Browsing by Author "Tein, When-Yen"
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Item An engineering approach for estimating seismic power spectra(1992) Tein, When-Yen; Spanos, Pol D.The aseismic design of large structures may need to account for the nonstationary and the multivariate aspects of strong ground motions. These aspects influence both the time and the spatial variation of the dynamic response of the structure. Current techniques for treating this problem have considerable limitations. An approach is developed in the thesis for addressing this issue based on energy considerations. Estimates of evolutionary seismic auto- and cross-spectra are obtained. It is assumed that the available accelerograms are realizations of broad-band stochastic processes. By analyzing the output of lightly damped linear systems excited by these processes, estimates are obtained for the associated auto and cross spectral density functions. The proposed estimation techniques have two main advantages. Firstly, the evolutionary auto- and cross-spectra can be estimated without having to assume a specific form for the evolutionary spectrum. Secondly, with the evolutionary auto- and cross-spectra determined, the moments of structural responses to these stochastic seismic models, which are functions of time, can be approximated from the spectra without having to resort to simulation. The proposed method is further extended to provide for situations where not enough data are available to accurately assess the stochastic character of the ground motion. Based on a stochastic interpretation of ground accelerations, a weighted least squares method is used to estimate the coefficients of a commensurate model from the responses of a lightly damped linear system excited by these accelerograms.Item APPROXIMATE HARMONIC ANALYSIS OF MARINE RISERS(1987) Tein, When-YenA model of a marine riser system which is appropriate for the study of its dynamic behavior in deep water conditions is developed. This model is used to estimate the steady-state riser response to harmonic excitation. In this regard, the finite element method yields a linear discrete multi-degree-of-freedom structural model; its stiffness matrix varies with time due to riser top tension fluctuations. Further, Morison's equation is used for estimating the hydrodynamic load on the riser. Due to the nonlinearity of the drag term appearing in this equation, the riser equation of motion becomes nonlinear. An approximate analysis procedure based on the concepts of equivalent linearization and of time averaging leads to efficient determination of the riser maximum stress. A continuous beam model under constant tension which is equal to the average of the top and the bottom riser tension is used to provide an estimate of its natural frequencies. Numerical results from a variety of parameter studies are reported.