Foundation Impedances

Abstract

The response to wave loading of a steel-jacket platform supported on flexible foundations may be significantly different from that of the platform supported on rigid base. This happens because the natural frequency and mode shape in the two cases can be quite different from each other and the frequency parameter has important bearing on the dynamic magnification of the structural response. The resistance of pile to vibratory motion is provided by the steel pile along with its interaction with the surrounding soil medium. While in the case of vertical motion the resistance developed by the pile is given by end bearing, skin-friction or by a combination of the two, in case of the pile subjected to horizontal forces the resistance is provided by horizontal bearing of the pile against the soil. The vibratory motion of the soil-pile subsystem is restricted because of significant loss of energy in soil medium. The damping of the pile motion occurs because of: (a) the radiation of wave energy into the far field and (b) the loss of energy due to internal friction of soil. Thus the accountancy of both the radiation damping and the material or hysteresis damping are essential in an accurate estimation of dynamic response of pile-supported structures.