Vibrations resulting from earthquakes, machinery, or unanticipated shocks may be very damaging and costly to structures. To avoid such damage, designers need a structural system that can dissipate the energy caused by these vibrations. Using elastically buckled struts may be a viable means to reduce the harmful effects of unexpected vibrations. Post-buckled struts can support high axial loads and also act as springs in a passive vibration isolation system by absorbing or dissipating the energy caused by external excitation. When a base excitation is applied, the buckled strut may act to reduce the dynamic force transmitted to the system, thus reducing the structural damage to the system. Several models of buckled and pre-bent struts are examined with different combinations of parameters and end conditions. The models include pinned or fixed columns supporting loads above their buckling load, and columns with an initial curvature supporting various loads. The varying parameters include external damping, internal damping, and stiffness. The columns will be subjected to simple harmonic motion applied at the base or to a multi-frequency base excitation. The response of each model is measured by the deflection transmissibility of the supported load over a large range of frequencies. Effective models reduce the motion of the supported load over a large range of frequencies. / Master of Science

The current design code governing bridge structure resistance to vessel impact loads in the U.S. is the American Association of State Highway and Transportation Officials' (AASHTO) Guide Specification and Commentary for Vessel Collision Design of Highway Bridges. The code stipulated method, based on Meir-Dornberg's equivalent static load method, is usually not warranted because of insufficient data on the impact load histories and wide scatter of the impact force values. The AASHTO load equations ignore certain fundamental factors that affect the determination of impact forces and bridge dynamic responses. Some examples of factors that are omitted during standard impact force analysis are: impact duration, pier geometry, barge-barge and barge-pier interactions, and structural characteristics of bridges. The purpose of this research is to develop new methods and models for predicting barge impact forces on piers. In order to generate research information and produce more realistic flotilla impact data, extensive finite element simulations are conducted. A set of regression formulas to calculate the impact force and time duration are derived from the simulation results. Also, a parametric study is performed systematically to reveal the dynamic features of barge-bridge collisions. A method to determine the quasi upper bound of the average impact force under any given scenarios is preposed. Based on the upper bounds of the average impact forces, an impact spectrum procedure to determine the dynamic response of piers is developed. These analytical techniques transform the complex dynamics of barge-pier impact into simple problems that can be solved through hand calculations or design charts. Furthermore, the dependency of the impact forces on barge-barge and barge-pier interactions are discussed in detail. An elastoplastic model for the analysis of multi-barge flotillas impacting on bridge piers is presented. The barge flotilla impact model generates impact force time-histories for various simulation cases in a matter of minutes. The results from the proposed model are compatible with the respective impact time-histories produced by an exhuaustive finite element simulation. All of the proposed methods and loading functions in this study are illustrated through design examples. Accordingly, the research results may help engineers to enhance bridge resistance to barge impacts and also lead to economic savings in bridge protection design.

The purpose of this dissertation is to analyze dynamic behavior of a stressed power system and to correlate the dynamic responses to a near future system voltage abnormality. It is postulated that the dynamic response of a stressed power system in a short period of time-in seconds-contains sufficient information that will allow prediction of voltage abnormality in future time-in minutes. The PSSE dynamics simulator is used to study the dynamics of the IEEE 39 Bus equivalent test system. To correlate dynamic behavior to system voltage abnormality, this research utilizes two different pattern recognition methods one being algorithmic method known as Regularized Least Square Classification (RLSC) pattern recognition and the other being a statistical method known as Classification and Regression Tree (CART). Dynamics of a stressed test system is captured by introducing numerous contingencies, by driving the system to the point of abnormal operation, and by identifying those simulated contingencies that cause system voltage abnormality. Normal and abnormal voltage cases are simulated using the PSSE dynamics tool. The results of simulation from PSSE dynamics will be divided into two sets of training and testing set data. Each of the two sets of data includes both normal and abnormal voltage cases that are used for development and validation of a discriminator. This research uses stressed system simulation results to train two RLSC and CART pattern recognition models using the training set obtained from the dynamic simulation data. After the training phase, the trained pattern recognition algorithm will be validated using the remainder of data obtained from simulation of the stressed system. This process will determine the prominent features and parameters in the process of classification of normal and abnormal voltage cases from dynamic simulation data. Each of the algorithmic or statistical pattern recognition methods have their advantages and disadvantages and it is the intention of this dissertation to use them only to find correlations between the dynamic behavior of a stressed system in response to severe contingencies and the outcome of the system behavior in a few minutes into the future.

