Floor response spectra from spectrum compatible motions

Floor response spectra are commonly used for the design of equipment, piping systems, control intruments or other sub-systems located in nuclear power plants. An artificial or modified real time-history of motion, compatible with a set of ground response spectra, is often used to generate these floor response spectra. This paper shows that even using a modified real time history, closely matching a given ground response spectrum, large variations of the floor response spectra can result. The paper discusses the cause of such variations and aims at finding ways to deal with them. A reactor building similar to that used in CANDU nuclear power plants, is chosen as an example. The reactor building is idealised by a simple stick model with five lumped masses. The N/S component of the El Centro (1940) earthquake time-history is modified by superposition of output waves to make it compatible with a given ground response spectrum. Floor response spectra are determined, at a number of locations, using the modified El Centro motion. To account for the variation of structural frequencies, due to uncertainties of material properties and approximations in the modelling technique used, the frequency is varied slightly and the floor response spectra are computed for each frequency. A comparison of the resulting floor response spectra shows that by changing the structural frequency slightly, not only can the frequencies of peak response be shifted, but also the value of the peak can be changed considerably. It is therefore, necessary to smooth the floor response spectra and to broaden the peaks