A scaling method for generating floor response spectra

Abstract

Efficient and accurate determination of Floor Response Spectra (FRS) is critical in seismic risk analysis and design. Scaling methods, which allow the generation of FRS corresponding to new Ground Response Spectra (GRS) by multiplying existing FRS with appropriate scaling factors rather than by performing re-analyses of the structure, are efficient and economical approaches. However, in many practical situations, it is challenging to generate FRS not only by scaling but also by analyzing structural responses due to the lack of structural model information.A recent breakthrough in generating FRS using a direct spectra-to-spectra method prompted the development of the scaling method presented in this paper. The analytical formulation of the direct method provides a strong physical insight into the essential characteristics of FRS, which allows the identification of dynamical information of significant equivalent modes of the underlying structure from the available FRS and GRS. Scaling factors are then determined in terms of the dynamical information (modal frequencies, modal damping ratios, and modal contribution factors) and input GRS. Numerical examples of a typical service building in nuclear power plants show that the FRS obtained by this scaling method agree very well with the results obtained from a large number of time history analyses. It is also demonstrated that this method provides accurate FRS for various damping ratios when the interpolation methods recommended in the current standards are not applicable. The proposed method, which is efficient, accurate, and convenient to implement, allows engineers to use as much of the available results of previous analyses as possible without performing a complete dynamic analysis, which is time consuming and introduces extra costs.