Steady State Response of Thin-walled Members Under Harmonic Forces

Title: Steady State Response of Thin-walled Members Under Harmonic Forces
Authors: Mohammed Ali, Hjaji
Date: 2013
Abstract: The steady state response of thin-walled members subjected to harmonic forces is investigated in the present study. The governing differential equations of motion and associated boundary conditions are derived from the Hamilton variational principle. The harmonic form of the applied forces is exploited to eliminate the need to discretize the problem in the time domain, resulting in computational efficiency. The formulation is based on a generalization of the Timoshenko-Vlasov beam theory and accounts for warping effects, shear deformation effects due to bending and non-uniform warping, translational and rotary inertial effects and captures flexural-torsional coupling arising in asymmetric cross-sections. Six of the resulting seven field equations are observed to be fully coupled for asymmetric cross-sections while the equation of longitudinal motion is observed to be uncoupled. Separate closed form solutions are provided for the cases of (i) doubly symmetric cross sections, (ii) monosymmetric cross-sections, and (iii) asymmetric cross-sections. The closed-form solutions are provided for cantilever and simply-supported boundary conditions. A family of shape functions is then developed based on the exact solution of the homogeneous field equations and then used to formulate a series of super-convergent finite beam elements. The resulting two-noded beam elements are shown to successfully capture the static and dynamic responses of thin-walled members. The finite elements developed involve no special discretization errors normally encountered in other finite element formulations and provide results in excellent agreement with those based on other established finite elements with a minimal number of degrees of freedom. The formulation is also capable to predict the natural frequencies and mode-shapes of the structural members. Comparisons with non-shear deformable beam solutions demonstrate the importance of shear deformation effects within short-span members subjected to harmonic loads with higher exciting frequencies. Comparisons with shell element solution results demonstrate that distortional effects are more pronounced in cantilevers with short spans. A generalized stress extraction scheme from the finite element formulation is then developed. Also, a generalization of the analysis procedure to accommodate multiple loads with distinct exciting frequencies is established. The study is concluded with design examples which illustrate the applicability of the formulation, in conjunction with established principles of fatigue design, in determining the fatigue life of steel members subjected to multiple harmonic forces.
CollectionThèses, 2011 - // Theses, 2011 -