Abstract
The spread foundations are expected to generate restoring forces such that the seismic loads do not significatnly increase for a level of horizontal displacement of the structure after yielding, because of the lifting of the footing edges and the plastic behavior of the ground in contact with the footing. Therefore, those structures with spread foundations are expected to be advantageous in terms of the seismic performance owing to the elongation of the natural period and enhanced damping caused by energy absorption, which are effectively similar to the seismic isolation mechanism. Moreover, those effects can be enhanced with the increase of the seimic ground motion amplitude, making the dynamic performance even more preferable under extreme seismic events. In this paper, the characters of the effective seismic isolation of the spread foundation are evaluated by thoretical assessment using the equivalent linearization method and by nonlinear dynamic analysis. It is successfully shown that the variation of the dynamic amplification factors caused by the nonlinearity of the spread foundations can be represented by the change of th equivalent stiffness and equivalent damping, and that the abrupt change of the dynamic response amplitude of the structure observed in the numerical simulation can be well accounted by the bifurcation of the dynamic amplification curves derived by the nonlinear amplification factors.