Estruturas como torres e chaminés industriais são bastante vulneráveis ao fenômeno de desprendimento de vórtices, devido à sua esbeltez e forma rombuda. Além disso, devido ao baixo amortecimento estrutural que possuem, essas estruturas também têm maiores chances de atingir grandes amplitudes de deslocamento, o que é causado pelo efeito de captura. Apesar de esse tipo de estrutura ser considerado simples dos pontos de vista estrutural e aerodinâmico, o estudo das vibrações transversais nessas estruturas é bastante complicado, pois envolve a interação entre tópicos complexos da mecânica dos fluidos e estrutural, tornando a determinação confiável da resposta estrutural um dos problemas mais difíceis da Engenharia do Vento. Ao longo das últimas cinco décadas, diversos pesquisadores vêm estudando esse fenômeno, buscando uma abordagem que consiga considerar todos os tópicos que envolvem o mecanismo de vibração por desprendimento de vórtices. Entretanto, apesar dos esforços, os modelos existentes para a verificação da resposta da estrutura são de caráter empírico, sendo os dois mais aceitos pela comunidade científica o modelo de comprimento de correlação de Ruscheweyh e o modelo matemático espectral de Vickery e Clark, o qual foi posteriormente aprimorado por Vickery e Basu. Primeiramente, são estudados em detalhe esses dois modelos e seus métodos derivados, os quais são apresentados em normas e códigos. Após isso, é feita uma proposta de cálculo de dimensionamento do deslocamento do topo de tais estruturas baseada no modelo de Vickery e Basu e adaptada às necessidades da NBR- 6123. E finalmente, são apresentados dados de 42 estruturas, as quais atingiram grandes amplitudes de vibração em seu topo. Essas estruturas foram dimensionadas segundo as diretrizes de cada um dos métodos estudados neste trabalho, o que demonstrou o bom desempenho do Método II do Eurocódigo, do Método do CICIND e da Proposta III-B para a NBR-6123. / Structures like towers and industrial chimneys are quite vulnerable to the vortex shedding phenomenon, due to their slenderness and non-aerodynamic form. Furthermore, due to their low structural damping, these structures are also more likely to reach large displacement amplitudes, which is caused by the lock-in effect. Although these structures are considered as simple from structural and aerodynamic points of view, the study of cross-wind vibrations in these structures is quite complicated, as it involves the interaction of complex topics of fluid and structural mechanics, turning a reliable determination of the structural response into one of the most complicated problems in Wind Engineering. Over the past five decades, several researchers have been studying this phenomenon, seeking an approach that could consider all topics involving the vibrating mechanism by vortex shedding. However, despite the efforts, the existing models for predicting the response of the structure are empirical, with the two most accepted by the scientific community being the Ruscheweyh’s correlation length model and the Vickery & Clark’s spectral mathematical model, which was further enhanced by Vickery & Basu. Firstly, these two models and their derivative methods, which are reported in standards and codes, are studied in detail. After, a calculation proposal for predicting the top displacement of such structures is presented, which is based on the Vickery & Basu model and adapted to the needs of NBR-6123. Finally, data for 42 real structures which have reached large vibration amplitudes at their tops is presented. These structures were designed according to the guidelines for each of the methods studied in this work, which demonstrated the good performance of the Eurocode II Method, of the CICIND Method and of the NBR-6123 III-B Proposal.

