Predicting the earthquake ductility demand through a rigid-plastic approach

Assessing the earthquake ductility of seismic resistant structures usually requires a non-linear dynamic analysis involving both elastic and plastic motion of the structure. A simpler way to estimate the inelastic displacements can be neglecting the elastic motion altogether and referring to a rigid-plastic model. The latter may in fact give a good estimate of the maximum plastic displacement for elastoplastic oscillators of a comparatively short period in the elastic range. The same model also bounds the plastic response for sufficiently high periods of the oscillators. For medium-period oscillators, however, the rigid-plastic approximation needs to be corrected. Recently, the authors presented a simple procedure to predict in which ranges of periods the rigid-plastic approximation can be adopted as it stands. Subsequently, they also provided an empirical formula to obtain a suitable correction to apply outside these ranges. Both contributions make the rigid-plastic approach ready to be applied in practice. By referring to some real earthquakes, the present paper applies this approach to various elastoplastic oscillators. The results found show that the rigid-plastic approach proposed by the authors gives quite good –and almost always conservative- predictions of the maximum inelastic displacements of the elastoplastic oscillators.