The authors concern includes two main points in the subject of design response spectra generation for secondary systems in nuclear power plant structures: the consideration of inelastic behavior in the secondary systems materials and the production of uniformly probable design response spectra. One works with a previously developed secondary system model attached to primary structure model tuned to the frequency range of a nuclear power plant building. A global ductility factor is formulated relating the plastic to the overall work done by the seismic external forces on the secondary system. This factor together with a particular definition of the yielding factor allows one to determine elastic to inelastic spectrum transpose factors. A methodology is proposed to generate uniformly probable coupled response spectra for multiply supported inelastic secondary systems. The seismic excitation is prescribed by a target power spectrum density function of the ground acceleration and an internal pressure condition is added to the seismic action. Examples illustrate the application of this proposed methodology.