“Estas vibrações são as primeiras do seu tipo para as pontes tipo viga….elas mostram a alta estabilidade aerodinâmica e confiabilidade da estrutura" (Anishyuk e Antonova, 2010). Esta é uma citação do porta-voz da empresa responsável pela construção da ponte, com vãos 120m e de 7 km de extensão, em Volgograd, na Rússia – que oscilou tão violentamente sob velocidades baixas do vento em maio de 2010, que os motoristas ficaram enjoados e a ponte foi fechada. As filmagens do movimento da ponte podem ser encontradas facilmente na internet, e causam uma visão perturbadora. O que é mais perturbador é que a empresa responsável pelo seu projeto e construção pôde ser tão ignorante da história das aerodinâmicas de pontes e conceitos aerodinâmicos básicos.As vibrações induzidas por vórtices que tinham atormentadas a Ponte Volgograd são agora atenuadas por um sistema avançado de dispositivos de amortecimento mecânico. Se as medidas de segurança tivessem sido tomadas durante a fase do projeto, a necessidade desses dispositivos de amortecimento poderia ter sido evitada. Uma variedade de dispositivos aerodinâmicos passivos tem apresentado ser extremamente eficaz na supressão das vibrações induzidas por vórtices em velocidades baixas do vento para um número de pontes de grande vão Investigações adequadas em túnel de vento realizadas durante a fase do projeto podem alertar os projetistas da existência de instabilidades aerodinâmicas inerente no projeto, e passos podem ser tomados para modificar o formato do tabuleiro para otimizar o seu desempenho dinâmico sob cargas de vento. Investigações experimentais da efetividade na supressão das vibrações induzidas por vórtices, por uma série de dispositivos aerodinâmicos passivos, têm sido realizadas no Túnel de Vento Prof. Joaquim Blessmann, em Porto Alegre. Dispositivos foram projetados com base nas modificações feitas a outros projetos de pontes que sofreram com as vibrações indesejadas da velocidade baixa do vento, tanto no túnel de vento durante a fase do projeto, durante a construção ou após a conclusão da ponte em grande escala. Dispositivos foram testados usando um modelo dinâmico de uma ponte da vida real, e os resultados indicaram que algumas modificações simples a geometria da seção transversal do tabuleiro podem ter um efeito significativo na sua resposta. / “These vibrations are the first of their kind for beam-type bridges….they show the high aerodynamic stability and reliability of the structure” (Anishyuk and Antonova, 2010). This is a quote taken from the spokesman of the company responsible for the construction of the 7km long reinforced concrete bridge in Volgograd, Russia – whose multiple continuous 120 meter spans oscillated so violently under low-wind speed conditions in May of 2010 that motorists became seasick and the bridge was closed. Footage of the bridge’s movement can easily be found on the internet, and makes for disturbing viewing. What is even more disturbing is that the company responsible for its design and construction could be so ignorant to the history of bridge aerodynamics and basic aerodynamic concepts. The vortex-induced vibrations which had plagued the Volgograd Bridge are now mitigated by an advanced system of mechanical damping devices. If proper precautions had been taken during the design stage of the bridge, the necessity of these damping devices could have been avoided. A variety of passive aerodynamic devices have been shown to be extremely effective in the suppression of low wind speed vortex-induced vibrations for a number of long-span bridges Proper wind-tunnel investigations undertaken during the design stage can alert designers to the existence of aerodynamic instabilities inherent to the design, and steps can be taken to modify the shape of the deck to optimise its dynamic performance under wind loads. Experimental investigations of the effectiveness of range of passive aerodynamic devices to suppress vortex-induced vibrations have been conducted at the Professor Joaquim Blessmann Wind-Tunnel, Porto Alegre. Devices were designed based on modifications made to other bridge designs which suffered from unwanted low wind speed vibrations, either in the windtunnel during the design stage, during erection or after completion of the full-scale bridge. Devices were tested using a dynamic section model of a real-life bridge deck design. Results indicate that some simple modifications to the cross-section geometry of the deck can have a substantial effect on its response.

