Dizon', 'given': 'Abel Bermie Roberto'}, 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z96Q1V58
<p>FRAME3D, a program for the nonlinear seismic analysis of steel structures, has previously been used to study the collapse mechanisms of steel buildings up to 20 stories tall. The present thesis is inspired by the need to conduct similar analysis for much taller structures. It improves FRAME3D in two primary ways.</p>
<p>First, FRAME3D is revised to address specific nonlinear situations involving large displacement/rotation increments, the backup-subdivide algorithm, element failure, and extremely narrow joint hysteresis. The revisions result in superior convergence capabilities when modeling earthquake-induced collapse. The material model of a steel fiber is also modified to allow for post-rupture compressive strength.</p>
<p>Second, a parallel FRAME3D (PFRAME3D) is developed. The serial code is optimized and then parallelized. A distributed-memory divide-and-conquer approach is used for both the global direct solver and element-state updates. The result is an implicit finite-element hybrid-parallel program that takes advantage of the narrow-band nature of very tall buildings and uses nearest-neighbor-only communication patterns.</p>
<p>Using three structures of varied sized, PFRAME3D is shown to compute reproducible results that agree with that of the optimized 1-core version (displacement time-history response root-mean-squared errors are ~〖10〗^(-5) m) with much less wall time (e.g., a dynamic time-history collapse simulation of a 60-story building is computed in 5.69 hrs with 128 cores—a speedup of 14.7 vs. the optimized 1-core version). The maximum speedups attained are shown to increase with building height (as the total number of cores used also increases), and the parallel framework can be expected to be suitable for buildings taller than the ones presented here.</p>
<p>PFRAME3D is used to analyze a hypothetical 60-story steel moment-frame tube building (fundamental period of 6.16 sec) designed according to the 1994 Uniform Building Code. Dynamic pushover and time-history analyses are conducted. Multi-story shear-band collapse mechanisms are observed around mid-height of the building. The use of closely-spaced columns and deep beams is found to contribute to the building's “somewhat brittle” behavior (ductility ratio ~2.0). Overall building strength is observed to be sensitive to whether a model is fracture-capable.</p>https://thesis.library.caltech.edu/id/eprint/9321Improving Seismic Collapse Risk Assessments of Steel Moment Frame Buildings
https://resolver.caltech.edu/CaltechTHESIS:06012018-015306089
Authors: {'items': [{'email': 'kennybuyco@gmail.com', 'id': 'Buyco-John-Kenneth', 'name': {'family': 'Buyco', 'given': 'John Kenneth'}, 'orcid': '0000-0002-8182-7119', 'show_email': 'YES'}]}
Year: 2018
DOI: 10.7907/2SFH-WP06
<p>It is important to be able to accurately assess seismic risk so that vulnerabilities can be prioritized for retrofit, emergency response procedures can be properly informed, and insurance rates can be sustainably priced to manage risk. To assess the risk of a building (or class of buildings) collapsing in a seismic event, procedures exist for creating one or more mathematical models of the structure of interest and performing nonlinear time history analysis with a large suite of input ground motions to calculate the building's seismic fragility and collapse risk. In this dissertation, three aspects of these procedures for assessing seismic collapse risk are investigated for the purpose of improving their accuracy.</p>
<p>It is common to use spectral acceleration with a damping ratio of 5% as a ground motion intensity measure (IM) for assessing collapse fragility. In this dissertation, the use of 70%-damped spectral acceleration as an IM is investigated, with a focus on evaluating its sufficiency and efficiency. Incremental dynamic analysis (IDA) is performed for 22 steel moment frame (SMF) models with 50 biaxial ground motion records to formally evaluate the performance of 70%-damped spectral acceleration as an IM for highly nonlinear response and collapse. It is found that 70%-damped spectral acceleration is much more efficient than 5%-damped spectral acceleration and much more sufficient with respect to epsilon for all considered levels of highly nonlinear response. Its efficiency and sufficiency compares also compares well with more advanced IMs such as average spectral acceleration.</p>
<p>When selecting input ground motions for nonlinear time history analysis, most engineers select ground motion records from the NGA-West2 database, which are processed with high-pass filters to remove long-period noise. In this dissertation, the extent to which these filters remove actual ground motion that is relevant to nonlinear time history analysis is evaluated. 52 near-source ground motion records from large-magnitude events are considered. Some records are processed by applying high-pass filters and others are processed by record-specific tilt corrections. Raw and NGA-West2 records are also considered. IDA is performed for 9-, 20-, and 55-story steel moment frame models with these processed records to assess the effects of ground motion processing on the calculated collapse capacity. It is found that if the cutoff period (Tc) is at least 40 seconds, then applying a high-pass filter does not have more than a negligible effect on collapse capacity for any of the considered records or building models. For shorter Tc (e.g. 10 or 15 seconds), it is found that the filters sometimes have a large effect on calculated collapse capacity, in some cases by over 50%, even if Tc is much larger than the building’s fundamental period. Of the considered ground motions, simply using the raw, uncorrected records usually yields more accurate results than using ground motions that have been processed with Tc less than or equal to 20 seconds.</p>
