Floor response spectrum method of multiply supported piping system assisted by time history analysis

The uniform support motion (USM) method is generally used for multiply supported nuclear piping system. This method uses a uniform response spectrum (URS) which envelopes all of the individual response spectra and can result in considerable overestimation of seismic responses. An alternate method is the independent support motion (ISM) approach. This approach can also result in overestimation when the maximum responses by multiple excitations are combined by the absolute sum rule, while this may result in underestimation when the maximum responses by the multiple excitations are combined by the square root of sum of squares rule. Then authors have developed a new method of the ISM approach named SATH (Spectrum Method Assisted by Time History Analysis) to achieve a more realistic combination of the maximum responses by the multiple excitations. In the SATH method, both of floor response spectra and floor acceleration time histories are used as seismic input data. The maximum modal responses by the multiple excitations are combined with the effect of correlation coefficients. In order to account for the correlation coefficients, the time history analysis of an oscillator having each of modal frequencies of the piping system is performed using each of the floor acceleration time histories. The correlation coefficients are calculated from the covariance and the standard deviations of time history responses of the oscillators. In this paper, the necessity of taking the effects of the correlation coefficients into account in the ISM approach is discussed, and then the advantage and the applicability of the SATH method to an actual design is confirmed.