Modal Spectral Calculation of Floor Response Spectra for Secondary Systems

Abstract eng:
The calculation of the dynamic response of a mechanical system considered as a secondary system attached to a structure can be a very time consuming task for complex systems. This paper deals with the calculation of the response spectra at the attachment points between the primary and the secondary system, which will be called “Floor Response Spectra (FRS).” The computed floor response spectra at the attachment points of the primary system are intended to be used as input for a second step analysis of the mechanical system. Here in, a modal spectral procedure for the calculation of FRS on the primary system is presented. The method developed considers the natural vibration frequencies, mode shapes, and modal participation factors of the primary structure to describe the primary system and combines them with a physical representation of a single degree of freedom secondary system considering a spectral procedure with non-classical damping where the crossover terms of the coupling of the two systems are neglected to obtain the FRS. Parametric studies of the importance of the coupling terms and of the correlation of the response between the primary and the secondary systems are presented. A comparison of the results of this approach with response spectrum and time history analysis results obtained using the complete model (primary plus secondary systems together) are presented. As a practical application the procedure is used for the analysis of a steam turbine - generator and its foundation for a thermo-electrical power plant where the steam turbine–generator and the reinforced concrete foundation are considered as the secondary and the primary system, respectively.

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