<p>For an existing building with unknown design plans, one might perform a collapse risk assessment using an archetype model for which the specific member sizes are assumed based on the relevant design code and building site. In this dissertation, the sensitivity of seismic collapse risk estimates to design criteria and procedures are evaluated for six 9-story and four 20-story post-Northridge SMFs. These SMFs are designed for downtown Los Angeles using different design procedures according to ASCE 7-05 and ASCE 7-10. Seismic risk analysis is performed using the results of IDA with 44 ground motion records and the results are compared to those of pre-Northridge models. It is found that the collapse risk of 9-story SMFs designed according to performance-based design vary by 3x, owing to differences in GMPEs used to generate site-specific response spectra. There is generally less variation in the collapse risk estimates of 20-story post-Northridge SMFs when compared to 9-story post-Northridge SMFs because wind drift limits control the design of many members of the 20-story SMFs. Differences in collapse risk between pre- and post-Northridge SMFs are found to be at least 4x and 8x for the 9- and 20-story models, respectively. Furthermore, in response to four strong ground motion records from large-magnitude events, some of the 9-story and all of the 20-story pre-Northridge SMFs experience collapse and most of the post-Northridge SMFs experience significant damage (MIDR > 0.03).</p>https://thesis.library.caltech.edu/id/eprint/10994Achieving Higher Fidelity Building Response through Emerging Technologies and Analytical Techniques
https://resolver.caltech.edu/CaltechTHESIS:07262017-074416397
Authors: {'items': [{'email': 'massari.8@osu.edu', 'id': 'Massari-Anthony-Thomas', 'name': {'family': 'Massari', 'given': 'Anthony Thomas'}, 'orcid': '0000-0002-6561-4674', 'show_email': 'NO'}]}
Year: 2018
DOI: 10.7907/Z9HH6H7N
The integration of sensor technology into the built environment has created an opportunity for a new approach to infrastructure development and management. Using collected data and principles of general physics, we discuss means and methods of using low cost dense instrumentation to perform damage detection, structural identification, and the benefits of cyber physical systems to community resilience. A nonlinear damping strategy for braced frame structures is introduced incorporating capped levels of damping forces. The study shows the effect of having control of damping forces in nonlinear analysis and the importance of limiting energy dissipation to rational levels. The issue of sliding mass is also studied to determine the contribution to energy loss and the effect to overall response. The results indicate a need to incorporate this effect in stiff structures with intentionally decoupled mass such as data centers. Finally, a discussion on dual system structures under plastic deformation in a post event deformed configuration is presented. A suggested displacement based method for design is suggested for implementation into future editions of the building code.https://thesis.library.caltech.edu/id/eprint/10352Reduced-Order Model for Dynamic Soil-Pipe Interaction Analysis
https://resolver.caltech.edu/CaltechTHESIS:06012020-154218098
Authors: {'items': [{'email': 'kien.nguyen.tru@gmail.com', 'id': 'Nguyen-Kien-Trung', 'name': {'family': 'Nguyen', 'given': 'Kien Trung'}, 'orcid': '0000-0001-5761-3156', 'show_email': 'YES'}]}
Year: 2020
DOI: 10.7907/mekk-dc25
<p>Pipelines are very vulnerable infrastructure components to geohazard-induced ground deformation and failure. How soil transmits loads on pipelines and vice versa, known as soil-pipe interaction (SPI), thus is very important for the assessment and design of resilient pipeline systems.</p>
<p>In the first part, this work proposes a simplified macroelement designed to capture SPI in cohesionless soils subjected to arbitrary loading normal to the pipeline axis. We present the development of a uniaxial hysteresis model that can capture the smooth nonlinear reaction force-relative displacement curves (FDCs) of SPI problems. Using the unscented Kalman filter, we derived the model parameter κ that controls the smoothness of the transition zone from linear to plastic using published experimental data. We extended this uniaxial model to biaxial loading effects and showed that the macroelement can capture effects such as pinching and shear-dilation coupling. The model input parameters were calibrated using finite element (FE) analyses validated by experiments. The FDCs of the biaxial model were verified by comparison with FE and smoothed-particle hydrodynamic (SPH) simulations for different loading patterns: cyclic uniaxial, 0-shaped, 8-shaped, and transient loading. Accounting for smooth nonlinearity, hysteresis, pinching, and coupling effects, the proposed biaxial macroelement shows good agreement with FE and SPH analyses, while maintaining the computational efficiency and simplicity of beam-on-nonlinear-Winkler foundation models, as well as a small number of input parameters.</p>
