A study of errors involved in seismic analysis of secondary systems.

Abstract

The study deals with the sources of error in seismic modal analysis of primary-secondary coupled systems using the 'combination of modal properties' approach. Four sources of error are studied: non-classical damping, evaluation of modal properties of the combined system, combination of modal responses and response contribution from the high frequency modes. Harmonic response of a non-classically damped system provides a valuable insight into its response to a general load. For a non-harmonic load, the error involved in diagonalization depends on its frequency content. The dependence of the error on the applied load increases with the severity of non-classical nature. Use of approximate mode shapes obtained by perturbation is an attempt to transform the equations of motion using a transformation matrix that is different from the exact-mode-shape matrix. The error involved in an attempted modal superposition analysis for such a case mainly stems from the neglect of off-diagonal terms in the transformed mass, stiffness and damping matrices. The phenomenon of closely spaced modes in primary-secondary coupled systems and consequent correlation between modal responses is discussed. (Abstract shortened by UMI.)