This paper reports the main results of an extensive parametric study using numerical simulations and computing displacement ductility demand of nonlinear single-degree of freedom (SDOF) systems and multi-degree of freedom (MDOF) systems for a set of 164 registered ground motions. The objective of this study is to propose values of strength reduction factors for rocking behavior for seismic analysis. In the first part focused on SDOF systems, non-linear seismic responses obtained with a hysteretic model simulating rocking are statistically compared with the ones related to established hysteretic models for ductile structures. Similar to established hysteretic models, results confirm that the frequency has little influence on the ductility demand if it is below 2 Hz and a substantial influence if it is above 2 Hz. Moreover, they show that the other parameters, especially the hysteretic behavior model, have only little influence on the displacement ductility demand. Surprisingly, displacement ductility demand is found to be practically independent of the additional viscous damping ratio. Finally, a relationship between displacement ductility demand and strength reduction factor for rocking systems is proposed. The second part shows that the results obtained for SDOF systems are also valid for MDOF systems.