SDOF displacement ductility demands based on smooth ground motion response spectra

This paper investigates the effect of the differences between “smooth” (e.g., design) earthquake ground motion acceleration response spectra and individual ground motion linear-elastic acceleration response spectra on nonlinear single-degree-of-freedom (SDOF) lateral displacement ductility demands. A significant amount of previous research exists on the development of relationships between the lateral strength and peak displacement of SDOF systems based on statistical studies. Traditionally, the lateral strength of the SDOF systems in these studies has been determined from individual ground motion acceleration response spectra, requiring that the structure strength is recalculated for each ground motion used in an ensemble. While the relationships developed using this approach are important, in practical seismic design applications, the lateral strength of a structure does not vary with the individual ground motions considered in an ensemble. Rather, the lateral strength is often determined using smooth design response spectra or site response analyses as prescribed by model building seismic design provisions. This paper focuses on this issue. It is shown that peak SDOF displacement ductility demand relationships developed based on individual ground motion spectra can lead to unconservative designs, particularly for survival-level, soft soil, and near-fault conditions.