Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 88-91). Also available via the Internet.

Thesis (M. Eng.(Mechanical Engineering))--University of Pretoria, 2000. / Summaries in English and Afrikaans. Includes bibliographical references.

In estimating econometric time series models, it is assumed that the parameters remain constant over the period examined. This assumption may not always be valid when using data which span an extended period, as the underlying relationships between the variables in these models are exposed to various exogenous shifts. It is therefore imperative to examine the stability of models as failure to identify any changes could result in wrong predictions or inappropriate policy recommendations. This research proposes a method of estimating the location of break points in linear econometric models with endogenous regressors, estimated using Generalised Method of Moments (GMM). The proposed estimation method is based on Wald, Lagrange Multiplier and Difference type test statistics of parameter variation. In this study, the equation which sets out the relationship between the endogenous regressor and the instruments is referred to as the Jacobian Equation (JE). The thesis is presented along two main categories: Stable JE and Unstable JE. Under the Stable JE, models with a single and multiple breaks in the Structural Equation (SE) are examined. The break fraction estimators obtained are shown to be consistent for the true break fraction in the model. Additionally, using the fixed break approach, their $T$-convergence rates are established. Monte Carlo simulations which support the asymptotic properties are presented. Two main types of Unstable JE models are considered: a model with a single break only in the JE and another with a break in both the JE and SE. The asymptotic properties of the estimators obtained from these models are intractable under the fixed break approach, hence the thesis provides essential steps towards establishing the properties using the shrinking breaks approach. Nonetheless, a series of Monte Carlo simulations conducted provide strong support for the consistency of the break fraction estimators under the Unstable JE. A combined procedure for testing and estimating significant break points is detailed in the thesis. This method yields a consistent estimator of the true number of breaks in the model, as well as their locations. Lastly, an empirical application of the proposed methodology is presented using the New Keynesian Phillips Curve (NKPC) model for U.S. data. A previous study has found this NKPC model is unstable, having two endogenous regressors with Unstable JE. Using the combined testing and estimation approach, similar break points were estimated at 1975:2 and 1981:1. Therefore, using the GMM estimation approach proposed in this study, the presence of a Stable or Unstable JE does not affect estimations of breaks in the SE. A researcher can focus directly on estimating potential break points in the SE without having to pre-estimate the breaks in the JE, as is currently performed using Two Stage Least Squares.

The detection of damage in structures through the use of vibrational methods offers particular advantages, which makes it an attractive method to use in specific applications. In this work the advantages and some of the possible applications of vibrational damage detection methods will be discussed. A study of the field of damage detection using vibration techniques is undertaken. Available methods are categorised in general groups according to the underlying principles. The principle, on which each group functions, as well as the advantages and disadvantages of each, concerning the practical application thereof, is explained. The goal of this work, which entails developing a damage detection method using large amounts of raw data directly and combining some of the most favourable properties of the different groups to detect damage, are set. The new method is developed and compared both numerically as well as experimentally to two methods, chosen from the literature because of similar methodology and their reputation for effective damage detection. The methods will be tested numerically with respect to accuracy, sensitivity and multiple damage detection ability. Finally experimental data is gathered and used to verify the methods damage detection ability. The new method provides a different approach to damage detection, by combining an available vibration detection method with the maximum available amount of data in order to increase the damage detection ability. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted

The unit root revolution in time series modeling has created substantial interest in non- stationarity and its implications for empirical modeling. Beyond the original interest in trend vs. di¤erence non-stationarity, there has been renewed interest in testing and modeling structural breaks. The focus of my dissertation is on testing for departures from stationarity in a broader framework where unit root, mean trends and structural break non-stationarity constitute only a small subset of the possible forms of non-stationarity. In the fi¦rst chapter the most popular testing procedures for the assumption, in view of the fact that general forms of non-stationarity render each observation unique, I develop a testing procedure using a resampling scheme which is based on a Maximum Entropy replication algorithm. The proposed misspecification testing procedure relies on resampling techniques to enhance the informational content of the observed data in an attempt to capture heterogeneity 'locally' using rolling window estimators of the primary moments of the stochastic process. This provides an e¤ective way to enhance the sample information in order to assess the presence of departures from stationarity. Depending on the sample size, the method utilizes overlapping or non-overlapping window estimates. The e¤ectiveness of the testing procedure is assessed using extensive Monte Carlo simulations. The use of rolling non-overlapping windows improves the method by improving both the size and power of the test. In particular, the new test has empirical size very close to the nominal and very high power for a variety of departures from stationarity. The proposed procedure is then applied on seven macroeconomic series in the fourth chapter. Finally, the optimal choice of orthogonal polynomials, for hypothesis testing, is investigated in the last chapter. / Ph. D.

