Studies of soil-structure and fluid-structure interaction
Prasad, Anumolu Meher
Veletsos, Anestis S.
Doctor of Philosophy
This dissertation deals with two distinct topics: (1) The effects of soil-structure interaction, both kinematic and inertial, on the dynamic response of a variety of base-excited foundations and of simple structures supported on such foundations; and (2) the effects of fluid-structure interaction for relatively simple structural systems subjected to forces induced by waves and currents. A fundamental step in the analysis of a base-excited structure-foundation system is the evaluation of the transfer functions of its foundation motion. Defined for harmonically excited massless foundations, these functions relate the amplitudes of the components of foundation motion to those of the free-field ground motion at some reference or control point. These functions are evaluated for surface-supported circular and rectangular rigid foundations and for embedded square foundations considering a spatially varying, horizontal free-field ground motion. Consideration is also given to more complex ground motions defined stochastically by a local power spectral density function and a spatial incoherence function. An approximate analyses based on the Iguchi-Scanlan averaging technique is employed. The structures examined are considered to have one lateral and one torsional degree of freedom in their fixed-base condition. The response quantities examined include the ensemble means of the peak values of the lateral and torsional components of the foundation input motion and of the associated structural de formations. These responses are evaluated over wide ranges of the parameters involved and are compared with those obtained for no soil structure interaction and for kinematic interaction only. Simple, physically motivated interpretations are given for the observed differences. The studies of fluid-structure interaction include comprehensive analyses of the differences in the responses of simple models of offshore structures computed by the standard and extended versions of Morison's equation for the hydrodynamic forces, and of the effects and relative importance of the numerous parameters involved. The responses are also evaluated by the equivalent linearization technique and Penzien's decoupling technique, and the interrelationship and accuracy of these approaches are elucidated. In addition, the decoupling technique is generalized to include consideration of a current of constant velocity, and a simple modification is proposed which improves the accuracy of this procedure. A simple approximation is included for the hydrodynamic modal damping values of multi-degree-of-freedom, stick-like systems.