Floor Spectra for Nonclassically Damped Structures

Abstract

An approach is presented for the generation of seismic floor response spectra for structures which cannot be modeled as classically damped. Although the nonclassically damped structures do not possess classical modes, the proposed approach can still employ prescribed ground response spectra directly as input for the generation of floor spectra. The approach requires the solution of a complex eigenvalue problem which provides the information about the equivalent modal frequencies and damping ratios of the structure. The floor response spectrum value is expressed in terms of these equivalent modal parameters. The seismic input is required to be defined in terms of psuedo acceleration and relative velocity ground spectra for the equivalent modal frequencies and damping ratios of the structure and also at the oscillator frequency and its damping ratio. To verify the applicability of the proposed approach in view of simplifying assumptions made in this development, a numerical simulation study has been conducted. The results obtained by the proposed direct approach and the time history approach for an ensemble of acceleration time histories as inputs compare very well with each other, thus validating the applicability of the proposed approach.