The design spectrum used in earthquake resistant design of structures is made by modifying the response spectrum. The inelastic design spectrum is obtained from dividing the elastic design spectrum by response modification factors which mainly depend on the permissible level of ductility. The design spectrum in ATC3-06 is specified based on peak ground acceleration and peak ground velocity. But the design spectrum in UBC-1988 is specified based on peak ground acceleration only.
The first object of this study is to evaluate the inelastic design spectra for various response modification factors by observation of the ductilities and the number of yield events, the second object of this study is to compare and evaluate the design spectra which are specified in ATC3-06 and UBC-1988. The ductility requirements and the number of yield events are calculated from inelastic analysis based on yield resistance of the inelastic systems. Namely, the design base shear obtained by multiplying the design spectrum by total weight of structure is considered as the yield resistance. Earthquake records used as input ground motion are classified into three groups such as high, medium and low ratio of peak ground acceleration to peak ground velocity according to there characteristics.
Consequently, the inelastic design spectrum with large response modification factor is expected to bring about large ductilities and may yield events for short period structures. The ductility requirements an the yield events expected for the design spectrum specified in UBC-1988 are remarkably different according to a/v ratios in comparison to those for the design spectrum in ATC3-06 for long period structures, therefore the design spectrum in UBC-1988 is not expected to control the yield displacement properly in comparison with the design spectrum in ATC3-06 for long period structures.