Molten carbonate fuel cells (MCFCs) have become an attractive emerging technology for stationary co-generation of heat and power. From a technical perspective, dynamic operation has a significant effect on the fuel cell life cycle and, hence, economic viability of the device. The scope of this thesis is to present an improved understanding of the system behaviour at transient operation that can be used to design a more robust control system in order to overcome the cost and the operating lifetime issues. Hence, a comprehensive multi-component multidimensional transient mathematical model is developed based on the conservation laws of mass, momentum, species, energy and electric charges coupled through the reaction kinetics. In essence, this model is a set of partial differential equations that are discretized and solved using the finite-volume based commercial software, ANSYS FLUENT 12.0.1. The model is validated with two sets of experimental results, available in open literature, and good agreements are obtained. The validated model is further engaged in an extensive study. First, the MCFC behaviour at high current densities or oxidant utilization, when the mass transfer becomes dominant, is investigated using peroxide and superoxide reaction mechanisms. In brief, both mechanisms predicted the linear region of the polarization curve accurately. However, none of these mechanisms showed a downward bent in the polarization curve. A positive exponent for the carbon-dioxide mole fraction is probably essential in obtaining the downward bent (“knee”) at high current densities which is in contrast to what has been reported in the literature to date. Next, a sinusoidal impedance approach is used to examine the dynamic response of the unit cell to inlet perturbations at various impedance frequencies. This analysis is further used to determine the phase shifts and time scales of the major dynamic processes within the fuel cell. Furthermore, numerical simulation is utilized in order to investigate the underlying electrochemical and transport phenomena without performing costly experiments. Results showed that the electrochemical reactions and the charge transport process occur under a millisecond. The mass transport process showed a comparatively larger time scale. The energy transport process is the slowest process in the cell and takes about an hour to reach its steady state condition. Furthermore, the developed mathematical model is utilized as a predictive tool to provide a three-dimensional demonstration of the transient physical and chemical processes at system startiv up. The local distribution of field variables and quantities are presented. The results show that increasing the electrode thickness provides a higher reaction rate, but may lead to larger ohmic loss which is not desirable. The reversible heat generation and consumption mechanisms of the cathode and anode are dominant in the first 10 s while the heat conduction from the solid materials to the gas phase is not considerable. The activation and ohmic heating have the same impact within the anode and cathode because of their similar electric conductivity and voltage loss. Increasing the thermal conductivity of the cathode material will facilitate the process of heat transport throughout the cell. This can also be accomplished by lowering the effects of heat conduction by means of a cathode material with a smaller thickness. In addition, a thermodynamic model is utilized to examine energy efficiency, exergy efficiency and entropy generation of a MCFC. By changing the operating temperature from 883 K to 963 K, the energy efficiency of the unit cell varies from 42.8 % to 50.5 % while the exergy efficiency remains in the range of 26.8% to 36.3%. Both efficiencies initially rise at lower current densities up to the point that they attain their maximum values and ultimately decrease with the increase of current density. With the increase of pressure, both energy and exergy efficiencies of the cell increase. An increase in this anode/cathode flow ratio lessens the energy and exergy efficiencies of the unit cell. Higher operating pressure and temperature decrease the unit cell entropy generation. / UOIT

Hydraulic circuits with fast dynamic response are often characterized by low power efficiency; on the other hand, energy-efficient circuits under certain circumstances, can demonstrate slow transient responses. Continuously rising energy costs combined with the demand on high performance has necessitated that hydraulic circuits become more efficient yet still demonstrate superior dynamic response. This thesis introduces a new hydraulic circuit configuration which demonstrates high dynamic performance and high efficiency. A pump-controlled hydraulic motor system was used as the basis of the study because of its high circuit efficiency. This is primarily because there is no power loss between the pump and motor. To improve the dynamic response of the pump, a DC motor was designed to control the pump swashplate (and hence flow rate) directly. The pump and DC motor were mathematically modeled and their parameters were experimentally identified. Based on the model and experimental results, a nonlinear PID controller was designed for the DC motor. By means of the DC motors quick dynamic response (in the order of 10 ms), the DC motor controlled pump demonstrated a fast dynamic response with a rise time of 15 to 35 ms depending on the pump pressure. As the dynamic response speed of the pump flow rate was increased, overshoot of the hydraulic motor output also increased. To reduce this overshoot, a bypass flow control circuit was designed to bypass part of the flow during the transient. Due to the unique operating requirements of the bypass flow control system, a PID controller with a resetable integral gain was designed for the valve to reduce the rise time of the bypass control valve. The feasibility ("proof of concept") of the bypass flow control concept was first established using simulation techniques. The simulation results showed that the bypass flow control system could significantly reduced the overshoot of the hydraulic motor rotational speed. The bypass controller was applied to the experimental test circuit. The transient results for the pump-controlled motor system with the bypass flow control are presented under a constant resistive and an inertial load. The test results showed that the bypass flow control could reduce the overshoot of the hydraulic motor rotational speed by about 50%. The relative efficiency of the circuit with the bypass flow control system was 1% to 5% lower for the particular pump-controlled system that was used in this study. For a pump/motor that does not demonstrate significant flow ripple of the magnitude experienced in this study, the relative efficiency would be the same as the pump/motor system without bypass. It was concluded that the proposed bypass control system, combined with the DC motor-swashplate driven pump, could be used to create an energy efficient circuit with excellent dynamic transient responses.

