After a brief review of the principal effects of soil-structure interaction on the response of structures subjected to earthquakes, a simple practical procedure is presented for evaluating these effects. The procedure is next applied to a study of the response of a proposed 400-ft. steel gravity platform (for a 300' water depth location), and the results of the study are discussed. The structure investigated consists of three large-diameter legs of tubular construction interconnected by a truss-type bracing system and supported on three circular pads.
In evaluating the response of structures to earthquakes, it?s normally assumed that the motion which is experienced by the base of the structure is the same as the free-field ground motion. The latter term refers to the motion which would take place at the level of the foundation if no structure was present. Strictly speaking, this assumption is valid only for structures supported on rigid ground.
For structures supported on soft soil, the foundation motion is generally different from the free-field ground motion and may include a rocking component in addition to a lateral or translational component. The rocking component may be particularly significant for tall structures.
Two factors are responsible for the difference in the responses of a rigidly supported structure and an elastically supported identical structure. First, the elastically supported structure has more degrees of freedom than the rigidly supported structure and hence different natural frequencies and modes of vibration. Second, a substantial part of the vibrational energy of the elastically supported structure may be dissipated into the supporting medium by radiation of waves and by hysteretic action in the soil itself. There is, of course, no counterpart of this effect of energy dissipation in a rigidly supported structure.
The soil-structure interaction effects investigated herein represent the difference in the responses of a structure computed by (1) assuming the motion of its foundation to be the same as the stipulated free-field ground motion, and (2) considering the modified or actual foundation motion, including the effects of energy dissipation in the supporting medium. This difference depends on the characteristics of the free-field ground motion, as well as on the properties of the structure and the supporting soils.
The first objective of this paper is to identify the principal effects of soil-structure interaction on the response of structures to earthquakes, and to review briefly a simple practical procedure for providing for these effects in design. The procedure, which utilizes response spectra for fixed-base systems, has provided the basis of the relevant provisions for earthquake-resistant building design recommended recently by the Applied Technology Council, and is believed to be ideally suited for preliminary design studies.
