The target spectrum which has been used most frequently for the seismic analysis of structures is the uniform hazard response spectrum (UHRS). The joint occurrence of the spectral values in different periods, in the development of UHRS, is a key assumption which remains questionable. The conditional mean spectrum (CMS) has been recently developed by Baker et al. as an alternative for UHRS. The CMS provides the expected response spectrum conditioned on the occurrence of the target spectral acceleration value in the period of interest which can be accounted as an improvement of the UHRS. In order to enhance the CMS, the correlation between the peak ground velocity (PGV) and the spectral acceleration values has been investigated in the current study, and finally, a newer form of target spectrum has been proposed. It is shown that the emerged new spectrum, named Eta-based conditional mean spectrum (E-CMS), is more efficient than the conventional CMS in order to enhance the UHRS. The nuclear industry design guidelines (i.e. Nuclear Regulatory Commission Guides 1.165 and 1.208) provide an alternative procedure based on UHRS for defining the design spectrum which has been compared with the proposed CMS and E-CMS. The results show that the alternative procedure might be somehow conservative for stiff structures such as nuclear facilities.