Eccentric discharge of granular solids is widely considered one of the most serious design conditions for thin-walled metal silos, and one which has been the cause of very many silo disasters in the past. Yet the reasons for these consequences have not been very well understood, given the serious difficulties inherent in measuring or modelling flow patterns of granular solids, wall pressures and the associated structural response. To this end, this thesis presents a programme of theoretical and computational analyses which investigate the effects of a very wide range of different discharge flow patterns from silos, including both concentric and eccentric flows. The critical effects of changes of flow channel geometry, silo aspect ratio, changes of plate thickness and geometric and material nonlinearity are explored in detail. The codified procedures and pressure distributions for concentric and eccentric discharge of the EN 1991-4 (2007) European Standard are analysed first on a number of example silos custom-designed according to EN 1993-1-6 (2007) and EN 1993-4-1 (2007), followed by the development and investigation of a more complete mixed flow pressure theory. The computational analyses presented in this thesis are thought to be the first of their kind.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Rock falls are the cause of the majority of mining-related injuries and fatalities in deep tabular South African mines. The standard process of entry examination is performed before working shifts and after blasting to detect structurally loose rocks. This process is performed by a miner using a pinch bar to ‘sound’ a rock by striking it and making a judgement based on the frequency response of the resultant sound. The Electronic Sounding Device (ESD) developed by the CSIR aims to assist in this process by performing a concurrent prediction of the structural state of the rock based on the acoustic waveform generated in the sounding process. This project aimed to identify, develop and deploy an effective classification model to be used on the ESD to perform this assessment. The project was undertaken in three main stages: the collection of labelled acoustic samples from working areas; the extraction of descriptive features from the waveforms; and the competitive evaluation of suitable classification models. Acoustic samples of the sounding process were recorded at the Driefontein mine operation by teams of Gold Fields employees. The samples were recorded in working areas on each of the four reefs that were covered by the shafts of the mine complex. Samples were labelled as ‘safe’ or ‘unsafe’ to indicate an expert’s judgement of the rock’s structural state. A laboratory-controlled environment was also created to provide a platform from which to collect acoustic samples with objective labelling. Three sets of features were extracted from the acoustic waveforms to form a descriptive feature dataset: four statistical moments of the frequency distribution of the waveform formed; the average energy contained in 16 discrete frequency bands in the data; and 12 Mel Frequency Cepstral Coefficients (MFCCs). Classification models from four model families were competitively evaluated for best accuracy in predicting structural states. The models evaluated were k-nearest neighbours, self-organising maps, decision trees, random forests, logistic regression, neural networks, and support vector machines with radial basis function and polynomial kernels. The sensitivity of the models, i.e. their ability to avoid predicting a ‘safe’ status when the rock mass was actually loose, was used as the critical performance measure. A single-hidden-layer feed-forward neural network with 15 nodes in the hidden layer and a sigmoid activation function was found to best suited for acoustic classification on the ESD. Additional feature selection was performed to identify the optimised form of the model. The final model was successfully implemented on the ESD platform. / AFRIKAANSE OPSOMMING: Rotsstortings is die oorsaak van die meerderheid van mynbouverwante ongelukke en ongevalle in diep tabulêre Suid-Afrikaanse myne. Die standaard proses van pretoegang ondersoeke om strukturele los rotse te erken, word uitgevoer voor enige werkskof en na skietwerk. Dit word gedoen deur ‘n myner wat ‘n breekyster teen die rots kap en ‘n oordeel vel op die frekwensie weergawe van die gevolglike klank. Die ‘Elektroniese Klinking Toestel’ (Electronic Sounding Device, ESD) is ontwikkel deur die WNNR met die doel om die proses te ondersteun. Dit word gedoen deur ‘n gelyktydige voorspelling van die strukturele toestand gebaseer op die akoestiese golfvorm gegenereer in die proses van klinking. Die projek stel ten doel om ’n effektiewe klassifikasie-model te identifiseer, te ontwikkel en toe te pas in die ESD om hierdie assessering uit te voer. Die projek vind in drie stadiums plaas: die insameling van geëtiketteerde akoestiese monsters van die werkareas; die ekstraksie van beskrywende kenmerke van die golfvorms en die mededingende evaluering van geskikte klassifiseringsmodelle. Klinking akoestiese monsters is opgeneem by Driefontein mynbouoperasie deur spanne van Gold Fields se werknemers. Die akoestiese monsters is opgeneem in werkareas van elk van die vier goudriwwe wat deur die skagte van die mynkompleks gedek word. Monsters is as ‘veilig’ of ‘onveilig’ geëtiketteer as aanduiding van die ekspert se oordeel van die rots se strukturele toestand. ‘n Laboratorium gekontroleerde omgewing is ook geskep om ’n platform te skep vanwaar akoestiese monsters met objektiewe etikettering waargeneem word. Drie stelle van kenmerke is onttrek van die akoestiese golfvorms om ‘n beskrywende datastel van kenmerke te vorm: vier statistiese momente van die frekwensie verspreiding van die gevormde golfvorm; gemiddelde energie ingesluit in sestien diskrete frekwensiebande in die data; en twaalf ‘Mel Frequency Cepstrum Coefficients’ (MFCCs). Klassifikasie modelle van die vier modelsamestellings was kompeterend geëvalueer vir die beste akkuraatheid in voorspellings van strukturele toestande. Klassifikasie modelle het k-naaste bure, selforganiserende kaarte, besluitnemingsbome, lukrake woude, logistieke regressie, neurale netwerke en steun-vektor masjiene met radiale basisfunksie en polinominale kerne. Die meting van die sensitiwiteit van die modelle, met betrekking tot die vermoë van die modelle om veilige voorspellings te beperk wanneer die rotsmassa los is, was gebruik as ’n kritiese werksverrigtingsmeting. ‘n Enkel-verskuilde-laag neurale netwerk met 15 nodes in die verskuilde laag en ’n sigmoïde aktiveringsfunksie is gevind as die mees geskikte vir die ESD. Addisionele keuse van kenmerke is uitgevoer deur die geoptimiseerde vorm van die model te identifiseer. Die model was suksesvol geïmplementeer op die ESD platform.

