The available earthquake records are classified into five groups according to their site stiffness and epicentral distance as the grouping parameters. For the groups thus defined, normalized response spectra are obtained for single-degree-of-freedom and massless oscillators. The effectiveness of the grouping scheme is examined by studying the variance of response quantities within each group. The implicit parameters of average frequency and significant duration are obtained for each group and their effect on the response spectra is studied. Correlation analyses between various ground motion characteristics such as peak displacement, velocity, acceleration and root mean square acceleration are carried out for each group.

Smoothed design spectra for relative and pseudo velocities and relative acceleration responses of single degree of freedom oscillators and the velocity and acceleration responses of massless oscillators are proposed for each group. Methods to predict relative velocity and relative acceleration spectra directly from the pseudo velocity spectra are presented. It is shown that the relative spectra can be reliably estimated from the pseudo spectra. The site dependent design spectra are defined for a wide range of oscillator periods and damping ratios.