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No description available.

Over the past thirty years, extensive research has been conducted with the objective of reducing wind damage to structures. Wind tunnel simulations of wind loads have been the major source of building codes. However, a simple comparison of pressure coefficients measured in wind tunnel simulations with full-scale measurements show that the simulations, in general, underpredict extreme negative pressure coefficients. One obvious reason is the lack of consensus on wind tunnel simulation parameters. The wind in the atmospheric surface layer is highly turbulent. In simulating wind loads on structures, one needs to simulate the turbulent character besides satisfying geometric and dynamic similitudes. Some turbulence parameters that have been considered in many simulations include, turbulence intensities, integral length scales, surface roughness, and frequency spectrum. One problem with these parameters is that they are time varying in the atmospheric boundary layer and their averaged value, usually considered in the wind tunnel simulations, cannot be used to simulate pressure peaks. In this work, we show how wavelet analysis and time-scale representation can be used to establish an intermittency factor that characterizes energetic turbulence events in the atmospheric flows. Moreover, we relate these events to the occurrence of extreme negative peak pressures. / Ph. D.

The understanding of how structural components endure loads, in particular variable loads, is that these components gradually, over some period of time depending on the nature of the loading and the material, develop a microcrack. After some additional time and loading, the microcrack grows to a size that might be detected. Beyond that point, the microcrack propagates in a manner that can be reliably predicted by computer analysis codes. Consequently, one can define different stages for the life of a structural component. These are: 1) the period prior to the formation of a microcrack, 2) the period of microcrack growth, and finally 3) the period of crack growth. To date, structural health monitoring approaches that seek to detect cracks offer no insight into the extent of deterioration occurring in the initial stage that is a precursor to the formation of the microcrack or its growth. However, an approach that would facilitate monitoring the extent of the deterioration that takes place during this stage promises to improve life prediction capabilities of structural components. The challenge, thus, is to develop quantitative assessment of damage accumulation from the earliest stages of the fatigue process and to provide a structure's signature that is dependent of the damage stage. One such signature is the structure's response to forced excitation. The realization of such a goal would help in advancing structural health monitoring procedures using interrogative system identification techniques and determine sensitivities of physical parameters to damage. Additionally, vibration-based spectral quantities are related to physical properties of the structure under test. In this thesis, nonlinear response to parametric excitation is exploited for nonlinear system identification of metallic and composite beam-mass systems before damage initiation through intermediate states of damage progression to failure. Parametric identification procedure combines linear and higher order spectral analysis of vibration measurements and perturbation techniques for the derivation of the approximate solution of the system nonlinear governing differential equation. The possibility of using optical Fiber Bragg Grating sensors technology for damage localization is also assessed. Spectral moments and quantities obtained from fiber optic strain measurements are evaluated near and away from cracks to assess the relation between these moments and cracks. Variations in parameters representing natural frequency, damping and effective nonlinearities for different levels of progressive damage in a beam-mass system have been determined. Their percentage variations have been quantified to establish their sensitivities to damage initiation. The results show that damping and effective nonlinearity parameters are more sensitive to damage conditions than the natural frequency of the first mode. Crack localization is assessed by means of optical fiber technology for a composite beam-mass system. The results show that noise levels in fiber optic signals are high in comparison to strain gage signals. Of particular interest, however, is the observation that the nonlinear response is more pronounced near the cracks than away from them. / Ph. D.

In this work, the random vibration of higher-order nonlinear beams and composite plates subjected to stochastic loading is studied. The fourth-order nonlinear beam equation is examined to study the effect of rotary inertia and shear deformation on the root mean square values of displacement response. A new linearly coupled equivalent linearization method is proposed and compared with the widely used traditional equivalent linearization method. The new method is proven to yield closer predictions to the numerical simulation results of the nonlinear beam vibration. A systematical investigation of the nonlinear random vibration of composite plates is conducted in which effects of nonlinearity, choices of different plate theories (the first order shear deformation plate theory and the classical plate theory), and temperature gradient on the plate statistical transverse response are addressed. Attention is paid to calculate the R.M.S. values of stress components since they directly affect the fatigue life of the structure. A statistical data reconstruction technique named ARMA modeling and its applications in random vibration data analysis are discussed. The model is applied to the simulation data of nonlinear beams. It is shown that good estimations of both the nonlinear frequencies and the power spectral densities are given by the technique. / Ph. D.

