Simplified methods for predicting seismic basemat uplift of nuclear power plant structures

This paper presents (a) the result of an analytical study on the seismic response of a nuclear power plant structure considering basemat uplift effect, and (b) two simplified methods subsequently developed for the prediction of the maximum amount of basemat uplift. From the result of the analytical study, these semi-empirical methods were developed whereby estimates can be made of the maximum amount of basemat uplift based on the linear response result. The methods provide a convenient means for structural design purposes without having to perform costly nonlinear time history analysis for cases where the amount of uplift is within 50% of the basemat dimension. Both upper-bound and lower-bound estimates predicted by the simplified methods are given, and they are compared to results of nonlinear time history analyses in order to examine their accuracy. The lower-bound estimate is based on the technique of energy balance between linear and nonlinear soil rocking strain energies. The upper-bound estimate is based on the energy balance technique but the result is further corrected using the instantaneous secant rocking stiffness of the foundation. General observation on the result of analytical study are presented. The results indicate the general trend that for cases where the amount of basemat is not substantial, the linear analysis and nonlinear analysis give comparable results in peak base shears, peak overturning moments, and in-structure response spectra; however, as the amount of basemat uplift increases, peak base shears and peak overturning moments predicted by nonlinear analysis are smaller than those predicted by linear analysis; the in-structure response spectra obtained by non-linear analysis gives lower peaks in low frequency range and higher peaks in high frequency range as compared to the linear analysis result. (orig.)