No description available.

The overall objective of this thesis is to develop a methodology which can be used to investigate the in-service performance of reinforced concrete members subject to shear loading, in order to update assessments of the shear capacity (and therefore the remaining life) of reinforced concrete bridges. To achieve this end tests have been carried out on two types of reinforced concrete members under different types of loading, with the principal response measured being the relative displacement of the top and bottom faces of the member, which has been referred to throughout as the “through-depth displacement”. The first member tested was a two-span continuous beam containing some web reinforcement in its central shear spans. This was loaded in a series of cycles to progressively increasing peak loads, with a few cycles to lower peak loads being carried out after the application of the higher peak loads.

As estruturas, particularmente na engenharia civil, podem apresentar ruína quando atingem sua capacidade resistente ou quando perdem sua estabilidade, sendo, portanto atribuição básica do engenheiro de estruturas o estudo de ambas as situações. A instabilidade de uma estrutura pode surgir de dois modos, a saber: por ocorrência de uma bifurcação de equilíbrio ou por ocorrência de um ponto limite, também conhecido por snap-through, onde o aumento do carregamento provoca uma diminuição da rigidez da estrutura, até que esta se anula no ponto limite (REIS; CAMOTIM, 2012). Estruturas como arcos, treliças e calotas esféricas abatidas, presentes em grandes coberturas, são tipos de estruturas que podem apresentar esta instabilidade, em que há a passagem dinâmica da estrutura para uma configuração de equilíbrio afastada e estável, saltando para essa configuração pós-crítica envolvendo grandes deslocamentos e inversão da curvatura. Se, no entanto, o carregamento é dinâmico, como, por exemplo, harmônico, a resposta do sistema adquire uma grande riqueza de possíveis comportamentos, em função da amplitude e frequência desse carregamento. As respostas podem resultar vibrações periódicas de vários períodos diferentes, quase periódicas, caóticas etc. Este trabalho tem como objetivo fazer um estudo da estabilidade estática e dinâmica do problema da treliça simples de duas barras (treliça de Von Mises) e do arco abatido senoidal, de comportamento elástico linear, com o estabelecimento das equações de equilíbrio na configuração deformada, i.e., levando em conta a não linearidade geométrica. A avaliação da resposta, bem como a caracterização de sua estabilidade, se dará pela apresentação das cargas críticas de instabilidade do sistema perfeito, exibição do comportamento de pós-instabilidade e, com a integração numérica do modelo matemático, o estudo geométrico dado pelos planos de fase, mapas de Poincaré, diagramas de bifurcação e fronteira de estabilidade. / Structures, particularly in civil engineering, can ruin when they reach their strength capacity or when they lose their stability. So, it is the basic assignment of the structural engineer to study both situations. The instability of a structure can arise in two ways, namely: by the occurrence of bifurcation of equilibrium or by the occurrence of a snap-through, where an increase of the loading causes a decrease in structure stiffness, until the stiffness is annulled in the limit point (REIS, CAMOTIM, 2012). Structures such as arches, trusses and domes, present in large roofs, are types of structures that may present this kind of instability, in which there is the dynamic passage of the structure to a far away stable equilibrium configuration, jumping to this post-critical configuration involving large displacements and reversal of the curvature. If, however, the load is dynamic, such as harmonic, the response of the system acquires a great wealth of possible behaviors, depending on the amplitude and frequency of this loading. The responses may result in periodic vibrations of several different periods, almost periodic, chaotic, etc. This work intends to study the static and the dynamic stability of the Von Mises truss and the shallow arc of linear elastic behavior, with the establishment of the equilibrium equations in the deformed configuration, i.e., taking into account the geometric non-linearity. The evaluation of the response, as well as the characterization of its stability, will be done by numerical integration of the mathematical model and geometric study of the phase planes, Poincaré maps, bifurcation diagrams and stability border.