The full-scale value of the Reynolds number associated with wind loads on structures is of the order of 10^7. This is further complicated by the high levels of turbulence fluctuations associated with strong winds. On the other hand, numerical and wind tunnel simulations are usually carried out at smaller values of Re. Consequently, the validation of these simulations should only be based on physical phenomena derived with tools capable of their identification. In this work, two physical aspects related to extreme wind loads on low-rise structures are examined. The first includes the statistical properties and prediction of pressure peaks. The second involves the identification of linear and nonlinear relations between pressure peaks and associated velocity fluctuations. The first part of this thesis is concerned with the statistical properties of surface pressure time series and their variations under different incident flow conditions. Various statistical tools, including space-time correlation, conditional sampling, the probability plot and the probability plot correlation coefficient, are used to characterize pressure peaks measured on the top surface of a surface-mounted prism. The results show that the Gamma distribution provides generally the best statistical description for the pressure time series, and that the method of moments is sufficient for determining its parameters. Additionally, the shape parameter of the Gamma distribution can be directly related to the incident flow conditions. As for prediction of pressure peaks, the results show that the probability of non-exceedence can best be derived from the Gumbel distribution. Two approaches for peak prediction, based on analysis of the parent pressure time series and of observed peaks, are presented. The prediction based on the parent time series yields more conservative estimates of the probability of non-exceedence. The second part of this thesis is concerned with determining the linear and nonlinear relations between pressure peaks and the velocity field. Validated by analytical test signals, the wavelet-based analysis is proven to be effective and accurate in detecting intermittent linear and nonlinear relations between the pressure and velocity fluctuations. In particular, intermittent linear and nonlinear velocity pressure relations are observed over the nondimensional frequency range fH/U<0.32. These results provide the basis for flow parameters and characteristics required in the simulation of the wind loads on structures. / Ph. D.

A parametric identification procedure is proposed that combines the method of multiple scales and higher-order statistics to efficiently and accurately model nonlinear systems. A theoretical background for the method of multiple scales and higher-order statistics is given. Validation of the procedure is performed through applying it to numerical simulations of two nonlinear systems. The results show how the procedure can successfully characterize the system damping and nonlinearities and determine the corresponding parameters. The procedure is then applied to experimental measurements from two structural systems, a cantilevered beam and a three-beam frame. The results show that quadratic damping should be accounted for in both systems. Moreover, for the three-beam frame, the parametric excitation is much more important than the direct excitation. To show the flexibility of the procedure, numerical simulations of ship motion under parametric excitation are used to determine nonlinear parameters govening the relation between pitch, heave, and roll motions. The results show a high level of agreement between the numerical simulation and the mathematical model with the identified parameters. / Master of Science

Technological/engineering design based learning (T/E DBL) provides a context in which students may utilize content knowledge and skills to develop prototype solutions to real-world problems. In science education, design based science (DBS) utilizes technological/engineering design based approaches in science education as a means for enhancing the purpose of and relevance for scientific inquiry by contextualizing it within the goal of developing a solution to a real-world problem. This study addressed the need to investigate the ways in which students utilize higher order thinking skills, demonstrated through the use of knowledge associated with declarative, schematic, and strategic cognitive demand when in engaged in DBS activities. The purpose of this study was to determine what relationships exist between engagement in DBS and changes in students' depth of understanding of the science concepts associated with the development of design solutions. Specifically, the study determined how students' abilities to demonstrate an understanding of the science concepts, required by assessments of different cognitive demand, change as they were engaged in a design-based science unit associated with heat transfer. Utilizing two assessment instruments, a pre/post-1/post-2 test and content analysis of student design portfolios based on Wells (2012) and utilizing Li's (2001) system to code student responses, the following research question was addressed: What changes in students' science concept knowledge (declarative, schematic, and strategic demand) are evidenced following engagement in design based learning activities? Although the results are not generalizable to other populations due to the limitations associated with the study, it can be concluded that design based learning activities incorporated in science courses can foster higher order thinking. Results from the study suggests that students' abilities to demonstrate their understanding of certain science concepts through higher order thinking, including utilizing concept knowledge strategically in open-ended problem solving, increased following engagement in design based learn activities. Results have implications in technological/engineering design education, in science education, and in integrative STEM education. Implications include the utility of design portfolios as both an assessment instrument and learning tool to ensure that concept knowledge is explicitly connected to and used in the design activity. / Ed. D.