Hydraulic circuits with fast dynamic response are often characterized by low power efficiency; on the other hand, energy-efficient circuits under certain circumstances, can demonstrate slow transient responses. Continuously rising energy costs combined with the demand on high performance has necessitated that hydraulic circuits become more efficient yet still demonstrate superior dynamic response. This thesis introduces a new hydraulic circuit configuration which demonstrates high dynamic performance and high efficiency. A pump-controlled hydraulic motor system was used as the basis of the study because of its high circuit efficiency. This is primarily because there is no power loss between the pump and motor. To improve the dynamic response of the pump, a DC motor was designed to control the pump swashplate (and hence flow rate) directly. The pump and DC motor were mathematically modeled and their parameters were experimentally identified. Based on the model and experimental results, a nonlinear PID controller was designed for the DC motor. By means of the DC motors quick dynamic response (in the order of 10 ms), the DC motor controlled pump demonstrated a fast dynamic response with a rise time of 15 to 35 ms depending on the pump pressure. As the dynamic response speed of the pump flow rate was increased, overshoot of the hydraulic motor output also increased. To reduce this overshoot, a bypass flow control circuit was designed to bypass part of the flow during the transient. Due to the unique operating requirements of the bypass flow control system, a PID controller with a resetable integral gain was designed for the valve to reduce the rise time of the bypass control valve. The feasibility ("proof of concept") of the bypass flow control concept was first established using simulation techniques. The simulation results showed that the bypass flow control system could significantly reduced the overshoot of the hydraulic motor rotational speed. The bypass controller was applied to the experimental test circuit. The transient results for the pump-controlled motor system with the bypass flow control are presented under a constant resistive and an inertial load. The test results showed that the bypass flow control could reduce the overshoot of the hydraulic motor rotational speed by about 50%. The relative efficiency of the circuit with the bypass flow control system was 1% to 5% lower for the particular pump-controlled system that was used in this study. For a pump/motor that does not demonstrate significant flow ripple of the magnitude experienced in this study, the relative efficiency would be the same as the pump/motor system without bypass. It was concluded that the proposed bypass control system, combined with the DC motor-swashplate driven pump, could be used to create an energy efficient circuit with excellent dynamic transient responses.

Terrorism has been an international threat to high occupancy civilian structures, government buildings, and military installations for many years. Statistical data from past terrorist attacks show that transportation infrastructure has been widely targeted, and a bombing of an ordinary highway bridge is a realistic scenario. Recent threats to bridges in the U.S. confirm this concern and have caught the attention of the bridge engineering community. Given that many ordinary highway bridges in the United States support critical emergency evacuation routes, military transportation plans, and vital economic corridors, the loss of a key bridge could result in severe national security, economic, and socioeconomic consequences. Therefore, in this research, a simplified procedure is developed to predict blast loads on bridge columns, and an understanding of the mechanisms that cause damage and ultimately failure of blast-loaded reinforced concrete bridge columns is advanced. To that end, computational fluid dynamics models are constructed and validated using experimental data. These numerical models are used to characterize the structural loads experienced by square and circular bridge columns subjected to blast loads, which is followed by the formulation of a simplified load prediction procedure. Additionally, nonlinear, three-dimensional, dynamic finite element models of blast-loaded reinforced concrete bridge columns are developed and validated using qualitative and quantitative data from recent experimental tests. The results of these analyses illustrate the fact that circular columns cannot be assumed to experience less base shear demand than a square column simply because they experience less net resultant impulse. Furthermore, the column response models developed in this research are used to identify and explain the mechanisms that lead to the spalling of side cover concrete off blast-loaded reinforced concrete members observed in recent experimental tests. Therefore, the results of this research advance the understanding of the structural loads on and the resulting response of reinforced concrete bridge columns subjected to blast loads, and as such these contributions to the structural engineering community enhance the security of the U.S. transportation infrastructure. / text

The Alpha Magnetic Spectrometer is an International Space Station Experiment that features a unique nonlinear support system with no previous flight heritage. The experiment consists of multiple straps with piecewise-linear stiffness curves that support a cryogenic magnet in three-dimensional space inside of a vacuum chamber. The stiffness curves for each strap are essentially bilinear and switch between two distinct slopes at a specified displacement. This highly nonlinear support system poses many questions in regards to feasible computational methods of analysis and possible response behavior. This thesis develops a numerical model for a multi-bilinear-spring support system motivated by the Alpha Magnetic Spectrometer design. Methods of analysis applied to the single bilinear oscillator served as the foundation of the model developed in this thesis. The model is developed using MATLAB and proves to be more computationally efficient than ANSYS finite element software. Numerical simulations contained herein demonstrate the variety of response behaviors possible in a multi-bilinear-spring support system, thus aiding future endeavors which may use a support system similar to the Alpha Magnetic Spectrometer. Classic nonlinear responses, such as subharmonic and chaotic, were found to exist.