<p>Next, this work presents analytical solutions for computing frequency-domain axial and in-plane soil impedance functions (SIFs) for an infinitely long rigid circular structure buried horizontally in homogeneous elastic half-space. Using Hankel— and Bessel—Fourier series expansion, we solved a mixed-boundary-value problem considering a harmonic displacement at the structure boundary and traction-free boundary condition at the half-space free surface. We then verified our analytical solutions using results obtained from FE simulations. The SIFs of a buried structure in a homogeneous elastic half-space calculated by these two approaches are in perfect agreement with each other. In addition, we used analytical solutions and FE simulations to comprehensively investigate factors that affect the SIFs in homogeneous and two-layered half-spaces, respectively. The parametric study shows that SIFs of buried structures in elastic half-space primarily depend on frequency of excitation, shear modulus and Poisson's ratio of the half-space, burial depth and radius of the structure. In a two-layered soil domain, SIFs depend also on material contrast and the distance from the structure location to the interface between soil layers.</p>
<p>Lastly, it demonstrates how the SIFs obtained previously can be incorporated into a reduced-order model to analyze SPI problems, specifically a straight pipe subjected to Rayleigh surface wave propagating through homogeneous and heterogeneous elastic half-spaces. Calculated displacement time histories at the control points are shown to agree well with those computed by direct two-dimensional FE analyses.</p>https://thesis.library.caltech.edu/id/eprint/13762Matching Waveform Envelopes for Earthquake Early Warning
https://resolver.caltech.edu/CaltechTHESIS:11112020-213135157
Authors: {'items': [{'email': 'beckyheeroh@gmail.com', 'id': 'Roh-Becky', 'name': {'family': 'Roh', 'given': 'Becky'}, 'orcid': '0000-0002-3905-0086', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/hw8k-zx98
<p>Current earthquake early warning (EEW) algorithms are continuously optimized to strive for fast, accurate source parameter estimates for the rupturing earthquake (i.e. magnitude, location), which are then used to predict ground motions expected at a site. However, they may still struggle with challenging cases, such as offshore events and complex sequences. An envelope-based two-part search algorithm is developed to handle such cases. This algorithm matches different templates to the incoming observed ground motion envelopes to find the optimal earthquake source parameter estimates.</p>
<p>The algorithm consists of two methods. Method I is the standard grid search, and it uses Cua-Heaton ground motion envelopes as its templates; Method II is the extended catalog search, and its templates are waveform envelopes from past real and synthetic earthquakes. The grid search is intended for robustness and provides approximate average solutions, whereas the extended catalog search matches envelopes considering the station’s specific site and path effects. In parallel execution, Methods I and II work together – either by confirming each other’s solutions or accepting the solution with stronger fits – to provide the best parameter estimates based on waveform-based data.</p>
<p>The main advantage of the two-part search algorithm is its ability to find parameter estimates of reduced uncertainties using the P-wave data from a single station. Many algorithms wait until multiple stations are triggered to reduce tradeoffs between the magnitude and location. This waiting time, however, is detrimental in EEW, for it jeopardizes the warning time that can be issued to nearby regions expected to experience strong shaking. The use of a single station would virtually eliminate this waiting time, maximizing the warning time without the cost in accuracy of the estimates.</p>
<p>Because EEW is a race against time, further actions are taken for more rapid estimation of the earthquake source parameters. A Bayesian approach using prior information has the potential to reduce uncertainties that arise in the initial time points due to tradeoffs between the magnitude and location. This essentially increases the confidence of the initial parameter estimates, allowing alerts to be issued faster. A KD tree nearest neighbor search is also introduced to reduce latency in the time it takes to find the best-fitting solutions. In comparison to an exhaustive, brute-force search, it cuts the searching time by only examining through a fraction of the total database.</p>
<p>An envelope-based algorithm examines the shape and relative frequency content and makes appropriate judgments, just as a human seismologist would; it also addresses the issue of data transmission latencies. Overall, this algorithm is able to interpret the complexity of earthquakes and assess the features they hold to ultimately communicate information of significant ground shaking to different regions.</p>https://thesis.library.caltech.edu/id/eprint/13998