Effect of Base Uplift on Dynamic Response of SDOF Structures

In this study, a parametric investigation for the case of a single-degree-of-freedom (SDOF) oscillator and a harmonic excitation is performed. The simplest case of a rigid base on a rigid foundation is examined first. Based on the results of this analysis, an empirical foumula is established, which permits the computation of the maximum of deformation. Using this formula, an approximate method for the estimation of earthquake-response spectra for uplifting structures utilizing linear-repsonse spectra is proposed. This method gives reasonably good results for all periods. The analysis is then extended to the case of flexible foundations. The behavior of the system described here is nonlinear overall, although the equations of motion can be linearized in each of the two regimes of the response, during full contact and after uplift. The details of the study are described, and the results are discussed.