For a bi-linear SDOF system subjected to a specific wave form of the ground acceleration, the unique yielding pseudo spectral acceleration and spectral displacement (Say, Sdy) together with various inelastic responses (Sai, Sdi) can be obtained via nonlinear time history analyses, respectively, by tuning the different levels of peak ground acceleration as various input ground motions. Meanwhile, the corresponding elastic responses (Sae, Sde) of a linear SDOF system with the identical mass, viscous damping and elastic stiffness as those of the bi-linear one can also be determined through linear time history analyses under the same excitations. The proposed NSAD format shown on the diagram of the elastic force ratio, Ω=Sae/Say I Say, versus the ductility ratio, (μ= (Sdi/Sdy), is a dimensionless plot of the seismic demands suitable to the engineers who are familiar with the conventional force-based design using linear structural analysis. In this paper, more than two hundred ground motions recorded in the Chi-Chi earthquake, Taiwan (1999) were chosen as the seismic inputs for the establishment of the NSAD format. The characteristics and applications of the NSAD format on the performance-based seismic design of the bridge structures were discussed, and realistic procedures for the methodology were proposed.
The results obtained shows that the NSAD format can help the engineers evaluate the multiple-level seismic demands not only with a well precision but also with a great convenience.